bluuebunny/crossref_metadata_2025 huggingface dataset:
A new method for measuring the virtual object diameter of high-field electron sources
Authors: Gesley Mark
Published: July 2002

A new method for measuring the virtual object diameter dν of field emission sources based on the spectral analysis of their flicker noise is described. The nature of the source and the various parameters which influence its size are considered. It is emphasized that the cathode surface does not coincide with the source plane which has a Gaussian current distribution. Extractor electrode aberrations do not make a significant contribution to the virtual source diameter. Extensions of the concentric sphere model of the diode region which modify the spatial electric field distribution do not lead to a larger estimate of the source size. From this discussion comes the relation linking the source diameter dν to the resolution length interval lδ which is defined at the cathode surface. By matching the frequency exponents of the theoretical and measured spectra characteristic frequencies are found which yield lδ and hence provides an estimate of dν. For thermal-field emission from a tungsten cathode at 1000 K in ultrahigh vacuum a value of dν ≂21±9 Å is obtained.
High-gain lateral p
n
p bipolar transistors made using focused ion beam implantation
Authors: Clark William M. , Utlaut Mark W. , Reuss Robert H. , Koury Dan
Published: July 2002

We report the fabrication of lateral pnp bipolar transistors using focused ion beam (FIB) implants of boron and phosphorus for the collector and base, respectively. The implants of B+, P+, and P++ were all at a dose of 1×1013 /cm2 and a beam voltage of 75 kV. These implants defined spaces between the emitter and collector regions of 0.5–1.50 μm; which, after diffusion and zero voltage depletion width effects were considered, produced effective on-wafer device basewidths of ∼0.2 μm. For the best devices, values of hFE near 100 were obtained with good junction characteristics and at peak collector currents of 10 μA/μm of device width.
Low-temperature alkali metal/III–V interfaces: A study of metallization and Fermi level movement
Authors: Cao Renyu , Miyano K. , Kendelewicz T. , Lindau I. , Spicer W. E.
Published: July 2002

The interfaces of alkali metals (Cs and Rb) on GaAs and InP(110) surfaces prepared at 110 K low temperature have been studied using photoemission. At low temperature, multilayers of alkali metals with laminar growth can be obtained. The overlayer metallization is investigated by following the density of states near the Fermi cutoff, the free electron plasma loss, and the Fermi level movement at the semiconductor surfaces. Several criteria of metallicity are proposed. It is found that one monolayer of Cs or Rb is not metallic, and full metallicity is established at around two monolayers of coverage. The Fermi level movement relative to the semiconductor band edges as a function of alkali metal coverage has been closely followed. The semiconductor band bendings at low coverages are attributed to the surface-donor states originating from alkali metal atom chemisorption. The Fermi level stabilization at these interfaces occurs when the overlayers become metallic. This pinning behavior is explained in terms of the metal-induced gap states. The Fermi level pinning at the room temperature interfaces is also discussed and compared with the low-temperature behavior.
Properties of II–VI semiconductor films grown by photoassisted molecular-beam epitaxy
Authors: Harper R. L. , Han Jeong W. , Hwang S. , Lansari Y. , Giles N. C. , Cook J. W. , Schetzina J. F.
Published: July 2002

We report the successful use of photoassisted molecular-beam epitaxy for the preparation of p-type CdTe epilayers and p-type modulation-doped HgCdTe multilayers in which arsenic is used as the substitutional dopant. The CdTe:As epilayers were grown at substrate temperatures which ranged from 230 to 180 °C. The resulting CdTe:As layers exhibited room-temperature hole concentrations ranging from 7×1015 to 6.2×1018 cm−3, as determined by van der Pauw Hall effect measurements. This highest doping level is nearly two orders of magnitude greater than that obtained for bulk p-type CdTe. The mobility of the CdTe:As epilayers ranged from 27 to 74 cm2/V s, at room temperature. The arsenic acceptor ionization energy in CdTe:As was found to be ∼58–60 meV using low-temperature photoluminescence measurements. The modulation-doped HgCdTe samples exhibited hole concentrations ranging from 5×1016 to 1×1018 cm−3 and hole mobilities of ∼200–325 cm2/V s at temperatures <200 K.
Investigation of short period AlAs/GaAs superlattices as conduction-band barriers
Authors: Paulus M. J. , Huang C. I. , Bozada C. A. , Cheney M. E. , Dudley S. C. , Stutz C. E. , Evans K. R. , Jones R. L.
Published: July 2002

Short period AlAs/GaAs superlattices, or pseudoalloys, are investigated as insulators in n+ GaAs–insulator–n GaAs structures, or superlattice barrier capacitors (SLBC’s), and as tunneling barriers in superlattice double barrier diodes (SLDBD’s). The energies of the lowest superlattice states in the SLBC’s and the energies of the quantum well quasibound states in the SLDBD’s are determined using thermionic emission analysis. Depending on the GaAs and AlAs layer thicknesses, the lowest states in the SLBC’s may be Γ states in the GaAs layers or X states in the AlAs layers. It is also possible for the barrier to behave as a random alloy. We observe all three cases. The SLDBD’s exhibit strong negative resistance at low temperatures and we find evidence that the superlattice barriers (SLB’s) behave as ideal Γ band structures, regardless of the layer thicknesses. Second current peaks which are attributed to resonant states in the SLB’s appear in the SLDBD I–V curves when the GaAs layers are thicker than 5 monolayers.
Microlithographic behavior of transition metal oxide resists exposed to focused ion beam
Authors: Koshida N. , Ichinose Y. , Ohtaka K. , Komuro M. , Atoda N.
Published: July 2002

