On the Cover:

From the front cover and continuing on to the back, the images shown are,

respectively: (1) Two-dimensional (2D) electron energy loss spectra (EELS)

from a silicon oxide-nitride-oxide (ONO) stack, in which the horizontal

dimension corresponds to energy loss, and the vertical represents a spatial

coordinate; that is, each 2D spectrum represents a collection of EELS

spectra spatially-resolved across the interfaces. From “Nanoscale Structural/

Compositional Characterization of Ultra-Thin Films; (2) Tensile test specimen

— MEMS polysilicon thickness 4 micrometers; (3) Magneto-optical image

of domain patterns in a new “synthetic antiferromagnet” Co/Ru/Co trilayer

showing two different types of domains separated by novel non-180 degree

walls. Similar trilayers are used in high density recording media; (4) Color

representation of one normal mode of magnetization vibration calculated for

a 50 nm thick film of Permalloy with 20 nm defects; and (5) A synchrotron

x-ray scattering pattern of a nanostructured thin film for electronic applications.

NIST Website: www.nist.gov

Voice Phone: 301–975–2000
i

Materials Science and

Engineering Laboratory

FY 2002 Programs and

Accomplishments

MSEL Materials

for Micro- and

Optoelectronics

Frank W. Gayle

Deputy Chief, Metallurgy Division

NISTIR 6971

March 2003

National Institute of

Standards and Technology

Arden L. Bement, Jr.

Director

Technology

Administration

Phillip J. Bond

Undersecretary of

Commerce for Technology

U.S. Department

of Commerce

Donald L. Evans

Secretary
ii

Certain commercial entities, equipment, or materials may be identified in this document

in order to describe an experimental procedure or concept adequately. Such identification

is not intended to imply recommendation or endorsement by the National Institute of

Standards and Technology, nor is it intended to imply that the entities, materials, or

equipment are necessarily the best available for the purpose.
iii

Table of Contents

Table of Contents

Executive Summary ............................................................................................... 1

Technical Highlights .............................................................................................. 3

Pore Size Distributions in Low-k Dielectric Thin Films from X-ray Porosimetry .............. 4

Direct Measurement of the Reaction Front in Chemically Amplified Photoresists ............ 6

Nanoscale Structural/Compositional Characterization of Ultra-Thin Films ...................... 8

Advanced Metallizations for Sub-100 Nanometer Electronics ..................................... 10

Carbon Nanotubes: Characterization and

Application for Nanometer Scale Devices .................................................................. 12

Performance-Limiting Defects in GaN ...................................................................... 14

Strain in Compound Semiconductor Photonic Systems ............................................... 16

Magnetization Dynamics in Films with Nanometer Scale Microstructure ...................... 18

Ballistic Magnetoresistance: A New Phenomenon with

Great Potential for Magnetic Sensors ........................................................................ 20

High-Speed Contact at the Head-Disk Interface in a

Magnetic Hard Disk Drive: Experiment and Modeling .............................................. 22

Projects in Materials for Micro- and Optoelectronics .......................................... 25

Microelectronics

Characterization of Porous Low-k Dielectric Constant Thin Films ............................... 26

Dielectric Metrology for Polymer Composite Films in the Microwave Range ................ 27

Polymer Photoresists for Next-Generation Lithography .............................................. 28

Superconformal Film Growth: Measurements and Modeling ..................................... 29

Micrometer-Scale Reliability: Stress Voiding and Electromigration .............................. 30

Phase Equilibria and Properties of Dielectric Ceramics .............................................. 31

Processing of Low Temperature of Co-Fired Ceramics .............................................. 32

Solder and Solderability Measurements for Microelectronics ...................................... 33

Mechanism of Sn Whisker Growth from Electrodeposits ............................................ 34

Solder Reliability: Lead-Free Solder........................................................................ 35

Micrometer-Scale Reliability: Mechanical Behavior of Thin Films ............................... 36

Electronic Packaging and Components: Packaging Reliability .................................... 37

Electronic Packaging and Components: Acoustic Loss in Piezo Crystals ..................... 38
iv

Table of Contents

Optoelectronics

Combinatorial Materials Research: Interconnects to Optoelectronic Materials ............. 39

Optical and Structural Characterization of Optoelectronic Semiconductors .................. 40

Micrometer-Scale Reliability: Strain in Photonic Semiconductors ................................ 41

Magnetic Materials

Magnetic Properties and Standard Reference Materials ............................................. 42

Magnetic Properties of Nanostructured Materials ...................................................... 43

Nanomagnetodynamics ............................................................................................ 44

New Materials for Ultra-High Density Data Storage .................................................. 45

Nanotribology .......................................................................................................... 46

Metrology for Nanoscale Properties: Brillouin Light Scattering .................................. 47

Related Projects

Nanoscale Characterization: Electron Microscopy ..................................................... 48

Phase Diagrams in High Temperature Superconductors ............................................. 49

Organizational Charts .................................................................................................... 51
1

Executive Summary

Executive Summary

Introduction

U.S. microelectronics and related industries are in

fierce international competition to design and produce

smaller, lighter, faster, more functional, and more reliable

electronic products more quickly and economically

than ever before. At the same time, there has been a

revolution in recent years in new materials used in all

aspects of microelectronics fabrication.

Since 1994, the NIST Materials Science and

Engineering Laboratory (MSEL) has worked closely

with the U.S. semiconductor, component, packaging,

and assembly industries. These efforts led to the

development of an interdivisional MSEL program

committed to addressing industry’s most pressing

materials measurement and standards issues central to

the development and utilization of advanced materials

and material processes. The vision that accompanies

this program — to be the key resource within the

Federal Government for materials metrology development

for commercial microelectronics manufacturing — may

be realized through the following objectives:

■ Develop and deliver standard measurements and data;

■ Develop and apply in situ measurements on materials

and material assemblies having micrometer- and

submicrometer-scale dimensions;

■ Quantify and document the divergence of material

properties from their bulk values as dimensions are

reduced and interfaces contribute strongly to

properties;

■ Develop models of small, complex structures to

substitute for or provide guidance for experimental

measurement techniques; and

■ Develop fundamental understanding of materials

needed in future micro- and opto-electronics.

With these objectives in mind, the program presently

consists of projects led by the Metallurgy, Polymers,

Materials Reliability, and Ceramics Divisions that

examine and inform industry on key materials-related

issues. These projects are conducted in concert with

partners from industrial consortia, individual companies,

academia, and other government agencies. The program

is strongly coupled with other microelectronics programs

within government and industry, including the National

Semiconductor Metrology Program (NSMP) at NIST.

The present report describes the major technical

activities and accomplishments in the area of Materials

for Micro- and Optoelectronics within MSEL in

FY2002 (October 2001 through September 2002).

In this report, we have tried to provide insight into

how MSEL research programs meet the needs of our

customers, how MSEL capabilities are being used to

solve problems important to the national economy and

the materials metrology infrastructure, and how MSEL

interacts with its customers to establish new priorities

and programs. Feedback and suggestions on how

we can better serve the needs of our customers and

encourage increasing collaboration to this end are

welcomed.

Mission

Our mission is to provide critical leadership in the

development of measurement methods and standards,

as well as fundamental understanding of materials

behavior needed by the U.S. microelectronics and

optoelectronics industries to remain competitive in

the changing global marketplace. As an essential part

of this mission, we are responsible not only for

developing new measurement methods with broad

applicability across micro- and optoelectronics classes

and industries, but also for working with individual

industry groups to develop and integrate measurements,

standards, software tools, and evaluated data for

specific, technologically important applications.

Establishing Priorities

As part of the development of this technical program,

a wide range of research opportunities are examined.

Choices for the research portfolio are based on several

criteria: the match to the NIST mission, the magnitude

and immediacy of industrial need, the degree to

which NIST’s contribution is critical for success, the

anticipated impact relative to the investment, ability to

respond in a timely fashion with high-quality output,

and the opportunity to advance mission science. This

requires that MSEL establish its research priorities

through extensive consultation and collaboration with

customers in U.S. industry and with counterparts in

the international metrology community using a variety

of methods including workshops, technical meetings,

standards committee participation, and individual

consultation with our customers. Materials metrology

needs are also identified through industry groups

and roadmaps, including the International Technology

Roadmap for Semiconductors (ITRS), the IPC Lead-free

Solder Roadmap, the National Electronics Manufacturing

Initiative (NEMI) Roadmap, the Optoelectronics

Industry Development Association (OIDA) roadmaps,

and the National [Magnetic Data] Storage Industry

Consortium (NSIC).

This program deals almost exclusively with rapidly

evolving technologies, where advances in measurement

science are needed to understand the limitations on
2

Executive Summary

system behavior. MSEL focuses its efforts to address

these needs in order to have the best chance to have

an impact on the course of technology.

Research Portfolio

Although there is increasing integration within various

branches of microelectronics and optoelectronics, the

field can be considered to consist of three main areas.

The first, microelectronics, includes needs ranging from

integrated circuit fabrication to component packaging

to final assembly. MSEL programs address materials

metrology needs in each of these areas including, for

example, lithographic polymers and electrodeposition

of interconnects, electrical, mechanical, and physical

property measurement of dielectrics (interlevel,

packaging, and wireless applications), and packaging

and assembly processes (lead-free solders, solder

interconnect design, thermal stress analysis, and

co-fired ceramics).

The second major area is optoelectronics, which

includes work that often crosses over into electronic

and wireless applications. Projects currently address

residual stress measurement in optoelectronic films,

performance of III-V optical materials, and wide

bandgap semiconductors. In particular, the wide

bandgap program has grown as a new collaboration

with the NIST Electronic and Electrical Engineering

Laboratory (EEEL). Building on the existing projects

on metal interconnects for GaN (Metallurgy Division)

and on interface and bulk defects in GaN (Ceramics

Division), the EEEL/MSEL program is developing

a comprehensive suite of measurement methods for

characterizing interface and bulk defects limiting the

application of GaN and related materials.

The third area is magnetic data storage where the

market potential is vast and growing and the technical

challenges extreme. NSIC plans to demonstrate a

recording density of 1 terabit per square inch —

40 times today’s level — by 2006. To reach these

goals, new materials are needed that have smaller grain

structures, can be produced as thin films, and can be

deposited uniformly and economically. New lubricants

are needed to prevent wear as spacing between the disk

and head becomes smaller than the mean free path of

air molecules. Some measurements require calibration

of magnetometers using certified magnetic standards

in several different shapes and magnetic strengths,

and with a wide range in magnetic character. These

standards are now being produced under this program.

MSEL is working with the magnetic recording industry

to develop measurement tools, modeling software, and

standards to help achieve these goals. Staff expertise at

MSEL spans all fields relevant to magnetic data storage,

including materials science, electrical engineering,

physics, mathematics and modeling, manufacturing

engineering, chemistry, metrology, and computer

science, with the Materials Science and Engineering

Laboratory, the Electronics and Electrical Engineering

Laboratory, the Physics Laboratory, the Information

Technology Laboratory, and the Manufacturing

Engineering Laboratory working as partners in

this effort.

In 2001, two new major projects were initiated

within Magnetic Data Storage. As part of the National

Nanotechnology Initiative, a major collaboration

among MSEL, the Information Technology Laboratory

(ITL), and the Electronics and Electrical Engineering

Laboratory (EEEL) is developing new measurement

methods and models for magnetic damping, needed

by the magnetic recording industry to improve

switching speed. In 2002, a long-term project

on GMR thin films was refocused into Spintronics,

the use of spin-polarized electrons for new devices

and magnetic imaging. Through an extensive network

of university and industrial collaborators, the process

measurement and control capabilities of the MSEL

Magnetic Engineering Research Facility are being used

to develop an understanding of the materials structure

and processing issues in the creation and transfer of

spin-polarized electrons.

MSEL output exists in a variety of forms, from a

fundamental understanding of micro- and optoelectronics

behavior to measurement techniques conveyed through

scientific literature and oral presentations, standard

reference materials, evaluated data and online databases,

software tools, and sensors for on-line process control.
3

Technical Highlights

Technical Highlights

The following Technical Highlights section includes

expanded descriptions of research projects that have

broad applicability and impact. These projects generally

continue for several years. The results are the product

of the efforts of several individuals. The Technical

Highlights include:

■ Pore Size Distributions in Low-k Dielectric Thin Films

from X-ray Porosimetry

■ Direct Measurement of the Reaction Front in

Chemically Amplified Photoresists

■ Nanoscale Structural/Compositional Characterization

of Ultra-Thin Films

■ Advanced Metallizations for Sub-100 Nanometer

Electronics

■ Carbon Nanotubes: Characterization and Application

for Nanometer Scale Devices

■ Performance-Limiting Defects in GaN

■ Strain in Compound Semiconductor Photonic Systems

■ Magnetization Dynamics in Films with Nanometer

Scale Microstructure

■ Ballistic Magnetoresistance: A New Phenomenon

with Great Potential for Magnetic Sensors

■ High-Speed Contact at the Head-Disk Interface in a

Magnetic Hard Disk Drive: Experiment and Modeling
4

Pore Size Distributions in Low-k Dielectric Thin Films

from X-ray Porosimetry

NIST is working to provide the semiconductor

industry with detailed information on the nanoscopic

pore size distribution of porous thin films destined

as low-k dielectric materials for the next generation

of integrated circuits. The electronics industry has

chosen the introduction of nanometer scale pores

into interlayer dielectric films as the method of

lowering the effective dielectric constant. While these

modifications change the dielectric constant favorably,

other important parameters such as physical strength

and barrier properties will also change, often in an

unfavorable way. A new method has been developed

to calculate the pore size distribution from x-ray

reflectivity measurements on thin films in a controlled

environment of solvent vapor.

The rapid and sustained advances in ultra-large-scale

 integrated circuit performance have been driven,

to a large extent, by miniaturization of the circuitry.

At these nanoscopic sizes, interlayers with extremely

low dielectric constants (low-k) are imperative to reduce

cross-talk and to increase processor speed. While the

candidate materials differ in their base chemistries, a

common theme emerges in the push to develop low-k

dielectric materials: nano-scale porosity must be

introduced in a controlled manner to further reduce the

dielectric constant. Techniques are needed to accurately

and non-invasively characterize the porosity in these

films while attached on a silicon substrate.

Capillary porosimetry is a prime candidate to

measure the pore size distribution (PSD) of the

nanoporous films. Bulk materials have been

characterized in this manner by measurement of the

mass gain of the material when exposed to a controlled

pressure of solvent vapor. As the pressure of the probe

solvent increases, pores become filled hierarchically by

size as is described by the appropriate thermodynamic

analysis such as the Kelvin equation:

that an equilibrium amount of adsorption occurs. The

value of P/Po can be varied either by mixing volumes of

solvent saturated and dry air at a constant temperature,

or by blanketing the sample in air that has reached

solvent saturation at a low temperature and increasing

the temperature of the thin film. Under such conditions,

a standard XR scan gives accurate values of the total

density that is a combination of wall density and solvent

filled pores. The mass uptake as a function of partial

pressure is calculated from these results.

As an example, the reflectivity data from the porous

hydrogen silsesquioxane (HSQ) and the silica xerogel

films are shown in Figure 1. Each of the two samples

has several data sets plotted together to show the effect

of solvent adsorption. A critical edge is clearly visible at

low qz values as the sharp drop in log(Reflectivity) from

the initially flat reflectivity curve. In Figure 1, four curves

are shown for each sample, varying from the sample

exposed to dry air (left most curve) to sample in

saturated toluene vapor (right most curve). There is a

progression of the critical edge to higher qz as the vapor

pressure increases, due to the increased electron density

of the film as the smaller pores become saturated with

liquid toluene.

ln( / )

2

RT P P0

V

r m − γ = Figure 1: XR curves of a porous organosilsesquioxane

(XLK) and xerogel films under controlled partial

toluene vapor pressure.

At P/Po = 1, toluene will condense in all of the

accessible pores or open pores by definition, so the

uptake at saturation, Wo, is a measure of the porosity.

It should be emphasized that this porosity only reflects

those pores that the toluene can infiltrate. There may

also be closed pores not accessible to toluene vapor

and therefore not reflected in the XR porosity.

Therefore, a matrix density that is a combination of

dense wall material and inaccessible closed pores can

be calculated.

where the pore radius, r, can be calculated from

the partial solvent vapor pressure, P/Po, known

thermodynamic properties of the solvent such as

surface tension, γ, and molar volume, Vm, temperature,

T, and gas constant, R.

X-ray reflectivity (XR) has been extensively used

in the Polymers Division to measure the thickness and

density of 1 μm nanoporous thin films. A method of

x-ray porosimetry (XRP) has been developed to create

a controlled solvent environment around the thin film so

Technical Highlights
5

Figure 2 presents the adsorption/desorption data

for a porous methyl silsesquioxane (MSQ) spin-on

glass (SOG) film in which P/Po is varied through the

substrate temperature. The solid lines are cubic spline

fits through the data and accurately mimic the shape

of the curves. The data points have considerably less

scatter than the data in which the isothermal technique

is employed. The hysteresis loop is very well defined

with data outside of the loop coinciding accurately.

If the principle of temperature–pressure equivalency is

valid, the temperature control method clearly increases

the accuracy of the measurement.

a special substrate, such as the piezoelectric surfaces

required for both the quartz crystal microbalance

and surface acoustic waves techniques. XRP can

be done on any smooth substrate, including the Si

wafers used in the semiconductor industry. In many

ways, XRP is similar and complimentary to optical

methods such as ellipsometric porosimetry (EP).

However, the EP analysis requires knowledge of

the optical constants, for both the matrix and the

adsorbate. With EP it is also necessary to invoke

the additional assumption that the polarizabilities

are additive. XRP does not require these additional

assumptions.

Another potential advantage of XRP is the ability

to quantify not only the average film density, but also

the density profile normal to the film surface. X-ray

reflectivity has been used to extract non-uniform

density profiles in a series of low-k films. It may be

possible to use XRP to extract pore size distributions

as a function of depth into film. To prevent dielectric

breakdown, it is desirable to have low porosity or very

small pores near the surfaces with the majority of the

porosity localized in the center of the thin film. XRP

could prove to be very useful for characterizing these

types of structures.

“The SANS/SXR measurements have become a key

metric in our low-k dielectric materials characterization

and screening process.”

Dr. Jeffrey Wetzel

Manager

Low-k Materials Technology

International SEMATECH

“The NIST x-ray reflectivity program is useful,

perhaps even critical, to the industry.”

Dr. Abner Bello

Technical Staff Member

Defect and Thin Film Characterization

Laboratory Applied Materials, Inc.

For More Information on this Topic

B.J. Bauer, W.L. Wu (Polymers Division, NIST)

Hae-Jeong Lee, Christopher L. Soles, Da-wei Liu,

Barry J. Bauer and Wen-li Wu, “Pore size distributions

in low-k dielectric thin films from X-ray porosimetry,”

Journal of Polymer Science, Polymer Physics Edition,

(in press).

Figure 2: XR toluene vapor adsorption data from MSQ

SOG films by varying the substrate temperature.

The pore size distribution can be calculated by

use of the Kelvin equation and other thermodynamic

considerations from a point-by-point differentiation,

as shown in Figure 3.

Figure 3: PSD of MSQ SOG films by varying the substrate

temperature (where AU = arbitrary units).

There are several advantages of XRP over other

techniques to determine pore size in ultrathin films.

Some methods require that the film be deposited on

Technical Highlights
6

Direct Measurement of the Reaction Front in

Chemically Amplified Photoresists

The continual device performance increases by the

semiconductor industry has been largely driven by

the fabrication of smaller structures with lithography.

As feature sizes approach sub-100 nm, the

photolithographic process must be controlled with

tolerances of (2 to 5) nm, dimensions comparable

to the molecular size of the polymer chains in the

photoresist imaging material. New experimental

methods are needed to measure transport and

materials science phenomena over nanometer

length scales to provide critically needed data for

the understanding, design, and control of new

lithographic materials and processes. In collaboration

with IBM and the University of Texas, we directly

measured the spatial evolution of a reaction front,

within a photoresist, with nanometer resolution

using neutron and x-ray reflectometry and a

deuterium-labeled photoresist polymer.

The semiconductor industry is rapidly approaching

 the need to fabricate sub-100 nm structures to

continue performance increases in integrated circuits.

Photolithography remains the enabling technology for

the fabrication of integrated circuit patterns. Although

industry is able to commercially produce 130 nm

features, lithographic materials and processes are

not fully available for the production of sub-100 nm

structures. New materials must be able to produce

structures with dimensional tolerances of (2 to 5) nm,

dimensions near the size of the polymer chain molecules

in the imaging layer.

1 and 2. The patterns are generated in the resist by

exposure to UV radiation through a mask. In the

exposed areas, the PAG decomposes forming an acid

species. Upon baking, the acid diffuses and catalyzes

a deprotection reaction rendering the insoluble resist

soluble in a developer. The soluble regions are then

removed with the aqueous base developer. Control

over this process is dominated by the events in the

transitional region between exposed and unexposed

areas of the photoresist.

Figure 1: At left, SEM image of a lithographically

fabricated structure with a nominal critical dimension of

150 nm and 300 nm pitch. At right, schematic illustration

of patterning through a mask to create the structure.

