Improved silicon dioxide electret for silicon-based integrated microphones

Capacitor microphones integrated on silicon chips are being designed to operate at lower bias voltage but still require either a voltage multiplier circuit or an electret. Conventional electret materials are difficult to process with silicon and their processing temperature limitations results in their unpopularity in the fabrication of silicon integrated microphones. Attempts to use electrets of silicon dioxide and silicon nitride have not been successful due to poor charge stability. A better technique was found by overcoating the silicon dioxide with dielectrics. This paper will describe films of silicon dioxide overcoated with tantalum pentoxide using sputtering and electron-beam deposition. The charge retention characteristic of tantalum pentoxide are highly dependent on forming conditions. Hence, the processing conditions and various annealing methods will be described and related to charge stability. Variations in charge density and distribution on exposure to elevated temperature and humidity will be reported