bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ surface composition and structure of InGaN and GaN thin films by time-of-flight mass spectroscopy of recoiled ions
Authors: Kim E. , Berishev I. , Bensaoula A. , Schultz J. A.
Published: July 2002

Time-of-flight mass spectroscopy of recoiled ions (TOF–MSRI) is used to determine the surface chemical composition and termination of GaN and InGaN thin films grown by gas source and electron cyclotron resonance molecular beam epitaxy (GS–MBE and ECR–MBE). We show that using TOF–MSRI all the critical growth steps, the nitridation, the buffer layer and the epilayers can be optimized in real time. In the case of GS–MBE, the ammonia pressure can be, reproducibly and easily, adjusted to achieve the highest N surface composition at the minimum corrosive ammonia flow. For InGaN the total TOF–MSRI ion counts drop with increasing In content. Such an observation can be applied to evaluate the thin film surface morphology in addition to its surface composition. Finally, we present preliminary data showing the use of TOF–MSRI for in situ GaN surface structure determination. We also show that by using a reflectron ion analyzer, much higher ion counts and better resolution can be achieved than a conventional electrostatic sector system. With such a modification, dopant level sensitivities should be achievable and data rates compatible with closed loop process control algorithms become possible.
Temperature-composition determination based on modeling of optical constants of III-V compound semiconductors measured by spectroscopic ellipsometry
Authors: Grassi Elena , Johnson Shane R. , Beaudoin Mario , Tsakalis Kostas S.
Published: July 2002

A general procedure to fit optical constants, using a transfer function model with temperature-and/or-composition-dependent coefficients, is presented. The model is further inverted by a simple algorithm to retrieve temperature and composition information from optical measurements obtained by spectroscopic ellipsometry. The method was applied to fit: (1) the complex index of refraction of the system AlXGa1−xAs at 600 °C, for values of X between 0 and 1. (2) Two data bases of complex dielectric constants, for near-lattice-matched InGaAs and InAlAs, and around temperatures of 500 °C. The parameters of the model are determined with a least squares algorithm with recursive “whitening” of the error, which shows fast convergence to a near-optimal solution, even when handling a large number of parameters. The level of accuracy achieved makes this method an adequate sensor for temperature, composition, and thickness control during molecular beam epitaxy growth.
Low-temperature molecular beam epitaxy of GaAs: A theoretical investigation of antisite incorporation and reflection high-energy diffraction oscillations
Authors: Natarajan K. , Venkat R. , Dorsey D. L.
Published: July 2002

Surface dynamics dominate the incorporation of charged, AsGa+, and neutral, AsGa0, antisite arsenic, and the temporal variation of reflection high-energy electron diffraction (RHEED) intensity in the low-temperature molecular beam epitaxy of (100) gallium arsenide (GaAs). A rate equation model is proposed which includes the presence and dynamics of a physisorbed arsenic (PA) layer riding the growth surface. The PA layer dictates the incorporation and concentration of AsGa+ and AsGa0. Additionally, it influences the RHEED oscillations (ROs) behavior and the RO’s dependence on its coverage through its contribution to the reflected intensity. The model results for the dependence of AsGa+ and AsGa0 concentrations on beam equivalent pressure (BEP) and growth temperature are in good agreement with experimental data. The experimental observations can be explained based on the saturation of the PA coverage at one monolayer and the competing rate processes such as the AsGa incorporation into and evaporation from the crystalline surface. Using the same kinetic model for the temporal behavior of the surface, the contribution of the PA layer to the RHEED intensity is computed based on kinematical theory of electron diffraction. The experimental observation of the ROs during growth at high and low temperatures with no ROs in the intermediate temperature range of 300–450 °C is in good agreement with our model results. At low temperatures, the surface is covered by the PA layer whose step density depends on that of the subsurface crystalline GaAs. Thus, a temporal variation of the step density of subsurface crystalline GaAs results in ROs, but with a different step height, that of the PA layer, of 2.48 Å. At high temperatures, the crystalline GaAs is exposed to the RHEED beam due to the evaporation of the PA layer and the ROs appear due to periodic step-density oscillations with a step height of 1.41 Å, which is the Ga–As crystalline interplanar distance. At intermediate temperatures, the surface is partially covered by the PA layer resulting in RHEED reflection contributions from both surfaces covered by the PA layer and crystal. Due to the very different interplanar distances between the crystalline GaAs and the PA layers, complete destructive interference of the RHEED intensity results at a 0.5 surface coverage of the PA layer. The RO dependence on the As BEP is also presented and discussed.
Use of spectroscopic ellipsometry for feedback control during the growth of thin AlAs layers
Authors: Beaudoin M. , Johnson S. R. , Boonzaayer M. D. , Zhang Y.-H. , Johs B.
Published: July 2002

