Improving on the Electret: An Introduction to MEMS Microphones. Mems микрофон
MEMS Microphone – a breakthrough innovation in sound sensingDate: 16-02-17
This article focuses on MEMS Microphone, its application and advantages. The article highlights the advantages of MEMS microphone over traditional solution. The article also introduces readers to MEMS Microphone construction and its key parameters. The article also focused on how MEMS Microphone has enable breakthrough innovations in consumer, medical, security and Automotive application. Finally article also highlights the leading position which STMicroelectronics has acquired through its innovative technology and supply chain management.
MEMS Microphone is a solid state integrated IC which can sense voice in the same way as ECM Microphone [Electret Condenser Microphone]. They are getting increasing popular in modern devices such as Mobile Phone, Tablet, Laptop, Smart TV, Automotive voice recognition, gaming and Remote controller etc.
According to IHS iSuppli, the market for MEMS microphones for consumer electronics and mobile handsets is forecast to grow revenue at a CAGR of 23% between 2010 and 2014. The increased popularity of MEMS Microphone is attributed to its reliable monolithic structure, high tolerance of mechanical vibration, small footprint and height and optional digital output.
In addition, MEMS microphones enable dramatic advancements in sound quality in multiple-microphone applications. Such microphone arrays, facilitated by the small form factor, superior sensitivity matching and frequency response of ST’s microphones, enable the implementation of active noise and echo cancelling, as well as beam-forming, a sound-processing technology that helps isolate a sound and its location. These features are invaluable with the increasing use of cell phones and other devices in noisy and uncontrollable environments.
MEMS Microphone Construction
There are mainly two types of MEMS microphones – Analog which convert sound into corresponding voltage output and Digital which gives a digital output typically pulse density modulation [PDM].
MEMS microphone basically is an acoustic transducer. • Transduction principle is the coupled capacity change between a fixed plate (back-plate) and a movable plate (membrane) • The capacitive change is caused by the sound, passing through the acoustic holes, that moves the membrane modulating the air gap comprised between the two conductive plates • The back-chamber is the acoustic resonator • The Ventilation hole allows the air compressed in the back chamber to flow out and consequently allowing the membrane to move back
Key Parameters of MEMS Microphone
Sensitivity: • The sensitivity is the electrical signal at the microphone output to a given acoustic pressure as input. The reference of acoustic pressure is 1Pa or even 94dBSPL @ 1kHz** • Sensitivity is typically measured: • for Analog microphones in mV/Pa or even dBV = 20 * Log (mV/Pa / 1V/Pa) • for Digital microphones in %FS or even dBFS = 20 * Log (%FS / 1FS)
Directionality: • The directionality indicates the variation of the sensitivity response with respect to direction of arrival of the sound • The STMicroelectronics MEMS microphones are OMNI-Directional which means that there is no sensitivity change at every sound source position in the space • The directionality can be indicated in a Cartesian axis as sensitivity drift vs. angle or in a polar diagram showing the sensitivity pattern response in the space
Signal to Noise Ratio [SNR]: • The signal-to-noise ratio specifies the ratio between a given reference signal to the amount of residual noise at the microphone output • The reference signal is the standard signal at the microphone output when the sound pressure is 1Pa @ 1kHz. In other words the microphone sensitivity • The noise signal (residual noise) is the microphone electrical output at the silence. This quantity includes both the noise of the MEMS element and the ASIC • Typically the noise level is measured in an anechoic environment and weighting-A the acquisition. The A-weighted filter corresponds to the human ear frequency response
Dynamic Range and AOP: • The dynamic range is the difference between the minimum and maximum detectable sound by the microphone without distortion: • The maximum signal that the microphone can “listen” without distortion is also called acoustic overload point (AOP). For both analog and digital ST microphones the AOP is 120dBSPL as sound pressure • The minimum signal that a microphone can “listen” depends on its SNR. In other words, the minimum signal is equivalent to the residual noise in terms of dBSPL
Frequency response: The frequency response of a microphone in terms of magnitude: • Indicates the sensitivity variation across the audio band. Or even, this parameter describes the deviation of the output signal from the reference 0dB • Typically the reference for this measurement is the exactly the sensitivity of the microphone à 0dB = 94dBSPL @ 1kHz • The typical frequency response of a microphone shows a roll-off at low frequency due to ventilation hole and an rise up at high frequency due to Helmholtz effect • The frequency response of a microphone in terms of phase: • Indicates the phase distortion introduced by the microphone. In other words the delay between the sound wave moving the microphone membrane and the electrical signal at the microphone output • This parameter includes both the distortion due to the membrane and the ASIC
ST’s Microphone offers Best acoustical performances - Best frequency response, Omni-directional polar pattern, Best Sensitivity , Reduced phase rotation, Optimized audio quality (SNR > 60dB), Precise unit-to-unit-matching. It also offers Highest reliability and robustness, Superior stability of humidity, temperature and dust parameters.
