Electret Charging Method Based on Soft X-ray Photoionization for MEMS Transducers

An electret charging method employing soft X-ray irradiation for microelectromechanical systems (MEMS) electret transducers is investigated. Soft X-rays can penetrate obstructions/narrow gaps and ionize gas molecules inside air gaps. This allows embedded electrets even after fabrication processes and vertical electrets on the sidewalls of high-aspect-ratio structures to be charged. The charging properties of polymer electrets subjected to soft X-ray irradiation were systematically examined. The surface potential of embedded electrets increased linearly with increasing irradiation time. The potential could be precisely controlled by the applied bias voltage. The surface potential of vertical electrets after charging was uniform to a depth of 20-30 times the gap opening. Since the photoionization rate depends on the collision diameter of the gas molecules with X-rays, a high charging rate was realized by using Xe gas. In the present charging method, photoionization was found to play a dominant role in generating electrons from gas molecules. Experiments performed at different gas pressures revealed that the effects of photoelectric phenomena and carrier displacement could be neglected. The present charging method was found to have a charge stability as high as that of corona charging. These results indicate that the present charging method has the potential for designing various new MEMS electret transducers.