bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of steps and kinks in catalytic activity
Authors: Ford L. P. , Blowers P. , Masel R. I.
Published: July 2002

In the literature, there is the idea that steps and kinks are the active sites for chemical reactions, but the experimental data are far from convincing. In this article we see if there is a correlation between step atom density, van Hardeveld and Hartog coordination numbers, or the electronic coordination number and reactivity for a number of simple decomposition and hydrogenolysis reactions on platinum as measured by temperature-programmed desorption. We have examined reactions of ethylene, nitric oxide, and methanol on (111), (110)-(1×1), (110)-(2×1), (100)-hex, (100)-(1×1), (210), (511), and (331) platinum surfaces. We have done a statistical analysis of our data to see if any of the correlations are non-negligible. We find that, in general, stepped surfaces have different reactivity than close-packed planes, but some stepped surfaces are more active than Pt(111) while others are less active than Pt(111). There are negligible correlations between step atom density and catalytic activity for our reactions. Similarly, there are negligible correlations between reactivity and the lowest or average van Hardeveld and Hartog coordination numbers. Calculations were done to understand the variations. There are correlations between the lowest electronic coordination number and reactivity for only the reaction of methanol to carbon dioxide, and between the average electronic coordination number and reactivity for the reaction of methanol to methane. The first correlation is not surprising considering the special nature of the rate-limiting step of this reaction. The second correlation raises many yet-unanswered questions. We find that, in general, stepped surfaces relax to more stable geometries. The relaxation process raises the electronic coordination number of the surface atoms. After relaxation, there is not a large difference between the coordination of atoms of platinum stepped surfaces and Pt(111).
Characterization of magnesium fluoride thin films deposited by direct electron beam evaporation
Authors: Dumas L. , Quesnel E. , Robic J.-Y. , Pauleau Y.
Published: July 2002

Magnesium fluoride films have been deposited on quartz and silicon substrates by direct electron beam evaporation. The structure, composition, and mass density of films were investigated as functions of the deposition temperature ranging from ambient temperature to 300 °C. The composition of films determined by infrared spectroscopy, and the residual stresses in films calculated from the change of the radius of curvature of Si substrates, were studied as functions of the aging time of films in room air and deposition temperature. The aging behavior is analyzed and discussed in connection with the microstructure and mass density of films.
Mechanical properties of titanium nitride coatings deposited by inductively coupled plasma assisted direct current magnetron sputtering
Authors: Lim Ju-Wan , Park Heung-Sik , Park Tae-Hyeong , Lee Jung-Joong , Joo Junghoon
Published: July 2002

TiN coatings were deposited on M2 high speed steels by an inductively coupled plasma (ICP) assisted sputtering technique. The structure and mechanical properties such as hardness, Young’s modulus, and adhesion strength of the coatings were investigated. For ICP sputtering, rf power was applied using a rf coil installed in the deposition chamber. Prior to the deposition, the substrate was pretreated by ICP in the same deposition chamber in order to improve the adhesion strength of the coating. The ICP power was varied from 0 to 600 W, and the hardness of the coatings was found to increase with increasing ICP power. When the ICP power was larger than 300 W, the hardness of TiN was above 6000 HK0.01, which was one of the highest hardness values of TiN reported in the literature. The adhesion strength of the coating was also high enough for industrial applications. The results from structural investigations indicated that the high hardness is a property of the dense structure as well as the high compressive residual stress of the coatings.
Oxide-free phosphate films on copper probed by core and valence-band x-ray photoelectron spectroscopic studies in an anaerobic cell
Authors: Rotole John A. , Sherwood Peter M. A.
Published: July 2002

This article reports the results of a study of the formation of phosphate films formed directly on the surface of metallic copper without the presence of an oxide layer. The experiments were performed using aqueous electrochemical treatment in an anaerobic electrochemical cell designed to allow an oxide-free metal surface to be exposed to electrochemical treatment without having to expose the electrode to the atmosphere. The electrochemical treatment was performed using negative voltages (versus a saturated calomel electrode) in 5 M orthophosphoric acid. When the experiments were performed outside the anaerobic cell with as-received metallic copper, the treatment leads to the removal of oxide to leave a metallic surface with negligible amounts of oxide. In the anaerobic cell, however, a film consisting of a mixture of Cu(I) metalphosphate and orthophosphate is formed directly bonded to the metal. When this film is exposed to air for ten days, the surface film, still directly bonded to the metal, is oxidized to Cu(II) orthophosphate, this film remaining in a stable condition. The surface chemistries were identified by valence-band photoemission interpreted by cluster calculations. The work illustrates how, by starting with an oxide-free metal surface and performing the treatment in an inert environment in an anaerobic electrochemical cell, one can achieve unusual surface chemistries.
Electronic properties of strained-layer superlattices
Authors: Osbourn G. C.
Published: July 2002

