Sometimes there are restrictions on the temperature that the substrate can be exposed to when depositing a film. APCVD or LPCVD films can be deposited at these lower temperatures, but the deposition rate is usually quite low because Ks decreases exponentially with temperature.

A method developed for these needs is Plasma-Enhanced Chemical Vapor Deposition, or PECVD. In addition to using a thermal source to provide the energy needed for the chemical reactions to occur during the deposition, a plasma source is used. Non-thermal energy to enhance processes at lower temperatures.

Plasma consists of electrons, ionized molecules, neutral molecules, neutral and ionized fragments of broken-up molecules, excited molecules and free radicals.

Free radicals are electrically neutral species that have complete bonding and are extremely reactive. (e.g. SiO, SiH3, F)

The net result from the fragmentation, the free radicals, and the ion bombardment is that the surface processes and deposition occur at much lower temperatures than in non-plasma systems.

In plasma-enhanced deposition, the wafers are placed on the lower plate electrode. Heat can be supplied to the wafers through a heater beneath the electrode.

The reactant gases, such as silane and oxygen for the deposition of silicon dioxide, are fed through inlets.

A glow discharge, or plasma, is sustained between the lower and upper electrodes. This is formed by applying a high electric field, often at 13.56 MHz, to a low pressure gas (between 50 mtorr and 5 torr), creating ions and free electrons.

The plasma is sustained when high-energy electrons strike and ionize atoms and molecules. In the plasma, interactions with the high-energy electrons cause the reactant gases to dissociate and ionize into a variety of species.

By supplying additional energy from the plasma to the reactant gases, the reactions needed for deposition can occur at temperatures much lower than those needed when only thermal energy is provided.

In fact, room temperature depositions are even possible with this method. Usually, though, PECVD depositions of dielectrics are done in the $200-350^0 C$ range.