bluuebunny/crossref_metadata_2025 huggingface dataset:
Highly selective etching of silicon nitride over silicon and silicon dioxide
Authors: Kastenmeier B. E. E. , Matsuo P. J. , Oehrlein G. S.
Published: July 2002

A highly selective dry etching process for the removal of silicon nitride (Si3N4) layers from silicon and silicon dioxide (SiO2) is described and its mechanism examined. This new process employs a remote O2/N2 discharge with much smaller flows of CF4 or NF3 as a fluorine source as compared to conventional Si3N4 removal processes. Etch rates of Si3N4 of more than 30 nm/min were achieved for CF4 as a source of fluorine, while simultaneously the etch rate ratio of Si3N4 to polycrystalline silicon was as high as 40, and SiO2 was not etched at all. For NF3 as a fluorine source, Si3N4 etch rates of 50 nm/min were achieved, while the etch rate ratios to polycrystalline silicon and SiO2 were approximately 100 and 70, respectively. In situ ellipsometry shows the formation of an approximately 10-nm-thick reactive layer on top of the polycrystalline silicon. This oxidized reactive layer suppresses etching reactions of the reactive gas phase species with the silicon.
Defect formation upon reactive direct-current magnetron sputtering of GeO2 films
Authors: Njoroge Walter , Lange Thomas , Weis Hansjörg , Kohnen Birgit , Wuttig Matthias
Published: July 2002

Defect formation upon reactive direct-current magnetron sputtering of GeO2 films has been studied using x-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) analysis, as well as atomic force microscopy and optical microscopy. The density of the defects shows a clear correlation with oxygen flow rate. High defect densities are observed for flow rates above approximately 27.5 sccm, where a transition of film structure is observed. XRD analysis identifies the defects as Ge crystallites. Defect formation is intimately related to the formation of whiskerlike structures on the target in the vicinity of the erosion zone and arcing in the same area. SEM imaging in conjunction with EDX allows a detailed understanding of defect formation.
Diagnostics of an inductively coupled CF4/Ar plasma
Authors: Hioki Kazuya , Hirata Hajime , Matsumura Shosaku , Petrović Zoran Lj. , Makabe Toshiaki
Published: July 2002

Experimental data for radiative and metastable state densities in pure argon and in mixture of 5% CF4 in argon are obtained to study the effect of CF4 on kinetics of inductively coupled plasma (ICP). The measurements were made by a combination of optical emission spectroscopy and computer tomography for radiative levels and by laser absorption for metastable state profiles. The data were obtained in an inductively coupled rf (13.56 MHz) plasma with a single coil and full cylindrical symmetry. At low pressures of 5 and 15 mTorr the data in pure argon and in the mixture do not differ much, both in the magnitude and in the radial and axial profiles. However, at 50, 100, and 300 mTorr there is a dramatic change when CF4 is introduced. The metastable density increases due to the smaller electron induced quenching while axial and radial profiles are quite different. Unfortunately introduction of CF4 spoils the degree of azimuthal isotropy. The observed results and explanations based on the important role of metastables and stepwise processes in kinetics of high electron density ICP are supported by measurements of plasma density by using a double probe.
Titanium oxide films on Si(100) deposited by electron-beam evaporation at 250 °C
Authors: Jang H. K. , Whangbo S. W. , Kim H. B. , Im K. Y. , Lee Y. S. , Lyo I. W. , Whang C. N. , Kim G. , Lee H.-S. , Lee J. M.
Published: July 2002

Titanium oxide films with a thickness of a 400 nm were deposited on p-type Si(100) at 250 °C by electron-beam evaporation where titanium dioxide was evaporated in oxygen environment at a pressure of 2×10−6–4×10−5 Torr. Effects of oxygen flow rate (FO2) between 0 and 40 sccm on properties such as crystallinity, surface roughness, and chemical states of the films have been investigated. Oxygen resonance backscattering spectroscopy shows that all films are oxygen rich, i.e., the ratio of oxygen to titanium of the films ranged from 2.25 to 2.3. X-ray diffraction patterns show that these films grown at 250 °C are polycrystalline of anatase TiO2. Ti K-edge x-ray absorption near-edge spectroscopy (XANES) spectra show that the films have mixed phases of anatase and rutile TiO2. More than 70% of the films is anatase TiO2 and its proportion of the films is decreased with increasing FO2. XANES spectra from the films grown at FO2=0 sccm are very similar to that of the powdered anatase TiO2. X-ray photoelectron spectroscopy indicates that there exist Ti2+, Ti3+, and Ti4+ charge states in the film grown at FO2≦10 sccm but there exist only Ti3+ and Ti4+ charge states in the film grown at FO2>10 sccm.
Surface reduction of Cr–V2O3 by CO
Authors: Toledano David S. , Henrich Victor E. , Metcalf Patricia
Published: July 2002

