Microelectronic devices are made by depositing many layers of thin films onto silicon wafers. For example, thin films of arsenic for silicon are deposited by chemical vapour deposition $($CVD$)$ of arsine $($AsH$3)$ vapour onto the surface of a silicon wafer. The chemical reaction for this CVD process is$,$

AsH$3($g$)->$ As$($s$)+3/2$ H$2($g$)$

The process operates at a pressure of $300$ Pa and a temperature of $600$ oC$.$ In many CVD reactors, the gas phase over the thin film is not mixed. Furthermore, at $600$ oC the surface reaction is very fast. Consequently, the molecular diffusion of AsH$3$ vapour to the surface of the wafer can be assumed to control the rate of As$($s$)$ formation.

A mixture of arsine and hydrogen gas continuously flows into the reactor at a molar ratio of $4$:$1$ H$2$:AsH$3.$ A diffuser $6$ cm above the wafer provides a quiescent gas space over the growing As$($s$)$ film.