Current imaging layers, chemically amplified

photoresists, are multi-component materials consisting

of a polymer resin initially insoluble in an aqueous base

developer solution, a photoacid generator (PAG), and

other additives. A schematic diagram of the fabrication

process and an example structure are shown in Figures

Figure 2: Schematic diagram of the reaction-diffusion

process in chemically amplified photoresists. The

transitional region between exposed and unexposed

regions is particularly important for critical dimension

and roughness control.

The initial development of chemically amplified

photoresists was a key technological breakthrough

furthering the continued use of optical photolithography

for the fabrication of sub-micrometer features. The

importance of chemically amplified photoresist concepts

is illustrated by the industry-wide use of these materials

in the fabrication of state-of-the-art devices today and

into the foreseeable future. With the imminent need

for sub-100 nm feature sizes, nanometer level control

over the spatial evolution of the deprotection reaction

front and the subsequent development steps is needed.

The current level of understanding of the acid-catalyzed

reaction-diffusion process is not sufficiently detailed to

achieve this goal.

Technical Highlights
7

To meet the need for spatially detailed data, we have

developed, in collaboration with the IBM T. J. Watson

Research Center and the University of Texas at Austin,

an experimental methodology to measure directly the

spatial evolution of the deprotection reaction front

through each processing step. By using model

photoresist polymers with a deuterated protection

group, neutron and x-ray reflectivity (NR, XR)

measurements are able to follow the compositional

and density profiles of the reaction-diffusion process

in an idealized transitional region in a bilayer film stack.

Neutron contrast with the reaction is possible because

the reaction products involving the deuterated protection

group are volatile as illustrated in Figure 3.

control over the fabrication of sub-100 nm structures.

In addition, the strategy developed here is general and

can be adapted for the study of candidate photoresist

materials being developed for future applications.

Figure 4: A) schematic diagram of the processing steps

including the application of each film layer, exposure to UV

radiation, a post-exposure bake (PEB) and development.

B) and D) show the NR data and compositional depth

profile and C) and E) show the XR data and density

depth profiles from each step.

For More Information on this Topic

E.K. Lin, C.L. Soles, R.L. Jones, J.L. Lenhart,

W.L. Wu (Polymers Division, NIST); S.K. Satija

(NCNR, NIST)

E.K. Lin, C.L. Soles, D.L. Goldfarb, B.C. Trinque,

S.D. Burns, R.L. Jones, J.L. Lenhart, M. Angelopoulos,

C.G. Willson, S.K. Satija, and W.L Wu, “Direct

Measurement of the Reaction Front in Chemically

Amplified Photoresists,” Science, 297, 372, (2002).

Figure 3: General deprotection reaction in a chemically

amplified photoresist. Deuterated molecular parts

with strong contrast to neutrons are shown in red.

PHOSt = polyhydroxystyrene, PBOCSt = poly

p-tert-butoxycarboxystyrene.

The NR and XR results from a series of bilayer

samples are shown in Figure 4. NR and XR

measurements were taken after each processing

step from the spin-coating of each film layer through

exposure and bake to the dissolution of the upper layer

and any deprotected polymer. The data show that the

deprotection reaction front broadens with time within

the photoresist. The initial interfacial width was

approximately 4 nm wide whereas the reaction front

was nearly 20 nm wide. Upon development, the data

show that the developed film remained sharply defined

with a surface roughness of approximately 4 nm as

well. Although the reaction front was much wider

than the final interfacial width, the selectivity of the

dissolution process allows for the fabrication of

well-defined nanostructures with diffusive

transport mechanisms.

The spatial detail afforded by the developed

methodology can play an important role in not only the

qualitative description of the reaction-diffusion process,

but also with the quantitative determination of physical

parameters and changes in any physical or chemical

properties over the nanometer length scales required for

“[This work] opens a window of opportunity to

construct structure-property relationships between

chemical transport mechanisms and ultimate resist

resolution. It may also lead to insights into the ultimate,

intrinsic resolution limits and critical dimension control

of polymer-based imaging materials.”

E. Reichmanis and O. Nalamasu

Bell Labs, Lucent Technologies,

Science, 297, 349, (2002)

Technical Highlights
8

Technical Highlights

Nanoscale Structural /Compositional Characterization

of Ultra-Thin Films

Many modern device structures are critically

dependent on the properties of ultra-thin layers

approaching 1 nm in thickness. Examples of such

layers include interfaces and reaction layers and

gate dielectrics in CMOS devices. As a result,

comprehensive understanding of the structure

(amorphous or crystalline) and the chemistry

(composition, chemical bonding, etc.) of these

ultra-thin films is one of the most important aspects

of successful device engineering. The present work

demonstrates the application of high-resolution

transmission electron microscopy combined with

spatially resolved electron energy-loss spectroscopy

to the analysis of (i) silicon oxide-nitride-oxide

heterostructures and (ii) zirconium oxide ultra-thin

films on silicon.

Silicon oxynitrides are leading candidates to replace

 pure thermally grown SiO2 as gate dielectrics

because they suppress boron penetration from

the poly-Si gate, enhance reliability, and reduce

hot-carrier-induced degradation. At the same time,

silicon oxide-nitride-oxide multilayers (ONO stacks)

attract considerable interest as possible charge storage

structures in non-volatile memory devices. The critical

structural and compositional parameters that affect

electrical performance of ONO-based devices include the

physical density of the amorphous oxide/nitride layers

and the depth distributions of both oxygen and nitrogen

atoms. However, few systematic studies that analyze the

effect of processing conditions on these parameters in

stacked ONO structures have been reported.

In this study, we applied spatially-resolved electronenergy loss spectroscopy (EELS) in a transmission

electron microscope (TEM) to analyze elemental

distributions in differently processed ONO stacks

(Figure 1) while physical densities of individual layers

were measured using x-ray reflectometry. The results

of structural/compositional analysis were correlated with

the electrical performance of ONO-based flash-memories.

The ONO stacks for this study (Figure 1) were

provided by Tower Semiconductor Ltd. Typical profiles

of O-K and N-K characteristic intensity distributions,

obtained using an imaging filter in a fixed-probe TEM,

are shown in Figure 2. The O-K intensity decreases to

the noise level in the middle of the nitride layer. The

nitrogen signal disappears in the oxide layers, but

produces small peaks both at the Si/SiO2 and poly-Si/

SiO2 interfaces. The presence of nitrogen at the Si/SiO2

interface in the oxy-nitride films has been reported

previously, based on similar EELS measurements, and

was attributed to nitrogen segregation during film

processing. Such nitrogen segregation could explain the

improved electrical breakdown strength of the bottom

oxide observed after nitride deposition. However, we

found that the amount of nitrogen at both interfaces

Figure 1: Typical high-resolution TEM image of an

ONO stack.

Figure 2: O-K and N-K spectrum-lines. The interfaces in

the HRTEM image were set parallel to the energy-dispersion

direction of the spectrometer. Each 2D spectrum has

two dimensions: horizontal, corresponding to energy loss,

and vertical, representing a spatial coordinate; that is,

each 2D spectrum represents a collection of EELS spectra

spatially-resolved across the interfaces. The individual

spectra are shown on the left. The spectrum-lines were

processed to remove background, and the intensities under

each edge were integrated over an energy window of 2 eV.

The profiles of integrated intensity distributions as a function

of spatial coordinate are shown on the right.
9

increased with increasing irradiation time for the

analyzed area, which suggested that the nitrogen

distribution might be affected by the electron beam.

The effect of the electron beam on the nitrogen

segregation was further tested using EELS spectrumimaging in a dedicated scanning TEM (STEM). In these

experiments, spectrum-images provided a collection of

spatially-resolved (along the interface) line-scans across

the ONO stack; the line-scans were separated in time by

about 2.5 s. The first line-scan yielded no significant

nitrogen signal at either the Si/SiO2 or the poly-Si/SiO2

interfaces. However, the next line-scan revealed a small,

but detectable, nitrogen signal at the Si/SiO2 interface,

while the nitrogen signal from the nitride layer was

reduced (Figure 3); the magnitude of this effect

decreased with decreasing radiation dose. These results

suggest radiation-induced nitrogen segregation to both

the Si/SiO2 and poly-Si/SiO2 interfaces.

In another example, we applied TEM/EELS to the

analysis of ZrO2 ultra-thin films on silicon. Metal oxides,

such as ZrO2, are key candidates for gate dielectrics to

replace SiO2 and SiOxNy in CMOS transistor technology.

Despite a large effort invested in the characterization of

ultra-thin ZrO2 films, many issues related to the effect of

processing conditions on both morphology and thermal

stability of ZrO2 films on silicon remain the subject of

a continuing debate. The present work is part of a

systematic study of these effects in gate dielectrics,

which is conducted in collaboration with the UCLA.

The ZrO2 films for this study were deposited using a

state-of-the-art atomic layer CVD (ALCVD) system

and subjected to annealing in different atmospheres.

The high-resolution TEM images of 2.5 nm ZrO2

films deposited on the HF-treated silicon surface are

presented in Figure 4. The as deposited film is largely

crystalline and exhibits a rough morphology; the latter

was attributed to a discontinuous nucleation of the ZrO2

film on the HF-treated Si surface. (A much smoother

morphology was observed for films deposited on

thermally grown 1 nm SiO2 layer.) The film includes

an interfacial layer having a thickness of about 1 nm.

Spatially resolved EELS spectra indicate a lack of any

significant Zr content in this layer, suggesting a silicon

oxide rather than a silicate phase. Although the presence

of a small amount of Zr in the layer still could not be

ruled out, the near-edge structure of both the Si-L2,3

and O-K edges for this interfacial layer were also

consistent with a silicon oxide.

Figure 3: Integrated nitrogen intensity as a function of

line-scan number in the EELS spectrum-image acquired in

the dedicated STEM. The nitrogen signal at both Si/SiO2

and poly-Si/SiO2 interfaces increases significantly with

increasing scan number. A corresponding decrease in the

nitrogen signal from the nitride layer is consistent with the

beam-induced nitrogen diffusion /segregation.

The implications of the beam-induced nitrogen

segregation in the ONO stacks are several. First,

similar artifacts could be present in other spectroscopic

techniques that involve high-energy probes, such as

secondary ion-mass spectroscopy (SIMS); therefore,

the SIMS data on nitrogen segregation in ONO

reported in the literature may need to be revisited.

Second, reasons other than nitrogen segregation can

be responsible for the improved dielectric properties of

the bottom oxide observed after nitride deposition in the

present ONO stacks. Additionally, our results indicate

that electron-beam-induced damage can be a serious

obstacle to EELS analysis at nanoscale resolution.

More studies of radiation-induced damage in industrially

relevant materials are needed to identify potential

artifacts.

Figure 4: Upper: HRTEM image of a ZrO2 film

deposited on HF-treated silicon at 460°C. Lower:

Spatially-resolved EELS spectra from the same area.

For More Information on this Topic

Contact: I. Levin

Technical Highlights
10

Technical Highlights

Advanced Metallizations for Sub-100 Nanometer Electronics

silver deposition was demonstrated this year in

the Metallurgy Division and is the first example of

superconformal feature filling with a metal other than

copper. “Superfilling” is characterized by bottom-up

deposition as shown in Figure 1.

Filling of trenches and vias with silver and copper

is quantitatively explained by the Curvature Enhanced

Accelerator Coverage Mechanism (CEAC) developed

at NIST. Electrochemical and surface analytical

measurements on planar substrates were used in

FY2002 to establish a one-to-one correlation between

catalyst coverage and the metal deposition rate.

In the case of silver deposition, adsorbed selenium

was found to catalyze the deposition rate. For the

thiol /disulfide-polyether-halide additives used in copper

superfilling, the sulfonate-end group of the thiol or

group of the thiol or disulfide molecules was found to

destabilize the passivating polyether surface film thereby

accelerating the local metal deposition rate.

The insight provided by uncovering the CEAC

mechanism has enabled the development of a new

two-step process for superconformal film growth;

in contrast to standard electrodeposition techniques,

the catalyst is deposited on the substrate prior to the

metallization. Specifically, the patterned substrate is first

Electrodeposited copper is rapidly replacing aluminum

in device interconnect technology because of its lower

electrical resistivity, superior electromigration behavior,

and the ability to fill fine features without the formation

of seams or voids. As feature dimensions go below

100 nm difficulties in maintaining performance are

anticipated. In FY2002 we have pushed the quantitative

limits of the Curvature Enhanced Accelerator Coverage

(CEAC) mechanism developed in the Metallurgy

Division of NIST. Exploring the mechanism has

yielded new electrochemical processing routes for

improved metallization and is guiding the development

of advanced metallizations beyond copper as well as

new processing routes such as surfactant catalyzed

chemical vapor deposition.

T his Metallurgy Division project is meeting the

 microelectronics industry’s need for improved

device metallization by exploring advanced materials

and processes for superconformal film growth.

In FY2002 we have:

■ Established the generality of the CEAC model for

superfilling by electrodeposited silver and other metals;

■ Developed a new processing method for eliminating the

delay time for superfill to begin during electrodeposition,

thereby increasing the aspect ratio and decreasing the

width of trenches that can be superfilled; and

■ Extended the CEAC mechanism to quantitatively

predict superfill during chemical vapor deposition.

Our current measurement and modeling effort is

focused on copper and silver metallizations, the latter

metal having the lowest resistivity of any element, thus

having potential on-chip applications. Superconformal

Figure 1: Superconformal filling of vias during silver (left)

and copper (right) electrodeposition (NIST Metallurgy

Division, using patterned wafers from International SeMaTech).

Figure 2: Time evolution of trench filling after different

“derivitization” steps. The concentration of SPS or MPSA

catalyst used for the 30 s surface derivitization steps are

indicated. All specimens were transferred to a catalyst-free

electrolyte for copper deposition.
11

“derivitized” or “dosed” with a submonolayer coverage

of catalyst and then transferred for electroplating

in an electrolyte that does not contain the catalyst.

For an optimum catalyst coverage, superconformal

filling of trenches and vias occurs as shown in

Figure 2 (50 μmol/L SPS). If the catalyst coverage

is too low or high, conformal or subconformal

deposition occurs, resulting in void formation during

feature filling (0.5, 500 μmol/L SPS, 1 mmol/L

MPSA). The filling behavior is completely analogous

to that obtained using a single (conventional)

electrolyte containing both catalytic and inhibiting

species. The new process provides unambiguous

verification of the CEAC mechanism of superconformal

film growth. Restricting the catalyst to the surface prior

to metal deposition also enables the rate differentiation

provided by the CEAC mechanism to be increased

relative to the conventional process. From a technical

perspective, the two-step process offers an interesting

solution to the difficult control issues associated with

catalyst destruction and related aging effects which are

known to occur in the “conventional” single-electrolyte

superfilling process currently used in industry.

The generality of the CEAC mechanism has now been

extended to include the first quantitative prediction of

superconformal chemical vapor deposition (CVD). More

recently, this work has enabled a direct comparison of the

CEAC mechanism with detailed experiment results on iodine

catalyzed copper CVD as shown in Figure 3.

Project results from FY2002 have firmly established

the generality of the Curvature Enhanced Accelerator

Coverage (CEAC) mechanism, which we developed in

FY2001 to explain superconformal electrodeposition of

copper. The output also clearly demonstrates the predictive

power of the CEAC formalism.

Three different electrode-shape change algorithms

have been developed. They show excellent agreement

with experimental results.

These algorithms, in the form of software, are

available from the Metallurgy Division website and

are currently being examined by Texas Instruments

and IMEC. In addition, Motorola (B. Melnick and

M. Freeman), International SeMaTech (C. Witt) and

Hynix Semiconductor (S.G. Pyo) are now active

collaborators in our experimental and modeling efforts.

The first two provide patterned wafers for NIST

electrodeposition studies; the last provides results from

iodine catalyzed chemical vapor deposition experiments

for NIST modeling.

Selected Project Publications for FY2002

D. Wheeler, D. Josell and T.P. Moffat, Modeling

Superconformal Electrodeposition Using the Level Set

Method, J. Electrochem. Soc., submitted July 2002.

T.P. Moffat, D. Wheeler, C. Witt and D. Josell,

Superconformal Electrodeposition Using Derivitized

Substrates, Electrochemical and Solid-State Letters,

submitted May 2002.

D. Josell, S. Kim, D. Wheeler, T.P. Moffat, S.G. Pyo,

Superconformal Deposition by Iodine-Catalyzed

Chemical Vapor Deposition, J. Electrochem Soc.,

submitted May 2002.

B.C. Baker, M. Freeman, B. Melnick, D. Wheeler,

D. Josell, T.P. Moffat, Superconformal Electrodeposition

of Silver from a KAg(CN)2-KCN-KseCN Electrolyte,

J. Electrochem. Soc., in press.

D. Josell, B. Baker, C. Witt, D. Wheeler and

T.P. Moffat, Via Filling by Electrodeposition:

Superconformal Silver and Copper and Conformal

Nickel, J. Electrochem. Soc., in press.

T.P. Moffat, B. Baker, D. Wheeler, J.E. Bonevich,

M. Edelstein, D.R. Kelly, L. Gan, G.R. Stafford,

P.J. Chen, W.F. Egelhoff, and D. Josell, Superconformal

Electrodeposition of Silver in Submicron Features,

J. Electrochem. Soc. 149, C423 (2002).

D. Josell, D. Wheeler and T.P. Moffat, Superconformal

Electrodeposition in Vias, Electrochemical and Solid-State

Letters 5, C49 (2002).

D. Josell, D. Wheeler and T.P. Moffat, Superconformal

Deposition by Surfactant-Catalyzed Chemical Vapor

Deposition, Electrochemical and Solid-State Letters 5,

C44 (2002).

D. Josell, D. Wheeler, W.H. Huber, J.E. Bonevich

and T.P. Moffat, A Simple Equation for Predicting

Superconformal Electrodeposition, J. Electrochem.

Soc. 148, (12) C767 (2001).

Our findings have also been conveyed to U.S.

industry, academia and other national laboratories

through more than ten external presentations in the

last year.

For More Information on this Topic

D. Josell, D. Wheeler, B. Baker, T.P. Moffat

Figure 3: Superconformal filling of vias during iodine

catalyzed chemical vapor deposition. Experimental via

results are from Hynix Semiconductor, and model predictions

for a trench are from the Metallurgy Division of NIST.

Technical Highlights
12

Carbon Nanotubes: Characterization and

Application for Nanometer Scale Devices

The Materials Reliability Division believes that

research on carbon nanotubes and their applications

can significantly support economic opportunities in

nanotechnology. Carbon nanotubes have a unique set

of characteristics that offer promise for a variety of

nanometer scale applications. The carbon–carbon

bond is among the strongest known atomic bonds,

and produces amazing mechanical strength. Because

of their outstanding thermal conductivity, nanotubes

are also being considered for thermal transport

applications. In addition, recent reports envision

nanotubes performing multiple functions in future

computers: both as switching elements and as

interconnections. We are, therefore, developing

nanometer scale measurement, manipulation, and

bonding techniques to enhance future nanometer

scale devices.

Technical Description

The Materials Reliability Division has begun research

 on carbon nanotubes in two application areas:

as probes in advanced imaging and as materials for

mechanical and electrical structures. Starting this

research required us to develop an understanding of

the different types of nanotubes available, their sources,

and techniques for handling them. Because nanotubes

are such new materials, and new to this Division, a brief

introduction is given.

Carbon nanotubes are essentially graphite sheet

structures that are rolled into tubes. They can be single

sheet tubes, referred to as single-walled, or they can be

multiple concentric tubes, referred to as multi-walled.

Several types of nanotubes are available commercially in

quantities of the order of a gram. One manufacturer’s

catalog lists five types of multi-walled nanotubes and

one type of single-walled. The type that we used for

probe tips for atomic force microscopy (AFM) differs

slightly. These are a number of multi-walled nanotubes

twisted together and referred to as nanotube ropes.

These ropes are large enough to view in an optical

microscope. Figure 1 shows various diameter

multi-walled nanotubes.

Accomplishments

This year, FY02, we have developed methods to handle

the nanotubes and used them in several experiments.

We have attached nanotubes to an AFM probe, to map

surface conductance on a nanometer scale, and to a

mechanical strain-testing device mounted in a scanning

electron microscope (SEM) that allows imaging of the

nanotube as it is strained. We have also joined with the

Electronics and Electrical Engineering Laboratory to

explore the high-frequency response of nanotube

AFM probes. Within the Division, we are also working

on multiscale modeling, so that designers can model,

for example, the effect of imposed deflection on

the attachment of a nanotube to its mount within

some device.

Figure 1: Transmission electron microscope (TEM) image

showing multi-walled nanotubes of different diameters.

These are large-diameter tubes, from a set obtained for

testing as AFM probes. Single-walled tubes are much

smaller, from 1 to 5 nm in diameter.

Developing the manipulation and attachment processes

has required the largest effort so far. To attach the

tubes, the first handling step is to remove impurities

from the bulk nanotubes with solvents. Next a drop

of solvent with nanotubes is dispensed on a substrate

such as a glass slide and allowed to dry, leaving the

tubes lying flat. Adhesive tape is then used to pick up

and hold several nanotubes for access and manipulation

under the optical or scanning electron microscope.