The electronic properties of semiconductor superlattices can be intentionally varied by changing their quantum well structures, i.e., by changing their layer thicknesses and their layer alloy compositions. New flexibility in varying superlattice electronic properties occurs in strained-layer superlattices, since lattice-mismatched materials can be used for the individual layers. The flexible choice of layer materials and layer thicknesses in strained-layer superlattices makes possible the tailoring of various superlattice electronic properties to desired values. Calculated values for several adjustable material properties in ternary strained-layer superlattices are presented to illustrate this tailorability. The potential advantages of new types of superlattices with more complicated unit cells are also discussed.
Application of orthogonal experimental design to LEED crystallography
Authors: Yang W. S. , Jona F. , Marcus P. M.
Published: July 2002

A description is given of the first application to LEED crystallographic problems of an optimization scheme called orthogonal experimental design (OED) which economizes on selection of trial values. The operational procedures required by the OED method are described, and their use in a LEED intensity analysis of the Si{001}2×1 surface is discussed. This first application to what is undoubtedly one of the most difficult problems in quantitative LEED crystallography was an essential ingredient in finding a satisfactory set of structural parameters for the Si{001}2×1 surface.
Plasma oscillations of layered electron gases in semiconductor heterostructures
Authors: Olego D. , Pinczuk A. , Gossard A. C. , Wiegmann W.
Published: July 2002

The plasma oscillations of layered electron systems in modulation doped GaAs–(AlGa)As heterostructures were investigated by inelastic light scattering. These experiments are the first to probe the dispersion of the plasma frequency in multiple semiconductor heterostructures. The measured dispersions are linear in the in-plane component of wave vector. These results confirm predictions of the layered electron gas model.
Read-only memory with electron-beam programmable floating-gate transistors
Authors: Shaver D. C.
Published: July 2002

Floating-gate transistors can be used as electron-beam programmable nonvolatile switches. These switches can provide customization, temporary test access, or fault-avoiding interconnections for large integrated circuits. To demonstrate the feasibility of using electron-beam programming, a 128 K bit nMOS EPROM was fabricated. It contains over 200 000 electron-beam programmable switches which are used for the data bits, for address definition in the row and column selectors, and for deselection of defective subsystems. Such large numbers of customizing links are feasible since the programming yield is very high and a programming rate of > 1000 links/s is easily obtained. This paper discusses the design of the electron-beam programmable ROM, design and process considerations for the individual floating-gate devices, and results on yield, programming speed, reversibility, and nonvolatility.
Fabrication of a high density storage medium for electron beam memory
Authors: Oro James A. , Wolfe J. C.
Published: July 2002

A new concept for archival electron beam memory is being investigated. The storage medium consists of a two dimensional array of columns supported by a thin, electron transparent, membrane. Information is written by melting selected columns with an electron beam. The increase in cross-sectional area of the resulting droplets provides contrast for electron transmission readout. In this paper, we describe a fabrication process for this storage medium. Gold columns are electroplated into a resist mold defined by electron beam lithography. Proximity effects are controlled by membrane and trilevel resist techniques. Two dimensional arrays of gold columns with 50 nm diameter, 100 nm period and 3:1 aspect ratios have been fabricated on 250 nm thick polyimide membranes.
Computer simulations of submicron FIB system optics
Authors: Thompson W. , Honjo I. , Utlaut Mark , Enge H.
Published: July 2002

The design of the optical elements for a focused ion beam (FIB) system having a 50 mm spot size over a 1 mm square field requires extensive computational analysis. We discuss the mathematical techniques applied to the components of interest in this submicron FIB system; the electrostatic lenses, the mass analyzer, and the electrostatic deflectors. The results of ion trajectory calculations predicted for the whole FIB column by the computer code snow are presented. The aberration coefficients to third order and a parametric study of a stigmatic Wien filter whose design includes entrance and exit fringe field effects will be considered. We also cover our optimization algorithms for selecting lens and deflector elements which demonstrate minimal chromatic and spherical aberrations and distortions. A spot symmetry and spot location map for the final 1 mm square field and its 50 nm image constraint is shown for mixed electronic configurations of dynamic focus, dynamic distortion, and dynamic stigmation correctors. A comparison of the computer predictions to measured values of lens parameters is given for a typical liquid metal source and its extractor lens. The equipotentials in the vicinity of a representative lens is plotted with emphasis on the dielectric-conductor interface in order to demonstrate the significance of stressed electric fields to the hardware designer.
Sb and Al chemisorption on GaAs(110)
Authors: Kai-Ming Zhang , Yong-Nian Xu
Published: July 2002

In the present paper the chemisorption of Al and Sb on the GaAs(110) surface is studied theoretically. In the case of higher coverage we designed a cluster model with Al4 sitting upon the GaAs(110) surface. The computer renormalization technique is used, and some pseudo-s-orbitals are adopted to fit the molecular orbitals of cluster Al4 by the least square method. The charge self-consistent extended Hückel method is used with a cluster model simulating the substrate GaAs(110). The calculated results show that differences do exist between column III and V elements on GaAs(110). The covalent character for the Sb–GaAs bond is quite obvious, whereas the Al overlayer cluster interacts only weakly with the substrate, so that covalent bonds can be formed only in the case of lower coverage.
Structural chemistry of the cleavage faces of compound semiconductors
Authors: Duke C. B.
Published: July 2002