Basic properties of transition metal oxide films as a high-resolution, negative working ion resist have been studied experimentally for focused ion beam (FIB) exposure. Thin amorphous films of WO3, MoO3, V2O5 and a mixture thereof were formed on Si wafers and exposed to a 20–70 keV Ga+ FIB. Development after writing was performed by chemical etching. The resist sensitivity and the contrast were measured as a function of the accelerating voltage of the FIB. The dry etching durability of the delineated resist patterns and their selectivity with respect to Si were also evaluated for SF6 plasma etching. The details of the characteristics can be explained in terms of the physical and chemical properties of the resist material.
Deep antisite-complex levels in Hg1−xCdxTe
Authors: Hanke M. , Hennig D. , Kaschte A.
Published: July 2002

Two types of deep levels are observed by deep-level transient spectroscopy in undoped Hg1−xCdxTe controlling performance of infrared devices. The arrangement of defect atoms has been modeled within a five-atom cluster embedded in the ideal Hg1−xCdxTe crystal and off-diagonal elements of the impurity potential matrix have been included explicitly. An explanation of the upper level as a Te-antisite defect and of the lower one as a Hg–Te-double-antisite defect is proposed.
Synchrotron radiation induced chemical vapor deposition of thin films from metal hexacarbonyls*
Authors: Mancini Derrick C. , Varma Shikha , Simons John K. , Rosenberg Richard A. , Dowben P. A.
Published: July 2002

We have studied the initial stages of synchrotron radiation [SR] induced chemical vapor deposition [CVD] of metal-containing thin films from metal hexacarbonyl gases. We have measured the dependence of the initial deposition rate upon gas pressure at room temperature. Substrates were exposed to SR for single fills of the electron storage ring at constant pressure of Mo(CO)6, Cr(CO)6, or W(CO)6 gas. Deposition was monitored in situ by Auger electron spectroscopy using the SR as the excitation source. The presence of metal, carbon, and oxygen in the deposited films was observed, and the results are consistent with previous studies. Deposition was found to be isolated to areas of the substrate exposed to the SR beam. We discuss these results as they relate to the use of SR as a means to induce CVD and the possibility of patterned deposition using a masked SR source.
Sliding friction measurements of physisorbed monolayers: A comparison of solid and liquid films
Authors: Krim J. , Chiarello R.
Published: July 2002

The quality factor and frequency of vibration of a quartz crystal oscillator decreases slightly when a film adsorbs on its surface. Simultaneous measurements of the shifts in quality factor and frequency allow one to quantitatively deduce the degree to which the film ‘‘slips’’ due to the oscillatory motion of the underlying substrate, and also the force which would be required to slide the film along the surface at constant speed. Experiments have been carried out for krypton monolayers undergoing solidification on a smooth gold surface and on an atomically rough silver surface. The force per unit area required to slide a liquid monolayer is roughly the same for each surface. Less force is required to slide a solid monolayer along the smooth surface, while the opposite is true for the rough surface.
The influence of post-exposure bake on linewidth control for the resist system RAY-PN (AZ PN 100) in x-ray mask fabrication
Authors: Grimm J. , Chlebek J. , Schulz T. , Huber H.-L.
Published: July 2002

Statistically designed experiments for optimization in processing the high sensitive negative tone Hoechst resist RAY-PN (AZ PN 100) were used to establish robust processes for two different applications of the resist: (1) pattern replication on a wafer and (2) during the mask copy process under 40-mbar He environment via x-ray lithography. Minimization of linewidth change with respect to a 1:1 pattern transfer was achieved through manipulation of the following variables: exposure dose, post-exposure bake (PEB) time and temperature, and development time. A two-stage sequential strategy was employed. After specifying the response and the primary variables, the first stage of the sequential approach was based on a full factorial design. In the second stage, the influence of PEB parameters for wafers as well as for x-ray mask blanks were studied using a central composite design. Finally, the response surfaces for the pattern replication on x-ray mask blanks are demonstrated and the optimal process parameter set for a linewidth control within 50 nm for 0.5- to 3.0-μm feature size values are given.
Preliminary analysis of electron-beam positioning errors in Lepton EBES4
Authors: Waggener H. A. , Peters D. W. , Chen G. , Rose C. M. , Fowlis D. C. , Chitayat A. , Caracci J.
Published: July 2002

A study was undertaken to quantify and minimize mechanically induced sources of electron-beam positioning errors in Lepton EBES4. Stage performance was improved by replacing the x axis rotary motor and ball-screw drive with a brushless, sinewave, proprietary linear motor drive. Vibrational and modal analyses indicated that the system’s present structural resonances have a negligible contribution to e-beam positioning errors. Thermal drift of the interferometer is the major contributor to pattern placement errors. Results from a vibrational analysis performed during stage scanning indicated that the crossed roller bearing stage has low compliance with a maximum yaw value of ±0.05 μrad. MARKET analyses of arrays covering 100 mm×100 mm indicated a maximum x or y residual error of ≤50 nm with a ‖mean‖+3σ of ≤50 nm. This level of image placement accuracy is consistent with that required for an advanced e-beam tool for reticle writing, submicron direct write applications, writing optical phase shift masks, and writing submicron 1× x-ray masks. A ZerodurTM metrology platform is being implemented on the second EBES4 system. Use of Zerodur will further improve image placement accuracy by reducing the impact of temperature fluctuations.
Investigation of emitter tips for scanning tunneling microscope-based microprobe systems
Authors: Staufer U. , Muray L. P. , Kern D. P. , Chang T. H. P.
Published: July 2002

