Understanding MEMS Microphone Technology and Design

What is a MEMS microphone?

MEMS (Micro Electro Mechanical System) microphones are miniature devices that provide high-fidelity sound. They are small and can be tightly integrated into electronic products such as smartphones, smart speakers, and electronic consumer products such as headphones. Now, MEMS microphones can not only record ordinary environmental sounds, but also have functions such as stereo, active noise reduction, directivity (beaming), and voice recognition. These audio functions can be achieved by increasing the number of microphones in the device. For example, the latest smartphones can have up to 6 MEMS microphones. The excellent performance makes MEMS microphones widely used, thus generating a large market demand.

What is a capacitive MEMS microphone and how does it work?

All microphones (traditional and MEMS) sense sound waves via a flexible diaphragm. Under the pressure of the sound waves, the diaphragm moves. Most MEMS microphones on the market today use capacitive technology to detect sound. Capacitive MEMS microphones work by measuring the capacitance between a flexible diaphragm and a fixed backplate. The change in air pressure caused by the sound waves causes the diaphragm to move. The position of the backplate does not change because air can pass through the small holes in the backplate. As the diaphragm moves, the distance between the diaphragm and the fixed backplate changes, which ultimately causes a change in the capacitance between the two. This change in electrical signal can be recorded and analyzed.

Figure 1: Different types of capacitive MEMS microphones

There are different types of capacitive MEMS microphones, such as:

• ŸSingle back panel microphone

• ŸDouble-layer back panel microphone, with the back panels on both sides of the diaphragm

• The back plate of a double-layer diaphragm microphone is encapsulated between two layers of diaphragms, and there can be a vacuum between the diaphragms.

MEMS Microphone Design

MEMS microphone designers need to study and optimize key performance indicators such as frequency response, sensitivity, signal-to-noise ratio (SNR), total harmonic distortion, and equivalent input noise. SNR is a key performance indicator, and different capacitive MEMS microphones improve SNR by increasing the signal (double-layer backplate and double-layer diaphragm) or reducing noise (vacuum sealing between the two diaphragms).

The design, modeling and study of capacitive MEMS microphones and their performance characteristics can be performed in MEMS+®, part of the CoventorMP® MEMS design platform. MEMS+ provides nonlinear and multi-physics parameter models of different MEMS structures that can be combined into a complete MEMS microphone design. In addition, it is possible to integrate MEMS+ microphones into Cadence Virtuoso® circuit simulation software, which will allow us to simulate the MEMS microphone and its ASIC with specific IC bias conditions.

Figure 2: Example of a MEMS+ model for a dual-backplate MEMS microphone

Future MEMS Microphone Design

In today's era of artificial intelligence, MEMS design solutions based on innovative automatic optimization technologies have appeared on the market. For example, a team from the Institute of Electronic Design Automation at the Technical University of Munich has studied and demonstrated the automatic optimization design of MEMS microphones based on MEMS+, including their readout circuits .