Interesting irreversible effects have been observed involving surface reduction of Cr-doped V2O3 by CO. Adsorption characteristics of CO on the paramagnetic insulating phase of (V0.985Cr0.015)2O3(0001) were examined by ultraviolect photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy, low-energy electron diffraction, and Auger electron spectroscopy, along with thermal desorption of the adsorbate and exposure of reduced surfaces to O2. When V2O3:Cr is exposed to CO at 273 K, little interaction is observed up to 103 Langmuir; however, higher exposures result in significant coverage of a C-containing moiety, a large amount of electron transfer to V cations at energies near EF, and a corresponding decrease in the occupation of states near the top of the O 2p band. Heating above 400 K desorbs the C-containing species, but the surface becomes further reduced, with increased charge transfer to V cations, suggesting that adsorbed CO reacts strongly with lattice oxygen and desorbs as CO2. During repeated CO adsorption/desorption cycles, the degree of surface reduction and the magnitude of O→V charge transfer continues to increase. However, no energy shift of the O 2p band is observed upon reduction. Comparison of the electronic structure of V2O3:Cr surfaces reduced by CO exposure and by inert-gas ion bombardment shows both similarities and differences. Both types of surfaces display a higher initial sticking coefficient for CO than do stoichiometric surfaces. However, while the electronic structure of ion-bombarded V2O3:Cr is largely restored to that of the stoichiometric surface by exposure to O2, that of CO-reduced V2O3:Cr is not. UPS indicates that O2, rather than reoxidizing CO-reduced V2O3:Cr, adsorbs as O− or (O2)−; the surface remains highly disordered as well. The restoration of the original surface structure during high-temperature (> 800 K) annealing is found to be independent of the adsorbed oxygen species.
Deposition and composition of silicon oxynitride films
Authors: Kuiper A. E. T. , Koo S. W. , Habraken F. H. P. M. , Tamminga Y.
Published: July 2002

Silicon oxynitride (SiOxNy) films have been grown by a low-pressure chemical vapor deposition (LPCVD) process from mixtures of SiH2Cl2, N2O, and NH3 at 820 °C. The overall layer composition can be varied by adjusting the N2O/NH3 gas flow ratio. Rutherford backscattering and Auger analysis of the films indicated a uniform composition throughout the layer, irrespective of the nature of the substrate. Both the thickness and the composition of these oxynitride films can conveniently be measured with ellipsometry; the oxygen to nitrogen ratio can be derived reliably from the value of the refractive index. It is inferred that LPCVD oxynitrides are homogeneous on an atomic scale, i.e., the silicon atoms are randomly surrounded by oxygen and nitrogen atoms, and are therefore not to be conceived of as a physical two phase mixture of silicon oxide and silicon nitride. Their stability in metal–oxynitride–oxide–silicon structures is found to improve with increasing oxygen content as regards flatband voltage shift upon temperature-bias stress.
High performance positive photoresists
Authors: Furuta A. , Hanabata M. , Uemura Y.
Published: July 2002

An attempt was made to improve the sensitivity and heat resistance of positive photoresist applicable to very large-scale integrated (VLSI) semiconductor processing. It was found out that, in general, when the lithographic sensitivity of resist was made higher, film thickness retention and heat resistance were lowered. A study was made on how to raise sensitivity without decrease in film thickness retention and heat resistance. It was necessary to design a new type of novolac resin that has a molecular structure and a molecular weight different from the existing materials. Many kinds of cresol-formaldehyde type novolac resins were synthesized and evaluated. Such items as the isomeric structure of cresol, the position of the methylene bond, and the influence of molecular weight were investigated. As a result of the optimization of these items, a number of different resist materials that exhibit improved characteristics in sensitivity, film thickness retention, and/or heat resistance were obtained. It is also remarkable that the introduction of the new polymer design enabled to make the allowance of the operating conditions much wider at various points of resist work, including prebaking temperature, light exposure time, developing time, and developing temperature, compared to the present commercially available products. This new material not only makes it possible to raise the throughput of the stepper exposure machine and developing machine, but the wider allowance of processing conditions assures higher yield of the circuits.
Stability of alignment marks for x-ray masks
Authors: Acosta R. E. , Maldonado J. R. , Fair R.
Published: July 2002

IBM’s x-ray lithography program uses optical alignment to position the mask relative to the wafer. Optical alignment places the constraint of high optical transparency for the B-doped Si and B–N membranes presently used in the program. This problem may be solved by using alignment marks located outside the x-ray exposure area, with the substrate’s transparency in the alignment region increased by (a) thinning of the B–Si, (b) eliminating the B–Si and supporting the alignment marks on polyimide, or (c) removing the supporting background completely. In any of these solutions the stability of the alignment marks becomes a separate issue from that of the stability of the mask itself. Processes developed to fabricate alignment marks by all three schemes are described, and measurements performed to ascertain the stability of the marks are reported.
SiO2 planarization technology with biasing and electron cyclotron resonance plasma deposition for submicron interconnections
Authors: Machida Katsuyuki , Oikawa Hideo
Published: July 2002

Bias electron cyclotron resonance (ECR) plasma deposition technology is proposed to planarize submicron interconnections with a high aspect ratio (height/space of interconnection) in which rf bias is applied to the substrate of the ECR plasma deposition system. The technology has the following advantages. First, the concave region of the narrow gap between submicron interconnections with a high aspect ratio above 1.0 can be planarized and formed at the same insulator thickness as that of the convex region. This is due to high directionality of ECR plasma particles. Second, both deposition and etching rates can be controlled by adjusting the gas flow rates as well as the rf and microwave power. Third, an rf bias can be applied from the initial stage of the planarization process. This is because of sputter-etching with O2 ions without Ar, thus enabling a shorter planarization time. Using the bias-ECR plasma deposition technology, the 0.5 μm line/space Al (0.5 μm thickness) interconnection surface can be perfectly planarized.
Microwave plasma etching of Si and SiO2 in halogen mixtures: Interpretation of etching mechanisms
Authors: Pelletier J. , Cooke M. J.
Published: July 2002

