NORTHWESTERN UNIVERSITY MECHANICAL ENGINEERING DEPARTMENT

ME 381 – Introduction to MEMS Prof. Horacio D. Espinosa

FINAL PROJECT

Micromachined Vibrating Gyroscopes: Design and Fabrication

Kimberly S. Elliott Parag Gupta Kyle B. Reed

Raquel C. Rodriguez

December 6, 2002.

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TABLE OF CONTENTS Abstract ...............................................................................................................................1 I. Introduction ......................................................................................................................1 II. Operation Principles of Vibrating Gyroscopes ..............................................................1 III. Performance and Design Issues ....................................................................................3 IV. HARPSS Vibrating Ring Gyroscope .............................................................................4 A. Fabrication ..........................................................................................................5 B. Performance ........................................................................................................6 C. Applications ........................................................................................................6 D. Limitations ..........................................................................................................6 V. Draper Tuning Fork Gyroscope ......................................................................................7 A. Fabrication ..........................................................................................................8 B. Performance ......................................................................................................10 C. Applications ......................................................................................................10 D. Limitations ........................................................................................................10 VI. Conclusions .................................................................................................................11 References .........................................................................................................................11 Biographical Sketch of Group Members ..........................................................................12 LIST OF FIGURES 1. Schematic of a Vibrating Gyroscope 2. Vibrating Ring Gyroscope 3. Flexural Modes of Vibration 4. Fabrication process for HARPSS gyroscope 5. SEM view of the anchor of a poly ring gyroscope 6. Voids and keyholes from polysilicon refill process 7. RIE lag effect. 8. (a) Coriolis Effect (b) Basic Tuning Fork 9. Schematic Drawing of Draper’s Tuning Fork Gyroscope 10. Dissolved Wafer, Silicon on Glass Process

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ABSTRACT In this paper, a review is given of the

current state of the art in micromachined vibrating gyroscopes. Following a brief introduction to their basic operating principles, two case studies are discussed: a vibrating ring gyroscope designed at the University of Michigan and a tuning fork design by Draper’s Laboratory. The first is produced using high aspect-ratio polysilicon micromachining technology (HARPSS) and the second uses single-crystal silicon electrostatically bonded to glass. Performance parameters and design issues are introduced and discussed. I. INTRODUCTION

The emergence of micromachining technology has generated the possibility to produce precision gyroscopic sensors that present important advantages over their macro-scale counterparts, such as lower cost and lower power consumption. These benefits have fueled an intensive worldwide research in the area, which explains why the performance of gyroscopes has improved by a factor of 10× every two years since 1991, when the Charles Stark Draper Laboratory demonstrated the first micromachined gyroscope [1]. The high demand for microgyros is due to their extensive applications, which range from automotive ride stabilization and rollover detection to virtual reality and military implementations.

II. OPERATION PRINCIPLES OF VIBRATING GYROS

A gyroscope follows Newton’s laws of motion, the first of which states that, in order to change the velocity vector of a moving mass, the application of a force is required. The second law states that the greater a mass the more resistant it is to change its velocity vector. A gyroscope is thus constructed by taking a body and suspending it about its center of mass in a frictionless support that allows for three degrees of angular freedom while having three degrees of constraint (i.e. for linear motion); such a device can therefore provide information about the angular orientation of its frame with respect to its moving mass [2].

Gyroscopes can be classified in three

basic types [3]: a) Spinning mass: is the classical

gyroscope that has a mass spinning steadily with free movable axis (so called gimbal). When the gyro is tilted, the gyroscopic effect causes precession (motion orthogonal to the direction tilt sense) on the rotating mass axis, hence a change in angle can be detected.

b) Optical: lets laser ray reflect around many times within the enclosure. If the enclosure rotates, the duration between the emission of the laser to eventual reception will be different. The laser go-around can be done by mirrors inside the enclosure or by a coil of optical-fiber.

c) Vibrating Gyroscope: here a vibrating element, when rotated, is subjected to the Coriolis effect that causes secondary

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vibration orthogonal to the original vibrating direction. By sensing the secondary vibration, the rate of turn can be detected.

The classical macro-scale gyroscope

employs a spinning wheel. Unfortunately, fabricating microscopic, low-friction bearings needed for this classical approach is impractical. Instead, an entirely different approach using a proof-mass mounted on a spring suspension is utilized in microgyroscopes [4]. Rather than spin as a conventional gyroscope rotor, the proof-mass vibrates back and forth in translational motion, as shown in Figure 1.