Spectroscopic ellipsometry (SE) is an accurate in situ method for determining the composition and thickness of III–V semiconductor layers during growth. In order to achieve this control, an optical constant database is used to compare the modeled and experimental ellipsometric data. This procedure is very effective for controlling thickness and composition when the film has been growing for some time but is usually unreliable until several minutes into the growth (corresponding to ∼50 nm). To use SE for the control of thinner layers (1–20 nm), a different approach has to be used. A new strategy is proposed which consists in looking at the raw SE signal in a limited wavelength range where the signal varies almost linearly with the film thickness. For AlAs grown on GaAs, it is found the phase part of the SE signal for light between 2.5 and 2.8 eV varies nearly linearly with the AlAs thickness. A series of AlAs/GaAs multiquantum well structures are grown and analyzed. The thickness determined by this use of the ellipsometric data are in close agreement with independent thickness measurements obtained from high resolution x-ray diffraction.
Morphology, polarity, and lateral molecular beam epitaxy growth of GaN on sapphire
Authors: Piquette E. C. , Bridger P. M. , Bandić Z. Z. , McGill T. C.
Published: July 2002

Gallium nitride was grown on sapphire (0001) substrates by radio frequency plasma assisted molecular beam epitaxy. The surface morphology was characterized during growth by reflection high energy electron diffraction, and ex situ by scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray diffraction. It is found that surface morphological features are linked to domains of specific wurtzite crystal polarity, (0001)Ga face or (0001̄)N face, for Ga-rich growth. For growth on AlN buffer layers, we commonly observe films which consist of largely (0001)Ga polarity material, as confirmed by selective etch tests, with a varying coverage of (0001̄)N-face inversion domains threading along the growth direction. For growth near stoichiometric conditions, the growth rate of the N-face domains is slightly lower than that for the Ga-face matrix, which results in the formation of pits with inversion domains at their centers. For samples grown by first depositing GaN under N-rich conditions, followed by growth under Ga-rich conditions, a different morphology is obtained, exhibiting large hexagonal flat terraces observable by SEM and AFM. The apparent grain size of these films is increased substantially over films grown using a single step approach. The cross sectional SEM images of the two-step films show a network of voids and columns at the interface between the N-rich and the Ga-rich layers, above which micron-scale islands form and coalesce via lateral growth. Lateral growth may result in reduced defect density and improved crystal quality. The asymmetric x-ray peak (112̄4) width is reduced to approximately 280 arcsec in the two-stage GaN films.
InGaAs/GaAs/AlGaAs GRIN-SCH quantum-well lasers grown by solid-source molecular-beam epitaxy using CBr4 doping
Authors: Gratteau Navid , Lubyshev D. , Miller D. L.
Published: July 2002

We have investigated the growth by solid-source molecular-beam epitaxy of InGaAs/GaAs/AlGaAs GRIN-SCH quantum-well lasers emitting near 940 nm, comparing carbon (from CBr4) to beryllium doping of the p-type material. The cladding layers, containing 45% aluminum, were grown at temperatures between 570 and 710 °C, while the single quantum wells were grown at 550 °C. The C- and Be-doped lasers exhibited similar characteristics for all substrate temperatures. For lasers with cladding layers grown at 570 °C, we obtained a relatively low threshold current density of 91 A/cm2 extrapolated to infinite stripe length for a 200-μm-wide stripe and an internal quantum efficiency as high as 95% with uncoated facets. At 690 °C, we obtained a threshold current density as low as 61 A/cm2, which compares reasonably well with the best reported value on near-(100) substrates of 45 A/cm2. In continuous-wave operation, we reached an output power of 575 mW/facet for a 200 μm×500 μm stripe with uncoated facets, and 49 mW/facet and a differential efficiency of 0.4 W/A per facet for a 5 μm×500 μm stripe.
Quantitative study of chemical mechanical planarization process affected by bare silicon wafer front surface topography
Authors: Shan Xu C. , Liu Jerry , Xia Yong
Published: July 2002

Chemical mechanical planarization (CMP) is an enabling technology in the fabrication of advanced semiconductor devices. The surface topography of the starting bare silicon substrate before CMP can have a significant impact on the results of the CMP process, specifically on the uniformity of the oxide film final thickness. The quantitative studies of this phenomenon are just beginning. In this article, a laser based optical scanning measurement was used to determine the front surface topography of the bare silicon substrates prior to thermal oxide growth. Fast two-dimensional mapping of the surface height variation is obtained with very high spatial and height resolution. The one-dimensional cross-section profile extracted from the measurement is demonstrated to have good correlation with the stylus based profiler measurement. A thermal oxide layer was grown and then polished in a high efficiency planarization process on a group of substrates, which had different levels of surface topography variations on the starting surface. A signature match between the oxide thickness variation and the silicon substrate front surface topography was identified. The resultant correlation reveals the potential impact on CMP process window from the starting material due to the degradation of within-die and die-to-die uniformity. Consequently, the product yield could be threatened. The optical topographical measurement using this laser scanning measurement is demonstrated here to be capable of providing fast, vital and unique information critical to achieving satisfactory results in the oxide CMP process.
Effects of ramp-up rates on the salicide process
Authors: Tan C. C. , Lu L. , Chen S. Y. , Shen Z. X. , See A. , Chan L. H. , Chua L. H. , Chan T. K. L.
Published: July 2002

