bluuebunny/crossref_metadata_2025 huggingface dataset:
Pulsed supersonic molecular beam growth of AlN
Authors: Ballarotto V. W. , Kordesch M. E.
Published: July 2002

Non-skimmed pulsed supersonic free jets of trimethylaluminum (TMA) and ammonia are used to grow thin films of AlN. The translational kinetic energy of the precursors used for growth trials is 130 meV for TMA and 160 and 500 meV for ammonia. Effects of varying each pulse rate, substrate temperature, and deposition time on film composition and growth rate are studied. The resulting films were nearly always oriented with the non-polar (10⋅0) plane parallel to the silicon substrate (001) plane or to the sapphire substrate basal plane. The measured lattice parameter ratio c/a is 1.594. Film growth only occurred in a narrow range of pulse rates. Films can be grown over the temperature range of 900–1200 K. An average growth rate of 0.115 μm/h is measured. AlN films could also be grown when the substrates were not directly in the beam path. The role of precursor flux on film growth is discussed. Additional post-deposition analysis includes Fourier transform infrared spectroscopy, reflectance spectroscopy, scanning electron microscopy, and Auger electron spectroscopy.
Plasma enhanced selective area microcrystalline silicon deposition on hydrogenated amorphous silicon: Surface modification for controlled nucleation
Authors: Smith L. L. , Read W. W. , Yang C. S. , Srinivasan E. , Courtney C. H. , Lamb H. H. , Parsons G. N.
Published: July 2002

Selective deposition of μc-Si on hydrogenated amorphous silicon is demonstrated using time-modulated silane reactant flow in a low temperature plasma enhanced process. Alternating cycles of thin silicon layer deposition and atomic hydrogen exposure result in silicon layers on receptive surfaces, with no net deposition on nonreceptive areas of the substrate. Selective deposition could be useful to form self-aligned contacts in hydrogenated amorphous silicon (a-Si:H transistor applications. However, a problem commonly observed in low temperature selective deposition is that the selective process tends to etch amorphous silicon, harming the devices. We describe a technique involving Mo metallization that stabilizes the a-Si:H surface with respect to hydrogen plasma exposure and allows selective μc-Si deposition on a-Si:H in device structures, while avoiding deposition on the top SiNx insulator material. Surfaces and subsequent selective nucleation and growth were characterized using atomic force microscopy, x-ray photoelectron spectroscopy, and Auger electron spectroscopy, which revealed the presence of Mo incorporation in the a-Si:H surface remaining after complete removal of the metal layer. A direct comparison of selective deposition experiments on films prepared with and without Mo treatment demonstrate that the metallization stabilizes nucleation of microcrystalline silicon on amorphous silicon surfaces.
Imaging and direct manipulation of nanoscale three-dimensional features using the noncontact atomic force microscope
Authors: Ramachandran T. R. , Madhukar A. , Chen P. , Koel B. E.
Published: July 2002

We report on a remarkable image contrast reversal in noncontact atomic force microscope (NC-AFM) imaging of nanosized three-dimensional (3D) particles. We show that the image contrast of such 3D particles can switch from positive to negative as a function of NC-AFM imaging conditions and this occurs during, both, in situ ultrahigh vacuum imaging and imaging in air. Our results indicate that the contrast reversal can arise from a tip-sample interaction force-gradient-dependent instability of the NC-AFM feedback loop. Exploiting the above instability to induce selective tip-sample contact, we propose and demonstrate a protocol for the controlled nanomanipulation of 5 nm diameter gold particles, in air at room temperature using the NC-AFM. The contrast reversal phenomenon is proposed to be universal, suggesting its potential applicability to nanomanipulation in a variety of materials systems.
High-rate etching of GaAs using chlorine atmospheres doped with a Lewis acid
Authors: Franz Gerhard
Published: July 2002

The etching behavior of GaAs versus BCl3/Cl2 is analyzed in a capacitively coupled plasma and a plasma excited by electron cyclotron resonance using optical emission spectroscopy (OES). For both types of discharges, the etch rate of the semiconductor can be parametrized by using the OES signal which is corrected by actinometry. Exhibiting open surfaces between 10 and 17 cm2, high-rate etching in both types of discharges is combined with very steep sidewalls and excellent radial uniformity.
Suppression of notching by lowering the bias frequency in electron cyclotron resonance plasma with a divergent magnetic field
Authors: Morioka H. , Matsunaga D. , Yagi H.
Published: July 2002

“Notching,” which is a kind of local side etching caused by charging of pattern structures, is a serious obstacle to achieving tight critical dimension (CD) control in fabrication beyond quarter micron devices. Although sidewall protection with increased polymer deposition on the sidewall can reduce notching, it tends to enhance the so-called proximity effect, which is the variation of etched profiles observed when the pattern spacing is varied. Therefore, notching has to be suppressed without extra sidewall protection to achieve tight CD control. To solve this problem, we studied the effects of rf bias frequency for both continuous mode and pulse modulated mode electron cyclotron resonance plasma sources with divergent magnetic fields, and found out that lowering the bias frequency can reduce the notching by itself. We found that the notch depth is markedly decreased by lowering the bias frequency from 13.56 MHz to 400 kHz under most conditions. In continuous mode plasma, however, this improvement becomes minimal when the pressure is decreased to reduce the proximity effect. On the other hand, using a pulsed plasma source having a 100 μs cycle and 25%–50% duty, we succeeded in suppressing notching even in cases of 1 mT and lower. Consequently, we have achieved a vertical etched profile with minimal proximity effect.
Process controlled microstructural and binding properties of hard physical vapor desposition films
Authors: Oechsner H.
Published: July 2002

