bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrogen pumping simulation for cryopumps
Authors: Nesterov S. , Vasiliev J. , Wagner L. C. , Boiarski M.
Published: July 2002

When designing a cryopump, it is desirable to have some method of predicting cryopump performance for the different cryopanel geometries that are being considered. Monte Carlo simulation is a method that has been used for evaluating the pumping speed of a cryopump when it has not yet accumulated any gas. The simulation of hydrogen pumping has its own challenges due to the typical location of the hydrogen pumping sites. As gases accumulate on the pumping surfaces, the passageways for hydrogen gas are restricted. In this article we describe the work that was done, using Monte Carlo simulation, to study the hydrogen pumping speed and capacity of a 200 mm cryopump as it accumulates hydrogen, water and argon. The hydrogen pumping speed is predicted as a function of the amount of other gases that have been accumulated.
Application of the footing effect in the micromachining of self-aligned, free-standing, complimentary metal–oxide–semiconductor compatible structures
Authors: Ayón A. A. , Ishihara K. , Braff R. A. , Sawin H. H. , Schmidt M. A.
Published: July 2002

The footing or notching effect is observed when silicon or polysilicon layers on buried dielectric films are overetched during dry processing. This effect is considered an undesirable feature for most applications, even though it is usually negligible in conventional reactive ion etching of submicron features due to the low current fluxes and small dimensions. However, the etching of wide trenches (several microns or more) using newer high density plasma etchers, can exhibit a notching effect that extends laterally a few microns. The minimization and suppression of notching depends on achieving a balance between the deposition of passivating films and silicon etching. Therefore, we review the dependence of the footing effect on etching conditions in a time multiplexed deep etcher. The microfabrication of cantilevered structures using the footing effect is demonstrated by the micromachining of self-aligned, released electrostatic actuators. Silicon etching, structure release and sidewall passivation or dielectric isolation deposition were done in situ using very large scale integrated complimentary metal–oxide–semiconductor (CMOS) compatible plasma chemistries only. Thus, the low-temperature, soft-mask scheme presented here can be easily integrated in the microfabrication of intelligent sensors and actuators. The measured pull-in voltage for a cantilevered beam 1000 μm long and 3.2 μm thick, of the order of 80 V, agrees with predicted values. The electrostatic actuators prepared in this fashion, subsequently underwent 105 pull-in cycles of wear testing of up to 100 V without failure, confirming the robustness of this approach in the microfabrication of CMOS-compatible actuators and suspended structures.
Effects of coil dc potential on ion energy distribution measured by an energy-resolved mass spectrometer in ionized physical vapor deposition
Authors: Kusano E. , Saitoh T. , Kobayashi T. , Fukushima K. , Kikuchi N. , Nanto H. , Kinbara A.
Published: July 2002

Energy distribution of Ar+ and Ti+ ions and plasma conditions have been investigated for various dc potentials of the rf inductive coil in ionized physical vapor deposition. The sputtering cathode used in the experiment is a conventional magnetron sputtering source with a Ti target (55 mmφ) which is coupled with a rf coil (60 mm φ, made of Cu). The mass spectrometer used to measure ion energy distributions is an energy-resolved type plasma monitor. The coil dc potential is controlled by changing the resistance of the resistor in the termination inductance-capacitance-resistance (LCR) circuit connecting the coil to the ground. By increasing the resistance of the LCR circuit, the peak of the Ar+ energy spectra shifts to a lower energy. In addition, it is found by the probe measurements that the plasma potential decreases with increasing the termination resistance. The changes in the peak energy in the ion energy spectra show a good agreement with the change in the plasma potential; as a result of the change in the plasma potential, the energy distribution of ions accelerated toward the grounded substrate by the sheath shifts to a lower energy in accord with a plasma potential change. In this experiment, plasma potential is strongly affected by the dc self bias of the rf inductive coil induced by a current flow from the ground to the coil through the termination resistance because in the apparatus used the coil was placed inside the chamber.
Investigation on multilayered chemical vapor deposited Ti/TiN films as the diffusion barriers in Cu and Al metallization
Authors: Hu J. C. , Chang T. C. , Chen L. J. , Yang Y. L. , Chen S. Y. , Chang C. Y.
Published: July 2002

A novel multilayered chemical vapor deposition (CVD) Ti/TiN structure was found to be a more effective diffusion barrier in Cu metallization than TiN alone. The Ti and TiN films were deposited by plasma enhanced CVD and low pressure CVD, respectively. In order to reduce the concentration of chlorine in the films, NH3 plasma posttreatment was applied to multilayered CVD-Ti/TiN films. The resistivity of the film was reduced from 240 to 120 μΩ cm using NH3 plasma posttreatment. Cu was electroplated on the multilayered CVD-Ti/TiN films. X-ray diffraction patterns showed a small (002) peak and strong (111) peak from the Cu film. The leakage current was kept low during the device application test indicating the Ti/TiN film possessed an enhanced barrier property over the TiN film alone.
On-line patterned wafer thickness control of chemical-mechanical polishing
Authors: Smith Taber H. , Fang Simon J. , Stefani Jerry A. , Shinn Greg B. , Boning Duane S. , Butler Stephanie W.
Published: July 2002