Increases in the sheet resistance of TiSi2 with a decreasing linewidth has been attributed to incomplete transformation from high resistance C49–TiSi2 phase to the desired low resistance C54–TiSi2 phase. In this article, we investigate the effects of increasing the ramp up rate of the first rapid thermal anneal (RTA1) in the salicide process. Electrical testing and micro-Raman spectroscopy were used to investigate the C49-to-C54 phase transformation on poly lines with different linewidths. Samples were annealed with two different ramp-up rates in RTA1. From micro-Raman spectra, it is observed that increased ramp-up rate during RTA1 results in more C54–TiSi2 formed during the second rapid thermal anneal (RTA2). It is also shown that higher ramp-up rates result in lower sheet resistance for deep submicron poly lines.
High density plasma deposited phosphosilicate glass as pre-metal dielectrics for advanced self-aligned contacts in sub 0.25 μm device technology
Authors: Yu Jengyi E. , Qiao Jianmin , Jin Bo , Gopalan Prabhuram , Feng Jeff G.
Published: July 2002

High density plasma deposited phosphosilicate glass (PSG) films were evaluated for an application as pre-metal gap-fill dielectric material. A high deposition rate (∼600 nm/min) PSG film with 9 at. % of P and a deposition-to-etch ratio of 5:1 was developed for a void-free gap fill down to 0.04 μm. Chemical-mechanical polish, reactive ion etch, and defect test of the PSG film have revealed that as-deposited PSG has performed similar to the annealed one, and PSG has an excellent film stability. A significant device yield improvement was obtained using this 9 at. % PSG process.
Optimizing Pd–Ge ohmic contact to GaAs through microstructure control
Authors: Radulescu F. , McCarthy J. M.
Published: July 2002

The microstructure evolution and the corresponding solid state reactions that take place during the formation of the Pd–Ge ohmic contact on GaAs were determined. By combining constant-heating-rate differential calorimetry (DSC) and cross-sectional transmission electron microscopy (XTEM) to study the behavior of the Pd (50 nm)/Ge (150 nm)/Pd (20 nm) thin film stack on GaAs, we identified four solid state reactions that take place during annealing: Pd–Ge interdiffusion, hexagonal Pd2Ge formation, PdGe transformation, and excess Ge crystallization. Specimens heated to temperatures that coincided with the DSC peaks were quenched in a He atmosphere and the resulting microstructure was characterized by XTEM and specific contact resistivity measurements. Based on these correlations, the mechanisms that contribute to the formation of optimal microstructure for ohmic contact were identified.
Enabling technologies for forming and contacting shallow junctions in Si: HF-vapor cleaning and selective epitaxial growth of Si and SiGe
Authors: Raaijmakers Ivo J. , Sprey Hessel , Storm Arjen , Bergman Timo , Italiano Joe , Meyer Doug
Published: July 2002

Future generation devices with critical dimensions of less than 130 nm will have source/drain areas with junction depths of less than about 70 nm and a sheet resistance of around 3 Ω/sq. Conventional technologies used to form and contact such shallow and low resistance source/drain areas are concluded to no longer be feasible in manufacturing. Elevated source/drain technology is shown to be very attractive for manufacturing sub-130 nm devices. In this article we describe two critical processes to form such elevated source/drains. First, a novel HF-vapor clean chemistry for native oxide removal is described. The etch chemistry uses acetic acid vapor as a catalyst to initiate and control etching with HF vapor. Excellent repeatability and selectivity are achieved. Second, in situ doped selective epitaxial growth (SEG) of Si and SiGe is addressed. The advantages of adding Ge to the epitaxial film are discussed. Issues like microloading and facet formation are also discussed and are demonstrated as solvable. Vacuum integration of the above two mentioned processes eliminates the need for a high temperature H2 bake. The elimination of the H2 bake and the addition of Ge to enable SEG at lower temperatures are demonstrated to substantially decrease the thermal budget, increase throughput, and eliminate queue time in the factory. These improvements make elevated source/drain technology technically and economically feasible for the manufacturing of 130–70 nm devices.
Nondestructive technique for the characterization of deep traps at interlayer interfaces in thin-film multilayer semiconductor structures
Authors: González-Hernández J. , Prokhorov E. , Gorev N. B. , Kodzhespirova I. F. , Kovalenko Yu. A.
Published: July 2002

In this article we present a low-frequency capacitance-voltage technique for determining the effective concentration of vacant deep traps at the film–buffer layer interface in a thin-film GaAs structure. This effective concentration is an index of the overall effect of vacant deep traps in both the buffer layer and substrate on the current transport in the film. The technique can be used for quality control of film–buffer layer–substrate GaAs wafers prior to device making.
Extreme ultraviolet mask defect simulation
Authors: Pistor Tom , Neureuther Andrew
Published: July 2002

This article investigates the printability of defects embedded inside multilayer mirrors used for masks in extreme ultraviolet lithography. Models for buried defects of various shapes and sizes and their subsequent distorting of the deposited films are developed and used to assess the effect on both the fields at the mask and the aerial image at the wafer. Because most of the reflection occurs in the top 10–20 layers and thus it is hypothesized that simulating with a truncated multilayer will still yield insight into the printability of the defects. The relationship between point defects and line defects of the same height is explored. The intensity minimum in the aerial image for defects is found to vary periodically with only a quarter wave change in the height of the defect due to phase considerations. Defocus can enhance a defect’s printability and an isolated line with only 2 nm height and one quarter of a feature width may print. Nonprinting isolated defects are shown to become killer defects when feature interactions are considered.
Magnification correction by changing wafer temperature in proximity x-ray lithography
Authors: Aoyama H. , Mitsui S. , Taguchi T. , Tanaka Y. , Matsui Y. , Fukuda M. , Suzuki M. , Haga T. , Morita H.
Published: July 2002

