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.

Silicon nitride corrugated diaphragms of 2 mm x 2 mm x l p m have been fabricated with 8 circular corrugations, having depths of 4 1 0 , or 14 pm. The diaphragms with 4-pm-deep corrugations show a measured mechanical sensitivity (increase in the deflection over the increase in the applied pressure) which is 25 times larger than the mechanical sensitivity of flat diaphragms of equal size and thickness. Since this gain in sensitivity is due to reduction of the initial stress, the sensitivity can only increase in the case of diaphragms with initial stress. A simple analytical model has been proposed that takes the influence of initial tensile stress into account. The model predicts that the presence of corrugations increases the sensitivity of the diaphragms, because the initial diaphragm stress is reduced. The model also predicts that for corrugations with a larger depth the sensitivity decreases, because the bending stiffness of the corrugations then becomes dominant. These predictions have been confirmed by experiments. The application of corrugated diaphragms offers the possibility to control the sensitivity of thin diaphragms by geometrical parameters, thus eliminating the effect of variations in the initial stress, due to variations in the diaphragm deposition process andlor the influence of temperature changes and packaging stress.

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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.

A new piezoelectric silicon accelerometer design is presented. The design allows sensing in three directions. The accelerometer is designed to have a detection level of and a resonance frequency of 10 kHz.

Zinc oxide based piezoelectric accelerometers were fabricated by bulk micromachining. A high yield was obtained in a relatively simple process sequence. For two electrode configurations a direction selectivity better than 100 was obtained for acceleration in the vertical direction and a selectivity of 3 for acceleration in the horizontal direction, respectively. The charge sensitivity in the vertical direction is better than 0.1 pC g −1 and a noise level of 1 × 10 −4 g Hz −1/2 was estimated.

The performance of a smgle-wafer fabrxated &con condenser mxrophone has been unproved by mcreasmg the stress and the acoustic hole densrty of the backplate and by decreasmg the rllaphragm tluckness The best muxophones show a sensitrvlty of 5 0 mV Pa-', which corresponds to an open-cmznt sensrtmty of 10 mV Pa-' for a nucrophone capantance of 6 6 pF The measured frequency response IS flat mtbm &-2 dB from 100 Hz to 14 kHz, wluch IS better than the reqmrements for a hearmg-ald rmcrophone. The operahng voltage of these microphones IS only 5 0 V, which IS about 60% of the collapse voltage The measured noise level of the microphones IS 30 dBA SPL, which LS approxunately as low as reqmred for a hearing-ald nncrophone (<29 5 dBA SPL)

A troublesome phenomenon encountered durmg the reahzatlon of free-standmg mlcrostructures, for example, beams, diaphragms and micromotors, IS that mltlally released structures afterwards stick to the substrate This effect may occur durmg wafer drying after the etching process has been completed, as well as during normal operation as soon as released structures come mto contact with the substrate In this paper the most Important types of attractive forces are dIscussed with respect to thetr possible influence on the performance of mtcromachmed structures It IS concluded that the mam reason for stlckmg of PECVD slhcon mtnde mlcromachmed structures IS adsorptron of water molecules The water molecules, adsorbed on both surfaces, attract each other as soon as the surfaces come mto contact It IS shown that a chemical surface modlficatlon, m order to achieve hydrophobic surfaces, IS an effective method for avoldtng adsorption of water, and therefore reduces sticking Sticking of mlcromachmed structures during drying IS reduced by rmsmg \nth a non-polar hqmd before wafer drying