Two MOEMS Microphone Designs for Acoustic Sensing Applications
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Two MOEMS Microphone designs for acoustic sensing applications Berit S. Avset, Kari A. H. Bakke, Britta. G. Fismen, Ib-Rune Johansen, Henrik Rogne, Håkon Sagberg*, Dag T. Wang SINTEF ICT, Forskningsveien 1, NO-0314 Oslo, Norway *also at: Department of Physics, University of Oslo, P.O.Box 1048, NO-0371 Oslo, Norway ABSTRACT Two different micro-optical microphones are presented. The first is based on interferometric read-out of a deflecting membrane, based on the Fabry-Perot principle. The sensing device is produced in Silicon by combining standard CMOS and MEMS processing. This ensures that high accuracy alignment and low cost production can be obtained. Optical sensor elements with a ring structure are included on one of the Fabry-Perot surfaces whereas the other surface is an optically transparent membrane. Light from an uncollimated light source such as a semiconductor laser or a LED, will form a ring pattern when transmitted through the membrane. The other microphone principle presented is based on a diffractive lens. The structure has a metallic ring pattern separated from a reflecting membrane with an air gap. When the air gap is an odd number of quarter wavelengths of the impinging light, these surfaces form a binary phase diffractive lens. By placing a light source and a detector in the focal plane of the lens, the measured intensity will be highly sensitive to the position of the membrane. Similarly to the first microphone this device can be mass produced at low cost using micromachining techniques. Results from prototype devices will be presented, proving both principles and showing excellent properties compared to expensive commercially available condenser microphones. INTRODUCTION In the past decade there has been an increasing interest and effort to utilize silicon MEMS techniques to miniaturize sensors and thereby improve their performance and expand their application areas. One example is the development of optical microphones. Traditionally displacement sensors such as microphones have been based on capacitor structures and impedance measurements. This has a number of disadvantages related to high voltage biasing, isolation between layers, alignment and positioning of membrane relative to back electrode, high requirements to preamplifiers, and non-linear response, all resulting in costly and complicated solutions. Optical displacement sensors are able to solve many of the major problems of capacitive sensors. There are no problems with high voltage biasing or need of electrical isolation. Interferometric sensors are able to achieve equal or better sensitivity than capacitive displacement sensors with less demand on electronics, but these solutions have been relatively expensive due to problems with alignment and positioning. Optical displacement sensors are described in a number of publications such as [1-3]. Several different types of optical microphones are described, and among these, solutions based on Fabry-Perot interferometers.
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The challenge of Fabry-Perot interferometers is
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