bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical metrology: At the root of manufacturing control
Authors: Bartelink Dirk J.
Published: July 2002

Competitive pressures in the integrated circuit (IC) industry are now requiring short time-to-market product introductions. With the most successful of these products ultimately reverting to commodity status, it is also becoming important to be able to convert from flexible manufacturing within a facility set up for engineering assisted processing to low-cost volume production as experience is gained. Computer-based design for manufacturability (DFM) of the IC process is the only effective way to meet this challenge, but today the unavailability of detailed information about process tool margins is preventing true DFM of process technology from becoming a reality. Statistical metrology is a method consisting of a short-loop process flows and parametric electrical test that can supply the margin performance data of specific process tools needed for DFM. Test structures are designed with large numbers of test sites and with a variety of sensitivities to the process variations. Each structure uses the minimum tool set and process flow necessary for electrically measurable parameters so as to ease the inversion of statistically confounded data. By utilizing technology computer-aided design (TCAD) tools to simulate the short process flow and by assigning statistical distributions to individual steps, a best fit to the data provides useful calibration data of key TCAD parameters. These parameters can then be used to simulate the process margin of a full process and, through DFM, make the process optimally robust for manufacturing.
Reflectance anisotropy of reconstructed GaAs(001) surfaces
Authors: Morris S. J. , Bass J. M. , Matthai C. C. , Milman V. , Payne M. C.
Published: July 2002

We have performed ab initio pseudopotential calculations of three alternative structures of the GaAs(001) surface to examine the dependence of the reflectance anisotropy spectrum (RAS) upon the precise surface configuration. Spectra were calculated based upon a hypothetical (2×1) surface, a (2×4)-β surface using experimentally measured atom positions, and a (2×4)-β surface using atom positions given by total-energy minimization. It was found that the RA spectra depended significantly upon the atom positions, and in particular, that proper surface relaxation, including the Ga layer below the surface, was necessary in order to account for the low-energy (2.5 eV) feature in the experimentally observed spectrum.
Atomic force microscopy studies of contact-electrified charges on silicon oxide film
Authors: Sugawara Y. , Fukano Y. , Uchihashi T. , Okusako T. , Morita S. , Yamanishi Y. , Oasa T. , Okada T.
Published: July 2002

Microscopic contact-electrified charges on a silicon oxide layer formed on a p-type Si substrate were investigated in air using an atomic force microscope with a biased conductive cantilever. We could successfully image the dissipation of the contact-electrified charges after a single contact. We found that the sign of the contact-electrified charges is reproducible and controllable by the polarity of the bias voltage. We also found that negative charges are more easily deposited on the silicon oxide surface than the positive charges. Furthermore, we found that the charge dissipation as a function of the time after contact electrification takes two steps for the negative charges. At the first step the charge dissipation is nearly same for both the repulsive and the attractive force measurements, while at the second step the dissipation for the attractive force measurement is much faster than that for the repulsive force measurement.
Evaluation of surface damage on GaAs etched with an electron cyclotron resonance source
Authors: Ko K. K. , Pang S. W. , Brock T. , Cole M. W. , Casas L. M.
Published: July 2002

Surface damage induced by dry etching on GaAs with an electron cyclotron resonance source has been studied using both electrical measurements and surface analysis techniques. It is found that the unalloyed contact resistance extracted from the transmission lines is very sensitive to the etch-induced damage, and it increases significantly with ion energy and ion flux but decreases with etch temperature. Conducting wires that are 40 to 1000 nm wide have been etched down to 1.3 μm deep with vertical profile and smooth surface morphology. The extracted sidewall damage depth of these wires ranges from 2.7 to 20.4 nm and it increases with ion energy and ion flux but decreases with etch temperature. At 200 W rf power, the sidewall damage depth decreases from 13.1 to 4.0 nm after removing 10 nm of the etched surface using low-energy reactive chlorine species. Results from cross-sectional transmission electron microscopy show a higher defect density and a shallower defect depth for samples etched with higher rf power. A lower defect density and a deeper defect depth are found at higher etch temperature. Auger electron spectroscopy shows that after etching with the Cl2/Ar plasma, the stoichiometry is not changed even at high microwave power or high etch temperature.
Low-voltage electron beam lithography on GaAs substrates for quantum wire fabrication
Authors: Steffen R. , Faller F. , Forchel A.
Published: July 2002

We have studied the benefits of low-voltage electron beam lithography (EBL) in polymethyl methacrylate (PMMA) positive electron beam resist for the fabrication of quantum wire etch masks on GaAs substrates. In order to determine the reduction of electron scattering effects for low-voltage EBL, we have compared point exposures in 100-nm-thick PMMA for energies between 2.3 and 15 keV. The exposure distribution functions were fitted by combinations of Gaussians and exponentials, showing significant energy-dependent deviations from the widely used double-Gaussian function. The results indicate that on GaAs substrates, backscattering at 2.3 keV is reduced by about two orders of magnitude compared to 15 keV exposures. InGaAs/GaAs quantum wires have been fabricated using low-voltage EBL, followed by an aluminum-liftoff step and wet-chemical etching. In sharp contrast to wires delineated at 25 keV, the resulting structures show no width variations at the field edges caused by proximity effects. The photoluminescence spectra show a wire width dependent blue shift up to 8 meV for 18-nm-wide structures, caused by lateral quantization.
Effects of substrate temperature and bias potential on hydrogen plasma etching of silicon
Authors: Ishii Masahiko , Nakashima Kenji , Hayakawa Tetsuo , Tajima Ichiro , Yamamoto Minoru
Published: July 2002

