Soft-X-ray-charged vertical electrets and its application to electrostatic transducers

A novel charging method for vertical electrets in narrow gaps using soft X-rays has been developed. Electrets can be charged up to the depth 20~30 times of the gap opening. With the present charging technology, MEMS electret transducers can be fabricated using a single wafer without assembling process. We demonstrate performance of vertical electrets using early prototype of in-plane accelerometer. Under 18.7 ¿m_p-p external oscillation at 500 Hz, 30 mV output has been obtained without external bias voltage. Surface potential for 80 ¿m-deep vertical electrets is estimated to be 52 V.