`
`
`
`
`Steven Nasiri, 408-988-7339 x108, snasiri@invensense.com
`3150A Coronado Drive, Santa Clara, California 95054
`
`
`Abstract
`
`Gyroscopes are expected to become the next “killer” application for the MEMS industry in the coming years. A multitude of
`applications already have been developed for consumer and automotive markets. Some of the more well known automotive
`applications such as vehicle stability control, navigation assist, roll over detection are only used in high-end cars, where cost is
`not a major factor. Examples of consumer applications are 3D input devices, robotics, platform stability, camcorder stabilization,
`virtual reality, and more. Primarily due to cost and the size most of these applications have not reached any significant volume.
`This paper provides a top-level review of various vibrating mass gyroscopes and examines the technology and packaging
`methodology for top four commercially available gyroscopes companies. Advancement in MEMS technology, fueled by the
`optical bubble, such as, wafer-scale-integration, and wafer-scale-packaging will be reviewed. New opportunities for design and
`development of the next generation of low-cost and high-performance gyroscopes based on the latest MEMS technologies are
`discussed.
`
`Introduction
`
`Micromachined inertial sensors, consisting of accelerometers and gyroscopes, are one of the most important types of silicon-
`based sensors. Microaccelerometers alone have the second largest sales volume after pressure sensors. It is believed that
`gyroscopes will soon be mass-produced at similar volumes once manufacturers are able to meet a $10 price target. Applications
`for gyroscopes are very broad. Some example for these applications are; automotive; vehicle stability control, rollover detection,
`navigation, load leveling/suspension control, event recording, collision avoidance; consumers, computer input devices, handheld
`computing devices, game controllers, virtual reality gear, sports equipment, camcorders, robots; industrial., navigation of
`autonomous (robotic) guided vehicles, motion control of hydraulic equipment or robots, platform stabilization of heavy machinery,
`human transporters, yaw rate control of wind-power plants; aerospace/military; platform stabilization of avionics, stabilization of
`pointing systems for antennas, unmanned air vehicles, or land vehicles, inertial measurement units for inertial navigation, and
`many more.
`
`This paper presents a review of silicon MEMS gyroscopes (rate sensors), their production status, and challenges towards
`fabrication of the next generation of lowcost gyroscopes. Following a brief introduction to gyroscope operating principles and
`performance specifications, the present status in the commercialization of micromachined rate sensors are discussed.
`Inertial sensors have seen a steady improvement in their performance and their fabrication technology, and today,
`microaccelerometers are among the highest volume MEMS sensors for the automotive. While the performance of gyroscopes
`has improved by a factor of 10 every two years, their costs have not dropped as was originally predicted. The initial drive for
`lower cost, greater functionality, higher levels of integration, and higher volume had slowed down during the optical bubble, when
`the sensor market was over taken with high potential returns promised by the telecom market. Although the telecom boom had
`slowed the wide spread development in gyroscopes, it poured billions of dollars into development of next generation MEMS
`technologies, equipment, modeling tools, foundries, and micromachine experts. This paper will discuss some of these
`advancements in MEMS development and their potential use in the creation of the next generation of advanced, integrated
`MEMS gyroscopes that can meet the market cost expectations, and further their performance.
`
`Micromachined Gyroscope Technology
`
`Operating Principles and Specifications
`Almost all reported micromachined gyroscopes use vibrating mechanical elements (proof-mass) to sense rotation. They have no
`rotating parts that require bearings, and hence they can be easily miniaturized and batch fabricated using micromachining
`techniques. All vibratory gyroscopes are based on the transfer of energy between two vibration modes of a structure caused by
`Coriolis acceleration. Coriolis acceleration, named after the French scientist and engineer G. G. de Coriolis (1792–1843), is an
`apparent acceleration that arises in a rotating reference frame and is proportional to the rate of rotation, Fig. 1.
