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MEMS Microphones

Image Source: bestforbest/Stock.adobe.com

By Tenner Lee for Mouser Electronics

Published December 2, 2022

Consumers are demanding ever-increasing mobility and optimized form factors for electronics. This demand has led to advances in the integration of disparate circuit components as well as constant performance improvements of single devices that serve multiple applications. With the onset of the pandemic and the explosion of video conferencing or video/voice communication, connectivity from anywhere with multiple applications has become a necessity. Microelectromechanical systems (MEMS), a class of components that saw rapid growth in the 1970s and 1980s, enable smaller and more powerful mobile devices for consumers. Integrating and implementing MEMS microphones provides advanced acoustic capabilities in these devices.

MEMS Overview

What Are MEMS?

MEMS are minuscule devices that integrate mechanical elements, sensors, actuators, or switches on a common chip. Although definitions vary, MEMS devices typically have dimensions less than 1mm in length and are built on a silicon substrate due to their fabrication process. MEMS devices inherit and use a wide variety of semiconductor manufacturing processes (e.g., lithography, wet and dry etching). Thus, MEMS devices leverage the maturity and infrastructure of the semiconductor industry, resulting in low-cost, high-reliability, and high-performance devices. Although MEMS and ICs share a general fabrication process, differences exist. MEMS fabrication processes deviate by including deeper etching, a release process to allow mechanical structures to separate from the bulk substrate, and consideration for stiction and parasitic elements. Overall, designers must consider a different set of fabrication conditions for MEMS, with a focus on mechanical properties (e.g., Young’s modulus, residual stress, and fatigue limit).

MEMS are commonly found in electronic devices that are used in everyday applications. Notable MEMS devices include accelerometers, gyroscopes, and pressure sensors. MEMS play a key role in the general trend of device miniaturization and the cutting-edge performance capabilities of current devices. One such class of MEMS devices is the MEMS microphone.

MEMS Microphones

MEMS microphones are acoustic devices that are relevant to all audio applications that require small packaging dimensions, high performance, low power consumption, and reliable performance. MEMS microphones operate as transducers that convert acoustic pressure into electrical signals. MEMS, like many other device components, can be divided into different types based on signal (analog compared with digital), output (single compared with differential [analog MEMS]), and operating principal (capacitive compared with piezoelectric).

These types of MEMS microphones are listed in Table 1 for reference, with notes on their advantages.

Table 1: Types of MEMS microphones and performance notes (Source: Author)

MEMS Microphone Applications

Laptops, cell phones, video teleconferencing stations, and noise-canceling headphones are examples of devices that use at least a single MEMS microphone. In many cases, devices such as in-ear headphones would not be as advanced as they are now without MEMS microphones. Advances in MEMS microphones provide noise cancellation, audio recording, and audio playback with minimal distortion and high-performance capabilities—all in an extremely small form factor.

Integrating and Implementing MEMS Microphones

Size, Weight, Power, and Cost

Mobile phones, hearing aids, wireless stereo earphones, and internet-connected devices are just a few applications that require small, high-performing, and affordable microphones. For these applications, traditional electret condenser microphones are simply not suitable. Capacitive MEMS microphones, however, fulfill these requirements and can offer improved reliability, sensitivity, improved signal-to-noise ratio (SNR), less distortion, and greater dynamic range. For these reasons, many of the MEMS microphones in use today employ capacitive technology to measure sound. Capacitive MEMS microphones work by measuring the capacitance between a fixed backplate and a flexible membrane that moves in response to sound waves.

Capacitive MEMS microphones are characterized by six different features:

• Packaging and enclosure
• An acoustic port
• A membrane
• A back plate
• A back cavity
• A front cavity

These six characteristics affect how well the microphone performs (Figure 1). When examining size, weight, power, and cost, fundamental limits and trade-offs exist due to how these features affect the microphone. When compared with general microphones, MEMS microphones are extremely compelling due to better size, weight, cost, and power envelopes. However, designers must consider application use cases, which can dictate microphone selection.