The next operation is to prepare the target to which the

nanotube will be attached. The target is mounted on a

micromanipulator. We have used a micromanipulator

controlled by manual micrometers under an optical

microscope and one driven by piezo-stepper motors

in the SEM. To practically view thick-walled nanotubes

optically, an optical microscope, with a very long

working distance and a very bright oblique illuminator,

was obtained.

To mount a nanotube to an AFM tip under the

optical microscope, the AFM tip is mounted on

the micromanipulator and then touched to a

carbon impregnated conductive adhesive. A small

submicrometer-sized bead of adhesive is subsequently

attached to the tip as it is pulled away. The AFM tip is

Technical Highlights
13

Technical Highlights

then positioned to an appropriate nanotube, and the

nanotube is applied to the tip. A nanotube rope attached

to an AFM tip is shown in Figure 2. The bead of

adhesive can be seen at the end of the pyramidal AFM

tip. From this image it is clear that the sensing portion

of the nanotube is much smaller than the end of the

AFM tip. Claims of an order of magnitude increase in

image resolution have been reported in the literature.

Figure 2: Scanning electron microscope (SEM) image of

nanotube attached to an AFM tip with applied adhesive.

The nanotube is actually a rope of several tubes about

100 nm in diameter.

Attaching nanotubes in the SEM begins with

a sharp tungsten probe mounted in a piezo-driven

micromanipulator. The tungsten probe is brought in

contact with the nanotube, and the electron beam is

focused and left stationary where the two come together,

producing a carbonaceous deposit. This technique,

called electron beam deposition (EBD), produces a

strong bond on the nanometer scale. Next, the nanotube

Figure 4: SEM image of attaching a nanotube to a MEMS

device, in progress. Development of carbon deposits can

be seen near the edges of the MEMS under the nanotube.

The nanotube rope is about 100 nm in diameter. The tip of

the tungsten probe that maneuvers the nanotube can be seen

in the lower left corner of the image.

Figure 3: SEM image of tungsten probe (bottom of image)

attaching a 20 mm long nanotube rope to the end of an AFM

tip (upper left) using EBD. After this nanotube rope was

attached, it was pulled until the tube separated from the

tungsten probe.

Future Research

More details on our research on the use of

nanotubes as probes in advanced imaging appears later

in this report. Future research on nanotubes as

mechanical materials includes developing systematic

procedures for mechanical measurements on nanotubes

of different types. In addition, we plan on applying these

attachment techniques to mechanical measurements of

other submicrometer tube or fiber-like materials, such as

collagen fibrils, and developing a practical modeling

capability for nanotube-device attachments.

For More Information on this Topic

P. Rice, D. Read

is moved to the AFM tip or other device for attachment,

again using EBD to bond the two together. We have

made preliminary tensile measurements using this

procedure by viewing the deflection of the AFM

cantilever as the tungsten probe is pulled away from

the AFM cantilever. We have found the EBD bond

can survive micronewton forces (Figure 3).

Microelectrical mechanical systems (MEMS) are

developing rapidly. Nanotubes can be used with MEMS

as mechanical and electrical components. Mechanical

and electrical characterization of the bonding techniques

is critical to developing the full potential of nanotubes on

MEMS. We can easily apply the micromanipulation

procedures we have developed to test various bonds

(Figure 4).
14

Technical Highlights

The III-V nitrides and GaN, in particular, are

emerging new semiconductors for electronic devices,

including light-emitting diodes (LED) and laser

diodes in the blue-green and UV wavelengths, UV

detectors and ultrahigh power switches. The main

challenges in this technology are optimization of

optical output efficiency and stability of metal contacts.

One objective of the MSEL project on GaN is to

quantify the effect of GaN defects, in particular

of threading dislocations, on these properties. In

FY2002 a mechanism for the degradation of electrical

contacts through threading dislocation-mediated phase

transformations was determined through detailed

characterization using transmission electron microscopy.

Doping methods to disable the mechanism may lead to

greatly improved stability, reliability, and performance

of commercial GaN devices.

Direct bandgap GaN and its alloys with AlN and

 InN are receiving considerable attention as

promising semiconductors for electronic devices

operating at high temperatures, high frequency and

high power and for optoelectronics applications in the

blue-green and UV wavelengths. Therefore, electrical

contacts to the nitrides (ohmic contacts and Schottky

barriers) which are stable at elevated temperatures are

needed.

As bulk crystals of these materials are not available,

GaN is epitaxially grown on a substrate, usually on

(0001) sapphire or (0001) 6H-SiC. Because of a large

lattice mismatch between the substrates and GaN, high

densities of various crystallographic defects are present

in the active GaN layers. Among the defects that can

cross the epitaxial GaN layer and reach a surface are

threading dislocations, inversion domains, nanopipes

and prismatic planar defects.

In order to achieve desirable characteristics of ohmic

contacts, different combinations of different metal layers

have been studied. For example, the commonly accepted

contact to p-type GaN is Ni /Au which gives good

electrical characteristics after annealing at 400-600°C.

For higher temperatures, the electrical characteristics

worsen and severe degradation of contact morphology

is observed. Degradation of such electrical contacts are

associated with dislocations in GaN, island formation

in the contact layers, and decomposition of GaN. In

FY2002, NIST developed a mechanism for contact

degradation that is based on heterogenerous nucleation

of GaN decomposition at threading dislocations/metal

interface, followed by wetting of the threading

dislocation by liquid Ga. The morphology of the metal/

GaN interface is an important issue: if the Ga metal

were to penetrate through a thin GaN layer and reach an

Performance-Limiting Defects in GaN

underlying layer in a device, the device could be rendered

inoperable. Furthermore, the stability of interfacial

morphology is a general concern for contacts to all

semiconductors.

Reaction and interdiffusion in the Au/Ni/GaN

system as a function of annealing temperature from

400 to 900°C has been investigated by several groups,

mostly using Rutherford backscattering (RBS), Auger

spectroscopy and x-ray scattering. Only rarely has

this microstructural evolution been directly observed

using transmission electron microscopy (TEM).

In collaboration with researchers from the Electrical

Engineering Department at Howard University, we

investigated electrical and microstructural behavior of

Ni/Au bilayers as a function of annealing temperature,

with emphasis on structural evolution at a TEM level.

The as-deposited layers consisted of 50 nm of Ni and

35 nm of Au, with Ni in contact to n-type GaN. Ohmic

contacts with a specific contact resistivity, ρs, as low as

6.9 × 10-6 Ω-cm2 for a doping level of 5.0 × 1017 cm-3

were obtained after annealing the sample for 10 s at

800°C. Two very important conclusions were achieved

from the TEM investigation:

1. A strong reaction between the metal contact layer

and the threading dislocations core structure leads

to decomposition of GaN along dislocation lines.

Such behavior will be a major factor limiting annealing

temperature and operation temperature/time of devices.

2. Interdiffusion between Ni and Au results in reversal

of Au and Ni phases across the metal film. Thus,

after annealing the phase primarily responsible for

the formation of the energy barrier of the contact

between GaN and metal contact is Au-based.

Figure 1: SEM image of Au/Ni/GaN/sapphire annealed

at 600 and 800°C for 10 sec.

Figure 1 shows a SEM image of Au/Ni/GaN

annealed at 600 and 800°C for 10 sec. For the

imaging, we used a cross-sectional ion milled TEM

specimen where two differently annealed specimens

were glued face to face. Whereas at 600°C the metal

film is continuous, at 800°C severe degradation of

the metal contact occurred. In many places the film is
15

separated from GaN, and deep V-shaped pits can be

found penetrating GaN. Such pits are detrimental to

the near-surface electrical performance of devices.

TEM analysis of these specimens showed that

the V-shaped pits are always associated with the

threading dislocations. Figure 2 shows bright and dark

field images of the 600°C-annealed specimen. The pits

are clearly seen in the bright field image and marked

by triangles. The weak beam imaging using different

g-vectors established that each pit is connected to a

place where a threading dislocation exits GaN. The

threading dislocations vary in character (e - edge,

s - screw and m - mixed).

Figure 2: Bright field (upper) and weak-beam dark field

(bottom) images of the 600°C-annealed specimen.

Bright and dark field images of the 800°C-annealed

specimen are shown in Figure 3. The pits associated

with the threading dislocations in the specimen are

significantly deeper and wider. The metal film in some

places is lifted from the GaN surface, and in other

places, the surface of GaN is exposed. It appears that

the surface of GaN, between the dislocations, was not

degraded and remained flat (see arrows in Figure 3).

This phenomenon, the elevated temperature chemical

instability of GaN defects exposed to either metal films

or free surfaces, is very general in nature and has

importance for GaN-based device design. Bulk GaN

has the following decomposition reaction above 800°C:

GaN 6 Ga(l)+N2(g). Nevertheless, GaN films can

survive higher temperatures due to the high stability of

the (0001) surface. In addition, the reaction is sluggish

due to kinetic difficulties recombining nitrogen atoms

into N2 molecules on the GaN surface. At locations

where a dislocation exits GaN, the atomic arrangement

is of lower stability and therefore triggers the

decomposition of GaN (Figure 4). Nitrogen atoms

may diffuse into the metal, recombine into molecules

and form N2 gas. Such gas accumulates under the

metal film and exerts enough pressure to cause metal

film de-cohesion (see Figure 3).

Figure 3: Bright field (upper) and weak-beam dark field

(bottom) images of the 800°C-annealed specimen.

Figure 4: Schematic drawing illustrating decomposition of

GaN along a dislocation line and resulting in the formation

of N2.

For More Information on this Topic

L.A. Bendersky, A.V. Davydov, W.J. Boettinger

Technical Highlights
16

Strain in Compound Semiconductor Photonic Systems

The photonics technology industry develops products

that often contain strain. Sometimes the strain is

intentionally introduced, as in the case of highly

strained quantum wells, while at other times it is

undesirable, as in the case of oxide-confined vertical

cavity surface-emitting lasers (VCSEL)s. Industry

seeks to better understand and control strain

development in such systems. We approach this

problem in the Materials Reliability Division with two

efforts — one concentrating on high spatial resolution

measurement of strain using electron microscopy

methods; and the other on theoretical modeling of

development of strain energy during heterostructure

growth using Green’s Functions and related boundary

element methods.

Background

Strain is a major factor in the compound semiconductor photonics industry. It can be

intentionally used for the self-assembly of quantum

dots (QDs) or for tuning the energy band structure of

heteroepitaxial layers. Deleterious effects of strain can

lead to delamination of buried oxide apertures in vertical

cavity surface-emitting lasers (VCSELs) or shifting of

operating wavelengths for InGaAsP/InP-based devices.

We have begun an effort, which is partially ATP-funded,

to understand and control strain development in III-V

semiconductor materials systems. One aspect of the work

involves developing experimental methods for measuring

elastic strains with spatial resolution in the range of tens

of nanometers. This involves the use of electron diffraction

in the scanning electron microscope (SEM) and also in

the transmission electron microscope (TEM). At present,

this is being applied to strains induced by phase transition

arising from the wet oxidation of AlGaAs layers confined

between GaAs layers. Such oxides are used as optical

apertures for VCSELs and, upon formation, result in

volumetric compressive strains in the semiconducting layers

in excess of 6%.

The second aspect of our contribution to solving this

problem is to develop a multi-scale theoretical method

for modeling the self-assembly of InAs QDs on GaAs

substrates. This involves the combined use of Green’s

functions (GF) and boundary element methods

(BEMs) incorporating anisotropic elasticity. Strain, in

this case, occurs from the mismatch in natural lattice

parameters between the two materials. We determine

both numerically and analytically the elastic strain energy

distribution in these systems. A nice aspect of the work

is that we are able to provide experimental data for direct

incorporation into the theoretical model using electron

microscopy, due to the well-defined conditions often

present during epitaxial growth of crystals.

Figure 1: SEM image of multilayer structure, with oxide

growth front visible. EBSD patterns obtained from positions

indicated by line segments. Image to far right is an indexed

pattern. Scale bar = 200 nm.

Strain Measurement with Electron Diffraction

We describe the SEM method of electron backscatter

diffraction (EBSD) here due to some recent advances in

our lab. The advantage of using a SEM method over a

TEM method for strain determination lies primarily with

the fact that less specimen preparation is required. This

reduces the likelihood of significantly changing the initial

strain state and, therefore, removes the need to model

the preparation-induced relaxation. The main disadvantage

of the SEM method is that strain sensitivity is not as good

(detectability ~ 0.1% at best) as that possible by use of

TEM. We estimate the EBSD spatial resolution for strain

measurement in these systems to be ~ 50 nm.

Figure 1 shows a cross-sectional SEM image of an

AlGaAs/GaAs multilayer structure, with an aluminum oxide

layer partially grown in. The diffuseness of the EBSD

patterns is an indication of the amount of strain present

around the oxide growth front. More specifically, pattern

diffuseness is a direct measure of the magnitude of the

elastic strain gradient present within the sampling volume

of the electron beam. The strain is greater near the growth

front. With automated beam scanning and pattern

collection, we are able to determine the spatial extent of

the distortion field caused by the oxidation. Preliminary

measurements suggest that the distortion field associated

with a single oxide layer extends more than 1 μm beyond

the position of the front. The diffraction measurements

are made only in crystalline portions of the specimen and,

hence, do not sample the oxide strain directly, but rather

the resulting effect on AlGaAs and GaAs layers in the vicinity.

More quantitative determinations of elastic strain

are made by measuring the widths of the bright bands

visible in the EBSD patterns, as shown in Figure 2.

One bandwidth is directly proportional to the Bragg

angle for the diffracting planes.

Technical Highlights
17

Figure 2: Indexed EBSD pattern used for elastic strain

determination.

We have applied our theory to InAs QDs in GaAs. We

have calculated EERR for a QD growing on the free

(001) surface for a buried seed dot as well as a seed

dot on the surface (Figure 3). We find that the grown

QD reduces the EERR of a new QD. For the grown QD

at the surface, the EERR for the new dot is lower in the

<100> and <010> directions. However, the change

in EERR is small when the grown QD is at the surface.

In contrast, the effect of a buried QD on EERR of the

new QD is very pronounced.

Energetics of Self-Assembly

of Quantum Dots

We are developing advanced theoretical techniques

using Green’s functions (GF) and boundary element

methods (BEMs) to model the self-assembly of arrays of

quantum dots in anisotropic semiconductors. We consider

the energetics of growth of a small QD in the strain field

of an existing “grown” QD. We introduce a new parameter

Γ, the elastic energy release rate (EERR), that determines

the growth of a QD. The EERR, as in fracture mechanics,

is defined as the elastic relaxation energy per unit volume

of growth and is given by dW/dVQD, where W is the elastic

strain energy and VQD is the volume of the growing QD.

Assuming uniform misfit strain in QDs,

 , (1)

We find that the maximum of EERR occurs vertically

above the grown QD only for some values of the depth

of the QD (Figure 4). This shows that, for QDs

covered by a thick spacer medium, a vertical array of

self-assembled QDs is energetically favorable. In the

case of a thin covering, an oblique stacking of QDs

could occur. We also find that there is an optimum

depth of the buried QD for the formation of a new

QD vertically above it.

Figure 3: Formation of a new QD under the influence

of a grown surface QD (a) or buried QD (b).

Figure 4: Variation of the EERR for formation of a

new QD with locations at different depths of the buried

QD (Figure 3b): (a) h = 0.1a; (b) h = 0.6a.

For More Information on this Topic

Electron diffraction strain measurement: R.R. Keller;

GF/BEM modeling: V.K. Tewary

where the first term is the work done by external

traction, the second term is the work done by the

intrinsic traction at the boundary of QDs, and the

last term represents the energy of the reference state.

We use eq. (1) to evaluate W by using GF and a

special BEM that requires numerical discretization only

along the boundaries of the uncapped QDs and the

interfaces between the substrate and QDs. Our BEM

is computationally more efficient than the conventional

BEM and the domain-based numerical techniques such

as the finite-element method.

Equation (1) takes into account the strain created by

the growing QD. This effect has been neglected in the

earlier papers that consider only the strain field of the

“grown” QDs. Equation (1) also accounts for the fact

that the material parameters of QDs are different from

the host solid and include the continuity conditions at

the interface between QDs and the host solid. These

contributions have been neglected in earlier calculations

using Green’s functions.

∑∫ = ∂Ω −

N

n

ijkl ij l k n

C n u dS

1

0

2

1 ( ε )

∫ +

D

Cijkl ij kldV 0 0

2

1 ε ε

∫∂ = D i i W Tu dS 2

1

Technical Highlights
18

Technical Highlights

Magnetization Dynamics in Films with Nanometer Scale Microstructure

the width of the susceptibility peak corresponding to

the magnetization precession frequency as illustrated in

Figure 1.

Magnetization damping is essential for data storage

and retrieval at GHz data rates in disk drives, MRAM

chips and other magnetoelectronic devices. In disk

drives, the write head materials, media and read head

must all be able to perform well on nanosecond time

scales. For measurement of magnetic damping

parameters, ferromagnetic resonance (FMR) is the

dominant technique, but the raw linewidth data is

often clouded by the presence of inhomogeneities.

Our modeling of ferromagnetic resonance spectra in

inhomogeneous thin films takes a novel approach to a

problem that for several decades has been solved only

for weak inhomogeneities. Our new model involves

solving for the normal modes of an inhomogeneous film

and calculating the absorption spectrum from all of the

modes. This model provides the theoretical basis for

studying the intrinsic damping processes through FMR

linewidth measurements, and ultimately for delivering

the needed data on magnetic damping to industry.

Magnetic data storage has become the most

 prevalent form of data storage in the world, far

surpassing paper documents in worldwide capacity.

Annual hard drive storage capacity shipped is

approximately 1 × 1019 bytes with a 60% per year

growth rate. To handle anticipated storage at GHz

data rates, it is important to understand and control

magnetization damping, the process that allows

magnetization to come to equilibrium by dissipating heat.

To develop high-data-rate hard drives, it will be important

to understand and control damping in the magnetic

materials used in write heads, media and in read heads.

For magnetic random access memory (MRAM) chips,

damping is a limiting factor for the speed of these chips

because reliable switching of the magnetic “bit” requires

waiting for the magnetization to come to equilibrium

before the next switching event.

The Nanomagnetodynamics Project is a cooperative

project between the Metallurgy Division in MSEL and

the Magnetic Technology Division in EEEL. The work

is designed to address magnetization dynamics, and

magnetization damping in particular, and to deliver to

industry the data and metrology needed to develop

high-data-rate magnetic devices. This highlight describes

a recent theoretical breakthrough by MSEL staff that

will enable reliable magnetization damping metrology.

The primary measurement used to determine the damping

properties of magnetic materials is ferromagnetic resonance

(FMR) linewidth. Ferromagnetic resonance, like electron

spin resonance (ESR) and nuclear magnetic resonance

(NMR) involves precession of the magnetization around

an equilibrium direction at a frequency that depends on

the applied field. The damping rate is determined from

Measurements of damping in ferromagnetic materials

are complicated by the fact that in addition to damping,

inhomogeneity contributes to the FMR linewidth.

Reliable interpretation of FMR linewidth data therefore

requires separation of the linewidth into components

arising from damping and inhomogeneity.

Unlike ESR and NMR in paramagnetic materials, the

spins in ferromagnetic materials are strongly coupled by

short-range exchange interactions and long range dipolar

interactions. To account for magnetic interactions, it is

convenient to describe the vibrations of the magnetization

in terms of spin waves, or in quantum mechanical language,

“magnons.” Spin waves (magnons) are the collective

vibrational normal modes of the magnetization in a uniform

film just as sound waves (phonons) are the collective normal

modes of a uniform crystal lattice. The analogy with sound

waves is useful in describing the interaction of spin waves

with defects. If a sound wave encounters a defect where

the elastic properties are different, a portion of the incoming

sound energy will be scattered into sound waves traveling

in different directions, but with the same frequency. For

spin waves, the uniform precession that is driven by a

microwave field in an FMR experiment can be scattered

into short-wavelength spin waves with the same precession

frequency as the uniform mode. The quantum mechanical

Hamiltonian describing a system with defects can be written

where αk

† and αk are the magnon (or phonon) creation

operators for waves at frequency ωk and Ak,k is a

coefficient describing the strength of the defect that

produces scattering.

Figure 1. a) Magnetization precession. Damping causes

the magnetization to come to equilibrium as shown by the

red spiral. b) A typical FMR signal, taken at 2.5 GHz in a

10 nm thick film of Permalloy. Damping is measured through

the line width ∆H.

= ∑ + ∑ +

k

k,k k k k,k k k

k

k k k ( ) †

'

*

' '

†

'

† H h ω a a h A a a A a a
19

The “two-magnon” model was first derived in the

1950’s to describe the effects of defects on linewidth

measurements in bulk ferrite materials. Until the

present work, it was the most sophisticated model of

inhomogeneities in FMR.

The physical picture presented by the two magnon

model is that inhomogeneities cause energy to be

transferred from the uniform precession mode to

other spin wave modes having the same frequency.

