bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized model of the metal/n-GaN Schottky interface and improved performance by electrochemical Pt deposition
Authors: Cojocari Oleg , Hartnagel Hans L.
Published: December 2006

A modified model of the Schottky interface is proposed, which includes a near-surface layer (NSL) in the depletion region of the semiconductor. An important effect of the NSL is the ability to make the value of the Schottky barrier strongly voltage dependent, in agreement with experimental behavior. The proposed model can therefore qualitatively explain the observed peculiarities of Schottky contacts to the GaN and related materials. Pt∕n-GaN Schottky contacts were fabricated by both electrochemical deposition and e-beam evaporation techniques. The use of electrochemistry resulted in significantly better performance of Schottky contacts. A comparative study of evaporated and electroplated contacts justifies the NSL model.
Chemical lithography
Authors: Yao Peng , Schneider Garrett J. , Murakowski Janusz , Prather Dennis W.
Published: December 2006

In this article, the authors present and demonstrate a novel, versatile lithography method with high resolution that they call chemical lithography (ChemLith). The concept is based on the fact that most of the commonly used photoresists change their solubility upon an acid-catalyzed chemical reaction. In photolithography, photoacid generator is mixed in the resist formula and the acid is generated by photon-initiated reactions. Using photons sets the fundamental limit on the feature size for photolithography. To overcome this limit, they propose to physically introduce the catalyzing acid (proton source) to the desired position on the resist surface. This concept can be implemented in two different ways, using a template in a manner similar to nanoimprint lithography or using an atomic force microscope tip similar to nanoprobe based nanolithography. They have achieved nanoscale feature size in both cases, which confirms the potential of ChemLith as an advanced lithography technique.
Demonstration of spatially programmable chemical vapor deposition: Model-based uniformity∕nonuniformity control
Authors: Sreenivasan Ramaswamy , Adomaitis Raymond A. , Rubloff Gary W.
Published: December 2006

A new chemical vapor deposition (CVD) reactor design was developed to intentionally induce spatially nonuniform film thickness deposition patterns on a single wafer. A segmented showerhead design allows individual regions of a wafer to be exposed to different precursor concentrations simultaneously during a run resulting in different thickness profiles on the wafer and a thickness gradient at the boundaries between segment regions. Different recipes were cycled through each of the segments in a sequence of deposition experiments to develop a model relating precursor concentration to film thickness in each segment region. As a demonstration of spatial programmability, the system was reprogramed using this model to produce uniform thickness amongst the segments; intersegment uniformity approaching 0.60% (thickness standard deviation) was demonstrated. Potential applications of this reactor design to combinatorial CVD are discussed.
Direct-write electron-beam lithography of an IR antenna-coupled microbolometer onto the surface of a hemispherical lens
Authors: Middlebrook C. T. , Zummo G. , Boreman G. D.
Published: December 2006

This article describes a method for performing direct-write lithography of an IR antenna-coupled microbolometer onto the surface of a hemispherical lens. Antennas on a dielectric half-space receive power more efficiently from the substrate side than from the air side. The use of a hemispherical lens facilitates reception through the substrate as well as elimination of trapped surface waves that would normally occur in the substrate. Using direct-write lithography onto the surface of the hemispherical lens eliminates the potential of an air gap between the antenna and lens. Additionally, the accuracy of alignment between the antenna and the center of the lens is controlled at the lithographic step. As a result, there is increased responsivity is observed in the antenna-coupled microbolometer when illuminated from the substrate-side compared to air-side illumination.
Effect of the addition of argon to reactive nitrogen gas on field emission properties of amorphous carbon nitride films
Authors: Li Junjie , Mimura Hidenori , Neo Yoichiro , Gu Changzhi , Li Haijun , Chen Shuxia
Published: December 2006

Amorphous carbon nitride films (a-CNx) were deposited as the electron field emitters by rf magnetron sputtering method at room temperature. The effect of adding Ar to reactive nitrogen gas on the field emission of a-CNx films is investigated. The addition of Ar increases the proportion and size of sp2-bonded clusters in the films and an overfull Ar addition decreases the N content in the film, measured by photoluminescence, x-ray photoelectron spectroscopy, and Raman spectra. Field emission testing results show that the Ar addition effectively improves the field emission ability of a-CNx films. Related to the change of chemical bonding structure and components in the films, the authors found that the enhanced field emission of the films was attributable to the increased proportion of sp2-bonded clusters, including sp2 C–N and sp2 C–C bondings, and the decreased N content in the film. As a conduction part of the amorphous network in the films, these rich as-formed sp2-bonded clusters not only enable the tunneling in the film to make electron emission easier but also lead to degrading the electron emission barriers and raising the Fermi level to enhance electron emission.
Field electron emission characteristics of diamond films with different grain morphologies
Authors: Lu X. , Yang Q. , Chen W. , Xiao C. , Hirose A.
Published: December 2006

