bluuebunny/crossref_metadata_2025 huggingface dataset:
Residual stresses and fracture properties of magnetron sputtered Ti films on Si microelements
Authors: Ljungcrantz H. , Hultman L. , Sundgren J.-E. , Johansson S. , Kristensen N. , Schweitz J.-Å. , Shute C. J.
Published: July 2002

The influence of different deposition parameters on the microstructure, the residual stress, and the mechanical properties of thin Ti films has been investigated. Ti films were deposited onto Si (001) wafers with a native oxide layer, held at a temperature <250 °C, by dc magnetron sputtering. The film thickness was 1 μm. The microstructure of the films, as determined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), was varied by changing the Ar sputtering pressure PAr between 0.8 and 8.3 mTorr, and by applying a negative substrate bias Vs between 0 and 300 V. The residual stresses in the films were determined by both a beam curvature technique and the XRD sin2 ψ method. For Vs=0, the residual stress in the films changed abruptly from tensile to compressive as PAr decreased below 2.5 mTorr. A maximum tensile stress of 0.4 GPa was obtained at 3 mTorr. The compressive stress reached at PAr=0.8 mTorr was 0.4 GPa. TEM showed accompanying structural changes from columnar structure with a high amount of voids in the grain boundaries to a dense structure at PAr=0.8 mTorr. For PAr=3 mTorr, where the largest tensile stress was obtained, the application of a negative substrate bias resulted in Ar ion bombardment that caused compressive stresses to develop for Vs≳75 V and a stress saturation value of 0.6 GPa was reached for Vs≳100 V. TEM showed a dense film structure for Vs≳75 V. The primary energetic particles bombarding the surface were, Ti atoms in the absence of an applied bias and Ar+ ions for sputtering with an applied bias. The energy of the energetic particle in each case was a function of the deposition condition and for both stress transitions, the energy was shown to be of approximately the same magnitude, ∼15 eV/deposited Ti atom. The Young’s modulus of the Ti film was experimentally estimated, and found in fair agreement with previously reported bulk data. Fracture tests were performed on Ti-coated Si cantilever beams in situ in a scanning electron microscope. The fracture strength of coated cantilever beams were typically 10%–15% lower than that of uncoated beams.
Rational pattern design for in vitro cellular networks using surface photochemistry
Authors: Hickman James J. , Bhatia Suresh K. , Quong Judy N. , Shoen Paul , Stenger David A. , Pike Christian J. , Cotman Carl W.
Published: July 2002

The ability to create patterns of specific silane monolayers by deep ultraviolet lithography has been previously demonstrated, and preliminary attempts have been made to use these patterns to control adhesion and outgrowth of neurons and other types of mammalian cells. Here we report characterization of the mechanisms involved in these photoinitiated processes and their utility in various strategies for creating patterns for biologically relevant systems. We have divided the mechanisms into three general classes. The first is surface photolysis of the silane monolayer, which appears to proceed by a purely photochemical mechanism. The second mechanism involves direct photochemical conversion of a terminal functional group on a silane monolayer into a species with altered properties, e.g., the conversion of a thiol to a more oxidized form that inhibits the subsequent adhesion of proteins. The third is a photolytic degradation of the monolayer. The mechanisms have been probed by x-ray photoelectron spectroscopy, ellipsometry, and wettability measurements. One result of these investigations has been the development of better strategies to create patterns. Controlled growth of hippocampal neurons on high resolution patterns is presented as demonstration of the efficacy of these strategies in spatially dictating cell adhesion. These results have important implications in designing in vitro culture systems to study well-oriented neuronal systems.
Optimization of the configuration of the vacuum pumps in the KEK 2.5-GeV linac
Authors: Saito Y. , Kakihara K. , Horikoshi G.
Published: July 2002

A pressure-distribution analysis was carried out for a vacuum unit of the KEK 2.5-GeV linac in order to improve the pressures at the radio frequency (rf) windows and accelerator columns. The characteristic matrix ([G]) of the vacuum network geometry including the pumps was calculated for several configurations of the pumps. The results show that three additional pumps must be installed near to the rf window, so as to suppress any pressure rise due to outgassing in the long waveguides. A distributed configuration of the pumps causes more substantial reduction of the pressures at every node. However, it cannot cause a drastic decrease in the pressure at the accelerator-column center, because of its poor conductance. Pressure measurements at the nodes have also been carried out for these configurations.
A spectroscopic investigation of phosphate adsorption onto iron oxides
Authors: Nooney M. G. , Murrell T. S. , Corneille J. S. , Rusert E. I. , Hossner L. R. , Goodman D. W.
Published: July 2002

Phosphate adsorption on a thin Fe2O3 film prepared in ultrahigh vacuum (UHV) was carried out in a liquid flow cell and subsequently returned to UHV for spectroscopic examination. Variations in exposure time to phosphatic solutions of differing pH were used to determine the adsorption/desorption kinetics, speciation of the adsorbed phosphate, and the absolute phosphate coverage. Rapid uptake of phosphate occurred in the first 10 min of exposure resulting in coverages of 3×1014 molecules per cm2, followed by a 20 min induction period, and then continued phosphate adsorption. Auger electron spectroscopy (AES) indicated that he maximum phosphate coverage realized in this study was ∼1×1015 molecules cm−2. X-ray photoelectron spectroscopy (XPS) showed that phosphorus shifted to more oxidized states during longer exposures to phospatic solutions.
Fabrication of large-sized polystyrene shells
Authors: Kubo Uichi , Nakano Hitoshi
Published: July 2002