This paper reports the preparation and characterization of field emitter tips for use in a scanning tunneling microscope aligned field emission (SAFE) microprobe system. With the tip being at close proximity to the extraction electrode, new demands are imposed on the emitter tips: (1) a low extraction voltage, (2) a well-defined emission pattern, preferably a single lobe emission, and (3) a high angular emission density. A combined field ion–field electron emission microscope equipped with a special stage for mounting a small aperture in close proximity to the emitter tip, which was used to simulate the first element of the electro-optical system of the SAFE microprobe, was used to analyze different tip preparation techniques. A low-temperature field-assisted thermal annealing process has been developed to routinely produce sharp W 〈111〉 tips well suited for SAFE operation. Tips having an effective tip radius of less than 500 Å, an emission half cone angle of less than 10°, and a peak angular emission density of 7 μA/sr at a total emission current of 1 μA were successfully prepared and used in the SAFE microprobe system.
Self-aligned high electron mobility transistor gate fabrication using focused ion beams
Authors: Atkinson G. M. , Kubena R. L. , Larson L. E. , Nguyen L. D. , Stratton F. P. , Jelloian L. M. , Le M. V. , McNulty H.
Published: July 2002

A new gate fabrication technique has been developed based on focused ion beam exposure and reactive ion etching of a polymethylmethacrylate (PMMA)/Ge/PMMA multilevel resist structure. The focused ion beam exposes the thin PMMA imaging layer that is transferred directly to the germanium layer using reactive ion etching (RIE). The underlying resist is etched first in oxygen at high pressure, providing an undercut, bowl-shaped profile, followed by an extremely low pressure, anisotropic oxygen process, which etches a narrow stem down to the base of the resist, resulting in a chalice-shaped profile. Removing the germanium layer allows the profile to be used to lift off the Ti/Pt/Au gate. This process is compatible with our self-aligned gate high electron mobility transistor (HEMT) process and provides an alternative technique for gate fabrication below 75 nm. Focused ion beam gate structures have been fabricated with 60 nm gate length and 0.15 μm2 cross section using this technique and shown that the resulting gate profile can be used to shadow a 200 nm thick Ohmic contact evaporation to form the self-aligned structure with a source/drain spacing of 225 nm. It has also been determined that ion bombardment damage to the HEMT channel during RIE can be eliminated by limiting the plasma potential to less than 45 V.
Interaction of titanium with single crystal silicon during rapid electron beam heating
Authors: Mahmood F. , Ahmed H. , Suleman M. , Raman V. K.
Published: July 2002

The formation of titanium disilicide by the solid phase reaction of titanium with single crystal silicon has been studied using an electron beam heating method which gives well-controlled, rapid time-temperature cycles. Rapid heating of titanium coated silicon in the temperature range 750–1000 °C gives rise to titanium disilicide films having a low value of resistivity of about 15 μΩ cm. It is also reported that heating at relatively high temperatures does not preclude the formation of metastable titanium disilicide phases. Auger electron spectroscopy combined with Ar+ ion sputter depth profiling of samples, which had been exposed to a number of different heat cycles, revealed that impurities such as oxygen and carbon, adsorbed in the titanium films, are driven towards the top surface. Cross-sectional transmission electron microscopy analysis of a fully reacted sample showed that rapid heating resulted in a smooth interface between the silicide and the silicon substrate.
Optical analysis of metalorganic vapor phase epitaxy grown ZnS/ZnSe/GaAs(100) heterostructures: Carrier diffusion and interface sharpness
Authors: Hermans J. , Wagner V. , Geurts J. , Woitok J. , Söllner J. , Heuken M. , Heime K.
Published: July 2002

Metalorganic vapor phase epitaxy grown epitaxial layers of ZnS and ZnSe on GaAs(100) are investigated by Raman spectroscopy, far-infrared reflectance spectroscopy, and x-ray diffractometry. This combination allows a separate determination of crystalline quality, stress, and free carrier concentration. Furthermore, the interface sharpness can be deduced. While for optimized growth conditions the interfaces are sharp within less than 1 nm, we observe the presence of free carriers in nominally undoped epilayers. They are interpreted in terms of a diffusion of Ga atoms from the substrate into the epilayer. It is shown, that the concentration can be drastically reduced by a decrease of the growth temperature. Concentrations below 1016 cm−3 are achieved for growth at 640 K
Evaluation of polyencapsulation, oxygen leak, and low energy ion bombardment in the reduction of secondary ion mass spectrometry surface ion yield transients
Authors: Corcoran S. F. , Felch S. B.
Published: July 2002

The accurate determination of dopant concentrations in the near-surface region of secondary ion mass spectrometry (SIMS) depth profiles is often complicated by surface ion yield transients. These ion yield transients, which result mainly from the implantation of the ion yield enhancing primary ion beam, can lead to errors in the quantification of impurity species as well as distortions in the dopant profile shape. Polyencapsulation, oxygen flooding, and matrix normalization have each proven to be useful in the reduction and elimination of these surface ion yield transients. The use of low primary beam energies has been shown to have the additional benefit of reducing profile broadening due to cascade mixing. In this study, three methods of transient reduction were evaluated in the SIMS characterization of ultra-shallow p+ junctions in Si (<1000 Å) using oxygen bombardment and positive secondary ion detection.
Platinum silicide junction spiking in arsenic doped polysilicon transistors
Authors: Grivna Gordon , Kirchgessner Jim , Carlson Jack
Published: July 2002