Figure 1: Schematic of a vibrating gyro

In this basic configuration, the proof

mass is first put into oscillation in the x-axis (drive axis), parallel to the substrate. Once in motion, the proof-mass is sensitive to angular rotation about the z-axis, perpendicular to the substrate. This rotation

thus induces a Coriolis acceleration in the y-axis (sense axis).

All vibrating gyroscopes rely on the phenomenon of the Coriolis acceleration. This acceleration is experienced by a body undergoing linear motion in a frame of reference that is rotating about an axis perpendicular to that of the linear motion. The resulting acceleration, which is directly proportional to the rate of turn, occurs in the third axis that is perpendicular to the plane containing the other two axes [5]. This coupling is the Coriolis effect and is given by:

Fc = 2mv × Ω (1) which is the force produced when a vibratory mass m, moving at a velocity v, is placed in a rotation, Ω [6].

Because the Coriolis effect manifests in a direction orthogonal to the vibratory motion, two degrees of mechanical freedom are required in a micromachined gyroscope, one for the drive and one for the sensing motion. As illustrated in Figure 1, any microgyroscope can be broken down into four main parts: the proof mass, an elastic spring, a dashpot and some method to measure the displacement of the mass. The proof mass is used to generate the inertial force after angular rotation is experienced by the gyro, and the spring is necessary to mechanically support the mass and return it to its original position after the acceleration ceases. The dashpot is usually the air captured inside the device and is important for controlling the motion of the mass (damping). Finally, the sense method is essential for detecting the induced motion (which is typically very small) by means of converting it into some electrical output.

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The two most common sensing methods are capacitive and piezoresistive. The first simply involves measuring the change in capacitance between the proof mass and a fixed electrode when a displacement is originated. The second method relies on the property that piezoresistive materials have of experiencing a change in resistivity as a direct result of elongation or contraction. Thus by measuring such change in resistivity of piezoresistive materials deposited on the beams (or spring) that support the mass, its motion can be detected.

III. PERFORMANCE AND DESIGN ISSUES

The most important factors in

determining the performance of a gyroscope are: scale factor, zero-rate output (ZRO), resolution and drift [7]. Scale factor is defined as the amount of change in the output signal per unit change of rotation [V/(°/s)]. The ZRO, on the other hand, represents the output of the device in the absence of angular rate. This output is the sum of white noise and a slowly varying function. The white noise defines the resolution [(°/s)/√Hz], and the peak-to-peak value of the slowly varying function defines the drift of the gyroscope [°/s].

Vibratory gyroscopes can be operated open or closed loop. In the open loop mode, the response is not instantaneous because some time is required for the amplitude of the sense mode to reach its steady state value. This time is approximately equal to 2Q/ω [8], and it limits the bandwidth of the sensor to a few

hertz. To obtain larger bandwidth, gyroscopes can be operated with a slight mismatch in the resonant frequencies of the sense and drive modes, but at the cost of reduced sensitivity [9].

On the other hand, in the closed loop mode of operation, the amplitude of the sense mode is continuously monitored and driven to zero. The bandwidth can then reach higher values than the open loop mode even with matching resonance and is only limited by the electronics. With matched resonant modes, assuming the electronic noise Vn has a white spectrum around the resonant frequency and that the detection circuit has a bandwidth BW, the minimum detectable electronic signal for a capacitive device can be expressed by [1]

BWVC

CC

Q nrest

parasrestz ⋅

+⋅∝Ω

δω )(

(min) (2)

This equation illustrates how lowering the resonant frequency, parasitic capacitance and noise of the circuit, and increasing the quality factor and Coriolis-induced capacitance change, would improve the resolution. It is important to note that, although the resonant frequency should be minimized, it also must be maintained above environmental noise (>2 kHZ) [1].

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IV. HARPSS VIBRATING RING GYROSCOPE

In the past few years, research has

been done on vibratory gyroscopes since there are many applications for these devices. Researchers at the University of Michigan have developed a vibrating ring gyroscope, schematically shown in Figure 2. This device consists of a vibrating ring, semicircular support springs, and drive, sense, and control electrodes, which are located around the structure. It is fabricated through the high aspect-ratio combined poly and single-crystal MEMS technology (HARPSS).

