OVER THE LAST COUPLE OF DECADES, THE MARKET for Micro-Electro-Mechanical Systems (MEMS) has been rapidly expanding. While the first commercial applications for MEMS technology were pressure sensors used by the automobile and medical industries, these days MEMS has a large range of other applications spread across numerous market sectors. Many of today’s most common MEMS applications are found in consumer electronics (CE), such as smart phones, tablets, and video game systems. Additionally, a variety of other applications, including wearable electronics for the health and fitness markets, also incorporate MEMS, and as the use of these devices increases, the demand for MEMS should become even more ubiquitous.
Because nearly all MEMS devices are built on silicon wafers, many of the same fabrication process tools—including vacuum—used by the integrated circuit (IC) industry are used to fabricate MEMS. The MEMS industry has also developed a handful of specialized process tools that use vacuum, which aren’t used by the semiconductor industry. Given this, vacuum plays an integral role in MEMS development, and as the MEMS market continues to flourish, the vacuum industry stands to benefit from this growth.
The Rise of Micromachines
MEMS are basically tiny machines: microscopic structures and devices that combine mechanical, optical, and fluidic elements with electronics. Typically smaller than a grain of sand, the size of MEMS devices can range from less than one micron up to several millimeters. While some MEMS devices are fairly simple structures without any moving parts, others are incredibly complex, featuring multiple moving structures integrated with microelectronics.
MEMS technology was first developed in the 1970s and early 1980s and was initially called silicon micromachining. The MEMS label was created in the ‘90s, when the U.S. Department of Defense began actively investing in the technology. The first commercial application of MEMS technology was microsensors used to detect strain in steel, and these devices were used primarily in the automobile and medical markets.
According to Dr. Michael Huff, the founder and director of the MEMS and Nanotechnology Exchange (MNX), which offers MEMS design and fabrication services, the first generation of these sensors was developed following a discovery by an employee of Bell Telephone.
“There was a gentleman at Bell Telephone Labs named Charles Smith, who saw that there is a piezoresistive effect in silicon, meaning that if you strain it, you’ll see a change in resistance,” said Huff. “Not too long after that, people started gluing pieces of silicon to steel plates as strain gauges. Later, we learned that you could do what’s called an anisotropic etching of silicon, allowing you to make thin membranes out of silicon with controlled dimensions, which were important for mechanical types of sensors such as pressure transducers.”
Following these initial discoveries, this early version of MEMS technology was further refined and adopted for commercial use in the automotive and medical industries.
“People soon realized they could put the silicon strain gauges directly into the diaphragm itself, and when that happened, these devices started hitting the market quite quickly,” said Huff. “They were used as pressure transducers for automobile applications—mainly manifold air-pressure sensing—and medical applications. When I worked for a large healthcare company, we discovered you could replace a manometer—basically a macro-scale pressure sensing device— with a silicon transducer, which are a lot cheaper and more reliable. Plus, they’re disposable, so you didn’t have to sterilize them after each use.”
A variety of MEMS sensors developed by Bosch Sensortec for use
in consumer electronics, safety systems, industrial technology,
and logistics. Photo courtesy of Bosch Media Service.
Over the next few decades, the MEMS industry created myriad new versions of this miniaturized technology for use in a wide range of different sectors. While the MEMS market is currently expanding like never before, Huff believes we’ve only scratched the surface of the technology’s potential applications.
“I’ve been involved with MEMS technology for 30 years,” said Huff. “The market is growing, and it’s growing faster than the traditional IC market. The number of MEMS applications out there is phenomenally large, and I think right now we’re just touching the tip of the iceberg of what’s possible.” The two major categories of MEMS devices include microsensors and microactuators. Microsensors, such as accelerometers and gyroscopes, detect information from the local environment. Microactuators, such as microvalves and micropumps, perform certain actions based on information they receive.
According to the MNX website, www.memsnet. org, the MEMS industry has developed microsensors for “almost every possible sensing modality, including temperature, pressure, inertial forces, chemical species, magnetic fields, radiation, etc.” The different types of microactuators are just as diverse, including “microvalves for control of gas and liquid flows, optical switches and mirrors to redirect or modulate light beams, independently controlled micromirror arrays for displays, microresonators for a number of different applications, micropumps to develop positive fluid pressures, microflaps to modulate airstreams on airfoils, as well as many others.”
