Optical MEMS technology redefines what’s possible in microphone performance.
The optical microelectromechanical systems (MEMS) market is the fastest-growing segment of the overall MEMS market. Driven in part by explosive demand for lower-power optical circuit switches in AI data centers as well as by LiDAR in autonomous vehicles, the market for optical MEMS is estimated at a 15% CAGR through 2031, according to the analyst firm Yole Group.
Optical MEMS microphones represent a more recent commercialization of this architectural approach. They’re especially instructive because it’s easy to compare their performance metrics with the capacitive architecture that has dominated MEMS microphones since the early 2000s.
Capacitive MEMS microphones
As an evolutionary advancement over electret condenser microphones (ECMs), which aren’t solder-reflow compatible and typically have poor part-to-part matching, capacitive MEMS microphones resolved many of the drawbacks of ECMs, particularly in production.
Capacitive MEMS microphones have at least one membrane (or diaphragm) and a perforated backplate a few microns from the membrane. Due to the inherent limitations of this architecture, it’s exceptionally difficult to further reduce noise and increase the dynamic range of capacitive MEMS microphones.
And when we’re talking about microphones, noise—often classified as signal-to-noise ratio (SNR) as well as dynamic range—matters.
While the SNR and dynamic range of capacitive MEMS microphones have improved over the last 20 years, state-of-the-art digital capacitive MEMS microphones have hit a performance wall of 73 dB SNR and a dynamic range of 101 dB.
Optical MEMS microphones
Optimal audio recording setups require the ability to capture both very quiet and very loud environments. These conditions require a wide dynamic range, defined as the difference between the lower limit of the noise floor and the upper limit of the acoustic overload point (AOP). Having both a low noise floor and a high AOP, the digital optical MEMS microphone delivers over 20 dB higher dynamic range than the digital capacitive MEMS microphone. This approximates a huge 10X advantage in dynamic range for optical MEMS microphones.
The MEMS structure in an optical MEMS microphone is a simply supported diaphragm with a reflective coating and a diffractive optical element (DOE). Consequently, there’s no noise contribution from air movement through small backplate holes. Also, the gap between the diaphragm and the DOE is in the order of tens of microns, which is an order of magnitude larger than that of capacitive microphones.
Optical MEMS microphones deliver important acoustic advantages (Source: sensiBel)
Features and results
The main features of the optical MEMS microphone architecture are as follows:
- There is no significant squeeze film damping in the system.
- The diaphragm can move tens of microns, so there is no practical mechanical limit to the AOP,
- The robust design eliminates the capacitive microphone’s narrow gap, reducing the risk of failure, particularly from stiction.
- As the MEMS element is electrically passive, there’s no voltage bias on the MEMS. In contrast, in capacitive MEMS devices, the diaphragm and backplate can adhere when the electrostatic attraction between them exceeds the system’s restoring forces.
In measurable terms, this means the newest digital optical MEMS microphone achieves 80 dB SNR (14 dBA noise floor) and a 146 dB AOP, representing a dynamic range of 132 dB, which is a breakthrough for digital MEMS microphones.
The resulting audio recordings are crisp and clear in very loud or very soft environments, as well as everywhere in between.
By Mike Tuttle, Customer Applications and Head of Products at sensiBel
www.sensibel.com