Fabrication of Electret Using Soft X-ray Irradiation for Silicon Microphone

To develop an ultraminiature high-performance microphone for the next generation, we have been studying a silicon microphone. We previously fabricated a single-crystalline silicon microphone using original MEMS technology. The prototype microphone showed excellent acoustic characteristics such as high sensitivity and wide frequency range. Aiming at low-voltage operation, we have recently developed a novel electret charging technique using soft X-ray irradiation. This technique leads to removing the 48-V bias voltage. Experimental results show that an internal inorganic dielectric film keeps electric charges and has excellent heat resistance. This technique exhibits promise for developing a low-voltage operation silicon microphone for purposes ranging from broadcasting to consumer use.