Polysilicon emitter bipolar npn transistors, silicided with platinum, exhibited emitter-base leakage after the final passivation thermal cycle. Transmission electron microscopy studies revealed platinum silicide (PtSi) spiking through the polysilicon into the emitter-base region, indicating an apparent release of Pt during high temperature (>400 °C) processing. A technique was developed to study the phenomenon using unpatterned monitor wafers. Observations from a series of tests indicate arsenic dose, Pt thickness, Pt sinter temperature, PtSi capping material, postsilicide formation temperatures, and exposure to oxygen during high temperature treatments (after silicide formation) as the major factors influencing PtSi spiking.
Molecular beam epitaxy growth of high performance AlGaAs/GaAs quantum well top emitting lasers
Authors: Wang Y. H. , Hasnain G. , Tai K. , Wynn J. D. , Weir B. E. , Dutta N. K. , Cho A. Y.
Published: July 2002

Planar, gain-guided top emitting AlGaAs/GaAs multiquantum well lasers are fabricated using molecular beam epitaxy and subsequently characterized. The device is composed of graded-index separate confinement cladding layers sandwiched between two-step composition followed by doping profiles doped distributed Bragg reflectors (DBRs). The resultant laser is found to exhibit continuous wave operation up to 90 °C, a characteristic temperature greater than 210 K, a threshold current density as low as 1.18 kA/cm2, and a continuous wave (cw) power of 1.5 mW operated at room temperature. Uniformities of threshold, voltage, outpower, and emitted wavelength for vertical cavity surface emitting lasers operated in cw conditions over a large area are described. A large variation in emitted wavelength, 200 Å over a 1 cm long row, is observed, as opposed to a 10% variation in the threshold current.
Influence of growth interruption on I–V characteristics of AlAs/GaAs double barrier resonant tunneling diodes
Authors: Reddy V. K. , Neikirk D. P.
Published: July 2002

We have examined the influence of growth interruption on the electrical characteristics of thin barrier, high-current density AlAs/GaAs resonant tunneling diodes (RTDs) grown by molecular-beam epitaxy. Interrupt schedules were determined by independent reflection high-energy electron diffraction measurements of specular spot intensity oscillations during prototypical device growth sequences. Our data suggests that interface roughness at inverted and normal interfaces does not play a significant role in determining the perpendicular transport characteristics of high current density AlAs/GaAs RTDs.
Patternwise esterification at room temperature: A novel wet and dry developable, deep ultraviolet photoresist
Authors: Mutsaers C. M. J. , Nijssen W. P. M. , Vollenbroek F. A. , Kraakman P. A.
Published: July 2002

A deep ultraviolet photoresist based on 1,1′(dodecamethylene)bis(3-diazo-2,4-piperidinedione) and poly(3-methyl-4-hydroxystyrene) can be esterified at room temperature if the ultraviolet exposure is performed in a dry environment. No water-free bake treatment during or after the exposure is necessary to obtain a high degree of esterification, as in the case of conventional diazonaphthoquinone/novolak based photoresists. Patternwise esterification results in 0.45 and 0.35 μm lines and spaces in the wet developable image reversal and the dry developable submicron positive dry etch resist process, respectively, using a 248 nm contact exposure.
Comparison of growth front profile of strained layers grown by migration-enhanced epitaxy and molecular-beam epitaxy using reflection high-energy electron diffraction
Authors: Chen Y. C. , Bhattacharya P. K. , Singh J.
Published: July 2002

We have compared the growth front smoothness of strained layers grown by migration-enhanced epitaxy and molecular-beam epitaxy by looking at the oscillation maxima of reflection high-energy electron diffraction intensities. We find that for growth of InxGa1−xAs on GaAs with x less than ∼0.15, migration-enhanced epitaxy improves the surface quality, while for larger amounts of mismatch (x≳0.15) the surface profile during migration enhanced epitaxy growth is worse than during conventional As-stabilized molecular-beam epitaxial growth. Results of Hall mobility, photoluminescence measurements, and modulation-doped transistor characteristics are consistent with the diffraction intensity studies.
Scanning tunneling microscopic and transmission electron microscopic studies of cytochrome c551 denaturation at the air–water interface
Authors: Liu Zhong-Fan , Manivannan Ayyakkannu , Inokuchi Hiroo , Yanagi Hisao
Published: July 2002

The morphologies of pseudomonas-cytochrome c551 in directly deposited film, mixed Langmuir–Blodgett (LB) film with an amphiphilic azobenzene derivative, and unfolded LB film have been studied using the scanning tunneling microscopy (STM) and transmission electron microscopy. In the first two cases, the protein molecule shows a nearly globular structure with an external diameter of ∼5 nm. Occasionally dimer- and trimerlike structures are also observed in such films. In contrast, the third case gives a rodlike structure whose dimension in length falls into a range of 21.5–29.1 nm, indicating the remarkable unfolding of cytochrome c551 at the air–water interface. Both tertiary and secondary structures of this protein have been virtually lost in such unfolding process. The polypeptide chains of cytochrome c551 molecules in the unfolded LB film are predominantly nearly fully extended, and highly aggregated in a local region, forming a crystalline or a twisted ropelike structure. Based on the high-resolution STM images of the unfolded cytochrome c551, a number of intramolecular structures including the locations of heme and several amino acid residues have been discussed.
Intersubband transitions in pseudomorphic InGaAs/GaAs/AlGaAs multiple step quantum wells
Authors: Li H. S. , Chen Y. W. , Wang K. L. , Lie D. Y. C.
Published: July 2002

