Electrical and structural studies of plasma-polymerized fluorocarbon films

Plasma-polymerized fluorocarbon films up to 8 μm in thickness were prepared by high-frequency glow discharge deposition, the purpose being to investigate the materials´ charge storage (electret) properties. Film compositions were characterized by infrared and by X-ray photoelectron spectroscopies. Under `mild´ plasma conditions, materials with high fluorine concentration (F/C<1.9) could be obtained, while films with lower F/C were found to be partially oxidized. Besides surface potential decay rate of corona charged specimens, other electrical measurements performed include those of complex permittivity and of thermally stimulated depolarization. The latter revealed relaxations (designated γ and β) at -80°C and +50°C, attributed to local motions of chain segments and to rotation of carbonyl dipoles, respectively. While fluorine-rich films have low dielectric loss and relatively good charge stability (characteristic charge decay time τ>1700 h), a systematic decrease in τ and increase in tanδ could be observed for specimens with decreasing F/C ratio