A simple method to fabricate a large-sized polystyrene shell has been developed for use in future inertial confinement fusion experiments. The fabrication technique is based on the emulsion method. Important improvements to the previous technique are that the water phase of the emulsion is gelled by cooling and an oil phase is not stirred in a formation of a water/oil emulsion. In addition, the densities of water and oil phases are adjusted to remove the effect of gravity. The fabricated shells had diameters up to 6.2 mm.
Growth processes of particles in high frequency silane plasmas
Authors: Watanabe Yukio , Shiratani Masaharu , Kawasaki Hiroharu , Singh Sanjay , Fukuzawa Tsuyoshi , Ueda Yoshio , Ohkura Hiroshi
Published: July 2002

Growth processes of particles in high frequency silane plasmas are studied as a parameter of discharge frequency (3.5–28 MHz) or by modulating the amplitude of discharge voltage (125–275 V). Except for the 28 MHz case, particles tend to grow through three phases of nucleation and subsequent initial growth, rapid growth, and growth saturation. A detailed study for 6.5 MHz explains the following features: morphology of particles shows that coagulation of particles plays a crucial role in the rapid growth phase; a coagulation rate of 200 s−1 observed in the rapid growth phase is extremely high compared to a thermal collision rate of 5 s−1 between particles; coagulation almost stops when decreasing the discharge power by about one-fourth at the middle of the rapid growth phase; two size groups of particles with narrow size dispersions coexist during and after the rapid growth phase. For 28 MHz, while, as compared to 6.5 MHz, particles appear early after the initiation of discharge and their density is high by about two orders, their growth rate in the subsequent phase is quite low. To properly explain most rapid growth features, a model, taking into account coagulation between oppositely charged particles, is proposed.
Reaction of dimethylzinc and diethylzinc on the As-rich GaAs(100)-c(4×4) surface
Authors: Lam H.-T. , Venkateswaran N. , Vohs J. M.
Published: July 2002

The reactions of dimethylzinc (DMZ) and diethylzinc (DEZ) on the As-rich c(4×4) reconstruction of GaAs(100) were studied using temperature programed desorption. Both DMZ and DEZ dissociatively adsorbed on the c(4×4) surface to form adsorbed Zn atoms and alkyl groups. The primary reaction pathway for surface methyl groups formed via dissociative adsorption of DMZ was desorption as methyl radicals between 500 and 720 K. A small fraction of the methyl groups also underwent dehydrogenation to form adsorbed hydrogen atoms and deposit carbon on the surface. In contrast, for surface ethyl groups formed from DEZ, the primary reaction pathway was β-hydride elimination to produce ethylene. Comparison of the results obtained in this study with those reported previously for the Ga-rich (4×2) reconstruction of GaAs(100) indicates that surface Ga atoms enhance the dehydrogenation activity of GaAs(100) surfaces.
Wurtzite GaN surface structures studied by scanning tunneling microscopy and reflection high energy electron diffraction
Authors: Smith A. R. , Ramachandran V. , Feenstra R. M. , Greve D. W. , Shin M.-S. , Skowronski M. , Neugebauer J. , Northrup J. E.
Published: July 2002

We report studies of the surface reconstructions for both the Ga-face and the N-face of wurtzite GaN films grown using molecular beam epitaxy. N-face reconstructions are primarily adatom-on-adlayer structures which can be formed by room temperature submonolayer Ga deposition. These structures undergo reversible order–disorder phase transitions to 1×1 in the temperature range of 200–300 °C. Ga-face reconstructions, on the other hand, require annealing to high temperatures (600–700 °C) in order to form, and in most cases they are stable at those temperatures. The film polarity is found to be determined by the initial nucleation stage of the film growth.
Dynamic images of plasma processes: Use of Fourier blobs for endpoint detection during plasma etching of patterned wafers
Authors: Rietman Edward A. , Lee John Tseng-Chung , Layadi Nace
Published: July 2002

By monitoring various process parameters (e.g., applied rf power, flow rate of gases, etc.) as a function of time, we show that Fourier series decomposition of the values of those parameters, at each time step, plotted on polar coordinates, gives closed curves representing the state of the plasma and the activity on the wafer. A change of the shape of the “blob” is a signature of the endpoint. The technique was successfully applied on TiN/poly-Si, WSix, and doped poly-Si gate etch, and contact etch processes. Moreover, the method is reproducible from wafer to wafer and can be used easily by inexperienced operators to spot endpoint in plasma processes.
Thickness-dependent crystallinity of sputter-deposited titania
Authors: DeLoach J. D. , Aita C. R.
Published: July 2002

