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

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

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

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 ¿m have been realized. Measurements on a microphone with a 2×2 mm diaphragm and a 1 ¿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 rang