Silicon-based inorganic electrets for application in micromachined devices

Layers of SiO₂ and Si₃N₄ have good mechanical properties for application in micro-machined electret capacitor microphones. These materials are investigated as single or double layers in terms of chargeability and long-term charge stability. The main emphasis is put on the miniaturization of electret layers. The lateral dimensions of the electrets are reduced to 2 mm and the charge decay characteristics under different environmental conditions are described. According to the experimental data, multilayer samples of silicon dioxide and nitride possess good chargeability and higher charge stability compared to the well investigated single layers. Typically, the double layers loose ~10% of their surface potential when annealed for 200 min at 300°C and show a peak of the thermally stimulated current at 430°C. It can also be shown that the miniaturization of samples does not necessarily cause a faster charge decay