bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrared spectroscopy of formate on oxygen-predosed Cu(100): Broadband reflectance and low-frequency vibrations
Authors: Lin K. C. , Tobin R. G. , Dumas P.
Published: July 2002

The adsorbate-induced broadband infrared reflectance change and low-frequency vibrational modes of formate on an oxygen-predosed Cu(100) surface near room temperature have been measured with synchrotron-based infrared spectroscopy. In contrast to CO, O, and H on the same surface, no broadband reflectance change is detected over the frequency range 200–2000 cm−1. The difference may be associated with the lack of a formate-derived electronic state near the Fermi level. Two vibrational bands are observed, at 365 and 380 cm−1, attributed to oxygen-metal stretch vibrations; previous electron energy loss spectroscopy at lower resolution detected only a single band. We suggest that the doublet may indicate the coexistence of two formate species with different binding sites.
Surface nanostructure determination by x-ray photoemission spectroscopy peak shape analysis
Authors: Tougaard S.
Published: July 2002

Factors that contribute to the uncertainty in quantitative analyses of surfaces by x-ray photoemission spectroscopy and Auger electron spectroscopy are considered. Quantification is usually based on the convenient but quite arbitrary assumption that the sample is homogeneous within the outermost few nanometers. This assumption can lead to uncertainties of several hundred percent in the analysis and, as a consequence, a meaningful quantification based on measured peak intensities alone is not possible. In contrast, the contribution to the uncertainty from other factors is much smaller. It is further pointed out that, when many factors contribute roughly equally to the error, even considerable improvements in the uncertainty of a single factor, have only little influence on the total error. It is therefore clear that in the future effort must be concentrated on the development of practical techniques to enhance the knowledge on the in-depth composition since, without this, no improvement can be expected even if a substantial improved accuracy of other factors is obtained. One such technique that relies on analysis of both the peak intensity and the peak shape is discussed and this technique seems to reduce the uncertainty considerably, to a typical level of 10%–20%, depending on the solid and surface morphologies.
Secondary ion mass spectroscopy resolution with ultra-low beam energies
Authors: Clegg J. B. , Smith N. S. , Dowsett M. G. , Theunissen M. J. J. , Boer W. B. de
Published: July 2002

The depth resolution attainable in secondary ion mass spectroscopy (SIMS) depth profiling is shown to be greatly enhanced by the use of a floating ion gun, which is capable of delivering high primary ion currents (ca. 100 nA) at impact energies down to 250 eV. With specially grown, genuinely abrupt GaAs (Si,Al) delta layers and Si0.8Ge0.2/Si structures we have achieved SIMS profile full width at half maximum data as low as 1.3–2.2 nm (species dependent). Approximating the leading edges by exp(z/λL), where z is depth, we achieve λL (growth length) values ≊0.2 nm (i.e., 0.5 nm per decade of concentration). The trailing edge data, exp(−z/λT), exhibit the expected elemental dependence due to differences in preferential sputtering and more complex effects, with λT (decay length) ranging from 0.6 nm for Si to 1.8 nm for Al. This analytical performance, which can be obtained in combination with at least 1 part per million atomic elemental sensitivity, represents a major improvement in SIMS depth profiling capability.
Mechanism of selective SiO2/Si etching with fluorocarbon gases (CF4, C4F8) and hydrogen mixture in electron cyclotron resonance plasma etching system
Authors: Doh Hyun-Ho , Kim Jung-Hun , Lee Seok-Hyun , Whang Ki-Woong
Published: July 2002

A mixture of fluorocarbon gases having high C/F ratio is commonly used to obtain the high selective etching of SiO2 to Si in high-density plasmas. We investigated the etch rate of SiO2 and Si in an electron cyclotron resonance (ECR) plasma etching system as functions of gas pressure, gap distance between the ECR layer and the substrate, and hydrogen percentage in fluorocarbon gases such as CF4 and C4F8. The selectivity increases considerably in the case of the C4F8 plasma as the gas pressure, hydrogen percentage, and the gap distance increase. Actinometric optical emission spectroscopy (OES) and appearance mass spectrometry (AMS) were employed to study the mechanism of selective SiO2 etching. Ar was used as the actinometer, and the behavior of the F atom and various fluorocarbon radicals was investigated for CF4 and C4F8 ECR plasmas. The following results were obtained. First, the CF2 radical behavior investigated by OES coincides well with that by AMS. Second, the behavior of CF2 radical density as a function of hydrogen percentage in the CF4 ECR plasma is very similar to that in the rf capacitively coupled plasma (RFCCP), but its increasing rate with H2 addition is smaller by an order of magnitude compared to using the CF4 in RFCCP system. Third, the dominant polymer precursor which is important for the selective SiO2 to Si etching is CF in the C4F8+H2 ECR plasma, and as the hydrogen percentage increases, the decreasing rate of the relative concentration of atomic F is higher in the CF4 ECR plasma than in the C4F8 ECR plasma. Therefore, it is suggested that the F abstraction reaction in gas phase process such as CF2+H→CF+HF is more dominant than the F scavenging reaction such as CF2+F+H→CF2+HF in the C4F8 ECR plasma in the case with hydrogen addition. Fourth, the densities of all the fluorocarbon radicals increase contrary to the decrease of atomic F when the pressure increases from 2.5 to 10 mTorr when the H2 addition is used. Last, the relative concentration of the F atom decreases as the distance from the ECR layer increases axially, but the fluorocarbon radicals do not show considerable changes.
Photothermal imaging by scanning thermal microscopy
Authors: Oesterschulze E. , Stopka M.
Published: July 2002