The influence of the ion-to-atom flux ratio and the ion energy on the texture development and the grain size is exemplified for well controlled ion beam assisted and plasma beam deposition of TiN films. The dependence of the atomic binding configuration in hard carbon and nitride films on the energy input during film growth is also addressed with special emphasis on a possible synthesis of the hypothetical β-C3N4 phase.
Atomic force microscopy and scanning tunneling microscopy/spectroscopy investigations of molybdenum ditellurides
Authors: Saidi A. , Hasbach A. , Raberg W. , Wandelt K.
Published: July 2002

The van der Waals surfaces of the layered material α-MoTe2 and the high temperature phase β-MoTe2 were investigated by means of atomic force microscopy (AFM) and scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS). On both samples atomically resolved images of the surface structures were obtained. The combined STS/STM measurements of β-MoTe2 provide unambiguous evidence that the contribution from the subsurface layer dominates the tunneling current. Furthermore, from the combined AFM and STM data we arrive at a model for a new structure modification of this surface. This modification represents an interesting kinetically controlled intermediate state of the transition from β- to α-MoTe2.
Tungsten silicide composition analysis by Rutherford backscattering spectroscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy
Authors: Bradbury C. A. , Fillmore D. K.
Published: July 2002

While Rutherford backscattering spectroscopy (RBS) is the accepted method for determining the composition of tungsten silicide films, the systems are expensive and not widely available. Furthermore, the depth resolution of RBS is of the order of 150 Å, which does not allow for the compositional analysis of thinner films. Also, chemical information is not available from RBS. These are critical issues as the geometries of ultralarge-scale integration devices become smaller. However, with special care applied to data acquisition and reduction, both Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) provide composition agreement with RBS. Additionally, AES and in particular XPS provide chemical information about thin films that is unavailable from RBS analysis. To demonstrate that AES and XPS provide composition analysis of thin films comparable to RBS, post-anneal tungsten silicide films deposited on silicon with both chemical and vapor deposition processes were analyzed by AES, XPS, and RBS. With care taken in data acquisition and reduction, silicon-to-tungsten ratios were comparable among the three techniques. Moreover, using AES and XPS, surface oxide thickness and interfacial layer composition can be determined.
Transition metal nitride formed by simultaneous physisorption and thermal evaporation; TiN/Si(100)
Authors: Ahn S. , Han J. Y. , Seo J. M.
Published: July 2002

It has been confirmed by x-ray photoelectron spectroscopy that the Ti nitride can be athermally formed without substrate disruption by simultaneous NH3 exposure and Ti evaporation to a clean Si(100)-2×1 held at 40 K under ultrahigh vacuum. One of the critical points in the present nitride formation at 40 K is solving the kinetic constraint problem by simultaneous adsorption of consisting elements. Compared to TiN formed by other techniques, this athermally formed Ti nitride does not contain oxygen contamination but has TiN precursors, TiNxHy. Postheat treatment partially converts TiNxHy to Ti nitride and induces the excessive metallic Ti to be mixed with the previously formed Ti nitride. When the Ti nitride is exposed to the air, Ti in the Ti nitride is preferentially oxidized and forms the Ti oxide on the top side. By mild-annealing it up to 550 °C under ultrahigh vacuum, the oxygen is transferred from Ti oxide to the Si substrate and the original Ti nitride is gradually recovered. Even for annealing up to the temperature high enough to desorb the oxidized Si, the Ti nitride hardly desorbs.
Silicon epitaxy by low-energy plasma enhanced chemical vapor deposition
Authors: Rosenblad C. , Deller H. R. , Dommann A. , Meyer T. , Schroeter P. , von Känel H.
Published: July 2002

A new technique for semiconductor epitaxy at low substrate temperatures is presented, called low-energy dc plasma enhanced chemical vapor deposition. The method has been applied to Si homoepitaxy at substrate temperatures between 400 and 600 °C and growth rates between 0.1 and 1 nm/s, using silane as the reactive gas. The quality of the Si films has been examined by reflection high-energy electron diffraction, scanning tunneling microscopy, cross-section transmission electron microscopy, and high-resolution x-ray diffraction. Two effects have been identified to lead to the formation of stacking faults after an initial layer of defect-free growth: (1) substrate bombardment by ions with energies in excess of 15 eV, and (2) hydrogen adsorption limiting the surface mobility of Si atoms and silane radicals. Both result in the accumulation of surface roughness, facilitating the nucleation of stacking faults when the roughness reaches a critical level. Defect introduction can be eliminated effectively by biasing the substrate during growth and by decreasing the hydrogen coverage, either by admixing small amounts of germane to the silane or by using a sufficiently high plasma density.
Blue CuxS coated ZnS:Ag,Cl phosphor as potential field emission display material
Authors: Yang Su-Hua , Yokoyama Meiso
Published: July 2002

