CMOS-compatible electrets for application in micromechanical transducers

Double layer membranes of thermally grown SiO₂ and chemically deposited Si₃N₄ as well as perforated backelectrodes covered with the double layer and double layers of the same composition on a whole wafer were investigated in terms of chargeability and charge stability. Although the processing of the membranes and the backelectrodes slightly impaired the charge storage, all of the samples showed high charge retention at room temperature, at 300°C and under elevated humidity of 95% at 60°C. The perforation of the backelectrodes reduced the chargeability and accelerated the penetration of moisture into the electret layer, resulting in an increased charge decay under high humidity. The KOH etching step, needed for the fabrication of freestanding membranes, led to a faster voltage drop at 300°C