An experimental study of the etching of Si with SF6 /halogen and halogen mixtures has been performed in a multipolar electron cyclotron resonance plasma with independent rf biasing. At constant total pressures and at a constant ion energy (75 eV), the anisotropy and the etch rate of silicon have been measured as a function of halogen percentages in the mixtures. The results are explained in terms of a recently developed diffusion model for plasma etching. Fluorine, chlorine, and bromine mixtures, contrary to iodine, etch Si, and SiO2. The bromine and chlorine adsorption on silicon appears to be monolayerlike, whereas fluorine adsorption is clearly of the multilayer type. Experimental results on etching anisotropy and selectivity with respect to SiO2 are also analyzed with emphasis on the peculiar behavior of bromine.
Fabrication of closely spaced quantum dot diodes
Authors: Randall J. N. , Reed M. A. , Kao Y.-C.
Published: July 2002

Quantum dot devices have been proposed as the basic structure for an integrated circuit technology with extremely high functional density [R. T. Bate, Sci. Am. 258, 96 (1988)]. Electronic transport has been reported through quantum dot diodes which are resonant tunneling devices with lateral dimensions small enough to produce zero-dimensional (0D) confinement effects [M. A. Reed, J. N. Randall, R. J. Aggarwal, R. J. Matyi, T. M. Moore, and A. E. Wetsel, Phys. Rev. Lett. 60, 535 (1988); G. Faini, A. Ramdane, F. Mollot, and H. Launois, (to be published) NATO Workshop Spain 1990; S. Tarucha, Y. Hirayama, T. Saku, and T. Kimura, Phys. Rev. B 41, 5459 (1990)]. We seek improvements in the switching characteristics of these 0D tunneling devices as well as processing techniques to fabricate them very close together in an attempt to study field coupling between dots. We will discuss the processing issues involved with fabricating very closely spaced quantum dot diodes. We are building pairs of quantum dot diodes which are approximately 1500 angstroms in diameter and spaced a few hundred angstroms apart. e-beam lithography is used to define the dot patterns. The use of nonalloyed InGaAs contacts, which is essential to providing ohmic-type contacts of this small diameter, [J. N. Randall, C. H. Yang, Y. C. Kao, and T. M. Moore, J. Vac. Sci. Technol. B 7, 2007 (1989); P. Gueret, P. Buchmann, K. Daetwyler, and P. Vettiger, Appl. Phys. Lett. 55, 1735 (1989)], presents complications for the reactive ion etching of these structures. The task of independently contacting two very small and closely spaced contacts requires high resolution lithography and accurate re-alignment techniques. The e-beam lithography techniques used to accomplish this task will be presented. Finally some transport measurements through isolated quantum dot diodes will be presented.
An investigation of the reactive ion etching of polysilicon in pure Cl2 plasmas by i
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i
t
u ellipsometry and quadrupole mass spectrometry
Authors: Thomas D. J. , Southworth P. , Flowers M. C. , Greef R.
Published: July 2002

The simultaneous application of in situ ellipsometry and quadrupole mass spectrometry to the reactive ion etching of polysilicon/SiO2/Si(100) multilayers in pure Cl2 plasmas at 13.56 MHz is described. At 100 mTorr and 50 W the surface texture of the unetched polysilicon is retained, but significant roughening is induced when the power is raised to 100 W. Water vapor is shown to play a critical role in the roughening process and is probably due to micromasking by hydroxide groups at the polysilicon surface. Detailed models of the ellipsometric results for roughened surfaces are derived and correlated with their scanning electron microscopy images. When CClx (x=1–4) species are present in the plasma, due to the erosion of a carbon-containing resist, roughening of polysilicon is prevented at 100 W and the etch rate of the underlying SiO2 is increased. Accompanying this increase is a shift in the product distribution for SiO2 towards higher SiCl4 but lower O2. In contrast the etch rate and product distribution for polysilicon remain almost unaffected by the presence of resist.
Dependence of structural and optical properties of In0.23Ga0.77As/GaAs quantum wells on misfit dislocations: Different critical thickness for dislocation generation and degradation of optical properties
Authors: Grundmann M. , Lienert U. , Christen J. , Bimberg D. , Fischer-Colbrie A. , Miller J. N.
Published: July 2002

The critical layer thickness Lc for dislocation generation and the characteristic thickness for consequent modification of the optical properties of strained quantum wells are shown to be remarkably different. We visualize the misfit dislocations directly using scanning cathodoluminescence and transmission electron microscopy. The dislocations are found to be asymmetrically distributed within the (001) interface plane. The critical thickness for In0.23Ga0.77 As on GaAs is determined to be Lc ≊15 nm in agreement with the theory of Matthews and Blakeslee [J. Cryst. Growth 27, 118 (1974)]. The deterioration of quantum efficiency, a change of recombination dynamics, the increase of spectral broadening and a shift of the band gap occur at much larger thickness than Lc since these quantities are barely affected by low dislocation densities. The relaxation is heterogeneous; strained and fully relaxed domains coexist. Using double crystal x-ray diffraction we find that partially relaxed layers with thickness larger Lc have no longer tetragonal but monoclinic crystal symmetry in quantitative agreement with the asymmetry of the dislocation density. Along the dislocation lines the lattice planes show an anisotropic tilting. The investigation of recombination dynamics and excitonic lifetime in this material system shows a change of the character of recombination dynamics from almost purely radiative to increasingly nonradiative transitions upon an increase of dislocation density. The delayed onset of degradation of the quantum efficiency and the drop of recombination lifetime is quantitatively explained by a diffusion model for charge carriers.
Properties of Schottky diodes on n-type Hg1−xCdxTe
Authors: Leech Patrick W. , Kibel Martyn H.
Published: July 2002

