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Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures

Issue 5 • Date Sep 1992

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Displaying Results 1 - 25 of 33
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Recent developments in ohmic contacts for III–V compound semiconductors

    Page(s): 2113 - 2132
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    Recent advances in the technology and understanding of ohmic contacts for a variety of III–V compound semiconductor material systems are reviewed. Special attention is focused on factors and critical issues involved in making low resistance and reliable ohmic contacts. The solid‐phase regrowth mechanisms of key metallization systems are described. In addition, special techniques to improve the ohmic contacts are discussed. Finally, the reliability issues of ohmic contacts are addressed. View full abstract»

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  • Microscopic uniformity in plasma etching

    Page(s): 2133 - 2147
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    As we enter the era of ultra‐large‐scale integrated circuit manufacture, plasma etching grows more important for fabricating structures with unprecedented dimensions. For feature sizes below 1 μm and aspect ratios (depth/width) much larger than one, etching rates have been observed to depend on aspect ratio and pattern density. Such dependencies tend to increase the cost of manufacturing because even small changes in device design rules, cell design, or wafer layout can result in time‐consuming, new plasma process development. In addition, microscopically nonuniform etching affects the trade‐off between chips lost from failure to clear and chips lost by damage from overetching. Although aspect ratio and pattern dependent etching have been observed for a large variety of material systems and processing conditions, the fundamental causes underlying these effects are poorly understood. Partly, this results from use of confusing and conflicting nomenclature and a lack of careful, quantitative comparisons between experiment and theory. In this article we review recent literature on microscopic uniformity in plasma etching and carefully define terminology to distinguish between aspect ratio dependent etching (ARDE) and the pattern dependent effect known as microloading. For ARDE, we use dimensional analysis to narrow the range of proposed mechanisms to four which involve ion transport, neutral transport, and surface charging. For microloading, we show that it is formally equivalent to the usual loading effect, where the reactant concentration is depeleted as a result of an excessive substrate load. View full abstract»

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  • Influence of the desorption and growth temperatures on the crystalline quality of molecular‐beam epitaxy InAlAs layers

    Page(s): 2148 - 2152
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    The influence of molecular‐beam epitaxy growth conditions, especially oxide desorption and growth temperature, on the final quality of InAlAs layers grown on (100) InP substrates, has been analyzed by conventional and high‐resolution transmission electron microscopy observations. InP thermal cleaning prior growth at 500 °C has been found to produce rough interfaces and extended defects which propagate up to the surface, while thermal cleaning at 530 °C results in abrupt interfaces and improved epilayer structure. The increase of growth temperature up to 530 °C, also improves the crystalline quality of InAlAs. View full abstract»

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  • Effect of electron cyclotron resonance generated hydrogen plasmas on carbon incorporation and interfacial quality of GaAs and AlGaAs grown by metalorganic molecular‐beam epitaxy

    Page(s): 2153 - 2156
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    The authors have investigated the effect of in situ hydrogen plasmas generated by electron cyclotron resonance on growth rate and carbon incorporation in GaAs and AlGaAs grown by metalorganic molecular‐beam epitaxy using triethylgallium and trimethylamine alane. Carbon backgrounds in GaAs grown at 500 °C were found to increase with increasing microwave power over the range 100–200 W and also with increasing H2 flow at 175 W. Hydrogen plasmas did not significantly increase the growth rate for either GaAs or AlGaAs even at growth temperatures as low as 375 °C. As at higher temperatures, carbon levels were not reduced in GaAs grown at low temperatures though some improvement was observed for AlGaAs grown at 375 °C. Plasmas were found to be much more effective for cleaning substrates prior to growth. At 500 °C, low microwave powers and moderate exposure times were found to produce the lowest levels of interfacial C, O, and Si contamination. View full abstract»

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  • Molecular‐beam epitaxy and migration‐enhanced epitaxy growth modes of GaAs on pseudomorphic Si films grown on GaAs(100) substrates