`
`
`Align EX1037
`Align v. 3Shape
`IPR2022-00144
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`0001
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`acor = 2V x Ω
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`Z
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`Ω
`Rate of
`Rotation
`
`Vibratory gyroscopes were demonstrated in the early
`1980’s. An examples of this type of devices is quartz
`tuning forks like the Quartz Rate Sensor by Systron
`Donner. Although quartz vibratory gyroscopes can yield
`very high quality factors at atmospheric pressure with
`improved level of performance, due to use of quartz as
`the primary material, their batch processing is not
`compatible with IC fabrication technology. In the late
`1980’s, after successful demonstration of batch-
`fabricated silicon accelerometers, some efforts were
`initiated to replace quartz with silicon in micromachined
`vibratory gyroscopes. Charles Stark Draper Laboratory
`demonstrated one of the first batch fabricated silicon
`micromachined rate gyroscopes in 1991.
`
`In general, gyroscopes can be classified into three
`different categories based on their performance: inertial-
`grade, tactical-grade, and rate-grade devices. Table 1
`summarizes the requirements for each of these categories. Over the past decade, much of the effort in developing
`micromachined silicon gyroscopes has concentrated on “rate-grade” devices, primarily because of their use in automotive
`applications. Automotive applications generally requires a full-scale range of at least 50°-300° /s and a resolution of about 0.5°-
`0.05 °/s in a bandwidth of less than 100 Hz depending on the application. The operating temperature is in the range from -40 to
`85° C.
`
`V
`
`X
`
`INVENSENS
`
`Y
`
`Moving
`Object
`
`Figure1- Coriolis accelerometer concept
`
`Tuning fork gyroscopes contain a pair of masses that are driven to oscillate with equal amplitude but in opposite directions.
`When rotated, the Coriolis force creates an orthogonal vibration that can be sensed by a variety of mechanisms. The Draper Lab
`gyro, figure 2, uses comb-type structures to drive the tuning fork into resonance, and rotation about either in-plane axis results in
`the moving masses to lift, a change that can be detected with capacitive electrodes under the mass.
`
`Vibrating-Wheel Gyroscopes have a wheel that is
`driven to vibrate about its axis of symmetry, and
`rotation about either in-plane axis results in the
`wheel’s tilting, a change that can be detected with
`capacitive electrodes under the wheel, Figure 3. It
`
`Table 1; Performance requirement for different type of gyroscopes
`
`Figure 2 - The first working prototype of
`the Draper Lab comb drive tuning fork
`
`is possible to sense two axes of rotation with a single vibrating
`wheel. A surface micromachined polysilicon vibrating wheel gyro,
`Figure 4, has been designed at the U.C. Berkeley Sensors and
`Actuators Center that demonstrated this capability.
`
`0002
`
`
`
`
`
`
`Figure 3 – Schematic design concept for Robert Bosch
`vibrating wheel. This design provides X/Y-axis sensing
`capability, and is being produce in production using poly silicon
`fabrication
`
`
`Figure 4. This polysilicon surface-micromachined vibrating
`wheel gyro was designed at the Berkeley Sensors and
`Actuators Center.
`
`Wine Glass Resonator Gyroscopes. A third type of gyro is the wine glass resonator. Fabricated from fused silica, this device is
`also known as a hemispherical resonant gyro. Researchers at the University of Michigan have fabricated resonant-ring gyros in
`planar form. In a wine glass gyro, the resonant ring is driven to resonance and the positions of the nodal points indicate the
`rotation angle. The input and output modes are nominally degenerate, but due to imperfect machining some tuning is required.
`
`Challenges with Designing a MEMS Gyroscope
`
`This section reviews some of the different choices that can be made for design of a vibrating gyroscope. Table 2 provides a
`summary of various design choices possible for a vibrating gyroscope, with over 2500 potential combinations. Most of research
`and development activities at the university level, with the University of California Berkeley being the most active, have been on
`surface micromachined gyros. One of the primary drivers for this has been DARPA and military interest for development of a
`single chip integrated six-axis inertial measurement units (IMU).
