Katsuro Sugawara

Kinetic studies on thermal decomposition of MOVPE sources using fourier transform infrared spectroscopy

A kinetic study was done on the decomposition of source materials used in metalorganic vapor phase epitaxy (MOVPE) such as trimethylgallium, trimethylindium, tertiarybutylarsine, tertiarybutylphosphine and dimethylzinc. The purpose of this study was to construct reaction models with accurate rate constants, which are required for the numerical analysis of MOVPE process and computer assisted process optimization. For the measurements we employed a quartz-tube cracking reactor and Fourier transform infrared spectrometer (FT-IR) as the gas monitoring system. First, the decomposition rate of each source was measured and the ability of the system to determine source gas decomposition rates was validated. Next the effect of substrate surfaces on the decomposition rates and the effect of gas mixing were examined. We observed that surface reaction rates were not negligible in the decomposition of some sources, and that the decomposition rates of group III sources increased when they were mixed with group V sources. The results of this study showed that the effect of substrates and gas mixing need to be properly accounted for in numerical models to accurately simulate epitaxial growth process.

Chemical Vapor Deposition Reactor Design Using Small‐Scale Diagnostic Experiments Combined with Computational Fluid Dynamics Simulations

A numerical method for designing chemical vapor deposition (CVD) reactors is proposed, which consists of three steps: extraction of the chemical mechanism and reaction rates using small‐scale, well‐defined experiments; prototyping a large‐scale reactor using this chemistry combined with computational fluid dynamic (CFD) simulations (we call these virtual experiments) and experimental optimization of a prototype reactor designed from the virtual experiments. This design methodology was validated using the model CVD process of low‐pressure CVD (LPCVD) of tungsten silicide from and in the temperature range of 130–360°C. A tubular hot‐wall reactor was used as the small‐scale reactor for diagnosing the gas‐phase and surface chemical mechanism. The CFD code FLUENT was used for the numerical simulations, and no fitting parameters were used in chemical reaction mechanisms. Simulations of the radial film growth rate and Si/W composition profiles on a 5 in. wafer in a cold‐wall reactor agreed well with experimental measurements. Such comparisons indicate that numerical reactor design can replace currently used empirical design methods.

Erratum: Misfit Dislocations in the Bicrystal System of Silicon–Boron-Doped Silicon

Arrangement of atoms at the interface of a bicrystal system with small lattice parameter differences has been investigated through the observation of the creation of misfit or interfacial dislocations. Specimens were prepared by epitaxial deposition of pure-silicon films onto boron-doped silicon substrates. The conditions required to create misfit dislocations were investigated as a function of the film thickness for both cases of thin and thick substrates. The well-defined critical film thickness was found, and smaller values were found for the thick substrate specimens. Results are interpreted in terms of the van der Merwe theory. Effect of heat treatment upon the misfit dislocation distribution was examined. It was observed that dislocations reacted to form networks in a complicated manner. The dislocation climb rate was negligibly small compared to the diffusion length of boron impurity.

Conformal deposition of WSix films on micron‐sized trenches: The reactivity of film precursors

Tungsten silicide films (WSix) were deposited from WF6 and SiH4 by low pressure chemical vapor deposition (LPCVD) using a tubular reactor system. At the deposition temperature of 150 °C, films having low concentrations of residual fluorine (∼1.5×1021 cm−3) deposited quite conformally on micron‐sized trenches. The sticking probability of the film precursor was determined from the step coverage quality, as a function of deposition temperature ranging from 120 to 390 °C. While the sticking probability remained constant above 270 °C, it changed with an activation energy of 7±2 kcal/mol in the temperature range 120–240 °C. The decreased probability of sticking improved the step coverage quality at low temperatures.

Deposition of WSix Films from Preactivated Mixture of WF6/SiH4

WSix films were formed on a substrate from thermal chemical vapor deposition (CVD) of WF6 and SiH4. Chemical reactions were initiated upstream of the substrate by a preheater, and chemical reactions are radical chain reactions which produce preactivated cursors that are deposited, causing film growth. Because chemical reactions occur upstream of the substrate, film formation occurred even at temperatures as low as 40° C. Compared with films deposited at the same substrate temperature, without preheating, the Si content increased by 50%, the interfacial concentration of residual fluorine decreased by one order of magnitude, and the sticking probability of the precursors on the substrate was the same. The sticking probability was shown to depend solely on the substrate temperature, even for varying degrees of preheating. Deposition on a low-temperature substrate of the preactivated precursors provides a means to deposit conformal WSix films with low interfacial concentration of fluorine.

Existence of extinction temperature in WSix film growth from WF6 and SiH4: An indication of the role played by radical chain reactions

Low pressure chemical vapor deposition (LPCVD) was carried out to deposit tungsten silicide films (WSix) from WF6 and SiH4 in a low temperature range from 80 to 390 °C, using a tubular reactor system. Drastic decrease of the deposition rate occurred at an extinction temperature Tex. Increase of the reactor size in the range from 4 to 22 mmφ decreased Tex from 140 to 80 °C. Above Tex, the sticking probability of the film forming species (η) and the film composition, x of WSix , did not depend on the reactor diameter. Dependence of Tex on the reactor diameter and independence of η and x above Tex from the reactor diameter indicates that a radical chain process dominates CVD‐WSix process to form film forming species.