Diamond films with different grain morphologies were prepared in order to compare their field electron emission properties. For the diamond films with well-oriented grains, the field electron emission properties are enhanced when the grain shape changes from flat to flattened pyramid and then to pyramid due to the increase in the field enhancement factors. Compared with the well-oriented diamond films, the diamond film with randomly oriented grains exhibits larger emission current due to the increase in the effective emission area. For the diamond film with cauliflowerlike grains, the nanotip structures improve the electron emission properties significantly through an increase in both the field enhancement factor and the effective emission area.
Controlled electroplating for high-aspect-ratio zone-plate fabrication
Authors: Holmberg A. , Lindblom M. , Hertz H. M.
Published: December 2006

The authors report a method for monitoring, control, and end-point detection of electroplating in nanostructures. The method is demonstrated on nickel plating into polymer molds, which is an important process in the fabrication of soft x-ray zone-plate diffractive optics. The lack of reproducibility presently limits the achievable nickel aspect ratio and, thus, reduces the zone-plate diffraction efficiency. The reported method provides reproducible plating via real-time control of the plating rate. It combines in situ light transmission measurements with current measurements to determine the thickness of the growing layer. The accuracy of the thickness prediction was better than ±4% (1σ) for 100–300nm nickel layers. Furthermore, a slight change in the light transmission signal indicates when a gratinglike zone-plate structure is slightly overplated and the plating should be stopped. This end-point detection provides the optimal filling of high-aspect-ratio molds for improved diffraction efficiency.
Accurate resonant frequency spacing of microring filters without postfabrication trimming
Authors: Holzwarth C. W. , Barwicz T. , Popović M. A. , Rakich P. T. , Ippen E. P. , Kärtner F. X. , Smith Henry I.
Published: December 2006

Optical microring resonators are compact building blocks for many microphotonic systems, as they enable narrow-band filtering. Of particular interest are integrated banks of microphotonic filters, with accurately spaced operational frequencies, for simultaneously processing multiple channels of information. Accurate spectral spacing has not yet been reported as it requires subnanometer control of the average dimensions of the optical resonators used to build high-performance microphotonic filters. Here, the authors present a technique enabling the relative dimensional control needed to achieve accurate spectral spacing, without postfabrication trimming. The authors fabricated eight-channel second-order microring-resonator filter banks with accurate spectral spacing using scanning-electron-beam lithography. The microrings are made of silicon-rich silicon nitride, and the subnanometer relative dimensional control is achieved through carefully calibrated adjustments of the electron-beam dose. When combining the dose alternations with small changes in microring radii, the authors demonstrate filter banks with accurate channel spacings ranging from 90to180GHz. We show that we can accurately control a 2.7nm change in average width of the ring waveguide to 0.11nm, despite a 6nm step size in the scanning-electron-beam lithography system.
Control of an electrospinning jet using electric focusing and jet-steering fields
Authors: Bellan Leon M. , Craighead H. G.
Published: December 2006

Electrospinning can be used to deposit a wide variety of nanoscale polymeric fibers that have electrical, optical, or biological properties of interest. While there have been many studies of material properties, the typical deposited nanofibers are in the form of a randomly oriented mat. The authors are interested in forming functional devices utilizing the properties of the individual nanofibers. To this end they have used electric fields to both confine and steer an electrospun polymer jet for controlled deposition of functional materials. They have used an electrode between the electrospinning tip and grounded sample to suppress the chaotic whipping mode, thereby focusing the characteristic spot size of the deposited fibers to a smaller diameter. The same electrode setup was then modified to produce a time-varying steering field. Using this system, they have deposited isolated electrospun polymer fibers in a controlled fashion. They have also demonstrated that it is possible to terminate electrospun fibers using the modulated electric field.
Gold nanoparticle wires made using RNA-based self-assembly
Authors: Cumming D. R. S. , Bates A. D. , Callen B. P. , Cooper J. M. , Cosstick R. , Geary C. , Glidle A. , Jaeger L. , Pearson J. L. , Proupín-Pérez M. , Xu C.
Published: December 2006

Self-assembly or bottom-up techniques based on molecular recognition are attracting a great deal of interest for nanofabrication. One such technology is based on the use of self-assembling tecto-RNA that has been shown to be able to make structures with nanoscale features (<10nm) over large areas of the order of several microns. The technique is extremely flexible in that different RNA designs can be used to create dramatically different patterns, and the technology has the potential to scale up so that it might be used to pattern whole chips, or even wafers. In a step towards using molecular recognition techniques to build devices such as electronic circuits or sensors it is necessary to be able to add device structures or link functional units together. The authors show that the tecto-RNA method can be extended to attach gold nanoparticles using DNA linkers. The self-assembling properties of the RNA can in turn be used to link together the gold nanoparticles to form wires and clusters in a Mg2+-ion controlled, as opposed to electrostatically aggregated, process. Using this method they have fabricated a gold nanoparticle wire between two nanoelectrodes and evaluated the electrical properties of the resultant device.
Silicon photodiodes for low-voltage electron detection in scanning electron microscopy and electron beam lithography
Authors: Silver C. S. , Spallas J. P. , Muray L. P.
Published: December 2006