Si masked by patterned SiO2 was etched using an electron cyclotron resonance hydrogen plasma. The dependence of the etch rate and the etched profile on substrate temperature and dc bias potential has been investigated, and an etch mechanism has been considered. The substrate temperature was varied between room temperature and 400 °C, and a bias potential between −100 and +100 V was applied. The etch rate of Si had the highest value at the lowest substrate temperature and decreased with increasing temperature. For the bias potential, the etch rate of Si had its peak value at 0 V. It decreased with increase of the potential which was either positive or negative. Si was also etched laterally when the substrate temperature was low. For the negative bias potential, the etched Si had a terrace on the border of a SiO2 mask. It is considered that the temperature dependence of the etch rate and the etched profile is caused by the action of hydrogen atoms, and the bias potential dependence is caused by the action of hydrogen ions. Thus, we conclude that both hydrogen ions with moderate energy and hydrogen atoms play an important role in Si surface etching.
Investigation of spontaneous ordering in GaInP using reflectance difference spectroscopy
Authors: Luo J. S. , Olson J. M. , Bertness K. A. , Raikh M. E. , Tsiper E. V.
Published: July 2002

Reflectance difference spectroscopy (RDS) is applied to the study of optical anisotropy in spontaneously ordered GaInP grown by metalorganic chemical-vapor deposition. The degree of ordering in GaInP has been associated previously with a shift of the band-gap energy ΔE0, and a crystal-field valence-band splitting ΔC. Theoretically and experimentally, both quantities are approximately proportional to the square of the order parameter, which varies from 0 to 1 for disordered and perfectly ordered GaInP, respectively. In this study, we examined a number of GaInP layers grown under conditions that yield a wide range of band-gap energies. The main spectral feature in all samples is a bulk-related, asymmetric peak at E0 with a long tail that extends well below E0 and a sharp, high-energy cutoff at E0+ΔC. The intensity of this peak is proportional to √ΔE0 and is therefore linear with the order parameter. By annealing GaInP in PH3/H2 mixtures, we find that the RD spectral features for energies between E0+ΔC and 3 eV are mainly surface induced. Evidence for a bulk-related RDS peak at E1 is also found.
Influence of GaAs(001) substrate misorientation towards {111} on the optical properties of InxGa1−xAs/GaAs
Authors: Rich D. H. , Rammohan K. , Tang Y. , Lin H. T. , Goldman R. S. , Wieder H. H. , Kavanagh K. L.
Published: July 2002

Local variations in the optical properties of thick In0.13Ga0.87As films grown on GaAs(001) substrates misoriented toward {111} planes have been studied with polarized and spectrally-resolved cathodoluminescence (CL) imaging. The degree of anisotropic relaxation and density of dark line defects (DLDs) in CL was found to depend on the choice of the substrate miscut orientation. An enhanced anisotropy in DLD density and strain relaxation was found for a misorientation towards (111)A relative to that for a misorientation towards (111)B. Local variations and spatial correlations in polarization anisotropy, band-gap energy shifts, luminescence efficiency, and defect-induced long-wavelength luminescence were examined.
Homoepitaxy and controlled oxidation of silicon at low temperatures using low-energy ion beams
Authors: Al-Bayati A. H. , Todorov S. S. , Boyd K. J. , Marton D. , Rabalais J. W. , Kulik J.
Published: July 2002

Homoepitaxy and controlled oxidation of silicon at low temperatures have been achieved using a dual source, mass-selected, low-energy, ion beam deposition system. For Si homoepitaxy, 28Si+ ions in the energy range 8–40 eV were used to grow films on Si{100} in the temperature range of 50–750 °C. The films were analyzed in situ by reflection high-energy electron diffraction and Auger electron spectroscopy (AES) and ex situ by high-resolution transmission electron microscopy, Rutherford backscattering spectrometry, and atomic force microscopy. For silicon oxidation, films of SiO2 on Si{100} at room temperature were grown by using 25 eV beams of 28Si+ and 16O+. Fast switching of the magnetic sectors allows deposition of these ions in alternating pulses. The pulse increments used were 1×1014 cm −2 for Si+ and 4×1014 cm−2 for O+. Analysis of the oxide films by in situ AES and ex situ x-ray photoelectron spectroscopy show that the films are SiO2, that the suboxides are localized at the interface, and that there is no limitation to the thickness of the oxide films that can be grown. The effects of ion energy and substrate temperature, contamination, and surface damage on the growth mechanism are discussed.
Optical probe microscope for nondestructive metrology of large sample surfaces
Authors: Marchman H. M. , Griffith J. E. , Trautman J. K.
Published: July 2002