    Page(s): 2157 - 2162
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    GaAs layers were grown by molecular‐beam epitaxy and by migration‐enhanced epitaxy (MEE) on pseudomorphic Si films grown on GaAs(100) substrates. The Si interlayer thickness (tSi) was varied from 0 to 3 monolayers (ML), and the effect on the GaAs growth mode was investigated by observing the behavior of the intensity of the specular spot of reflection high‐energy electron diffraction patterns. From these measurements it was concluded that the surface migration of Ga atoms is disturbed by the Si atoms on the growing surface. The disturbance increased, at the growth temperature of 520 °C, with increasing the Si interlayer thickness to the point that for tSi≥0.4 ML, the two‐dimensional (2D) growth changed to a three‐dimensional one. By increasing the growth temperature, the growth mode improved but the Si surface segregation increased, as detected by secondary ion mass spectrometry. The effect of the thermally activated Si segregation process on the GaAs growth mode is discussed. Using MEE at a growth temperature of 480 °C, the Si segregation was eliminated and a 2D growth mode was obtained. View full abstract»

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  • Adsorption and reaction of diethylzinc on GaAs(100)

    Page(s): 2163 - 2169
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    The reactions of diethylzinc (DEZ) on the GaAs(100) surface have been studied using temperature programmed desorption, x‐ray photoelectron spectroscopy (XPS), and high‐resolution electron energy‐loss spectroscopy. DEZ dissociated on the GaAs(100) surface below room temperature forming adsorbed zinc atoms and ethyl groups. The adsorbed zinc metal interacted weakly with the GaAs(100) surface and desorbed near 530 K. The ethyl species formed via DEZ dissociation bond to surface Ga atoms. The resulting surface ethyl‐Ga complexes were stable to approximately 500 K at which point they reacted to form ethylene, hydrogen, and smaller amounts of ethyl radicals and ethane. At high surface ethyl coverages a fraction of the ethyl‐Ga species reacted between 300 and 500 K to form gaseous galliumalkyls. XPS results demonstrate that the decomposition of the adsorbed ethyl groups did not result in carbon deposition onto the surface. View full abstract»

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  • New approach to low temperature deposition of high‐quality thin films by electron cyclotron resonance microwave plasmas

    Page(s): 2170 - 2178
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    An electron cyclotron resonance microwave plasma processing system consisting of a plasma chamber and a processing chamber as well as a new apparatus called the ‘‘species selector and energy controller (SSEC)’’ are described in detail. Silicon dioxide (SiO2) films fabricated using this system without and with the SSEC are used for various experiments to demonstrate that the SSEC can effectively suppress the damaging effects resulting from the bombardment on the substrates and the on‐growing films by energetic particles and photons produced in the plasma during film deposition, as well as effectively reduce the upstream diffusion of the reactant gas from the processing chamber to the plasma chamber, thus suppressing the formation of microdust particles due to the heterogeneous gas phase reaction. The film growth for the films fabricated without the SSEC is due mainly to mass‐limited reaction and that with the SSEC is due to surface rate‐limited reaction. The electronic properties of the SiO2 films fabricated with the SSEC at temperatures higher than 250 °C approach those of high‐quality thermally grown silicon oxides. The effects of the SSEC on the film properties and the film growth mechanism are also discussed.     View full abstract»

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  • Monte Carlo‐fluid model of chlorine atom production in Cl2, HCl, and CCl4 radio‐frequency discharges for plasma etching

    Page(s): 2179 - 2187
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    Chlorine chemistries are often used for the radio‐frequency (rf) discharge plasma etching of compound semiconductors, metals, and silicon. A variety of gas mixtures are used as Cl atom donors, many of which have different electron transport coefficients. In this article we computationally investigate 13.56 MHz rf discharges sustained in He/Cl2, He/HCl, and He/CCl4 gas mixtures in the context of plasma etching. The study is performed using a Monte Carlo‐fluid hybrid model of rf discharges. We find that the Cl atom production efficiency is surprisingly similar in these mixtures, while the details of the electron transport (sources of ionization, locations of attachment, electronegativity) dramatically differ. We also find that even at the low pressures of interest (0.25–1 Torr) attachment in He/HCl mixtures is dominated by vibrationally excited HCl, in analogy to high‐pressure discharge devices. View full abstract»

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  • Formation and damage of sidewalls after Cl2/CH4 based reactive ion beam of InP

