SILICON MICROPHONES

New work in transducer engineering is directed toward the use of silic on technologies for the fabrication of tiny microphones. For such devi ces, a number of conventional transduction methods are suitable, such as the condenser, piezoelectric, and piezoresistive principles. Under study an: also microphones based on sound-wave-induced modulation of t he drain current in a field-effect transistor or acoustic sensors, dep ending on the modulation of light propagation in a micromachined waveg uide. Experimental silicon sensors according to all these principles h ave recently been built, with some of them showing already good acoust ic properties. Most advanced are two-chip condenser microphones with e xternal biasing. Electret biasing and one-chip designs made with sacri ficial-layer techniques are also under study. Future work will be conc erned with the improvement of the characteristics of all silicon micro phones and the integration of amplifiers or signal-processing electron ics on the microphone chip. Advantages of silicon microphones are thei r small size (membrane areas similar to 1 mm(2)), low vibration sensit ity due to small diaphragm thickness (<1 mu m), the possibility of int egrating sensor and amplifier on a single chip, and potentially low co st due to batch processing.