The electrical properties of Schottky diodes formed by Ag, Au, Cu, Pd, Pt, Sb, and Ti on n-Hg1−xCdxTe (x=0.6–0.7) have been measured using current-voltage and capacitance-voltage techniques. Both LPE grown (111) Hg0.3Cd0.7Te and metal-organic chemical-vapor deposition grown (100) Hg1−xCdxTe epitaxial layers were used with surface preparation either by chemical etching or by air exposure. The current–voltage characteristics of contacts fabricated on chemically etched surfaces have been described by a thermionic emission-recombination model. For these etched surfaces, the barrier heights produced by the metals Ag, Au, Cu, and Sb on (111) Hg0.3Cd0.7Te were in the range φ′b=0.74 V to 0.79 V while for Pd and Pt, φb=0.69±0.01 V. In close agreement, Pt contacts on etched (100) surfaces exhibited a barrier height of φ′b=0.70±0.01 V for stoichiometries of x=0.6–0.68. The effects of air exposure on diode characteristics were most significant for the Au contacts due to an inability of the metal to reduce interfacial TeO2 or to react appreciably with the Hg1−xCdxTe. The interfacial reaction of Au and of Pt layers with Hg0.3Cd0.7Te has been studied by Auger depth profiling.
Lateral dopant profiling in semiconductors by force microscopy using capacitive detection
Authors: Abraham David W. , Williams C. , Slinkman J. , Wickramasinghe H. K.
Published: July 2002

Recently, high-resolution mapping of dopant concentration has been demonstrated with the scanning capacitance microscope (SCM). Here, we demonstrate that a similar measurement can be made with the atomic force microscope using the previously demonstrated capacitive force sensing mode. By applying appropriate bias to the force tip, depletion-induced capacitive variation is mapped over regions of varying dopant density. This method has a predicted sensitivity comparable to the SCM, and in addition allows imaging of trapped charge, as well as an independent measurement of the surface topography. Results of first-order model calculations are presented which give estimates as to the limits in sensitivity and resolution of this method
Reactivity, growth mode, and kinetics of the Fermi level pinning at the Ni/GaAs(110) interface
Authors: Landesman J. P. , Jezequel G. , Olivier J. , Larive M. , Thomas J. , Taleb-Ibrahimi A. , Bonnet J. E.
Published: July 2002

The formation of GaAs(110) interfaces with Ni was investigated with soft x-ray photoelectron spectroscopy at the Super-ACO synchrotron radiation source at L.U.R.E. in Orsay (France). Ni was deposited on p-GaAs(110) surfaces cleaved in ultrahigh vacuum (UHV), either by direct evaporation at substrate temperatures of 300 K [room temperature (RT)] or 20 K [low temperature (LT)] or by evaporation on top of an Ar layer condensed at 20 K on the semiconductor surface. This Ar layer was further desorbed by annealing around 100 K, and the Ni could thus be brought smoothly onto the GaAs(110) surface. This method allows the formation of nonreacted interfaces with a metal which, when deposited directly at RT or LT, brings a strong degree of surface disruption. Ga 3d and As 3d core level spectra were recorded on surfaces with overlayers from 0.01 to 10 Å, together with the valence band (VB) spectra in normal conditions (photon energy hν=50 eV) and in resonant conditions (hν=67 eV, corresponding to the photoemission threshold for the Ni 3p core levels). A reaction component was observed on the Ga 3d spectra for Ni thicknesses as small as 0.2 Å on the direct interfaces formed at RT as well as LT (direct interface meaning interface formed by direct evaporation). No reaction with the GaAs surface could be detected when the Ni is deposited on the Ar buffer layer, even for a thickness of 15 Å. The valence band spectra indicate that NiAs is formed for direct interfaces. At LT, this reaction is limited to the first atomic layer, but it is observed to extend up to 10 Å thick layers for deposition at RT. The evolution of the intensity of the core level and VB spectra clearly indicates the formation of NiAs clusters at RT. The evolution of the Schottky barrier height differs for the two types of deposition. For the direct evaporation, Ef saturates [on p-type GaAs(110)] at 500 meV above the valence band maximum (VBM), the kinetics being slower at LT due to photoelectron perturbation of the band bending (photovoltage effect); for deposition on the Ar buffer layer, both the final position (300 meV above VBM) and the kinetics are different.
Low temperature epitaxy of HgTe, CdTe, and HgCdTe using flow modulation techniques
Authors: Bhat I. B. , Ehsani H. , Wang W. S. , Ghandhi S. K. , Karam N. H.
Published: July 2002

In this article, we report on the low temperature growth of HgTe, CdTe, and HgCdTe by flow modulation epitaxy (FME). Dimethylmercury, dimethylcadmium, and methylallyltelluride were used as the precursors in this ‘‘layer-by-layer’’ growth approach. Growth of CdTe at one monolayer per cycle was obtained at temperatures above 250 °C. The growth rate was invariant with temperature and reactant pressures. HgTe growth was carried out by FME at 140 °C at atmospheric pressure by precracking the Hg and Te alkyls at 350 °C. HgCdTe layers were grown by interdiffusing sequential layers of CdTe and HgTe.
Thin film interference effects in phase shifting masks causing phase and transmittance errors
Authors: Ronse Kurt , Jonckheere R. , Baik Ki-Ho , Pforr R. , Van den hove L.
Published: July 2002

