Simulation of Mass Transport for Deposition in Via Holes and Trenches

The results of Monte Carlo computer simulations of near‐surface mass transport of the gas‐phase species within three dimensional structures, such as via holes and trenches on the wafer surface, in the context of low‐pressure chemical vapor deposition (LPCVD) are presented. Issues treated include: ( i ) step coverage of deposited films, ( ii ) the relative importance of surface mobility vs. re‐emission of adsorbed reaction precursors at the surface in achieving conformal coverage in a chemical vapor deposition (CVD) process, and ( iii ) the effective pressure of byproduct within via holes during selective CVD of tungsten. Unlike previous efforts which neglect re‐emission in favor of surface diffusion, or assume Knudsen diffusion for mass transport in vias and trenches, our approach allows simultaneous consideration of direct and re‐emitted transport. Surface diffusion is found to be unnecessary in explaining commonly observed step coverage of deposited films, whether with uniform coverage or with re‐entrant angle. In contrast, near surface precursor transport by re‐emission, as characterized by reaction probability or sticking coefficients less than 1, plays an important role in controlling step coverage in these processes. An increase in the sticking coefficient is the origin of the degradation of step coverage.