We describe the design and implementation of a tip-scanning omnidirectional force-sensing probe microscope. Chemically etched optical-fiber probes are employed for nanometer resolution imaging of structures on large samples, such as lithographic masks and semiconductor wafers. The use of light-emitting optical-fiber probes has had significant implications regarding the important issues of probe shape and fine scanner nonlinearity in scanned probe metrology. The fiber probe shapes are especially suited for metrology of high-aspect-ratio features often encountered in integrated circuit fabrication. Position monitoring, to avoid fine scanner nonlinearities, can be achieved without Abbé offset errors because the optical signal used to detect the tip motion is emitted directly from the apex of the fiber probe. The force-sensing technique in this system also allows detection of lateral forces from sidewalls in addition to shear forces for tip-to-sample regulation. The optical probe microscope system has an intrinsically higher throughput capability than other types of probe microscopes because feature imaging and probe positioning are done optically so that only the line profile measurement is done with the probe. This system should also allow future acquisition of spatially resolved optical properties of the sample in addition to pure topography.
Initial results for InGaAs films grown on InGaAs substrates
Authors: Hoke W. E. , Lyman P. S. , Carter J. R. , Hendriks H. T. , Bonner W. A. , Lent B.
Published: July 2002

InGaAs films have been deposited on semi-insulating InxGa1−xAs (x=0.04–0.05) substrates with improved material properties compared to similar InGaAs films grown on GaAs substrates. For near lattice matched conditions the films grown on InGaAs substrates have a smooth surface morphology compared to a dislocation-induced cross hatch morphology on GaAs substrates. The resulting film double crystal x-ray linewidths are considerably narrower. The InGaAs film photoluminescence intensity is stronger with a narrower x-ray linewidth due to the elimination of lattice mismatch dislocations. Also the Hall mobilities are higher for the films grown on InGaAs substrates.
Ellipsometry study of the nucleation of Si epitaxy by electron cyclotron resonance plasma chemical vapor deposition
Authors: Li M. , Hu Y. Z. , Irene E. A. , Liu L. , Christensen K. N. , Maher D. M.
Published: July 2002

The formation and evolution of Si nuclei on the Si(100) surface at 600 and 700 °C were observed in a microwave electron cyclotron resonance plasma chemical vapor deposition system, using both real-time in situ single wavelength and spectroscopic ellipsometry combined with high-resolution cross-sectional transmission electron microscopy and secondary ion mass spectroscopy. The deposited Si layers are epitaxial, and decidedly different nucleation behavior is seen at 600 and 700 °C. The experimental ellipsometry results were compared with simulations and the results show that temperature has a profound effect on the initial nucleation and growth, and the different interface structures that are observed are attributable to impurities.
Improved emission stability of carburized HfC〈100〉 and ultrasharp tungsten field emitters
Authors: Yu Ming L. , Hussey Brian W. , Kratschmer Ernst , Chang T. H. Philip , Mackie William A.
Published: July 2002

We have evaluated the cold-field-emission characteristics of HfC〈100〉 and ultrasharp tungsten emitters. We found that proper acetylene treatment improved both the angular current confinement and the emission stability of thermally cleaned HfC〈100〉 tips. Stable emission exceeding 10 μA/sr for over 1 h and angular confinement to a 3° semicone angle have been observed. The improvements are probably related to the modified work function and surface chemical composition induced by the acetylene treatment at the tip apex. Carburization of W〈100〉 and W〈111〉 tips also significantly improved the emission current stability. This study indicates the usefulness of surface processing in the development of cold-field emitters.
A focused ion beam secondary ion mass spectroscopy system
Authors: Crow G. A. , Christman Locke , Utlaut M.
Published: July 2002

This article describes a Ga+ focused ion beam secondary ion mass spectroscopy system, and measures several quantities of interest to aid in interpreting secondary ion mass spectroscopy results. We have measured sputter yields and rates, estimated the instrument efficiency, and calculated useful yields and practical sensitivities for a variety of elements used in the semiconductor industry. We have performed measurements at the system base pressure, and have also introduced oxygen and iodine to determine any enhancement effects.
Fabrication issues for the prototype National Institute of Standards and Technology SRM 2090A scanning electron microscope magnification calibration standard
Authors: Newell Brian L. , Postek Michael T. , van der Ziel Jan P.
Published: July 2002

A new National Institute of Standards and Technology scanning electron microscope magnification calibration standard has been fabricated and distributed in production prototype form. The SRM 2090A samples contain structures ranging in pitch from 3000 to 0.2 μm and are useful at both high- and low-accelerating voltages. The design and fabrication of the samples has incorporated many of the improvements suggested through two previous prototype series. This article discusses optimization of the lithographic techniques in the fabrication process. We used electron beam lithography for fabrication because of the 0.1 μm features required on the samples. After liftoff, unexpected linewidth differences were measured among the smallest features. These differences were reduced through modification of the electron beam writing paths. We propose that the resist heating effect caused an axial dependence among features fabricated using the original electron beam writing paths. In addition, the susceptibility of the present prototype design to electron beam proximity effects was analyzed. Proximity effects from the 9 μm range of backscattered electrons have little effect on the pitch structures of the prototype but may impact development of the prototype as a linewidth standard.
Surface investigations by scanning thermal microscopy
Authors: Stopka M. , Hadjiiski L. , Oesterschulze E. , Kassing R.
Published: July 2002

A scanning thermal microscope has been developed which is capable of imaging thermal properties of materials with high spatial resolution. First results indicate a lateral resolution less than 200 nm. The microscope employs a miniaturized thermal probe whose tip is formed as a thermocouple. The probe is laser heated to generate a thermovoltage. A sample approaching the heated tip leads to a heat flow from the tip to the cooler sample surface and thus to a decrease of the measured voltage. In the initial experiments we scanned the tip above the sample surface with open feedback loop and mapped the thermovoltage at each location of the scan range. Furthermore, we closed the feedback loop keeping the thermovoltage constant and measured the z displacement of the piezoelectric tube carrying the probe. All these measurements yield topographical as well as thermal information of the sample surface.
Si nanostructures fabricated by electron beam lithography combined with image reversal process using electron cyclotron resonance plasma oxidation
Authors: Kurihara K. , Iwadate K. , Namatsu H. , Nagase M. , Murase K.
Published: July 2002

