As technologies advance, they often become smaller. This is partly the case with microphones and is certainly the case with the invention of the MEMS microphone.
What is a MEMS microphone? A MEMS (micro-electromechanical systems) mic is a pressure-sensitive diaphragm etched into a silicon wafer via MEMS processing. MEMS mics are largely based on electret capsules and typically have onboard preamps and analog-to-digital converters. MEMS mics are also known as mic chips or silicon mics.
In this article, we'll discuss MEMS mics in greater detail, touching on their construction, applications, and MEMS technology in general.
What Is A MEMS Microphone?
A MEMS microphone is a mic that is produced using micro-electromechanical systems processing techniques. MEMS mics are also commonly referred to as microphone chips, silicon microphones,
These mics are effectively etched into a semiconductive silicon wafer. A pressure-sensitive moveable membrane (diaphragm) is etched behind a stationary perforated plate. The perforated stationary plate and the diaphragm act together to form a capacitor (much like the design of a condenser microphone).
Like most MEMS technologies, MEMS microphones are fabricated on production lines using semiconductor silicon wafers and highly automated processes. Different layers of different materials are stacked on top of the silicon wafer, and then unneeded material is etched away.
When the etching is done, the transducer element of the MEMS mic has a movable diaphragm, a stationary but perforated plate, and the housing around it.
An ASIC (application-specific integrated circuit) is designed to fit with the transducer element of a MEMS microphone. It uses a charge pump to place a fixed charge between the stationary plate and the microphone membrane. ASICs are purpose-built microchips.
In this way, the basic MEMS transducer closely resembles an electret condenser microphone (ECM). To learn more about MEMS mics and ECMs, skip ahead to the section MEMS Microphones Vs. Electret Condenser Microphones.
Together, the transducer component and the ASIC are placed on a printed circuit board (PCB) and protected with a mechanical cover. This cover is designed with a small hole to allow sound into the microphone element. Sound must pass through the protective case and then through the perforated plate to reach the MEMS diaphragm.
Most MEMS microphones used today contain a second semiconductor die (integrated circuit) designed to function as an audio preamplifier.
A digital MEMS microphone design will typically have an additional metal-oxide-semiconductor (CMOS) chip that acts as an analog-to-digital converter. These chips effectively take the amplified analog audio signals and convert them into digital data. These ADCs allow digital MEMS microphones to be more easily integrated with digital products.
The most common format for digital encoding within MEMS microphones is pulse density modulation (PDM). PDM allows communication with a single data line and a clock. Receivers of PDM signals, like the MEMS mics themselves, are inexpensive and readily available.
How Do MEMS Mics Work?
So now that we know the basics of what a MEMS microphone is, it's time to look into how they work.
Before we get started, let's go over the components of a MEMS mic:
To cover all our bases, let's take a hypothetical digital MEMS mic so we can discuss the analog-to-digital converter as well.
- MEMS transducer element: included the diaphragm/membrane; perforated stationary plate, and housing.
- Printed circuit board (PCB): includes the ASIC polarizing unit; mic preamplifier, and analog-to-digital converter.
- Mechanical cover.
Sound waves enter the MEMS microphone through its cover and pass through the perforated housing and plate before reaching the diaphragm/membrane.
Sound waves cause varying sound pressure at the diaphragm and a difference in pressure between the front and rear of the diaphragm. This pressure difference causes the diaphragm to move in congruence with the sound waves.
But this diaphragm movement does nothing to create a mic signal unless there is a charge between the conductive diaphragm and the stationary plate. The plate and diaphragm essentially function as a capacitor and require a charge to function properly. The ASIC provides this charge.
Once charged, the plate and diaphragm may produce a voltage. Since they act as a capacitor, any change in capacitance will cause an inversely proportionate change in voltage. The capacitance is a function of the distance between the plate and diaphragm, so as the diaphragm oscillates back and forth, an AC voltage (mic signal) is produced.
This voltage needs amplification to be useful as an audio signal, so a separate integrated circuit (semiconductor die), included in the PCB, amplifies the signal.
In an analog MEMS mic, the amplified signal would then be outputted from the MEMS mic and sent to where it needs to go.
However, with a digital MEMS mic, there is an added process in which an ADC converts the analog signal (typically via PDM) before outputting the digital audio signal.
There are a few extra details to run through to better understand how MEMS microphones are designed to function. Let's begin by looking at a simple cross-sectional diagram of a MEMS mic:
The horizontal lines represent the layering of material, with the empty space representing the etching.