Directional acoustic patterns using MEMS Microphone
An omnidirectional microphone response is generally considered to be a perfect sphere in three dimensions. The smallest diameter microphone gives the best Omni-directional characteristics at high frequencies. This is the reason that makes the MEMS microphone the best Omni-directional microphone But MEMS Microphone can also be used in array to modify the response according to desired acoustic patterns
As an application example
The signals of a couple of microphones are processed** to shape the response along the x direction
The physical and acoustic parameters of ST’s MEMS microphones perfectly fit the challenging requirements of distant-speech interaction systems. The small form factor allows the researchers to easily embed entire arrays of microphones in the walls, desks, or speech-enabled appliances of the automated home, while the microphones’ excellent acoustic characteristics, coupled with sophisticated signal-processing technologies, will make it possible to identify and capture an individual speaker from several meters away, in a crowded room with music playing.
The distant-speech interaction capability will not only dramatically change the way people interact with technology, but can make a real difference for those who can’t easily move around, such as the elderly or the motor-impaired. In addition to the home scenarios, the distant-speech interaction systems can find use in robotics, tele-presense, surveillance and industry automation.
STMicroelectronics advantages: ST MEMS microphones are available in plastic packages. The patented technology breakthrough saves space and increases durability in consumer and professional voice-input applications, from mobile phones and tablets to noise-level meters and noise-cancelling headphones.
While other MEMS microphone manufacturers still produce devices with metallic lids, ST leads the way with industry-unique, innovative plastic packages. ST’s MEMS microphones are suitable for assembly on flat-cable printed-circuit boards that simplify design in today’s space-constrained consumer devices. The patented package technology allows equipment manufacturers to place the ‘sound hole’ either on the top or the bottom of the package to ensure the slimmest possible design and shortest acoustic path from the environment to the microphone. While the microphones with the sound hole on the top (top-port) suit the size and sound-inlet position requirements of laptops and tablets, the bottom-port microphones are mostly used in mobile phones.
ST has recently introduced - MP34DT01- first MEMS microphone in the market that couples the advantage of a top-port sound-inlet position with unparalleled signal-to-noise ratio (SNR) of 63 dB and flat frequency response in the full audio band of 20–20,000 Hz. The device’s best-in-class SNR makes it also suitable for applications beyond typical consumer applications, such as phonometers – sound-level meters that require high dynamic range. ST’s MEMS Microphone can keep frequency response even after the reflow soldering.
ST ‘s industry-unique capability to manage the whole supply chain and leading-edge MEMS production capacity enables short product-development cycles and time-to-volume for high-performance, cost-competitive silicon acoustic devices.
Conclusion: MEMS microphone are entering new application areas such as voice-enabled gaming, automotive voice systems, acoustic sensors for industry and security applications, and medical telemetry. Its unique construction, performance and form factor has made possible what was unthinkable earlier. The readers are encouraged to use MEMS Microphone to take a lead over their competition and ST is committed to support its customers.