    Page(s): 2188 - 2191
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    The cause of anisotropic etching of a reactive ion etching (RIE) process for InP/InGaAsP involving a mixture of Cl2, Ar, CH4, and H2 is investigated, applying Auger photoelectron spectroscopy and transmission electron spectroscopy. The extend of sidewall damage is given. An InP/InGaAsP heterostructure quantum wirelike structure is fabricated and the effect on the photoluminescence intensity and wavelength is reported. Successful device fabrication using RIE is demonstrated. View full abstract»

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  • Pattern profile control in magnetron reactive ion etching of poly‐Si

    Page(s): 2192 - 2196
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    The effect of adding SF6 or C2H6 to Cl2 plasma to control the pattern profile in poly‐Si etching has been studied using a magnetron reactive ion etching system. It is found that the sidewalls of patterns etched with pure Cl2 plasma are either sloped or vertical depending on whether a radio‐frequency (rf) power of 700 W (0.9 W/cm2) or 600 W(0.77 W/cm2) is applied. By increasing the content of SF6 the angle of the sidewall taper of the pattern etched at 700 W rf power reaches 90°. Adding C2H6 makes it possible to adjust the slope of the sidewall over a range from 77° to 90° at a rf power of 600 W. These observed results are attributed to plasma polymerization. View full abstract»

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  • In situ passivation of GaAs after BCl3/Cl2 reactive ion etching

    Page(s): 2197 - 2200
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    Reactive ion etching (RIE) of compound semiconductors is well‐known to produce damaged surfaces that reduce quantum yield and produce excessive leakage currents. Therefore, it is desirable to develop procedures to passivate or remove such process‐induced damage. From a manufacturing perspective, it would be ideal to perform such postetch passivation in situ. In this work, we use real‐time photoluminescence monitoring and ex situ x‐ray photoelectron spectroscopy to evaluate the effectiveness of in situ H2 plasma passivation of GaAs after RIE in a BCl3/Cl2 plasma. Because of reactor wall contamination, we conclude that in situ passivation, at least for this chemical etching system, is not practical; two chambers are needed, one for etching and another for passivation. However, RIE damage is removed effectively by first stripping the Cl‐contaminated GaAs surface using an acid solution and then exposing the resultant oxidized surface to the H2 plasma. This procedure results in an elevated quantum yield that remains stable in air indefinitely. View full abstract»

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  • Formation of SiO2/GaAs(100) interfaces by electron stimulated oxidation of ultrathin Si overlayers: Subcutaneous oxidation processes

    Page(s): 2201 - 2210
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    Recent studies of chemical vapor deposition of SiO2 films on GaAs substrates show the existence of subcutaneous oxidation processes which degrade the electrical properties of the interface. To form SiO2/GaAs interfaces this work presents a different approach based on a direct writing technique that consists in the growth of the SiO2 film by electron beam stimulated oxidation of ultrathin Si layers deposited onto the GaAs(100) surfaces. Modeling of the interface chemical evolution is performed by a detailed quantitative analysis of the Auger intensities. Subcutaneous oxidation is found to play an important role for Si films as thin as 1 monolayer (ML), and its kinetics is discussed by comparison with that of the SiO2 growth. However, while a Ga oxide film clearly appears below the Si oxide layer for 1 Si ML deposits, no signs of Ga oxidation are found for Si films 2.5 ML thick, that providing an estimate for discussing the minimum thickness required to grow abrupt SiO2/GaAs interfaces. View full abstract»

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  • Oxidation of silicon in an oxygen plasma generated by a multipolar electron cyclotron resonance source

    Page(s): 2211 - 2216
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    Silicon dioxide films with breakdown field strength ≳12 MV/cm and fixed charge density ∼3×1010 cm-2 were grown using an oxygen plasma generated by an electron cyclotron resonance source at 16 °C. This oxidation temperature is much lower than previously reported plasma oxidation of silicon, while the breakdown field and fixed charge density for these oxides are comparable to thermally grown oxide at high temperature. The oxidation rate was studied as a function of microwave power, pressure, rf power, source to sample distance, and oxidation time. The oxidation rate increases with microwave power but decreases with source distance and rf power. At pressure ≪5 mTorr, the oxidation rate increases rapidly with decreasing pressure. The oxide thickness increases with the oxidation time but the oxidation rate decreases. These oxide films were found to have oxygen to silicon ratio of 2 using x‐ray photoelectron spectroscopy. From the ellipsometry measurements, these films show a refractive index around 1.47. These results indicate that high quality oxide films with properties that are similar to that of thermal oxide can be grown at low temperature. View full abstract»

