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

A charging model is proposed for an electret film made of potassium-ion-doped SiO2 that can be formed between the two opposing micro-electrodes of a micro-electrostatic actuator, separated by a microscopic gap. On the basis of experimental evidence that charging only occurs in the positively biased electrode during the charging process and that the charging polarity is negative, we assumed that the cause of the electret charges is negatively charged oxygen ions residing in the SiO2 film, which arise as a consequence of potassium ion depletion. This assumption was supported by SIMS (Secondary Ion Mass Spectroscopy) analyses that indicate the presence of a depletion region of potassium ions near the interface of the silicon and the oxide film on the positively biased electrode. Calculations of the charged potential using Poisson’s equation with the spatially distributed negative charges in the depletion region showed good agreement with the measured surface potential. It appears that our charging model can consistently elucidate potassium ion electret technology.