Charge storage in double layers of silicon dioxide and silicon nitride

Silicon dioxide has long been studied for applications as electret layer in micromachined devices. However it could already be demonstrated that double layers of silicon dioxide and silicon nitride possess higher charge stability than well investigated single layers. In this contribution experimental results for double layers of SiO₂ and Si₃N₄ with different compositions will be reported and discussed. The investigated layers were prepared by thermal oxidation of silicon substrate (SiO₂) and APCVD (Si₃ N₄). The samples were charged by the point-to-grid corona method. The charge decay was observed by measuring the surface potential under different environmental conditions (elevated temperature and humidity) which will be described briefly. Besides results from TSC measurements will be presented and compared to those of a corresponding single layer of SiO₂. In addition to improved electret properties the internal stress in the investigated double layers can be adjusted by a proper thickness ratio of oxide layer to nitride layers. Therefore double layers of silicon dioxide and nitride seem to be promising electrets for fabrication of integrated sensors and actuators