Soft X-ray Charging Method for a Silicon Electret Condenser Microphone

A novel charging method using soft X-ray irradiation for forming an electret of a silicon condenser microphone was developed. Soft X-rays can penetrate through the microphone diaphragm and are suitable for ionizing gas inside the microphone element. The surface potential of a positively biased SiO2/Si3N4 dielectric film negatively increased with irradiation time, since induced anions and electrons are dragged toward the film by an applied electric field. The charging rate is inversely proportional to the bias voltage and electrode-gap distance. Charged electrets have excellent retention properties; therefore, the method is promising for the realization of silicon-electret condenser microphones.