U.S. Department ofCommerce

Carlos M. Gutierrez, Secretaiy

Technology Administration

Robert Cresanti, Under Secretaiy of Commercefor Technology

National Institute ofStandards and Technolog}'

William Jeffrey, Director

Certain commercial entities, equipment, or materials may be identified in this document in order to describe an

experimental procedure or concept adequately. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment used are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 1052

Natl. Inst. Stand. Technol. Spec. Publ. 1052, 186 pages (August 2006)

CODEN: NSPUE2
NIST Special Publication 1052

Accomplishments

in Nanoteciinology

Compiled and Edited by:

Michael T. Postek,

Assistant to the Director

for Nanotechnology,

Manufacturing Engineering

Laboratory

Joseph Kopanski,

Program Office

and

David Wollman,

Electronics and Electrical

Engineering Laboratory

U. S. Department of Commerce

Technology Administration

National Institute of Standards and

Technology

Gaithersburg, MD 20899

August 2006

National Institute of Standards and Teclinology • Technology Administration • U.S. Department of Commerce
Acknowledgments

Thanks go to the NIST technical staff for providing the information outlined on this report.

Each of the investigators is identified with their contribution. Contact information can be

obtained by going to:

http ://www. nist.gov

Acknowledged as well, is the invaluable assistance and support provided by Mark Bello of

the NIST Public and Business Affairs Office, Thelma Allen, Douglas Smith and Anne Chaka of

the NIST Program Office. Bessmarie (Beamie) Young is also thanked for her contribution to the

design of the cover for this document. Mylene Ouimette and Use Putman of the NIST

Information Services Division are thanked for their assistance in getting the document prepared

for publishing and Geoff Holdridge of the National Nanotechnology Coordination Office and

WTEC is thanked for his invaluable assistance in document formatting and preparation.
Table of Contents

Table of Contents

Introduction 1

1 - Fundamental Nanoscale Phenomena and Processes

Nanoscale Science,

Joseph A. Stroscio and Daniel T. Pierce (PL) 5

Nanotechnology with atom optics, Jabez J. McClelland PL) 7

Nanoscale Characterization of Polymeric Materials Degradation,

Tinh Nguyen, Xiaohong Gu, and Jon Martin (BFRL) 9

Nanomechanical Characterization of Nanoparticle-filled Polymeric Coatings and Composites,

Stephanie Scierka and Peter Votruba-Drzal (BFRI.) 11

Creating Visual Models of Nanoworlds, Judith Devaney (ITL) 13

Brillouin Light Scattering,

Ward Johnson, Sudook Kim, Colm Flarmery, and Pavel Kabos (MSEL) 15

Nanolithography: Next-Generation Polymer Photoresists,

Vivek M. Prabhu (MSEL) 16

Modeling for Organic Conduction, Carlos A. Gonzalez (CSTL) 17

Spin Electronics, William H. Rippard and Matthew R. Pufall (EEEL) 19

Nanomagnetodynamics, Robert D. McMichael (MSEL) 21

Nanomagnetism, John Unguris and Mark Stiles (PL) 22

Synthesis, Characterization, and Manipulation of Magnetic Nanoparticles,

Guangjun Cheng and Angela R. Hight Walker (PL) 24

Nanomagnetodynamics: Fundamental Properties, Thomas J. Silva, (EEEL) 25

Bioelectronics

Michael Gaitan, Darwin Reyes-Hernandez, and Brian Polk, EEEL, Laurie Locascio,

and Sam Forry CSTL 27

Single Molecule Nanovials,

Michael Gaitan EEEL, and Laurie Locascio and Wyatt Vreeland, CSTL 29

Optical Manipulation of Nanocontainers and Nanotubes,

William D. Phillips, Kristian Helmerson (PL) 31

NIST Accomplishments in Nanotechnology

iii
Table of Contents

Fundamental Properties of Quantum Dots, Kevin Silverman (EEEL) 33

Multiscale Modeling of Nanostructures, Vinod Tewary and David Read (MSEL) 35

Designing the Nanoworld: Nanosystems and Nanooptics, Gamett W. Bryant (PL) 36

Precision Metrology for Nanoscale Quantum Technologies,

John Lawall and Gamett W. Bryant (PL) 38

Control of Cold Quantum Gases for Atomic Clocks and Quantum Information Processing,

Paul S. Julienne and Lite Tiesinga (PL) 40

Quantum Computing with Trapped Ions, David Wineland and Tom O'Brian (PL) 42

Neutral Atom Quantum Computation, William D. Phillips, James Porto (PL) 44

2 - Nanomaterials

Polymer Miscibility of Imidazolium Compatibilized Nanotubes,

Jeffrey Oilman (BFRL) 45

New Materials to Protect Firefighters from Bum Injuries,

Jeffrey Oilman and Kuldeep Prasad (BFRL) 46

Characterization and Photoreactivity of Titanium Dioxide Nanostmctures,

Stephanie Scierka and Joarmie Chin (BFRL) 47

Optical Properties of Carbon Nanotubes, John Lehman (EEEL) 49

Characterization of Single Walled Carbon Nanotubes, Angela R. Hight Walker (PL) 50

Stability of Nanowires, Geoffrey B. McFadden (ITL) 51

Semiconductor Quantum Dot Structure Metrology, Alexana Roshko (EEEL) 53

Chemistry and Stmcture of Nanomaterials, Daniel A. Fischer (MSEL) 55

Mechanical Metrology for Small-Scale Structures,

Edwin R. Fuller, Jr. and George D. Quinn (MSEL) 56

Highly Charged Ion Modified Materials,

Joshua M. Pomeroy and John D. Oillaspy (PL) 57

Nanoscale Metrology of Ill-Nitride Semiconductor Films and Nanowires,

Norman Sanford and John Schlager (EEEL) 58

Mathematical Modeling of Nanomagnetism,

Michael J. Donahue and Donald O. Porter (ITL) 60

Magnetic Materials For Nanoscale Sensors and Ultra-High Density Data Storage,

William F. Egelhoff, Jr. (MSEL) 62

NIST Accomplishments in Nanotechnology

iv
Table of Contents

3 - Nanoscale Devices and Systems

Evanescent Microwave Probing and High-resolution Microwave Power Imaging,

Michael Janezic, James Baker-Jarvis, and Pavel Kabos (EEEL) 63

Chemical Sensing with Nanowires & Nanotubes, Richard E. Cavicchi (CSTL) 64

III-Nitride Semiconductor Nanowires: Growth and Characterization,

Kris Bertness and Norman Sanford (EEEL) 65

Measuring & Understanding Charge Transport in Molecules,

Roger van Zee, (CSTL) 66

MOS Device Characterization and Reliability Project,

John S. Suehle and Eric M. Vogel (EEEL) 68

Nanoelectronic Device Metrology Project, Curt A. Richter and Eric M. Vogel (EEEL) 70

Nanomagnetodynamics: Devices, Stephen E. Russek, (EEEL) 72

Nanoscale Engineered Magnetic Sensors,

David P. Pappas, (EEEL) 74

Single Photon Sources and Detectors, Richard Mirin (EEEL) 76

Terahertz Technology, Erich Grossman and Robert Schwall (EEEL) 78

Nanoscale Fabrication, James Beall (EEEL) 80

Johnson Noise Thermometry, Wes Tew, Sam Benz, Sae Woo Nam (EEEL) 82

Quantum Communications, Sae Woo Nam (EEEL) 84

Quantum Computing Using Superconducting Phase Qubits, Ray Simmonds (EEEL) 86

Quantum Sensors, Kent Irwin (EEEL) 88

Single Electronics for Standards and Metrology,

Mark Keller and Neil Zimmerman (EEEL) 90

Quantum Voltage Metrology, Samuel Benz and Yi-hua Tang (EEEL) 92

Architectures for Fault-tolerant Quantum Computing, Emanuel Knill (ITL) 94

Synthesis of Quantum Logic Circuits, Stephen S. Bullock (ITL) 96

Quantitative single-molecular pair fluorescence resonance, Lori S. Goldner (PL) 98

Bacteriophage/Quantum-dot nanocomplex to detect biological target

in clinical or environmental isolates, Jeeseong Hwang (PL) 100

NIST Accomplishments in Nanotechnology

V
Table of Contents

4- Instrumentation Research, Metrology and Standards for Nanotechnology

Chemical Sensor Microscopy for Nanotechnology,

Tinh Nguyen and Xiaohong Gu (BFRL) 101

Nanomechanical Properties of Polymer Thin Films, Christopher M. Stafford (MSEL) 103

Nanomechanical Metrology, Douglas T. Smith and Lyle E. Levine (MSEL) 104

Improving Image Resolution in Nanotechnology, Alfred Carasso (ITL) 105

Optical Metrology for Nanolithography,

John H. Burnett, Simon G. Kaplan, and Eric L. Shirley (PL) 107

True Three-Dimensional Tomography Utilizing Bayesian Statistical Inference,

Donald Malec and Juan Soto (ITL) 109

Structure and Dispersion Measurements of Polymeric Building Materials,

Li-Piin Sung (BFRL) Ill

Synthetic X-ray Spectrum Images for 3D Chemical Imaging at the Nanoscale,

John Henry J. Scott and Nicholas Ritchie (CSTL) 113

Carbon Nanotube Metrology, Paul Rice and Tammy Oreskovic (MSEL) 115

Nanotribology and Surface Properties, Stephen Hsu (MSEL) 116

Neutron Reflectometry for Highly Accurate Nanometer Metrology,

Joseph A. Dura (MSEL) 117

Gradient Reference Specimens for Advanced Scanned Probe Microscopy,

Michael J. Fasolka and Duangrut Julthongpiput (MSEL) 118

Quantitative Nanomechanical Properties, Donna C. Hurley (MSEL) 119

Highly Charged Ions in EUV Lithography, John D. Gillaspy and Joshua M. Pomeroy (PL) 120

Electrical Test Structure Metrology, Michael Cresswell and Richard Allen (EEEL) 122

Molecular Nanomagnets, Brant Cage and Stephen E. Russek, (EEEL) 124

Nanomagnetic Measurements, John Moreland, (EEEL) 126

Nanoparticle Measurements for Biological Applications, John Moreland, (EEEL) 128

Imaging Methods for Nanoparticle and Cell Systems to Investigate Toxicity,

Cynthia J. Zeissler and Peter E. Barker (CSTL) 130

NIST Accomplishments in Nanotechnology

vi
Table of Contents

5 - Nanomanufacturing

Development of High-resolution Variable Pressure Reference Scanning Electron Microscope,

Andras E. Vladar, Michael T. Postek and John S. Villarrubia (MEL) 133

Scanning Probe Microscopy for Nanoscale Measurements,

Ronald Dixson, Joseph Fu, George Orji, and Theodore Vorburger (MEL) 135

Scanning Probe Microscopy for Nanoscale Measurements: Atom-Based Height Standards,

Joseph Fu, Ronald Dixson, George Orji, and Theodore Vorburger (MEL) 137

Atom-Based Dimensional Metrology, Rick Silver (MEL) 139

Critical Dimension Metrology using Scanning Electron Microscopy and a Model-Based Library,

John S. Villarrubia and Andras E. Vladar (MEL) 141

SI traceable force for scanning probe microscopy and instrumented indentation,

Jon R. Pratt (MEL) 143

Phase-sensitive Scatterfield Optical Imaging for sub- 10 nm Dimensional Metrology,

Rick Silver (MEL) 145

Advanced Control Systems and Positioning for Nanoscale Measurements and Standards,

Nicholas G. Dagalakis (MEL) 147

Molecular Measuring Machine, John A. Kramar (MEL) 149

Agile Nanoassembly to Prototype, Test, and Manufacture Functional Nanodevices,

Thomas W. LeBrun (MEL) 151

Development of Nano-tip Electron Guns for the Scaiming Electron Microscope,

Andras E. Vladar and Michael T. Postek (MEL) 153

Measurement Strategy for Line Edge and Linewidth Roughness Metrology,

John S. Villarrubia and Andras E. Vladar (MEL) 155

Combinatorial Metal Selection for Catalytic Growth of ZnO Semiconductor Nanowires,

Albert V. Davydov (MSEL) 157

Electrical Properties of On-Chip Intercoimections, Daniel Josell and T.P. Moffat (MSEL) 158

Horizontal Growth and In Situ Assembly of Oriented Semiconductor Nanowires,

Babak Nikoobakht and Stephan J. Stranick (CSTL) 159

Organic Electronics, Eric K. Lin (MSEL) 161

NIST Accomplishments in Nanotechnology

vii
Table of Contents

6 - Major Research Facilities and Instrumentation Acquisition

The NIST Advanced Measurement Laboratory Nanofab,

Robert Celotta (CNST), Russ Hajdaj (CNST) and Eric M. Vogel (EEEL) 163

Probing Nanoscale Disorder with Neutron Tunnehng Spectroscopy,

Robert M. Dimeo and Dan A. Neumann (MSEL) 164

Probing the Nanostructure of Lipid Membranes with the AND/R (Advanced Neutron

Diffractometer/Reflectometer), M. Mihailescu and David Worcester (MSEL) 165

Appendix 1

Cross Reference to NIST Nanotechnology Focus Areas 167

Appendix 2

Key Words 173

NIST Accomplishments in Nanotechnology

viii
Introduction

Introduction

The National Institute of Standards and Technology (NIST) is the Nation's measurement and standards

laboratory, estabhshed by the U.S. Congress in 1901. Today, NIST's mission is to promote U.S. innovation

and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life. To enable accurate and precise measurements at the nanoscale, NIST is conducting scientific research into fundamental nanoscale properties, developing

appropriate standards, and is providing calibrations and traceability to basic standard international (SI) units. NIST seeks to fulfill its mission by working closely with industry, academia, and other government agencies.

NIST's work in nanotechnology is in coordination with the National Nanotechnology Initiative (NNI), which

is administrated by the Subcommittee on Nanoscale Science, Engineering, and Technology (NSET) of the National Science and Technology Council. (See the NNI web site at www.nano.gov.) Under the NNI, NIST is developing a broad range of nanometrology (measurements) and nanomanufacturing techniques necessary for the successful commercialization of nanotechnology. In recent testimony before Congress, Dr. Tom Cellucci,

president of Zyvex Corporation stated: "Much ofour leadership position and the jobs generatedfor Americans

can be directly attributable to NIST. NISTfulfills a vital role in bringing the promise ofnanotechnology to the American people. NIST is responsiblefor developing the measurements, standards, and data critical to private industry's development ofproducts for a potential market that is estimated to exceed a trillion dollars

in the next decade.

"

NIST, as an agency of the U.S. Department of Commerce, also works to help facilitate international trade by working with international standards organizations and other national metrology institutes. Key issues in the future for U. S. success in international trade, with respect to nanotechnology, will be traceability of

measurements and "harmonization" of international measurements and standards.

This document includes a list of selected NIST accomplishments in nanotechnology for the period of fiscal years 2004 and 2005. These accomplishments are grouped into the NNI's Program Component Areas (PCAs),

which are defined below. The NNI has established the PCAs as the major subject areas for research and

development vital to the successful commercialization of Nanotechnology. A bibliography of NIST

publications in nanotechnology Irom the years 2000 to 2005 is also included on the accompanying compact

disk.

NNI Program Component Areas

The NNI sets out four goals detailing the overarching strategy and plans of the initiative. Program

Component Areas (PCAs) are the major subject areas under which related NNI projects and activities are grouped. The PCAs relate to areas of investment that are critical to accomplishing the NNI goals. These areas

cut across the interests and needs of the participating agencies, and by grouping together work that is taking

place within multiple agencies, advancement may be further expedited. The PCAs are intended to provide a means by which the NSET Subcommittee, the Office of Management and Budget (OMB), Congress, and

others may be informed of the relative investment in these key areas, and by which the agencies funding R&D

can better direct and coordinate their activities. Detailed investments and agency plans for each PCA will be

included in the annual NNI supplement to the President's budget. The PCAs are relatively broad and are expected to continue to adequately describe the activities within the NNI for the next several years. The PCAs

are defined as follows:

• Fundamental nanoscale phenomena and processes

Research leading to the discovery and development of ftindamental knowledge pertaining to new

phenomena in the physical, biological, and engineering sciences that occur at the nanoscale.

NIST Accomplishments in Nanotechnology

1
Introduction

Elucidation of scientific and engineering principles related to nanoscale structures, processes, and

mechanisms.

• Nanomaterials

Research aimed at discovery of novel nanoscale and nanostructured materials and at a comprehensive

understanding of the properties of nanomaterials (ranging across length scales, and including interface

interactions). Also, R&D leading to the ability to design and synthesize, in a controlled manner,

nanostructured materials with targeted properties.

• Nanoscale devices and systems

R&D that applies the principles of nanoscale science and engineering to create novel, or to improve

existing, devices and systems. This PCA includes the incorporation of nanoscale or nanostructured

materials to achieve improved performance or new functionality, and possibly greater levels of

miniaturization. The systems and devices themselves may, but need not necessarily, be less than 100

nanometers in size to meet this definition.

• Instrumentation research, metrology, and standards for nanotechnology

R&D pertaining to the tools needed to advance nanotechnology research and commercialization,

including next-generation instrumentation for characterization, measurement, synthesis, and design of

materials, structures, devices, and systems. This PCA also includes R&D and other activities related

to development of standards, including standards for nomenclature, materials, characterization and

testing, and manufacture.

• Nanomanufacturing

R&D aimed at enabling scaled-up, reliable, cost effective manufacturing of nanoscale materials,

structures, devices, and systems. Includes R&D and integration of ultra-miniaturized top-down

processes and increasingly complex bottom-up or self-assembly processes.

• Major research facilities and instrumentation acquisition

Establishment of user facilities, acquisition of major instrumentation, and other activities that develop,

support, or enhance the Nation's scientific infrastructure for the conduct of nanoscale science,

engineering, and technology research and development. Includes ongoing operation of user facilities and networks.

• Societal Dimensions

Various research and other activities that address the broad implications to society, including benefits

and risks, such as: - Research directed at environmental, health, and safety impacts of nanotechnology development

and risk assessment of such impacts - Education-related activities, such as development of materials for schools, undergraduate

programs, technical training, and public outreach - Research directed at identifying and quantifying the broad implications of nanotechnology for

society, including social, economic, workforce, educational, ethical, and legal implications

Individual PCAs may encompass basic research through application-oriented efforts, including

experimental, theoretical, modeling, and simulation approaches. There is a gradation within the PCAs in the

scale or level of organization, ranging from fundamental nanostructure phenomena through nanomaterials,

devices and systems, tools and instrumentation, and manufacturing. The last two PCAs listed above are

different in nature, in that they provide for state-of-the-art research facilities and for addressing societal implications. The basic nature of the infrastructural research at NIST supports all of the PCAs including

Societal Dimensions.

NIST Accomplishments in Nanotechnology

2
Introduction

Instrumentation, Metrology, and Standards/Nanomanufacturing

The NNI gives NIST a leadership role in the Instrumentation Research, Metrology, and Standards for

Nanotechnology PCA. The nanometrology PCA is pervasive, crosscutting, and unifying to all the NNI PCAs.

Instrumentation provides the necessary data upon which scientific conclusions can be based, and correct metrology provides the ability to properly and accurately interpret these data. Researchers in NIST's seven

laboratories are also developing the traceable measurements, standards, data, and models that will provide the enabling infrastructure to facilitate the commercialization of nanotechnology. NIST's nanotechnology work is also making possible new, quantum-based realizations of the fundamental units in support of international metrology and, ultimately, international trade. NIST works with all the other federal agencies in exploiting

nanotechnology in areas such as homeland security and environmental protection to the benefit of the missions

of all the partnering agencies. NIST also shares the responsibility for the Nanomanufacturing PCA with the National Science Foundation and functions to provide the needed metrology necessary to exploit the research

and discoveries in nanotechnology providing a bridge between discovery and products for the Nation.

Unique NIST Resources

NIST measurements and standards capabilities are critical to maintaining US industry's technological

leadership. Four key resources are available for nanotechnology and nanometrology research: the Advanced

Measurement Laboratory, the Center for Nanoscale Science and Technology, the Center for Neutron Research,

and the Advanced Chemical Sciences Laboratory. These resources are strongly augmented by nanometrology

research in all of the technical laboratories across the entire NIST campus.

• Advanced Measurement Laboratory (AML). The AML is a 49,843 m^ (536,500 SF) state-of-theart laboratory complex opened in FY 2004. This laboratory provides NIST with an ultra controlled

metrology environment (temperature, vibration, air quality, humidity, and power) for nanomanufacturing and nanometrology. The AML is the most environmentally stable laboratory of

its kind in the world and houses NIST's most advanced metrology tools. The complex of five buildings includes a state-of-the-art nanofabrication facility. This entire laboratory complex will enable NIST and its partners to produce and disseminate world-class measurement standards enabled

by next-generation metrology capabilities beyond currently obtainable levels.

• NIST Center for Nanoscale Science and Technology (CNST). The Center enables science and

industry by providing essential measurement methods, instrumentation, and standards to support all phases of nanotechnology development, from discovery to production.

• NIST Center for Neutron Research (NCNR). A national user facility that serves more than 2,000

scientists annually, the NCNR is the only cold (i.e. low energy) neutron facility with comprehensive

capability in the United States - providing tools essential to study the complex biological, polymeric

and composite materials that are at the forefront of nano-materials research.

• Advanced Chemical Sciences Laboratory (ACSL). The ACSL was buih to house programs in biotechnology, analytical, environmental, clinical, and other related chemical measurements and

standards. The state-of-the-art ACSL features advanced designs that will help NIST meet the 21st- century needs for accurate chemical measurements, standards, and methods used for pharmaceutical

manufacturing, medical diagnosis, pollution monitoring and clean up, and other research in nanotechnology.

Michael T. Postek

Assistant to the Directorfor Nanotechnology

NIST Accomplishments in Nanotechnology

3
Introduction

Common NIST Abbreviations Used in this Document

• BFRL Building and Fire Research Laboratory

• CSTL Chemical Science and Technology Laboratory

• EEEL Electronics and Electrical Engineering Laboratory

• ITL Information Technology Laboratory

• MEL Manufacturing Engineering Laboratory

• MSEL Materials Science and Engineering Laboratory

• PL Physics Laboratory

• TS Technology Services

• CNST Center for Nano Science and Technology

• NCNR NIST Center for Neutron Research

Please note that in the author line for each project, the numbers in parentheses refers to the NIST Division within the Laboratory.

Further information about these and other topics can be found at:

http://www.nist.gov/public_affairs/guide/index.htm

NIST Accomplishments in Nanotechnology

4
1 - Fundamental Nanoscale Phenomena and Processes

Nanoscale Science

Development and application ofnew, nanometer scale

measurement andfabrication tools to explore the

quantum properties ofnanostructures, atoms, and

electron systems and their interactions. The quantum

properties ofnanostructures, confined electron sys- tems and their interactions with atoms, the effects of

reduced size and dimensionality, the effects ofelec- tron spin, and thefundamentals ofnanoscale magnetism are probed using advanced scanning tunneling

microscopy and spectroscopy. Bottom upfabrication

ofatomic scale structures, atom by atom, using novel

techniques developed at NIST are combined with tra- ditional MBEfabrication technologies.

Joseph A. Stroscio and Daniel T. Pierce, PL (841)

In this program we are investigating systems and fabrication technology at the forefront of nanoscale science

and technology using scanning tunneling microscopy

(STM) and spectroscopy techniques. In addition to using

the STM probe tip to measure physical properties, we are also using it to manipulate atoms to form new quantum

structures. Nanofabrication technologies are being combined with traditional fabrication methods using molecular beam epitaxy to create future atomic scale devices. In this report we highlight two recent achievements, in- volving the novel dynamics of atoms observed in atom

manipulation [1], and the electronic structure of 1- dimensional single atom chain systems [2]. Atom Dynamics during Atom Manipulation

Manipulation of single atoms with the scanning tun- neling microscope is made possible through the controlled and tunable interaction between the atoms at the end of the STM probe tip and the single atom that is being manipulated. In the STM tunneling junction used for atom manipulation, a host of interactions that depend on

the electric potentials, the tunneling current, and tipadatom proximity effects that come into play in the atom

manipulation process. Understanding these interactions

and their optimization is central to understanding the atom manipulation process and is required for the effi- cient and reliable atom manipulation needed for large- scale construction of atomic scale devices.

Imaging with the STM places the probe tip suffi- ciently far from the sample so that tip-sample interac- tions do not initiate atom movement, and hence one can

image a stationary single atom. In order to move a sin- gle atom, we turn on the tip-adatom interaction by bringing the tip closer to the adatom by means of adjusting the tunneling junction resistance (the junction resistance is set by the ratio of the tunneling voltage to the tunneling

current). It is useful to view the interaction as creating a highly localized potential well that traps the adatom under the tip. A great deal of information about the adatom

motion can be learned from recording the tip height trace during the manipulation process. A new type of image, we call a "manipulated atom image" is obtained by re- peating this tip height trace measurement by rastering the

tip over the surface, as in the normal STM topographic

mode, except now with the adatom trapped in the moving tip-induced potential well. Details on the scale of a small fraction of a lattice constant are observed in the manipulated atom image, as shown in Fig. 1

Figure 1. (A) Manipulated atom image ofCo over Cu(lll) sur- face. Tunnel current, 50 nA; sample bias, -5 mV; T = 4.3 K. The

labels A, B, and C denotefee, hep, and top sites, respectively. (B) Tunnel current recorded during manipulated atom image going

through the hep andfee sites as indicated by the horizontal line in

(A). The arrow shows the increased noise in the tunnel current corresponding to the position of the hep site.

Our understanding of the manipulated atom image

follows from the dynamics of the adatom motion hopping between fee and hep sites on the Cu(l 1 1) surface. The manipulated atom image for Co on Cu( 111) can be

thought of as a binding site image for the Co atoms, with

the additional contrast changes between the fee and hep

sites yielding information concerning the difference in the surface potential energy for these two sites. Using

the motion of the Co atom to image the local binding

sites and give information on the local surface potential

is an example of a new class of measurements based on

single atom transducers sensing local environments,

NIST Accomplishments in Nanotechnology

5
1 - Fundamental Nanoscale Phenomena and Processes

which we refer to as an "atom based metrology".

Audio frequency components of the tunnel current can be used as a real-time diagnostic of the atom manipulation process. For Co on Cu( 1 11 ), a periodic rasp- ing sound is heard during the acquisition of a manipulated atom image, see video on website

http://realex.nist. gov:8080/ramgen/hiphopatoms.rm. An

examination of the tunneling current signal shows that

this occurs when the Co atom is placed over the hep site (see Fig. 1). It is this increase in noise density that gives

rise to the "rasping" sound. An examination of the tun- neling current noise properties shows that the Co atom is switching between the favored fee site and the metastable hep site, and it is this switching phenomena that is responsible for the periodic rasping sound.

Insight into the switching dynamics is obtained from

analyzing the two-state random telegraph noise (RTN)

measured in the tunneling current signal. From an

analysis of the residence time distributions in the RTN

we arrive at a switching rate for the Co atom to go from

the fee to hep site, and vice-versa, as shown in Fig. 2. The transfer rates show two distinct regimes; a transfer

rate independent of tunneling current, voltage and temperature that is ascribed to quantum tunneling between

the two wells, followed by a transfer rate with a strong

power law dependence on current or voltage, indicative

of vibrational heating by inelastic electron scattering.

I 1 1 1 L

-10 -1 Sample Bias (mV)

Figure 2. Transfer rate versus sample bias at constant tip height, obtained by measuring the distribution ofresidence time in the hep andfee states from two-state random telegraph noise in the tunnel current. Junction resistance 150 kilohms; T = 2.3 K. Rhcp, red circles: Rf^c, black squares. Solid red line shows a power-law

fit to the initial threshold region; blue horizontal line shows the average transfer ratefor the low-bias region for the hep transfer

rate. End States in One-Dimensional Chains

served a new kind of electronic state at the ends of a one- dimensional nanostructure. This "end state" is a direct consequence of the lower dimensionality of the structure. Such end states can be thought of as zero-dimensional

analogs to two-dimensional states that occur at the sur- face of a crystal. To study electronic states in one- dimension, we fabricate atomic chains by depositing

gold on stepped silicon surfaces (Fig. 3). The energetics

of the stepped surfaces drives the formation of atomic

chains that cover the entire surface. Perfect chains can be up to 70 atoms in length, limited only by atomic defects.

Figure 3. 8 nm ^ 19 nm STM topography images ofthe same

sample area at positive (top left) and negative (bottom left) biases reveal a contrast reversal over the end atoms. A tight binding

model shows the density ofstates along a four-atom chain (right) with atomic position along the horizontal direction and energy along the vertical direction. Red shows regions ofhigh density of

states. At +0.5eV the density ofstates is localized over the center two atoms while at -0. 7eV localized states arefound over the end

atoms.

To characterize this new kind of electronic state, we

made spatially resolved scanning tunneling spectroscopy

measurements along finite chains to map the density of

states. These measurements reveal quantized states that form in isolated chain segments. Furthermore, a transfer of spectral weight from the filled to the empty states over

the atoms at the ends of the chains is directly attributable

to the formation of end states. A comparison to a tightbinding model demonstrates how the formation of end

electronic states transforms the density of states and the quantized levels within the chains. The end states effectively lower the energy levels of the filled states within

the chains, suggesting a possible driving force for their formation.

Selected Publications

1. J. A. Stroscio and R. J. Celotta, Science 306, 242

(2004) .

2. J. N. Crain and D. T. Pierce, Science 307, 703

(2005) .

The ability to fabricate structures on the nanoscale

and measure their electronic properties presents the opportunity to discover new phenomena. We have obNIST Accomplishments in Nanotechnology

6
1 - Fundamental Nanoscale Phenomena and Processes

Nanotechnology with atom optics

The ultimate goal ofnanotechnology is the absolute

control over matter on an atom-by-atom basis. Only

with this level ofcontrol can a wide range ofnew

quantum electronic andphotonic devices befabricated with the necessary accuracy and high yield to make the next generation ofelectronic and information processing architectures possible. Conventional

nanofabrication techniques, such as optical and elec- tron beam lithography, andfocused ion beam milling,

have made great progress in recent years, but it is clear that they will not serve at the single-atom level. We are developing new, radical approaches to atomby-atom nanofabrication that make use ofthe exquisite control afforded by atom optics - the use oflaser

beams, electric fields and magneticfields to cool,

trap, ionize, accelerate, implant, and otherwise manipulate single atoms.

Jabez J. McClelland, PL (841)

In order to controllably fabricate material at the sin- gle-atom level, it is necessary to have some way to relia- bly access a single atom whenever it is needed. In es- sence, we need a source of "atoms on demand." One

way to achieve this is by freezing atoms on a surface and

collecting them when needed, as is done with lowtemperature scanning-tunneling-microscopy-based atom

manipulation. As an alternative, we are exploring the use of laser cooling and trapping of neutral atoms to real- ize a completely deterministic source of atoms. Such a

source, we have discovered, has unique properties that make it ideally suited for deterministically implanting

single ions in host materials with nanometer-scale resolution.

Our most significant recent accomplishment has been

the demonstration in the laboratory of a deterministic source of single Cr atoms with 99% fidelity - that is, if an atom is "requested" from the source at any random

time, it will produce one atom (rather than zero or more

than one atom) with a probability of 99%.

The source is based on a magneto-optical trap

(MOT), which uses six mutually orthogonal laser beams

and a set of magnetic field coils that produce a field minimum at a central point in space. Atoms from an

evaporation source are cooled by the laser beams and

captured by the light force, which is made spatially dependent by the magnetic field gradient. The result is a population of very cold atoms trapped in the center of

the MOT.

C3^

Count rate (rrrs ') Count rate (ms ')

Figure 1. A magneto-optical trap with feedback-controlled load- ing maintains a population ofexactly one atom with 99% probability. A key ingredient in making the MOT into a deterministic source of atoms is very high efficiency fluores- cence detection. This allows us to observe the arrival and departure of individual atoms in the trap with good

signal-to-noise. With this high detection efficiency, we

are able to implement a fast feedback control loop,

which essentially turns on loading only when the trap is empty and dumps the trap contents if it by chance contains more than one atom. With a moderate amount of

optimization of the feedback loop parameters, we have

demonstrated 99% single-atom fidelity for random sam- NIST Accomplishments In Nanotechnology
1 - Fundamental Nanoscale Phenomena and Processes

pling of trap contents, and also forced periodic ejection

at rates up 10 Hz with fidelity over 90%. These results have been published in recent journal articles [1-2].

The next phase of our project involves extraction and

focusing of the single atoms with nanometer resolution. In order to accomplish this, we will take advantage of the extremely low temperatures (-120 |aK) achievable in a MOT. These cold temperatures result in a very compact

phase space volume for the atoms in the source, which

allows very tight focusing of extracted atoms if a suitable

set of optics is implemented. When an atom is called

for, we will photoionize the atom in the trap with a fast laser pulse and extract it with a moderate electric field. Ion ray tracing calculations, which take into account the temperature and spatial extent of the source, show that with relatively simple electrostatic lenses, we will be

able to focus the extracted ions to spot sizes that ap- proach 1 nanometer.

5 nm

Figure 2. Ion ray tracing calculations show that single atoms from an ionized magneto-optical trap (MOT) can befocused to the nanometer-scale.

In a parallel development, we have also been investi- gating the implementation of our deterministic source with other atoms that might enable new technologies if they can be implanted singly with good fidelity. We

have identified erbium as such an atom because as a rare earth it has technologically important optical properties

when implanted in a host material. For example, erbium

dopants provide the optical gain in fiber lasers used ex- tensively in the telecommunications industry. An ex- perimental program has begun to demonstrate laser cool- ing and trapping of erbium for the first time, and recently

the first fluorescence measurements have been made in preparation for this.

Figure 3. An atomic beam oferbium atoms is illuminated by a

laser beam in the first step toward realizing a deterministic source ofsingle erbium atoms.

Selected Publications

1. S. B. Hill and J. J. McClelland, Appl. Phys. Lett. 82,

3128 (2003).

2. S. B. Hill and J. J. McClelland, J. Opt. Soc. Am. B,

21, 473 (2004).

NIST Accomplishments in Nanotechnology

8
1 - Fundamental Nanoscale Phenomena and Processes

Nanoscale Characterization of Polymeric Materials

Degradation

Development ofreliable methodologies to predict

long-term performance has been the ultimate goal of

both users and producers ofpolymeric materials used

in various industries including building and automotive. This project aims to provide nanoscale physical

degradation ofpolymeric coatings exposed to the en- vironments. This information is essentialfor understanding the degradation mechanism and developing

accurate prediction modelsfor polymeric coatings.

This work is part ofthe NIST/Industry Consortium on

Service Life Prediction ofCoatings, which has been

going on for ten years.

Tinh Nguyen, Xiaohong Gu, and Jon Martin,

BFRL (861)

Polymeric coatings are used extensively in outdoor

structures, both for aesthetic and functional purposes. However, these materials undergo degradation during

service. In this research, polymer-coated samples are exposed to outdoor environments and NIST-developed

accelerating devices that can precisely vary UV light

intensity and wavelengths, temperatures, and relative humidities. In addition to traditional techniques that pro- vide data for kinetics and mechanistic studies, and for verifying service life prediction models, atomic and

nanoscale techniques such as positron annihilation spec- troscopy (PAS) and atomic force microscopy (AFM) are

also emloyed to provide early physical changes of coatings. This highlight presents some examples on the use of AFM for studies of nanoscale physical degradation of

polymeric coatings exposed to the environments.

One example to demonstrate the utility of AFM for studies of automotive coatings photodegradation is illustrated

in Figure 1 . These are AFM height images of a commercial polyurethane coating on a car bumper before (top)

and after exposure (bottom) to ultraviolet (UV) radiation

in an artificial weathering device. The line profile and

RMS roughness data (right, top) indicate that the coating

surface before exposure is very smooth, with no evi- dence of pinholes or pits. After exposure, pits ranging

from 1 50 nm to 400 nm in diameter and 2 nm to 5 nm in depth have developed on this sample surface. One particularly useful application of AFM for quantitative studies of early degradation of coatings is its abiUty to measure the enlargement and deepening of pit

formation with exposure at the nanoscale level.

Figure 1. 5 /urn x 5 /mi, 3-D AFM height images (left) and 2-D

images and line profiles (right) ofa commercial automobile polyiirethane-coated bumper before and after exposure to UV radiation.

This is demonstrated in Figure 2 for a urethane coat- ing exposed for 6160 h in an artificial UV device at 70 %

RH and 50 °C. For this measurement, the AFM images were taken at the same location after each exposure time.

The pit depth is observed to increase nearly linearly with

time, but its diameter enlarges rapidly at first then slows

down. This quantitative data is useful for kinetics studies

of the degradation and allows us to assess the effects of

surface morphological changes on coatings appearance.

!

- ir

Figure 2. Imaging and measurement ofpit enlargement and

deepening by AFMfor a coating exposed to the environments; a) 3-D image, b) 2-D image with a line crossing two pits, c) corre- sponding line profile showing pit width and depth, and d) depth

and diameter ofthe large pit asfunction ofexposure time.

These results and extensive nanoscale evidence

clearly indicate that photodegradation of a polymeric

coating is not a uniform thickness loss (ablation) process but is a heterogeneous phenomenon with localized pitNIST Accomplishments in Nanotechnology

9
1 - Fundamental Nanoscale Phenomena and Processes

ting, which grows with time. This information is essential for developing accurate models to predict long-term

performance of coatings.

AFM can also provide valuable information on sur- face microstructure changes caused by weathering, one example is shown in Figure 3 . These are height and

phase images of a 70 % (mass fraction) poly(vinylidene

fluoride) (PVDF) and 30 % copolymer of PMMA and

poly(ethyl acrylate) (PMMA-co-PEA) before and after exposure to an UV source.

" Before exposure After UV exposure

Figure 3. J jum x 1 /urn AFM height and phase images ofsurface ofafilm containing 70% PVDF and 30% PMMA-co-PEA before and after exposure to UV.

Before exposure the surface is covered with an amorphous material. However, the amorphous material

is removed after UV exposure, revealing the fine lamellae structure. From AFM results, contact angle, and

spectroscopic data, it has been determined that the en- richment of crystalline PVDF material on the surface is responsible for the excellent UV resistance of these coatings.

In Corrosion Environment; 120 Thiclt PU Coating on Steel, 30 d at 24 "C

4 |am

Figure 4. AFM height images ofpolymer-coated steel in NaCl,

showing nanoscale pitformation.

Preferential Degradation of Hydrophilic Regions

BFTIR Spectra of 70/30 PEA/PS Blend Exposed to I M HCl Vapor at 25 °C.

AFM linages and Line Profile for a ?SI PEA (30/70) Blend Exposed to IM HCl Vapor for 3 h at 25 "C.

AFM is useful for detection of nanoscale pits formed

in protective coatings, as shown in Figure 4. These

height images and line profile show that nanopits have

developed in this polyurethane coating after 30 d exposure to a corrosive environment. This result is the first evidence to verify NIST conceptual and mathematical

models proposed several years ago that corrosive ele- ments from the environments migrate to the metal sur- face underneath an intact film is through pathways de- veloped during exposure, and that the pit formation occurs at the "hydrophilic" domains in the films.

The postulation that pitting occurs in the hydrophilic

regions of a polymer film is verified by Figure 5, which

shows AFM images and IR changes of a blend film of an

inert PS and a hydrolysable polyetheyl acrylate (PEA)

after exposing to HCl vapor. The PEA matrix is hydrolyzed and removed, which is seen as pits in the film.

Figure 5. AFM image ofa PS/PEA blend exposed to hydrolysis

environment showing the removal ofhydrolysable PEA matrix, which is seen as pits.

Although both photo and hydrolytic degradation is believed to occur in the degradation-susceptible regions

in a film, infonriation about the location and nature of

these nanometer regions is unknown. Work is on-going

to answer these questions.

NIST Accomplishments in Nanotechnology

10
1 - Fundamental Nanoscale Phenomena and Processes

Nanomechanical Characterization of Nanoparticle-filled

Polymeric Coatings and Composites

The polymeric coatings industries seek experimental

protocols to access theformulation, casting, resulting

optical and nanomechanical properties, and measure

the subsequent photodegradation effects ofpolymeric

nanocomposites. Project objectives are to develop

experimental protocols for measuring the nanomechanical properties ofnanoparticlefilled polymers

through establishing relationships between the extent ofdispersion, photodegradation effects, and changes

in nanomechanical properties ofpolymeric materials.

Stephanie Scierka and Peter Votruba-Drzal,

BFRL (861)

Inorganic fillers, such as Titanium Dioxide (Ti02) and

Zinc Oxide (ZnO), have historically been added to polymeric building materials to enhance both the appear- ance properties and the mechanical durability. A com- mon misconception is that these fillers are considered to be inert despite their semi-conductor electronic structure.

Little regard has been given to understanding potentially

important effects related to their quantum mechanical

properties. For example, recent research has revealed

Ti02 films exhibit measurable photocatalytic activity

when exposed to ultraviolet (UV) irradiation. This property may be used to engineer polymer nanocomposite

coatings that self-sterilize by degrading chemical and

bacterial contaminants by demineralization. The chal- lenge of this project is to establish experimental protocols to access the formulation, casting, resulting optical

and nanomechanical properties, and measure the subsequent photodegradation effects of polymeric nanocomposites. Successful development of experimental protocols may lead to the novel engineering of polymeric

nanocomposite coatings such as those that maximize the

beneficial photocatalytic activity for sterilization but

minimize the negative effects of photodegradation, and

enhance fire resistance, and mechanical strength.

Recent accomplishments include the presentation of

our research results at the 2004 Fall Materials Research

Society Meeting and the 2005 Adhesion Society Meeting. At those meetings we reported the results from

model epoxy nanocomposite thin films containing one of

three types of titanium dioxide (TiOi) particles that were

degraded using an integrating sphere-based ultraviolet weathering chamber. Instrumental Indentation Testing

(IIT) was used to measure nanomechanical changes in the surface region (top 1000 nm) of thin films resulting

from UV exposure. Attenuated Total Reflectance- Fourier Transform Infrared Spectroscopy (ATR-FTIR) and

0 2 10' 4 10" 6 10" 8 10" 1 10^

Incident Dose (J)

Figure 1. Extent of UVdegradation in the epoxy system as monitored by ATR-FTIR. An increase in the oxidation products is shown by the C=0 stretch at 1 762 cm '. Error bars represent the estimated uncertainty of95 % confidence intervals.

Differential Scanning Calorimetry (DSC) were used to support the mechanical results with chemical and ther- mal data. The addition of nanoparticles increased the

elastic modulus of the thin films. Exposure to UV radiation increased the elastic modulus of the nanocomposite

thin films. The unfilled epoxy was the most photosensitive sample tested, exhibiting the highest rates of chemical oxidation (Figure 1 ), the largest decrease in the glass

transition (Tg), and the greatest increase in elastic modulus with increased exposure (Figure 2).

0 5

0 3

01 -

Epoxy

Filler A 25%

Filler B 2 5%

Filler C 2 5%

-1 1—I—

I

1—I—I—I—TTT—1"

1

kA

J I I I I I I I I I I I I I I i_: 0 2 10" 4 10" 6 10" 8 10" 1 10^

Incident Dose (J)

Figure 2. Relative increase in the elastic modulus ofthefirst 1000 nm ofthe polymeric nanocomposites due to UV exposure. Error bars represent the estimated uncertainty of95 % confidence

intervals. Lines are added as a visual guide.

The rate of increase in the elastic modulus was re- duced with increasing volume fraction of nanoparticles.

Similar trends were observed in the nanocomposite

films, but the rates of change were much lower than the

unfilled epoxy and decreased with increasing volume

NIST Accomplishments in Nanotechnology

11
1 - Fundamental Nanoscale Phenomena and Processes

fraction of nanoparticles. The addition of particles re- sulted in an increase in surface roughness with additional

exposure and increased the uncertainty in measurements

for the filled samples, making it difficult to quantify dif- ferences in the rate of change due to photoreactivity.

These results were received with considerable interest from industrial representatives at the meetings. Future

studies will use a more photostable matrix where accel- erated rates of degradation can be linked to particle reactivity and will include characterization of particle dispersion. These methods of characterization may be in- cluded in Phase IV of the Service Life Predication of

Polymeric Coatings Industrial Consortium within the Building and Fire Research Laboratory.

Selected Publications

1 . Scierka, S., Drzal, P.L., Forster, A.L., Svetlik, S., "Nanomechanical Properties of UV Degraded

Ti02/Epoxy Nanocomposites, Materials Research

Society Proceedings Volume 841, 2004.

2. Drzal, P.L., Scierka, S., Forster, A.L., Svetlik, S., "Instrumented Indentation of Model Photodegraded

Ti02/Epoxy Nanocomposites", Proceedings of the

28"^ Annual Adhesion Society Meeting, 2005.

NIST Accomplishments in Nanotechnology

12
1 - Fundamental Nanoscale Phenomena and Processes

Creating Visual Models of Nanoworlds

The place where quantum and macro effects meet, the

nanoworld is full ofthe unexpected. In such an environment, visual models oflaboratory arid computational experiments can be critical to comprehension.

We are developing an immersive visual modeling sys- tem that enables scientists to easily view and interact

with their data in multiple ways in real time.

Judith Devaney, ITL (891)

Computational and laboratory experiments are generating increasing amounts of scientific data. Often, the complexity of the data makes it difficult to devise a priori methods for its analysis, or the data is from new

landscapes, such as the nanoworld, where we have little experience. Moreover, there may be ancillary data, from

databases for example, that would be helpful to have

available. We are developing visual analysis capabilities

in an immersive environment that allow scientists to in- teract with data objects in a three-dimensional landscape

rather than simply viewing pictures of them. Fully immersive computer graphics include one or more large

rear projection screens to encompass peripheral vision,

stereoscopic display for increased depth perception, as well as head tracking for realistic perspective based on

the direction the user is viewing. With visual exploration, scientists can easily perceive complex relationships

in their data, quickly ascertaining whether the results match expectations. Real time interaction adds to the

potential for speeding the process of insight.

In a fast moving field like nanotechnology, it is important to be able to create and interact with new visual

models quickly. Our visual environment is built for generality, flexibility and speed. Rather than a single monolithic program, it is a collection of tools designed to work

together to create, display, and interact with visual models. We have created three main categories of tools: in- frastructure software, representation software, and scene

interaction software. We join programs together using

Unix pipes and filters for creation and transformation.

We construct Dynamically Shared Objects (DSO's) for functionality, and scenegraph objects for ease of place- ment. The Program DIVERSE

Our visual environment centers around a core infra- structure program called DIVERSE (Device Independent

Visualization Environment-Reconfigurable, Scalable,

Extensible). DIVERSE, which was developed by Virginia Tech with support and technical contributions by

NIST, is an interface that facilitates the development of

immersive computer graphics programs for use on a wide variety of graphics displays. DIVERSE provides a

toolkit to load in previously compiled objects, called DSOs. DSOs can be used to describe the graphics dis- play, input devices, navigation techniques or interaction

styles for a given visualization. Using collections of

DSOs, applications can be reconfigured without recompiling. The same DSO that defines how a wand controls an object can be used both in an immersive environment

and on a laptop computer. With the ability to import the output of conventional visalization programs, and access

to the increasing capabilities of commodity graphics

cards, we have a very rich environment in which to ex- press and communicate visual models.

Figure 1. Image ofa quantum dot createdfrom the output ofa computer simulation ofthe optical properties ofnanostructures. The spheres represent s-orbitals.

Representation

Representation refers to the process of transforming raw data into a visual geometric format that can be

viewed and manipulated. The key to quickly and easily

visualizing scientific data in an immersive environment

is the capability provided by the set of tools used to convert raw data to immersive data. We have created a variety of tools to make glyphs, color them, place them,

and render them to a desired level of transparency and

other properties. While our system is based on our own

internal representation, we take advantage of representations computed by other packages by providing tools to take the output of common packages and convert them

into the format that places them in our environment.

NIST Accomplishments in Nanotechnology

13
1 - Fundamental Nanoscale Phenomena and Processes

Interaction

For efficient scientific exploration, it is important to have user interaction that is both easy to set up and easy

to adapt to differing needs. Individual DSOs that add new functionality to our visualization software system

are loaded at run-time, and their behavior is cumulative.

Together, a set allows for a huge range of combinations

data visualization modes. We have developed a wide

range of DSOs that allow a user to interact with the objects to be viewed. This includes functionality to move

objects around, select individual objects or sets, assign

functionality to the selections, interact with outside soft- ware, bring data into the system, send data out of the system, and load or unload objects during a visualization. They can also interactively select the level of detail

in a scene. Individual DSOs can add simple capability to a scenegraph, such as adding a particular light source or an object to represent a pointer for the user to select objects. DSOs can also add tasks to the objects of a scenegraph. When selected, an object can be assigned the task of turning on or off another object or itself, or executing

a command to dynamically interact with another DSO or an external program. Objects can change visibility, initi- ate new simulations, define clipping planes, and so on. DSOs provide a variety of ways to navigate through the environment, including changing the scale at will. They

allow individual scenes as well as time sequences (i.e., movies) to be viewed.

The output of an interaction can be saved as a simple

image, as a movie, or the interaction itself can be saved

as an experience that can be replayed at a later time.

Applications to the Nanoworld

We have successfiilly applied our visual analysis

techniques and tools to the study of a variety of nanoscale phenomena. Among these are visual analyses of

s-orbitals for the simulation of electronic and optical properties of complex nanostructures such

as semiconductor nanocrystals and quantum dots

(see Figure 1),

electric, magnetic, and energy field vectors from

the simulation of optical scattering by metamagnetic materials (see Figure 2),

intermediate voltage electron microscope measurement approaches to attain three-dimensional

chemical images at nanoscale-resolution, and

Figure 2. Image ofthe electric, magnetic, and energyfield vectors in a single time step in the simulation ofresonant optical

scattering by metamagnetic materials.

Selected Publications

1. J. E. Devaney, S. G. Satterfield, J. G. Hagedom, J. T. Kelso, A. P. Peskin, W. L. George, T. J. Griffin, H.

K. Hung, R. D. Kriz, Science at the Speed of

Thought, in Ambient Intelligencefor Scientific Discovery, (Y. Cai, ed.). Lecture Notes in Artificial In- telligence 3345, February 2005.

2. J. Sims, G. Bryant, H. Hung, Intrinsic Surface States

in Semiconductor Nanocrystals: HgS Quantum Dots,

American Physical Society, Los Angeles, CA, March

21-25,2005.

3. R. D. Kriz, J. Kelso and A. Ray, Visual Interpretation

and Analysis of HPC Nanostructure Models using

Shared Virtual Environments, Modeling and Simulation 1 (4), 2002, pp. 9-10.

4. J. Kelso, L. E. Arsenault, S. G. Satterfield and R. D.

Kriz, DIVERSE: A Framework for Building Extensible and Reconfigurable Device Independent Virtual

Environments, in Proceedings ofthe Virtual Reality

2002 Conference, Orlando, FL, March 24-27, 2002.

dynamics of molecular interactions leading to the formation of smart gels.

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

Brillouin Light Scattering

Brillouin light scattering is being developed as a tool for characterizing acoustic and magnetic waves at gigahertzfrequencies in nanostructures and thin-film

materials. Measurements and modeling in 2004focusedprimarily on interactions ofspin waves in ferromagneticfilms and acoustic wave propagation in nanopatterned polymers.

Ward Johnson, Sudook Kim, Colm Flannery

MSEL (853); and Pavel Kabos, EEEL (818)

Brillouin light scattering (BLS) is an experimental

technique that measures the intensity of spectral components of light that is inelastically scattered by vibrational waves (phonons) or spin waves (magnons) in a material. Fabry-Perot interferometric techniques are used to ac- quire accumulated spectra through repeated mechanical

sweeping of the etalon spacing.

BLS is the only laboratory technique that is currently

available for directly detecting magnons of finite wave

number. Because of this capability and because of

innovations in interferometric techniques, BLS has been

increasing applied to problems in magnetics over the

past couple of decades. It also has been extensively used

for charactizing phonons and determining elastic constants of thin films. In recent years, BLS has begun

to be employed on other systems with nanoscale

dimensions, including patterned thin-film structures on

substrates. The power of BLS for characterization of

magnetic and elastic properties at the nanoscale arises from the gigahertz operating frequency range (which

includes modes localized in nanostructures),

noncontacting detection, selectivity with respect to wave

vector and frequency, and ability to detect modes at the thermal level of excitation.

In this work, BLS measurements and modeling of

magnetic and vibrational modes are being pursued on

several material systems, including ferromagnetic films,

nanopatterned polymeric films, nanoporous lowdielectric-constant (Iow-a:) dielectrics, spin-momentumtransfer devices, molecular rotors, and carbon nanotubes.

Over the past year, we developed and applied

methods for measuring magnons that arise directly or

indirectly from microwave pumping of metallic

ferromagnetic films in a static magnetic field (Figure 1). BLS spectra were studied as a function of scattering

angle, microwave power, and laser power to provide

information on magnon-magnon interactions in NigiFeig

(Permalloy). The results demonstrate, for the first time,

detection of nonzero-wave-number magnons arising

from the decay of pumped uniform-precession magnons

in metallic thin films. The metrology that we have

developed will serve as a powerful new tool for characterizing interactions of magnons that limit the speed of magnetic-storage devices, spin-valve sensors, and other thin-film magnetic devices.

BLS measurements and modeling of vibrational

modes in nanoimprinted polymeric lines were performed

in collaboration with Colorado State University,

University of Akron, and NIST Polymers Division. In addition to bulk and surface modes, the spectra revealed

flexural and Sezawa modes that are localized in the imprinted lines. These results suggest the possibility of

characterizing anisotropic elastic constants on a scale of

tens of nanometers in these structures and other

nanoscale lines on substrates. Especially since elastic constants in nanostructures are expected to deviate from

bulk values, such information is important for modeling

mechanical integrity and elastic interactions, which will play a critical role in the fabrication and performance of

a variety of next-generation electronic devices.

A

V

——

,+A (0

Figure 1. Scattering oflight offspin waves in a ferromagnetic

thin film with microwave excitation.

NIST Accomplishments in Nanotechnology

15
1 - Fundamental Nanoscale Phenomena and Processes

Nanolithography: Next-Generation Polymer Photoresists

Photolithography, the process used to fabricate inte- grated circuits, is the key enabler and driverfor the

microelectronics industry and the workhorse technology for nanoscale fabrication. Lithographic feature

sizes have reached sub 100 nm length scales, but significant challenges have arisen forfuture size de- creases because both the image resolution and the

thickness ofthe imaging layer are approaching the

macromolecular dimensions characteristic ofthe

polymers used in the photoresistfilm. Unique highspatial resolution measurements are developed to identify the limits on materials andprocesses that

challenge the development ofphotoresists for next- generation lithography.

Vivek M. Prabhu, MSEL (854)

Photolithography is the driving technology used by

the microelectronics industry to fabricate integrated cir- cuits with ever decreasing sizes. In addition, this fabri- cation technology is rapidly being adopted in emerging

areas in optoelectronics and biotechnology requiring the rapid creation of nanoscale structures. In photolithography, a designed pattern is transferred to the silicon substrate by altering the solubility of areas of a polymerbased photoresist thin film through an acid catalyzed

deprotection reaction after exposure to radiation through

a mask. To fabricate ever smaller features, next generation photolithography will be processed with shorter wavelengths of light requiring photoresist films less than

1 00 nm thick and dimensional control to within 2 nm.

To advance this key fabrication technology, we work

closely with industrial collaborators to develop and apply

high-spatial resolution and chemically specific measurements to identify the ultimate materials limits for advanced photoresists. We seek to also identify and to understand changes in material properties, interfacial behavior, and process kinetics at nanometer scales that can

significantly affect the patterning process. We apply and advance unique measurement methods

to provide structural measurement of fabricated nanoscale structures and new insight and detail into the complex physico-chemical processes used in advanced

chemically amplified photoresists. These methods in- clude x-ray and neutron reflectivity (XR, NR), small

angle x-ray and neutron scattering (SAXS, SANS), near- microscopy (ATM).

Figure 1. Left: Nanoscale structures fabricated with photolithography. The nominal line width is 180 nm. Right: Schematic of

the photolithographic process to create nanoscale structures.

edge x-ray absorption fine structure spectroscopy

(NEXAFS), solid state nuclear magnetic resonance (NMR), quartz crystal microbalance (QCM), fluores- cence correlation spectroscopy (FCS), and atomic force

Recent accomplishments include: profiling the distri- bution of water at photoresist-liquid interfaces for immersion lithography and the developer counterion distri- bution in ultrathin films immersed in a developer solution; photoresist component segregation with NEXAFS

spectroscopy; quantification of the post-exposure bake

time on the reaction-diffusion of photoacid 3D deprotection volume; and quantified the effects of developer and

additives on the final resolution of lithographic features using a model reaction-front bilayer geometry. Each of

these factors greatly influences the resolution of the final nanoscale structure.

This past year, SEMATECH selected NIST to apply

these fundamental measurements to identify materials sources of the fabrication limits of advanced 193 nm

photoresists. This knowledge is needed for the evaluation of future nanofabrication technologies based on photolithography. In addition, Intel has assigned a scientific staff member to work with the Polymers Division/MSEL

to identify materials issues in the resolution of EUV

photoresist materials.

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

Modeling for Organic Conduction

Recently, there has been increased interest in molecular electronics due to the prospect ofdesigning effi- cient electronic nano-scale components that could

exhibitfunctionalities not achievable with silicon devices (e.g. optical emitters). Despite the significant

progress made in the development oftechniques to fabricate and characterize molecular conductors, the

fact remains that molecular electronics is currently more ofan art than a science characterized by a significant lack ofreproducibility. The scientific and

technological objective ofthis project is to evaluate/develop theoretical and computational tools that

model the electrical activity and transport properties

oforganic molecules used in nanoelectronic devices.

Carlos A. Gonzalez, CSTL (838)

The decrease in the size of transistors by a factor of

two every eighteen months, the trend known as Moore's

law, has led to the fabrication of microelectronic devices

that are increasingly faster, more powerful, and ubiquitous. However, it is projected that the physical limits of

even the most advanced lithographic techniques under

development will be reached in the foreseeable future. Beyond that time, continued increases in circuit density

will require a paradigm shift in the design, make-up, fabrication, and operation of electronic components. Molecular electronic is seen by many as a viable alternate component technology. Molecules, either singly or in small ensembles, perform the function of the electronic

components. Significant progress has taken place in the

field. Molecular field-effect transistors (IBM, Lucent),

reversible molecular switches (Hewlett-Packard), molecular negative-differential resistors (MEC, Inc), and

molecular rectifying-diode have all been discovered and

characterized, and Hewlett-Packard recently announced

a prototype molecular-memory device with a density of

-10 GBits/cm". Despite this progress, it has become

clear that there is a marked need for the development of

robust theoretical and experimental tools that could be

used on a routine basis in the design of novel materials

leading to the development of reliable electronic devices

based on molecular electronics. In this project, reliable and efficient theoretical methodologies leading to the understanding of the fundamental mechanisms governing

electrical conductance in organic materials at the molecular level are developed and compared to experimental data. Theoretical studies of the electronic transport of

molecular wire circuits are conducted using a timeindependent Landauer scattering formalism based on a

first-order expansion of the system non-equilibrium

Green's function (NEGF). In this model, the molecular

bridge is bound to the metallic electrodes by means of

thiolate groups at the head and tail of the bridge. Possi- ble effects of the system's chemical stability, conformational changes of the organic conductor, and excited

states on the transport properties of the device are also

investigated. In addition, methods to predict the effects of electrostatic interactions at the molecule-electrode

junction on the Current-Voltage curve are currently being examined. These effects could lead to technologically important phenomena such as molecular rectifica- tion and negative differential resistance. In this research,

a simple methodology to study trends in conductance of

molecule-metal junctions based on Density Functional Theory calculations (DFT) of modified quasi-molecular Green

functions in a capacitor-like electric field was developed.

This method, called the Green Function Condensed-toFragments model (GFCF), is based on a series of assumptions about the voltage spatial profile and the moleculesurface chemisorptive coupling in metal-molecule inter- faces that seem to be validated for a number ofjunctions.

The method assumes that the voltage drops entirely at the

interfaces and that the junction conductance can be ap- proximately factorized as a product of contact and molecular contributions. The main advantage of this approximate

methodology rests on the fact that it is very simple to use, computationally efficient, and its results can be analyzed in terms of familiar chemical concepts such as molecular orbitals and dipole moments. The model has been used in the calculation of I-V curves of a series of the 7t-conjugated

oligomers: 4-phenylethynyl-l-[(4-thiol)-phenyl-ethynyl]

benzene (OPE), 2-amino-5-nitro-4-phenylethynyl-l-[(4-

thiol)-phenylethynyl] benzene (NH2-OPE-NO2), and 2- fluoro-4-phenylethynyl- 1 -[(4-thiol)phenylethynyl]benzene

(F-OPE). The results of these calculations were compared

to the corresponding experimental I-V curves. These three oligo(phenylene-ethynylene)s are highly conjugated sys- tems with similar electronic structure, and possible differ- ences in their conductance behavior should be only the result of substitution in the central ring. The results obtained

in this work indicate that the I-V curves exhibited by substi- tuted n-conjugated oligomers attached to gold electrodes can be interpreted qualitatively with the help of the quasiGreen function matrix projected onto fragments of the iso- lated molecule. This conclusion is supported by the reasonable qualitative agreement between the computed and ex- perimental I-V curves in the case of OPE, F-OPE, and NO2-

OPE-NH2, especially at voltages lower than 1 .3 V (see Fig.

1).

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

Figure 1. Comparison oftheoretical (red line) and experimental

(black dots) I- V curves for OPE 's. A similar qualitative agreement in the case of satu- rated hydrocarbons such as C12H25SH, oligo(phenylene

vinylenes) as well as in a series of substituted platinum(II) acetylides indicates that the GFCF method is transferable to different molecular systems and that its reliability just depends on the voltage spatial profile of

the system. The discrepancies between the theoretical and experimental I-V curves observed at larger voltages can be attributed to limitations of the method in this voltage regime. However, it is very encouraging to see

that even at these voltages the simple theoretical scheme

used here predicts the right trends for the three molecules

studied. The results of our calculations lead to the conclusion that the I-V switching observed in N02-0PENH2 correlates well with significant rearrangements in the electronic structure of the molecule (mainly through

the pz atomic orbitals) reflected in the behavior of the corresponding Green functions with the voltage. In the case of OPE and F-OPE these drastic changes are not observed and the systems behave as simple molecular

turmeling wires. The results of this research pose the

possibility of using standard quantum-chemical calculations as a "screening tool" to aid in the rational design of

other molecular systems that could exhibit novel and

technologically important I-V behavior. We are currently testing the reliability and transferability of the model on a larger variety of molecular systems for which

reliable experimental data is available.

Figure 2. Changes in the excitation energy ofOPE as a function ofthe ring torsion angles.

We are also investigating the nature of the interaction

between the molecule and the metal at the interface. In

particular, we are applying highly correlated ab intio molecular orbital theory in order to determine possible

changes in the electronic structure of the molecule due to chemisorption to the metallic surface. Properties such as geometrical parameters, chemical stability, binding energies and UV-vis spectra (see Figure 2) have been computed and the results compared to available experimental

data.

This research is conducted within the recently estab- lished NIST Center for Theoretical and Computational

Nanosciences. Within the context of this center it is en- visioned that the theoretical models developed and vali- dated in this work will provide a set of important tools

that will complement the experimental studies conducted

by researchers in the nascent molecular electronics in- dustry as well as in research centers at NIST, National

Labs and Universities.

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

Spin Electronics

The Spin Electronics program is creating thefoundationsfor the development ofnew magnetoelectronic

technologies that utilize the electron spin instead ofits charge. It investigates the transfer ofspin angular

momentumfrom electrons to magnetic thin films to induce magnetization dynamicsfor applications as microwave oscillators in high-speed signal processing

and switching ofdiscrete memory elements.

William H. Rippard and Matthew R. Pufall,

EEEL (818)

Wireless communications devices are ubiquitous,

ranging from simple radios to complex structures such as

cell phones and wireless Internet systems. All these devices are based upon the transmission and reception of

electromagnetic signals, with higher frequencies being

required for high data-transmission rates. Common oscillators for wireless applications operate in the gigahertz

regime but are large (several millimeters on a side) and

must be added onto semiconductor chips after their manufacture, increasing component cost. Further, magnetic data storage technologies require novel methods of

high-speed operation of nanoscale memory elements.