Silicon photodiode (SPD) detectors can be used for secondary electron detection in miniature electron beam columns, where small apertures, tight lens spacings, and short working distances make traditional detectors impractical. Monte Carlo simulations presented in this article suggest that in these configurations, SPDs have an advantage over traditional secondary electron detectors due to their potentially higher collection efficiency. Responsivity results presented here for SPDs with 40 and 60Å passivation layers show measurable responsivity for electron energies down to 80eV. High contrast images obtained using a SPD mounted with the Novelx miniature electron beam column and biased with a floating voltage to accelerate low-energy electrons demonstrate the effectiveness of SPDs for secondary electron detection. Based on these simulations and results, it is believed that SPDs present a reliable, inexpensive solution for secondary electron detection in miniature electron beam columns, as well as a robust mechanism for system calibration in column arrays for lithography.
Actinic extreme ultraviolet lithography mask blank defect inspection by photoemission electron microscopy
Authors: Lin Jingquan , Neuhaeusler Ulrich , Slieh Jawad , Brechling Armin , Kleineberg Ulf , Heinzmann Ulrich , Oelsner Andreas , Valdaitsev Dima , Schoenhense Gerd , Weber Nils , Escher Matthias , Merkel Michael
Published: December 2006

A new method for the actinic inspection of defects inside and on top of extreme ultraviolet (EUV) lithography multilayer-coated mask blanks is presented. The experimental technique is based on photoemission electron microscopy supported by the generation of a standing wave field inside and above the multilayer mask blank when illuminated near the resonance Bragg wavelength at around 13.5nm. Experimental results on programed defect samples based on electron beam lithographic structures or silica balls overcoated with an EUV multilayer show that buried defects with a lateral size down to 50nm are detectable. Furthermore, phase structures as shallow as 6nm in height on a programed phase grating sample have been detected by this technique. The contrast of the phase defect structures has shown to be strongly dependent on and controlled by the phase of the standing wave field at the mask blank surface, and thus can be optimized by tuning the inspection wavelength.
High brightness inductively coupled plasma source for high current focused ion beam applications
Authors: Smith N. S. , Skoczylas W. P. , Kellogg S. M. , Kinion D. E. , Tesch P. P. , Sutherland O. , Aanesland A. , Boswell R. W.
Published: December 2006

A high brightness plasma ion source has been developed to address focused ion beam (FIB) applications not satisfied by the liquid metal ion source (LMIS) based FIB. The plasma FIB described here is capable of satisfying applications requiring high mill rates (>100μm3∕s) with non-gallium ions and has demonstrated imaging capabilities with sub- 100-nm resolution. The virtual source size, angular intensity, mass spectra, and energy spread of the source have been determined with argon and xenon. This magnetically enhanced, inductively coupled plasma source has exhibited a reduced brightness (βr) of 5.4×103Am−2sr−1V−1, with a full width half maximum axial energy spread (ΔE) of 10eV when operated with argon. With xenon, βr=9.1×103Am−2sr−1V−1 and ΔE=7eV. With these source parameters, an optical column with sufficient demagnification is capable of forming a sub-25-nm spot size at 30keV and 1pA. The angular intensity of this source is nominally three orders of magnitude greater than a LMIS making the source more amenable to creating high current focused beams, in the regime where spherical aberration dominates the LMIS-FIB. The source has been operated on a two lens ion column and has demonstrated a current density that exceeds that of the LMIS-FIB for current greater than 50nA. Source lifetime and current stability are excellent with inert and reactive gases. Additionally, it should be possible to improve both the brightness and energy spread of this source, such that the (βr∕ΔE2) figure-of-merit could be within an order of magnitude of a LMIS.
Estimation of scattered particle exposure in ion beam aperture array lithography
Authors: Parekh V. , Ruiz A. , Ruchhoeft P. , Nounu H. , Litvinov D. , Wolfe J. C.
Published: December 2006

In ion beam proximity lithography, ions that are incident on the nominally opaque regions of a stencil mask can scatter into the open windows and escape, exposing a wide area of the substrate. Since these ions can lose much of their initial energy in the mask, the scattered particle exposure is concentrated near the resist surface. The resulting loss of contrast can be mitigated to some extent by using aperture array lithography (AAL) where a mask of reduced density minimizes the number of windows from which a scattered ion can escape. Even so, the problem worsens as the pitch of an array, printed by multiple, offset exposures of the AAL mask, shrinks below about 250nm. The only solution is to increase the mask thickness, hence the window aspect ratio, to reduce the escape angles of the scattered particles. In this article, the authors characterize an effective background dose η in the first 75nm of poly(methylmethacrylate) resist for 30keV He+ ion exposures of 0.6μm thick masks with 45, 80, and 110nm circular windows on 150, 300, and 400nm pitches, respectively. They project that η would be (6.8±0.8)%(σ) of the primary ion dose for the printing of dense arrays, with period equal to twice the window diameter, over this range of feature sizes.
Hybrid mold reversal imprint for three-dimensional and selective patterning
Authors: Peng C. , Pang S. W.
Published: December 2006

