A micro-machined capacitive electret microphone

Silicon-based electret materials are presented for the application in micro-machined sensors. Various dielectric layer systems of silicon dioxide and silicon nitride are investigated and compared to the well known single layer silicon dioxide. The superior material system silicon dioxide/silicon nitride is optimised in terms of charge stability, mechanical behaviour and processing capabilities. Only the advantageous combination of low stress and good electret properties allows the realisation of electret membranes for silicon microphones Electret membranes were realized by means of an anisotropic etch process. Various measurements of charge stability were performed and evaluated. For the first time the application of an inorganic electret membrane in a micro-machined capacitive silicon microphone is presented and results are shown. An equivalent noise level of 32 dB(A) could be achieved for an electret microphone with a 4 mm(2) membrane area.