The major features of the reconstructed atomic geometries of the (110) surfaces of zincblende structure compound semiconductors are characterized by two parameters: the relative displacement perpendicular to the surface of the anions and cations in the outermost layer Δ1,⊥ and the angle ω1 between the plane determined by the chains of these species in the reconstructed surface and the truncated bulk surface. Analysis of the surface sructures of nine compound semiconductors reveals a linear correlation between Δ1,⊥ and the bulk cubic lattice constant a0. A linear correlation between ω1 and a0 also is suggested, but is less firmly established. If the sum of either the Pauling covalent radii or the nonlocal pseudopotential core radii are substituted in place of a0 comparable correlations are achieved. The structures of two previously unexamined surfaces, InAs(110) and AlP(110), are shown to be predicted correctly by these correlations.
Angle-resolved photoemission study of GaAs(100) surfaces grown by molecular beam epitaxy
Authors: Salmon L. G. , Rhodin T. N.
Published: July 2002

Angle-resolved ultraviolet photoemission spectroscopy (ARUPS) using synchrotron radiation has been used to advance our understanding of the surface electronic structure of reconstructed GaAs(100) surfaces. ARUPS spectra were taken using a variable energy polarized photon source (ℏω=15–100 eV). GaAs(100) surfaces with (1×2), c(8×2), and (4×6) reconstructed structures were grown in situ by molecular beam epitaxy (MBE) and characterized by in situ Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). Surface state peak positions were measured across the surface Brillouin zone and the peak positions and dispersion characteristics were determined. The results were consistent with occurrence of the backbonded surface states for ideal GaAs(100) surfaces described by Pollman and Pantelides. Some surface state structures not predicted by them were also observed. The (1×2) As-stabilized surface showed surface states with bonding energies of −6.0 and −9.0 eV corresponding to the two backbonded surface states predicted theoretically for the ideal As-terminated GaAs(100) surface. Three other surface state bands were observed at −1.0, −3.0, and −12.0 eV which were not predicted. All five surface states continued to be observed on the c(8×2) and (4×6) Ga-rich surfaces. Two additional surface states were also observed at binding energies of −10.0 and −14.0 eV for the Ga-rich (4×6) surface. The surface state at −9.0 eV was not observed for the less Ga-rich c(8×2) surface and neither deep surface state was present for the (1×2) As-rich surface. In addition, 1–9 monolayer Ge single-crystal epitaxial overlayers were grown on the three clean reconstructed GaAs surfaces. Photoelectron emission from the GaAs-derived surfaces states was strongly attenuated. The surface reconstruction of the GaAs was removed by Ge overlayers three monolayers or greater in thickness after which the Ge overlayer subsequently developed into a stable (1×2) two-domain structure. The number of Ge overlayers required to overcome the reconstruction features and to initiate stable overlayer growth depended on the nature of the initial GaAs surface reconstruction.
A Monte Carlo study of Si(111) surface oxidation
Authors: Goodnick S. E. , Porod W. , Grondin R. O. , Goodnick S. M. , Wilmsen C. W. , Ferry D. K.
Published: July 2002

A theoretical study of the initial oxidation stages of Si(111) surfaces is undertaken. A Monte Carlo simulation is used to study the individual surface processes that have been suggested in the literature. The relative importance of these mechanisms for the chemical composition of the oxide layer as well as for the initial stages of the formation of SiOx is investigated.
Cleaning chemistry of GaAs(100) and InSb(100) substrates for molecular beam epitaxy
Authors: Vasquez R. P. , Lewis B. F. , Grunthaner F. J.
Published: July 2002

The widely accepted sample-cleaning procedure for MBE substrates, involving the thermal desorption of native oxide-passivation layers in UHV, is examined using high resolution x-ray photoelectron spectroscopy. Oxide composition, carbon contamination, and substrate surface stoichiometry are followed for a variety of temperatures, for GaAs, Inx Ga1−xAs, and InSb substrates. The oxide desorption mechanism observed involves thermal reduction of As+5 to As+3 resulting in the further oxidation of the GaAs substrate surface and the additional formation of Ga2O3 and As0. At higher temperatures, first the As2O3 desorbs followed by the loss of Ga2O3. Carbon-contamination levels were minimized (<0.5% of a monolayer) by the etch removal of the chemical passivating layer by a HC1/EtOH strip immediately prior to vacuum introduction of the sample. The etch is done in inert-atmosphere conditions and gives total removal of both arsenic and gallium oxides, yet leaves the substrate stoichiometry unmodified. This method was quite effective for the Inx Ga1−xAs (x=0–1) system; however, halide-passivation films gave better results on InSb.
Spatial resolution limits in electron beam nanolithography
Authors: Kyser David F.
Published: July 2002

Computer simulation of high energy primary electron scattering and subsequent generation of ‘‘fast’’ secondary electrons in thin film targets is demonstrated with Monte Carlo techniques. A hybrid Monte Carlo model is utilized to calculate the generation and subsequent spatial trajectory of each secondary electron in the target. The three-dimensional spatial distribution of energy dissipation by such fast secondary electrons is shown to be the fundamental resolution limit for electron beam lithography with high voltage beams (100 keV) and thin film polymer targets. The dependence of resolution on beam voltage and film thickness is presented, and quantitative comparison is made between these new Monte Carlo calculations and the limited amount of experimental data available in the scientific literature.
A zone plate shadow camera for 100 keV x rays and 6 MeV protons
Authors: Stone G. F. , Ceglio N. M.
Published: July 2002

