Introduction to MEMS (Microelectromechanical Systems)

This article explores basic characteristics and common applications of a technology that has been incorporated into a wide variety of high-performance electronic devices.

I always appreciate a name that is truly informative and, in this respect, the term “microelectromechanical systems” (MEMS) does not disappoint—it’s as much a concise definition as it is a name.

So what does MEMS mean?

MEMS refers to technology that allows mechanical structures to be miniaturized and thoroughly integrated with electrical circuitry, resulting in a single physical device that is actually more like a system, where “system” indicates that mechanical components and electrical components are working together to implement the desired functionality. Thus, it’s a micro (i.e., very small) electrical and mechanical system.

Mechanical to Electrical to (Micro)mechanical

Mechanical components and systems are generally considered to be less technologically advanced than comparable solutions based primarily on electrical phenomena, but this doesn’t mean that the mechanical approach is universally inferior. The mechanical relay, for example, is far older than transistor-based devices that provide similar functionality, but mechanical relays are still widely used.
Nevertheless, typical mechanical devices will always have the disadvantage of being hopelessly bulky in comparison to the electronic components found in integrated circuits.

 The space constraints of a given application may cause electrical components to be favored or required, even when a mechanical implementation would have resulted in a simpler or higher-performance design.

MEMS technology represents a conceptually straightforward solution to this dilemma: if we modify the mechanical devices such that they are not only very small but also fully compatible with integrated-circuit manufacturing processes, we can, to a certain extent, have the “best of both worlds.”

This is a physical gear and chain. This machinery moves and functions as you would expect a gear and chain to move and function. However, the links in the chain are about 50 µm long—i.e., less than the diameter of a human hair. Image courtesy of Sandia National Laboratories.

What Makes a MEMS?

In the previous section, I stated that MEMS technology is a conceptually straightforward solution. As you might expect, coming up with the idea of a microscopic mechanical device is much easier than actually building it.

We use the verb “to machine” to describe the work of turning a piece of metal into a mechanical component such as a gear or a pulley. In the MEMS world, the equivalent term is “to micromachine.” The tiny mechanical structures in a MEMS device are fabricated by physically modifying silicon (or another substrate material) using specialized techniques about which I know almost nothing. These silicon mechanical structures are then combined with silicon integrated circuits, and the resulting electromechanical system is enclosed in packaging and sold as a single device.

As explained in a paper on MEMS published by Loughborough University in England, MEMS devices make use of micromachined structures, sensors, and actuators. Sensors allow a MEMS to detect thermal, mechanical, magnetic, electromagnetic, or chemical changes that can be converted by electronic circuitry into usable data, and actuators create physical changes rather than simply measure them.

Examples of MEMS Devices

Let’s look at an example of the functionality and internal structure of a MEMS device.

Micromachined cantilever switch beams. Image courtesy of Analog Devices.

This graphic conveys the physical structure of micromachined cantilever switch beams. There are four switch beams and each one has five contacts (the use of multiple contacts is a technique for reducing on-state resistance). The switch beams are actuated by an applied voltage.

Image courtesy of Analog Devices.

Here we see the MEMS switch (on the right) and the associated driver circuitry (on the left), interconnected and housed in a QFN package. The driver circuitry allows a typical digital device, such as a microcontroller, to effectively control the switch because it does everything necessary to generate a ramped, high-voltage actuation signal that promotes effective and reliable switch operation.

MEMS Applications: When Are MEMS Devices Used?

MEMS technology can be incorporated into a wide variety of electronic components. The companies that make these components would presumably claim that a MEMS implementation is superior to whatever was used before the MEMS version became available. It would be difficult to verify enough of these claims to justify a generalized statement along the lines of “MEMS devices offer significantly better performance than non-MEMS devices.” However, my general impression is that in many situations MEMS is indeed a significant step forward and, if performance or ease of implementation is a priority in your design, I would look at MEMS devices first.
In the context of electrical engineering, MEMS technology has been incorporated into four product categories:

• Audio
• Sensors
• Switches
• Oscillators

There might be some less-common products that don’t fit into one of these categories; if you’re aware of something that I overlooked, feel free to let us know in the comments.

Audio

In the audio domain, we have MEMS microphones and MEMS speakers. The basic characteristics of a MEMS mic are conveyed by the following diagram.

Sensors

Sensors are the dominant application of MEMS techniques; there are MEMS gyroscopes, inclinometers, accelerometers, flow sensors, gas sensors, pressure sensors, and magnetic-field sensors.

Switches

Electrically controlled switches are, in my opinion, a particularly interesting application of MEMS technology. The ADGM1004, which I wrote about in this article, is easy to control, works with signal frequencies from 0 Hz to over 10 GHz, has less than 1 nA of leakage current in the off state, and provides an actuation lifetime of at least one billion cycles.

Combining a micromachined resonator with excitation circuitry and sustaining circuitry results in a MEMS oscillator. If you'd like to investigate an actual MEMS component, you can check out a news article from 2017 in which I discussed the SiT2024B MEMS oscillator from SiTime.

Diagram courtesy of SiTime.

Oscillators

I don’t have much experience with MEMS oscillators, but I think that they might be an excellent choice in demanding applications; in the abovementioned article on the SiT2024B, I point out that based on SiTime’s information, a MEMS oscillator can seriously outperform quartz-based oscillators.

Conclusion

Many electronic devices incorporate MEMS technology, and it’s likely that you’ll come across a MEMS component sooner or later—if not every time you design a board. I hope that this article has provided a good overview of what MEMS is and how it is used in electronic design.