Phase shifting masks have proven their potential to enhance resolution and depth-of-focus in optical lithography. Nevertheless, many questions still have to be solved, before the phase shifting concept can be introduced in production lines. Controllable mask fabrication is a major concern. This article addresses the influence of physical material properties and process steps in the attempt to fabricate phase shifting masks with acceptable phase shift and transmittance. Spectrophotometer measurements and computer simulations showed the need of matched refractive indices for all transparent materials in the reticle. Second, the sensitivity of the lithographic performance to these shifter deviations was investigated by aerial image simulations. These predicted a high sensitivity to phase errors, especially for negative resists. Using a negative tone top surface imaging resist process and a positive tone wet developed resist, experiments confirmed these predictions. Special attention was paid in finding an accurate and efficient method to measure the transmittance and phase shift deviations of a reticle experimentally.
Fermi-level inhomogeneities on the GaAs (110) surface imaged with a photoelectron microscope
Authors: Kim Changyoung , King Paul L. , Pianetta Piero
Published: July 2002

A photoelectron microscope operating with a retarding field analyzer can exploit core level energy shifts in order to image Fermi-level variations of semiconductor surfaces. Fermi-level maps of cleaved n- and p-type GaAs (110) resolved to better than 10 μm indicate lateral variations in the surface Fermi level which are often quite abrupt. In agreement with earlier, lower resolution work [J. M. Palau, E. Testemale, and L. Lassabatere, J. Vac. Sci. Technol. 19, 192 (1981)],1 Fermi-level topography is found to be highly correlated with surface roughness as characterized by scanning electron microscope (SEM), optical microscope, and stylus profilometer. Defective fracture surfaces have two distinct characters within SEM resolution: linear defects (steps and ledges) that may extend over millimeters and smooth glassy areas. Both types of surface defects shift the Fermi level towards midgap. The largest defect derived pinnings encountered to date result in the Fermi level lying 0.5 eV above the valence-band maximum for both n- and p-type GaAs. Submonolayer In evaporations reduce Fermi-level variations by shifting surface Fermi levels in formerly unpinned regions into the gap.
Work function, electron affinity, and band bending at decapped GaAs(100) surfaces
Authors: Chen W. , Dumas M. , Mao D. , Kahn A.
Published: July 2002

Work function and band bending measurements are performed on n- and p-type GaAs(100) surfaces prepared by molecular-beam epitaxy and As decapping. The band bending is almost independent of the decapping temperature and surface reconstruction [(1×1)→C(4×4)→C(2×8)→(4×2)]. The work function and electron affinity undergo large and systematic variations of up to 400 meV which consistently establish maxima and minima of these quantities for the (2×4)-C(2×8) and (4×2)-C(8×2) structures, respectively. These variations are due to surface dipoles induced by charge exchanges between the cation and anion dangling bonds in the top two layers of the crystals. These charge exchanges are fully explained by applying a simple electron counting model to each reconstruction.
Effect of interface roughness and scattering on the performance of AlAs/InGaAs resonant tunneling diodes
Authors: Förster A. , Lange J. , Gerthsen D. , Dieker Ch. , Lüth H.
Published: July 2002

The effect of the indium concentration on the quality of the AlAs/InGaAs interfaces in resonant tunneling diodes is studied. It is shown by high-resolution transmission electron microscopy that the interface quality of the normal interface (AlAs on InGaAs) nearly achieves even at low growth temperatures TG=500 °C the quality of the corresponding AlAs/GaAs interface at the optimum substrate temperature. The improvement in the interface quality and the higher potential barrier in the InGaAs system leads to higher peak-to-valley ratios (PVRs) of the diode relative to that in the GaAs system. In addition, current–voltage characteristics of resonant tunneling diodes are calculated as an integral over the transmission coefficient and the supply function. Scattering effects are included by folding the transmission probability with a Lorentzian. The theoretical simulations give further insight into the physics of the diodes.
Molecular-beam epitaxy on exact and vicinal GaAs(1̄1̄1̄) substrates
Authors: Yang K. , Schowalter L. J. , Laurich B. K. , Campell I. H. , Smith D. L.
Published: July 2002

GaAs films grown on exact (1̄1̄1̄) substrates in the √19 ×√19 reconstruction regime always show facets. The facets are composed of vicinal surfaces which are inclined by small angles (∼2°) from the exact (1̄1̄1̄) plane. The comparison between surface morphologies of films grown on vicinal substrates tilted toward different directions shows that the most stable steps run along the 〈110〉 directions and step downward toward the [112̄], [2̄11], and [12̄1] directions. In the √19 ×√19-surface-reconstruction growth regime, smooth films can be grown only on substrates tilted toward the [21̄1̄] direction. This result suggests that a vicinal surface, which is tilted toward the [21̄1̄] direction, may have less surface-free energy than the exact (1̄1̄1̄) surface. Characterization of superlattices grown on vicinal GaAs(1̄1̄1̄) substrates, tilted 3° toward the [21̄1̄] direction, are also presented.
Sidewall passivation during the etching of poly-Si in an electron cyclotron resonance plasma of HBr
Authors: Haverlag M. , Oehrlein G. S. , Vender D.
Published: July 2002

