The Future of the MEMS Inertial Sensor Performance, Design and Manufacturing

Michael Perlmutter1, Stephen Breit2

1 Civitanavi Systems s.r.l. Via del Progresso 5, 63827 Pedaso (FM) ITALY

2 Coventor, Inc., 1000 Centre Green Way Suite 200, Cary, NC, 27513 USA

Inertial Sensors and Systems 2016 Karlsruhe, Germany

978-1-5090-2515-2/16/$31.00 ©2016 IEEE P10 Abstract The market for high-performance inertial sensors, tactical grade and above, has been dominated to date by macro-scale devices such as HRGs, RLGs and FOGs. While the size, power requirements, and cost of these sensors have decreased considerably over the past two decades and further reductions can be expected, the tactical IMU size remains on the order of 0.5l, and the cost remains in the $10k range. During the same period, micro-scale inertial sensors based on MEMS technology have been introduced for automotive and consumer electronics applications and are now produced at the rate of several million per day. MEMS IMUs have become truly ubiquitous, however they have so far fallen short of the requirements for high-performance applications. Nevertheless, there have been continuing reductions in their size, power and particularly cost, to less than $3 for an IMU. There is great interest in understanding future trends for MEMS and their potential threat to traditional sensors for tactical and navigation grade applications.

One author has many years of experience in developing macro-scale high-performance sensors and IMUs. The other author has many years of experience in supplying design tools to leading MEMS organizations worldwide. We will first describe trends that are affecting MEMS inertial sensors, including commoditization, consolidation of advanced CMOS manufacturing, market demands for increasing package- and chip-scale integration, the Internet of Things (IoT), and the entry of mainstream CMOS foundries to MEMS manufacturing. We’ll speculate on the direction that could lead to greatest business success for foundries and their customers. We will then survey promising approaches to improving the performance of MEMS sensors and we will conclude with speculations on the roadmap and timeline for a possible crossover for MEMS in high- performance applications.

Introduction High-performance (tactical grade –1 deg/h day-to-day bias stability – and above) inertial sensors have always been macro-scale devices. While macro-scale devices that meet high-performance specifications have shrunken in size and cost over recent decades, they remain relatively large and costly compared to inertial sensors micro-machined on silicon wafers, known as MEMS. Over the past two decades, MEMS inertial sensors have become highly successful and are now manufactured at very high volume and very low cost. MEMS performance, though improving, remains at or below tactical grade. This paper explores whether MEMS inertial sensors can ultimately displace macro-scale sensors in high-performance applications.

[2] 2. A Brief History of MEMS Commercialization An excellent general introduction to MEMS can be found in [1]. After nearly two decades of academic and commercial R&D, the first commercialization of a MEMS inertial sensor was the introduction by of a single-axis for crash detection in cars which entered volume production in 1993 [2]. Bosch introduced the first MEMS gyro electronic stability control in cars, entering volume production in 1997. Automotive applications of MEMS generally, and MEMS inertial sensors in particular, drove the industry from 1993 through the mid 2000s.

The introduction of the Nintendo Wii in 2006 marked a breakthrough of MEMS inertial sensors into consumer electronics. The first-generation Wii had a 3-axis accelerometer in its handheld controller. The ability to sense a user’s arm motions was key to a new generation of computer games not possible with competing gaming systems. There had been other applications of MEMS in consumer electronics prior to the Wii, but sensing made the product a run-away hit.

The introduction of the Apple iPhone in 2007 closely followed the Nintendo Wii and, it’s fair to say, was a watershed event in modern life generally and MEMS in particular. The first- generation iPhone incorporated a 3-axis accelerometer. Initially, it enabled a critical feature of the device: automatic rotation of the display from portrait to landscape mode. Subsequent versions, starting with the iPhone 4 [3], included a so-called 9-axis Inertial Measurement Unit (IMU), consisting of a 3-axis MEMS accelerometer, a 3-axis MEMS , and 3-axis magnetometer (based on non-MEMS technology). The improved motion sensing capabilities made possible more sophisticated applications such as augmented reality. A 9-axis IMU is now a mandatory feature of all smart phones.