Magnification errors arising from wafer processing cause significant overlay errors in lithography. We have devised a new, simple method that involves changing the wafer temperature to correct magnification errors on an x-ray stepper. A wafer is first heated in the wafer orientation unit, and then it is allowed to cool as it is transferred to the wafer stage due to the temperature difference between the wafer and the air in the chamber. For a given cooling time, the reproducibility of the wafer temperature is less than 0.3 °C; and the maximum available temperature change is 1.5 °C. The key point is that the placement of patterns on a wafer immediately after the chucking remains fixed regardless of further changes in the wafer temperature, provided that the force holding the wafer to the stage is strong enough. The maximum magnification error that can be corrected by this method is about 4 ppm. This is determined by the maximum available temperature change (1.5 °C) and the coefficient of linear expansion for Si. The results of overlay tests revealed the correction controllability to be below 0.2 ppm. When this correction method was used, the total overlay accuracy was found to be 38 nm (3σ), which includes mask placement error, etching process error, and so on.
Buried heterostructure complex-coupled distributed feedback 1.55 μm lasers fabricated using dry etching processes and quaternary layer overgrowth
Authors: Söderström D. , Lourdudoss S. , Carlström C.-F. , Anand S. , Kahn M. , Kamp M.
Published: July 2002

An attractive process for fabricating buried heterostructure complex-coupled distributed feedback (BH-CC-DFB) lasers emitting at 1.55 μm has been developed. It combines low damage chemically assisted ion beam etching (CAIBE) for grating definition, subsequent overgrowth of a quaternary GaInAsP layer (λPL=1.22 μm) by low-pressure metalorganic vapor phase epitaxy, reactive ion etching for mesa definition and semi-insulating InP:Fe regrowth by hydride vapor phase epitaxy. CAIBE improves grating uniformity, GaInAsP layer overgrowth increases gain coupling and semi-insulating regrowth facilitates lateral mode confinement and thermal dissipation. The as-cleaved BH-CC-DFB lasers fabricated with the above combination show a statistical single mode yield of 87% and a very high side-mode suppression ratio up to 55 dB. A −3 dB bandwidth of 10 GHz is also demonstrated.
Multilayer hexamethyldisiloxane film as bottom antireflective coating for ArF lithography
Authors: Wang L. A. , Chen H. L.
Published: July 2002

A new multilayer bottom antireflective coating (BARC) layer composed of hexamethyldisiloxane (HMDSO) film stack is demonstrated for ArF lithography. The required optical constant of each layer can be tuned by varying the gas flow rate ratio of oxygen to HMDSO in an electron cyclotron resonance plasma enhanced chemical vapor deposition process, and the deposition can be completed in one step without changing to other materials. The measured swing effect is shown significantly reduced by adding such a multilayer BARC on both Al/Si and silicon crystal substrates. The capability of applying the multilayer BARC to various highly reflective substrates is discussed. Additionally, the multilayer BARC is shown to be capable of providing larger thickness-controlled tolerances than a single-layer BARC.
Stencil reticle development for electron beam projection system
Authors: Kawata S. , Katakura N. , Takahashi S. , Uchikawa K.
Published: July 2002

Electron beam (EB) stepper, which is the electron beam projection system (EBPS), has been developed as one of the next generation lithography (NGL) systems for the 100 nm technology generation and beyond. An EB lithography system similar to EBPS is SCALPEL® (scattering with the angular limitation projection electron-beam lithography). EBPS and SCALPEL® use the scattering contrast principle for the wafer exposure. The development of electron beam scattering reticles is a key issue for the EB lithography system development. The SCALPEL® system uses the scattering membrane reticle (SCALPEL® mask). The scattering stencil reticle (EB reticle) has been investigated as the reticle for EB steppers. 75 and 200 mm test EB reticles have been developed. The 75 mm test reticle is fabricated with a stress controlled silicon-on-insulator wafer. The fabrication process condition for the 200 mm test reticle is not fixed. The status of risk issues for the EB reticle development is presented. The image placement measurements on the membrane are in agreement with the simulation results. Stencil patterns, consisting of 70 nm lines and spaces, have been exposed with the test reticles using a 100 kV test column.
Electron optical system for the x-ray mask writer EB-X3
Authors: Saito Kenichi , Kato Junichi , Shimizu Akira , Morita Hirofumi , Matsuda Tadahito , Nakayama Yoshinori
Published: July 2002

An electron optical system that yields a calculated beam edge resolution of 21 nm at a beam voltage of 100 kV and a current density of 50 A/cm2 was developed for the EB-X3 x-ray mask writer. Based on the system, we investigated a way to increase the current density. The introduction of a zoom lens into the system provided a calculated beam edge resolution of 35 nm at 100 A/cm2. Problems arising from the high beam voltage and high current density, such as the melting of the first shaping aperture and microdischarges in the electron gun, were also examined. Optimization of the aperture and its holder through thermal simulations reduced the temperature of the first shaping aperture to 753 °C at 100 A/cm2. Through electron trajectory calculations, an electron gun was designed in which reflected electrons are prevented from charging the insulators; and this enabled us to completely eliminate microdischarges.
Effect of Au overlayer on Ni contacts to p-type GaN
Authors: Kim Jong Kyu , Lee Jong-Lam , Lee Jae Won , Park Yong Jo , Kim Taeil
Published: July 2002