A combined scanning tunneling microscopy (STM) and scanning thermal microscopy (SThM) setup is introduced that is capable of mapping simultaneously topographical as well as local thermal properties of sample surfaces. The miniaturized thermocouple probe used in the microscope images the thermal conductivity of the sample if operated in the statical mode. To investigate the dynamical thermal behavior of the sample as well, the thermocouple is applied in the ac or photothermal mode. In this case, the resultant amplitude and phase information detected with a lock-in amplifier depend on the local thermal properties of the sample, which is well known from classical macroscopic photothermal investigation techniques. The microscope was employed for the investigation of thermal properties of (111) textured diamond films with high lateral resolution.
Molecular dynamics simulation of multilayer homoepitaxial deposition on face-centered-cubic(100) metal surfaces
Authors: Kelchner Cynthia L. , DePristo Andrew E.
Published: July 2002

Two features are observed to be important in the homoepitaxial deposition of 50 monolayers on Pd(100) and Cu(100) at 80 K. First, a fourfold hollow site that is missing one or more of its four supporting atoms on the surface (i.e., an overhang site) can be stable. This increases the surface roughness by allowing defects in the growing surface. Second, multiatom rearrangements occur during growth. These events decrease the local surface roughness by filling deep holes in the surface. Neither of these two features is included in currently used kinetic growth models.
Dependence of atomic layer-deposited Al2O3 films characteristics on growth temperature and Al precursors of Al(CH3)3 and AlCl3
Authors: Yun Sun Jin , Lee Kyung-Ho , Skarp Jarmo , Kim Hae-Rim , Nam Kee-Soo
Published: July 2002

The materials characteristics of Al2O3 films grown on a Si (100) substrate by traveling wave reactor atomic layer deposition were investigated in the growth temperature ranging from 250 to 500 °C. The Al2O3 films grown using Al(CH3)3 trimethylaluminum (TMA) and H2O as precursors were characterized and also compared with the films grown using AlCl3 and H2O. Both samples grown with different precursors revealed identical chemical binding state of oxidized Al and very flat surface morphology. In the study of impurity incorporation, the films grown using TMA showed the C and H count rates of secondary ion mass spectrometry (SIMS) approximately six and 10 times higher than those of the film grown using AlCl3, respectively. For the Al2O3 films grown using TMA, the results showed that the impurity contents and the growth rate of the films decreased and the refractive index increased with the growth temperature. The content of impurities could be also lowered by increasing the N2 purge time after TMA pulse. However, the effect of purge time increase on the impurity incorporation was very small compared to that of growth temperature. The refractive indices were 1.64 and 1.68 at the growth temperatures of 250 and 400 °C, respectively. The rough estimation of H content using refractive indices and SIMS data indicated that the H content decreased from approximately 8.5 to 3.0 at. % as the substrate temperature increased from 250 to 400 °C. The Al2O3 film grown using AlCl3 and H2O at 500 °C, which contained approximately 0.5 at. % Cl, revealed a refractive index of 1.65.
X-ray photoemission spectroscopy study of UV/ozone oxidation of Au under ultrahigh vacuum conditions
Authors: Krozer Anatol , Rodahl Michael
Published: July 2002

(111) textured Au films were oxidized under UHV conditions by exposing them to UV radiation and ozone. The Au films were characterized by x-ray photoelectron spectroscopy. The gold surface becomes heterogeneous upon oxidation: approximately 0.8 monolayer (ML) of the adsorbed oxygen is present as chemisorbed species. The rest of the adsorbed oxygen forms, on the average, a few ML thick gold oxide with a stoichiometry close to Au2O3. Both the oxide and the chemisorbed oxygen disappear slowly when heated to 150 °C. During isothermal decomposition at 200 °C, the oxide is transformed to chemisorbed oxygen prior to desorption. Both the chemisorbed oxygen and the gold oxide can be removed by rinsing with water. The surface is hydrophilic after the rinse. When contaminated gold films are exposed to UV/ozone, impurity oxides appear at higher binding energies than the binding energies of oxygen on clean gold samples. The impurity oxides are thermally much more stable than Au2O3.
Comparison of the temperature dependence of the properties of ion beam and magnetron sputtered Fe films on (100) GaAs
Authors: Bernstein S. D. , Wong T. Y. , Tustison R. W.
Published: July 2002

The effects of substrate temperature on the crystallographic, electrical, and mechanical properties of ion beam (IBS) and magnetron sputtered (MS) Fe films on (100) GaAs were studied and compared. Films were deposited at substrate temperatures ranging from 100 to 500 °C. Given the large differences between the two deposition techniques, the film properties were remarkably similar. A higher deposition rate was achieved for magnetron sputtering, and the deposition rate increased with increasing temperature for both techniques. For both deposition techniques, as the substrate temperature was increased the films changed from random or (110) oriented to a preferred (200) orientation, with IBS films having a higher degree of preferred orientation. The full-width at half-maximum of the (200) rocking curves are comparable for both techniques and varied from 0.5° to 0.9°. Both techniques show an initial decrease in resistivity with increasing temperature, reaching a minimum resistivity approaching that of bulk Fe at about 200 °C. With further increases in temperature, the resistivity increases. Resistivity of IBS films was lower than that of MS films. The stress of MS films was tensile (0.5–1.5 GPa) at all deposition temperatures while the stress for IBS films was compressive (−1 GPa) at low temperatures and became tensile (0.5 GPa) at high temperatures.
Fabrication of high performance GaN modulation doped field effect transistors
Authors: Bardwell J. A. , Foulds I. , Lamontagne B. , Tang H. , Webb J. B. , Marshall P. , Rolfe S. J. , Stapledon J. , MacElwee T. W.
Published: July 2002