3. Current Industry Trends Affecting MEMS We can identify at least four industry trends that affect the future of MEMS inertial sensors: commoditization, proliferation of MEMS technology, consolidation of advanced CMOS suppliers, and increasing integration of heterogeneous technologies through chip-scale packaging. We will describe each of these four trends in more detail and then discuss likely consequences of the trends.

Commoditization of MEMS motion sensors: Market research firms that follow the MEMS industry, such as Yole Development [4] and IHS iSuppli [5], state that the shipment volume of MEMS continues to grow exponentially [4],[5]. The growth in unit volume is

[3] driven not only by mobile devices (smart phones and tablets) but by new applications such as wearables (activity sensors, for example) and IoT devices. Their projections show continued growth in units shipped for at least the 5 years. However, IHS data shows that total revenue from MEMS-based motion sensors in consumer electronics has decreased since 2014 and is projected to continue dropping through the end of this decade [6]. The implication is that unit prices for MEMS-based motion sensing components are dropping faster than volume is increasing, leading to the overall decline in revenue.

Proliferation of MEMS know-how and manufacturing technology: Due to the commercial success of MEMS, a whole eco system for MEMS development exists today that was not available to the industry pioneers. The eco system includes suppliers of materials, manufacturing equipment, metrology equipment, testing equipment and services, and packaging technology and services, not to mention design software. The existence of this eco system greatly lowers the barrier to entry for new entrants in both time and cost. While pioneering companies spent a decade or more going from initial feasibility to production, newer entrants can get there in a few years.

Consolidation of advanced CMOS manufacturing: Since the 1960’s, semiconductor technology nodes have followed Moore’s law. Staying competitive in CMOS means staying on Moore’s law, and this has gotten progressively more costly with each new technology node, leading to consolidation of advanced CMOS manufacturing. Currently, there are only 4 companies capable of manufacturing the latest CMOS logic nodes [7]. This means that a lot of older fabs and equipment are becoming obsolete each year. MEMS manufacturing does not require the latest equipment, so it offers a way of extending the productive life of this fully amortized capital equipment.

Increasing chip-scale integration, aka More than Moore integration: Chip-scale integration means combining heterogeneous technologies at the die and package levels. Benefits of chip-scale integration include reduced form factor, lower power consumption, higher performance, more functionality, and lower manufacturing cost. Some markets, such as wearable devices can only be accessed through chip-scale integration. Chip-scale integration has been happening for a while in MEMS motion sensors. MEMS have progressed from single- to dual- to tri-axis sensing in a packaged, board-mountable component. The same progression has occurred for MEMS . MEMS IMUs now include multiple 3-axis sensors, as well as analog/mixed-signal and

[4] digital CMOS ICs, and memory to provide 6- and 9-axis digital output. This trend is only intensifying. The has over 30 dies in Apple’s S1 package. Interestingly, the 6- axis MEMS IMU is separately packaged. At the Consumer Electronics Show (CES) 2016, Intel announced the Intel Curie, a single System-in-Package component with 13 dies including a 6-axis MEMS IMU.

Combined, these industry trends provide strong motivation for mainstream CMOS foundries such as TSMC and GlobalFoundries to enter the MEMS market. Both companies have publically stated these motivations and their intentions of supplying MEMS. This will likely mean strong new competition for today’s leading suppliers and further commoditization. Business will become even more competitive for MEMS suppliers, causing some to consolidate or exit and others to pursue niche strategies, such as focusing on high performance, to survive. MEMS consumers, i.e. system integrators, will benefit from even lower costs and a wider selection of components if not suppliers.

4. Are MEMS Inertial Sensors a Disruptive Technology? We will explore this question through the lens of the technology adoption framework described by Clayton Christensen in his classic book The Innovator’s Dilemma [9]. According to Christiansen, suppliers of an incumbent technology focus on pleasing their best customers, which inevitably means evolutionary refinements to the technology that push its performance. Smaller, less-demanding markets are ignored, leaving opportunities for new entrants to meet the market needs with a new technology that is initially lower cost and lower performing than the incumbent technology. As time goes on, the performance of the new technology improves to the point where it starts taking over the core market of the incumbent technology. Christiansen’s book gives a number of real-world examples, the most relevant to MEMS being the evolution of hard disk storage technology. Succeeding generations of lower cost, lower performing drives were introduced by new suppliers. In each generation, the new suppliers succeeded initially by serving a new, less-demanding market that was not served by the suppliers of incumbent technology. Ultimately, the newer technology displaced the incumbent technology (and suppliers) even at the high end of the market.