We present a gauge study of an on-line metrology system for chemical-mechanical polishing and a 600 wafer run by run (RbR) control experiment enabled by on-line wafer measurement. The variability, reliability, and accuracy of the on-line metrology system are found to be very good. We show that a simple control approach provides a root-mean-squared error of less than 100 Å. In contrast, using pilot wafers and sheet film equivalents to control a process results in a 39% decrease in performance, and that using fewer sites may increase variability and lead to an incorrect controlled thickness. We outline how an 8%–80% improvement in throughput, as well as several reductions in cost of ownership, are possible using on-line metrology in conjunction with run by run control.
Raman scattering and infrared absorption in multiple boron-doped Ge dots
Authors: Liu J. L. , Wu W. G. , Tang Y. S. , Wang K. L. , Radetic T. , Gronsky R.
Published: July 2002

Multiple boron-doped Ge quantum dots are investigated. The structure, which consists of 20 periods of Ge quantum dots stacked with 6 nm Si spacers, is grown on a Si (100) substrate by solid source molecular beam epitaxy. Cross-sectional transmission electron microscopy and atomic force microscopy are used to characterize the structural properties of these Ge dots. Raman spectrum shows a downward shift of the Ge–Ge mode, which is attributed to the phonon confinement in the Ge dots. From polarization dependent Raman spectra, the strong inter-sub-level transition in the Ge quantum dots is observed. The transition is further confirmed by Fourier transform infrared spectroscopy using a waveguide geometry. The observed peak at 5 μm in the infrared absorption spectrum is consistent with that in the Raman spectrum and attributed to the transition between the first two heavy hole band states of the Ge quantum dots. The polarization dependence measurement is used to study the nature of the transitions. This study suggests the possible use of Ge quantum dots for infrared detector application.
Analysis of high-index Si(001) surfaces by reflectance difference spectroscopy
Authors: Mantese L. , Xue Q. K. , Sakurai T. , Aspnes D. E.
Published: July 2002

We report surface-induced optical anisotropy spectra of high-index Si(115), (114), and (113) surfaces obtained using reflectance difference spectroscopy. Air-oxidized surfaces show sharp derivative-type features that are step-induced and located near the critical point energies of bulk Si, consistent with those of lower-index Si(001) surfaces. Clean reconstructed surfaces are characterized by a broad feature near 3 eV that tends to decrease in amplitude upon H exposure and a step-induced structure near the (E0′,E1) transition of bulk Si. In contrast, H exposure of Ge-covered surfaces tends to sharpen and enhance lower-energy structures. The derivative-type features located near the bulk critical point energies of Si can be described in terms of electronic states localized by the finite penetration depth of light.
Effect of growth rate on the nucleation of α-Fe2O3 on α-Al2O3(0001) by oxygen-plasma-assisted molecular beam epitaxy
Authors: Yi S. I. , Liang Y. , Chambers S. A.
Published: July 2002

We have investigated the role of growth rate on the nucleation of epitaxial α-Fe2O3 on α-Al2O3(0001). We show that a slow growth rate (∼1–2 Å/min) must be employed during growth of the first 30 Å in order to form relaxed, three-dimensional islands. A higher growth rate (∼0.1–0.3 Å/s) can then be used. Island coalescence occurs, resulting in a laminar surface for film thicknesses as low as a few hundred Å. If a higher initial growth rate is used, three-dimensional island growth is kinetically impeded, and film relaxation occurs by misfit dislocation generation. The film surface then roughens on a more macroscopic length scale, giving rise to a poor quality surface.
In situ formation, reactions, and electrical characterization of molecular beam epitaxy-grown metal/semiconductor interfaces
Authors: Chen L. C. , Caldwell D. A. , Finstad T. G. , Palmstro/m C. J.
Published: July 2002

In situ electrical characterization is used to study the interface properties and the contact penetration during reactions at metal/semiconductor interfaces. Ni contacts were formed in situ by deposition through a removable molybdenum shadow mask on molecular beam epitaxy-grown n-type GaAs(100) c(4×4) As-rich surfaces. Annealing at 300 °C resulted in NixGaAs (x≈3) formation. Subsequent exposure of the NixGaAs to an As4 flux at 350 °C resulted in the formation of NiAs at the surface and the epitaxial regrowth of GaAs at the Ni3GaAs/GaAs interface. The Schottky barrier height (φbn=0.68 V, as deposited) increased with NixGaAs formation (φbn=0.87 V) and decreased slightly with subsequent As4 exposure (φbn=0.85 V). A thin buried n+ marker layer was used to determine changes in the metal/semiconductor interface position from in situ capacitance–voltage measurements. The marker-layer movement demonstrated consumption and subsequent regrowth of GaAs beneath the contact. The ideality factor obtained from current–voltage measurements for the contacts on regrown GaAs was ⩽1.11, which is indicative of the high electrical quality of the regrown GaAs.
Plasma diagnostics of magnetic field assisted ionized magnetron sputtering
Authors: Joo Junghoon
Published: July 2002

