Stabilization of positive charge in SiO/sub 2//Si/sub 3/N/sub 4/ electrets

The stability of charge storage in electrets is a key parameter for their use in practical devices. For this purpose a new charging procedure for SiO₂/Si₃N₄ thin film electret on a silicon carrier has been developed. It combines a heat-treatment at 450degC with a hexamethyldisilazane treatment before charging of the electret. The retention of a positive charge and the stability of its spatial trapping are investigated. The optimized technological parameters provide a lifetime of the charge on open air of about 200 years. No spatial charge retrapping has been observed after fabrication of the electret. A set of experiments has been carried out to characterize the properties of the charged electrets and to stabilize the charge retention using heat treatment at temperatures up to 450degC after charging, which is important for their packaging at elevated temperatures