`
` Design Style
`
`Optical Gyro
`
`Ring Laser Gyro
`
`Vibrating Gyro
`
`Z-axis Sensor
`
`X/Y-axis
`
`Vibrating Mass
`
`Vibrating Ring
`
`Linear Vibration
`
`Rotary Vibration
`
`Dual Mass Tuning Fork
`
`Gyroscopes are much more challenging sensor products than acceleration or pressure sensors. Gyroscopes are basically two
`high performing MEMS devices
` Application
`integrated into one single device
`that have to work together to
`produce results. They are a self-
`tuned resonator in the drive axis,
`and a micro-g sensor
`in
`the
`sensing
`axis. The
`absolute
`magnitude of the Coriolis force
`sensed is orders of magnitude
`lower
`than any high volume
`production MEMS accelerometer.
`Capacitive sensors are generally
`used for measuring these small
`changes
`of
`capacitance.
`Gyroscope performance is very
`sensitive
`to
`all
`potential
`manufacturing
`variations,
`packaging,
`linear acceleration,
`temperature, etc. To achieve high
`performance and lowcost, lots of
`care must be taken during the
`initial
`design.
`Gyroscope
`designers must achieve a solution
`that can be insensitive to most of
`these potential variations.
`
`Single Mass
` MEMS Technology
`
`Bulk Silicon
`
`Poly Silicon
`
`Mixed Process
`
` Actuation Mechanism
`
`Parallel Plats
` Coriolis Sensor
`
`Electro-Static
`
`Electro-
`
`Piezoelectric
`
`Torsional Plates
`
`Comb Drive
`
`Electro-Static
`
`Electro-Magnetic
`
`Piezoelectric
`
`Parallel Plats
`
`Comb Fingers
`
`
`
`Table 2: Shows the various options, >2500, that could be used for designing a gyroscope.
`
`0003
`
`
`
`Gyroscope Packaging Challenges
`
`One of the most difficult decisions that can have the biggest effect on the cost is the choice for the final package. Generally,
`packaging is one of the highest components of the final cost for most types of MEMS sensors. In majority of cases, sensor
`designers and MEMS experts are not packaging experts. MEMS designers are primarily focused on the design and development
`of the sensor element, with the objective of demonstrating performance on the bench. The task of taking the MEMS sensor
`element and package it is the packaging engineer’s problem. In order to have the lowest cost MEMS product packaging issues
`must be addressed up front in the initial phase of design cycle. MEMS by definition is a mechanical feature that can be
`manufactured in batch processing with little cost differential. It is very unlikely that a lowcost solution can be realized without
`addressing packaging issues properly on the outset.
`
`Path to High-Performance and Low-Cost Gyroscopes
`
`The main challenge for the MEMS gyroscope industry has been achieving high-performance and low-cost MEMS solutions.
`Although there are several high-performance MEMS gyroscopes in production already, they are still fairly costly for most
`applications. Today’s automotive gyroscopes cost more than $60 for more demanding applications like vehicle dynamic control,
`and navigation assist, and are slightly below than that for less demanding ones like roll-over detection. The automotive market
`has developed many safety and comfort
`related
`applications that relies on rate-sensors. Today they are
`only offered on the high-end cars, where cost is less of a
`concern. There are also many consumer applications that
`are waiting for smaller and more affordable gyroscopes,
`like input devices, camcorder stability control, and more. To
`benefit from all these high volume applications, a smaller
`size and lower cost gyroscopes are needed.
`
`25
`
`10
`
`5
`
`25
`
`20
`
`15
`
`10
`
`Volume
`(millions)
`
`0.1
`
`50
`
`1
`
`20
`
`05
`
`7.5
`ASP ($)
`
`5
`
`3
`
`for MEMS
`the price curve
`review of
`A quick
`accelerometers, figure 5, shows that high volume occurred
`once unit prices achieved $3 target prices. The initial
`manufacturers of these type of sensors, for the first few
`years were Lucas-Nova Sensors, and EG&G-IC-sensors.
`In spite of millions of dollars invested in automation,
`capacity build up, and quality control systems, they were
`only able to reduce their cost to just under $10, and could not meet market demand for $3. Analog, Motorola, and few other
`companies recognized the market needs and focused their development from the beginning on fabrication of low cost
`accelerometers. They succeeded in the design and development of a new generation of accelerometers products that broke the
`price barrier. This allowed the market for these products to reach its full potential. All the original companies that failed to
`recognize the need were forced out of the market and are no longer producing accelerometers.