The effects of an axially applied axial magnetic field on ionized magnetron sputtering with an internal inductively coupled plasma (ICP) antenna were diagnosed with a Langmuir probe, an impedance probe, and optical emission spectroscopy (OES) to find an operation regime for low energy ionized deposition for low defect density films. A very weak axial magnetic field (Bz) of 20 G was found to reduce plasma potentials by 45% while keeping the ion current density at 80% of the value without Bz. Also based on OES measurements, the plasma density was increased by three times with 5 G of Bz but the coil sputtering was markedly reduced by increasing Bz. However, the radial uniformity of ion saturation current with Bz was about 2.5%–10.4% which was less than the 1.3%–5.5% observed without Bz (std. dev./avg.×100%). However, when the substrate was moved close to ICP coil by 1 cm from 5 cm above the ICP antenna, the radial uniformity was improved to 0.9%–5%, which is thought to be due to a balance between E-mode and H-mode discharge.
A comparative study on inductively-coupled plasma high-density plasma, plasma-enhanced, and low pressure chemical vapor deposition silicon nitride films
Authors: Yota J. , Hander J. , Saleh A. A.
Published: July 2002

Silicon nitride films have been deposited using inductively-coupled plasma high-density plasma chemical vapor deposition (HDP CVD), plasma-enhanced chemical vapor deposition (PECVD), and low pressure chemical vapor deposition (LPCVD) methods. Characterization and comparison of the three films were performed using Fourier-transform infrared spectroscopy, secondary-ion mass spectroscopy, Rutherford backscattering spectrometry, and hydrogen forward-scattering spectrometry, in addition to wet-etch rate and stress measurement studies. It was found that silicon nitride films deposited using HDP CVD method have several advantages over the silicon nitride films that were deposited using the LPCVD and PECVD methods. The HDP CVD silicon nitride film can be deposited at much lower temperatures (⩽400 °C) than LPCVD silicon nitride, and has substantially less hydrogen (5.5 at. %) than the PECVD film. In addition, the PECVD film contains some oxygen in the film. The wet-etch rate of HDP CVD silicon nitride film is comparable to that of LPCVD film and is significantly less than that of PECVD film in both hot phosphoric acid and buffered HF solutions. The stress of the HDP CVD film is similarly compressive to the PECVD silicon nitride, and not as highly tensile as that of LPCVD silicon nitride.
History of industrial and commercial ion implantation 1906–1978
Authors: Yarling C. B.
Published: July 2002

This article reviews the history of ion implantation, the genealogy of commercial implanter manufacturing companies, and identifies some of the people who have helped to make the industry what it is today. It will concentrate on the time period from 1906, when the first ion implanter was “invented,” until 1978, when ion implanters are considered to have come of age. It is clear that the ion implanter has enabled semiconductor technology to travel the International Technology Roadmap for Semiconductors, where microns of junction depths in the mid-1960’s are now in the submicron regime at the start of the new millennium. However, new shallow doping technologies that may eventually replace some implant steps are now being developed by several companies.
InAsSb strained-layer superlattices for long wavelength detector applications
Authors: Osbourn G. C.
Published: July 2002

InAsSb strained-layer superlattices (SLS’s) are proposed as novel III–V semiconductor materials with the potential for long wavelength intrinsic detector applications. Theoretical studies of the band gaps of various InAs0.4Sb0.6/InAs1−xSbx SLS’s with x > 0.6 have been carried out. The results indicate that the wavelength response of various SLS’s with x ≳ 0.73 can be extended to 12 μm at 77 K through the intentional use of layer strains. These new structures offer the metallurgical and processing advantages of III–V semiconductors for 12 μm detector applications. Further advantages include a weaker dependence of the SLS band gap on composition and reduced band-to-band tunneling in the SLS compared to bulk Hg0.8Cd0.2Te.
Plasma-assisted etching mechanisms: The implications of reaction probability and halogen coverage
Authors: Winters Harold F. , Coburn J. W.
Published: July 2002

Evidence is presented which indicates that the way in which energetic ion bombardment accelerates the etching of Si in a fluorine environment is by a direct acceleration of the product formation step and not by damage-enhanced chemistry. The use of fluorine coverage to characterize the etching process is discussed in detail and the assumption that the etch rate of Si can be decoupled into a spontaneous and an ion-assisted contribution is questioned. Finally, examples of the influence of ion bombardment on Al(100)–Cl2, Cu(100)–Cl2 at 310 °C, and Nb–XeF2 are used to emphasize that some of the conclusions reached for the Si–XeF2 system are not generally applicable to all gas–solid combinations.
Growth mechanism of thin oxide films under low-energy oxygen-ion bombardment
Authors: Todorov S. S. , Fossum E. R.
Published: July 2002

Bombardment of silicon surfaces by low-energy oxygen ions has been investigated as a possible process for growing films of SiO2 at room temperature. Broad ion beams of energy 40–200 eV and variable oxygen content have been used to grow ultrathin oxides of extremely uniform thickness. The ion beam oxides are similar to thin thermal oxides in many respects—composition, chemical binding, optical, and electrical properties. The dependence of the thickness and quality of the oxide films on ion dose, ion energy, and substrate temperature have been investigated. The obtained thickness is observed to vary only slightly with increasing substrate temperature up to 650 °C which indicates nonthermal process kinetics. The ion-beam oxides reach a limiting thickness of 40–60 Å which is largely independent of ion dose and is also found to be insensitive to ion energy. The observed oxidation is explained on the basis of surface implantation and radiation-enhanced diffusion and reaction processes. Limited thicknesses are observed even when sputtering is negligible because of the decreasing effective penetration of the ions due to the swelling of the target which accompanies the conversion of Si to SiO2. Thus the film grows until the oxide–semiconductor interface moves beyond the current-ion penetration depth after which oxidation effectively stops. This model is equally applicable to high-energy, high-dose oxygen-ion implantation for production of buried oxides in silicon-on-insulator technology where it is observed that oxide growth occurs predominantly at the upper interface.
Step-and-scan lithography using reduction optics
Authors: Buckley Jere D. , Galburt Daniel N. , Karatzas Charles
Published: July 2002