We have fabricated and tested a zone plate coded imaging camera for use with high energy x rays and charged particles. The camera system has demonstrated a higher order resolution capability of 4 μm with 100 keV x rays and 9 μm with 6 MeV protons. The unique components of this zone plate shadow camera are its thick zone plate coded aperture and recording medium for protons. The coded aperture is a free-standing, 75 μm thick, gold micro-Fresnel zone plate with 30 μm minimum zone width. It was fabricated by extending reactive ion etching and gold microplating techniques to ‘‘thick’’ structures. The proton recording medium is CR-39, a polycarbonate plastic, which serves as a high resolution nuclear track detector highly sensitive to protons.
Optimization of the optical parameters in variable shape electron beam lithography
Authors: Jones G. A. C. , Rao V. R. M. , Sun H. T. , Ahmed H.
Published: July 2002

Several different configurations of shaped electron beam lithography systems have been proposed for VLSI fabrication. In order to obtain high throughput a high current density is required in the electron beam and electron–electron interaction comes into play. Lorentz interaction effects and chromatic aberrations then limit the resolution. Theoretical and experimental studies have already shown the extent by which the edge resolution deteriorates for specific optical configurations, but only preliminary work has been published on optimizing optical parameters to obtain the best edge resolution. A detailed study of the effects of varying key parameters in the shaping optics has been carried out. Köhler illuminated dual aperture variable shaped electron beam configurations have been studied for a wide range of demagnifications of the shaped image. In particular, the shaping optics have been investigated for magnifying, 1:1, and demagnifying configurations coupled with the effect of varying the size of the shaping apertures. Both experimental and theoretical studies by computer modelling have been carried out. The results show that there are specific combinations of optical parameters that lead to a minimum of edge blurring for a given beam current.
A simplified focusing and deflection system with vertical beam landing
Authors: Kuroda Katsuhiro , Ozasa Susumu , Komoda Tsutomu
Published: July 2002

A focusing and deflection system with vertical beam landing and reduced aberrations was developed for direct electron beam lithography. The system consists of two magnetic lenses and a magnetic deflector. The excitations between the lenses are inverse to each other. The deflector, which has saddle-type coils, is set inside the first lens. The pivot point introduced by the deflector and the first lens coincides with the focal point of the second lens to achieve a vertical beam landing. These arrangements and the operating conditions also enable minimized aberrations, especially elimination of transverse chromatic aberration. In computer simulation, the system gives a 2 mrad/mm landing angle coefficient and a 0.1 μm aberration disk after refocusing at each deflective point, when a converging semiangle of 2.5 mrad is used in a 3.2×3.2 mm2 deflection area at a 30 kV beam energy. The converging angle gives a current density of 10 A/cm2 using the LaB6 electron gun with 5×105A/cm2sr brightness. In the simulation, characteristics were also calculated for beam misalignment, quantization of cosine-distributed deflection coils, and so forth. These results show the system to be feasible, free of serious errors, and suitable for realization. Experimental results also showed good performance. The system has been applied in variably shaped electron beam lithography machines and has successfully been used for 0.5 μm pattern drawing within 0.1 μm overall accuracy.
Deep-UV spatial-frequency doubling by combining multilayer mirrors with diffraction gratings
Authors: Hawryluk A. M. , Smith Henry I. , Ehrlich D. J.
Published: July 2002

A multilayer dielectric mirror was deposited over an aluminum grating of 198 nm period on a quartz substrate. This was then used as a parent mask to expose a grating of 99 nm period in PMMA by deep-UV spatial-frequency doubling with an ArF laser (λ=193 nm). The multilayer mirror suppressed the zero order permitting the doubled pattern to be exposed over a depth of field of several micrometers. Holographic lithography and spatial-frequency doubling are compared. It is shown that the latter is preferred if λ/Δλ<105.
Direct pattern fabrication on silicone resin by vapor phase electron beam polymerization
Authors: Morita Masao , Imamura Saburo , Tamamura Toshiaki , Kogure Osamu , Murase Kei
Published: July 2002

A new technique for high resolution lithography with a dry-developed multilayer resist system is demonstrated. The technique involves a direct pattern fabrication by electron beam-induced vapor phase graft polymerization. The selective pattern formation in irradiated areas was performed using PMMA or silicone resin as a base film and styrene as a grafting monomer. The base film patterns can be fabricated by dry etching with the mask of grafted patterns. However, because of isotropic behavior in the growth of grafted polymer, the base film thickness to be directly etched with the mask of graft-polymerized film is severely limited by the resolution to be achieved. The combination of thin silicone resin as a directly etched layer using graft-polymerized film and thick organic polymer as the bottom layer can solve the conflict between resolution and overall resist thickness, because the silicone resin has an excellent durability against oxygen RIE of organic polymers. Submicron pattern formation with a high aspect ratio was demonstrated with this dry-developed multilayer resist system.
Modulation-doped FET threshold voltage uniformity of a high throughput 3 inch MBE system
Authors: Abrokwah J. K. , Cirillo N. C. , Helix M. J. , Longerbone M.
Published: July 2002