Fabrication of GaN devices presents a number of processing issues. Since the AlGaN/GaN material and sapphire substrates are transparent in the ultraviolet, light scattering can degrade the quality of the lithography. GaN and AlGaN are chemically inert materials, highly resistant to wet etching. Schottky contacts are fabricated with noble metals (Pt, Au) which present adhesion problems. The approaches to solving these difficulties and the routes to fabrication of high performance GaN-based modulation doped field effect transistors are presented. The light scattering has been addressed by the deposition of amorphous Si on the backside of the wafers. Mesa etching for device isolation has been performed with both chemically assisted ion beam etching and a newly developed wet etching technique. Finally, metal adhesion has been greatly improved by the initial deposition of a thin sputtered Pt layer, followed by a thick layer of Pt/Au by e-beam evaporation. A 100 μm wide device showed a maximum dc current density of 946 mA/mm, a peak transconductance of 160 mS/mm, a short circuit current gain cutoff frequency of 15.6 GHz, and a maximum oscillation frequency of 49.4 GHz.
Spatial quantization in GaAs–AlGaAs multiple quantum dots
Authors: Reed M. A. , Bate R. T. , Bradshaw K. , Duncan W. M. , Frensley W. R. , Lee J. W. , Shih H. D.
Published: July 2002

We present results of the fabrication and investigation of totally spatially localized crystalline structures. Low temperature photoluminescence exhibits structure that is best explained by a bottleneck for hole energy loss. This bottleneck is believed to be a direct consequence of the modification of the band structure by the fabrication-imposed potential and is believed to be the first evidence for total spatial quantization in a fabricated heterojunction system.
Gallium arsenide and aluminum gallium arsenide reactive ion etching in boron trichloride/argon mixtures
Authors: Scherer A. , Craighead H. G. , Beebe E. D.
Published: July 2002

GaAs and Al0.3Ga0.7As were reactive ion etched in a mixture of boron trichloride and argon. The effects of the independent variables such as the time, power, pressure, and gas composition on the etch depth as well as the quality of the resulting etched surfaces were analyzed through a multiple linear regression approach. This provided second-order equations with an excellent ability to describe experimental results. The etching conditions for GaAs and AlGaAs were compared and optimum parameters for equal rate etching of GaAs/Al0.3Ga0.7As layers with straight walls were obtained. These were then sucessfully applied to the high-resolution structuring of multiple quantum well layers, and structures as small as 40 nm were etched. The addition of oxygen to the etch gas produced a high-quality etch for GaAs with >5:1 rate selectivity over Al0.3Ga0.7As.
Chemical processes involved in the etching of silicon by xenon difluoride
Authors: Dagata J. A. , Squire D. W. , Dulcey C. S. , Hsu D. S. Y. , Lin M. C.
Published: July 2002

The spontaneous etching of silicon by xenon difluoride has been investigated under single collision conditions using multiphoton ionization mass spectrometry for the detection of radical products. SiF2 is the only gas phase radical observed with an apparent activation energy of 6.7±0.5 kcal/mol. Electron ionization techniques were used to determine an apparent activation energy for gas phase production of SiF4 of 5.6±0.8 kcal/mol. The identification of SiF2 as the sole reactive desorbing species during the steady-state etching process is useful for understanding many aspects of primary surface processes and secondary gas phase reactions in etching.
Cleaning and nitridation of GaAs surfaces in multipolar plasmas investigated by i
n s
i
t
u photoemission and spectroscopic ellipsometry
Authors: Friedel P. , Landesman J.-P. , Boher P. , Schneider J.
Published: July 2002

We describe the results of a study of the interactions of multipolar plasmas (with H2 or N2 as the gas) with GaAs (100) surfaces. Using two complementary in situ techniques [x-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry], we show the behavior of the surfaces in terms of electric properties during a treatment. In particular, on the p-doped surfaces, the overall movement of the Fermi level, as detected by the two techniques, indicates a depinning during the H2 plasma treatment and a lowering of the band bending. XPS results show that during these phases the deoxidation of the surface is complete together with the removal of the amorphous As and carbon contaminations. The native nitridation (in the N2 plasma) also modifies the electric properties of the surface, and the results of the two techniques are in good agreement, but the physical interpretation of these effects is not yet made.
Focused ion beam technology and applications
Authors: Melngailis John
Published: July 2002

