In this paper we describe our investigations of the electrical conductivity of the silicon dioxide-air interface. It appears that this conductivity is caused by the adsorption of water vapour on the oxide surface and strongly depends on the relative humidity of the surrounding air. Considering this fact we have decided to investigate the possibility to reduce the surface conductivity by means of a chemical modification of the oxide surface, which reduces the adsorption of water vapour on it. To measure the conductivity we have used a so-called open-gate FET structure. The performance characteristics of this structure and the experimental results obtained with it are presented. We have found that by using silane agents like HMDS and DCDMS the surface conductivity of silicon dioxide can be reduced with at least a factor 1000 and 10000 respectively.

It 1s known that the deflection of a diaphragm 1s determmed by two mechamsms, bendmg moments or bendmg stress and tensile forces or membrane stress Usually the influence of tensile forces 1s not taken mto account when calculating the mechamcal properties of thm diaphragms Hence the mathematical descrlptlon thus obtamed will only be valid if the deflection of the diaphragm is small compared with its thickness and if lateral stress is absent In this paper we will consider uniformly loaded circular diaphragms, which are assumed to be lsotroplc, and present the results of a study on the influence of bendmg stress and tensile stress, which together determine the diaphragm deflectlon Startmg from theoretical conslderatlons, a sunulatlon program 1s developed of which several results are presented and discussed

This paper describes the realization process of small-size electrets by using techniques generally applied in the fabrication of integrated circuits and microsensors. In the first part of the paper, attention is paid to the different electret decay mechanisms and their relative contribution to the overall stability of miniaturized electrets. Then, a process is described by which polymer electrets such as Teflon-FEP and PTFE can be deposited and shaped in a predefined pattern on a silicon wafer. In the third part, results on the application of new materials, especially silicon dioxide (S i o s) , for use in electret applications, are presented. It appears that after an appropriate treatment of the oxide surface, its charge-stability is at least equal to that of polymer electrets such as Teflon-FEP and PTFE.

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

In this paper the theory and experimental results of integrated electret-based silicon sensors for the detection of sound and pressure are presented. A silicon electret microphone for use in hearing-aid microphones is described, with an experimentally determined sensitivity of 19 mV/Pa in the frequency range of 50 H a to 10 kHz. Also the realization of a pressure sensor, developed for application in catheter-tip blood-pressure measurements, is discussed. This type of electret sensor appeared to have a pressure sensitivity of 1.6 pV/Pa in the required frequency range of 0 to 100 Hz.