Optimization and Microfabrication of High Performance Silicon-Based MEMS Microspeaker

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.

Review of Recent Development of MEMS Speakers

“…On one hand, to achieve high SPL at low frequencies and thus improve the acoustic performance over a wide frequency range, the fundamental drum mode frequencies are typically designed at around 2 kHz to 3 kHz [ 16 , 34 , 35 ]. On the other hand, from the acoustic point of view, the drum mode vibration with the deformed emissive surface and higher harmonics stimulation due to nonlinearities will distort the acoustic wavefront, therefore causing sound distortions and deteriorating the sound quality [ 2 ]. Thus, some special diaphragm designs other than edge clamped diaphragms have been developed, such as rigid diaphragms with radial rib structures supported by suspension beams [ 2 ] and circular diaphragms supported by four flexible dual-curve actuators [ 21 ], in which piston mode vibrations at low frequencies can be employed to generate the sound while the drum mode vibrations can be shifted to high frequencies to avoid the sound distortion of MEMS speakers.…”

“…As shown in Figure 3 b, when the current flows through coils, Lorentz force will be generated due to the interaction between the external magnetic field and the electric current, thus bending the acoustic diaphragm. For a planar concentric coil with turns carrying an electric current , the Lorentz force generated by a magnetic field with a flux density can be expressed as [ 2 ]: where is the length of the coil, is the radius of the i th turn, and is the radial component of the magnetic flux density on the coil plane corresponding to the i th turn.…”

“…Currently, speakers in those mobile electronic devices are dominated by conventional speakers with bulky moving coils, which are still challenging to be batch fabricated since voice coils and permanent magnets must be assembled [ 1 ]. The miniaturization of these conventional speakers also has a negative impact on the sound quality and reaches some limits due to the employed materials and the fabrication approaches [ 2 ]. For example, the plastic or polymer diaphragms of conventional speakers are too soft to be used as high-quality radiator surfaces [ 3 ].…”

“…For example, the plastic or polymer diaphragms of conventional speakers are too soft to be used as high-quality radiator surfaces [ 3 ]. The simplification of the mechanical suspensions and electromagnetic parts in the miniaturization would lead to reduced bandwidths and increased nonlinearities, thus deteriorating the sound quality [ 2 , 4 ]. It is also difficult for conventional manufacturing technologies to achieve high dimensional precision and good reproducibility in the miniaturization of speakers.…”

“…Various materials and fabrication approaches have also been explored for developing MEMS speakers [ 15 , 16 , 17 ]. The performances of MEMS speakers have been evaluated and compared with conventional speakers in terms of several key specifications, such as device footprint, output sound pressure level (SPL), power consumption, bandwidth, and total harmonic distortion (THD) [ 2 , 9 , 10 , 17 ]. Among them, the SPL and bandwidth are two widely used parameters to evaluate the acoustic performance of MEMS speakers.…”