As we see in the above cross-section, the stationary plate is perforated, allowing sound to reach the diaphragm below. In this case, the polarizing ASIC chip is attached to the plate, but this is not always the case.
The back chamber in this example is closed off, which would mean that the MEMS mic is a pressure microphone (the diaphragm is only open to sound waves on one side). So this particular oversimplified mic example would have an omnidirectional pattern.
The back chamber also acts as an acoustic resonator, helping to tune the mic properly.
The ventilation hole is included to allow the back chamber to be at ambient pressure (whatever that may be in the environment).
Note that perforations behind the diaphragm (or in the outer protective shell) could be included in the design to yield a directional polar pattern.
For an in-depth article on how microphone transducers work in general, check out my article How Do Microphones Work? (A Helpful Illustrated Guide).
What Is Micro-Electromechanical Systems Technology?
Micro-electromechanical systems technology is, generally speaking, miniaturized mechanical and electro-mechanical elements made using the techniques of microfabrication.
The fabrication of MEMS technology involves the deposition of material layers, patterning by photolithography, and etching to produce the required cavities and shapes. In these ways, MEMS technology is very similar to semiconductor device fabrication and is even considered an evolution of semiconductor fabrication.
MEMS technology is very small. The components used are typically between 1 to 100 micrometres thick. The general size of a single MEMS device ranges between 20 micrometres to a millimetre.
MEMS devices consist of a central data-processing unit (such as a microchip integrated circuit). There are also typically several other components within the MEMS device that interact with the environment (such as microsensor or, in the case of a MEMS mic, the diaphragm/membrane).
To learn about MEMS technology in more detail (and from someone who understands it much better than I), check out the following link here.
What Are MEMS Mics Used For?
MEMS microphones are largely used in consumer-grade products that require some sort of microphone. Their small size and simple ADCs make them an easy (and inexpensive) addition to devices that require a mic but not an overly accurate mic.
MEMS microphones are often involved in the design of:
- Smarthome devices
- Desktop & laptop computers
- Hearing aids
Related reading on My New Microphone: What Kind Of Microphones Are Used In Cell Phones?
MEMS Microphones Vs. Electret Condenser Microphones
The MEMS microphone designed is largely based on the design of the electret condenser microphone capsule.
Both mic types include the following design details:
- Work on electrostatic principles.
- Capacitor-based capsules with one movable plate (diaphragm) and one stationary plate.
- A method of permanently charging the capsule (ASIC or electret material.
Both these mic types are also very common and typically used in consumer and professional electronics that require microphones.
Let's now look at the differences between these two commonly-used microphones. In particular, we'll discuss the benefits each microphone type has over the other.
Starting with the typical MEMS microphone:
- Smaller size
- Analog PCB and ADC incorporated within the package
- Lower-impedance (much better for noisy environments)
- More resistant to mechanical vibration
- MEMS microphone technology is being developed rapidly
And now for electret condenser microphones:
- Many legacy designs have electret microphone capsules
- ECM connections include pins, wires, SMT, solder pads and spring contacts which makes them much more flexible for design within various applications
- Better protection from dust and moisture, partly due to their larger physical size
- ECM products are available with intrinsic directionality: including omnidirectional, bidirectional, unidirectional, or even noise-cancelling
- Wider operating voltage range, allowing them to function with loosely regulated voltage rails
- Electret technology extends to professional-grade studio, measurement, and film microphones
Are microphones analog or digital? By the nature of their design, microphones are analog transducers that convert sound waves (mechanical wave energy) into audio signals (electrical energy). Most of the time, mics output an AC voltage (analog audio signals). However, there are digital mics on the market with built-in analog-to-digital converters that out digital audio.
To learn more about digital and analog microphones, check out my article Are Microphones Analog Or Digital Devices? (Mic Output Designs).
What is the sensitivity of a microphone? Microphone sensitivity can be thought of in three ways:
- Sensitivity rating: the microphone's output level at a given sound pressure level.
- Mic sensitivity level in Windows OS: the volume/gain of the inputted microphone in Windows OS.
- Mic sensitivity: the way in which a microphone reacts to smaller nuances in sound pressure level. The transient response has a lot to do with this “sensitivity.”
To learn more about microphone sensitivity and transient response, check out the following My New Microphone articles:
• What Is Microphone Sensitivity? An In-Depth Description.
• What Is A Good Microphone Sensitivity Rating?
• What Is Microphone Transient Response & Why Is It Important?
Choosing the right microphone(s) for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Microphone Buyer's Guide. Check it out for help in determining your next microphone purchase.