A new image reversal process has been developed for Si nanodevice fabrication that uses electron beam lithography and electron cyclotron resonance (ECR) plasma techniques. This process is based on Si oxidation with an ECR oxygen plasma through the openings in resist mask patterns. Si on SiO2 is selectively etched by either Cl2-based ECR plasma etching or KOH anisotropic etching by using a plasma oxide mask. ECR plasma formed silicon oxide with a thickness of 2–3 nm was found to be an excellent etch mask for these etching techniques. Highly directional ECR oxygen plasma keeps the change in the resist linewidth and edge roughness small enough for nanofabrication. Furthermore, the linewidth of reversed Si patterns can be reduced by SF6 addition to Cl2 in ECR plasma etching. This image reversal process successfully achieves 10-nm-scale Si wires and pillars.
Fabrication of 0.2 μm large scale integrated circuits using synchrotron radiation x-ray lithography
Authors: Deguchi K. , Miyoshi K. , Ban H. , Matsuda T. , Ohno T. , Kado Y.
Published: July 2002

In the last few years, we have made dramatic improvements in the key components of synchrotron radiation x-ray lithography including a high-brightness and compact synchrotron radiation source, high-accuracy steppers, precise masks, and highly sensitive chemically amplified resists. These advances have been used in the test fabrication of complementary metal–oxide–semiconductor large scale integrated circuits using separation by implanted oxygen technology in the 0.2 μm regime. Synchrotron radiation lithography was used at the active, gate, contact, and four-level metallization levels or at the gate level only. Fabricated gate array large scale integrated circuits, including a 48×48 bit multiplier, have excellent and fully functional characteristics. This article describes lithographic performance under device fabrication conditions, demonstrating an excellent critical dimension control of 0.02 μm (3σ), an overall overlay better than 0.15 μm (3σ) (including metallization levels), and a throughput of 12 wafers (6 in.) per hour.
Two-dimensional profiling of large tilt angle, low energy boron implanted structure using secondary-ion mass spectrometry
Authors: Cooke G. A. , Pearson P. , Gibbons R. , Dowsett M. G. , Hill C.
Published: July 2002

Experimentally determined two-dimensional dopant maps of implants into semiconductors are required for the calibration and verification of process simulation tools used in very large scale integrated (VLSI) circuit design. Direct measurement with currently available techniques is not possible owing to the physical size of the areas in question. Using a specially fabricated structure and a modified secondary-ion mass spectroscopy instrument, it has been possible to measure profiles with high spatial resolution and sensitivity. In this article we present the results of an investigation of a complex boron implant into silicon, as used in advanced VLSI P-type metal–oxide–semiconductor source–drain regions, and compare it with results from process simulators.
Junction metrology by cross-sectional atomic force microscopy
Authors: Alvis Roger , Mantiply Bryant , Young Michael
Published: July 2002

In this article we address practical aspects of the extension of cross-sectional atomic force microscopy to junction metrology. Examining an arsenic (As+) ion implantation split lot, four specimens were epoxied into a single stack. Thus, each sample was exposed to the same preparation conditions. Using a noncontact mode of image acquisition, we verified that a 120 keV implantation resulted in deeper doping profiles than a 35 keV implant. A shift of the source and drain junctions with respect to the gate was also observed as the angle of implant was changed. Correlating the measured junction depths of each sample with secondary-ion-mass spectroscopy profiles, we determined that the detection limit of the junction etch was in the range of 1017–1018 cm−3. The absolute accuracy of this technique was partially limited by tip interaction with the large topography introduced by etching.
Extendibility of synchrotron radiation lithography to the sub-100 nm region
Authors: Deguchi Kimiyoshi , Miyoshi Kazunori , Oda Masatoshi , Matsuda Tadahito , Ozawa Akira , Yoshihara Hideo
Published: July 2002

This article discusses the resolution of synchrotron radiation lithography in the sub-100 nm region, taking into consideration the mass production of large-scale integrated circuits, under attainable conditions for the x-ray mask, proximity gap, and resist processes. Resolution and exposure latitude for line-and-space patterns are markedly improved by using a mask with a contrast of only 2.5. Resolutions of 90, 80, 70, and 60 nm can be achieved with proximity gaps of 30, 20, 15, and 10 μm if a high-contrast resist and a low-surface tension developer are used. The latitude will be 10% for pattern sizes as small as 70 nm when the proximity gap is narrower than 15 μm. The effects of mask duty [which is defined to be the ratio of the absorber (line) width to the pattern pitch, i.e., duty cycle] on the optimum exposure dose and mask linearity are also evaluated.
Atomic structures of Ag2Te studied by scanning tunneling microscopy
Authors: Ohto M. , Tanaka K.
Published: July 2002

Atomic structures of Ag2Te in the low- and high-temperature phases have been studied using a scanning tunneling microscope in air and an x-ray diffraction system. In the low-temperature phase having a monoclinic lattice, (001) and (01̄0) atomic images are obtained. In the superionic high-temperature phase, which is stable at temperatures above 145 °C, (100) surfaces of the Te cubic lattice are observed. In both phases, surface atomic reconstructions are not detected.
Effect of the Ti/TiN bilayer barrier and its surface treatment on the reliability of a Ti/TiN/AlSiCu/TiN contact metallization
Authors: Ouellet L. , Tremblay Y. , Gagnon G. , Caron M. , Currie J. F. , Gujrathi S. C. , Biberger M.
Published: July 2002