Intersubband transitions from the ground state to the first and second excited states in pseudomorphic AlGaAs/InGaAs/GaAs/AlGaAs multiple step quantum wells have been observed. The step well structure has a configuration of two AlGaAs barriers confining an InGaAs/GaAs step. Multiple step wells were grown on GaAs substrate with each InGaAs layer compressively strained. During the growth, a uniform growth condition was adopted so that inconvenient long growth interruptions and fast temperature ramps when switching the materials were eliminated. The sample was examined by cross-sectional transmission electron microscopy, an x-ray rocking curve technique, and the results show good crystal quality using this simple growth method. Theoretical calculations were performed to fit the intersubband absorption spectrum. The calculated energies are in good agreement with the observed peak positions for both the 1→2 and 1→3 transitions.
Analytical evaluation of the energy deposition function in electron-beam lithography in the case of a composite substrate
Authors: Raptis I. , Glezos N. , Hatzakis M.
Published: July 2002

In this article the energy deposition function in the case of e-beam lithography is calculated analytically. The distribution of electrons as a function of depth and energy is calculated first using a method based on the Boltzmann transport equation which is easily applicable in the case of multilayer substrates. Next the lateral distribution of the electrons is calculated and each contribution (primary, secondary, and backscattered electrons) is considered separately. Energy dissipation results are used as input to a cell removal model for the resist development simulation.
Fast proximity effect correction: An extension of PYRAMID for thicker resists
Authors: Cook Brian D. , Lee Soo-Young
Published: July 2002

In previous years PYRAMID, a hierarchical, pattern shape modification scheme for proximity effect correction in electron-beam lithography, was presented. PYRAMID has been successful in correcting circuit patterns with a minimum feature size of 0.1 μm exposed in 200 nm poly(methylmethacrylate) (PMMA) on silicon. Current research has focused on improving PYRAMID so that it can handle thicker resists (e.g., 500 nm), which have a much greater proximity effect than do thinner resists. As a result, some of the assumptions and approximations that were made in the previous version of PYRAMID are no longer valid. Several modifications and improvements have been implemented in an effort to allow PYRAMID to correct circuits in thicker resists. The principle modification was the implementation of a new, table-based convolution method (termed CDF convolution). Using this method, it is possible to calculate exact local exposure values due to circuit elements located within the local correction window (3.6 μm×3.6 μm) during the correction. Other improvements include modifications to the junction correction and inner maximal rectangle adjustment phases of PYRAMID, as well as the addition of an option to iterate the correction. This article discusses these modifications and improvements and presents preliminary experimental results indicating successful correction of circuits exposed in 500 nm PMMA on silicon.
Alkaline soluble polysiloxane electron-beam resist for 0.2 μm rule 256 Mbit dynamic random access memory fabrication
Authors: Hashimoto Kazuhiko , Matsuo Takahiro , Endo Masayuki , Sasago Masaru , Nomura Noboru
Published: July 2002

In order to improve pattern resolution and reduce the proximity effect in electron-beam (EB) direct-writing lithography, a novel silicon-containing negative EB resist for bilayer resist application has been developed. This new negative resist can be developed by an aqueous alkaline solution such as a tetramethyl ammonium hydroxide, due to an alkaline soluble matrix polymer in the resist. Swelling of the resist patterns was prevented by use of the alkaline developer. This newly developed EB resist has high sensitivity below 10 μC/cm2, and also high resolution capability. 0.2 μm line and space patterns were successfully fabricated. The etching selectivity ratio of this new negative resist versus novolak resin in an oxygen plasma was greater than 50, because of the high silicon content in this resist. Therefore, it was easy to obtain high fidelity pattern transfer. Experimental 0.2 μm rule 256 Mbit dynamic random access memory patterns were successfully obtained in a bilayer resist system on uneven topography of the processed wafer by using a conventional variable-shaped EB machine with 20 keV accelerating voltage. A noncharging resist process and proximity effect correction were also adopted to improve pattern accuracy.
Structural characterization of encapsulated Au/Zn/Au ohmic contacts to p-type GaAs
Authors: Lin X. W. , Liliental-Weber Z. , Washburn J. , Piotrowska A. , Kaminska E.
Published: July 2002

Conventional transmission electron microscopy and high-resolution electron microscopy were used to study the cross sections of Au/Zn/Au ohmic contacts to a [001]-oriented p-type GaAs substrate. The metal contacts fabricated by sequential vapor deposition were capped with an Al2O3 thin film to form a closed system when subjected to a 420 °C anneal for 3 min. For an as-deposited specimen, the Zn layer was found to react with the two adjacent Au layers and the reaction products were three room-temperature phases of Au3Zn. One of these phases is identified for the first time and its structure is characterized. Upon annealing, these phases remained stable and no interfacial reaction between the metal contact and GaAs was observed, as opposed to an uncapped contact. The interface between the annealed contact and GaAs exhibited the same degree of uniformity and planarity as that of the as-deposited specimen. This study demonstrates that the formation of metallic protrusions into GaAs, as commonly found in an uncapped contact, is not necessary for the transition from rectifying to ohmic behavior of the contact.
Cesium on GaP(110) surfaces: A confirmation of the metal-induced gap states-plus-defects model
Authors: Linz R. , Clemens H. J. , Mönch W.
Published: July 2002

