Research and development of miniaturized electrets

Abstract

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.

On the charge storage and decay mechanism in silicon dioxide electrets

“…The potentials V, are normalized to their initial potential Knit right after the thermal treatment, as mentioned in Section 3. The conclusions of our earlier paper [7] are consolidated by the recent measurements. The electric field of the electret, charged to 663 V, amounts to 6 MV/cm, which is very close to the reported theoretical maximum field strength of 7 MV/cm [lo].…”

“…From the results of experiments, described in our previous paper [7] concerning Si02 electrets, it was concluded that the cancellation of lateral surface conduction would result in prolonged charge retention. Because the silanol groups at a Si02 surface were reported either to be the cause of the conductivity due to mobile protons of those groups or to attract, by their polar nature, physically absorbable water molecules, it was thought and shown that substitution of these surface groups sufficiently reduced the surface conductivity, according to the following mechanism.…”

“…The life time of an electret is assumed to be equal to the time constant of the charge decay, i.e., the period in which the electret charge has decayed to 37% of its initial value [18]. In that case, the lower limit for the electret life time of the 663 V electret can be fixed a t 40 y instead of 20 y, formerly estimated [7]. The estimated life time of the 256 V electret of This behavior, supported by other measurement results, was the motive for the investigation of the decay mechanism, as discussed in Section 2.2.…”

“…Our previous investigations [7] led to the conclusion that an untreated Si02 layer is a poor electret not because of its bulk conductivity, but due to its large lateral surface conduction. It has been shown, however, that such a Si02 layer may be used as an excellent electret if the surface conductivity has been reduced by an appropriate chemical surface modification.…”

Charging mechanism of electret film made of potassium-ion-doped SiO2

“…C Electrets are materials that provide a semipermanent electric potential. While the first electret was discovered by the polarization of carnauba wax, 1 a wide variety of organic [2][3][4][5][6] and inorganic 7,8 dielectric materials have since been used as electret materials and their charging mechanisms have been investigated as well. 9,10 However, applications of electret films seem to be quite limited, especially in the area of microdevices; only electret condenser microphones are commercially available at the present time.…”