Fabrication of a piston-type condenser microphone with structured polysilicon diaphragm

This work focuses on the fabrication of a piston-type silicon condenser microphone. The proposed device is basically composed of a movable polysilicon diaphragm and a fixed gold backplate, making up the condenser electrodes. The movable electrode is realized by a rigid plate supported by small flexible springs. In forder to manufacture plates which are rigid and light at the same time, polysilicon membranes stiffened by vertical ribs are investigated. The fabrication technology developed for this purpose is a variation of the HEXSIL process. Both the piston and the springs are manufactured through the same polysilicon deposition, thus providing a simple process for the fabrication of the structured diaphragm. The fixed electrode is fabricated on the same chip by electroplating a thick layer of gold. The resulting process employs a combination of bulk and surface micromachining techniques. In this paper the process architecture used in the fabrication of the first prototype is presented and some related technological problems are discussed as well. Preliminary results on the functional characterization of the microphones are finally reported.