Acoustic silicon sensors

Abstract

Future work in transducer engineering will be directed toward the use of silicon technologies for the fabrication of tiny acoustic sensors. For such devices, a number of transduction methods are suitable, for example the condenser, piezoelectric, and piezoresistive principles. Feasible also are microphones based on sound-wave-induced modulation of the drain current in a field-effect transistor, or acoustic sensors depending on the modulation of light propagation in a micromachined waveguide. Experimental silicon sensors according to all these principles have already been built, but their properties are often not yet satisfactory. Most advanced are two-chip condenser microphones with external biasing; electret biasing, and one-chip designs made with sacrificial-layer techniques are also under study. Future work will concern the improvement of the characteristics of all silicon microphones and the integration of amplifiers or signal-processing electronics on the microphone chip. Optical silicon microphones, as described above, will be particularly interesting in the future because of the rapid development of optical communications. Advantages of silicon microphones are small size (membrane areas∼1 mm2), low vibration sensitivity due to small diaphragm thickness (<1 μm), the possibility of integration of sensor and amplifier on a single chip, and potentially low cost due to batch processing.