The formation of Cs–GaP contacts was studied at room temperature and at 80 K by recording Auger electrons, energy distribution curves (EDCs) of photoelectrons excited by He i radiation [ultraviolet photoemission spectroscopy (UPS)], and by measuring the contact potential with a Kelvin probe in the dark [contact-potential difference technique (CPD)]. While the samples held at room temperature show a saturation coverage of one monoatomic Cs layer, at low temperatures homogeneous growth of metallic films was achieved. The position of the Fermi level with respect to the valence-band maximum was obtained as the difference between the ionization energy from UPS and the work function from CPD measurements. This procedure accounts for any possible influence of surface photovoltage. After the occurrence of a Fermi edge in the EDCs recorded at 80 K, the Fermi level was found to become pinned on both p- and n-GaP(110). The corresponding barrier height φBn measures 1.00 eV. This result excellently agrees with what is predicted by the model, that the continuum of metal-induced gap states (MIGS) determines the barrier heights in abrupt Schottky contacts, and definitely rules out the Schottky–Mott model also for GaP. The barrier heights published so far for GaP Schottky contacts may again be explained by the MIGS-plus-defects model.
Effects of oxygen on the sublimation of alkaline earths from effusion cells
Authors: Hellman E. S. , Hartford E. H.
Published: July 2002

The stability of the atomic flux from effusion cells is essential for the application of molecular-beam epitaxy (MBE) to new materials systems. In particular, the possible effects of oxygen background pressure on source stability have been a concern to those applying MBE to the growth of oxides, such as the high Tc superconductors. We have studied the effects of oxygen exposure on the sublimation of the alkaline earth elements Mg, Ca, Sr, and Ba from effusion cells, using deposition rate measurements and quadrupole mass spectrometry. Sublimation of Mg, Ca, and Sr is suppressed exponentially by oxygen with a characteristic pressure comparable to the source equilibrium vapor pressure. Ba sublimation is increased linearly by oxygen background. A surprising finding is that immediately after an exposure to oxygen, Mg, Ca, or Sr flux from an effusion cell is increased over a period of several minutes. We suggest an explanation for this phenomenon in terms of changes in the emissivity of oxide coated source material. The implication is that the stability of these sources is better at moderate oxygen pressures than at low oxygen pressures.
Fabrication of aspheric high numerical aperture reflective diffractive optic elements using electron beam lithography
Authors: Mikolas D. , Bojko R. , Craighead H. G. , Haas F. , Honey D. A. , Bare H. F.
Published: July 2002

Electron beam lithography and reactive ion etching have been used to fabricate four level aspheric reflecting focusing diffractive optical elements. A fast (f/2), 1 mm diam, four-phase-level, reflecting, off-axis imaging diffractive optical element, without spherical aberration, coma, and astigmatism, has been fabricated on silicon. Alignment and stitching errors have been held to less than 40 nm and the smallest pattern feature is 0.6 μm. The silicon grating is coated with gold to enhance reflectivity with an efficiency of 73%. The quality of diffractive optical elements is limited by the fidelity of the fabrication steps, and methods are demonstrated for proximity effect correction, alignment, high selectivity mask creation, and reactive ion etching that can be used for high quality diffractive optic element fabrication of essentially arbitrary type.
New method to measure dI/dV and d2I/dV2 of the tunneling current–voltage characteristics
Authors: Yamada H. , Sumomogi T. , Endo T.
Published: July 2002

In the case of the I/V curve consisting of an almost rectilinear line which includes small dI/dV and d2I/dV2 term, a lock-in amplifier is generally used for the detection and measurement of these derivatives. The digital technique of using an A/D converter and the numerical analysis are powerless for the detection and measurement of small derivatives, because these small derivatives are buried under the least significant digit voltage. In this research, we propose a new method to detect and measure these small derivatives employing an analog subtraction circuit and an amplifier for the pretreatment of A/D conversion. This method effectuates the fast and simple scanning tunneling spectroscopy.
Surface roughness observation by scanning tunneling microscopy using a monolithic parallel spring
Authors: Zhang H.-C. , Sasaki A. , Fukaya J. , Aoyama H.
Published: July 2002

A new scanning tunneling microscope (STM) and its application to surface roughness observation are described. Precision two-dimensional positioning of our STM instrument was achieved with a two-dimensional parallel spring driven by two stacked piezoelectric actuators (PZT), so that the mutual interference effect between x- and y-axis displacements is very small. The PZT for the z-axis displacement and control was mounted on the monolithic parallel spring. Therefore, the mutual interference effect of three-dimensional displacements is negligible. We have applied this STM to observation of surface roughness. A block gauge and a gold thin film evaporated on glass plates are observed at the constant-current mode and the zero bias voltage.
Local probing instrumentation at Advanced Technologies Center: Surface and force devices with tunneling sensor
Authors: Moiseev Yu. N. , Panov V. I. , Savinov S. V. , Yaminsky I. V.
Published: July 2002

Force and surface devices with tunneling sensor—scanning tunneling and atomic force microscopes—are constructed. Thermal drift, seismic and electronic noise are eliminated in precise experiments. The tunneling sensor is proved to be advantageous for large scale images and force versus distance measurements with high dynamic range and subnanometer resolution. The influence of adsorbate on scanning tunneling microscope (STM) images of graphite is demonstrated. The holes 2 nm in diameter are produced on graphite exposed at air using STM.
Observation and surface modification of electropolymerized poly(N-methylpyrrole) using atomic force microscopy
Authors: Yano T. , Nagahara L. A. , Hashimoto K. , Fujishima A.
Published: July 2002