The etching of poly-Si (n+ and undoped) in a radio-frequency-biased electron cyclotron resonance plasma of HBr was studied. Etch rates of Si, oxide, and photoresist were obtained by ellipsometry as a function of the bias voltage and two substrate temperatures (15 and 50 °C) at 5 mTorr pressure. The etch rate of poly-Si depends on the doping level, with n+ Si etching faster than intrinsic Si. High selectivities of Si over both oxide and photoresist can be achieved at low bias voltages. Using angle-resolved x-ray photoelectron spectroscopy, it is shown that a carbon and bromine containing layer is deposited on the sidewall of the poly-Si during the etching process when a photoresist patterned wafer is used. The thickness of the sidewall film decreases with increasing substrate temperature and increases at the bias voltage is raised. The thickness of this sidewall film influences the anisotropy of the process, with some undercutting occurring at high temperature and low bias voltage.
Relation between atomic structure and surface-stress anisotropy: Calculations for the clean Si(001) surface
Authors: Da̧browski J. , Pehlke E. , Scheffler M.
Published: July 2002

The surface-stress tensor is an important quantity entering the elastic theory of interaction between atomic steps on solid surfaces. For the clean Si(001) surface the difference between its diagonal elements, the surface stress anisotropy Δσ, comes out by about a factor of two smaller in experiment than compared to the value obtained from previous first-principles calculations. This discrepancy was usually assigned to the presence of surface defects; as other plausible causes one could quote unrealistic atomic surface structures used for the calculations or even a failure of the local-density approximation (LDA) on which the calculations were based. This constituted a serious deficiency in the understanding of the Si(001) surface. We demonstrate that, when first-principles LDA calculations are carried out for the perfect Si(001) surface with the correct character of reconstruction, the calculated and measured Δσ agree within the experimental and numerical uncertainties. We conclude that standard LDA calculations can properly describe elastic properties of the Si(001) surface and that the effect of surface defects on the measured surface stress anisotropy is, at least for the measurements reported in the literature, much smaller than it has been previously anticipated.
First principles studies of band offsets at heterojunctions and of surface reconstruction using Gaussian dual-space density functional theory
Authors: Chen Xiaojie , Mintz Abner , Hu Jinsong , Hua Xinlei , Zinck Jenna , Goddard William A.
Published: July 2002

The use of localized Gaussian basis functions for large scale first principles density functional calculations with periodic boundary conditions (PBC) in 2 dimensions and 3 dimensions has been made possible by using a dual space approach. This new method is applied to the study of electronic properties of II–VI (II=Zn, Cd, Hg; VI=S, Se, Te, Po) and III–V (III=Al, Ga; V=As, N) semiconductors. Valence band offsets of heterojunctions are calculated including both bulk contributions and interfacial contributions. The results agree very well with available experimental data. The p(2×1) cation terminated surface reconstructions of CdTe and HgTe (100) are calculated using the local density approximation (LDA) with two-dimensional PBC and also using the ab initio Hartree–Fock (HF) method with a finite cluster. The LDA and HF results do not agree very well.
Simulation of atomic force microscope tip–sample/sample–tip reconstruction
Authors: Markiewicz Peter , Goh M. Cynthia
Published: July 2002

The image obtained with the atomic force microscope is a convolution of the tip and sample. A numerical algorithm which has been previously reported enables the removal of the tip geometry, hence exposing a more accurate picture of the sample. The efficacy of such a scheme is explored with the use of simulations of the tip–sample interaction using simple geometric considerations. Two examples that illustrate the limitations of image analysis by such procedures are presented.
Nanostructuring of alkanethiols with ultrasharp field emitters
Authors: Müller H. U. , David C. , Völkel B. , Grunze M.
Published: July 2002

We have investigated the electron beam exposure of self-assembled monolayers (SAM) of alkanethiols on gold. SAM resist systems are of growing interest for proximal probe lithography due to their high chemical selectivity though being as thin as a monolayer. For this purpose a custom built proximal probe lithography device was used. The electrons emanate from an ultrasharp tungsten tip, providing extraordinary field emission properties—small emission angle and low emission energy. In contrast to common scanning tunneling microscope (STM) lithography our system can be operated not only in tunneling mode, but also in true field emission mode, i.e., at much larger tip-to-sample distances (typically 10–100 nm). Therefore the latter mode allows much higher writing speeds, as the tip does not have to follow the sample topography as accurately as in the STM mode. The first results obtained by exposure in the field emission mode are presented.
Notching as an example of charging in uniform high density plasmas
Authors: Kinoshita Takashi , Hane Masami , McVittie James P.
Published: July 2002

Numerical simulation was used to study both surface charging and ion trajectory distortion during submicron patterning in high density plasma etching. The plasma was assumed uniform and the cause for the surface charging was the directionality difference between ions and electrons. The role of ion transit time effects on the ion energy distribution function was also considered, while the effect of discharging currents such as through insulators was not included. Using a Monte Carlo sheath simulator, a Poisson equation solver, and an ion/electron trajectory simulator, the steady state potential distribution and ion trajectories were calculated for various line-and-space structures and plasma conditions where notching, which is a local sidewall etching, has been observed after the overetching part of polysilicon etching processes. The results show significant positive charging at the bottom of high aspect ratio spaces which depends on the ion energy distribution function. Notching at the bottom of an outermost polysilicon line before a wide space is the result of ion deflection toward the line which has the lower potential from receiving more electrons from a side facing the wide space. The simulator was validated using previously reported electron cyclotron resonance etcher notching results. It was found that the calculated potential difference between the bottom of a space and the adjacent line shows the same qualitative dependence on the wide space width as experimental notch depth results show. In addition, these potential differences are large enough to substantially increase the ion flux at regions where notching occurs for the ion energy distribution function for the plasma conditions used. Thus this shows aspect ratio dependent charging effects which are not dependent on initial plasma nonuniformity.
Physical characterization of two-dimensional doping profiles for process modeling
Authors: Alvis Roger , Luning Scott , Thompson Liliana , Sinclair Robert , Griffin Peter
Published: July 2002

