What are MEMS Microphones?


Microelectromechanical systems microphones (also known as MEMS) can be described as one of the smallest types of microphones in the market. Their development led to very small microphones with really high performances. The MEMS microphones are known to offer good sensitivity, low power consumption, higher SNR and above that they are available in petite sizes.  One more important aspect of the 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 that are fabricated on semiconductor production lines using silicon wafers and highly automated processes.  Layers of different materials are deposited on top of a silicon wafer, and then 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 easily 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 creates a change in 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 microphones 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 easier in the case of MEMS microphone 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 overall 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 noise floor, absolute sensitivity and vibration rejection.


Additional Resources & Source Texts