High sensitivity capacitive MEMS microphone with spring supported diaphragm

Abstract

Capacitive microphones (condenser microphones) work on a principle of variable capacitance and voltage by the movement of its electrically charged diaphragm and back plate in response to sound pressure. There has been considerable research carried out to increase the sensing performance of microphones while reducing their size to cater for various modern applications such as mobile communication and hearing aid devices. This paper reviews the development and current performance of several condenser MEMS microphone designs, and introduces a microphone with spring supported diaphragm to further improve condenser microphone performance. The numerical analysis using Coventor FEM software shows that this new microphone design has a higher mechanical sensitivity compared to the existing edge clamped flat diaphragm condenser MEMS microphone. The spring supported diaphragm is shown to have a flat frequency response up to 7 kHz and more stable under the variations of the diaphragm residual stress. The microphone is designed to be easily fabricated using the existing silicon fabrication technology and the stability against the residual stress increases its reproducibility.

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.…”

“…Since many small size audio applications prefer a low voltage operation, the microphone sensitivity needs to be increased by reducing the diaphragm stiffness alone. The diaphragm stiffness can be reduced by using a low stress material, perforated diaphragm or as a combination with corrugated or spring type diaphragm [4,[6][7][8][9][10][11][12]. However, the reduction in diaphragm stiffness will cause the reduction in its operating frequency range and pull-in voltage.…”

Design and characterization of high sensitive MEMS capacitive microphone with fungous coupled diaphragm structure

“…In one approach the material, including silicon nitride, polysilicon, aluminum, and polyimide (Scheeper et al 1991;Torkkeli et al 2000;Pedersen et al 1997;, and fabrication process of the diaphragm is optimized to increase the sensitivity while in the other, the structure is redesigned to obtain the same goal. The main problem in all mentioned structures (Ying et al 1998;Li et al 2001; Mohamad et al 2008;Kim et al 2006;Yang 2010), is that by an increase in the mechanical sensitivity the pull-in voltage significantly decreased, especially in spring supported diaphragm. In this paper a novel structure for diaphragm is proposed which can increase both mechanical sensitivity and effective area with no negative effect on pull-in voltage.…”

“…In comparison to the normal diaphragms, the corrugated diaphragm has larger effective area, after the deflection, as well as higher mechanical sensitivity. Mohamad et al (2008) and Kim et al (2006), tried to increase both the mechanical sensitivity and the effective area, using some spring structures in their designs.…”

Effective diaphragm area of spring-supported capacitive MEMS microphone designs

“…Several designs of high sensitivity capacitive microphones have been demonstrated including corrugated, low-stress polysilicon, and spring supported based diaphragm [2][3][4][5][6][7] . However, the current analytical analysis is based on assumption that the small diaphragm movement is flat and piston-like to ease the microphone modeling and calculation involved 8, 9 . This paper highlights the non-flat deflection of a microphone diaphragm and demonstrates its effects on several types of microphone designs.…”