Silicon Condenser Microphones

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 [1], 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 [2]. They were implemented as two-chip sensors, consisting of a membrane chip and a backplate chip [3] and later as single-chip sensors [4]. 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 [5]. 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.

Literature

  • [1] G. M. Sessler, J. Audio Engineer. Soc. 44, 16-22 (1996).
  • [2] D. Hohm and G. M. Sessler, in Proc. of the 11th Int. Congress on Accoustics (Paris, 1983), Vol. 6, pp. 29–32.
  • [3] D. Hohm, in Fortschritte der Akustik – DAGA 1985, pp. 847-850; D. Hohm and G. Hess, J. Acoust. Soc. Am. 85, 476-480 (1989).
  • [4] P. R. Scheeper, A. G. H. van der Donk, W. Olthuis, and P. Bergveld, J. Microelectromech. Systems 1, 147-154 (1992).
  • [5] C. Thielemann and G. M. Sessler, Acustica–acta acustica 83, 715–720 (1997).
  • [6] R. Kressmann, M. Klaiber, and G. Hess, Sensors and Actuators A 100, 301-309 (2002).
  • [7] M. Füldner, A. Dehé, and R. Lerch, IEEE Sensors Journal 5, 857-863 (2005).