Silicon condenser microphones
Silicon microphones are made on silicon wafers with the methods of micromachining. These methods are borrowed from microelectronics and consist of lithography-, doping-, deposition- and etching processes. A large number of microphones with very reproducible properties can be produced on a single wafer, as Fig. 1 shows.
While several microphone types, such as piezoelectric or FET-modulating transducers, may be implemented in silicon , the most successful variety is the capacitive silicon microphone. This consists of a membrane separated by an air gap from a rigid back electrode. If the system is polarized with a small dc-bias, excitation of membrane vibrations by a sound wave generates an electric output signal proportional to the sound pressure.
Such silicon condenser microphones were first proposed in 1983 . They were implemented as two-chip sensors, consisting of a membrane chip and a backplate chip  and later as single-chip sensors . The latter are made with a sacrificial-layer technology where the air gap is obtained by removing an oxide layer originally deposited between membrane and back plate by an etching process. A typical single-chip microphone is shown in Fig. 2 . In recent years, several variations of such microphones have been discussed [6,7]
Typical silicon microphones have membrane areas of about 1 mm2, membrane thicknesses of 0.2 to 0.4 µm, resonance frequencies in the near ultrasonic range and sensitivities of approximately 10 mV/Pa. They are furthermore shock resistant, insensitive to vibration, and may be operated permanently at temperatures up to 100°C and up to 260°C for short periods. Thus, they can be produced as SMD devices for printed circuit boards. Silicon microphones are now finding applications in mobile phones, notebooks, PDAs, digital cameras, MP3-players etc and are already made in quantities of 300 million annually.
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