Traditional magnetic recording and magnetic random

access memory (MRAM) technologies are encountering

problems as they seek to push dimensions below 50 na- nometers and speeds above 1 gigahertz.

This project concentrates on spin-momentum transfer (SMT) from electron currents to multilayer, ferromagnetic films. SMT is a newly discovered phenomenon that appears in nanometer-scale magnetic devices. We are studying devices that use SMT to induce coherent magnetic precession. The precession frequency can be tuned

from 1 gigahertz to more than 40 gigahertz by changing

the current amplitude or magnetic field angle applied to the structures. Spin-polarized currents can also be used

to switch small magnetic elements used in nanoscale

magnetic recording and MRAM technologies. These new

techniques may enable more efficient switching of sub50-nanometer structures at speeds above 5 gigahertz with

considerably less power and better selectivity.

We are using elecfron-beam-lithographed point contacts and nanopillar structures to achieve the high current

densities needed to induce magnetic excitations in multilayer films. For sufficiently high current densities and

applied magnetic fields, there is an abrupt increase in the

resistance of a point-contact junction. The resistance step

is attributed to precessional dynamics induced by the

SMT effect. We have found that SMT is a generic effect occurring for a wide range of experimental conditions:

for in-plane fields, out-of-plane fields, and intermediate

angles as well as for a number of different device structures. We have also discovered that SMT occurs in a number of different and previously unexplored alloys of

Co, Fe and Ni.

The origin of the SMT effect is conservation of angular momentum. When a dc current flows perpendicular to the plane (CPP) of a GMR "spin-valve" device, electrons

are spin-polarized by the "reference" magnetic layer. Inelastic electron scattering then leads to the transfer of

spin angular momentum to the "sense" magnetic layer.

This transfer of angular momentum can result in coherent oscillations in the sense layer ranging from a few

gigahertz to more than 40 gigahertz, the same range used

for wireless applications. These new devices are only 40

nanometers in diameter and compatible with standard

semiconductor processing, making the new technology

attractive for applications.

Work is now focusing on developing tunable oscilla- tors and on investigating the fiandamental mechanisms

that govern the interaction between magnetization and

spin current.

Tunable Coherent Spin Waves Generated by Direct Current

We have successfiilly excited high-frequency magnetic oscillations in a magnetic multilayer structure with

a direct current. The oscillations are caused by the torque

exerted on the magnetization by the electron spins in the

current. The current is injected at high density through a 40 nanometer contact into the magnetic multilayer. At a

certain critical value, the current induces precessional

oscillations in one of the two magnetic film layers. These

oscillations, detected as a voltage change due to the GMR effect, range in frequency from 1 to 40 gigahertz,

with spectral widths as small as 2 megahertz. The small

widths imply that the oscillations have lower damping

and greater coherence than most known magnetic excitations. The frequency of the oscillations increases with an

externally applied magnetic field. For in-plane fields,

frequencies as high as 40 gigahertz were obtained. An

increase in current causes the oscillation frequency to decrease, with a tunable range of 1 to 5 gigahertz.

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

Figure 1. Current controlled high frequency excitations and

scanning-electron-microscope image ofa nanopillar device.

Spin-Transfer Switching of Magnetic Devices

Using Pulsed Currents

We have demonstrated magnetization reversal in

1 00-nanometer-sized magnetic thin-film devices with

ultrashort spin pulses. The devices have two magnetic

layers separated by a nonmagnetic spacer layer. The devices have two stable states: parallel and antiparallel

alignment of the magnetizations of the free and fixed

layers. The state can be read by using the GMR effect. A

current pulse is applied to the device, causing reversal of

the free-layer magnetization. We studied rapid reversal due to current pulses whose durations ranged between

100 picoseconds and 100 nanoseconds. The pulse duration required for reversal decreases with increasing cur- rent amplitude; for the highest currents applied, reversal occurred in less than 300 picoseconds. This is the shortest reversal time reported yet for SMT-based switching.

Such a result is promising for applying SMT as a method

of writing magnetic data in MRAM and magnetic hard

disk drives, which will be required to operate at gigahertz rates in the near future.

Telegraph Sv^itching Induced by SpinMomentum Transfer

We have also demonstrated that a high density spinpolarized direct current passing through a small patterned magnetic element induces two-state random tele- graph switching of the magnetization via the SMT effect. The magnetization undergoes large, 40 to 90 degree an- gular rotations between two states at rates up to 2 gigahertz for a wide range of currents and applied magnetic

fields. The switching involves the collective motion of a

large number of spins. Such a collective, dynamic source

for random telegraph noise is substantially different from

that those found in superconducting devices and semiconductor circuitry, where isolated atomic-level defects

are usually responsible for two-state fluctuations.

Frequency Modulation and Phase Locking in

Spin-Transfer Microwave Oscillators

Initial work on SMT focused only on the continuous wave output of the new devices. However, such pure

tones transmit no information. Instead, the outputs must

be modulated, for instance in amplitude (AM) or fre- quency (FM), in order to transmit data. We are now able

to modulate the current passed through the device. As

the current is modulated at frequencies much less than

the natural oscillation frequency set by the device, side- band lobes appear on both sides of the original carrier frequency. The details of the outputs can be understood

in terms of standard communications theory. The devices can also be "injection-locked" to an external drive signal

close to their natural oscillation frequencies. In this scenario, the devices are forced to oscillate at the same fre- quency as the injected signal. Over this locking region

the relative phases of the two signals can be controlled

by the dc current passed through the device. We are in- vestigating locking and phase-shifting architectures for possible directional micro-wireless communications.

I

'

1 X

M odu lation

>

c response

T3

Ampli ij Li

9 6 9.7 9.8 9.9 10

F requency (G H z)

Figure 2. Output ofthe device with DC current injection only

(single peak at 9.8 GHz); device output with additional modulation included at 50 megahertz and amplitude of400 microamperes

(peak at 9.8 GHz with satellites at 9. 75 and 9.85 GHz).

NIST Accomplishments in Nanoteclinology

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Nanomagnetodynamics

In order to pursue the rapid development path set out for hard drives and magnetic memory, industry needs

the ability to measure and control magnetization on nanometer length scales and nanosecond time scales. This projectfocuses on the metrology ofdynamics and

damping in magnetic thin films, especially on the ef- fects ofnano-scale dimensions and defects in ferromagnetic resonance measurements.

Robert D. McMichael, MSEL (855)

We develop ferromagnetic resonance (FMR) tech- niques to measure the static and dynamic properties of

thin films and patterned arrays of technologically important ferromagnetic metals and their interfaces with nor- mal metals. The primary results include measurements

of interfacial effects in magnetic damping, which governs behavior on the sub-nanosecond time scale, and

assessment of the magnetic homogeneity of thin films or patterned arrays of magnetic bits. These results are communicated to the magnetic data storage industry through

conference presentations, journal articles and site visits. We have initiated a new investigation into the properties of patterned magnetic film edges. The importance of

edges in the switching of magnetic thin film devices has

been recognized, but no techniques for characterizing

magnetic edge properties have yet been developed. Our

initial models show that microwave frequency ferromagnetic resonance measurements will be sensitive to the edge properties. These edge properties will be useful in the micromagnetic design of non-volatile magnetic RAM

memory.

0

\

Applied Field (T)

H

*

0.5 \

Figure 1. Evolution oftheferromagnetic resonance spectrum as a function ofappliedfield in a 16 nm thick strip of Permalloy. The lowfrequency trace is due to the edge mode. Other modeling

has shown the edge mode to be sensitive to edge conditions. A continuing thrust of this project has been the development of models that describe broadening of the

FMR linewidth by defects. These models, which are

specifically designed for thin films, account for both in- homogeneity and magnetic interactions. Our earlier models were restricted to "local" defects such as anisot- ropy. One important application of these models pro- vides a method for measurement of anisotropy variations

and axis alignment in perpendicular recording media.

This year, we have capped off the model development

with a model for the nonlocal effects of film roughness.

Testing of the roughness-linewidth model involved

making variable roughness substrates, characterizing the roughness by atomic force microscopy (AFM) and

measuring nearly conformal Permalloy overlayers in FMR. Using only the AFM microstructural data and the known properties of Permalloy, and assuming conformal

roughness, our model predictions agreed extremely well

with the measurements. Allowing one free parameter

describing thickness variations, the agreement with the experiment is nearly perfect. Validation of this model is the first example of linewidth modeling based entirely on measured microstructure.

0 10 20 30 40 50 60 70 80 90 100 M angle O

Figure 2. FMR line broadening measured in a Permalloyfilm and calculated almost entirelyfrom the microstructure of the un- derlayer shown in the inset.

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

Nanomagnetism

To make magnetic devices in the nanometer regime,

manufacturers need to be able to characterize the structures they build and to accurately model their

behavior. We are developing the capability ofmeasuring the magnetic properties ofnanostructures in the

presence ofappliedfields and with applied currents. With these capabilities, we are measuring the changing properties ofdevices as their size is reduced. In

parallel, we are developing models that quantitatively

describe the physics captured by these devices.

John Unguris and Mark Stiles, PL (841)

Magnetic Nanoconstrictions

Small, patterned magnetic thin film structures play a

critical role in emerging magneto-electronic technologies, which range from non-volatile random access memories to magnetic field sensors. One potential advantage of miniaturizing these devices is that the size and geometry of the patterned structure may be used to shape the internal magnetic nanostructure and control the device operation. Imaging this internal nanostructure is a measurement challenge, since the structures consist of

only a small amount of magnetic material and can easily

be perturbed by conventional magnetic imaging methods

such as Magnetic force Microscopy (MFM).

One such structure is the magnetic nanoconstriction.

Nanoconstrictions make ideal structures for reproducibly

trapping magnetic domain walls, and furthermore, since

the local current density in the constriction can be large, the domain walls can be manipulated using current in- duced spin torques. Magnetic nanoconstrictions there- fore provide a means of building magnetic devices that can be switched using current pulses, without applying

external magnetic fields.

Figure 1. SEMPA images ofvarious magnetic nanoconstrictions. A magnified image ofone ofthe constrictions is shown along with a corresponding OOMMF micromagnetics calculation.

To image the structure of domain walls trapped and

manipulated in nanoconstrictions, we have used the NIST Scanning Electron Microscopy with Polarization

Analysis (SEMPA) facility. Samples with various ge- ometries and sizes were prepared using electron beam

lithography. We have been completely successful noninvasively imaging static domain walls and domain

structures after applying current pulses. We have found

that the domain wall's shape depends sensitively upon

the constriction geometry.

Imaging Magnetic Sensors

Recent advances in thin film and multilayer magnetism offer the exciting possibility of creating a new generation of magnetic sensors that are small, inexpensive

and as sensitive as currently used, more cumbersome

sensors, such as SQUIDs. Development of this new

generation of sensors requires understanding and controlling the magnetic nanostructure in order to enhance

sensitivity and eliminate potential sources of magnetic

noise. We are working with other groups at NIST to test and measure possible new materials and devices for use as sensors. One potentially useful device is a zigzag shaped

magnetic sensor. The device's magnetic sensitivity is based on anisotropic magnetoresistance (AMR) and

therefore requires the magnetization to be tilted with

respect to the current passing through the device. In the zigzag sensor the shape anisotropy forces the magnetization to tilt relative to the current. We have used SEMPA

to measure how well various zigzag geometries accomplish this angular biasing and to look for potential sources of magnetic noise.

0 2 4 6 8 10

Figure 2. SEMPA image ofa zigzag magnetic sensor element compared with the OOMMF micromagnetics simulation. Line

scans through the center ofthe elements show the magnitude of

NIST Accomplishments In Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

the magnetic oscillations. SEMPA imaging is especially useful since it not only

images the magnetization directly, but also provides a

quantitative measurement of magnetization directions

that can be compared directly with the results of micromagnetic calculations. The SEMPA images also reveal

potential magnetic trouble spots such as trapped magnetic singularities which can interfere with reproducible

magnetic response, as well as defects that can produce

magnetization-related noise. Work is currently underway to allow imaging electrically active magnetic sensors which will allow a better understanding of noise sources and failure modes in real devices.

Current Induced Magnetic Switching

In a magnetic multilayer, changing in the relative ori- entation of the magnetizations of two layers changes the current flowing through the multilayer. This effect, called giant magnetoresistance, is used in magnetic sensors, magnetic random access memory (MRAM), and

read heads in magnetic disk drives. Currently, an inverse

effect, where the current changes the magnetic configuration, is being studied for possible applications. In this case, a current passing through a multilayer exerts a torque on the magnetizations of different layers causing

the magnetizations to rotate.

Large enough currents passing through such multilayers can switch the magnetizations of the layers between parallel and antiparallel or cause one to rapidly precess around the other layer. If the size of the current necessary to reverse magnetizations can be reduced sufficiently, spin-transfer torques could provide a way to switch bits in magnetic random access memory

(MRAM). In other devices, it should be possible to make current-controlled oscillators. We have developed quantitative models for these

spin-transfer torques to help the development of devices

based on this effect. In particular, we recently developed

an analytic formula for the torque as a function of the device geometry and the magnetic configuration. Such a formula will allow rapid simulation of prototypes for device optimization. To test these results, we have car- ried out extensive micromagnetic simulations as a function of current and applied magnetic field to compare

with existing experimental results. We find that these

simulations explain most, but not all of the experimental

results quantitatively. The differences between the simulations and the experiments suggest both new measure- ments and necessary extensions of the model.

This work has been described in four papers over the

last year, building on three prior publications. It has

been reported in five invited talks in academia, conferences, and industry, with four more invited talks scheduled in the near future. Presently, we are extending the modeling to describe structures with non-uniform magnetization.

Relative Chanae m Resistance

H 0.06

0.04

< 0.02

0

-0.3 0 0.3 0,6 0.9 1.2 Current density (10*^ A/'cm^)

Figure 3. Resistance of a magnetic nanopillar at 300 K as a function ofcurrent and appliedfield. The current is sweptfrom negative to positive values. Green lines indicate configuration changes

when thefield is swept in the opposite direction.

Selected Publications

1. Zigzag Shaped Magnetic Sensors, F. C. S. da Silva,

W. C. Uhlig, A. B. Kos, J. Aumentado, M. J. Donahue, J. Unguris, D. P. Pappas, J. Appl. Phys. 85, 6022

(2004).

2. Current-Driven Switching in a Single ExchangeBiased Ferromagnetic Layer, T. Y. Chen, Y. Ji, C. L. Chien, and M. D. Sfiles, Phys. Rev. Lett. 93, 026601

(2004).

3. Boltzmann Test of Slonczewski's Theory of Spin

Transfer Torque, J. Xiao, A. Zangwill, and M. D.

Stiles, Phys. Rev. B 70, 172405 (2004).

4. Spin Configurations and Classification of Switching

Processes in Ferromagnetic rings down to sub- 100

nm dimensions, M. Klaui, C.A.F. Vaz, T.L. Monchesky, J. Unguris, E. Bauer, S. Cherifi, S. Heun, A. Locatelli, L.J. Heyderman, Z. Cui and

J.A.C. Bland, J. Magn. Magn. Mater. 272-276, 1631

(2004).

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

Synthesis, Characterization, and Manipulation of

Magnetic Nanoparticles

Magnetic nanoparticles have potential applications in catalysis and magnetic recording, as well as in medical sensors and biomedicine. The ability to reproducibly synthesize, characterize, and manipulate

these nanoparticles is criticalfor successfully realiz- ing these applications.

Guangjun Cheng and Angela R. Right Walker,

PL (844)

The nanoscale synthesis of inorganic materials has

advanced significantly in the last few years. Our group

uses a refined thermo-decomposition method to synthesize magnetic cobalt nanoparticles with controllable

sizes and shapes. We have, for instance, synthesized

monodispersed colloidal mixtures of cobalt nanoparticles

in the epsilon (hep) crystalline form, the bulk, roomtemperature ferromagnetic phase of Co. X-ray powder

diffraction (XRD) and transmission electron microscopy

(TEM) have been used to characterize the magnetic

nanoparticles.

Figure 1. TEM image ofmonodispersed 15 nm cobalt nanopartcilesWe have also produced magnetic-field-induced

(MFI) assemblies of Co nanoparticles. Under the influ- ence of a magnetic field, Co nanoparticles dispersed in a

colloidal solution assemble into linear chains along the

direction of the applied field. After the removal of the magnetic field, the MFI chains fold into a three- dimensional (3D) super-coiled structure. Optical Microscope and TEM have been used to characterize these morphologies. A superconducting quantum interference

device (SQUID) magnetometer is used to measure the magnetic properties of MFI assembhes.

•200;nrn" '•X- '.--

"^^^Ij

Figure 2. TEM image ofMFI chains ofcobalt nanopartciles.

We have been able to create MFI chains as long as 1 cm in length, corresponding to approximately one million magnetic dipole nanocrystals stacked head to toe along the magnetic field lines. Moreover, we found that reversal of the direction of the magnetic field leads to a

propeller-like rotation of the chains by 1 80 degrees

around their two-fold axis to again align the supra dipole

along the magnetic field direction. Surprisingly, the other possibility in which each of the individual magnetic dipoles reorient does not occur, demonstrating that the individual magnetic dipoles are strongly coupled to each other, even more than they are to the external magnetic field. Removal of the magnetic field causes rapid

collapse of the linear structures and these fioppy chains

fold into a three-dimensional, globular, coiled structure. Light agitation leads to fragmentation, yielding predominantly small ring structures To fully realize the biological potential of these magnetic nanoparticles, methods are being developed to improve their bio-compatibility by coating them with gold

or silver. These bimetallic nanoparticles are expected to maintain the magnetic properties of Co, while the Au or Ag coatings improve their biocompatibility. Co/Au and

Co/Ag nanoparticles are prepared by growing Au or Ag

on the pre-synthesized Co nanoparticles. MagnetoRaman spectroscopy developed in our laboratory and

SQUID magnetometry are used to characterize these bioinert particles.

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

Nanomagnetodynamics: Fundamental Properties

This project is developing instruments, techniques,

and theoryfor the understanding ofthe high-speed

response ofcommercially important magnetic materials. Techniques used include linear and nonlinear

magneto-optics andpulsed inductive microwave magnetometry. Emphasis is on highfrequency (above 1 gigahertz), time-resolved measurementsfor the study

ofmagnetization dynamics under large-field excitation. Research addresses the nature ofcoherence and

damping in ferromagnetic systems and their effects on

thefundamental limits ofmagnetic data storage. The

project provides results ofinterest to the magnetic

disk drive industry, developers ofmagnetic randomaccess memory, and the growing spin-electronics

community.

Thomas J. Silva, EEEL (818)

Advances in magnetic information storage are vital to economic growth and U.S. competitiveness in the world

market for computer products and electronic devices.

Our primary customers are the magnetoelectronics in- dustries involved in the fabrication of magnetic disk

drives, magnetic sensors, and magnetic random-access

memory (MRAM).

Data-transfer rates are increasing at 40 percent per

year (30 percent from improved linear bit density, and 10

percent from greater disk rotational speed). The maximum data-transfer rate in nanometric devices is currently

200 megabytes per second, with data-channel performance of over 1 gigahertz (in the microwave region), with

corresponding magnetic switching times of less than 1 nanosecond. At these rates, a pressing need exists for an

understanding of magnetization dynamics, and measurement techniques are needed to quantify the switching

speeds of commercial materials.

The current laboratory demonstration record for stor- age density is over 30 gigabits per square centimeter

(200 gigabits per square inch). How much further can

longitudinal media (with in-plane magnetization) be

pushed? Can perpendicular recording, patterned media

with discrete data bits, or heat-assisted magnetic re- cording extend magnetic recording beyond the super- paramagnetic limit at which magnetization becomes

thermally unstable? We are developing the necessary

metrology to benchmark the temporal performance of new methods of magnetic data storage.

Our technical strategy is to identify future needs in

the data-storage and other magnetoelectronic industries,

develop new metrology tools, and do the experiments

and modeling to provide data and theoretical underpinnings. We concentrate on two major problems in the magnetic-data-storage industry: ( 1

) data-transfer rate, the problem of gyromagnetic effects, and the need for large

damping without resorting to high fields, and (2) storage

density and the problem of thermally activated reversal of magnetization. This has led to the development of

instrumentation and experiments using magneto-optics

and microwave circuits. Microwave coplanar

waveguides are used to deliver magnetic-field pulses to materials under test. In response, a specimen's magnetization switches, but not smoothly. Rather, the magnetization vector undergoes precession. Sometimes, the magnetization can precess nonuniformly, resulting in the generation of spin waves or, in the case of small devices,

incoherent rotation.

Figure 1. Mike Schneider next to cryogenic and roomtemperature pulsed inductive microwave magnetometers.

Metrology

We use several methods to detect the state of magnetization as a function of time.

The magneto-optic Kerr effect (MOKE) makes use of

the rotation of polarization of light upon reflection from

a magnetized film. We have used MOKE with an optical

microscope to measure equilibrium and nonequilibrium

decay of magnetization in recording media.

The second-harmonic magneto-optic Kerr effect (SHMOKE) is especially sensitive to surface and interface magnetization. We have used SHMOKE for timeresolved vectorial measurements of magnetization dynamics and to demonstrate the coherent control of magnetization precession.

In our pulsed inductive microwave magnetometer

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

(PIMM), the changing magnetic state of a specimen is deduced from the change in inductance of a waveguide.

This technique is fast, inexpensive, and easily transfer- able to industry. It may also be used as a time-domain

permeameter to characterize magnetic materials. Since

the development of the PIMM at NIST, similar systems

have been built at several industrial research laboratories

and universities.

While these instruments have immediate use for the

characterization of magnetic data-storage materials, they

are also powerful tools for the elucidation of magnetodynamic theory. The primary mathematical tools for the analysis of magnetic switching data are essentially phenomenological. As such, they have limited utility in aid- ing industry in its goal to control the high-speed switching properties of heads and media. We seek to provide

firm theoretical foundations for the analysis of timeresolved data, with special emphasis on those theories

that provide clear and unambiguous predictions that can

be tested with our instruments.

Surface Dependence for Rotatable Anisotropy

We found that there is a surface contribution to the

rotatable anisotropy in Permalloy. Contributions to the magnetic anisotropy for which the magnetization direction itself is the source of symmetry breaking are generally categorized as "rotatable anisotropy," and act to en- hance the gyromagnetic frequency. In a comprehensive

study intended to determine the physical origins of ro- tatable anisotropy in single-layer Permalloy films, the pulsed inductive microwave magnetometer was successfully used to measure the anisotropy energy density as a function of magnetization angle. We found that the component of the energy density that is independent of magnetization angle, and therefore results in a rotatable ani- sotropy, is inversely proportional to film thickness.

These results have significant implications for data stor- age applications, where the thickness of the magnetic

sensor layer in a disk drive must be substantially reduced

to accommodate higher areal densities of data.

Large-Angle Magnetization Dynamics

We examined the relationship between nonlinear

magnetic response and the change in the Gilbert damping parameter a for patterned and unpattemed thin Per- malloy (Ni8oFe2o) films subjected to pulsed magnetic

fields. An improved magnetization-vector-resolved tech- nique based on the second-harmonic magneto-optic Keneffect (SHMOKE) was used to measure magnetization

dynamics after pulse field excitation. The magnetization

excitations were achieved with pulsed fields that were

aligned parallel to the hard axis of thin Permalloy films

while a DC bias field was applied along the easy axis. At

low-bias fields, a is inversely related to the bias field, but

there was no significant reduction in the absolute value

of the magnetization, as might be expected if there were

significant spin-wave generation during the damping

process. We discuss the discrepancies between data obtained by ferromagnetic resonance, where spin-wave

generation is prevalent, and pulsed field studies, and

conclude that fundamental differences between the two

techniques for the excitation of the ferromagnetic spin

system might explain the different proclivity toward

spin-wave generation manifest by these two experimental methods.

^ 140

3> 120- ^ 100

c

*-4— g

»

COo

80-1 60

40

20

0-1 -20

H^= 20A/m

H = 1320 A/m

1.2

1.0

0.8

0.6

0.4

0.2

0.0 At (ns)

Figure 2. Vector-resolved, time-domain SH-MOKE signalfor

magnetization dynamics in Permalloy measured at low applied

bias field ///, at which the damping is significantly enhanced. The

data show no substantial evidence ofinhomogeneities during the magnetization switching process, demonstrating that spin-wave

generation does not explain the enhancement ofdampingfor this particular experimental geometry.

Selected Publications

1 . J. P. Nibarger, R. Ewasko, M. Schneider, and T.J. Silva,

"Dynamic and Static Magnetic Anisotropy in Thin-Film

Cobalt Zirconium Tantalum," J. Magn. Magn. Mater.

286, 356-361 (February 2005).

2. M. L. Schneider, A. B. Kos, and T. J. Silva, "Finite Coplanar Waveguide Width Effects in Pulsed Inductive

Microwave Magnetometry," Appl. Phys. Lett. 85, 254-

256 (July 2004).

3. M. L. Schneider, A. B. Kos, and T. J. Silva, "Dynamic

Anisotropy of Thin Permalloy Films Measured by Use

of Angle-Resolved, Pulsed Inductive Microwave

Magne-tometry," Appl. Phys. Lett., in press.

4. M.L. Schneider, T. Gerrits, A.B. Kos, and T.J. Silva,

"Contributions to the Gyromagnetic Damping in Pulsed

Inductive Microwave Magnetometry," submitted.

5. R. Bonin, M.L. Schneider, and J. P. Nibarger, T.J. Silva,

"Dependence of Magnetization Dynamics on Magnetostriction in NiFe Alloys," submitted.

6. T. Gerrits, M.L. Schneider, and T.J. Silva, "Observation

of Enhanced Damping in Ni8oFe2o/Cu Bilayers," submitted.

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Bioelectronics

Experiments in cell biology have been virtually un- changedfor many decades, involving manual handling ofcell cultureflasks, changing the cell media

(feeding the cells), exposing the cell culture to a com- pound ofinterest, and observing cell response and

viability using a microscope. For this reason, cell bi- ology experiments are typically very time consuming

and labor intensive. Recently, the trend has been to conduct studies that involve large arrays ofsmall cell cultures by automating these steps as much as possible using robotic instrumentation, but the experiments

are still essentially the same. A true paradigm shift is to pattern or isolate single cells, or small cell populations, and to probe their individual response using

optical, electronic, and mechanical methods. The

technical approach ofthis research effort is use thin film electrodes to capture single cells from solution by

dielectrophoresis, to anchor them patterned selfas- sembled monolayers, and to interface sensitive single

molecule optical measurement methods to probe single cell response.

Michael Gaitan, Darwin Reyes-Hernandez, and

Brian Polk, EEEL (812), Laurie Locascio, and

Sam Forry CSTL (839)

Recent scientific reports describe the ability to stimulate electronically and probe single cell activity and to transport, sort, and position single cells. These capabilities have resulted in significant advances in biological

sciences and medicine. An excellent example is patch

clamp technology that has revolutionized the field of

electrophysiology. Advances in microfabrication methods have recently led to the development of patch clamp

arrays and other automated on-chip techniques; however,

the widespread adoption of more complex integrated

systems for biologically relevant measurements contin- ues to face technological hurdles. These include complicated materials integration issues (maintaining a biocompatible environment for cells within the in vitro measurement systems, developing stable and drift-free electrodes for accurate measurements in varied buffer

solutions, etc.), the difficulty of accurately determining

the electrical/electromagnetic response of integrated

electronic/MEM S/fluidic systems, and issues related to reproducible fabrication of integrated devices. By ad- dressing these critical measurement infrastructure needs,

NIST will accelerate the development of powerftil new

bioelectronic platforms and techniques.

Our technical approach is to integrate microelectronic, MEMS, and microfluidic systems with cells in order to achieve a new level of control over the elec- tronic/biological interfaces under study. We will develop

methods to adhere and grow cells in defined patterns in a

biological hybrid in vitro environment that incorporates

microelectronic circuits, both electrochemical and rf, to stimulate and sense cell activity. MicroChannel networks

will be used to transport the biological specimens to exact locations and to deliver precise amounts of chemicals

or drugs to the local cellular environment. These tech- niques will allow us to apply precise electrical, electro- magnetic, and chemical stimulation to the cells and to measure their metabolic, electrical, and physiological

responses. We will focus our initial research efforts on

the study of retinal (neuronal) cells and will later progress to other cell systems.

Two weeks growth of retinal cells on poly(ethylenelmine).

Cells are covering all the polycation area.

Figure 1. Two weeks growth ofretinal cells on

poly(thyleneimine). Cells are covering all the polycation area.

Accomplishments

A method was developed to adhere retinal cells on

micropattemed polyelectrolyte multilayer (PEM) lines adsorbed on poly(dimethylsiloxane) (PDMS) surfaces using microfluidic networks. PEMs were patterned on

flat, oxidized PDMS surfaces by sequentially flowing

polyions through a microchannel network that was

placed in contact with the PDMS surface. Polyethyleneimine (PEI) and poly(allylamine hydrochloride)

(PAH) were the polyions used as the top layer cellular adhesion material. The microfluidic network was lifted off after the patterning was completed, and retinal cells were seeded on the PEM/PDMS surfaces. The traditional practice of using blocking agents to prevent the adhesion

of cells on unpattemed areas was avoided by allowing

the PDMS surface to return to its uncharged state after the patterning was completed.

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Figure 2. Fluorescent images ofthe cytoskeleton and nuclei of

R28 cells on polystyrene (A), PAH (B), and PEI (C). The cy- toskeleton is denoted by the red color, whereas the nuclei are observed in blue color. The morphology on polystyrene and the larger areas ofPAH and PEI are similar. Cells are elongated

when patterned in smaller areas as observed on the PEM lines

((B) and (C)). Two weeks growth ofretinal cells on

poly(thyleneimine). Cells are covering all the polycation area.

The adhesion of rat retinal cells on the patterned

PEMs was observed 5 h after seeding. Cell viability and

morphology on the patterned PEMs were assayed. These

materials proved to be nontoxic to the cells used in this study regardless of the number of stacked PEM layers.

Phalloidin staining of the cytoskeleton revealed no aparent morphological differences in retinal cells compared

with those plated on polystyrene or the larger regions of

PEI and PAH; however, cells were relatively more elon- gated when cultured on the PEM lines. Cell-to-cell

communication on adjacent PEM lines was observed as interconnecting tubes containing actin that were a few hundred nanometers in diameter and up to 55 i^m in length.

This approach provides a simple, fast, and inexpensive

method of patterning cells onto micrometer-scale features.

Selected Publications

1. D. R. Reyes, E. Perruccio, S. P. Becerra, L. E. Locascio, M. Gaitan, "Patterning Retinal Cells on Polyelectrolyte Multilayers," The 7th International Conference on Miniaturized Chemical and BioChemical Analysis

Systems (^TAS 2003), Oct 05-09, 2003, Squaw Valley, California, pp. 713-716 (18-FEB-2004).

2. J. Shah, J. Geist, L. Locascio, M. Gaitan, R. Mulpuri,

W. Vreeland, "Surface modification of PMMA for application of wall coating polymers for microchip elec- trophoresis", Electrophoresis, (Approved by ERB)

3. D. R. Reyes, L. E. Locascio, M. Gaitan, "Polyelectrolyte Multilayers", Encyclopedia of Biomaterials and

Bioengineering, Marcel Dekker, New York, pp. 1-15

(2005).

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Single Molecule Nanovials

The biotechnology and healthcare industries require

measurements oflarge sample arrays as part oftheir

combinatorial approach to disease recognition and

drug development. For example, manufacturers of

technologies such as DNA andprotein arrays are

striving to make larger arrays that can more effi- ciently support larger combinatorial measurements.

As the size ofthe sensors is decreased, the sample size

(and hence the number ofmolecules in the sample)

also decreases. Since biological molecules are ex- pected to have variation in their behavior, a question

arises concerning whether the statistical variation of

biological molecules will affect the accuracy ofthose

measurements; e.g., is there a minimum sample size that is required to yield a result that is comparable to traditional ensemble measurements. The ability to measure the structure andfunction ofsingle biomolecules will yield the statistical behavior oftheir

large populations. The primary objective ofthis work

is to develop methods to encapsulate small populations or single DNA, RNA, andprotein molecules in 20-100 nm vesicles that will enable them to be

moved and held in position for optical characterization. This method could not only enable new methods

for single molecule measurements and single molecule

standard reference material, but also enable direct

observation and measurement of nanoparticleformation.

Michael Gaitan, EEEL (812), and Laurie Locascio and Wyatt Vreeland, CSTL (839)

The semiconductor electronics industry has driven

the development of fabrication tools that are capable of

patterning structures that are smaller than cellular dimensions (i.e., on the order of 100 nm). In combination with

micromachining methods developed by MEMS research,

it is possible to create three-dimensional structures that are commensurate with the size of biomolecules. We

will utilize nanofabrication methods and enhanced sur- face coatings to develop novel solid-state structures for the control and manipulation of single biomolecules.

These methods will be based on electrofluidic, electro- mechanical, optical, and magnetic transport of biomolecules in confined nanometer-scale environments.

We have developed and shown that monodispurse

vesicale populations can be formulated with size control over the range of 20 - 1 00 nm by controlling the flow

rates in fluidic networks. The vesicles are formed by

self-assembly of phospholipid molecules that are diluted

in a water miscible solvent stream that is hydrodynamically focused into a water stream. Furthermore, other

molecules that are diluted in the water stream will be- come encapsulated in the vesicles as they form. If the concentration of those molecules is sufficiently small,

single molecules will be encapsulated. Since the method

is in a planar format, one can for the first time directly

observe nanoparticle formation. Ongoing work is to ac- curately characterize this formation process, to incorporate the capability to modulate the type and the number

of molecules entrapped in the liposome nanovials, and to interface the stream with optical techniques to study sin- gle molecule dynamics downstream.

Figure 1. (a) Schematic ofliposome nanovialformation process

in the microfluidic channel. Color contours represent the concentration ratios ofIPA to aqueous buffer, (b) 3-D color contour map

ofDilClSfluorescence intensity atfocused region during lipo- someformation.

Accomplishments

A method has been developed to form liposome

nano-vials in a continuous flow system. Microfluidics is used to elicit control over the spontaneous self-assembly

of liposomes from a solution of dissolved phospholipids.

In this work, we hydrodynamically focus a stream of

lipid tincture at a microchannel cross-junction between

two aqueous buffer streams. In a typical procedure, iso- propyl alcohol (IPA) containing the dissolved lipids plus

a fluorescent dye (DilClS) flows through the center inlet channel, and an aqueous phosphate buffered saline solution flows through the two side inlet channels. DilClS is a membrane-intercalating dye that exhibits enhanced ver

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fluorescence when trapped in a lipid membrane as compared to the fluorescence of the dissolved dye. The flow

rates of the IPA and buffer channels are adjusted to control the degree of hydrodynamic focusing and the width

of the center stream, thus controlling the IPA/buffer dilu- tion process.

Figure 2. Photomicrograph (a) ofthe hydrodynamicallyfocused

stream ofisopropanol injected into a water buffer. The isopropanol rapidly diffuses into the water and the boundary that is ob- served due to the mismatch ofthe dielectric constant ofthefluids disappears. In (b) a redfluorescent membrane intercalating dye

molecule is diluted in the isopropanol that is imbedded in the phospholipid membrane when the liposomes areformed. In panel (c), a green water soluble dye marker is mixed with the water buffer.

Figure 3. Fluorescent photomicrograph ofa single vesicle formed using the technique. In panel (a), the membrane ofthe liposome is imaged using the red membrane intercalating dye

marker. In panel (b) the water aqueous interior ofthe liposome is imaged using the green fluorescent water soluble dye maker. The

number ofwater soluble molecules that will be captured in the liposome nanovial can be controlled by changing the concentration of those molecules in the water buffer stream.

Selected Publications

2. A. Jahn, W. N. Vreeland, M. Gaitan, L. E. Locascio,

"Controlled Vesicle Self-Assembly in Microfluidic

Channels with Hydrodynamic Focusing," JACS 126,

2674-2675, 2004.

1. M. Gaitan, "Nanotechnology in Integrative Systems:

Introduction to Integrative Systems," Nanoscale Sci- ence and Technology, Kluwer Academic Press, pp. 373-388 (17-AUG-2004).

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Optical Manipulation of Nanocontainers and Nanotubes

Storage, transport, and manipulation oftiny amounts

ofmaterial are importantfor applications that include

drug delivery, micro-analysis, and the creation of

model analog ofbiological systems. We are developing techniquesfor the production and manipulation of

nanocontainers, "artificial cells" such as liposomes

andpolymerosomes as well as nanotubes based on

lipid orpolymer layers. These cells and tubes can

store and transport materials, and be used as nanoscale reactor vessels.

William D. Phillips and Kristian Helmerson, PL

(842)

The cell is, arguably, Nature's factory for the nanomanufacturing of proteins and other macromolecules

necessary for sustaining life. Much of this work is car- ried out by small numbers of molecules in many confined reaction compartments of nanometer dimensions.

These nanoscale compartments are integrated into reaction networks, which allow for the transport of information in the form of the chemical compounds. Many

groups, including ours at NIST, are working on the development of biomimetic cells and cell networks. The

goal is to create reaction networks consisting of wellcharacterized nano-reaction vessels for directed delivery

and controlled reaction of chemical compounds.

The process of self-assembly is used widely by Nature for building complex, fianctional structures out of

nano-scale building blocks. Amphiphillic molecules

(molecules with a section that is hydrophilic and a section that is hydrophobic), such as lipids, can self- assemble into bilayer membranes that can then be

formed into shapes with sizes ranging from several tens of nanometers to hundreds of microns. When the membranes assemble into liposomes (spherical structures consisting of a lipid bilayer membrane that separates the aqueous interior environment from the aqueous external environment) they are a natural choice as containers for performing biochemical reactions in ultra-small vol- umes.

Giant liposomes are approximately 1 0 luim in diameter, enclosing about a pico-liter of volume. The membrane is similar in size and composition to that of a cell and therefore giant liposomes should be a natural container for complex biochemical reactions to occur in. A

number of groups are investigating the use of giant lipo- somes as ultra-small containers for biochemical reactions. The basic idea is to bring two liposomes into contact and then induce the membranes to fuse such that the contents of each liposomes mix. We use optical trapping

to bring two giant liposomes into contact and then use an

optical scalpel (a focused, pulsed UV laser beam), aimed

at the contact point between the lipid membranes, to in- duce fusion of the two liposomes into one (see Figure 1). With this technique have demonstrated a controlled

chemical reaction using liposomes as containers. One or two figures should be included in most cases.

Figure 1. Schematic diagram, and photographic images oftwo

liposomes, brought into contact with optical tweezers and induced

to fuse by an optical scalpel.

Polymersomes, which are vesicles similar to lipo- somes but have a self-assembled membrane composed of

amphiphillic polymers, can also be used as biocompatible nano-containers. Similar to liposomes, polymersomes

can be trapped and manipulated with optical tweezers. We have also, demonstrated fusion of polymersomes

initiated by the pulsed UV laser, but the process is typically much slower than the fusion of liposomes, often

requiring several UV pulses. One potentially useful fea- ture of polymersomes, however, is that the polymers can be cross-linked to form even more stable structures.

Lipid nanotubes have recently been observed between live cells and are purported to be conduits for in- tercellular organelle transport. Lipid nanotubes can also be "pulled" from lipid membranes. Researchers in Sweden are using micropipettes inserted into lipid membranes to stretch nanotubes and connect them to other

liposomes, thereby forming complex networks of channels and containers for processing chemical reactions. Recently, we have demonstrated that optical manipulation techniques, such as optical tweezers, could be used

to stretch lipid membranes to form nanotubes (see illus- trations A-D in figure 2). Although the lipid nanotubes

formed are relatively stable, they are manifestations of

strongly distorted lipid membranes and hence, they are not extremely robust. If the lipid nanotubes are released

or the system perturbed, then the nanotubes would tend

to collapse back to a structure that minimizes the energy

of the system, which in this case would be the initial liposome.

Similar to liposomes, the membranes of polymer- somes can be stretched to form nanotubes. This approach

is particularly appealing because the polymersomes are readily cross-linked. We incorporate a surfactant in the polymer membrane, which modifies the visco-elastic

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properties of the membrane, such that nanotubes can be

readily pulled from the membrane using optical tweezers. We have been able to pull nanotubes up to a ~1 cm

in length and were only limited by our sample chamber.

We subsequently stabilize our pulled nanotubes by free radical polymerization of polymers. The cross-linked

nanotubes are extremely robust. They can be washed and

removed from solution, which we did to prepare them

for imaging by transmission electron microscopy (TEM).

The inset of figure 2 is a TEM image of a cross-linked

polymer nanotube. The outer diameter of this cross- linked polymer nanotube is -80 nm.

The polymer membranes that we pull on with optical tweezers to form nanotubes appear to be as easy to manipulate as lipid membranes, especially with regard to forming network structures such as a Y-junction. Figure

2 contains a composite image of a network composed of

polymer nanotubes and polymersomes, imaged via a

fluorescent membrane dye. Optical tweezers were used

to pull nanotubes from polymersomes and attach them to other polymersomes (see illustration A-F in figure 2). The several Y-junctions apparent in this image were

formed by either pulling an additional nanotube from a polymersome that already had a nanotube attached to it and the two nanotubes subsequently merged to form the

junction, or by pulling a nanotube from the membrane of

an existing nanotube.

Figure 2. The illustration on the left shows the procedurefor

creating polymersome-nanotube networks using optical tweezers. The tweezers directly pull on the polymer membrane to stretch out

a nanotube, which can then be inserted into the membrane ofanother vesicle. Thefluorescence micrograph is a composite image

ofa polymersome-nanotube networkformed by optical tweezers. The structures can be made highly stable by subsequently crosslinking the polymers. The inset (upper right) is a TEM image ofa

cross-linked polymer nanotube.

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Fundamental Properties of Quantum Dots

Self-assembled semiconductor quantum dots (QDs) are remarkablyflexible nanostructures that exhibit unique

optical and electronic properties due to their size (-1000

nm^J compared to bulk semiconductors and quantum

wells. They exhibit atomic-like properties due to their three-dimensional electronic confinement, which offers substantial opportunities for unique and improved devices. Quantum dots have yielded superior performance

when incorporated into conventional devices such as semiconductor lasers (low threshold current density and

low temperature dependence) and amplifiers (reduced

cross-gain modulation and large bandwidth). They are

also employed as single photon sources and detectors for

such applications as quantum computing and ctyptography. However, many ofthe fundamental optical, electronic, and structural properties ofquantum dots have

not been measured, yet measurements are sorely needed

for the accurate modeling and optimization of QD devices. We have developed and applied tools to measure

semiconductor quantum dot properties, including the dipole moment, homogeneous lineshape, and carrier

dynamics.

Kevin Silverman, EEEL (815)

Spectral hole burning (SHB) is a powerfiil tool for measuring the optical properties of semiconductor quantum dots (QDs). We employ two complementary tech- niques, ultrafast transient SHB and high-resolution CW

SHB, to measure the linewidth and relaxation dynamics

of these nanostructures from cryogenic to room temperature.

Around room temperature, the homogenous linewidth

of the QD transition is long and significant cross- relaxation between QDs at different energies occurs. This affects the threshold current density in QD lasers and limits the isolation between channels in a QD semiconductor optical amplifier. We have developed a twocolor, differential transmission (DT) measurement capability to temporally and spectrally resolve the carrier dynamics in QDs resonant and non-resonant with pump

excitation. It uses an RF chopping scheme whereby the pump beam is an optically narrowband signal chopped at low frequency and the probe beam is broadband, modulated at a high frequency. Both are coupled to a waveguide sample containing QDs, allowing us to in- crease the interaction length with the weakly absorbing

QDs. After interaction with the sample, the light is sent

to a spectrometer, detected with a photodiode, and the

DT signal is demodulated.

From the rise and decay times observed in the DT

measurements, we determined conclusively, for the first time, that carrier transfer at room temperature occurs between InGaAs/GaAs QDs resonant with pump excitation and those QDs initially empty. In addition, the width of the initial spectral hole produced an estimate of

12 nm for the homogeneous broadening in the QDs.

1180 1200 1220 1240 1260 Wuve length (nm)

Figure 1. Pump-probe spectroscopy ofInGaAs/ GaAs quantum

dots showing pump (dotted) spectrum and differential transmission ofprobe (solid line at various delay times.

The temperature dependence of the escape rate was

also measured using degenerate DT spectroscopy, with an optical gating technique, in which the probe is separated from the pump by means of a non-linear optical

gate. The results indicated that single carriers initially

escape to the wetting layer before being recaptured by

other dots. The escape rate could be modeled with a 4

LO phonon absorption or a thermal model, yielding a

barrier for escape of between 128 and 175 meV for the QDs studied.

At cryogenic temperatures, the interaction between

QD excitons and acoustical modes in the host semiconductor is dramatically reduced. Here, semiconductor

QDs act as sources of single photons-on-demand, indis- tinguishable photons and entangled pairs of photons. Of

fundamental importance to these applications is the co- herence time of the QD exciton and its relationship to the

radiative lifetime. Here, narrow linewidth sources are required to resolve the relatively narrow spectral holes. For this we employed two tunable laser diodes as the pump and probe sources. In addition, we added a hetero- dyne detection scheme, which is necessary when operating at the picowatt power levels required to avoid satu- rating the transition. Fig. 2 displays the spectral hole

burned by a low intensity pump laser at a sample temperature of lOK. From a lorentzian fit to this data, we NIST Accomplishments in Nanotechnology

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determined the linewidth of the transition to be 0.74 |aeV

which corresponds to a dephasing time (T2) of 1.7 ns. The dephasing time is of critical importance to quantum

computing applications since it sets an upper limit on the time that coherent qubit manipulations can occur.

MeV

-2-1012

Laser Detuning (GHz)

Figure 2. Low-intensity spectral hole at lOK. The linewidth ofthe

transition here is 0. 74 /ueVfull-width at half-maximum.

We have also measured the lifetime (Tl) of the QD

exciton in this sample with the method of time-resolved

single photon counting (fig. 3). An exponential fit to this data yields a radiative lifetime of 900 ps. Comparing this with our previous measurement we find the relation Tl =

2*T2 holds in this case and the QDs interrogated here

are purely radiatively broadened. Being radiatively

broadened ensures that the QD is well isolated from the semiconductor matrix that surrounds it and may fairly be

dubbed an 'artificial atom'.

The measurement of fundamental optical and elec- tronic properties of quantum dots places NIST at the

forefront of important data generation as input to the design and simulation of QD devices. Future generation

communication systems will benefit in a variety of per- formance measures.

900 1 , , r— ! 1 1 r

Time (ns)

Figure 3. Time-resolved photoluminescence trace ofthe QD

sample at lOK. Solid line is a convolution ofthe instrument re- sponsefunction and an exponential decay of900 ps.

The measurement of fundamental optical and electronic properties of quantum dots places NIST at the

forefront of important data generation as input to the design and simulation of QD devices. Future generation communication systems will benefit in a variety of

performance measures.

Selected Publications

1. K.L. Silverman, R. Mirin, S.T. Cundiff, "Lateral coupling of InGaAs/GaAs quantum dots

investigated using differential transmission

spectroscopy", Phys. Rev. B 70, 205310-1 - 5 (2004).

2. J.J. Berry, Martin Stevens, R. P. Mirin and K.

L. Silverman, "High-resolution spectral hole

burning in InGaAs/GaAs quantum dots.", Applied Physics Letters 88: 061 14 (2006).

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Multiscale Modeling of Nanostructxires

We have developed a multiscale modelfor nanostructures in solids. The model relates the physical processes at the interatomic level to measurable lattice distortions at the nanometer level and macroscopic

stresses and strains. The model links the subnano (interatomic), nano (nanostructures), and macro length

scales by integrating the powerful techniques ofmolecular dynamics, lattice-statics Green 's function, and

continuum Green 's functions.

Vinod Tewary and David Read, MSEL (853)

Technical description

Mathematical modeling is a very important tool for understanding the mechanical behavior of nanomaterials

and for research and design of devices based upon

nanostructures. A nanostructure needs to be modeled at the following scales: (i) the core region of the nanostructure (sub-nanometer) where the nonlinear

effects may be significant, (ii) the region of the host solid around the nanostructure (nanometer), and (iii) free surfaces and interfaces in the host solid (macro). A

nanostructure causes lattice distortion in the host solid

that manifests as strain throughout the solid. The strain is essentially a continuum-model parameter whereas the

lattice distortions are discrete variables that must be

calculated by using a discrete lattice theory. Hence one

needs a multiscale model that relates the discrete lattice distortions at the microscopic scale to a measurable

macroscopic parameter such as strain.

Conventional models of nanostructures are based

upon either the continuum theory, which is not valid close to the defect, or molecular dynamics (MD) that is CPU intensive and usually limited to small crystallites,

which may introduce spurious size effects. We need a computationally efficient multiscale model that links the length scales from sub-nano to macro and can be used on

an ordinary desktop. Such a model will be a valuable

tool for research and engineering designs as well as for teaching as it can provide quick answers to 'what if -type

questions.

Our model is based upon the lattice-statics Green's

fiinction (LSGF) G that reduces asymptotically to the continuum Green's function (CGF). The displacement

field in this model containing N atoms is given by

u(l) = (1/N) IkG(k) F(k) exp(ik l)

using MD without making the linear approximation. For

small k, G(k) reduces to CGF. Thus, for large 1, the above equation reduces to macroscopic continuum

theory while the discrete lattice effects are retained in F(k). Thus, our model is truly multiscale as it seamlessly links the discrete atomistic effects in F(k) to macroscopic scales through the GF. Even for a millionatom model, the calculation of GF takes only a few CPU

seconds on a standard 3 GHz desktop.

Accomplishments

Initial conference presentations and a journal paper on this new approach have been well received. We are now applying our approach to more challenging systems

with potential industrial applications such as Ge quantum

dots in Si and InAs quantum dots in GaAs. We are now

able to model quantum dots of actual sizes, up to about 8 nanometers, on a desktop computer. To model such a

large quantum dot, it is necessary to include at least a

million atoms in the host lattice. An attempt to model

such a system using MD would need huge computational

effort. It is usual to model such quantum dots using the simple continuum theory. Figure 1 shows our calculated displacement field for an 8 nm diameter Ge quantum dot

in a million atom model of Si host in [100] and [110]

crystallographic directions. For comparison, results predicted by the isotropic continuum model are also shown. The difference is significant near the QD/ host

interface which is the region of practical interest.

06

« 04

w$. / —Continuum model

Multiscale (100) -^MuBlscale (111)

SiGe quantum dot

Size MRvl

Atomic displacements

-60 -40 -20 0 20 40 60

PoSUor along (100) or (111). LHit = aat(Si)/4

Figure 1. Displacementfield through a section ofan 8 nm di- ameter Ge quantum dot in Si.

where I is a lattice site, k is a reciprocal space vector,

and F(k) is the Kanzaki force which is calculated by

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Designing the Nanoworld: Nanosystems and Nanooptics

Developing and exploiting precision metrologyfor

quantum and nanotechnology requires nanoscale

modeling ofultrasmall structures, devices, the optics

ofthese structures, their dynamical operation, and

their response to probes. Atomic/nanoscale simulations ofthe electronic properties and nanooptics of

complex nanosystems at the nano/molecular interface,

including semiconductor nanocrystals, selfassembled

dots, nanodot arrays and solids, metallic nanoparticles, and bio/nanohybrids, are being carried out to provide the foundation neededfor the metrology and

design ofnanolasers, nanodetectors, biomarkers and

sensors, quantum devices, and nanomaterials.

Gamett W. Bryant, PL (842)

Theory is used to extend the fundamental understanding of systems at the atomic/nanoscale interface as necessary to interpret experiment, explore new applications

in nanoscale and quantum technologies, and motivate new and enhanced precision metrology for these

nanotechnologies. We are developing the theoretical understanding needed to create nanooptics structures for applications as, for examples, nanosensors and biolabels,

single photon sources and qubits for quantum information processing, and as the nanoscale optical communication structures needed in emerging quantum and nanoscale technologies.

Semiconductor nanocrystals and quantum dots have

been intensely studied for their potential as tailorable,

man-made, artificial atoms. Artificial molecules and sol- ids fabricated from nanocrystals and quantum dots offer intriguing new possibilities, not possible in natural molecules and solids, because quantum-dot molecules and

solids are not limited by the rules of chemical binding

that determine natural molecules and solids. Nanodot

nanosystems require nanodots with precisely determined

geometry, nanodots tailored with internal structure for enhanced functionality, and arrays of close-packed

nanodots (quantum-dot molecules and solids). To pro- vide an understanding of these nanosystems, models of

nanodots (such as the atomic model for an InAs self- assembled dot shown in figure 1 ) must describe variations in composition on the atomic scale, realistic shapes

and faceting, coupling between closely spaced nanodots,

and nanoarchitectures of dots. Atomic scale theory is needed to describe these nanostructures. We have developed an empirical tight-binding theory of nanodot nanosystems that allows us to study individual nanocrystals

and quantum dots, and, importantly, the complex

nano/biohybrids and nanodot nanoarchitectures that are

built from nanodots. Ab initio models for small dots are used to validate the calculations for complex structures.

Figure 1. Atomic modelfor an InAs self-assembled dot. We have used the theory to model the individual

dots that serve as the building blocks for complex stmctures to determine how the electronic structure of these

nanodots can be tailored by shape and internal structure. Surface effects significantly influence the functionality

of semiconductor nanocrystals, especially

nano/biohybrids being used as nanosensors. Passivation

with ligands or high band-gap semiconductor shells is necessary to reduce trap densities, enhance quantum

yield and increase photostability. Considering CdS

nanocrystals with unrelaxed, unfaceted surfaces, we have

shown that passivated dots with saturated dangling bonds

have no surface states in the fundamental band gap and

all near-band-edge states are quantum-confined internal

states. When only the surface cations are passivated, an

anion-derived surface state band and a band of backbonded surface states exits near the valence band edge.

When only surface anions are passivated, a broad band

of mixed surface/internal states exists between the conduction band edge and the onset of internal states. Because of this strong sensitivity to passivation, explicit

models for surface effects are necessary for precise modeling of the electronic states. We also showed that cap- ping the CdS dot with more than two monolayers of ZnS

significantly reduces the influence of the surface on the

internal electronic structure and optical properties. Such

information is critical for designing complex nanostructures with optimal optical response.

Nanoarchitectures of dots could push classical computing almost to the atomic regime and provide coherent

quantum structures needed for quantum computing. An

understanding of how dots couple in these structures is essential. We have shown that interdot coupling in dot

NIST Accomplishments in Nanotechnology

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molecules (the atomic-scale charge density of a typical electronic state in a double-dot molecule is shown in figure 2) and arrays modifies significantly level ordering

and state symmetries. For ID arrays of stacked dots, the dispersion of electron bands results from the mixing of

the electron envelope functions. In contrast, hole bands

depend on the atomic character of the hole states, especially between dots. Antisymmetric interdot mixing of

low-energy hole states is favored for widely spaced dots. Symmetric mixing is favored for closely spaced dots. The character of this mixing critically impacts the optical response of these states. Interdot coupling also affects the maximum spin polarization that can be realized in hole states. This polarization mixing has consequences

for nano/quantum device applications for the generation

of polarized photons, entangled photons, and quantum

computing with entangled spins.

Figure 2. Atomic-scale charge density of a typical electronic state

in a double-dot molecule.

Nanoscale simulations of optical fields near nanosys- tems are also being carried out. Results are being used to design nanoprobes and nanocavities for use in precision

nanooptics metrology. Results are also being used to design and model the nanooptics highway, that is, collections of nanoparticles used for nanooptical communication to generate, transport and collect photons on the nanoscale, well below the diffraction limit that governs

the classical transport of photons. Nanooptics highways

will be critical for the transport of excitations in quantum

devices and in the metrology of these devices.

The optical properties of molecules are dramatically

influenced by coupling to plasmon resonances of nearby

metallic structures. This can lead to harmful effects such

as fluorescence quenching and to useful effects such as surface enhanced Raman scattering. The demonstration

of single molecule sensitivity via SERS has stimulated

great interest in using the electromagnetic response of

metallic nanoparticles as nanoscale sensors and probes.

The extreme near-field enhancements required for single

molecule Raman scattering is thought to occur in the gaps between metal nanoparticles. We have studied the

optical properties of coupled metallic nanoparticles to investigate their potential in plasmon enhanced spectroscopies and to identify plasmonic nanoprobe structures with optimal response. We have shovm that changing the dimensions of the nanoparticles or increasing local interparticle coupling is a major source of field enhancement. Coupling pairs of

nanoparticles generates a redshift of the plasmonic re- sponse and an enhancement of the near field signal in the cavity between the nanoparticles. The redshift in pairs of

particles is similar to the redshift in an isolated particles

with the same total length, but the cause of the shift is different. The redshift in coupled pairs is connected with

the coupling at the cavity, while the redshift in isolated

particles is connected with depolarization. The local field enhancement at the cavity of a pair is more than an order

of magnitude larger than near the end of an isolated particle. We have shown how this field enhancement can be

tailored by the particle size and shape, interparticle separation, and the geometry of the particle ends. Such in- formation will greatly aid the design of structures optimal for nanoscale sensing or for nanooptical communication.

NIST Accomplishments in Nanotechnology

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Precision Metrology for Nanoscale Quantum Technologies

Advanced optical metrology is needed to provide the

characterization tools criticalfor the continuing development ofemerging nanotechnologies and quantum (coherent) technologies. We are developing and

exploiting precision metrology at the interface between atomic and nanoscale systems, focusing on precision optical metrology and nanooptics, to further the

understanding ofthe nanooptics and nanomechanical

properties ofnanoscale and quantum-coherent sys- tems.

John Lawall and Gamett W. Bryant (842)

Accurate measurement of very small distances is essential for characterizing structures used in nano and

quantum technologies and for industries such as semiconductor fabrication. For some kinds of measurements,

such as those needed in large chip fabrication, the ability to precisely measure distances of centimeters is important. We have been developing advanced, high-precision

optical techniques for displacement measurements. We

recently demonstrated the ability to measure displacements up to 25 mm with the lowest uncertainty ever re- ported. To characterize micromechanical (MEMS) and

nanomechanical (NEMS) systems, the challenge is even

greater. Femtometer displacements at GHz speeds need

to be measured.

We have demonstrated a technique for measuring

displacements with an accuracy of 10 picometers (about

1/1 0th the size of an atom) over a range of 25 mm. The

corresponding relative uncertainty is 4 x 10"'", the smallest ever reported. The method employs Fabry-Perot in- terferometry, and offers unique advantages over ap- proaches previously used. The key idea is that the reso- nant frequencies of two modes of a scanning Fabry-Perot

cavity are measured simultaneously, using optical tech- niques to measure the absolute frequency and radiofrequency techniques to measure the difference fre- quency. The associated measurement redundancy imposes a strict bound on many systematic sources of error. In addition, the spatial filtering provided by the resonant

cavity provides control over the exact geometry of the

laser light that is not possible in more conventional ap- proaches - and is essential when making measurements

with this level of accuracy. Another remarkable advantage of the technique is that the light can be extinguished

or blocked as the target is moved, which is not the case

in the traditional approach where sinusoidal fringes are counted during the motion. Finally, the Fabry-Perot cavity under interrogation is located remotely from the laser

system and coupled via an optical fiber link, making the system particularly attractive for remote sensing applications.

< > Av~ 16THz

Figure 1. The length ofa scanning Fabry-Perot cavity is measured by simultaneously probing three resonances, two at 633 nm

(red) and one at 613 nm (orange). In the case illustrated above,

the order number N is measured to be N = 732 635, and the dif- ference in order numbers probed by the red and orange lasers is 25 164. A natural extension of this work relates the displacement measurement directly to the SI definition of the meter via replacement of the iodine-stabilized helium- neon measurement laser with a cesium clock. This effort

is a modem realization of an idea, first put forth by Zoltan Bay at NBS in 1970, of using a microwave meas- urement of the mode spacing within a Fabry-Perot cavity

to measure displacements free of the systematic error induced by diffraction. In addition to probing two adja- cent modes of a Fabry-Perot cavity with red light at 633

nm, as discussed above, we probe another mode with

orange light from a helium-neon laser at 612 nm. A precise measurement of the mode spacing using the light at 633 nm allows us to determine the exact number of

modes in the 1 6 THz interval between the red and orange

lasers, as shown in the figure. We then beat light from

the helium-neon lasers against light from a visible fre- quency comb that is locked to a cesium clock. The re- sulting measurements of the cavity length are not compromised by diffraction, and are related directly to the

definition of the second, and by extension the meter. We are now beginning to exploit advanced optical

metrology tools, such as the high precision displacement

techniques that we have developed, to probe nanoscale

and quantum coherent phenomena that underlie nano and

quantum technologies involving nanomechanical sys- tems, nanooptical systems, and coupled nanooptomechanical systems. Nanomechanical systems are currently

used as ultrasensitive force and mass sensors. They offer the potential for imaging large molecules, improving our

understanding of quantum measurements, and being used

to read out quantum logic gates based on individual

magnetic nuclei. We are planning to construct nanomechanical devices at the AML nanofabrication facility and

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1 - Fundamental Nanoscale Phenomena and Processes

to explore them in the regime where they exhibit quantum behavior. This will involve initial cooling by cryogenic means and then further cooling by techniques

reminiscent of quantum optics. One possible approach

exploits both nanomechanical and nanooptical tech- niques by integrating a quantum dot into the mechanical

resonator and effecting sideband laser cooling. Another

approach involves coupling the resonator to a Cooper

pair box. Both approaches have been predicted to allow

cooling to the vibrational quantum ground state of the

oscillator. We will pursue the most viable approaches

that allow us to reach the quantum limit and begin to probe and exploit quantum effects with nano/mesoscopic

structures.

Classical optical metrology and communication use macroscopic source, detectors, lenses, mirrors,

waveguides, etc. However, classical optics does not work at subwavelength distances. Nanoscale manipulation of photons for communication in nano/quantum devices and for optical metrology with proximal nanoprobes and nanosensors demands very different tools to control and guide photons. Photons will be manipulated on the nanoscale by converting them to material excitations (plasmons in metallic nanoparticles, electronic excitations in quantum dots), transporting the excitations

along nanooptical guides made of coupled nanoparticles,

and then converting the excitations back to photons.

We plan to develop approaches for nanooptical metrology and communication based on the use of metallic

nanoparticles and semiconductor quantum dots as nano- components for generating, transporting, guiding, focusing, storing and detecting photons on the nanoscale.

This will include exploiting metallic nanoparticles and

structures to locally enhance fields, via the plasmonic

excitations of the metallic structures, to achieve fields strong enough for high sensitivity single molecule detection and analysis. We will develop nanoantennas and

nanoguides, made by integrating metallic nanoparticles,

nanocrystals and quantum dots, to controllably transport

light from source to nanosample to detector via the material excitations in the nanoantenna or guide. A typical

nanoantenna or guide could have metallic nanoparticles

to enhance fields, nanocrystals to control excitation

transfer and quantum dots to provide single-photon generation and detection.

In the quantum limit, a single photons or plasmon

would be used to probe or communicate with individual

nanostructures, qubits, atoms, molecules, etc. We plan

to pushing nanooptical metrology and communication to

this limit. Control of photon and plasmon generation

and manipulation down to the single quanta limit will be

required. This opens up the ultimate quantum limit

where the coherent evolution of quantum processes can

be intiated, probed, or controlled via single photonic or plasmonic quanta without the decoherence due to other

quanta as would happen if a classical field were used. Achieving this limit will be critical for the measurement

of quantum processes in chemical and biological structures, quantum devices, and quantum information processors.

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Control of Cold Quantum Gases for Atomic Clocks and

Quantum Information Processing

The need is for greatly improved primary standards

for time andfrequency andfor the development of

practical devices that exploit the revolutionary potential ofquantum information processing. To achieve

these ends, this project will develop theoretical tools and modelsfor characterizing coherent quantum mechanical manipulation ofsingle atoms and their inter- actions in ultracold quantum gases.

