Recent Trends in Structures and Interfaces of MEMS Transducers for Audio Applications: A Review

Abstract

In recent years, Micro-Electro-Mechanical Systems (MEMS) technology has had an impressive impact in the field of acoustic transducers, allowing the development of smart, low-cost, and compact audio systems that are employed in a wide variety of highly topical applications (consumer devices, medical equipment, automotive systems, and many more). This review, besides analyzing the main integrated sound transduction principles typically exploited, surveys the current State-of-the-Art scenario, presenting the recent performance advances and trends of MEMS microphones and speakers. In addition, the interface Integrated Circuits (ICs) needed to properly read the sensed signals or, on the other hand, to drive the actuation structures are addressed with the aim of offering a complete overview of the currently adopted solutions.

Keywords: ICs; MEMS; electromagnetic; electrostatic; interfaces; microphones; piezoelectric; speakers; structures; trends.

Figure 1

MEMS microphones and speakers market forecast.

Figure 2

Schematic 3D view and cross section of an electromagnetic speaker employing the structure proposed in [41].

Figure 3

Schematic 3D view and cross section of an electromagnetic speaker employing the structure proposed in [42].

Figure 4

Schematic 3D view and cross section of a capacitive microphone.

Figure 5

Schematic 3D view and cross section of a piezoelectric speaker exploiting the $d_{31}$ mode.

Figure 6

Schematic 3D view and cross section of a piezoelectric speaker exploiting the $d_{33}$ mode.

Figure 7

Schematic 3D view and top view of a piezoresistive microphone based on the “fish ear” structure proposed in [20].

Figure 8

Schematic 3D view and cross section of a thermoacoustic speaker.

Figure 9

Microscope image of (a) the fabricated capacitive MEMS microphone in [11], with (b) an enlarged view of the dual-anchor structure.

Figure 10

Scanning Electron Microscope (SEM) image of a fabricated biomimetic piezoelectric MEMS directional microphone [25]. $L_D$ (880 µm) and $W_D$ (1200 µm) are each individual diaphragm’s length and width, respectively. The inset shows in detail the planar interdigitated structure implemented to exploit the $d_{33}$ mode.

Figure 11

Microscope photograph of the top (a) and cross-section (b) view of a fabricated electrostatic MEMS speaker [85]. The diaphragm was removed in (b) for obtaining a clearer view of the structure.

Figure 12

Block diagram of the readout chain of a MEMS microphone.

Figure 13

Schematic representation of a (a) CC approach and (b) CV approach based pre-amp circuit for MEMS microphones.

Figure 14

Schematic representation of the pre-amp circuit used as readout for an ElGoFET MEMS microphone. $V_{DD}$ is the supply voltage, $I_{DS}$ is the bias drain-to-source current of the FET device while $i_{sig}$ is the signal current.

Figure 15

Schematic representation of the pre-amplifier circuit for a piezoresistive MEMS microphone.

Figure 16

Block diagram of the driving chain for a MEMS speaker.

Figure 17

Schematic representation of a standard class D amplifier for audio applications.