P.R. Scheeper

A review of silicon microphones

Silicon micromachining has successfully been applied to fabricate piezoelectric, piezoresistive and capactive 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 mm2) 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 will result in a further reduction of the noise.

Fabrication of silicon condenser microphones using single wafer technology

A condenser microphone design that can be fabricated using the sacrificial layer technique is proposed and tested. The microphone backplate is a 1- mu m plasma-enhanced chemical-vapor-deposited (PECVD) silicon nitride film with a high density of acoustic holes (120-525 holes/mm/sup 2/), 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. >

Investigation of attractive forces between PECVD silicon nitride microstructures and an oxidized silicon substrate

A troublesome phenomenon encountered during the realization of free-standing microstructures, for example, beams, diaphragms and micromotors, is that initially released structures afterwards stick to the substrate. This effect may occur during wafer drying after the etching process has been completed, as well as during normal operation as soon as released structures come into contact with the substrate. In this paper the most important types of attractive forces are discussed with respect to their possible influence on the performance of micromachined structures. It is concluded that the main reason for sticking of PECVD silicon nitride micromachined structures is adsorption of water molecules. The water molecules, adsorbed on both surfaces, attract each other as soon as the surfaces come into contact. It is shown that a chemical surface modification, in order to achieve hydrophobic surfaces, is an effective method for avoiding adsorption of water, and therefore reduces sticking. Sticking of micromachined structures during drying is reduced by rinsing with a non-polar liquid before wafer drying.

Improvement of the performance of microphones with a silicon nitride diaphragm and backplate

The performance of a single-wafer fabricated silicon condenser microphone has been improved by increasing the stress and the acoustic hole density of the backplate and by decreasing the diaphragm thickness. The best microphones show a sensitivity of 5.0 mV Pa−1, which corresponds to an open-circuit sensitivity of 10 mV Pa−1 for a microphone capacitance of 6.6 pF. The measured frequency response is flat within ±2 dB from 100 Hz to 14 kHz, which is better than the requirements for a hearing-aid microphone. The operating 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 is approximately as low as required for a hearing-aid microphone ( <29.5 dBA SPL).

A silicon condenser microphone with a silicon nitride diaphragm and backplate

A new condenser microphone design, which can be fabricated using the sacrificial layer technique, is proposed and tested. The microphone backplate is a 1 mu m PECVD silicon nitride film with a high density of acoustic holes (120-525 holes mm-2), covered with a thin Ti/Au electrode. Microphones with a 1.5*1.5 mm diaphragm show a flat frequency response between 100 Hz and 14 kHz and a sensitivity of about 2 mV Pa-1 using a bias voltage of 16 V. These values 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.

Modelling of silicon condenser microphones

Several models concerning the sensitivity of capacitive pressure sensors have been presented in the past. Modelling of condenser microphones, which can be considered to be a special type of capacitive pressure sensor, usually requires a more complicated analysis of the sensitivity, because they have a strong electric field in the air gap. It is found that the mechanical sensitivity of condenser microphones with a circular diaphragm, either with a large initial tension or without any initial tension, increases with increasing bias voltage (and the corresponding static deflection), whereas the mechanical sensitivity of other capacitive pressure sensors does not depend on the static deflection. It is also found that the mechanical sensitivity increases with increasing input capacitance of a preamplifier. In addition, the open-circuit electrical sensitivity and, consequently, the total sensitivity too, also increases with increasing bias voltage (or static deflection). However, the maximum allowable sound pressure at which the diaphragm collapses, an effect that has to be taken into account, decreases with increasing static deflection in most cases, ulthnately resulting in an optimum value for the bias voltage. The model for microphones with a circular highly tensioned diaphragm has been verified successfully for two microphone types.

PECVD silicon nitride diaphragms for condenser microphones

The application of plasma-enhanced chemical vapour deposited (PECVD) silicon nitride as a diaphragm material for condenser microphones has been investigated. By means of adjusting the SiH4/NH3 gas-flow composition, silicon-rich silicon nitride films have been obtained with a relatively low tensile stress. Aluminium can be etched selectively with respect to the silicon nitride films. Using aluminium as a sacrificial layer, 300 × 300 μm silicon nitride diaphragms have been made. Admittance measurements on silicon nitride capacitances have shown that the insulating properties are sufficiently good for application as a microphone diaphragm.

Fabrication of a subminiature silicon condenser microphone using the sacrificial layer technique

The application of the sacrificial layer technique for the fabrication of a subminiature silicon condenser microphone with a plasma-enhanced chemical vapor deposited silicon nitride diaphragm has been investigated. Square diaphragms with dimensions from 0.6 to 2.6 mm and a thickness of 1 mu m have been realized. Measurements on a microphone with a 2*2 mm diaphragm and a 1 mu m airgap have shown that a sensitivity of 1.4 mV/Pa for low frequencies can be achieved with a low bias voltage (-2 V). The sensitivity decreases for high frequencies. This effect is probably due to the small airgap. Therefore, microphones with wider airgaps have to be developed to achieve a flat frequency response for the entire audio frequency range.<<ETX>>

Optimization of Capactive Microphone and Pressure Sensor Performance by Capacitor-electrode Shaping

In many designs of capacitive microphones or pressure sensors the electrode size is chosen to be equal to the diaphragm size. In this paper it will be discussed whether an electrode size or shape that differs from that of the diaphragm is attractive for obtaining a maximum value for the sensor sensitivity and the signal-to-noise ratio. A theoretical analysis will be given for circular diaphragms and electrodes, from which it can be shown that for maximum sensitivity the electrode should be located at the centre of the diaphragm, with a radius depending on the value of the amplifier input capacitance.

Amplitude-modulated electro-mechanical feedback system for silicon condenser microphones

For silicon condenser microphones, narrowing the air gap as a part of the miniaturization process causes 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. A feedback system is presented, which uses an amplitude-modulated 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 10.