Scanning 1:1 optical lithography has a long record of success that has been greatly abetted by the relatively straightforward nature of 1:1 ring-field optics and by the fact that mask and wafer can be mounted to a common scanning carriage. However, as lithography requirements progress into the submicron regime, there is strong motivation to employ reduction optics in order to keep the difficulty of the mask-making task within reasonable bounds. A new ‘‘step-and-scan’’ lithography tool achieves this goal and combines the best features of scanners and steppers by sequentially scanning 20×32.5-mm subfields. Key features of this system are a ring-field projection system, with a 4:1 reduction ratio, and a scanning system in which the reticle and wafer are carried by separate stages driven at different but precisely synchronized velocities.
On the design and effective strength of stigmators for electron beam lithography
Authors: Gesley M. , DeVore W.
Published: July 2002

Past expressions for stigmator strength are applied to experimental data using three different stigmators; two having magnetic focusing with low geometric aspect ratios (bobbin length to diameter), the other using electrostatic focusing with a larger aspect ratio. While the functional dependence of the variables is confirmed, the measured effective lengths are found to be much less than predicted from heuristic formulae, which assume either the quadrupole field is defined by the physical length of the stigmator, or worse, by a longer length due to a fringe-field effect. This fringe-field assumption becomes progressively worse as the aspect ratio decreases or when the stigmator is placed near ferrous material. The use of a two-stigmator column design to reduce blanker-induced beam motion is also discussed. The stigmator located in the final lens is used to form an anastigmatic column section between the conjugate blanker and substrate. The upper stigmator independently stigmates the beam at the target.
Temperature dependent transport measurements on strained Si/Si1−xGex resonant tunneling devices
Authors: Gennser Ulf , Kesan V. P. , Iyer S. S. , Bucelot T. J. , Yang E. S.
Published: July 2002

Temperature dependent transport through strained Si/SiGe hole resonant tunneling diodes has been studied. A quantitative measure of the different current components due to the conduction through the light hole and heavy hole resonant states at any particular bias and temperature has been obtained, and energy values for states in the SiGe well have been extracted. We also show that the quenching of the negative differential resistance at room temperature in hole resonant tunneling diodes is due to thermally assisted tunneling through higher resonant states.
Etching of indium tin oxide in methane/hydrogen plasmas
Authors: Adesida I. , Ballegeer D. G. , Seo J. W. , Ketterson A. , Chang H. , Cheng K. Y. , Gessert T.
Published: July 2002

The reactive ion etching of the transparent conductor, indium tin oxide (ITO), in methane/hydrogen plasmas has been characterized. It is shown that ITO can be selectively etched on GaAs and AlGaAs. Anisotropic structures and gratings with submicrometer dimensions in ITO are presented. Application of the etching process to the fabrication of highly sensitive metal–semiconductor–metal (MSM) photodetectors with interdigitated ITO fingers is demonstrated.
Improved growth uniformity in molecular-beam epitaxy: Alternative strategies
Authors: Aers G. C. , Wasilewski Z. R.
Published: July 2002

We use computer modeling to evaluate alternative strategies for the improvement of molecular-beam epitaxy (MBE) growth uniformity. Using GaAs growth in a V80H MBE system as an example, we evaluate the effect of shifting the effusion cell away from the substrate on layer uniformity, Ga melt temperature, and yield. Four different crucible systems are compared. We find that, for the V80H system, pulling back a straight wall crucible brings substantial uniformity improvement only when the crucible orifice is capped with an annular disk. This modification is not necessary for a conical crucible or straight wall crucible with a conical insert. The increase in the cell operating temperature due to the pull back is found to be about 50 °C for a 10 cm cell shift for all the crucible designs. Depending on the cell, such a shift can improve layer uniformity over a 3 in. wafer by up to a factor of 5.
Low stress and optically transparent chromium oxide layer for x-ray mask making
Authors: Trube Jutta , Yabe Hideki , Aya Sunao , Marumoto Kenji , Matsui Yasuji
Published: July 2002

This article describes the properties of dc-sputtered chromium oxide layers such as internal stress, density, growth rate, and microstructure. Besides these mechanical properties, the composition of the layers and the optical properties such as transmissivity and refractive index were also investigated. Transparent, low-stress, and smooth chromium oxide layers were obtained by optimizing the sputtering conditions: gas pressure, O2 content in the working gas, dc-power density, and annealing temperature. The most critical parameter is the O2 content in the working gas. Using these optimized chromium oxide layers in the mask fabrication process as a masking, as well as an etch stop layer for the W etching, the etching behavior during absorber patterning in a subtractive x-ray mask production process was improved.
PROXECCO—Proximity effect correction by convolution
Authors: Eisenmann Hans , Waas Thomas , Hartmann Hans
Published: July 2002