Three-dimensional (3D) nanostructures are needed in many applications including sensors, actuators, and microanalysis systems. In this work, a novel technology combining UV lithography, hybrid mold, and reversal imprint for fabricating 3D structures is developed. A hybrid mold made from quartz is used. The mold has structures patterned by lithography and dry etching. The quartz mold also has selected patterns formed by 50nm thick Cr. A layer of UV definable SU-8 polymer is spin coated onto the hybrid mold and patterned by optical lithography. The mold with the patterned SU-8 layer and no residue is then transferred to a substrate with topography by reversal imprint with temperatures as low as 50°C and pressures of nominally 2MPa. Depending on the dimensions of patterns on the mold compared to the ones on the substrate and the imprint pressure, patterns can be selectively transferred to substrates through reversal imprint. This technology greatly simplifies the fabrication process and provides more flexibility in building complex 3D structures.
Integrated multi-electron-beam blanker array for sub-10-nm electron beam induced deposition
Authors: Zhang Yanxia , Heerkens C. T. H , van Bruggen M. J. , Kruit P.
Published: December 2006

An integrated multi-electron-beam blanker array is proposed for the multi-electron-beam source reported by van Bruggen et al. [J. Vac. Sci. Technol. B 23, 2833 (2005)], which aims at the throughput improvement of sub-10-nm electron beam induced deposition. The integrated blanker array consists of a current limiting aperture array, a blanker array, and a microaperture-lens array. The integrated blanker array generates 100 individually controlled beamlets, projecting the virtual source image in the principle plane of the field lens. The electrostatic cross-talk, charging, and contamination are reduced by the grounded current limiting aperture plate above the blankers. The blanker array and microaperture-lens array are fabricated on a first wafer, while the current limiting aperture array is fabricated on a second, 100μm thick wafer. The wafers will be bonded with an alignment accuracy of approximately 200nm. The first test chips, where the blankers are grouped and controlled by external circuitries, are under fabrication and key processes are tested.
Micromachined piezoresistive proximal probe with integrated bimorph actuator for aligned single ion implantation
Authors: Persaud A. , Ivanova K. , Sarov Y. , Ivanov Tzv. , Volland B. E. , Rangelow I. W. , Nikolov N. , Schenkel T. , Djakov V. , Jenkins D. W. K. , Meijer J. , Vogel T.
Published: December 2006

The authors report a microfabrication procedure of self-actuated piezoresistive scanning probes (SAPSPs). They are designed for a SAPSP instrument that is integrated with an ion beam for aligned single ion implantation in ultrahigh vacuum. The novelty of the design is an integrated hollow pyramid, instead of a previously mechanically hand mounted pyramid [J. Vac. Sci. Technol. B 23, 2798 (2005)]. The pyramid has dual purpose. First it collimates the ion beam and suppresses secondary particles from the back side of the cantilever, so that secondary particles from the target material can be used for single ion detection. Second the pyramid also provides an atomic force microscope tip for the scanning probe. A crucial step in the fabrication is the back side opening via etching for the hollow pyramid. The fabrication procedure will be discussed in detail.
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-wide nanogaps fabricated with single-walled carbon nanotube shadow masks
Authors: De Poortere E. P. , Stormer H. L. , Huang L. M. , Wind S. J. , O’Brien S. , Huang M. , Hone J.
Published: December 2006

The authors present a method for producing nanometer-scale gaps, based on metal evaporation through a suspended single-walled carbon nanotube acting as a shadow mask. 83% of the nanogap devices display current-voltage dependencies characteristic of direct electron tunneling. Fits to the current-voltage data yield gap widths in the 0.8–2.3nm range for these devices, dimensions that are well suited for single-molecule transport measurements.
Nanoscale floating-gate characteristics of colloidal Au nanoparticles electrostatically assembled on Si nanowires
Authors: Jeon H.-S. , Cho C.-W. , Lim C.-H. , Park B. , Ju H. , Kim S. , Lee S.-B.
Published: December 2006

Nanoscale floating-gate characteristics of colloidal Au nanoparticles electrostatically assembled on Si nanowires have been investigated. Colloidal Au nanoparticles with ∼5nm diameters were selectively deposited onto the lithographically defined n-type Si nanowire surface by 2min electrophoresis between the channel and the side gates. The device transfer characteristics measured at room temperature showed hysteresis, with the depletion mode cutoff voltage applied by the side gates shifted by as much as 1.5V, with the source-drain bias at 1.4V. The results demonstrate that the electrostatic assembly of colloidal Au nanoparticles is a useful method for the fabrication of Si nanowire based nanoscale floating-gate nonvolatile memory structures.
Thin membrane self-alignment using nanomagnets for three-dimensional nanomanufacturing
Authors: Nichol Anthony J. , Arora William J. , Barbastathis George
Published: December 2006

