7.4.4 Plasma-enhanced CVD (PECVD)
In PECVD a plasma is initiated in the CVD chamber, usually by supplying an RF voltage to the platen on which the wafer sits – the chamber geometry is similar to a reactive ion etch chamber. Ions are accelerated from this plasma onto the wafer surface, so that the CVD reaction is initiated not only by heating the wafer, but also by the energy imparted as the ions land. This allows high-quality film deposition at much lower wafer temperatures and higher deposition rates than unenhanced CVD, with the added advantages of film density and structure that come with ion bombardment during deposition. If performed in a reactive atmosphere, the presence of reactive radicals can ensure complete reaction of the growing film, so PECVD has found many applications in growing relatively high-quality oxide material. PECVD has the added feature that a film can be encouraged to grow from the base of a hole, and to ignore its walls, making it the only technique we have seen that gives active control of step coverage.