Abstract
We present the fabrication and characterisation of a low-cost, miniaturised, yet highly sensitive microphone, consisting of a nitride membrane and a diffractive Fresnel lens micromachined on the same chip. The membrane and the reflective Fresnel lens form an interferometer, and the displacement is read by measuring light intensity at the lens focal point. A micromachining process was developed in order to control accurately the distance between the membrane at its idle position and the Fresnel lens, so that the microphone can be operated without online self calibration. The microphone was primarily developed for high-end audio applications, but work to extend its use to photo acoustic detection of gas is under progress.