In this study, we sputter deposited titania films of thickness ranging from 256 to 705 nm on unheated substrates and studied changes in phase constituency as a function of film thickness. X-ray diffraction, and infrared spectroscopy were used for post-deposition analysis. The results show that the thinnest films consisted of anatase, rutile, and an amorphous structure. As film thickness increased, anatase formed at the expense of the amorphous constituent, whereas the amount of rutile per unit film volume remained constant. We hypothesized that if the thickness-related crystallographic changes were caused by bulk annealing effects due to in situ plasma heating during deposition, then it should be possible to reproduce these changes, ex situ, by furnace annealing. The thinnest films were annealed at three temperatures: 300, 500, and 700 °C. There was no change in phase composition at 300 °C. Metallurgical recovery of both crystalline phases occurred at 500 °C. At 700 °C, rutile grew at the expense of anatase, which completely disappeared. The results showed that post-deposition annealing caused crystallographic changes that were inconsistent with the thickness-dependent crystallographic changes in the as-grown films. We suggested that the latter was caused by enhanced surface, not bulk diffusion, possibly due to an increase in substrate temperature during deposition.
Silicon field emitter cathodes: Fabrication, performance, and applications
Authors: Temple D. , Palmer W. D. , Yadon L. N. , Mancusi J. E. , Vellenga D. , McGuire G. E.
Published: July 2002

This article gives an overview of fabrication and performance of a class of vacuum microelectronic devices, namely, silicon tip-on-post field emitter arrays (FEAs). Experimental data illustrating the device performance are presented in the context of requirements for field emission flat panel display and microwave power amplifier applications. Critical geometrical parameters of the device are discussed, and a fabrication process flow designed to optimize these parameters is described. Equipment and methods for testing electrical performance of the FEAs and results thus generated are presented. Specifically, emission current versus gate voltage characteristics for arrays with tips formed using anisotropic (crystallographic–orientation–dependent) or isotropic etching techniques, uniformity of these characteristics across a 4 in. diameter substrate, stability of emission current in ultrahigh vacuum conditions, and changes in emission current upon exposure to active gases at varying pressure are discussed.
Structure engineering for hillock-free pure aluminum sputter deposition for gate and source line fabrication in active-matrix liquid crystal displays
Authors: Voutsas Apostolos T. , Hibino Yoshi , Pethe Rajiv , Demaray Ernest
Published: July 2002

In this article we present the results of a study aimed at developing hillock-free, pure-Al thin-film material suitable for the fabrication of gate and source lines in thin film transistor active-matrix liquid crystal displays. Strong Al(111) texture was shown to be a key attribute for achieving good resistance to hillock formation. To obtain this strong Al(111) texture, we explored the incorporation of a thin metal layer, under the Al film, and we showed that Ti could be an appropriate candidate for further optimization. Key variables affecting the quality of Al were found to be the roughness of the glass substrate, the thickness of the titanium, the background vacuum quality, and the sputtering temperature. By optimizing the deposition process for both Al and Ti layers, we showed that aluminum films with very strong (111) texture (rocking curve full width at half maximum 0.86°), very smooth surface (root mean square surface roughness 1.8 nm), and a uniform, columnar grain size (0.3–0.5 μm) could be deposited by dc magnetron sputtering. In addition to improving the aluminum microstructure, optimization of the gate dielectric process was also performed with the aim of reducing hillock formation. Combined optimization of the sputtering and gate dielectric deposition processes yielded an overall reduction in hillock density of 3.5–4 orders of magnitude, without the need for anodic oxidation or other means of capping the aluminum film. By combining Al/Ti dc sputtering, dry-etching technology for one-step patterning of the composite Al/Ti film, and gate insulator deposition at 300 °C we demonstrated, for the first time, gate metallization technology based on pure Al with excellent step coverage and ultralow hillock density (<103 cm−2).
Scanning tunneling microscopy study of Fe(CO)5 and Fe(C5H5)2 adsorption on Si(111)7×7 and B/Si(111)√3×√3
Authors: Thibaudau F. , Masson L. , Chemam A. , Roche J. R. , Salvan F.
Published: July 2002

The adsorption at room temperature of ferrocene and iron pentacarbonyl on Si(111)7×7 and B/Si(111)√3×√3 R30° have been studied. On Si(111)7×7, the adsorption sites have been identified by means of scanning tunneling microscopy. We propose a ferrocene adsorption model on Si(111)7×7, i.e., a di-sigma bridging by the molecule between an adatom and a restatom site similar to that proposed for the ethylene. This process is in agreement with the lack of reactivity of this molecule on the B/Si(111)√3×√3 R30° surface. For the iron pentacarbonyl, we have found evidence of a dissociative adsorption on nucleophilic sites. At a higher temperature, an exposure to iron pentacarbonyl lead to the growth of good quality iron silicide. Whereas, silicide carbide is formed on exposure to ferrocene. The films obtained can be explained by means of the chemisorption process at room temperature.
Improvement of bonding strength between Au/Ti and SiO2 films by Si layer insertion
Authors: Nagata Hirotoshi , Shinriki Takashi , Shima Kaori , Tamai Masumi , Min Haga Eungi
Published: July 2002

In the fabrication of broad band optical waveguide modulators, a certain technology must be used to prepare 15–20-μm-thick Au electrodes on the SiO2 surface covering the waveguides. A thin transition metal film is commonly inserted between the Au and SiO2 to improve adhesive strength. The transition metal film used in this study is Ti. Here, the SiO2 surface is precoated with Si prior to the Ti film formation, demonstrating a significant improvement in adhesive strength. The Si layer works as a barrier against oxidation of the Ti film due to the SiO2, leading to a homogenous film growth of metallic Ti.
Polycrystalline nanopore arrays with hexagonal ordering on aluminum
Authors: Li A. P. , Müller F. , Birner A. , Nielsch K. , Gösele U.
Published: July 2002