The effect of Au overlayer on Ni contacts to p-type GaN was investigated by comparing the change of electrical properties with interfacial reactions at the interface of Ni/p-type GaN. The contact resistivity at the interface of Ni/p-type GaN was decreased by two orders of magnitude at 600 °C by incorporating the Au overlayer on the Ni contact. This originates from the outdiffusion of Ga atoms from p-type GaN to the Au overlayer, and subsequently produces Ga vacancies acting as acceptors for electrons below the contact. Consequently, net hole concentration increases below the contacts, resulting in the reduction of the Schottky barrier height for hole transport, via the decrease of contact resistivity.
Resolution of the multiple aperture pixel by pixel enhancement of resolution electron lithography concept
Authors: Kampherbeek B. J. , Wieland M. J. , Kruit P.
Published: July 2002

For the sub 100 nm integrated circuit generations we investigate the potential of a new lithography concept: multiple aperture pixel by pixel enhancement of resolution. The wafer is illuminated by a large number of electron beams which are triggered by an equal number of light beams. The light beams are switched on and off by an optical mask. The electron beams are focused by parallel electric and magnetic fields [P. R. Malmberg, T. W. O’Keeffe, and M. M. Sopira, J. Vac. Sci. Technol. 10, 1025 (1973); J. P. Scott, ibid. 15, 1016 (1973)]. By means of computer simulation we determine the point spread function of the electron beam at the wafer. The requirements on the magnetic field form and the alignment are evaluated. It is concluded that the resolution can be better than 1:1 image projection. For 100 nm lines and spaces there is a wide process window.
Double-layer inorganic antireflective system for KrF lithography
Authors: Xu Min , Ko Tze-Man
Published: July 2002

A novel bottom antireflective method is described to resolve the problems in conventional antireflective methods such as insufficient antireflection and dependence on substrate materials and structures. The proposed antireflective method is composed of two layers. A high photoabsorption layer at the bottom absorbs most of the light reflected from the substrate and an interference layer at the top suppresses the surface reflection from the bottom layer. This new method can achieve extremely low substrate reflectivity (<0.5%) for all substrates, and it provides a larger process window on film thickness than conventional methods. Silicon oxynitride (SiOxNy:Hz) film was chosen as the bottom antireflective coating material. A new plasma-enhanced chemical-vapor deposition scheme with SiH4, N2, N2O, and He as reactant gases was used to fabricate the SiOxNy:Hz films in order to obtain a wide variation of film optical properties. The compositional characteristics of the films were investigated by x-ray photoelectron spectroscopy. The complex indices of refraction (n-ik) and thicknesses of the films were measured by variable-angle spectroscopic ellipsometry. The film structure was analyzed by Fourier transform infrared spectroscopy. Comparing this method with the conventional one using SiH4 and N2O as reactant gases, it provided a much wider range of film compositions that could easily meet the different requirements in many different process applications. In order to produce high-quality silicon oxynitride films and to enhance the extent of reaction between SiH4 and N2, the effects of substrate temperature, plasma power, and reactor chamber pressure were studied to find the optimum deposition conditions. Moreover, correlations of the n and k values with the SiOxNy:Hz stoichiometry were obtained. While the n values could be correlated with a linear relation to the atomic concentration ratio of [O]/([O]+[N]), the k values were determined exclusively by the silicon contents in the films.
Mechanistic feature-scale profile simulation of SiO2 low-pressure chemical vapor deposition by tetraethoxysilane pyrolysis
Authors: Labun Andrew H. , Moffat Harry K. , Cale Timothy S.
Published: July 2002

Simulation of chemical vapor deposition in submicron features typical of semiconductor devices has been facilitated by extending the EVOLVE [T. S. Cale, T. H. Gandy, and G. B. Raupp, J. Vac. Sci. Technol. A 9, 524 (1991)] thin film etch and deposition simulation code to use thermal reaction mechanisms expressed in the Chemkin format. This allows consistent coupling between EVOLVE and reactor simulation codes that use Chemkin. In an application of a reactor-scale simulation code providing surface fluxes to a feature-scale simulation code, a proposed reaction mechanism for tetraethoxysilane [Si(OC2H5)4] pyrolysis to deposit SiO2, which had been applied successfully to reactor-scale simulation, does not correctly predict the low step coverage over trenches observed under short reactor residence time conditions. One apparent discrepancy between the mechanism and profile-evolution observations is a reduced degree of sensitivity of the deposition rate to the presence of reaction products, i.e., the by-product inhibition effect is underpredicted. The cause of the proposed mechanism’s insensitivity to by-product inhibition is investigated with the combined reactor and topography simulators. This is done first by manipulating the surface-to-volume ratio of a simulated reactor and second by adjusting parameters in the proposed mechanism such as the calculated free energies of proposed surface species. The conclusion is that simply calibrating mechanism parameters to enhance the by-product inhibition can improve the fit to profile evolution data; however, the agreement between with reactor-scale data and simulations decreases. Additional surface reaction channels seem to be required to simultaneously reproduce experimental reactor-scale growth rates and feature-scale step coverages.
Process integration issues for doping of ultrashallow junctions
Authors: Current M. I. , Foad M. A. , Murrell A. J. , Collart E. J. H. , de Cock G. , Jennings D.
Published: July 2002

