Effective diaphragm area of spring-supported capacitive MEMS microphone designs

Abstract

Capacitive (condenser) MEMS microphones have been developed using various design and fabrication techniques to improve performance. Mechanical sensitivity of a condenser MEMS microphone can be increased by reducing the residual stress of the diaphragm using several design approaches including corrugated diaphragms, and in recent years, various spring type diaphragms. The electrical sensitivity of the condenser microphone is proportional to the deflection of the diaphragm, however, the parabolic deflection of the diaphragm has reduced its effective diaphragm area, thus the sensitivity of parallel plate type capacitor on a condenser MEMS microphone. This paper presents the numerical analysis on the effective diaphragm area of several condenser MEMS microphone designs of 1.1mm x 1.1mm square. The analysis shows that the effective area of a spring-supported diaphragm is about 20% higher, and its capacitance change thus electrical sensitivity, is about 150% higher than a fully clamped flat diaphragm of an equal size. In addition, a flat deflection and higher effective diaphragm area of a spring-supported diaphragm can be achieved by carefully designed spring mechanisms.

Modelling and Optimisation of a Spring-Supported Diaphragm Capacitive MEMS Microphone

“…Weigold et al [13] was using a spring-supported, thin polysilicon diaphragm fabricated on silicon on insulator (SOI) wafer to achieve a sensitivity of -47 dB (ref 1 V/Pa) with the amplifier circuit. Another high sensitivity of up to 8.2 mV/Pa was reported by Fuldner et al [9] using a spring type diaphragm membrane of 1 mm in diameter and low bias voltage of 1 V. Mohamad et al [12,14] has also demonstrated that a specially designed spring-supported diaphragm can easily achieve up to 100 times higher mechanical sensitivity and 1.5 times higher capacitance change compared to the edge-clamped diaphragm of the same size.…”

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“…Diyaframın boyutu ve gerginliği, mikrofonun akustik performansının belirlenmesinde çok önemli bir rol oynar (Kim, Lee, Lee, Lee, & Park, 2007). Diyafram ile arka plaka arasındaki hava boşluğu bir kapasitör görevi görür ve diyaframın hareketi kapasitansı değiştirerek bir elektrik sinyali üretir (Mohamad, Lovenitti, & Vinay, 2008). Kapsül olarak adlandırılan bu sistem bir kondansatör görevi görmektedir.…”

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