The use of an AlSiCu/TiN bilayer for the metallization of 1.0-μm-diam and 1.4-μm-deep straight wall contacts to 0.2-μm-deep n+ and p+ diffusions, results in a n+/p− and a p+/ṇ− junction leakage lower than 10 pA even after nine heat treatments (60 min each) at 450 °C. However, there is a very important degradation of the contact chain resistance statistics at small contact size. On the other hand, the use of a Ti/TiN bilayer barrier under the AlSiCu/TiN interconnect maintains a n+/p− and a p+/n− junction leakage lower than 20 pA and prevents the degradation of the contact chain resistance statistics after the nine heat treatments. It is finally demonstrated that a vent in nitrogen followed by a momentary air exposure of the Ti/TiN bilayer barrier results in larger contact resistance than a vent in nitrogen followed by a one hour long air exposure of the Ti/TiN bilayer barrier before the deposition of the AlSiCu/TiN interconnect.
Thermal stability of W, WSix, and Ti/Al ohmic contacts to InGaN, InN, and InAlN
Authors: Vartuli C. B. , Pearton S. J. , Abernathy C. R. , MacKenzie J. D. , Shul R. J. , Zolper J. C. , Lovejoy M. L. , Baca A. G. , Hagerott-Crawford M.
Published: July 2002

W, WSi0.44, and Ti/Al contact properties were examined on n+In0.65Ga0.35N, InN, and In0.75Al0.25N. W was found to produce low specific contact resistance (dc∼10−7 Ω cm2) ohmic contacts to InGaN, with significant reaction between metal and semiconductor occurring at 900 °C mainly due to out diffusion of In and N. WSix showed an as-deposited dc of 4×10−7 Ω cm2 but this degraded significantly with subsequent annealing at ≥600 °C. Ti/Al contacts were stable to ∼600 °C (dc∼4×10−7 Ω cm2 at ≤600 °C). The surfaces of these contacts remained smooth to 800 °C for W and WSix and 650 °C for Ti/Al. InN contacted with W and Ti/Al produced ohmic contacts with dc∼10−7 Ω cm2 and for WSixdc∼10−6 Ω cm2. All remained smooth to ∼600 °C, but exhibited significant interdiffusion of In, N, W, and Ti, respectively, at higher temperatures. The contact resistances for all three metalization schemes were ≥10−4 Ω cm2 on InAlN, and degraded with subsequent annealing.
Solid source molecular beam epitaxy of GaInAsP/InP: Growth mechanisms and machine operation
Authors: Wamsley C. C. , Koch M. W. , Wicks G. W.
Published: July 2002

This article reports on the growth of GaInAsP/InP by solid source molecular beam epitaxy the incorporation efficiencies of the group V dimer and tetramer molecular species, and the functioning of the MBE system. The incorporation efficiency of As4 is found to be close to that of As2 while the incorporation efficiency of P4 is only half that of P2. Room temperature and low temperature photoluminescence measurements demonstrate that material grown using As4 is of better quality than that grown using As2. After the use of P4, a tenfold increase in the pressure burst occurs upon warming of the liquid nitrogen shrouds, and is believed to be caused by the formation of deposits containing white phosphorus. The atomic As to P incorporation ratio is approximately 10, when using P2, and exhibits a substrate temperature dependence.
Phase instability of n-CdS grown by molecular-beam epitaxy
Authors: Yamada T. , Setiagung C. , Jia A. W. , Kobayashi M. , Yoshikawa A.
Published: July 2002

The metastable nature of molecular-beam epitaxy grown CdS layers was studied. Undoped CdS layers were grown on (100) GaAs substrates/(100) ZnSe buffer layers, and crystal structures were characterized by x-ray diffraction and transmission electron microscopy. Undoped films showed both zincblende and wurtzite structures depending on the growth condition; higher substrate temperatures and smaller VI/II beam intensity ratios preferentially form wurtzite structures. The Ga doping significantly affected the crystal structure and Ga doped CdS layers preferentially formed wurtzite structures. Thermal instability of the CdS film was observed through various ex situ annealing experiments. The wurtzite structure phase was developed from the zincblende structure CdS layers by thermal annealing where the annealing temperature was lower than the growth temperature.
Surface segregation of arsenic and phosphorus from buried layers during Si molecular beam epitaxy
Authors: Hobart K. D. , Kub F. J. , Jernigan G. G. , Thompson P. E.
Published: July 2002

Unintentional doping of silicon films grown on heavily doped buried layers of arsenic and phosphorus during silicon molecular beam epitaxy has been studied by spreading resistance profiling and x-ray photoelectron spectroscopy. The measurements indicate that dopant surface segregation during growth is responsible for the unintentional doping. The segregation phenomenon has been characterized over a growth temperature range of 450–750 °C. The growth on heavily doped buried layers has implications for device applications such as Si/SiGe heterojunction bipolar transistors. Epitaxial processes to circumvent this effect and create abrupt doping profiles are explored.
Fabrication of silicon field emitters by forming porous silicon
Authors: Kim Donghwan , Kwon Sang Jik , Lee Jong Duk
Published: July 2002