The commercialization history of MEMS inertial sensors suggests that they have already taken several steps in the technology adoption pattern described by Christensen. MEMS accelerometers and gyroscopes first succeeded commercially in new, low-end markets not served by the incumbent technologies. In the late 1990s, automotive electronic stability

[5] control (ESC) was an entirely new market, enabled by MEMS gyroscope technology. Existing macro-scale devices were too large and costly. The performance requirements for ESC were quite low compared to tactical-grade applications. MEMS suppliers such as Analog Devices, Bosch, and Motorola (later called FreeScale Semiconductor and now part of NXP) dominated the European and North American automotive markets. In the mid- 2000s, new MEMS suppliers InvenSense and ST Microelectronics succeeded at first by supplying MEMS-based gyroscopes for consumer electronics applications. The initial performance and cost requirements for consumer electronics were even lower than for automotive applications. InvenSense, for example, was founded in 2004 and its first product was a MEMS gyroscope for image stabilization in consumer-grade cameras introduced in 2007. Today, a new initiative funded by the EU government is exploring ways that lower-cost consumer-grade MEMS can be used in automotive applications. In other words, lower-cost consumer MEMS may displace higher-cost automotive MEMS.

Are MEMS inertial sensors a disruptive technology? The answer derived from the preceding examples is most certainly “yes”. Successive generations of MEMS have already followed Christensen’s technology adoption pattern. In the automotive and consumer spaces, successive generations of MEMS have succeeded by enabling new markets for inertial sensors, from safer cars, to new computer games, to better cameras, to smart phones that have displaced built-in navigation systems in cars.

5. Speculations on the Future: High Performance MEMS Could MEMS technology take another step in Christensen’s technology adoption pattern and displace the incumbent inertial sensing technologies for high-performance applications, as suggested conceptually in Figure 1? Here we define high performance as better than tactical grade, approaching inertial/navigation grade. To assess this possibility, we will consider technology, market and regulatory concerns. The technology concern is whether the MEMS-based IMUs can be improved to achieve high performance. The market or business concern is whether new markets can be identified that will serve as entry points for suppliers of MEMS-based IMUs. Lastly, the regulatory concern is how ITAR will affect the distribution and hence commercial success of high-performance MEMS-based IMUs.

At this time, the incumbent gyroscope technologies in tactical- and inertial/navigation- grade (we define inertial/navigation grade as 0.1 to 0.01 deg/h day-to-day bias stability) IMUs and Inertial Navigation Systems (INS) are based on optical sensors – Ring Laser

[6] Gyroscopes (RLGs) and Fiber Optic Gyroscopes (FOGs). Both of these gyroscope technologies entered low-volume production in tactical-grade instruments (in the late 1970s for RLGs and the early 1990s for FOGs) and gradually became the technologies of choice for inertial/navigation-grade applications. Some systems currently in production are based on Hemispherical Resonating Gyroscope (HRG) technology, however the total number of these systems is very small compared to those based on RLG and FOG technologies.

Today there are MEMS-based systems available at the low end of tactical grade performance (day-to-day bias stability of 10 deg/h) with occasional announcements by a few companies indicating systems that will perform at the same level as current tactical grade RLGs and FOGs. Earlier this year, Northrop Grumman announced that it has been selected by the U.S. Defense Advanced Research Project Agency (DARPA) to develop a navigation-grade IMU based on MEMS technology. Sensors in Motion also recently announced a Symmetrical Resonating Gyroscope (SRG) with day-to-day bias repeatability of 1 deg/h, bias in-run stability of 0.05 deg/h and Angular Random Walk of 0.005 deg/rt-h. This is clearly tactical grade performance. However, the cost of these high-performance MEMS-based systems is still quite high compared to MEMS IMUs for automotive and consumer applications.

Figure 1: Conceptual diagram of MEMS vs RLG/FOG performance vs. time based on similar diagrams in [9]. Here, performance may be any relevant Figure of Merit such as bias instability.