`
`Figure 5; Shows automotive accelerometers price elasticity
`
`There is a real market need for lowcost gyroscopes not unlike the accelerometers a decade ago. Once the cost targets are met
`this family of MEMS sensor will enjoy a drastic increase in volume and will become the next high-volume application for the
`MEMS industry.
`
`Vibrating Gyroscope Production Status
`
`This section provides some insight into the gyroscope designs, performance, and packaging of the top four vibrating gyroscope
`manufacturers worldwide, representing more than 95% of unit volumes shipped for this class of sensors.
`
`Robert Bosch has been the most active in design and fabrication of silicon vibratory gyroscopes. They have 50% of the gyro
`market for the automotive VDC, and related applications today. They are producing in high volume, with several million units
`shipped already. Bosch has developed both Z axis and X./Y axis rate sensors. Its Z-axis design, shown in figure 6, was
`introduced in 1998 and uses electromagnetic drive with capacitive sensing. The X/Y-axis gyro is a rotary vibrating mass, which
`was briefly discussed earlier and the schematic of this design is shown in figure 3. The MEMS sensor element along with its
`custom ASIC and all the discrete component are packaged in a hermetically sealed metal can, figure 7 and 8 which is then place
`inside its automotive style plastic housing, figure 9, with integral connectors and mounting brackets.
`
`0004
`
`
`
`
`Figure 6; Bosch silicon dual mass tuning fork design, with
`Z-axis rate sense, in plan vibration and in plan sensing
`
`
`Figure 7; Bosch MEMS sense element along with all
`supporting electronics, in a metal header package
`
`
`
`
`Figure 8; Bosch metal header cross section, showing the
`permanent magnet suspended over the sensor chip.
`
`Figure 9; Bosch’s gyroscope package with
`connector and mounting brackets
`
`integral
`
`BEI Systron Donner is a major manufacturer of rate sensors for automotive. Their gyroscope is designed based on using one-
`piece quartz inertial sensor, figure 10. These micromachined inertial sensing elements measure angular rotational velocity, using
`tuning fork vibratory principals and piezoelectric actuation and sensing. These sensors generate a signal output proportional to
`the rate of rotation sensed. Each sensor element is packaged in a hermetically sealed metal headers, figure 11, which are then
`combined with discrete electronic to produce the finished modules products, figure 12.
`
`
`
`
`
`Figure 10; Systron Donner
`
`quartz sensing element
`
`
`Figure 11; individual quartz
`elements are packaged in a
`metal header
`
`Figure 12; Packaged sensor element
`are packaged with electronics for
`automotive
`
`
`
`Silicon Sensing Systems a joint venture between Sumitomo and British
`Aerospace, has brought to market an electromagnetically driven and sensed
`MEMS gyro, figure 14, with a permanent magnet sits above the MEMS device,
`figure 15. Current passing through the conducting legs creates a force that
`resonates the ring. This Coriolis-induced ring motion is detected by induced
`voltages as the legs cut the magnetic field.
`
`Analog Devices has been working on MEMS gyros for many years, and has
`patented several concepts based on modified tuning forks. The company has
`recently introduced the ADXRS family of integrated angular rate-sensing gyros, in
`which the mass is tethered to a polysilicon frame that allows it to resonate in only
`one direction. Capacitive silicon sensing elements interdigitated with stationary
`silicon beams attached
`to
`the substrate measure
`the Coriolis-induced
`displacement of the resonating mass and its frame, Figure 13.
`
`
`
`Figure 13. The iMEMS ADXRS angular rate-
`sensing gyro from Analog Devices
`
`0005
`
`
`
`
`
`
`Figure 14. The resonant ring at the heart
`of the Silicon Sensing Systems gyro is
`shown here as an SEM image. Both
`electromagnetic drive and sensing are
`accomplished with a permanent magnet
`in the center (not shown).
`
`Figure 15. Shows the Silicon Sensing
`Sensor package with a permanent
`magnet mounted directly on top of the
`sensor element in a metal can header
`for sealing under vacuum.