Paul S. Julienne and Eite Tiesinga, PL (842)

Cold gases of neutral atoms offer unprecedented opportunities for ultraprecise control of quantum processes

for individual atoms or collections of atoms. Recent

experimental progress opens up opportunities for dra- matic breakthroughs in atomic clocks and quantum in- formation processing. The task of this project is to develop theoretical modeling and predictive capability for using ultracold quantum gases for these purposes.

Typical atom traps can hold on the order of 1 million

atoms at temperatures on the order of 1 (iK in a trap of

10 \xm to 1 mm in size. It is also possible to use a set of

directional laser beams to impose an optical lattice struc- ture on the trapped atoms. Such a lattice, shown schematically in figure 1 below, can confine individual atoms

to a volume on the order of 30 nm in single trapping

cells spaced a few hundred nm apart. When a quantum

gas of bosons undergoes a Mott insulator transition, for example, it is possible to occupy a wide region of the

lattice uniformly with exactly one atom per site or, in some cases, two atoms per site.

Figure 1. Optical lattice structure imposed on trapped atoms

using a set ofdirectional laser beams.

Lattices with no more than one atom per site are very

promising for atomic clocks, since the confinement to a

single quantum state completely eliminates the Doppler

effect, and single cell confinement eliminates the clock

frequency shift due to atomic collisions. Lattices also offer prospects for entangling gate operations for quantum information processing by coherent quantum manipulation of atomic interactions in cells containing two

atoms.

The collisions and interactions between cold atoms

play a very significant role in cooling and trapping of the gases and in the loading and control of atoms in optical

lattices. Some collisions are very beneficial, aiding the cooling of the gas or the control of desired quantum

properties. Other collisions are harmful, and result in loss of trapped atoms or the decoherence of quantum

properties. The long-term goal of this project is to develop quantitative theory for characterizing the interac- tions and collisions of specific atomic species that can form cold, neutral atomic gases. These models also characterize how optical and magnetic fields can modify

key properties. We have previously developed quantitative models for Li, Na, K, Rb, and Cs atoms, and more

recently, have begun work on Cr, Ca, Sr, and Yb. All of

these species have been cooled or trapped in the ultra- cold regime. These models have been developed in conjunction with experimental groups at NIST and other

laboratories, and are consistent with all the known data on these systems.

Most recently we have developed quantitative theory

to explain and account for the dynamical properties of

Feshbach resonance states in the alkali-metal species,

both for free-space collisions and for interactions in opti- cal lattice cells. Figure 2 shows a schematic example of

a Feshbach resonance for collisions of ^^Rb atoms in one of its Zeeman sublevels near a magnetic field of 1

5

mT. The figure shows how the collision probability varies versus magnetic field strength and collision energy

(color contour) and how the binding energy of a molecular bound state of the atom dimer varies (red line). Such

magnetically controllable properties have been widely

used by experimental groups to control and modify the properties of a variety of species of ultracold bosonic and

fermionic quantum gases, for example, to achieve BoseEinstein condensation or to create a Bose-Einstein condensate of dimer molecules comprised of two fermionic

atoms. One recent project we completed considered a new

architecture for quantum information processing using a

set of fixed marker atoms individually confined in opti- cal lattice cells. By bringing a movable messenger atom

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

to a lattice site and performing a two-atom logic gate

operation, then moving the messenger atom to other

sites, a distant marker atom can be entangled with the

original marker atom and used for quantum computation.

We showed how the moving and bringing together of the atoms could be done dynamically. We also showed how

particular Feshbach resonance states of

^ ''Rb atoms might

be used to construct the needed two-atom gate operations. This work was done in collaboration with a group

at the University of Irmsbruck, Austria.

4 \

6 1 mT)

Figure 2. Schematic example ofa Feshbach resonance for colli- sions of ^'Rb atoms in one ofits Zeeman sublevels near a magneticfield of 15 mT.

It is known that cold, trapped atoms can be combined

into molecules by a time-dependent sweep of the magnetic field that moves the field to a domain where there

is a bound molecular dimer state. In a collaborative effort with Oxford University, we have developed general

models of this process for atoms in free space or in lat- tice cells. Using the example of the 15.5 mT ^^Rb reso- nance shown in the Figure, we showed how a universal

theory of the lifetime of the bound state could be calculated in general from the known parameters of the reso- nance. The lifetime increases by orders of magnitude

when the bound state is very weakly bound close to the point of resonance. Our predictions were verified quantitatively by an experimental study at the University of

Colorado.

A Feshbach resonance near 830 mT for fermionic

species ^Li has been used by a number of groups to study

the paring of unlike fermionic spin states in a quantum

degenerate gas analogous to Cooper pairing and to

switch the gas between such paired states and a molecular Bose-Einstein condensate of dimer molecules. We

have worked closely with an experimental group at the University of Innsbruck to interpret and analyze data that allows us to construct a highly quantitative predictive

model of ^Li atom interactions that will be extremely

useful to researchers in this field. The next generation of atomic clocks will be based on optical frequency instead of microwave atomic transitions. This is because of the 10000 times faster oscilla- tion fi'equency of optical clocks as compared to the present Cs microwave standard. In particular, cold, trapped

Ca, Sr, and Yb atoms offer excellent candidates for opti- cal frequency atomic clocks. The alkaline-earth-metal

atoms have atom cooling properties very different from

the alkali-metal atoms. We have started a program of

first principles calculation of the properties of these sys- tems relevant to using them as clocks.

In particular we have calculated the optical spectrum

when light is used to photoassociate two Ca atoms to an

excited molecular state. The optically allowed atomic

transition gives rise to a spectrum similar to that known

for alkali-metal atoms, only simpler. However, the Ca

atoms have a weak intercombination line transition that

is not present for alkali-metal species. This line is the

basis for the atomic clock transition in Ca, as is true for the analogous line for Sr or Yb. We have calculated the intercombination line photoassociation spectrum. We

also have predicted that the molecular intercombination

lines in these species can be used as optically controlla- ble Feshbach resonances that should allow control of

scattering properties similar to what has been done with

alkali-metal atoms. This is important, since the lack of

magnetic structure in the ground state of Ca, Sr, or Yb

does not permit magnetic Feshbach control.

One of the largest contributors to the error budget of

an atomic clock comes from a shift in the clock transi- tion frequency due to collisions between the atoms. We

are initiating a study to understand and calculate the collisional shifts that will affect optical frequency atomic

clocks. This includes cataloging the physical processes

that will contribute, and identifying the possible magnitude of each. This will guide fiiture experimental studies

that will try to quantify the actual magnitude of such

shifts, and will aid the development of predictive models

calibrated by experimental data.

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Quantum Computing with Trapped Ions

This project, part ofthe NIST Quantum Information Program, strives to demonstrate a working prototype andfully scalable quantum computer ofapproximately 10 to 20 quantum bits (qubits) using laser- cooled trapped ions. Quantum computing has the potential to revolutionize information processing,with

dramatic impacts in such areas as cryptography, database searching, complex systems modeling, and

many other applications. Within the past year, this project has successfully demonstrated all the requirementsfor a fully scalable quantum computer, as well

as demonstrating application ofquantum computing

principles to fundamental metrology. This project

serves the interests ofthe intelligence and defense

communities, as well as the longer-term interests of

the commercial information technology sector.

David Wineland and Tom O'Brian, PL (847)

The NIST trapped ion quantum computing project is arguably the most successful quantum computing re- search effort in the world. This project uses the team's

long expertise in quantum state engineering and measurement of atomic ions confined in linear electromagnetic traps. Over approximately the past year, the project

has demonstrated all the five DiVincenzo criteria for a

fully-scalable quantum computer, has achieved several world's first demonstrations of crucial quantum computing achievements such as quantum teleportation of in- formation on massive particles and robust error correction, and has successfully applied quantum computing

techniques to enhanced spectroscopy and early demonstrations of a quantum logic atomic clock. The program

attracts top researchers and collaborators from across the world, and has received intense interest from the U.S.

intelligence and defense communities and from U. S. information technology companies interested in adopting

the NIST technology.

The main thrust of the project is to demonstrate a working, scalable, prototype quantum computer consist- ing of about 10 to 20 trapped ion qubits. The project currently uses ^Be* ions for most experiments, although

other species are used for related experiments. The team

is consistently working with collections of four to five

ions, with a clear path to scaling to larger numbers of

qubits. Over the past two years, the team has achieved all of the DiVincenzo criteria for a practical, scalable quantum computer: a scalable system of well-defined qubits; a method to reliably initialize the quantum system; long

coherence times; existence of universal gates; and an

efficient measurement scheme. Most researchers consider the trapped ion approach to have the clear lead in quantum computing research among many other ap- proaches (neutral atoms, superconducting Josephson

Junctions, nuclear magnetic resonance, etc.).

This project represents close collaborations between

the Time and Frequency Division (experiment) and the Mathematical and Computational Sciences Division

(theory), as well as close interactions with the Quantum

Electrical Metrology Division on the fabrication of microscale ion traps. The project is funded through internal NIST support and the intelligence community.

Accomplishments

• Demonstration of quantum teleportation of information on massive particles (^Be^ ions), simultaneously

with a group fi-om Innsbruck. Previous demonstrations of quantum teleportation used photons. Teleportation of quantum information among massive

particles may be a key to efficient transfer of quantum information in large-scale quantum computers -

effectively serving as the "wiring" of a quantum

computer.

nature

Quantum

teieport^on

with atoms

oil explanation

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Visual recognition

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Enhanced quantum state detection efficiency. A key

process in both quantum computing and metrology

using quantum information processing is highfidelity detection of particular quantum states. By

encoding the state of one qubit with an additional

(ancilla) qubit and measuring both qubits, the state of the original qubit is detected with greater fidelity in the presence of noise. These results suggest that errors in individual qubit measurements need not

NIST Accomplishments in Nanotechnology

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1 - Fundamental Nanoscale Phenomena and Processes

limit scalable quantum computers, when the scheme

is applied to more ancilla qubits.

Trap chip

Figure 2. Six-zone alumina wafer ion trap The team is conducting

research on developing traps to support two-dimensional ion ar- rays to support more efficient quantum computing with larger

numbers ofqubits.

• Demonstration of broadly-applicable robust quantum error correction. Measurement noise is unavoidable in quantum processing for computing and

communications. A scalable quantum computer re- quires techniques to control and correct for the er- rors introduced by such noise. The team conducted

the first experimental demonstration of a robust, scalable quantum error correction scheme using

three ^Be^ ions in a linear, multi-zone trap. An en- coded one-qubit state is protected against spin-flip errors by means of a three-qubit quantum error- correcting code. In principle, the approach enables a quantum state to be maintained by means of re- peated error correction, an important step towards

scalable fault-tolerant quantum computation using

trapped ions.

• Demonstration of entangled state spectroscopy for three particles. The ultimate Heisenberg limit for measurement on a system of particles is obtainable

only when the particles are entangled. Then the sensitivity of the measurement for N entangled particles

increases by N"^ compared to unentangled particles.

The team entangled three ^Be^ ions and showed that

1/2 the phase sensitivity increased by the factor of 3 (within experimental uncertainty) as expected. Such

Heisenberg-limited measurements could be used to reduce the required time for averaging atomic clocks

to their ultimate uncertainty by a factor of N when

the N particles are entangled. For example, the current NIST primary frequency standard (atomic

clock) requires about 1 0 days of averaging of continual measurements on about one million unentangled cesium atoms to achieve the ultimate noiselimited uncertainty. If the million atoms could be entangled, the measurement time to reach the noiselimited uncertainty would be reduced by about a factor of one million to about one second. While this early demonstration of entangled state spectroscopy on three ions is obviously far removed from routine

entanglement of one million atoms, the demonstration clearly points to the value of quantum information processing techniques for fiindamental metrology as well as for quantum computing.

• Based on the work described above as well as many other accomplishments in quantum computing

and quantum metrology over many years, team

leader Dr. Dave Wineland won the Frederick Ives Medal (highest award presented by the Optical Society of America) and the Presidential Rank Award for Distinguished Service presented to the handful of

highest-achieving senior professionals in the Federal

government.

Selected Publications

1 . M. D. Barrett, J. Chiaverini, T. Schaetz, J. Brit- ton, W. M. Itano, J. D. Jost, E. Knill, C. Langer, D. Leibfried, R. Ozeri, and D. J. Wineland, Deterministic quantum teleportation ofatomic

qubits. Nature 429, 737 (2004).

2. T. Schaetz, M. D. Barrett, D. Leibfried, J. Brit- ton, J. Chiaverini, W. M. Itano, J. D. Jost, E.

Knill, C. Langer, and D. J. Wineland, Enhanced

Quantum State Detection Efficiency through

Quantum Information Processing, Phys. Rev.

Lett. 94,010501 (2005).

3. J. Chiaverini, D. Leibfried, T. Schaetz, M. D.

Barrett,R. B. Blakestad, J. Britton, W.M. Itano,

J. D. Jost, E. Knill, C. Langer,R. Ozeri, and D. J. Wineland, Realization ofquantum error correction, Nature 432, 603 (2004).

4. D. Leibfried, M. D. Barrett, T. Schaetz, J. Britton, J. Chiaverini, W. M. Itano, J. D. Jost, C. Langer, and D. J. Wineland, Towards Heisenberg-Limited Spectroscopy with Multiparticle

Entangled States, Science 304, 1476 (2004)

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Neutral Atom Quantum Computation

Quantum information processing has applications to commercial information transmission, national security intelligence, and advanced device development

through secure transmission, decryption, and modeling ofquantum systems. We are developing the tools

to use neutral atoms as qubitsfor quantum processing. Optical lattices and optical tweezers are to be

usedfor storage as well as manipulation andprocessing ofthe atomic qubits.

William D. Phillips and James Porto, PL

(842)

Quantum information stores and manipulates qubits,

which differ fundamentally from the ordinary bits of

familiar, classical information. While ordinary information bits can be zero OR one, qubits can be both zero AND one, at the same time in quantum superposition.

The possibility of entangling many qubits gives quantum

processing a computational power that is exponentially

greater than that of classical computing for some problems. For example, the factoring of large integers is a famously hard problem. The time to factor grows as an

exponential of the size of the number, so that as far as anyone knows no present or conceivable classical computer could factor a very large number in any reasonable

time. By contrast, a large scale quantum computer, if made, could factor numbers, using Shor's algorithm, in polynomial time.

The difficulty of factoring numbers ensures the security of public-key encryption schemes, so the possibility

of easy factoring impacts commercial communication as well as national intelligence. Another side of quantum

information is that transmission of information as qubits

guarantees that the message cannot be intercepted without it being evident to the communicators. Small quantum processors would provide the capability of making

quantum repeaters to enable quantum secure transmission over long distances. Finally, quantum information

processing allows the efficient simulation of quantum

devices that are exponentially difficult to simulate on a

classical computer. This would provide an important

tool for the design of new devices, and understanding of

solid-state systems.

We intend to use single Rb atoms as qubits, with the

internal hyperfine states of the atoms being the qubit

states. The atoms are first laser cooled to microkelvin

temperatures, trapped and further cooled so as to un- dergo Bose-Einstein condensation, then held in an optical lattice, (an array of optical microtraps formed by in- tersecting laser beams) that can hold single atoms at single, addressable locations. Optical tweezers, (single fo- cused laser beams) will move atoms from one location to another, allowing coUisional interactions that will entangle the atoms and effect the quantum computation.

So far, we have produced large arrays of atoms, demonstrated the confinement of single atoms on single lat- tice sites, developed a superlattice method for tightly

confining the atoms while keeping them separated by

enough to allow individual optical addressing, and demonstrated the ability to move the atoms in a state dependent manner. We are collaborating with an international

group of researchers on the theory of realizing atomic

quantum processors, and on optical tweezer manipulation, rapid entanglement processes, and the conversion of

information from photons ("flying" qubits used for in- formation transmission) to atoms (material qubits used

for information processing).

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^^"^H^^^^^^HI^Non-a di a ba t i c

Adiabatic foading puts atoms > 99.5% in the ground state.

Figure 1. The time-dependence ofthe diffraction pattern ofatoms

releasedfrom an optical lattice reveals thefidelity ofputting at- oms into the motional ground state ofthe individual lattice-site optical traps. The near-constancy ofthe pattern in time shows

that "adiabatic " loading has better than 99.5%fidelity, while

deliberately nonadiabatic loading results in a strong time dependence.

NIST Accomplishments in Nanotechnology

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2 - Nanomaterials

Polymer Miscibility of Imidazolium Compatibilized Nanotubes

The conductivity, strength, toughness, andflammability properties ofpolymers may all be substantially

improved by the addition ofcarbon nanotubes

(CNTs). However, the effective utilization of CNTs in nanocomposite applications may depend on the ability

to homogeneously disperse them into the polymer matrix. Furthermore, extensive interfacial interaction is required to achieve load transfer across the CNTpolymer interface, to prevent reaggregation during

subsequent processing, and to enable other enhanced

properties in the nanocomposite. Nanotubes preferentially aggregate into bundles, where adjacent tubes are held together by strong van der Waals attractions.

Jeffrey Oilman, BFRL (866)

One approach to achieving good dispersion of nanotubes in polymers is to take advantage of van derWaals

interactions between the aromatic surface of the CNT

and compatible hydrocarbon groups on a surfactant. For

many applications, this non-covalent compatibilization

approach may be a more facile and practical processing

method than the alternative, which is to introduce covalently bonded functional groups on the ends and side- walls of the CNT's. Up to now, two types of surfactants have been studied: non-ionic surfactants used with or- ganic solvents for dispersion in epoxy resins, and ionic

surfactants, such as sodium dodecyl sulfate (SDS), which

can be used with water soluble polymers. Unfortunately,

SDS undergoes acid hydrolysis at 40 °C and has an onset temperature for decomposition of 190 °C; these properties limit its use for melt blend processing, since most

polymers are melt processed at 200 °C or above. We

have demonstrated that imidazolium salts are excellent

cationic treatments for layered silicates (clays); they enable high temperature curing and melt processing of

polymer clay nanocomposites due to their high thermal

stability and excellent polymer compatibility when one

of the imidazolium alkyl groups is a C- 16 aliphatic chain. In addition, we have also been investigating the use of imidazolium intercalated graphite to prepare polymer graphite nanocomposites. In this project, we are extending this approach to the study of the miscibility of

imidazolium treated nanotubes in commodity polymers,

The effect of adding 0.5 % (mass fraction) 1 ,2-dimethyl3-hexadecylimidazolium tetrafluoroborate (DMHDImTFB) to PS/CNT is shown in Figure 1. In the TEM image on the left very few unaggregated CNTs can be observed; instead nanotube agglomerates (~1 |j.m) dominate. In contrast, when DMHDIm-TFB modified CNTs

are used, well dispersed single nanotubes can be observed in the TEM (right).

Median- 109 ± t mn

2O0 400 EOO

Nearest ReigMjor dstance inm) 0 :!DC 400 aOC Nearest neflhbor d&(3nce inni)

Figure 1 . TEM images of PSMW'NT (without DMHDItn TPB) (a) and PSMW'NT (1:1) D^-IHDIm TFBfb). Figure 1. a) TEM images ofPS/MWNT (without DMHDIm) and

b) PS/MWNT(1:1) DMHDIm TFB.

In the process of investigating new approaches to compatibilize nanoadditives, such as clays and nanotubes, to polymers, we have also developed several tech- niques for monitoring and measuring the degree of dis- persion achieved. A laser scanning confocal micrograph

of a polystyrene/nanotube nanocomposite is shown in figure 2. A fluorescent dye was blended in the polymer

matrix, but the fluorescent intensity is quenched in the

vicinity of the nanotubes. Thus, by measuring the spatial

variation of the fluorescent intensity we can determine

the degree of dispersion of the CNTs in the polymer matrix.

NIST Accomplishments in Nanotechnology

45
2 - Nanomaterials

New Materials to Protect Firefighters from Bum Injuries

Firefighters are being seriously burned andfatally

injured while wearing their protective clothing system.

Nanocompositefabrics may have the ability to dissi- pate heat energy awayfrom the skin and protect

against burn injury.

Jeffrey Gilman and Kuldeep Prasad, BFRL (866)

Calculations of thermal performance of protective

clothing, using a model developed at NIST, have demonstrated that fabrics with unidirectional heat conduction

properties have the potential to protect firefighters from

serious bum injuries (Figure 1).

Anisotropic Anisotropic ,, „ , Isotropic k =io*i< l<j=100*k Kuldeep Prasad

Figure 1. Surface temperature as a function oftime.

Estimates of the thermal properties of carbon nanotubes (CNTs) indicate that the ratio of the thermal conductivity along the axis to that across the diameter of the nanotube may be as much as to five orders of magnitude

(Table I).

Table I. Comparison of Thermal Conductivity

Material Thermal Conductivity (W/mK)

CNT 2500 - 37000

graphite 100-200

polymers 0.01-20

Unfortunately, our preliminary results, obtained us- ing infrared imagery, have indicated that fabrics made

from polymer/nanotube composites (containing on the order of only a few percent nanotubes) do not possess

the requisite properties to prevent the formation of local hot spots; presumably because the exchange of thermal

energy between the polymer matrix and nanotubes is ineffective (right hand side of Figure 2). On the other

hand, our experiments indicate that materials consisting

predominately of aligned carbon nanotubes can provide

the necessary unidirectional thermal conductivity (left hand side of Figure 2).

Figure 2. Comparison showing the effect ofalignment of

carbon nanotubes on unidirectional conductivity

We will use our imidazolium based ionic liquids to prepare nanotube liquid crystal solutions and a highly

aligned, naturally occurring polymer (silk) as a spinning

dope to prepare nanotube fibers. The general idea is to compatibilize the nanotubes to the silk using the imidazolium treatment developed in our laboratory so that the nanotubes and silk are blended within the fiber structure. The silk will give the flexibility to the thermally conductive nanotube phase in the fiber. Our expectation is that

this will allow us to increase the nanotube content of the

fibers to the point where a significant enhancement in thermal conductivity can be achieved. We will also in- vestigate the possibility of using -OH and -NH2 fiinc- tionalized nanotubes to improve compatibility and,

thereby, to enhance the efficiency of thermal energy ex- change between the nanotubes and the silk. Once we

have perfected the technique for making macroscopic

fibers from carbon nanotubes, we will prepare several samples for testing on BFRL's firefighter mannequin.

The test materials will be placed on problem areas (where the fabric is forced to bend or crease to conform

to the shape of the body) of the mannequin and the temperature distributions will be determined from quantitative analyses of images obtained from IR photography.

NIST Accomplishments in Nanotechnology

46
2 - Nanomaterials

Characterization and Photoreactivity of Titanium Dioxide Nanostructures

There is a wide range ofphotoreactivity observed in TiO} nanostructures, depending on the method of

preparation. At present, it is difficult to control the

photoreactivity ofthese materials since the properties

controlling photoreactivity are not well understood.

To compensatefor this lack ofpredictability, each

industrial user ofTiOj has developed qualitative or at

best, semi-quantitative methods, to evaluate photoreactivity. Project objectives are to obtain a comprehensive understanding ofthefundamental properties and

mechanisms controlling TiOj photoreactivity using an

integrated metrology and scientific approach with

facilities and expertise unique to BRFL and NIST.

Stephanie Scierka and Joannie Chin, BFRL (861)

Background

High volumes of semiconductor metal oxides such as titanium dioxide (Ti02) are utilized each year as fillers and UV absorbers for building materials such as paints, sealants and bulk plastics. A common issue shared by

each of the abovementioned applications is that their service life and durability is affected by the photocatalytic properties of these semiconductor metal oxides.

This property of Ti02 nanostructures can also be exploited to deliberately destroy harmful organic materials,

such as viruses, bacteria and toxic chemicals.

The major impact of this research would be to estab- hsh quantitative, scientifically based techniques for the measurement of photoreactivity in Ti02 and other semiconductor nanomaterials.

Approach

Techniques for the measurement of photoreactivity

that will be developed and investigated in this program

include the following:

• Photoconductivity

• Electron Paramagnetic Resonance (EPR)

• Chemical assays and probes, including methyl

viologen, horseradish peroxidase and isopropyl

alcohol.

Each of these techniques yields specific information on each of the three primary processes in a photocatalytic reaction, which are (a) photo-induced charge carrier

ductor, and (c) reaction of charge carriers and activated

species with adsorbed materials on the semiconductor

surface. Information obtained using each of these analytical techniques will contribute toward a more comprehensive understanding of the entire photoreactive process.'

Another thrust in this program involves the characterization of the surface physical and chemical properties

of the nanomaterials themselves, particularly that which

affects the adsorption of and subsequent reactivity of the nanoparticles with reagents.

Results

An industrial workshop was held in October 2004

with representatives from the coatings, polymers, catalysis, and Ti02 manufacturing sectors. The workshop dis- cussion highlighted the needs of each industry and re- sulted in a general proposal to develop a test for both

durable and catalytic materials that produces a rating for photocatalytic activity on raw materials. It was deter- mined that each industry would work to develop an in- dustry-wide performance test, specific to each application. Experimental results to date show there are differ- ing mechanisms for specific Ti02 nanoparticles and ap- pear to depend on the method of analysis used. Work is continuing.

Photoconductivity: The measured conductance of a Ti02 nanostructure film increases upon ultraviolet (UV)

exposure. The extent of the increase in conductance depends upon the Ti02 nanoparticle and surface treatment. Calcination of the Ti02 nanoparticles caused a decrease

in the measured conductance. An optimum amount of

humidity is necessary for maximum conductance.

2.00E-08

Figure 1. Photoconductance of Ti02 nanoparticles upon UV

irradiation.

generation in the semiconductor, (b) interfacial charge

transfer to surface species on the surface of the semicon- Assays: A methyl viologen assay was used to moniNIST Accomplishments in Nanotechnology

47
2 - Nanomaterials

tor the photoreactivity of several Ti02 nanomaterials.

These results were unlike that obtained from a horseradish peroxidase assay, which measures the peroxide concentration. It appears that the production of peroxide

does not correlate directly with electron and hole production. Further interpretation of the results is underway.

Methyl Violgen Assay

-0-5300 400 500 600 700 800 900

wavelength (nm)

Carboxy Proxyl Spin Trap

'mC

' 01 i

A1

\ C2

A C1

Time (min)

Figure 4. Carboxy proxyl spin trap concentration in Ti02 suspensions as a function of UV irradiation time.

Figure 2. Methyl viologen assay for a series of Ti02 nanomaterials. The photoreactivit}' is measured at 602 nm.

Leuco Crystal Violet Assays

275 375 475 575 wavelength (nm)

675

Figure 3. Horseradish peroxidase assayfor a series of TiOj

nanomaterials. Peroxide concentration is measured at 590 nm.

EPR: EPR spectroscopy measures free radicals and

their concentrations. Our current experiments are using

nitroxide spin traps to titrate the free radicals generated

in aqueous TiOi suspensions exposed to UV irradiation. Given spin traps are reported to monitor specific free

radicals. Preliminary results show that spin traps can

follow free radical generation and that free radical generation does vary with Ti02 nanomaterial. Each spin

trap appears to be measure a different free radical or set of free radicals as the shown by the change in the de- crease in concentration for given TiOi nanomaterials.

Experiments are continuing.

Amino Proxyl Spin Trap

0- -

A

A

7A

B1

A1

A

Time (min)

Figure 5. Amino proxyl spin trap concentration in TiOj suspensions as a function of UV irradiation time.

NIST Accomplishments in Nanotechnology

48
2 - Nanomaterials

Optical Properties of Carbon Nanotubes

The goal is to establish the next generation ofstan- dards for optical power measurements at NIST (and

elsewhere). The science: to lower barriers to identifi- cation oftube size, spacing, species and dielectric

function ofcarbon nanotubes (preferably by rapid,

inexpensive, optical techniques).

John Lehman, EEEL (815)

Our group helps industry, defense, and the medical

community use lasers safely and cheaply. We have

demonstrated the first thermal detectors coated with car- bon nanotubes and, in collaboration with the National

Physical Laboratory (NPL), measured relevant detector

properties for building the next generation of standards

for laser power and energy measurements at NIST and

elsewhere. Figure 1 depicts a pyroelectric detector

coated with carbon single-wall nanotubes (SWNTs).

This detector is the first of its kind and allows us to di- rectly measure the optical and thermal properties of vari- ous tube compositions and topologies. We have also

built the first detectors coated with carbon multiwall

nanotubes (MWNTs) grown on the detector platform.

Related to this work, we have shown qualitative results

for cleaning SWNTs by laser vaporization, and begun

fabrication of MWNT-coated coupons for emissivity

measurements with the National Institute of Advanced

Industrial Science and Technology (AIST).

We have provided pyroelectric detectors to NPL for evaluation of detector properties from 0.8 fim to 14 ]xm. In collaboration with AIST, we have undertaken measurement of blackbody emissivity at 300 K of MWNT

nanotube samples. This work is relevant to optical and

thermal properties of CNTs in general, and specifically

to the design of robust blackbody emitters. We have undertaken UV-laser damage cleaning studies of SWNT

samples for the design of radiometer and thermal detector absorbers. In the past year we have generated three publications (see below) and presented our work at the Conference on Lasers and Electro Optics (CLEO), as well as NSF and NASA workshops.

established ourselves among the leaders in this developing field. Applications range from laser-based manufacturing tools to semiconductor lithography to healthcare

and laser surgery.

Figure 1. A pyroelectric detector coated with carbon single-wall nanotubes (SWNTs).

Selected Publications

1. J. H. Lehman, C. Engtrakul, T. Gennett, A. C. Dillon, "Single-wall carbon nanotube coating on a pyroelectric detector," Appl. Opt., 44, 483-488, (2004).

2. J.H. Lehman, R. Deshpande, P. Rice, B. To, A.C.

Dillon, "Carbon multi-walled nanotubes grown by

HWCVD on a pyroelectric detector. Infrared Physics

and Technology, in press, accepted Dec 04.

3. A. C. Dillon, A.H Mahan, J. L. AUeman, K.M.

Jones, S-H. Lee and J.H. Lehman*, "Hot-wire

Chemical Vapor Synthesis for a Variety of Nanomaterials with Novel Applications," Thin Solid

Films, in press, accepted Dec 04.

Thermal-detector coatings based on CNTs will resist damage and aging at shorter wavelengths and higher

power and energy. Resistance to aging is particularly

important at UV wavelengths, where we plan to support

calibration of blue and UV laser sources and detectors

for applications such as high resolution DVD and chemical and biological agent detection systems. We have

worked collaboratively with NPL, AIST, and NREL and

NIST Accomplishments in Nanotechnology

49
2 - Nanomaterials

Characterization of Single-Walled Carbon Nanotubes

Raman spectroscopy is a powerful toolfor the characterization ofcarbon nanotubes. Phonons, in particular

the one corresponding to the radial breathing mode,

are now used routinely to determine the diameter of

the nanotubes. The resonant character ofthese modes

can be employed to extract the nanotubes chirality.

These informations allow a determination ofthe semiconducting and metallic properties ofthe carbon

nanotubes.

Angela R. Hight Walker, PL (844)

and bundles of single-walled carbon nanotubes. We have

demonstrated single nanotube sensitivity by resonant enhancement of the Raman scattering from the radial breathing mode.

Measurements are underway to examine the temperature

dependence of these Coulomb interactions. The response

of the carbon nanotubes with different diameters was

observed by varying the incident laser wavelength and as a function of temperature. We observed a temperature

dependence of the optical interband transitions in carbon

nanotubes that is consistent with theoretical prediction.

The multibody effects in the optical spectra of singlewalled carbon nanotubes (SWCNT) were investigated by means of resonant Raman spectroscopy. Resonance en- hancement of the Raman scattering intensity of the radial breathing mode in SWCNTs is used as a probe of tube

diameter and of one-dimensional electronic structure. The newly constructed, confocal magneto-Raman microscope at NIST permits continuously tunable laser excita- tion from the NIR to the UV. The novel Raman facility

consists of a microscope capable of working over a wide

range of temperatures {T = 4.2-300 K) and magnetic

fields (H = 0-8 T) coupled to a triple grating spectrometer with ultimate Raleigh rejection capabilities, thereby

permitting low-frequency or Terahertz Raman spectros- copy. From such measurements we have obtained the

direct optical transition energies involved in the resonant

process. We observe that the measured resonance energy

iSo 200 ?5o 300 Bo lis 2o5 250 300 Bo

Ramon shift (cm )

Figure 1. The resonant dependance ofthe radial breathing mode

ofSWCNT.

for these modes deviate from the optically-allowed inter- band transitions predicted by calculation of the singleparticle electronic excitations. These results are inter- preted as arising from excitonic effects in the resonant Raman scattering of single-walled carbon nanotubes.

Our initial investigations were carried out on isolated

100 1.25 1.50 1.75 2,00 1.2 1.4 1.6 1.8 2.0 1.4 1.6 1.8 2.0

Figure 2. Temperature dependance ofoptical interband transition.

For future work, we plan to develop the capability to routinely use optical spectroscopic techniques for the characterization of carbon nanotubes. We will continue our exploration of the temperature and magnetic field dependence of the resonant Raman scattering in bundles

of carbon nanotubes. We shall extend this to isolated

carbon nanotubes to identify those properties that are

intrinsic to the carbon nanotubes and those that result from interaction between nanotubes. Diameter-selective

DNA-wrapped carbon nanotubes will also be investi- gated. The use of photocurrent spectroscopy to complement the resonant Raman spectroscopy in probing the one-dimensional electronic structure of the nanotubes

will be explored. This latter technique will also find potential applications in the studies of carbon nanotubebase optoelectronic devices.

NIST Accomplishments in Nanotechnology

50
2 - Nanomaterials

Stability of Nanowires

Metallic or semiconductor nanowires are important

components in many electronic technologies. At small

length scales many materials exhibit unusual electrical, chemical, and thermal properties that are not observed in the bulk. Applications include novel elec- tronic devices rangingfrom high efficiency lasers and

detectors to exotic single electron transistors and cel- lular automata. The tendency ofnanowires to fragment into nanospheres due to area-minimizing surface

instabilities acts as a limit to the length ofnanowires

that can be used in nanodevices. This instability can

also be used beneficially as a mechanism for the self- organization ofchains ofnanospheresfrom unstable

nanowires. We have studied analytically the effect of

surface tension anisotropy on this instability in order

to help understand and control nanowirefragmentation. Our analysis predicts the wavelengths ofthe

instability as a function ofthe degree ofanisotropy of

the surface energy ofthe nanowire.

Geoffrey B. McFadden, ITL (891)

At the small length scales that characterize nanostructures, the importance of surface effects relative to vol- ume effects becomes significant. Typical surface effects

that can be important at the nanoscale include surface

energy or capillarity, surface diffusion, surface adsorption, and surface stress and strain. In particular, the ef- fects of capillarity must be taken into account in order to understand the tendency of nanowires to fragment when

the rate of surface diffusion of atoms is high enough to allow shape changes to occur over practical time scales.

As shown by Plateau in his classical studies of capillary instabilities, a cylindrical interface with an isotropic surface free energy is unstable to volume-preserving axi- symmetric perturbations whose wavelength exceeds the circumference of the cylinder. Such perturbations lower

the total energy of the cylinder, leading to the breakup of

the cylinder into a series of drops or bubbles. The stability of a liquid jet was subsequently studied by Lord

Rayleigh, who argued that the length scale of the instability is determined by the perturbations having the fast- est growth rate; the phenomenon has generally come to be known as the Rayleigh instability. The Rayleigh instability arises in a number of diverse applications, such

as Inkjet printing, two-phase flow, quantum wires, fiber spinning, liquid crystals, and polymer blends.

Because of the underlying crystal lattice, the surface

energy of a liquid-solid or vapor-solid interface is generally anisotropic and depends on the orientation of the

local normal vector at each point of the interface. The

surface energy of a solid-solid interface between two

crystals is also anisotropic in general, with the additional

corrtplication that the surface energy also depends on the

direction cosines that characterize the relative orientations of the two crystals. Here we have considered a model of this type in which the surface energy depends

only on the local normal vector.

Figure 1. Equilibrium shapes for materials with cubic anisotropy. These shapes are energy minimizing surfaces that are the anisot- ropic versions ofsoap bubbles (the isotropic case). An observation that partially motivates this work is the apparent stability of elongated nanowires that are grown in a bridge configuration or epitaxially on a het- erogeneous substrate The nanowires (alternatively called nanorods or quantum wires) are "one- dimensional" crystals with dimensions as small as one nanometer high, a few nanometers wide, and can be as long as a micron. There are long-standing studies on

experimental techniques to grow nanowires, and the sta- bility of these nanowires is beginning to come under

study. Another motivation for the work is the recent observation that the Rayleigh instability of a nanowire

can be used to produce self-organized chains of nanospheres with interesting electrical and opto-electronic

properties. In either case it is desirable to develop models to predict the length scales of the instabilities in order

to assess the geometry of the resulting structures.

Continuum modeling of nanowires provides some

guidance as to their expected stability, though the strict applicability of continuum models is limited if the length

scales approach atomic dimensions. There are a number

of possible mechanisms that could stabilize a nanowire,

including elastic interactions between the wire and the

substrate, quantum electronic shell effects, and surface

energy anisotropy. A useful model of the surface energy

anisotropy for a cubic material is given by the expression

y(«;t,«v.«2) = To + 4 £ («/ + «/ + /?/)]. In the above

figure we show examples of 3D equilibrium shapes cor- NIST Accomplishments in Nanotechnology

51
2 - Nanomaterials

responding to this surface energy. The shapes are smooth

for -1/18 < s < 1/12. For e < 0, the shapes resemble

rounded cubes, with [110] edges first forming at e = -

1/18. As e decreases below -1/18, the edges extend to- ward the [111] directions, merging to form a comer for e = -5/68. For e > 0 the shapes are octahedral, with [100] comers first forming at e = 1/12. These equilibrium

shapes are most easily computed using the ^-vector for- malism of Hoffman and Cahn, which produces a closedform expression for the equilibrium shape in terms of the surface energy. This is also useful in formulating the

variational problem for the stability of a nanowire.

We consider differentiable surface energies with ani- sotropies mild enough that the surface of the wire is smooth and does not exhibit any missing orientations. In order to examine the stability of the wire using a variational approach, we employ a general energy functional

that describes the total surface energy of the system. This

expression and the constraint of constant volume of the wir are perturbed about the two-dimensional equilibrium

shape. The higher order terms in this perturbation ex- pansion produce a condition for stability. For constant volume, if the perturbation increases the energy, the equilibrium state is stable, otherwise it is unstable.

Figure 2. Capillaiy instability ofa nanowire with an anisotropic surface energy with three-fold symmetry about the wire axis.

For small levels of anisotropy, we evaluate the stability of an isolated nanowire approximately using asymptotics. For larger amplitudes of anisotropy, we compute

solutions numerically. We find that surface tension ani- sotropy can either promote or suppress the Rayleigh instability, depending on the orientation of the nanowire

and the magnitude and sign of the anisotropy. For general surface energies we derive an associated eigenproblem whose eigenvalues govem the stability of the wire.

The eigenproblem is described by a pair of coupled sec- ond-order ordinary differential equations with periodic

coefficients, which generally lack closed-form solutions. We have applied the analysis to a number of examples,

including the above case of a cubic material. We have

computed the stability of the wire to general perturbations when the axis of the wire is in a high symmetry

orientation such as [001], [01 1], or [1 1 1].

In addition to determining the stability of an isolated

wire, we have also examined how both the anisotropy of

the surface energy of the wire and the interaction of the wire with a substrate affects the stability of the rod. The

equilibrium configuration of a wire in contact with a

substrate has an elegant description that can be obtained

by again appealing to the Hoffman-Cahn ^-vector for- malism. This approach determines the contact angles in terms of the surface energies of the phases that meet at the contact line. Using general anisotropic surface energies we have then derived an associated eigenproblem

that describes the stability of the system. The problem is described by a pair of coupled second-order ordinary

differential equations with periodic boundary conditions

along the axis of the rod and boundary conditions arising

from the contact angles between the wire and substrate. We have considered the effects of the overall orientation

of the crystal relative to the substrate and examined a range of contact angles. The substrate is assumed to be

rigid with an isotropic surface energy. We applied the analysis to a number of examples, in- cluding the case of a cubic material, and compute the

stability of the wire to perturbations when the axis of the wire is aligned parallel to the high symmetry orientations

[001], [01 1], and [111]. We assumed a weak anisotropic

surface energy to eliminate missing orientations on the

wire. The magnitude and the sign of the anisotropy de- termine the relative stability in comparison to the iso- tropic case. In general as the contact angle tends to 90

degrees the wire becomes more stable, which is analogous to the stability of a 3D planar film.

Selected Publications

1. K. F. Gurski and G.B.McFadden, The Effect of Anisotropic Surface Energy on the Rayleigh Instability,

Proceedings ofthe Royal Society (London) A 459

(2003), pp. 2575-2598.

NIST Accomplishments in Nanoteclinology

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2 - Nanomaterials

Semiconductor Quantum Dot Structure Metrology

Self-assembled semiconductor quantum dots (QDs) continue to grow in importance in the optoelectronics marketplace. They have an atomic-like density ofstates that

offers substantial opportunitiesfor unique and improved

devices. These include ultra-low-threshold laser diodes

with low temperature dependence, semiconductor optical

amplifiers with reduced cross-gain modulation and large

bandwidth, and mid-wave and long-wave infrared

photodetectors. However, there are many aspects ofQD

growth that are not well understood, in particular dot

nucleation, dot size and shape, and the effect ofcapping

the dots. The optical and electrical properties ofthe QDs

and resulting devices depend strongly on the shape and

composition. Control over nucleation sites would enable

production ofregular arrays of QDs. In addition, the

lateral uniformity ofdot density, which is importantfor

device yield andperformance, has not been investigated

until now. We have measured many ofthese structural properties through nanoscale probing using atomicforce

microscopy (AFM) and transmission electron microscopy (TEM).

Alexana Roshko, EEEL (815)

We performed studies of the shape of InGaAs quantum dots grown on (001) GaAs substrates by molecular

beam epitaxy (MBE). Samples were investigated by

high resolution TEM and AFM. From cross-sectional

TEM, we observed a shape transition for QDs -8.5 nm

in height. Dots less than 8.5 nm are pyramidal; taller dots are multifaceted, allowing accommodation of the increased volume without increasing the QD base length.

While similar dots have been previously observed for both the InAs/GaAs and InGaAs/GaAs dots, the dot

shapes found here do not fit any previous models for the

dot facet planes, which we identified as nearly {111}

planes. The addition of a 20 nm GaAs cap layer dramatically changed the dot dimensions, increasing the diameter of the dot base by factors of 3 to 5. We have demonstrated that large lateral variations in dot density and height occur for InGaAs quantum dots

grown by both MBE and metal-organic chemical vapor

deposition (MOCVD). Atomic force microscopy was

used to examine the density and height distributions of

self-assembled QDs. AFM images were taken at a pro- grammed array of points on the surface of 50 mm diameter wafers, and systematic analysis of the images was

used to determine the average dot density and height

from each image. It was found that an image scan size of 3 X 3 |xm2 was optimal for measurements of QD density.

For dots grown by MBE, the standard deviation in dot density was found to vary from 1 5 to 30 % of the average dot density across the central 26 x 26 mni2 re- gion of the wafers. Investigations of the QD density

found variations of up to 25 % for more local length

scales on the order of micrometers. These density variations, on both the large and local scales, do not correlate with observable step edges on the sample. Nor do the

large-scale variations appear to be due to nonuniformities in temperature, as differences of more than

1 5 °C across the wafers would be required to produce the observed variations. Also, the variations do not typically

follow a concentric pattern across the wafers, as would

be expected if they were due to temperature gradients.

The standard deviation in dot height was 10 % or less of

the average height, which was about 10 nm. Similar

results were found for MOCVD-grown QDs. However,

MOCVD samples were found to have broader distribu- tions of QD size than those grown by MBE. The majority of MOCVD-grown dots are less than 5 nm in height,

but there is a continuous distribution of larger dots, with

heights up to 30 nm or more. Dots grown by MBE have

nearly Gaussian size distributions and a maximum variation in height of less than 1 5 nm. An inverse relation- ship was found between the dot height and density dis- tributions on several MBE samples, suggesting that a uniform distribution of QD material was deposited on

the wafers, but that the nucleation of dots was nonuniform. We have also examined the influence of growth

temperature, vicinal substrates, arsine partial pressure (in

MOCVD), and wafer rotation rate (MBE) on the dot dis- tributions.

The scientific community was surprised at the extent of the density non-uniformity of quantum dots across a wafer revealed by our studies. Work by others had concentrated on the variation in QD density as a function of

growth conditions as measured on small specimens, held

in the center of the MBE stage during growth. At least one research group made a change in the way they grow

quantum dots as a result of our findings. The search for means to control the density and position of QDs re- mains a dominant theme in the industry, and is part of

our further study.

NIST Accomplishments in Nanotechnology

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2 - Nanomaterials

Selected Publications

1. A. Roshko, T.E. Harvey, S.Y. Lehman, R.P Mirin,

K.A. Bertness and B.L. Hyland, "GaAs Buffer Layer

Morphology and Lateral Distributions of InGaAs

Quantum Dots", submitted to J. Vac. Sci. Tech. B,

2004.

NIST Accomplishments in Nanotechnology

54
2 - Nanomaterials

Chemistry and Structure of Nanomaterials

Successful nanoscale materials fabrication is empowered by a detailed knowledge ofthe chemistry and

structure ofsurface bound molecules; e.g., the optimization ofself-assembled monolayers, molecular templates, micro-electro-mechanical system lubricants,

andfunctionalized nanotubes. We have developed

Near-Edge X-ray Absorption Fine Structure

(NEXAFS) spectroscopy techniques that are ideally

suited to the non-destructive measurement ofchemical bond concentration, rehybrization and orientation,

with sub-monolayer molecular sensitivity in diverse

nanoscale materials. Furthermore, NEXAFS can dis- tinguish chemical bonding in the light elements, measure the orientation ofinterfacial molecules, and

measure surface and bulk chemistry independently

and simultaneously.

Daniel A. Fischer, MSEL (852)

We have utilized NEXAFS to observe, verify, and

quantify the reversible cis-trans molecular conformation

transformation from hydrophobic to less hydrophobic

states. The right panels of Figure 1 (upper and lower)

show the polarization-dependent NEXAFS anisotropy

behavior of the C-F and C-C peaks, which reverse with

in situ UV light exposure, highlighting the reorientation

of the semi-fluorinated segments.

Application of NEXAFS spectroscopy to the study of

electronic structure and chemical composition is illus- trated in Figure 2 for various chemically fiinctionalized,

single-walled carbon nanotubes. Upon peroxide fiinc- tionalization, the C=C ring resonance is greatly diminished on extensive sidewall functionalization, indicating

loss of extended conjugation and disruption of nanotube

electronic structure. The C=0 peak intensity is greatest

for peroxide chemistry. NEXAFS spectroscopy supports

a model of peroxide funtionalization, shown in Figure 3.

Materials having low energy surfaces are used in many applications, for example, in non-wetting surfaces

or marine coatings for fouling resistance. We have pro- duced a photo-responsive polymer surface by combining

the reversible photo-switching properties of azobenzene

with the self-assembly behavior and low surface energy

properties of semi-fluorinated segments to create a fluoroazobenzene molecule surface. Upon UV exposure,

this surface switches between a hydrophobic and a less hydrophobic orientation, as shown in Figure 1 (left upper

and lower panels). For such surfaces, one could imagine

applications ranging from low cost surface patterning to polymer surfaces that would adsorb biological macromolecules on cue.

• - e=9a

9 ZS6 300 SOS 310 Energy (cV)

J r

liglit( A==3<i»nm)' i2S0 ns 300 W5

Energy (cV)

Figure 1. UV light reorientsfluorobenzene semi-fluorinated seg- ments (green) downwards, i.e., to a less hydrophobic state.

I'l I' 1

1

1 I r 1 1 1'

1 1 1 1 1 I

I

I 1 1 1

1

1 I 1 1 I I 270 280 290 300 310 320

Incident Photon Energy (eV)

Figure 2. Carbon NEXAFS ofoxidized /funtionalized nanotubes

Figure 3. Model ofperoxidefuntionalization ofnanotubes.

NIST Accomplishments in Nanotechnology

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Mechanical Metrology for Small-Scale Structures

Mvriad industrial and biological systems are com- posed ofsmall-scale structures for which the mechanical behavior is not accurately known. Optimizing the performance and reliability ofthese systems

requires either mechanical property measurements on

specimens ofthese structures harvestedfrom the appropriate phases or interfaces ofthe system, or the

ability to test these structures in situ. We have developed a standardized testing configuration and methodologyfor localized measurements ofstrength and

fracture toughness ofmaterials and interfaces at the

micrometer to nanometer length scale.

Edwin R. Fuller, Jr. and George D. Quinn MSEL

(8520)

This project develops methodology to: (1) measure

mechanical properties of microstructures for myriad in- dustrial and biological systems that cannot be fabricated

in bulk samples; (2) study small-scale mechanical phe- nomena that may be controlled by surface effects, e.g., the influence of surface stresses on crack nucleation and

extension; and (3) obtain quantitative mechanical property data of materials and interfaces for designing smallscale structures and components and for assessing their mechanical reliability. To address these goals, well

characterized testing configurations must be developed

for small-scale measurements of strength and crack extension. We are pursuing three tasks: (1) specimen design and finite-element analysis; (2) specimen fabrication; and (3) mechanical testing and fracture analysis

(fractography). Work in the Ceramics Division this year

has focused on the first and third areas. Two collaborations were established in the fabrication task: one with

James A. Beall of the Quantum Electrical Metrology

Division (817) in NIST Boulder, and one with North- western University.

Significant progress has been made in the design of a compressively loaded test configuration with a welldefined, tensile gage section. Such a specimen can be

loaded using a depth-sensing nanoindenter as a universal

testing machine, thereby giving a record of both applied

force and point displacement. One of these specimens,

fabricated by James Beall from a silicon wafer by deep

reactive ion etching (DRIE), is shown in the right side of

Fig. 1 . The configuration is similar to a theta specimen,

except that the geometry is hexagonal. When a load (per

unit thickness) is applied to the top beam, a uniform uniaxial tensile stress results in the middle gage section.

Finite element analysis gives (horizontal) gage section

stresses on the order of 1.25 GPa for 50 mN/|im of ap- plied load. For a 2 N applied load, these 100 mm thick

specimens generate 500 MPa of tensile stress in the gage

section. The left side of Fig. 1 shows a reconstructed

failed specimen. The insert shows the fracture surface of

the gage section, and the two [111] cleavage facets that were formed. Alternate geometries, including a round

theta specimen, have also been tested and modeled.

Figure 1: Prototype specimen design.

To extend this technique to a wide variety of materials and systems, general fabrication techniques are being

developed. As an example, focused-ion-beam (FIB)

milling, performed in collaboration with Northwestern

University, has been used to produce hex specimens that are an order of magnitude smaller than the DRJE silicon. Figure 2 shows such a hexagonal theta specimen, fabri- cated by FlBing, of a lamellar, directionally solidified eutectic (DSE) of Nio.5Coo.5O and Zr02. When applied

to this type of system, the test method enables direct measurement of interfacial mechanical properties between the two DSE phases.

Figure 2: Hexagonal theta .specimen oj a DSE made by FIBing.

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Highly Charged Ion Modified Materials

With the diminishing size ofelectronic device elements comes the increasing need to precisely and reproducibly control the transport properties ofmaterials at the

nanometer scale. This need is a consequence ofthe

fact that the electronic properties ofa single device

element must be well matched to other components in order to be incorporated into a greater microelectronic circuit. The materials and systems thatfunction

at one size scale may notfunction at another. We are

using highly charged ions to modify' materials to create new composites in a way that allows us to tune

spatialfeatures and transport properties to values that

lie in ranges not accessible with traditional materials.

Joshua M. Pomeroy and John D. Gillaspy, PL

(842)

The ability to precisely and reproducibly control the

electronic transport properties of a material is paramount

to its inclusion in any engineered device or system. It is natural then, that understanding and controlling the electronic properties of materials receives a great deal of

attention from researchers in all branches of science.

Examples of this need can be found in industries such as storage media where the size of hard drive read elements

must shrink below 100 nm in dimension. At this nanoscale dimension, metallic devices have too low a resis- tance area (RA) product to be successfully incorporated

into larger systems, while the RA product of oxide based

tunnel junctions is too high. The RA product is a direct indicator of the ability of electrons to flow perpendicular

to the plane of a given material. At the NIST Electron

Beam Ion Trap (EBIT) Facility, we have designed a way

to use highly charged ions (HCI) to create a new type of

composite material with a tunable RA product.

Techniques using highly charged ions to create composite materials take advantage of the large damage

zones that are localized on the surface. Damage zones from singly charged ions are due to the kinetic energy

transfer to the lattice from many collisions distributed in a column that penetrates hundreds of nanometer into the

bulk. When highly charged ions are used, a massive

charge transfer occurs when the HCI is about 1 nm

above the surface, creating a very high density of electronic energy on the surface. This energy disrupts the equilibrium of the lattice and a damage zone is created.

The characteristic properties of the individual damage zones determine the properties of the composite material, and are dependent on the material system, kinetic energy, and ion charge state.

Figure 1. An STM image ofa 25 nm feature created on a gold

surface with a highly charged xenon ion.

In order to understand the properties of the individual

damage zones, we have studied the variation of damage

zones on different materials systems as a function of ion

kinetic energy and charge state. For example, a feature created by a single xenon HCI on a gold surface is found

to displace several thousand of times its own mass (Fig- ure 1). Features of this type provide unique opportunities to study quantum behavior of nanometer sized fea- tures critical to efforts in quantum computing and quantum optics without expensive low-yield nano-fabrication

techniques.

Immediate technological impact can be achieved by

exposing multilayers to HCIs, tuning the size of the damage zones (by controlling charge state) to control

transport between the layers. Using data collected from

studies of individual damage zones, we have modeled

selected materials systems and found that by using carefully selected material-ion combinations, composite materials with tunable electrical transport can be created

and easily integrated into existing technologies. Experiments are currently underway to demonstrate this capability by developing a composite material with a

tunable RA product, an immediate need for the highdensity magnetic storage industry.

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Nanoscale Metrology of Ill-Nitride Semiconductor Films

and Nanowires

The Ill-nitrides comprise the only known semiconductor material system that will enable the development

ofelectrically pumped blue-UV lasers and LEDs.

These optoelectronic devices are vital components of

systemsfor a wide range ofapplications including

solid state lighting, detection ofchemical/biological

agents, cancer diagnoses, and water decontamination.

Rftelecommunications and electric power distribution

also benefitfrom the robust nature ofIll-nitrides to withstand high electrical power and high temperatures. In spite ofthe promise ofthese materials, there are complicating issues concerning the impact of

nanoscale defects, strains, and phase segregation on

their electrical and optical behavior and overall device efficiency. Interestingly, the wide bandgap semiconductors are one ofthefew instances ofa material

system where device applications and strong eco- nomic demands have emerged even though there is limited understanding ofthefundamental structural

and electronic properties responsiblefor their successes. In particular, progress in blue-UV laser development is hindered by thefact that 2D epitaxial

growth ofIll-nitride structures is accompanied by a

high density ofnanoscale structural imperfections and

defects. On the other hand, one-dimensional III- nitride nanowire crystals (50-100 nm in diameter, micrometers to tens ofmicrometers in length) have re- cently been shown to exhibit exceptional structural

purity and extremely low defect density. Even with the

improved crystalline properties that the Ill-nitride nanowires offer over their thin-fdm 2D counterparts,

the nature ofthese nanowires makes them extremely

challenging to characterize and to fabricate into use- ful devices. To optimize these materials (2Dfilms and

ID nanowires), this project develops and applies cor- related metrology methods to examine the interplay of

spectroscopic, structural, and compositional effects on

the nanoscale.

Norman Sanford and John Schlager, EEEL

(815)

Due to nonradiative recombination mediated by defects, the external quantum efficiency of LEDs is rela- tively low. It must be increased for Ill-nitride technology to truly impact global conversion to solid state lighting. Furthermore, InGaN is a uniquely quantum system

whereby the insolubility of indium results in precipitates

of nanocrystalline inclusions of InN that exhibit quantum

confinement. These naturally occurring 'quantum dots' are thought to be responsible for luminescence and

lasing action, not the composition of the material. Therefore, understanding the local phase segregation, the composition dependence on InN dot radii, the interaction

of carriers with defects and piezoelectric effects, and the pathways of radiative and non-radiative recombination

processes are the key issues in optimizing LEDs and la- sers.

366 nm map

due to GaN

both images: 30 ^m x 30

*->

455 nm map ^. due to InGaN

quantum well

Figure 1. Multiphoton NSOMPL image ofburied InGaN quan- tum well and surrounding GaN. We developed two complementary techniques for optically probing nanoscale features in Ill-nitride layers

and nanostructures: near-field scanning optical microscopy (NSOM) and confocal photoluminescence spec- troscopy, both implementing multi-photon pumping.

Multi-photon luminescence (MPL) spectroscopy offers a means by which the uniformity of an active, buried layer,

e.g. a quantum well in a laser or LED structure, can be

probed regardless of the thickness of the surrounding

layers. Standard direct above-bandgap photolumines- cence (PL) spectroscopy is problematic since the pump

beam could be absorbed elsewhere in the structure before exciting the regions of interest. Using MPL spectroscopy, with ultrafast pumping for photon energies near or above midgap, even embedded structures may be

examined, since the surrounding layers are transparent to the pump.

NIST Accomplishments in Nanotechnology

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The intense optical field of the ultrafast pump pulses near 800 nm wavelength enables electrons to be pro- moted to the conduction band via virtual state processes. Subsequent relaxation to the valence band, possibly also involving exciton, donor, acceptor, or defect levels,

yields the luminescent output. The NSOM uses a "look

through" geometry whereby the pump beam is focused

through the bottom of the sample substrate. For data collection, the relative positions of the pump beam and the tapered fiber tip are held constant while the sample is transversely scanned between them. For the present

NSOM tips used (tapered UV-transparent optical fibers with aluminum apertures), the spatial resolution is roughly 100 nm. Significant spatial fluctuations (-50%)

in the photoluminescence (PL) signatures from both In- GaN (near 400 nm wavelength) and the surrounding

GaN (near 360 nm) layers were observed. The PL fluc- tuations from these two layers (at these two distinct wavelength bands) of the sample are uncorrelated. Fol- low-up metrology, including correlated cathodoluminescence scans over the same targeted scan areas, is in progress to help pin down the sources of the PL fluctuations.

Mulfi-photon confocal spectroscopy (MFCS) was

also developed in our laboratory, enabling us to probe

material samples at cryogenic temperatures and measure

the temporal decay of the PL, two features not possible

in the NSOM configuration. The spatial resolution of

the confocal spectroscopy system is roughly 250 nm or

better. Furthermore, the high numerical aperture optics

used with MFCS offers high optical throughput and the

ability to separately scan depth-resolved spectra. The

combination of NSOM with MFCS allows correlation of

spatially resolved, time-resolved, and low-temperature

spectroscopy to aid in identification and understanding

of defects, strains, and alloy fluctuations in both 2D and

ID semiconductor structures.

Wide bandgap semiconductors offer revolutionary

improvements to the global utility infrastructure, since

adoption of solid state lighting technology by 2025 could

reduce by 50% the world's usage of electricity for lighting, reduce global power consumption by 1 0%, and

vastly reduce power plant emissions and environmental

pollutants. The advent of high-purity ID structures in these materials is also opening up possibilities for entirely new optoelectronic and electronic device applications. The economic and societal impact of a more thor- ough understanding of nanoscale effects can thus be enormous through the leveraging of increases in device

efficiency and consequently reduced time to widespread

adoption of solid state lighting.

Selected Publications

1. N. A. Sanford et al., "Fabrication and analysis of

GaN nanorods grown by MBE," phys. stat. sol. (c), DOl 10.1002/pssc.200461602 (2005).

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Mathematical Modeling of Nanomagnetism

Measurement, understanding, and control ofmagnetic

phenomena at the nanoscale each require the support

ofmathematical models ofthe physics involved, and

software that correctly implements and makes predictions based on these models. The Object-Oriented

MicroMagnetic Framework (OOMMF) project provides this capability in a public domain package of

portable software components organized in an extensible framework. OOMMF software is widely used

and cited in the physics and engineering literature. Current objectives are to continue to expand the fea- tures supported by components in the OOMMF

framework, with focus on high priority items, such as thermal effects and spin transjer, motivated by their

relevance to nanoscale sensor and spintronics work.

Michael J. Donahue and Donald G. Porter, ITL

(891)

Many existing and developing applications of

nanotechnology make use of the phenomena of magnetism at the nanoscale. Some of the most familiar and

successful examples are technologies widely used by the information storage industry such as magnetic recording

media, GMR sensors for read heads, and magnetic RAM

(MRAM) elements. Mathematical modeling and computational simulation continue to support advances in in- formation storage technology such as the advanced patterned magnetic recording media that promise to achieve

recording densities of 1 Tb per square inch.

Nanomagnetism modeling is playing a role in other

nanotechnology efforts as well. For example, modeling was critical to the development of a fully magnetic logic

gate and shift register was accomplished at University of

Durham and recognized by the Institute of Physics as one of the top ten stories in physics for 2002. Other efforts aim to develop logic and information processing

devices that carry information in the form of the spin of a charge carrier, so called spintronics devices. In biotechnology, the use of paramagnetic beads to locate and position biological macromolecules is under study. In materials science, the probing capability of ferromagnetic resonance is used to characterize and measure material

properties, where a model-based understanding of

nanomagnetodynamics is a key to interpreting experimental results. Also relevant to materials science is the

effort to develop improved sensor designs capable of

detecting magnetic fields that are both smaller in magnitude and more localized in space.

In each of these areas, the NIST Object-Oriented MicroMagnetic Framework (OOMMF) system is in use to enable nanoscale science and engineering. OOMMF is a

public domain package of portable software components

organized in an extensible framework to enable computational simulation of magnetic systems.

Figure 1. Top: Image of2000 nm diameter ring offerromagnetic

material produced by a magneticforce microscope. Bottom:

Magnetization pattern computed by OOMMF based on known

parameters chosen to match the experimental work

One recent example of the use of OOMMF to support

nanotechnology R&D is depicted in Figure 1 . An image

of 2000 nm diameter ring of ferromagnetic material was

produced by a magnetic force microscope (MFM). Such

ring elements are proposed for both storage and sensor

applications, where their utility is critically dependent on

their precise behavior. The colors of the image represent

the strength of stray magnetic field sensed at each location scanned above the ring. An MFM directly measures

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NIST Accomplishments in Nanotechnology

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2 - Nanomaterials

stray field; it does not directly measure the magnetization pattern found in the ring itself. A measured stray

field does not uniquely determine what magnetization

pattern produced the stray field. Also illustrated is a magnetization pattern computed by OOMMF software

based on known parameters chosen to match the experimental work. The pattern of arrows represents the magnetizafion pattern predicted by the model, and it can be

confirmed that the predicted pattern is consistent with

the measured stray field. In complementary roles like

this, modeling is able to indirectly deduce details of a nanoscale system that are not within the capability of

direct measurement.

Many of the equations governing nanomagnetism

were established long ago. The notable Landau-Lifshitz

equation dates back to 1935, and William Fuller Brown

established the fundamentals of micromagnetic modeling

in articles and books published from the 1940s to 1970s.

At that time, the applications of the theory were mostly

limited to simple geometries that could be attacked analytically. It was not until the 1990s that the widespread

availability of significant and inexpensive computing

power made application of micromagnetic modeling to practical problems a reality.

Unfortunately, the accurate solution of the relevant equations in software is a more difficult task than many

physicists and engineers first realize. By the mid-1990s,

many researchers and their students had produced their own micromagnetic simulation programs to support their own work. While their publications were careful to list details of their experiments and analyses, their homegrown software would typically only merit a high-level

description. Usually the equations the software was

meant to solve would be noted, but no reports of soft- ware testing were provided, and there was no opportunity to review its performance. The problems solved by

these computer codes were typically precisely matched

to a particular experiment, and it was rare that two independent teams would perform computer simulations of

precisely the same scenario to enable comparison.