The two-magnon model is expected to be valid when the

spin wave amplitudes are small compared to the uniform

precession amplitude, so scattering from spin wave

modes back to the uniform precession mode is weak

and spin wave–spin wave interactions are not important.

Our approach to the problem of ferromagnetic

resonance in inhomogeneous thin films treats the very

common case where the Ak,k coefficients are too large

for the two-magnon model to be valid. To do this, we

start with the Hamiltonian for films with inhomogeneities

given above and find the eigenmodes and eigenvalues

directly. This process involves constructing a model

microstructure of the film, calculating the Ak,k ́ coefficients, limiting the Hamiltonian to approximately

1000 spin wave modes and computationally diagonalizing

the Hamiltonian matrix having elements Ak,k + ωkδk,k ́.

The resulting eigenmodes of the inhomogeneous film

are mixtures of the spin waves and the uniform,

precession mode. Each eigenmode has a finite coupling

to a uniform driving field that depends on the amount of

the mode that it contains. Figure 2 shows the

eigenmode spectrum and an eigenmode for a typical

calculation.

Figure 2: Eigenmode spectrum of an inhomogeneous

thin film. Each eigenmode couples directly to the uniform

FMR driving field to give the simulated FMR spectrum

shown. The background image is the magnetization pattern

corresponding to one of the eigenmodes. The image width

is 5 mm and the defect size is 20 nm.

The physical picture presented by this model is

very different from that of the two-magnon model.

k ≠ 0

k = 0

k = 0

k = 0

Inhomogeneities do not merely couple the spin wave

modes, they cause mixing of the spin wave modes into

a new set of eigenmodes, each of which is observable

in an FMR experiment. In this picture, it is clear that the

linewidth caused by inhomogeneities is due to a spreading

of the FMR intensity in frequency, not to any additional

damping process.

Figure 3 shows a comparison between our

eigenmode calculation, the two-magnon model and

experimental linewidth measurements for a 20 nm film of

Permalloy deposited on polycrystalline NiO. The coupling

to antiferromagnetic NiO produces an inhomogeneous

“exchange bias” field and a uniaxial anisotropy field

that varies in strength and direction from grain to grain.

These intentionally created “defects” are turned off by

deposition of a thin layer of Ta on the NiO, decoupling it

from the Permalloy.

Figure 3: Comparison of the two-magnon model and the

eigenmode model of FMR line width with Permalloy films:

one made inhomogeneous by NiO and a control sample.

The development of this new model of FMR in

inhomogeneous films is important in several respects.

■ It gives a clear physical picture of the magnetization

dynamics in inhomogeneous thin films.

■ It clarifies that the linewidth due to inhomogeneities

does not constitute a different kind of damping.

■ It provides the theoretical basis for interpreting FMR

linewidth that will allow us to study the intrinsic

damping processes and, ultimately, to deliver the

necessary data to industry.

For More Information on this Topic

For further information on magnetization dynamics,

damping and ferromagnetic resonance, please contact

R.D. McMichael.

Andrew Kunz and R.D. McMichael, “Normal Mode

Mixing and Ferromagnetic Resonance Linewidth.” IEEE

Trans. MAG, (in press).

Technical Highlights
20

Technical Highlights

Ballistic Magnetoresistance:

A New Phenomenon with Great Potential for Magnetic Sensors

A sharp Ni wire is placed about 20 μm from a flat Ni

substrate. In an electrochemical cell, an electrodeposition

potential is applied between the wire and substrate. A fine

column of Ni grows out from the wire until it meets the

substrate. When electrical contact is made between the

column and the substrate the electrodeposition potential

drops immediately to zero and the electrodeposition stops.

At the point of contact with the substrate, the column has

a diameter of ~10 nm. When the wire and substrate

are magnetized in parallel, electrical current easily flows

through the nanocontact. However, when the magnetization

is antiparallel, as illustrated in Figure 3, electrons tend

to be reflected at the nanocontact because they have

the wrong spin to pass easily through this region, and

a sharp increase in electrical resistance is observed.

This effect is known as magnetoresistance, and values

as large as 700% have been observed in this type of

magnetic nanocontact.

Magneto-encephalography: Imaging Brain Activity

Figure 2: Magnetic sensors are the only technology

available for real-time imaging of brain activity. Current

sensors are bulky and provide low resolution. Advanced

sensors could provide much higher resolution and provide

improved surgical guidance.

Magnetic sensors play a central role in many important

technologies ranging from data storage to health care

to homeland security. A common need among these

technologies is higher sensitivity and smaller size.

One possible route to higher sensitivity and smaller size

is the introduction of the Ballistic Magnetoresistance

(BMR) effect as a method of sensing magnetic fields.

Although BMR is, at present, a poorly understood and

poorly characterized effect, it has shown very large

magnetoresistance values in nanoscale devices and

could potentially lead to dramatic improvements in

magnetic sensors. NIST has initiated a program aimed

at clarifying, controlling, and utilizing BMR for advanced

magnetic sensor applications.

Magnetic sensors play a central role in many

 important technologies. To name just a few,

computer hard disk drives use magnetic sensors to read

stored data, magneto-encephalography uses magnetic

sensors to image brain wave activity, and bio-hazard

detectors use magnetic sensors to detect magnetic

beads that are bio-tagged for biological agents such as

anthrax, botulism, and cholera.

An array of 37 magnetic sensors: An image of epileptic activity:

Figure 1: Computer hard disk drives use a magnetic

sensor to read data.

A common need among these technologies is

sensors with higher sensitivity and smaller size. With

higher sensitivity and smaller size, more data could be

stored on a hard disk, higher resolution imaging of

brain activity would be possible, and earlier warnings

of bio-contamination could be given.

The recent discovery of the Ballistic Magnetoresistance

effect (BMR) holds the promise of both greatly increased

sensitivity and greatly reduced size for magnetic sensors.

Figure 3 illustrates how a typical sample is fabricated.

Figure 3: An illustration of magnetic nanocontacts formed

by electrodeposition that exhibit very large values of

magnetoresistance (N. García, et. al., 2001).
21

To put this result in context, the sensors that read data

from hard disk drives currently have a magnetoresistance

of only about 15%, almost 50 times smaller. Moreover,

the size of the nanocontact in BMR samples is ~ 50

times smaller than the hard disk drive sensors. Thus,

the BMR effect holds the potential for magnetic sensors

of greatly increased sensitivity and greatly reduced size.

The term “ballistic” means that the nanocontact is

too small for electrons to flip their spin in transit, as

they would in a macroscopic Ni sample.

Unfortunately, it often takes many years to go from

laboratory demonstration of a concept to a commercially

successful device. While BMR looks promising, it is

certainly at an early stage of development. The samples

investigated so far have poor reproducibility, poor stability,

and poor reliability. The techniques of fabrication are

poorly understood, and there is no theory that can

explain the magnitude of the BMR effect.

Fortunately, there is the precedent of the Giant

Magnetoresistance (GMR) effect to serve as a model.

When GMR was discovered in 1989, it was subject

to very similar shortcomings. Many experts thought it

would never be a commercially successful technology.

However, the technical community, assisted by a strong

effort at NIST, began to address the issues, and

gradually the problems were solved. The result is that,

today, every hard disk drive produced in the world uses

GMR technology in the read head.

Clearly, the sample geometry illustrated in Figure 3 is

not suitable for fabrication of hard disk read heads. The

industry’s manufacturing practices require lithographic

processing on wafers, and a fundamentally different

approach would be prohibitively expensive. In light of

this need, initial work at NIST has focused on techniques

for achieving BMR in planar geometries on wafers.

Our first attempt is illustrated in Figure 4. Although

the geometry looks ideal, the BMR achieved so far is

only 14%. We believe the problem lies in the method of

pinhole formation. Consequently, we are exploring novel

approaches to controlling pinhole size, such as the use

of the Coulomb explosion that occurs when a highly

charged ion like Xe44+ contacts a surface. By varying

the charge on the ion, the size of the pinhole can be

finely tuned.

A different and much more successful planar

geometry on a wafer is illustrated in Figure 5. In this work,

a Ni film ~10 nm thick is deposited on an insulating

wafer and patterned into the form of a nano or micro

gap. Electrodeposition is used to bridge the gap with a

Ni nanocontact. In such samples, we have been able to

achieve a much higher BMR value of 350%. However,

problems with reproducibility, stability, and reliability

remain. We are investigating these shortcomings with

scanning electron microscopy (SEM) with polarization

analysis, transmission electron microscopy (TEM),

Lorentz microscopy, and scanning tunneling microscopy

(STEM). The field of BMR is so new that very little

microscopy has been carried out to date.

Figure 5: An illustration of Ni films with lithographically

patterned gaps for electrodeposition of magnetic

nanocontacts on wafers.

We believe that with these studies, it is likely that

the shortcomings of BMR materials can be addressed,

and BMR technology can move towards a new

generation of ultrasensitive nanoscale magnetic sensors

that will have wide-ranging impact on many important

technologies.

“The technical challenges facing the data storage

industry in the next 10 years are so broad and so deep

that the industry cannot do it alone. We need

researchers like you here at NIST to help us.”

Dr. Sining Mao

Director, Read-Head Research

Seagate Technology

June 14, 2002

For More Information on this Topic

Contact: W.F. Egelhoff, Jr.

M. Muñoz, G.G. Qian, N. Karar, H. Cheng,

I.G. Saveliev, N. García, T.P. Moffat, P.J. Chen, L. Gan,

and W.F. Egelhoff, “Ballistic Magnetoresistance in a

Nanocontact between a Ni Cluster and a Magnetic Thin

Film.” Appl. Phys. Lett., 79, p. 2946, (2001).

Figure 4: A planer geometry for BMR. Pinholes are

created by dielectric breakdown at thin spots in an Al2O3

film when the electrodeposition potential is applied.

Technical Highlights
22

Technical Highlights

High-Speed Contact at the Head-Disk Interface in a

Magnetic Hard Disk Drive: Experiment and Modeling

speed. The impact force is measured with an acoustic

emission (AE) sensor and the deformation volume is

obtained with an atomic force microscope (AFM).

For the experiments reported here, the height of the

ridge is 2000 nm and the angle of incline is 0.003°.

There is considerable difficulty in alignment, z-height

control, and force calibration. Figure 1 shows a typical

electrical signal from the AE sensor during an impact.

The intensity of the signal is converted to force which

is about one newton or less. The calibration of the AE

signal is carried out using both the ball-drop method

and pencil–lead break method. A modified ball drop

method was developed to improve the accuracy of the

calibration. Figure 2 shows the setup: one ball drops

onto another ball to create the signal. The bottom ball

is pressed into the disk and the area of contact is

measured with an optical microscope.

As magnetic data storage technology moves

towards higher areal density with lower flying

heights and higher rotational speeds, the propensity

of random contacts at the head-disk interface is

bound to increase. The tribological performance of

the head-disk interface will have significant impact

on the performance and durability of the hard disk

drive. A high-speed impact test method has been

developed for evaluating nanometer-thick lubricant

film/carbon overcoat materials on hard-disk surfaces,

and a three-dimensional finite element model has

been constructed to calculate the stress concentrations

and the effect of material parameters on the resulting

deformation. The availability of an experimental

technique and a model enables effective screening

of different material chemistries and lubricant

combinations to improve the level of protection

for hard disk technology.

Development of newer generations of hard disk

 drives with higher data storage capacity poses

many technological challenges. The magnetic disk drive

consists of a multilayer disk that stores the data and

an air-bearing slider that contains a magnetic head for

reading and writing. As the areal density of data storage

increases towards 155 Gb /cm2, the physical spacing

between the head and disk surfaces needs to be reduced

from (10 to 15) nm to 3.5 nm. The rotational speed

needs to increase to 15,000 rpm (45 m /s) from

7200 rpm. The combination of the low flight height

and high speed makes occasional contacts inevitable

due to disk waviness, spindle wobble, and disk surface

roughness. Sometimes catastrophic collisions (avalanche)

result in the loss of data.

Besides improving the spindle and surface

roughness, new overcoats and lubricants are being

investigated to improve the protection of the disk

against such high-speed collisions. Actual durability

field trials to evaluate new materials under such

conditions are time-consuming and expensive as well

as statistics-based. Thus, there is an urgent need to

develop an accelerated test methodology to evaluate

materials under controlled conditions.

The experimental research includes development of

a one-pass, high-speed impact test instrumentation and

test method to evaluate materials on a hard disk surface.

The basic concept is to artificially create a ridge (500 to

2000) nm high with a 0.002° to 0.01° angle of incline

on the disk surface by controlled scratching at the

substrate side of the disk to create a ridge on the top

surface with its multilayers intact. A 3 mm diameter ruby

ball collides with the ridge under 7200 rpm rotational

Figure 1: Acoustic emission signal from an impact event.

Figure 2: Ball-dropping apparatus (left) and data (right)

for the force calibration.
23

By dropping one ball on top of a second ball

positioned at the pre-formed deformation zone, the

signal transmits through the known contact area

between the ball and the disk. The acoustic signal from

the collision is recorded with the sensor placed on the

other side of the disk. The resulting calibration curve

indicates a sensor constant of (3.2 ± 0.4) N/V under

these experimental conditions.

A series of impact tests was conducted on disk

samples with different monolayer (1 nm) coatings.

The transmitted force measured for a blank disk is

(0.92 ± 0.23) N. For perfluoro-polyether (PFPE),

the transmitted force is (0.61 ± 0.04) N and for PFPE

+ 5% phosphazine, the transmitted force is (0.31 ±

0.07) N. This ranking corresponds with industrial field

experience in terms of robustness of lubricant coatings.

The computational component of this research

investigates the dynamic problem of a high-speed

spherical asperity impacting on a flat surface of a hard

disk at a shallow angle. The objective is to ascertain

the result of such an impact and to investigate the

complex interactions of multilayer materials such as a

hard disk. We seek to obtain the values of contact

duration, contact zone size, time history solutions of

stress field (including residual stress), damage/plastic

yield zone, contact area, and contact force for a given

set of attributes including the layered structure of the

disk. Due to the time-dependent nature and the difficulty

of the problem (such as the geometric complexity, the

materials’ nonlinear, nonuniform and path-dependent

properties, and inertial effects), analytical closed form

solutions are not possible. A numerical method

using finite element simulation was used. It should be

emphasized that this approach is continuum in nature

as implied by the conventional finite element theory.

However, as the thicknesses of the layers are reduced

to a few nanometers, the effect of surface mechanical

stresses present at the free surface and interfaces

becomes significant and, thus, cannot be ignored.

Using the guidelines described above, we

constructed a 3D finite element model. The model

includes only the top half of the disk due to the

shallowness of the impact. The slider is modeled by

a two-mass system linked by a spring simulating a

nanoasperity on the head surface.

For a given layered structure and geometry,

asperity size, and impact velocity, the model is capable

of predicting sliding contact events, including contact

duration, zone size, forces, penetration depth, underlying

plastic zone, and energy transfer. The simulations use

realistic values from current disk design: asperity size

= 30 nm in radius, impact speed = 10 m/s, and (5,

30 and 50) nm thicknesses for the overcoat, media layer

and substrate, respectively. The results are given below.

Figure 3 is a contour plot of the maximum principle

stress field created by two collisions. Each collision

occurs on a time scale of approximately 10 ns. The

in-situ stress field under the asperity demonstrates

compression ahead and tension behind the penetration.

For More Information on this Topic

Contact: M. Bai, T.J. Chuang, S.M. Hsu, Z.C. Ying

“Simulation of a Single Asperity Impact at Head-Disk

Interface,” Chuang, T.-J., and Hsu, S.M., ASME/STLE

Proceedings on the Frontiers of Magnetic Hard Disk

Drive Tribology and Technology, Cancun, Mexico;

also submitted to ASME Trans. Journal of Tribology.

Figure 3: Contour plat of the maximum principle

stress field created from two collisions between an asperity

(represented by a semi sphere in the figure) and substrate.

Figure 4: Plastic zones developed underneath the collision

sites. Also shown are the deformed craters on the top surface.

The plastic deformation zone created in the substrate

as a result of two collisions between the asperity and

substrate is shown in Figure 4. When the asperity has

left the disk surface after a collision, there exists a welldefined zone of residual stress, indicating that elastic

recovery causes stress redistribution. The stress field

distribution is influenced by the different mechanical

properties of the individual layers as noted in Figure 3.

The simulation also gives an indication of the energy

partition as a result of collision under an adiabatic

assumption. Results suggest that under a short

duration, relatively light normal load, the momentum

transfer due to velocity change results in a substantial

temperature rise at the interface. The energy attributable

to frictional dissipation is about 40 fJ per impact for a

given friction coefficient of 0.3. The energy from plastic

deformation is only about 7 fJ when a portion of the

kinetic energy from the moving slide is transferred onto

the disk. It is clear that the dominant mechanism of

energy consumption is due to heat generated from

friction, an outcome consistent with experimental

observations.

Technical Highlights
24
25

Projects in Materials for

Micro- and Optoelectronics

Projects in Materials for

Micro- and Optoelectronics

Project Overview
26

Characterization of Porous Low-k Dielectric Constant Thin Films

NIST is working to provide the semiconductor

industry with unique on-wafer measurements of the

physical and structural properties of porous thin films

important to their use as low-k dielectric materials.

The methodology utilizing several complementary

experimental techniques to measure the average

pore and matrix morphology has been expanded to

include pore size distributions. New methods have

been developed to measure closed pore content and

matrix morphology in films having complex structures

with any morphological type.

Barry J. Bauer and Wen-li Wu

The next generation of integrated circuits will require

 feature sizes that demand new low-k interlayer

dielectric materials. To address problems with power

consumption, signal propagation delays, and crosstalk

between interconnects, dielectric constants will have

to reach unprecedented values. One avenue to low-k

dielectric materials is the introduction of nanometer

scale pores into a solid film to lower its effective

dielectric constant. While these modifications change

the dielectric constant favorably, other important

parameters such as physical strength and barrier

properties will also change, often in an unfavorable

way. Characterization of the pore structure is crucial to

rapid development of new dielectric materials, and new

measurement methods are necessary.

The small sample volume of 1 μm films and the

need to characterize the films on silicon wafers narrows

the number of available measurement methods. We

have been providing standard characterization using a

combination of small angle neutron scattering (SANS),

high-resolution x-ray reflectivity (HRXR), and ion

scattering techniques. These measurements are

performed directly on films supported on silicon

substrates so that processing effects can be investigated.

New methods are required, however, to provide more

detailed information necessary to complete the product

development.

During the past year, we have continued our

efforts on characterization of current industriallyrelevant materials through International SEMATECH,

and by new collaborations with Dow Chemical and IBM.

More significantly, we have perfected new techniques

for more thorough and accurate film characterization.

International SEMATECH, a consortium of

microelectronics companies, provided more than

20 separate samples for characterization by NIST.

The structural information provided by NIST is placed

into a master database that includes data from a variety

of sources, detailing both the structural and material

properties needed to evaluate candidate materials.

Member companies make extensive use of the data to

guide the development of low-k dielectric materials for

integration into future devices.

Finally, we have evaluated and perfected new

measurement methods to more thoroughly and

accurately characterize pore and matrix morphology.

Capillary porosimetry has been applied to HRXR

measurements to determine pore size distributions,

rather than simple averages. By the use of a carrier gas

and control of vapor pressure through thermal methods,

the accuracy and speed of the measurements have

greatly improved. Based on these findings, an automated

measurement system has been designed and is under

construction with the ultimate goal of providing a

measurement technique that can be transferred to the

materials suppliers and chip manufacturers.

Figure 1: Representative mass gain and pore size

distribution calculated from HRXR porosimetry

measurements (AU = arbitrary units).

New techniques have been developed that measure

SANS from films that have been infused by mixtures of

normal toluene and toluene in which hydrogens have

been replaced by deuterium. The match point measures

where the neutron contrast matches that of the wall

material. By filling the open pores with contrast match

fluids, SANS reveals the closed pore content and size.

The relative open and closed pore content is becoming

increasingly important to low-k applications.

Contributors and Collaborators

B.J. Bauer, H.J. Lee, R.C. Hedden, C.L. Soles,

H. Wang, D. Liu, W.L. Wu (Polymers Division, NIST);

C. Glinka (NIST Center for Neutron Research);

J. Wetzel, J. Lee (International SEMATECH);

B. Landes (Dow Chemical); M. Baklanov (IMEC);

D. Gidley (University of Michigan); W. Chen,

E. Moyer (Dow Corning); S. Yang (Lucent)

Microelectronics
27

Dielectric Metrology for Polymer Composite Films

in the Microwave Range

Novel dielectric hybrid materials based on organic resins

and ferroelectric ceramics with high dielectric constants

have been identified by the industry as key dielectrics

for advancing miniaturization and functional performance

of microwave electronics. The objective of this project is

the development of broadband dielectric metrology for

such materials and fundamental understanding of the

high frequency relaxation mechanism in relation to

structural and molecular attributes.