In this article the authors present the use of nanomagnets as fine alignment features for structures assembled by folding nanopatterned membranes. The membranes are patterned with arrays of nanomagnets that passively align to other arrays. They analyze the scaling of the force between magnets and calculate the required structural compliance for the magnetic forces to actuate the membranes. A model of the alignment system that captures the magnetic forces and the membrane dynamics has been developed. They show that an external magnetic field provides magnetic saturation of the nanomagnets and aligns the membranes rotationally. The magnetic dipole approximation used in the model was confirmed experimentally by patterning cobalt and iron nanomagnets on silicon nitride cantilevers and actuating in an external magnetic field. Self-alignment between the two arrays of cobalt nanomagnets was achieved confirming the magnetic alignment principle.
Actinic inspection of extreme ultraviolet programed multilayer defects and cross-comparison measurements
Authors: Goldberg Kenneth A. , Barty Anton , Liu Yanwei , Kearney Patrick , Tezuka Yoshihiro , Terasawa Tsuneo , Taylor John S. , Han Hak-Seung , Wood Obert R.
Published: December 2006

The production of defect-free mask blanks remains a key challenge for extreme ultraviolet (EUV) lithography. Integral to this effort is the development and characterization of mask inspection tools that are sensitive enough to detect critical defects with high confidence. Using a single programed-defect mask with a range of buried bump-type defects, the authors report a comparison of measurements made in four different mask inspection tools: one commercial tool using 488nm wavelength illumination, one prototype tool that uses 266nm illumination, and two noncommercial EUV “actinic” inspection tools. The EUV tools include a dark field imaging microscope and a scanning microscope. Their measurements show improving sensitivity with the shorter wavelength non-EUV tool, down to 33nm spherical-equivalent-volume diameter, for defects of this type. Measurements conditions were unique to each tool, with the EUV tools operating at a much slower inspection rate. Several defects observed with EUV inspection were below the detection threshold of the non-EUV tools.
Study of overlay metrology in atomic force microscope lithography (overlaying lithography with atomic force microscope)
Authors: Li Xiaona , Han Li , Gu Wenqi
Published: December 2006

Atomic force microscope (AFM) has been considered as a useful lithographic tool of the future nanofabrication. In this article, overlay metrology in AFM lithography was investigated. A method of overlay metrology with cross marks was designed and applied to write two-layer patterns. The alignment arithmetic was developed on the basis of mark-registration method of electron beam lithography. A set of vernier was designed to measure the overlay precision. Further analysis about the overlay errors was presented. Experimental results indicated that the overlay metrology can obtain overlay precision less than 30nm and a higher precision will be realized by improving the system.
From nanoscale displacement sensing and estimation to nanoscale alignment
Authors: Gao Jun , Picciotto Carl , Wu Wei , Tong William M.
Published: December 2006

In this article, the authors present theoretical background and practical procedures for linking displacement sensing and estimation to nanoscale alignment. The authors discuss the application of nanoscale displacement sensing and estimation (nDSE)-based overlay metrology tool. The authors propose a new look at what constitutes displacement and alignment. From there, the authors develop several frameworks to bridge these two concepts based on a set of new pseudodisplacement concepts and on the careful selection of references. Direct displacement-measurement-based alignment (DDMA) is the first method the authors have used to generate reliable alignment results. The authors achieved sub-100-nm (down to 21nm) alignment accuracy. Indirect displacement-measurement-based alignment (IDMA) is the most accurate method in theory. IDMA truly utilizes nDSE to achieve precision coplacement between different layers and processes. IDMA and DDMA, along with a group of other nDSE-based algorithms/procedures, are vibration tolerant, making them ideal as overlay alignment tools for nanoimprint lithography.
Robust shadow-mask evaporation via lithographically controlled undercut
Authors: Cord B. , Dames C. , Berggren K. K. , Aumentado J.
Published: December 2006

Suspended shadow-mask evaporation is a simple, robust technique for fabricating Josephson-junction structures using scanning electron-beam lithography. The basic process entails the fabrication of an undercut structure in a resist bilayer to form a suspended “bridge,” followed by two angle evaporations of superconducting material with a brief oxidation step in between. The result is two overlapping wires separated by a thin layer of oxide. Josephson junctions with sub-50-nm diameters are of particular interest in quantum computing research. Unfortunately, standard shadow-mask fabrication techniques are highly variable at linewidths below 100nm, due to the difficulty of simultaneously fabricating a narrow line and a large undercut region. While most previous processes used poly(methylmethacrylate) (PMMA) for the top (imaging)layer and either lower-molecular-weight PMMA or a PMMA/methacrylic acid copolymer for the bottom (support) layer, the authors’ process uses a PMMA/poly(methylglutarimide) (PMGI) bilayer. The advantage of using PMGI as the support layer is that it develops in aqueous base solutions, while PMMA is insensitive to aqueous solutions and only develops in certain organic solvents. This allows the two layers to be developed independently, ensuring that the imaging layer is not biased during the development of the support layer and allowing the process to achieve the full resolution of the PMMA imaging layer, which can be extremely high. Additionally, the extent of the undercut in the support layer can be precisely controlled by defining it lithographically, rather than simply varying the PMGI development time as in previous processes. Although PMGI is sold as a “liftoff resist” and widely assumed to be electron insensitive, their experiments have shown that this is not the case. Instead, when dilute developer and low electron doses are used, PMGI behaves very much like a conventional photoresist. By exploiting this behavior, as well as its high electron sensitivity with respect to PMMA, the authors were able to define undercuts by defining low-dose regions adjacent to their features, exposing the underlying PMGI separately. In this manner, it is possible to create well-controlled undercut regions as large as 600nm. Extensive modeling of both the exposure and development processes was used to verify their results. By using a Monte Carlo simulation of electron scattering to simulate the electron exposure and mass-transfer relationships to simulate the process of developing the undercut region, the authors were able to produce a model that closely matches experimental results. With the process fully characterized, it is possible to produce nearly any linewidth/undercut combination, limited only by PMMA resolution and the mechanical stability of large overhang structures. This robustness, combined with the high resolution of the PMMA imaging layer, will allow the reliable fabrication of many interesting devices and circuits based on nanoscale Josephson junctions.
Plasma oxidation of polyhedral oligomeric silsesquioxane polymers
Authors: Eon D. , Raballand V. , Cartry G. , Cardinaud C. , Vourdas N. , Argitis P. , Gogolides E.
Published: December 2006

