MEMS Microphone Technology and Design

What is a MEMS microphone?

MEMS (Micro-Electro-Mechanical Systems) microphones are miniature devices that provide high-fidelity sound sensing in small size. They can be tightly integrated into electronic products, such as smartphones, smart speakers, and consumer electronics, such as headphones. Nowadays, MEMS microphones are not only capable of recording common ambient sounds but also have features such as stereo sound, active noise cancellation, directivity (beam gathering), and voice recognition. These audio functions can be achieved by increasing the number of microphones in a device, such as up to six MEMS microphones in the latest smartphones. The excellent performance of MEMS microphones has led to a wide range of applications and thus a large market demand.

What is a condenser MEMS microphone? How does it work?

All microphones (conventional and MEMS microphones) sense sound waves through a flexible diaphragm. Under acoustic pressure, the diaphragm is displaced. Most MEMS microphones on the market today use capacitive technology to detect sound. Capacitive MEMS microphones measure the capacitance between a flexible diaphragm and a fixed back plate. The change in air pressure from the sound waves causes the diaphragm to shift. Air can pass through the small holes in the backplate, so the position of the backplate does not change. During the movement of the diaphragm, the distance between the diaphragm and the fixed backplate changes, which eventually leads to a change in the capacitance value between the two, and this change in the electrical signal can be recorded and analyzed.

• Different types of capacitive MEMS microphones exist, such as
• Single backplane microphones
• Double-backed microphones with a split backplate on both sides of the diaphragm

Double-layer diaphragm microphones where the backplane is encapsulated between two diaphragms and the membrane can be vacuumed between them.

MEMS Microphone Design

MEMS microphone designers need to study and optimize key performance metrics such as frequency response, sensitivity, signal-to-noise ratio (SNR), total harmonic distortion, and equivalent input noise. SNR is a crucial performance metric. Different capacitive MEMS microphones improve SNR by increasing signal (double-layer backplane 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+®, a component of the CoventorMP® MEMS design platform. MEMS+ provides nonlinear and multi-physics parametric models of different MEMS structures that can be composed 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 MEMS microphones and their ASICs with specific IC bias conditions.

The Future of MEMS Microphone Design

MEMS design solutions based on innovative automatic optimization techniques are already available in the market today in the era of artificial intelligence. For example, a group from the Institute of Electronic Design Automation at the Technical University of Munich has studied and demonstrated the automated optimization design of a MEMS microphone based on MEMS+, including its readout circuit.