In electron-beam lithography the proximity effect limits achieved resolution. A possible approach for proximity effect correction is the transform based method which uses deconvolution. This method requires high computing power and data reduction algorithms because of the small grid size. The fine grid is mainly needed for preserving pattern, not for the resolution of the correction. The new method called PROXECCO avoids this disadvantage by separating the calculation into correction related and pattern related steps. The grid size is spatially independent from pattern and can be as coarse as needed for correction. This results in a quadratical gain in computing time for the transformation. The pattern dimensions are kept exactly maintained and data reduction is not needed anymore.
Fabrication of submicron apertures in thin membranes of silicon nitride
Authors: Amar A. , Lozes R. L. , Sasaki Y. , Davis J. C. , Packard R. E.
Published: July 2002

A technique for manufacturing submicron apertures used for investigating hydrodynamic quantum phenomena in the superfluids 4He and 3He is described. The apertures are fabricated in ∼100 nm thick suspended membranes of silicon nitride by using e-beam lithography and reactive ion etching.
Lateral field-emission devices with subtenth-micron emitter to anode spacing
Authors: Oro James A. , Ball Daniel D.
Published: July 2002

Field-emission vacuum microelectronic devices, which operate at a low voltage, require sharp emitters and minimal emitter to anode spacing. Using high resolution electron beam lithography in combination with metal lift-off, we have fabricated lateral multitip diodes with emitter to anode spacings ranging from 0.03 to 0.5 μm. This is the smallest emitter to anode gap spacing of any lateral structure reported in the literature. The devices were tested in the sample chamber of our scanning electron microscope (SEM) which has been adapted with fixed probe tips and associated electrical feed throughs. The SEM’s secondary electron detector was used to confirm field emission by detecting electrons generated during device operation. The electrical data show low voltage operation and the linearity of the Fowler–Nordheim plots provides another confirmation of field emission. We have also fabricated triodes with 0.5 μm emitter to anode spacing by adding a columnar gate electrode to the basic diode structure.
New characterization method of ion current-density profile based on damage distribution of Ga+ focused-ion beam implantation in GaAs
Authors: Assayag G. Ben , Vieu C. , Gierak J. , Sudraud P. , Corbin A.
Published: July 2002

A new method is reported for characterizing focused ion probe current distributions based on the comparison between damage simulations and transmission electron microscopy observations. Several focused-ion beam operation conditions were modeled, such as low-to-high source emission currents and variable beam acceptances. At low current and small acceptance, the ion spot exhibits a nearly Gaussian profile, otherwise larger tails are evidenced which can be modeled either by Pearson or ‘‘bi-Gaussian’’ distributions. The sensitivity of the procedure to the tail extension is highlighted.
Ultra-shallow boxlike profiles fabricated by pulsed ultraviolet-laser doping process
Authors: Ishida Emi , Sigmon Thomas W. , Weiner Kurt H. , Frost Michael R.
Published: July 2002

Ultra-shallow, boxlike impurity profiles are produced using gas immersion laser doping (GILD) and then analyzed by spreading resistance profilometry (SRP) and secondary ion mass spectrometry (SIMS) to determine the impurity distribution. At high concentrations, the profiles obtained by SRP exhibit the expected boxlike shape over the entire range of junction depths: The measured concentration within the junction region is uniform while the dopant gradient at the junction exceeds 1 decade/5 nm. In comparison, the same profiles analyzed using high primary ion energy SIMS show a broader transition at the metallurgical junction. Caused by knock-ons and ion mixing during the sputtering process, the inaccuracy is markedly reduced by lowering the acceleration energy of the primary Cs+ ion beam. At lower concentrations (<1019/cm3), profiles analyzed by SRP exhibit shallower junctions than expected. Electrical measurements of diodes and Hall structures show that high-quality, ultra-shallow n+p, np, and pn junctions are fabricated with good dose control using GILD. For complete characterization of GILD, accurate measurement of both chemical and electrically active dopant profiles are required. At present, neither SIMS nor SRP provides an entirely accurate impurity profile.
Imaging of surface electrostatic features in phase-separated phospholipid monolayers by scanning Maxwell stress microscopy
Authors: Inoue Takahito , Yokoyama Hiroshi
Published: July 2002

The scanning Maxwell stress microscope (SMM) is a versatile electric force microscope, which has been developed here to image a variety of electrical features of surfaces and thin films. By the SMM, imaging the surface potential distribution over phase-separated Langmuir–Blodgett monolayers of phospholipid deposited on metal substrates has been successful. For monolayers deposited at a surface pressure corresponding to liquid expanded–liquid condensed phase separation, the presence of circular domains with diameters around 1 μm has been clearly visualized, giving direct insight into the electrostatic interactions in the lipid monolayers.
Cross-sectional scanning tunneling microscopy of semiconductor vertical-cavity surface-emitting laser structure
Authors: Zheng J. F. , Ogletree D. F. , Walker J. , Salmeron M. , Weber E. R.
Published: July 2002