Nanopore arrays with 6×108–5×1010 cm−2 pore densities were fabricated by self-organized anodization on aluminum. A two-step anodization process was used to oxidize aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays were obtained within domains of a few micrometers, which are separated from neighboring domains with different orientation of the pore lattice by domain boundaries, i.e., the nanopore arrays show characteristics analogous to two-dimensional polycrystalline structure. The interpore distance can be controlled by changing the electrolyte and/or the applied voltage.
Photoresponse of photodiode arrays for solid-state image sensors
Authors: Lee J. S. , Hornsey R. I.
Published: July 2002

A closed-form analytical expression for diffusion-limited photocurrent in a closely spaced photodiode array is presented with reference to modern CMOS imaging arrays. The analysis makes use of the fact that the current density at the peripheral edges of the photodiode in a closely spaced array has a maximum value near the surface and decays approximately in a linear fashion with the depth into the substrate. The approximated lateral current density at the peripheral edge is then used to derive an expression for the overall peripheral photocurrent. The final expression shows good agreement with the experimental results.
The stabilization of metastable phases by epitaxy
Authors: Farrow R. F. C.
Published: July 2002

Recent developments in the field of molecular beam epitaxy (MBE) have rekindled interest in the subject of pseudomorphism: the stabilization of metastable phases by epitaxy. These developments have led to the growth of metastable films of α-Sn, α-Sn:Ge alloys, and BaxCa1−xF2 solid solutions. Unlike the early examples of pseudomorphism, these films are relatively thick, i.e., ≳0.5 μm. This has permitted the first quantitative study of strain anisotropy in pseudomorphic films by the technique of double-crystal x-ray diffraction. The growth and investigation of such films using MBE techniques is a new and fertile area of materials research driven by the potentially useful properties which the films exhibit. In contrast with the relative ease with which α-Sn, α-Sn:Ge, and fluoride solid solutions can be grown by MBE, attempts to prepare metastable InSb1−xBix solid solutions have encountered major problems due to the surface segregation of competing liquid eutectic phases on the growth surface. These developments are reviewed and future directions of research in the field of pseudomorphic growth by MBE are discussed.
Molecular beam epitaxial growth of CdTe, HgTe, and Hg1−xCdxTe alloys
Authors: Summers C. J. , Meeks E. L. , Cox N. W.
Published: July 2002

Preliminary results are presented of the growth of CdTe, HgTe and Hg1−xCdxTe layers with x-values between 0.9 and 0.17 by molecular beam epitaxy (MBE). The growths were performed in a MBE system equipped with both binary (CdTe) and elemental Cd, Hg, and Te sources on (111) orientated A faced CdTe wafers. Growth of CdTe layers on (100) orientated GaAs and InP layers was also obtained.
Experimental results from fast electron pattern generator: A variable shaped beam machine
Authors: de Chambost E. , Allanos B. , Frichet A. , Perrocheau J.
Published: July 2002

Fast electron pattern generator (FEPG) is a variable shaped beam lithographic system specially designed for direct writing on wafers and for mask fabrication. Experimental results are given: gold gratings on silicon patterned with 10×0.3 μm spots, 1 μm gate field-effect transistor (FET), masks 1×. All these results performed with a 5 A/cm2 current density are consistent with a 1 μm technology. When the spot current exceeds 5 A/cm2, defects due to the space charge effect appear. Thermal effects on glass substrates are pointed out.
Characterization of CdTe, (Cd,Zn)Te, and Cd(Te,Se) single crystals by transmission electron microscopy
Authors: Rai R. S. , Mahajan S. , McDevitt S. , Johnson C. J.
Published: July 2002

CdTe, (CD,Zn)Te, and Cd(Te,Se) crystals grown by the Bridgman technique have been characterized by transmission electron microscopy. Results indicate that the Te precipitates are seen in all the crystals, but their density and size are lowest and largest in the case of Cd(Te,Se) crystals. In addition, dislocations, stacking faults, and microtwins are observed in as-grown CdTe, (Cd,Zn)Te, and Cd(Te,Se) crystals. Arguments have been developed to rationalize these observations and their ramifications on crystal perfection are discussed.
A tunneling atomic force microscope with inertial tip-to-sensor approach
Authors: Probst O. , Grafström S. , Kowalski J. , Neumann R. , Wörtge M.
Published: July 2002

An atomic force microscope is presented which uses the tunneling effect for signal detection, the tunneling tip-to-sensor approach being based on an inertial sliding mechanism. Concentric tube piezo arrangements of both the sample and slider mounts serve for thermal drift compensation. Particular emphasis lies on the description of the sliding device, sawtooth voltage generation and control for the slider motion, and tip feedback electronics. The image of a 4.7×4.7 μm2 surface section of an optical storage disk illustrates the large-scale scanning performance of the instrument.
Facetless Bragg reflector surface-emitting AlGaAs/GaAs lasers fabricated by electron-beam lithography and chemically assisted ion-beam etching
Authors: Tiberio R. C. , Porkolab G. A. , Rooks M. J. , Wolf E. D. , Lang R. J. , Larsson A. , Forouhar S. , Cody J. , Wicks G. W. , Erdogan T. , King O. , Hall D. G.
Published: July 2002

