Electret transducer having an electret of inorganic insulating material

Abstract

An electret transducer in which the electret material is an inorganic insulator sheet with electric charges in it and is juxtaposed to an electrode, and either the sheet or the electrode is mechanically deformable to change the distance between the two, thereby changing the electric charge induced in the electrode by the charged electret sheet. In a preferred form, the electret material comprises a multilayer sheet of insulating materials. The method of forming the transducer includes at least the steps of vapor-depositing a thin layer of inorganic material on a substrate and subjecting the exposed surface of the insulating material to a source of electric charge for a time sufficient to implant charges in the insulating material. If the insulating material is to be deformable, it should be deposited on a thin layer of metal which, in turn, has been deposited on a thicker substrate. A spacer layer of metal is deposited on top of the charged insulating material. Thereafter, selected areas of the substrate and of the spacer layer are chemically removed to leave the thin metal layer and the charged insulating material free of support except around the periphery of such open area. The ability of the insulating material to accept and hold a charge may be improved by depositing the insulating material as two separate layers of different materials, one immediately on top of the other.