Ions of kiloelectron volt energies incident on a solid surface produce a number of effects: several atoms are sputtered off, several electrons are emitted, chemical reactions may be induced, atoms are displaced from their equilibrium positions, and ions implant themselves in the solid, altering its properties. Some of these effects, such as sputtering and implantation are widely used in semiconductor device fabrication and in other fields. Thus the capability to focus a beam of ions to submicrometer dimensions, i.e., dimensions compatible with the most demanding fabrication procedures, is an important development. The focused ion beam field has been spurred by the invention of the liquid metal ion source and by the utilization of focusing columns with mass separation capability. This has led to the use of alloy ion sources making available a large menu of ion species, in particular the dopants of Si and GaAs. The ability to sputter and to also induce deposition by causing breakdown of an adsorbed film has produced an immediate application of focused ion beams to photomask repair. The total number of focused ion beam fabrication systems in use worldwide is about 35, about 25 of them in Japan. In addition, there are many more simpler focused ion beam columns for specialized uses. The interest is growing rapidly. The following range of specifications of these systems has been reported: accelerating potential 3 to 200 kV, ion current density in focal spot up to 10 A/cm2, beam diameters from 0.05 to 1 μm, deflection accuracy of the beam over the surface ±0.1 μm, and ion species available Ga, Au, Si, Be, B, As, P, etc. Some of the applications which have been demonstrated or suggested include: mask repair, lithography (to replace electron beam lithography), direct, patterned, implantation doping of semiconductors, ion induced deposition for circuit repair or rewiring, scanning ion microscopy, and scanning ion mass spectroscopy.
Reduction imaging at 14 nm using multilayer-coated optics: Printing of features smaller than 0.1 μm
Authors: Bjorkholm J. E. , Bokor J. , Eichner L. , Freeman R. R. , Gregus J. , Jewell T. E. , Mansfield W. M. , Mac Dowell A. A. , Raab E. L. , Silfvast W. T. , Szeto L. H. , Tennant D. M. , Waskiewicz W. K. , White D. L. , Windt D. L. , Wood O. R. , Bruning J. H.
Published: July 2002

We demonstrate high resolution reduction imaging in the soft x-ray spectral region using multilayer-coated reflective optics. In particular, a Schwarzschild objective was used at 20:1 reduction with 14 nm radiation to image line and space features from a transmission mask onto a resist-coated silicon wafer with a resolution better than 0.1 μm. The mirrors of the objective were coated with Mo/Si multilayers to provide nearly 40% reflectance at near-normal incidence for the 14 nm radiation. Our results demonstrate that multilayer coatings are capable of enhancing the reflectance of optical components at soft x-ray wavelengths without significantly degrading their imaging performance.
Proximity effect correction at 10 keV using ghost and sizing for 0.4 μm mask lithography
Authors: Muray Andrew , Lozes Richard L. , Milner Kathy , Hughes Greg
Published: July 2002

An investigation of proximity effect correction at 10 keV using ghost and sizing indicates 0.4 μm design targets can be achieved on a raster scan lithography system. It is shown through simulation and experiment that the proximity effect can be effectively eliminated by use of ghost. Sizing provides an additional degree of freedom to control process latitude and wall angle.
Focused ion beam induced deposition of platinum
Authors: Tao Tao , Ro JaeSang , Melngailis John , Xue Ziling , Kaesz Herbert D.
Published: July 2002

Focused ion beam induced deposition of platinum from a precursor gas of (methylcyclopentadienyl)trimethyl platinum has been demonstrated. This organometallic compound is solid at room temperature with a vapor pressure of 0.054 Torr. Ga+ ions at 30–40 keV have been used. The resistivity and composition of the film and the deposition yield have been measured as a function of ion current density, line dose, substrate temperature, geometry, and supplemental hydrogen pressure. Yield varies from 0.2 to 34, and resistivity varies from 70 to 700 μΩ cm depending on the conditions. The resistivity and content of the carbon impurity are reduced as the ion current increases: the lowest resistivity is observed at the highest current density corresponding to 0.222 nA at scan speed 500 cm/s repeated over a 350 μm long line. The minimum linewidth achieved so far is 0.3 μm. Transmission electron microscopy shows the Pt film to be amorphous, and Auger analysis gives the film composition 46% Pt, 24% C, 28% Ga, and 2% O. The addition of hydrogen gas supplied to the same area by a second nozzle is found to have little effect on yield or resistivity. Although the deposition of gold from an organometallic precursor on a substrate at elevated temperature results in low resistivity films, in the case of platinum this is not observed; in fact, deposition yield goes to zero as the substrate temperature is raised. The deposition of platinum at near grazing incidence of ion beam on a cleaved silicon surface has been observed. Such deposition differs from that of normal incidence, tending to form discrete islands at about 0.25 μm dimensions.
A technique for stable adhesion of DNA to a modified graphite surface for imaging by scanning tunneling microscopy
Authors: Lyubchenko Yuri L. , Lindsay S. M. , DeRose J. A. , Thundat T.
Published: July 2002

A procedure has been developed in which mercurated DNA is covalently bonded to a modified graphite surface. This technique allows for stable adhesion of the DNA which enables one to image it with the scanning tunneling microscope. The method is outlined in this paper and images are presented of a partially modified graphite surface and one completely modified with DNA attached. The DNA images demonstrate B-helix periodicity.
Clusters formed in laser-induced ablation of Si, SiC, Pt, UO2 and evaporation of UO2 observed by laser ionization time-of-flight mass spectrometry and scanning tunneling microscopy
Authors: Tench R. J. , Balooch M. , Bernardez L. , Allen Mike J. , Siekhaus W. J. , Olander D. R. , Wang W.
Published: July 2002

