Question

Chemical Vapor Deposition (CVD) of a Silicon Thin Film onto a Wafer: Solid, nonporous silicon thin films can be laid down onto wafer surfaces by the following chemical vapor deposition ( CVD) process: SiH 4 (g) ? Si (s) + 2 H 2 (g) .

It is desired to deposit a very thin film of Si on a wafer surface. Usually, CVD processes are carried out at very low system pressure and high temperature using a reactor like the one shown in Figure 1 -1 . Often CVD reactors are designed to provide a quiescent (still) gas phase over the developing film, so that a uniform thin film is laid down. Consequently, the molecular diffusion of the chemical vapor SiH 4 to the surface of the growing Si film is often the rat e-limiting process.

(a) Propose differential material balances in terms of flux and concentration gradient. Propose reasonable boundary/initial conditions. Use the term " Delta" to describe the diffusion path length between surface of the wafer and inlet gas diffuser. Since the film is very thin, you may assume that"Delta" does not change much as the Si film thickness increases.

(b) Is this process modeled as steady state or pseudo -steady state?

(c) Develop an equation to predict the flux of SiH 4 onto the surface of the wafer as a function of the bulk phase mole fraction of SiH 4 vapor,"Delta" , the diffusion coefficient for SiH 4 vapor in H 2 gas, and the total molar concentration of all gas-phase species present.

(d) Determine the diffusion path length in the diffusion-limited CVD reactor necessary to set the Si film formation rate to 0.5 micro m/min at a temperature of 900 degrees celcius, 0.005 bar total system pressure, and mole ratio of H 2 of SiH 4 of 99:1 in the feed gas stream. The density of solid silicon is 2.32 g/cm 3 . Use the Fuller -Schettler -Giddings correlation to estimate the binary gas-phase diffusion coefficient of SiH 4 in H 2 , using 32 as the "atomic volume" for the Si contribution to SiH 4 .

Answer 1