We report results of the characterization of a high throughput 3 in. MBE system which we have used to fabricate high quality, uniform modulation-doped field-effect transistors (MODFET’s). MODFET threshold voltage of 14.6% and hence Nd2 product uniformity of 11.3% standard deviation across a 3 in. MBE wafer was achieved. MODFET’s with 77 K mobility over 100 000 cm2/V s were fabricated. Silicon was used as the n-type dopant of the (Al,Ga)As/GaAs heterostructures. Successful calibration of the Si cell temperature resulted in controlled doping in the range 1014 cm−3 to mid-1018 cm−3 in GaAs. AlxGa1−xAs (x≂0.3) was also doped successfully up to 2.5 ×1018 cm−3. Doping and thickness uniformity of 2% for both (Al, Ga)As and GaAs over the central 2.5 in. of 3 in. substrates was achieved using substrate rotation.
Investigation of the surface structure of GaAs(110) by high energy ion channeling
Authors: Gossmann H.-J. , Gibson W. M.
Published: July 2002

The first application of high energy ion channeling for the structure determination of the atomically clean GaAs(110) surface is reported. It is found that the surface Ga and As atoms have small lateral displacements (≤0.1 Å) from ideal bulklike sites, whereas a normal component of the first layer shear vector as large as 0.7 Å is compatible with the experimental data. The implications of these results with respect to current LEED models are discussed.
Influences of molecular reflection on the lift-off pattern edge quality
Authors: Arai K. , Yanagawa F. , Kurosawa S.
Published: July 2002

One of the problems in defining LSI patterns by the lift-off technique is burrs, which appear at film pattern edges. The cause of burrs formed at the SiO film pattern edges is discussed. Burrs result from indirectly deposited thin layer residues on the resist pattern side walls. This layer is called a ‘‘side wall layer.’’ A model experiment is carried out and it is shown that the reflection of evaporated SiO molecules is the cause of indirect deposition. SiO molecule reflection probability during deposition is measured. The side wall layer is simulated using the reflection probability. It is concluded that reflection at the substrate surface and reflection at the vacuum chamber wall contribute to an equal degree to the side wall layer formation. Several methods of suppressing the side wall layer, which are also applicable to materials like Pb with higher reflection probability, are discussed. Cooling the substrate or the vacuum chamber, or setting up a shielding board in the chamber are predicted to be effective.
New coatings for high temperature materials protection
Authors: Bennett M. J.
Published: July 2002

The development, characterization, and performance evaluation of two new thin (≲20 μm) ceramic coatings for high temperature materials protection is reviewed. These coatings were silica produced by two vapor deposition procedures and ceria formed by sol–gel technology. The current position regarding the use of these coatings in the UK nuclear industry is described.
A step-and-repeat x-ray exposure system for 0.5 μm pattern replication
Authors: Hayasaka T. , Ishihara S. , Kinoshita H. , Takeuchi N.
Published: July 2002

A new step-and-repeat x-ray exposure system has been developed to establish practical submicron x-ray lithography. Penumbra and run-out error have been determined on the basis of resolution consideration and alignment accuracy analysis. The fundamental system parameters have been derived for constructing an efficient and accurate exposure system. To realize such a system, certain new technologies have been developed. (1) A high power Si–Kα x-ray source brings a large x-ray flux onto the wafer through a silicon nitride mask membrane. (2) The optical mark and gap detection method is connected to a precise alignment mechanism placed on an x–y table designed for wafer stepping. (3) This construction ensures atmospheric environment exposure. (4) The exposure system operation is fully automatic thanks to microprocessors. A 0.5 μm pattern has been accurately replicated, and a ±0.15 μm alignment accuracy has been achieved using this exposure system.
Application of polymer–bisazide composite system negative resists to electron beam lithography
Authors: Tanigaki Katsumi , Suzuki Masayoshi , Ohnishi Yoshitake
Published: July 2002

Pattern profiles achieved in composite resists of a polymer and a bisazide compound tend to be rectangular, in spite of electron backscattering from substrates. This phenomenon is explained by the bisazide concentration dependence of the composite resist sensitivity to electron beam and the depth profile of bisazide distribution in a polymer film. An optimum amount of bisazide compound, needed to obtain the highest sensitization, exists. It does not depend on molecular weight, but on the kind of polymers used. An excessive amount of bisazide compound incorporation is found to reduce sensitivity. Bisazide compounds distribute to a great extent in the vicinity of a substrate during a prebaking process. These factors reduce the backscattering problem taking place in electron beam lithography to obtain resist patterns with vertical walls.
Reactive ion etching of SiO2 with vertical sidewalls and its application to ion-implantation masks for bubble devices
Authors: Gokan H. , Mukainaru M.
Published: July 2002