Cluster formation is traditionally observed by mass spectrometry, which has the disadvantage that the detection sensitivity often decreases with increasing mass. Alternatively, one may collect the clusters onto an atomically flat substrate and identify them by scanning tunneling microscopy (STM). Both techniques were used here. For the first technique, a Nd:YAG laser (frequency quadrupled to 266 nm, 5 ns pulse width) focused onto spots of 4–100 μm diameter was used to ablate refractory materials, and a reflectron time-of-flight tube served to mass-analyze the plumes. The observed mass spectra for Si, Pt, SiC, and UO2 varied in the distribution of ablation products among atoms, molecules, and clusters, depending on laser power density and target material. For the second technique, cleaved surfaces of highly oriented pyrolytic graphite were positioned either 10 cm away from materials ablated at 10−5 Torr by 1–3 excimer laser (308 nm) pulses of 20 ns duration, or 1 m away from materials vaporized at 10−8 Torr by 10 Nd:Glass laser pulses of 1 ms duration. The deposited material was then analyzed by STM in air. Clusters up to 30 Å in diameter were observed for all materials, making the use of STM at room temperature for mass analysis questionable.
Annealing behavior of Al–Y alloy film for interconnection conductor in microelectronic devices
Authors: Lee Y. K. , Fujimura N. , Ito T. , Nishida N.
Published: July 2002

Progress in patterning technologies and computer-aided circuit designs have brought us to the threshold of submicron dimensions in the fabrication of very large scale integrated circuits. For metallization at submicron dimensions, several requirements must be satisfied. In the present study we have prepared pure Al and Al–Y alloy films with different yttrium concentration by electron-beam evaporation and evaluated several annealing characteristics for the Al–Y/(100)Si system by using a transmission electron microscope, scanning electron microscope, and measuring the electrical resistivity. We found that in the Al–0.7 wt % (0.2 at. %) Y alloy film annealed above 300 °C the electrical resistivity decreases to a value similar to that of a pure Al film, that is, 2.9 μΩ cm, and the same concentration of yttrium is also sufficient to minimize the generation and growth of annealing hillocks. These results are explained by the compositional and microstructural changes occurring in the Al–Y/(100)Si system.
Measurement of x-ray absorption spectra of photoresists using a silicon lithium detector
Authors: Wells G. M. , Taylor J. W. , Cerrina F. , Pearson D. , MacKay J.
Published: July 2002

We have developed a simple experimental technique for measuring the absorption spectrum of x-ray photoresists in the region from 1000 to 3000 eV. Knowledge of the absorption characteristics of a photoresist allows selection of the most efficient range of energies to use for performing x-ray exposures. The measurements were performed using the Medical Physics beamline on the University of Wisconsin Aladdin synchrotron source. The beamline is equipped with a lithium-drifted silicon detector that provides an energy resolution of 150 eV. To avoid saturation of the Si(Li) detector, special ‘‘microbeams’’ of approximately 50 circulating electrons in the storage ring were employed. The beam intensity was monitored using a photodiode. The operation of the detector and associated diagnostic equipment will be described. Four by four millimeter square, 1.4 μm thick silicon nitride membranes were used as substrates for the resist films. Photoresist absorption was measured by comparing the transmission of silicon nitride membranes with and without photoresist coatings. Experimental results for both conventional and chemically amplified positive and chemically amplified negative x-ray photoresists are presented.
Analysis of the possibility of performing microelectronic microwave vacuum devices with extended interaction on field emitter arrays
Authors: Sinitsyn N. I. , Gulyaev Yu. V. , Golant M. B. , Nefyodov I. S. , Torgashov G. V. , Zakharchenko Yu. F. , Zhbanov A. I.
Published: July 2002

The possibility of microminiature microwave vacuum devices with extended interaction to be designed on the base of field emitter arrays is considered theoretically. The optimal designs of planar slow-wave systems on dielectric substrates and of edge field emitter arrays electron guns for such systems are defined. High coupling resistances achieved in the above structures with low acceleration voltages in a wide frequency range and electron beams formed with suitable electron trajectory spread prove the assimilation of microwave range to be possible by such devices, including the millimeter-wavelength band.
Approach to obtain high quality GaN on Si and SiC-on-silicon-on-insulator compliant substrate by molecular-beam epitaxy
Authors: Yang Z. , Guarin F. , Tao I. W. , Wang W. I. , Iyer S. S.
Published: July 2002

Crystalline SiC thin layers have been grown on 125 mm silicon-on-insulator (SOI) substrates as a promising and economical substrate for the growth of GaN epilayers. Through the use of an AlN/GaN strained superlattice buffer layer, high quality GaN layers as thin as 2000 Å on Si(111) substrates have been achieved. X-ray diffraction curves with a full width at half-maximum (FWHM) as narrow as 25 arcmin were obtained. The associated low-temperature photoluminescence (PL) spectrum showed a dominant bound-exciton peak with a FWHM of 8 meV. We have further combined these two techniques to synthesize the first GaN on SiC on a SOI structure.
Surface roughness characterization of soft x-ray multilayer films on the nanometer scale
Authors: Yu J. , Cao J. L. , Namba Y. , Ma Y. Y.
Published: July 2002

The soft x-ray reflectivity of multilayer films is affected by the surface roughness on the transverse nanometer scale. Scanning tunneling microscopy (STM) is an ideal instrument for providing high-lateral-resolution roughness measurements for soft x-ray multilayer films that cannot be obtained with other types of instruments on the transverse nanometer scale. The surface roughnesses of Mo/Si, Mo/C, and W/Si soft x-ray multilayer films prepared by an ion-beam-sputtering technique were measured with a STM on the vertical and transverse attributes. The film roughnesses and average spatial wavelengths added to the substrates depend on the multilayer film fabrication conditions, i.e., material combinations, number of layers, and individual layer thickness. These were estimated to lead to a loss of specular reflectivity and variations of the soft x-ray scattering angle distribution. This method points the way to further studies of soft x-ray multilayer film functional properties and can be used as basic guidance for selecting the best coating conditions in the fabrications of soft x-ray multilayer films.
Characterization of oxide desorption from InSb(001) substrates
Authors: Liu W. K. , Santos M. B.
Published: July 2002