Jan Obrzut

We have developed an accurate broadband

measurement technique that enables dielectric

measurements at microwave frequencies of up to several

GHz. The broadband test methods currently in use

by the industry are based upon a lumped element

approximation and are limited to frequencies below a

few hundred megahertz. Our new technique is based on

the observation and theoretical analysis of fundamental

mode propagation at high frequencies in thin film

dielectrics terminating a coaxial air-filled transmission

line. The 3D vector plot of the electric field of the first

fundamental mode, simulated inside a high-k specimen,

is shown in Figure 1.

The new broadband methodology enables

measurements at frequencies of 100 MHz to 10 GHz

with the precision required by new electronic technologies.

In partnership with the Institute for Interconnecting and

Packaging Electronic Circuits, IPC, we initiated a standard

test method development and chair the IPC-D37-d test

method subcommittee for Embedded Passive Devices.

We guided the design of the test protocol and made

arrangements with co-sponsoring member companies

for round robin evaluation.

Our capability to measure the dielectric relaxation

times in the sub-nanosecond regime and the relaxation

strength for wide range of dielectric permittivity values

is used to quantify dispersion, alignment, and structure

in hybrid materials. Using the broadband study, we

demonstrated that the composites of organic polymer

resins filled with ferroelectric ceramics exhibit a

dominant intrinsic high-frequency relaxation behavior.

Such dielectric properties were beneficial in eliminating

the electromagnetic noise in processors and logic

devices. We have confirmed that the fastest dielectric

relaxation process is controlled by the dipolar dynamics

of the polymer matrix while the dielectric loss arises

from the difference in coupling between the relaxed

and unrelaxed dipoles. Our new numerical models can

quantitatively correlate the dependence of the real and

imaginary parts of the complex permittivity on the

volume fraction of the ceramic filler.

We have developed constrained regularization

algorithms to obtain a continuous distribution function

of relaxation times. The method is able to resolve

multiple relaxation processes without any a priori

assumptions about their character. It may be generalized

to different types of fundamental relaxation processes

and not restricted to a superposition of Debye-like

functions. This approach will help judge the identity

between microscopic and macroscopic relaxation

behavior of polymer composite systems. It also allows

us to link broadband dielectric spectroscopy with other

techniques such as time domain spectroscopy, photon

correlation spectroscopy and NMR.

Contributors and Collaborators

O. Anopchenko, N. Noda and J. Obrzut (Polymers

Division); B. Rust (Mathematical and Computational

Sciences Division); R.Croswell (Motorola);

R. Charbonneau (Storage Tech.); D. McGregor

(DuPont); T. Hubbing (University of Missouri);

R. Greenlee (Merix); J. Murry (Litton); Tom Newton

(IPC); R. Foly (DOD); J. Peiffer (3M); L. Patch

(NCMS); D. Senk (Shipley); R. Sheffield (Nortel)

Figure 1: The vector plot of the electric field, E,

of the first fundamental mode inside the film specimen

(ε* = 34.6 – j1.6) terminating an AP-7 coaxial line.

The arrows indicate the direction and strength of the

electric field E.

In contrast to the assumption of the lumped element

model, the field in the specimen section is not uniform.

At a resonant frequency of about 8.2 GHz, the magnitude

of the electric field, E, shows two maxima localized near

the edge of the specimen, while the minimum magnitude

of field E extends along the specimen diameter. The

corresponding plot of the magnetic field, H, is parallel to

the propagation direction. Both E and H fields appear to

decay rapidly at the boundary between the high-k dielectric

and air. The results of numerical simulations confirm

our earlier experimental analysis that, at microwave

frequencies, the film specimen becomes a transmission

line with propagation direction along the film diameter

rather than its thickness.

Microelectronics
28

Polymer Photoresists for Next-Generation Lithography

Photolithography, the process used to fabricate

integrated circuits, is the key enabler and driver for the

microelectronics industry. As lithographic feature sizes

decrease to the sub-100 nm length scale, significant

challenges arise because both the image resolution

and the thickness of the imaging layer approach

the macromolecular dimensions characteristic of

the polymers used in the photoresist film. Unique

high-spatial resolution measurements are developed

to reveal limits on materials and processes that

challenge the development of photoresists for

next-generation sub-100 nm lithography.

Eric K. Lin

Photolithography is the driving technology used by

 the microelectronics industry to fabricate integrated

circuits with ever decreasing sizes. In this process, a

designed pattern is transferred to the silicon substrate by

altering the solubility of areas of a polymeric photoresist

thin film through an acid catalyzed deprotection reaction

after exposure to radiation through a mask. To fabricate

smaller features, next-generation photolithography will be

processed with wavelengths of light requiring photoresist

films less than 100 nm thick and dimensional control

to within 2 nm. Material and transport issues arise

when fabricating structures over these length scales.

To date, the materials and processes needed to fabricate

sub-100 nm features have not been selected.

To advance this key fabrication technology, we

work closely with industrial collaborators to develop

and apply high-spatial resolution and chemically specific

measurements to understand changes in material

properties, interfacial behavior, and process kinetics

at nanometer scales and to provide high-quality data

needed in advanced modeling programs.

Over the past year, we have applied and enhanced

unique measurement methods and computer simulation

tools to provide new insight and detail into the complex

physico-chemical processes used in advanced chemicallyamplified photoresists. These methods include x-ray and

neutron reflectivity (XR, NR), small angle neutron and

x-ray scattering (SANS, SAXS), incoherent elastic

neutron scattering (IENS), near-edge x-ray absorption

fine structure (NEXAFS), combinatorial methods, x-ray

standing waves with fluorescence (XSW), nuclear

magnetic resonance (NMR), molecular dynamics (MD)

simulations, and atomic force microscopy (AFM). In

particular, XR and NR studies with specially designed

model materials were used to profile the reaction front

with nanometer resolution (see Highlights), NEXAFS

measurements have been extended to depth profile the

chemical composétion of the top 5 nm of a polymer film

surface, SAXS measurements have been performed in

transmission on patterned structures to complement

earlier SANS studies, and AFM techniques have been

adapted to begin study of the mechanical properties of

interfacial polymer layers and structures.

Figure 1: NEXAFS carbon-edge spectra from the bulk and

surface of a model photoresist before (blue /black) and after

exposure and bake (green /red). A delay time of 5 min was

imposed after exposure and before a bake. The reaction is

incomplete only at the surface (peak signature at arrows)

corresponding to surface base (yellow) contamination as

illustrated.

Our experimental and theoretical methods have been

used to understand important technical problems in the

development of next-generation materials. These topics

include: the spatial evolution of the reaction-diffusion

process as a function of temperature and time; the domain

size and distribution of photoacid generator molecules

within a model photoresist polymer matrix; the microscopic

dynamics of photoresist polymers in thin films; the

segregation and distribution of small molecule

contaminants, solvents, and moisture in ultrathin polymer

layers; investigation of the anti-reflective coating-photoresist

interface for material sources of compromised line-profile

control; development contributions to the measured

line-edge roughness; and the structural characterization

of critical dimensions and line-edge roughness in

lithographically-fabricated patterned structures.

Contributors and Collaborators

E.K. Lin, C.L. Soles, J.L. Lenhart, R.L. Jones,

V.M. Prabhu, F.W. Starr, J.F. Douglas, P.M. McGuiggan,

D.L. VanderHart, W.L. Wu (Polymers Division);

D.A. Fischer, S. Sambasivan, J. Woicik (Ceramics

Division); S.K. Satija, C.J. Glinka (NCNR); D.L. Goldfarb,

M. Angelopoulos (IBM T.J. Watson Research Center);

B.M. Trinque, S.D. Burns, C.G. Willson (University of

Texas); R. Puligadda, Y. Wang, D. Guerrero (Brewer

Science)

Microelectronics
29

Microelectronics

Superconformal Film Growth: Measurements and Modeling

State of the art manufacturing of microelectronic

devices relies on superconformal deposition of copper

for on-chip interconnects. Superfilling of trenches and

vias results from the interplay of rate inhibiting and

accelerating adsorbates. NIST has recently identified

the mechanism that explains “bottom-up” filling of

submicrometer features. A quantitative capability for

predicting the influence of adsorbates on superconformal

film growth has been demonstrated. The mechanism

has also been shown to apply to the electrodeposition

of elements beyond copper, as well as CVD processing.

Thomas P. Moffat and Daniel Josell

T he mechanism of superfilling or “bottom-up” filling

 of submicrometer features has been explained. The

chief tenets are: a) the deposition rate is proportional

to catalyst coverage and b) the catalysts “floats” or

remains segregated at the interface during film growth.

Superfilling follows as a natural consequence of these

stipulations. Namely, the catalysts coverage increases

(decreases) as the local surface area decreases during

deposition, thereby giving rise to “bottom-up” filling

and a bright surface finish.

Figure 1: “Bottom-up” filling of vias with silver.

Quantitative description of the curvature enhanced

accelerator coverage (CEAC) mechanism enables the

simulation of feature filling. Three different electrode

shape-change algorithms have been developed: 1) a simple

model based on the solution of a single first-order

differential equation, 2) a parameterized interface tracking

code, and 3) a level set model, which incorporates a

rigorous description of all concentration fields. Good

agreement was found between these formalisms and feature

filling experiments. A summary of the “simple model”

simulations for filling trenches of variable aspect ratio as

a function of applied potential and catalyst concentration

is shown in Figure 2.

The generality of the CEAC mechanism has been

verified by demonstrating superfilling of metals other

than copper, i.e. silver electrodeposition from an alkalin

electrolyte, as well as the first quantitative predictions of

superfilling by iodine catalyzed chemical vapor deposition

(CVD).

All kinetic parameters were determined from

voltammetric (i-V) and chronoamperometric (i-t)

experiments performed on planar substrate. Correlation

between the reactivity and catalyst coverage was determined

by surface analysis following metal deposition or,

alternatively, by dosing the surface with a prescribed

quantity of catalyst prior to the metal deposition

experiment. The second scheme not only substantiates

the CEAC mechanism but may also be implemented in

manufacturing in order to circumvent the challenging

control issues associated with catalyst consumption

during electrolyte aging.

Figure 2: Feature fill/fail bondaries as a function of

processing parameters: catalyst concentration, etc.

Contributors and Collaborators

D. Wheeler, J. Bonevich, W. Egelhoff (MSEL);

G. McFadden (ITL); S. Coriell, B. Baker, C. Yang,

L. Richter, S. Robertson (CSTL); J. Mallett, C. Witt

(International SeMaTech); B. Melnick, M. Freeman

(Motorola); S.G. Pyo (Hynix Semiconductor)

Figure 3: A one-to-one correlation between catalyst

coverage (red) and the deposition rate (blue).
30

Micrometer-Scale Reliability:

Stress Voiding and Electromigration

The microelectronics industry continues to strive

for better device performance through a combination

of dimensional scaling and incorporation of new

materials for chip-level interconnects. Reliability of

the resulting new systems is poorly known at present.

We conduct studies to understand the mechanisms

of thermal strain — and electric current — induced

degradation phenomena, with emphases on the

roles of localized stress and localized variations

in microstructure on interconnect lifetimes.

Robert R. Keller and Roy H. Geiss

Technical Description

The two primary reliability-limiting phenomena that

 chip-level interconnects undergo are stress voiding

(SV) and electromigration (EM), occurring during

thermal- and electric current-induced stressing,

respectively. Both involve the development and

subsequent relaxation of stress and take the form of

surface topography development and void formation.

Consequent device failure occurs in the form of

passivation cracking, short circuiting, and open

circuiting. Previous studies have demonstrated that

even small variations in localized microstructure can

have large impacts on these types of failure.

We began an effort about a year and a half ago

that shows that both mechanical and diffusion-based

degradation processes can occur due to alternating

current (AC) stressing at low frequencies and high

current densities. Interconnect damage under such

conditions appears to occur due to thermomechanical

fatigue resulting from Joule heating of metal

interconnects well-adherent to oxidized silicon

substrates. Thermal expansion mismatch strains lead to

large stresses in the metal and cause cyclic deformation.

This work has suggested both a new testing technique

as well as a potentially new reliability threat to narrow

interconnects. AC stressing can be used to perform

fatigue testing of very small structures in this manner.

The reliability threat appears as we consider such

testing in metals encapsulated by soft dielectrics,

which may not suppress the surface damage. Here,

we report new observations of microstructural changes

accompanying AC stressing of Al-alloy interconnect lines.

Accomplishments

We conducted a quasi in situ test whereby a line was

characterized for surface structure and crystallographic

orientation distribution prior to testing, as well as during

periodic (every 15–60 s) test interruptions. Figure 1

shows a SEM image and the corresponding orientation

map obtained by automated electron backscatter diffraction

(EBSD) after less than one minute of stressing. The

topography in the large grain towards the left of the field

of view can be explained by the schematic to the far left.

Given the observed orientation, threading dislocations

could glide on the indicated slip system leaving slip

offsets at the surface. During cycling, the severity of

the offsets increases considerably.

Figure 1: Dislocation mechanism for topography formation

induced by AC stressing in Al-1Si.

Figure 2: Grain growth induced within first minute of

AC stressing, prior to occurrence of topography.

An unexpected observation is shown in Figure 2,

where the orientation maps show that significant grain

growth can occur even within the first minute of

stressing, prior to the formation of slip offsets. This

demonstrates that the effect induced by low-frequency

cycling of high-current density is not simply mechanical

fatigue, but that diffusion must also play a significant

role. Interestingly, we have seen this effect only in the

aluminum alloy. Similar ongoing studies on copper

done by our colleagues at the Max-Planck-Institut have

shown no evidence of grain growth in the absence of

topography development. Rather, that work shows grain

growth that takes place concurrently with or after the

formation of slip offsets.

Contributors and Collaborators

R. Mönig, C. Volkert (Max-Planck-Institut für

Metallforschung)

Microelectronics
31

Phase Equilibria and Properties of Dielectric Ceramics

Ceramic compounds with exploitable dielectric

properties are widely used in technical applications

such as actuators, transducers, and resonators or

filters for wireless communications. The commercial

competitiveness of next-generation devices depends

on new ceramics with improved properties and/or

reduced processing costs. Experimental phase

equilibria determination, integrated with systematic

chemistry-structure-property studies (experimental,

theoretical, and modeling) contribute toward the

fundamental understanding and rational design of

these technologically important materials.

Terrell A. Vanderah, Benjamin P. Burton,

Eric J. Cockayne, and Igor Levin

Ternary oxides in the Bi2O3-ZnO-Nb2O5

 system exhibit high-dielectric constants (ε),

relatively low dielectric losses, and compositionally

tunable temperature coefficients of capacitance (τc ).

Such properties combined with sintering temperatures

of less than 950oC render these materials attractive

candidates for capacitor and high-frequency filter

applications in multilayer structures co-fired with silver

electrodes. The system features two structurally distinct

ternary compounds, Bi1.5Zn0.92Nb1.5O6.92 (ε = 145,

τc = –400 MK–1) and Bi2Zn2/3Nb4/3O7 (ε = 80,

τc = +200 MK–1), which exhibit very dissimilar

dielectric properties and form temperature-stable,

commercially important mixtures. However, the absence

of structural information for both phases has precluded

understanding of the unusual dielectric properties;

in particular, Bi1.5Zn0.92Nb1.5O6.92 ceramics exhibit

dielectric relaxation, attributed to a dipolar glass-like

mechanism, while no such behavior is observed for

Bi2Zn2/3Nb4/3O7. Recent studies combined electron,

x-ray, and neutron powder diffraction techniques to

elucidate the crystal structures of these compounds

which feature pyrochlore and zirconolite-like structures,

respectively. The results reveal that displacive disorder

in Bi1.5Zn0.92Nb1.5O6.92 is responsible for the high

dielectric constant and the relaxation phenomenon.

Subsolidus phase relations have been determined

for the BaO:TiO2:Ta2O5 system and the BaO:Ta2O5

subsystem, which are pertinent to the processing

of Ta2O5-based ceramics for cellular base station

resonators and filters. BaTiO3 dissolves a considerable

amount of Ta5+ by forming Ti4+ vacancies. The

formation of Ba3Ti4Ta4O21, a member of the hexagonal

A3M8O21–type ternary oxides, was confirmed as well

as its solid solution. Several new compounds have been

found, including four members of the orthorhombic

“rutile-slab” homologous series, BaTinTa4O11+2n,

with n-values 3,5,7,9. Three ternary phases with

close-packed [Ba,O] layer structures related to that

of 6L Ba4Ti13O30 were found: 13L Ba18Ti53Ta2O129,

7L Ba10Ti27Ta2O69, and 8L Ba6Ti14Ta2O39. The

crystal structures of the 13L and 7L phases were

determined by single-crystal x-ray diffraction. Phases

with tetragonal tungsten bronze related structures

occur over large compositional ranges, both within the

ternary and along the BaO-Ta2O5 binary.

PbZr1-xTixO3 (PZT) is an important piezoelectric

material that has applications in transducers. A firstprinciples-based effective Hamiltonian was developed for

Zr-rich PZT. This model is the first that reproduces the

correct sequence of phase transitions in this system:

orthorhombic to rhombohedral to cubic, as the

temperature increases.

Other important transducer materials include relaxor

ferroelectrics. An effective Hamiltonian was developed

for the relaxor ferroelectric PbSc1/2Nb1/2O3 (PSN),

based on first-principles calculations. This model allows

one to simulate the structure and dielectric properties

of PSN as a function of cation ordering, temperature,

and applied fields, in systems representing as many as

300,000 atoms. Molecular dynamics methods allow

the polarization dynamics and frequency dependence of

the dielectric permittivity to be studied. The local electric

field at the Pb sites due to the charged Sc and Nb ions

has been calculated (see Figure 1) and incorporated into

the model. The results establish a clear link between

chemical disorder and dielectric response, and they

indicate “nanoscale texture,” i.e., the size and

geometrical arrangement of domains play a fundamental

role in the observed macroscopic properties.

Contributors and Collaborators

R.S. Roth, W. Wong-Ng (MSEL); J.E. Maslar (CSTL);

R. Geyer (EEEL); S. Bell (TRAK Ceramics, Inc.);

C.A. Randall (Penn State); J.C. Nino (Penn State);

K. Leung (Sandia); L. Bellaiche (University of Arkansas);

U. Waghmare (JNCASR, Bangalore, India); S. Prosandeev

(Rostov State University, Russia); V. Zelezny and

J. Petzelt (Czech Academy of Sciences)

Figure 1: Local electric fields in a model for the relaxor

ferroelectric PSN. Experiments suggest that relaxor

ferroelectrics contain chemically-ordered regions in a

disordered matrix and that such inhomogeneities may be

responsible for the relaxor behavior. Computations show

that chemically-ordered regions in PSN (circles) have

much lower local fields than the disordered matrix.

Microelectronics
32

Processing of Low Temperature Co-Fired Ceramics

Low temperature co-fired ceramics (LTCC) are

rapidly becoming the technology of choice for RF

components and modules for portable wireless

applications. LTCC technology has demonstrated that

it can enable the high density integrated packages

meeting the requirements for portable wireless.

Commercialization of these modules requires the

accurate prediction of dimensional changes during

sintering in order to design passive integrated

components (resistors, capacitors and inductors)

in the LTCC module.

John E. Blendell and Jay S. Wallace

Low temperature co-fired ceramics (LTCC) are

 finding increasing use in portable wireless

applications due to their high dielectric constant, low

loss and high Q. These features allow a reduction in

the size of modules for high-frequency applications.

In addition, LTCC can include integrated passive

components, they are environmentally robust, and

they can be rapidly produced. These factors lead to a

significant cost advantage for LTCC modules. Because

dimensional tolerances are critical for high-frequency

applications, accurate predictive models of dimensional

changes during sintering are necessary for the

commercialization of LTCC modules.

Modeling the evolution of the microstructure, and

the dimensional changes that take place during sintering,

is being carried out using a combination of phase field

methods and object-oriented finite element analysis.

This allows accurate modeling of sintering processes

with arbitrary boundary conditions. The finite element

models are being used to solve for the stress state of

the powder compact. These stress profiles will be

used in the phase field model to predict mass flow and

microstructural evolution during firing. Preliminary

two-particle models have been constructed to study

the shrinkage in a glass phase. These models will

be extended to multiple particle models to allow the

interactions that can cause rearrangement of the

particles to take place.

An important aspect of this research is the

verification of the models developed. We have

constructed an optical hot stage microscope for direct

examination of the surface of the LTCC tapes during

sintering. The movement of markers placed on the

surface has shown that there is anisotropic shrinkage,

Figure 1. Not all markers move towards a common

center, but rather there is local rotation of the markers,

indicating local variations in shrinkage.

Optical transmission through the unsintered tapes

was measured, Figure 2. The measurements show

density variations on the 100 μm scale, consistent with

the sintering results. The transmission measurements

are being extended into the near IR region using an IR

diode laser to improve the resolution of the density

variation data.

Figure 1: Hot stage optical microscope measurement of

surface marker movement showing anisotropic shrinkage.

Figure 2: Optical transmission of unsintered tape showing

variations due to particle packing variations.