The ZnS:Ag,Cl phosphor shows a blue luminescence due to electron–hole recombination at donor–acceptor pairs. Ag dopant acts as the acceptor and Cl co-dopant acts as the donor. For conductivity treatment, the CuxS conductive layer is coated on the particle’s surface of ZnS:Ag,Cl by a wet chemical reaction. The low voltage phosphor film is prepared by sedimentation. Analyses by x-ray diffraction patterns and scanning electron microscopy photographs observe a preferential orientation phenomenon of the phosphor film. Electron probe microanalysis reveals that the Cu ions in the CuxS coated ZnS:Ag,Cl are homogeneously distributed. For CuxS coated at room temperature, the luminance intensity of the phosphor film is increased with the coating time and coating solution concentration. However, the luminance of phosphor film will degrade due to the oxidized phosphor’s surface. A proper heat treatment enhances the luminance of the phosphor film. A high luminance is observed from the CuxS coated ZnS:Ag,Cl phosphor and it is compatible for field emission display applications.
Fermi level pinning on HF etched silicon surfaces investigated by photoelectron spectroscopy
Authors: Schlaf R. , Hinogami R. , Fujitani M. , Yae S. , Nakato Y.
Published: July 2002

A widely used approach to obtain smooth oxide-free and (partially) H-terminated silicon (Si) surfaces is to immerse Si wafers into CP4A (a mixture of H2O, HNO3, CH3COOH and HF in a volume ratio of 22:5:3:3) and/or HF solutions of varying concentrations. It is usually assumed that such treatments result in a dramatic reduction of the surface density of states and that, therefore, no surface band bending can occur. In our experiments we investigated the electronic surface structure of a number of CP4A/HF treated n- and p-Si wafers with varying doping densities by x-ray photoelectron spectroscopy (XPS). XPS allows a straightforward detection of surface stoichiometry as well as one of band bending and surface photovoltages (SPV) on semiconductor materials because the positions of the core level peaks directly depend on the position of the Fermi level within the band gap at the surface. Our experiments show that on all surfaces investigated Fermi level pinning still exists after the samples were immersed in the CP4A/HF solutions and that the pinning states are located close to the conduction band. Most of the samples also showed SPV when measured under illumination. The measurements also show that up to 36.6% of the surfaces are covered by F atoms depending on the treatment and the doping density. From the amount of band bending we estimated the density of surface states present on the various samples.
Processing of porous GaAs at low frequency sparking
Authors: Gudiño-Martínez A. , Rosendo E. , Vidal M. A. , Navarro-Contreras H. , Rojas-López M.
Published: July 2002

We report on the preparation of photoluminescent porous GaAs by the application of high voltage spark discharges at low repetition rates (20 Hz) in air and in argon atmospheres. The spark-processed porous (spp) samples were characterized by the observation of their visible photoluminescence (PL) when illuminated with UV monochromatic radiation. In contrast to previous work on spp-GaAs at high sparking frequencies we find that the PL of samples prepared at low sparking frequency is highly reproducible from sample to sample. Important differences are observed in the initial PL spectra of the spp-GaAs according to the atmosphere of preparation under similar conditions. After prolonged air exposure both the spp-GaAs prepared in air and in argon show two similar broad peaks at energy positions 2.5 and 3.1 eV. Raman results indicate that the PL might not be associated to any size dependent mechanism. We present evidence that oxygen compounds formed by the exposure to air of the samples play a role in the PL excited in the spp-GaAs. This is reinforced by x-ray photoelectronic spectroscopy measurements that indicate that the spp-GaAs is covered by an oxidized film.
Accurate and rapid determination of thickness, n and k spectra, and resistivity of indium–tin–oxide films
Authors: Zhang K. , Forouhi A. R. , Bloomer I.
Published: July 2002

The transparent conductor, indium–tin–oxide (ITO), is an important thin film component of flat panel displays. An optimum ITO film should be both highly transparent to visible wavelengths and at the same time, conductive. In practice, however, a tradeoff exists between these two attributes, making it difficult to produce a film that simultaneously meets both demands. In order to achieve the optimum balance between these properties, an effective method of characterizing ITO films is necessary. In this article we will present results of a new measurement technique that simultaneously determines, thickness, the spectra of the refractive index n and extinction coefficient k from 190 to 1100 nm, and the energy band gap of ITO films deposited on either transparent or opaque substrates. In addition, we will demonstrate how the film’s resistivity can be correlated to the film’s extinction coefficient. This technique is based on wide-band spectrophotometry, combined with spectral analysis that incorporates the Forouhi–Bloomer dispersion equations for n and k [Forouhi and Bloomer, Phys. Rev. B 34, 7018 (1996); 38, 1865 (1998)]. The measurement technique is nondestructive and takes 1–2 s.
Molecular dynamics simulation of Cu and Ar ion sputtering of Cu (111) surfaces
Authors: Kress J. D. , Hanson D. E. , Voter A. F. , Liu C. L. , Liu X.-Y. , Coronell D. G.
Published: July 2002

