Abstract
Audio applications such as mobile communication and hearing aid devices demand a small size but high performance, stable and low cost microphone to reproduce a high quality sound. Capacitive microphone can be designed to fulfill such requirements with some trade-offs between sensitivity, operating frequency range, and noise level mainly due to the effect of device structure dimensions and viscous damping. Smaller microphone size and air gap will gradually decrease its sensitivity and increase the viscous damping. The aim of this research was to develop a mathematical model of a spring-supported diaphragm capacitive MEMS microphone as well as an approach to optimize a microphone’s performance. Because of the complex shapes in this latest type of diaphragm design trend, analytical modelling has not been previously attempted. A novel diaphragm design is proposed that offers increased mechanical sensitivity of a capacitive microphone by reducing its diaphragm stiffness. A lumped element model of the spring-supported diaphragm microphone is developed to analyze the complex relations between the microphone performance factors and to find the optimum dimensions based on the design requirements. It is shown analytically that the spring dimensions of the spring-supported diaphragm do not have large effects on the microphone performance com pared to the diaphragm and backplate size, diaphragm thickness, and air-gap distance. A 1 mm2 spring-supported diaphragm microphone is designed using several optimized performance parameters to give a –3 dB operating bandwidth of 10.2 kHz, a sensitivity of 4.67 mV/Pa (–46.5 dB ref. 1 V/Pa at 1 kHz using a bias voltage of 3 V), a pull-in voltage of 13 V, and a thermal noise of –22 dBA SPL.
Highlights
The silicon capacitive microphone has been studied and shown to potentially replace the existing and widely used piezoelectric microphone due to its high sensitivity, long term stability and ability to withstand a high temperature soldering process [1,2]
A lumped element model of the spring-supported diaphragm microphone is developed to analyze the complex relations between the microphone performance factors and to find the optimum dimensions based on the design requirements
A lumped element model of the spring-supported diaphragm microphone is derived to analyze the complex relations between the microphone performance factors and to find its optimum parameters setting based on the design requirements
Summary
The silicon capacitive microphone has been studied and shown to potentially replace the existing and widely used piezoelectric microphone due to its high sensitivity, long term stability and ability to withstand a high temperature soldering process [1,2]. 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. A lumped element model of the spring-supported diaphragm microphone is derived to analyze the complex relations between the microphone performance factors and to find its optimum parameters setting based on the design requirements. 2) Four doubly clamped L-shaped springs in Figure 1(c) (see Figure 6)
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