Thermal desorption of oxide layers from InSb(001) substrates was studied using reflection high energy electron diffraction (RHEED), x-ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron spectroscopy. Surfaces of as-loaded substrates prepared using either of two common chemical etchants were composed of oxide layers containing In2O3 and Sb2O5. As the substrate temperature was raised, a multistage desorption process was observed. Most of the Sb2O5 was reduced during the first stage, leaving behind a thin oxide layer consisting predominantly of In2O3. In2O3 was desorbed during the second stage, as the substrate temperature was raised further in the presence of an Sb flux. Indium droplets condensed on the surface before a smooth morphology ensued. We suggest that the reduction of In2O3 produced In droplets which reacted with the supplied Sb flux to form InSb. Annealing produced smooth, stoichiometric surfaces that gave rise to well-defined, Sb-stabilized pseudo-(1×3) RHEED patterns. InSb films grown on such substrates exhibited good structural and electrical properties.
Three-dimensional siloxane resist for the formation of nanopatterns with minimum linewidth fluctuations
Authors: Namatsu Hideo , Takahashi Yasuo , Yamazaki Kenji , Yamaguchi Toru , Nagase Masao , Kurihara Kenji
Published: July 2002

Linewidth fluctuation of resist patterns is a serious problem in fabricating nanodevices when lithographic resolution is improved to the nanometer scale. As a resist material for reducing linewidth fluctuations, we evaluate hydrogen silsesquioxane (HSQ) with a three-dimensional framework from the standpoints of resist patterning and its ability to reduce linewidth fluctuation. Infrared analyses indicate that SiH bonds in HSQ are broken by electron-beam irradiation, and consequently, the crosslinking required for negative tone patterning is generated. By applying a TMAH developer suitable for the dissolution of the siloxane bonds in HSQ, we improve contrast and reduce the thickness loss of the lightly exposed resist area. In addition, the HSQ resist has relatively high sensitivity for resist materials without any reactive groups. The etching durability sacrificed for the attainment of high sensitivity is improved by oxygen plasma treatment. No damage, such as pattern-shape deformation after the oxygen plasma treatment, is observed. Linewidth fluctuation due to edge roughness of resist patterns results from aggregates in the resist polymer. Aggregates in HSQ are small owing to its three-dimensional framework. In addition, the excellent development property of HSQ avoids any influence from polymer aggregates on development. Consequently, linewidth fluctuation can be reduced to less than 2 nm. This leads to a decrease in the variation of gate capacitance in single-electron transistors.
High efficiency, dual collection mode near-field scanning optical microscope
Authors: Stranick Stephan J. , Richter Lee J. , Cavanagh Richard R.
Published: July 2002

We have developed a near-field scanning optical microscope that provides simultaneous transmission and reflection mode measurements while concurrently recording a topograph of the sample surface. In this microscope design, an ellipsoidal cavity is used to collect and reimage the light reflected from a near-field optic. Reimaging of the near-field optic away from the mechanical components of the microscope eliminates any hindrance that they would otherwise cause. The near-field optical microscope configuration described in this article allows for the efficient, symmetric collection of reflected and transmitted light. This instrumental design has been optimized for chemical problems that require molecular characterization on the nanometer scale. Images of ∼100 nm Au particles demonstrate the utility of this instrument.
Investigation of Ta–RuO2 diffusion barrier for high density memory capacitor applications
Authors: Yoon Dong-Soo , Baik Hong Koo , Lee Sung-Man , Park Chang-Soo , Lee Sang-In
Published: July 2002

The properties of both oxygen indiffusion and oxidation resistance in a Ta+RuO2 layer for high density memory devices were investigated by using Rutherford backscattering spectroscopy, four point probe, x-ray diffraction, x-ray photoelectron spectroscopy, and planar transmission electron microscopy. The Ta+RuO2/Si system sustained up to 800 °C without an increase in resistivity. The Ta+RuO2 diffusion barrier showed a Ta amorphous microstructure and an embedded RuOx nanocrystalline structure in the as-deposited state. The Ta+RuO2 film showed the formation of RuO2 phase by reaction with the indiffused oxygen from atmosphere after annealing in an air ambient. The Ta+RuO2 diffusion barrier showed that Ta is sufficiently bound to oxygen in the as-deposited state, but RuO2 consists of Ru and Ru–O binding state. The Ta–O bonds showed little change compared to the as-deposited state with increasing annealing temperature, whereas Ru–O bonds significantly increased and transformed to conductive oxide, RuO2. Therefore, the Ta layer deposited by RuO2 addition effectively prevented the indiffusion of oxygen up to 800 °C and its oxidation resistance was superior to various barriers reported by others.
On-wafer photoacid determination and imaging technique for chemically amplified photoresists
Authors: Dentinger Paul M. , Lu Bing , Taylor James W. , Bukofsky Scott J. , Feke Gilbert D. , Hessman Dan , Grober Robert D.
Published: July 2002