Wafer production in Bosch’s new semiconductor manufacturing
facility, which produces ICs and MEMS technology.
Photo courtesy of Bosch Media Service.
When integrated together with microelectronics, these two types of MEMS devices can perform an incredible array of functions. MEMS microsensors gather and measure environmental data, and their electronic components covert this data into an electric signal that is sent to the microactuators. The microactuators respond by moving, pumping, filtering, or regulating their environment depending on their dedicated purpose. Such technology is truly amazing in that these miniature devices are able to sense, actuate, and control at the micro level, but they generate effects on the macro level—and many times do so much better and more efficiently than their macro-scale counterparts.
These days there are a massive number of commercial applications for MEMS stretching across a diverse range of markets, such as automotive, CE, healthcare, defense, construction, aeronautics, and industrial manufacturing. Some of the most common products featuring MEMS technology include inkjet printers, automobile air bag deployment and rollover control systems, microphones for mobile devices, blood pressure sensors, digital light processors (DLP) for digital film projection, wearable devices for health and fitness, and a variety of different devices used in space exploration.
Overall, the MEMS sector did nearly $12 billion in business in 2013, with more than 10 percent growth from 2012, according to a study by Yole Development. In the years ahead, Yole estimates that the MEMS market will nearly double and reach $22.5 billion by 2018.
Consumer Electronics Creates Big Demand
Of the numerous MEMS applications, the largest demand is currently for microphones, especially those used in smartphones and tablets. These microphones have proven exceptional for canceling ambient noise, offering high-definition audio quality for video recording, and improving the accuracy of voice command functions. For the first time ever, the market for MEMS microphones is set to exceed $1 billion in 2014, according to a report by IHS Technology. This is in large part due to the exploding popularity of mobile devices, such as Apple’s iPhone and iPad.
“The largest market for MEMS right now is microphones that go into smart phones and tablets,” said Huff. “There are multiple billions of these types of devices being produced because there are so many cell phones and tablets out there.”
Similar MEMS microphones are also being used in the automotive market, offering improved voice command functions and hands-free calling. Outside of microphones, MEMS technology is also being used to provide for more intuitive user interface functions in mobile smart devices.
“There are also MEMS applications that allow your cell phone or pad to reorient its screen depending on how it’s positioned with respect to gravity,” said Huff. “These are MEMS-based inertial sensors— three-axis accelerometers. Now you’re also starting to see three more axes added to sense rotational changes—gyroscopes— for a total of six axes. These are being used in smart phones and tablets and are basically GPS-like devices.”
Buoyed by the burgeoning growth of MEMS in mobile electronic devices, another arena that is rapidly expanding and may play an even bigger role in the future is wearable electronics for the health and fitness markets. From smart watches that monitor heart rate to armbands that track physical activity, these wearable devices are attracting serious attention—and already bringing in substantial revenue. According to IHS, MEMS-based wearables are set to generate $41.3 million in revenue in 2014, and by 2016, that figure is forecasted to reach $91.5 million.
One of the most closely watched examples of such wearables is the new Apple Watch, which includes a heart monitor with two types of optical sensing systems: one that detects visible light and one that detects infrared. The Apple Watch is set to hit the market in early 2015, and if it and other wearable technologies take off in a big way, the demand and revenue opportunities for MEMS should expand as well.
Viewing a Bosch MEMS accelerometer through a scanning
electron microscope, with a tiny insect added for scale.
Photo courtesy of Bosch Media Service.
MEMS and Vacuum
Because the fabrication of MEMS evolved from the process technology used to create integrated circuits, many of the same techniques and tools used in semiconductor labs are also used in MEMS manufacturing. For instance, the three basic building blocks of MEMS fabrication are deposition, lithography, and etching. Given this, vacuum plays a crucial role in the production of MEMS.
“We use as much vacuum equipment as the IC industry—and perhaps even more,” said Huff. “We incorporate a lot of the same semiconductor process tools to fabricate MEMS, and whether it’s physical vapor deposition (PVD) or chemical vapor deposition (CVD), we always use vacuum,” said Huff. “For instance, PVD—particularly e-beam— uses very high vacuum levels. We also use traditional low-pressure CVD equipment that is connected to vacuum pumps, which are usually more of a roughing-type of pump. Basically anything you see in semiconductor labs is also used in our manufacturing.”