In 1996, NIST challenged this research community

with a standard problem, inviting all those with magnetic modeling simulation software to attempt to compute some properties of a magnetization reversal similar

to those routinely reported in the literature at the time,

but all starting from the same assumptions. The results were alarming. The number of significantly different solutions almost matched the number of programs. Subsequent research has discovered flaws in both the pro- grams and the problem. An important value of OOMMF

is that it provides a transparent benchmark against which

any research team developing their own magnetic modeling software can compare their results.

Figure 2. Part ofthe graphical user interface for OOMMF.

Those researchers for whom OOMMF provides all required modeling capabilities can, of course, can also choose to use OOMMF in place of developing their own

software. In addition, OOMMF is structured as an extensible framework of software components, so those

users who have needs beyond OOMMF's current capabilities can often extend it to meet their needs without

the need to rewrite a new software package from scratch. OOMMF has been remarkably successful in achieving its objectives. OOMMF software has been

downloaded more than 8,000 times. A growing number

of peer-reviewed research publications cite use of

OOMMF; 185 are listed on the OOMMF web site.

Current plans for OOMMF are to supply additional

extension modules and to make improvements to the framework necessary to support the nanomagnetic simulations required in emerging research areas. For example, NIST research in the development of ultra-low field magnetic sensors requires reliable modeling of thermal

effects, a component that OOMMF has until now not provided. There is also considerable interest in repre- senting the effects of spin transfer in nanomagnetic sys- tems. In each of these cases, research is active into de- termining the correct models for these phenomena.

Further information on the OOMMF project, include

software downloads, documentation, and extensive ref- erences, can be found at http://math.nist.gov/oommf/.

NIST Accomplishments in Nanotechnology

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2 - Nanomaterials

Magnetic Materials for Nanoscale Sensors and Ultra-High Density

Data Storage

Magnetic sensors play a central role in many important technologies rangingfrom health care to homeland security. A common need among these technologies is greater sensitivity through nano-structured

materials. In ultra-high density data storage, one of

the most pressing needs is for nano-structured media

that store data at ever-increasing densities. Improved

methodsfor the magnetic isolation ofgrains in ultrathin films are a key need. We have initiated research

programs in both areas.

William F. Egelhoff, Jr., MSEL (855)

Over the past decade, NIST's Magnetic Engineering

Research Facility (MIRF) has made important and

widely recognized contributions to the thin magnetic

films used as read heads for hard disk drives. MIRF is one of the most versatile facilities in the world for the

fabrication and analysis of novel magnetic thin films. This versatility is illustrated by two new areas of re- search that have recently been initiated. One is magnetic

sensors and the other is magnetic media. The common

link is that both require nano-structured thin films.

Figure 1. The Magnetic Engineering Research Facility. We are now 1.5 years into the Magnetic Sensors

Competence Program. We have evaluated a series of

complex magnetic alloys that in bulk form are very sensitive to small magnetic fields but had not previously

been studied in thin-film form (e.g., Ni77Fei4Mn5Cu4).

Our approach is to carry out the metrology needed to optimize these materials in thin-film form.

We have made several surprising discoveries. First, these alloys are uniformly less sensifive in thin-film

form. Second, some of the bulk sensitivity can be recov- ered by nano-layering the magnetic thin films with non- magnetic thin films. Third, conventional thin-films sen- sor designs contain a design flaw that allows magnetic

irregularities to reduce the potential sensitivity by over a

factor of 1 00. Fourth, we have found a way to use nanostructuring of the thin films to reduce these irregularities

sharply and achieve a factor of 40 improvement in sensitivity. Fifth, an analysis of the physics of the nanostructuring immediately suggests that opportunities exist for significant further gains.

In the area of novel magnetic media we have been

collaborating very closely with Seagate. Seagate is the world's leading manufacturer of hard-disk drives, and

for the past two years they have been sending a physicist

from their research labs to work with us, first on ballistic magnetoresistance, and then on novel magnetic media.

We have recently demonstrated a new method for magnetically decoupling grains in layered cobalt- palladium (CoPd) materials, a leading candidate for the next generation of magnetic media. We have found that

if we deposit a gold film on top of the CoPd and anneal

the sample in air an unexpected phenomenon occurs as a

result of rapid diffusion of atoms along grain boundaries.

Specifically, Co atoms on the grain boundaries are re- placed by gold atoms from the surface. As a result the grain boundaries are demagnetized, and each grain can

magnetically switch independently of its neighbors. This

method provides the best magnetic isolation of grains to date and may be key to development of fiature media.

Annealed at 300° C, 1 hour.

miiltilaver

Magnetization

can rotate abruptly

Figure 2. Grains isolated magnetically by Au diffusing into grain

boundaries.

NIST Accomplishments in Nanotechnology

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3 - Nanoscale Devices and Systems

Evanescent Microwave Probing and High-resolution Microwave

Power Imaging

Evanescent Microwave Probing: The development of new man-made electronic materials with properties

not previously seen in nature, in addition to the growing interest in the electrical properties ofbiological

materials, has led to a critical need to develop accurate measurement methodsforfully understanding the

interaction ofthese materials with electromagnetic

fields. Conventional methods do not have the neces- sary sensitivity to characterize these materials at the

micrometer and nanometer scales, so NIST researchers are developing an evanescent microwave probe

system to perform quantitative, high-frequency, di- electric measurements on electronic and biological

materials. High-resolution Microwave Power Imaging: NIST is developing metrologyfor quantitative

imaging and measurement ofelectromagneticfield

components and material characterization ofnanoscale electrical and magnetic devices by establishing accurate spatially resolved waveform andfrequencydomain metrology.

Michael Janezic, James Baker-Jarvis, and Pavel

Kabos, EEEL (818)

realize for measurements below 10 micrometers. The

alternative is needle-type probe, which can be used below 1 micrometer, but are difficult to model. Therefore we developed a new theoretical model that accurately

models the relation between the resonance characteristics of the needle probe and the dielectric properties of the material under test.

coupling loops

to network analyzer

X/A coaxial resonator

c

probe

dielectric sample

Figure 1. Evanescent microwave probe system.

We have developed a prototype evanescent micro- wave probe system that can quantitatively measurement

the complex permittivity of dielectric materials. By making use of evanescent fields, the measurement resolution

is dependent on the dimensions of the probing structure, rather than the wavelength of the incident wave. As a

result, near-field spectroscopic techniques are capable of

both imaging and characterizing the electromagnetic

properties of a wide range of materials at small spatial resolutions.

Recent improvements to the evanescent microwave

probe system include broadening the frequency range of

the method by making use of the probe's higher-order

resonant modes. Previously, this method has only been

employed at a single frequency, but measurements made

on several standard dielectric materials showed that data

from the higher-order modes was comparable to other resonator methods.

To improve the resolution of the evanescent micro- wave probe method, we developed a new theoretical model for needle-type probes. Traditionally, sphericaltipped probes are used as evanescent microwave probes

because the interaction between the probe and sample is easy to model. However, spherical probes are difficult to

0.000

N- -0.005

1422 Rexollte 7980 Fused Silica Vistal Alumina

1 10 100 1000 Gap (lam)

Figure 2. Resonantfrequency as afunction ofthe gap between

the probe and dielectric material.

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3 - Nanoscaie Devices and Systems

Chemical Sensing with Nanowires and Nanotubes

Process control and sensing rely on the ability to detect specific chemical species. The ability to detect

and quantify the presence ofspecific species gives

optimal control. Using nanowires and nanotubes as sensing units offers the possibility to build sensors capable ofmonitoring multicomponent mixtures with

compact devices.

Richard E. Cavicchi, CSTL (836)

The novel properties of nanowires and carbon nanotubes show great promise for use in chemical sensing

devices. Nanowires have a high fraction of their constituent atoms on the surface and therefore have electri- cal properties that are especially sensitive to the chemical environment in which they operate. We have been

investigating growth and characterization of these materials, as well as implementation into sensing devices.

We have investigated lower temperature growth of

tungsten nanowires. Tungsten nanowires are important

for field emission and conductivity sensing devices. Existing methods require temperatures in excess of 800 °C

to produce nanowires. Lower temperature growth is important for preserving the integrity of process steps for integrated devices, for example the use of aluminum

metallization.

We have demonstrated a general method for the fabrication of tungsten nanowires by hydrogen plasma

treatment of sputtered tungsten films and metal wire

filaments at 500 °C. This work also revealed a surprising nanostructure termed a "nanoduct" consisting of hol- low rectangular cross-section tubes of tungsten. These

structures were found to grow in confined geometries

and a model was proposed for the growth process.

ZnO is a promising candidate material for nanolaser de

in which the wavelength of light is comparable to the devi

size. A nanolaser would detect species adsorbed on its sur by a change in the resonance factor Q of the lasing due to absorption of light. There is also interest in ZnO nanowire

field emission and solar cells. Using zinc powder as a sou

material in a vapor transport growth process, we have gro'

vertically oriented faceted ZnO nanowires on substrates a1 peratures as low as 500 °C, much lower than the 800 °C tc

1 000 °C of the widely used vapor-liquid-solid growth met

These nanowires could serve as the basis for surface-emiti

nanolasers.

Figure 2. ZnO nanowires grown by vapor transport growth.

Carbon nanotubes have demonstrated high thermal

conductivity along the length of the nanotube. Recently we have grown nanotubes on micromachined calorimeter

sensors and grown chemically sensitive coatings over the nanotubes. The nanotubes greatly enhance the surface to volume ratio, while maintaining high thermal contact to the sensor.

Figure 1. Tungsten "nanoduct" grown in a hydrogen plasma.

NIST Accomplishments in Nanotechnology

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Ill-Nitride Semiconductor Nanowires:

Growth and Characterization

We are growing nanowires in wide bandgap III- nitride alloys. The eventual goal ofthis work is the

realization ofelectrically pumped UV lasers and

LEDs in these nanoscale structures. In fact, the III- nitrides are the only known semiconductor materials

that will enable the development ofsuch devices,

which address a wide range ofapplications including

detection ofchemical and biological agents, biomedical procedures such as cancer diagnoses and treat- ment, and compact systems for water decontamination. We are carrying out nanowire growth optimization studies and are pursuing structural and spectroscopic studies to understand defect and recombination

effects.

Kris Bertness and Norman Sanford, EEEL

(815)

We have grown isolated GaN and AlGaN nanowires

using gas-source molecular beam epitaxy (MBE). To the

best of our knowledge, we are the third group in the world to have done so. The wires exhibited photolumi- nescence (PL) of comparable intensity to that from bulk

free-standing HVPE GaN (indicating very low defect

density), a relaxed c lattice constant, and the tendency to align in response to external electric fields. The

nanowires ranged from 50 to 250 nm in diameter, were

well separated, and grew to lengths ranging from 2 to 6

micrometers. They formed spontaneously, without metal

catalysts, under conditions of low Ga flux combined with

high nitrogen flux (with RF-plasma dissociation) at temperatures near 820 °C. The growth substrates were

Si(l 1 1) wafers covered with thin AIN buffer layers. The

growth axis of the wires oriented along the [0001] direction, and X-ray diffraction using the (0002) reflex yielded a lattice parameter c = 0.5 1 852 nm +/- 0.00002

nm, indicating that the wires were fully relaxed. Fieldemission scanning elecfron microscope images showed

that the wires had hexagonal cross-sections. Nanowires

were harvested and dispersed by immersing as-grown

specimens in a toluene bath under ultrasonic agitation

and dispensing droplets of the nanowire/bath slurry onto

sapphire (or other) substrates. Evaporation of the solvent

yielded dispersed nanowires. Unpolarized PL (325 nm

cw excitation) at 3 K from both as-grown and dispersed

samples showed the donor-bound exciton peak near

3.4723 eV dominating the spectrum, as expected for low-strain GaN. The dispersed NWs displayed a prefer- ence for settling on metal patterns versus bare regions of

sapphire substrates. Longer wires (roughly 6 ^m) required application of a voltage between metal patterns

during dispersal to display alignment. These alignment

techniques may prove useful to isolating individual

nanowires for optical and electrical measurements.

Figure 1. Field-emission scanning electron microscope image of

GaN nanowires.

The nanowire topology itself is very attractive for a number of important applications. For example, in nearfield scanning optical microscopy (NSOM), nanowire

lasers could provide an active tip with unprecedented

brightness compared to conventional NSOM that utilizes passive tapered fiber tips (typical tip aperture is roughly

100 nm in diameter), which suffer from unavoidably

high optical loss. Additionally, the structure and emission wavelength of the nanowire laser lends itself well to integration with medical catheters for in-vivo cellular imaging technologies and localized photodynamic ther- apy. In these applications, the UV nanowire emission

permits the optical excitation of fluorophore-tagged cancer cells, and the small diameter of the emission aperture

affords a high degree of spatial selectivity in locating the cancerous region. Furthermore, ultra compact UV

sources are required for homeland security needs in the spectroscopic detection of anthrax and other biotoxins.

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Measuring & Understanding Charge Transport in Molecules

Molecular electronics is afield that many predict will

have important technological impacts on the elec- tronic and communication systems ofthefixture. In molecular electronic systems, molecules perform the

functions ofelectronic components. We are characterizing the structural properties of and the conduction

mechanisms through, molecules and developing methods that reliably and reproducibly measure the electrical properties ofmolecular.

Roger D. van Zee, CSTL (836)

The drive to increase electronic device performance,

with the associated push to ever smaller device dimensions, has lead industry observers to conclude that sili- con-based technology will reach a point of diminishing

gains in the near future. This, in turn, has generated in- terest in alternative technologies. It is hoped that the tremendous flexibility available with organic synthetic

chemistry and self-assembly techniques can be harnessed

to produce devices comprised of single or small numbers

of molecules. The CSTL team of researchers in molecular electronics is integrating a range of techniques that will provide key information on electronic structure and

electron transport in cmolecular electronic systems.

The program takes an integrated approach. The conduction or current-voltage properties of molecule are measured using scaimed-probe microscopes for specific molecular species. Spectroscopic measurements are then

performed to compare whether these measured current- voltage properties correspond to intrinsic molecular

properties. Theoretical calculations are used to interpret the results from both of these experiments. Finally, these

composite data are compared to the performance of device prototypes made from similar materials.

Obtaining a detailed understanding of electronic trans- port properties in molecules requires the ability to correlate structure and transport mechanisms. Here we have

used a "standard" test molecule to provide reference

measurements. This molecule affords a means of probing

the impact of molecular orientation on tunneling behavior. Asymmetry in the current-voltage curves show that, in the case of a "standing up" phase, the tunneling behavior is rectified by the asymmetry of the transport

junctions, while in the "laying down" molecule, this asymmetry is almost completely removed (Cf Figure 1). This can be explained by differences in metal surface

potentials of the tip and sample surface.

S 200 c

^ 150

(-bias)/

/

c

t 150

f

(-biao

1 ninl 5 nm

Figure 1. Orientation dependence on tunneling behavior in dodecanethiol showing asymmetric I-V behavior in molecules stand- ing up on the surface (a) vs. more symmetric behavior in molecules laying down

The conductance measurements described above sug- gested that geometry and conduction were linked. However, molecular conductance should also be linked to the electronic states of the molecules. To investigate whether

these electronic structure properties are related to con- ductance measurements, ultrafast laser spectroscopies

have been used to study the geometry and electronic

structure of test. These data are used to draw energy

level diagrams, such as the example shown below, and

also provide prediction of hole- and electron-injection

barriers (<!>). Future experiments will compare these optical measurements to device prototype conductance

measurements.

5

4

^0

LJJ

0t, elec

CD

t, hole

-Fermi

Figure 2. Energy level alignment diagramfor oligo(phenyleneethynylene) on gold.

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In order to interpret and understand the results from

the current-voltage and electronic structure measurements, theoretical calculations are required. Particular

attention has been focused to the electrostatics at the molecule-metal interface, which has been found to affect significantly the transport properties of the system.

These calculations allow one to visualize how the electrons and fields are distributed along and around the molecule (Cf Figure 3). This algorithm has been used to study the possible sources that lead to asymmetric cur- rent-voltage in a series of organic thiolates connected to gold electrodes. In order to develop efficient and robust

models to gather a qualitative picture of electron conductance through molecular bridges, a simple algorithm has

been implemented. Results of the calculations are in good agreement with the experimental curves shown in Figure 1.

V=1,5(\/)

Y(a.u.)

Figure 3. Charge distribution and electrostatic potential profiles for a prototypical molecule (benzene dithiolate) attached to gold

electrodes.

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MOS Device Characterization and Reliability Project

The CMOS FET (Field Effect Transistor) is approaching physical channel lengths of 10 nm and gate dielectric thickness of 1 nm. To scale to these nanometer

dimensions requires new materials such as high permittivity gate dielectrics and metal gate electrodes.

The MOS Device Characterization and Reliability

MOSDCR) Project develops the electrical, reliability,

andphysical characterization techniques necessary to address the metrology issues for CMOS devices

posed by these new materials.

John S. Suehle and Eric M. Vogel, EEEL (812)

Introduction

The International Technology Roadmap for Semiconductors (ITRS) states that an effective thickness of 1 nm will be required by 2010. Due to increased power

consumption, intrinsic device reliability, and circuit instabilities associated with Si02 of this thickness, a high

permittivity gate dielectric (e.g. Hf02) with low leakage

current and at least equivalent capacitance, perfonnance,

and reliability will be required. Furthermore, because of

the depletion of the polysilicon gate associated with the high electric fields in the device, metal gate electrodes

will also be required. Electrical, reliability, and physical

characterization techniques need to be developed and

enhanced to address issues associated with alternate di- electrics and metal gate electrodes.

Electrical Characterization and Reliability

Electrical characterization of Metal Oxide Semiconductor (MOS) capacitors and Field Effect Transistors

(FETs) has historically been used to determine device

and gate dielectric properties such as insulator thickness, defect densities, mobility, substrate doping, bandgap,

and reliability. The strategy of this effort is to develop

robust electrical characterization techniques and methodologies to characterize charge trapping kinetics, Vi

instability, defect generation rates, and time-dependent

dielectric breakdown (TDDB) for both patterned device

samples and blanket films obtained from our collaborators. Many issues such as tunnel/leakage current and spatially dependent properties associated with metal oxide

and silicate dielectrics are also present in ultra-thin oxide

and oxide-nitride stacked dielectrics. Therefore, many of

the characterization schemes will first be developed on

the simpler ultra-thin oxide and oxide-nitride dielectrics and then be applied to the metal oxide and silicate di- electrics for a variety of high-k samples subject to differ- ent deposition and gate electrode processes.

A series of experiments was conducted to characterize the energy dependence of interface trap density in n

and p-channel Hf02 FETs supplied by IBM. The study

demonstrated that the mobility reduction observed in n- channel devices is due to positions of interface traps in the upper half of the Si energy band.

!«f02 nFETs

at i laean D||i

-0.2 0.0 0.2 Eem • Ei (eV)

10'

lHf02 pFETs

100 kHz

-C,2 0.0 0.2

(b)

Figure 1. Energy distribution ofinterface traps as determined by

the rise and fall time dependence ofcharge-pumping currents for Hf02-gatedMOSFETs. (a) n-MOSFET. (b) p-MOSFET.

The JEDEC standard, JESD-92, was published describing several techniques that can be used to detect soft breakdown in ultrathin gate oxides, where detection can be difficult. The standard also includes guidelines for designing test structures and data analysis procedures.

A mechanistic study of progressive breakdown in small area ultra-thin gate oxides was completed. One

breakdown mode features a conducting filament that is stable until hard breakdown occurs, and a second mode

features a filament that continually degrades with time.

The acceleration factors are different for each mode, in- dicating different physical mechanisms are involved in the evolution and formation of the final hard breakdown

event. Unstable filaments that result from the first soft breakdown progressively degrade and change physical

structure until their leakage current becomes unacceptably large. A set of voltage and temperature acceleration

parameters different from oxide wear-out is necessary to project the leakage current with time.

Reverse short channel effect (RSCE) was observed in high-K (Hf02 on SiON buffer, AI2O3 on SiON buffer)

gated submicron n-MOSFETs. SiOa or SiON control samples show normal short channel effect (SCE) behavior. The possible causes such as inhomogeneous channel

doping profile and gate oxide thickness variation near S/D ends have been ruled out. The results indicate that the channel length dependent interface trap density is the main cause of the RSCE, while oxide charge also plays a

role.

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Physical Characterization

Project staff are optically characterizing advanced

oxynitrides, oxide/nitride stacks, and more complex compounds, such as hafiiium silicon oxynitride and hafniumaluminum silicate using spectroscopic ellipsometry. Because many of these materials have bandgaps just below 6

eV, the measurement range obtainable with the VUV el- lipsometer enables the investigation of processing related

structure in the index of refraction, which shows up in the absorbing region above the bandgap energy. Project work

is directed at determining preferred structural models,

spectroscopic index of refraction values, or preferred optical dispersion functions for each of these film systems,

and the variability of these parameters due to differences

in fabrication processes. Work is also directed at extract- ing accurate bandgap values and interpreting the processing related structure found in the dielectric function.

Analysis has been done primarily with software developed

at NIST for SE, which allows the addition of the latest published or custom-developed optical response models

for each material system investigated.

-14 -12 .10 ^ .6 -4 .2 0

Blndrnfl En«rgy

Figure 2. Soft X-ray photoelectron spectroscopy spectra ofHfSiO

and HfSiON thin films, which gives information about the valence band edge, but not about the full band-gap. Inclusion ofnitrogen

into the silicate network decreases the valence band offset (about

1.6 eV) by introducing states in the gap. A shift in the 02p edge is also observed. We develop fundamental understanding of the physical

properties of high-K dielectric films via comparison of

structural, optical, and electrical characterization. These

multimethod studies utilize techniques such as transmission electron microscopy (TEM), soft X-ray photoemission (SXPS), X-ray absorption spectroscopy (XAS), grazing incidence angle X-ray diffraction (GIXRD), capacitance-voltage (C-V) and current-voltage (I-V) analysis, as well as SE, reflectivity, and FTIRATR. The crystalline

phase of various high-K dielectric films can depend on

film thickness, stress, grain-size, and impurities. The degree of crystallinity, interface roughness, chemical homogeneity, and stoichiometry can have a direct effect on the

dielectric properties and the leakage current across such

dielectric layers. Strain, interface properties, and the

effects of surface contamination species can also have important effects. We have begun developing measurement techniques

needed to characterize candidate gate metals for next generation CMOS with high-k gate dielectrics. Fabricated test structures and extracted work function values using the

internal photoemission, flatband voltage as a function of

oxide thickness method, and Scanning Kelvin Probe Microscopy (SKPM). A mask set containing a variety of test structures with which to calibrate and compare these measurement methods has been designed and procured.

The test structures contain up to four levels of different interleaved metal layers, MOS capacitors, and sheet resistors.

Selected Publications

1. E. M. Vogel, G. A. Brown, "Challenges of Electrical Measurements of Advanced Gate Dielectrics in MetalOxide-Semiconductor Devices," 2003 International

Conference on Characterization and Metrology for ULSI Technology, Mar 24-28, 2003, Austin, Texas,

pp. 771-781 (30-SEPT.-2003)

2. J.-P. Han, E.M. Vogel, E.P. Gusev,C. D'Emic, C.A.

Richter, D.W. Heh, J.S. Suehle, "Asymmetric Energy

Distribution of Interface Traps in n- & p- MOSFETs

with HfO^ Gate Dielectric on Ultra-thin SiON Buffer

Layer, IEEE, Electron Device Letters, March 2004.

3. J. S. Suehle, B. Zhu, Y. Chen, and J. B. Bernstein,

Detailed Study and projection of Hard Breakdown

Evolution in Ultra-thin gate Oxides, Microelectronics

Reliability, 45, p. 419 2005.

4. N. V. Nguyen, J. E. Maslar, Jin-Yong Kim, Jin-Ping

Han, Jin-Won Park, D. Chandler-Horowitz, and E. M.

Vogel, "Crystalline Quality of Silicon-On-Insulator

Characterized by Spectroscopic Ellipsometry and Ra- man Spectroscopy," Appl. Phys. Lett., vol. 85, p. 2765, 2004.

5. S-E Park, S. Jeliazkov, J. J. Kopanski, J. Suehle, E. Vogel, A. Davydov, and H-K Shin, "Electrical Characterization of MOS structures and Wide Bandgap

Semiconductors by Scanning Kelvin Probe Microscopy", 2005 MRS Spring Meeting, San Francisco,

CA.

NIST Accomplishments in Nanotechnology

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3 = Nanoscaie Devices and Systems

Nanoelectronic Device Metrology Project

The CMOS FET (Field Effect Transistor), which is the current basis of ULSI (Ultra-Large-Scale Integrated)

circuits, is beginning to showfundamental limits as- sociated with the laws ofquantum mechanics and the

limitations offabrication techniques. Therefore, it is expected that entirely new device structures and computational paradigms will be required to augment

and/or replace standardplanar CMOS devices. The

Nanoelectronic Device Metrology (NEDM) Project

develops the metrology to enable these beyond-CMOS

nanotechnologies (such as Si-based quantum devices,

and molecular electronics).

Curt A. Richter and Eric M. Vogel, EEEL (812)

Introduction

The NEDM Project investigates and is developing

metrology for two specific areas of nanotechnology: (1)

Si-based quantum electronics, and (2) molecular electronics. Molecular electronics is based upon bottom-up

fabrication paradigms, while Si-based nanoelectronics

are based upon the logical continuation of the top-down

fabrication approaches utilized in CMOS manufacturing.

Organic semiconductors for use in pervasive organic

electronics is a third emerging technology under investi- gation in the NEDM. These technical approaches bracket

the possible manufacturing techniques that will be used

to make future nanoelectronic devices.

Molecular Electronics

In molecular electronics, our major objective is a NIST standard suite of molecular test structures and a fundamental understanding of charge transport through

molecules and molecular ensembles. We are developing

robust molecular test structures in order to use them to measure the electrical properties of molecules. Specifically, we are developing nanofabricated test-structures

for assessing the electrical properties and reliability of

moletronic molecules. These in-house test structures and

prototypical ME devices obtained from leading researchers outside of NIST are thoroughly assessed to determine

if they are viable test vehicles. In addition to the complexity of the nanofabrication of test structures, the chal- lenges associated with measuring the electrical properties (such as current-voltage and capacitance-voltage as

fiinctions of temperature and applied fields) of these

small molecular ensembles are daunting. The mxcasured

electrical properties will be correlated with systematic

characterization studies by a variety of advanced analytical probes and the results used in the validation of predictive theoretical models.

Figure 1. Electrically measuring molecules: the concept. Two of the most critical elements to forming a successful ME device structure are the formations of a highquality molecular monolayer and a top-metal/molecule

interface. The NEDM is investigating ways to improve

the characterization and control of these top and bottom

molecular interfaces. To do this, the NEDM is optimizing self-assembly of molecular films onto both metals

such as gold and directly onto Si surfaces. The direct attachment of organic molecules to the silicon surface is of increasing importance for emerging molecular elec- tronics applications as devices incorporating molecules

chemically bonded to silicon are amenable to integration

with existing Si processing techniques. In order to determine optimal top-metallization techniques, we are developing new characterization approaches to deter- mine metal/molecular interactions. Empirically, new

deposition techniques and new top-metals (such as or- ganic conductors) are being assessed to determine their

viability as top metal contacts. We have a significant collaboration with Hewlett

Packard (HP) Research Labs. By using a crossbar test structure consisting of a molecular monolayer sandwiched between a series of perpendicular metal wires, collaborators at separate facilities recorded nearly identi- cal electrical measurements. This step, along with others taken to eliminate potential sources of error, ensures that the measured behavior is directly attributable to the device and not the experimental set up. Electrical (currentvoltage, or IV) measurements of crossbar devices containing eicosanoic acid exhibit a controllable, two-state

NIST Accomplishments in Nanotechnology

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switching behavior that is due to the presence of the molecular layer. However, the molecular monolayer is not

the sole cause. Rather, the switch-like behavior most

likely arises from the interaction of the molecules with

the electrodes. This example illustrates that the properties of molecular electronic devices are often determined

not by the molecule alone, but by the entire device that consists of both the molecules and the attachment electrodes. This two-state behavior was independently measured in two separate laboratories, indicating that it is not

a measurement artifact and illustrating that these devices

are robust enough to ship via conventional methods and

remain active. In addition to IV measurements, what

well may be the first capacitance-voltage (CV) measurements of molecular monolayer-based devices were

taken at NIST. These CV curves also show two-state

behavior.

We have also developed an improved solution-based

method for the direct attachment of long-chain aliphatic

molecules to Si. In this method, ultraviolet (UV) radiation is used to assist the attachment of alcohols to the hydrogen-terminated Si(l 1 1) surface to form molecular

monolayers successfully. To investigate the quality of

these organic monolayers, they were physically and

chemically characterized with infrared spectroscopy,

spectroscopic ellipsometry, and contact angle measurements. The electrical properties of these organic films were probed by using IV and CV measurements obtained

from a metal-organic-silicon test structure fabricated by

post-monolayer metal deposition. The effect of differing

alkane chain length on the electrical properties was in- vestigated, and the CVs are in agreement with fraditional theory for a metal-insulator- semiconductor.

Silicon Nanoelectronics

The focus of the Si-based nanoelecfronics is the physical and electrical metrology of the basic building

blocks of silicon quantum electronic devices (e.g., quantum layers, wires, and dots of silicon surrounded by sili- con dioxide). By identifying and addressing the critical metrology issues associated with these basic building

blocks, the basis of metrology for future Si-based ULSI

nanotechnology will be defined.

tially enhanced transport behavior. A systematic study of

the inversion electron transport properties of SiNWs with

different channel geomefries has shown that a SiNW

device exhibits enhanced inversion channel current density. The extracted electron inversion mobility of the 20

nm width nanowire channel (1000 cm^A'^s) is found to be

2 times higher than that of the reference MOSFET of

large dimension (W >l|j,m). The enhancement is attrib- uted to the possible suppression of inter-valley phonon

scattering due to strain in the SiNW FET caused by the oxidation process.

poiy-Si

Figure 2. A silicon nanowire transistor.

Selected Publications

1 . Richter, C.A., D.R. Stewart, D.A.A. Ohlberg, R.S. Williams,; "Electrical Characterization of

Al/AlOx/Molecule/Ti/Al Devices," Appl. Phys. A,

80, 1355-1362. March 2005.

2. S.-M. Koo, A.-F. Fujuwara, J.-P. Han, E. Vogel, C.

Richter, and J. Bonevich, "High Inversion Current in SiHcon Nanowire Field Effect Transistors." Nano

Lett., Vol. 4, 2107-21 1 1, Nov. 2004.

3. C. A. Richter, C. Hacker, L. J. Richter, E. M. Vogel,

"Molecular Devices Formed by Direct Monolayer

Attachment to Silicon," Solid State Electronics, Vol.

48, pp. 1747-1752 (17-JUNE-2004).

4. "Metrology for Emerging Research Devices and

Materials," Eric Vogel, 2005 International Confer- ence on Characterization and Metrology for ULSI

Technology, Univ. Texas-Dallas, Richardson, Texas,

March 15-18, 2005.

A major goal is to fabricate silicon nanowire transistors using both top-down (lithography and etching) and

bottom-up (placing grown nanowires using a variety of

techniques) processing. These structures will be used to establish the relationship between the key fabrication

conditions, physical properties (such as the geometry of

the nanowire), and final electrical properties of these

silicon devices. We have demonstrated that silicon nanowire (SiNW) field effect transistors (FETs) fabri- cated by a standard 'top-down' approach exhibit substanNIST Accomplishments in Nanotechnology

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Nanomagnetodynamics: Devices

NIST is developing measurementsfor magnetic devices used in the magnetic data storage and magnetoelectronics industries for advanced magnetic re- cording systems, magnetic solid-state memories, and

magnetic sensors. Broadband electrical measurements

are being developed to characterize nanoscale devices

based on giant magnetoresistance and spin-dependent

tunneling.

Stephen E. Russek, EEEL (818)

The data storage industry is developing smaller and

faster technologies that require sub-hundred-nanometer

magnetic structures to operate in the gigahertz regime.

New techniques are required to characterize these magnetic structures on nanometer-size scales and over a wide

range of time scales varying from picoseconds to years. For example, the response of an 80-nanometer magnetic

device, used in a read head or a magnetic random access memory (MRAM) element, may be determined by a 5- nanometer region that is undergoing thermal fluctuations

at frequencies of 1 hertz to 1 0 gigahertz. These fluctuations give rise to noise, non-ideal sensor response, and

long-term memory loss. Advances in technology are dependent on the discovery and characterization of new

effects such as giant magnetoresistance (GMR) and spindependent tunneling (SDT). NIST has developed new

techniques for characterization of magnetic thin films

and device structures on nanometer size scales and gigahertz frequencies.

Device Magnetodynamics — NIST has fabricated test structures that allow the characterization of small magnetic devices at frequencies up to 40 gigahertz. The re- sponse of submicrometer magnetic devices such as spin

valves, magnetic tunnel junctions, and GMR devices

with current perpendicular to the plane (CPP), are measured in both the linear-response and nonlinear-switching

regimes. We have measured the sensors using micro- wave excitation fields and field pulses with durations

down to 100 picoseconds. We have compared measured

data to numerical simulations of the device dynamics to determine the ability of current theory and modeling to predict the behavior of magnetic devices. We are developing new techniques to control and optimize the dynamic response of magnetic devices. These include the engineering of magnetic damping by use of rare-earth doping and precessional switching, which controls switching using the timing of the pulses rather than pulse

amplitude.

Magnetic Noise and Low-Field Magnetic Sensors —

NIST has developed new techniques to measure both the low-frequency and high-frequency noise and the effects of thermal fluctuations in small magnetic structures. Understanding the detailed effects of thermal magnetization

fluctuations will be critical in determining the fiindamental limit to the size of magnetic sensors, magnetic data

bits, and MRAM elements. High-frequency noise is measured in our fabricated structures and in commercial

read heads. High-frequency noise spectroscopy directly measures the dynamical mode structure in small magnetic devices. The technique can be extended to measure

the dynamical modes in structures with dimensions as small as 20 nanometers. The stochastic motion of the magnetization during a thermally activated switching process is measured directly, which will lead to a better understanding of the long-time stability of high-density

magnetic memory elements.

In-Situ Magnetoconductance and Quantitative Magnetometry —NIST has developed new techniques to measure the electronic and magnetic properties of magnetic thin-film systems in situ (as they are deposited).

One such technique, in-situ magnetoconductance measurements, can determine the effects of surfaces and inter- faces on spin-dependent transport in a clear and unambiguous manner. The effects of submonolayer additions of

oxygen, noble metals, and rare earths on GMR are being

studied.

Magnetoresistive Scanning System

Implemented

We built a high-frequency magnetoresistive (MR)

scanning system to probe high-frequency electric and

magnetic fields above high-peed circuits. The system can

use either commercial recording heads or NISTfabricated high-frequency MR sensors. The spatial resolution can be as small as 100 nanometers depending on

the type of probe. A special set of high-frequency struc- tures — which included co-planar waveguide tapers,

shorts, and opens — were fabricated to allow imaging of

structures that had spatially varying microwave fields in three dimensions. The structures were imaged in three modes: low frequency magnetic fields (1 kilohertz),

high-frequency electric fields (1 to 4 gigahertz), and

high-fi"equency magnetic fields (1 to 4 gigahertz).

Low-Frequency Noise Measurements on Commercial Magnetoresistive Sensors

Low-frequency noise was measured in the frequency

range from O.I hertz to 10 kilohertz on a variety of

NIST Accomplishments in Nanotechnology

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commercially available magnetic sensors. The types of

sensors investigated include those implemented with

anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR)

effect devices. The 1/f noise components of electronic

and magnetic origin were identified by measuring sensor

noise and sensitivity at various applied magnetic fields. For the GMR sensors, both electronic and magnetic

components contribute to the overall sensor noise. Maximum noise occurs at the bias field that gives maximum sensitivity. The noise of TMR-based sensors is primarily due to resistance fluctuations in the tunnel barrier, having little to no field dependence. The best lowfield detectivity of the sensors that has been measured is on the order of 100 picotesla per root-hertz at 1 hertz. These magnetic sensor noise data are part of a database

of low-field sensor performance.

128 Jim 128 urn

(d)

128 iJjn 128 (im

Figure 1. Image offields above a coplanar waveguide obtained

using a high-band^'idth spin-valve recording head: (a) optical

image, (b) low-fi-equency magnetic image, (c) 1 gigahertz capacitive image, (d) 1 gigahertz magnetic image.

In-Situ Observation of Nano-Oxide Formation in

Magnetic Thin Films

In-situ conductance and reflection high-energy elec- tron diffraction (RHEED) measurements were taken dur- ing the oxidation of 20-nanometer-thick Co and Co-Fe

layers. The conductance shows an initial drop with exposure to oxygen followed by a period of increasing conductance, indicating an increase in specular reflection of

electrons at the oxide interfaces. RHEED measurements

show a blurring of the (1 11) face-centered-cubic (FCC)

texture with exposure to oxygen, indicating the formation of an amorphous oxide during the initial conductance drop and conductance increase. After the conductance begins to decrease again, a new diffraction pattern appears in the RHEED data, indicating the formation of

CoO with a FCC (1 1 1) texture but with a different lattice spacing. These studies shed light on the physical mechanisms for the giant magnetoresistive effect used in magnetic sensors.

Current Density Distribution Determined through

In-Situ Conductance Measurements

The sheet conductances of top-pinned spin valves and

single-material films were measured in situ as the thin- film layers were grown. The data were fit to a Boltzmann

transport calculation. The electrical conductivity and

electron mean free paths were determined for each material by measuring the in-situ conductance of thick, sin- gle-material films. The electron transmission probabilities were deduced for each interface from the theoretical

fits to the multilayer data. From these interfacial trans- port parameters the ratio of current density to electronic

field, or effective conductivity, was calculated as a fiinc- tion of position for the completed spin valve. The distri- bution of current in the spin valve was not very sensitive

to the overall amount of diffuse scattering at the inter- faces. Spin valves are used in modem magnetoresistive

read heads in disk drives.

0.10

0,08 -

So.06

oO.(

0.02 -

0.00

CoFe Ru CoFe t = 6 nm

5 nm

10 15 20 25 30 35 Thickness (nm)

Figure 2. In-situ conductance measurements ofa spin-valve taken during growth. The data can befit to determine the local current

density.

Selected Publications

1. N. Stutzke, S.E. Russek, D.P. Pappas, and M.

Tondra, "Low-Frequency Noise Measurements on Commercial Magnetoresisfive Magnetic Field Sensors," J. Appl. Phys., in press.

2. A. McCallum and S.E. Russek, ''In-Situ Observation

of Nano-Oxide Formafion in Magnetic Thin Films,"

IEEE Trans. Magn. 40, 2239-2241 (July 2004).

3. A. McCallum and S.E. Russek, "Current Density Distribution in a Spin Valve Determined Through In Situ Conductance Measurements," Appl. Phys. left. 84,

3340-3342 (April 2004).

NIST Accomplishments

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Nanoscale Engineered Magnetic Sensors

In 2004 we embarked on an interdisciplinary program

on low-noise, nanoscale magnetic sensors with improved detection levels. The program includes development ofnew amorphous and nanocrystalline materials, imaging ofmagnetic domains, noise measure- ment in sensor devices, and micromagnetic modeling.

David P. Pappas, EEEL (818)

Magnetoresistive magnetic field sensors have wide application in research and industry. Currently, cryogenic

superconducting quantum interference devices are used

for the measurement of ultra-low magnetic fields. They

provide important information in many areas of basic

research, medical magnetic field monitoring, and security. However, it is necessary to develop devices operating at room temperature that are scalable, linear, and

have comparable sensitivity to realize the maximum

benefit of such devices. The most promising candidates

for development are magnetoresistive technologies.

Magnetic noise is a critical problem in the development

of these devices; such noise can be two orders of magnitude higher than the intrinsic Johnson, shot, and 1/f noise

of the device. New materials, measurement techniques,

and device architectures are required to characterize and

reduce the effect of low- and high-frequency magnetic

fluctuations. New Zigzag Magnetoresistive Field

Sensors

We developed new "zigzag" magnetoresistive sensors that, because of their shape, are sensitive to magnetic fields parallel to their axes but insensitive to fields perpendicular. The sensors are made of thin films of

Ni-Fe, 35 micrometers long and 5 micrometers wide,

with nanoscale design elements at the edges. The devices

sense magnetic fields using a small current down their centerlines. They are able to distinguish positive and

negative fields because their resistance is an odd function of field.

In general, anisotropic magnetoresistive sensors must

be biased, with their magnetization at an angle with re- spect to the current direction. The new sensors are fabri- cated in a zigzag pattern that pins the magnetization at alternating positive and negative 45 degrees to the direc- tion of the current flow. This novel approach provides a

built-in magnetization bias determined by the corrugation of the edges. Since the angle of magnetization of the magnetic domains is controlled only by the sensor shape.

and not by adjacent biasing fields, the devices may be

scaled to nanometric dimensions.

When the sensor is exposed to a magnetic field ori- ented in the direction of the current, the angle of magnetization relative to the current in both sets of the domains decreases. This results in an increase of electrical resistance in each domain and in the entire device. For

magnetic fields in the negative current direction, the magnetization angle increases and the resistance decreases. For magnetic fields in the perpendicular axis, the magnetization angle increases in one set of magnetic

domains and decreases in the other, resulting in no net change in device resistance.

The sensors are simpler in design and likely will be

cheaper to make than conventional magnetic sensors used in portable devices, which typically include multiple aluminum strips that alternate diagonally across the

sensor. The new zigzag sensors are expected to have less electronic noise by confining the current to the center of

the device and by eliminating edge imperfections that can result in nanoscale magnetic fluctuations.

, Such sensors could be used to measure magnetic

fields in applications such as compasses, weapons detection, medicine, and non-destructive evaluation of structural materials.

Figure 1. Micrograph ofa zigzag magnetoresistive element obtained by use ofa scanning electron microscope with polarization

analysis (SEMPA). With respect to the long axis ofthe sensor (the axis ofcurrent), green indicates magnetization at an angle of +45

degrees, orange at an angle of-45 degrees. The magnetic domain

patterns were also accurately modeled with NIST's Object Ori- ented MicroMagnetic Framework (OOMMF).

Magnetodynamic Imaging

New techniques are being developed to dynamically

image magnetic nanostructures. We have successfiilly

used Lorentz microscopy to image the magnetic structure

in a spin-dependent tunneling device. We are extending Lorentz imaging to allow dynamical observation of the magnetic structure in magnetic films and active devices

at frequencies from 1 hertz to 10 kilohertz. Fluctuations

in the magnetic structure, which often correspond with

defects in the device, give rise to noise and stochastic

behavior that can limit their performance as sensors and

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memory elements.

We designed a new scanned-probe-imaging system

based on dynamic ballistic electron magnetic microscopy

that will be able to image fluctuation in magnetic struc- tures on sub-nanometer length scales. This technique

uses spin-dependent scattering of the electrons emitted

from a scanned turmeling tip to detect local magnetic

structure. This technique is capable of imaging buried

magnetic structures and fluctuations in active giantmagnetoresistive and spin-dependent tunneling devices.

Figure 2. Lorentz microscopy image ofa 1 00-micrometer Ni- Fe/Co-Fe tunneljunction. Disorder in both the magnetically hard Co-Fe layer and the magnetically soft Ni-Fe layer can be observed.

Nano-magnetic Imaging for Forensic Applications

Magnetic tapes are still the prevalent method for storing audio and video data in much of the world. Examination of media from tape media for authenticity and

validation is critical in a wide range of investigations,

including criminal and terrorism, for example. In this work, we have built a system that can image both audio

cassette and VHS tapes in real time (i.e. as the tape is being played back) with sub-micrometer resolution

downtrack.

This allows investigators to identify whether or not

tapes are copies or if they were altered after the re- cording in questions. An image of a typical recording

stop event on a cassette tape is shown in the Figure 3, illustrating the high resolution of this technique and also

the capability to recover the data along a given pice of

the track. The short burst of high frequency occurs as the

tape slows down, and then the gradual lowering of the head back onto the tape can be seen near just after. These

images are taken with a 256 linear array of sensors that glide just above the tape to map out the magnetic fields. The sensors are nano-engineered to have low noise and a

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pice ofthe track.

dynamic range that corresponds to that needed for these

tapes. In addition, the magnetic response of each of the 256 sensors has been individually calibrated. This results

in a very accurate image that allows for unprecedented

accuracy in the analysis of entire tapes, computerized

screening, and data recovery of partially erased data.

Selected Publications

1. F. C. S. da Silva, W.C. Uhlig, A.B. Kos, S. Schima,

J. Aumentado, J. Unguris, and D. P. Pappas, "ZigzagShaped Magnetic Sensors," Appl. Phys. Lett. 85,

6022-6024 (December 2004).

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Single Photon Sources and Detectors

Emitters and detectors ofsingle photons on demand are importantfor low light level metrology and quantum key

distribution, a secureform ofcommunication. They are

also critical components ofquantum computing systems

and experiments that will test the fundamental limits of

precision in optical metrology. Epitaxial InGaAs/GaAs

quantum dots are attractive as single photon emitters for

their ease offabrication and inclusion with monolithic

microcavities, short spontaneous emission lifetimes, and

the possibility ofelectrical injection. We are developing

single photon emitters based on InGaAs quantum dots

(QDs), with the ultimate goal ofelectrically pumped operation. We are also developing QD-based single photon detectors.

Richard Mirin, EEEL (815)

For practical implementation, single photon sources should operate at high temperatures. We have demonstrated photon antibunching from a single, self- assembled InGaAs quantum dot at temperatures from 5 K up to 135 K, and single photon emission up to 120 K. We grew InGaAs quantum dots using molecular beam

epitaxy and isolated single dots with etched mesas. The

second-order intensity correlation g(2)(0), which is a measure of the independence of single photon emitters, was derived by measuring the coincidence photon counts

in a Hanbury Brown-Twiss interferometer. For temperatures up to 100 K, g(2)(0) was measured to be approximately linear with temperature and less than 0.26. At

120 K, g(2)(0) increases to about 0.47, which is slightly

less than the second-order intensity correlation expected

from two independent single emitters. At 135 K, g(2)(0) was 0.67, which still indicates nonclassical light emission that is equivalent to having three independent single

emitters. Contributions to the emission other than that from the uncharged single exciton were separated by

analysis of additional peaks in the photoluminescence

spectra. If we were able to eliminate the contribution

due to the additional peaks, we would obtain a g(2)(0)

value of 0.087 at a quantum dot temperature of 100 K.

There are two reasons why the single InGaAs quantum dot decreases in emission at higher temperatures,

thereby requiring higher pump intensities and subsequently degrading the performance as a single photon

source. Experiments suggest that carrier transfer to the wetting layer is a contributing factor, which can be miti- gated using larger quantum dots with less quantum con- finement energy or wider bandgap barriers. Secondly,

optical phonon scattering becomes more pronounced as

the temperature increases. The problem could be less- ened by promoting faster radiative recombination times

of the quantum dot excitons by the use of a microcavity.

Single electron-hole pair injection by electrical means

should improve the performance of an InGaAs quantum

dot single photon source by removing the possibility of

charged exciton or biexciton formation. The photon

repetition rate of the electrically pumped source will be

controlled by the frequency of an applied AC voltage,

allowing precisely defined emission of single photons.

The results above represent the highest reported temperature for non-classical light emission from the In- GaAs/GaAs system. The single photon source is being

developed for applications including ulfra-secure communication, the testing of frindamental theories of quantum optics, and high-accuracy radiometry in which opti- cal power and energy measurements are calibrated by

counting photons.

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Figure 1. Second-order intensity correlation ofsingle photon

source measured as afunction oftemparture. (a) 5 K, (b) 50 K,

(c) WOK, (d) 120 K, and(e) 135 K. We are also developing a type of single photon detector that is based on a quantum-dot, optically gated field- effect transistor in III-V semiconductor material. It will be able to operate at temperatures that are obtainable

with solid state cooling, unlike the state-of-the-art single

photon detectors based on superconductors. Such a device has the potential for high quantum efficiency (>

90%) and low noise, and it is compatible with conventional semiconductor fabrication and processing methods. In our device, the barrier material adjacent to the

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QD is optically excited to create electron-hole pairs. A

hole is captured by the QD and stored there. In effect, the QD is an engineerable trap and can store charge for a long time. We have observed persistent photoconductivity in our detectors at very low light levels. Next generation devices are in fabrication and testing.

Selected Publications

1. R. Mirin, "Photon antibunching at high temperature

from a single InGaAs/GaAs quantum dot," Appl.

Phys. Lett. 84, 1260 (2004

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Terahertz Technology

We develop metrological methods, components, and

technologyfor imaging and spectroscopy from about

100 GHz to several terahertz (THz). Wefabricate our own nanoscale detectors that we invent, engineer, and

evaluate.

Erich Grossman and Robert Schwall, EEEL (815)

Imaging in the approximately 100 GHz to 2 THz range offers the possibility of detection of concealed weapons and

remote detection of contraband {e.g., explosives under

clothing) without the use of ionizing radiation. Spectroscopy in this frequency range has additional applications in industrial processing and remote identification of chemicals. We have developed unique capabilities in detectors,

optics, and metrology methods. These capabilities are being

used to construct improved imaging systems and to provide

government agencies with unbiased evaluations of systems

and components developed in industry.

Figure 1. An optical image ofa representative hand gun used in

test.

Figure 2. An active mm-wave image ofthe representative hand

gun.

Accomplishments

Real-Time Imaging with Slot-Ring Antenna-Coupled

Uncooled Millimeter-Wave Bolometer Arrays — Imaging

applications in the terahertz spectral range require large

numbers of pixels, and an appropriate balance between sensitivity and cost. One approach to achieving all three goals

simultaneously is to use lithographic antennas to couple the radiation into nanometer-sized detectors. In this case, the antenna engineering challenge is to develop an array- compatible antenna with the highest possible directivity in a completely planar monolithic structure. This is the motivation behind a new antenna design we have developed, in which slot-ring antennas with finite ground planes are pat- terned on electrically thin substrates. Because the application is imaging, sensitivity to radiation from only one side of the substrate is desired, so a planar backshort is employed to reflect radiation from the backside of the substrate into the forward direction. With the backshort at the proper position behind the plane of the slot-ring anterma,

constructive interference occurs between the directly re- ceived radiation and that reflected by the backshort, in- creasing the directivity. Measured beam patterns are highly

circular and exceptionally narrow (22° at -3 dB) from a completely planar antenna. They can be arrayed at a spacing of 1 .5 wavelengths with negligible measured effect on

performance. A 120-element array for 95 GHz has been

fabricated and used to obtain real-time images.

Figure 3. Scanning electron micrograph ofSi02 encapsulated Nb

microbolometer. Thefigure insert shows a schematic ofthe cross- section ofthe bolometer and the encapsulating oxide.

• Actively Illuminated Scanned 40kpixel 95 GHz

Imaging System — Fundamental considerations dictate the use of actively illuminated imaging systems for indoor applications, and active illumination favors a scarmed architecture. We have completed design and fabrication of all the major components — line source, detector array, lenses and

scanner of a real-time whole-body imaging system. System

test will commence in late 2004 and continue through 2005.

• Characterization ofSb-Heterostructure Quantum Tunneling Diodes as Submillimeter Wave Detectors —We

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have measured the diode I(V) and noise of Sbheterostructure quantum tunneling diodes fabricated from

epitaxial layers of InAs and AlGaSb by an industrial collaborator. These devices exhibit exceptionally high nonlinearity in the 1(V) characteristic, which produces the rectifi- cation without bias. Thus the device does not suffer from

the 1/f noise associated with a dc bias. Our measurements

indicate that these devices have sensitivity that challenges

that of other existing room-temperature detectors in this frequency range, and that they could be attractive candidates in the millimeter and submillimeter wave region.

Selected Publications

1. A. Luukanen, E. N. Grossman, H. P. Moyer, and J. N. Schulman, "Low-frequency noise properties of

quantum turmeling Sb-heterostructure millimeter wave diodes," submitted to Appl. Phys. Lett. (2004).

2. A. Luukanen, E. N. Grossman, H. P. Moyer, and J. N. Schulman, "Noise and THz rectification characteristics of zero-bias quantum tunneling Sbheterostructure diodes," Submitted to Int'l. J. High

Speed Electronics (2004).

3. A. J. Miller, A. Luukanen, and E. N. Grossman,

"Micromachined anteima-coupled uncooled microbolometers for terahertz imaging arrays," Proc. of

SPIE, 5411, pp. 18-24 (2004).

4. E. N. Grossman, A. Luukanen, and A. J. Miller

"Terahertz active direct detection imagers," Proc. of

SPIE, 5411, pp. 68-77 (2004).

5. A. Luukanen, R. H. Hadfield, A. J. Miller, and E. N.

Grossman, "A superconducting antenna-coupled microbolometer for THz applications," Proc. of SPIE,

5411, pp. 121-126 (2004).

6. A. Luukanen, A. J. Miller, and E. N. Grossman,

"Active millimeter-wave video rate imaging with

staring 120-element microbolometer array," Proc. of

SPIE, 5410, pp. 195-201 (2004).

7. E. N. Grossman, T. E. Harvey, and C. D. Reintseraa,

"Controlled barrier modification in Nb/NbOx/Ag

metal insulator metal tunnel diodes," J. Appl. Phys.

91(12), pp. 10134-10139 (June 15, 2002).

8. S. Nolen, T. E. Harvey, C. D. Reintsema, and E. N.

Grossman "Properties of cavity-backed slot-ring an- tennas at 95 GHz," Proc. SPIE 4373, pp. 49 (2001).

9. E. N. Grossman, S. Nolen, N. G. Paulter, and C. D. Reintsema, "Concealed weapons detection system

using uncooled, pulsed, imaging arrays of millime- ter-wave bolometers," Proc. SPIE, 4373, pp. 7

(2001).

10. N. G. Paulter, E. N. Grossman, G. N. Stenbakken, B.

C. Waltrip, S. Nolen, and C. D. Reintsema, "Design

of an active mm-wave concealed object imaging sys- tem," Proc. SPIE., 4373, pp. 64 (2001).

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Nanoscale Fabrication

Our goal is to maintain state-of-the-art nanofabrication capabilities in NIST Boulderfacility for the use of

the NIST staffand collaborators.

James Beall, EEEL (817)

Our facilities for fabricating integrated circuits are essential to nearly all of the work in the Group. We maintain a research-class facility specialized in the fabrication of

complex nanoscale superconducting circuits, nanoscale

electronics and MEMS structures. Beginning with computer-aided design, we use our electron-beam and optical

lithography to make structures smaller than 40 nm and

complex circuits containing as many as 107,000 Joseph- son junctions. Our tools are housed in 200 m^ of "class 100" clean room space, which was improved greatly in 1999. We have recently added tools for fabricating

MEMS that are essential for the creation of many of our

ultra-sensitive instruments, and for micromachined ion

traps for quantum computing and future atomic clocks.

Figure 1. Maggie Crews inspects a mask generated in our optical pattern generator.

Our facilities are used for a variety of diverse projects

including those by most of the NIST organizations in Boulder. They include the fabrication of Josephson array voltage standards, a prototype capacitance standard

based on single electron turmeling, prototype Johnson

noise thermometry, microcalorimeters for materials

analysis and space observation including the most complex superconducting system ever made, quantum computing using Josepson phase qubits, quantum computing

using ion traps, miniature atomic clocks, terahertz imaging systems for homeland security, revolutionary single

photon counters for fundamentally secure communications, and novel nanoscale magnetic imaging probes.

Figure 2. View ofclean room lithography bay with digital pattern

generator and wafer stepper recessed into the wall on the right

and wet processing on the left. Optical lithography produces

linewidths smaller than 500 nm and is often combined with elec- tron beam lithographyfor structures as small as afew tens ofnm.

Our facilities are available as an "open-shop" operation.

After appropriate training, all of our staff can personally use them to fabricate the devices needed for their re- search. We keep the facility and the processes flexible and under the control of each individual user to avoid

constraining research and to allow maximum creativity.

Our past accomplishments are testimony to the success of this approach.

In the past two years we have upgraded our optical pat- tern generator to a modem instrument capable of making

masks with feature sizes of 1 .5 micrometers. Having this

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in-house mask-making facility allows very rapid turn- around, which improves productivity and encourages

creativity. This capability is rare for a facility of this size and sophistication. For patterning at the 0.5 [im scale, we use optical lithography in a recently acquired I-line stepper and a UV contact aligner. Electron-beam lithography enables registration and patterning at less than 50 nm. Our capability in electron beam lithography is being

enhanced with the addition of a field emission scanning

electron microscope with wafer-scale stages and load

lock.

Our thin-film deposition and etching tools are similar to those found in a semiconductor fabrication facility, but

they have been optimized for normal and superconducting metal fabrication. We have general purpose and

dedicated sputtering and e-beam systems to deposit multilayers of metals. We use electron cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition

(CVD) of SiO: for circuit insulation. Dry etching is per- formed in standard RIE and plasma etching tools. HI

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Figure 3. Access bayfor diffusion furnaces used to fabrication many MEMS structuresfor integrated structures that make state- of-the-art standards and measurement technologies possible.

We have tools necessary to fabricate structures for microelectrical-mechanical systems (MEMS). We use a set of research-scale tube fumace reactors (5" capable), for wet and dry silicon oxidation, solid-source diffusion of

boron, low-stress silicon nitride/polysilicon low-pressure

chemical vapor deposition (LPCVD), and LPCVD of

low-temperature oxide. We have added two silicon dryetch tools for micromachining of Si. We use xenon di- fluoride gas to rapidly and isotropically etch silicon for a

sacrificial layer release of surface micromachined structures. A deep reactive ion etch system allows highly ani- sotropic etching of Si for through-wafer etching and

definition of high aspect ratio Si structures.

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Johnson Noise Thermometry

We use a nanoscale quantum-based voltage source to reduce the uncertainty ofJohnson noise thermometers, with the ultimate goal ofcreating an intrinsic quantum-based electronic temperature standardfor

use by the NIST Chemical Science and Technology

Laboratory. The quantum-basedpseudorandom noise sources that we use arefabricated with nanoscale di- mensions in our laboratory.

Wes Tew, CSTL (836), Sam Benz, EEEL (817),

and Sae Woo Nam, EEEL (815)

Gas-based and fixed-point methods of precision thermometry are either hmited to specific fixed temperatures or re- quire considerable effort to reduce uncertainties below parts

in 10^. An electronic quantum-based method would provide

another approach as well as advantageous features such as improved linearity of temperature calibrations. It would

also provide an improved realization of the Kelvin thermodynamic temperature scale, and a direct link between temperature and electrical standards, possibly providing a route

to a redetermination of the Boltzmann constant. More importantly, this quantum-based Johnson noise thermometry

(JNT) method is a new paradigm to realize the thermodynamic temperature scale through electrical and quantumbased standards. This JNT approach to create an "electronic Kelvin" is analogous to the NIST watt-balance program to realize an "electronic kilogram."

Our customers and collaborators include the NIST Chemical Science and Technology Laboratory, the temperature

calibration laboratories of other national measurement insti- tutes, and industrial applications requiring long-term temperature stability or with temperature sensors in difficult or remote locations.

In order to meet these needs and to create a quantumbased electronic temperature standard, we have developed new technology in a number of different areas. We

have constructed low-noise cross-correlation electronics,

developed a nanoscale quantum voltage noise source (QVNS) specifically for this application, and performed

the first calibrations of the electronics using the QVNS.

We have compared the QVNS synthesized pseudo-noise

voltage waveforms with the voltage noise of resistors in

triple point cells of both gallium and water. We have also devised a novel ratiometric method that uses the QVNS

to compare the voltage noise of resistors at different temperatures.

Accomplishments

• NIST has performed many comparisons between the Johnson noise voltage of a resistor in a gallium cell and the synthesized pseudo-noise waveforms of the quantum volt- age noise source. Similar measurements have been done

with a water triple-point cell. Measurements were made

using the custom built cross-correlation electronics system.

The computer-controlled optically-interfaced system correctly sampled at the 50 MHz sampling rate and stored and

processed the waveforms in real time. This correlation elec- tronics is different from other systems because we digitally process the signal in an FPGA before sending it to a computer. This is necessary to reduce the data rate to the computer.

Frequency (kHz)

Figure 1. Log-log plot showing the measured spectrum ofa QVNS-synthesizedpseudo-noise waveform. The spectrum was

measured with the JNT cross-correlated electronics and shows the power (<V^>) spectrum S„ ofthe 1258 tones synthesized by the QVNS. Each bin has a width oflHz and is an average of200

samples. The power spectrum is in arbitrary units based on the

digitizer bins.

• NIST fabricated numerous circuits specifically to improve the performance and optimize the circuits for the JNT

program. We have tested the circuits with appropriate

pseudo-noise waveforms and demonstrated proper cross- correlation using an FFT spectrum analyzer. Benz also per- formed extensive measurements of input-output coupling

for the JNT circuits and devised a novel unipolar bias method to decrease the coupling by about 40 dB. These

results allowed us to take the next step and measure the arrays at much smaller voltages and higher bandwidth using

cross-correlation electronics.

• NIST has continued development of the Johnson noise

thermometry system. Unwanted distortion was observed

when the Josephson signal was measured with the cross- correlation electronics. We were able to significantly reduce

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this distortion by using inductive chokes on the input of the

electronics. The cause of the distortion appears to be mixing

in the fnst FET amplifier stage of 100 MHz to 400 MHz

delta-sigma modulator tones synthesized by the Josephson

array. With this dramatic improvement, the Josephson synthesized noise waveform and the gallium and water triple

point cells were directly compared. We found agreement to 2 parts in 10^ with a Is uncertainty of 1 x 10"^ between the voltage noise of a 100 W resistor in a triple-point gallium

cell (T9o= 302.916) and a pseudo-noise waveform with the same average power that is synthesized by a quantized volt- age noise source. We estimate the temperature of the resistor to be 302.5 K ± 0.3K la uncertainty based on the uncertainty fi-om the cross-correlation). With better characterization of our JNT system, we expect to achieve relative accuracies of parts in 10^ for arbitrary temperatures in the range between 27 K and 1000 K.

• We have proposed using a new "ratiometric method"

that takes advantage of the linearity of the Josephson waveforms to independently compare different noise powers. The Johnson noise power at a known temperature is first balanced with a synthesized noise power from the QVNS.

The process is then repeated by balancing the noise power

from the same resistor at an unknown temperature. When

the two noise power ratios are combined, a thermodynamic

temperature can be derived where the scaling is accomplished by the ratio of the two QVNS spectral densities. Using this method, preliminary results of the ratio between

the gallium triple point and the water triple point were used

to demonstrate the accuracy of the measurement system

with a standard uncertainty of 0.04 %.

• This year we have been able to measure the ratio of the gallium and water triple-point temperatures to within an

accuracy better than 100 \xKfK. using the ratiometric method. This was accomplished by making improved Jo- sephson circuits with smaller common mode signals, using

flip-chip on flex packaging developed by the Quantum

Voltage Project, and improving the output transmission

lines and electrical connections in the electronics by elimi- nating distortion-producing poor contacts. Future efforts

will focus on understanding the frequency dependence of

the absolute measurements, which probably are due to the frequency response of the transmission line. These errors should be correctable or able to be reduced significantly. In

the next year we also plan to perform more absolute and

relative mode measurements at higher temperatures (273 K

to 1000 K) through comparisons with an ITS-90 calibrated platinum resistance thermometer.

Selected Publications

1 . S. Nam, S. Benz, P. D. Dresselhaus, W. L. Tew, D.

R. White, and J. M. Martinis, "Johnson noise ther- mometry using a quantum voltage noise source for calibration," Digest 2004 Conference on Precision

Electromagnetic Measurements, 27 June - 2 July

2004, London, England, pp. 664-665 (2004).

2. S. Nam, S. Benz, P. D. Dresselhaus, and J. Martinis,

"Johnson noise thermometry measurements using a quantized voltage noise source for calibration,"

IEEE Transactions on Instrumentation and Measurement, 52, 2, pp. 550-554 (April 2003).

3. S. P. Benz, P. D. Dresselhaus, and J. Martinis, "An

AC Josephson source for Johnson noise thermometry," IEEE Trans. Instrum. Meas., 52, 2, pp. 545-549

(April 2003).

