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A Critical Review of MEMS Gyroscopes and Commercialization Status

By Paul Perry,2014-04-30 23:23
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A Critical Review of MEMS Gyroscopes and Commercialization Status

    A Critical Review of MEMS Gyroscopes Technology and Commercialization Status

     Steven Nasiri, 408-741-5443, snasiri@invensense.com

    19500 Via Real Dr. Saratoga, Ca 95070

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 (17921843), is an

    apparent acceleration that arises in a rotating reference frame and is proportional to the rate of rotation, Fig. 1.

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 acor = 2V x ; Donner. Although quartz vibratory gyroscopes can yield Z very high quality factors at atmospheric pressure with

    ; improved level of performance, due to use of quartz as Rate of the primary material, their batch processing is not Rotation compatible with IC fabrication technology. In the late V 1980’s, after successful demonstration of batch-

    fabricated silicon accelerometers, some efforts were X initiated to replace quartz with silicon in micromachined

    vibratory gyroscopes. Charles Stark Draper Laboratory

    demonstrated one of the first batch fabricated silicon Moving micromachined rate gyroscopes in 1991. Object Y INVENSENS E In general, gyroscopes can be classified into three Figure1- Coriolis accelerometer concept 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.

    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 Table 1; Performance requirement for different type of gyroscopes 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

    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. Figure 2 - The first working prototype of

    the Draper Lab comb drive tuning fork

     Figure 3 Schematic design concept for Robert Bosch Figure 4. This polysilicon surface-micromachined vibrating vibrating wheel. This design provides X/Y-axis sensing wheel gyro was designed at the Berkeley Sensors and capability, and is being produce in production using poly silicon Actuators Center. fabrication

    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).

    Gyroscopes are much more challenging sensor products than acceleration or pressure sensors. Gyroscopes are basically two high performing MEMS devices

    integrated into one single device Application Optical Gyro Ring Laser Gyro Vibrating Gyro that have to work together to Gyro produce results. They are a self- Design Style tuned resonator in the drive axis, X/Y-axis Z-axis Sensor

    Sensor and a micro-g sensor in the

    sensing axis. The absolute Vibrating Ring Vibrating Mass magnitude of the Coriolis force

    sensed is orders of magnitude Linear Vibration Rotary Vibration lower than any high volume

    production MEMS accelerometer. Dual Mass Tuning Fork Single Mass Capacitive sensors are generally

    used for measuring these small

    changes of capacitance. MEMS Technology Bulk Silicon Poly Silicon Mixed Process Gyroscope performance is very

    sensitive to all potential Actuation Mechanism Electro-Static Electro-Piezoelectric manufacturing variations, Magnetic packaging, linear acceleration,

    Parallel Plats Torsional Plates Comb Drive temperature, etc. To achieve high

    performance and lowcost, lots of

    care must be taken during the Coriolis Sensor Electro-Static Electro-Magnetic Piezoelectric initial design. Gyroscope

    designers must achieve a solution Parallel Plats Comb Fingers that can be insensitive to most of

    these potential variations.

    Table 2: Shows the various options, >2500, that could be used for designing a gyroscope.

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 2525applications that relies on rate-sensors. Today they are

    only offered on the high-end cars, where cost is less of a 20concern. There are also many consumer applications that

    are waiting for smaller and more affordable gyroscopes, 15Volume like input devices, camcorder stability control, and more. To (millions)10benefit from all these high volume applications, a smaller 10size and lower cost gyroscopes are needed. 551A quick review of the price curve for MEMS accelerometers, 0.10figure 5, shows that high volume occurred once unit prices 50207.553achieved $3 target prices. The initial manufacturers of ASP ($)these type of sensors, for the first few years were Lucas-

    Nova Sensors, and EG&G-IC-sensors. In spite of millions Figure 5; Shows automotive accelerometers price elasticity 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.

    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.

     Figure 6; Bosch silicon dual mass tuning fork design, with Figure 7; Bosch MEMS sense element along with all Z-axis rate sense, in plan vibration and in plan sensing supporting electronics, in a metal header package

     Figure 8; Bosch metal header cross section, showing the Figure 9; Bosch’s gyroscope package with integral permanent magnet suspended over the sensor chip. connector and mounting brackets

    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 Figure 11; individual quartz Figure 12; Packaged sensor element elements are packaged in a are packaged with electronics for quartz sensing element

    automotive metal header

    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 Figure 13. The iMEMS ADXRS angular rate-silicon beams attached to the substrate measure the Coriolis-induced sensing gyro from Analog Devices displacement of the resonating mass and its frame, Figure 13.

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    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 Figure 18; Vertical integration of the MEMS mirrors with drive

    electronics

    Conclusion

    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

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    4. S.Nasiri patent, MEMS Mirrors and MEMS arrays, Pat# 6,480,320

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