In ionized physical vapor deposition, used in Cu interconnect technology, the interaction of energetic ions with the growing Cu film is sensitive to both the impact angle and the energy. Detailed information, such as the angle and energy dependence of the sputter yield and sticking probability, is required for realistic feature scale modeling of film coverage in the metallization of micron-sized features (vias and trenches) in integrated circuits. Here we describe the results of molecular dynamics (MD) simulations of sputtering of Cu (111) surfaces by Cu and Ar ions suitable for incorporation into feature scale simulations. For each impact angle and energy considered (10–100 eV for Cu ions and 50–250 eV for Ar ions), the following averaged properties were calculated: sputter yield (number of Cu atoms sputtered per impact), sticking probability, thermal accommodation coefficient, average reflection angle of the impact ion, and average emission angle of the sputter products. The calculated sputter yields and energy threshold at normal incidence for both Ar and Cu sputtering of Cu are in good agreement with experiment and other MD simulations. Detailed comparisons are also made with previously reported sputter yields calculated with binary collision theory.
Precise, scalable shadow mask patterning of vacuum-deposited organic light emitting devices
Authors: Tian P. F. , Bulovic V. , Burrows P. E. , Gu G. , Forrest S. R. , Zhou T. X.
Published: July 2002

We demonstrate a scheme to pattern vacuum-deposited, small molecular weight organic light emitting diodes (OLEDs). Both the organic thin films and the metallic electrodes are patterned by shifting the position of a single shadow mask which accompanies the substrate throughout the deposition process. A full color, stacked OLED (SOLED) has been fabricated using this technique. The substrate movement relative to the mask was manually controlled to a root mean square accuracy of ±8 μm using a mask translating fixture. The performance of the patterned SOLED is comparable with that of devices fabricated by conventional, low tolerance methods. The limits to the display fill factor and resolution using this technique are discussed. The technique is generally applicable to all structures requiring precise patterning of vacuum-deposited thin films.
Role of boundary conditions at the anode in the development of an electric field induced avalanche in a gas at low pressure
Authors: Pletnev V. , Zukotynski S.
Published: July 2002

A dc glow discharge configuration was investigated that includes a flat semitransparent anode situated between two symmetrically positioned cathodes. The transparency of the anode increases the electron path length for ionizing collisions. This leads to a significant increase in the number of free electrons in the discharge volume. The simplicity of the discharge configuration makes it possible to model the electron avalanche in its early stage with proper boundary conditions and without the use of adjustable parameters in both elastic and nonelastic scattering cross sections. The results obtained for hydrogen indicate that the number of free electrons in the discharge can be increased by several orders of magnitude in comparison with a similar discharge configuration but with an opaque anode.
Using a quartz crystal microbalance for low energy ion beam etching studies
Authors: Doemling M. F. , Lin B. , Rueger N. R. , Oehrlein G. S. , Haring R. A. , Lee Y. H.
Published: July 2002

A quartz crystal microbalance (QCM) has been used for etching yield measurements in a low energy ion beam system. The goal is to obtain etching yields for ion energies below 150 eV for various ion chemistries and target materials. Typical beam currents are about 0.5 μA, and the mass change per unit time on the QCM is much smaller than that for typical QCM applications. A number of problems with the application of a QCM were encountered and a description of how they were overcome is presented in this article. Quantitative etch yield results for the etching of two different photoresists and SiO2 down to 25 eV ion energy are presented.
Appearance potential mass spectrometry: Discrimination of dissociative ionization products
Authors: Singh Harmeet , Coburn J. W. , Graves David B.
Published: July 2002

Appearance potential mass spectrometry (APMS) has recently gained importance for detection and quantitative measurements of reactive radical species in plasmas using line-of-sight sampling of radicals. In this work, we have investigated the assumption that the extraction efficiency of ions produced by direct ionization of radicals, and ions produced by dissociative ionization of the parent molecule used as the reference signal, are equal in the ionizer of the mass spectrometer. We find that the dissociative ionization products are extracted with much lower efficiency (2–50 times smaller for the cases studied) than the direct ionization products. This is expected due to the excess kinetic energy of the dissociatively ionized products as a result of the Franck–Condon effect. Use of this procedure will thus lead to an overestimation of the radical number density by a factor of 2–50, depending on the nature of the parent and the daughter ion. We recommend an alternate procedure for APMS that utilizes an inert gas direct ionization signal as a reference for calibration of the radical signal to calculate the radical number density. The biggest uncertainty in the radical number density estimation with the recommended procedure is expected to be the uncertainty in the published cross sections.
1/f
noise in p-type amorphous silicon
Authors: Johanson Robert E. , Kasap S. O. , Gaspari Franco , Yeghikyan Davit , Zukotynski Stefan
Published: July 2002

We have measured conductance fluctuations in four samples of p-type hydrogenated amorphous silicon, two doped at 10−4 and the other two at 5×10−2, at temperatures between 22 and 200 °C. The noise power density varies for the most part as 1/fα in the frequency range 2 Hz to 1 kHz, although deviations from a strict power law are observed. In all samples, the magnitude of the noise trends higher with temperature typically increasing by a factor of 5 over the temperature range. α also increases with temperature from near unity to over 1.4. The magnitude of the noise decreases as the Fermi level moves toward the valence band with increased doping. The dependence on doping and temperature is inconsistent with generation-recombination noise. Above 180 °C for the 10−4 doped samples, the noise fails to scale as the square of the bias current at low frequencies.
Core-level satellites and outer core-level multiplet splitting in Mn model compounds
Authors: Nelson A. J. , Reynolds John G. , Roos Joseph W.
Published: July 2002