We report the fabrication and characterization of facetless Bragg reflector surface-emitting AlGaAs/GaAs lasers. Both first-order (120-nm period) and second-order (240-nm period) gratings were fabricated by electron-beam lithography and chemically assisted ion-beam etching (CAIBE). These grating pairs provide the optical feedback of the laser, eliminating the need for cleaved or etched mirror facets. Specifically, this work includes: the fabrication and testing of a variable pitch grating-laser array which demonstrates optical emission peaks with 5-Å separation for adjacent lasers; demonstration of facetless Bragg reflector lasers with 120/240-nm grating pairs that show lower threshold currents, higher quantum efficiencies, and improved beam width compared to conventional facetless second-order grating lasers; and a demonstration of grating surface-emitting diode lasers with hybrid first-order and nonresonant, 120/307-nm, grating pairs that produced a directed beam at 45° with respect to the substrate. The fabrication technology and optical performance of these devices are presented.
Metrology of electron-beam lithography systems using holographically produced reference samples
Authors: Anderson Erik H. , Boegli Volker , Schattenburg Mark L. , Kern Dieter , Smith Henry I.
Published: July 2002

Metrology in an electron-beam lithography system is typically carried out by a combination of beam scan and laser-interferometer-controlled sample motion. The high-resolution technique presented in this paper avoids the stage motion by using a holographically produced grid, which is essentially a permanently recorded interference pattern. This grid can be at least as accurate as the interferometer, and no stage motion, with the potential for additional error sources, is required to map out the distortion in the deflection field. The quality of the grid is critical since it is the reference to which the distortion is compared. With careful control of the holographic exposure system, high-quality low-distortion orthogonal grids were fabricated. We have produced grids with a period of 200 nm and orthogonality of a few arc seconds using an UV laser holographic system. Once the grid is processed to produce a high-contrast signal for either back-scattered or transmitted electrons, both scanning and signal-processing techniques are needed to measure the distortion. If the errors are larger or similar in size to the grating period a moiré technique can be used. Unfortunately, when the distortion is much less than the period this technique does not have the required resolution. Therefore, two new techniques have been developed. The first uses a lock-in amplifier to measure the phase difference between the expected signal and the actual measured signal. The second uses digital image processing to compare the expected and measured images. With both of these techniques resolution of about one beam step (≊6 nm) has been achieved.
Optimization of the capacitance–voltage profiling method based on inverse modeling
Authors: Iniewski K. , Salama C. A. T.
Published: July 2002

The doping profile in semiconductors can be extracted very accurately from C–V measurements using the inverse modeling approach. This method requires very extensive numerical calculations which makes its practical implementation very difficult for process characterization purposes. To minimize the execution time of this technique a dynamic mesh algorithm has been developed. Using this algorithm the decrease in the calculation time of the doping profile parameters of over one order of the magnitude was obtained. The improved computational method was verified using simulation examples and experimental results for ion implanted metal–oxide semiconductor capacitors.
Exciton transport and localization in type-II GaAs/AlAs superlattices
Authors: Gilliland G. D. , Wolford D. J. , Bradley J. A. , Klem J. , Jaros M.
Published: July 2002

We have measured directly the transport of excitons in a type-II short-period GaAs/AlAs superlattice. We find the transport is laser-power dependent, and, at low powers or long times, is diffusive. At low powers, we find diffusion constants which increase monotonically from 2×10−3 cm2/s at 1.8 K to ∼7.0 cm2/s at 30 K. Our results confirm the existence of exciton localization and thermally activated hopping. Implications of these results are discussed.
Barrier modulated semiconductor quantum wires
Authors: Knipp P. A. , Reinecke T. L.
Published: July 2002

Calculations of the electronic states and transition energies of a novel kind of barrier modulated semiconductor quantum wire have been made by solving the two-dimensional Schrödinger equation numerically. Laterally confined electronic states are found with energy spacings up to 10 meV for wire widths down to 250 Å. The results for the transition energies are shown to be in good agreement with recent photoluminescence data for GaAs/InGaAs/GaAs quantum wires using the wire widths measured in scanning electron microscopy. This agreement indicates that these wires have negligible optically inactive dead layers and that their electronic and optical properties are well understood.
Free-electron laser internal photoemission measurements of heterojunction band discontinuities
Authors: McKinley J. T. , Albridge R. G. , Barnes A. V. , Ueda A. , Tolk N. H. , Coluzza C. , Gozzo F. , Margaritondo G. , Martin D. , Morier-Genoud F. , Dupuy C. , Rudra A. , Ilegems M.
Published: July 2002

Internal photoemission (IPE) measurements of the conduction band discontinuities at GaAs/amorphous-Ge interfaces have been carried out at the Vanderbilt Free-Electron Laser (FEL). IPE is a simple and direct measurement capable of unprecedented accuracy. The FEL is uniquely equipped for semiconductor spectroscopy due to its brightness, tunability, and spectral range, 1–8 μm (0.15–1.2 eV). IPE uses FEL photons of sufficient energy to optically pump electrons over the conduction band discontinuity, producing a photocurrent. No complex modeling is required to extract the discontinuity from the experimental data, since it coincides with photocurrent threshold energy. Both linear and Fowler curve fits of the threshold region agree within an accuracy of ±5 meV. The IPE measurements are in agreement with previous conventional vacuum photoemission measurements that were limited to ±100 meV accuracy. Unlike conventional vacuum photoemission, IPE does not have the requirement that interfaces must be grown and measured in situ. Since FEL-IPE utilizes subband gap photons, it can be applied to buried interfaces, as well as interfaces under bias. Because of its accuracy and simplicity, systematic use of this technique over a broad range of semiconductor interfaces promises to resolve long-standing disagreements between competing semiconductor band-lineup theories.
Influence of water vapor on nanotribology studied by friction force microscopy
Authors: Binggeli M. , Mate C. M.
Published: July 2002

