The MEMS technology seems to byte bigger and bigger chunks from the conventional semiconductor market. And although the accelerometer is the first thing that pops into mind when you think of an example, the most widespread MEMS representative is most probably the microphone.

For quite a while now, MEMS technology microphones have been on the market. They are mostly present in small handheld devices (mobile phones) and their appearance was primarily motivated by the continuous drive of having smaller and lighter end products. We will first go through a list of the advantages that these new mics have over the old conventional electret ones, and then we will have a look at a few particularities.

Since electronics first appeared as a profession, engineers have always been confronted with the prospect of miniaturization. Now-a-days, when there is a constant pressure to minimize even more, to reduce PCB area and finished product weight and dimensions, MEMS microphones came to help particularly with these aspects. The omnipresent mobile phone is perhaps the best example of a product where engineers have to design with minimum component footprint on the PCB and where achieving a flat and thin end-product is a declared design goal. They are greatly helped by the MEMS technology, as microphones manufactured under the new patented methods provide a volume about 10 times smaller than electret counterparts!

From the layout point of view, this is a great achievement. Not only that, but combining CMOS and MEMS technology is already achievable and what we can see on some of these new generation microphones is the integration of additional circuitry in the same package. Typically this would be either an amplification stage, or an amplification stage followed by an AD converter and some logic to shift digital data out of the microphone.
Just compare this:

to this:

and you will get an idea of how much simpler the design becomes. Therefore, the real-estate gain on the PCB comes not only from the smaller footprint of the microphone itself, but also from removing the need of additional circuitry…and in case of ultra-compact designs, even an 0402 or 0201 package matters.

Smaller volume also means smaller weight, and this is not something to be disregarded in compact design. For the overwhelming majority of the projects, the final product has very tight weight specifications which designers usually struggle to meet. Although it is more a concern for the mechanical engineers, reducing weight by using MEMS microphone instead of electret type might bring the end product weight down by that extra needed gram or so.

PCB size is not the only factor affecting the cost of the final product which can be influenced by these microphones. Traditionally, electret microphones were not capable of withstanding very high temperatures without having their electrical or acoustic characteristics negatively influenced. From a manufacturing process point of view, this means they could not be soldered on the PCB through reflow, where temperatures as high as 260 Celsius are required for at least 10 seconds. In many cases, all other components on the board could be reflowed through the oven, but then a special station had to be inserted in the process flow in order to solder this particular component (either manually or automated). MEMS microphones have eliminated this disadvantage, being capable not only to withstand the soldering temperatures of lead free reflow process, but also bringing extended operational temperature range in the design (the automotive range of -40Celsius to +85Celsius can be easily obtained with MEMS; the MSM4C family from Memstech is actually specified to be operational up to 100Celsius). Below, you may see a recommended reflow temperature profile for Analog Devices’ ADMP421, where Tp is specified as 260Celsius for lead-free soldering:

However, no matter how good these devices are and how important the cost-savings would be, we all know everything comes down to the most important electrical parameter…price! Probably the first available MEMS microphones were very expensive. But surely that must be because the technology was new and because demand was still small (apparently, even the first mouse for a PC was priced initially at $10K). Having been for a while on the market, however, means the cost of a MEMS microphone is not only comparable, but in many cases also lower than the one for conventional electret type mics. A rapid search on Digikey yields prices of even less than one euro per piece for significant quantities (and this is obviously not the best you could get).

MEMS microphones are produced in a new technology, and each company that designs and manufactures them works with different patented methods. Knowels Acoustics, for instance, designed both a back-plate and a diaphragm on the surface of the silicon wafer, together with all necessary electrical connections. Using a chemical process, the diaphragm is physically “detached” partially from the rest of the assembly, and it is thus capable of vibrating in resonance with the ambient sound. Since the diaphragm and the back plate actually represent a capacitor, changing the distance between them in the rhythm of the sound actually generates a charge variation in the same pattern, which can be converted to voltage variation signal further down the processing chain:

Akustica uses an air cavity inside the microphone itself in order to achieve the sound-to-electricity conversion. The company also combines MEMS and CMOS design on the same wafer in order to manufacture what they advertise to be the world’s smallest microphone:

One important aspect in the design of MEMS microphones is to enable sound to reach the vibrating or resonating element inside the device. This is done through an acoustic port hole operated in the housing of the MEMS device. Innovative design means that a good variety of packaging solutions are available for the product engineer, depending on the location of the acoustic device in the final product. In mobile phones, for instance, its ideal position would be near the user’s mouth, on the same size of the PCB with the keypad. The drive for obtaining thinner and slimmer phones, however, has pushed the microphones on the other side of the board, together with the other “high profile” electronic components. This arrangement proved a bad move since it significantly lengthened what is called the acoustic path from the microphone to the outside world. To account for this aspect, some MEMS microphone manufacturers have provided the acoustic port hole on the bottom side of the microphone package. This means it may safely be mounted on the opposing size of the PCB, as long as a matching acoustic hole is provided through the board exactly under its own package acoustic hole. This way, it is possible to mount the microphone on the opposing side of the keyboard, and still obtain a short acoustic path from the environment to the microphone (the SPM0404LE5H from Knowels Acoustic, for instance, has this useful feature).

There is a good number of MEMS microphones manufacturers on the market. One of the most known names there is Analog Devices, with more than 20 years experience in MEMS. This apparently helps them to bring on the market the device with the smallest profile (not smallest footprint, but smallest height). They can achieve an amazing 1.1mm worst case height for their ADMP421. Wolfson Microelectronics, Knowels Acoustic and Memstech are closely behind with a height of 1.1 mm but in nominal case. Their advantage, however, is represented by the standard case which they have adopted (for some of their products, at least): 3.76mm x 2.95mm x 1.1mm. This means their products are footprint compatible, and OEMs always like to have a second source for every component on the PCB. There also is another standard package size, slightly larger in footprint: 4.72mm x 3.76mm x 1.1mm, but also here Analog Devices is better by 0.1mm in height, providing a nominal of 1mm with its ADMP401. Akustica brings on the market a MEMS microphone in a tiny package of 2mm x 2mm x 1.25mm. It is slightly higher than the others, but its footprint is minimal.