Figure 1: MEMS microphone general design and structure. Mechanical design determines the performance of the MEMS microphone. Highlighted features of the MEMS microphone all play a role in how the MEMS performs. (Source: Author)

Dynamic Range

The dynamic range of a MEMS microphone is characterized by the acoustic overload point (AOP) and the noise floor of the device (Figure 2). A key factor to high-performance microphones is to ensure that the dynamic range does not limit performance of the device and covers the full range of signal levels desired. When properly designed, the dynamic range of the microphone will indicate the measurement range of the microphone from the softest audible signal to the loudest without significant distortion (or sound pressure levels [SPLs]).

Figure 2: Illustration of dynamic range. Note that typically the usable dynamic range of the microphone will be less due to application requirements that bound both at high SPL (due to distortion requirements) and low SPL (due to SNR requirements). (Source: Author)

The AOP of a MEMS microphone describes the distortion performance of the MEMS microphone at higher SPLs. The AOP provides the SPL where the total harmonic distortion (THD) exceeds 10%. A higher AOP is better and directly translates to the ability to capture louder sounds more accurately at a given distortion level. If the device under design requires high-quality audio performance at higher SPLs, a large AOP is required. Certain datasheets may specify the AOP_PEAK or the AOP_RMS; in these cases, designers can add a 3dB value to convert AOP_RMS to AOP_PEAK.

To calculate the dynamic range of an analog MEMS microphone, you need the maximum voltage swings on the output voltage. Always verify that the voltage swing matches the dynamic range.

V_o = 2 x 10((S_dBV+(AOP_p-94dBSPL))/20)

For digital MEMS microphones, consider the analog-to-digital converter (ADC) when discussing dynamic range. The ADC needs to have enough bits to fully represent the complete signal swing. To calculate the ADC level (for a pulse code modulated signal), use the following equation:

DB = (6.02N + 1.78)

Noise and Distortion

When using a MEMS microphone, account for noise and distortion. Noise and distortion represent two different factors for design performance. Distortion in a MEMS microphone is generally given by THD as a percentage that represents the ratio of energy found in higher harmonics to the fundamental frequency or first harmonic. Following is the equation for THD:

THD_F = (√V₂² + V₃² + ... + V_n²) / V₁

Note that you must specify the number of higher-order harmonics (n) when describing THD and you should account for them when examining any given microphone. THD is measured at the output of the microphone; thus, a lower THD level means that the output is a more accurate representation of the desired acoustic signal. Careful design is essential to minimize the distortion depending on the application and acoustic signal profile. Certain algorithms may be more susceptible to distortion; likewise, certain notes may sound more distorted to the human ear at the same THD level.

Noise in a MEMS microphone can come from many different sources, including self-noise. Like all sensors, MEMS microphones suffer from noise due to electronic components (Johnson noise), quantization noise (ADC), and others. Noise sources also come from mechanical elements (i.e., how the microphone membrane is designed, debris ingress into the device). Finally, noise can come from environmental factors: Wind is one of the most obvious sources of noise. To reduce noise from environmental factors, the placement of the sensor as well as the design of the enclosure of the MEMS sensor during integration are vital. Techniques such as noise cancellation are also viable and can help improve overall device performance.

Noise in a MEMS microphone is measured as equivalent input noise (EIN), expressed as dBSPL at the output of the sensor.

EIN = 94 - SNR

Sensitivity

Sensitivity is a key factor in high-performance MEMS microphones. One important design consideration is ensuring that the microphone is sensitive enough to efficiently convert acoustic pressure to an electrical signal with enough SNR (Figure 3). In general, sensitivity is measured from a reference acoustic pressure of 94dBSPL (1Pa); however, sensitivity is measured differently between analog and digital microphones. For analog microphones, because we are measuring voltage, sensitivity is expressed in dBV/Pa or mVRMS/Pa. For digital microphones, sensitivity is measured as dBFS (decibels per full scale).

You must account for sensitivity with AOP, dynamic range, and EIN because sensitivity alone does not specify enough to judge performance.

Figure 3: A high-performance MEMS microphone should be sensitive enough to convert acoustic pressure to an electrical signal efficiently. (Source: Author)

Acoustic and Electrical Speciications

Acoustic and electrical specifications for MEMS microphones center on performance, reliability, and consistency metrics. When considering applications, reliability and consistency must sometimes take precedence over performance. For example, in applications that require multiple microphones (e.g., stereo), consistency of frequency response/sensitivity may be more important than overall AOP.