Contributors and Collaborators

P. Schenck, B. Hockey (Ceramics Division);

J. Warren (Metallurgy Division); M. Locatelli

(University of Washington, Seattle, WA)

Microelectronics
33

Microelectronics

Solder and Solderability Measurements for Microelectronics

Solders and solderability are increasingly tenuous links

in the assembly of microelectronics as a consequence

of ever-shrinking chip and package dimensions and

the international movement toward environmentallyfriendly, lead-free solders. Through collaboration with

the NEMI Pb-Free Assembly Project, we are

providing the microelectronics industry with

measurement tools, data, and analyses that address

national needs in the implementation of lead-free

solders. We also work with industrial standards

organizations to provide measurement methods and

guidance for integration of lead-free solders and new

component types into assembly processes.

Maureen E. Williams, William J. Boettinger,

Carol A. Handwerker, Ursula R. Kattner,

and Frank W. Gayle

T he U.S. microelectronics industry has clearly

 articulated the measurement needs for Pb-free solders

and for solderability and assembly. For example, the

urgency for materials data for Pb-free solders has been

specified in the 1997 IPC, 1999 ITRS, 2000 NEMI, and

2000 IPC Lead-Free Solder Roadmaps. The pressure from

the Japanese consumer product market and from the EU

to produce lead-free microelectronics continues to increase.

In addition, the lack of understanding and control of current

standard solderability measurements has inhibited the

development of improved measurements necessary for new

solders and for new packaging schemes. These industrial

needs are addressed through this NIST project.

Since 1999, NIST has served a major role in the

NEMI Pb-Free Assembly Project to identify and move

Pb-free solders into practice. Last year, NIST held a

workshop cosponsored by NEMI, NSF, and TMS, on

Modeling and Data Needs for Lead-Free Solders. The

report from this workshop is serving as a roadmap for

research on the reliability of lead-free solders. In FY2002,

the NEMI Lead-free Task Force and the National

Institute of Standards and Technology (NIST) have

worked together to respond to the identified needs by:

■ Identifying the most important lead-free solder data

for the microelectronics community;

■ Developing “a guide to recommended practices” for

measuring the mechanical, thermal, electrical and

wetting properties of lead-free solders. This document

is being distributed to all interested parties through the

NEMI and NIST websites and through NIST in hard

copy form;

■ Gathering into a single database existing physical and

mechanical property data that have been developed by

researchers around the world;

■ Critically reviewing existing materials property data for

the most important alloys for use in finite element

analyses; and

■ Providing a list of literature references on alloys,

processing, reliability, environmental issues, and

components for assisting in the implementation of

lead-free solders.

In a second major activity with the NEMI Lead-Free

Assembly Project, NIST completed the microstructurebased failure analysis of thermally-cycled assemblies as

part of the project’s full-scale reliability trials with the

new national standard Pb-free solder. The NEMI Final

Report will be issued in early FY2003.

NIST is also working in collaboration with the IPC

Standards Committees (most closely with members from

Celestica, Lucent, Raytheon, Rockwell, and Shipley-Ronel)

to establish reproducible solderability test standards for

board assembly. Activities include: 1) providing benchmark

experiments for the wetting balance tests to predict

on-line solderability for a wide range of surface finishes,

lead materials, and solder alloys; 2) developing a “best

practice” for globule solderability test for small components

and pads; and 3) evaluation of the effect of component

lead geometery on industrial solderability tests.

Figure 1: Metallographic cross-section of a lead-free SnAgCu

solder joint showing fracture in the solder adjacent to the

interface with the component following accelerated thermal

cycling. Failure location is the same as for Sn-Pb eutectic

solder, which this alloy is replacing as a standard alloy.

Highlights

■ Sources of uncertainty have been established for

wetting balance solderability tests leading to increased

repeatability and reproducibility of tests.

■ Recent flux studies performed at NIST led to a change

in test procedures for the IPC/EIA/JEDEC J-STD

002 solderability standards.

■ Research at NIST helped to separate the role of solder

melting temperatures from solder paste wetting

dynamics and the effect of air reflow.

Contributors and Collaborators

L.C. Smith (NIST); K.-W. Moon (University of

Maryland); D. Napp (National Center for Manufacturing

Sciences and associated consortium members);

R. Gedney (NEMI Lead-Free Task Force and

associated consortium members)
34

Microelectronics

Mechanism of Sn Whisker Growth from Electrodeposits

The electrodeposition of metallic alloys has been

central to the growth of the electronics industry.

This is largely due to the exceptional properties

exhibited by electrodeposited material as well as

the favorable economy of scale associated with

electrodeposition processes. A technology important

to electronics manufacturing is the electrodeposition of

Sn-based coatings and surface finishes to guarantee

solderability. With the rapid move to Pb-free solder

surface finishes, new test methods are needed for

predicting the tendency of such coatings to form

Sn whiskers that can lead to catastrophic failures.

Christian E. Johnson, Maureen E. Williams,

and Kil-Won Moon

I n FY2001, the National Electronics Manufacturing

 Initiative (NEMI) Pb-Free Task Force, of which NIST

is a member, identified a serious concern of OEMs,

contract manufacturers, and component manufacturers

with the conversion from Sn-Pb to Pb-free surface

finishes for circuit boards and component leads. It is

well known that the use of pure Sn protective surface

finishes can cause serious problems: tin whiskers

(1 mm diameter and several mm long) can grow from

the plated tin surface and cause electrical shorts and

catastrophic failure. Historically Pb was added to

Sn plate to prevent whisker growth, and, thus, the

“whisker problem” disappeared. With the rapid move

to environmentally-friendly Pb-free assemblies, the

microelectronics industry must remove Pb from the

surface finishes. For many applications, an electroplated

Sn-based surface finish remains a preferred surface for

ease of processing. However, the growth of whiskers is

currently a perilous drawback.

Based on these needs, this NIST project is focusing

on: 1) understanding the effect of alloying elements

other than Pb on the tendency for whisker formation,

2) developing techniques to separate possible physical

and microstructural factors affecting whisker formation,

and 3) assisting the NEMI Standardization Technology

Integration Group (TIG) with both test development

and understanding the root causes of whisker

formation. The Sn-Cu system was chosen as the most

versatile for electroplating, since it is compatible with

Sn-3.9Ag-0.6Cu and Sn-0.7Cu, the Pb-free solders

selected as the new national standard alloys by NEMI

for reflow soldering and for wave soldering, respectively.

Whiskers are generally believed to grow to relieve

residual stress in electroplated Sn. However, the origin

of this stress has not been determined definitively.

Therefore, as a starting point, we focused on the

relationship of microstructural change and whisker

formation as a function of Cu additions. Increased Cu

content reduced the grain size of the Sn deposit from

0.65 μm to 0.2 μm. Furthermore, Cu in deposits forms

Cu6Sn5 intermetallic compounds (IMC) at the Sn

grain boundaries. In the absence of Cu additions,

no whiskers were observed after one year of room

temperature aging. However, with Cu additions, whiskers

were observed within two days increasing to a density

of 50/mm2 after one year for the highest Cu levels.

Our current hypotheses are that a smaller grain size

produces a higher level of intrinsic stress and that the

presence of IMC particles at the grain boundaries may

prevent relaxation of the stress. As a consequence, the

stress level in the Sn-Cu deposits is relieved through

whisker growth. To investigate this idea, cantilever beam

test coupons were made from phosphor bronze and a

commercial bright electrolyte. Using Stoney’s Equation,

an average planar stress of the deposited film was

calculated from the deflection of the beam as a function

of time. Preliminary results of pure Sn and Sn-Cu

deposits indicated that the presence of the IMC particles

in the Sn-Cu deposits inhibited stress relaxation. The

compressive stress in the pure Sn deposit went from

an initial level of 14 MPa to zero after four days. In

contrast, the initial compressive stress in the Sn-Cu

deposit was 32 MPa and after 100 days was still at

21 MPa. Whiskers were present after four days on the

Sn-Cu sample, but after 100 days, the pure Sn sample

was still whisker-free.

Figure 1: Calculated stress levels with and without Cu

additions.

NIST is providing leadership in two NEMI

whisker projects: identifying the fundamental growth

mechanism and developing an accelerated test method.

Our goal at NIST is to provide an understanding of the

fundamentals of the whisker formation, which will ensure

that the test developed by the NEMI group will have

widespread applicability.

Contributors and Collaborators

W.J. Boettinger, G. Stafford (NIST); R. Gedney,

I. Boguslavsky, P. Swaminath (NEMI project members)
35

Solder Reliability: Lead-Free Solder

The worldwide “green” movement in the electronics

industry to replace lead–tin eutectic solders with

lead-free solders creates a need for critical data on

the industry’s new lead-free solder compositions

for these design and reliability models. Members of

the Materials Reliability Division are working with

members of the Metallurgy Division, the Colorado

School of Mines, and the National Electronics

Manufacturing Initiative (NEMI) to develop and

disseminate such data on lead-free solders.

Tom Siewert

Technical Description

The worldwide “green” movement in the electronics

 industry to replace lead–tin eutectic solders with

lead-free solders creates a need for critical data on the

industry’s new lead-free solder compositions for these

design and reliability models. In fact, the NEMI website

describes their view of the situation as, “The NEMI

Lead-free Assembly Project was launched in 1999 to

help North American companies develop the capability to

produce lead-free products by 2001, with an eye toward

total lead elimination by 2004. The goals and focus of

the project were determined by the findings of NEMI’s

1998 roadmap and an industry task force formed by

NEMI to investigate process and material considerations

of lead-free electronics assemblies.” Their program

plan is being implemented by the Lead-free Assembly

Project. Project work is organized into five groups;

however, our interests fit with only two: the Alloy Group

(whose mission is to select a lead-free alloy, pursue an

industry standard, and gather data) and the Reliability

Group (who is analyzing data from a round robin that

compared the performance of lead and lead-free alloys).

Development of our database started in April 2000

after we became members of the NEMI Lead-free Alloy

Group, and we learned that modelers and production

engineers need more data before they can switch their

production lines to lead-free solders. The long history

in the use of current lead-based solders means that

these data sets are quite complete and widely available.

The modelers and production engineers need equally

complete sets of data on the various lead-free alternatives,

so they can make informed decisions for their production

applications. Researchers are rapidly developing

corresponding data on lead-free alloys, but the data are

widely distributed among the various technical journals

and proceedings. In addition, it is beginning to appear

as though differences in test procedures (e.g., loading

rates and dwell times) may make some of the data

inconsistent from laboratory to laboratory. Finally,

individual researchers may be repeating some of the

work of others, while other critical data needs are being

overlooked. Making the existing data more widely

available addresses all of these issues.

The database focuses on the three lead-free

compositions that seem to have the widest interest:

near the tin–silver–copper eutectic (Sn-4Ag-0.8Cu),

near the tin–silver eutectic (Sn-4Ag), and near the

tin–copper eutectic (Sn-0.8Cu). We have added data

for eutectic tin–lead composition (Sn-37Pb) for

comparative purposes, and have added data for other

lead-free alloys as we find them. We have not added

data for electronic materials other than solder.

Accomplishments

Our team continues to work with this NEMI

Lead-free Alloy Group to gather into a single database

the existing physical and mechanical property data that

have been developed by researchers around the world.

The most recent version of the database (4.0) is

posted on the Materials Reliability Division’s website at:

http://www.boulder.nist.gov/div853/. We have reported

on the availability of the database in posters at national

technical meetings, and the interest in the data is shown

by this website being one of the top 5 sites for our

Division (based on number of hits in the past year).

Feedback from users of the database is guiding the

addition of new data. We are also publishing a Standard

Practice Guide that describes standardized procedures

to facilitate the comparison of data between laboratories

and to permit the combination of data from different

sources into a single, comprehensive database.

We have developed the

capability of tensile-testing

solder samples that have

test dimensions on the

order of the sizes of solder

balls used in flip-chip

packages. The sample

shown at right has a gauge

length of 500 μm and a

rectangular cross section that is 250 μm by 200 μm.

A hydraulic tensile machine was outfitted with a control

system that can reliably apply strain rates of 10–5 s–1

to the specimen. The strain is measured by capturing

video images of the gauge section at magnifications of

up to 750x. The images are then processed using image

correlation techniques to measure the strain across the

whole gauge section. Studies can now be conducted on

the effects of phase sizes vs. structure size, lead free

solder properties, and/or strain rate effects.

Contributors and Collaborators

T. Quinn, Y.-W. Cheng, C. McCowan,

C. Handwerker (Metallurgy Division and Chair of

NEMI Lead-free Alloy Group); Members of the

NEMI Alloy and Lead-free Alloy Task Groups;

J.C. Madeni, S. Liu (Colorado School of Mines)

Microelectronics
36

Microelectronics

Techniques for characterizing the mechanical behavior

of thin films are being developed and applied. This is

necessary since thin-film microstructures and properties

are usually quite different from those of the same

materials in bulk form. While the general principles

of conventional mechanical testing apply to thin films,

special test equipment and techniques are required.

The ultimate goal of this effort is to test specimens

produced by semiconductor fabrication equipment and

similar in size to features on integrated circuit chips.

David T. Read

Technical Description

Thin films are an essential component of all advanced

 electronic devices. Understanding of failure modes

in these devices, especially interface delamination,

requires knowledge of the mechanical behavior of the

films. Techniques for measuring such behavior are being

developed and applied. Because the films are formed by

physical vapor deposition, their microstructures, and,

hence, their mechanical properties, are quite different

from those of bulk materials of the same chemical

composition.

The objectives are:

■ To develop experimental techniques to measure basic

tensile properties, fatigue behavior, and fracture

resistance in thin-film specimens fabricated and

sized like materials used in actual commercial devices;

■ To relate thin-film mechanical behavior to microstructure;

■ To extend test techniques from their present level

(1 μm thick, 10 μm wide) to smaller specimens that

are similar in size to the conductive traces used in

contemporary VLSI circuits (widths of 0.1 to 1 μm).

While the general principles of conventional

mechanical testing are applicable to thin films,

conventional test equipment and techniques are not.

Because vapor-deposited films are of the order of 1 μm

thick, the failure loads are of the order of gram-forces

or less, and the specimens cannot be handled directly.

Accomplishments

Last year we reported the application of our

recently developed force-probe technique to two new

types of materials: 1) aluminum interconnect layers

made in a commercial CMOS (complementary metal

oxide semiconductor) fabrication facility, obtained

through the MOSIS service, and 2) polycrystalline

silicon made at Sandia National Laboratories. This

year, we have demonstrated that the same technique is

applicable to polymer materials used in electronic

devices. We tested a commercial photodefinable polymer,

a polyimide, between room temperature and 200°C.

The tensile section was 0.6 x 10 x 190 μm. Our

success with these specimens from the three main

families of materials, metals, ceramic-like, and polymers,

and positive reports from other laboratories using similar

techniques, has made us optimistic that standardized

testing of thin films may be near.

The cured polyimide was very well behaved, with results

as expected and consistent with the manufacturer’s product

bulletin; the elongation to failure was over twice the

minimum value given by the manufacturer. This establishes

that in this material, no “thin film” effects alter the tensile

properties, down to a thickness of 0.6 μm. Changes in

polymer mechanical properties in films thin enough to

constrain the arrangement of the chain-like polymer

molecules have been anticipated and may still occur

for thinner specimens.

Micrometer-Scale Reliability: Mechanical Behavior of Thin Films

Figure 1: Stress-strain curve of photodefinable polyimide

at room temperature showing hysteresis loops.

Three other efforts were pursued in this project:

continuation of our testing of CMOS aluminum;

fabrication and testing of microtensile specimens

representative of copper interconnect lines in

microelectronics devices; and extension of our

microtensile test technique to exotic materials such as

carbon nanotubes. While CMOS aluminum specimens

down to 2 μm wide were successfully fabricated, this

set of specimens behaved in a very brittle manner, more

brittle than the previously tested set. We are applying

advanced analytical tools to find the cause of this

behavior. Our preliminary results on copper are similar

to those for other metals we have tested, but the details

are currently covered by a nondisclosure agreement with

the supplier of the specimens. Our initial experiments

with carbon nanotubes are described elsewhere.

Contributors and Collaborators

Y.-W. Cheng, J.D. McColskey (NIST); R. Emery,

T. Scherban (Intel); B. Yeung, (Motorola); C. Litteken

(Stanford University)
37

Microelectronics

Electronic Packaging and Components: Packaging Reliability

We are developing methods to examine materials and

interfaces in electronic packaging applications and

elucidate the damage mechanisms. Our current focus

is on advanced packaging structures and embedded

passive materials using electron-beam moiré to

measure mechanical strain, and thermal microscopy

to measure heat flow and thermal properties on

increasingly smaller size scales.

Andrew Slifka and Elizabeth Drexler

Technical Description

The microelectronics industry is moving rapidly

 toward higher-density components of smaller

size using less expensive materials. One move in this

direction is the advent of embedded and integrated

passive components in printed circuit boards (PCBs)

and another is the use of PCBs in various types of

grid array packages. These organic-based PCBs can

have a large coefficient of thermal expansion (CTE)

in comparison with many materials found in their

proximity. This CTE mismatch can reduce the reliability

of electronic packaging systems by causing localized

stress. The CTE mismatch in flip-chip packages is

also a source of failure and fatigue.

We are investigating the damage induced from CTE

mismatches between organic materials, organics and metals,

organics and ceramics, and organics, metals and ceramics

to determine the initiation of damage and the ultimate

failure mechanisms in these systems. Electron-beam moiré

is employed to measure displacements and calculate

mechanical strain due to thermomechanical loading.

Thermal microscopy is used to measure changes in

interfacial thermal resistance in order to detect the onset

of thermomechanical damage at that interface before

any surface manifestation is visible.

We are developing new measurement methods,

both thermal and mechanical, using scanned-probe

microscopy (SPM) in order to characterize packages

at increasingly smaller size scales.

Accomplishments

Building on the preliminary measurements made in

FY01, we developed the thermal SPM into a quantitative

technique. We used an industrial embedded resistor

(Figure 1) to make comparative measurements between

the SPM thermal system and the IR microscope. Similar

temperature differences can be achieved with both the

SPM and IR systems, although two different heating

techniques are used due to geometric constraints.

Improved spatial resolution is motivating the move to

the SPM. With increased miniaturizing in electronic

packaging, the IR microscope no longer has the needed

resolution to detect materials and interfaces in future,

and even some current, packages.

Figure 1: Schematic of the materials contained in the

embedded resistor specimen and their relative positions.

Mapping of the change in interfacial thermal

resistance in an embedded resistor specimen, shown

in Figure 2, among the various material combinations

shows the increase in degradation with respect to

thermal cycling (–55 to 125°C). The interface between

the TiO2-filled epoxy (laser barrier) and the ceramic

embedded resistor shows signs of damage within

5 thermal cycles. Corresponding electron-beam moiré

data show that maximum strains are found in the laser

barrier and in the resin-rich regions of the printed

circuit board, particularly between the resistors,

where unfilled resin collects during lamination.

In collaboration with the Jet Propulsion Laboratory,

we are looking at techniques to mitigate the destructive

consequence of CTE mismatches in flip-chip packages.

Strains in specimens were measured before, after, and

without the microwave process.

Contributors and Collaborators

J. Felten (DuPont Technologies, Research Triangle

Park, NC); V. Shah (MicroFab Technologies, Plano, TX);

R. Snogren (SAS Circuits, Inc., Littleton, CO);

N. Budraa (Jet Propulsion Laboratory, Pasadena, CA)

Figure 2: Thermal SPM image showing the laser barrier,

resistor, and copper (from top to bottom), 8°C, full scale.

37
38

Microelectronics

Electronic Packaging and Components:

Acoustic Loss in Piezo Crystals

eliminated. Acoustic resonance measurements are

performed using tone-burst direct piezoelectric coupling

with planar electrodes above and below the sample.

Q–1 and resonant frequencies are determined through

analysis of free-decay waveforms in the time domain.

The configuration of the sample, supporting structure,

and copper electrodes are shown in Figure 1.

Langasite, langatate, and langanite are

piezoelectric materials that show promise of

providing characteristics superior to quartz for a

variety of electronic oscillator and filter applications.

The acoustic loss of these crystals is being studied in

our laboratory to identify and quantify the dominant

physical mechanisms that degrade performance,

including the intrinsic phonon–phonon mechanism.

Through this work, the project seeks to provide

guidance to other researchers in minimizing the

acoustic loss and determining the most promising

material to pursue among this group of compounds.

Ward Johnson and Sudook Kim

Technical Description

Langasite (La3Ga5SiO14), langatate

 (La3Ga5.5Ta0.5O14), and langanite

(La3Ga5.5Nb0.5O14) have attracted significant

attention in recent years as candidate materials

for improved electronic oscillators and filters. The

potential advantages of these crystals over quartz

include higher piezoelectric coupling (which enables

devices to be made smaller), lower acceleration

sensitivity, and higher Q (which reduces phase noise

and enables higher-frequency operation). These

materials also have no phase transition below the

melting point, which allows devices to be operated

at high temperatures and provides the potential of

producing large-diameter wafers (for surface-acousticwave devices) more easily and cheaply than with quartz.