Scanning tunneling microscopy (STM) studies of a semiconductor vertical-cavity surface-emitting laser (VCSEL) device viewed in cross section on an atomic scale are reported. The strained layer In0.2Ga0.8As/GaAs multiple-quantum wells (MQWs) in the laser active region are imaged with atomic resolution. For the first time, the interface roughness of In0.2Ga0.8As/GaAs MQWs is revealed by imaging spectroscopically different individual indium and gallium atoms. It was found that STM images can directly map the interface electronic structure of Al0.67Ga0.33As/GaAs multiple layers in the Bragg reflectors of the VCSEL. The images reflect enhanced or reduced interface electron concentration in regions with a spatial extension of ∼100 Å. The bias effect is also discussed in the imaging of heterostructures.
Studies on the scanning tunneling microscopy images of adsorbates with the method of exciton dynamics and their application
Authors: Li Y. P. , Huang Xintang , Zhang Haifeng , Huang Wenhao
Published: July 2002

By the method of exciton dynamics, direct formulas were deduced about the relationship between the tunneling current and the position of the tip for both single and dimer adsorbates when the purely coherent memory function is independent of time, and the application environment for these practical models were discussed. In addition, they were also used to investigate the effect of the substrate’s thickness on the scanning tunneling microscope (STM) images and interpret the STM images of Au(110) for both 2×1 and 3×1 reconstructed surfaces. The results are in good agreement with experiments and other theory.
Current characteristics for the scanning tunneling microscope
Authors: Stamp A. P. , McIntosh G. C. , Liu Xue-Wen
Published: July 2002

A theoretical investigation of the characteristics of the tunneling current in the scanning tunneling microscope (STM), taking into account the atomic structure of a nanoprotrusion tip ending in a single atom and nanostructure features of the sample, is presented. The current is determined from the total wave function for the tip-sample system. Scattering theory is used to determine the total wave function in terms of the local Green’s functions. Because of the lateral confinement of the electrons by the mesoscopic size of the probing tip, the current through the STM exhibits a Gaussian spatial dependence. It is shown that the constant current mode of operation does not give an accurate description of the topographical features of the potential and hence the surface of the sample.
Theory of scanning tunneling microscopy
Authors: Liu Xue-Wen , Stamp A. P.
Published: July 2002

To overcome the drawbacks of the transfer Hamiltonian method of Bardeen, which is predominantly used in the theory of the scanning tunneling microscope (STM), we develop a new system of formulation in three dimensions accounting for the explicit correlation between the measured current and the electronic states of both the tip and the sample surfaces with the coupling between them being taken into account. It is found that the contrast or the corrugation of the STM image can be essentially modified by the nature of tip electronic state and that the resolution of the STM is determined by the coupling between the tip and the sample along with the tip electronic state. Comparison with other theories are given.
Corrugation reversal in scanning tunneling microscopy
Authors: Chen C. Julian
Published: July 2002

An intriguing corrugation reversal in scanning tunneling microscopy (STM) images has been observed on several metal surfaces as well as on graphite. With a sudden change of tip condition, under the same tunneling conditions, the STM image can switch from positive (atoms being protrusions) to negative (atoms being depressions) and vice versa. In both cases, the absolute values of corrugation amplitude can be one order of magnitude greater than what is expected from the electron charge density near the Fermi level. We explain this effect as a manifestation of tip electronic states. If m≠0 states dominate the tip electronic states near the Fermi level, images with inverted atomic corrugation are expected. For an m=0, l≠0 tip state, noninverted images with large corrugation amplitude are expected. The type of image (i.e., inverted or noninverted) is expected to be independent of the tip–sample distance. The absolute values and the distance dependence of corrugation amplitudes expected from the tip–state effect are in agreement with experimental observations. Alternative explanations are discussed in conjunction with experimental observations.
Influence of a ferromagnetic tip on the Abrikosov vortex lattice in NbSe2 studied by low-temperature scanning tunneling microscopy
Authors: Behler S. , Pan S. H. , Bernasconi M. , Jess P. , Hug H. J. , Fritz O. , Güntherodt H.-J.
Published: July 2002

We use a high-vacuum low-temperature scanning tunneling microscope to study the Abrikosov vortex lattice in NbSe2 at 2.7 K. Vortex lattices imaged with a ferromagnetic Ni tip in an external field show a higher vortex density than lattices imaged with a nonmagnetic tip in the same field. The additional field induced by the Ni tip can be extracted from the vortex density and is found to be about 10 mT, independent of the external field. In zero external field, the Ni tip generates vortices with a density corresponding to 10 mT. The scanning motion of the Ni tip neither affects single vortices, nor the complete vortex lattice. We attribute this to the homogeneity of the tip stray field.
Study and control of molecule–surface interaction at the atomic level: Sb4 on Si(001)
Authors: Mo Y. W.
Published: July 2002

Scanning tunneling microscope (STM) is used to study the reaction kinetics of Sb4 on Si(001). Four distinct types of precursor states are observed which can be converted to the final state of chemisorption by either thermal annealing or by a STM tip-induced conversion process. The STM tip can also reversibly rotate the orientation of individual Sb dimers, as well as displace them. A mechanism based on the polarizability is proposed for the interaction between the STM tip and the Sb clusters. A new image–anneal–image method is developed for studying surface kinetics in which the same adsorbates are imaged before and after thermal annealing. This allows direct observations of the thermal reaction of individual adsorbates while avoiding potential STM tip effects on the reaction. Using this method, the detailed reaction path of Sb4 on Si(001) is directly determined. The precursors are found to be immobile before conversion, contrary to the popular conception about precursors.
Investigation of scanning tunneling microscopy tunneling barrier signals in air and water
Authors: Song J. P. , Mo/rch K. A. , Carneiro K. , Thölén A. R.
Published: July 2002