Copolymers containing polyhedral oligomeric silsesquioxane (POSS™) units have been developed to be used as photoresist components in a bilayer resist scheme for 193nm lithography. This article reports on the behavior of these new POSS based materials under oxygen plasmas. The authors demonstrate using in situ ellipsometry and in situ x-ray photoelectron spectroscopy that during the first seconds in the plasma a silicon oxide layer is formed on the top surface of the POSS materials. This superficial layer prevents etching and material consumption. An ion-enhanced oxidation model is proposed to describe and explain the experimental data and further investigate POSS etching mechanisms in oxygen plasma. The model shows that the oxide formation rate is reduced exponentially with the oxide thickness. It also predicts that thickness loss has its main roots in the layer densification that occurs when the oxide is formed and shows that the oxide formation is ion enhanced and thus favored at −100V compared to 0V bias.
Wafer-scale fabrication of polymer distributed feedback lasers
Authors: Christiansen M. B. , Schøler M. , Balslev S. , Nielsen R. B. , Petersen D. H. , Kristensen A.
Published: December 2006

The authors demonstrate wafer-scale, parallel process fabrication of distributed feedback (DFB) polymer dye lasers by two different nanoimprint techniques: By thermal nanoimprint lithography (TNIL) in polymethyl methacrylate and by combined nanoimprint and photolithography (CNP) in SU-8. In both techniques, a thin film of polymer, doped with rhodamine-6G laser dye, is spin coated onto a Borofloat glass buffer substrate and shaped into a planar waveguide slab with first order DFB surface corrugations forming the laser resonator. When optically pumped at 532nm, lasing is obtained in the wavelength range between 576 and 607nm, determined by the grating period. The results, where 13 laser devices are defined across a 10cm diameter wafer substrate, demonstrate the feasibility of NIL and CNP for parallel wafer-scale fabrication of advanced nanostructured active optical polymer components, with a yield above 95%.
Precise positioning of single-walled carbon nanotubes by ac dielectrophoresis
Authors: Banerjee Sarbajit , White Brian E. , Huang Limin , Rego Blake J. , O’Brien Stephen , Herman Irving P.
Published: December 2006

The precise placement of single-walled carbon nanotubes (SWCNTs) in device architectures by ac dielectrophoresis involves the optimization of the electrode geometry, applied voltage and frequency, load resistance, and type of nanotube sample used. The authors have developed a toolkit to controllably integrate SWCNTs in device structures by the use of floating potential metal posts and appropriate electrode geometries, as designed using electric field simulations, and used it to fabricate structures such as crossed nanotube junctions.
Evidence for internal stresses induced by nanoimprint lithography
Authors: Ro Hyun Wook , Ding Yifu , Lee Hae-Jeong , Hines Daniel R. , Jones Ronald L. , Lin Eric K. , Karim Alamgir , Wu Wen-li , Soles Christopher L.
Published: December 2006

The thermal embossing form of nanoimprint lithography is used to pattern arrays of nanostructures into three different polymer films. The shape of the imprinted patterns is characterized with nanometer precision using both x-ray scattering and reflectivity techniques. The time dependent response of the pattern shape at temperatures near the glass transition temperature reveals large levels of residual stress induced by the imprinting process. During the imprint, large shear fields are generated as the viscous polymer flows into the mold. If these shear distortions do not have time to relax during the imprinting, internal stresses are frozen into the final pattern. At elevated temperatures in the freestanding structures (once the mold has been separated from the imprint), there is an accelerated reduction in pattern height in the reverse direction from which the material originally flowed into the mold. Factors that influence this residual stress include the relative molecular mass or viscosity of the resist and the amount of time the pattern is annealed at high temperature in the presence of the mold.
Technique to automatically measure electron-beam diameter and astigmatism:BEAMETR
Authors: Babin S. , Gaevski M. , Joy D. , Machin M. , Martynov A.
Published: December 2006

