Deposition is limited by the reaction rate at the surface of the wafer (in epitaxy there are large flows, thus there is no limitation of mass transport)
Etching takes place (in parralel with the film growth) due to the presence of HCl
Si + 2HCl = SiCl2 + H2, the total reaction rate can be written as a function of partial pressures: R=k1pSiCl2-k2pHCl
The end product is a crystal → the surface of a film will look as below with terrace and kinks and steps (terrace-kink surface)
in order to a have a perfect crystal, the terrace needs to be completed entirely, before a new one is grown
in order for adsorbed particles to find all the vacancies, it is imperative for them to have sufficient energy for surface diffusion (high temperature is needed)
if too many molecules arrive on the surface and there is no time for diffusion → agglomerates are created
Temperature influence on deposition rate: same as in the CVD case
at low temperatures deposition rate ~ e-E/kT
at high temperatures deposition rate becomes independent of temperature, because the reaction rate is so high, that the deposition depends only on the mass transport to the surface
Substrate preparation