4. S. W. Nam, S. P. Benz, J. M. Martinis, P. D. Dresselhaus, W. L. Tew, and D. R. White, "A ratiometric

method for Johnson noise thermometry using a quandzed voltage noise source," Temperature: Its Measurement and Control in Science and Industry,

7, (1), pp. 37-42, Dean C. Ripple Ed., Proc. The

Eighth International Temperature Symp., AIP Conf

Proc. 684, New York: Melville (2003).

5. S. P. Benz, J. M. Martinis, S. W. Nam, W. L. Tew,

and D. R. White, "A new approach to Johnson noise

thermometry using a Josephson quantized voltage source for calibration," Proc. TEMPMEKO 2001,

the 8th IntT. Symp. on Temperature and Thermal

Measurements in Industry and Science, B. Fellmuth,

J. Seidel, and G. Scholz, Eds., Berlin: VDE Verlag,

pp. 37-44 (April 2002).

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Quantum Communications

We develop and apply single-photon detectors (optical/infrared) for metrology and science. Our work is based on nanoscale detectorsfabricated in our own

laboratory.

Sae Woo Nam, EEEL (815)

New quantum-based communication and measurement

systems that use single and correlated photons have been

developed; however, the current tools to calibrate the components in these systems are inadequate for the emerging applications. For accurate calibration, a detector capable of determining the number of photons in a

single pulse of light is needed. We are developing highefficiency detector systems with this capability in the three telecommunication windows (850 nm, 1310 nm,

and 1550 nm). These new instruments will also aid in the understanding and advancement of new photon sources such as the single-photon turnstile (SPT). They can be

used as a spectroscopic diagnostic tool or characterization tool for optical elements, optical materials, and optical systems at ultra-low light levels. A near-term use of

the detectors is for initial research in Quantum Key Distribution, a method of communication where security is assured by the laws of quantum mechanics rather than by

mathematical complexity.

Figure 1. Array offour transition edge sensor (TES) singlephoton

detectors. The larger squares are 50 pim on a side and the smaller ones are 25 /um. The arrows arefor alignment.

Two distinct strategies are being pursued in this project.

The first, which is best suited for counting photons at telecommunication wavelengths or measuring the energy

of a single photon, is a refinement of superconducting

transition-edge sensor (TES) detector technology that was previously developed by the Quantum Devices

Group and researchers at Stanford University. In this device a thin film of tungsten is cooled to its superconducting transition temperature (-100 mK). The tungsten

is biased at its transition from the normal to superconducting state such that the input of a small amount of

heat from the arrival of a single infrared photon, dra- matically changes in the tungsten resistance. The change

in resistance is proportional to the heat absorbed.

These TES detectors can be operated in two distinct modes, depending on the photon source properties. For

single-wavelength (monochromatic) sources such as la- sers and spontaneous parametric down-conversion

(SPDC) crystals (used for quantum information and metrology) the sensor response is directly proportional to absorbed photon number (photon number discrimination

mode). For broadband sources (such as white-light or multiple fluorescence emissions) the energy-resolving

ability of the TES devices allows photon-counting spec- troscopy to be performed across the entire near infrared/

optical band (2 )im to 200 nm). In this mode, the photon

number is discarded in favor of determining the energy

(i.e., color) of the incoming photons (spectroscopic

mode).

Over the past year we have increased the quantum effi- ciency of the TES detector system from less than 20 % to over 80 %. This was done by incorporating the tungsten

film in a carefully designed "optical cavity" that prevents

the incident photon from passing through the detector or refiecdng off its face and increases the probability that a photon incident on the detector will be absorbed in the tungsten. Additional work is underway to eliminate

losses in the fiber optic components that couple photons

to the detector, made to increase the system detection

efficiency. Our goal is to get as close as possible to 100% efficiency.

Conventional semiconductor-based single-photon detectors suffer from high rates of "dark counts" (false positives), at the communication wavelengths of 1300 nm

and 1550 nm, which severely limit single-photon applications. The TES detectors have no intrinsic dark counts,

allowing them to be operated at much lower light levels than any existing metrological instrument. As the effi- ciency of TES systems has increased this has led to the investigation of previously unknown sources of dark

counts in fiber optic systems. One such source at 1,550

nm is now thought to be the black body radiation inci- dent on a fiber at room temperature.

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Their unparalleled sensitivity allows these detectors to be

a significant tool in the development of quantum-based

communication protocols and the evaluation of the security of optical quantum cryptographic systems.

The second technical strategy involves the use of a dif- ferent type of superconducting detector. These detectors, called superconducting single photon detectors, or SSPDs, have been previously investigated at the University of Rochester. They incorporate a very narrow (~I00

nm) serpentine line of metallic superconductor such as Nb or NbN carrying a bias current very close to its criti- cal current. When a photon strikes the superconductor a small "hot spot" is created locally, reducing the critical current to a value below the bias current. This develops a

voltage across the superconductor that can easily be detected. While the detector is not photon-number resolv- ing (one or more photons will create the hot spot), it is extremely fast, producing pulses of order a few tens of

picoseconds. Such a device would be potentially very

useful in a quantum communication system. At the present time, these devices have very low quantum effi- ciency (a few percent) and are incapable of long-term

unattended stable operation. We are initiating work to address both of these issues.

Accomplishments

• Development ofa processfor reliably fabricating

thin film tungsten detectors with a controllable transition temperature —Previously tungsten films for TES had

been successfully made at Stanford, at Munich Technical

University, and at NIST, but only with low yield and

widely varying properties. During 2004 a development

program using statistical process control methodology was completed that enables us to deposit films with high

yield and controllable Tc.

• Increase in speed of TES detectors by 2 to 3 x — Im- provements in the etching process used to pattern the TES detectors has resulted in improved edge definition and devices that are 2 to 3 times as fast as earlier- generation sensors.

• Increase in TES system quantum efficiency from

<20% to >80 % — The reliable process for fabricating

tungsten films has enabled us to explore a variety of configurations for increasing quantum efficiency. A structure incorporating a metallic mirror and dielectric layer

below the tungsten and an anti-reflection layer on top has

yielded 93 %+ absorption of 1,550 nm photons in the tungsten and an overall system efficiency over 80 %.

• Fabrication ofthin, high-Tc NbNfilms — The development of SSPDs has been limited by the availability of

thin (~4 nm) high Tc (-10 K) films. Previously these

films were available from only one institution in Moscow, Russia. In 2004 we succeeded in fabricating a series of films which initial tests indicate have large area

Tc, equivalent to those from Moscow.

Selected Publications

1. S. Nam, A. J. Miller, and D. Rosenberg, "Lowtemperature optical photon detectors for quantum in- formation applications," Nuclear Instruments and

Methods in Physics Research A 520, pp. 523-526

(2004).

2. D. Rosenberg, S. W. Nam, A. J. Miller, A. Salminen, E. Grossman, R. E. Schwall, and J.M. Martinis,

"Near-unity absorption of near-infrared light in tungsten films," Nuclear Instruments and Methods in Physics Research A 520, pp. 537-540 (2004).

3. G. DiGiuseppe, M. Atature, M. D. Shaw, A. V. Ser- gienko, B. E. A. Saleh, M. C. Teich, A. J. Miller, S. W. Nam, and J. Martinis, "Direct observation of

photon pairs at a single output port of a beam-splitter

interferometer," Physical Review A (Atomic, Molecular, and Optical Physics) 68(6): pp. 1 63817-1-4,

(December 2003).

4. A. J. Miller, S. W. Nam, J. M. Martinis, and A. V.

Sergienko, "Demonstration of a low-noise nearinfrared photon counter with multiphoton discrimination," Appl. Phys. Lett., 83 (4): pp. 791-793 (28

July 2003).

5. J. M. Martinis, "Electronics for arrays of transition edge sensors using digital signal processing," IEEE

Transactions on Applied Superconductivity 13(2): pp. 618-21, (June 2003).

6. B. Cabrera, J. M. Martinis, A. J. Miller, S. W. Nam,

and R. W. Romani, "TES spectrophotometers for near IR/optical/UV," Proc, 9th IntT Workshop on Low Temperature Detectors, 22- 29 July 2001,

Madison, WI, pp. 565-570 (2002).

NIST Accomplishments in Nanotechnology

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Quantum Computing Using Superconducting Phase Qubits

We develop techniques for making highly coherent

quantum systems using integrated circuits. This in- cludes developing new high-fidelity measurement

techniques for quantum systems. Ultimately, we would

intend to help make large-scale quantum information

processing systems a reality. All ofthe nanoscale cir- cuits for our work are made in our laboratory.

Ray Simmonds, EEEL (817)

The integrated circuit components of classical computers

are rapidly approaching the so-called "quantum limit." In- stead of avoiding quantum effects, we have the opportunity

to exploit them as a means for more effective computation.

A quantum computer has the ability to use the intrinsic properties of quantum systems to naturally perform parallel

processing during a calculation. This allows a quantum

computer to solve problems considered intractable for classical computers. Three such problems have been of considerable interest: discrete logarithms, factorization, and search

algorithms for large databases. The practical significance of

building a successful large-scale quantum computer is tre- mendous. It could

• Provide a powerful tool for encryption. A quantum computer is seen as the only instrument that could break the most secure encryption codes in use today. This is an immensely important subject for national security.

• Solve highly complex (many-body) problems in a rea- sonable amount of time. This will become increasingly important for the chemical and biological sciences.

• Provide rapid search engines to help navigate us through

the information age.

• Allow us to simulate large quantum systems efficiently.

In a conventional computer, information is often stored as

electrical charge on tiny capacitors. The presence or ab- sence of charge stored on a single capacitor represents a

(classical) bit, which can represent two (classical) information states "0" and "1." All logical computations are done

using groups and combinations of this binary information.

A quantum bit or "qubit" is described in terms of two quan- tum states denoted by "|0>" and "|1>". Remarkably, a quan- tum bit can be placed in a state that is a mixture of both

"|0>" and "|1>"! Even more remarkable is the fact that multiple qubits can be placed in a massive mixture of all combinations oftheir possible states, a phenomenon known as entanglement.

Entanglement is the "magic" of quantum mechanics and

allows a quantum computer to stir up quantum information

in order to produce a meaningful calculation with incredible speed. Whether or not quantum computing becomes practical, our work will produce new knowledge for the precise

measurement of quantum systems. Through our research with quantum-mechanical superconducting circuits, we are learning how to directly control and measure quantum sys- tems in new ways. We have already shown (as described

below) the ability to detect previously unknown nanoscale

quantum systems that could never be seen before. By gaining the ability to control quantum systems directly, we are exploring untouched regimes of nature and may find ways

to direct unforeseen advancements in nanotechnology.

Although this project began only three years ago,we have

made significant progress over this short period. The first qubit design used a current-biased Josephson junction made

from niobium with an amorphous aluminum-oxide tunnel

barrier. It was successfiil in showing Rabi oscillations by

varying the power of the microwave drive. The energy re- laxation time was 300 ns, and the Rabi decay time was es- timated to be 10 ns. The next qubit showed a reduction in excess tuimeling events caused by quasiparticle heating.

1"

' S

) jiiiJiil/lA/VVwwvH.

-11,32 -0.31 -0 3 4).29 -0.2S

Figure 1. An example ofqubit spectroscopyfor qubits (a) and (b) showing the transition frequency col 0for various flux biases to the qubit loop. Notice the small "splittings " indicative ofmicroscopic

quantum systems residing within the tunneljunction coupling to our superconducting quantum bit. Top inset shows Rabi oscilla- tions awayfrom resonators.

Accomplishments

Developed a New Flux Biased Josephson Phase Qubit — We developed a new qubit design that is well isolated from the external environment while still providing an ex- tremely sensitive readout. In addition, this qubit does not generate quasiparticles during measurement. This system

has shown Rabi oscillations with up to 75 % visibility with

100 ns decay times. We have also seen energy relaxation times as long as 500 ns.

• Discovered Spurious Resonators Within Josephson

tunnel Junctions — Using our new improved qubit we have

developed spectroscopic measurements of the qubit transi- tion fi-equency over a wide range of possible operating flux NIST Accomplishments in Nanotechnology

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3 - Nanoscale Devices and Systems

biases. In doing so, we discovered nanoscopic spurious

resonators within the tunnel junctions of the qubit. EHmination of these resonators in future Josephson junctions could

improve the performance of all superconducting devices.

Measure Delay (ns)

Figure 2. (a) A fabricated coupled qubit circuit, (b) Simultaneous measurement oftwo coupled qubits during state oscillations.

• Developed a Methodfor Characterizing Tunnel Junctions and Phase Qubits — Using a low frequency nonlinear

current bias, we have developed an effective way to measure extremely small quasiparticle currents in the current- voltage characteristics of tunnel junctions fabricated using

various novel methods. These measurements are then correlated with qubit spectroscopy, Rabi oscillations, and energy

relaxation times in order to characterize the performance of

these new tunnel junctions and the qubits that incorporate them.

strongly coupled interactions between phase qubits and

other quantum systems.

• Obsei-ved Quantum Oscillations Between a Josephson

Phase Qubit and a Nanoscopic Resonator — We have detected coherent quantum oscillations between Josephson

phase qubits and two-level resonator within the tunnel barrier of a superconducting phase qubit. These results reveal a new'aspect of the quantum behavior of Josephson junctions,

and they demonstrate the means to measure two-qubit inter- actions in the time domain. The junction-resonator interac- tion also points to a possible mechanism for decoherence

and reduced fidelity in superconducting qubits.

• Coiipled-Qubit Interactions — Through the first ever simultaneous measurement of two superconducting

qubits, we have recently witnessed two coupled phase

qubits entangle themselves by performing coherent state oscillations. This is a tremendous step forward! Soon, we

hope to have the ability to perform simple logic operations between two qubits, the building blocks for a full scale quantum computer.

Selected Publications

1 . K. B. Cooper, M. Steffen, R. McDermott, R. W.

Simmonds, S. Oh, D. A. Hite, D. P. Pappas, and J. M. Martinis, "Observation of quantum oscillations between a Josephson phase qubit and a microscopic

resonator using fast readout," cond-mat/0405710 ac- cepted by Phys. Rev. Lett. (2004).

2. R. W. Simmonds, K. M. Lang, D. A. Hite, D. P. Pappas, J. M. Martinis, "Decoherence in Josephson

phase qubits from junction resonators," Phys. Rev.

Lett. 93, 077003 (2004).

3. J. M. Martinis, S. Nam, J. Aumentado, K. M. Lang,

C. Urbina, "Decoherence of a superconducting qubit

due to bias noise," Phys. Rev. B 67, 094510 (2003).

4. J. M. Martinis, S. Nam, J. Aumentado, and C. Urbina, "Rabi oscillations in a large Josephsonjunction qubit," Phys. Rev. Lett. 89, 11 7901 (2002).

• Developed a New, Faster Method to Read Out the Phase Qubit — We have implemented a new qubit state- measurement technique that is a faster order of magnitude

than our former method. In this new method, with a temporal resolution of less than 5 ns, a flux bias pulse is applied

to the qubit so that the |1> state, if occupied, is suddenly

presented with a very small energy barrier, through which it rapidly tunnels. This new advance has allowed us to monitor rapid qubit state variations, opening the door to tracking

NIST Accomplishments in Nanotechnology

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Quantum Sensors

NIST has developed detector systems that are redefining the measurement abilities ofcurrently available

technology, often by orders ofmagnitude, and contin- ues to develop groundbreaking detector systems for

both industry and research groups. Our sensors are based on quantum phenomenafor spectroscopy, imaging, and other precision measurementsfor wavelengthsfrom dc through gamma rays. We integrate

these sensors with custom superconducting and room

temperature electronics, cryogenic structures and

software to create complete measurement systems.

Applications ofthis measurement capability include

materials analysis, astronomy, and homeland defense.

We design some ofour sensors to make them capable

ofchemically analyzing samples smaller than 100 nm.

Kent Irwin, EEEL (817)

By operating at temperatures below 1 K, a new generation

of photon detectors is achieving unparalleled sensitivity in measuring very small differences in the energy of photons.

At these low temperatures, they are able to take advantage

of quantum phenomenon, including quantum interference (SQUIDs), quantum phase transitions (superconducting

transition-edge sensors), and quantum tunneling (normalinsulator-superconductor tunnel junctions). As a world

leader in developing these new detector systems, NIST has

developed transition edge sensor (TES) bolometers for use

in a variety of applications. These devices utilize a strip of

superconducting material, biased in its transition from nor- mal to superconducting states, as an extremely sensitive thermometer. This thermometer is attached to an absorber

that is isolated from a cold (-100 mK) heat sink by a micromachined structure. The heat deposited by incident photons is then measured to accurately determine their energy.

Applications of our nanoscale sensors include ultra-high- resolution X-ray spectroscopy for materials analysis for the semiconductor industry, and the development of large scale arrays of quantum sensors for high-throughput imaging and

spectroscopy of electromagnetic radiation for materials

analysis and astronomy. In the International Technology

Roadmap for Semiconductors, improved X-ray detector technology is identified as one of the most important metrology needs for the semiconductor industry to help address analysis requirements for small particles and defects. NIST's TES microcalorimeter X-ray detectors have been

identified as a primary means of realizing these detector advances, which will greatly improve in-line and off-line metrology tools that currently use semiconductor energydispersive spectrometers (EDS). In addition, the astronomy

community has an ever increasing need for instruments

capable of supplying extremely high energy-sensitivity

coupled with large-format arrays for imaging and photon

collection. TES detector arrays promise to greatly expand

the abilities of astronomers to study objects ranging from

solar flares to supernova remnants to the formation of galaxies. The Quantum Sensor project has formed collaborations to transfer our TES technology into astronomical in- struments with several institutions, including NASA, Stan- ford University, the Lockheed-Martin Solar Astrophysics

Laboratory and the UK Astronomy Technology Center.

Accomplishments

• NIST has developed x-ray detectors of extremely high energy resolution and demonstrated a complete x-ray spec- troscopy system. This detector and spectrometer have been

made possible by our broad expertise in such fields as su- perconductivity, device fabrication including silicon micromachining, superconducting electronics, cryogenic engineering, and low-noise, room temperature electronics. A significant accomplishment has been to demonstrate

world-record energy resolution for an EDS detector of 2.4 eV at 5900 eV (see Figure 2), which is over 30 times better than the best high resolution semiconductor-based detectors

currently available. We have used the system to identify

nanomicrometer particles of materials such as W on Si substrates, an identification problem that is impossible with

standard EDS detectors and of great importance to the semiconductor industry. It has also demonstrated energy

shifts in the EDS X-ray spectra of materials such as Al, Fe, and Ti, depending on their chemical bonding state, thus allowing differentiation between a particle of Al and AI2O3,

for example.

S 2000 >

-; 1500

ou

QW

a

1000-

500

OKa NIST K3670 glass Si{Li)EDS

NIST peal EDS

(real-time analog processing) Mg Ka

ZnLa

'.SiKa

CKa/

; Ni Lot' ;FeLa,.y

Jl^Lu -I

Al Ka

>

10000

5000

c

3

oU

00Q

500

0

1000 1500 2000 Energy (eV)

Figure 1. Comparison ofresolutions ofNIST microcalorimeter EDS to standard semiconductor EDS.

• We have made significant progress towards fabricating a practical on-chip, solid-state microrefrigerator using

Normal metal - Insulator - Superconductor (NIS) turmel NIST Accomplishments in Nanotechnology

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3 - Nanoscale Devices and Systems

junctions. Presently, cryogenic instruments that operate near 100 mK use complex, expensive adiabatic demagnetization refrigerators and dilution refrigerators. The thin-film microrefrigerators are compact and readily integrated onchip with devices requiring sub-kelvin cooling such as microcalorimeter X-ray spectrometers for X-ray microanalysis. We have fabricated an NIS refrigerator designed to operate at bath temperatures of 300 mK, which is accessible with relatively simple and inexpensive ^He refrigerators.

These are the largest working NIS refrigerators made to date and the only ones fabricated with photolithographic

techniques.

5875 5880 5885 5890 5895 5900 5905 5910

Energy (eV) Figure 2. World-record 2.4 eV energy resolution achieved with NIST microcalorimeter at 6 keV.

• We have demonstrated the first multiplexed readout of

an X-ray microcalorimeter array, using time-division

SQUID multiplexing to read out many detectors through a

single amplifies. This breakthrough establishes a clear path

to the instrumentation of kilopixel microcalorimeter arrays, which will have wide-ranging implications for fields as diverse as materials analysis and X-ray astronomy. In the experiment, the multiplexed TES detectors measured the energy of 5.9 keV X-rays emitted from an ''"^Fe source with

an average resolution of 6.94 ± 0.05 eV, which represents

only a small and well-understood degradation in resolution from the non-multiplexed performance of the detectors. We

believe that we can multiplex 32 detectors with an energyresolution degradation of only 0.1 eV. This would allow a

square, 1024-detector array to be read out with only 32 sig- nal channels. We have demonstrated the first microwave SQUID

multiplexer. This breakthrough opens a path to reading out very large arrays of SQUIDs for magnetoencephalography

and non-destructive evaluation, and CCD-scale arrays of

low-temperature detectors. In this circuit, the outputs of

many SQUID amplifiers are simultaneously monitored by a

single high electron mobility transistor (HEMT) amplifier

channel. Each SQUID amplifier is placed in a high-Q reso- nant circuit with a different resonant frequency, and excited

by a comb of microwave signals at each of the resonant

frequencies. The amplitude of the reflected microwave sig- nals at each frequency is a function of the input signal at each SQUID. In an initial experiment, we demonstrated the

low-noise readout of SQUIDs using this microwave reflec- tometer approach, and multiplexed two SQUIDs simultaneously near 500 MHz. Future work will focus on operation at 5 GHz and developing technology for much larger SQUID

arrays. Because of the large bandwidth available with mi- crowave measurements, it should be possible in the future

to read out many thousands of SQUIDs in a single coaxial

cable, and to instrument arrays of SQUIDs with thousands

or millions of pixels.

We have developed a surface micromachining method

for producing freestanding pixels above the surface of a

silicon wafer. This process has been integrated into the Xray TES fabrication process, and TES devices have been

produced and cooled down.

• We have delivered a full, 1,280-pixel multiplexed

submillimeter bolometer array to the SCUBA-2 project.

This is the first true submillimeter camera available to the community.

Selected Publcations

1 . A. M. Clark, A. Williams, S. T. Ruggiero, M. L. ven

den Berg, and J. N. Ullom, "Practical electron- tunneling refrigerator," Appl. Phys. Lett. 84 (4): pp. 625-627; 26 (January 2004).

2. J. N. Ullom, W. B. Doriese, G. C. Hilton, J. A.

Beall, S. Deiker, W. D. Duncan, L. Ferreira, K. D.

Irwin, CD. Reintsema, and L. R. Vale, "Characterization and reduction of unexplained noise in super- conducting transition-edge sensors," Appl. Phys.

Lett. 84(21): pp. 4206-4208 (May 24, 2004).

3. J. P. Pekola, R. J. Schoelkopf, and J. N. Ullom,

"Cryogenics on a Chip," Physics Today 57(5): pp. 41 (May 2004).

4. K. D. Irwin and K. W. Lehnert, "Microwave SQUID

multiplexer," Appl. Phys. Lett. 85 (I I): pp. 2107-

2109 (2004).

5. W. B. Doriese, J. A. Beall, S. Deiker, W. D. Duncan,

G. C. Hilton, K. D. Irwin, C. D. Reintsema, J. N. Ullom, L. R. Vale, and Y. Xu, "Time-division multiplexing of high-resolution TES X-ray microcalorimeters: Four pixels and beyond," Appl. Phys. Lett., 85:4762 (2004).

6. J. Beyer, D. Drung, and K. D. Irwin, "Bias conditions of DCSQUID for a time-domain SQUID multiplexer," Review of Scientific Instruments, 75, pp. 502 (1994).

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Single Electronics for Standards and Metrology

NIST has developed novel integrated circuits for stan- dards and metrology based on the unique properties

ofelectronic devices that can manipulate and detect

individual electrons. In our laboratory wefabricate

these circuits that have all three dimensions on the

nanoscale.

Mark Keller and Neil Zimmerman, EEEL (8 1 7)

This project addresses three different needs: a fundamental representation of capacitance, a fundamental repre- sentation of electrical current, and general applications of

nanoscale single-electron tunneling (SET) devices, with a

particular emphasis on future integrated nanoelectronics.

For the first need, NIST is developing intrinsic stan- dards based on fundamental physical principles, such as the

volt, based on the Josephson effect, and the ohm, based on

the quantum Hall effect. The present representation of the SI farad is through silica-based artifact capacitors. Although

these capacitors are of high quality, they are susceptible to

drift in time and may depend on other parameters such as temperature, pressure, and frequency. The metrology community needs a capacitance representation that is based on

fundamental physical principles rather than on properties of

individual physical artifacts.

For the second need, at present, there is no fundamental

representation of current; the representation of current is via

the representations of voltage and resistance. Though these

representations are based on fundamental physical principles and are of high quality, the representation of current is dependent upon them. An independent representation of

current could provide significant additional confidence in the coherency of the representations of the SI electrical units through closure of the "metrology triangle" V = I R

with all measurements based on fundamental constants.

For the third need, various classes of future nanoelectronics beyond CMOS are projected to work with one or a few electrons. These include molecular electronics, semiconductor-based integrated circuits using single-electron

memory or logic, and quantum computing (QC). We are using our expertise to elucidate properties of transfer of

superconducting "Cooper pairs," which are the basic

mechanism for one realization of QC circuits. We are also addressing an endemic problem in single-electron logic, the "charge offset" phenomenon, which makes it difficult or impossible to integrate multiple SET-based devices to- gether.

Figure 1. Atomicforce microscope image ofan electron counter,

the heart ofa new capacitance standard based on counting electrons. The standard, shown in the schematic, consists ofthe elec- tron counter, a capacitor, and a single-electron electrometer (not shown) to monitor the process. The electron counter, based on

seven nanometer-scale tunneljunctions in series, can "pump"

electrons onto the capacitor with an error rate ofless than 1 elec- tron in l(f.

Accomplishments

Electron Counting Capacitance Standard Demonstrated —The components of the prototype ECCS standard demonstrated by NIST are illustrated in the figure. The electron counter can "pump" electrons onto the capacitor with an

error rate of less than one electron in 10^. The electron pumping is monitored with an SET-based electrometer fab- ricated on the same chip as the pump, with a charge sensitivity better than 10"^ electrons. The capacitor operates at cryogenic temperatures and uses vacuum as the dielectric,

resulting in a frequency-independent measure of capacitance. To operate the ECCS, approximately 100 million

electrons are placed, singly, on the capacitor. The voltage

across the capacitor is measured, resulting in a calibration of the cryogenic capacitor. This capacitance can then be

transferred to room temperature by use of a standard ac bridge measurement technique. The result is a value of ca- pacitance with a repeatability of one part in 10^.

• Quasiparticle Poisoning in Cooper Pair Transistors— In collaboration with Professor Michel Devoret (Yale), we

have experimentally verified a model for erratic quasiparticle turmeling in superconducting Cooper pair transistors (these are the superconducting versions of the SET). This

phenomenon, quasiparticle "poisoning," will limit the operation of the superconducting Cooper pair pump as well as

NIST Accomplishments in Nanotechnology

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charge-based qubits. Our work showed that the gap energies in the leads versus the island played a significant role

in determining these poisoning rates. In addition, we have

confirmed a "remote poisoning" phenomena, whereby a voltage-biased junction device can cause poisoning in other devices on the same chip, even if it is electrically isolated. This has important implications for the electrometry that we

will use in the Cooper pair pump, as well as in charge-based

qubits.

• rf-SET demonstrated —NIST has demonstrated operation of a rf-SET, a version of an SET electrometer read out by microwave reflectometry. This approach greatly in- creases the bandwidth of the electrometer (to 3 MHz in our

present setup) and improves the charge resolution (to 50

1/2 |ie/Hz ). This is the basic technology we will use to per- form the passive electron counting experiments. Future work will focus on expanding the bandwidth and narrowing

the charge resolution to perform better than the quantum

Umit.

• SETElectronics Developed to Probe GaAs Quantum

Dots for Single Photon Source — The past two years have

produced significant progress towards the development of a single-photon turnstile, a device designed to generate single

photons on demand using semiconductor quantum dots (QD) and single-electron principles. A key step towards this goal is to measure the tunneling of single electrons onto

individual quantum dots. We have succeeded in making

these measurements by integrating an SET electrometer over a low density field of GaAs quantum dots. A sche- matic cross-section of the device is shown below, along

with a scanning electron micrograph of the SET electrometer. Using these devices, we have clearly identified the addition of single electrons onto single quantum dots located below the SET. We have counted up to 3 electrons added to a single dot, and have measured the energy spectrum associated with adding electrons on this dot.

• Si-based Pumps Turnstiles, CCDs Demonstrated —

Our work in collaboration with a group in NTT, Japan, has

made a great deal of progress in investigating Si-based devices that can control the motion of single electrons In the

past two years, we have demonstrated such control in three

different types of devices: pumps, turnstiles, and CCDs.

The most promising device for the future is the CCD, both

because we have recently demonstrated in subsequent work

that such devices can be made with improved reliability and

homogeneity, and because we demonstrated that this device

has the potential to run at higher speed. In one of our publications we showed that the CCD can pump single electrons

at rates as fast as 100 MHz, corresponding to 15 pA; this is an improvement by about a factor of 5 over previous results

in metal-based pumps. However, we have to note that we

were not able to measure error rate in detail in the CCDs;

this work is now in progress.

(a) Gate

AuGe/NI n-GoAs Contact I

AJGoAs ' InGoAs QDs

i-GoAs

. n-GoAs

SET Island

Figure 2. (a) Schematic cross-section ofour SET electrometer

integrated on a GaAs chip containing self-assembled quantum

dots, (b) SEM micrograph ofan SET electrometer. We have identi- fied single-electron tunneling events into individual quantum dots located beneath the electrometer.

Selected Publications

1 . X. Jehl, M. W. Keller, R. L. Kautz, J. Aumentado,

and J. M. Martinis, "Counting errors in a voltagebiased electron pump," Phys. Rev. B 67, 165331-1 - 165331-9.

2. N. M. Zimmerman, M. A. El Sabbagh, and Y. Wang, "Larger value and SI measurement of the improved cryogenic capacitor for the electron-counting

capacitance standard," IEEE Trans. Instrumentation

and Measurement 52, pp. 608-1 1 (2003).

3. A. Fujiwara, N. M. Zimmerman, Y. One, and Y.

Takahashi, "Current quantization due to single- electron transfer in Si-wire charge-coupled devices,"

Appl. Phys. Lett. 84, pp. 1323 (2004).

4. K. D. Osbom, M. W. Keller, and R. P. Mirin, "Single-electron transistor spectroscopy of InGaAs self- assembled quantum dots," Physica E 21, pp. 501-

505 (2004).

5. J. Aumentado, M. W. Keller, J. M. Martinis, and M.

H. Devoret, "Nonequilibrium quasiparticles and 2e

periodicity in single-Cooper-pair transistors," Phys. Rev. Lett. 92, pp. 066802 (2004).

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Quantum Voltage Metrology

NIST is meeting the nation 's voltage metrology needs

by developing state-of-the-art precision dc and ac

voltage standard systems and by providing highperformance voltage measurementsfounded on nanoscale quantum-based superconducting circuit and sys- tem technology. In ourfacility wefabricatefor our

standards the most complex superconducting nanoscale circuits in the world.

Samuel Benz and Yi-hua Tang, EEEL (817)

Accomplishments

• In early 2004, the NIST voltage metrology laboratory

moved into the new Advanced Metrology Laboratory

(AML) to take advantage of its superior control of environmental parameters such as temperature and humidity.

This challenging move demonstrated that our nanoscale

quantum-based voltage measurement system is very ro- bust and we verified that its performance was equivalent

to that in the existing measurement laboratory.

• NIST has increased by tenfold the junction density of

Josephson arrays. This was achieved by vertically stack- ing 10 superconducting Josephson junctions on top of

each other at a spacing of 45 nm. The biggest challenge was to vertically etch the stacked thin films so that the junctions in the stacks have the same area and achieve

the same electrical characteristics. This was achieved

and demonstrated by measuring the electrical characteristics of 1000 stacked arrays with and without a 9 GHz

microwave bias. The resulting voltage steps were flat; that is, they produced a precision constant voltage, based on the Josephson effect, over a large current range of 1 mA. This large operating range indicates impressive uniformity for these nano-stacked arrays and significant progress toward our goal of using nanotechnology to increase the performance of ac and dc voltage standard

systems. Higher junction density is required to increase

the output voltage as well as the operating bandwidth of

the superconducting circuits whose quantum mechanical

properties enable precise synthesis of arbitrary waveforms.

• Using nano-stacked Josephson junction arrays, NIST

has more than doubled the precision stable output volt- age for programmable voltage standard circuits. The

stacks are made by alternately sputtering multiple normal

metal layers of molybdenum disilicide between super- conducting niobium layers. The stacks are then interconnected in series with superconducting wiring in order to create a long series-connected array ofjunctions for the maximum output voltage. For the past few years, the record output voltage for programmable Josephson volt- age standards has been limited to about 1 V. These new

fabrication techniques have allowed us to double and

triple the number ofjunctions in the circuit, which, combined with a small increase in the drive frequency from

16 GHz to 19 GHz, has allowed us to reach 2.5 V maximum output voltage. The circuit can produce precision

output voltage over a current range of 1 mA and over a frequency range from 14 to 19 GHz. The operating current and frequency ranges are also large compared to previous programmable circuits and indicate good uniAn international agreement signed in 1 990 redefined

the practical volt in terms of the voltage generated by a superconductive integrated circuit developed at NIST

and the Physikalisch-Technische Bundesanstalt in Germany. This circuit contains thousands of superconducting Josephson junctions, all connected in a series array and biased at a microwave frequency. The voltage developed by each junction depends only on the frequency

and a fundamental physical constant; thus, the circuit never needs to be calibrated. This allows any standards

or commercial laboratory to generate highly accurate

voltages without the need to calibrate an artifact stan- dard. This advance has improved the uniformity of volt- age measurements around the world by about a hundredfold. These systems are rapidly becoming essential for meeting legal and accreditation requirements in commercial, governmental, and military activities.

The U.S. electronics instrumentation industry maintains its world-leading position through the development

and deployment of accurate, flexible, and easy-to-use

instruments. This increasingly sophisticated instrumentation places mounting demands for higher accuracy volt- age metrology both in calibration and testing laboratories

and on production lines and factory floors. NIST meets

this need by providing some customers who need immediate realization of the highest possible in-house accu- racy with their own quantum voltage standard systems,

and other customers with a calibration service for their Zener voltage references. NIST provides the foundation

for voltage metrology that enables the U.S. electronics

instrumentation industry to compete successfully in the global market. Through this strong voltage metrology

foundation, NIST also supports scientific research in high accuracy voltage measurements for the standards

and superconductive electronics communities and the U.S. military.

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formity for the 67,410 total junctions. This is the first successful application of stacked Josephson junctions,

and the results suggest that further output vohage improvements may result from taller stacks and larger ar- rays. 50 nm

Figure 1. Transmission electron microscope (TEM) image ofa

two-junction stack with molybdenum-disilicide barriers and nio- bium outer and middle electrodes. The image shows that the MoSiJ deposits uniformly the niobium, even when the niobium is as

thin as 20 nm. (Image by John Bonevich-MSEL, NIST)

• This year we have developed and combined three new technologies to create an improved, more robustly

packaged programmable voltage standard with higher

performance. First, we built a new circuit that produces

50-times higher voltage resolution. We also developed a

flexible, microwave-compatible cryopackage that improves electrical contact reliability. By combining these two breakthroughs with previously demonstrated

stacked-junction fabrication technology, we have assembled a robust, cryopackaged programmable voltage stan- dard that produces a record 2.6 V output with 77 )u.V resolution using the same electronics. The new standard

will replace an older programmable circuit in the voltage

calibration lab that was capable of 1 V output and 4 mV

resolution. The novel superconducting integrated circuit of this new system uses a ternary-logic design to achieve

the increased voltage resolution. As compared to previ- ous double-stacked circuits and the older non-stacked

programmable circuit, the double-stacked junction uniformity was improved, effectively increasing the operating margins two-fold to 2 mA for full chip operation.

The new cryopackage uses a "flip-chip on flex" technology that we hope will improve the service life and reli- ability of our Josephson systems, because directly sol- dered connections to the chip replace less reliable press

contacts. The system is currently being implemented in the voltage calibration service.

Figure 2. A 1 cm x 1 cm superconducting integrated circuit with 67,406 double-stacked SNS Josephson junctions for the 2.6 V

high-resolution PJVS operating at 18.5 GHz.

Selected Publications

1. S. P. Benz and C. A. Hamilton, "Application of the Josephson effect to voltage metrology," invited paper, Proc, IEEE 92, (10) (October 2004).

2. M. Ishizaki, H. Yamamori, A. Shoji, P. D. Dresselhaus, and S. P. Benz, "A programmable Josephson

voltage standard chip using arrays of

NbN/TiN/NbN/TiN/NbN double-junction stacks operated at 10 K," Digest, 2004 Conference on Precision Electromagnetic Measurements, 27 June-2 July

2004, London, England, pp. 8-9 (June 2004).

3. Y. Chong, P. D. Dresselhaus, and S. P. Benz,

"Thermal transport in stacked superconductornormal metal-superconductor Josephson junctions,"

Applied Physics Letters 83, (9), pp. 1794-1796 (1 September 2003).

4. P. D. Dresselhaus, Y. Chong, J. H. Plantenberg, and

S. P. Benz, "Stacked SNS Josephson Junction Arrays for Quantum Voltage Standards," IEEE Trans. Applied Superconductivity, 13, (2), pp. 930-933

(June 2003).

5. C. J. Burroughs, S. P. Benz, and P. D. Dresselhaus,

"AC Josephson voltage standard error measurements

and analysis," IEEE Trans. Instrumentation and

Measurement, 52, (2), pp. 542-544 (April 2003).

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Architectures for Fault-tolerant Quantum Computing

In theory, quantum computers can efficiently simulate

quantum physics, factor large numbers and estimate

integrals, thus solving computational problems that are otherwise intractable. In practice, quantum computers must operate with noisy devices called "gates

"

that tend to destroy thefragile quantum states needed

for computation. The goal offault-tolerant quantum

computing is to compute accurately even when gates

have a high probability oferror each time they are

used. We are developing architectural concepts and

error remediation strategies which will enable the

development ofpractical quantum computing devices.

Emanuel Knill, ITL (891)

Research in quantum computing is motivated by the

great increase in computational power offered by quantum computers. There are a large and still growing number of experimental efforts whose ultimate goal is to demonstrate scalable quantum computing. Scalable

quantum computing requires that arbitrarily large computations can be efficiently implemented with little error

in the output.

One of the criteria necessary for scalable quantum

computing is that the level of noise affecting the physical

gates is sufficiently low. The type of noise affecting the gates in a given implementation is called the error model. A scheme for scalable quantum computing in the presence of noise is called afault-tolerant architecture. In view of the low-noise criterion, studies of scalable quantum computing involve constructing fault-tolerant architectures and providing answers to questions such as

the following:

Ql : Is scalable quantum computing possible for error model E?

Q2: Can fault-tolerant architecture A be used for scalable quantum computing with error model E?

Q3: What resources are required to implement quantum computation C using fault-tolerant architecture A

with error model E?

To obtain broadly applicable results, fault-tolerant ar- chitectures are constructed for generic error models. In such cases, the error model is parameterized by an error probability per gate (or simply error per gate, EPG),

where the errors are unbiased and independent. The

fundamental theorem of scalable quantum computing is the threshold theorem which answers question Q 1 as

follows:

If the EPG is smaller than a threshold, then scalable quantum computing is possible.

Thresholds depend on additional assumptions on the

error model and device capabilities. Estimated thresh- olds vary from below 10"^ to 3 x 10 \ with 10"^ often

quoted as the target EPG for experimental realizations of

quantum computing.

Figure 1. The new NIST architecturefor quantum computing

relies on several levels oferror checking to ensure the accuracy of

quantum bits (qubits). The image above illustrates how qubits are grouped in blocks to form the levels. To implement the architec- ture with three levels, a series ofoperations is performed on 36

qubits (bottom row) each one representing a 1, a 0, or both at once. The operations on the nine sets ofqubits produce two relia- bly accurate qubits (top row). The purple spheres represent qubits

that are either used in error detection or in actual computations. The yellow spheres are qubits that are measured to detect or correct errors but are not used in final computations.

Many experimental proposals for quantum computing

claim to achieve EPGs below 1 0"^ in theory. However,

in the few cases where experiments with two quantum

bits (qubits) have been performed, the EPGs currently

achieved are much higher, 3 x 10"^ or more in ion traps

and liquid-state nuclear magnetic resonance (NMR) ex- periments, for example.

In our work we have provided evidence that scalable quantum computing is possible at EPGs above 3x10'.

While this is encouraging, the fault-tolerant architecture

that achieves this is extremely impractical because of

large resource requirements. To reduce the resource

requirements, lower EPGs are required.

We have developed a fault-tolerant architecture, called the C/Q architecture that is well suited to EPGs

between 10""* and 10"^. We have analyzed the resource requirements for this architecture and compared it to the

state of the art in scalable quantum computing.

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The Architecture

Fault-tolerant architectures realize low-error qubits

and gates by encoding them with error-correcting codes.

A standard technique for amplifying error reduction is concatenation. Suppose we have a scheme that, starting

with qubits and gates at one EPG, produces encoded

qubits and gates that have a lower EPG. Provided the

error model for encoded gates is sufficiently well behaved, we can then apply the same scheme to the en- coded qubits and gates to obtain a next level of encoded

qubits and gates with much lower EPGs. Thus, a concatenated fault-tolerant architecture involves a hierarchy

of repeatedly encoded qubits and gates. The hierarchy is described in terms of levels of encoding, with the physical qubits and gates being at level 0. The top level is used for implementing quantum computations and its qubits and gates are referred to as being logical. Typically, the EPGs decrease superexponentially with number of levels, provided that the physical EPG is below

the threshold for the architecture in question.

The C4/C6 architecture differs from previous ones in five significant ways. First, we use the simplest possible

error-detecting codes, thus avoiding the complexity of

even the smallest error-correcting codes. Error correction is added naturally by concatenation. Second, error correction is performed in one step and combined with

logical gates by means of error-correcting teleportation.

This minimizes the number of gates contributing to er- rors before they are corrected. Third, the fault-tolerant architecture is based on a minimal set of operations with

only one unitary gate, the controUed-NOT. Although

this set does not suffice for universal quantum computing, it is possible to bootstrap other gates. Fourth, verifi- cation of the needed ancillary states (logical Bell states) largely avoids the traditional syndrome-based schemes.

Instead, we use hierarchical teleportations. Fifth, the highest thresholds are obtained by introducing the model

of postselected computing with its own thresholds,

which may be higher than those for standard quantum

computing. Our fault tolerant implementation of postselected computing has the property that it can be used to prepare states sufficient for (standard) scalable quantum

computing.

The properties of the proposed architecture were de- termined with several months of calculations and simulations on large, conventional computer workstations. Although the new architecture has yet to be validated by

mathematical proofs or tested in the laboratory, it pro- vides some evidence that scalable quantum computation

may be closer to our reach that previously believed.

Figure 2. Errors for a CNOTgate implementation at levels 0, 1 and 2. The errors are conditional on nofaults having been detected. The error bars are 68% confidence intervals. As can be

seen, errors decrease rapidly with increasing level at EPGs of3%

or below. Extrapolation suggests that this behavior persists for

even larger EPGs. At high EPGs, the "no fault" condition hap- pens rarely. Nevertheless it is possible to complete a quantum

computation with polynomial overhead by using many trials to prepare relatively error-free states that can then be used to im- plement error-corrected logical gates with high success probabilities. Summary

We have given evidence that accurate quantum computing is possible with error probabilities above 3% per

gate, which is significantly higher than what was previously thought possible. However, the resources required

for computing at such high error probabilities are excessive. Fortunately, they decrease rapidly with decreasing

error probabilities. If we had quantum resources comparable to the considerable resources available in today's

digital computers, we could implement non-trivial quantum computations at error probabilities as high as 1% per

gate.

Selected Publications

1 . E. Knill, Quantum Computing with Realistically

Noisy Devices, Nature 434 (03 March 2005), pp. 39-

44.

2. http://www.nature.com/nature/links/050303/050303-

4.html

3. http://math.nist.gov/quantum/

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Synthesis of Quantum Logic Circuits

Design automation is an important techniqueforfinding efiicient classical circuits. Given a specification

ofthe Boolean function which the target circuit should

implement, a synthesis program automatically determines a sequence ofgates realizing this function.

Quantum logic synthesis aims to build a similar tool- set for quantum circuits, seeking the fewest number of

one and two-qubit processes to achieve a target quantum computation. We have developed a technique of

automatic quantum circuit synthesis for unstructured

qubit evolutions improving a construction from the

mid 1990s by a factor ofmore than one hundred. In

addition, we have shown the new construction to be

within a factor oftwo ofoptimal. For multi-level

quantum logics (qudits) similar advances have led to thefirst circuits ever with optimal asymptotics.

Stephen S. Bullock, ITL (891)

While classical computers manipulate bits which

carry values of 0 or 1 , quantum computers manipulate

quantum bits (qubits) which are state vectors of twolevel quantum systems. If the quantum computer is not exchanging energy with the outside environment, these

qubit state vectors are rotated during the computation.

Typical quantum algorithms call for implementing such

a rotation, mathematically a unitary matrix, and then

observing the qubits. Thus, while efficient Boolean cir- cuits realize complicated functions on bits using a small

number of logic gates, efficient quantum circuits break

complicated unitary matrices into a small number of

simple factors. These factors (quantum gates) typically

correspond to manipulating one or two quantum bits.

The past two years have seen marked advances in the design of universal quantum logic circuits. Such circuits implement any possible unitary evolution by appropriately tuning their gate parameters. The new techniques

are also overtly constructive. Unitary matrices implementing nontrivial quantum computations are large, e.g. requiring 2" x 2" matrices for n qubits. The new quantum circuit synthesis algorithms rely on well-known matrix decomposition such as QR or the Cosine Sine Decomposition. For ten qubits, commercial software on a 2.5GHz PC requires a few seconds for these factorizations.We illustrate basic quantum circuit design with an

example. Each circuit in the figure below applies the same two-qubit computation, namely multiplying the 01

and 10 states by the complex number / while leaving

00 and 1 1 unchanged. Each qubit is represented by a

single line or rail in the circuit. The boxes denoted

5 and H indicate single qubit operations, i.e. particular

2x2 unitary matrices. The gate spanning both qubits is a quantum controlled not, or CNOT. CNOT flips the

target qubit (carrying the inverter) when the control qubit

(black slug) carries 1 , so that these down-target CNOTs

exchange 10 andl 1

.

S_

Figure 1. Two quantum circuits for performing the same task.

Although their target computation is the same, either diagram might be better suited to a given implementation

of quantum computing. For example, if CNOTs are costly while one-qubit gates are implemented more easily, then the circuit at left is preferable. Alternately, if one-qubit gates are expensive but CNOTs are cheap,

then the circuit at right is preferable. In practice, CNOTs

tend to be more expensive than one-qubit rotations, and

three-qubit gates tend to be more rare and difficult to implement than CNOTs. Further, while it is possible to build any unitary using exclusively one qubit gates and

CNOT, simply employing one-qubit gates does not suffice. Thus, the following discussion of universal quantum circuits focuses on minimizing CNOT counts.

In fact, it is not obvious that CNOT and one-qubit

gates suffice to build any unitary evolution. This was

settled in 1995 in a landmark paper of Barenco, Bennett,

Cleve, DiVincenzo, Margolus, and Shor. If n is the number of qubits on which the unitary operator acts, then

these authors showed that 48 n M" CNOTs suffice, in addition to many one-qubit gates. Shortly thereafter,

Knill argued that some multiple C4" gates must be re- quired for reasons of dimension. The result has a parallel in classical circuits. Namely, given a random bit- valued function on n bit strings, approximately 2 " / 2

strings will take on a value of 1 . Thus, we expect

2 " / 2 gates are required to distinguish for which bit strings the circuit should return a nonzero value. A Unitary-Universal n Qubit Circuit

In summer of 2004, we discovered a new unitaryuniversal n qubit quantum circuit requiring roughly

(1 / 2)4" CNOTs, an improvement by a factor of two

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3 O O

2 -7 O

2 O 0 2 1 0

2 2 0

2

11 .7 2 1 O

2 1 1 2 1 2

Figure 2. Recursively generated treefor automated quantum circuit synthesis for three qutrits.

over the best known circuit at the time and a factor of

100 over the 1995 circuit. Moreover, work from the summer of 2003 had sharpened Knill's bound, actually

valid for any two-qubit gate, to a specific bound of

(1 / 4)4" CNOTs. Hence, the present circuit may never

be improved by more than a factor of two.

The outline for deriving the circuit is as follows. The

key step is to use the Cosine-Sine Decomposition (CSD)

for matrices. The CSD splits any unitary matrix into three factors; the circuit elements outline in the box below represent the first and last. The slash in the circuit represents a multi-line carrying an arbitrary number of

qubits, meaning U may be any unitary matrix on any

number of qubits. Moreover, the circuit elements having

the square box controls correspond to uniformly controlled rotations, a circuit block for which particularly

CNOT-efficient circuits are known. Hence, the diagram

employs the CSD to reduce construction of the n qubit

U into four simpler n - 1 qubit unitaries , V^, V^,

and . This allows for a recursive construction which

terminates with hand-optimized two-qubit circuits.

Figure 3. Basic quantum circuit synthesis decomposition.

Circuits for Quantum Multi-Level Logics

In addition, the efficiency of the circuit for quantum

multi-level logics (qudits, d =level) has improved. One

may think of a qudit by way of analogy: a qubit state is a quantum superposition of 0 and 1 , a qudit state is a quantum superposition of 0 ,\,...,d -I . Knill's lower

bound also stated that Cd ^" two-qudh gates were re- quired for any generic d" x d" unitary U . Yet the

best known constructive procedure required Cn^d^"

gates. In fall of 2004, we produced a new Cd ^" construction, closing this gap. The circuit exploits a variant

of the QR matrix decomposition. The original asymptotically optimal qubit circuits, due to researchers at the University of Helsinki, also leaned on a QR technique

and a Gray code cancellation. Rather than generalize the Gray code cancellation to base d numbers, the new cir- cuit relies on a recursively generated tree. This tree describes which entries of the unitary matrix to construct with quantum gate elements at which time. A sample

tree, for 3 qutrits (i.e., J = 3, n = 3 ) is shown above.

Lessons Learned

These specialized techniques for generic unitary matrices might help in optimizing other quantum circuits.

• Just as Boolean factorizations are important for

classical circuit design, so too matrix decompositions are important in quantum circuit design.

• It is possible to exploit parallels to classical logic

synthesis. For example, one may view the side factors of the CSD as multiplexors, applying an

n - 1 qubit unitary matrix dependent on the most

significant qubit.

• Novel aspects of quantum circuits must be explored

thoroughly. For example, at the heart of the qudit

circuit is a subcircuit capable of solving the hard

problem of initializing a generic quantum memory

state. In contrast, the classical problem of initiali- zation trivially requires at most n bit flips.

Selected Publications

1. V. Shende, I. Markov and S. Bullock, A Practical Top-down Approach to Quantum Circuit Synthesis,

http : //www,arxiV. org/abs/quant-ph/0406 176 .

2. S. Bullock D.P. O'Leary and G. Brennen, Asymptotically Optimal Quantum Circuits for d-level Systems,

http://www.arxiv.org/abs/quant-ph/04 10116.

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Quantitative Single-Molecular Pair Fluorescence Resonance

Energy Transfer

Recently available single-molecule sensitive optical

diagnostics have tremendous impact on our understanding ofdisease and the basic processes oflife, but are limited by artifacts and significant uncertainties

in the measurement process. We demonstrate a quantitative methodfor the measurement ofsingle molecular-pairfluorescence resonance energy transfer

(spFRET) that enables us to elucidate molecular

structures on a length scale of2 nm to 8 nm.

Lori S. Goldner, PL (844)

Tremendous technical developments have recently

made it possible to detect, identify, track, and manipulate

single biomolecules in an ambient environment or even

in a live cell. Single molecule approaches have changed

the way many biological problems are addressed and are leading daily to new insights in the basic processes of

life. The ability of single molecule approaches to avoid

ensemble averaging and to capture transient intermediates and heterogeneous behavior renders them particularly powerful in elucidating mechanisms of molecular

machineries; what they do, how they work individually,

how they work together, and finally, how they work in- side live cells

Fluorescence resonance energy transfer, or FRET, is a technique widely utilized to elucidate molecular struc- ture and dynamics and is especially noted for its sensitivity to inter or intra molecular distances. Indeed, the term

"spectroscopic ruler" has often been applied to this tech- nique, in which a change in the distance between two

fluorophores (donor and acceptor dyes) changes the spectroscopic properties of the dyes in a calculable fashion. It is widely appreciated that FRET also depends on

the relative orientation of the two dyes, but since the

relative orientation is often unknown or fluctuating in time, this dependence is also widely ignored. The resuh

is that FRET, which in principle might provide a highly

accurate measurement of either distance or relative ori- entation of a single dye pair, in practice provides neither

accurately and is generally only used for observing structural changes.

In this work [1], we describe how accurate single molecular-pair FRET (spFRET) measurements can be used

to directly validate or identify specific molecular structures. This is a two part problem: first, we show how to accurately measure the probability of energy transfer, E. This problem is, in principle, already solved in the litera- ture but we give a specific recipe for accurate measurement of E, enabled by single molecule techniques, that has not been used before in this context. Once E has

been measured, there is still the problem of calculating or measuring the dependence of E (for a specific dye

pair) on the inter-dye distance and relative orientation. This involves knowing the spectral overlap of the two

dyes and the quantum-yield of the donor dye , both of

which are notoriously difficult to measure with high ac- curacy. Here we demonstrate a single-molecule tech- nique for solving this problem accurately and in situ.

0.0 0.2 0.4 0.6 O.i FRET efficiency1.0

Figure 1. Top: An average "bent helix " structure ofRNA complex

under study here, determinedfrom 8 minimum-energy NMR

structures. Dyes molecules are shown in blue (donor) and magenta (acceptor) attached on the 5' ends ofthe RNA. Bottom: A

histogram ofaverage spFRET efficiencies showing structural heterogeneity.

These two problems, (1 ) accurate measurement of E and

(2) accurate determination of the spectral overlap and

donor quantum-yield have been widely discussed in the

literature, so that the requirements for accurate measurements are well understood and straightforward; the problem is that these measurements are sufficiently diffi- NIST Accomplishments in Nanotechnology

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cult and time-consuming that the experimentalist often

settles for a simple qualitative observation of structural change. We have developed an in situ solution that permits highly accurate measurements of E for individual

FRET pairs and a method for direct comparison with

structural models.

We demonstrate our technique with a study of an RNA

complex that regulates the replication of a DNA plasmid

in the bacteria E.-coli. For this system, NMR structural data gives 8 possible structures; using this new technique, we can distinguish between these structures and

determine if there are conversions between structures or

if the structures are static.

This work will be extended to include the study of HTVgenome/drug interactions and other RNA/DNA/protein

complexes for which an improved understanding of

structure and dynamics will enable rational design of

drugs or more accurate understanding of basic biological

processes.

Selected Publications

1 . Yim, P. B., Zhang, Z., DeJong, E. S., Carroll, J. M.,

Marino, J. P., and Goldner, L. S. Single RNA Kissing Complexes Studied by Fluorescence Resonance

Energy Transfer. Proc. SPIE, in press. 2005. Biomedical Optics.

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Bacteriophage/Quantum-Dot Nanocomplex to Detect Biological

Target in Clinical or Environmental Isolates

In a time ofbio-terrorism threats it is necessary to have new methods availablefor specifically targeted

biological pathogens. Different challenges need to be

addressed when trying to identify pathogens in the

outside environment. A detection system needs to be

rapid, highly sensitive, and specific.

Jeeseong Hwang, PL (844)

Currently, the most advanced technique to detect bacteria rehes on the labeHng of targets with green fluores- cence protein (GFP) gene. For instance, Oda et.al. used

GPP genes to express and fluorescently detect E. coli (M. Oda, M. Morita, H. Unno, Y. Tanji, Applied and

Environmental Microbiology 70, 527-534,2004). This

procedure requires expensive laboratory equipment and

expertise in molecular biology laboratory. In addition, the difficulty in detecting bacteria with this approach

occurs when the expression of GFP expression is low,

requiring very costly high-sensitivity fluorescence detection techniques, such as single molecule imaging. Fur- thermore, GFP photobleaches rapidly, allowing fluores- cence measurements for only a few seconds to a minute

under ordinary microscopy conditions. When the minimum number of phage per bacteria to cause infection is only a few, bacterial detection with GFP expression will be very difficult due to the low fluorescence signal and

fast photobleaching rate of GFP.

Researchers at NIST and NIH have developed a new

technique to detect bacteria strains using fluorescent

quantum dots (QDs) and genetically engineered bacte- riophage. The technique centers on combining functionalized (e.g. surface-coated with specific proteins, peptides, etc.) QDs with modified bacteriophage genetically

engineered to express surface-coat-molecule(s) These

modified bacteriophage are also adapted to be highly

specific to given target biological samples, for instance

strains of bacteria. The novel combination of phage

quantitatively labeled with QDs will enable the detection

and quantification of low abundance targets present

down to the single copy level of target biological samples fi-om clinical or environmental isolates. The unique

optical characteristics of QDs such as photostability,

size-dependent spectral properties on the same nanometer scale as its linked bacteriophage makes the combination highly suitable for imaging and discovery of single

pathogenic targets including deadly bacterial strains and

tumor cells.

Selected Publications

1. J. Hwang, R. Edgar, M. McKinstry, G. Giulian, C.

Merril, and S. Adhya, Invention Disclosure, "Bacte- riophage/Qunatum-Dot Nanocomplex to Detect Biological Targets in Clinical and Environmental Isolates," NIST Docket #05-007 (2005).

(a) (b) (c)

luanlum Dol

Phage Virus

•

m

m

100 nm 1

Figure 1. (a) A schematic ofthe detection concept. The capsid region ofa bacteriophage virus is genetically modified to express biotin moieties to conjugate with streptavidin QDs. (b) TEM image ofphage/QD complexes. Multiple QDs are bound to phage viruses,

(c) Fluorescence image ofbacteria targeted with phage/QD nano-complexes. Quantitative analysis ofthe results evaluates the specificity ofthe T4 phage viruses to a E. coli bacterial strain. NIST Accomplishments in Nanotechnology

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Chemical Sensor Microscopy for Nanotechnology

Nanoscale chemical mapping has immense potential

as a device and materials characterization toolfor

current and developing nanotechnologies. However,

techniques used to achieve nanoscale chemical imaging are still in early stages ofevolution. This research

combines Chemical Force Microscopy (CFM) and

moisture-enhanced probe-sample interactions as a

viable methodfor measuring the surface chemical

properties ofmaterials with nanometer spatial resolution.

Tinh Nguyen and Xiaohong Gu, BFRL (861)

Nanometer scale chemical mapping has immense potential as a materials characterization tool for current and

emerging nanotechnologies. Despite this critical need,

techniques to achieve nanoscale chemical information

are still in infancy stage. The use of chemicallyfunctionalized probe in an AFM, called Chemical Force

Microscopy (CFM) has been shown as an excellent ap- proach to characterize chemical heterogeneity at the nanoscale spatial resolution. Much of current research

using CFM has been performed in solvents. However,

for polar polymeric materials and biomaterials, solvent is not a desirable medium. By using a combination of hydrophilic AFM tips and elevated relative humidity (RH)

at the tip-sample environment, we have successfully developed a method to image materials chemical heterogeneity in air at a nanoscale spatial resolution.

The first example to demonstrate this method is shown in Figure 1, which was highlighted in Today's

Chemist at Work in 2003. The sample was a patterned

self-assembled monolayer (SAM), consisting of COOH

regions (thin stripes) and CH3 matrix material. These are height and phase images taken by tapping mode AFM

using a NIST-patented RH chamber attached to the AFM

and an unmodified Si tip. Under normal ambient conditions, Si02 on Si surface is fully hydroxylated (approximately 50H groups per nm^. Therefore, Si tips should be

regarded as OH-terminated probes. Except for the high

spots of the surface defects, the height images are essentially featureless at low or high RHs. However, the phase

images at different RH levels reveal the bright COOH

regions separated by the CH3 areas. Further, the image

contrast difference increases with increasing RH.

Figure 2 presents another example of using the present approach to enhance AFM phase image contrast

Technique: Phase Imaging AfM: AFM Tip; OHtenninated Si tip; Sample; Pancmcd SAM with altcraatiog "sthpcs" of hydrophihc (COOH) and hydrophobic (CH3) materia!.

Figure 1. Height and phase AFM images ofa patterned SAM,

showing the effect ofRH on the phase image contrast. (In each RH, height image is on the left andphase image is on the right.)

Figure 2. Effect ofRH on phase image contrastfor different hydrophilic/hydrophobic gradients; (unmodified Si tip; stripe: hydrophihc; matrix: hydrophobic).

of chemically-heterogeneous samples. These images were taken using tapping mode AFM for a gradientpatterned SAM sample, whose schematic is included for

clarity (center illustration). The thin stripes are the hydrophihc COOH regions and the matrix (thick stripes) is the chemical gradient material. These images were also obtained using tapping mode AFM and an unmodified Si

tip. At low RH, the phase contrast between the stripes

and the matrix is poor, even for the regions that have the highest surface free energy differences. However, the

contrast between the same two regions is greatly in- creased at 95 % RH. At high RH, the contrast even in the regions having small surface free energy difference can

also be observed. Because the chemical structure and

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chain lengths of the stripe and the matrix are similar, the contrast observed in Figures 1 and 2 is believed to be due

mainly to the hydrophilicity difference between the two

regions.

The use of fiinctionalized carbon nantube (CNT) tip to probe chemical heterogeneity using the present ap- proach is illustrated in Figure 3. These are AFM phase

images taken at four different RHs for a Si02/0DS (noctyldimethylchlorosilane) pattern SAM using oxygenplasma treated CNT tip and unmodified Si tip. In this figure, the darker stripes are the Si02 material and the

lighter matrix is the hydrophobic CH3, and all images were taken at the same location.

30 p.m X 30 \im

Figure 3. AFMphase image contrast as a function ofRHfor

plasma-treated CNT tip (a) and Si tip (b) on a SiO^-Si/CH}-

terminated patterned SAM; stripe is Si-Si02 substrate and matrix

is CHj-terminated SAM.

Clearly, the image contrast obtained by the plasmatreated CNT tip is greatly enhanced at 50% RH but drastically decreases at 73 % or 95 %RH. For the Si tip, the RH-enhanced contrast remains high even at high RHs.

The use of this approach for chemical imaging by

contact mode AFM is shown in Figure 4. The sample was a Si02/0DS patterned SAM and the probe was an unmodified Si3N4 tip. Data on the adhesion force as a function of RH for the SiOa substrate and CH3-

terminated SAM are included for comparison. The image

contrast is greatly enhanced at 53 % RH and decreases

slightly at 90 % RH, consistent with adhesion force data. One example on the use of this method to probe

chemical heterogeneity in polymeric materials is shown

in Figure 5. These are images taken using tapping mode

AFM of a cryo-fractured PS-PEO (polyethylene oxide)

block copolymer surface. The dark domains are the hydrophilic PEO material and the bright areas are the hydrophobic PS. The phase images clearly show that high

RH not only enhances the contrast between the hydrophilic and hydrophobic regions but also enlarges the hydrophilic areas.

a

0 20 -VO 60 80 too Relative Humidity (%)

Figure 4. Effect ofRH on image contrast (a) and adhesion force

(b) obtained by AFM contact mode for a Si02/0TS patterned

SAM; stripe is the Si02 substrate and matrix is ODS.

Figure 5. AFMheight andphase images ofPS/PEO block co- polymerfractured surface at two RH levels.

The above examples and additional data obtained by

both functionalized and unfunctionalized tips firmly

demonstrate that the approach of using elevated RH in the tip-sample environment and hydrophilic AFM tips is a viable method to probe materials chemical heterogeneity in air.