Silicon dioxide patterns having vertical sidewalls have been obtained without pattern width loss by reactive ion etching in CF4. The polymerization effect, caused by fluorine deficiency in the plasma, is found to influence not only the morphology of the etched resist surface but also the SiO2 etching profiles. The polymerization is greatly reduced by depressing the temperature rise during etching. A temperature-controlled cathode and a heat sink material are used for this purpose. The polymerization is further reduced by increasing flow rate, by increasing pressure and by decreasing power density. The SiO2 patterns having vertical sidewalls are obtained under the no-polymerization etching conditions. These SiO2 patterns have been successfully applied to ion-implantation masks for 16 Mbit bubble devices.
A bilevel resist for ion beam lithography
Authors: Milgram A. , Puretz J.
Published: July 2002

A focused ion beam of gallium was used to implant films of baked photoresist, spin-on glass (SOG) and the bilayer combination of spin-on glass on baked photoresist. Plasma etch conditions were found which gave substantial differences between the implanted and nonimplanted SOG so that the SOG acted as a negative resist. The implanted SOG in the SOG-photoresist bilayer was patterned and then used as an etch mask during O2 reactive ion etching of the photoresist. In this manner, rectangular profile structures of height 2.0 μm and width 0.4 μm were formed where the line width variation along the patterned line is a small fraction of the line width. This bilayer system is shown to be suitable for use in optical trilayer patterning. A new processing sequence for microstructure fabrication is proposed (called dual processing) whereby the bilayer structure is used sequentially in both focused ion beam implantation and trilayer processing (electron beam or optical) to achieve fine features of less than 0.5 μm size within prescribed regions that can be micrometer size.
Safety considerations for plasma aluminum etching
Authors: Corn Glenn , Baldwin David G.
Published: July 2002

This paper evaluates personnel exposure to hazardous chemical compounds during the operation and maintenance of a plasma aluminum etcher. Since chemical exposure occurs mainly through inhalation and skin contact, breathing zone samples and chemical residues from the reactor and cold trap were analyzed. Data were collected during normal operation, cleaning. and routine maintenance. The results indicate that the concentration of volatile compounds in the worker breathing zone were at least an order of magnitude lower than recommended limits. Carbon tetrachloride was detected while recharging a reservoir. The concentration of 0.88 ppm was below the maximum recommended 8 h average concentration of 5 ppm. The chemical residue from the cold trap contained a suspected carcinogen: 1,1,2-trichloro-1,3-butadiene (2.0% of sample). Hexachlorobutadiene, which is structurally similar to 1,1,2-trichloro-1,3-butadiene, is a strongly suspected carcinogen with a threshold limit value of 0.02 ppm.
Characterization of diazo-type photoresists using e-beam exposure and metal–ion-free developers
Authors: Yamashita Hiroshi , Todokoro Yoshihiro
Published: July 2002

The performance characteristics of diazo-type positive photoresists are explored using e-beam exposure and metal–ion-free developers. For Tokyo Ohka OFPR1000, resist profiles with straight sidewalls are obtained while maintaining sufficient resist thickness. The effects of prebake and developer temperature on the solubility rate of OFPR1000 are investigated. A higher contrast is obtained with the resist prebaked at 65 °C and developed at 23 °C. Higher prebake temperatures and lower or higher developer temperatures result in a lower contrast. Submicron resolution can be obtained.
Two layer resist systems for hybrid e-beam/deep-UV lithography
Authors: Takasu Yasuhiro , Todokoro Yoshihiro
Published: July 2002

Two layer resist systems for hybrid e-beam/deep-UV lithography are described which consists of positive type RE5000P (Hitachi Chemical) or negative type CMS-EX (Toyo Soda) and PMMA. The systems have no interfacial problems between the top layer resist and the bottom layer resist. The thin RE5000P and CMS-EX have very high UV absorption coefficients below 250 nm and form a high contrast mask for optical pattern transfer to the thick PMMA layer. The sensitivity of these systems for e-beam lithography is 1 μC/cm2 (RE5000P/PMMA) and 2.5 μC/cm2 (CMS-EX/PMMA) which are one order of magnitude higher than the sensitivity of the AZ1350J/PMMA system. High image resolution and linewidth control with reduced proximity effects have been demonstrated.
Summary Abstract: The influence of surface reconstruction on the initial stages of silicon molecular beam epitaxy
Authors: Gossmann H.-J. , Feldman L. C.
Published: July 2002

High energy ion scattering/channeling is used to investigate the initial stages of interface formation (overlayer thickness up to ≊10 Å) during Si molecular beam epitaxy. Changes in the geometry of the Si substrate surface, i.e., reordering, and of the Si overlayer are measured as a function of Si coverage, deposition temperature, and substrate reconstruction. It is found that room temperature deposition reorders the Si(100)-2×1 substrate but not the Si(111)-7×7. On both surfaces, however, deposition at 300 K results in a highly imperfect overlayer. Epitaxial temperatures of ≊790 K for Si(111)-7×7 and of ≊570 K for Si(100)-2×1 are obtained.
Self-aligned dual surface lithography
Authors: Krusius J. P. , Nulman J. , Perera A.
Published: July 2002