In an atmospheric environment and in water, the tunneling barrier signals are usually very low compared with those measured in the ultrahigh vacuum. The tunneling barrier data presented in this paper show that the barriers in air and in distilled water have a statistical character. The barrier mean value depends not only on the tip and specimen, but also on the medium in the tunneling gap. By repeated scannings of specimen surfaces, it is found that the barrier heights connected to individual specimen locations are reproducible. This is illustrated by repeated scans on a (TiN on W)/water interface obtained with a tungsten tip. The areas of TiN and W are clearly distinguished. Measurements of tunneling barrier heights are therefore suited for nucleation studies of surface coatings and for the detection of different material components in a specimen.
Monolayer growth oscillations and surface structure of GaAs(001) during metalorganic vapor phase epitaxy growth
Authors: Reinhardt F. , Jönsson J. , Zorn M. , Richter W. , Ploska K. , Rumberg J. , Kurpas P.
Published: July 2002

Reflectance anisotropy spectroscopy (RAS) was used to study metalorganic vapor phase epitaxial growth of GaAs(001). In previous studies, GaAs(001) surfaces have been identified as c(4×4)-like under pregrowth arsenic stabilized conditions. During growth the RAS spectra showed significant differences with respect to the pregrowth spectra. Moreover, the RAS signal at a fixed photon energy has been shown to exhibit an oscillatory behavior when growth is initiated. The period of these oscillations has been identified as the time needed for the growth of exactly one monolayer. In this study, the dependence of the oscillations (amplitude and period) on temperature and trimethylgallium (TMGa) partial pressure is investigated and discussed together with the corresponding RAS spectra taken at V/III ratios between 5 and 300. Three different types of RAS spectra are identified during growth. Firstly, at high temperatures and low TMGa partial pressures, the surface anisotropy is nearly the same as in the c(4×4)-like pregrowth state, with a minimum at 2.5 eV (type I spectra). This indicates that the surface during growth is essentially covered with a double layer of As. Secondly, at low temperatures or high TMGa partial pressures a different structure with a sharp minimum at 2.65 eV is observed (type III spectra). This might be related to a surface largely covered with TMGa fragments. Finally, intermediate between these extreme epitaxial conditions, RAS spectra with a minimum at 2.0 eV and nearly vanishing anisotropy around 2.5 eV are found. This indicates that the As dimers of the pregrowth surface have largely disappeared and a new surface structure, possibly consisting of Ga dimers, is now present. Oscillations have their maximum amplitude at the boundary between type II and type III spectra in a pTMGa-T diagram. At this boundary the growth rate becomes nonlinear and starts to saturate with increasing pressure of TMGa.
Compression of field-emission angular distribution using a cathode shield
Authors: Thomson M. G. R.
Published: July 2002

A conventional field-emission electron gun consists of a pointed cathode and an extraction electrode at a positive potential which produces a strong electric field at the cathode surface. Electrons are field-emitted from the cathode, pass through an axial hole in the extractor electrode, and must then be accelerated to full energy. The field emission current density at the cathode surface is very sensitive to changes in the electric field strength. The distribution of electric field on the end of the cathode can be changed by an additional nearby electrode if its size is not large compared to the cathode radius. A cylindrical shield electrode has been proposed which surrounds the cathode, and for convenience can be at the same potential as the cathode. This configuration has been examined theoretically using second-order finite element analysis. The results of this analysis show that the change in distribution of the electric field on the cathode will depend upon the relative dimensions of the cathode and the shield, and, for a significant change in emission distribution, the shield radius must not be larger than approximately five times the cathode radius. The addition of the shield leaves the virtual source size unchanged, so the brightness of the source is approximately proportional to the axial current density. It has been suggested that combined cathodes and shields can be fabricated by ion milling to form a pointed cathode in the end of a shield.
Mechanism for anisotropic etching of photoresist-masked, polycrystalline silicon in HBr plasmas
Authors: Cheng C. C. , Guinn K. V. , Donnelly V. M.
Published: July 2002

After etching in a helical resonator HBr plasma, polysilicon (poly-Si) micron-size features masked with photoresist (PR) were investigated with angle-resolved x-ray photoelectron spectroscopy and scanning electron microscopy to spatially resolve surface chemical compositions. The poly-Si sidewalls and trench bottoms, and the PR sidewalls are covered with one to two monolayers of SiBr and SiBr2. No SiBrx species are present on top of the PR surfaces. No carbon or oxygen were detected on the poly-Si sidewalls, suggesting that line-of-sight deposition of carbon from sputter erosion of the corners and sides of the PR does not occur. Because Br atoms react very slowly with poly-Si, relative to the ion-assisted etch rates, anisotropic etching does not require sidewall passivation from products of PR erosion. A SiBrx layer does form on the PR sidewall, however, and could play a role in suppressing lateral erosion of the mask and improving profile control with HBr versus Cl2 plasma etching.
Imaging fibers by atomic force microscopy
Authors: Curtin Carter Margaret M. , McIntyre N. Stewart , Davidson Ross , King Hubert W.
Published: July 2002