Physical characterization of doping profiles in two dimensions holds great promise for both high quality analysis of specific structures and for general physical model verification. This latter activity enables the calibration of process simulators and could lead to accurate predictive simulation of modern integrated circuit devices. We used both one- and two-dimensional analytical techniques [secondary-ion-mass spectroscopy (SIMS) and transmission electron microscopy (TEM)] to quantitatively characterize implanted and rapid-thermal-annealed dopant profiles at a polysilicon gate edge. The samples were given self-aligned arsenic implants of 1×1015 ions/cm2 at 35 and 120 keV and at 0° and 20° angles of incidence. The implant was followed by a 30 s/1000 °C rapid thermal anneal. SIMS profiles were used to calibrate 1D simulations and the TEM micrographs in the 1D regions far from the mask edge. Quantitative TEM micrographs near the gate edge were then compared with two-dimensional simulations of final doping distributions, and the implications of discrepancies are discussed.
Scanning local-acceleration microscopy
Authors: Burnham N. A. , Kulik A. J. , Gremaud G. , Gallo P.-J. , Oulevey F.
Published: July 2002

By adapting a scanning force microscope to operate at frequencies above the highest tip–sample resonance, the sensitivity of the microscope to materials’ properties is greatly enhanced. The cantilever’s behavior in response to high-frequency excitation from a transducer underneath the sample is fundamentally different than to its low-frequency response. In this article, the motivations, instrumentation, theory, and first results for this technique are described.
Sensor nanofabrication, performance, and conduction mechanisms in scanning thermal microscopy
Authors: Luo K. , Shi Z. , Varesi J. , Majumdar A.
Published: July 2002

A new nanofabrication procedure has been developed for making thermocouple probes for high-resolution scanning thermal microscopy. Thermocouple junctions were placed at the end of SiNx cantilever probe tips and were typically 100–500 nm in diameter. Cantilever bending due to thermal expansion mismatch was minimized for Au–Ni, Au–Pt, and Au–Pd thermocouples, by carefully choosing thermal probe materials, film thicknesses, and deposition conditions. A spatial resolution of 24 nm was demonstrated for thermal microscopy although the noise-equivalent limit of 10 nm was estimated from experimental data. Using thermo-power measurements, a simple model was developed to calculate the tip-sample thermal resistance. Model-based calculations, correlations between topographical and thermal features, as well as experiments in different gaseous and humidity environments indicate that the dominant tip-surface heat conduction is most likely through a liquid film bridging the tip and the sample surface, and not through the surrounding gas, solid-solid point contact, or near-field radiation. Dynamic measurements within a 100 kHz bandwidth showed a time constant of about 0.15±0.02 ms which was attributed to the thermal time constant of the whole cantilever. Calculations suggested the RC electrical time constant and the thermal time constant of the thermocouple junction to be on the order of 10 ns which, however, could not be experimentally probed.
Nanofabrication of electrodes with sub-5 nm spacing for transport experiments on single molecules and metal clusters
Authors: Bezryadin A. , Dekker C.
Published: July 2002

Electron-beam deposition is used to fabricate free-standing carbon nanoelectrodes separated by a gap of less than 5 nm. Fabrication is carried out under direct visual control in an electron microscope. After coating the carbon electrodes with a thin metal film (e.g., AuPd) such structures can be used to study electrical transport properties of single molecules (e.g., conjugated polymers) or metal nanoclusters. The fabrication process of the nanowires is described in detail. Furthermore, we suggest electrostatic trapping as a new method to bridge the electrodes with a single conducting nanoparticle in a controlled way. This principle was tested successfully on Pd nanoclusters and carbon nanotubes.
Resist processes for low-energy electron-beam lithography
Authors: Schock K.-D. , Prins F. E. , Strähle S. , Kern D. P.
Published: July 2002

Low-energy electron-beam lithography processes have been studied, a new resist system has been proposed, and preliminary tests have been performed. The interaction between electrons and e-beam resist and its effect on the exposure dose and the penetration depth of the electrons have been studied as a function of electron energy. A silylation process for low energy e-beam lithography has been tested and applied to a new bilayer resist scheme for low-energy electron-beam exposure.
Nanochannel fabrication for chemical sensors
Authors: Stern Margaret B. , Geis Michael W. , Curtin Jane E.
Published: July 2002

In a novel chemical sensor, the chemical charge coupled device (CCD), electrostatic fields in nanocapillary channels smaller than a Debye length will be used to separate and concentrate ions in solution with a predicted detection limit of <1× 10−13 M. Conventional integrated circuit techniques are used to deposit thin dielectric and amorphous-Si films on a Si substrate and to lithographically define channel and reservoir structures. Hollow Si3N4 nanochannels with heights between 20 and 100 nm, widths between 0.5 and 20 μm, and lengths up to 5 mm have been fabricated by wet chemical etching of a sacrificial amorphous-Si layer in tetramethylammonium hydroxide. Initial modeling of a three-phase chemical CCD predicts the ability to select and concentrate ionic constituents by many orders of magnitude, according to their diffusion coefficients.
Surface roughness of SiO2 from a remote microwave plasma enhanced chemical vapor deposition process
Authors: Rack M. J. , Vasileska D. , Ferry D. K. , Sidorov M.
Published: July 2002