We have used atomic force microscopy (AFM) to investigate the surface morphology of poly(N-methylpyrrole) after electropolymerization on indium tin oxide and highly oriented pyrolytic graphite (HOPG) surfaces. We have found that the polymer surface became rougher with higher current density or lower solution temperature. In situ observations of the initial stages of electropolymerization revealed that an extremely weak adhesion exists between the polymer and HOPG. As a result, the AFM tip–sample interaction could easily move the polymer, and we have proposed that this can be used for patterning surfaces.
Arsenic gas-phase doping of polysilicon
Authors: Ransom C. M. , Jackson T. N. , DeGelormo J. F. , Kotecki D. , Graimann C. , Sadana D. K.
Published: July 2002

Polysilicon layers have been doped with high uniform concentrations of arsenic in short processing times using gas-phase doping. The polysilicon layers were doped at total system pressures from 1 to 760 Torr with arsine or tertiarybutylarsine (TBA) as gas-phase dopant sources. An arsenic concentration as high as ≳4×1020 atm/cm3 in 100 nm polysilicon layers was measured after a 60 s diffusion in 2.4% arsine in He at 1000 °C at 760 Torr. Using tertiarybutylarsine (TBA) at 3 Torr, a sheet resistance of 2.5 kΩ/⧠ was measured in a 100 nm polysilicon layer after 12 min at 900 °C. Doping at temperatures as low as 770 °C also gave high arsenic concentrations. Finally, a 6 μm deep DRAM trench capacitor was uniformly doped with arsenic to a concentration of 8×1019 atm/cm3 using gas-phase doping and a two-step polysilicon deposition process.
Some preliminary observations of the rapid thermal oxidation of porous silicon, and the rapid thermal nitriding of oxidized porous silicon
Authors: Shieh Sy-Yuan , Evans James W.
Published: July 2002

Porous silicon, produced by the anodic oxidation of p-type single crystal wafers of silicon in hydrofluoric acid/ethanol electrolytes has been subjected to rapid thermal oxidation. For comparison purposes, and also to generate porous oxide for subsequent nitriding, other samples of porous silicon were subjected to conventional furnace oxidation. By TEM, EDS, and ESCA it was shown that the structures and compositions of the oxides produced by the two routes were similar. It was further demonstrated that oxidized porous silicon could be nitrided by rapid thermal processing. The resulting structure, although still porous, showed a much diminished etching rate in hydrofluoric acid, compared to the original oxide.
Microhardness measurements by scanning tunneling microscope
Authors: Barbato G. , Desogus S. , Germak A. , Picotto G. B. , Xhomo E.
Published: July 2002

A scanning tunneling microscope (STM) operating in air was used to image an array of Vickers indentations made on hardened steel, having diagonal lengths ranging from 1 to 70 μm. The morphology of the indentations shows that the boundary of the indentation is not sharply defined. Therefore, accurate STM-based microhardness measurements need image analysis methods for measuring either the indentation area or a parameter well correlated with the geometry of the indentation. Three methods have been investigated. Two methods use different methodologies to estimate the level of the undisturbed surface of the tested material before the indentation is made, and the indentation area is subsequently calculated. The third one analyzes locally the slope of the indentation boundary area. The area is obtained from the profile of the indentation, as in the traditional optical measurements. The repeatability of the indentation area measurements obtained by using the different methods, has been evaluated by imaging the indentations at different sampling areas. The influence of the sample orientation and tip scan rate on the measurement repeatability has also been studied.
Scanning tunneling microscope and atomic force microscope study of epitaxially grown palladium crystallites on graphite
Authors: Kojima Isao , Kurahashi Masayasu
Published: July 2002

Scanning tunneling microscope (STM) and atomic force microscope (AFM) have been applied to the study of an epitaxial growth of Pd on graphite. Pd deposited at 450 °C nucleates to crystallize and grow epitaxially along the [111] direction. The STM images of the topmost layer of the (111) face exhibited a long periodic structure with a periodic distance of about 2.7 nm. Codeposition of Pd and Cu on graphite at 550 °C created clear images of the atomic arrangement together with the periodic structure. The periodic length was found to exactly correspond to a distance of 10 atoms. Auger measurements of each crystallite showed that its surface layer mostly contains Pd atoms although almost equal amount of Pd and Cu was deposited. The AFM observations also showed images with similar periodicity to those from the STM. On the basis of the STM and the AFM results, the possibility of the reconstruction of the surface is discussed.
Scanning tunneling microscope and tunneling stabilized magnetic force microscope characterization of magnetic nanocrystalline materials
Authors: Nogués J. , Rodell B. , Rao K. V.
Published: July 2002