[7] We see a number of promising approaches that could result in MEMS inertial sensors with significantly improved performance

 Improved sensitivity – as we gain understanding of the fundamental mechanisms that affect the sensitivity of the individual sensors and the manufacturing processes, their sensitivity will increase as will the yield of sensors capable of higher sensitivity.

 Better long-term stability – will be achieved by understanding and acting on the various materials and processes that control long-term stability of the sensors. In addition there will be great progress made in the electronics used in today’s MEMS sensors that will result in much better long-term stability.

 More sensors in smaller volume – We expect that a reduction in the short-term noise of MEMS inertial sensors can be achieved by combining the output of several sensors. If the noise is uncorrelated then such noise can be reduced by the square root of the number of sensors used. Several research teams using individual sensors have demonstrated this and we expect that MEMS manufacturers will soon take advantage of this technique provided there is an adequate pull in the market for sensors with lower ARW. In addition, we envision that suppliers could place multiple sensors with different sensitivity characteristics on one die to offer optimal performance throughout the desired dynamic range.

 Novel assembly techniques – If single-die designs for 3-axis sensing cannot achieve sufficient accuracy, sensitivity, stability and dynamic range, we must rely on novel assembly techniques to provide sensor clusters with two or three orthogonal dies to reduce the cost of such systems. Prof. Andrei Shkel from the UC Irvine and his collaborators have proposed various novel folded MEMS IMU configurations that show great promise in providing multi axis systems in very small volumes.

Perhaps the most promising approaches that we expect to result in MEMS sensors with higher performance involve applying some type of in-situ calibration technique. It should be pointed out that we expect that producers will strive first to understand the physical mechanisms underlying the various error sources and then using such knowledge reduce these error sources as much as can be achieved before resorting to some type of calibration techniques.

[8] There are two calibration techniques that appear to hold the most promise:

 Mechanical calibration. In this method the inertial MEMS sensor or perhaps a complete 3-axis system is mounted on a mechanical calibration/rotational stage. This stage is similar to a single axis rotation table that can be rotated in a bi- directional way. In the ideal embodiment of this technique the inertial sensor is manufactured directly on such a rotational stage so no additional mounting or attachment step is required. By applying a small known, fixed and repeatable bi- directional rotational bias to such an inertial sensor or system, the unit can be continuously calibrated and bias errors from turn-on to turn-on as well as in-run can be subtracted from the motions/signals of interest.

 Electronic Calibration. In this novel method applicable to sensing elements with symmetrical sense and drive modes, the electrical excitation/bias applied to the drive/sense electrodes is periodically reversed. By applying such a known and repeatable bi-directional bias, the unit can be continuously calibrated and in-run bias errors as well as from turn-on to turn-on can be subtracted from the sensor output. This calibration method requires no mechanical modifications to the sensing element and might be the least expensive and thus hold the most promise.

The prices of the consumer- and industrial-grade MEMS inertial sensors have continued to decrease dramatically as this technology has developed into a multi-billion euro and multi billion units-per-year business. Products such as the Google Glass and smart watches by Apple and others use 9-axis inertial sensor combos that have an estimated average sales price below $2/unit. We have also observed that various models of smart phones from the same manufacturer have different brands of MEMS inertial sensors thus indicating very tough competition among consumer MEMS suppliers. Optical gyros have also benefited from continuing price reductions as all such manufactured sensors benefit from cost decreases of an ever-increasing manufacturing volume. What we see now is that the higher performance MEMS-based IMUs are benefiting from their advantages in size, weight and power consumption vs. FOG and RLG sensors. Surprisingly, they have not yet been able to compete effectively on price on most high-performance applications despite the seeming possibility of lower prices due to their semiconductor heritage. One reason for this might be the fact that currently only small specialty companies and traditional military inertial navigation defense contractors manufacture them. Perhaps not until there is a new

[9] commercial application that induces one of the high volume MEMS manufacturers to enter the market for high performance sensors, will we see a great reduction in their price.