However, because MEMS devices differ considerably from IC technology in their mechanical nature, the MEMS industry has also developed several specialized processes and tools, including vacuum, which you won’t find in IC manufacturing.
“We have a few specialized tools—not more than a dozen or so—that are unique to the MEMS industry,” said Huff. “For example, with the etch systems, we use vacuum systems that are kind of unique to MEMS technology called deep reactive ion etching (DRIE). This technology allows us to etch through a whole silicon wafer, creating a nearly vertical hole through the wafer similar to a drilling process. That technology uses a roughing pump backed up with a turbo pump, so it’s an etch system that basically runs at a fairly low vacuum level.”
Additionally, vacuum is widely used in MEMS packaging and testing. The highly complex mechanical functions of many MEMS devices require a specialized environment that only vacuum packaging can provide. Huff noted that such packaging equipment is typically a very specialized area of MEMS, incorporating IC technology that’s augmented with some level of vacuum packaging capability.
“With many MEMS devices, such as inertial sensors, you can’t have these packaged in an ambient environment with normal pressures because this would damp the sensor too much,” said Huff. “In order for these things to operate and be more sensitive, you typically want to package them in a vacuum environment. Traditional IC guys don’t care about vacuum packaging because they can basically squeeze plastic over the IC chip and everything still functions fine.”
Once a device is fabricated and packaged, vacuum plays a crucial role in testing the functionality of MEMS technologies.
“Apart from packaging, the whole testing process to ensure these devices function properly— whether it’s done as part of the manufacturing process or experimental testing—involves vacuum and is a very important part of the equation.”
Bosch Sensortec’s BME280 MEMS sensor, which combines
pressure, humidity, and temperature measurement into a
single device. Photo courtesy of Bosch Media Service.
Differences from IC
Outside of the differences in process technology, the MEMS industry also places a higher value on certain things the semiconductor field isn’t typically concerned with. For example, MEMS often produces thicker structures—thicker thin films— than most ICs. Huff noted that it’s not unusual for MEMS fabricators to deposit 10 or 20 microns of polysilicate on a substrate, which is something you won’t see in the IC field. Moreover, unlike the IC industry, the MEMS field is highly interested in the mechanical properties of the thin films.
“As long as the thin film isn’t delaminating from the wafer, IC guys don’t really care about the mechanical properties of the films; they’re more interested in the electrical properties,” said Huff. “Since we’re making micromechanical structures, the film stress and the mechanical material properties matter a whole lot for us. So we’re always playing with our processes in order to get more predictable mechanical properties for the film, whether it’s nitride, polysilicate, metals, or whatever.”
Another factor that sets MEMS apart from the IC field is the fact that the MEMS industry isn’t concerned with using the largest silicon wafers possible. Where the IC industry is talking about moving toward 450 mm wafers, the MEMS industry is still using comparatively small wafers for some of its most popular applications.
“We typically don’t care so much about having the largest wafers possible, like the IC industry does,” said Huff. “The largest MEMS application right now is microphones, and those are manufactured on 150mm wafers, which tells you something about the economics of this industry.”
The Standardization Challenge
While the MEMS field has long been able to borrow manufacturing techniques and tools from semiconductor industry, as the MEMS market grows and the devices become more and more complex, the IC processing technology is proving inadequate. The biggest obstacle for the widespread commercialization of MEMS is the lack of standardized design tools and founding processes Many of the latest MEMS devices require highly customized manufacturing methods, and fabrication processes that work for one MEMS application typically can’t be used for others. Because of this, it often takes much more time and money to develop MEMS devices compared to semiconductors, which have well-established design rules and standardized foundry processes.
“IC fabrications have converged to certain standard processes, which can be used to implement all kinds of circuit functions, while MEMS fabrication is much more customized and diversified among different applications and different foundries,” notes the MEMS Foundry Engagement Guide. “For example, pressure sensors and inkjet printing nozzles are fabricated by bulk micromachining, and airbag accelerometers and micromirror projection arrays are fabricated by surface micromachining. There is currently no library of design rules available for MEMS.”