MEMS Microphones, the Future for Hearing Aids
Driven by aging populations and a pronounced increase in hearing loss, the market for hearing aids continues to grow, but their conspicuous size and short battery life turn many people off. As hearing loss becomes ever more common, people will look for smaller, more efficient, higher quality hearing aids. At the start of the hearing aid signal chain, microphones sense voices and other ambient sounds, so improved audio capture can lead to higher performance and lower power consumption throughout the signal chain.
Microphones are transducers that convert acoustical signals into electrical signals that can be processed by the hearing aid’s audio signal chain. Many different types of technologies are used for this acoustic-to-electrical transduction, but condenser microphones have emerged as the smallest and most accurate. The diaphragm in condenser microphones moves in response to an acoustic signal. This motion causes a change in capacitance, which is then used to produce an electrical signal.
Electret condenser microphone (ECM) technology is the most widely used in hearing aids. ECMs implement a variable capacitor with one plate built from a material with a permanent electrical charge. ECMs are well established in today’s hearing industry, but the technology behind these devices has remained relatively unchanged since the 1960s. Their performance, repeatability, and stability over temperature and other environmental conditions are not very good. Hearing aids, and other applications that value high performance and consistency, present an opportunity for a new microphone technology that improves on these shortcomings, allowing manufacturers to produce higher quality, more reliable devices.
Microelectromechanical systems (MEMS) technology is driving the next revolution in condenser microphones. MEMS microphones take advantage of the enormous advances made in silicon technology over the past decades—including ultrasmall fabrication geometries, excellent stability and repeatability, and low power consumption—all of which have become uncompromising requirements of the silicon industry. Until now, the power consumption and noise levels of MEMS microphones have been too high to make them appropriate for use in hearing aids, but new devices that meet these two key specifications are now enabling the next wave of innovation in hearing aid microphones.
MEMS Microphone Operation
Like ECMs, MEMS microphones operate as condenser microphones. MEMS microphones consist of a flexibly suspended diaphragm that is free to move above a fixed backplate, all fabricated on a silicon wafer. This structure forms a variable capacitor, with a fixed electrical charge applied between the diaphragm and backplate. An incoming sound pressure wave passing through holes in the backplate causes the diaphragm to move in proportion to the amplitude of the compression and rarefaction waves. This movement varies the distance between the diaphragm and the backplate, which in turn varies the capacitance, as shown in Figure 1. Given a constant charge, this capacitance change is converted into an electrical signal.Figure 1. Capacitance of MEMS microphone varies with amplitude of acoustic wave.
The microphone sensor element is constructed on a silicon wafer using similar manufacturing processes to other integrated circuits (ICs). Unlike ECM manufacturing technologies, silicon manufacturing processes are very precise and highly repeatable. Each MEMS microphone element fabricated on a wafer will perform like every other element on that wafer—and like every element on different wafers produced across the many years of the product’s lifetime.
Silicon fabrication uses a series of deposition and etching processes in a tightly controlled environment to create the collection of shapes in metal and polysilicon that form a MEMS microphone. The geometries involved in the construction of MEMS microphones are on the order of microns (µm). The holes in the backplate through which sound waves pass can be less than 10 µm in diameter and the diaphragm thickness can be on the order of 1 µm. The gap between the diaphragm and the backplate is on the order of several microns. Figure 2 shows a SEM image of a typical MEMS microphone transducer element, looking at it from the top (diaphragm) side; Figure 3 shows the cross section through the middle of this microphone element. In this design, sound waves enter the microphone through the cavity in the bottom of the element and pass through the backplate holes to excite the diaphragm.Figure 2. SEM image of MEMS microphone.Figure 3. Cross section of a MEMS microphone.