This brings us to consideration of potential new high-volume commercial applications that require tactical grade – or near tactical grade performance, however, at a price point that is a small fraction of current tactical grade systems. Some of these new commercial applications for MEMS based inertial sensors may be:

 Urban personal navigation  Indoor navigation  Subterranean personal navigation  Search and rescue robots  Autonomous land vehicles  Autonomous subsea vehicles  Professional and amateur sports  Augmented and virtual reality (AR, VR)  Implantable prostheses  Guided microsurgery

The military continues to require an ever-larger number of inertial systems with ever-better accuracy. Various types of precision and guided munitions would greatly benefit from low cost tactical grade systems especially with lower CSWaP (Cost, Size, Weight and Power).

In addition to the general trend in the military for lower CSWaP there has been a transformational push and demand for more accurate inertial sensors driven by the desire for a reduced reliance on GNSS (Global Navigation Satellite Systems) such as GPS. This reduced reliance or perhaps an elimination of the reliance on GNNS has been a significant motivation for the development of more accurate MEMS inertial sensors with a stated objective by the US Defense Department of achieving GPS-level timing and positioning performance without GNSS. In addition, many military applications desire/demand persistent position and navigation in environments where GNSS was never designed for use such as undersea, underground and indoors. These applications would require navigation-grade MEMS sensors for true inertial navigation without the aid of external signals than can be jammed, spoofed or turned off.

Finally, we would be remiss in not considering how ITAR controls will affect high- performance MEMS IMUs. Many of these improved MEMS sensors will be derived from

[10] lower performance sensors whose export is not controlled. However, when such MEMS based sensors achieve day-to-day bias of better than 0.5 deg/h over a couple of weeks they will become ITAR controlled. This might greatly limit their use in non-military applications, and therefore limit markets that might otherwise justify high-volume, low-cost production.

6. Conclusion We have identified four industry trends that affect the future of MEMS inertial sensors: commoditization, proliferation of MEMS technology, consolidation of advanced CMOS suppliers, and increasing integration of heterogeneous technologies through chip-scale packaging. We have also shown evidence that MEMS inertial sensors are a good example of a disruptive technology that is initially lower performing than the incumbent technology, but which has advantages in lower cost, smaller size and lower power consumption. However, the performance of any technology improves as it matures.

We have identified a large number of new applications that will demand high-performance, low-cost inertial sensors. In addition, there is a strong desire for less reliance on GNSS in the US military thus providing yet another strong motivation for higher performance MEMS inertial sensors to provide GNSS-free navigation. Although we do not know which of these applications might provide the break-through impetus for high performance MEMS inertial sensors we expect that one or more of them might serve as entry points for suppliers of high performance MEMS-based IMUs. We speculate that the cost of such sensors will only approach that of current industrial-grade sensors when the manufacturing is done by one of the high-volume MEMS manufacturers.

Finally we point to a number of promising approaches for improved performance such as including more sensors in a smaller package, using novel assembly techniques and incorporating electronic or mechanical calibration techniques to improve bias stability.

References [1] S. D. Senturia, “Microsystem Design”, Kluwer Academic Publishers, 2001

[2] P. Walter, “The History of the Accelerometer”, Sound and Vibration, January 2007

[3] St.J. Dixon-Warren, “Motion Sensing in the iPhone 4”, MEMS Journal, December 2010 (http://www.memsjournal.com/2010/12/motion-sensing-in-the-iphone-4-mems- accelerometer.html)

[11] [4] J.C. Eloy, “Understanding the major trends in the MEMS industry…and what will happen”, MEMS and Sensors Industry Group MEMS Executive Congress US 2014, November 2014

[5] J. Bouchaud, “MEMS: looking back at 2014 and 5 years outlook”, MEMS and Sensors Industry Group MEMS Executive Congress US, November 2014

[6] J. Bouchaud, “The Ups and Downs of the MEMS and Sensor Market”, MEMS and Sensors Industry Group MEMS Executive Congress US, November 2015

[7] R. Courtland, “Transistors Could Stop Shrinking in 2021”, IEEE Spectrum, http://spectrum.ieee.org/semiconductors/devices/transistors-could-stop-shrinking-in- 2021, July 2016

[8] J. Morrison and D. Yang, “Inside the Apple Watch: Technical Teardown blog”, https://www.chipworks.com/about-chipworks/overview/blog/inside-the-apple-watch- technical-teardown-blog, April 2015

[9] C. Christensen, “The Innovator’s Dilemma”, Harvard Business School Press, 1997.

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