A new class of microstructures, such as the opposed gate–source transistor, requires the careful alignment of submicron features on two opposite surfaces of a substrate. Concepts for self-aligned methods for accomplishing dual-surface lithography have been explored for the first time. Self-alignment is based on the integration of the exposure mask into the microstructure and subsequent exposure through the substrate. Electrons, ions, and x rays have been explored for exposing lines as narrow as 200 nm. Both theoretical Monte Carlo results and experimental data on fabricated structures are given. Self-aligned dual-surface lines with a width of 200 nm have been demonstrated across a 3-μm-thick silicon membrane. Ion, electron, and x-ray exposure through the substrate limits the thickness of the membrane to about 100 nm, 1 μm, and 10 μm, respectively for reasonable particle energies and submicron resolution.
An optimized positive resist for electron-beam direct writing: PER-1
Authors: Iida Yasuo , Tanigaki Katsumi
Published: July 2002

Optical resists have been optimized for e-beam direct writing in order to enhance process compatibility. Drawbacks of optical positive resist (such as AZ-2400) for e-beam application are the following: (1) It is readily converted to a negative image when exposed to the high doses required for fiducial mask detection (multiple scans over a localized region), and (2) this negatively converted region is difficult to remove. The ratio Dgi/Dp is proposed as a measure of rating positive resists for ease of conversion. As this ratio becomes larger, the likelihood of forming a negative image in the resist becomes smaller. The values for AZ-2400 and RE-5000p were 15 and 40, respectively. Conversion mechanisms were examined and two important factors were found: (1) The sensitizer contribution to negative image conversion was small, and (2) methylene group density in novolak was a dominant factor. Considering the above factors, a new positive novolak e-beam resist PER-1 has been developed with a Dgi/Dp ratio of 285. Quarter-micron patterns are easily delineated in PER-1 with a variable-shaped e-beam system.
An electron image projection stepper
Authors: Livesay W. R. , Anderson L. B.
Published: July 2002

An electron image projection system has been developed which provides subfield or die-to-die alignment without mechanical motion of the mask or the wafer. This is made possible by a novel thin foil detection system which allows conventional alignment marks (i.e., etched grooves) and backscattered electron signals to be used for die-to-die alignment. Due to the fact that the wafer forms part of the imaging system in a photocathode projection system, any wafer flatness variation aggravates image distortion. This has made it very difficult to achieve accurate level-to-level overlay using a global alignment technology. The new alignment technique described provides a means of detecting alignment marks surrounding each die or subfield. The alignment marks can be placed in the scribe streets of the wafer, thereby not occupying valuable active areas. The foil detector is placed between mask and wafer and is biased in order to not distort the imaging field. Experimental results of image distortion and alignment sensitivity measurements indicate feasibility of this technique.
Molecular beam epitaxy of gallium arsenide using direct radiative substrate heating
Authors: Hellman E. S. , Pitner P. M. , Harwit A. , Liu D. , Yoffe G. W. , Harris J. S. , Caffee B. , Hierl T.
Published: July 2002

Conventional mounting techniques in molecular beam epitaxy using indium to solder GaAs substrates to molybdenum blocks result in a rough back side which is incompatible with integrated circuit processing. We present the design of a new substrate holder which allows direct radiative heating of substrates without use of indium bonding. Temperature measurement and control are done by radiative coupling to a shielded thermocouple which sits just behind the GaAs wafer. Excellent temperature uniformity is achieved over a 3 in. wafer by a molybdenum holder ring design which minimizes contact with the substrate and holds the wafer against the back of the ring to eliminate shadowing of the substrate from the heater. No coating of the wafers is necessary for temperature control. Stress related defects and growth induced wafer warpage are not observed. Deep level transient spectroscopy, photoluminescence, and mobility measurements show the clear superiority of GaAs layers grown using direct radiative substrate heating compared to conventional indium bonding.
Application of x-ray topographic methods to thin film electronic materials
Authors: Weissmann Sigmund
Published: July 2002

X-ray methods are presented which, besides offering a topographic survey of the defect structure in electronic materials, give quantitative information on the induced strain distribution. Complete strain tensor analyses are obtained by the back-reflection divergent beam method and by the method of computer-aided rocking curve analysis (CARCA). The latter is based on the double-crystal diffractometer principle. An example of the strain analysis by CARCA is given showing the strain dependence on the thickness variation of an InGaAsP film grown on a substrate of InP. Valuable information on the density of interface dislocation is obtained by combining these methods with lattice curvature measurements using scanning topography with automatic Bragg angle control. The elucidation of the strain distribution in depth becomes enhanced by application of x radiation of various wavelengths.
Geometrical design of an alignment mark for maskless ion implantation in GaAs
Authors: Morita Tetsuo , Miyauchi Eizo , Arimoto Hiroshi , Takamori Akira , Bamba Yasuo , Hashimoto Hisao
Published: July 2002

