MEMS Microphones

What are MEMS Microphones?

Microelectromechanical systems microphones (MEMS) are among the smallest microphones on the market. Their development led to very small microphones with really high performance. MEMS microphones offer good sensitivity, low power consumption, and higher SNR. They are available in petite sizes. Another important aspect of MEMS microphones is that they exhibit almost no change in performance after reflow soldering and have excellent temperature characteristics.

How Are They Fabricated?

MEMS microphones use acoustic sensors fabricated on semiconductor production lines using silicon wafers and highly automated processes. Layers of different materials are deposited on top of a silicon wafer. The unwanted material is then etched away, creating a moveable membrane and a fixed backplate over a cavity in the base wafer. The sensor backplate is a stiff perforated structure that allows air to move quickly through it, while the membrane is a thin, solid structure that flexes in response to the change in air pressure caused by sound waves. Changes in air pressure created by sound waves cause the thin membrane to flex while the thicker backplate remains stationary as the air moves through its perforations. The movement of the membrane changes the amount of capacitance between the membrane and the backplate, which is translated into an electrical signal by the ASIC.

Issues & Advantages

MEMS microphone design involves many of the same issues as conventional microphones, but the scale difference changes their relative importance. For example, the diffraction effects are still an issue for MEMS microphones because the packaging is still microscopic. Because the size of the MEMS microphone diaphragm (usually 2 mm or less) is much smaller than any audio length wavelength of interest (< 17 mm), the shape of the diaphragm is not an issue; however, just like the conventional microphones, diffraction effects (the increase at the pressure at the face of the microphone relative to the free field) from packaging may still be dominating effect on frequency response. On the other hand, achieving a smooth frequency response is usually more accessible in the case of MEMS microphones because of the simple mechanical structure. System resonances can be designed well above the frequency of interest, and damping can be introduced to tame the resonance of the fundamental mode of the microphone diaphragm.

The Conclusion

Generally, one associates large-diaphragm microphones with low noise floors and better performance. What is interesting is that although a single MEMS-scale microphone element may have worse performance than a conventional size, for a given total area and roll-off frequency, it can be shown that an array of very thin, low-mass diaphragms will outperform a single, large diaphragm in terms of the noise floor, absolute sensitivity, and vibration rejection.