Process integration and in-line metrology are now the key agenda items for formation of ultrashallow junctions for ultralarge scale integration devices. Integration of the complete process sequence from selection of the device layer material through introduction and activation of dopants involves dozens of interacting decisions. This article highlights some issues associated with the control of channeling, damage annealing of preamorphized and molecular ion implants, outgassing of oxygen from product wafers during rapid thermal processing cycles and conditions for control of beam divergence and doping microuniformity near gate stack structures.
High-resolution two-dimensional dopant characterization using secondary ion mass spectrometry
Authors: Ukraintsev V. A. , Chen P. J. , Gray J. T. , Machala C. F. , Magel L. K. , Chang M.-C.
Published: July 2002

A quantitative two-dimensional (2D) boron dopant profiling for submicron technology has been demonstrated using secondary ion mass spectrometry (SIMS). A novel test structure incorporating a moiré pattern has been developed to improve SIMS lateral resolution. This approach significantly simplifies 2D SIMS test chip manufacturing, data acquisition, and analysis. As a result, 2D dopant profiling with a lateral resolution limited by the photomask pixel size (10 nm) and sensitivity of 3×1017 cm−3 has been realized on commercial equipment. 2D dopant profiles are reproducible within 10 nm. The measured profiles were compared with 2D Monte-Carlo and calibrated TSUPREM4 simulations and showed good agreement.
Effects of molecular properties on nanolithography in polymethyl methacrylate
Authors: Dobisz Elizabeth A. , Brandow Susan L. , Bass Robert , Mitterender Jeffrey
Published: July 2002

High-resolution lithographic performance of polymethyl methacrylate (PMMA) of molecular weights (MWs) of 50, 100, 496, and 950 K is compared. A chain scission model is used to analyze the behavior of the four molecular weight resists. The chain scission model is combined with an empirical dissolution model to successfully describe the edge profile of a bar pattern. Isolated linewidth data for the 100 and 496 K resists both fit a Monte Carlo code generated linespread function that was convolved with a Gaussian of standard deviation 9 nm. The width was comparable to that in the 950 K resist, but a factor of 3 narrower than that found for the 50 K resist. The higher molecular weight, 496 and 950 K resists showed more developer induced swelling than the lower molecular weight resists. In fact, the developer induced swelling limited the ability to develop 40 nm gratings in the 496 and 950 K resists. Reduction in developer strength produced some improvement. Etching of the supporting resist structure in the gratings was also observed, particularly in the 50 and 100 K resists. The 50 K MW resist exhibited the worst grating contrast upon development. Grating enhanced etching relative to 10 μm bar areas exposed with comparable area dose was observed. A 40 nm period grating was defined in the 100 K resist.
Field emission carbon thin film and its lifetime and stability
Authors: Thuesen L. H. , Li Tolt Z. , Fink R. L. , Yaniv Z. , Marrese Colleen M. , Bandy Steve , Nishimoto Clifford
Published: July 2002

Tests were performed on FEPET, Inc. carbon thin film cathodes to characterize lifetime, resistance to environmental conditions, and performance stability. The cathodes were found to exhibit half lifetimes on the order of 7000 h in sealed glass envelopes. Operation in oxygen and water were found to degrade the cathode performance severely. Xenon had no effect on the cathode lifetime or stability, demonstrating its resistance to ion sputtering. During characterization in vacuum systems, initial turn on was quick and stable. Carbon thin films are suitable cathode materials for display, microelectronic, and ion propulsion uses.
Homoepitaxial diamond (001) thin film studied by reflection high-energy electron diffraction, contact atomic force microscopy, and scanning tunneling microscopy
Authors: Takami Tomohide , Kusunoki I. , Nishitani-Gamo M. , Ando T.
Published: July 2002

A surface of the homoepitaxial diamond (001) thin film produced by microwave plasma chemical vapor deposition was observed by reflection high-energy electron diffraction (RHEED), contact atomic force microscopy (contact-AFM) and scanning tunneling microscopy (STM) on the same sample. It was checked by the RHEED observation that most areas of the sample surface had a 2×1 or 1×2 reconstructed structure, which indicates that most areas of the surface were covered with hydrogen termination. By contact-AFM a pseudoperfect surface with rare atomic-scale defects and steps was observed. By STM, on the other hand, a 2×1/1×2 double-domain structure was observed and the surface had many steps and defects. One should be careful because a contact-AFM image does not ensure true atomic-resolution since the multitip effect is dominant.
Smart pyrometry for combined sample temperature and reflectance measurements in molecular-beam epitaxy
Authors: Bertness K. A.
Published: July 2002

Improved accuracy of measurements of semiconductor wafer temperature is demonstrated in a normal-incidence optical reflectance spectroscopy system that has been modified to allow rapid switching between the measurement of wafer reflectance and wafer emission. The resulting optical system is capable of “smart” pyrometry, in which the emissivity is determined each time the temperature is measured. Measurements during heteroepitaxial growth show that the data from smart pyrometry typically differ from those of conventional pyrometry by 5–10 °C, and occasionally by as much as 20 °C for multilayer structures of AlGaAs and GaAs. Theory, experimental procedures, and calibration procedures are discussed.
Real-time composition control of InAlAs grown on InP using spectroscopic ellipsometry
Authors: Beaudoin M. , Grassi E. , Johnson S. R. , Ramaswamy K. , Tsakalis K. , Alford T. L. , Zhang Y.-H.
Published: July 2002

