Optimization and Microfabrication of High Performance Silicon-Based MEMS Microspeaker

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Abstract
A novel structure of electrodynamics microelectromechanical systems (MEMS) microspeaker designed for mobile electronics is proposed in this paper. The originality of the device lies on the use of a rigid silicon membrane suspended by highly flexible silicon springs, contrary to most MEMS and non-MEMS microspeakers, which use polymer diaphragms. Important rigidity of the membrane and high linearity of the magnetic actuation conferred outstanding sound quality. The design of the silicon springs enabled large out-of-plane displacement of the membrane, which improved the bass rendering and the acoustic intensity over the whole bandwidth. The low density of silicon material helped to reduce the mobile mass and thus improved the microspeaker efficiency. A prototype with a membrane diameter of 15 mm and a thickness of 20 μm is microfabricated and characterized. The silicon springs enabled out-of-plane displacement of more than 300 μm. Acoustic intensity of 80-dB SPL is measured at 10 cm with 500-mW input power. This sound pressure level is obtained at frequencies from 330 Hz up to 70 kHz. Thanks to the membrane backside microstructure, most of the membrane proper modes are shifted out of the audible bandwidth. The measured electroacoustic efficiency is almost three times better than that of conventional microspeakers.

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