To simulate the nanotribological effects of adsorbed water molecules and the capillary condensation of water around contacting asperities, we have studied the friction and adhesive forces acting on the tip of a friction force microscope in contact with a flat surface as function of humidity. On the hydrophilic surface of silicon oxide strong capillary formation occurs around the tip. In contrast, on the hydrophobic surface of silicon oxide covered with a perfluoropolyether lubricant, the water capillary formation around the tip is greatly suppressed. Friction and adhesive forces decrease substantially with increasing humidity during sliding of the tip across both sample surfaces, implying that the surfaces are lubricated by adsorbed water.
Step height measurement using a scanning tunneling microscope equipped with a crystalline lattice reference and interferometer
Authors: Fujii Toru , Suzuki Masatoshi , Higuchi Toshihiro , Kougami Hiroshi , Kawakatsu Hideki
Published: July 2002

A crystalline lattice is used as a scale with an atomic pitch which may provide a resolution as high as an x-ray interferometer when the lattice is clearly observed. The scanning tunneling microscope (STM) which uses a lattice reference for correction is more compact than a STM using a correction method which has similar resolution (i.e., interferometry). Step height measurement requires higher resolution than the linewidth measurement in the XY plane. The scheme for the correction in the XY plane requires a two-dimensional lattice image. However, the motion of the probe in the XZ plane for step height measurement does not provide the two-dimensional image needed for the lattice reference correction since the motion in the Z direction is regulated by surface profiling. To ensure the accuracy of displacement, the instrument must be calibrated by an interferometer. We have developed a new scheme for the correction of the Z directional motion using a lattice reference and an instrument designed for the scheme which has a guide mechanism for straightness of motion and is equipped with interferometers for a length standard. The possibility of step measurement using a lattice reference is demonstrated.
Moiré patterns in scanning tunneling microscopy images of layered materials
Authors: Kobayashi Katsuyoshi
Published: July 2002

Moiré patterns observed in scanning tunneling microscopy of lattice-mismatched systems are studied theoretically. It is found that there are two cases showing the moiré patterns. One is explained by only the electronic-structure effect. In the other case, the effect of the lattice displacement is important. As prototypes of the two cases, scanning tunneling microscopy images of monolayer-graphite and transition-metal–dichalcogenide surfaces are investigated by numerical calculations.
Geometric focusing technology applied to SiO2 etching
Authors: Donohue J. F. , Kent M. , Sampson A.
Published: July 2002

With the current downward trend in operating pressures, the role of ion and neutral densities in maintaining elevated etch rates has become increasingly important in the field of plasma dry etch. Many advanced source technologies have been developed that achieve higher densities and greater reagent production, but they often attain enhancements by imposing additional power requirements on an etch system. This article will show that in some advanced chamber designs, changes in electrode geometry can enhance ion density and/or dissociation and thus improve etch rate, without increasing the radio frequency power demand. The term ‘‘geometrical focusing’’ is used to describe the effects observed on a plasma after modifications were made to the grounded electrode of a commercial etch chamber. The improvements were recorded both in open areas and dense submicron contacts. In the future, more work is planned to investigate the effects of further modification to the focusing design, and more experimentation to determine the optimum process range over which this technology is applicable.
X-ray photoemission spectroscopy analysis of oxide etch polymers in inductively coupled high density plasmas
Authors: Park Jae Hyun , Chung Seoung Woo , Kim Jae Jeong , Kim Woo Shik
Published: July 2002

In ultralarge scale integrated devices, many new requirements appeared for contact processes. These are highly selective oxide etching and less microloading in quarter micron contact holes. High density plasma sources have many advantages to meet these requirements, such as low pressure and proper process range for quarter micron devices. In this study the composition and structure of oxide etch polymer in inductively coupled plasma sources were studied by x-ray photoemission spectroscopy to analyze the characteristics of high density plasma. The experiments for this study were performed in a high density plasma chamber employing inductively coupled plasma sources. Three kinds of postetch treatments were used for the analysis of the composition and the characteristics of the oxide etch polymer. The depth profile and composition data of the polymer residue were obtained by x-ray photoemission spectroscopy analysis.
Nanometer-scale resolution of calixarene negative resist in electron beam lithography
Authors: Fujita J. , Ohnishi Y. , Ochiai Y. , Nomura E. , Matsui S.
Published: July 2002

