Chapter 5.5 - Plasma-Enhanced Chemical Vapor Deposition

Abstract

The chemical activation in plasma-enhanced chemical vapor deposition (PECVD) is achieved by supplying electrical power to a gas at reduced pressure, typically between 10 mtorr and 10 torr. At these pressures, the application of a sufficiently high voltage creates a visible glow, called glow discharge plasma. The plasma consists of about equal concentrations of ions and electrons. The visible glow is caused by charge recombination processes and the relaxation of electronically excited atoms and molecules. The electrical power is coupled into the gas through the mediation of plasma electrons. The energetic electrons in the plasma ionize the gas, if only to a minor extent—about one part per million. A much larger fraction of the gas, about 1 percent, is chemically activated by the electrons. The increased chemical activity of the gas results primarily from dissociation of the molecules into smaller species, called radicals. Radicals are chemically unsaturated and therefore capable of chemical reactions at high rates. Gas flow velocities in most PECVD systems are so low that neutral radicals created in the plasma find their way to the surface of the film by diffusion.