An improved method of reducing the level of contaminants (e.g., fluorine) absorbed in films deposited within a substrate processing chamber. A seasoning layer is deposited within the substrate processing chamber to cover contaminants that may be absorbed within walls or insulation areas of the chamber interior. The deposited seasoning layer is more stable than prior art seasoning layers and is thus less likely to release the absorbed contaminants into the substrate processing chamber during the subsequent deposition of films. In a preferred embodiment, the seasoning layer is formed from a mixed frequency PECVD process in which the low frequency RF signal is supplied at a high power level to increase ion bombardment and enhance film stability. The increased bombardment favors the formation of stable SiF bonds between silicon and fluorine atoms in the lattice structure of the film rather than unstable SiF.sub.2 or other bonds. When residual fluorine atoms (e.g., fluorine atoms absorbed within the chamber walls) are incorporated into the deposited seasoning layer, fewer loosely bonded fluorine atoms are incorporated into the layer than in prior art silicon oxide seasoning layers. Fewer loosely bonded fluorine atoms in the seasoning film results in fewer contaminants being incorporated into films deposited over substrates in subsequent processing steps.