Atomic scale surface as well as magnetic domain pattern studies have been carried out on nanocrystalline Fe86Zr7B6Cu and Fe73.5CuNb3Si13.5B9 melt cast ribbons by using scanning tunneling microscope and tunnel stabilized magnetic force microscope (TSMFM) techniques. In the micrometer range for the Fe–Cu–Nb–Si–B alloy we observe at the surface the typical smooth wavy and hilly structures expected for amorphous alloys. However, in nanocrystalline Fe–Zr–B–Cu we find uniformly distributed ellipsoidal bumps aligned along the direction of ribbon casting. At a nanometric range, on the other hand, both the alloys are found to contain homogeneously distributed nanocrystallites of 10±2 nm diameter as determined from an autocorrelation function analysis of our data. Such a determination of the nanocrystallite size with the implication that particle size is significantly smaller than the magnetic exchange length explains the unusually soft magnetic properties of nanocrystalline materials. Furthermore the smooth morphology of the nanocrystallites suggests that they are embedded homogeneously throughout the ribbon in an amorphous matrix. In our TSMFM studies for the Fe–Cu–Nb–Si–B ribbon we observe no magnetic contrast. In the case of the Fe–Zr–B–Cu ribbon, however, elliptic-shaped magnetic patterns are observed which probably arises from stray fields concentrating at the depressions between clusters of nanocrystals.
Diamond-coated tips and their applications
Authors: Liu N. , Ma Z. , Chu X. , Hu T. , Xue Z. , Pang S.
Published: July 2002

Scanning tunneling microscope (STM) has recently demonstrated its potential application as a powerful tool for nanometer-scale surface modification. We have successfully fabricated new types of diamond-coated STM tips by using microwave plasma chemical vapor deposition (MPCVD). The reactant is a gaseous mixture of methane and hydrogen, sometimes adding oxygen. While natural diamond is a good insulator, the electrical conductivity of diamond-coated tips made by MPCVD technique is always high enough for STM experiments. These tips are quite stable, hard enough to make machining on metal or other solid surfaces in nanometer scale. We have applied the diamond-coated STM tip to so-called controlled machining on the metallic surfaces, such as gold, silver, and platinum films, and even polycrystal platinum, single crystal nickel, and palladium. Further investigations, including the results caused by different tips, applications for nanometer-scale processing and engineering, etc., have been discussed. This technique might become a potential useful tool for nanometer science and technology in the future.
Manipulation of rectangular arrangement of Se-ring-type molecules on graphite (highly oriented pyrolytic graphite) surfaces
Authors: Czajka R. , Kasuya A. , Horiguchi N. , Nishina Y.
Published: July 2002

A rectangular lattice arrangement of Sen (n=5–8) ring clusters has been fabricated for the first time on the c plane of graphite (highly oriented pyrolytic graphite) crystal and has been examined by a scanning tunnel microscope (STM) image which was found to consist of a 0.72 nm×0.85 nm lattice spacing with individual molecules of 0.53±0.05 nm in diameter. Each lattice point consist of Se-ring-type molecules, most probably six-membered. The STM images were taken with the bias voltage VT between probe and sample as a parameter. For −0.7 V<VT<+0.7 V the patterns of the rectangular lattice remained similar, whereas at VT≊−0.7 V the regular arrangement suddenly collapsed to a random distribution of larger clusters, their diameter consisting of a range up to 5 nm anywhere. The process of the phase change in the lattice system is not reversible with respect to the bias voltage.
Scanning tunneling microscopy study of C60 adsorption on 2H–MoS2 (0001) surface
Authors: Chen Ting , Sarid Dror
Published: July 2002

Scanning tunneling microscope images of submonolayer to monolayer coverages of C60 on cleaved 2H–MoS2 (0001) surfaces show unique adsorption and initial growth patterns that are not observed with similar coverages of C60 on metal surfaces. Intramolecular contrast of the adsorbed C60 show both the local density of states of the molecules and underlying crystalline lattice features and is sensitive to the applied bias voltage. While the interactions between the adsorbed C60 and MoS2 surface are found to be essentially van der Waals, more reactive processes upon adsorption are evidenced by the observation of locally ordered, 1-nm-sized features (craters), with reversed contrast to those C60 that are bound to the substrate via van de Waals interaction. These interactions and the mechanism of the voltage-dependent intramolecular contrast will be discussed. These results will be compared with those obtained on metallic and other semiconductor substrates.
Inhomogeneities of phase separated Langmuir–Blodgett films studied by atomic force microscopy
Authors: Chi L. F. , Fuchs H. , Johnston R. R. , Ringsdorf H.
Published: July 2002

Micromorphology of phase separated Langmuir–Blodgett (LB) films is investigated by atomic force microscopy from submicrometer scale up to 100 μm scale. Several novel phenomena are revealed: (a) the existence of grains with the size of 20–100 nm in diameter in the liquid expanded phase; (b) the existence of a rough domain boundary with fingerlike structures and (c) an inhomogeneous distribution of holelike defects in the liquid condensed domains. Aging effects are studied systematically, which give the hint for the existence of water on transferred LB films. The results are relevant to the understanding of the early stages of the domain formation and the two-dimensional nucleation processes.
Investigation of molecular chains structure of polyimide Langmuir–Blodgett films by atomic force microscopy*
Authors: Yang Xiao-Min , Min Guang-Wei , Gu Ning , Lu Zu-Hong , Wei Yu
Published: July 2002

Atomic force microscopy was applied to investigate the microscopic molecular structures of polyimide Langmuir–Blodgett (LB) films. The polyimide chains of a monolayer film on graphite were observed. It was found that the polyimide LB film with a groovy surface occurs due to the orientation of polyimide chains. By horizontal deposition the polyimide chains show a zigzag structure, while by vertical deposition the polyimide chains show a straight-line structure. Moreover, the polyimide chains are always oriented along the direction of deposition. It is suggested that the polyimide chains are oriented by the LB technique.