PLASMA DEPOSITION OF LOW-STRESS ELECTRET FILMS FOR ELECTROACOUSTIC AND SOLAR-CELL APPLICATIONS

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