Spectroscopic ellipsometry (SE) was shown to be an accurate in situ method for determining the composition and thickness of III–V semiconductor layers during growth. In order to fully exploit the potential of SE for real-time in situ control, one needs to acquire a database of optical constants. In this article, we present the acquisition and parameterization (both composition and temperature) of a fully dynamic database and its use in the real-time composition control of InAlAs grown on InP. This is accomplished by acquiring SE data from growing films of different compositions, while the temperature is controlled using feedback from band-gap thermometry. The layer compositions are assessed by fitting high-resolution x-ray diffraction patterns with a simulation based on dynamical diffraction theory. In order to improve the stability during real-time control, the database was parameterized using a transfer function model. The parameterized database was then used, in real time, during growth to control the InxAl1−xAs film composition (x) to within ±0.003.
Structural and optical characterization of InGaN/GaN multiple quantum wells grown by molecular beam epitaxy
Authors: Ng H. M. , Moustakas T. D. , Ludwig K. F.
Published: July 2002

InGaN/GaN multiple quantum wells (MQWs) were grown on (0001) sapphire substrates, which were first coated with thick GaN or AlN films, at relatively low temperatures (650 °C). At this growth temperature, we found by x-ray diffraction that the MQWs show distinct higher order superlattice peaks, indicating that the interfaces between wells and barriers are abrupt with little interdiffusion between the layers. However, photoluminescence and cathodoluminescence studies provide evidence that at such low growth temperatures, the inhomogeneities in the InGaN alloys are enhanced due to spinodal decomposition in these alloys.
A Study of Potassium Dihydrogen Phosphate (KDP) Crystal Surfaces by XPS
Authors: Engelhard Mark , Evans Cheryl , Land T. A. , Nelson A. J.
Published: July 2002

Potassium dihydrogen phosphate KH2PO4 (KDP) is a transparent dielectric material best known for its nonlinear optical and electro-optical properties (Refs. 1 and 2). Because of its nonlinear optical properties, it has been incorporated into various laser systems for harmonic generation and optoelectrical switching. In addition, KDP is particularly suitable for use in large-aperture laser systems such as that located at the National Ignition Facility (NIF) because it can be grown as a single crystal to large size (Refs. 3 and 4). Despite the importance of this material, surface composition and surface electronic structure data were found to be nonexistent. X-ray photoemission spectroscopy was thus used to characterize the composition and surface structure of (100) and (101) native crystals.
Functional Specialization in Rhesus Monkey Auditory Cortex
Authors: Tian Biao , Reser David , Durham Amy , Kustov Alexander , Rauschecker Josef P.
Published: July 2002

Neurons in the lateral belt areas of rhesus monkey auditory cortex prefer complex sounds to pure tones, but functional specializations of these multiple maps in the superior temporal region have not been determined. We tested the specificity of neurons in the lateral belt with species-specific communication calls presented at different azimuth positions. We found that neurons in the anterior belt are more selective for the type of call, whereas neurons in the caudal belt consistently show the greatest spatial selectivity. These results suggest that cortical processing of auditory spatial and pattern information is performed in specialized streams rather than one homogeneously distributed system.
Structural Mechanism for Statin Inhibition of HMG-CoA Reductase
Authors: Istvan Eva S. , Deisenhofer Johann
Published: July 2002

HMG-CoA (3-hydroxy-3-methylglutaryl–coenzyme A) reductase (HMGR) catalyzes the committed step in cholesterol biosynthesis. Statins are HMGR inhibitors with inhibition constant values in the nanomolar range that effectively lower serum cholesterol levels and are widely prescribed in the treatment of hypercholesterolemia. We have determined structures of the catalytic portion of human HMGR complexed with six different statins. The statins occupy a portion of the binding site of HMG-CoA, thus blocking access of this substrate to the active site. Near the carboxyl terminus of HMGR, several catalytically relevant residues are disordered in the enzyme-statin complexes. If these residues were not flexible, they would sterically hinder statin binding.
Interferon-γ-Mediated Site-Specific Clearance of Alphavirus from CNS Neurons
Authors: Binder Gwendolyn K. , Griffin Diane E.
Published: July 2002

Recovery from viral encephalomyelitis requires immune-mediated noncytolytic clearance from neurons by mechanisms assumed to be the same for all neurons. In alphavirus encephalomyelitis, antibody clears infectious virus from neurons in all regions of the central nervous system (CNS), but CD8 T cells contribute to elimination of viral RNA. To understand the role of T cells in clearance, we infected antibody knockout mice with Sindbis virus. Virus was cleared from spinal cord and brain stem neurons, but not from cortical neurons, and required both CD4 and CD8 T cells. Infection with cytokine-expressing recombinant viruses suggested that T cells used interferon-γ, but not tumor necrosis factor α, in clearing virus and that populations of neurons differ in responsiveness to this effector pathway.
Oblique Stepwise Rise and Growth of the Tibet Plateau
Authors: Tapponnier Paul , Zhiqin Xu , Roger Françoise , Meyer Bertrand , Arnaud Nicolas , Wittlinger Gérard , Jingsui Yang
Published: July 2002