A new method to fabricate silicon field emitter array (FEA) by forming porous silicon is proposed. Silicon FEAs have been successfully fabricated on n-type and p-type substrates using this method. In this method, porous silicon is formed around the masking discs in HF solution to make tips. Then the porous silicon is thermally oxidized to form the insulator between the gate metal and the silicon substrate. The final gate hole diameter is nearly equal to the masking disc diameter, and the gate leakage current is less than 0.5% of the anode current. For a silicon FEA made on an n-type substrate with a gate hole diameter of 1.4 μm and for a silicon FEA made on a p-type substrate with a gate hole diameter of 1.8 μm, an emission current of 1.1 μA per tip and 1.0 nA per tip is obtained at a gate voltage of 100 V, respectively.
Ar, N2, and Cl2 electron cyclotron resonance plasmas measured by time-of-flight analysis: Neutral kinetic energies and source gas cracking
Authors: Goodman Rory S. , Materer N. , Leone Stephen R.
Published: July 2002

Neutral mean kinetic energies, ion intensities, and neutral source gas cracking from Ar, N2, and Cl2 electron cyclotron resonance (ECR) plasmas, are measured by modulated beam time-of-flight (TOF) analysis. The TOF distributions are characterized by a two component form consisting of an effusive Maxwell–Boltzmann distribution and a fast Gaussian component, that accounts for nonthermal species produced in the source. The mean kinetic energies of neutral species are found to range between 0.04 and 0.45 eV, depending on species and plasma conditions. Mean kinetic energies increase at a nearly constant rate, with decreasing pressure from 8.0×10−2 to 2.5×10−2 Pa with constant applied microwave power. At pressures below 2.5×10−2 Pa, the neutral mean kinetic energies sharply increase. This sharp increase in neutral mean kinetic energy is attributed to an abrupt increase in the ion flux out of the source. The increase in kinetic energy can be separated into two contributions, (i) thermal at higher pressures and (ii) nonthermal at lower pressures. This effect is much stronger for atomic neutrals than for molecular neutrals, where internal degrees of freedom can accept energy in momentum transfer collisions. Cracking of N2 and Cl2 is also examined as a function of source pressure at constant microwave power. The N:N2 flux ratio from the ECR source varies between 0.2 and 1.4. The Cl:Cl2 flux ratio varies from 10 to 16, indicating a very high degree of dissociation in the plasma. Both flux ratios decrease with increasing source pressure. The total flux of Cl increases with increasing source pressure over the entire range, while the N atom flux peaks at ∼1.3×10−2 Pa and decreases on either side of this pressure.
T-top forming simulation using percolation theory
Authors: Kamon K. , Nakazawa K. , Yamaguchi A. , Matsuzawa N. , Ohfuji T. , Kanzaki K. , Tagawa S.
Published: July 2002

In KrF or ArF resist processing, a chemically amplified resist is widely used for ultralarge scale integrated device fabrication. Decomposition (positive resist) or cross linking (negative resist) is amplified by an acid catalytic reaction during post-exposure baking (PEB). T-top forming becomes a serious problem in these resists. In resist simulation, to take these characteristics into account, percolation theory is introduced. The acid and product distributions during PEB are iteratively calculated. Thus, we can conclude that the acid and product distribution in resist films are time dependent. Moreover, a resist simulator that can take into account macroscopic feature changes from microscopic molecular structural change is necessary. From resist surface observation and slow positron annihilation measurements, free volume generation is confirmed. A new resist process model, including prebake, PEB, and development for chemically amplified resists is established by the cluster model. CPU time is 1 min each for a three dimensional image and for development, which is fast enough for practical evaluation use. The defocus dependence of the resist profile agrees well with the experiment. For chemically amplified resists, decomposition or cross linking proceeds vertically rather than horizontally. Thus, a rectangular resist profile can be obtained. Simulation results based upon this model can describe T-tops or resist bridges.
Sub-10 nm imprint lithography and applications
Authors: Chou Stephen Y. , Krauss Peter R. , Zhang Wei , Guo Lingjie , Zhuang Lei
Published: July 2002

New developments, further details, and applications of imprint lithography are presented. Arrays of 10 nm diameter and 40 nm period holes were imprinted not only in polymethylmethacrylate (PMMA) on silicon, but also in PMMA on gold substrates. The smallest hole diameter imprinted in PMMA is 6 nm. All the PMMA patterns were transferred to a metal using a liftoff. In addition, PMMA mesa’s of a size from 45 nm to 50 μm were obtained in a single imprint. Moreover, imprint lithography was used to fabricate the silicon quantum dot, wire, and ring transistors, which showed the same behavior as those fabricated using electron (e)-beam lithography. Finally, imprint lithography was used to fabricate nanocompact disks with 10 nm features and 400 Gbits/in.2 data density—near three orders of magnitude higher than current critical dimensions (CDs). A silicon scanning probe was used to read back the data successfully. The study of wear indicates that due to the ultrasmall force in tapping mode, both the nano-CD and the scanning probe will not show noticeable wear after a large number of scans.
Nucleation and growth of GaN layers on GaAs, Si, and SiC substrates
Authors: Ploog Klaus H. , Brandt Oliver , Yang Hui , Yang Bin , Trampert Achim
Published: July 2002

