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What's New in MEMS?

By Mike Rosa, PhD.

While the MEMS market has matured in recent years, there’s still a lot of healthy growth ahead as new device technologies come to market and segment stalwarts (IMUs, RF MEMS, etc.) see device volumes rise with further adoption in end-user applications. Here, we briefly review 2016 and discuss some recent additions to the MEMS segment: MEMS-based fingerprint sensor (FPS) and light detection and ranging (lidar) devices.

In 2016, MEMS device segment revenue was around $13 billion (CAGR ~ 13%, through 2021) with unit volume at just under 17 billion (CAGR ~ 21%, through 2021), as shown in figure 1.

The majority of that device revenue was generated by growth in the consumer and automotive markets, each representing >50% and 25% of total device revenues, respectively. This is hardly surprising as mobile applications of MEMS have increased dramatically in recent years and the push towards advanced driver-assistance systems (ADAS) has also bolstered the use of MEMS in automotive applications.

Figure 1. Gyroscopes, accelerometers and the digital compass once led the MEMS revolution, but today’s device volume forecasts are dominated by the consumer communications sector, with microphones and RF filter devices constituting greater than two-thirds of all MEMS device volume by 2021. (Source: Yole Développement, 2016, “Status of MEMS Industry”)

Fingerprint Sensors

Let’s first take a look at the consumer segment and specifically, the increasingly prolific FPS device found on so many of today’s mobile devices. FPS devices are used in a number of security-centric applications. According to Yole Développement, their 2016 market value was $2.9B with roughly 800M units shipped. FPS devices are forecast to increase to over 2 billion units with a market value approaching $3.7B by 2022.

Currently, there are four different fundamental operating FDS technologies: optical, capacitive, thermal and ultrasonic (see figure 2).

Figure 2. There are four well-known techniques for making FPS devices: optical, capacitive, thermal and ultrasonic. For today’s mobile applications, the capacitive type is most widely used, although piezo-based ultrasonic technologies are quickly gaining share in those markets. (Source: Yole Développement, 2017)

The most prolific device technologies used today are capacitive-based. You’re probably most familiar with the capacitive FPS device in your smartphone or laptop computer.While each FPS technology has different pixel counts and densities and overall levels of effectiveness, it’s the sensing area that ultimately determines utility and volume adoption in the consumer market. In this regard, capacitive and ultrasonic technologies beat out the competition with somewhat conservative sensing areas of ~28mm2 and ~36mm2, respectively. Combined with pixel counts north of 10K, pixel densities >500 ppi and extremely low power consumption in standby mode, it’s easy to see why these two technologies are prime candidates for use in mobile products.

Although capacitive devices already enjoy broad adoption, piezo-based ultrasonic devices are just now entering the market. With increased sensitivity capable of imaging both epidermal and dermal layers of the finger, piezo-based devices offer greater security and are impervious to the effects of dust and moisture on the scanning surface.

Today, piezo-based FPS chips can be fabricated using either aluminum nitride (AlN), scandium-doped aluminum nitride (ScAlN) with scandium concentrations in the range of 20–30%, or lead zirconium titanate (PZT) piezo-materials, each with an increased electromechanical coupling coefficient (kt2). While the CMOS compatibility of the AlN family of films is preferred, there are device architectures that might enable a PZT-based device by taking advantage of CMOS/ sensor integration based on wafer bonding. Various deposition technologies can be utilized depending on the film parameters of interest, but FPS chips will likely rely on physical vapor deposition (PVD) as a high-productivity manufacturing approach.

So, that leaves one big question: is this a 200mm or 300mm play? For now it seems 200mm (and small gen panel) is the way forward; however a 300mm play in the future should not be ruled out for this high-volume, increasingly ubiquitous device.

MEMS-Based Lidar

Light detection and ranging (lidar) mechanisms are another device-type in growing demand. They are currently used in geographic information sensor, military, CMOS image sensor and robotics applications. However, Yole Développement forecasts significant growth over the next five years in lidar mechanisms being widely used in the evolving autonomous vehicle area (see figure 3).

Lidar is a technique whereby laser light is used to illuminate a specific target and measure its distance from the source. 

Figure 3: Automotive applications are forecast to grow substantially by 2022. (Source: Yole Développement, 3D Imaging and Sensing Technologies Report, 2017)

Encased in a spinning “can” mounted on the roof of a car, it is the mechanism by which many present-day autonomous vehicles “see” their surroundings. While effective, these “cans” are not attractive. Nor are they cheap: prices range from $10K–$30K per unit, and can climb up to $75K for the most advanced models.

To address this cost issue, several companies are touting discrete, MEMS-based lidar solutions with price points targeted below $100. With die sizes on the order of 25mm2, these devices are small 2D-scanning mirror architectures operated by a series of electromechanical comb-drive structures. There are several R&D efforts underway to develop new and interesting technologies in this space. The Berkeley Sensor & Actuator Center (BSAC), for example, has its own approach for MEMS-based lidar.

Another lidar solution is an optical phased array (OPA)—a die-based mirror composed of many smaller reflective micromechanical elements—which can carry out sophisticated beam forming, steering and tracking of multiple objects. Each mirror element in the OPA is ~2μm wide and 35μm long. The full 1D mirror device is capable of scan angles >22° at scanning speeds in excess of 500 kHz with operating voltages <10 V. While similar to the MEMS-based lidar devices with mirror elements manipulated by electromechanical comb structures, these comb-drive structures can be as thin as 300nm with equivalent 300nm spacing.

So, while metal deposition will be important in forming high-quality mirror surfaces for OPA devices, it seems that deep reactive ion etch (DRIE) will be the key enabling process, especially if device designs call for reliable submicron process control.

While the future of lidar looks bright, there is ample competition between component OEMs looking to enable this exciting segment of the automotive industry. There is much work underway in this sector involving detection of vehicles, objects, pedestrians and even occupants. For example, recently, Bloomberg[1] reported that Texas Instruments is introducing new automotive chips using radar technology that could challenge technologies such as lidar. So we will likely see many players in this field with chip-based detection systems, from large suppliers to new upstart companies trying to leverage new technologies.

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