We report a systematic study of the Mn 2p, 3s, and 3p core-level photoemission and satellite structures for Mn model compounds. Charge transfer from the ligand state to the 3d metal state is observed and is distinguished by prominent shake-up satellites. We also observe that the Mn 3s multiplet splitting becomes smaller as the Mn oxidation state increases, and that 3s–3d electron correlation reduces the branching ratio of the S:7S5 states in the Mn 3s spectra. In addition, as the ligand electronegativity decreases, the spin-state purity is lost in the 3s spectra, as evidenced by peak broadening. Our results are best understood in terms of the configuration–interaction model including intrashell electron correlation, charge transfer, and final-state screening.
Ionized sputter deposition using an extremely high plasma density pulsed magnetron discharge
Authors: Macák Karol , Kouznetsov Vladimir , Schneider Jochen , Helmersson Ulf , Petrov Ivan
Published: July 2002

Time resolved plasma probe measurements of a novel high power density pulsed plasma discharge are presented. Extreme peak power densities in the pulse (on the order of several kW cm−2) result in a very dense plasma with substrate ionic flux densities of up to 1 A cm−2 at source-to-substrate distances of several cm and at a pressure of 0.13 Pa (1 mTorr). The pulse duration was ∼100 μs with a pulse repetition frequency of 50 Hz. The plasma consists of metallic and inert gas ions, as determined from time resolved Langmuir probe measurements and in situ optical emission spectroscopy data. It was found that the plasma composition at the beginning of the pulse was dominated by Ar ions. As time elapsed metal ions were detected and finally dominated the ion composition. The effect of the process parameters on the temporal development of the ionic fluxes is discussed. The ionized portion of the sputtered metal flux was found to have an average velocity of 2500 m s−1 at 6 cm distance from the source, which conforms to the collisional cascade sputtering theory. The degree of ionization of the sputtered metal flux at a pressure of 0.13 Pa was found to be 40%±20% by comparing the total flux of deposited atoms with the charge measured for the metal ions in the pulse.
Oxidation and chemical state analysis of polycrystalline magnetron sputtered (Ti, Al)N films at ambient and liquid N2 temperatures
Authors: Seal S. , Kale A. , Sundaram K. B. , Jimenez D.
Published: July 2002

In order to improve the functional properties of hard coatings, recent investigations have been directed towards Ti–N based multicomponent materials. The nitride (Ti, Al)N, in particular, with a Ti:Al ratio of 1:1 seems to be a promising alternative to the widely used TiN, exhibits better oxidation resistance and hence improved performance over that of TiN. (Ti, Al)N coatings were dc sputter deposited onto 316SS substrates under ambient and liquid nitrogen temperatures. As deposited films were oxidized in a vertical fused-silica tube furnace in pure O2 flowing atmosphere at temperatures ranging from 700 to 900 °C. Scanning electron microscope and atomic force microscope images reveal information about the particle size and film thickness. X-ray photoelectron spectroscopy was employed to study the chemistry of the top few atomic layers in addition to compositional analysis and information on the details of chemical bonding. The difference in film stoichiometry are compared at two different deposition conditions thus reflecting their behavior under oxidizing conditions.
Growth of refractory oxide films using solid oxygen sources in a molecular beam epitaxy apparatus
Authors: Stall R. A.
Published: July 2002

Refractory oxide films of Al2O3, SiO2, MgO, and Al2MgO4 have been grown in a molecular beam epitaxy (MBE) system using As2O3 and Sb2O3 as the oxygen source. On the growth surface, elements such as Al whose oxide is more stable than the group V oxide will reduce the group V oxide and form a refractory oxide. With the growth temperature held above 350 °C the group V oxide is volatile and therefore will not be appreciably incorporated into the film as verified by ESCA measurements. This technique allows for MBE growth of oxides with low background pressures because the group V oxides stick well to the liquid nitrogen cooled cryopanels. Metal–insulator and semiconductor–insulator superlattices are possible by opening and closing the group V oxide cell shutter. The wide variety of oxide films possible with this technique could find applications to microwave, optical, magnetic, and display devices.
A UHV-compatible round wafer heater for silicon molecular beam epitaxy
Authors: Finegan S. N. , Swartz R. G. , McFee J. H.
Published: July 2002

An ultrahigh vacuum-compatible round wafer heater for silicon molecular beam epitaxy (MBE) is described. The assembly incorporates a hot tantalum filament and is capable of heating a single 2 in. diameter silicon wafer to a temperature of 1200 °C at a rate of 350 °C/min with a total power dissipation of 600 W. Best results are obtained when a silicon diffuser is positioned between the wafer and the filament. In this case, the observed temperature variation across the wafer is ≤16 °C, and virtually slip-free epitaxial layers can be grown. The quality of epitaxial layers grown using this heater is, in general, the highest we have yet observed for silicon MBE material, with line dislocation density below 103/cm2.
Formation of silicon and water cluster ions in pulsed-laser stimulated field desorption
Authors: Tsong T. T.
Published: July 2002

Multiatom cluster ions of 3 to 16 atoms/cluster have been observed in pulsed-laser stimulated field desorption of silicon emitters. In a time-of-flight mass spectrum fine structures can be found in the mass line of every ion species, which are produced by isotope mixing in the clusters, and which will facilitate a correct identification of the charge state of the ions. The most abundant cluster ion species are Si2+4, Si2+5, and Si2+6. The numbers 4–6 may be called the magic numbers of the desorption process since they correspond to highly symmetric small units of atoms existing in Si surface layers. Most of the energy distributions of the ion species contain an exponentially decaying low energy tail which is produced by spontaneous dissociation of larger cluster ions, possibly due to photodissociation of larger cluster ions by the same laser pulses used for desorption. Water cluster ions of the form (H2O)mH+ (m=1–12) can be observed in pulsed-laser field desorption of ‘‘field condensed’’ water on a metal emitter surface. The appearance energy decreases monotonically with the cluster size, and also depends on the condition of the surface.
Characterization of photochemical processing
Authors: Hirose M. , Yokoyama S. , Yamakage Y.
Published: July 2002