Huff echoes this sentiment when comparing the differences between MEMS and IC fabrication. “If you’re making ICs, there are already design tools and standardized founding processes that enable you to know almost exactly what you’re going to get,” said Huff. “In a year, you could develop a new chip design for some application. It’s a very predictable engineering exercise for the IC industry, whereas in MEMS you have startups that have an idea, and then you have to figure out how to design it, how to design it for manufacturability, and then they have to figure out how to make the thing and come up with a customized process for it—that’s a much higher hurdle to cross.”
The good news is that in spite of the hurdles faced by the MEMS industry, many are already working diligently on standardization. According to Huff, MNX has thus far completed almost 3,000 different MEMS product developments, incorporating nearly every type of fabrication process and producing nearly every type of application. Based on the data collected from all of these different runs, MNX has identified specific patterns that Huff believes will allow the company to develop reusable process modules that can be used for a number of different MEMS devices.
“Using these process modules, you won’t have to reinvent the wheel with every new application,” said Huff. “Of course, you may have to put together some new process modules to enable a custom device to be made, but it will be a heck of a lot easier, cheaper, and less time consuming than if you had to develop a completely customized process sequence from scratch—which is basically where most people in MEMS are at right now. This is something we’re working on right now, and we’ve filed patents on it. We’re not quite ready to release this to prime time yet, but we’re definitely making progress.”
Alissa M. Fitzgerald, Ph.D., founder and managing member of the MEMS product development firm A.M. Fitzgerald & Associates, also believes that standardization is key to the successful evolution of MEMS manufacturing. In her whitepaper “No Rest for the Weary: MEMS Manufacturing Must Press On, With Big Markets Ahead,” Fitzgerald noted that standardization efforts should initially focus on MEMS material properties.
“The industry needs to develop best practices for characterizing MEMS materials, methods to employ those practices to stabilize and monitor material quality, and to define design rules for working with the materials,” writes Fitzgerald. “Given the variety of MEMS devices and the different ways they employ materials, standards should be tailored to device type, such as pressure sensor, accelerometers, gyroscopes, etc. Standardized materials, and design rules for working with them, will fully enable simulation-driven design, which is the key to a truly fabless industry. Streamlining the MEMS development cycle is essential to reducing time to market and the capitalization needed by MEMS companies.”
The latest generation of MEMS acceleration
sensors from Bosch provide airbag control in
a vehicle’s front, sides, and rear.
According to Huff, while MEMS is currently an important segment of the vacuum market, due to the fact that the industry often uses dated and rebuilt vacuum equipment from the semiconductor field, its full potential as a growth driver for the vacuum industry has yet to be realized.
“I think that the growth of MEMS is definitely having a positive effect on the vacuum industry,” said Huff. “I won’t say it’s having a really huge effect yet because the MEMS guys are still using older IC equipment for the most part. So if you ask if this is good for the vacuum industry, I would say, ‘Yes.’ But is it going to make everyone in the vacuum industry a billionaire overnight? No.”
That said, with the recent surge in demand for MEMS from the Consumer Electronics market and the potential for its use in wearable devices and other advanced sensing technology, the MEMS industry could soon become much more integrated into the mainstream. And whether we realize it or not, MEMS is already a big part of our daily lives—and it stands to become even more so in the future.
“If I had to guess, I’d say the average person in the US probably owns around 10 MEMS devices right now,” said Huff. “They might not know they own them, but they do. You have a few in your cell phone, a few in your car… but if you had people using wearable devices and other applications controlling your house and office environment and things like that, it’s kind of easy to envision that this number could grow to a few hundred or even a thousand sensors per person. If you imagine all of the possible applications, then this could become a much different market at that point.”
With such vast potential for increased growth and revenue, it seems highly likely that MEMS fabrication will eventually become much more standardized. At that point, the rapid commercialization of MEMS that many have long predicted was just around the corner will finally come to fruition. And if this happens, the effect MEMS has on the vacuum industry could become exponentially greater.
“If we can get this industry to the point where it’s much more predictable, cost effective, and time effective for developing products,” said Huff, “then I think it would be a complete game changer, not only for MEMs, but for a lot of other industries, including vacuum.”