Values of the acoustic loss Q–1 reported in

the literature vary widely and may not represent the

intrinsic limits of these materials. Therefore, current

data provide little guidance in establishing a focus of

further research. This project seeks to remedy this

situation by identifying the dominant contributions to

Q–1 in langasite, langatate, and langanite. The known

physical mechanisms that can significantly contribute

to Q–1 include dissipation through external mechanical

contact, defect anelasticity, nonlinear coupling to other

modes, and intrinsic coupling to thermal phonons

(phonon–phonon loss through the Akheiser mechanism).

Accomplishments

In FY02, an experimental system was constructed

for measuring Q–1 and frequencies of resonant shear

modes of Y-cut plano-convex disks in vacuum at

temperatures from 150 K to 1000 K. The plano–convex

geometry of the sample has the effect of localizing the

vibrations of certain modes near the center, so damping

from mechanical support at the edges is essentially

Figure 1: Plano-convex single-crystal disk supported on

sapphire spheres and centered laterally by larger alumina

spheres. Driving electrodes are above and below sample.

Measurements of Q–1 in langatate have been

performed as a function of frequency, temperature,

external mechanical contact, and vibrational amplitude.

As shown in Figure 2, Q–1 varies significantly between

samples which are taken from the same crystal boule.

The dominant contributions to Q–1 in these measurements

have not yet been identified. However, the contribution

from external damping has been determined to be

insignificant, except for the lowest mode near 2 MHz. At

higher amplitudes, a nonlinear contribution is observed.

Figure 2: Q–1 versus resonant frequency in a Y-cut

plano–convex disk of langatate at ambient temperature.

Contributors and Collaborators

R. Smythe (Piezotechnology, Inc., Orlando, FL)

This project is partially supported by a grant from

the U.S. Army Research Laboratory.
39

Combinatorial Materials Research:

Interconnects to Optoelectronic Materials

GaN and related nitrides have been identified as

important materials for the realization of solid-state

lighting, UV/blue lasers, LEDs and photodetectors.

The engineering of improved electrical contacts to

these devices is of particular industrial importance.

This project develops a strategy for industry to reduce

contact resistance, increase visible light transmittance,

and improve thermal and mechanical stability of ohmic

contacts to a variety of optoelectronic devices. Due to

the large number of possible materials combinations

and process variables involved, a combinatorial

approach is valuable.

Albert V. Davydov and Leonid A. Bendersky

T ypical electrical contacts to p-type GaN devices

 consist of thin film Au/Ni bi-layers, but contact

resistance and reliability are far from ideal. The goal of

this project is to establish techniques for industry to use

to rapidly develop improved contacts for GaN and other

compound semiconductor devices. A combinatorial

approach (looking at Au/Ni ratio and metal film

thickness) integrated with thermodynamic and diffusion

modeling is being used to quantify the relationship

between physical properties such as electrical resistance

and optical transmittance of metal contacts and the

fundamental microstructural and compositional

properties of the metal-to-GaN interface.

Thermodynamic information and diffusion data

are being evaluated for the Au-Ni-Ga-N system to help

interpret the metallurgical reactions in the Au/Ni/GaN

structures. To date, we have measured the thermal

stability of the GaN compound and modeled the

P-T-composition phase diagram for the Ga-N system.

In addition, the Ni-Ga-N phase diagram has been

evaluated experimentally and thermodynamically and

presented at the CALPHAD-2002 Meeting.

A combinatorial array of Ni/Au bi-layer contacts of

varying compositions has been fabricated by physical vapor

deposition (e-beam evaporation) on commercial GaN

wafers. The thickness of each metal layer varied discretely

from 5 to 85 nm using a motion-controlled shutter system,

producing an array of 88 elements (Figure 1(a)). The

composition and microstructure of the contacts were

examined with XRD (with Prof. I. Takeuch, UMD), optical

microscopy, SEM and AFM. The transparency of metal

layers to visible light was determined by reflectometry and

transmission measurements. From transmission and XRD

measurements (Figure 1b, c), we have identified library

elements with optimum as-deposited Au/Ni compositions

that are highly transparent and of good crystalline quality

with epitaxial metal layers.

In FY2003, we will fabricate combinatorial

libraries with an extended compositional range to

develop thermodynamic stability data for GaN with

other metal contact materials as well as continue to

establish the analysis techniques for quantifying phase

transformations and interdiffusion in these GaN/metal

contact systems and developing the relationships

between phase transformations, microstructures,

and electrical performance.

Figure 1: (a) Array of as-deposited Au/Ni bi-layered

metallization (88 elements) on 2-inch GaN/sapphire wafer;

(b) Optical transmission imaging of the above sample at

l = 450 – 713 nm; (c) XRD mapping of Au 111 reflection

from all 88 Au /Ni elements.

Contributors and Collaborators

W.J. Boettinger, D. Josell, A.J. Shapiro (Metallurgy

Division, MSEL); P.K. Schenck, J.E. Blendell (Ceramic

Division, MSEL); I. Takeuchi (University of Maryland);

V.A. Dmitriev (Technologies and Devices International, Inc.)

Optoelectronics
40

Optical and Structural Characterization of

Optoelectronic Semiconductors

Photonics manufacturers need to better understand

how composition, strain, inhomogeneity and defects

affect the performance of optical emitters and

detectors made from III-V compound semiconductors.

In the Optical and Structural Characterization of

Optoelectronic Semiconductors Project, we are

measuring critical properties of thin film structures

relevant to device performance by a variety of

spectroscopic techniques. The project is currently

focused on deep UV to near IR (250 nm to 850 nm)

emitting materials: AlxGa1–xAs, AlxGa1–xN and

InyGa1–yN.

Lawrence Robins and Grady White

Medium to wide bandgap III-V semiconductors enable

 display, lighting, and data storage technologies,

based on their optical emission and their ability to detect

at deep UV to near IR wavelengths. Progress in the

development of GaN, AlxGa1-xN and InyGa1-yN devices

(250 nm to 550 nm) has been hampered by problems

with materials growth and structure, such as film/

substrate lattice mismatch, high defect and impurity

concentrations, and structural/compositional

inhomogeneity. AlxGa1-xAs (650 nm to 850 nm) is a

better developed technology, but the industry still needs

improved accuracy in measurements of alloy composition

and residual strain. We have, therefore, undertaken

several initiatives to address these needs for improved

metrology methods and accurate measurements.

1) A calibration curve was determined for the dependence

of the photoluminescence (PL) emission energy on

Al fraction (x) in AlxGa1-xAs films for x < 0.37, near

room temperature, based on reflection high energy

electron diffraction (RHEED) and wavelength dispersive

spectroscopy (WDS) measurements of x.

2) Correlated Raman and PL measurements were

performed on AlxGa1-xAs films as a function of applied

biaxial stress to 350 MPa. Two primary Raman

peaks, described as AlAs-like and GaAs-like, were

observed in films with x = 0.2 and x = 0.5. The

differing stress and composition dependencies

of the Raman and PL peaks allow us to determine

both the Al fraction and the magnitude of the stress.

Measurements suggest that the stress coefficient

of each Raman peak is approximately constant as a

function of x, with the AlAs-like peak showing a larger

stress coefficient than the GaAs-like peak.

3) The ordinary refractive index, no(E), and absorption

coefficient, α(E), were measured in the 200 nm to

2500 nm range for several AlxGa1-xAs films by

spectroscopic transmittance, T(E), and reflectance,

R(E), correlated with discrete-wavelength prismcoupled waveguide-mode measurements of no. EDS

measurements of x enabled determination of the x

dependence of no(E) and α(E). Transmittance,

T(E), and reflectance, R(E), spectra of a

representative sample are plotted in Figure 1.

Figure 1: Transmittance and reflectance measurements

(red) and model (blue) for a AlxGa1-xN sample.

Contributors and Collaborators

C.E. Bouldin, A.J. Paul, M.D. Vaudin (Ceramics);

J.T. Armstrong, R.B. Marinenko (Surface and

Microanalysis); K.A. Bertness, N.A. Sanford

(Optoelectronics); A.V. Davydov, A.J. Shapiro

(Metallurgy); J.G. Pellegrino (Physical and Chemical

Properties); A.V. Dmitriev, D.V. Tsvetkov (TDI);

S. Keller, U.K. Mishra, S.P. DenBaars (UCSB)

Figure 2: Ordinary refractive index and absorption

coefficient obtained in the optimization procedure.

Optoelectronics

The index–film thickness product no(E)df was

determined from the spacing of the interference fringes

in T(E) and R(E); similarly, α(E)df was determined at

energies near the bandgap from the absorption cutoff of

T(E). To determine the individual factors no, α, and df,

a two-Sellmeier-term model for no(E) was adjusted to

best match the no values from the discrete wavelength

waveguide mode measurements, while simultaneously

fitting the T(E) and R(E) data. The no(E) and α(E)

curves obtained from this multiple optimization

procedure are shown in Figure 2.
41

The compound semiconductor photonics industry

seeks to both measure and control strain that

develops during the course of device fabrication.

In the Materials Reliability Division, we are

developing electron diffraction methods for

measuring and mapping elastic strain distributions.

The methods are applied to phase transition-induced

strains for the case of oxide formation in vertical

cavity surface-emitting lasers, and to lattice

parameter mismatch strains for self-assembled

quantum dots.

Robert R. Keller and Roy H. Geiss

Technical Description

Vertical cavity surface-emitting lasers (VCSELs)

have, as an integral component, oxide layers to act as

apertures resulting in record low threshold currents and

high efficiencies. These apertures are grown by selective

wet oxidation of AlGaAs layers (compositions typically

contain 98–100% AlAs) situated between GaAs layers.

Upon transformation from AlGaAs, the aluminum oxide

undergoes a large volume contraction in excess of

6%. The exact magnitude of the contraction is not

well-characterized as the theoretical volume change

exceeds 20% while the few experimental measurements

suggesting 6–7% are based on TEM images of layer

thickness changes. The contraction can be manifested

as elastic strain in adjacent GaAs layers, dislocations in

GaAs and in the interfaces, and also as delamination.

We are using TEM and SEM electron diffraction

methods to measure elastic strain states via determination

of lattice spacings and angles. While the TEM method

provides superior spatial and strain resolutions, the

SEM method is not subject to the difficulties associated

with thin foil preparation, namely site-specificity and

stress relaxation induced during thinning. Here, we

report progress on SEM measurements by electron

backscatter diffraction (EBSD).

Accomplishments

The EBSD method has seen little successful

application to the measurement of elastic strains due

primarily to the low signal-to-noise inherent in the

detection system. We have made exciting progress

along two avenues for strain measurement in oxidized

AlGaAs/GaAs multilayer structures. We started with the

most direct measure of strain available in the diffraction

patterns, namely the spacing of the Kikuchi-like bands

of high intensity. These bandwidths are proportional

to Bragg angles and therefore provide a direct link to

interplanar spacings in the crystal. Figure 2 of the

Micrometer-Scale Reliability: Strain in Photonic Semiconductors

Technical Highlight on this subject shows an example

of an indexed pattern with a computer-simulated pattern

overlaying an experimental result. Rather than measure

bandwidths directly from the data, we have begun to

apply simple image mathematics to accentuate the

differences between patterns showing strain and those

showing no strain.

We have, to date, been able to detect very small

differences in strain from locations within the crystal

that are only tens of nanometers apart. Quantification

of these strains is underway.

The other aspect of EBSD-based strain

measurement makes use of automated collection of

diffraction patterns, followed by analysis of pattern

sharpness. If the elastic strain varies considerably within

the volume of material sampled by the electron beam

(of approximate diameter 50 nm), then there will be a

continuously changing lattice parameter that causes a

continuously changing Bragg angle. In the pattern, this

effect broadens the edges of the Kikuchi-like bands and

makes the pattern more diffuse. A larger strain gradient

leads to more diffuseness. The extent of diffuseness

can be quantified and this effect mapped as shown in

Figure 1. Here, each pixel represents a different beam

position, and greater diffuseness is represented by a

darker intensity. In this figure, we concentrate on the

colored pixels within the GaAs layers only.

Figure 1: Crystallographic orientation map with darker

pixels indicating greater pattern diffuseness or strain.

The strain field around the aluminum oxide

growth front (indicated schematically in brown) can

be qualitatively seen to extend over a few GaAs/AlAs

bilayers in directions normal to the layers. It extends

even further in directions parallel to the layers as

indicated schematically by the dashed white oval.

We are in the process of evaluating means for

quantifying the relationship between EBSD pattern

diffuseness and magnitude of elastic strain gradients.

Contributors and Collaborators

A. Roshko, S. Lehman, K. Bertness (Optoelectronics

Division, EEEL)

Optoelectronics
42

Magnetic Properties and Standard Reference Materials

Prior to FY02, two SRM’s were issued: a nickel

sphere (SRM 772a) and a nickel disc (SRM 762).

The measurements for the complete series of magnetic

SRMs are made using the Thorpe–Sentfle method

shown above. This technique has the advantage of not

requiring knowledge of the field gradient anywhere in

the apparatus. As that is the largest source of error,

higher accuracy is possible.

We measure the susceptibility by physically moving

the sample to various positions from the field center to

some distance outside of the magnet measuring the

magnetic force and then integrate that force vs. distance

relationship.

Two new SRMs were developed this year. The first

was a 1 mm diameter yttrium iron garnet (YIG) sphere

for use in calibrating the more sensitive ranges of

magnetometers (SRM 2853). In addition, it will be

useful in calibrating very sensitive instruments such

as alternating gradient magnetometers and SQUID

magnetometers. Because of its high resistivity, the

yttrium iron garnet sphere should also be useful for

calibrating magnitude and phase in alternating current

magnetometers. The second SRM is a platinum cylinder

paramagnetic susceptibility standard (SRM 764a).

This will be useful in calibrating the zero field point

and linearity of magnetometers due to its paramagnetic

nature, and it will also be useful for calibrating the field

scale after the moment scale has been calibrated.

Contributors and Collaborators

A.J. Shapiro, R.V. Drew; S.D. Leigh (ITL);

Industrial and academic collaborators: L.J. Swartzendruber,

D.E. Matthews, G.E. Hicho, L.C. Smith, L.H. Bennett,

E. Della Torre (George Washington University)

Figure 2: Thorpe-Sentfle Method for Absolute

Susceptibility Measurement.

Figure 1: Absolute magnetometer.

Users of magnetic materials, including the recording

industry, the permanent magnet industry, and the

manufacturers of electric motors and transformers of

all types, need standard reference materials (SRM’s)

to calibrate instruments used to measure the critical

magnetic properties of their starting materials.

The magnetic materials group is in the process of

developing such standards. Methods for improving

the efficiency and accuracy of measuring and

characterizing magnetic materials of industrial

importance, including measurement of magnetic

susceptibility over wide frequency ranges, are also

under investigation.

Robert D. Shull and Robert D. McMichael

I

n collaboration with scientists from universities,

 industry, and other Divisions at NIST, magnetic

materials important to the scientific and industrial

communities are prepared and methods for the improved

measurement of their properties are developed. Standard

Reference Materials for the calibration of existing and

planned instruments used in the measurement of magnetic

properties are developed and produced. An equally

important parameter to know how to measure is time

dependence of magnetization. Consequently, methods

for the characterization of accommodation and aftereffect

in magnetic recording and permanent magnetic materials

are developed. Models are also developed for determining

the most efficient methods to fully characterize the

hysteresis and magnetostriction in magnetic materials.

Commercial instruments for the measurement of

magnetic properties are relative instruments that rely

on known samples for their calibration. To produce

standard samples for such instruments, an absolute

magnetometer was developed and assembled at NIST.

A picture of this instrument is shown below.

Magnetic Materials
43

Magnetic Materials

Magnetic Properties of Nanostructured Materials

In the past decade, there has been a remarkable

improvement in the technology for materials

preparation resulting in present day’s capability

of controlling morphology and features at the

nanometer level. In magnetism, such control allows

the fabrication of nm-thick composite materials of

dissimilar magnetic behavior, leading to materials with

novel bulk magnetic character and unusual properties.

We are providing materials metrology necessary for

U.S. industry to understand the behavior of these new

materials and take advantage of the novel properties.

Alexander J. Shapiro and Robert D. Shull

G reater information storage on recording discs

 is being accomplished by reducing the size of

the regions in which information is stored down to

nanometer sizes. Unfortunately, when those regions

become too small, the material ceases to be ferromagnetic

(FM) and, instead, becomes superparamagnetic and no

longer retains magnetic information. In order to exceed

the so-called “superparamagnetic limit,” the recording

industry has recently introduced magnetic media containing

two layers of a ferromagnet separated by a sub-nanometer

thick layer of Ru. In this structure, the thickness of the

nm-diameter regions has been effectively increased by

providing magnetic coupling between the two ferromagnetic

layers via the in-between Ru. Consequently, the volume

of the element has been maintained large, even though

the lateral dimensions of the recording “bit” have been

decreased, thereby enabling retention of the ferromagnetic

state. However, for this new type of media, it is unknown

how long information will be retained in those “bits”

and what sort of fields are necessary for reversing their

magnetization. We have addressed this problem by imaging

the magnetic domains in this new type of media using the

magneto-optic indicator film (MOIF) technique developed

in our laboratory (Figure 1).

The first conclusion from the MOIF observations

was that, surprisingly, the magnetization vectors in the

two FM layers in this 3-layer structure were not colinear

as originally assumed. In Figure 1, the lack of colinearity

is seen by the 100 degree angle between the magnetization

vectors (one white and the other black) of the two FM

layers. As a consequence of this non-colinearity, the

magnetization reversal mechanism now involves the

cooperative behavior of both Co layers simultaneously,

leading to different coercive fields and switching speeds.

Figure 2 (e-h) shows that this non-colinearity is due to

the presence of pinholes in the 0.5 nm thick Ru.

Figure 2: MOIF image of the remagnetization process of

a Co/Ru (a-d) and a Co/Ru/Co (e-h) thin film containing no

pinholes in a field oriented along the total magnetization vector.

H is directed vertically in the figure at increasing magnitude

from 4 mT to 4.8 mT (a-d) and from 2 mT to 54 mT (e-h).

For the Co/Ru/Co multilayer with no pinholes, the

two FM layers have magnetization vectors which are colinear.

With pinholes present, the greater their number, the greater

their effect on the magnetic spin misalignment shown in

Figure 1. Since most sub-nanometer thin films contain

pinholes, these results clearly demonstrate the importance

of knowing the effect of these defects. One of the

ramifications of the non-colinearlity is the presence of time

dependence in the coercivities of the stored information, a

particularly deleterious situation. Information safely stored

now may not be safely stored tomorrow. Two papers have

been published presenting these results and a third is in

progress to clarify the situation.

Contributors and Collaborators

Alexander J. Shapiro, Robert D. Shull, Robert D.

McMichael, William F. Egelhoff (NIST); Valerian I.

Nikitenko (Johns Hopkins University); Vladimir S.

Gornakov (ISSP, Russian Academy of Sciences, NIST)

Figure 1: MOIF image of the remagnetization process of

a Co/Ru/Co homogeneous structure containing pinholes

in the Ru in a field oriented along the total magnetization

vector. (a) :oH = 0 mT after magnetization in a field of

+60 mT; (b) :oH = –9.4 mT; (c) :oH = –9.4 mT after a

5 sec. wait; (d) :oH = -13.0 mT. The inserts in (a) and

(d) show the intensity change of the MO signal.
44

Magnetic Materials

Nanomagnetodynamics

For GHz data rate hard drives, magnetic memory

chips and other high-frequency magnetic devices,

magnetization damping will be an important factor

affecting performance. Another consequence of

damping is thermal fluctuations that cause sensor

noise and data loss in media. We provide industry

with: 1) an improved understanding of magnetic

damping mechanisms, 2) improved magnetic

damping measurements and standards, and

3) data and methods for controlling magnetic

damping in technically relevant materials.

Robert D. McMichael

T his project is concerned with the metrology and

 mechanisms of magnetization damping, a process

where magnetic energy is dissipated as the magnetization

comes to equilibrium. The same mechanisms that allow

damping also allow thermal energy to cause magnetization

fluctuations that cause noise in sensors such as disk

drive read heads and data loss in recorded media.

Damping mechanisms are recognized as being important

both for GHz data rate writing and reading processes

and for long-term stability of stored data. A convenient

measurement of damping is the ferromagnetic resonance

(FMR) linewidth, a measurement that is complicated by

the presence of inhomogeneities.

In FY2002, our accomplishments include:

1) a very successful model of ferromagnetic resonance in

inhomogeneous films (see highlight), 2) demonstrations

of the effects of intentional defects, 3) measurements of

damping in thin magnetic metal films by spin transport

into normal metals, and 4) measurements of the very low

damping in high moment, soft materials under development

for read heads.

We have measured the intrinsic linewidth of

Permalloy films, and we have found that it follows one

of several proposed phenomenological forms. This data

has been very valuable to theorists working on physical

damping mechanisms.

One novel physical damping mechanism is damping

via spin-polarized electron transport. It had been shown

elsewhere that additional damping can be created by

polarized electrons propagating into Pt or Pd surface

layers. We have shown that the enhanced damping can

be achieved with as little as 0.5 nm of Pt, and we have

shown that the effect is substantially reduced by a Cu

spacer between the Permalloy and Pt that is as thin as

0.5 nm. The abrupt decrease with a thin Cu spacer

suggests that the damping enhancement has a contact

component in addition to the spin transport component.