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Nanomechanical Properties of Polymer Thin Films

There are numerous drivers for the use ofultrathin

polymericfilms, most notably the microelectronics

industry wherefeature sizes of22 nm are anticipated

by 2010. However, the ability to measure the physical

and mechanical properties ofpolymeric materials on

this length scale remains a daunting task. We employ

our buckling-based metrology to measure the elastic

modulus (resistance to deformation) ofnanostructured

thin films as well as ultrathin polymerfilms.

Christopher M. Stafford, MSEL (854)

Nanotechnology promises to revolutionize a growing

set of materials applications ranging from electronics to drug delivery to ballistic protection. However, the quest

to engineer materials on the nanoscale {e.g., in the form

of ultrathin films) is met with the daunting task of measuring the physical and mechanical properties of these systems. Understanding the mechanical properties of

nanofilms is especially critical in the fabrication of

MEMS, NEMS, and electronic devices. Current methodologies such as indentation can probe the bulk mechanical properties of materials but are challenged by

thin polymer films as well as multilayered structures. We have established a buckling-based metrology that measures the elastic modulus of relatively stiff polymers

films supported on soft elastic substrates. This metrology exploits a buckling instability that occurs in laminates upon compression. The wavelength of the instability, d, can be related to modulus ratio of the film and

substrate, E/Es, through the following equation:

A. = l/rh

l-v

3(l-v

1/3

This year, we applied this metrology to measure the mechanical properties of nanoporous low-^ dielectrics in development for semiconductor applications. The

modulus of these low-A: films has been well correlated with their resilience to the chemical mechanical polishing (CMP) used during processing. The materials studied

have pores (air) ranging in size from 5 nm to 50 nm en- capsulated in a continuous matrix of organosilicate

(MSSQ). The modulus of the series of nanostructured

films decreases monotonically with increasing porosity

(Figure 1), and these data agree with semi-analytical

models describing the porosity dependence of a materials' modulus.

Figure 1. Modulus (Ej) as a function ofporosity (P) for a series ofnanoporous organosilicate films. We also applied this metrology to measure the elastic modulus of ultrathin polystyrene (PS) films. The data are shown in Figure 2. For A > 30 nm, the apparent modulus

is constant; we observe a transition in the modulus as a

fiinction of film thickness that occurs at ~ 30 nm. We

have generated several arguments for the observed de- crease in mechanical integrity of ultrathin polymer films. Regardless of the origins of this transition, the data alone

are compelling and need to be accounted for in future theories and models for how fundamental physics of

polymer thin films impacts their mechanical properties.

The data also highlight issues concerning the fabrication

and stability of sub-30 nm features patterned in thin

polymer films

C3 OhO

4-

2-

I ' 1 ' 1

i

• ISOOxIO'm^

20 40 60 h (nm)

100

Figure 2. Modulus (Ej) as a function ofthickness (h) for PSfilms oftwo different molecular mass.

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Nanomechanical Metrology

The needfor reliable, standardized mechanical test methods applicable to nanometer-scale volumes of

material has grown dramatically in recent years. Nanoindentation has become the method ofchoicefor

quantitative determination ofmechanical properties of

small volumes ofmaterial, but standardization efforts have laggedfar behind the application ofthe techniqut. Over 1000 commercial nanoindentation instruments are currently in use, with no traceableforce

calibration and an extremely limited choice ofstan- dardized test methods. We develop standard refer- ence materials (SRMs), traceable calibration trans- ducers, and robust, reliable methodsfor obtaining,

analyzing and modeling nanoindentation data.

Douglas T. Smith and Lyle E. Levine, MSEL

(852)

Nanoindentation is the most commonly used method

for determining hardness and elastic properties of small

volumes of materials. In this technique, a diamond in- denter is pushed into a specimen surface, and the force on and displacement into the surface are recorded. The

technique is capable of providing information on the

elastic and plastic deformation of a specimen for indentations as shallow as a few nanometers, and requires

minimal specimen preparation effort. However, there

are only a very limited number of accepted test methods

available, and no means to traceably calibrate or verify

the performance of nanoindentation instruments. This

leads to large interlaboratory variations in results, and a dearth of reliable data for understanding or predicting

nanoscale mechanical behavior.

In many mechanical test methods, including nanoindentation, a force is applied to a specimen, and dis- placement is measured. Traceable displacement measurement by interferometry is well established. Force

measurement is more problematic, because the SI unit

for force is still based on an artifact kilogram mass. The

Microforce Competence Program at NIST has developed

a primary realization of force, traceable to electronic and

length SI units, for force calibration in the range 1 mN to 10 nN. Transfer force cells have now been developed

that allow force calibration, traceable to NIST, for commercial nanomechanical test equipment such as nanoindentation instruments and atomic force microscopes.

One such cell, shown in Figure 1 , was calibrated against

the NIST primary force balance to an uncertainty less than 0.5 % for forces in the range 0.05 mN to 5.0 mN.

That cell was then mounted, as if it were a specimen, in several popular commercial nanoindentation instruments.

For forces above 2 mN, the recorded nanoindenter force was within the 1 % uncertainty required by most draft standards for nanoindentation machines. However, for lower applied force, the error increased dramatically, and

at 0.05 mN, the recorded force was almost 18 % below

the actual applied force.

Figure 1. Sl-traceableforce cell.

In addition, mechanics at the nanoscale is inherently

difficult to model accurately. Finite element modeling

(FEM) can effectively capture the elastic behavior of

macroscopic structures but includes no accurate failure

criteria, since this depends upon atomic-scale behavior.

Classical atomistic simulations can handle enough atoms

(millions to billions) to model such events but their potentials are inaccurate for large strains. Quantummechanics-based simulations using density fiinctional theory (DFT) are extremely accurate and handle the chemistry exactly, but such simulations are so CPU in- tensive that they can handle only a few hundred atoms. A combination of all three techniques is required to ac- curately model mechanical behavior at the nanoscale.

Over the past year, we have developed techniques to handle such multiscale modeling for quasistatic applications such as nanoindentation. At the macroscale, FEM

is used to simulate the elastic behavior of a nanomechanical system. The FEM mesh is fine enough to use

the predicted elastic displacement fields to generate

boundary conditions and initial atom positions for an

atomistic simulation using classical potentials. The use of classical potentials in a large simulation cell allows us

to correctly propagate the long range stresses to the criti- cal regions where bond distortions are large or where

chemistry effects need to be explored. In these critical regions, we embed a DFT simulation.

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Improving Image Resolution in Nanotechnology

Current nanoscale electron microscopy images re- main ofrelatively low quality. To address this issue we are developing improved mathematical toolsfor

image analysis, and the use ofsuch tools to provide

measurable increases in resolution in state-of-the-art scanning electron microscopy. One very major diffi- culty lies in the large image sizes, often on the order

of 1024x1024 pixels, or larger. This presents formidable computational challenges. Many new techniques

are based on nonlinear partial differential equations,

and typically require thousands ofiterations, and several hours ofCPU time, to achieve useful results. Real-time image processing algorithms are exceedingly rare and very highly sought after.

Alfred Carasso, ITL (891)

A fundamental problem in scanning electron microscopy (SEM) is the fact that the shape of the electron beam that produced the image is seldom known to the microscopist. Therefore, image deblurring must proceed

without knowledge of the actual point spread function

that caused the blur. Such so-called blind deconvolution

is fraught with difficulty, and little authoritative discussion of this subject is to be found in most image processing textbooks.

Nevertheless, in recent years, considerable progress was achieved at NIST in developing mathematical tech- nologies that lead to real-time image processing algorithms. In addition, a unique new capability has been

created, the so-called APEXmethod, that can achieve

useful blind deconvolution of 1024 x 1024 SEM imagery

in about 60 seconds on current workstations. Because of

its manifold applications, this technology is the subject

of intense and continuing research and development.

The APEX Method

The APEX method is an FFT-based direct blind deconvolution technique that can process complex high

resolution imagery in seconds or minutes on current desktop platforms. The method is predicated on a re- stricted class of shift-invariant blurs that can be ex- pressed as finite convolution products of twodimensional radially symmetric Levy stable probability

density functions. This class generalizes Gaussian and

Lorentzian densities but excludes defocus and motion

blurs. Not all images can be enhanced with the APEX

method. However, we have shown that the method can

be usefully applied to a wide variety of real blurred images, including astronomical, Landsat, and aerial images,

MRI and PET brain scans, and SEM images. APEX

processing of these images enhances contrast and sharp- ens structural detail, leading to noticeable improvements

in visual quality.

A B

Figure 1. APEXblind deconvolution ofstate ofthe art Scanning

Electron Microscope imagery produces measurable increases in sharpness. (A) Original 1024x1024 Tin sample micrograph has

Lipschitz exponent a = 0.40 and TV norm = 13000. (B) Sharpened

image has a = 0.29 and TV norm = 34000.

A B

Figure 2. APEXsharpening ofSEM imagery. (A) Original

1024x1024 Magnetic Tape sample has a = 0.35 and TV norm =

14000. (B) Sharpened image has a = 0.26 and TV norm = 39000.

Application to SEM Imagery

Recently, a new Hitachi Scanning Elecfron Microscope was acquired by the NIST Nanoscale Metrology

Group, capable of producing higher quality imagery than

had previously been possible. A major challenge for our deconvolution algorithms was to demonstrate measurable increases in sharpening of such state of the art imagery. Two sharpness measures were used, the image

Lipschitz exponent a, and the image discrete total variation or TV norm. Image sharpening increases the TV

norm, due to the steepening of gradients, while it decreases the Lipschitz exponent as finer scale features become resolved. Examples of such sharpening are shown in Figures 1 and 2. In Figure 1 A, the original

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1024x1024 Tin sample micrograph has TV norm of

13000 and Lipschitz exponent a = 0.40. The APEXsharpened Figure IB has TV norm = 34000 with a =

0.29. In Figure 2A, the original 1024x1024 Magnetic

Tape sample has TV norm = 14000 with a = 0.35. The

APEX-processed Figure 2B has TV norm = 39000 with a = 0.26. These very substantial sharpness increases are

typical of those obtained in numerous other test images.

Measuring Image Smoothness

Most commonly occurring imagesf(x,y) are not dif- ferentiable functions of the variables x and y. Rather,

these images display edges, localized sharp features, and

other fine-scale details or texture. Many digital imageprocessing tasks require prior specification of the correct mathematical function space in which the true image

lies. If an image is incorrectly postulated to be too smooth, the processing algorithm may produce an overly

smoothed version of the true image in which critical in- formation has been lost.

During the last 10 years, a very considerable amount

of image analysis research has been based on the as- sumption that most images belong to the space of functions of bounded variation. However, it has been subsequently discovered that such so-called total variation

(TV) image processing sometimes results in unacceptable loss of fine-scale information. This phenomenon is now known as the staircase effect. In papers published

in 2001, French researchers Gousseau, Morel, and

Meyer, showed that most natural images are, in fact, not of bounded variation, and that TV image-processing

techniques must inevitably smooth out texture.

Correct characterization of the lack of smoothness of

images is a fundamental problem in image processing. It turns out that so-called Lipschitz spaces are the appropriate framework for accommodating non-smooth images.

The Lipschitz exponent a for the given image, where

0 < a < 1, measures the fine-scale content of that image,

provided the image is relatively noise free. Heavily tex- tured imagery has low values for a, while large values of a indicate that the image is relafively smooth. Estimating an image's Lipschitz exponent is a delicate problem.

We have developed a new, computationally efficient,

method for estimating a. It merely requires blurring the image by convolution with a specific singular integral

kernel, and evaluating the discrete norm of the differ- ence between the blurred and original images. The rate at which this norm tends to zero, as the kernel ap- proaches the Dirac (5-l\inction, is directly related to the Lipschitz exponent a. Since the required convolutions can be accomplished by FFTs, very minimal computational effort is thus needed to implement the resulting

procedure. In addition, this approach has the advantage

of allowing consideration of substantially wider

Lipschitz spaces than is mathematically possible with

existing procedures, thereby encompassing a much wider

class of images.

Significant potential applications of this technology

include the routine monitoring of image sharpness and

imaging performance in electro-optical imaging sys- tems, the performance evaluation of image reconstruction software, the detection of possibly abnormal fine- scale features in some medical imaging applications, and

the monitoring of surface finish in industrial applications. In addition, specifying the correct Lipschitz space

wherein an image lies can be used to solve the blind image deconvolution problem in a way that preserves texture, i.e., fine detail, in a recovered image. We recently

have developed a new method, the Poisson Singular In- tegral (PSI) method, which yields an excellent approximation to optimal image filtering for a very wide class of

images.

Selected Publications

1 . S. Carasso, The APEX Method in Image Sharpening

and the Use of Low-exponent Levy Stable Laws,

SIAMJournal on Applied Mathematics 63 No. 2

(2002), pp. 593-618.

2. S. Carasso, D. S. Bright and A. E. Vladar, The

APEX Method and Real-time Blind Deconvolution

of Scarming Electron Microscope Imagery, Optical

Engineering A\ No. 10 (2002), pp. 2499-2514.

3. Carasso, Singular Integrals, Image Smoothness, and

the Recovery of Texture in Image Deblurring, SIAM

Journal on Applied Mathematics, 64 (5), (2004), pp. 1749-1774.

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Optical Metrology for Nanolithography

Responding to a NISTproposal, the semiconductor

manufacturing industry is seeking new, high-index

lens materials for 193 nm immersion lithography in order to achievefeatures sizes as small as 32 nm. The

materials must have high indices ofrefraction, have

isotropic optical properties, have transmission cutoff

wavelengths shorter than 193 nm, and have high optical quality. NISTfirst identified several promising

classes ofmaterials, and is pursuing a project to optically characterize candidate materials in these and

related classes that meet these specifications. This is a joint experimental and theoretical effort.

John H. Burnett, PL (842), Simon G. Kaplan, and

Eric L. Shirley (844)

The Rayleigh criterion for resolution in optical sys- tems is k^AJNA , where is a prefactor (>0.25) that depends on optical engineering, A is the wavelength of

light, and NA = n sin 6 is the numerical aperture, in- volving n , the index of refraction in, and 6 , the half- cone angle for light incident at the focus. For conventional optical lithography these parameters have been

pushed to near their theoretical limits, with the exception

of the index. To drive resolution further, the industry

has now turned to increasing the index. .While the index

in the above resolution criterion is that at the image in the photoresist, as shown in Figure 1 the NA is limited

by the smallest index of the resist, of the fluid above the

resist, and of the last lens element.

We were the first to point out that the index of the

last lens element is the key bottleneck for decreasing

feature sizes. We initiated a program to identify potential high-index optical materials and to establish their

feasibility for incorporation in lithography tools. The

industry now accepts our claim that these materials could

potentially enable one further generation of feature sizes,

to 32 nm, using conventional optical lithography.

On theoretical grounds, wide-band-gap optical materials are limited to Group-I or Group-II oxides and fluorides. Our calculations and measurements show that the indices of fluorides are too low to be useful. However, we have established that the alkaline-earth oxides, magnesium aluminum spinels, and several other related oxide compounds have the requisite high indices and band

gaps. As a result of our identification of these promising

classes of high-index oxide materials, the semiconductor

lithography industry is considering redesigns of the li- thography tools to take advantage of the improved resolution that could be achieved.

Although we have identified these classes of materials for the industry, a wide variety of materials within

these classes remain to be explored. To be usefiil, the

optical properties of the materials must meet certain tight

specifications on, e.g., the absorption, index, index dis- persion, thermal optical coefficient, stress-optical coefficients, and intrinsic birefringence. The material classes we have proposed permit wide compositional variations

involving stoichiometry, atomic substitution, and solid solutions of related materials. Thus these material sys- tems offer substantial phase space to potentially tailor optical properties to meet the requirements.

Because there exist little or no experimental or theoretical data on the optical properties of these materials at deep ultraviolet wavelengths, this project is in its in- fancy. However, the required state-of-the-art facilities to make these measurements have been developed for this and related projects. Preliminary results for some of

these materials indicate that their properties may be in the acceptable ranges. In association with the semiconductor industry, we are working with ultraviolet materials suppliers to explore the fiill range of these materials

to identify the most promising candidates and demonstrate feasibility.

The most quantitative output of our work must come

from measurement, but measurement results are inter- preted and/or confirmed using parallel theoretical simulation and modeling. As a simple example, Figure 2 il- lustrates theoretical results for the index of refraction as a function of photon energy for magnesium, calcium,

strontium and barium oxide. While the results differ somewhat from experimental values, the trends and ap- proximate values of the indices of refraction illustrates the use of a theory that can operate independently from

experimental input. This is valuable for searching

through the large phase space of possible new materials

and material combinations for promising candidates to attempt to grow. One multi-dimensional region that we

intend to search next is in magnesium aluminum spinels

with varying degrees of magnesium versus aluminum

richness and degrees of inversion.

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light ray

light rav/

V krts

( fluid "fkni )

resist m= n„,i,sm (9, = riQ^ sin &f = siti 0^

Figure 1. Schematic oflight rays traveling through final lens element, immersion fluid, and resist. The NA is limited by the smallest index.

I ft I > 1 1 1 1 1—,—I 1 012 3 45678 pholon cncrsY (cV)

Figure 2. Theoretical indices ofrefraction versus photon energy for magnesium, calcium, barium and strontium oxide. The downturn indicates the onset ofabsorption. The photon energy corre- sponding to 193.4 nm is indicated.

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True Three-Dimensional Tomography Utilizing Bayesian

Statistical Inference

Scanning Transmission Electron Tomography (STEM)

has the capability ofproducing projected images

based on more than a single axis ofrotation. In order

to take advantage ofthis feature, Bayesian statistical methods need to be extendedfrom the current practice

ofreproducing a series oftwo-dimensional slices with

ad hoc post-processing, to producing directly true three-dimensional images using the STEMprojections

to determine the smoothness ofthe 3D image. This

goal is achieved by developing software based on 3D

geometry and extending current 2D Bayesian methods

to 3D. In addition, full Bayesian inference, necessitating Monte Carlo methods, is utilized to assess thefi- nal quality ofthe reconstructed image.

Donald Malec and Juan Soto, ITL (898)

The Scanning Transmission Electron Microscope

(STEM) offers the possibility of obtaining highresolution, three-dimensional image reconstruction of

material or biological samples at the nanometer scale. Image reconstruction is based on using a series of twodimensional projections obtained as entry and exit en- ergy differentials of electron beams aimed though a sample at various tilt angles. At NIST, there are a number of ongoing projects associated with both overcoming

problems and capitalizing on unique features of STEM

in order to convert these projections back into images.

This project entails extending Bayesian two-dimensional

image methods to three-dimensional and providing associated estimates of precision.

Bayesian methods in the literature are based on re- producing two-dimensional images (slices) from projections based on rotating a projection bed (an array of detectors) around a single axis of rotation. By obtaining a

grid of these points, pseudo three-dimensional images

can be reconstructed based on a series of stacked slices as in medical imaging from CAT scans. These slices can

be further processed to fill in the gaps based on judgment. Forming two-dimensional slices is the current

level of practice. These methods are based on twodimensional geometry and cannot be trivially extended

to 3D. Imaging directly into three dimensions has several advantages. Among them is a more direct use of the nearby continuity of nearby features of a solid and, most

importantly, the ability to use novel or unsystematic tilt series angles.

This project achieves three aims:

1 . To extend state-of-the-art Bayesian methods of

image estimation from two-dimensional slices to true three dimensions.

2. To obtain measures of precision of areas of

these images using full Bayesian inference

methods.

3. To implement and develop more efficient geometrical algorithms needed to implement the fuU-Bayes approach

The following example illustrates the issues dealt with here. Consider a fictitious solid that contains the

letters of word "FOCUS" in two orientations (see Figure

1). The left half of this figure represents the first layer of

the solid's depth and the right half the second layer.

Figure 1. Afictitious solid containing the letters ofthe word

"FOCUS" in two orientations. The left image represents thefirst layer ofthe solid 's depth and the right image the second layer.

Suppose that this solid is covered by an opaque, uniform layer of a known substance, rendering its interior

invisible. An ideal electron beam (i.e., when multiple

scattering can be ignored) can be aimed through the substance as a way of visualizing the underlying structure without destroying the coating or the solid. In an ideal

setting a series of tilt angles results in a series of pro- jected images at each angle. The image in Figure 2 is a simulation of the solid based on 121 tilt series of 1

equi-spaced tilt-angles, between 0 and ti, around the depth and height axes. For each tilt series, the energy

degradation is captured on a 100 x 100 grid. The degradation of the energy is based on both the underlying ge- ometry due to the letters and Poisson noise based on a dosage of 10^ electrons per pixel.

NIST Accomplishments In Nanotechnology

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!

i

1 1 il \

1 1 1 E

1

i i 1 1 8 1 s i B

1

i

f i i tr r *

»

ai f Jw d ^ W ^ W OBI ^ ^ ^

ir

^TM ^?i JJ** o* aoi p9 g» fek obi "w

^ Si

•»•• 6*« S«« SP* am

^

am Ha"•Iw Sis »\»

Ml £SiiC iM f*^ iiHi 93l9 OEM ooa XS2 fD XEI EiW wmt *v WW

•hb ^**m ISSIB 13 fid

% J8 Im

\ \ ^ ^ § M t ^ ^ /

\ S ^ k 1 1 t f f I /

f 1 1 1 i ! 1 1 1 \ !

Figure 2. Simulation ofthe solid based on 121 tilt series of11 equi-spaced tilt-angles, between 0 and n, around the depth and

height axes.

The original solid can be faithfully recaptured using

the high-quality sample images above. More realisti- cally, available projections may be of lower quality and

resolution, possibly because of the fragility of material or

the time and expense of collecting detailed projections.

The next simulation is a result of projecting the same

solid onto an 1 1 x 5 series of tilt angles equi-spaced between 0 and 71. The projection bed in this case consists

of only a grid of 7 by 7 sensors. In addition, the dosage

is only 1 000 resulting in much more noise due to Poisson

error. A realization is given in Figure 3

.

i1}

1 ! 1 i ^

1

Hl

/

^ E:«

BES8SSES ^TM

1 ^

, J r 1 IS,

1^

^4

wmmm j W an

„^ i

mem maani

ikAm wamm

\k wk

1. Il E ^

1 ! 1

I"

1

Figure 3. Simulation based on projecting the same solid onto an 11x5 series oftilt angles equi-spaced between 0 and k. The projection bed consists ofa grid of 7 by 7 sensors. Also, the dosage is only 1000 resulting in much more noise due to Poisson error.

Using this information as input, the 3D Bayesian

method can still produce an estimated image. Doing so

results in the image of Figure 4 using the posterior mean.

Figure 4. 3D Bayesian estimate based upon simulation results of

Figure 3.

Figure 5. Posterior standard errors for each voxel in the example. Dark shade indicates a relatively larger error. As mentioned, estimates of the accuracy of the image

can also be made based solely on the projected data with no knowledge of the true image. For this example, the

resulting posterior standard errors for each voxel are shown in Figure 5, where a dark shade indicates a rela- tively larger error. As can be seen, in general the edges

are less accurate than the center. This is to be expected

since all angles of rotation are about the center resulting

in more data that incorporates the center voxels. However, the interaction between the letters and surrounding

space is clearly evident in the accuracy estimates. Obtaining these estimates can be used informally, or per- haps with additional methodological development, as a way of determining where to obtain more projections to most efficiently reduce error.

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Structure and Dispersion Measurements of

Polymeric Building Materials

Almost all commercially viable polymeric construction products have nanoscale pigments dispersed

throughout the polymeric matrix. Poor dispersion of

these pigments is qualitatively known to adversely

affect the appearance, service life, and mechanical

properties ofthese products. Current methodsfor as- sessing pigment dispersion in industry are subjective,

indirect, unreliable, and limit to large micron-size

aggregate dilute suspension. The objective ofthis project is to develop non-destructive, light scattering

techniquesfor characterizing nanoscale pigment dis- persion in liquid (uncured) and in solid (cured) polymeric matrices and relate these dispersion properties

to the appearance and weathering service life of

polymeric materials.

Li-Piin Sung, BFRL (861)

We have developed methodologies for quantitative assessment of pigment dispersion using small angle neutron scattering (SANS) beam in NCNR's cold neutron

facility. To develop a more practical and non-destructive

method for characterizing the dispersion of pigments in uncured and cured polymeric materials, a state-of-the-art

light scattering materials characterization laboratory was

established. By correlating with the SANS measurements

and microcopy measurements, an efficient and accurate measurement using light scattering metrology will be

developed. Collaboration with theoreticians from university especially in applying particle dispersion theory is ongoing to derive a quantitative dispersion parameter.

With the completion of BFRL light scattering materials characterization laboratory (FY04), we now have the new capabilities of characterizing the morphology (nanodomain due to dispersion in mixing or phase separation)

and measuring structure dispersion for various processing conditions in uncured and cured coatings. Current

research efforts will focus on metal oxide nanoparticulate systems because of their importance to a range of

industries, but the methods developed will be generic in nature. In this way, we will establish a framework for determining structure-property relationships for nanoparticles and nanoparticle-polymer systems.

examined. Several trials have attempted to improve

nanoparticle dispersion by chemical surface treatments

/physical mixing of nano-Ti02. Figure 1 shows parts of

SLS and DLS results. From SLS results, the dispersion

parameters such as particle cluster size and the interac- tions of particle-particle and particle-matrix can be de- termined. From DLS, autocorrelation function and re- laxation time of the nanoparticle in solvent matrix can be

characterized in different dispersion states. Data analysis

is in progress and more experiments are underway to better understand the dispersion kinetics/dynamics.

(a) (b)

Figure 1. (a) SLS Results ofvarious nanoparticles suspense, (b) DLS results for nano-Ti02 pigments dispersed in the toluene.

(C) Black - slow (D) Black - quench

Figure 2. Examples ofdifferent pigment dispersion due to differ- ent annealing conditions. Slow and quench indicated slow andfast cooling conditions. Image size: 60 jim x 60 /jm.

We have conducted a series of particle dispersion

measurements using small angle neutron scattering

(SANS), angular-resolved static and time-resolved dynamic light scattering (SLS &DLS) techniques. Studied

particle size ranges from 20 nm to 800 nm, and two major types of particles (Ti02 and polymer spheres) were

In addition to studying the particle dispersion in sus- pension state, we have also studied particle dispersion

and microstructure/morphology of pigmented coatings

under different processing conditions and the effect of

dispersion and microstructure on the durability (UV degradation). The controlled parameters are cooling rate, particle size, mixing time, pigment volume concentration, mixing chemistry. . .etc. Figure 2 shows pigment

dispersion of two types of pigmented PVDF coatings

(blue and black pigments) under different annealing conditions. The white dots in the confocal images are the NIST Accomplishments in Nanotechnology

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pigments, and the images clearly show that different

cluster sizes and spatial distribution of particles. Figure

3 shows SANS studies of nanostructure for different chemical and physics mixing of polymer coatings and

the UV degradation results. Detailed information can be

found in the recent publications. Dispersion and microstructure play a big role on durability (UV degradation).

q(nni ')

(C)

Figure 3. (a) SANS results ofPVDF coating by different mixing

methods. (B) UV degradation results on three coatings with differ- ent microstructures. (C) SANS results ofPVDF coatings under

different annealing conditions and UV degradation results (lines) after two-month indoor UVdegradation.

Figure 4. Surface morphology and light scattering profiles from

masked and exposed PVDF coatings. The 5-axis sample stage of

LS-I is also shown here. We have also investigated the nano- to micro size surface morphology changes of coatings with different

particle dispersion due to the UV degradation and

scratch test. Figure 4 shows an examples of surface morphology changes after 1 2 years exposed in the Florida

and its corresponding surface scattering feature using the new light scattering instrument (LS-I). The LS-I provides the capability of probing surface and subsurface

microstructure changes due to UV degradation.

With the capability of accurate dispersion measurements, we can provide the direct link between these dis- persion properties (particle and cluster size, interactions,

degree of dispersion) to the appearance (surface roughness and gloss) and weathering service life of polymeric

materials. In addition to the dispersion characterization

measurements, the newly developed LS instrument (LSI) characterizes surface roughness and microstructure of

polymer films, and will have a fiill capability of color

and gloss measurement upon acquiring a customized

Scanning Wavelength (White light) Illumination System.

The techniques and measurement methods developed in

this study will support dispersion measurements in the BFRL nanoscale metrology project and the characterization of polymer surface project.

This project, which is strongly supported by the in- dustry (members of NIST/Industry Polymer Interphase

Consortium and NIST/Industry Service Life Coating

Consortium), provides essential scientific-base data for the development of methodology and measurement protocols that can be used for evaluating surface appearance, dispersion state of coating constituents. A successful development of these methods will greatly improve

the commerce and reliability of products that depend on

surface appearance, dispersion state of coating constituents, including the automotive industry, coatings and

other building polymeric materials.

Design and application of this instrument were pre- sented in the Inter-Society Color Council (ISCC) and

Council for Optical Radiation Measurement (CORM)

jointed Technical Meeting in May 10-14 2004. Parts of

the results have been presented at Rohm & Haas Chemical Company, and a coating international meeting

(FSCT/ICE) in October 2004.

Selected Publications

1 . L. Sung, S. Vicini, D. L. Ho, L. Hedhli, C. Olmstead, and K. A. Wood, Polymer 45(19), 6639-

6646 (2004).

2. J. Faucheu, L. Sung, J. W. Martin, K.A. Wood,

FSCTICE 2004 Proceedings ofthe 82"' Annual

Meeting Technical Program, McCormick Place

North, Chicago, IL, October 27-29, (2004); in press, JCT Research (2005).

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Synthetic X-ray Spectrum Images for 3D Chemical Imaging at the

Nanoscale

Synthetic "phantom " x-ray spectrum image datasets are needed to develop and test reconstruction algorithms, visualization tools, and metrology softwarefor

3D chemical imaging. By generating simulated observables (X-ray spectra) from samples constructed in

silico with exactly known composition and geometry,

this project seeks to provide test data to judge the effi- cacy and accuracy ofdata processing systems at- tempting to reconstruct the unknown 3D distribution

ofelements within real samplesfrom measured X-ray

spectrum images.

John Henry J. Scott and Nicholas Ritchie CSTL

(837)

In the last 30 years, analytical electron microscopy

(AEM) and scanning electron microscopy with x-ray microanalysis (SEMXM) have advanced steadily via

innovative electron optics, better spectroscopic detectors,

improved computer control, and multi-modal data acquisition technology. The analyst now has access to a wealth of high-quality, multidimensional data about the sample. As a result, the challenge now is to devise

schemes to manage and exploit this flood of raw data

and to synthesize the results into meaningful solutions to real-world problems. Recently, researchers have begun

to explore the possibility of chemical tomography in the AEM, the determination of 3D elemental distributions in the sample based on tilt-series of electron energy-loss

(EELS) and X-ray emission data. One of the chief barriers to this work is that for thicker samples EELS maps

can show non-monotonic relationships between the observed signal in the 2D tilt images and the concentration

of the analyte in that projected pixel. This violates the projection requirement, a key assumption in many 3D

reconstruction algorithms. Because of this, most successfiil work to date has been limited to systems that (at least approximately) meet this requirement and do not

exhibit the full complexity of interactions possible in the AEM or SEM. To get past this hurdle, it is necessary to construct new models for 3D reconstruction in the AEM

and SEM that explicitly account for effects such as beam

spreading, multiple scattering, and through-sample self- absorption.

Following the lead of the medical imaging community, one useful step towards the creation of 3D reconstruction algorithms is the analysis of synthetic datasets from phantoms with known properties. Figure 1 shows

the results from 3D Monte Carlo simulations of three

cylinders (Cu, Al, and SiOa), each 600 nm

Id tilt = 0

Mont» C«rfo

trajectories

Figure 1. (a) AEM cylinder phantoms (600 nm diameter by 1 mm

long) shown in orthographic projection while at zero tilt; the blue arrow points toward the X-ray detector, (b) the same phantoms as seen from the perspective ofthe X-ray detector, showing how some

cylinders eclipse others during tilting, (c) view along the tilt axis, perpendicular to the incident electron beam; the blue arrow points

toward the X-ray detector, (d) 300 keV Monte Carlo electron

trajectories. 100 incident electrons shown striking the sample at 1 of32 horizontal beam raster locations with the sample at zero tilt.

NIST Accomplishments in Nanotechnology

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in diameter and 1 long. The electron beam (purple

line) was rastered over 32 pixels parallel to the x-axis

and the resulting X-ray spectra at the XEDS detector were calculated, resulting in spectrum-line profiles for each tilt from 0° to 1 80°. From the 5,760 spectra calculated, chemical sinograms were extracted by summing

the intensities in 1 30 eV wide windows over the Cu Ka,

O Ka, and Al Ka peaks. These data display both beam

broadening and self-absorption effects, but do not in- clude Bragg scattering or dynamical interactions, two

other effects that plague 3D AEM.

Figure 2 shows how a similar dataset was generated

for an even more complex sample, a 3D "Rubik's cube"

of various elements. Unlike the dataset shown in Figure 1 , this dataset consists of a complete 2048-channel

X-ray spectrum at each pixel in a 2D array. Thus while

this second dataset does not exhibit a tilt variable, it comprises a 2D image instead of a series of ID line profiles. Both types of dataset are important in refining a

practical 3D reconstruction algorithm.

The elemental map in Figure 2 (top right) shows

many of the desired physical effects that complicate 3D

reconstruction from electron-excited X-ray emission

data. As the primary electrons interact with the surface

of the sample at each pixel, they spread outward laterally

and downward vertically into the different phases in the cubes. Since the electron range for a 20 keV incident

beam in these materials is large compared to the size of

the subcubes, the resultant excitation volume is expected

to extend into neighboring cubes. To fiirther complicate

the analysis and eventual 3D reconstruction, after the Xrays are generated by the inelastically scattered electrons, they are differentially absorbed on their way out of

the sample toward the X-ray detector. For soft X-rays,

such as the N Ka X-rays shown in blue in Figure 2 (top

right), this self-absorption effect can be quite large. It is visible as a gradient of detected intensity, increasing

from the bottom of the image (farther from the detector,

longer escape path length) to the top (shorter escape path

length).

Although this project was begun just a few months

ago, it has already yielded several important datasets that have revealed the complex and dynamic nature of observed X-ray spectra in 3D samples and tilt series. Efforts are underway (in collaboration with the NIST

Mathematical and Computational Sciences Division) to produce parallelized versions of these simulators that would allow us to scale up the synthesis of phantom

datasets to more pixels and more tilts. In addition to the preliminary publications listed below, this work has been

presented at invited talks at multiple universities and at national microanalysis meetings.

Figure 2. (top left) Phantom sample consisting ofa "Rubik's cube " ofaluminum, silicon, silicon dioxide, titanium nitride, and

copper. This fictional sample contains many ofthe materials used

in modern semiconductor devices, admixed andjuxtaposed on the nanoscale to simulate the complexity oftrue 3D chemical imaging

problems. Each ofthe 27 smaller cubes is 100 nm on an edge, so

the entire assembly is 300 nm on an edge. The beam (purple vertical line) is shown striking the sample in the center ofthe SiOi

subcube. (top right) 3-color X-ray elemental map showing quantitatively the detected X-ray intensitiesfor the Cu KaX-ray emission line (red), the Si KaX-ray emission line (green), and the N KaX-ray emission line (blue) after excitation with a 20 keV

incident electron beam. Note the differential absorption ofthe Nitrogen K linefrom bottom to top (the X-ray detector is offthe top ofthe image) and the bleeding ofexcitation near the phase

boundaries. The X-ray map is 300 nm on a edge, (bottom) The same cube shown with the top, middle, and bottom layers verti- cally expanded and horizontally translated to reveal the 3D struc- ture more clearly.

Selected Publications

1 . N. W. M. Ritchie, "A New Monte Carlo Application

for Complex Sample Geometries", submitted to Sur- face and Interface Analysis, 2004.

2. J. S. Villarrubia, A. E. Vladar, B. D. Sunday, and M.

Bishop, Proc. SPIE 5375, 199-209, (2004).

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Carbon Nanotube Metrology

Characterization ofindividual carbon nanotubes is required to ensure that these unique materials meet

the demanding requirements offuture electronic,

optoelectronic, chemical sensing, and biomedical

applications. A principal challenge associated with

isolated tube characterization is optimization ofthe

connection between the nanotube and the test platform. To address this issue, we are developing

techniques to manipulate and attach carbon

nanotubes to specially designed test structures in the

scanning electron microscope. In addition, for

biomedical applications, preliminary studies on cell- nanotube interactions are underway to explore the

viability and affect o/in-vivo probes, sensors, and

scaffolds based on carbon-nanotubes.

Paul Rice and Tammy Oreskovic, MSEL (853)

We are developing techniques to measure mechanical

and electrical properties of isolated carbon nanotubes.

Figure 1 shows a test device fabricated to measure the

electrical characteristics of an individual multi-walled

carbon nanotube. The nanotube was attached, or welded,

to the device in the scanning electron microscope using an accepted technique referred to as electron beam deposition. In this example, a single tube extends from one

electrode to the other.

planned to measure contact resistance within the weld

and resistance changes due to distortions during welding.

Figure 1. SEM image ofcarbon nanotube test devicefor

electrical measurements. The nanotube extends across a 2 /Mn

gap-Understanding the properties of the contact requires

visualizing its structure on the atomic scale. Using highresolution microscopy, we have seen some interesting new structures of the weld. Previously, it was assumed

that the weld was a uniform material composed of

amorphous carbon. Using atomic force microscopy and

transmission electron microscopy, we have seen spurious

deposits of carbon far outside the weld. Figure 2 shows a nanotube welded to a chromium film with residual

carbon covering almost everything. Further work is

Figure 2. AFM image oj a multi-walled carbon nanotube welded

to a chromium film. The surface texture is residual carbon from

the weld process.

In addition to contact issues, preliminary studies have

been conducted to assess nanotube biocompatibility.

Cells were exposed to nanotube suspensions and

entangled mats. Figure 3 shows proliferation assay

results, indicating reduction in growth and metabolic

activity for cells grown directly on nanotubes. These

results indicate that cell attachment may not be as strong

in regions of high nanotube density. However, when the nanotubes were coated with a biologically compatible

surfactant, cell growth was not noticeably affected.

^ 10 -^Carbon ^'anolubc Mais -^Carbon Nanolubc Siispcnsi

- Surfactant Solutinn

- Control

Time (Days)

Figure 3. Resultsfrom alamar blue assay test for cell growth and

proliferation. As the cells grow and divide, they metabolize the Alamar blue compound, changing the medium in which the cells are growingfrom a blue color to clear. Greater percent reduction

indicates more vigorous cell growth.

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Nanotribology and Surface Properties

Accurate adhesion andfriction measurements at the

nanoscale are emerging as critical issues in device

industries and nanotechnology. Working with device

and magnetic storage industries and other agencies, we have designed and built instruments to meet these

needs. These instruments are housed in NIST's new

Advanced Measurement Laboratory(AML). This

state-of-the-artfacilityfeatures vibration isolation,

class 1000 clean room standards, and temperature

control.

Stephen Hsu, MSEL (852)

The Nanotribology and Surface Properties Project was initiated as a part of the MSEL Nanotechnology Initiative. The project develops measurement capability for adhesion, friction, and surface forces at the nanoscale.

Working with instrument makers, the Nanotribology

Group has designed and built a remarkable facility fea- turing equipment and instrumentation that establishes a new state of the art for conducting measurements and

observations of nanoscale events. Included in this facility are a nanoadhesion apparatus, a dual white light inter- ferometer microscope, a multiscale friction tester, and a multifunction apparatus combining an ultrahigh vacuum

scanning tunneling microscope with an atomic force microscope (UHV STM/AFM). Together, these instru- ments allow us to image, manipulate, and measure adhesion, friction, and other nanomechanical properties of

materials from the microscale to the nanoscale.

To resolve some of the instrumental challenges, we

have engaged instrument makers, such as Hysitron,

Veeco, and others, to solicit their input and collaborations. At the same time, we have established our own

capabilities in tip fabrication, cantilever spring constant

calibration, and tip characterization. Three new instru- ments were successfully installed as a result of these col- laborations: the NIST-Hysitron multiscale friction tester, the NIST nanoadhesion apparatus, and the first prototype

of an interferometer microscope from Veeco. We are continuing to work with our partners to develop next- generation instruments, as suggested by the Nanometrology Grand Challenge Workshop (NIST, January 2004).

Adhesion is an important issue in nanotechnology as well as in device manufacturing industries. Conventional adhesion measurement relies on large contact ar- eas and the accurate positioning of two surfaces. Force

measurements t>'pically are performed by strain gauges

and transducers with force sensitivities in the newton and

millinewton ranges. These conventional instruments are unsuitable for nanoscale contacts. To resolve this issue, the newly designed NIST apparatus, Figure 1 , allows

adhesion force measurements between two surfaces at the nanoscale level. In this device, capable of nanonewton force resolution, laser light is piped through an opti- cal fiber and directed at a quadrant photodetector. Because the length and stiffness of the fiber can be modified easily, a wide range of spring constants can be obtained, and the measurement of adhesion forces can be

achieved at multiple force levels. Thus, the NIST apparatus has established an exciting new opportunity to measure the influence of surface forces on adhesion, molecular interactions, and the compliance of surfaces, all critical data to device manufacturing industries.

Figure 1. Schematic operation ofthe NIST nanoadhesion apparatus.

With our new adhesion and nanofriction instruments, we are quantifying the effects of plowing and electrostatic charge on measurements. We continue to work with our extemal academic under the Nanotechnology Extramural

Initiative to develop friction measurement via three ap- proaches: friction-measuring MEMS devices, AFM's, and

ultrahigh vacuum friction measurement. These efforts have

been successful, and we have gained considerable understanding of how meniscus forces and electrostatic forces can exert significant effects on nanofi'iction measurements.

Enhanced by the superior vibration isolation and

clean room environment in NIST's new Advanced

Measurement Laboratory (AML), our UHV STM/AFM

system provides atomic imaging and force measurement

with an unprecedented resolution and accuracy. Combined with our conventional AFM, we are able to image

and manipulate surface features and measure a wide

range of material characteristics and properties important

in nanodevice operation.

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Neutron Reflectometry for Highly Accurate Nanometer

Metrology

To provide researchers with state-of-the-art nanometer metrology using neutron reflectometry methods.

Joseph A. Dura, MSEL (856)

Nanotechnology research requires accurate measurements of features on the nanometer level. We report

a comparison of techniques for measurements of 1.5 nm

to 8 nm Si02 films on Si at state-of-the-art precision and

accuracy. Si02 on Si is a well-understood system for which high quality ultra-thin samples are available. It is technologically relevant since at this thickness it serves

as a gate oxide in Si electronics. The International

Technology Roadmap for Semiconductors (ITRS) states the need for such measurements at a standard uncertainty

of « 1 %. The present comparison was organized under

the auspices of the Consultative Committee for Amount

of Substance (CCQM), the body authorized under the

International Committee for Weights and Measures

(CIPM) to oversee the fi-amework for the SI system for amount of substance. Neutron reflectometry (NR), graz- ing incidence x-ray reflectometry (GIXRR), x-ray photoelectron spectroscopy (XPS) and ellipsometry were

found to be the most accurate and precise methods.

Sets of ten samples for (100) and (111) orientation Si were prepared by thermal oxidation. A sample of each

wafer was characterized at the National Physical Laboratory (NPL) by XPS under carefully established reference

conditions. Ellipsometry, providing relative differences

in thickness across the wafer, was used to adjust the XPS

data to determine the reference thickness Jrt for each

sample. The Si02 thicknesses determined by each tech- nique at a given laboratory, d, were analyzed through the equation d = md^j + c. Here, m scales the measurement

against the XPS result, whereas c represents an offset

that incorporates the thickness of a contamination layer

containing adsorbed carbonaceous matter and water. In techniques that take into account the contamination, e.g. GIXRR and NR, c can represent a misrepresentation of

the contamination layer. XPS is unique in that the contamination has no effect on the result and c should be

zero. It is also unique in that while being linear, its length scale has a high uncertainty and needs calibration. This calibration was an essential part of the study and

showed that a 1 .4 % correction was required. For all techniques, the RMS scatter of the data about the linear

fit is an indication of precision. Accuracy is indicated by

consistent m values near 1 and c values near 0.

NR was measured on 4 samples with d as follows:

1.65 ± 0.15 nm, 2.00 ± 0.05 nm, 5.55 + 0.03 nm, and

8.04 ± 0.01 nm. The uncertainties encompass the fall range of acceptable fits and therefore represent a 95 %

confidence level. Fig. 1 shows the reflectivity vs. mo- mentum transfer, g=47isin0/A,, for the 8 nm sample, the best fit (blue), and two poorer but acceptable fits.

Figure 1. Neutron Reflectivity vs. Q. The inset is the scattering

length density profile determined by each fit. XPS measurements from the 13 additional laboratories achieved excellent reproducibility when employing

the reference geometry; the RMS scatter about the linear

fit vs. Jrt ranged fi-om 0.019 nm (NPL) to 0.136 nm. NR had the next lowest scatter of all measurements

(0.026 nm), indicating high precision. For comparison,

data from all techniques achieved mean RMS scatters of

0.15. A plot of m vs. c for all laboratories is reproduced

in Fig. 2. The high apparent accuracy of both NR and

XPS is indicated by their proximity to m = 1, c = 0.

0 ellipsometry • MEIS ARBS

TEM

4 GIXRR

* SIMS «NR

XPS

X NRA

0.7 08 09 1 0 1 1 1.2 1.3

gradient, m

Figure 2. Plots ofm and c obtainedfrom various techniques

(from Fig. 20A ofRef[l]).

Selected Publications

1. M. P. Seah, et al. Surface and Interface Analysis (in

press).

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Gradient Reference Specimens for Advanced Scanned

Probe Microscopy

Engineering ofnanomaterials, biomaterials, and nano

electro-mechanical systems hinges on techniques for

imaging complex nano-structures. In this respect, new Scanned Probe Microscopy (SPM) methods

promise nano-scale mapping ofchemical, mechanical

and electro-optical properties, but these techniques

generally offer qualitative information. Through a

suite ofreference specimens fabricated with a combinatorial design, we aim to calibrate image data from

emerging SPM methods, thereby advancing these

nanometrology tools.

Michael J. Fasolka and Duangrut

Julthongpiput, MSEL (854)

Recent years have seen the development of a new

generation of SPM techniques, which intend to measure

chemical, mechanical, and electro/optical properties on

the nanoscale. However, contrast in new SPM images is difficult to quantify since probe fabrication can be inconsistent and probe/sample interactions are not understood.

Our research at the NIST Combinatorial Methods Center

(NCMC) aims to provide a suite of reference specimens

for the quantification of next-generation SPM data. By

design, our specimens will gauge the quality of custommade SPM probes, calibrate SPM image contrast through "traditional" surface measurements (e.g., spectroscopy, contact angle) and provide information for understanding complex probe/sample interactions. Our

specimens are produced with bench-top microfabrication

routes and combinatorial gradient methods developed by

the NCMC. Here, combinatorial methods are key, since

they enable the fabrication of specimens that vary properties that govern SPM image contrast in a systematic,

independent manner. Moreover, as opposed to tradi- tional reference specimens, combinatorial samples pro- vide not one, but a multitude of calibration conditions.

Figure 1 illustrates principles of our specimen design

through a specific case useful for quantifying chemically

sensitive SPM techniques such as friction-force SPM, or Chemical Force Microscopy, which employs a custommade probe. The crux of this specimen is a "gradient

micropattern" (V-Dp): a series of micron-scale lines that continuously change in their chemical properties (e.g., surface energy) compared to a constant matrix. Two

"calibration fields" adjacent to the V-Dp directly reflect the chemistry of the lines and the matrix. Thus, tradi- tional measurements (e.g., contact angle) along the cali- bration fields ( 1 ) gauge local chemical differences in iheV-Dp and thereby (2) calibrate contrast in SPM images acquired along the V-|jp.

I Gradient i Matrix ' Micropattern ' Gradient

Calibration

[

\ Calibration

Field 1 / 1 Field

Figure 1. Schematic illustration ofour gradient reference speci- men for chemically sensitive SPM techniques.

Figure 2 demonstrates use of this specimen for cali- brating friction force SPM image contrast. We fabricate

this specimen via microcontact printing of a chlorosilane

self assembled monolayer (SAM) on a Si02 matrix. The

chemical gradient is achieved via a graded UVozonolysis of the SAM. The plot abscissa gives the dif- ference in friction force (contrast) between the lines and

matrix for SPM images collected along the V-|np. The

ordinate expresses the corresponding surface energy (y)

data (from contact angle measurements) collected along

the calibration fields. Thus, from a single specimen we

create a comprehensive calibration curve that relates SPM fiiction force to differences in surface energy. Moreover, the plot neatly illuminates the smallest y dif- ference sensed by the probe (red arrow), which is usefiil for gauging the quality of custom-made probes.

20- > 18-

16-

14-

(0 12-

cor 10-

8- «

6

Frictioi 4-

2-

0-10 I

20 30 40 SO 60

Y — Y (mJ/m')

'siO, 'sAM '

Figure 2. Preliminary calibration curve relatingfriction force SPM image contrast to differences in surface energy (y), as deter- mined from a single gradient reference specimen. The minimum

contrast point (red arrow) illuminates the sensitivity ofthe probe.

Currently, we are refining this reference specimen

design, and we are developing similar designs for other

advanced SPM methods.

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Quantitative Nanomechanical Properties

We are developing AFM-based methods to measure

mechanical properties on the nanoscale. Atomicforce

acoustic microscopy provides elastic-modulus values

either at a single point or as a map oflocal property

variations. The results further our understanding of

surfaces, thin films, and nanoscale structures.

Donna C. Hurley, MSEL (853)

New measurement tools are needed in the burgeoning

field of nanotechnology. In particular, information about

mechanical properties is critical in many applications. To

meet this need, we are developing methods based on the atomic force microscope (AFM). Atomic force acoustic

microscopy (AFAM) involves the vibrational modes of

the AFM cantilever when its tip is in contact with a sample. With AFAM, the indentation modulus Mof a

material can be determined. The small tip radius (-5-30

nm) enables images with nanoscale spatial resolution.

In FY04 we examined several issues concerning

quantitative AFAM. In one effort, we measured Mfor three nickel films about 50, 200, and 800 nm thick. M

ranged fi-om 2 1 0 GPa to 223 GPa, lower than that for polycrystalline nickel. Scanning electron microscopy

(SEM) showed nanocrystalline microstructure (grain

diameter <30 nm). The reducfion in Mcan be attributed

to an increase in intercrystalline material. Furthermore,

the average values of M for all three films were the same

within measurement uncertainty (~10 %). Thus AFAM

results were not influenced by the elastic properties of

the substrate, even in a 50-nm film. This contrasts with

nanoindentation, which must account for substrate properties to accurately measure submicrometer films. We also investigated relative humidity (RH) effects on AFAM measurements. We created a RH-controlled

chamber and used it on patterned self-assembled

monolayers (SAM) of n-octyldimethyl-chlorosilane on

silicon (Si) substrates. An AFM topography image and

AFAM relative-stiffness image are shown in Fig. 1 . The

hydrophobic SAM stripes are all but invisible in the high-resolution topography image (10 nm full scale), but

are clearly seen in the AFAM image. Regions covered

by the SAM appear more compliant (lower contact

stiffness) due to AFAM's sensifivity to variafions in the tip-sample adhesion. Further experiments with similar

samples will quantify humidity and adhesion effects. We also performed AFAM experiments to examine

how the tip radius R changes with use. Tests were made

with multiple cantilevers on a sample with known elastic

properties. The load was successively increased (up to ~5 |j,N) to break or plastically deform the tip. Values of

R measured from high-resolution SEM images (Fig. 2) were compared to values obtained from AFAM data

using a Hertzian contact-mechanics model. The AFAM

values of R were consistently smaller than the SEM

values. Further analysis and experiments are underway

to clarify this issue. The knowledge gained will refine our understanding of AFAM contact mechanics and thus

improve measurement accuracy and repeatability.

Figure 1. (a) AFM tapping-mode topography and (b) AFAM

relative-stiffness images ofSi with n-octyldimethylchlorosilane

SAM.

(a) (b)

Figure 2. SEM images ofAFM tip (a) before use and (b) after repeated AFAM experiments. Circled regions indicate tip wear.

The above results involve either quantitative singlepoint measurements or qualitative imaging. In FY04, we

worked to realize quantitative nanomechanical mapping.

This involved a new frequency-tracking circuit to find

the contact-resonance frequencies at each pixel. The

circuit uses a digital signal processor that enables rapid

data acquisition (typically ~20 min. for a 256 x 256

image). We have obtained resonance-frequency images

for a variety of materials. Future work will focus on

issues related to quantitative image interpretation {e.g.,

calibration, tip wear, and contact-mechanics models).

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Highly Charged Ions in EUV Lithography

International SEMA TECH has asked NIST to help in the extension ofphotolithography to circuit dimensions of30 nm and below through studies ofphoton

emission efficiency and surface damagefrom highly

charged ions. We have deployed the NIST Electron

Beam Ion Trap Facility to complete these tasks.

John D. Gillaspy and Joshua M. Pomeroy, PL

(842)

conferences, several peer-reviewed journal and book

articles, and a number of invited talks at international

meetings of EUV lithography researchers. Our work on

this topic led the leader of the NIST Plasma Radiation

Group to become chair of the International SEMATECH

Fundamental Data Working Group, and co-organizer

(with Intel) of the first in a series of EUV Source Modeling Workshops.

This research ended with the conclusion that the source models were indeed accurate in all areas tested,

and that further significant gains in efficiency were possible, particularly if tin atoms are used instead of xenon

atoms. The validated theories have been able to confidently guide light source development companies in improving the efficiency of the photon production by the plasma. The success in this area has been such that at the most recent International EUV Lithography Symposium (Fall 2004), the source efficiency problem was officially moved fi-om the #1 spot on the list of critical problems, to a spot that is so low on the list that it is no

longer even assigned a numerical ranking. The EUV

light source problem has been essentially solved.

Figurel. Light emission intensity, taken at the NIST EBITfacility, as a function ofphoton and electron energy; the red peak moves

into the desired photon energy range when tin ions are substituted for xenon ions.

Because of fundamental science studies undertaken

earlier, the NIST Plasma Radiation Group was familiar

with a peculiar type of surface damage that is significant

only when the ion charge state is high. This "potential

energy mediated damage" grows with ion charge, and

can become so large that it dominates the kinetic energy

damage by a large factor. At the beginning of NIST'

s

work on the light source efficiency problem, it became

clear that this exotic ion damage mechanism was largely

unknown by the semiconductor industry. NIST brought

this to the attention of the community, and embarked on

a directed study to investigate the magnitude of the mechanism on samples of actual EUV lithography multilayer optics and other materials that were expected to be

used in the construction of the lithography tools.

In a form of "high precision accelerated testing", we

Despite considerable interest in the development of

exotic nanotechnologies, the extension of conventional

photolithography to extreme ultraviolet (EUV) wavelengths is the mainstream technique selected by the semiconductor industry to continue miniaturization

deeply into the nano-regime (beyond the so-called "32

nm node"). Highly charged ions (HCIs) figure prominently in the list of fundamental physics issues that contribute to both the promise and the potential downfall of

EUV lithography.

On the positive side, HCIs show promise for providing an efficient source of the 13.5 nm light that is necessary for EUV lithography to become cost effective for mass manufacturing. On the other hand, HCIs are the most reactive form of matter on earth, so the potential

damage they could cause to critical materials in lithography tools is high. NIST was asked to deploy its Electron

Beam Ion Trap (EBIT) Facility for Highly Charged Ion Research to assist the large team of experts who are working on these issues around the world.

One key issue was determining the fundamental

physical limit in the light source efficiency. At the beginning of the NIST work, prototype light sources developed by industry and the National Laboratories had obtained very high power outputs, but were still over an

order of magnitude away from the efficiency needed for

practical viability. Because the efficiency obtained in the prototypes was so high already, there was serious concern that further increases might be limited by fimdamental physics. Industry planning (and R&D spending on the order of $ 1 B/year) was proceeding based on

predictions of complex plasma models that were virtually untested by well-characterized laboratory experi- ments. Researchers at NIST provided benchmark test data and coordinated worldwide fundamental data activity for the industry in this area.

Our work was communicated through approximately

1 ,000 pages of internal reports to industry, periodic teleNIST Accomplishments In Nanotechnology

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used the NIST EBIT facility to dose the materials with

small but well-characterized amounts of HCIs, and then

applied ultra-sensitive reflectivity and scanning tunnel- ing microscopy measurements to observe the collective damage from non-overlapping ion impacts, and even for some materials the effect of individual ions. This tech- nique has the advantage over conventional high-dose

accelerated testing of being able to reveal relatively

clearly the fundamental ion damage effects and mechanisms. This work revealed that the highly charged ions

used in xenon based light sources will cause over an or- der of magnitude more damage than singly charged ions. At the most recent International EUV Lithography Symposium, the problem of damage of source materials was

officially advanced to the #2 spot on the critical items

list.

In summary, NIST's specialized capabilities in the

area of highly charged ion physics played a role in a much larger worldwide effort which, together, brought

many pieces of a complex puzzle together to essentially

eliminate one critical problem, while elevating the significance of a different, previously underappreciated,

problem. The shift in priorities allows industry to better

allocate resources to focus on the problems that are truly

most important in order to improve the probability of

making EUV lithography a cost-effective tool for mass

manufacturing devices at length scales of 30 nm and below.

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Electrical Test Structure Metrology

A central goal ofthis project is to fabricate, calibrate,

and document a selection of Critical Dimension (CD)

reference materials with calibrated values as low as

70 nm and having designated referencefeatures with an expanded CD uncertainty less than ± 3 nm. A second area is research is a focus on lowerresistance interconnect material, in particular copper

which exhibits lower resistance and improved reliability over aluminum.

Michael Cresswell and Richard Allen, EEEL

(812)

Technical Approach

The key issues which, in the past, have prevented us from providing well-characterized and useful critical dimension (CD) reference materials to the semiconductor industry were our inability to produce reference fea- tures with 1) acceptable uniformity and 2) acceptably narrow dimensions. In this project our goal was to pro- duce reference materials that meet both of the require- ments. To achieve this goal we replicated the CD refer- ence features in the device layer of a 150 mm (110) SI- MOX (Separation by Implantation with Oxygen) wafers

with a device layer thickness of 1 50 nm. This device

layer is electrically isolated from the remaining thickness

of the substrate by a 390 nm thick buried oxide created

by oxygen implantation.

An image of the test chip, containing candidate refer- ence features, is transferred into a silicon dioxide film

that serves as a hard-mask. The edges of the features are aligned as close as possible to the <112> lattice directions. This allows for the etching of the features in the

silicon to be performed using a lattice-plane selective

etch, tetra-methyl-ammonium hydroxide (TMAH),

which etches (111) silicon-lattice planes at a rate 10 to 50 times more slowly than it etches other planes, such as the ( 1 1 0) surface of the wafer. Thus the (111) planes of

the reference-feature sidewalls behave as lateral etch

stops. Since these (111) planes are perpendicular to the (110) surface, the reference features have planar, vertical, (111) sidewalls.

We used a combination of two metrologies to achieve

acceptable uncertainties in the CD: these are lattice plane counts, as revealed by High Resolution Transmission Electron Microscopy (HRTEM), and Atomic Force Microscopy (AFM). We used a CD-AFM, which is specialized for CD measurement in that it uses what is referred to as a "boot shaped" tip. This allows for imaging

of the sidewalls of a vertical feature as well as the surfaces. This technical approach to CD calibration has not been implemented previously, as far as we know. Following a screening procedure using optical and scanning

electron microscopy, AFM images of the each of the selected reference features is performed. Following

AFM imaging, a selection of features is chosen for HRTEM imaging. HRTEM cannot be performed on all of the reference features as the preparation needed for HRTEM by necessity destroys the features. From the

lattice-plane count, a curve showing the relationship between the CD as measured by the AFM and the CD as revealed by the silicon lattice is acquired. CDs are as- signed to the remaining chips from this curve. As a result of this procedure, CD reference features with dimensions between 40 nm and 1 00 nm were pro- duced with the unprecedented expanded uncertainties of

less than 2 nm.

Interconnect fabrication processes involve a sequence

of several tens of lithographic operations, each requiring

close alignment of the patterned resist to features that have been etched after the previous such operation. Successful manufacturing typically requires alignment and

overlay accuracies in the low several nanometers range

Challenges associated with advanced interconnect in- clude the understanding of charge transport in very nar- row features. The charge transport is affected by the

interaction of the charge carriers with grain boundaries

and sidewalls. When the size of either the grains or lines

is significantly larger than the mean free path of the electrons, the conductivity of the wires is similar to that of

bulk. However, when these dimensions are less than or equal to the mean free path, the conductivity is expected

to increase significantly. There are two "features" of

conventional copper interconnect systems that complicate any experiments. The first feature is the barrier

layer that is needed to keep the copper from diffusing

into the surrounding insulator and silicon layers. This

barrier layer is usually comprised of a metal, such as tantalum, which is a poor conductor. The second feature

is that the planarization in the damascene patterning process can lead to dishing or erosion, which leads to the sheet resistance being a very non-linear function of di- mension. Decoupling the contributions to a measured

sheet resistance of these two effects is difficult, if not

impossible.

We are taking several approaches to address these is- sues. The first approach is to develop a model from

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which the sheet resistance can be calculated for an arbitrary set of dimensions, grain sizes, and scattering coefficients. The results from this model will be compared

with measurements of patterned and unpattemed copper

films at a range of temperatures. To simplify analysis of

the material, we will initially measure copper films with no barrier layer and later extend the measurements to films with barrier layers.

Figure 1. SEM image ofthe new test structure. (Lengths ofthe

lines shown are approximately 10 fim, and the widths rangefrom

about 45 nm to 210 nm.)

Accomplishments

Delivered to ISMT, in collaboration with NIST's

Manufacturing Engineering Laboratory (MEL) and In- formation Technology Laboratory (ITL), ISMT, and

VLSI Standards, ten chips containing NIST calibrated

CD reference features. These chips, which have been

distributed to ISMT's ten member companies, including

Intel, IBM, TI, AMD, and Freescale/Motorola, were delivered with sub- 100 nm CDs and expanded uncertainties

of 1 .5 nm. This represents a major improvement over a

prior delivery of reference materials from this project

with a minimum of 1 5 nm uncertainty. The improvement in uncertainty resulted fi-om the implementation of

a new type of HRTEM-target test structure, an improved

lattice-count/CD extraction procedure, the extensive use of SEM inspection to identify targets with superior CD

uniformity, and the use of advanced AFM to serve as the

transfer metrology.

Held a public meeting at which time these results were presented to SEMATECH, the SEMATECH member companies, and other interested parties. At that time,

SEMATECH indicated some interest in pursuing this area of research. CD reference materials team (with ITL leadership)

awarded ATP funding to extend work to non- semiconductor industry related nanotechnology applications. Four papers describing aspects of this work are

being presented at conferences.

Selected Publications

1 . N. Guillaume, W. K. Kahn, R. A. Allen, M. W.

Cresswell, M. E. Zaghloul, "Application of Conformal Mapping Approximation Techniques: Parallel .Conductors of Finite Dimensions," IEEE Transactions on Microwave Theory and Techniques, Vol.

53, No. 3, pp. 812-821 (Ol-JUN-2004).

2. R. A. Allen, M. W. Cresswell, C. E. Murabito, R. Dixson, E. H. Bogardus, "Critical Dimension Calibration Standards for ULSI Metrology," Characterization and Metrology for ULSI Technology: 2003,

International Conference on Characterization and

Metrology for ULSI Technology, Mar 24-28, 2003,

Austin, Texas, pp. 421-428 (30-SEP-2003).

3. R. A. Allen, B. A. am Ende, M. W. Cresswell, C. E. Murabito, T. J. Headley, W. F. Guthrie, L. W. Linholm, C. H. EUenwood, E. H. Bogardus, "Test

Structures for Referencing Electrical Linewidth

Measurements to Silicon Lattice Parameters Using

HRTEM," IEEE Transactions on Semiconductor

Manufacturing, Vol. 16, No. 2, pp. 239-248 (01-

MAY-2003).

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Molecular Nanomagnets

New methods are being developed to quantitatively

determine the high-frequency properties ofmolecular

nanomagnetsfor potential use in nanoscale magnetic

data storage and biomedical imaging.

