Heat-resistance evaluation of SiO2 electret for microphones

Microphones using MEMS (microelectromechanical systems) have been practically applied and widely used for mobile equipment, such as cellular phones. The major reason for adopting MEMS is that they can use a re-flow soldering process as a construction method, in addition to having desirable features, such as being small and thin. However, since a guaranteed 300-degree C heat resistance is required, almost no electret types have been commercialized. No material can bear such high temperatures as a high polymer film electret. The present MEMS microphone is externally biased or CMOS process compatible with a built-in charge pump. Research has been conducted on SiO₂, SiN, etc., which exhibit affinity in the Si process, as electret materials. Here, the heat resistances of differing thicknesses of SiO₂ film and SiN film that covered the SiO₂ film were evaluated. Results show that electret surface voltage becomes high, and the surface resists deterioration by heat, so that the TEOS film is thick. Moreover, there was no significant change in the SiN film thickness.