Glass textile fibers have been used to demonstrate an atomic force microscope technique presented in this article. This method can be applied for imaging the surface of any micron or larger sized fibers. The fibers are discovered to have a variable corrugated surface texture, rather than the previously supposed uniformly cylindrical form, with individual ridges about 5–10 nm high. Scanning electron microscopy images of equivalent glass fibers are included for comparison.
Region-wise proximity effect correction for heterogeneous substrates in electron-beam lithography: Shape modification
Authors: Lee Soo-Young , Liu Bin
Published: July 2002

P Y R A M I D, a hierarchical rule-based scheme for proximity effect correction in electron-beam lithography, has been demonstrated to enable rapid correction of circuit patterns exposed on homogeneous substrates with minimum feature size of 0.1 μm. The current version of PYRAMID modifies the shape of circuit primitives. So far, it has been limited to homogeneous substrates only. This article describes an extension of our pattern shape modification scheme for heterogeneous substrates. The space-varying nature of a heterogeneous substrate makes exposure estimation and proximity correction more involved than in case of a homogeneous substrate. As a result, the schemes developed for homogeneous substrates cannot be used directly used to deal with heterogeneous substrates. A simple but effective approach has been implemented in an effort to allow our shape modification scheme to correct patterns on heterogeneous substrates. The principal idea is to correct a circuit region-wise where each region is homogeneous. A transition zone is defined between two adjacent regions of a substrate and additional adjustment is done on primitives within transition zones to get smooth transition between regions. A fuzzy factor, adjustment factor, is introduced to make the convergence of correction faster. This article describes the region-wise proximity effect correction scheme proposed for heterogeneous substrates and presents preliminary simulation results indicating successful correction.
Study of luminescent porous polycrystalline silicon thin films
Authors: Han P. G. , Poon M. C. , Ko P. K. , Sin J. K. O. , Wong H.
Published: July 2002

Luminescent porous poly-Si thin films can be obtained by electrochemical etching of phosphorus-doped poly-Si films deposited by low-pressure chemical-vapor deposition. As-deposited poly-Si film has no photoluminescence but all porous poly-Si films, large area or micron-size patterns, show comparable orange-red photoluminescences to those obtained from crystal Si. High-resolution atomic force microscopy and scanning electron microscopy analyses show that all porous poly-Si films have smooth surfaces and uniform thicknesses, and are composed of Si grains (∼150 nm) with nanopores (∼20 nm) formed around the surfaces. The pores increase with anodization time, and grow preferentially along the poly-Si grain boundaries and the Si 〈100〉 crystal directions. The evolution of the microstructure is analogous to that of the etching of a coral ball layer due to sea water.
Influence of Coulomb interactions on choice of magnification, aperture size, and source brightness in a two lens focused ion beam column
Authors: Kruit P. , Jiang X. R.
Published: July 2002

Focused ion beam columns with liquid metal ion sources should be optimized to give the highest possible current in a probe of given dimension. A procedure for this optimization is presented, which includes the influence of Coulomb interactions between the particles in the beam. The column is first optimized without including the Coulomb effects and it is found that the optimized magnification and aperture size are independent of the gun brightness. The magnification approaches a constant value in the high current domain, where the gun lens aberrations seriously influence the probe size. The Coulomb effects can be characterized by pointing out a domain in the probe current-probe size plane which is inaccessible, no matter how high the brightness of the gun may be. The approximate position of this inaccessible area can be found with a relatively small number of calculations. It is then demonstrated how the beam current as a new function of probe size can be reoptimized by the choice of magnification and aperture size. Coulomb effects turn out to be dominating the probe size except for very small probes and for very large probes.
Ion implanted nanostructures on Ge(111) surfaces observed by atomic force microscopy
Authors: Chen Y. J. , Wilson I. H. , Cheung W. Y. , Xu J. B. , Wong S. P.
Published: July 2002

Epi-ready Ge(111) surfaces were implanted with cobalt ions to doses of 1016–5×1017 ions/cm2 at accelerating voltages of 40–70 kV. Cellular nanostructures were observed by contact mode and tapping mode atomic force microscopy (AFM). These are similar (at higher resolution) to those reported in earlier scanning electron microscope measurements. Image distortions observed in contact mode AFM are attributed to not only the effect of the tip size but also the change of the effective tip shape due to the softness and stickiness of the implanted surface layer. The variation of the root-mean-square roughness with ion dose (1016–1017 ions/cm2), accelerating voltage (40– 70 kV), and mean beam current density (15–150 μA/cm2) is presented and explained in terms of ion range and surface temperature.
Optical manipulation of a lasing microparticle and its application to near-field microspectroscopy
Authors: Sasaki Keiji , Fujiwara Hideki , Masuhara Hiroshi
Published: July 2002

A novel probe of a near-field scanning optical microscope (NSOM) is proposed. The probe is composed of a micrometer-sized spherical particle doped with laser dye, that acts as an optical cavity for lasing, and of a nanometer-sized particle attached to the microsphere. The high-intensity evanescent field is generated just outside of the lasing microsphere and scattered with the nm particle, which forms a minute light source for NSOM. Three-dimensional positioning control of the probe is performed with the accuracy of <10 nm by a laser manipulation technique. Photon tunneling from the lasing microsphere to an object was demonstrated as a drastic change of an emission spectrum depending on the microsphere-object distance. The lasing microspherical probe has the advantage of high sensitivity due to intracavity enhancement of the tunneling loss.