Because the geometries are tightly controlled during the manufacturing process, the measured performance from microphone to microphone is highly repeatable. Another advantage of using MEMS technology to build microphones is that the diaphragm is extremely small, resulting in very low mass and making a MEMS microphone much less susceptible to vibration than an ECM, which has a much more massive diaphragm.
Evolution, Repeatability, and Stability
MEMS microphones have developed to the point where they are now the default choice for many audio capture applications that require small size and high performance, but most commercial grade microphones are unsuitable for the hearing aid industry, which requires significantly smaller parts with lower power consumption; better noise performance; and improved reliability, environmental stability, and device-to-device repeatability. MEMS microphones technology is now at the stage where all of these can be offered: ultrasmall packages, very low power consumption, and very low equivalent input noise.
Tight controls in the silicon manufacturing process make the stability and device-to-device performance variation of MEMS microphones significantly better than that of ECMs. Figure 4 shows the normalized frequency response of several MEMS microphones of the same model; Figure 5 shows the normalized frequency response of various ECMs. The frequency response of each MEMS microphone is nearly identical, while that of the ECMs shows significant device-to-device variation, especially at high and low frequencies.Figure 4. Frequency response of several MEMS microphones.Figure 5. Frequency response of three sets of ECM microphones.
MEMS microphones also exhibit excellent stability across a wide temperature range. Figure 6 shows the change in sensitivity as the ambient temperature is varied between –40°C and +85°C. The black line shows less than 0.5-dB variation over the temperature range for the MEMS microphone, while the ECMs show up to 8-dB variation over temperature.Figure 6. Sensitivity to vibration vs. temperature: MEMS vs. ECMs.
MEMS microphone designs also have significantly improved power supply rejection compared to ECMs, with a typical power supply rejection ratio (PSRR) of better than −50 dB. The output signal and the bias voltage (power) share a common pin on an ECM, so any ripple on the power supply appears directly on the output signal. The exceptional PSRR of MEMS microphones allows freedom in the audio circuit design that is not possible with ECMs. This can result in reduced component count and system cost.
In tiny, battery-powered applications like hearing aids, every microwatt of power is critical. Microphones cannot be power cycled to save power when the hearing aid is operating, so the microphone’s active power consumption is of critical importance. Typical ECM microphones used in hearing aids can draw 35 µA when powered at typical Zn-air battery voltages (0.9 V–1.4 V). The current draw of MEMS microphones used in hearing aids can be less than half of that at the same voltages, enabling hearing aids to go longer between battery changes.
The latest generation of MEMS microphones has the excellent noise and power performance required by the hearing aid industry. Analog Devices has leveraged more than 20 years of experience in MEMS technology to build high-performance microphones that can be used in the hearing aid market. Typical omnidirectional MEMS microphones specify an equivalent input noise (EIN) of 27.5 dB SPL (A-weighted, 8 kHz bandwidth), which makes them suitable for hearing aid applications. The ⅓-octave EIN noise performance, typically used for specifying hearing aid microphones, is exceptionally good at low frequencies, as shown in Figure 7. This level of noise performance is achieved with only 17 µA current draw at typical hearing aid battery voltages. The microphones are available in tiny packages with less than 7.5 mm3 total volume, as shown in Figure 8.Figure 7. MEMS microphone ⅓-octave noise.
Figure 8. Omnidirectional MEMS microphones for hearing aids. a) bottom view. b) top view. c) bottom view of package that facilitates hand soldering.
New high-performance, low-power MEMS microphones demonstrate that they will be the next generation of microphone technology for hearing aids. MEMS microphones can compete in performance with many hearing aid ECMs and can surpass ECM technology in many areas, such as repeatability, stability, size, manufacturability, and power consumption. MEMS microphones are the future for hearing aids, and that future is here now.
An Introduction to MEMS Microphones
This technical brief explains the physical characteristics and the benefits of microphones based on MEMS technology.