Brant Cage and Stephen E. Russek (EEEL)

nanomagnets by varying the ligand structure and binding

them to various films. We are looking at new applications by incorporating the nanomagnets into molecular

devices and exploring how the nanomagnets relax nuclear spins in biological systems.

Broadband SQUID-Detected Electron

Paramagnetic Resonance Probe

High-frequency electron paramagnetic resonance (HF

EPR) is a powerful technique for the characterization of

magnetic materials. Measurements at 100 gigahertz and

above allow greater resolution in the determination of

magnetic energy levels, which are useful for the development of new materials for nanomagnetic data storage,

spin electronics, and biomagnetism. However, a serious

shortcoming of conventional HF-EPR is its inability to quantitatively measure magnetic moment. We have developed a new technique to make quantitative measure- ments using a commercial SQUID magnetometer.

Figure 2. Structure ofFe-8 molecular nanomagnets, after R. Sessoli and D. Gatteschi, Angew. Chem. Int. Ed. 42, 268-297

(2003). We are able to directly measure the change in magnetic moment of a specimen as microwave stimulation causes resonance at different values of applied magnetic

field. The apparatus uses a 95 or 141 gigahertz klystron

microwave source followed by an isolator, an attenuator

for saturation studies, and a directional coupler. A detec- tor/mixer monitors the fi-equency and both reflected and

incident power. A square-to-round waveguide transition

to thin-wall tubing is used to deliver the microwaves to a sample located inside the magnetometer. The probe as- sembly and sample are mechanically oscillated through

the SQUID pick-up loops to obtain the magnetic moment. Quantitative measurement of the degree of satura- tion at any value of magnetic field allows spin-lattice

relaxation times to be calculated in the low-power, satu- ration regime. (Conventional EPR studies require ap- proaching or exceeding the high-power, nonlinear re- gime.) This is important at high frequencies where the

available microwave power is usually limited.

Many technologies require or are enabled by the use of magnetic nanostructures such as molecular nanomagnets. The study of magnetic nanostructures will enable

data storage on the nanometer scale, a better understanding of the fundamental limits of magnetic data storage,

and new biomedical applications.

Figure 1. Brant Cage next to a magnetometer modified to simul- taneously measure magnetic moment and high-frequency EPR

spectra on nanomagnets.

We are developing new methods to characterize the magnetic properties of nanomagnetic structures such as molecular nanomagnets. One method is high-frequency

electron paramagnetic resonance (EPR), based on a su- perconducting quantum interference device (SQUID)

magnetometer, which can simultaneously measure lowfrequency magnetic properties and high-frequency characteristics, such as resonant absorption/emission of mi- crowaves in the frequency range of 95 to 141 gigahertz over a temperature range of 1 .8 to 400 Kelvin. Molecular nanomagnets, which are the smallest well defined

magnetic structures that have been fabricated, exhibit

quantum and thermal fluctuation effects that will necessarily be encountered as magnetic structures shrink into the nanometer regime. These systems, which contain

from 4 to 1 9 transition-metal atoms, form small magnets

with blocking temperatures of 0. 1 to 5 Kelvin. We are investigating new methods of manipulating these

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02

0.1

0

a M^=-ioA 1 0.1

G

4

2

I • T - 10 K 1 ^

1 A T - 4 K 1 „ \ All L -8^oji

0

e ^/ 1 j 1 n . 0 j)6 If 1 1 1 A 34567 B9 10 H -6

ij

4 K W

v = 141 GHz

T

' 1' 1 0 1 2 3 4 5

>i„H (T)

Figure 3. SQUID high-frequency EPR spectra ofFe-8 showing

resonant absorption corresponding to transitions between the quantized energy levels.

Characterization of Energy Levels and

Saturation in Fe-8 Molecular Nanomagnets

We completed a study of Fe-8 molecular nanomagnets using SQUID HFEPR. This work is the first to quantitatively measure the magnetization suppression of

molecular nanomagnets under microwave illumination.

By resonantly pumping low lying energy levels at 95

gigahertz it was possible to suppress the magnetization

by 80 percent, indicating the presence of an efficient "spin cascade" that allows energy to be efficiently trans- ported from the low-lying spin levels to high-energy levels. The exact mechanism of this spin cascade is still under investigation.

T., Weighted Image (Positive Contrast)

1

. 'iiii- •

• Commercial

• •

Superparamagnetic Concentration

Figure 4. MRI cross-sections ofFe-8 solutions in water, along

with pure water and standard Gd-based contrast agents. The

variation in intensityfrom that ofthe water samples indicates that Fe-8 effectively varies Tj. Work is done in collaboration with Denver Children 's Hospital and University of Colorado.

Development of Molecular Nanomagnets for

MRI Contrast Agents

On potential application of molecular nanomagnets is for magnetic resonance imaging (MRI) contrast agents.

The fluctuating moment of a molecular nanomagnet can have a large effect on the nuclear magnetic resonance (NMR) energy relaxation time (Ti) and dephasing time

(T2) of the surrounding protons in water or biological

tissue. The local changes in the relaxation times can be

used to highlight different tissues, the vascular system,

tumors, or neurological activity. Measurements of the

effects of Fe-8 on the NMR signal of protons in biological solutions have shown that Fe-8 can effectively modify both T] and T2. The detailed understanding of the magnetic properties and fluctuations of nanomagnetics

will lead to more effective contrast agents and new MRI

imaging modalities.

Selected Publications

1. B. Cage and S. E. Russek, "Design for a Multifrequency High Magnetic Field Superconducting Quantum Interference Device-Detected Quantitative Electron Paramagnetic Resonance Probe: Spin-Lattice

Relaxation of Cupric Sulfate Pentahydrate

(CuS04-5H20), Rev. Sci. Instrum. 75, 4401-4405

(November 2004).

2. B. Cage, S. E. Russek, D. Zipse, and N. S. Dalai,

"Advantages of SQUID-Detected Magnetic Reso- nance Over Conventional High-Frequency EPR for Characterization of Nanomagnetic Materials," J. Appl. Phys., in press.

3. B. Cage, A. Barker, S. E. Russek, C. Stoldt, C. Lanning, G. Ruchiera, R. Shandas, D. Zipse, and N.

S. Dalai, "Single Molecule Magnets Hold Promise as Water Soluble Iron-Based MRI Contrast Agents of

Size less than 5 nm," submitted.

4. B. Cage, S. E. Russek, D. Zipse, J. M. North, and N.

S. Dalai, "Determination of the Spin-Lattice Relaxation Time of Individual ms Levels of the Molecular

Nanomagnet Fe-8 using SQUID Detected High Fre- quency EPR," submitted.

5. B. Cage, B. F. Riddle, J. R. Baker-Jarvis, S. E. Russek, C. M. Ramsey, J. M. North, D. Zipse, and N.

S. Dalai, "A Compact, Inexpensive, High-Frequency

(50 GHz) Electron Paramagnetic Resonance (EPR)

Probe Design for Materials Characterization," submitted.

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Nanomagnetic Measurements

We develop micro-electromechanical systems

(MEMS)for nanomagnetic research. Microcantilever-based applications include scanned-probe microscopy; ultra-sensitive magnetometers with submonolayer sensitivity; torsional oscillator magnetometry andferromagnetic resonance spectroscopy of

submicrometer magnetic dots; and scanned probesfor

measuring microwave power above circuits.

John Moreland, EEEL (818)

devices. We are developing MEMS magnetometers with

integrated magnetic samples that can offer tremendous

gains in magnetic-moment sensitivity. Our micromachining facility is at the state of the art, providing the

tools necessary for bulk and surface micromachining on

Si wafers. In the near future we plan to demonstrate new

metrology instrumentation based on MEMS devices that will enable us to create instruments that have superior

performance compared to current magnetic-measurement

methods based on macroscopic modalities.

We are developing micro-electromechanical systems

(MEMS) technology for micrometer- and submicrometer-scale magnetic samples. Nanoscale samples are a challenge for conventional magnetometers, and new

methods are being employed to probe magnetism on this scale. Normally, measurements are made on arrays of

micromagnetic dots. Due to fabrication limitations, the

results are clouded by statistical variations in dot shape,

size, and spacing. More sensitive detectors are needed

that can measure magnetic properties of individual dots.

In particular, there is a need to understand atomicscale spin damping in ferromagnetic systems in order to improve the switching speed of magnetic devices. For

example, data-transfer rates for commercial disk drives

will soon require operational bandwidths in excess of 1 gigahertz. For switching times less than 1 nanosecond,

gyromagnetic effects dominate. One way to understand

damping is to investigate size effects as magnetic devices are reduced to submicrometer dimensions. Studies

of magnetic nanodots give a better understanding of spin

damping and aid in development of faster disk drives. We will provide new magnetometers based on highly

specialized MEMS chips fabricated at NIST The in- struments will be inexpensive, since MEMS can be

batch-fabricated in large quantities. Large-scale magnetic wafer properties can be transferred to smaller

MEMS magnetometers so that nanometer-scale measurements can be calibrated with reference to fundamental

units. Our focus is on developing torque and force magnetometers, magnetic-resonance spectrometers, and magnetic-resonance imaging (MRI) microscopes on MEMS

chips. We expect that this technology will lead to atomic

scale magnetic instrumentation for the measurement and

visuaHzation of fundamental magnetic phenomena.

In order to improve upon scanning probe microscopes, such as MFM, and keep pace with industry

needs, we are focusing on specialized MFM tips for imaging heads and media. Ultra-small tips are currently

being developed for magnetic-image resolution of

20 nanometers. We are looking at new technologies for

fabricating, controlling and measuring nanometer-scale

The Information Storage Industry Consortium's re- cording-head metrology roadmap calls for highresolution, quantitative magnetic microscopes and magnetometers that go beyond the limitations of current technology. Magnetic measurement systems have be- come increasingly complex. Our expertise in magnetism, probe microscopy, and cleanroom microfabrication

techniques helps move instruments from the development stage to routine operation in the industrial laboratory and on the factory floor.

In order to improve upon magnetic microscopes, we

are focusing on specialized magnetic-force-microscope

(MFM) tips for imaging heads and media. New ap- proaches based on MFM imaging are being developed to go beyond the current MFM resolution of 1 0 nanometers. We are developing ways to attach submicrometer

magnetic resonance particles to ultra-sensitive cantile- vers and to position particles a few nanometers from the sample surface.

Figure 1. Dong-Hoon Min preparing resonant cantileverfor measurement ofabsolute magnetic moment ofa magnetic thin film.We are developing new tools for measurements of

nanoscale magnetic phenomena and representations of

magnetic units for the next generation of data-storage

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magnetic structures near the probe tip. In particular,

MFM resolution can improve only with the development

of more sensitive cantilevers for measuring the small

magnetic forces associated with nanometer-scale magnetic probe tips. Conventional MFM is not an intrinsi- cally quantitative technique. Quantitative field mapping

can be done with tiny field probes based on mechanical

detection of magnetic resonance in the probe. We are developing ways to fabricate small magnetic-resonance

particles on ultra-sensitive cantilevers and position the

particles a few nanometers fi-om the sample surface for

field mapping with one-nanometer resolution.

Frequency Modulation Detection in MEMS InSitu Magnetometer

We have improved our in-situ magnetometer for monitoring the magnetic properties of thin-film mulfilayers during deposition. By tracking the frequency shift we can separate the magnetic signal from the mass load- ing and thermal effects. The demonstrated sensitivity for a Ni-Fe film deposition is less than 0. 1 nm. This is sufficient for fundamental studies of thin-film interface magnetism and magnetic exchange interactions in multilayer structures.

B

quency, which can be determined accurately with a fre- quency source based on an atomic clock.

S

.2

0 2 4 6 8 10 12 14 16 18 Film thickness (nrri)

Figure 2. Magnetometer response as a function offilm thickness measured with a quartz crystal micro-balancefor NiygFcg 2(3 nm)/Cu(3 nm) multilayerfilm.

Absolute Magnetic Moment

We have demonstrated the concept for an ultra-small magnetic moment standard reference material based on a

thin magnetic film deposited onto a cantilever detector. We performed microtorque magnetometry measurements

on a series of cantilevers with thin Permalloy (NigoFeio)

samples deposited onto them. These cantilevers and the Permalloy samples have been optimized for moments on

the order of a nano-joule per tesla. The method has

promise for relating all critical measurements pertaining

to the determination of the magnetic moment to fre0.63

0,61

0,59

00

<n 0.57

c=) 0.55

0.53

0,51

0,49

0.47

sample 1 (slope=26.2) sample 2 (slope=26.6) sample 3 (slope=28.2) sample 4 [slope=28,9)

14,0 15,0 16,0 17,0 [VHn] (10-^ m/A)

1,0 19.0

Figure 3. Plots of l/(f-fo') as a function ofreciprocal magnetic

field 1/Ho for 4 samples ofNiosFeo.2,,500 nanometers thick. The

inverse ofthe slope ofthe line is proportional to the magnetic

moment, and the inverse ofthe intercepts is proportional to the anisotropy energy. The average values obtainedfor the samples

are respectively 4.67 ± 0.46 nanojoules per tesla and 143. 7 ± 6.4 joules per cubic meter.

Selected Publications

1 . D. H. Min, A. McCallum, S. E. Russek, and J. M.

Moreland, "Sub-Monolayer In-Situ Micromechanical Torque Magnetometer with Integrated Fiber Optic Interferometer for UHV Applications," J. Magn.

Mater. 286, 329-335 (February 2005).

2. L. Gao, D. Q. Feng, L. Yuan, T. Yokota, R. Sabiri- anov, S. H. Lieu, M. Chabot, D. Porpora, and J. M.

Moreland, "A Study of Magnetic Interactions of

NigoFcio Arrays Using Ultra-Sensitive Microcantile- ver Torque Magnetometry," J. Appl. Phys. 95, 7010-

7012 (June 2004).

3. M. D. Chabot, J. Moreland, L. Gao, S. H. Liou, and

C. W. Miller, "Novel Fabrication of Micromechanical Oscillators with Nanoscale Sensitivity at Room

Temperature, J. MEMS, in press.

4. D. H. Min and J. Moreland, "Quantitative Measurement of Magnetic Moments with a Torsional Resonator: Proposal for an Ultra-Low Moment Reference

Material," J. Appl. Phys., in press.

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Nanoparticle Measurements for Biological Applications

We are developing techniquesfor magnetic manipulation and measurement ofsingle molecules and radiofrequency tagsfor magnetic resonance imaging.

John Moreland, EEEL (818)

Measurement tools and methods are needed for isolating,

manipulating and probing the behavior and structure of

single molecules. The semiconductor electronics industry has driven the development of fabrication tools that are capable of patterning structures on the order of 100

nanometers, smaller than cellular dimensions. Using micro-electromechanical systems (MEMS), we can create three-dimensional structures that are commensurate with

the size of biomolecules. Interactions of single molecules

with nanoscale mechanical structures, restriction ele- ments, and other single molecules are probed by magnetic, electronic, electromechanical and optical tech- niques.

This interdisciplinary NIST effort will result in a well

characterized platform integrated with atomic force microscope (AFM), fluorescence resonance energy transfer (FRET), optical microscopy, and electronics, thereby

enabling a wide variety of single-molecule studies. Determination of DNA structure will be performed by di- rectly interrogating ordered bases as they are threaded

through a well characterized nanopore.

Figure 1. Elizabeth Mirowski characterizing spin-valve magnetic

traps for microfluidic magnetic bead capture and sorting.

Instrumentation is being adapted to a new class of

microwave probe stations that use micromachined probe

chips to extend voltage and current probe measurements on microwave circuits with submicrometer spatial resolution in the 100 gigahertz range. We are extending this technology to biological and medical applications in an

effort to develop applications-specific magnetic particles

as well as highly specialized microchips. For example,

we are currently developing a chip-scale MRI system for ultra-high-resolution MRI of cell organelles.

High-Throughput Lab-on-Chip for Sorting Biologically-Compatible Magnetic Particles

We have integrated a novel microfluidic magnetic

trap platform with an external magnetic force microscope (MFM) to capture and sort magnetic particles into discrete positions in an array. The principle of the platform is to sequentially flow magnetically modified biological samples into the array and to sort them to pro- duce a matrix reference. The matrix can then be trans- ported to a location where each individual magnetic particle — and hence biological sample — can be probed to obtain details about its medical or forensic fiinctions,

such as gene sequence or physical structure in different physiological environments. Alternatively, the array can be used to sort particles of varying sizes and magnetic

susceptibilities, thereby providing standards for magnetic

particles used in medical applications such as contrast enhancers in magnetic resonance imaging or to coax to- gether severed nerve cells so that they can repair themselves and restore mobility to limbs.

* • « • • • ' 3 nlcioneter

particle

Figure 2. An array oftraps (1 micrometer x 3 micrometer rec- tangles) and randomly distributed magnetic particles (3 micrometer and 5 micrometer diameters). The white area is the magnetorobotic arm. The top image is before sorting and the bottom is after sorting.

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The platform consists of an array of magnetic trapping elements separated from the fluid sample by an optically transparent thin membrane. The magnetic parti- cles are trapped by the local field gradients produced by

the magnetic element with a force of approximately 100

piconewtons, which can ensure that the particle remains

in the desired position through rigorous transport. The

cobalt-coated MFM cantilever serves as a magnetorobotic arm that provides a translatable local magnetic

field gradient that captures and moves the particles with

nanometer precision. We integrated the electronics of the magnetorobotic arm with a digital camera and pro- grammed it to sort an initially random distribution of

particles by moving them within the array of magnetic

trapping elements. The forces acting on a 1 micrometer

diameter particle were measured by viscous drag in the

fluid to be on the order of 50 piconewtons, thereby al- lowing a sorting rate of approximately 1200 particles per

minute. Release of the particles from the cantilever is achieved by retracting the arm, leaving the particles in the solution, retained by the transparent membrane.

Modeling of MRI images for Single-Particle RF

Tag Applications

We developed a computer program for calculating

the static field profile near microscopic magnetic parti- cles with different magnetic moments. The program is based on solving Maxwell's equations with given

boundary conditions. The program also predicts the ef- fect of the field profile on MRI images for different imaging scenarios (phase versus frequency encoding). The

program goes beyond previous calculations to include

the limit below one volume pixel (voxel). Typical minimum voxel sizes for MRI instruments are around 300

micrometers. However, even if a magnetic particle is much smaller then a voxel, as is the case for micrometersized particles, its influence on the applied magnetic

field locally can be significant several voxels away due

to the narrow line width of the proton nuclear magnetic

resonance of water. By developing particles with specific

shapes and sizes and materials with nanometer dimensional control, very distinct changes in the MRI image

will occur and can be detected with research-grade MRI

systems used in medical centers today. The particles

would be used as tags for individual cells in the body to monitor in-vivo cell fiinctionality.

Figure 3. MRI image distortion due to the presence on individual magnetic polystyrene spheres about 1 micrometer in diameter. The data were taken at the Laboratory ofFunctional and Molecular Imaging at the National Institute ofHealth (NIH) and modeled

at NIST (inset).

Selected Publications

1. E. Mirowski, J. Moreland, S.E. Russek, and M.J. Donahue, "Integrated Microfluidic Isolation Plat- form for Magnetic Particle Manipulation in Biological Systems,"^;?/?/. Phys. Lett. 84, 1786-1788

(March 2004).

2. E. Mirowski, J.M. Moreland, A. Zhang, S.E. Russek,

and M.J. Donahue, "Manipulation and Sorting of

Magnetic Particles by a Magnetic Force Microscope on a Microfluidic Magnetic Trap Platform," submitted.

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Imaging Methods for Nanoparticle and Cell Systems

to Investigate Toxicity

Cost-effective and accurate imaging methods are needed to support studies ofnanoparticles and cell systems. This need spans a scalefrom afew nanometers to many tens ofmicrometers and may include liv- ing tissue. The capabilities ofconventional optical

microscopy combined with recent advances in desktop

computing power, were applied to nanoparticle location in human cells, ultimately to aid in the selection

and development ofthe most suitable imaging methods for nanoparticle toxicity studies.

Cynthia J. Zeissler and Peter E. Barker, PL (837)

Little is known about the toxicity of nanoparticles,

despite the rapid emergence of nanotechnology products.

The potential ability for nanoparticles to pass through

skin, and possible consequences, are of interest to manufacturers and users alike. Studies of nanoparticle trans- port in skin tissue are being developed at NIST, for which imaging capabilities and limitations will be of

paramount importance.

Nanoparticle and cell systems can be studied using

strongly fluorescent nanoparticles called quantum dots,

typically 1 nm to 20 nm in size. The fluorescence emission of quantum dots is readily detected by optical microscopy, however the ability to locate quantum dots to a

precision below the diffraction limit of light is a problem. Resolution limitations due to the diffraction of light can be approached by other methods such as transmission electron microscopy (TEM), but with trade-offs. Specialized approaches such as optical superresolution

are in development.

A variety of methods may be employed in a complementary fashion to try to address all imaging needs ranging from the nanoscale to the scale of relatively large

organic systems of many tens of micrometers. Factors involved in the decision point for selecting conventional

optical, specialized optical, or electron methods for nanotechnology research and development in general,

have bearing on more than just the basic imaging capabilities. These may include instrumentation costs, the extent to which live-cell system experiments are possible, which imaging artifacts may be present, whether or not chemical measurements can be included, timing considerations, and other factors. To demonstrate recent computing advances and for comparison to current and future advanced imaging

methods that may be employed in the toxicity program., a

capability baseline was explored. This involved a simple optical system, combined with three-dimensional

reconstruction methods utilizing recent desktop computting power improvements. Phase contrast and confocal

methods were not applied, so that full resolution, simplicity, and low cost could be demonstrated.

Test specimens were created by culturing SK-BR-3

and MCF-7 human cancer cells with various combinations of streptavidin-conjugated quantum dots having

emission bands of 585 nm, 605 nm, 655 nm and 705 nm.

Samples were fixed prior to imaging.

Optical sectioning was performed, which simply

takes advantage of the narrow depth of field inherent in high-resolution optical systems, and does not harm the specimens. Optical sections were three-dimensionally

reconstructed, using a desktop Pentium 4 2.8 GHz processor, 512 MB RAM and 32 MB video RAM system.

As computer speed and memory improve, the ability to image larger sections of tissue at once while retaining

full spatial resolution is expected to improve also.

Figure 1 is an example of a single optical section of

an SK-BR-3 cell, taken of 412 rmi transmitted light fi-om a 100 W tungsten-halogen lamp using a 30 second exposure time. This demonstrates that dominantly blue or

intense laser sources were not necessary to achieve resolution slightly better than 300 nm, except possibly for future time-resolved live-cell studies.

Figure 1. Optical section ofan SK-BR-3 cell cultured with

nanoparticles. The white scale bar is 5 jum long.

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Nanoparticle positions were localized relative to cell features by combining the transmitted light images with

epi-fluorescence images of the quantum dots. After re- construction and surface-modeling, images such as those

shown in Figure 2 were used to distinguish and locate nanoparticles outside and inside of the cells. Figure 2 mostly depicts a single cell, although with sufficient computer memory and speed, multiple cells can be

evaluated at once, of advantage to skin tissue studies.

Figure 2. Cross section through 3-D reconstructed cell (blue) showing internal arrangement oftwo different types ofnanoparticles (red and green). To aid visibility, the z-axis has been exag- gerated by about 2.5x and surface modeling has been applied

causing quantum dot locations on the cell exterior to be repre- sented by red and green balloons. Thefield ofview is nominally 25 fum X 25 iim x 6 fim.

Figure 3 shows a virtual cross-section through a three-dimensionally reconstructed image of an area in- side of a single cell, showing an example of the dimensions of the feature produced by quantum dot fluores- cence. Each voxel is 47 nm x 47 nm x 50 nm. Voxel

interpolation may be able to achieve a precision in quantum dot location close to the size of the quantum dot di- ameter, notably well below the optical resolution limit. The value of such precise quantum location by optical

methods relative to the diffraction-limited location of

cell features remains to be seen, since to utilize this in- formation with regard to subcellular components, combination with TEM methods may be required. Future

studies including focused ion beam (FIB) specimen

preparation combined with TEM are planned.

Figure 3. Cross section through quantum dotfluorescence

(white) within a cell (red). Each voxel is 47 nm x 47 nm x 50 nm. The slice shown is 5 fjm high, nominally 8 jum wide and 2 fim

thick.

These results demonstrate that a simple optical microscope using inexpensive white-light sources combined with a liquid crystal tunable filter, and 3-D reconstruction on a modem desktop system, can be used to image the location of quantum dot nanoparticles outside

and inside cells with a resolution of slightly better than

300 nm, consistent with the expected Rayleigh resolution

limit for the conditions used. However, also suggested is that voxel interpolation may improve quantum dot location precision to less than 50 nm, below the diffraction

limit. This presents a simplified but effective approach

applicable to imaging nanoparticle and cell systems.

The next testing phase will include skin cells such as keratinocytes and fibroblasts on scaffold systems, and

may include live systems, both of which are expected to offer special imaging challenges.

Selected Publications

4. Y. Xiao and P.E. Barker, Semiconductor nanocrystal

probes for human chromosomes and DNA, Minerva

Biotech. 16 (2004) in press.

5. Y. Xiao and P. E. Barker, Semiconductor nanocrystal probes for human metaphase chromosomes, A^m- cleic Acids Research 32:3(2004): 1-5.

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Development of High-resolution Variable Pressure Reference

Scanning Electron Microscope

High-resolution scanning electron microscope (SEM)

imaging and metrology ofnanostructures need the

best available instrumentation. The structures are

very small, comparable with the size ofthe imaging

electron beam or the excited volume; therefore SEMs

need to be pushed to their maximum capabilities. A

reference metrology SEM is currently under development to meet the requirements ofthe emerging

nanotechnology and the semiconductor industry. This new instrument is equipped with a field-emission elec- tron gun and a special electron optical column to provide well-focused electron beam in every working

mode. It has a custom-designed high-resolution laser

interferometer sample stage. Highly accurate metrology techniques, including model-based metrology

methods are currently under development.

Andras E. Vladar, Michael T. Postek and John S. Villarrubia, MEL (821)

There is currently no metrology SEM that exists,

which is fully characterized, capable of measuring full- size wafers and photomasks, has low measurement uncertainty and capable of serving as a Reference SEM.

NIST is now developing such an instrument. It is based on a new Variable Pressure SEM instrument, which is located in the new Advanced Metrology Laboratory. It has a thermal field emission electron gun with 1 to 3 nm

spatial resolution, large (200 and 300 mm wafer and 6

and 9 inch photomask) sample measurement capability,

and high-resolution laser interferometer stage. It works

with variable electron landing energy and variable vac- uum pressures. Its development and deployment of the Reference SEM is planned by CY 2006, to provide

highly accurate, traceable 2-dimensional size and 3- dimensional shape e-beam measurements.

Traceability will be facilitated with a laser interferometer

sample stage characterized with fast and smooth motion

and very high resolution (<0.2 nm) with very fast (>2

million data points/s) positioning readout. It will have

two image scanning modes: standing e-beam with

scanned stage and standing stage with scanned e-beam.

These will allow for the best calibration settings for large

and small distances, and sizes. It will have a variety of

possible signals and detectors and will work in oil-free,

clean vacuum. The instrument will be located on a vibration isolation platform, i.e., on a heavy concrete slab,

which floats on air springs. The laboratory will be in a new, clean laboratory with 0.25 °C temperature and 1 %

humidity control.

Development is needed to turn this instrument into one of the best dimensional metrology SEMs in the world. Hence, it will be highly scrutinized. This is necessary to find the best measurements settings and parameters, a thorough investigation of combined measurements

and optimization of the SEM itself will be carried out. An assessment of resolution, e-beam and signal transfer

characteristics, distortion and noise characteristics in various working modes will be performed. Full meas- urement uncertainty, accuracy & precision will be

documented. The accurate top-down and cross sectional

imaging and dimensional measurements will be sup- ported by extensive Monte Carlo modeling of SEM images of all measured structure. The modeling accurately accounts for the physical properties of 2- and 3- dimensional sample structures and the signal generation,

including instrument-dependent modeling of signal collection, amplification and noise characteristics. All these lead to, advanced and optimized edge algorithms and to highly accurate shape and size determinations.

The work in this project is in progress. It has been

shown that the variable vacuum capability, as it was re- cently published, gives unprecedented image quality and

resolution on otherwise difficult-to-image photomasks.

Figure 1 shows two images of the same location of a Cr

on quartz photomask. The image on the left is the evi- dence for severe sample charging in high-vacuum mode

even at low, 800 V accelerating voltage, the right one in low vacuum at a much higher, 3.4 kV accelerating volt- age. Even so, it shows much less sample charging. Note

the slight contamination mark left during the previous

high-vacuum imaging.

Figure 1. Severely charging image taken in high vacuum (left). The image on the right was taken in low vacuum with no apparent

charging (right).

Figure 2 shows two high-magnification images of the same photomask. Note the excellent resolution; even the Cr grains are well visible. These are excellent inputs for dimensional measurements. The left, tilted-view image

reveals the layered structure of the Cr film. The right

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Figure 2. High-resolution tilted (left) and top-down-view (right) images ofa Cr photomask.

The reference SEM, as one of the best SEMs existing

today has a specification for the highest achievable spatial resolution close to 1 nm. In many cases the spatial

resolution, i.e., the smallest resolvable distance between

features in the SEM image of these microscopes is not

limited by the focusing ability of the electron optical

column. Sample stage vibration and drift, electromagnetic fields, including charging, the extent of the infor- mation volume (that part of the excited volume where

the signal is coming from), and other factors limit the resolving power. Techniques that allow for imaging and

measurements at the best resolution are becoming important when the potential advantages of further improvements in the focusing ability of the instrument cannot be

realized due the above problems.

As the electron beam scans the sample in the X and Y

directions, the electronics of the microscope collects the

signal intensity for certain short periods of time (pixel

dwell time) and generates a two-dimensional intensity

distribution, a matrix, which can be viewed as an image.

The image produced by a sharply focused beam is a good representation of the sample as long as the sample

is sufficiently motionless and the beam scans linearly.

This is never the case with real microscopes when the images are taken at the highest resolution, because the electron beam and the sample stage always have unintentional motions; both have low frequency drift and other,

higher frequency motion. The result is a complex, compound distortion that is present in most real-life SEM

imaging and measurements. Barring other potentially

significant problems, like charging, contamination,

noise, focusing, this distortion could be the most difficult to get rid of If the compound motion of the electron beam and sample is mostly low frequency (drift), then

linear and non linear distortions dominate the problems

with the resulting image. If the compounded motion of

the electron beam and sample is mostly high frequency

and covers a larger area than the size of the finest focus's

spot, then the image is blurred, just like when an image was taken with a not fully focused beam.

The short-term drift of the electron beam is typically

very small, but external electromagnetic fields could move the beam and as a result, the edges of sharp objects

might show characteristic jaggedness. The sources of

these external electromagnetic fields can be found and

the fields can be reduced at their origin. It is also possi- ble to get satisfactory remedy through the use of high

magnetic permeability metal shields and compensatory

techniques.

Figure 3 shows the SEM sample stage motion during

a short (5 |as) period of time tracked by laser interfer- ometer. The approximately 1 60 pm resolution shows in details that the stage is not completely motionless at nanometer levels it covered a 1 .7 nm by 1 .7 nm area, while staying most of the time in a 1 nm^ area which was

not centered at the intended relative XO,YO coordinates.

X axis data nm

Figure 3. Short-time SEMsample stage motion tracked by laser interferometer.

This denotes an error that is in the size and location

of the information volume of the collected data, because

without fijrther information the best bet is to assign the data to their intended location. The knowledge of the stage position at these levels allows for compensation for stage drift and vibration and therefore considerable spatial resolution improvement.

These and other advanced and highly accurate metrology techniques, including model-based metrology

methods are currently under development and will be

implemented quickly when the new SEM arrives at NIST.

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Scanning Probe Microscopy for Nanoscale Measurements

Industrial users ofscanning probe microscopy need

accuracy in dimensional measurements at the nanometer scale. They rely on calibration standards to achieve this. Therefore, we are developing world

class traceable calibrations ofscanning probe microscopes with nanometer and sub-nanometer uncertainties. Wefocus here particularly on two recent ad- vances in linewidth measurement, one ofthem having

uncertainty at the nanometer level, for semiconductor

and nanomanufacturing applications.

Ronald Dixson, Joseph Fu, George Orji, and

Theodore Vorburger, MEL (821)

The NIST response to the need for traceable scanning

probe microscope (SPM) calibration standards is crosscutting and involves the development of special in-house

metrology instruments as well as methods for commercially available instruments and collaborations with external partners. One component is the development of

the calibrated atomic force microscope (C-AFM). This

instrument incorporates displacement inteferometry to achieve traceability to the SI meter. The C-AFM uses conventional conical tips and is thus best suited for pitch

and height measurements, which are the appropriate

measurands for scale calibration standards. This instru- ment provides traceable pitch and height measurements

for nano-scale applications. Pitch ranging up to 20 \im

has been measured. The standard uncertainty Uc ranges

from ~ 0.5 nm at smaller scales to ~ 0.1 % at the largest

scales. Step height, ranging from a few nanometers up

to several hundred nanometers, can be measured with Uc on the order of 0.5 % at the largest scales. We expect the

fourth generation instrument, recently reassembled with

a new scarmer, to exhibit even lower uncertainties.

For measurement of linewidth, especially on nearvertical structures, a critical-dimension AFM (CD-AFM)

is more appropriate. This type of instrument uses flared

tips and two dimensional feedback to image near-vertical

sidewalls. These instruments are typically used in semiconductor manufacturing. As part of our collaboration

with SEMATECH, the leading semiconductor industry

consortium, NIST was able to acquire a Veeco SXM320.

A complementary approach involving carbon nanotube

tips on a conventional AFM combined with image stitch- ing is also being developed. Recent accomplishments in both of the areas are rapidly advancing the state of

linewidth metrology with nanometer level uncertainties. NIST and SEMATECH have effectively worked together on linewidth standards. One component of this cooperation has been the single crystal critical dimension

reference materials (SCCDRM) project initiated by

NIST researchers in EEEL (Cresswell and Allen). The

2004 release of SCCDRM samples to the SEMATECH

Member Companies was recently completed, and CDAFM dimensional metrology played a central role. The

measurements on the SCCDRM samples were performed

by NIST scientist Ronald Dixson during his tenure as a Guest Scientist at SEMATECH. A Veeco Dimension

X3D, the current generation CD-AFM, was used for the SCCDRM measurements. Prior to the measurements,

the X3D was characterized and calibrated, and implemented as a reference measurement system (RMS) for traceable measurements of pitch, height, and width.

High resolution transmission electron microscopy

(HRTEM) was used as an indirect method of tip width

calibration for the SCCDRM samples. The AFM and

HRTEM results that were used for the tip calibration are shown in figure 1 . Features on the distributed samples

ranged in width from approximately 50 nm to 250 nm

with expanded uncertainties of about 2 nm (k=2) on most features. As a result of this project, CD-AFM

linewidth measurements can now be performed with a

1 nm (k=l) standard uncertainty.

300

200

E

100

0

0 100 200 300 HRTEM CD, nm

Figure 1. Regression ofCD-AFM width values on HRTEM values. The observed slope is consistent with unity - indicating that the two methods have consistent scale calibration. The average

offset between the results was used to correct the tip width cali- bration.

Carbon nanotubes, with small dimensions and

excellent wear resistance, offer considerable potential as AFM tips. For linewidth metrology of nano-scale

structures, however, nanotube tips have an important

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limitation. The nanotubes are mounted at a non-normal

angle with respect to the surface. Therefore, for features with near-vertical sidewalls, the tip will only be able to accurately image one sidewall at a time. One possible

solution to this is to image a specimen in two

orientations - one at 1 80 degrees with respect to the other - such that each sidewall of the feature is accessible in one of the images. If such images can then

be merged with sufficient accuracy, the composite image

may have metrological value.

Figure 2a. Conventional AFMslope image using nanotube tip taken on a near-vertical structure in initial orientation. Only the

right sidewall can be imaged accurately, as seen by the relative sharpness ofthe two edge regions The arrow shows the location ofa defect.

While image stitching is a well known technique, it has not been applied to scanning probe dimensional metrology. We are exploring the use of image stitching for linewidth metrology using nanotube tips. We have completed two comparisons of stitching results with cali- brated CD-AFM width measurements. The results agreed within the estimated 40 nm expanded uncertainty

of the stitching measurement. Our initial results, illus- trated in figures 2 and 3, were obtained using tips having

sub-optimal length, radius, and mounting angle. We are collaborating with suppliers to refine the carbon nanotube tips and are working on a new generation of this experiment. Publications at the SPIE Microlithography

Conference in both 2004 and 2005 and at the 2005 ULSI

Conference have resulted from this work.

Figure 2b. Conventional AFMslope image using nanotube tip taken on a near-vertical structure after the sample has been ro- tated by 180 degrees. The apparent right sidewall, which corre- sponds to the left edge ofthe structure in the original orientation, can now be imaged accurately. The arrow shows the location of

the same defect identified in figure 2a.

Figure 3. Composite image stitched togetherfrom the images in Figures 2a. and 2b. Both edges ofthe structure are now sharp.

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Scanning Probe Microscopy for Nanoscale Measurements:

Atom-Based Height Standards

Industrial users ofscanning probe microscopy (SPM)

need accuracy in dimensional measurements at the

nanometer scale. They rely on calibration standards

to achieve this. We are developing proceduresfor

using atom-based step height standards for calibrating the z-scale ofscanning probe microscopes at the

highest levels ofresolution ofthese instruments. A

draft documentary standard developed under an ASTM Subcommittee is thefinal piece in the chain of

developments to accomplish this with picometer (pm)

scale uncertainties.

Joseph Fu, Ronald Dixson, George Orji, and

Theodore Vorburger, MEL (821)

The semiconductor and other nanotechnology industries require physical standards with nanometer and subnanometer uncertainties for calibration of the scanning

probe tools they use to measure the spacings, linewidths,

and heights of the components they fabricate. We have

been developing procedures and standards to provide the

calibrations needed for all these quantities. Step heights

in particular need to be measured to sub-nanometer uncertainties, but until recently traceable step height stan- dards were only available commercially down to heights

of about 7 nm. We have developed procedures for industry to cali- brate the z-scale of their probe microscopes with pmlevel uncertainties using the 0.3 nm single-atom step

heights found on the ( 1 1 1

) lattice plane of Si. This de- velopment has taken place systematically over several

stages. Initially, we collaborated with the University of

Maryland to fabricate a number of prototypes of the

Si(l 11) steps using a process pioneered by them. We

then performed an independent, traceable calibration of

the average Si(l 1 1) step height using the NIST cali- brated atomic force microscope (C-AFM). This instru- ment incorporates displacement inteferometry in all three axes of motion to achieve traceability to the SI meter. An image of a Si(l 1 1) sample taken in the C-AFM is shown in figure 1 . Our resuh of 304 pm agreed within

10 pm of the accepted value of determined from x-ray

diffraction of the bulk Si material. We then developed a recommended value for the sur- face step height based on these two methods and on results from electron and surface x-ray diffraction. The

value recommended was 312 pm ±12 pm, with a cover- age factor k = 2.

Figure 1. Image ofSi(lll) step height sample with native oxide

taken on the C-AFM.

This work was performed at about the same time as an industrial comparison of five laboratories, including

NIST with a proposed calibration procedure using the

Si(l 1 1) steps as a standard. The data were provided to NIST by the laboratories and the step height results were

calculated by us using a single step-height algorithm and

associated software. The goal here was to investigate the

internal variability of the industrial and NIST measure- ments to determine if that variability would be consistent

with the pm-scale uncertainties desired for this type of

measurement. The results from the laboratories showed

that the industrial data coupled with a straightforward

step height algorithm from NIST could yield calibrations of SPMs using the 3 12 pm step heights with uncertainties of several percent. Images obtained fi-om industrial participants during this comparison are shown in figures 2 and 3.

Figure 2. An AFM image oj single atomic step on Si (111) taken as part ofthe industrial comparison.

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Figure 3. Another AFM image of single atomic step on Si(lll) taken as part ofthe industrial comparison. The boxes drawn on

the image illustrate the selection ofmeasurement areas to calculate the step heght.

Lastly, we have led the development of a documentary

standard that describes the experimental approach and

the step height algorithm needed to produce stable results for the measured step height. This standard is being

developed under ASTM Subcommittee E42-14. The

document, entitled "Standard Practice for Calibrating the z-magnification of an Atomic Force Microscope at Subnanometer Displacement Levels Using Si(ll 1) Single

Atom Steps", with ASTM Work Item # 7322, has undergone several drafts and is now being reviewed by ASTM

committee members.

Selected Publications

1. R. Dixson et al., Silicon Single Atom Steps as AFM

Height Standards, Proc. SPIE 4344, 157 (2001).

2. J. Fu et al.. Algorithms for calculating single-atom

step heights, Nanotechnology 10, 428 (1999).

3. N. G. Orji et al. Traceable pico-meter level step

height metrology. Wear 257, 1264 (2004).

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Atom-Based Dimensional Metrology

Customer needs andproject objective: NIST is re- sponsible to U.S. industryfor the development of

length intensive measurement capabilities and cali- bration standards in the nanometer scale regime. The new class ofscannedprobes have unparalleled resolution and offer the most promisefor meeting these

future needs ofthe evolving nanomanufacturing and

semiconductor industry. One important application of

the high-resolution SPM methods is in the development oflinewidth standards whose dimensions can be

measured and traced directly to the crystal lattice. These artifacts are intended to be dimensionally stable

to allow transfer to other measurement tools which

can measure the artifacts with dimensions known on

the nanometer scale.

Richard Silver, MEL (821)

This project is developing atom-based linewidth

measurement techniques and artifact standards to assist

in the calibration of linewidth metrology tools and the development of unique interferometry capabilities which

can be used in conjunction with accurately measured tips to measure feature position and dimensions. The re- search is intended to enable the accurate measurement of

dimensions by counting the atom spacings across a feature in a controlled environment or by measurements

based directly on the intrinsic crystal lattice spacing.

The technique is compatible with subsequent transfer of

artifacts to other measuring instruments after the atomicscale measurements have been completed.

The technical work is focused in a few related thrust

areas. The first area is the development of lithographically patterned three-dimensional structures using SPM

methods in UHV. These structures must be in written so

that they may found and measured in other tools. Macro

vision systems are a key aspect to enable interconnection

to the macro world. A significant part of this effort is focused on preparing substrates for both imaging and

fabrication at this scale. Wet chemical processing methods are used to prepare atomically ordered Si surfaces at ambient temperatures.

A second research thrust is the development of tech- niques for the preparation of SPM tips with reproducible

geometries and the characterization of the tip dimensions on the atomic scale. These well characterized tip probes

can be used to pattern samples which may be etched and

processed at a subsequent step.

The remaining thrust is for the development of new

interferometric techniques for sub-nanometer resolution

measurement. The new approach to interferometry enables measurements in the 20 picometer resolution range

with accuracy in a similar range. The interferometry

system is also designed for use directly on the STM to enable atomic scale measurements with new levels of

accuracy. An essential element of this project is the fabrication

of test artifacts and structures for the development of

high resolution imaging methods. It is imperative to enable fabrication methods for sub- 10 nm sized features. Several recent developments in optical microscopy, scat- terometry and SEM metrology require test samples with

critical dimensions below 10 nm. These test structures are simply not available at this time. In this project we

are developing the methods for fabrication of sub- 10 nm

sized features and etching methods to transfer these pat- terns into the silicon substrates.

Figure 1. 10 nmfeatures prepared using a UHVSTM.

Although the new class of scanned proximal probes

have unparalleled resolution and offer much promise for meeting the high resolution imaging and fabrication

needs of nanomanufacturing, the substrates onto which

the features are fabricated or imaged plays an essential

role. Before this resolution can be translated into precise, accurate dimensional measurement capabilities and

atom-based standard artifacts, the methods for preparing

atomically flat substrates with atomic order need to be

made routine and robust. Significant effort has resulted

in major accomplishments in this area of research which

were pubHshed in Applied Physics Letters.

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Figure 2. Thisfigure above shows AFM images ofSi substrates having three different miscut angles. The miscut angle plays a

central role in the final morphology.

One key to accurate metrology at this scale is the development of methods for the production of reproducible

tip shapes, the accurate measurement of tip geometry,

and models which interpret imaging data to extract real feature dimensions. NIST has developed the means for the reproducible production and characterization of SPM

tips, using tools such as field-ion-field-electron microscope (FIFEM).

Tip radius calculated from FIM image ofW(llO)

of tips optimized for STM measurements and fabrication on atomically ordered surfaces.

The verify the atomic spacings at the surface and to enable traceable measurements of sub-50 nm sized features, a new interferometry capability has been developed. This new approach uses a constant number of

wavelengths in the measurement path and continuously

changes the interferometer wavelength using a tunable

diode laser as the reference mirror is scanned. The laser frequency is constantly tracked and measurements with

20 picometer resolution have been demonstrated.

0 5O10OI5O2a>2a]3tlO^4O

Figure 4. These data are an STM image acquired synchronously

with the new diode laser interferometry system. The profile on the

right shows the atoms with their interferometer readings.

Recent research is focused on using the new Omicron

UHV scanning tunneling microscope to extend the li- thography processes described here to the atomic domain. In this recent work, atomic resolution images of

hydrogen terminated Si surfaces were obtained using

only room temperature processes. The research is to extend the sub- 10 nm lithography process to individual atoms.

Selected Publications

1 . J. Fu, H. Zhou, and R. M. Silver, "Dependence of

Morphology on Miscut Angle for Si (1 1 1) Etched in NH4F", Applied Physics Letters 82, No. 18. 2003.

2. R. M. Silver, S. Gonda, J. Jun, L. Howard, and C.

Jensen, "Atomic Resolution Measurements with a New Tunable Diode Laser Interferometer", Optical

Engineering 43, No. 1 , 2004.

3. P. V. M. Rao, C. Jensen, and R. M. Silver, "A Generic Shape Model for STM Tip Geometry," JVST

B, Mar/Apr (2004).

(*'+*'+/^"'(*''+f'+/'y'

Figure 3. Atomic resolution FIM image ofan STM tip.

The FIM instrument can also be used for field evaporation which produces tips with controlled microscopic

geometries. One focus of this project is the preparation

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Critical Dimension Metrology using Scanning Electron Microscopy

and a Model-Based Library

Tolerances for measurement accuracy and repeatabilityfor transistor gates are below the spatial resolution

of the scanning electron microscope. This results in measurement artifacts that can only be remedied by

accountingfor the sample-instrument interaction. We

have implemented a model-based library approach

that is proving to be more accurate and less sensitive

to secondary sample characteristics than the existing

measurement methods.

John S. Villarrubia and Andras E. Vladar, MEL

(821)

Transistor gates are typically the smallest manufactured features in an integrated circuit. As such, they tend

to be the most sensitive to manufacturing faults. Moreover, their size is closely linked to important device

properties, like speed, packing density, and off-state leakage current. Accordingly, manufacturers measure

these "critical dimensions" (CDs) in order to monitor for

drifts or faults in the manufacturing process. Transistors

with CDs of 30 nm are in mass production in 2005.

In a scanning electron microscope (SEM) secondary

electron image, the edges of the gate appear bright. (See

the intensity peak in the blue curve in Fig. 1 .) The usual

measurement procedure is to assign edge positions at the

location of these bright, so-called edge blooms. Edge

assignment is traditionally done using image processing

criteria, such as an intensity threshold crossing or the

position of the maximum slope in the intensity curve.

Measurements must have accuracy and repeatability

small compared to the device dimensions. In 2005, the

specifications require measurement bias below 3 nm and

repeatability better than 1 nm. However, the edge bloom

region is anywhere from a few nanometers to tens of

nanometers wide, owing to the finite spot size of the in- cident beam and, often more importantly, scattering of

electrons within the sample (Fig. 1 again). This means

the required accuracy and repeatability are smaller than

the spatial resolution of the measuring tool. Under this circumstance, not only is the accuracy of an image processing assignment of edge position suspect, but even the

repeatability of such an assignment becomes sensitive to small changes in the instrument condition or secondary

characteristics of the sample (e.g., wall angles or rounding of comers), changes that alter the shape of the intensity in the bloom even when they do not alter the actual edge position.

220 1—

^L_ . . • I . . \. 80 100 120 140 L60 X (nm]

Figure 1. Simulation ofelectron trajectories in the sample. The sample boundaries are indicated by the red line. Incident elec- trons scatter within the sample creating an extended "interaction volume, " indicated by the black trajectories. Intensity variation

(blue) as the incident beam is scanned across the edge) is a con- sequence ofthe different numbers ofsecondary electrons pro- duced at different landing positions.

To perform such measurements, we have developed

an alternative to the traditional measurement algorithms.

This approach, a model-based library (MBL) method,

has 3 components. The first is a physics-based model

that permits calculation of the image for a given sample

and imaging conditions specified by parameters. This

model uses a time-consuming Monte Carlo simulation of

electron trajectories. The second component is software

that can search, and if need be interpolate, a library of

precomputed model results (Fig. 2). This allows fast computation of model results once a library has been

constructed. The final component is a nonlinear least squares algorithm that solves for the set of parameters

that produces the best least squares match between

measured and calculated images. The parameters deter- mined in this way include the edge positions, from which

the desired feature size can be determined. As a bonus,

other aspects of the feature shape (e.g., sidewall angles)

that affect the image can also be determined.

The accuracy of the MBL technique was assessed by

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comparing results determined by MBL from top-down

images (the usual industrial imaging configuration) with

cross-section SEM measurements. Initial tests were per- formed at NIST using our laboratory SEM, measuring

first isolated and then later dense (i.e. closely spaced)

polycrystalline Si lines. Agreement between the two

techniques was better than 2 nm for linewidth and 0.2°

for sidewall angle (Fig. 3a), differences that could be

accounted for by line edge roughness.

0 3 6 9

side Wall Angle Cdeviation from vertioal, in degrees)

Figure 2. Schematic ofthe MBL method. A "library " ofedge

shapes (blue) and their corresponding computed images (black) is constructed ahead oftime. A measured unknown is compared to images in the library. The library is interpolated if necessary to obtain the best match. The parameters ofthe matching shape are

attributed to the unknown.

In a collaboration with International SEMATECH,

test patterns were fabricated in resist and polycrystalline

silicon, measured top-down in an industrial CD-SEM,

cross-sectioned, and remeasured in a laboratory SEM.

MBL analysis of the top-down images was performed.

Comparison with the cross section results indicated substantial agreement (see Fig. 3b and 3c), although there was some evidence of electron-beam induced resist shrinkage similar to that observed by others. MBL linewidth measurements at a given position on

a line were as much as 3 times more repeatable than

linewidths determined from the same images by regres- sion to baseline (one of the traditional methods). The

improvement is attributed to the fact that MBL is a fit- ting method that uses the intensity at many pixels to determine an edge position, while the standard image processing methods typically restrict consideration to a smaller set of pixels.

announced a product using their own implementation of

the MBL method at SEMICON West 2003. Hitachi, Ltd. sent an engineer to work with us for a year as a guest

researcher. This collaboration has resulted in a publication {Proc. SPIE, in press) showing that MBL is less sensitive than existing methods to SEM focus variation near best focus.

Figure 3. Agreement between MBL and cross-section measurements (a) for isolated polycrystalline ("poly ") Si lines in which

top-down measurements were performed with a laboratory SEM'

(b) for dense poly lines measured with a commercial CD-SEM,

and (c) for dense UV resist lines measured with a commercial CDSEM. In all cases the red lines are the cross sections predicted by

the MBL technique based upon top-down measurements, and the blue lines are edges assigned based upon the cross-sections.

Selected Publications i

1 . J. S. Villarrubia, A. E. Vladar, J. R. Lowney, and M. [ T. Postek, Proc. SPIE 4689, 304-3 12 (2002).

2. J. S. Villarrubia, A. E. Vladar, B. D. Bunday, and M.

Bishop, Proc. SPIE 5375, 199-209, (2004).

The major manufacturers of CD-SEMs have been

given free access to our simulation codes. Soluris, Inc.

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SI traceable force for scanning probe microscopy and

instrumented indentation

A dizzying variety ofsmall-scaleforce measuring in- struments are routinely employed by industrial and

government laboratories to evaluate the mechanical

performance ofthin-film metal, ceramic, andpolymer

coatings, microelectromechanical systems, and a host

ofbiological processes andfunctions. Forces as small as a single atomic bond can be detected and are used to help manufacturers choose materials and

processes that makeup everythingfrom next generation semiconductors to next year's automobiles. Until

recently, theseforce measurements were considered

largely qualitative. Now, the NIST smallforce metrology lab has begun to put these precise measurements on a firm quantitative basis, having developed new methods and instruments that allow SI traceable

calibration offorces below 10'^ N.

Jon R. Pratt, MEL (822)

The lowest force traceable to a NIST deadweight

force machine is 44 N. Below this value in the United

States, secondary laboratories, industrial metrologists,

and others responsible for ensuring force traceability

purchase or fashion their own primary standards of

force; a task most simply accomplished by obtaining

traceable mass artifacts. To date, this hierarchical structure has been satisfactory in the United States; however,

there is a growing push for standardization in the field of

nanomechanical testing (ASTM, ISO, and VAMAS all have working groups on the subject), where forces are two to three orders of magnitude smaller than the deadweight force produced by even the smallest available

traceable mass artifact in the United States, or 0.5 mg

(approx. 5 |aN). NIST is leading the way in the development of new standards to meet this challenge.

A new hierarchy of small force at NIST

In principle masses smaller than a milligram can be

calibrated by NIST, but such artifacts are difficult to handle, and the trend is for their relative uncertainty to increase in proportion to the decrease in mass. In fact, their uncertainty could very well be as large as the force

for a deadweight producing 1 nN. A more convenient

basis for small force standards is desired. To realize the unit of force using the SI definition of

the Newton, the force must be expressed in terms of

measured quantities that are themselves expressed in terms of some combination of the SI base units. Deriving a unit from realizations of the base units is typically

the path of least uncertainty (e.g., kg, m,s), but this needn't always be the case. For instance, electrical forces may be measured in terms of the SI unit of length

in combination with appropriate electrical units. Electri- caf units are themselves derived units in the SI, but they

may also be linked to practical representations of the volt and ohm based on the Josephson and quantized Hall effects. Because of this, they may be measured through a

large dynamic range with little loss of relative uncertainty, unlike the situation in mass. The block diagram

of Figure 1 lays out the new hierarchy of NIST small

force metrology—one where the dynamic range of electrical quantities is exploited to produce a primary realiza- tion of force based on a combination of length, capacitance, and voltage for forces below 10"^ N.

(Kg, m, s)

SI Base Units J (m)

(m)

Deadweight

force X

(10± 10-»)N

(Kg,m, s )

Quantum Invariants

Calculable

capacitor

(e, h)

Q.H.E. Ohm

~h/eJ.E. Volt V

[ -h/e J

electronic mg

NIST EFB

^ 1 „. tic

force or spring

constant artifact force or spring constant artifact

Instrumented

indentation machine

Atomic

force microscope

Figure 1. SI traceability in the NIST smallforce metrology laboratory. A Primary standard of small force between 1 0"^ N

and lO''^ N: The NIST Electrostatic Force Balance

At NIST, we can now realize a primary standard of

small force from electrostatics, and have constructed a

series of increasingly refined systems to realize force using a coaxial cylindrical capacitor arrangement. The

present version of this primary standard, referred to as the NIST Electrostatic Force Balance, or simply the EFB, is shown in the photo of Figure 2.

The EFB consists of an electrostatic force generator

that acts along a vertical axis (z-direction) aligned to local gravity within a few milliradians. Forces are generated when voltages are applied to a pair of nested, coaxial cylinders. The outer high-voltage cylinder is fixed

while the inner electrically-grounded cylinder is free to translate along a z-axis, varying the degree of overlap.

The capacitance of this geometry is a linear function of

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this overlap, and the electrical force is directed solely

along the cylinder axis.

The EFB operates as an electronic null balance and

generates electrical forces that may be calculated from

2 az

where F is the force in nN, dC/dz is the capacitance gra- dient in pF/mm, Uj is the voltage applied to the outer

electrode in V before loading, U2 is the voltage applied

to the outer electrode in V after loading, and is the

potential difference between the electrodes resulting

from surface field effects. Rather than directly measure

Vs, we reverse the polarity of the applied voltage and

take the mean of the computed electrical forces.

Interestingly, the EFB could be used to redefine a mass artifact in terms of electrical and length units. Recently we used a nominally 2 mg mass artifact as a 20

|iiN deadweight. One standard deviation of the electrical force data was 0.7 nN, for a relative standard uncertainty

better than 4 x 10"^, which is at the same level as that achieved by conventional mass calibration.

Force calibration of an SPM

At conventional force levels, force cells are equipped

with strain gage transducers that convert changes in mechanical force to changes in electrical resistance. At the force levels considered here, SPM cantilevers can be

doped with piezoresistive patterns that achieve the semiconductor equivalent of a strain gage fransducer, so that these cantilevers are fiinctionally equivalent to a force

cell. We probed the EFB with just such a cantilever,

recording its change in resistance as a fiinction of the

null force. The relative standard uncertainty of the sensitivity for this device was less than 1%.

Typical SPM cantilevers are passive devices; force

induced deflections are measured using an optical lever

that is part of the microscope, not part of the cantilever. Because of this, force calibration by direct comparison to the EFB is impractical, but an indirect comparison to a

piezoresistive cantilever is quite practical, and a scheme

is illustrated in Figure 3. As shown, the SPM simply

probes the piezoresistive cantilever as if it were a specimen, and the calibrated force readout is used to calibrate the output of the microscope's optical lever.

Ohm meter Volt meter

Figure 3. SPMforce calibration using a piezoresistive reference

cantilever. Micronewtonforce calibration has been achieved with

relative standard uncertainty less than 5%.

Selected Publications

1 . J. R. Pratt, D.T. Smith, D.B. Newell, J.A. Kramar,

, and E. Whitenton, 2004, "Progress towards Systeme ^ International d'Unites traceable force metrology for nanomechanics". Journal of Materials Research,

19(1), pp. 366-379.

Figure 2. The NIST electrostaticforce balance (a) outside view of vacuum system (b) front corner view ofbalance (c) side view of

balance

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Phase-sensitive Scatterfield Optical Imaging for sub- 10 nm

Dimensional Metrology

Customer needs andproject objective: There is a

substantial needfor high resolution, low cost, yet

h.igh throughput metrology capability in nanomanufacturing. This project proposes to advance optical

microscopy to unprecedented levels ofperform- ance through theoretical and experimental development ofa new technique we call "scatterfield

optical imaging". This new methodpromises to make possible optical measurements ofnanometer-sizedfeatures using high-throughput, low cost

optical methods with the potentialfor a significant impact on innovation and quality control in

semiconductor manufacturing and nanotechnology as well as providing the measurement basis

for new nanometer-scale calibration standards

well beyond the state-of-the-art.

Richard Silver, MEL (821)

Although optics are often thought to lose its effec- tiveness as a metrology tool beyond the Rayleigh resolution criterion, there is now evidence that optics can be

used to image and measure features smaller 10 nm in dimension. The resolution obtained by conventional optical microscopy is usually considered to be limited to approximately half of the wavelength of the light used

for illumination. However, another optical metrology

method, interferometry, can routinely achieve subnanometer resolution using visible light. The key to understanding the difference between these two methods is related to the complexity of the electromagnetic fields (illumination) and the targets used in the measurement.

In the case of interferometry, single plane waves, that is, waves having a simple sinusoidal oscillation in space

and time, are reflected from planar, mirrored surfaces,

with the phase information preserved. For microscopy,

the illuminating fields are much more complicated and

the samples often have significant three-dimensional

character.

Optical imaging and metrology with resolution well

beyond the wavelength is a possibility as a result of a new optical configuration and design, increased computation speed, high-resolution CCD array technology, improved signal processing of large arrays, and reliable theoretical scattering models. The ability to "engineer"

the illuminating field is key to this advanced measure- ment technique. It involves low numerical aperture (NA)

illumination optics and polar illumination schemes with

high angle to on-axis plane wave illumination. Another

key technological advance is the realization that phase

information, accessible by stepping through the ideal focus, provides a sensitive, complex optical signature for a given sample and optical configuration. The sample or

target design also plays a central role in the ability to increase optical sensitivity to dimensional changes. A

target structure which enhances the amplitude of the

scattered fields and accessible information content can be optimized with accurate simulation tools.

2.5

2

0.5

1

'1 1

160 nm

-1000 -500 0 500 1000 1500 Focus position, nm (-ve-below; +ve=Above)

Figure 1. Throughfocus focus metric profiles showing nanometer

sensitivity to linewidth. A key enabling technology that these improvements

rely upon is the unprecedented accuracy by which we

can now calculate the electromagnetic scatter from features. The exact profile of the scattered field is ex- tremely sensitive to small changes in the shape and size of the scattering feature. Once one can accurately calculate the scattering fianction and propagate it in the optical

system, one can in principle measure features near to nanometer dimensions. It is no longer necessary to be

limited by the concept of image-based edge detection

microscopy in optically-based dimensional metrology.

This is a radical departure from traditional methods of

optical imaging.

This project is now designing a new optical instru- ment with a key design element being access to the rear focal plane of the objective lens. An open or flexible optical layout has been designed which places an accessible conjugate back focal plane not in the imaging path.

This will enable specific, complex fill factors for the pupil plane which then determines the angles and intensities of the Kohler illumination. The instrument will also take advantage of the obvious gains obtained when

using shorter illumination wavelengths such as 1 93 nm

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illumination.

Figure 2. CCD image ofa 39 nm CD array with a cross section

profile shown on the right.

The sample plays an essential role in the resulting scattered intensity profile. Complex scattering profiles or "signatures" can be created or enhanced in a fundamental way by varying the sample geometry and spatial

fi-equency content. An important aspect in the advance

of these optical methods is the possibility of providing

the nanomanufacturing and semiconductor industries

with the capability of making simultaneous critical di- mension and overlay measurements. This would have an enormous impact on the quality, productivity, and cost of

layer to layer lithography type manufacturing processes.

The impact from the successful implementation and

development of this new measurement science will be

significant. Optics has some significant advantages over

other high resolution imaging techniques such as scan- ning electron microscopy (SEM) and atomic force microscopy (AFM). SEM requires high vacuum, is a serial scanning technique, with expensive instrumentation and

can be destructive in making measurements. AFM has

significant throughput, reliability, and cost of ownership

challenges. Optical metrology, on the other hand, is massively parallel, with high throughput, relatively low

instrumentation cost, and is non-destructive. The demonstration of accurate sub- 1 0 nm feature measurements

with nm sensitivity to changes in shape will be substantial.

The key deliverables of this project include a new

measurement technique capable of imaging sub- 10 nm

sized features with sensitivity to nm scale geometrical

changes. The method will be developed for specific applications in the nanomanufacturing and semiconductor

industries, as well as core applications in high-level

measurements for NIST standards. The fundamental optical limits are being explored and documented in a sys- tematic way. A new optical instrument is now being

developed with 193 run measurement wavelength capabilities. A parametric exploration of the sample and optical illumination space is underway, currently using

simulation tools and to be followed up with experimental

confirmation. This work is being performed to define

the key target and optical parameters which maximize

sensitivity to the sample properties of interest followed

by the optimization of those parameters.

Selected Publications

1. R. M. Silver, R. Attota, M. Stocker, M. Bishop, J. Jun, E. Marx, M. Davidson, and R. Larrabee, "Highresolution Optical Overlay Metrology", SPIE Vol.

5375 P. 78, 2004.

2. R. Attota, R. M. Silver, M. Bishop, E. Marx, J. Jun,

M. Stocker, M. Davidson, R. Larrabee, "Evaluation

of new in-chip and arrayed line overlay target designs", SPIE Vol. 5375 P. 395, 2004

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Advanced Control Systems and Positioning for Nanoscale

Measurements and Standards

Advance control andpositioning systems are needed

for high precision nanoscale measurement, manipulation, assembly and standards. We are experimenting

with arrays ofhigh precision MEMSpositioning

stages for scanning electron and scanning probe microscopy; multi-degree-of-freedom nanometer resolution optic displacement sensors; automated manipulation and assembly ofmicro and nano scale component; advance mathematical modeling and optimization; control algorithms andperformance measures.

