Study of degradation by heat and heat‐resistance property in electret condenser microphone

Abstract

This paper describes improvement of the heat resistance of a fixed electrode electret microphone for mobile equipment. First, hypotheses are presented on the cause of the thermal degradation and the corresponding charge behavior in the fixed electrode electret microphone and a model of the internal electric field is formed. The validity of the model is investigated by the thermal stimulated current (TSC) method. Next, the lifetime of the microphone is estimated by reference to the isothermal charge decay test (ICD method) in addition to the hypotheses. As a model, the charges are assumed to be shifted in the direction of greater depth with reduction of the surface potential. This phenomenon is accelerated by heat. The magnitude of the degradation depends on the process used for fabrication of the electret forming film. However, there is sufficient lifetime margin at room temperature. By applying this margin, the environmental characteristic may be improved if the charges are shifted to the interior or injected deeply to avoid the causes of degradation. As an example, for a general-purpose FEP film with a thickness of 12.5 µm, it is estimated that a lifetime of 1000 hours at 100 °C can be guaranteed by deep injection at depths up to about 8 µm from the electret surface. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 90(5): 1– 8, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20283