`
`
`Figure 16. The Silicon Sensing
`Systems gyro. This device measures
`29 by 29 by 18 mm and is used to
`stabilize the Segway Human Transport
`
`
`
`
`
`Summary of State of Production; Table 3 shows a summary comparison of the key features for the top four manufactures of
`vibrating gyroscopes products representing greater than 95% of total volume being shipped presently.
`
`Company Rate
`Axis
`Z
`
`Bosch
`
`MEMS
`
`Bulk
`
`Bosch
`
`BEI
`
`X/Y
`
`Z
`
`Polysilicon
`
`Quartz
`
`Drive
`
`Sense
`
`Electromagnetic Capacitive
`
`Electrostatic
`
`Capacitive
`
`Package
`Type
`Metal
`header
`Metal
`header
`Metal
`header
`Electromagnetic Electromagnetic Metal
`header
`Ceramic
`
`Piezoelectric
`
`Piezoelectric
`
`Z
`
`Bulk
`
`Silicon
`Sensing
`Capacitive
`Electrostatic
`Poly
`Z
`ADI
`Table 3; Summary of various gyroscopes in volume production presently
`
`
`Seal Ambient
`
`Atmosphere
`
`Vacuum
`
`Atmosphere
`
`Vacuum
`
`Atmosphere
`
`Wafer-Scale-Integration and -Packaging
`
`During the telecom bubble, optical MEMS took the spot light away from all other MEMS sensor developments including
`gyroscopes. Most MEMS sensor companies had pushed aside most of their sensor related development in favor of designing the
`next best optical components. MEMS mirrors were in the forefront of all these activities. With Nortel paying over 3.0 Billion
`dollars for Xros mirrors, it was logical for all MEMS companies and MEMS experts to move into the optical MEMS. Although, the
`telecom market did not materialize and most all MEMS optical companies are things of the past, venture capital firms and
`corporate telecom companies had invested billions of dollars in the MEMS industry. These investment dollars resulted in a more
`mature industry, with better infrastructure, more experts, and more advanced technologies. MEMS telecom companies were all
`trying hard to outperform their competition and be the first in the market with the most advance product. They generally were
`very protective of their IP and did not publish too many details on their design and development activities.
`
`Silicon bulk micromachining has benefited the most from these investment dollars. Many companies focused their development
`efforts on building bulk silicon mirrors. Transparent Networks (TNI) was one such company. Started in 2000, TNI raised $30M
`dollars to develop high port count, 1000x1000, optical switch product base on bulk silicon. TNI had to develop many new
`technologies for its integrated single silicon chip with 1200 mirrors. One of these technologies was wafer-scale-integration, figure
`18, where all the drive electronics were fabricated using a standard semiconductor foundry, and then they were integrated with
`the fabricated bulk silicon MEMS wafers. This enabled the fabrication of a 1200 integrated mirror arrays with four actuators each,
`figure 17, to be controlled with less than 120 connections and 2 watts of power. The key enabling technology was the mirror
`actuator mechanism called the “Nasiri-Platform.” One of the main design features of Nasiri-platform is its virtual-pivot and its
`ability to do angular amplification. While almost all MEMS mirrors where designed using dual gimbal design, by using the Nasiri-
`platform, TNI was able to drop the drive voltages from typically 400V to less than 120 volts, hence enabling electronic integration.
`
`0006
`
`
`
`The TNI mirror design has many features that are similar to vibrating gyroscopes such as, mechanical actuators, sensing
`capacitance, and the need for integrated electronics. Bulk silicon micromachine industry had never seen this level of
`sophistication in its 30+ years of history.
`
`Wafer-scale-packaging is another technology that has benefited from the optical boom. The most simple example of wafer-scale-
`packaging is addition of the protective cap over the active MEMS component that can provide the required sealing for further
`packaging and handling. Number of companies are already using this technology in production.