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  • Enhanced etching of Si(100) by neutral chlorine beams with kinetic energies up to 6 eV

    Page(s): 2217 - 2221
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    Enhanced etching of a room temperature Si(100) surface is observed during exposure to a neutral hyperthermal chlorine beam. Etching is monitored by mass spectrometric detection of silicon chloride products (SiClx) in the scattered flux. The etchant beam is produced by laser vaporization of cryogenic chlorine films; it consists of ≥93% molecular chlorine with a variable kinetic energy distribution depending on laser energy. The sustained etching rate, based on detection of SiCl3+, is independent of energy when the maximum Cl2 kinetic energy is less than 3 eV but increases by a factor of 3.6±1 when the maximum energy is ≊6 eV. No etch products are detected with thermal Cl2. The etching rate of the most energetic chlorine is greater than the thermal chlorine etching rate by a factor of ≥30. View full abstract»

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  • Oxygen magnetically enhanced reactive ion etching of silylated resist patterns

    Page(s): 2222 - 2229
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    One of the critical steps in top‐surface imaging lithographic processes such as diffusion enhanced silylated resist DESIRE is the dry development, i.e., the anisotropic oxygen plasma etching of the nonsilylated resist selective with respect to silylated resist. In this article the etch characteristics of the DESIRE process in a magnetically enhanced reactive ion etching etcher are described. The application of a magnetic field is shown to increase both the etch rate and the etch selectivity. The dependency of etch rates, etch selectivity, linewidth and residue formation on magnetic field, radio‐frequency (rf) power, oxygen pressure, and flow are reported. The expected linewidth variation with varying etch selectivity, due to sloped silicon profiles in the resist, is observed when the selectivity variation is due to varying rf power or oxygen flow. However, it is not observed when the selectivity variation is due to varying magnetic field. An explanation is given. The etch rate of silylated resist is shown to be dependent on the properties of the thin SiO2 top layer formed during oxygen etching, which is dependent both on etching and on silylation conditions. View full abstract»

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  • Latent image diffraction from submicron photoresist gratings

    Page(s): 2230 - 2233
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    Light scattering from latent images in photoresist is useful for lithographic tool characterization, process monitoring, and process control. In particular, closed‐loop control of lithographic processes is critical for high yield, low cost device manufacturing. In this work, we report use of pulsed laser diffraction from photoresist latent images in 0.24 μm pitch distributed feedback laser gratings. Gated detection of pulsed light scattering permits high spatial resolution probing using ultraviolet light without altering the latent image. A correlation between latent image and etched grating diffraction efficiencies is demonstrated and shows the value of ‘‘upstream’’ monitoring.   View full abstract»

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  • Diffraction effects in x‐ray proximity printing

    Page(s): 2234 - 2242
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    The influence of diffraction on the shape and size of features printed using x‐ray proximity printing with a collimated x‐ray source (measured beam divergence of ∼0.2 mrad full width at half‐maximum) at mask to wafer gaps of 25 μm and above is described. Three major conclusions can be drawn from the results: (1) Diffraction can distort the shape of a printed feature, and the systematic shape changes observed in resist images can be explained using simple scaling based on Fresnel diffraction; (2) The linewidth change with exposure dose is independent of feature type and size, and depends only on the square root of the mask to wafer gap; and (3) The bias of each printed feature varies when all of the feature types and sizes are printed at the same dose. Resist images of 2.0–0.25 μm contact holes printed at mask to wafer gaps ranging from 25 to 515 μm are presented. The square contact holes on the mask print diamond shaped at a Fresnel number of 2.5. Isolated lines, spaces, and line‐space arrays ranging from 1.0 to 0.25 μm were printed in thin PMMA (0.1 μm thick) resist at gaps ranging from 25 to 80 μm over a series of exposure doses. A 10% increase in exposure dose results in a 16, 20, and 30 nm change in linewidth at gaps of 25, 40, 80 μm, respectively. When the 1.0–0.25 μm lines, spaces, and line‐space arrays are printed with a single dose at a 40 μm gap, the bias variation among the features appears to be approximately ±20 nm. However, this bias measurement is very sensitive to the metrology technique used, and systematic errors in these absolute measurements may be a significant contributor to the observed bias variation. All the experimental results are compared to calculated aerial images. Exposures in thick PMMA (0.5 μm) show trends similar to those observed in thin resist. However, as the thickness of the resist increases,- - the dose at which the printed linewidth has zero bias decreases, and approaches the dose required to clear a large open area. View full abstract»