New nonpolymer materials, calixarene derivatives were tested as high-resolution negative resists for use in electron beam lithography. Arrays of 12-nm-diam dots with a 25 nm pitch were fabricated easily. The sensitivity of calixarene in terms of area dose ranged from 700 to 7000 μC/cm2, and the required dose for dot fabrication was about 105 electrons/dot. The standard area dose for calixarene is almost 20 times higher than that for polymethyl methacrylate (PMMA), but the electron spot dose for dot fabrication by calixarene is almost the same as that for PMMA and other highly sensitive resists such as SAL (chemically amplified negative resist for electron beam made by Shipley). The electron spot dose for such extremely small dots does not seem to depend on standard area dose, but any resist tends to require the same dose under exposure in a 50 keV electron beam writing system. We propose a qualitative exposure model that suggests a tradeoff of dose and dot size. The calixarene seems to be promising material for nanofabrication.
MxP+: A new dielectric etcher with enabling technology, high productivity, and low cost-of-consumables
Authors: Shan Hongching , Lee Evans , Welch Michael , Pu Bryan , Carducci James , Ke Kuang-Han , Gao Hua , Luscher Paul , Crean Gerard , Wang Rynn , Blume Richard , Cooper James , Wu Robert
Published: July 2002

Dielectric etch accounts for more than half of all the dry etches used in integrated circuit (IC) fabrication, and plays a very important role in fulfilling strict requirements of volume-manufacturing of IC circuits whose feature size is progressively decreasing. The challenge of meeting volume manufacture requirements is what MxP+ has achieved through a series of hardware and process innovations. By Pareto analysis of the wet clean time of the MxP chamber, we were able to define six major drivers to address three key issues: (1) reduce wet clean time, (2) eliminate system complexity, and (3) achieve technical excellence. Key components of the MxP+ that allow us to address them include a quartz gas distribution plate which prevents the aluminum particle formation, and the electrostatic chuck which eliminates the mechanical clamp system while reducing the particle contamination and wafer edge exclusion. The unique chamber liners of the MxP+ not only shield chamber walls, but also provided a wide process window. Process characterizations have been done for contact, via, nitride, mask open and self-aligned contact etches, and results show that the etcher is capable of etching dielectric films of 0.35 μm features of either high or low aspect ratios. Extended runs proved the process window of the chamber was very wide, the stability and the uniformity of the process were superior, and particle addition was very low. The etch rates of BPSG and TEOS are about 7500 and 4700 Å/m, respectively, when running at 1000 W on MxP+, which is about 30% to 40% higher than that of MxP. Also, through hardware and process innovations, no periodic dry cleans are needed for the MxP+ to maintain excellent particle performance. Furthermore, the cost-of-consumables is dramatically reduced and the new design of process kits extends its lifetime by a factor of 2 for a much reduced cost.
Normal incidence infrared modulators using intersubband transitions in InAs/GaSb/AlSb stepped quantum wells grown by molecular beam epitaxy
Authors: Du Q. , Alperin J. , Wang W. I.
Published: July 2002

Normal incidence infrared (3–5 μm) modulators based on AlSb/InAs/Al0.4Ga0.6Sb/GaSb/AlSb stepped quantum wells have been successfully grown on (100) GaSb substrates by molecular beam epitaxy. X-ray diffraction characterization showed sharp satellite peaks. The devices were fabricated and measured using a Fourier transform infrared spectrometer at 77 K. The largest normal incidence infrared absorption coefficient (α) of 3200 cm−1 at 5 μm has been obtained at 14 V applied reverse bias.
An evaluation of the intrinsic emittance of a field emitter
Authors: Liu Youfan , Lau Y. Y.
Published: July 2002

The beam quality of an idealized two-dimensional field emitter is evaluated. Under the assumption that the space-charge effects are negligible, a scaling law is constructed for the intrinsic emittance of an isolated emitter. This scaling law takes into account the Fowler–Nordheim (F–N) emission law and the effects of local field enhancement. It is given in terms of the height and width of the emitter, anode–cathode separation and voltage drop, as well as the F–N coefficients A and B.
Methodologies for in situ pyrometric interferometry monitoring and control of molecular beam epitaxy growth of AlAs/GaAs distributed Bragg reflectors
Authors: Lee H. P. , Li Y. , Sato D. L. , Zhou J. J.
Published: July 2002

This article discusses general methodologies for realizing a model-reference molecular beam epitaxy control system for binary AlAs/GaAs distributed Bragg reflector (DBR) structures using single wavelength pyrometric interferometry monitoring. A least-square (LS) phase estimation method for end point detection is described. Experimental results on DBR structures grown with closed-loop control show evidence of a self-compensating mechanism that tends to preserve the periodicity of each DBR pair. The potentials for applying the LS phase estimation method for iterative adaptive control and spectral reflectance monitoring are also discussed.
Photoemission oscillations as an in situ monitor of layer thickness with monolayer resolution
Authors: Zinck J. J. , Chow D. H. , Schulman J.
Published: July 2002

Photoemission oscillations have been observed during the growth of InAs and AlSb on (001) GaAs substrates. The ability to observe photoemission oscillations is dependent on the growth conditions in a manner similar to reflection high-energy electron diffraction oscillations. Consequently, the mechanism responsible for photoemission oscillations is believed to be related to fluctuations in surface step density during deposition. We have used the photoemission oscillation technique to measure the AlSb barrier thicknesses during the growth of InAs/AlSb resonant tunneling diodes. Measured peak current densities are clearly better correlated with barrier thicknesses measured by photoemission oscillations than by barrier thicknesses assumed by time-based ‘‘dead reckoning.’’ Calculations of current/voltage characteristics of the resonant tunneling devices indicate that a substantial amount of As is incorporated in the barrier layer as a result of the growth conditions selected.
Properties and applications of the ‘‘epitaxial shadow mask molecular beam epitaxy technique’’
Authors: Malzer S. , Kneissl M. , Kiesel P. , Gulden K. H. , Wu X. X. , Smith J. S. , Döhler G. H.
Published: July 2002