Experimental and numerical analysis of rapid reaction to initiate the radical chain reactions in WSix CVD

Abstract Gas-phase reactions leading to radical chain reactions of tungsten hexafluoride (WF 6 ) and silane (SiH 4 ) in tungsten silicide (WSix) chemical vapor deposition (CVD) were studied using a hot-wall tubular reactor. To prevent film growth being limited by gas-phase diffusion or by surface reactions, a reactor diameter from 2.4 to 4 mm and pressures from 0.5 to 10 Torr were used. To extend the reaction zone to permit detailed measurement of the axial film growth-rate profile, relatively high axial flow velocities from 6 to 19 m s −1 were used. A two-dimensional numerical simulation was used to improve the accuracy of the analysis. The measured overall reaction rate, r , was independent of pressure and reactor diameter, indicating that it could be expressed by a simple first-order reaction of WF 6 , r = k gr C WF6 , where C WF6 is the WF 6 concentration. An Arrhenius plot of k gr gave an activation energy of 28 kJ mol −1 . This relatively small activation energy confirms that the gas-phase reactions are controlled by radical chain reactions. The experimentally observed behavior that the reaction rate was independent of SiH 4 concentration may suggest that SiH 4 does not participate in the elementary reaction that activates WF 6 . One possibility is that thermal decomposition of WF 6 limits the initiation reaction

Finite Difference Analysis of Radial Phosphorus Dopant Distribution in Czochralski-Grown Silicon Single Crystals

The electrical properties of semiconductor devices are directly related to the dopant concentration in single crystals. Therefore, to make semiconductor devices with controllable electrical properties, it is important to control the dopant concentration accurately, both along the growth axis and in the radial direction. The dopant concentration along the growth direction can be described by the normal freezing equation and is well understood, but the factors controlling the dopant distribution in the radial direction are not well understood. We, therefore, made a detailed, quantitative analysis of the radial dopant distribution in Czochralski silicon crystals by solving the coupled Navier-Stokes, continuity, and energy equations for the silicon melt flow and temperature fields, and by solving the diffusion and segregation equations for the phosphorus distribution in the melt and in the crystal. Good agreement between measured and simulated results of the radial phosphorus concentration in silicon single crystals was obtained. The melt was exposed to a background gas of Ar into which PH 3 was added to counteract evaporation of the phosphorus from the melt. Simulated radial phosphorus concentration distributions compared well with measured radial distributions, and the PH 3 added to the background Ar gas increased the average melt dopant concentration and also improved the radial phosphorus concentration uniformity.

Theoretical Reactor Design from the Simple Tubular Reactor Analysis for Wsix CVD Process

We propose a methodology for systematic reactor design and optimization of operating conditions. The reactor can be designed systematically through the following steps: extraction of the chemistry from small-scale experiments, simulations for the virtual large-sized reactor configurations using the chemistry obtained from small-scale experiments, and final optimization by actual experiments on the real reactor manufactured according to the results of the simulation experiments. We call this technique ECONOMIX (Experiment, COmputer, kNOwledge MIXed CVD Process Design). The validity of this theoretical approach was confirmed for the WSix CVD process. The chemistry of the WSix CVD process was obtained from the tubular hot wall reactor. The rate constants of consecutive reaction were re-determined using two-dimensional simulation with consideration of temperature distribution. The growth rate and composition ratio were simulated for the cold wall reactor using reaction kinetics obtained by tubular hot wall reactor analysis. The predicted growth rates and composition ratios of Si/W showed good agreement with the experimental data. Therefore, the theoretical approach, ECONOMIX, is valid for the design of CVD reactor. We can design reactors and optimize operating conditions efficiently by this theoretical approach using computer simulation coupled with small-scale experiments.

Microdefects in Oxide Films Deposited on Featured Surfaces of VLSI Substrates by Thermal CVD of TEOS and O 2

Silicon dioxide films formed by tetraethylorthosilicate, (TEOS), oxidation have been used for passivation of transistors and integrated circuits. Microdefects tend to occur during film formation on featured surfaces of very large scale integration (VLSI) devices, and this degrades the integrity of the passivation films. To understand the mechanisms of microdefect formation better, we measured the density of microdefects in films formed by TEOS oxidation on featured VLSI substrates. The deposited film thickness was 450 nm and, after deposition, the films were annealed and wet etched. The microdefect density was measured as a function of the film thickness after etching, showing the higher density for thinner film. The formation of microdefects occurred because of the film stress resulting from the large shrinkage that occurred during annealing. This shrinkage is partly due to densification, but we believe another major factor is that shrinkage occurs because of the decomposition of silanol radicals and unreacted intermediate molecules deposited during film formation. By correlating the tendencies of microdefects to form in terms of the solid angle made between the deposition species and the film surface, we also observed that defects tended to occur mostly in small holes and narrow grooves. This is a result of stress‐induced etching in these structures because of the sharp curvature of the films.