A fundamental task of chemically amplified photoresists is to record the incident radiation by generating catalyst concentration gradients within the film. In many resists, the catalyst is a strong Brönsted acid which yields a latent image of pH within the exposed film. A number of mechanistic questions remain about acid generator efficiency and its mobility once generated and heated. We have developed a technique in which a pH-dependent fluorophore is incorporated into the resist (an undyed version of SAL 605 from the Shipley Company and similar formulations). The localized acid concentrations generated by exposure to x-rays are analyzed and imaged using fluorescence spectroscopy and microscopy. Initial experiments, the spectroscopic apparatus, and initial far-field imaging are reported elsewhere [S. J. Bukofsky, G. D. Feke, Q. Wu, R. D. Grober, P. M. Dentinger, and J. W. Taylor, Appl. Phys. Lett. 73, 3 (1998)]. In this article, several fluorophores are evaluated, and various criteria for successful imaging within the photoresist matrix are established. These criteria include pKa of the fluorophore, photostability, and functional groups that affect the performance of the acid within the film. The technique is used to show the relative efficiency of two photoacid generators in otherwise identical matrices, and the method has the potential for rapid photogenerated acid yield determination among a variety of photoacid generators. This technique can be used in thin imaging films and, due to the low level of fluorophore required (0.01–0.1 wt. %), for photoacid yield determination in optical photoresists without significantly altering the absorbance characteristics of the film. Initial near-field scanning optical microscopy images are shown and the potential of the technique for several lithographic applications, including measurement of the actual spread of the acid distribution during postexposure bake, is discussed.
Improved size uniformity of InAs quantum dots grown on a lateral composition modulated InGaAs surface
Authors: Wohlert D. E. , Cheng K. Y. , Chang K. L. , Hsieh K. C.
Published: July 2002

The properties of InAs quantum dots (QDs) deposited on compositionally homogenous and laterally modulated surfaces is investigated by photoluminescence (PL), atomic force microscopy (AFM), and transmission electron microscopy. We use solid source molecular beam epitaxy on (100)-oriented InP substrates to fabricate the samples. It is found that QDs grown on a laterally modulated surface are more uniform in size. This is implied by a decrease of 22% in the full width at half maximum (FWHM) in the PL signal at 77 K for InAs QDs deposited on the modulated surface as opposed to the homogenous surface for equal monolayer coverage of InAs. Similarly, plan view scans taken by ex situ contact AFM also show improved size uniformity of QDs grown on the laterally composition modulated surface as evidenced by a decrease in the standard deviation of area data compiled from the images. It is shown that the improvement in the geometrical uniformity of the quantum dots as depicted by the PL FWHM and AFM data is facilitated by an early onset of size self-equalization due to the lateral composition modulation.
Plasma doping for shallow junctions
Authors: Goeckner M. J. , Felch S. B. , Fang Z. , Lenoble D. , Galvier J. , Grouillet A. , Yeap G. C.-F. , Bang D. , Lin M.-R.
Published: July 2002

In this article we review the characteristics of ultrashallow junctions produced by plasma doping (PLAD). PLAD is one of the alternate doping techniques being developed for sub-0.18 μm devices. Here, we describe results from a wide range of experiments aimed at the production of ultrashallow junctions. For the results shown here, a BF3 plasma was used to provide the dopant ions that were implanted into 150 and 200 mm Si substrates using wafer biases ranging from −0.14 to −5.0 kV. The ultrashallow junctions formed with this technique have been examined with both secondary ion mass spectrometry and electrical profiling techniques. Good sheet resistance uniformity, charging performance, structural quality, and photoresist integrity have been obtained. When PLAD is used in the production of sub-0.2 μm gate length p-metal–oxide–semiconductor field effect transistors, one finds subthreshold swing, off-state leakage, and hot-carrier reliability that are similar to beamline-implanted ones. In addition, higher drive currents are seen in the plasma-doped devices.
Fabrication and transport measurements of YBa2Cu3O7−x nanostructures
Authors: Larsson P. , Nilsson B. , Ivanov Z. G.
Published: July 2002

We have developed a process to fabricate nanostructures in oxide materials, especially YBa2Cu3O7−x (YBCO) thin films. The fabrication process is based on a multilayer mask with an amorphous carbon layer as a key component. Electron-beam lithography, reactive ion etching, and ion-beam etching have been the supporting techniques for the process. Nanobridges ranging in width from 50 to 170 nm have been fabricated and characterized. The initial YBCO film was 50 nm thick and had a Tc of 86 K. Tc of the fabricated bridges decreased about 3–5 K with respect to the initial film. The level of the critical current density was 4–9×105 A/cm2 at 77 K and 1.6–2×107 A/cm2 at 4.2 K. No increase of the critical current density was observed as the cross-section area became smaller. The process has also been applied to fabricate 30–50 nm wide trenches in a-axis-oriented YBCO on top of a similar-oriented PrBa2Cu3O7−x (PBCO) layer. No supercurrent was observed in such junctions. The current transport was governed by Mott variable range hopping.
In search of optimum conditions for the growth of sharp and shallow B-delta markers in Si by molecular beam epitaxy
Authors: Wittmaack K. , Griesche J. , Osten H. J. , Patel S. B.
Published: July 2002