We have investigated the roughness of the top surface of silicon dioxide deposited via a remote plasma enhanced chemical vapor deposition (RPECVD) process in a microwave reactor. We find a roughening transition at a deposition temperature of approximately 250 °C. Above this temperature, the surface is fairly smooth (root mean square roughness ∼0.3 nm). Below this deposition temperature, the oxide surface becomes extremely rough. Rapid thermal annealing at 900 °C does not eliminate this roughness, which is very nonuniform at the nanometer scale. For very thin RPECVD oxide applications, oxide surface roughness could be a limitation. We have used our three-dimensional Poisson solver in order to investigate the effects of oxide surface roughness taken from actual atomic force microscopy measurements on the confining potential within the silicon inversion layer of a metal–oxide–semiconductor (MOS) field effect transistor. In order to assess the quality of our process and system, oxides are characterized electrically with MOS capacitors, and structurally with Fourier transform infrared spectroscopy, high-resolution cross-sectional transmission electron microscopy, and etch rates in HF containing solutions.
Low temperature plasma-promoted chemical vapor deposition of tantalum from tantalum pentabromide for copper metallization
Authors: Chen Xiaomeng , Frisch Harry L. , Kaloyeros Alain E. , Arkles Barry
Published: July 2002

A low temperature plasma-promoted chemical vapor deposition (PPCVD) process has been developed for the growth of tantalum (Ta) from the halide source tantalum pentabromide (TaBr5), using hydrogen as reducing agent, for incorporation in emerging integrated circuitry (IC) copper metallization schemes. Ta films were produced at substrate temperatures of 400–450 °C, reactor working pressures of 0.6–0.7 Torr, hydrogen carrier flow rate of 50 sccm, hydrogen reactant flow rates between 200 and 1200 sccm, and plasma power ranging from 20 to 100 W, corresponding to a power density of 0.11–0.55 W/cm2. The films were subsequently characterized by Auger electron spectroscopy (AES), Rutherford backscattering (RBS), x-ray diffraction (XRD), atomic force microscopy (AFM), four-point resistivity probe, scanning electron microscopy (SEM), and cross-section SEM. These studies indicated that the Ta films thus produced were carbon and oxygen free, contained bromine concentration below 2.5 at. %, and exhibited better than 75% step coverage in nominally 0.6 μm, 2.5:1 aspect ratio, trench structures.
Modeling solvent diffusion in photoresist
Authors: Mack C. A. , Mueller K. E. , Gardiner A. B. , Sagan J. P. , Dammel R. R. , Willson C. G.
Published: July 2002

A semi-empirical study into the effects of residual casting solvent on the lithographic properties of photoresist is described. Solvent content of a commercial i-line photoresist after postapply bake has been measured using a quartz crystal microbalance and using radio-labeled solvent with scintillation counting. Analysis of this data has led to a calibrated model of solvent diffusivity as a function of solvent content which can then predict solvent content as a function of depth into the photoresist for a given bake.
Reactive ion etch of patterned and blanket silver thin films in Cl2/O2 and O2 glow discharges
Authors: Nguyen Phucanh , Zeng Yuxiao , Alford T. L.
Published: July 2002

Silver (Ag) is being investigated as a potential metal for interconnect technology because it has the lowest bulk resistivity of any metal and higher electromigration resistance than aluminum. Silver dry etch is an important process step in the implementation of this metal as an interconnect. This article demonstrates both blanket and patterned etch of Ag films in a reactive ion-etch reactor using Cl2/O2 and O2 glow discharges. X-ray diffraction and scanning electron microscopy were used to analyze the postetch materials. Stress caused by volume expansion upon the formation of Ag2O, AgO, AgCl, and AgClOx, and possibly the sputtering and formation of volatile products, are also believed to induce the etch. The etch mechanism initiated by stress led to roughness and residues on the postetch surface for a blanket etch, and jagged edges for a line etch. It is not suitable to use the above chemistries for a blanket films etch; but for pattern etch, it showed suitable line formation after resist removal and clean. Further study will be needed to improve roughness, uniformity, and edges geometry of the line edges. Corrosion of Ag in chlorine ambient is an essential issue to be considered in future work.
Reduction of long range fogging effect in a high acceleration voltage electron beam mask writing system
Authors: Ogasawara Munehiro , Shimomura Naoharu , Takamatsu Jun , Yoshitake Shusuke , Ooki Kenji , Nakayamada Noriaki , Okabe Humiyuki , Tojo Toru , Takigawa Tadahiro
Published: July 2002

We have developed an antireflecting plate with a novel structure to reduce the long-range fogging effect, which is especially serious in mask writing by a high acceleration voltage electron beam mask writing system. This structure is characterized by an array of holes whose axes converge to the beam irradiation position. These holes can efficiently absorb the electrons scattered at a reticle by avoiding the collision of the electrons with the inner walls. The reflectance of this structure is about 30% of that of the plane structure for 30 keV electrons, whereas that of a parallel hole array structure is about 40%. The fogging effect was evaluated in a high accelerating voltage (50 kV) electron beam mask writing system equipped with this new antireflecting plate, and nm-level global uniformity in pattern size was achieved.