`
`
`
`
`
`
`
`
`
`
`
`
`Figure 17; TNI mirror with Nasiri-Platform actuation
`
`Conclusion
`
`Figure 18; Vertical integration of the MEMS mirrors with drive
`electronics
`
`Gyroscope is the next killer application for the MEMS sensor industry. There are many mature applications already developed
`and produce in limited volumes in automotive, consumer, industrial, medical, and the military market. Many high volume
`applications, in excess of 100 million per year, are waiting for the availability of a lowcost gyroscope. Today’s gyroscope products
`are not designed to address the growing cost demand, creating an opportunity for the new generation of gyroscopes. The new
`generation designs are primed to take advantage of all the latest development in the MEMS industry.
`
`Bulk silicon can be a good lowcost solution for gyroscopes, due to its maturity and inherent higher mass, if the new design can
`eliminate the need for the use of magnets, metal can packaging, and allow for integrated electronics. Even more savings can be
`possible if standard off-the-shelf plastic packaging with lead frames can be utilized for the final assembly.
`
`About the Author
`Mr. Nasiri is a 26 years veteran in the MEMS industry with expertise in MEMS design, Fabrication and Packaging. Presently he is
`the President and CEO of InvenSense Inc. The company is working on the design and development of next generation vibrating
`gyroscopes with integrated electronics and wafer-scale-packaging to achieve the lowest possible cost for MEMS resonating
`gyroscopes.
`Mr. Nasiri has been the cofounder and early stage executive in several successful Silicon Valley MEMS startup companies,
`where he had served as the VP of Operations and Engineering, including Honeywell-Sensym, GE-NovaSensor, Integrated
`Sensor Solutions, IMST (bought by Maxim), and most recently Transparent Networks, where he was the founder and Vice
`President of MEMS and Packaging.
`Mr Nasiri was the main driver and the managing director for the successful ISS-Nagano Gmbh (bought by Texas Instrument)
`startup in Germany, where he designed and developed his novel high-pressure sensor packages for under-the-hood automotive
`applications. These products and products based on his concept are being manufactured worldwide in of tens of million units
`annually.
`Mr. Nasiri has an MBA from Santa Clara University, and an MSME from San Jose State University, and BSME from University of
`California Berkeley. He has 10 patents issued and over 12 pending.
`
`References
`
`1. N. Yazdi, F. Ayazi, and K. Najafi. Aug. 1998. “Micromachined Inertial Sensors,” Proc IEEE, Vol. 86, No. 8.
`2.
`Jonathan Bernstein, Corning-IntelliSense Corp , MEMS inertial sensing technology
`3. S.Nasiri, Wafer Level Packaging of MOEMS Solves Manufacturability Challenges In Optical Cross Connect, 2003,
`4. S.Nasiri patent, MEMS Mirrors and MEMS arrays, Pat# 6,480,320
`5. T.B. Gabrielson. May 1993. “Mechanical-thermal noise in micromachined acoustic and vibration sensors,” IEEE Trans,
`Electron. Devices, Vol. 40:903-909.
`
`0007
`
`
`
`6.
`
`J. Bernstein et al. 8 June 1998. “Low Noise MEMS Vibration Sensor for Geophysical Applications,” Proc 1998 Solid
`State Sensor and Actuator Workshop, Hilton Head Island, SC:55-58. Extended version also in IEEE JMEMS Dec.
`1999.
`7. M. Lutz et al. June 1997. “A precision yaw rate sensor in sillicon micromachining,” Tech Dig 9th Intl. Conf Solid State
`Sensors and Actuators (Transducers ’97), Chicago, IL:847-850.
`8. Y. Gianchandani and K. Najafi. June 1992. “A bulk silicon dissolved wafer process for microelectromechanical
`systems,” J Microelectromech Syst:77-85.
`J. Bernstein et al. Feb. 1993. “A micromachined comb-drive tuning fork rate gyroscope,” Proc IEEE Micro Electro
`Mechanical Systems Workshop (MEMS ’93), Fort Lauderdale, FL:143-148.
`10. J.J. Bernstein and M.S. Weinberg. 5 March 1996. U.S. Patent #5,496,436, “Comb-Drive Micromechanical Tuning Fork
`Gyro Fabrication Method.”
`
`9.
`
`0008
`
`