In FY2002, four papers related to this project were

published, five contributed talks presented and three

invited talks given at Seagate Research, U.C. San Diego,

and a National Storage Industry Consortium meeting,

and two more invitations have been received to speak at

the 47th annual MMM conference in November 2002.

Figure 1: Increased FMR linewidth resulting from

intentionally-created defects created to clarify the effects

of nanoscale defects on FMR damping measurements.

Our modeling describes the effects of the random coupling

to the polycrystalline, antiferromagnetic NiO.

Figure 2: Effect of Pt overlayers on damping in Permalloy

films. The effect shows how spin polarized electron

transport across the interface contributes to damping.

Contributors and Collaborators

R.D. McMichael, A.B. Kunz, D.J. Twisselmann,

A.P. Chen, W.F. Egelhoff (Metallurgy Division, MSEL);

S.E. Russek (EEELM); E. McHenry, H. Okumura

(Carnegie Mellon University)
45

Magnetic Materials

New Materials for Ultra-High Density Data Storage

The objective of this program is to provide assistance

to U.S. companies in the ultrahigh density data

storage industry, which includes such products as

hard disk drives and magnetoresistive random access

memory chips. Our work provides U.S. companies

with significant competitive help by investigating the

scientific issues underlying the manufacturing process.

Often, this scientific understanding will point the way

to improved manufacturing processes. One such

example is illustrated in the figures below.

William F. Egelhoff, Jr.

H ard disk drives play a crucial role in the

 information revolution. The World Wide Web is

stored on millions of hard disk drives around the world.

The exponential growth of the Web has depended on

the exponential growth in hard disk storage capacity.

Maintaining these exponential growth rates is one of

the great challenges of the information age.

Today, giant magnetoresistance (GMR) is the current

technology used for read heads in hard disk drives. It is

unrivaled in its ability to read sub-micron-sized bits at

gigahertz frequencies. However, the time is not far off

when technologies better than GMR will be needed to

maintain the historical exponential growth rates in

storage density.

Our aim is to assist the U.S. companies that are

developing these new technologies. One promising new

technology is magnetic tunnel junctions (MTJs). MTJs

are magnetic thin films with a typical layer structure of

Co/Al2O3 /Co. Electrons tunnel from one Co layer to

the other through the Al2O3. One problem with MTJs

is that they are not highly reproducible.

To understand the origin of this problem, NIST

began a series of measurements at the Magnetic

Engineering Research Facility (MERF) to determine

the fundamental growth mechanisms in MTJ fabrication.

MERF is a unique facility specifically designed to

study surface and interface effects occurring during the

growth of magnetic thin films for ultrahigh density data

storage applications. The results at MERF identified

interdiffusion at the Al /Co interface as a problem in the

growth of MTJs, as illustrated in Figure 1.

When the Al is oxidized to make Al2O3 some of the

Al remains in metallic form in the Co. The extent of the

interdiffusion is highly susceptible to small changes in

experimental conditions. As a result, nominally identical

MTJs exhibit a lot of scatter in their performance data,

as illustrated in Figure 2.

NIST demonstrated at MERF that oxidizing

two atomic layers of the Co surface prior to the Al

deposition provided an excellent diffusion barrier and

that the Al reduced the Co oxide back to Co metal.

NIST transferred this technology to Read-Rite, which

found greatly improved manufacturing uniformity for

MTJs and an improved magnetoresistance when this

approach was used.

Magnetic Tunnel Junction Fabrication

Figure 1: An illustration of the intermixing that occurs

in the standard method of MJT fabrication when Al is

deposited on Co and the NIST modification using a thin

Co oxide diffusion barrier to prevent the intermixing.

Figure 2: Read-Rite Corp. data on magnetic tunnel junctions.

“The results were amazing. The scattered data

collapsed into a single point.”

Dr. H.C. Tong

Director of Advanced Read-Head

Research, Read-Rite Corp.

Contributors and Collaborators

W.F. Egelhoff, Jr., R.D. McMichael (NIST/MSEL);

M.D. Stiles (NIST/Physics); 6 U.S. Corporations, the

Naval Research Laboratory, 7 U.S. Universities, and

13 International Universities
46

Nanotribology

and retrieves/writes data and then retreats to the ramp

to park. This design minimizes head disk contacts.

However, because of the reduced flight height, waviness

of the disk, or rotational wobble, occasional contacts

between the head and disk do occur and sometimes

catastrophically. This kind of random contact takes

place under high speed flying conditions and is difficult

to measure.

A ridge is artificially created on a disk and a ruby

ball is used to collide with the ridge under high speed

conditions (see technical highlight). The impact force

and the deformation are measured. Preliminary test

results show the ranking of lubricants and overcoats

agree with field experience. This quick test enables

screening of different coating /lubricant combinations for

their effectiveness in protecting the disk. A finite element

model has also been successfully developed to predict

the stress concentrations and deformation for a hard

disk. The interaction of various mechanical properties

of different layers can be simulated.

Another aspect of improved protection is the

lubricant. The spacing allows only a monolayer of

molecules to protect the surface. Hydrocarbon molecules

hold promise to improve the bonding strength and

self-repairing properties of the lubricant film. Current

lubricant (perfluoropolyether) has limited solubility

of functional molecules. We have demonstrated that a

multi-component molecular assembly can be achieved by

using hydrocarbon molecules. However the performance

depends on film thickness. There is no known acceptable

method to accurately measure the film thickness of a

complex hydrocarbon film on an atomically rough

surface at 1 nm scale. A master calibration sample was

successfully developed using a circular barrier to contain

a solution with a known amount of solute on an actual

hard disk. The solution is frozen in liquid nitrogen and

slowly evaporated under controlled conditions. Uniformity

of the film is checked by FTIR. Samples of average film

thickness were then measured by various methods and

the correlation constants were obtained. This enables

the use of hydrocarbon molecules for hard disk

applications.

The bonding characteristics of lubricant molecules

with overcoats are important in shear resistance. An

electron spin resonance technique was developed to

measure unpaired electrons of overcoats quantitatively.

Working with MMC Corporation, a series of hard disk

carbon overcoats on a glass substrate was measured.

This technique is expected to provide a tool to control

surface reactivity/energy.

Contributors and Collaborators

M. Bai; MMC Corp, UC Davis, NSIC (IBM,

Seagate, Readrite, Maxtor), Pennzoil-Quaker State

The head-disk interface has become an important

scientific and technological issue in the data storage

industry. To reach 155 Gb/cm2 areal density and fast

data transfer, the spacing between the head and the

disk is shrinking to 3.5 nm for speeds approaching

40 m/s. Occasional contacts will test the strength

and robustness of the protective carbon and lubricant

coatings, each of which is only one nanometer thick.

Measuring and evaluating material properties and their

response to impact under these conditions has become

a significant technological need.

Stephen Hsu, Richard Gates, Patrick Pei,

Pu-Sen Wang, Jerry Chuang, and Dan Fischer

Ceramics Division researchers continue to work

 with the National Storage Industry Consortium

(NSIC) as members of the Tribology Working Group.

We have achieved significant progress in four project

areas: 1) the development of a high-speed impact test

to simulate head-disk collisions under ramp-load and

unload operating conditions; 2) the creation of a finite

element model for calculating stress concentrations

and deformation as a result of such impacts; 3) the

development of a master calibration sample for film

thickness measurements of complex hydrocarbon

molecules on hard disks in the one nanometer ± 0.2 nm

range; 4) the development of an electron spin resonance

technique to measure the unpaired electrons in carbon

overcoats with MMC Corporation.

These projects, taken together, represent our efforts

in addressing two crucial technological issues facing the

magnetic hard disk industry: how should the disk be

protected from occasional high-speed head-disk contacts

when the head is flying faster and lower; how should

we define surface reactivity and bonding in new carbon

overcoats while ensuring durability.

NSIC has set the ambitious goal of reaching

155 Gb/cm2 areal density by 2006 with a concurrent

increase in data transfer rate approaching 100 Gbit/s.

This translates to a 10-fold increase in capacity in

four years.

To increase the areal density, the flight height and

the carbon overcoat thickness must be reduced to

3.5 nm and 1 nm respectively. Since the protection of

the disk depends on the combination of the lubricant

and the carbon overcoat, thinner overcoats may reduce

the durability. NSIC is working on improved overcoats

and lubricants.

Head disk interface operation has shifted from

constant-start-and-stop mode to a ramp-load-andunload mode. The head is parked on the ramp outside

the disk. For read and write operations, the head flies in

Magnetic Materials
47

Magnetic Materials

Metrology for Nanoscale Properties: Brillouin Light Scattering

A Brillouin light scattering facility has been

constructed for measuring phonon and magnon

properties at gigahertz frequencies in micron-scale

volumes of thin-film materials. The current focus of

research using this system is on characterizing

interactions between magnetic modes and thermal

phonons which play a central role in determining the

switching times of magnetic-storage devices, spinvalve sensors, and other thin-film magnetic devices.

Ward Johnson and Sudook Kim

Technical Description

Brillouin light scattering is the inelastic scattering of

 incident photons with elastic waves (phonons) or

spin waves (magnons) in a material. This scattering can

involve either the generation of waves (Stokes process)

or annihilation of waves (anti-Stokes process) in the

material. Fabry–Perot interferometric techniques for

measuring the shifts in photon frequencies arising from

Brillouin scattering have evolved rapidly over the past

couple of decades, such that they now provide powerful

and practical methods of characterizing the dynamic

properties of bulk and thin-film materials. In our

division, these techniques are being pursued because

of their capability to characterize both elastic and spin

waves at gigahertz frequencies in thin-film material with

surface areas typically on the order of 50 μm in diameter.

The research is currently focused on the interactions

of magnons and phonons in ferromagnetic thin films.

This subject is of great importance with respect to

maximizing the speed of magnetic-storage devices,

spin-valve sensors, and other thin-film magnetic devices.

The coupling of directly excited spin waves to other

waves in the material determines the time to achieve

equilibration of the magnetization during a switching

event. Brillouin light scattering may provide a particularly

powerful tool for probing these interactions because the

detection generally can be switched between magnons

and phonons simply by rotating a polarizing filter in the

path of the scattered light. The plan of research includes

measurements of changes in the populations of magnons

and phonons induced by ferromagnetic resonant excitation

of permalloy (Ni81Fe19) thin films having varying amounts

of rare-earth doping, which affects the damping constants

of magnetic switching. Also, measurements will seek to

determine whether rare-earth doping affects the peak

line widths, which are proportional to the damping

constants of the individual acoustic and spin waves.

Accomplishments

Over the past year, we have assembled a

Brillouin-scattering system and placed it in operation.

Initial measurements of thermal magnons and phonons

in thin films of pure permalloy (Ni81Fe19) and Tb-doped

permalloy have been performed. Figure 1 shows typical

magnon and phonon spectra from Ni81Fe19 (1.2% Tb).

The central peak that extends off scale in both plots

arises from a direct reference beam. Apparent peaks

at frequencies near –2.7 GHz and +3.5 GHz in

Figure 1(b) are experimental artifacts arising from

elastic scattering off the sample. Peaks to the left and

right of the central peak arise, respectively, from Stokes

and anti-Stokes interactions with magnons and phonons

having various symmetries.

Figure 1: Brillouin spectra from a Ni81Fe19 thin film

doped with 1.2% Tb. (a) Magnon spectrum. (b) Phonon

spectrum.

Contributors and Collaborators

P. Kabos (NIST Radio-Frequency Technology

Division); S. Russek (NIST Magnetic Technology

Division); C. Patton (Colorado State University)

This project was partially funded by the National

Nanotechnology Initiative.
48

Related Projects

Nanoscale Characterization: Electron Microscopy

Electron microscopy is used to characterize the

structure and chemistry of materials at the nanometer

scale to better understand and improve their

properties. New measurement techniques in electron

microscopy are being developed and applied to

materials science research. The MSEL Electron

Microscopy Facility primarily serves the Metallurgy,

Ceramics, and Polymers Divisions as well as other

NIST staff and outside collaborative research efforts.

John E. Bonevich

A tomic-scale structure and compositional

 characterization of materials can lend crucial

insights to the control of their properties. For instance,

direct observation of local structures by transmission

electron microscopy (TEM) provides an important

information feedback to the optimization of crystal

growth and processing techniques. Various

characteristics may be observed such as crystal

structure and orientation, grain size and morphology,

defects, stacking faults, twins and grain boundaries,

second phase particles — their structure, composition

and internal defect structure, compositional variations

and the atomic structure of surfaces and interfaces.

The MSEL Electron Microscopy Facility consists of

two transmission electron microscopes, three scanning

electron microscopes, a specimen preparation laboratory,

and an image analysis /computational laboratory. The

JEOL3010 UHR-TEM has atomic scale resolution as

well as detectors for analytical characterization of thin

foil specimens. An X-ray detector (EDS) provides

compositional analysis and an energy-selecting imaging

filter (IF) allows compositional mapping at atomic

resolution. The JSM6400 is equipped with electron

backscattered diffraction/phase identification (EBSD)

and EDS systems for characterization of texture and

composition of materials.

FY2002 Highlights

■ ATP intramural research collaboration with the

Semiconductor Electronics and Electricity Divisions

(EEEL) to characterize quantum effects in confined

Si devices.

■ ATP intramural research collaboration with the

Optoelectronics Division (EEEL) to characterize

quantum dot III-V materials.

■ Measurement of size distributions in CeO2 nanoparticles

for use as an x-ray particle size standard reference

material.

■ Characterization of superconformal deposition of silver

in sub-100 nm features for on-chip interconnections.

The shape of self-assembled quantum dots

(QDs) play a major role in their electronic properties.

In conjunction with RHEED and AFM, TEM studies

of QD shape and faceting have revealed novel structures

in In0.44Ga0.56As (see Figure 1). Typical models for

QD shape include hexagonal dots as well as diamond

shaped pyramids. However, TEM results show unusual

facets. Instead of appearing with trapezoidal or triangular

cross-sections, the QDs have multiple facets at their

peaks and bases. The varied shapes have implications

for the control and reproducibility of the MBE growth

process of QDs.

Figure 1: Plan-view TEM of InGaAs QDs reveals “X”

features indicating the shape due to surface facets.

Some QDs have additional facets (arrowed).

Figure 2: Cross-sectional HRTEM of InGaAs QDs

indicating unusual faceting.

Contributors and Collaborators

R.D. McMichael, D. Josell, T.P. Moffat, J. Cline

(MSEL); A. Roshko, S. Lehman (EEEL); E. Vogel,

N. Zimmerman (EEEL); Prof. A. Stanishevsky

(University of Maryland, Institute for Plasma Research)
49

Related Projects

Phase Diagrams in High Temperature Superconductors

Phase diagrams serve as “road maps” for

processing high Tc superconductors. Of current

interest are the Ba2RCu3O6+x (R = Sm, Gd and Er)

superconductors. The critical single-phase regions of

Ba2+x(Nd1+x-yRy)Cu3O6+z for R = Gd, Y and Yb

were determined such that both flux-pinning and melting

can be tailored and optimized. The construction of

subsolidus phase diagrams of these systems, and the

role of phase equilibria and kinetics in the formation of

the Ba2YCu3O6+x phase, are deemed important for

the rapid advancement of second generation coated

conductor technology.

Winnie Wong-Ng and Lawrence Cook

NIST phase equilibrium research has continued

 to provide critical information pertinent to the

development of practical superconductors. DOE

has supported high Tc research through a program

of intensive R&D focused on wire and cable for

high-impact commercial applications. This effort

includes three principal groups of superconductors:

(1) 1st generation Bi-Pb-Sr-Ca-Cu-O (BSCCO)-based

wire/tape produced by the Ag-powder-in-tube (PIT)

technique; (2) 2nd generation Ba2RCu3O7-type

(Y-213 and R-213, R = lanthanide) coated-conductors

produced by rolling-assisted biaxially-textured

substrate/ion beam assisted deposition (RABiTS/

IBAD); (3) the recently discovered MgB2 material.

By mixing the smaller lanthanides (Gd, Y, Yb)

with the larger Nd in the Ba2–x(Nd1+x–yRy)Cu3O6+z

(Nd-213) superconductor, both the flux-pinning and

the melting properties can be tailored and optimized.

A size trend in the single phase solid solution prepared

in air was observed. Due to the closer match in size

of Nd3+with Gd3+ than Nd3+ with Y3+, and Nd3+

with Yb3+, the solid solution region is largest for

the Gd-substituted system and smallest for the

Yb-substituted system. We have successfully

characterized selected solid solution members by

x-ray, neutron, and electron diffraction, and by

differential thermal analysis (DTA). There is

considerable improvement of Jc(H) for samples with

partial Y-substitution at higher field at 77 K, compared

with that of Ba2NdCu3O6+z and Ba2YCu3O6+z.

This improvement is likely due to the increased flux

pinning as a result of doping of Nd3+ in the Ba2+ site.

The “BaF2 ex situ” process and the liquid-phaseepitaxy process are promising methods for producing

long-length high quality Y-213 superconductor.

Therefore investigation of the phase relationships in the

system BaF2-BaO-Y2O3-CuOx-H2O is another area of

concentration. Using specially constructed controlledatmosphere instrumentation and a strategic approach,

we succeeded in determining the presence of low

temperature liquids in this air-sensitive multicomponent

reciprocal Ba-Y-Cu //O,F system, which can be modeled

in compositional space as a trigonal prism. This

prism can be viewed as consisting of three tetrahedra

(BaO-Y2O3-CuOx-BaF2, BaF2-YF3-CuF2-CuOx,

and BaF2-Y2O3-CuOx-YF3). We have also initiated

a more detailed study of melt equilibria in the

BaF2-BaO-Y2O3-CuOx system. The role of Ba(OH)2

in this system, as related to low-temperature melting,

was identified as warranting further investigation.

All this information is important for both the “BaF2”

process and liquid phase epitaxy process.

To control film properties, it is important to

understand the details of Y-213 phase evolution from

amorphous “BaF2” films. To collect data at a faster rate

over a broader range, we have used a position-sensitive

detector on the high-temperature x-ray diffractometer.

Three sets of films with different thickness (provided by

R. Feenstra of the Oak Ridge National Laboratory) have

been studied. We found that the growth of the Y-213

phase is controlled by the reaction at the interface of

the Y-213/precursor, not by diffusion. Sufficient water

vapor pressure is critical for Y-213 formation. The

detailed phase evolution sequence, kinetics of phase

formation, and the texture of these films as a function

of processing parameters, will continue to be pursued.

A smaller effort was spent on the study of the

melting equlibria of the Pb-doped and Pb-free Bi-2223

superconductors. In collaboration with FSU, the

Pb-free 2223 phase was found to be in equilibrium

with eight phases in the Bi-Sr-Ca-Cu-O system.

Investigation continues to obtain a complete set of

subsolidus 4-phase volumes, which is necessary for

the determination of the primary phase field. We have

measured the enthalphy of formation of MgB2, at 298 K

by solution calorimetry. The vapor pressure of MgB2

up to 1100oC was also successfully measured using a

thermogravimetric effusion method.

As data accumulate, we anticipate increased use

of thermodynamic methods for data smoothing and

extrapolation, with the ultimate goal of producing a

comprehensive phase equilibria model for each of the

three major high Tc systems. By providing the phase

equlibria data as the basis for optimal processing, high

Tc technology will be advanced through reductions in

cost and improvements in performance.

Contributors and Collaborators

I. Levin, M. Vaudin, J.P. Cline (Ceramics); Q. Huang,

B. Toby (Reactor); L. Swartzendruber (Metallurgy);

A. Kearsley (Mathematical and Computational Sciences);

R. Klein (Chemistry); T. Holesinger (LANL); R. Feenstra

(ORNL); M. Rupich (AMSC); J. Storer (3M); J. Kaduk

(BP-Amoco); R. Meng (University Houston);

M. Suenaga (BNL); and PVPSS Sastry (FSU).
50
51

Organizational Charts

Organizational Charts

Materials Science and Engineering Laboratory

Polymers

E.J. Amis, Chief

C.R. Snyder, Deputy

Metallurgy

C.A. Handwerker, Chief

F.W. Gayle, Deputy

Ceramics

G.G. Long, Chief (Acting)

(Vacant), Deputy

Materials Reliability

F.R. Fickett, Chief

T.A. Siewert, Deputy

NIST Center for

Neutron Research

J.M. Rowe, Director

Director

Deputy Director

National Institute of Standards and Technology

Director

Boulder Laboratories

Baldrige National Quality

Program

Director for

Administration

& Chief Financial

Officer

Technology

Services

Advanced

Technology

Program

Manufacturing

Extension

Partnership

Manufacturing

Engineering

Laboratory

Chemical Science

and Technology

Laboratory

Materials Science

and Engineering

Laboratory

Electronics and

Electrical Engineering

Laboratory

Information

Technology

Laboratory

Physics

Laboratory

Building and

Fire Research

Laboratory

L.E. Smith, Director

S.W. Freiman, Deputy Director
52
52