A well-known recording component for simple, low-cost audio circuits is the electret microphone. Electrets belong to the category of condenser microphones, i.e., microphones that use a capacitive element to convert electrical signals from sound waves. Electret microphones are undoubtedly adequate in many applications; nevertheless, it’s good to be aware that you now have another option: MEMS.
MEMS stands for “microelectromechanical systems” and refers to (very) small devices that include moving parts. You’ve probably heard of MEMS gyroscopes or accelerometers, but it seems to me that MEMS microphones are not so well known.
In terms of the actual transducer element, a MEMS microphone is not fundamentally different from an electret. Both rely on a capacitive element that exhibits changes in capacitance corresponding to the air-pressure variations that we call sound.
But the capacitive element in a MEMS microphone is fabricated using (you guessed it) MEMS technology; in other words, the transducer is a microscopic component that fits right in with the microscopic semiconductor-based components in an integrated circuit. This innovation leads to microphone modules that are smaller and more user-friendly.
The above diagram gives you the general idea of how a MEMS mic is made. A single housing contains both the transducer and the signal processing circuit. One thing that separates MEMS microphones from typical ICs is the gap in the housing. We usually expect semiconductor devices to be sealed from the external environment, but in this case, we need something that allows sound waves to reach the transducer.
You might be inclined to approach MEMS microphones with the “if it’s not broken, don’t fix it” attitude. Why switch to MEMS when my electret is working just fine?
First, MEMS devices might provide enhanced audio performance. One manufacturer, Knowles, claims that MEMS mics offer “improved performance in varied environmental conditions.” My guess is that you would need to carefully consider your various design goals and constraints in order to determine if MEMS would be significantly better than electret in terms of audio quality.
Second, MEMS is smaller. In the world of consumer electronics, smaller equals better, though in your application the size of an electret microphone might be perfectly adequate.
Third—and this is the big one—MEMS microphones offer high levels of integration that enable impressive functionality combined with ease of use.
Complicated for the Manufacturer, Simple for the Board Designer
Electret microphones need a preamplifier circuit. Even when you have an IC specifically designed for this task (such as the MAX4466), you still need quite a few components:
Courtesy of Maxim Integrated.
MEMS microphones can eliminate this additional design effort by incorporating preamplifier circuitry into the microphone module. The output of the “microphone” is now a buffered (and, in some cases, amplified) analog audio signal. However, the output impedance might not be nearly as low as what you would see from a typical op-amp, so check the datasheet and design accordingly.
Courtesy of Knowles.
Actually, the integrated preamp is only the beginning. The signal-processing IC inside the microphone module is not limited to analog circuitry. So why not finish the job and digitize the analog waveform coming from the preamp? Now the “microphone” outputs digital audio data that can be sent directly to a microcontroller or digital signal processor.
Pulse density modulation is a common digital-MEMS-microphone output format.
MEMS microphones offer impressive features and, in certain applications, they are far superior to electret-based solutions. Keep them in mind for your next audio project.
Knowles :: SiSonic™ surface mount MEMS
Built on our CMOS/MEMS technology platform (originally launched in 2002), the SiSonic™ silicon-based microphone series is entering its fourth generation of development, with product shipments exceeding 5 billion units to date. The proven and evolving design series continues to support high-performance, high-density innovation in such applications as cell phones, digital still cameras, portable music players, and other portable electronic devices.
Design variables include ever-smaller sizes, lower profiles and mounting options, increased output capacities, and new digital audio options that eliminate analog noise. For manufacturers, surface mount designs eliminate off-line subassembly production costs. Customized designs are supplied on tape-and-reel and can be run through standard automatic pick-n-place equipment during in-line surface mount manufacturing.
The microphones can also be integrated with our patented IntelliSonic™ software and special porting designs to provide a precisely customized sound.