We report on the properties and applications of the epitaxial shadow mask (ESM) molecular beam epitaxy (MBE) technique. A summary of the results for various devices grown with this method is given as well as an outline for future applications in waveguide opto-electronic integrated circuits (OEICs). The ESM-MBE technique was first introduced for the in situ structuring of the lateral doping profile of n-i-p-i structures to provide selective ohmic contacts. The epitaxial mask, composed of a 9 μm AlGaAs layer with a 3 μm GaAs layer on top was grown on a semi-insulating GaAs substrate. Windows were etched into the mask by employing standard photolithographical techniques and wet chemical etching. For the regrowth the fact is exploited that, at a fixed position of the substrate, the molecular beams from the effusion cells are incident with different angles relative to the mask window. Therefore, the lateral doping profile can be structured in situ during the growth creating exclusive n- and p-doped regions which provide excellent selective and ohmic contacts to n-i-p-i structures. A continuous rotation of the substrate can be retained during the growth of undoped layers while the substrate has to be aligned for delta-doping the layers. With a simultaneous control of the shutter movement relative to the substrate rotation homogeneous doping layers can be achieved by the flash doping technique which means that the respective doping shutter is opened only for a short time when the substrate is in the optimum position. Numerous samples have been investigated and we conclude that employing this growth technique, a similar yield and crystal quality is achieved compared with standard growth techniques. In detail, this has been demonstrated by the growth of Bragg mirrors, which exhibit exactly the expected reflectance except for very narrow shadow mask windows (<20 μm) for which a 1% decrease in the growth rate was observed. Photoluminescence and absorption spectra from quantum well structures have confirmed that heterostructures have the same properties as samples grown on nonstructured substrates. The current–voltage characteristics of n-i-p-i modulator structures and light emitting devices exhibit an excellent p-n junction behavior. In a highly doped electro-optic n-i-p-i modulator device, for example, dark currents lower than 10 nA at Upn=−5 V have been achieved. Future applications for OEICs, like the growth of buried waveguide structures are in progress. Hereby, the ESM-MBE is employed to provide a lateral optical confinement by the in situ structuring of the lateral composition of the sample which results in regions with different refraction index.
Nondestructive test methodology for molecular beam epitaxy grown pseudomorphic high electron mobility transistor materials
Authors: Pao Y. C.
Published: July 2002

A new nondestructive differential mobility, dual-temperature (DMT) high electron mobility transistor (HEMT)-like material test methodology has been developed. This is the first nondestructive HEMT-like material test which is capable of determining the two-dimensional electron gas electrical properties of sheet charge density and mobility. The current work uses a magnetoresistance measurement system, based on the electromagnetic absorption spectroscopy operated at X band (i.e., 9–10 GHz) which provides resistivity and mobility data required for the DMT calculation. A systematic MBE growth experiment was conducted to verify this new test methodology, and the measurement results agreed well with the expected design values.
Improved modulation contrast of asymmetric Fabry–Perot field-effect transistor self-electro-optic effect devices by in situ thickness corrections
Authors: Kuo J. M. , D’Asaro L. A. , Kuo H. C. , Pei S. S. , Chang P. C.
Published: July 2002

We present an in situ optical method for thickness correction during the gas-source molecular beam epitaxial growth of asymmetric Fabry–Perot cavities. Separate growth corrections are made to center the spectrum of the mirror reflectivity, and to place the cavity resonance at the desired wavelength, all by measuring the reflectivity spectrum without breaking the vacuum. We choose to use an AlInP/InGaP quarter-wave mirror to demonstrate that the correction can be made after the growth of only three periods. By adding additional GaAs cap layer thickness and thereby adjust the positions of the Fabry–Perot minima of a AlGaAs/GaAs field-effect transistor self-electro-optic effect devices, improvement of modulation contrast from 3:1 to 9:1 is demonstrated.
Growth and characterization of high mobility two-dimensional electron gases
Authors: Coleridge P. T. , Wasilewski Z. , Zawadzki P.
Published: July 2002

GaAs/GaAlAs based two-dimensional electron gas structures, with peak mobilities up to 6.4 million (cm2/V s), have been grown in a V80H molecular beam epitaxy system. Unlike many systems in which layers with ultra-high mobilities have been grown, this is a general purpose machine with 3 inch wafer capability and is used extensively for a variety of other growth projects. Some of the procedures taken to achieve these high mobilities are presented and some of the special considerations involved in transport characterization are discussed.
Modeling and comparisons of field emitter devices with various geometries
Authors: Kang Jie Hun , Cho Jin Woo , Kim Jae Wan , Kim Jong Min
Published: July 2002

We introduce three different geometries of gated field emitter devices and their simulation results. Each geometry is defined as tip-on-post, flat cathode, and bottleneck, respectively. The optimized cathode height and width of tip-on-post with best emission characteristics are reported. The calculated electric field and emission current are compared at different aspect ratios of tip-on-post cathode. We demonstrate the feasibility of a diamond thin film for triode geometry application with less leakage current. The calculation of the electric field of a flat cathode is determined by variable geometric parameters. Additionally, the feasibility of the bottleneck type of field emission cathode is studied in this simulation.