Multiple boron delta spikes in silicon, with spacings between 4.3 and 20 nm, have been grown by molecular beam epitaxy at temperatures of about 100 °C (L1) and 400 °C (S4). The test samples were depth profiled by secondary ion mass spectrometry using 500 eV O2+ at normal beam incidence. The surface of S4 was quite smooth, with a root mean square roughness σ<0.1 nm. By contrast, L1 was rather rough, σ≅0.5 nm. The boron depth profiles of S4 revealed sharp peaks but pronounced tails on either side. The tails, which dominate the dopant distributions at concentrations below about 40% of the peak level, are attributed to defect-promoted boron diffusion during growth. Sample L1 showed boron spikes of larger width above the 10%–20% peak level, but a much more rapid, roughly exponential falloff on both sides. This sharpness of the dopant spikes implies the absence of boron diffusion during low-temperature growth. The “best” deltas (those with small width and sharp falloff) were obtained with boron contaminants of ambient origin that resided at the (oxidized) interface between the substrate and the silicon buffer layer. This observation suggests that boron atoms in silicon dioxide are rather immobile. Depth profile measurements on crystalline samples, either containing boron deltas or being uniformly doped with boron, revealed severe variations of the B+ signal over a depth of up to 25 nm at normal and oblique beam incidences (up to 50°, also with oxygen flooding). Silicon matrix signals measured in parallel did not show any variation beyond the transient depth. The initial overshoot of the B+ signal, observed just below the transient depth, sometimes exceeded the stationary signal by more than a factor of 2, and the signal undershoot in extended regions at larger depths was low by up to several 10%. This artifact calls for recalibration of previously reported profiles of shallow boron implantations in silicon.
Highly localized thermal, mechanical, and spectroscopic characterization of polymers using miniaturized thermal probes
Authors: Hammiche A. , Bozec L. , Conroy M. , Pollock H. M. , Mills G. , Weaver J. M. R. , Price D. M. , Reading M. , Hourston D. J. , Song M.
Published: July 2002

In this article, we demonstrate the versatility of use of cantilever-type resistive thermal probes. The probes used are of two kinds, Wollaston wire probes and batch-microfabricated probes. Both types of probe can be operated in two modes: a passive mode of operation whereby the probe acts as a temperature sensor, and an active mode whereby the probe acts also as a highly localized heat source. We present data that demonstrate the characterization of some composite polymeric samples. In particular, the combination of scanning thermal microscopy with localized thermomechanometry (or localized thermomechanical analysis, L-TMA) shows promise. Comparison with data from conventional bulk differential scanning calorimetry shows that inhomogeneities within materials that cannot be detected using conventional bulk thermal methods are revealed by L-TMA. We also describe a new mode of thermal imaging, scanning thermal expansion microscopy. Finally, we outline progress towards the development of localized Fourier transform infrared spectroscopy: here the probe, in this case operated in the temperature-sensing mode, detects the photothermal response of a specimen exposed to the beam and heated thereby.
Biospheric Primary Production During an ENSO Transition
Authors: Behrenfeld Michael J. , Randerson James T. , McClain Charles R. , Feldman Gene C. , Los Sietse O. , Tucker Compton J. , Falkowski Paul G. , Field Christopher B. , Frouin Robert , Esaias Wayne E. , Kolber Dorota D. , Pollack Nathan H.
Published: July 2002

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) provides global monthly measurements of both oceanic phytoplankton chlorophyll biomass and light harvesting by land plants. These measurements allowed the comparison of simultaneous ocean and land net primary production (NPP) responses to a major El Niño to La Niña transition. Between September 1997 and August 2000, biospheric NPP varied by 6 petagrams of carbon per year (from 111 to 117 petagrams of carbon per year). Increases in ocean NPP were pronounced in tropical regions where El Niño–Southern Oscillation (ENSO) impacts on upwelling and nutrient availability were greatest. Globally, land NPP did not exhibit a clear ENSO response, although regional changes were substantial.
Modulation of Oscillatory Neuronal Synchronization by Selective Visual Attention
Authors: Fries Pascal , Reynolds John H. , Rorie Alan E. , Desimone Robert
Published: July 2002

In crowded visual scenes, attention is needed to select relevant stimuli. To study the underlying mechanisms, we recorded neurons in cortical area V4 while macaque monkeys attended to behaviorally relevant stimuli and ignored distracters. Neurons activated by the attended stimulus showed increased gamma-frequency (35 to 90 hertz) synchronization but reduced low-frequency (<17 hertz) synchronization compared with neurons at nearby V4 sites activated by distracters. Because postsynaptic integration times are short, these localized changes in synchronization may serve to amplify behaviorally relevant signals in the cortex.
Evidence for Substantial Variations of Atmospheric Hydroxyl Radicals in the Past Two Decades
Authors: Prinn R. G. , Huang J. , Weiss R. F. , Cunnold D. M. , Fraser P. J. , Simmonds P. G. , McCulloch A. , Harth C. , Salameh P. , O'Doherty S. , Wang R. H. J. , Porter L. , Miller B. R.
Published: July 2002

The hydroxyl radical (OH) is the dominant oxidizing chemical in the atmosphere. It destroys most air pollutants and many gases involved in ozone depletion and the greenhouse effect. Global measurements of 1,1,1-trichloroethane (CH
3
CCl
3
, methyl chloroform) provide an accurate method for determining the global and hemispheric behavior of OH. Measurements show that CH
3
CCl
3
levels rose steadily from 1978 to reach a maximum in 1992 and then decreased rapidly to levels in 2000 that were lower than the levels when measurements began in 1978. Analysis of these observations shows that global OH levels were growing between 1978 and 1988, but the growth rate was decreasing at a rate of 0.23 ± 0.18% year
−2
, so that OH levels began declining after 1988. Overall, the global average OH trend between 1978 and 2000 was −0.64 ± 0.60% year
−1
. These variations imply important and unexpected gaps in current understanding of the capability of the atmosphere to cleanse itself.