Plasma deposition of low-stress electret films for electroacoustic and solar cell applications

We studied electric charge retention and internal stress in low-pressure plasma-deposited silicon-compound films (silicon nitride, oxide, and oxynitrides), prepared from SiH4/NH3/N2O mixtures in a dual-mode microwave/radio frequency (2.45 GHz/13.56 MHz) plasma at low (similar to 30 degrees C) substrate temperature. The internal stress is found to vary from tensile (similar to+150 MPa) to compressive (similar to-100 to -500 MPa) when the bombarding ion energy is increased. Minimum stress values occur for the substrate bias voltage of about -100 V, which also results in a high charge stability on the film surface. Further improvement of the electret properties is achieved by postdeposition annealing, by exposure to organosilicone vapors, and by using oxynitride layers. This is confirmed by the presence of deeper charge traps, as determined by the measurement of thermally stimulated discharge currents, and by chemical stabilization of the film surfaces, as revealed by x-ray photoelectron spectroscopy. The films are considered for novel miniaturized electret microphones and electret-enhanced solar cells. (C) 1996 American Vacuum Society.