We summarize our results on plasma-assisted molecular beam epitaxy of cubic GaN on GaAs(001) and on Si(001) and of hexagonal GaN on 6H-SiC(0001) with emphasis on the nucleation process. A two-step growth sequence must be used to optimize and control the nucleation and the subsequent growth independently. While a perfect epitaxial orientation exists for GaN-on-GaAs due to the coincidence lattice relationship of the two constituents. The same effect is impeded for GaN-on-Si by the growth of SixNy inclusions at the interface which act as nucleation cores for the formation of the hexagonal GaN phase. A suitable template, such as a thin GaAs or SiC insertion layer, avoids formation of the SixNy inclusions. Finally, growth of hexagonal GaN-on-6H-SiC without any buffer layer requires very careful adjustment of the N-to-Ga flux ratio and the substrate temperature, independently for the nucleation stage and for the subsequent layer-by-layer growth. The structural perfection and the optical properties of the resulting 1 μm thick GaN films then reach state-of-the-art quality even without a buffer template.
Diffusion of copper into polyimide deposited by ionized cluster beam
Authors: Kim N. Y. , Yoon H.-S. , Kim S. Y. , Whang C. N. , Kim K. W. , Cho S. J.
Published: July 2002

Copper was deposited by sputtering on two types of polyimide (PI) films, one is fabricated by ionized cluster beam deposition (ICBD) and the other is Kapton, commercially available from DuPont. Diffusion properties of copper into these two polyimide films were investigated by x-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy. In addition, the packing densities of these PI films were measured by a rotational analyzer type ellipsometer. We found that PI films deposited by ICBD (ICB-PI) have a higher packing density than Kapton. We also found that less copper is diffused into ICB-PI than into Kapton, that is, PI by the ICB method blocks Cu diffusion into PI. The diffusion coefficient of Cu/ICB-PI is 2.45×10−17–2.8×10−15 cm2/s at 300–390 °C, the activation energy of Cu diffusion into ICB-PI is 1.75 eV and D0 for ICB-PI is 1.92×10−1 cm2/s. The Cu atoms diffuse into PI as a form of clusters.
Depth control of focused ion-beam milling from a numerical model of the sputter process
Authors: Vasile Michael J. , Xie Jushan , Nassar Raja
Published: July 2002

A mathematical model of focused ion-beam milling is used to generate dwell times for the vector scanned pixel address scheme of a focused ion-beam deflection system. The model incorporates the absolute sputter yield of the solid as a function of the angle of incidence, and the relationship between the ion-beam current distribution and the pixel size of the deflection pattern. The object of this work is to be able to call for an arbitrary geometric shape to be ion milled and then have the numerical model compute the pixel dwell times for the deflection system such that the final cavity is sputtered. Experimental verification of the procedure was accomplished with parabolic troughs, hemispherical troughs, and cosine troughs. The term “trough” means a plane of symmetry in the ion-milled cavity. These same geometric shapes were also ion milled using a rotational axis of symmetry, yielding sinusoidal ring patterns, parabolic dishes, and hemispherical dishes. The absolute maximum depth for each of the cavities is consistently between 0.8 and 0.9 of the programed (a priori) depth. The discrepancy between the programed depth and the observed depth is attributed to a slowly decreasing ion-beam current over the duration of the milling and to redeposition.
Modeling of edge roughness in ion projection lithography
Authors: Henke Wolfgang , Torkler Michael
Published: July 2002

To investigate causes and cures for resist profile edge roughness in ion projection lithography a model for exposure and development of chemically amplified resist exposed with He+ or H+ ions is described. The APEX-E resist system was chosen as a paradigm system. Predominant factors increasing line edge roughness are image blur and low exposure dose. Both effects result in formation of ion clusters in nominally unexposed regions close to feature edges. These clusters can lead to statistically distributed development paths and consequently to line edge roughness.
Minimum ion-beam exposure-dose determination for chemically amplified resist from printed dot matrices
Authors: Bruenger W. H. , Torkler M. , Weiss M. , Löschner H. , Leung K. , Lee Y. , Hudek P. , Rangelow I. W. , Stangl G. , Fallmann W.
Published: July 2002

Matrices of 90 nm dots have been printed into high-sensitivity positive resist (UVII HS, Shipley) with an ion projection system (IMS Vienna) to investigate the influence of shot noise on the printing probability of dots. Dot defect probability increases with diminishing ion dose following a Poisson distribution which demonstrates that shot noise is the dominating effect. The minimum ion numbers per dot to generate 50% defect probability are Ncrit=115 for standard resist treatment. This corresponds to N=165 for smaller defect probabilities of 10−4. Resist sensitivity was decreased with postexposure bake temperatures of 110 °C instead of 140 °C to improve the resolution capability of the resist. Under these conditions, and an additional resist top coat, Ncrit=130 ions per dot have been measured. The article demonstrates that ion projection lithography is not limited by shot noise at minimum resolution elements of 90 nm diam. The corresponding exposure doses are 0.3 μC/cm2.
Lithographically induced self-assembly of periodic polymer micropillar arrays
Authors: Chou Stephen Y. , Zhuang Lei
Published: July 2002