Geometrical features of an alignment mark for focused ion beam (FIB) implantation in GaAs with precise alignment were studied. To optimize the etched shape on a GaAs wafer, intensity profiles of secondary electrons emitted by scanning the FIB across the mark were calculated from Monte Carlo simulation of implanted ion trajectories. The contrast and sharpness of the intensity profiles at the mark relate to the inclination angle of its sidewall and the etched depth. It was confirmed that the calculated results agreed with the experimental ones. For irradiation of 160-keV-Si FIB on the alignment mark with an 85° inclination angle, its etched depth of 2  μm gives the maximum contrast and sharpness. Under this condition, Si was implanted in the designated region with an alignment accuracy of 0.1 μm.
Lithographic approach for 100 nm fabrication by focused ion beam
Authors: Matsui Shinji , Mori Katsumi , Saigo Kazuhide , Shiokawa Takao , Toyoda Koichi , Namba Susumu
Published: July 2002

A bilevel resist process using P(SiSt90–CMS10) silicon containing resist as a top layer has been developed for Ga+ focused ion beam (FIB) lithography. A 100 nm linewidth pattern with 750 nm thickness has been demonstrated. Lithographic characteristics for 100 kV Ga+ FIB have been studied for PMMA positive resist and P(SiSt90–CMS10) negative resist. The results indicate that backscattering and proximity effects are negligible and that 100 kV Ga+ FIB resist sensitivity is about 100 times larger than that for 20 kV electron beam. Moreover, it has been observed that discontinuous lines, which may be caused by shot noise or by an oscillation at the end of the Taylor cone of Ga ion source, are produced at low dose for both PMMA and P(SiSt90–CMS10) resists.
Exposing submicron patterns with a variable shaped electron beam machine
Authors: Gu W. Q. , He M. H. , Han R. T.
Published: July 2002

There are several problems with exposing submicron patterns with the variable shaped electron beam machine. One is the proximity effect caused by the backscattered electrons from the resist and substrate. The other is the shape edge slope of the beam profile caused by the Coulomb effect, and the third is the variation of beam current density with beam size. The edge resolution depends on the shape edge slope and it is varied with the shaped beam current density and shaped beam size as discussed in this paper. Some experiments on the edge resolution have been performed by using the JEOL JBX6AII machine. Some problems such as underexposure, beam spot underlapping, and pattern infidelity in the submicron range are also discussed. It is indicated that a shape edge resolution of 0.15 μm is required for exposing 0.4 μm pattern. In order to decrease the influence of the proximity effect and improve the shape edge resolution, a series of measures on the column adjustment and the exposure parameter selection must be taken. An adjustment method which causes the increase of the shaped beam current density with a decrease of the shaped beam size has been used for partially correcting the proximity effect. Submicron patterns with 0.4 μm linewidth on the Cr plate coated with positive resist OEBR-1000 and on the wafer coated with EBR-9 have been obtained.
Atomistic numerical simulation of epitaxial crystal growth
Authors: Das Sarma S. , Paik S. M. , Khor K. E. , Kobayashi A.
Published: July 2002

Epitaxial crystal growth is studied via a molecular dynamics simulation and an atomistic energetics calculation. It is found that: (i) pseudomorphic growth can be more easily achieved if the lattice constant of the absorbate is smaller than that of the substrate; (ii) the best epitaxial growth for the Lennard-Jones (LJ) system is achieved for the face-centered cubic (fcc)(111) orientation at around one-half the melting temperature of the solid. We also investigate diffusion of a Si atom on the Si(111) surface using the Stillinger–Weber three-body potential.
Optimization of system parameters for throughput in a variable rectangular electron beam exposure system
Authors: Takamoto Kiichi
Published: July 2002

Optimization of system parameters for throughput is investigated under the condition that performances of an electron optical system and an X–Y table system, and large-scale integrated (LSI) pattern properties are given. The beam is deflected by a vector scanning method. The table is operated in a step and repeat mode. Exposure time is calculated from resist exposure time, settling time of beam deflection digital–analog converters, and table movement time. It is shown that there exist field sizes, aperture angles, and maximum beam sizes which minimize the exposure time. Optimum system parameters depend on LSI pattern properties. The concept of optimizing system parameters for throughput can be applied to system parameter determination in the development of a high throughput exposure system.
Submicron electron-beam lithography using a beam size comparable to the linewidth control tolerance
Authors: Rosenfield M. G. , Bucchignano J. J. , Rishton S. A. , Kern D. P. , Kettell L. M. , Molzen W. W. , Hohn F. J. , Viswanathan R. , Warlaumont J. M.
Published: July 2002

Presently available electron-beam lithography systems for submicron lithography have minimum beam sizes or edge resolution on the order of 0.1–0.2 μm. Satisfactory proximity correction, linewidth control, and profile quality are critically dependent on beam size. Moreover, variations in beam size about the expected size can result in severe degradation of linewidth control. In this paper, we will examine the relationships between linewidth control, beam size, and proximity correction and show that it is possible, but difficult, to successfully use electron-beam lithography under these conditions. High resolution vector scan electron-beam systems with known Gaussian beam sizes are used to produce features down to 0.25 μm in single and multilayer resist. It is shown that the beam size and forward scattering must be included in the proximity correction of submicron patterns. The effects of varying the beam size are also investigated and it is shown, for example, that an increase in beam size of only 0.02 μm can significantly reduce the developed resist linewidth of sub-half-micron features in 1.0 μm thick resist.