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  • Fabrication of ultrahigh quality vertical facets in GaAs using pattern corrected electron beam lithography

    Page(s): 2243 - 2250
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    Extremely high quality vertical structures, suitable for integrated optical components, have been fabricated in GaAs using electron beam lithography and chemically assisted ion beam etching. By using the directly exposed electron beam resist as etch mask all quality degrading intermediate pattern transfers were eliminated, resulting in high resolution and reproducibility. In order to obtain a vertical mask profile and high durability, pattern corrected exposure and heat treatment of the electron beam resist was employed. The pattern correction of the resist exposure was studied using an exposure simulation program. Calculations, based on an estimation of the facet roughness using a scanning electron microscope, suggest optical scattering ratios as low as 0.6% and 7.1% for external and internal reflection of 850 nm light, respectively.   View full abstract»

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  • 20 nm linewidth platinum pattern fabrication using conformal effusive‐source molecular precursor deposition and sidewall lithography

    Page(s): 2251 - 2258
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    In a manner suitable for large‐scale processing, vertical platinum structures having thicknesses (linewidths) down to 20 nm, heights up to 700 nm, an aspect ratio up to 13, and different geometric shapes have been fabricated on top of a silicon dioxide substrate by Pt deposition from the thermal decomposition of a platinum precursor molecule under gas phase collisionless conditions followed by ion‐assisted etching. Scanning electron microscopy analysis shows that the structures have very small grains, high uniformity, and very sharp contours, demonstrating a high degree of conformal deposition. Scanning Auger microscopy reveals platinum only on the structures. X‐ray diffraction and x‐ray photoelectron spectroscopy analysis of the as‐deposited platinum film show that the platinum film is polycrystalline and has no detectable impurity. Scotch tape test shows good bonding of the film. Mass spectrometric measurements suggest that substantial amounts of stable precursor fragments from partial decomposition of the parent precursor desorb from the substrate. The observed desirable film morphology and purity can be attributed to the favorable choice of template material, decomposition mechanism of the precursor, and the chemical beam conditions used. View full abstract»

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  • Use of diffracted light from latent images to improve lithography control

    Page(s): 2259 - 2266
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    As the microelectronics industry strives to achieve smaller device design geometries, control of linewidth, or critical dimension (CD), becomes increasingly important. Currently, CD uniformity is controlled by exposing large numbers of samples for a fixed exposure time which is determined in advance by calibration techniques. This type of control does not accommodate variations in optical properties of the wafers that may occur during manufacturing. In this work, a relationship is demonstrated between the intensity of light diffracted from a latent image consisting of a periodic pattern in the undeveloped photoresist and the amount of energy absorbed by the resist material (the exposure dose). This relationship is used to simulate exposure control of photoresist on surfaces having slight variations in optical properties, representative of those found in operating process lines. We demonstrate that linewidth uniformity of the developed photoresist can be greatly improved when the intensity of diffracted light from the latent image is used to control the exposure dose. Samples include a variety of photoresist materials and substrates with a wide range of optical properties. To verify the experimental observations, diffractions from the latent image grating structures is modeled using rigorous coupled wave analysis. The modeling is used to predict the diffraction from a latent image as a function of the substrate optical properties and the parameters of the latent image (i.e., linewidth, sidewall angle). Good agreement is obtained between theoretical and experimental observations. Conversely, the inverse problem is solved in which the parameters of the diffracting structure (the latent image) are determined from a measurement of the diffracted power. Therefore, the diffracted power can be monitored for the purpose of determining when the latent image will produce the proper CD upon development.   View full abstract»

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  • Simulation of the effect of thin film microstructure on current and temperature distributions in very large scale integrated metallization structures

