Silicon micromachining has successfully been applied to fabricate piezoelectric, piezoresistive and capacitive microphones. The use of silicon has allowed the fabrication of microphones with integrated electronic circuitry and the development of the new FET microphone. The introduction of lithographic techniques has resulted in microphones with very small (1 mm') diaphragms and with specially shaped backplates. The application of corrugated diaphragms seems a promising future development for silicon microphones. It is concluded from a noise consideration that the FET microphone shows a high noise level, which is mainly due to the small sensor capacitance. From this noise consideration, it can be shown that integration of a capacitive microphone and a preamplifier wig result in a further reduction of the noise.

A new condenser microphone design, which can be fabricated using the sacrificial layer technique, is proposed and tested. The microphone backplate is a 1 pm PECVD silicon nitride film with a high density of acoustic holes (120-525 holes/mm2), covered with a thin Ti/Au electrode. Microphones with a flat frequency response between 100 Hz and 14 kHz and a sensitivity of typically 1-2 mV/Pa have been fabricated in a reproducible way. These sensitivities can be achieved using a relatively low bias voltage of 6-16 V. The measured sensitivities and bandwidths are comparable to those of other silicon microphones with highly perforated backplates. The major advantage of the new microphone design is that it can be fabricated on a single wafer so that no bonding techniques are required.

Several models concemmg the senslhvlty of capaclhve pressure sensors have been presented m the past Modellmg of condenser nucrophones, which can be constdered to be a specml type of capacltlve pressure sensor, usually reqmres a more comphcated analysis of the senstttvlly, because they have a strong electnc field m the atr gap It IS found that tbe mechamcal sensltlvlty of condenser rmcrophones ~nth a ctrcular diaphragm, either mth a large uutlal tenston or without any mltml tensloo, Increases mth mcreasmg bias voltage (and the correspondmg static deflection), whereas the mechamcal sensltrvlty of other capacltrve pressure sensors does not depend on the static deflection It is also found that the mechamcal senslttv@ increases wtth mcreasmg input capacitance of a preamphfier In addltlon, the open-clrcmt electrical sensltlvlty and, consequently, the total sensltlvlty too, also Increases mth mcreasmg bras voltage (or stahc deflechon) However, the maxtmum allowable sound pressure at which the diaphragm collapses, an effect that has to be taken into account, decreases Hrlth mcreasmg static deflection in most cases, uhnnately resulting in an optimum value for the bias voltage The model for microphones with a nrcular highly tensloned diaphragm has been venfied successfully for two mlcrophone types *Present address Texas Instruments

\ AbstractIn this paper a study of the noise performance of electret microphone systems as a part of hearing aids is presented. The signalto-noise ratio of the microphone-preamplifier combination, containing a field-effect transistor (FET) and a high value resistive bias element in a hybrid configuration, is mainly determined by the noise generated in the preamplifier circuit.A theoretical analysis of the noise sources in a source follower is given. The dominating noise sources are the channel noise of the FET, the thermal noise of the gate bias element, and finally the noise due to the gate leakage current of the FET and its package. It is shown that for the systems investigated, the noise performance does not depend on the choice of the amplifying device (JFET or MOSFET) itself, but only on its packages. Besides this, it is found that it is necessary to keep the parasitic capacitances as small as possible and to make the resistance of the bias element as large as possible.

Miniaturization of sensors and actuators has become a n important topic during the last decade. For silicon condenser microphones, narrowing the air gap as a part of the miniaturization process causes, however, a reduction of the bandwidth of the microphone. By introducing an actuator electrode on the diaphragm for electro-mechanical feedback, it is possible to increase the bandwidth of condenser microphones with a narrow air gap. In this paper a feedback system is presented, which uses an amplitudemodulated actuator signal with a carrier frequency far above the audio-frequency range. With feedback, the bandwidth of the microphone is increased by at least a factor IO.