Thermally grown SiO2 and crystalline GaAs surfaces exposed to an etching gas under an ArF excimer laser irradiation have been studied by in situ x-ray photoelectron spectroscopy (XPS). Reaction kinetics of photochemical etching and resulting products on the solid surfaces have been revealed from the chemical shifts of adsorbates and substrate atoms. Etching products in SiO2/(NF3+H2) system are confirmed to be SiF4, N2O, and NO2. For the etching of GaAs in HCl, GaCl3 and AsCl3 are found to be the most probable products in the gas phase.
Heterojunction band discontinuity at the Si–Ge(111) interface
Authors: Mahowald P. H. , List R. S. , Spicer W. E. , Woicik J. , Pianetta P.
Published: July 2002

The interface formed when Si is evaporated on clean cleaved Ge(111) is studied using photoemission spectroscopy, Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) at growth temperatures of 20 and 300 °C to investigate the effect of the change in morphology at the interface between disordered and pseudomorphic growth. The interface is most likely abrupt at room temperature while intermixing of Ge is observed for the high temperature growth. From LEED and other considerations, the atomic structure at the interface is changed, but no measurable difference between the Si core level energy compared to the Ge core level energy is found for the two growth temperatures. This is interpreted as the absence of heterojunction band discontinuity change [neglecting possible strain effects on the valence band maximum]. This indicates further the absence of a modification of the band lineups due to slightly ionic bonds crossing the interface, which is called the atomic bonding dipole. The valence bands were also observed directly for the 300 °C growth with 130 eV photons and the heterojunction discontinuity was 0.4±0.1 eV.
GaAs/GaAlAs device structures prepared by molecular beam epitaxy using indium-free mounting techniques
Authors: SpringThorpe A. J. , Mandeville P.
Published: July 2002

A simple indium-free mount is described for the VG-Semicon V80-H MBE system. In conjunction with nonstandard substrate preparation techniques, the use of this mount has enabled GaAs/GaAlAs device structures to be prepared with consistently low (≤200/cm2) oval defect concentrations. Both 2DEG structures, with high 4 K mobilities, and GRINSCH lasers, with low thresholds, have been prepared to demonstrate the capabilities of the mounting procedures.
SiO2 films deposited on Si by dual ion beams
Authors: Minowa Y. , Ito H.
Published: July 2002

Uniform SiO2 films of 0.1∼1  μm thickness were deposited on Si substrates (5 in. in diameter) by dual ion beams (DIB) which consists of ionized cluster beams (ICB) and ionized gas beams (IGB). In the source of ICB, fine powder of SiO is vaporized to form a jet stream, which on being ejected from a multinozzle into a high vacuum chamber, is cooled and clustered by adiabatic expansion. The clusters thus obtained are partially ionized in an electron shower, accelerated, and made to bombard the Si substrate. On the other hand, in the source of IGB, O2 gas molecules ejected from a nozzle are partially ionized, excited in an electron shower, accelerated, and made to bombard the Si substrate too. These two beams collide and combine together on their way to the substrate or on its surface. The properties of the SiO2 films formed by the DIB mainly varied with oxygen partial pressure, oxygen ion current density, and deposition rate; with an oxygen partial pressure of 6.7×10−2 Pa, an oxygen ion current density of 6×10−7 A/mm2, and a deposition rate of below 1200 Å/min at a substrate temperature of below 200 °C, the SiO2 films obtained had an infrared spectrum peaking at 1060 cm−1 with a half-width at the half-maximum of 80 cm−1. It indicates that the DIB can produce at a very low temperature with a very high deposition rate almost the same quality SiO2 films as those obtained by thermal oxidation. This is considered to be possible because of the existence of the ionized and exited gas molecules and the migration of ionized clusters onto the surface of the substrate.
Characterization of localized atomic surface defects by tunneling microscopy and spectroscopy
Authors: Hamers R. J.
Published: July 2002

Tunneling microscopy and spectroscopy are used to characterize defects in Al overlayers on Si(111). At low coverages where Al and Si adatoms are both present, voltage-dependent scanning tunneling microscopy imaging allows them to be selectively imaged due to their different electronic structures. In (3)1/2-Al, Si adatoms substituting for Al give rise to a defect state at −0.4 eV which is strongly localized in space. Band-bending measurements indicate that the Si substitutional defects are electrically neutral. The results are interpreted in terms of a Mott–Hubbard model for a strongly correlated system.
Synthesis and characterization of an organogermanium resist: Poly(trimethylgermylmethyl methacrylate–co–chloromethylstyrene)
Authors: Mixon D. A. , Novembre A. E.
Published: July 2002