Accuracy and resolution of scanning electron microscopy and an electron-beam lithography system directly depend on beam diameter; it should be monitored and tuned frequently. A technique is described to determine beam size using an automatic procedure. In the developed method, a specially designed and fabricated test pattern is scanned using an e-beam. A spectrum of the signal is analyzed; beam diameter is automatically determined using a software program. Results of design, fabrication, and analysis of the beam calibration test pattern are presented.
True three-dimensional proximity effect correction in electron-beam lithography
Authors: Anbumony Kasi , Lee S.-Y.
Published: December 2006

Proximity effect in e-beam lithography is mainly due to the “nonideal” distribution of exposure (energy deposited in the resist). The proximity effect correction schemes developed so far employ a two-dimensional (2D) model, i.e., exposure variation along the resist depth dimension is not considered. The exposure distribution estimated by the 2D model can be significantly different from the actual exposure distribution, especially for the nanoscale patterns. In this article, a three-dimensional (3D) correction method which uses a 3D point spread function in controlling e-beam dose distribution within each circuit feature in order to achieve a certain desired 3D remaining resist profile after development is described. The dose to be given to each region of a feature is determined based on the estimated remaining resist profile (with the emphasis on the sidewall shape) through iterations. Simulation results demonstrating the potential improvements by the 3D correction are provided.
Thermal imprint with negligibly low residual layer
Authors: Bogdanski Nicolas , Wissen Matthias , Möllenbeck Saskia , Scheer Hella-Christin
Published: December 2006

Thermal imprint into polymer layers that are thin compared to the pattern height allows for imprints with extremely low and uniform residual layers where lift-off works without previous dry etching. The authors report about possible restrictions of this method such as unintended self-assembling and recovery of polymer underneath the imprinted structures. Both can be attributed to structure size properties and temperature influence and hinder a subsequent successful lift-off. Taking the example of two different temperatures and two different initial layer thicknesses the authors investigate their impact on the resulting structure shape and potential defects. In addition, to qualitatively estimate the residual layer thickness the authors apply lift-off and discuss the results with focus on the usability of this approach as a lithography technique.
Electrostatic chucking for extreme ultraviolet lithography: Simulations and experiments
Authors: Nataraju M. , Sohn J. , Veeraraghavan S. , Mikkelson A. R. , Turner K. T. , Engelstad R. L. , Van Peski C. K. , Orvek K. J.
Published: December 2006

The purpose of this research is to assess the effectiveness of electrostatic chucks in reducing low-spatial frequency mask (or reticle) flatness variations and to validate finite element (FE) models of the chuck-mask interaction. The flatness of a sample extreme ultraviolet lithography reticle and an electrostatic pin chuck were measured using a Zygo interferometer. The measured flatness data were entered into the FE models, and electrostatic chucking was simulated by applying an area-weighted average pressure on the reticle. The shape of the mask when clamped by the electrostatic chuck was then predicted using the FE model. To validate these predictions, experiments were conducted in which the previously measured reticle was electrostatically clamped using the pin chuck. These experiments were conducted in a vacuum chamber to minimize the effects of humidity. Interferometric plots of the chucked reticle surface were obtained and compared with the FE predictions. It was found that the measured and predicted shapes were in good agreement. Results from the experiments and simulations will be used to establish specifications for the chuck design and to identify the range of flatness variations that can be accommodated with electrostatic chucking.
Pattern noise in electron beam resists: PMMA, KRS-XE, TOK, HSQ
Authors: Miller Marshal A. , Poppe Wojtek J. , Neureuther Andrew R. , Liddle Alex , Harteneck Bruce
Published: December 2006

The variation in the printing of nominally identical contacts with electron-beam exposure is used to quantitatively determine the statistical variation in chemically amplified resists (KRS-XE with and without top coat, TOK) and non-chemically-amplified resists (PMMA and HSQ). Uniform 17×23 arrays of 24 and 32nm contacts were exposed at fixed doses with a 100keV electron beam. By looking at data observed from top view scanning electron microscopy images, a normal distribution was fitted to the fraction of contacts that printed versus dose to determine the standard deviation of the distribution relative to the dose at which 50% of the contacts printed. The top coat on KRS-XE increased contact uniformity and reduced the required dose. Quantitative analysis shows that PMMA contained as much noise as the chemically amplified resist systems, KRS-XE and TOK. Except HSQ, this normalized standard deviation ranged from 0.16 to 0.21 which is indicative that the contact hole printing process may be dominated by less than 40 events. HSQ exhibited lower standard deviation values, corresponding to over 1000 effective events.
Nanometer-precision pattern registration for scanning-probe lithographies using interferometric-spatial-phase imaging
Authors: Moon Euclid E. , Smith Henry I.
Published: December 2006

The authors propose a solution to drift and disturbances between a scanning-probe tip and a substrate that commonly distort scanned images and undermine effective lithographic patterning. An interferometric position detection method is employed to continuously suppress drift and control the tip-scanning trajectory with nanometer precision, relative to the substrate. An associated interferometric method is used to control tip height during approach to the substrate. Patterns with arbitrary geometries are written by means of a tap-imprint method, using probes with sub-0.7-nm tip diameters.
Reproduction of the Morpho blue by nanocasting lithography
Authors: Saito Akira , Miyamura Yusuke , Nakajima Masaki , Ishikawa Yoko , Sogo Kenji , Kuwahara Yuji , Hirai Yoshihiko
Published: December 2006

