The combination of a short residence time and confinement of high temperatures to the region near the wafer makes showerhead reactors appropriate for use in systems where high rate gas phase reactions are important. These are very similar assumptions to those underlying our plug flow treatment of cylindrical ducts. The radial velocity is just what we derived at the beginning of this section, by conserving total gas volume. The vertical velocity is linear in height, the radial velocity similarly linear in radius. Hc, the velocity is dominated by the radial component.
Diffusion is always necessary for deposition.
Since the time in the stream is only dependent on the height, the time for gas phase reactions is independent of the radial position. Since viscosity is the diffusion of momentum, and our overall analysis showed that diffusion dominates convection in the vertical direction, it seems likely that we can't. Let's take a look. Just as we had derived in general from our overall analysis, convection plays little role in vertical transport of mass and momentum. The stagnation flow solution is not relevant to the chamber.