    Page(s): 2267 - 2276
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    A two‐dimensional finite element program is used to solve current and temperature distributions in inhomogeneous aluminum and tungsten metal lines deposited over very large scale integrated topography. Microstructure depictions of both aluminum and tungsten lines are obtained using the process simulator simbad. Density information is then used to calculate conductivity profiles as input to the current and temperature solver simcat. Experimental results are used to calculate and correct for the cooling in the third dimension of the simulation and results are presented regarding the effect of substrate and passivation thickness on line heating. Two examples of the effect of topography and microstructure on the flow of current and heating within metal lines are presented. Aluminum and tungsten lines are deposited over a series of trenches of varying sidewall angle, and the line temperature and maximum current stress are plotted as a function of this angle. The lower density of the tungsten deposited on the sidewalls of the trench is determined to have a significant effect on the operating temperature of the metal line. As a second example the current distribution is compared between two aluminum lines deposited at different substrate temperatures over a 1 μm trench with vertical sides. View full abstract»

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  • Chemical vapor deposition tungsten via plug process development with polyimide interlevel dielectric in a multilevel metal system

    Page(s): 2277 - 2283
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    Both blanket and selective chemical vapor deposition (CVD) W via plug processes have been evaluated in a multilevel metallization system using a low (≤3.0) dielectric constant fluorinated polyimide as the interlevel dielectric (ILD) material. In the blanket CVD W deposition and etchback process, the sputter deposition conditions for the Ti–W ‘‘glue’’ layer prior to the CVD W deposition were found crucial to the success of the process. An in situ high temperature sputter etch and prebake eliminated a metal–polyimide ‘‘bubbling’’ problem. A Ti–W film with moderately tensile stress is required to prevent polyimide from ‘‘wrinkling’’ and ‘‘cracking’’ during the subsequent CVD W deposition step. In the selective W process, excellent selectivity resulted with polyimide ILD compared to silicon dioxide; however, tungsten growth ‘‘inhibition’’ on Ti–W or W metal in the polyimide via was experienced and was difficult to overcome.   View full abstract»

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  • Epitaxial CoSi2 formation on Si(001) from an amorphous Co75W25 sputtered layer

    Page(s): 2284 - 2292
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    The formation of epitaxial CoSi2 on (001) Si using the solid state reaction between an amorphous Co75W25 sputtered layer and Si has been studied. Auger electron spectroscopy depth profiling, Rutherford backscattering spectrometry, x‐ray diffraction, transmission electron microscopy, and resistivity and mobility measurements on Van der Pauw structures have been used to investigate the interaction between the amorphous alloy and Si. By employing anneals in vacuum between 500 and 600 °C for 60 or 120 min, Co diffuses out of the alloy into the substrate to form CoSi2. X‐ray diffraction measurements indicate that the greater part (about 75%) of the disilicide film is epitaxial. The CoSi2 film is unstrained at the growth temperature. At room temperature a tetragonal distortion of the silicide lattice is noted, which results from cooling from the growth temperature, and is caused by the difference in the thermal expansion coefficients of Si and CoSi2. After a selective etch to remove the remaining amorphous alloy, a second anneal at a higher temperature has been performed to improve the quality of the silicide. As a result of the second anneal, the Rutherford backscattering channeling minimum yield and the residual resistivity of the film decrease to values of about 25% and 2.6 μΩ cm, respectively. Only the epitaxial (A type) orientation of CoSi2 could be detected after the second anneal by x‐ray pole figure measurements. Both the vertical and the lateral distortions in the silicide lattice, measured at room temperature, appear to decrease during this anneal. The Debye temperature, obtained from electrical measurements, increases from 382 K, after Co outdiffusion, to 412 K after the second anneal.   View full abstract»

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  • Application of reflection electron microscopy in cross‐sectional study of multilayer semiconductor devices

    Page(s): 2293 - 2296
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    Reflection electron microscopy (REM) has been applied, for the first time, to study the cross‐sectional structures of multilayer semiconductor devices. An example is given of a GaAs/AlxGa1-xAs multiple quantum wells infrared detector. The technique was used during the device fabrication process as an in‐line monitoring method to provide a direct and precise dimensional measurement of a fabricated mesa so as to reach an optimum fabrication condition. The REM results are compared with those deduced from electrical measurements of the fabricated devices and a complete consistency is obtained. View full abstract»

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Aims & Scope

The Journal of Vacuum Science and Technology B is devoted to reports of original research, review articles, and Critical Review articles.

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Editor
Gary E. McGuire
International Technology Center