We observed, and believe it to be the first time, the self-formation of periodic, supramolecular (micrometer scale) pillar arrays in a thin, single-homopolymer film melt, which was originally flat on a plate. The self-formation was induced by placing a second plate (called a mask) a distance above the polymer film. The pillars, formed by rising against the gravitational force and surface tension, bridge the two plates. The pillar height is equal to the plate-mask separation, which is two to seven times the film’s initial thickness. If the surface of the mask has a protruding pattern (e.g., a triangle or rectangle), the pillar array can be formed only under the protruding pattern with the edge of the array aligned to the boundary of the mask pattern. We also discuss a model and novel applications of lithographically induced self-assembly.
Granulated metal nanostructure deposited by laser ablation accompanied by cascade drop fission
Authors: Kozhevin V. M. , Yavsin D. A. , Kouznetsov V. M. , Busov V. M. , Mikushkin V. M. , Nikonov S. Yu. , Gurevich S. A. , Kolobov A.
Published: July 2002

A new method is proposed for the fabrication of thin films consisting of closely packed nm-size metallic grains. The method is based on laser ablation of metallic (Cu) target and cascade fission of ejected liquid metallic drops charged in the laser torch plasma. It is suggested that cascade fission is limited by field emission current from the drop surface, which stops this process sharply when the Cu granule size diminishes down to about 10 nm. The fabricated films, free oxidized in air, are shown to consist of one or several layers of monodisperse spherical Cu grains, 8 nm in diameter, covered by about a 1 nm thick Cu2O layer.
Characteristics of Ga2O3(Gd2O3)/GaAs interface: Structures and compositions
Authors: Hong M. , Kortan A. R. , Kwo J. , Mannaerts J. P. , Krajewski J. J. , Lu Z. H. , Hsieh K. C. , Cheng K. Y.
Published: July 2002

Growth of a single crystal was found in the first few oxide layers on GaAs(100) substrate, electron beam evaporated from a Ga5Gd3O12 source. Reflection high-energy electron diffraction and x-ray diffraction studies show that the thin oxide film is epitaxially grown on GaAs with the surface normal (110) and in-plane axis [001] parallel to (100) and [011] of GaAs, respectively, and the crystallographic structure is isomorphic to Mn2O3. The chemical composition of the oxide film was determined by x-ray photoelectron spectroscopy to be unequivocally pure Gd2O3. Based on Gibbs free energies of formation for all possible pairs in Ga, As, Gd, and O, a model is proposed to explain the epitaxy and the growth of single-domain Gd2O3 on GaAs.
Human DNA Repair Genes
Authors: Wood Richard D. , Mitchell Michael , Sgouros John , Lindahl Tomas
Published: July 2002

Cellular DNA is subjected to continual attack, both by reactive species inside cells and by environmental agents. Toxic and mutagenic consequences are minimized by distinct pathways of repair, and 130 known human DNA repair genes are described here. Notable features presently include four enzymes that can remove uracil from DNA, seven recombination genes related to RAD51, and many recently discovered DNA polymerases that bypass damage, but only one system to remove the main DNA lesions induced by ultraviolet light. More human DNA repair genes will be found by comparison with model organisms and as common folds in three-dimensional protein structures are determined. Modulation of DNA repair should lead to clinical applications including improvement of radiotherapy and treatment with anticancer drugs and an advanced understanding of the cellular aging process.
Lack of Replicative Senescence in Cultured Rat Oligodendrocyte Precursor Cells
Authors: Tang Dean G. , Tokumoto Yasuhito M. , Apperly James A. , Lloyd Alison C. , Raff Martin C.
Published: July 2002

Most mammalian somatic cells are thought to have a limited proliferative capacity because they permanently stop dividing after a finite number of divisions in culture, a state termed replicative cell senescence. Here we show that most oligodendrocyte precursor cells purified from postnatal rat optic nerve can proliferate indefinitely in serum-free culture if prevented from differentiating; various cell cycle–inhibitory proteins increase, but the cells do not stop dividing. The cells maintain high telomerase activity and p53- and Rb-dependent cell cycle checkpoint responses, and serum or genotoxic drugs induce them to acquire a senescence-like phenotype. Our findings suggest that some normal rodent precursor cells have an unlimited proliferative capacity if cultured in conditions that avoid both differentiation and the activation of checkpoint responses that arrest the cell cycle.
Lack of Replicative Senescence in Normal Rodent Glia
Authors: Mathon Nicole F. , Malcolm Denise S. , Harrisingh Marie C. , Cheng Lili , Lloyd Alison C.
Published: July 2002

Replicative senescence is thought to be an intrinsic mechanism for limiting the proliferative life-span of normal somatic cells. We show here that rat Schwann cells can be expanded indefinitely in culture while maintaining checkpoints normally lost during the immortalization process. These findings demonstrate that senescence is not an inevitable consequence of extended proliferation in culture.
Requirement of a Centrosomal Activity for Cell Cycle Progression Through G
1
into S Phase
Authors: Hinchcliffe Edward H. , Miller Frederick J. , Cham Matthew , Khodjakov Alexey , Sluder Greenfield
Published: July 2002

Centrosomes were microsurgically removed from BSC-1 African green monkey kidney cells before the completion of S phase. Karyoplasts (acentrosomal cells) entered and completed mitosis. However, postmitotic karyoplasts arrested before S phase, whereas adjacent control cells divided repeatedly. Postmitotic karyoplasts assembled a microtubule-organizing center containing γ-tubulin and pericentrin, but did not regenerate centrioles. These observations reveal the existence of an activity associated with core centrosomal structures—distinct from elements of the microtubule-organizing center—that is required for the somatic cell cycle to progress through G
1
into S phase. Once the cell is in S phase, these core structures are not needed for the G
2
-M phase transition.