Some important top-level metrics for acoustic and electrical specifications include frequency response, directivity, and power supply rejection (PSR)/power supply rejection ratio (PSRR). Frequency response refers to the sensitivity of the microphone over different audio frequencies. When under consideration, this metric should be as flat as possible or as consistent as possible.

At higher frequencies, a Helmholtz resonance will appear, limiting the performance range of the microphone (Figure 4). Helmholtz resonance occurs when the air in a vessel or cavity vibrates at its natural frequency—in the same way that a note is produced when you blow over the mouth of a glass bottle. The frequency of the resonance is determined by the volume and geometry of the chamber and its opening.

Figure 4: Notional illustration of the frequency response from a MEMS microphone. The Helmholtz resonator results in resonance or increased sensitivity at the end of the illustration. (Source: Author)

Directivity or directionality indicates the sensitivity of the microphone over the angular extent of sound arrival to the sensor. A very directional microphone will only allow sound in a small region to be registered. Using multiple microphones as an array can improve directionality through receive beamforming.

PSR/PSRR indicates the ability of the microphone to reject noise added by the power supply. PSRR/PSR is measured as the residual noise of the microphone output with a spurious input signal at the supply voltage. A higher PSR/PSRR is always desired. The equations for PSR and PSRR are as follows:

PSR = 20 log log (V_out(217Hz))

PSR = 20 log log (V_out(217Hz)/V_in(217Hz))

Mounting

The placement of the MEMS microphone is important when designing for overall device capability and form factor. On a component level, the placement of one or many MEMS microphones will play a direct role in how the microphone will ultimately function beyond the values in the datasheet. Depending on design decisions in placement and mounting, microphone performance characteristics may be severely altered or degraded, or they may perform as expected. For example, if the microphone is placed near actuating and acoustically active components, expect poor performance such as distortion and increased noise, irrespective of MEMS microphone specifications. Design considerations are paramount to the MEMS device selection itself.

When determining the placement of the MEMS microphone, consider the following:

• The design of an acoustic sound channel with proper acoustic sealing (The channel length should be as short as possible and match acoustic port dimensions as closely as possible to avoid generating another Helmholtz resonator.)
• Mounting rigidity (The mounting and housing of the MEMS microphone can affect the frequency response by propagating vibrations to the component. Minimize transfer of acoustic noise to the microphone.)
• Proximity to other circuit components (maintain spatial isolation of the device)
• Device packaging and distance and shape of the device surface, corners, and edges (closer to the edge and surface is generally better)
• Isolation from acoustic and RF noise sources
• Isolation from thermal gradients
• Adjacency to the desired acoustic source

Specific applications require special considerations for microphone placements. For example, in stereo applications, microphones should be placed as far apart laterally as possible.

Interface

The interface for MEMS microphones can be subdivided by whether the microphone has an analog or a digital output. As discussed earlier, the interface for analog MEMS microphones is divided into single and differential outputs. Designers should account for the output impedance in analog MEMS microphones, which can run into hundreds of ohms. Additionally, designers should be careful not to attenuate the signal by mismatching impedances following the microphone. Another consideration is including a capacitor of at least 1µF for direct current filtering.

For digital MEMS microphones, things are a bit easier, but good interface practice is key. Designers need to consider digital output interfaces such as pulse density modulation and inter-IC sound, and may consider including source-terminating resistors.

For both analog and digital MEMS microphones, signal integrity requires matched impedances and complete absence of parasitic elements on trace lines. Designers must also avoid rejection of possible coupling of lines to noise sources (microstrip design of trace lines can be useful). In these cases, rejection of noise and interference in digital interfaces is superior to that of analog interfaces.

Conclusion

MEMS enable smaller, more powerful, and increasingly mobile devices for consumers. In this article, we learned about MEMS microphones, which provide advanced acoustic capabilities in these devices, with tips for integration and key parameters to watch (e.g., application use cases should always define what type of MEMS microphone is best). Still, there are many more topics that we did not discuss or that deserve more attention (such as phase distortion); you can find further information with more in-depth analysis in application notes.

Project and program technical lead for Machine Learning/Artificial Intelligence research and development. 15 years of experience leading, developing, managing projects, and advising/consulting on algorithm development/design, system optimization, and algorithm testing/validation. Graduate degree in electrical engineering with foundation in signal processing and EM.