- New MaxRF models eliminate GSM/TDMA burst noise and provide wide-band RF noise suppression
- UltraMini footprint - less than 11.5mm
- Slim UltraMini footprint - less than 8.5mm
- Digital mics eliminate analog noise
- Integrated designs with differential or switchable gain
- Bottom port for thinnest ever designs
- Multiple performance modes (sleep, low-power, standard mode) optimize voice-trigger applications by entering a low-power high-SNR sensing mode
|SPA||SPA1687LR5H-1||3.35mm x 2.50mm x 0.98mm||Analog Differential||Bottom||-38 dBV||65||+/- 1dB||325|
|SPA||SPA2629LR5H||3.35mm x 2.50mm x 0.98mm||Analog||Bottom||-38.0 dBV||65||+/- 3dB||165|
|SPH||SPH0641LM4H-1||3.50mm x 2.65mm x 0.98mm||Digital||Bottom||-26 dBFS||64||+/-1 dB||Low Power: 270 / Standard: 700|
|SPH||SPH0641LU4H-1||3.50mm x 2.65mm x 0.98mm||Digital||Bottom||-26 dBFS||64||+/- 1dB||Low Power: 270 / Standard: 700 / Ultrasonic: 1000|
|SPH||SPH0644HM4H-1||3.50mm x 2.65mm x 0.85mm||Digital||Top||-36 dBFS||58||+/-1 dB||900|
|SPH||SPH0644LM4H-1||3.50mm x 2.65mm x 0.98mm||Digital||Bottom||-37 dBFS||65.5||+/- 1dB||900|
|SPH||SPh2642HT5H-1||3.50mm x 2.65mm x 1.00mm||Analog||Top||-38 dBV||65||+/-1 dB||185|
|SPH||SPh2644LM4H-1||3.50mm x 2.65mm x 0.98mm||Digital||Bottom||-37 dBFS||65.5||+/- 1dB||900|
|SPH||SPH6611LR5H-1||3.50mm x 2.65mm x 0.98mm||Analog||Bottom||-38 dBV||65||+/- 1dB||185|
|SPK||SPK0415HM4H||4.00mm x 3.00mm x 1.06mm||Digital||Top||-26 dBFS||61||+/-3 dB||650|
|SPK||SPK0641HT4H-1||4.00mm x 3.00mm x 1.00mm||Digital||Top||-26 dBFS||64.5||+/-1dB||Low Power: 270 / Standard: 700|
|SPM||SPM0687LR5H||4.72mm x 3.76mm x 1.25mm||Analog Single Ended (SE) Differential (Diff)||Bottom||-40 dBV (SE) / -35 dBV (Diff)||70dB||+/- 1dB||365|
|SPQ||SPQ1410HR5H||3.76mm x 2.24mm x 1.10mm||Analog||Top||-42 dBV||59||+/-3 dB||160|
|SPU||SPU0410HR5H||3.76mm x 2.95mm x 1.10mm||Analog||Top||-42 dBV||59||+/- 3 dB||160|
|SPU||SPU0410LR5H||3.76mm x 3.00mm x 1.10mm||Analog||Bottom||-38 dBV||63||+/-3 dB||160|
|SPU||SPU0442HR5HB-1||3.76mm x 2.95mm x 1.10mm||Analog||Top||-42 dBV||59||+/- 1dB||185|
|SPU||SPU1410LR5H||3.76mm x 3.00mm x 1.10mm||Analog||Bottom||-38 dBV||63||+/-3 dB||160|
|SPV||SPV0842LR5H-1||2.75mm x 1.85mm x 0.90mm||Analog||Bottom||-38 dBV||62.5||+/- 1dB||165|
|SPV||SPV1840LR5H-B||2.75mm x 1.85mm x 0.9mm||Analog||Bottom||-38 dBV||62.5||+/- 3dB||60|
|SPW||SPW0442HR5H-1||3.10mm x 2.50mm x 1.00mm||Analog||Top||-42 dBV||59||+/-1 dB||165|
|SPW||SPW2430HR5H||3.10mm x 2.50mm x 1.00mm||Analog||Top||-42 dBV||59||+/- 3dB||110|