The Morpho butterfly’s blue, which is a specific color produced by nanostructures, is expected to serve to various industrial applications such as posters, displays, cosmetics, etc. The natural Morpho blue has recently been emulated successfully by an artificial stepped nanostructure, but the fabrication of the nanostructure is too expensive using conventional lithography method. To solve the problem, nanocasting lithography (NCL) is newly applied using UV curable polymer to replicate the stepped nanostructure and improve heat resistance for the following deposition process. After fabrication of the stepped polymer structure by NCL, TiO2 and SiO2 layers are deposited using the vacuum electron beam deposition and the Morpho blue structure is efficiently reproduced at low cost. The reflective characteristic of the reproduced structure is almost same as the natural Morpho blue.
Contribution of photoacid generator to material roughness
Authors: Fedynyshyn Theodore H. , Pottebaum Indira , Astolfi David K. , Cabral Alberto , Roberts Jeanette , Meagley Robert
Published: December 2006

The authors have developed an atomic-force-microscopy-based technique to measure intrinsic material roughness after base development. This method involves performing an interrupted development of the resist film and measuring the resulting film roughness after a certain fixed film loss. Employing this technique, the authors previously established that the photoacid generator (PAG) is a major material contributor of film roughness and that PAG segregation in the resist is likely responsible for nanoscale dissolution inhomogeneities. The additional roughness imparted on a test polymer by incorporation of a series of iodonium, sulfonium, diazo, and imido PAGs was measured. The roughness was then correlated to the inhibition properties of the various PAGs. This was accomplished both through a NMR technique that measures interaction of the PAG with the polymer and by evaluating the dissolution inhibition properties of the PAG through a percolation model. Several PAGs that result in significantly lower material roughness and thus the potential for significantly reduced linewidth roughness in resist imaging have been identified.
Scanning x-ray microscopy investigations into the electron-beam exposure mechanism of hydrogen silsesquioxane resists
Authors: Olynick Deirdre L. , Liddle J. Alexander , Tivanski Alexei V. , Gilles Mary K. , Tyliszczak Tolek , Salmassi Farhad , Liang Kathy , Leone Stephen R.
Published: December 2006

Electron-beam exposed hydrogen silsesquioxane cross-linking chemistry is investigated by scanning transmission x-ray microscopy (STXM) and atomic force microscopy (AFM). Using STXM, a maximum in the chemical contrast is obtained by measuring the x-ray absorption at 535.4eV, corresponding to the 1sK-edge transition in oxygen. An area-dependent and dose-dependent chemical conversion is observed for feature sizes between 150nm and 10μm and doses between 0.4 and 40mC∕cm2. The activated (cross-linked) regions extend beyond the exposure zones, especially for higher dosed exposures. With AFM, thickness changes in the latent images (e-beam exposed but undeveloped) are observed, which also display a dependence on exposed area. Potential mechanisms, involving chemical diffusion outside the exposure zone, are discussed.
Low-resistance Ohmic contacts developed on undoped AlGaN∕GaN-based high electron mobility transistors with AlN interlayer
Authors: Sun Yunju , Eastman L. F.
Published: December 2006

A low-resistance Ohmic contact on undoped GaN∕AlGaN∕AlN (10Å)∕GaN high electron mobility transistors is first demonstrated using a Ta∕Ti∕Al∕Mo∕Au metallization scheme. A contact resistance of 0.16±0.03Ωmm is achieved by rapid thermal annealing of the evaporated contact at 700°C for 1min followed by 800°C for 30s in a N2 ambient. Excellent edge acuity is also demonstrated for this annealed Ta∕Ti∕Al∕Mo∕Au Ohmic contact.
Technical note: A deformable phantom for dynamic modeling in radiation therapy
Authors: Kashani Rojano , Lam Kwok , Litzenberg Dale , Balter James
Published: January 2007

A deformable phantom was developed to aid in quality assurance for dynamic imaging and targeting techniques in radiation therapy. Made of simple materials combined with standard components for imaging and motion experiments, this phantom can be relatively easily constructed and used for both diagnostic imaging and dosimetry. Repeat imaging studies indicate that the phantom meets criteria of relative attenuation, deformation, and reproducibility of configuration, necessary for quality assurance of radiographic and tomographic targeting.
Mosaic Tetracycline Resistance Genes Are Widespread in Human and Animal Fecal Samples
Authors: Patterson Andrea J. , Rincon Marco T. , Flint Harry J. , Scott Karen P.
Published: December 2006

Mosaic tetracycline resistance genes comprising
tet
(O),
tet
(W), and
tet
(32) sequences were abundant in DNA extracted from pig and human fecal samples, accounting for 78% (50/64) and 46% (37/80) of genes amplified with a
tet
(O) primer set, respectively, in two samples. The nonmosaic
tet
(32) gene was isolated from a human saliva bacterium.