Nicholas G. Dagalakis, MEL (823)

Methods to build high precision complex three- dimensional nanoscale structures, sensors and devices

using techniques that allow scale-up is a central chal- lenge of nanomanufacturing. Parallel operation nano- manufacturing of these devices must be explored and

standardized in order to allow for the economy of scale needed for successful production. This project will ad- vance the science of manufacturing high precision 3D

nanoscale devices and metrology sensors by developing

mathematical modeling, simulation and optimization

tools, manipulation and metrology tools, control algorithms and performance measures, standards and fabrication techniques.

Project objectives:

• To develop mathematical modeling, simulation and

optimization tools.

• To develop manipulation and metrology tools.

• To develop control algorithms and performance

measures.

• To develop standards and fabrication techniques.

With collaboration from RPI/CAT researchers, one

degree of freedom and two degree of freedom arrays of

planar high precision MEMS micro/nano positioners

have been designed and fabricated based on NIST macro

and meso scale designs (see Figures 1 and 2). These

devices have various sizes and design flexures, thermal

actuators and two different moving mechanism stage

thicknesses.

With collaboration from RPI/CAT researchers we

have developed mathematical models of advanced

MEMS mechanisms and conducted various optimization

studies (Figures 3 and 4). Finite Elements models have

been developed and were used to conduct stiffness, dis- placement and stress analysis studies (see Figures 3 and

4). Experimental test results are being used in order to develop accurate mathematical models and controllers.

Figure 2. Scanning Electron Microscope picture ofa MEMS

Scale, Dual Parallel Cantilever Micro/Nano Positioner. The ex- ternal size is approximately 1.8 mm by 1.8 mm, the thickness is 10

micrometers, all circular notch flexures have a width of 7 micrometers.

Figure 2. Light microscope picture ofa 2x2 array ofMEMS scale X-Y axes Dual Parallel Cantilever Micro/Nano Positioners.

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Figure 3. Finite elements model simulation ofthe stage displacement, ofa MEMS Scale Dual Parallel Cantilever Micro/Nano Positioner, due to an inputforce.

Figure 4. Finite elements model simulation maximum von Mises stress concentration, at the outer surface ofthe pivot circular notch

flexure ofa Micro/Nano Positioner, due to an inputforce.

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Molecular Measuring Machine

Nanometer accuracy and resolution metrology over

technically relevant areas is becoming a necessityfor

the progress ofnanomanufacturing. We are developing the Molecular Measuring Machine (M^), a scannedprobe microscope (SPM) and Michelson in- terferometer based metrology instrument, designed to achieve nanometer measurement uncertaintyfor

point-to-point measurements over a 50 mm by 50 mm

working area. Both long-range measurements ofsubmicrometer pitch gratings over a distance of 10 mm,

and short-range, high-resolution measurements ofa

molecular crystal lattice have been accomplished.

The estimated relative measurement uncertainty so far

attainedfor pitch measurements is 6x lCr\ coverage

factor k = 2. We have also utilized the SPMprobe tip

positioning accuracy ofM^ along with scanning probe

oxidation lithography to create some simple nanometer dimension patterns that could serve as prototype

calibration standards for disseminating the SI unit of

length at this scale.

John A. Kramar, MEL (821)

Interest abounds and much hope has been placed in the current explosion of research efforts in nanotechnology and nanomanufacturing. Incredible devices have

been proposed in fields as diverse as medicine, information technology, and military applications. Several novel

devices have been demonstrated in prototype form or

limited initial production, for example artificial noses, molecular electronics memory and logic elements, and

biological probes including medical diagnostics and

therapeutics. For these impressive early ideas and suc- cesses to mature into a widespread, stable, and profitable

economic sector requires that the infrastructural base of

dimensional metrology be extended into this nanoscopic

regime. Metrology is also needed in commerce or trade

for quality assurance and communication of specifications and capabilities. Mass production requires process

control; process control requires accurate metrology.

The Molecular Measuring Machine (M^) is a metrology instrument designed to achieve sub-nanometer resolution with both the probe system and the metric, with

the goal of achieving 1 nm uncertainty for point-to-point

measurements within its 50 mm by 50 mm working area. The probe system is a scanning probe microscope (SPM)

and the metric is a Michelson interferometer. The in- strument embodies a realization of the International System (SI) unit of length, since the fi-equency of the light

Figure 1. Cut away view ofM^.

source for the interferometer is measured relative to a recommended radiation. In order to achieve the desired

motion resolution simultaneously with a large operating

range, piezoceramic-driven, flexure-guided fine-motion

stages are stacked on long-range, slideway-guided carriages. The range of the fine motion stages is 10 |am

while the range of the coarse motion is 50 mm. The in- terferometer system measures the combined motion of

the coarse and fine stages. The instrument is isolated fi-om environmental fluctuations and noise by temperature control, vacuum, and vibration isolation systems

(Fig. 1).

This instrument has been applied to the measurement

of submicrometer pitch gratings. This application makes

use of the SPM probe to resolve the sub optical wavelength features and the long distance, high resolution and

high accuracy metrology system to produce precise, low

uncertainty measurements. For example, the pitches of

nominal 200 nm and 400 nm gratings were measured

with uncertainty as small as 1 0 pm (coverage factor ^ = 2) to serve as reference standards for the production

of diffraction gratings for the x-ray spectrometer of the space-borne Chandra Observatory. Single SPM traces 10 mm in length were acquired in these measurements.

At the small end of the scale, lattice constants of a molecular crystal have also been measured. In this case, the challenge was to minimize noise from vibrations and

the control electronics, and from the heterodyne Michelson interferometer. Figure 2 is an image of the organic

conductor (TEET)[Ni(dmit)2]2 acquired under closed

loop servo control based on the interferometer measurements. The measured lattice parameters were within the experimental uncertainty, dominated by the nanometerlevel periodic uncertainty in the interferometers.

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Conference on Metrology, Inspection, and Process

Controlfor Microlithography XIII, SPIE Vol. 3677,

pp. 1017-1028(1999).

2. J. A. Kramar, J. S. Jun, W. B. Penzes, F. E. Scire, E. C. Teague, and J. S. Villarrubia, Recent Advances in Metrology, "Grating Pitch Measurements with the Molecular Measuring Machine," Characterization,

and Standardsfor Optical Digital Data Disks, Pro- ceedings of SPIE, Vol. 3806, pp. 46-53 (1999).

3. J. A. Kramar, Jay Jun, W. B. Penzes, V. P. Scheuerman, F. E. Scire, and E. C. Teague; "Molecular Measuring Machine Design and Performance," Proceedings ofthe 16'^ Annual Meeting of

the American Society ofPrecision Engineering, Vol.

25, pp. 19-22 (2001).

Figure 3. Prototype calibration artifact made by using

scanned probe oxidation litliograpliy and reactive ion etciiing.

Selected Publications

1 . John Kramar, Edward Amatucci, David Gilsinn, Jau- Shi Jun, William Penzes, Fredric Scire, E. Clayton

Teague, and John Villarrubia, "Toward Nanometer

Accuracy Measurements," Proceedings of the SPIE

NIST Accomplishments in Nanotechnology

150

X (nm)

Figure 2. Metrology image of (TEET)[Ni(dmit)2]2, an organic

conductor, acquired with Af.

Using the interferometer-controlled tip positioning

capability of M^, nanometer accuracy calibration arti- facts can be made. An example is of this is the use of

scanned probe oxidation lithography to create 70 nm

wide lines on a silicon substrate (Fig. 3). The writing process itself creates lines with approximately 3 nm

height, but the features can be transferred into the silicon producing up to 20 nm relief by using reactive ion etching. The ability to create such artifacts provides a means

for disseminating the measurement traceability embodied

in M^ to customer metrology instruments. It is envisioned that nanoimprint lithography may enable the effi- cient and cost effective replication of such artifacts, once

the scale fidelity of the replication process is verified.
5 - Nanomanufacturing

Agile Nanoassembly to Prototype, Test, and Manufacture

Functional Nanodevices

Today little ability exists to directly manipulate and

assemble nanoscale components, despite rapid ad- vances in imaging, measurement, andfabrication at the nanoscale. This limits the ability to prototype and

test many classes ofnanodevice, and is a significant

obstacle worldwide to both R&D and commercial

production ofthese devices. We are developing laser

tweezer-based systems to control the position and ori- entation ofnanocomponents and enable nanoassembly

from prototype to production. Natural, heuristic operator interfaces are a key part ofthe nanoassembly

system, as is automationfor efficient production.

Thomas W. LeBrun, MEL (821)

Light beams exert small forces on material objects,

and for objects smaller than tens of micrometers the

forces can be designed to effectively "grip" a particle in an optical beam and move it to a desired position. To be

useful for nanoassembly this elementary ability to grasp

point-like particles must be extended to allow full manipulation (position and orientation) of nanoscale components with a range of shapes, sizes, and material properties — and with other components in close proximity.

The technique also offers a number of unique advantages

over material probes: multiple particles can be simultaneously controlled because beams do not obstruct each

other, the number and behavior of the particles manipulated can be changed dynamically, particles can be manipulated in real-world environments such as aqueous

solutions or in-vivo, and the process can be made parallel.

To perform agile nanoassembly we have developed a scanning Optical Tweezer (OT) instrument that traps and

manipulates multiple particles simultaneously by timesharing — rapidly moving a single trap beam between

multiple particles. As the trapping beam rests on each

particle it is dithered at high speed to effectively draw a

trap that fits the shape of the particle being manipulated,

moving the trap moves the particle accordingly.

The position and orientation of each trap is controlled

in software and the operator uses a CAD-like graphical user interace (GUI), shown in figure 1 , to control each

particle independently. Note that objects smaller than a few hundred nanometers can be imaged but not well re- solved using traditional optical microscopy, so we have

used 3 micrometer spheres in fig. 1 . Using the GUI, the operator can draw a number of shapes such as point-like

traps, lines and poly lines, and perform operations including select, translate, and rotate — all in three dimensions (3D). The front end also allows displays images

and synthetic representations of the objects in the 3D

workspace, and allows the operator to vary the focal plane of the imaging CCD as required.

This provides an effective user interface, but the sys- tem must also scale to handle new and more complex

tasks, so it must be part of a larger modular architecture

designed to grow to include new capabilities, such as automation, teleoperation, or operation of many systems

in parallel. Therefore we have built a a modular net- work-based achitecture using message passing for inter- module communication. Modules for an operator inter- face (GUI), system control, and trap scanning are currently used, with modules for vision, scripting and auto- mation under development. The communications layer is based on the Neutral Messaging Language (NML) developed at NIST so that software modules that reside on

one computer during development (such as vision and

scanning), can be easily moved to independent processors as the system scales up. These systems can also be

combined to support complex fabrication operations in

parallel, for example by pipelining or by hierarchial inte- gration of many OT systems.

9

O

0

Q

1

1 /

* 1 *

%

[I °

Figure 1. Graphical user interfacefor nanoassembly.

To fabricate functional nanodevices with this device we are initially focusing on nanowire-based devices.

Nanowires are archetypal components that can fiinction as mechanical elements (rods), conductors (wires), optical elements (waveguides or lasers) and electronic ele- ments (diode junctions or transistors). Yet while we can

fabricate a wide variety of nanorods, from carbon nanotubes, to semiconductor nanowires and gold nanorods,

little ability exisits to assemble predesigned structures from these components.

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5 - Nanomanufacturing

ing p-type and n-type nanowires in contact. We have

assembled prototype crossbar structures with 1 |im alu- mina microwires as shown in figure 2. Much smaller

wires can be used, and the obstacles to efficiently fabri- cating devices are more technical than fundamental: imaging nanowires undergoing rapid rotational diffusion,

controlling adhesion, and drying structures for SEM imaging and testing without damage.

We are also investigating gold nanorod-based struc- tures for plasmonic devices for optical detection and spatial localization of electric fields over ranges much

smaller than an optical wavelength. The goal is to use

the automation capabilities under development to rapidly move from simple devices to more complex circuits in- corporating multiple types of nanowire. These devices

also promise very fast operation, but radiation from sin- gle conductors with the corollary crosstalk to adjacent

wires may limit performance. Using our technique we

can also investigate waveguide geometries and structures

for impedance matching, although these techniques

drawn from RF engineering may be more applicable to plasmonic wavelengths.

Many useflil structures can also be made from parti- cles and spheres, but this level of manipulation is already

well developed for spheres with diameters on the order

of a micrometer. To manipulate smaller particles trap

intensity can be increased, but undesirable heating and

damage generally results. This is a difficult problem to circumvent physically, but a solution can be engineered

using a fast control system to servo out Brownian motion, and maintain particles at the stiffest part of the trap. This allows smaller particles to be trapped and biological

systems to be trapped with reduced power, thereby

avoiding damage from radiative heating. This has the advantage of making the technique more amenable to nanobiological applications.

Optical trapping has an established history in biological research for manipulating cells and biological parti- cles (e.g. cell sorting, patch clamp, manipulation inside

cells), and more recently optical tweezers have been used

to measure Raman spectra of cells, and measure biological forces generated by single molecular motors tethered

to micron-sized spheres.

To investigate medical applications of this tool we

have have also begun studying the function of biological

nanoparticles (liposomes) used for gene therapy of cancer. When administered, the particles are taken up by

cancer cells, releasing DNA that is converted by the cell into a normally occurring protein that terminates the cell. But the process by which the particles enter the cells is not well understood.

Using our system we have trapped the biological

nanoparticles (fig. 2), and are beginning to study cell- nanoparticle interactions so that entry of a liposome into a cell can be controlled and measured. The immediate

aim of this work is to identify the mechanism of cellular uptake of these nanoparticles, but the long-term goal is development of what might be called single cell metrology — the accurate measurement of cellular properties

and function right down to the molecular level.

Figure 2. Clockwisefrom top left: an array of3 /um spheres

automatically selected andpositioned, 6 spheres spinning simultaneously, a crossbar structure, a 200 nm liposome and a larger

vesicle trapped in tandem.

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Development of Nano-tip Electron Guns for the Scanning

Electron Microscope

High-resolution scanning electron microscope (SEM)

imaging and metrology ofnanostructures need new

higher resolution electron sources. Experimental

nano-tip emitters have shown significant improvements as they offer higher brightness and smaller

electron source size, which translate into higher spatial resolution and better signal-to-noise ratio images.

Images taken with experimental nano-tips showed a minimum two-fold improvement over the original

resolution performance specification ofthe test SEM.

The tip lifetime wasfound to be many months in light

use.

Andras E. Vladar and Michael T. Postek, MEL

(821)

Through this work the most important questions and

problems associated with nano-tip-based electron guns

and their possible applications to SEMs were investi- gated. These were related to the tip making procedures,

the theoretical assessment of the Hitachi S-6000 critical dimension SEM's electron gun and to the preparation of

the SEM for insertion of nano-tips, and finally to the experiments with a nano-tip. Figure 1 shows an SEM

image of a nano-tip made at NIST that can be used as electron emitter.

Figure 1. SEM image ofa nano-tip.

The S-6000 SEM specifications call for a guaranteed

resolution of 1 5 nm on suitable samples. This includes

the decrease due to the non-ideal, practical electron opti- cal column, and unavoidable adverse effects from the environment, such as electric fields and mechanical vi- bration, etc. In some cases, the resolution performance

can be somewhat better than this 1 5 nm guaranteed

specification.

A trial run, designed as a transitional step toward the nano-tip experiments, was carried out with a new, sharp

tip. The tip did not go through the usual forming process,

after insertion, and as soon as it was turned on, it started

to* emit at around 3000 V. The tip noise was acceptable,

and the tip worked for many hours. The tip worked for several months, even though flashing was never per- formed. The vacuum in the gun region with occasional

bakes was excellent. On this microscope the gun region

is pumped with 3 ion-getter pumps. Note the lower extraction voltage for the sharp tip. This indicates that the

tip was very sharp, less extraction voltage was needed to be applied to get usefiil electron beam. The resolution

compared to the original tip has improved.

Since nano-tips are susceptible to damage, a tipmaking procedure had to be developed to produce tips

with predictable properties because characterization

methods may render the nano-tips useless. These, while

very valuable in the development, were not used for those tips that were put into the microscope. A new, fully

computerized, automatic tip making apparatus was designed and developed, see Figure 2. This apparatus

makes the tips fully automatically. It uses AC etching to clean the raw tip and DC etching to form well-shaped

sharp tips. At the end when the lower part of the wire

became separated at the neck that was formed at the re- gion close to the surface, a fast electronic circuitry

quickly turned off the etching current. This has pre- vented the blunting of the newly formed tip. The computer followed and recorded the etching current-to-time

and wet cell voltage-to-time curves and based on these curves a decision could have been made about the quality of the tip. This whole process lasted for one hour

from tip cleaning to final etch, and it produced nano-tips

with good yield.

Figure 2. Fully automatic computerized nano-tipfabrication apparatus. NIST Accomplishments in Nanotechnology

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A tip that had shown good electrical characteristics during its fabrication, and was thought to be a nano-tip was installed in the modified S-6000 CD-SEM. After

several long baking procedures when the vacuum of the cathode chamber has reached the low end of the 10' Pa

range, and it stayed stable even after opening the air-lock

valve, the gun was turned on. During the preliminary tip

tests, when only operating modes with extraction volt- ages below 600 V were allowed, only noisy but inter- pretable images appeared on the screen. When extraction

voltage was turned on in automatic mode, a stable image was obtained at 761 V. To our pleasant surprise, after a

short adjustment of the electron optical column the image showed better resolution than that was ever possible

with this microscope.

Figure 3 shows SEM images taken with the original,

"sharp" and nano-tips at various locations on the same

copper grid sample. The resolution is clearly the best with the nano-tip.

Figure 3. Copper grid sample images taken with the original tip

(left), sharp tip (middle) and nano-tip (right). TheJield-of-view is 3 /jmfor all images.

The objective lens aperture current, which is related

to beam current stayed steady within 1 1 %. The vacuum

measured by the current of IP3 pump degraded by about

30 %, which was normal. The vacuum of the electron

gun measured by the current IP 1 and IP2 remained essentially unchanged, i.e., the pumps maintained excellent vacuum around the tip. The extraction voltage remained

stable at 761 V. The cathode emission current was measured by a LabView-based measuring system, which is believed to be more accurate than the screen emission

reading. In approximately 2.5 hours it has changed from

close to 2 i^A to its half and then stayed relatively stable for the rest of the measurement (the emission current reading on the screen was 4 )j,A at the beginning and 2

\iA at the end). The tip now has been operational for more than many months, without noticeable performance

degradation. There were power outages several times,

and the gun chamber was not pumped for many hours.

Occasional baking procedures restored and kept the good vacuum in the gun chamber. The tip was just lightly

used, but it was turned on many times, and it always

worked well. Therefore, long-term reliability appears to be high.

Figure 4 shows an image taken on a gold-on-carbon

resolution sample. The image obtained with the nano-tip

demonstrates a resolution of about 6 nm. It is likely that with further tuning of all circuits and the electron-optical

column, it could be improved even ftirther.

Figure 4. SEM image taken on a gold-on-carbon resolution sample with 900 V accelerating voltage, 2 /uA emission current. The

gap marked with the arrow is approximately 6 nm wide. Thefield- of-view is 1. 75 fjm.

Based on the results of this project, it appears feasible

that nano-tip gun technology could replace the standard

electron source and thus provide increased instrument

performance. The result of this work clearly demonstrated that the improvements predicted by the theoretical calculations can actually be achieved in practice. The

lifetime and resolution of the nano-tip gun were surprisingly good, which opens the possibility for the development of a new higher-resolution electron gun technol- ogy. The work in this project is continuing and currently

long-term emission stability measurements are running.

It is expected that the next major step for the nano-tip

electron gun development project a series of tests will be

in a more modem cold field emission instrument to determine how far the resolution performance can be

pushed.

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Measurement Strategy for Line Edge and Linewidth

Roughness Metrology

With the sizes ofcritical integrated circuitfeatures

now only a few tens ofnanometers, the roughness of

device edges is becoming a substantialfraction of

their sizes. Some deleterious effects ofroughness are

already known, and others arefeared. This has re- cently led to specifications for roughness metrology,

but the artifacts to which such metrology is subject

and appropriate methods to avoid them were largely

unknown. This project addressed many ofthese issues

in a collaboration with International SEMATECH.

John S. Villarrubia and Andras E. Vladar, MEL

(821)

Although semiconductor electronics devices are designed with smooth edges, they don't get produced that way. Edges that were meant to be straight end up being

rough (Fig. 1 ). The amount of roughness depends upon a complex interplay between material properties and

manufacturing processes. In the semiconductor industry,

roughness has typically been measured using a standard

deviation metric—3 standard deviations of edge position

variation around the average edge for line edge roughness (LER) or 3 standard deviations of linewidth for linewidth roughness (LWR). LWR of 5 nm to 10 nm is not uncommon.

2.0kV 3.3mm x1S0k.SE(U) 300nm

Figure 1. Polycrystalline silicon lines, exhibiting rough edges.

This was not a significant problem when circuit fea- tures were larger, but with the physical size of transistor gates now close to 30 nm it is becoming a significant concern. Does the variation of transistor gate length (Fig.

2) degrade transistor performance? Do rough edges re- duce interconnect conductivity owing to diffuse scattering of electrons? Do high electric fields near rough edges

of contact holes have deleterious effects? The semiconductor industry concerns about roughness are in an early

stage, and there are still more questions than answers

specifications for roughness metrology only first ap- peared in the International Technology Roadmap for Semiconductors (ITRS) in 2001. However, there are some answers that give concrete reasons to substantiate concerns. For example, simulations and some measure- ments have associated LWR with high off-state transistor leakage currents and variable threshold voltages (the

voltage at which a transistor turns on).

Figure 2. Geometry ofa transistor gate, showing a rough-edged

gate electrode over doped regions ofthe silicon substrate. A thin non-conducting dielectric (not shown) separates the gate electrode from the substrate. The (in this case n-type) dopant is implanted

after the gate electrode is in place. The gate electrode serves as a

mask, preserving the opposite doping ofthe gate, and incidentally

guaranteeing that the dopant concentration edge is initially (be- fore diffusion) as rough as the gate edge.

For the last two years NIST has been engaged in a

collaboration with International SEMATECH to study

metrology of LER and LWR. The collaboration has idenfified a number of measurement artifacts in roughness

measurement as typically practiced, and has proposed

improved measurement methods to deal with them.

As noted, a standard deviation roughness metric is in general use in semiconductor electronics applications.

This is determined, for example, by measuring a length L

of line at intervals A, subtracting the average length, and

computing the standard deviation of the resulting set of

residuals. Measurement issues identified with this metric

include (1) its dependence upon the sampled length of

line and the chosen measurement spacing, (2) sampling

errors that result from the finite size of line segments

used to estimate roughness and (3) random error and

measurement bias that result from noise in the scanning

electron microscope (SEM) images used to measure

roughness.

The first issue is not a measurement artifact per se

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the true value of roughness varies with L and A, so it is not an error when measurements with different L and A

differ—but it may lead to faulty comparisons when it is ignored in practice. Thus, it is not legitimate to conclude

that sample A is rougher than sample B if A and B were measured with different L or A. The likelihood that such

erroneous comparisons will be made is increased by the

fact that different measurement tools have different capabilities, so may not customarily be programmed to perform the measurements in the same way. The industry's measurement specification is incomplete; it specifies a maximum allowable LWR and a measurement

repeatability, but while it implies a value for L it does

not specify A.

Sampling error is a consequence of the fact that in practice only a finite length of line can be measured. The

chosen segment has a particular configuration of rough

edges that will yield a particular value of the roughness.

A different randomly chosen sample will have a different roughness. The size of this uncertainty can be reduced by

averaging a number of samples for each measurement.

The industry specified a repeatability for roughness

measurements. This represents a required measurement

quality outcome, but how many samples must the meas- urement technician average to assure this outcome? We

derived a simple formula showing, among other things,

that the number depends upon the roughness power spectral density of the sample.

One of the more significant issues with the industry's current metric is the phenomenon of measurement bias. Bias is a non-random component of error (sometimes

called a "systematic" error). A simple measurement of

the standard deviation of edge positions or linewidths is subject to bias because noise in the image on average

increases this standard deviation. There is a false "noise

roughness" variance that adds to the actual roughness

variance. This results in an overestimate of the roughness

by the standard metric (black curve in Fig. 3). Measured

values of noise roughness are comparable to the roughness values specified by the industry for the current generation of devices. In practice, actual product is often far rougher, but if circuits can be made as smooth as the industry's goal, a substantial fraction of the measured

roughness would, in fact, be a measurement artifact. We

proposed an improved metric that corrects the measure- ment bias (red curve in Fig. 3).

Finally, in semiconductor electronics applications to- day, the industry almost exclusively relies upon the stan- dard deviation metric. We pointed out that there are a number of other possible roughness metrics, and under some plausible scenarios, one or more of these others might be more closely related to device performance.

2 4 6

Total Pixel Time ((is)

Figure 3. Bias in L WR measurement. The average roughness

value as determined by the usual standard deviation metric (black

line) increases as the noise level in an image increases. (High

noise in this graph corresponds to low total pixel dwell time, to the left ofthe graph.) All images were taken from the same part of

the sample, so the true roughness is unvarying, and the changes in average measured roughness must be a measurement artifact.

Critical dimension SEM suppliers have expressed an

interest in modifying their measurement algorithms to make them less susceptible to bias and other measure- ment artifacts that were brought to light in these studies. Revised language for roughness specifications based

upon these results is currently under consideration by

ITRS technical working groups.

Selected Publications

1. B. D. Bunday, M. Bishop, D. McCormack, J. S. Villarrubia, A. E. Vladar, T. Vorburger, N. G. Oiji, and

J. A. Allgair, "Determination of Optimal Parameters

for CD-SEM Measurement of Line Edge Roughness," Proc. SPIE 5375, 515-533 (2004).

2. J. S. Villarrubia and B. D. Bunday, "Unbiased Esti- mation of Linewidth Roughness," submitted to Proc. SPIE 5752 (2005).

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Combinatorial Metal Selection for Catalytic Growth of ZnO Semiconductor Nanowires

Semiconductor nanowires (NWs) offer a unique type

of nanoscale building block for creating next- generation lasers and chemical/biological sensors. Control of NW dimension and structural and elec- tronic properties is a major barrier to device fabrication. To address these issues, we are developing a

strategy for selecting metals for catalytic growth of

zinc-oxide (ZnO) nanowires using an approach based

on phase diagrams and high-throughput fabrication

and analysis.

Albert V. Davydov, MSEL (855)

Semiconductor NWs show significant promise for a wide variety of optoelectronic and electronic devices

including nano-lasers, detectors, and chemical and biological sensors. To advance commercialization of such

devices, it is necessary to control properties including

size, orientation, and structural and electronic defects of

the fabricated nanostructures. NWs are often produced

using a vapor-liquid-solid (VLS) approach, in which

nanometer size islands of catalytic metal act as nucleation and growth sites for semiconductor NW growth. The

choice of the metal catalyst can significantly affect the NW properties. We are developing a strategy for select- ing appropriate catalytic metals and applying this strat- egy to VLS growth of ZnO semiconductor. Selection and

screening of catalysts is achieved using thermodynamic

information from phase diagrams as well as a highthroughput (combinatorial) approach recently demonstrated for metallizations to wide-band-gap semiconductors [1]. We have examined the role of elemental noble metals

and their alloys on ZnO NW growth and properties. An

experimental library was designed using the ternary sil- ver-gold-copper phase diagram and included elemental,

binary and ternary compositions as well as two growth

temperatures (900 and 1000 °C). The metal library ele- ments were deposited on a single gallium ni- tride/sapphire substrate, which was subsequently annealing to fomi isolated meal droplets upon which the ZnO

NWs were grown. The resulting set of metal islands and

ZnO NWs was characterized using electron microscopy,

x-ray diffraction, and optical spectroscopy. It was found

that the ZnO NW growth was significantly influenced by

the nature of the catalytic metal droplets formed on the

substrate. For example, growth from the Au islands at 900 °C yielded only a thin ZnO nucleation layer with no NWs, while other metal compositions, which melt at lower temperatures than Au, produced a variety of NW

shapes, sizes and orientations at this temperature (Fig. 1). The approach also permitted rapid correlation of the optical properties of the nanowires with the composition of

the metal islands from which they were grown. For ex- ample, ZnO NWs fabricated from the silver islands at 900 °C produced the sharpest excitonic peak near 3.36

eV on the cathodoluminescence spectrum (comparable to that for bulk ZnO single crystal), indicating superior

crystalline structure. In contrast, the spectra were significantly broader for the NWs grown from the gold and

copper containing islands. A comprehensive assessment of the correlation of structural and spectroscopic data for the ZnO NW library is still underway. These initial re- sults demonstrate the power of the combined phase dia- gram/combinatorial approach for controlling semiconductor NW properties.

Figure 1. Selected scanning electron microscope images from the combinatorial library: a) ZnO wetting layer grown using gold

catalyst (note the absence ofNWs); b) ZnO NWs grown using

gold-copper alloy catalyst; c) ZnO NWs grown using silver catalyst. Growth temperature is 900 °C. The scale bar on the top image is 1 ;jm.

Selected Publications

1 . A.V. Davydov, A. Motayed, W.J. Boettinger, R.S.

Gates, Q.Z. Xue, H.C. Lee and Y.K. Yoo., Phys.

Stat. Sol.(c), DOI I0.1002/pssc.200461605 (2005).

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Electrical Properties of On-Chip Interconnections

Conductors in on-chip metallizations are reaching

dimensions where defective seed layers are impacting

manufacturing yields and electron scattering on sur- faces and grain boundaries is reducing electrical

transport. To meet these challenges we have quantified the sources ofthe resistivity increase ofwires

made ofsilver, the most highly conductive ofall metals, demonstrated seedless processing routesfor copper wires, and improved understanding and modeling

ofthe superfdlfabrication process.

Daniel Josell and T. P. Moffat, MSEL (855)

Reduction of transistor size in integrated circuits has

been accompanied by shrinkage of the on-chip intercon- nects that carry electronic signals. This is straining in- dustrial technology for copper seed deposition, a technical challenge noted in the International Technology

, Roadmap for Semiconductors. Such defects motivated our "seedless" ruthenium barrier process last year and

motivate this year's work on seedless copper superfill

using ruthenium or iridium barriers (Fig. 1 ) deposited by

Atomic Layer Deposition.

Figure 1. Trenches containing copper that was electrodeposited

directly on an ALD iridium barrier (thin bright layer).

Shrinking dimensions are also leading to degraded

electrical transport in industrial copper interconnects.

Silver wires for electrical studies (Fig. 2) were fabricated

using a superfill process developed in the Metallurgy

Division. Wire resistivity increased with decreasing wire

width (Fig. 3). The Fuchs-Sondheimer analysis for elec- tron scattering was extended to include specular and dif- fuse scattering and resulting modeling indicated the

Figure 2. Cross-sections oj electrodeposited silver wires after removing metalfrom thefield (transmission electron microscope).

increase arises from grain boundary as well as surface

scattering, with significant implications for mitigation.

(b) 300 nm Tall Ag Wires

5.0

£ 4.0 ua

.s 3.0 >

K 2.0

1.0

• Corrected Data

spec. 0.0, refl. 0.4 spec. 0.0, refl. 0.3 .... spec. 0.0, refl. 0.2

0.0 5.0 10.0 15.0

1/Wlre width, ^m'

20.0

Figure 3. Electrical resistivities of300 nm tall silver wires with predictions that accountfor surface scattering with varying amounts ofgrain boundary scattering. Resistivities for wires less than 100 nm wide are impacted by defects. We continue to meet the industrial need for feature

fill prediction. Experimentally determined deviations

from the ideal adsorbate behavior underlying superfill

have been added to our CEAC model. This has made our

predictions even more accurate.

Industrial outreach continues: code sent by request to

Intel, ST Microelectronics, and ATMI and an invited

article in the IBM Journal of Research and Development.

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Horizontal Growth and In Situ Assembly of Oriented

Semiconductor Nanowires

The positioning and directed assembly ofsemiconductor nanowires (NWs) is ofconsiderable current inter- estfor "bottom-up " approaches to the engineering of

intricate structures from nanoscale building blocks. In

this regard, we have developed an approach that results in the horizontal growth, in situ alignment, and

predictable positioning ofnanowires. This strategy

allows for the large scale production ofarrays of

nanowires and nanowire based devices.

Babak Nikoobakht and Stephan J. Stranick,

CSTL (837)

The vision of nanotechnology includes the possibility

of building powerful, extraordinarily compact devices

out of atomic-scale components. Nanowires (NWs),

long, thin crystals of metals or semiconductors, are one

such component. NWs can be used to not only link nanoelectronic devices, analogous to a conventional

wire, but they can also function as unique devices themselves, e.g., photodetectors and/or light-emitting ele- ments. A crucial barrier to realizing this vision is the inherent difficulty associated with manipulating components

so small that only the most sophisticated instruments can

track and possibly direct their movement. Even simple

manipulation, such as the alignment of NWs, presents an enormous technical challenge. To date, the most successful NW alignment methods have involved multi-step processes that include growth of a large number of the NWs on a suitable substrate, harvesting the NWs by

shearing them off of the substrate, mixing the NWs in a

solvent, aligning them using an applied field or by flow

through microchannels, and finally, critical point drying

to remove the solvent. Further, these time consuming

procedures do not address the need to place the NW at a predetermined position on a substrate.

In addressing some of these issues, a method was de- veloped in our laboratory by which in situ growth of

horizontally aligned, crystalline NWs at predetermined

(nanoscale) locations on a substrate became possible.

We discovered that an Au catalyst droplet 25 nm in size or smaller, at high temperature, in the presence of Zn and

O2 precursors grows horizontally to a ZnO NW with a unique direction on a a-plane sapphire substrate. In this process, Au droplet moves on the surface as NW grows

(Figure la). Atomic force microscopy also shows the

straight growth trajectory of NWs and their smooth, defect-free surface (Figure 1 b). We have found that the

size of the Au catalysts and their spacing are very essential in promoting this type of growth. To pattern a sur- face with Au droplets that serve as "seed" particles for the horizontal NW growth, soft lithography and electron- beam lithography (EBL) are used. Both techniques can be used to deposit Au on selected areas of a surface. Another advantage of this approach is the ability to control the width of the NWs within a range of 2 to 25 nm. This

is achievable by adjusting the lateral dimension of the deposited Au patterns, which results in Au particles 2-20

nm in size.

0 2 4 (uiti) 2 4

(f)

Figure 1. a) SEM image ofZnO NWs grown from Au nanodroplets. The brighter part of the NW is Au. b) AFM image ofZnO

NWs shows the smooth surface ofNWs and the unique growth

direction, c) Steps 1-4 show the preparation ofAu lines as catalyst, d) AFM image ofpolymer lines made in step 3. e) AFM im- age ofAu lines after removing polymer lines in step 3. f) Schematic growth ofNWsfrom an Au line.

Using soft lithography, the developed method begins

with patterning a sapphire substrate (1 120) with narrow Au lines, oriented perpendicular to the perfered growth

direction of the NWs. Au lines are fabricated by depositing 5- 6 nm of Au onto the substrate, followed by over

coating it with poly (ethyl methacrylate).

This is immediately followed by placing a micropatNIST Accomplishments in Nanotechnology

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teraed polydimethylsiloxane (PDMS) stamp on the polymer/substrate and removing it once the polymer is sohdified at 25° C. The polymer lines are nominally 1 .5

|a,m in width with a 400 nm spacing between them

(Fig. Ic, step 3 and Fig. Id). The patterned substrate is submerged in an Au etchant solution (KI/I2/H2O) for 20-

40 seconds (depending on the initial Au film thickness)

followed by rinsing with deionized water (18 MQ). The

Au film protected by the polymer lines (unetched) results

in parallel Au lines, which can be made narrower than

100 nm by increasing the etching time. The last step is to remove the polymer lines by washing with ethanol

(Figure le). The advantage of using narrow Au lines is that at high temperature, this morphology results in nanodroplets (nanoparticles) with the majority of them

forming lines. These are the nanodroplets that catalyze

the horizontal growth and result in formation of the or- ganized NWs, which schematically is illustarted in Fig- ure 1 f A SEM image, in Figure 2a, shows the alignment

of the NWs and their growth direction. SEM and elec- tron back scattering diffraction were used to determine

the growth direction of NWs( ± [1 1 OOJsap) relative to the two known facets of the sapphire wafer.

The width of the NW is a critical parameter in the application of NWs, as this determines the extent of

quantum behavior (confinement) in the direction trans- verse to the long axis of the NW. This effect is often seen in quantum wires with size regimes less than 10 nm,

and there are very few examples of fabricating NWs with

diameters within this size range. The developed ap- proach enables us to grow quantum wires with very

small diameters (Figure 2b). These quantum wires are grown from Au colloidal nanoparticles a few nanometers

in diameter.

Precisely positionable and addressable NWs are crucial requirements of nanodevice fabrication; however,

despite intense efforts in this direction, there has not been a practical solution to these issues. Our approach

takes us one step closer to fulfilling these requirements.

As a demonstration, EBL was used to precisely place Au

catalyst on a substrate. NWs can be grown at those areas

selectively (Figure 2c). These NWs later can be ad- dressed by EBL technique for nanodevice fabrication.

The significant potential impact of this work has been

recognized by the scientific community, as the highlights/vignettes have appeared on nearly a dozen

nanotechnology focused websites world-wide, ranging

from "popular scientific magazines" (e.g., US Tech) to science "foundation" (e.g., The Foresight Institute) venues. The dramatically reduced process time compared

with less effective physical alignment procedures mentioned above, together with the inherent control of NW

placement, provide a methods that should facilitate fabrication of nanoscale devices.

Figure 2. SEM image ofarrays ofZnO NWs all oriented in one

direction, b) ZnO NWs grown from Au colloidal NPs. Use ofcol- loidal particles also results in growth ofquantum wires, c) few

NWs originatedfrom an Au line, which wasfabricated by e-beam

writing.

This work will also serve as a basis for further re- finements including, for example, the use of conventional photolithography to form the Au droplet seed pattern, which could allow this approach to be widely used

in nanodevice fabrication on an industrial scale.

Selected Publications

1. B. Nikoobakht, C. A. Michaels, S. J. Stranick and

M. D. Vaudin, Appl. Phys. Lett. 85, 3244 (2004).

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Organic Electronics

Thefield oforganic electronics has dramatically

emerged in recent years as a technology encompassing a wide array ofdevices and applications including

embeddedpassive devices, flexible displays, organic

solar panels, and radiofrequency identification tags.

Device performance, stability, andfunction critically

depend upon charge transport and material interaction at the interfaces ofdisparate materials at nanoscale dimensions. We develop and apply nondestructive measurement methods to characterize the elec- tronic and interfacial structure oforganic electronics

materials with respect to processing methods, processing variables, and materials characteristics.

Eric K. Lin, MSEL (854)

als and interfaces. We are developing measurement

methods to provide the data and insight needed for the

rational and directed development of emerging materials

and processes. We have focused on several active organic electronics materials including dielectric measurements of nanocomposites polymer films for embedded passive devices

and advanced measurements of the orientation and struc- ture of organic semiconductors for transistor applications.

Figure 1. Schematic diagrams ofthe local structure ofpoly 3- hexylthiophene, an organic semiconductor near an interface (left) and the architecturefor an organic transistor (right).

To meet the metrology needs for embedded passive

devices, two new measurement methods were successfully developed and will find widespread use within in- dustry. A NIST-developed Test Method for Dielectric

Permittivity and Loss Tangent of Embedded Passive Materials from 100 MHz to 12 GHz was accepted and rec- ommended as a standard test method. In addition, a new

test was developed and applied for testing passive materials at high electric fields and voltages.

Finally, near-edge x-ray absorption fine structure (NEXAFS) spectroscopy was applied to several classes of organic electronics materials to investigate the elec- tronic structure, chemistry, and orientation of these molecules near a supporting substrate. NEXAFS pro- vides a powerful and sensitive method to probe the inter- facial structure that is critical to the performance of these

devices. For example, it was found that several organic

semiconductor molecules preferentially align "edge-on"

rather than "face-down" on solid substrates.

Organic electronic devices are projected to revolutionize integrated circuits through new applications that take advantage of low-cost, high volume manufacturing,

nontraditional substrates, and designed functionality.

The current state of organic electronics is analogous to the early stages of the silicon electronics industry with

the concurrent development of multiple material platforms and processes and a lack of measurement stan- dardization between laboratories. A critical need exists for new diagnostic probes, tools, and methods to address new technological challenges.

cheap dynamic signs wearable electronics

RFID tags flexible solar cells

Organic electronics present fundamentally different measurement challenges fi-om those identified for inor- ganic devices. The adoption of this technology will be

advanced by the development of an integrated suite of

metrologies to correlate device performance with the nanoscale structure, properties, and chemistry of materiNIST Accomplishments in Nanotechnoiogy

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6 - Major Research Facilities and Instrumentation Acquisition

NIST Accomplishments in Nanotechnology

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6 - Major Research Facilities and Instrumentation Acquisition

The NIST Advanced Measurement Laboratory Nanofab

To continue to respond to U.S. science and industry's

needsfor more sophisticated measurements and stan- dards in theface ofheightened global competition,

NIST is constructing one ofthe most technologically

advancedfacilities in the world—the Advanced Measurement Laboratory, orAML. The NISTAML Nanofab (NF) is one offive buildings in the AML at the

Gaithersburg, MD campus. The AML Nanofab will

provide researchers at NIST working on a variety of

semiconductor and other nanotechnology research the

ability to fabricate prototypical nanoscale test structures, measurement instruments, standard reference

materials, and electronic devices.

Robert Celotta (CNST) Russ Hajdaj (CNST) and

Eric M. Vogel, EEEL (812)

The AML contains 2 above ground instrument buildings, 2 completely below ground metrology buildings,

and 1 class 100 clean room building which will house

the NIST AML Nanofabrication Facility. The AML will provide NIST with superior vibration, temperature and

humidity control, and air cleanliness.

The NIST AML Nanofab has approximately 10,000

sq. ft. of Class 100, raised floor, bay and chase, clean room space. NIST has invested in a complete suite of new equipment (capable of processing 150 mm wafers)

that will be installed over the upcoming year. This in- cludes furnaces (2 banks of 4 tubes each), LPCVD (poly,

nitride, LTO), rapid thermal annealer, 3 reactive ion

etchers (SF6/O2, Metal, Deep), 3 metal deposition tools (thermal, e-beam, sputterer), contact lithography (frontand back-side alignment), e-beam lithography, focused

ion beam, and numerous monitoring tools (FESEM,

spectroscopic ellipsometer, contact profilometer, 4-point

probe, microscope with image capture, etc.).

The NIST AML Nanofab will be operated as shared

access user facility. This means that the staff of NIST

and its partners, subject to provisions, training, and user

fees, will be permitted to independently operate the equipment. The tools will be operated in a manner such

that a wide variety of materials can be processed. The

facility will be directed by NIST's Semiconductor Electronics Division. Unlike other nanofabs, the NIST AML

Nanofab is unique in that it is located next to the most

advanced metrology tools in the world, and its focus will be on fabricating nanoscale structures necessary for me

trology and standards in support of the semiconductor

industry, nanotechnology, biotechnology, and homeland

security.

, The NIST Advanced Measurement Laboratory

(AML) Nanofabrication Facility (NF) will:

Enable fabrication of prototypical nanoscale test structures, measurement instruments, standard refer- ence materials, electronic devices, MEMS, and biodevices critical to NIST's Strategic Focus Areas

(Nanotechnology, Homeland Security, Healthcare)

and the Nation's Nanotechnology Needs

Provide access to expensive nanofabrication tools,

technologies and expertise in a shared-access,

shared-cost environment to NIST and its partners

• Foster internal collaboration in Nanotechnology

across NIST's Laboratories

Foster external collaboration in Nanotechnology

with NIST's partners.

Figure L The NIST Advanced Measurement Laboratory

NIST Accomplishments in Nanotechnology

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6 - Major Research Facilities and Instrumentation Acquisition

Probing Nanoscale Disorder with Neutron Tunneling

Spectroscopy

To provide researchers with dynamical probes of

atomic and nanoscale disorder using high-resolution

neutron tunneling spectroscopic methods.

Robert M. Dimeo and Dan A. Neumann, MSEL

(856)

Structural disorder can have dramatic effects on macroscopic materials properties. Nanoscale disorder is described in terms of distributions of geometrical quantities

such as bond lengths and bond angles, which can be determined from diffraction measurements. Alternatively,

it can be quantified by the distribution of potential energies experienced by the atomic scale constituents of the system. This distribution is sharp for a highly ordered

(crystalline) system, and broad for a disordered system.

Rotational tunneling spectroscopy is a powerful tool to characterize the local molecular environment due to its exquisite sensitivity to the local potential. This tech- nique has been used to determine the distribution of rotational potentials felt by methyl iodide molecules adsorbed in a series of nanoporous silica glasses. The de- gree of confinement-induced disorder in the methyl io- dide can be directly related to the pore sizes in the glass.

The silica glasses used in this study have a range of

pore size distributions with nominal diameters, <d>, that vary from 2.5 nm to 14.4 nm. Liquid methyl iodide

(CH3I) was condensed into the porous hosts until 95 %

of the open pore volume was filled and then cooled to 5 K. Spectra were collected using the NCNR High-Flux

Backscattering Spectrometer with an energy resolution

of 0.8 laeV.

The rotational potential felt by methyl (CH3) groups

in methyl iodide (CH3I) arises solely from interactions

with the local environment including the neighboring

molecules. The energies of the transitions between rota- tional states can be found by solving the Schrodinger

equation for the model potential by V3(l-cos(30))/2 in the angular coordinate about the C-I axis, 9. In the limit of an infinitely high barrier, V3, the motion of the CH3

group is that of a torsional oscillator. However, if the

barrier is not too large, the wavefunctions will have sig- nificant overlap and quantum mechanical tunneling between the potential minima can occur. This results in a

splitting of the rotational ground state. The energy of

this splitting, which can be directly measured using cold neutron spectroscopy, has a nearly exponential dependence upon the barrier height. This strong dependence on

V3 is the reason why tunneling spectroscopy is so sensitive to the local molecular environment and, in particular, the quenched disorder of adsorbed molecular solids.

The neutron scattering spectra display broad asymmetric lines indicative of a distribution of potential barrier heights. These distributions can be extracted from

the neutron scattering data using a simple model in which a sum of Gaussian distributions of potential barri- ers (which yield an asymmetric tuimeling line-shape) is used to fit the spectra. A sum of two potential distribu- tions is sufficient to describe the data. One distribution is quite broad, indicating substantial disorder of the methyl

iodide solid, while the other is much narrower suggesting

a relatively high degree of order.

The relative weights of the two components show a pronounced dependence on <d>. Figure 1 shows that the

fraction of the total integrated intensity in the disordered

component of the tutmeling spectrum decreases as <d>

increases. This behavior can be described with a simple

model in which the relatively well-ordered component of

the scattering is attributed to CH3I molecules near the center of the pore while the highly disordered component

is associated with a ring of thickness t of disordered

molecules near the pore walls. Treating the pores as cir- cular cylinders, the fraction /of disordered molecules is just the ratio of the ring to circular areas:

/ = 29 It. {d)rVAd)

The solid line in Fig. 1 compares this model to the data for t = 1.41 (4) nm, which corresponds to ~ 3 molecular layers, and provides an excellent fit to the data.

d

- - 1 -

< 2t

t =1.41(4) nm

<d> (nm)

Figure 1. Fraction fversus mean pore diameter. The inset shows

a schematic ofa single pore.

NIST Accomplishments in Nanotechnology

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Probing the Nanostructure of Lipid Membranes with the AND/R

(Advanced Neutron Diffractometer/Reflectometer)

To provide researchers with probes ofbiological

nanostructure using neutron reflectometry methods.

M. Mihailescu and David Worcester, MSEL

(856)

Lipids are the major constituents of cell membranes.

Arranged as a two-molecule bilayer with a thickness of

about 5 nm, they insulate the inside of cells and cell or- ganelles from the outside. Proteins embedded in this bilayer provide the only conduits between the two sides. Intriguingly, the structures of the embedded proteins are shaped by lipid-protein interactions. To understand and

describe these interactions quantitatively, the structures of lipid bilayers of varying lipid compositions must be

determined. Neutron diffraction is a powerful tool for

this purpose. As an illustrative example, we present such

a study of bilayer membranes containing cholesterol. Due to their dual hydrophobic-hydrophilic character,

cell membrane lipids organize spontaneously into bilayers, causing the hydrophobic alkyl chains to be hidden in the bilayer center and the polar 'headgroups' to be ex- posed to water. The deposition of lipids onto a solid substrate (silicon, quartz, or glass) generally results in a

stack of bilayers aligned parallel to the substrate surface

(Fig. 1). The samples in this study were formed from

dimyristoylphosphatidylcholine (DMPC) lipids in mixture with 0.20-mole fraction of cholesterol. Cholesterol, an essential component of mammalian membranes, participates in many physiological processes and plays an

important role in organizing other lipids and membrane

proteins. There is increasing evidence that it can associate preferentially with saturated lipids to form ordered

domains called "rafts". These are believed to play an

important role in signaling across cell membranes.

Figure 1. A stack oflipid bilayers at 66 % relative humidity, and

T = 300 K (image courtesy ofF. Castro-Roman and R. Benzfrom

VC Irvine).

The usefulness of neutron diffraction for studies of

DMPC-cholesterol and other membranes arises because

membrane lipids and the surrounding water are rich in

hydrogen atoms that can be selectively replaced with

deuterium without changing the structure of the membrane. Deuterium nuclei coherently scatter neutrons much more strongly than protons. Selective deuteration

thus allows the deuterated atoms to be readily identified. The structural information sought in our experiments was how the lipid mass (scattering length density) is dis- tributed across the thickness of the membrane. Specular

diffraction on oriented lipid muhilayers, (i.e. diffraction where the neutron beam incident and emergent angles

relative to the sample surface are equal), produces a dif- fraction peak at Bragg angles related to the d spacing of

the bilayers.

The DMPC lipids were deuterium-labeled near the ends of the alkyl chains. 4000 bilayers, deposited on a

thin glass slide, were mounted vertically in the beam in a sealed sample chamber at room temperature and a controlled relative humidity of 66 %. Fig. 2 shows two sets of diffraction patterns collected in a specular set-up for two water-confrast conditions: 100 % H2O and a mixture containing a 0.50 mole fraction of D2O.

100%H2O 50%D2O

m

0 0.25 0.5 0.75 1 1.25 1.5 Oz IA"'l

Figure 2. Diffraction intensities for oriented multilayers of

DMPC + 0.20 molefraction ofcholesterol at 22 °C.

These data allow the position and extension of the

different submolecular groups to be resolved. The bi- layer thickness, as determined from the position of the

diffraction maxima, is 6/= 5.4 nm, including the water

associated with the lipid. Deuterium labeling at different positions along the lipids would allow a determination of

the positions of specifically labeled atoms in the bilayer

with a resolution of better than 0. 1 nm.

NIST Accomplishments in Nanotechnology

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2004 - 2005 NIST Accomplishments in Nanotechnology

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Appendix

Appendix 1 - Cross-Reference to NIST Nanotechnology Focus Areas

Nanometrology (Fundamental science and basic measurement capabilities)

Nanomechanics/ structure

Structure and Dispersion Measurements of Polymeric Building Materials,

Li-Piin Sung (BFRL) Ill

Nanomechanical Properties of Polymer Thin Films, Christopher M. Stafford (MSEL) 103

Nanomechanical Metrology, Douglas T. Smith and Lyle E. Levine (MSEL) 104

Nanoelectrical

Evanescent Microwave Probing and High-resolution Microwave Power Imaging,

Michael Janezic, James Baker-Jarvis, and Pavel Kabos (EEEL) 63

Electrical Properties of On-Chip Interconnections, Daniel Josell and T.P. Moffat (MSEL) 158

Nanochemical/ Nanobiological

Chemical Sensor Microscopy for Nanotechnology, Tinh Nguyen and Xiaohong Gu (BFRL) 101

Probing the Nanostructure of Lipid Membranes with the AND/R (Advanced Neutron

Diffractometer/Reflectometer), M. Mihailescu and David Worcester (MSEL) 165

Nanoassembly/fabrication

Nanoscale Science, Joseph A. Stroscio and Daniel T. Pierce (PL) 5

Nanotechnology with atom optics, Jabez J. McClelland (PL) 7

Agile Nanoassembly to Prototype, Test, and Manufacture Functional Nanodevices, Thomas W.

LeBrun(MEL) 151

Nanolithography: Next-Generation Polymer Photoresists, Vivek M. Prabhu (MSEL) 16

Combinatorial Metal Selection for Catalytic Growth of ZnO Semiconductor Nanowires, Albert V. Davydov (MSEL) 157

Dimensional IMetrology/ Innaging

Development of High-resolution Variable Pressure Reference Scanning Electron Microscope,

Andras E. Vladar, Michael T. Postek and John S. Villarrubia (MEL) 133

Scanning Probe Microscopy for Nanoscale Measurements, Ronald Dixson, Joseph Fu, George Orji,

and Theodore Vorburger (MEL) 135

Scanning Probe Microscopy for Nanoscale Measurements: Atom-Based Height Standards,

Joseph Fu, Ronald Dixson, George Orji, and Theodore Vorburger (MEL) 137

NIST Accomplishments in Nanotechnology

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Appendix

Atom-Based Dimensional Metrology, Rick Silver (MEL) 139

Critical Dimension Metrology using Scanning Electron Microscopy and a Model-Based Library,

John S. Villarrubia and Andras E. Vladar (MEL) 141

SI traceable force for scanning probe microscopy and instrumented indentation, Jon R. Pratt (MEL) 143

Phase-sensitive Scatterfield Optical Imaging for sub- 10 nm Dimensional Metrology, Rick Silver

(MEL) 145

Improving Image Resolution in Nanotechnology, Alfred Carasso (ITL) 105

Optical Metrology for Nanolithography, John H. Burnett, Simon G. Kaplan, and Eric L. Shirley

(PL) 107

True Three-Dimensional Tomography Utilizing Bayesian Statistical Inference, Donald Malec and

Juan Soto (ITL) 109

Advanced Control Systems and Positioning for Nanoscale Measurements and Standards,

Nicholas G. Dagalakis (MEL) 147

Molecular Measuring Machine, John A. Kramar (MEL) 149

Development of Nano-tip Electron Guns for the Scarming Electron Microscope, Andras E. Vladar

and Michael T. Postek (MEL) 153

Measurement Strategy for Line Edge and Linewidth Roughness Metrology, John S. Villarrubia

and Andras E. Vladar (MEL) 155

Nanocharacterization (Characterization of nanostructured materials)

Nanomechanics/ structure

Creating Visual Models of Nanoworlds, Judith Devaney (ITL) , 13

Nanomechanical Characterization of Nanoparticle-fiUed Polymeric Coatings and Composites,

Stephanie Scierka and Peter Votruba-Drzal (BFRL) 11

Mechanical Metrology for Small-Scale Structures, Edwin R. Fuller, Jr. and George D. Quinn

(MSEL) 56

Nanotribology and Surface Properties, Stephen Hsu (MSEL) 116

New Materials to Protect Firefighters from Bum Injuries, Jeffrey Oilman and Kuldeep Prasad

(BFRL) 46

Quantitative Nanomechanical Properties, Donna C. Hurley (MSEL) 1 19

NIST Accomplishments in Nanoteciinology

168
Appendix

Probing Nanoscale Disorder with Neutron Tunneling Spectroscopy, Robert M. Dimeo and

Dan A. Neumann (MSEL) 164

Neutron Reflectometry for Highly Accurate Nanometer Metrology, Joseph A. Dura (MSEL) 117

Nanoelectrlcal

Semiconductor Quantum Dot Structure Metrology, Alexana Roshko (EEEL) 53

Nanochemical/ Nanobiological

Nanoscale Characterization of Polymeric Materials Degradation, Tinh Nguyen, Xiaohong Gu,

and Jon Martin (BFRL) 9

Polymer Miscibility of Imidazolium Compatibilized Nanotubes, Jeffrey Oilman (BFRL) 45

Characterization and Photoreactivity of Titanium Dioxide Nanostructures, Stephanie Scierka and

Joannie Chin (BFRL) 47

Chemistry and Structure of Nanomaterials, Daniel A. Fischer (MSEL) 55

Synthetic X-ray Spectrum Images for 3D Chemical Imaging at the Nanoscale, John Henry Scott and Nicholas Ritchie (CSTL) 1 13

Gradient Reference Specimens for Advanced Scanned Probe Microscopy, Michael J. Fasolka and

Duangrut Julthongpiput (MSEL) 118

Nanoassembly/fabrication

Brillouin Light Scattering, Ward Johnson, Sudook Kim, Colm Flannery, and Pavel Kabos

(MSEL) 15

Highly Charged Ions in EUV Lithography, John D. Gillaspy and Joshua M. Pomeroy (PL) 120

Highly Charged Ion Modified Materials, Joshua M. Pomeroy and John D. Gillaspy (PL) 57

Carbon Nanotubes/ Nanowires

Optical Properties of Carbon Nanotubes, John Lehman (EEEL) 49

Characterization of Single Walled Carbon Nanotubes, Angela R. Hight Walker (PL) 50

Stability of Nanowires, Geoffrey B. McFadden (ITL) 51

Chemical Sensing with Nanowires & Nanotubes, Richard E. Cavicchi (CSTL) 63

Ill-Nitride Semiconductor Nanowires: Growth and Characterization, Kris Bertness and Norman

Sanfbrd (EEEL) 65

Carbon Nanotube Metrology, Paul Rice and Tammy Oreskovic (MSEL) 115

NIST Accomplishments in Nanotechnology

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Appendix

Nanoelectronics (Nanoscale electronics and optoelectronics)

Nanoelectronics

Organic Electronics, Eric K. Lin (MSEL) 161

Modeling for Organic Conduction, Carlos A. Gonzalez (CSTL) 17

Measuring & Understanding Charge Transport in Molecules, Roger van Zee, (CSTL) 64

Spin Electronics, William H. Rippard and Matthew R. Pufall (EEEL) 19

Nanoscale Metrology of Ill-Nitride Semiconductor Films and Nanowires, Norman Sanford and

John Schlager (EEEL) 58

Nanoelectronic Device Metrology Project, Curt A. Richter and Eric M. Vogel (EEEL) 70

Horizontal Growth and In Situ Assembly of Oriented Semiconductor Nanowires,

Babak Nikoobakht and Stephan J. Stranick (CSTL) 159

MOS Device Characterization and Reliability Project, John S. Suehle and Eric M. Vogel (EEEL) .. 68

Electrical Test Structure Metrology, Michael Cresswell and Richard Allen (EEEL) 122

The NIST Advanced Measurement Laboratory Nanofab, Robert Celotta (CNST), Russ Hajdaj

(CNST) and Eric M. Vogel (EEEL) 163

Nanomagnetics

Molecular Nanomagnets, Brant Cage and Stephen E. Russek, (EEEL) 124

Nanomagnetism, John Unguris and Mark Stiles (PL) 22

Nanomagnetodynamics, Robert D. McMichael (MSEL) 21

Nanomagnetodynamics: Devices, Stephen E. Russek, (EEEL) 72

Nanomagnetodynamics: Fundamental Properties, Thomas J. Silva, (EEEL) 25

Mathematical Modeling of Nanomagnetism, Michael J. Donahue and Donald G. Porter (ITL) 60

Nanomagnetic Measurements, John Moreland, (EEEL) 126

Synthesis, Characterization, and Manipulation of Magnetic Nanoparticles, Guangjun Cheng and

Angela R. Hight Walker (PL) 24

Magnetic Materials For Nanoscale Sensors and Ultra-High Density Data Storage, William F. Egelhoff, Jr. (MSEL) 62

Nanoscale Engineered Magnetic Sensors, David P. Pappas (EEEL) 74

NIST Accomplishments in Nanotechnology

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Appendix

Nanobiotechnology (and Nanochemistry)

Bioelectronics, Michael Gaitan, Darwin Reyes-Hernandez, and Brian Polk, EEEL (812), Laurie

Locascio, and Sam Forry CSTL (839) 27

Single Molecule Nanovials, Michael Gaitan EEEL (812), and Laurie Locascio and Wyatt Vreeland,

CSTL (839) 29

Optical Manipulation of Nanocontainers and Nanotubes, William D. Phillips, Kristian Helmerson

(PL) 31

Quantitative single-molecular pair fluorescence resonance, Lori S. Goldner (PL) 98

Bacteriophage/Quantum-dot nanocomplex to detect biological target in clinical or environmental

isolates, Jeeseong Hwang (PL) 100

Nanoparticle Measurements for Biological Applications, John Moreland, (EEEL) 128

Imaging Methods for Nanoparticle and Cell Systems to Investigate Toxicity, Cynthia J. Zeissler

and Peter E. Barker (CSTL) 130

Quantum Devices, (Computing, and Communications)

Quantum devices

Fundamental Properties of Quantum Dots, Kevin Silverman (EEEL) 33

Multiscale Modeling of Nanostructures, Vinod Tewary and David Read (MSEL) 35

Precision Metrology for Nanoscale Quantum Technologies, John Lawall and Gamett W. Bryant

(PL) 38

Nanoscale Fabrication, James Beall (EEEL) 80

Quantum Sensors, Kent Irwin (EEEL) 88

Single Electronics for Standards and Metrology, Mark Keller and Neil Zimmerman (EEEL) 90

Quantum Voltage Metrology, Samuel Benz and Yi-hua Tang (EEEL) 92

Quantum computing

Control of Cold Quantum Gases for Atomic Clocks and Quantum Information Processing,

Paul S. Julienne and Eite Tiesinga (PL) 40

Quantum Computing with Trapped Ions, David Wineland and Tom O' Brian (PL) 42

Quantum Computing Using Superconducting Phase Qubits, Ray Simmonds (EEEL) 86

Neutral Atom Quantum Computation, William D. Phillips, James Porto (PL) 44

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Appendix

Synthesis of Quantum Logic Circuits, Stephen S. Bullock (ITL) 96

Architectures for Fault-tolerant Quantum Computing, Emanuel Knill (EEEL) 94

Quantum communications

Designing the Nanoworld: Nanosystems and Nanooptics, Gamett W. Bryant (PL) 36

Single Photon Sources and Detectors, Richard Mirin (EEEL) 76

Terahertz Technology, Erich Grossman and Robert Schwall (EEEL) 78

Johnson Noise Thermometry, Wes Tew, Sam Benz, Sae Woo Nam (EEEL) 82

Quantum Communications, Sae Woo Nam (EEEL) 84

NIST Accomplishments in Nanotechnology

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Appendix

Appendix 2 - Keywords

A • (Nano) Manufacturing

Atom (Nano) Material

B

Biohazard

(Nano) Biological

(Nano) Biotechnology

Bottom up

C

Carbon Nanotubes

(Nano) Chemistry

(Nano) Chemical characterization

Combinatorial method/ library

D

Data

Dimensional metrology

E

(Nano) Electrical characterization

(Nano) Electronics

Environmental

F

(Nano) Fabrication/ Assembly/

Manipulation/ Modification

IInstrumentation

L

(Nano) Lithography

M

(Nano) Magnetics

(Nano) Materials characterization

(Nano) Mechanics

(Nano) Mechanical characterization

MEMS

Modeling

Molecule

Molecular beam epitaxy

N

Nanowires

Neutron characterization

O

(Nano) Fabrication

(Nano) Optical characterization

(Nano) Optics

(Nano) Optoelectronics

Organic

P

(Nano) Particles

Photons/ Photonics

Polymers

Q

Quantum communications

Quantum computing

Quantum devices

Quantum dots

NIST Accomplishments in Nanotechnology

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Appendix

R

Reference material

S

SI Units

Scanning probe microscopy (electron or

optical probe)

Scanning probe microscopy (physical probe)

Scanning Tunneling Microscopy

Semiconductor

Sensors

Societal Dimensions PCA (implications for)

Spectroscopy

Standard Reference Material

Standards

Standards organization

(Nano) Structure/ Structured

T

Traceability

X

X-ray characterization

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Appendix

NIST Accomplishments in Nanotechnology

175