This paper describes the synthesis of trimethylgermylmethyl methacrylate (GE) from germanium tetrachloride, and also the first reported syntheses of polymers containing the GE functionality. Copolymers with chloromethylstyrene (CMS), having GE contents of 0.68, 0.79, and 0.90 mole fraction, are prepared by free radical solution polymerization. These materials are shown to be sensitive negative-acting electron-beam and deep-UV resists having good resistance to O2 reactive ion etching (RIE). Throughout the study the organogermanium copolymers are compared with organosilicon copolymers containing trimethylsilylmethyl methacrylate (SI) and CMS, which differ solely at the metal atom center. Subtle differences attributed to steric effects are observed in the polymerization kinetics, glass transition temperatures, and lithographic response of these homologous materials. Additionally, for copolymers of equimolar metal–monomer content, the etch resistance of P(GE–CMS) is up to 25% greater than that of P(SI–CMS) for a specific O2 RIE condition. These organogermanium materials hold potential for use in submicron bilevel lithographic processing, similarly to the homologous organosilicon materials.
Ion implant compensation of tensile stress in thick silicon nitride films for x-ray masks
Authors: Qingyun D. , Liankui Z. , Mengzhen C. , Junru M.
Published: July 2002

High-energy ion implantation can compensate the tensile stress in thick silicon nitride films. We investigated the relationship between the compensated stress in the silicon nitride films and ion implantation parameters like ion energy, dose, and kind of ions, as well as the influence of different annealing temperatures. We developed the technique of stress compensation for silicon-nitride films and applied it in making silicon nitride x-ray masks. As a result of the application, we have made x-ray masks as large as 40×40 mm. Finally, we present an elementary description of the mechanism of the stress compensation.
Optical second harmonic generation from Ni(110) with adsorbed CO
Authors: Hamilton J. C. , Anderson R. J. M. , Williams L. R.
Published: July 2002

Optical second harmonic intensity from Ni(110) has been measured as a function of CO exposure, temperature, and excitation wavelength and compared with similar experiments on Ni(111). Second harmonic generation from Ni(110) is much weaker than from Ni(111); there is little or no azimuthal dependence of the second harmonic intensity; and the effect of CO on the second harmonic intensity is much less than for Ni(111). These observations are explained in terms of the unoccupied electronic levels of the two surfaces.
Model of epitaxial growth of GaAs on Si(100): Nucleation at surface steps
Authors: Alerhand O. L. , Kaxiras Efthimios , Joannopoulos J. D. , Turner G. W.
Published: July 2002

A microscopic model of epitaxial growth of GaAs on Si(100) is proposed, which explains how double-layer steps on the Si(100) surface act as nucleation sites in the initial stages of growth. On the flat terraces of Si(100), there is a strong tendency towards the formation of mixed layers of Ga and As, which inhibit growth of zincblende GaAs. The bonding topology of the surface steps supresses this mixing, and drives the growth of ideally bonded GaAs in a three-dimensional mode with islands forming along the steps edges. Total-energy calculations indicate that the proposed model for nucleation of GaAs at double-layer steps on the surface is energetically stable.
Effect of disorder on the Al/GaAs(001) interface
Authors: Donner S. K. , Caffey K. P. , Winograd Nicholas
Published: July 2002

Distinct differences in the nucleation of Al at room temperature on arsenic-stabilized (2×4)- and c(4×4)-GaAs(001) surfaces grown by molecular-beam expitaxy have been observed with reflection high-energy electron diffraction. The nucleation of Al is found to be dependent upon the amount of disorder on the (2×4)-GaAs(001) surface. Phase diagrams showing the MBE growth parameters required for the preparation of more ordered versus more disordered surfaces of GaAs(001) have been constructed. The presence of disorder has been found to be independent of the rate of cooling of the GaAs substrate to room temperature for Al deposition. Nucleation on the more-disordered (2×4) surface and on the c(4×4) surface are similar in the direction of twofold periodicity, but differ in the direction of fourfold periodicity. The presence of a strong Al metal adatom–adatom interaction of submonolayer coverages, formerly reported for Al/(2×4)-GaAs(001), may help to explain why the more-ordered (2×4)-reconstructed surface preferentially nucleates fcc Al(110).
Sub-0.2-μm lithography by using a variable-shaped electron beam assisted by a focused ion beam process
Authors: Hosono K. , Minami H. , Kusunose H. , Fujino T. , Nagahama K. , Morimoto H. , Watakabe Y.
Published: July 2002

Sub-0.2-μm lithography processes have been developed by using a variable-shaped electron beam (EB) assisted by a focused ion beam (FIB). This process has been applied to the mushroom-gate fabrication of low-noise high electron mobility transistors. By use of a 320 keV Si++ and a shaped EB of 0.1 μm×4.0 μm, the mushroom gate was fabricated in dimensions of ∼0.15 μm. The overlay errors were <0.2 μm both for ‘‘top’’ and ‘‘bottom’’ gates, which were respectively written by FIB and EB. The ‘‘bottom’’ gates were especially exposed for only 48 min per 15 000 devices together with the gate pads.
High-precision interferometric alignment using checker grating
Authors: Tabata M. , Tojo T.
Published: July 2002

