Chemical vapor deposition (CVD) is a process that is widely used to manufacture thin films for integrated circuits and other components in the electronics industry. Weak concentrations of reactant gases are fed into a reactor, which is typically held at a low operating pressure. The gases diffuse towards a silicon substrate, or wafer, positioned on top of a heated susceptor. The gases react with material on the substrate, creating a thin film of product material that has desirable electrical properties. High-quality films are those with a uniform chemical composition and thickness across the entire substrate area.

CVD is often used in the production of wafers for very large scale integrated circuits, or VLSIs. Researchers at Nihon University and the University of Tokyo recently used FLUENT to simulate this application. In their work, they used a fixed gas inlet stream for the CVD reactor, but varied the number and location of outlet holes in an attempt to optimize the film quality across the wafer. The source gases used were tungsten hexafluoride (WF 6 ) and silane (SiH 4 ). Using a radical chain-reaction scheme that involved over 6 reactions, a film of tungsten silicide (WSi x ) was created on the wafer. The reaction scheme was first validated for an existing reactor. The predicted deposition rate and film composition were in excellent agreement with data, differing by no more than 6%, so the researchers began work on the optimized CVD reactor.

The radial velocity distribution of the gas on the susceptor as a function of the gas outlet position. The most uniform gas velocity distributions occur for side outlets positioned above the susceptor plate.

To assess the quality of the films created for each of the outlet scenarios, several features of the flow-field were examined. The first was the radial velocity distribution of the gas near the susceptor plate. A centered outlet below this plate gave rise to a highly non-uniform profile, which could lead to inconsistent flow conditions where surface reactions take place, and have an adverse impact on the resultant film. Outlet holes on the side walls were found to give a far more consistent radial velocity profile. To determine the best configuration for the outlet holes on the side of the reactor, the deposition rate of WSi x as a function of radial position was examined next. The number of side holes was varied from 1 to 9 (uniformly spaced), and these results were compared to those using an annular slot for the outlet. As expected, as the number of holes increased, the uniformity approached that of the annular slot case. As a final test, the composition of the film as a function of radius was compared for the different outlet hole configurations, and once again, the 9-hole case produced the most uniform result. The final reactor design was characterized by no more than a 3% variation in film thickness across the wafer, supporting the use of FLUENT to optimize the CVD process for the manufacture of VLSIs.

The CVD reactor is shown with some of the important internal components. The cross-sectional view shows the orientation of the components inside the reactor.

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