Two end member models of how the high elevations in Tibet formed are (i) continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and (ii) time-dependent, localized shear between coherent lithospheric blocks. Recent studies of Cenozoic deformation, magmatism, and seismic structure lend support to the latter. Since India collided with Asia ∼55 million years ago, the rise of the high Tibetan plateau likely occurred in three main steps, by successive growth and uplift of 300- to 500-kilometer-wide crustal thrust-wedges. The crust thickened, while the mantle, decoupled beneath gently dipping shear zones, did not. Sediment infilling, bathtub-like, of dammed intermontane basins formed flat high plains at each step. The existence of magmatic belts younging northward implies that slabs of Asian mantle subducted one after another under ranges north of the Himalayas. Subduction was oblique and accompanied by extrusion along the left lateral strike-slip faults that slice Tibet's east side. These mechanisms, akin to plate tectonics hidden by thickening crust, with slip-partitioning, account for the dominant growth of the Tibet Plateau toward the east and northeast.
A Transgenic Model for Listeriosis: Role of Internalin in Crossing the Intestinal Barrier
Authors: Lecuit Marc , Vandormael-Pournin Sandrine , Lefort Jean , Huerre Michel , Gounon Pierre , Dupuy Catherine , Babinet Charles , Cossart Pascale
Published: July 2002

Listeria monocytogenes
is responsible for severe food-borne infections, but the mechanisms by which bacteria cross the intestinal barrier are unknown.
Listeria monocytogenes
expresses a surface protein, internalin, that interacts with a host receptor, E-cadherin, to promote entry into human epithelial cells. Murine E-cadherin, in contrast to guinea pig E-cadherin, does not interact with internalin, excluding the mouse as a model for addressing internalin function in vivo. In guinea pigs and transgenic mice expressing human E-cadherin, internalin was found to mediate invasion of enterocytes and crossing of the intestinal barrier. These results illustrate how relevant animal models for human infections can be generated.
Direct Detection of Galactic Halo Dark Matter
Authors: Oppenheimer B. R. , Hambly N. C. , Digby A. P. , Hodgkin S. T. , Saumon D.
Published: July 2002

The Milky Way galaxy contains a large, spherical component which is believed to harbor a substantial amount of unseen matter. Recent observations indirectly suggest that as much as half of this “dark matter” may be in the form of old, very cool white dwarfs, the remnants of an ancient population of stars as old as the galaxy itself. We conducted a survey to find faint, cool white dwarfs with large space velocities, indicative of their membership in the galaxy's spherical halo component. The survey reveals a substantial, directly observed population of old white dwarfs, too faint to be seen in previous surveys. This newly discovered population accounts for at least 2 percent of the halo dark matter. It provides a natural explanation for the indirect observations, and represents a direct detection of galactic halo dark matter.
Conductance Switching in Single Molecules Through Conformational Changes
Authors: Donhauser Z. J. , Mantooth B. A. , Kelly K. F. , Bumm L. A. , Monnell J. D. , Stapleton J. J. , Price D. W. , Rawlett A. M. , Allara D. L. , Tour J. M. , Weiss P. S.
Published: July 2002

We tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled molecules in either the ON or OFF switch state range from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. Conversely, when the surrounding matrix is poorly ordered, the inserted molecules switch more often. We conclude that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer.
RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension
Authors: Johnston Wendy K. , Unrau Peter J. , Lawrence Michael S. , Glasner Margaret E. , Bartel David P.
Published: July 2002

The RNA world hypothesis regarding the early evolution of life relies on the premise that some RNA sequences can catalyze RNA replication. In support of this conjecture, we describe here an RNA molecule that catalyzes the type of polymerization needed for RNA replication. The ribozyme uses nucleoside triphosphates and the coding information of an RNA template to extend an RNA primer by the successive addition of up to 14 nucleotides—more than a complete turn of an RNA helix. Its polymerization activity is general in terms of the sequence and the length of the primer and template RNAs, provided that the 3′ terminus of the primer pairs with the template. Its polymerization is also quite accurate: when primers extended by 11 nucleotides were cloned and sequenced, 1088 of 1100 sequenced nucleotides matched the template.
Highly Polarized Photoluminescence and Photodetection from Single Indium Phosphide Nanowires
Authors: Wang Jianfang , Gudiksen Mark S. , Duan Xiangfeng , Cui Yi , Lieber Charles M.
Published: July 2002

We have characterized the fundamental photoluminescence (PL) properties of individual, isolated indium phosphide (InP) nanowires to define their potential for optoelectronics. Polarization-sensitive measurements reveal a striking anisotropy in the PL intensity recorded parallel and perpendicular to the long axis of a nanowire. The order-of-magnitude polarization anisotropy was quantitatively explained in terms of the large dielectric contrast between these free-standing nanowires and surrounding environment, as opposed to quantum confinement effects. This intrinsic anisotropy was used to create polarization-sensitive nanoscale photodetectors that may prove useful in integrated photonic circuits, optical switches and interconnects, near-field imaging, and high-resolution detectors.