This paper presents a double-diffraction alignment method using a checker grating. An interferometric method, which utilizes a novel pair of grating marks located on a mask and a wafer, has been developed. A one-dimensional grating for the mask mark and a checker grating for the wafer mark were used. A 633-nm He–Ne laser was chosen for the alignment light, which is transparent to the photoresist on the wafer. Two correction mirrors corrected the chromatic aberration due to the difference in wavelength between the exposure light and the alignment light. The mirrors allowed only the two first-order diffracted beams to be projected at the mask mark. These two diffracted beams formed a moire image on the mask, and interfered with the mask grating. The zero-order diffracted-light intensity from the mask grating was detected as the alignment signal. This alignment signal was not influenced by defocusing because only the two diffracted beams took part in the image formation on the mask. The alignment-signal sensitivity must not be varied in order to achieve high-precision alignment. By using a new AGC (auto gain control) technique, the signal sensitivity was hardly varied by the optical signal fluctuation. The performance of this alignment method was experimentally evaluated using large-scale integrated processed marks; as a result, a higher than 0.15-μm (3σ) alignment accuracy has been obtained.
Fabrication of GaAs/GaAlAs transport devices using a deep submicron trench etching technique
Authors: Lee K. Y. , Hansen W. , Smith T. P. , Knoedler C. M. , Hong J. M. , Kern D. P.
Published: July 2002

This paper demonstrates a submicron trenching technique for defining low-dimensional electron systems in GaAs/GaAlAs heterostructures for transport measurements. A variety of electron systems can be defined between trenches as narrow as 200 nm etched into a suitable heterostructure. The use of narrow trenches gives the advantages of high resolution, simplicity, and flexibility, and also avoids the complications associated with negative resists and lift-off techniques. Using the trenching technique, we have fabricated Aharonov–Bohm ring devices with a Schottky gate over the entire ring. At 0.4 K, these devices exhibit clear interference effects which are dependent on the gate bias.
Thermal oxidation of silicon nitride and silicon oxynitride films
Authors: Kuiper A. E. T. , Willemsen M. F. C. , Mulder J. M. L. , Elferink J. B. Oude , Habraken F. H. P. M. , van der Weg W. F.
Published: July 2002

The oxidation behavior of low-pressure chemical vapor deposition silicon oxynitrides was investigated for layer compositions ranging from that of pure nitride to oxynitride with an atomic ratio of O/N=1. Oxidations were performed at 850–1000 °C in ambients having different H2O/O2 flow ratios. Rutherford backscattering spectrometry and elastic recoil detection were employed to quantify the increase in oxygen content of the films upon oxidation. Hydrogen profiles in oxidized samples were measured using nuclear reaction analysis and elastic recoil detection. We observed that the oxidation rate for either oxynitride composition is nearly two orders of magnitude smaller than that of silicon. Linear oxidation kinetics were measured for the conditions applied in this study, which are indicative of a reaction-controlled process. The activation energy of this process was determined to be 2 eV for silicon nitride and appeared to decrease with growing oxygen content in oxynitrides. The hydrogen profiles measured in oxidized samples showed a peak at the oxide/oxynitride interface. The size of this peak varies with oxidation conditions and film composition. A reaction mechanism for the oxidation of (oxy)-nitride is proposed that relates the observed hydrogen accumulation to Si–NH and Si–OH groups at the oxidizing interface. The measured acceleration of the oxidation rate induced by adding small amounts of HCl to the ambient is interpreted in terms of enhanced transport of NH species to the surface. This effect is greatest for pure nitride and diminishes with increasing oxygen content in oxynitride material.
X-ray stepper exposure system performance and status
Authors: Flamholz Alex , Rippstein Robert
Published: July 2002

An integrated x-ray align and expose system has been installed at Brookhaven National Laboratory National Synchrotron Light Source VUV beam line U2. The system includes the first commercially available x-ray align and expose system, the Karl Suss XRS-200 and matched beamline. Subsystem and aligned exposure tests were performed to characterize the entire system. Initial results are presented for key lithographic properties as well as throughput. Improvements and future activities are also described.
The effect of process coatings on the alignment signal in a proximity lithography system
Authors: Chen G. , Wallace J. , Palmer S. , Cerrina F. , Randall J.
Published: July 2002

The achievement of a high accuracy mask-to-wafer alignment system is one of the key steps in the development of submicron x-ray lithography technology. In this paper, we examine the effects of the depth of the wafer mark and of different process coatings (including photoresist) on the alignment signal in an alignment system that uses linear Fresnel zone plates. A mathematical model based on scalar diffraction theory has been developed to simulate the effect of various coatings on the diffracted signal. An experiment is also carried out to verify the model. From the prediction of the theoretical model and the experimental results, we discuss how the process coating may affect the alignment signals in our two state alignment system. In the light of these results, we deduce the optimal design of the wafer alignment marks in our alignment system.
A scanning tunneling microscope with a capacitance-based position monitor
Authors: Griffith J. E. , Miller G. L. , Green C. A. , Grigg D. A. , Russell P. E.
Published: July 2002

A serious impediment to precision metrology with scanning probe microscopes is the unreproducible, nonlinear behavior of the piezo ceramic actuators. We have developed a simple solution to the problem by measuring the position of the scanning head with capacitors. The circuit monitoring the gap between the plates has a linear response with slope 12 μm/V and noise of 0.1 nm/(Hz)1/2. The linearity of the system is verified by comparing the capacitor output with a scan of a grating that is periodic in two dimensions. We analyze the errors associated with the technique and show how to reduce them to acceptable levels.
Soft x-ray projection lithography
Authors: Ceglio N. M. , Hawryluk A. M. , Stearns D. G. , Gaines D. P. , Rosen R. S. , Vernon S. P.
Published: July 2002

Recent advances in x-ray optics have made possible the practical consideration of soft x-ray projection imaging for the fabrication of high density integrated circuits.