Due to symmetry factors, at least eight support springs are needed to result in a balanced ring with two identical flexural modes that have equal natural frequencies and are 45° apart from each other (see Figure 3) [8]. Each support spring has two electrodes, one to sense the motion and one to electrostatically drive the ring. This electrostatic force vibrates the ring in an in-

plane elliptically shaped primary flexural mode with fixed amplitude. When the device is rotated around its normal axis, energy is transferred to the secondary mode from the primary mode, which increases the amplitude in the secondary mode. This buildup is capacitively monitored.

Figure 3: Flexural Modes of Vibration [8].

The amplitude of the sense mode is

proportional to the rotation rate and is given by [8]:

zdrivegsense qQ

Aq Ω⋅⋅⋅=0

4ω

(3)

where

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Ag ≈ 0.37 angular gain of the ring structure (which depends on the geometry of the sensor and is very stable over temperature and lifetime of the device);

Q quality factor of the mechanical structure;

ω0 angular flexural resonance frequency;

qdrive vibration amplitude of the drive mode;

Ωz rotation rate.

The vibrating ring structure also compares favorably to other vibratory gyroscopes. It is less temperature sensitive since both vibration modes change similarly as the temperature changes. Also, the quality factor, Q, directly amplifies the sensitivity since the resonant frequency is the same in both modes [1]. Furthermore, environmental vibrations can only cause a non-desired response if the mass or stiffness of the ring is asymmetrical. If the ring is symmetrical, then the non-desired vibrations will be filtered out. The device can overcome built in asymmetries by electronically compensating to balance the structure [10].

A. FABRICATION PROCESS

The fabrication process for a gyroscope using HARPSS technology is shown in Figure 4. HARPSS is a mixed-mode fabrication technology that combines features of bulk micromachining with surface micromachining. First step, deposit and pattern a thin layer of LPCVD silicon nitride underneath the electrode bonding

pads. This serves as an isolation dielectric layer [8]. Next, dry etch into a silicon substrate using the STS Silicon Deep Reactive Ion Etcher (DRIE) to form deep trenches with smooth and straight sidewalls. Then deposit a sacrificial oxide layer and refill the trenches with polysilicon. Dope the polysilicon layer with boron to speed its etch rate [11]. Then, use a dry directional/isotropic SF6 silicon etch to release silicon sense electrodes that are as tall as the ring structure [11]. This involves a deep, directional etch followed by an isotropic SF6 silicon etch. Using HF:H2O , etch away the sacrificial oxide to create capacitive air gaps between the sense-electrodes and the ring structure [11].

Figure 4. Fabrication process for HARPSS

gyroscope [11].

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This technology has been

successfully used to fabricate numerous thick polysilicon vibrating gyroscopes. Typical dimensions of a gyroscope using this method are around 1.1mm ring in diameter and 80 µm tall [10].

B. PERFORMANCE

This device provides several important features required for high-performance gyroscopes, including small ring-to-electrode gap spacing (<1 µm) for increasing the sense capacitance; large structural height for increasing the radius and sense capacitance and reducing the resonant frequency; and a better structural material (polysilicon) for increasing the quality factor Q with an orientation-independent Young’s modulus [1].

An 80 µm tall prototype polysilicon ring gyroscope was tested open loop under a vacuum. The sensitivity of the device under poor vacuum and large parasitics was measured to be 200 µV/deg/sec [11]. The resolution was found to be approximately 1 deg/sec for a material quality factor of 1200. However, improvement in the material quality factor will reduce the resolution to 0.01 deg/sec in a 1 Hz bandwidth [11]. The minimum detectable signal that can be achieved in a 10 Hz bandwidth is 5x10-3 deg/sec [10]. C. APPLICATIONS

High-performance microgyroscopes are needed in many different applications, including inertial navigation, control,

defense, and avionics. HARPSS fabricated gyroscopes are best used in “rate grade” applications, which require a rotation rate resolution and bias stability of about 0.5 deg/s [10]. Applications in the automotive industry are traction control systems, ride stabilization, and roll-over detection. Consumer applications in electronics include stabilization of pictures in digital video cameras and inertial mice in computers. Applications that require improved performance, such as guidance of missiles, are not usually best suited with a HARPSS vibrating ring gyroscope.

D. LIMITATIONS

To obtain a high mechanical quality factor and improved resolution, both external and internal energy losses should be minimized. One of the typical problems encountered in the design of the HARPSS gyroscope that causes energy loss is the anchor problem. Excessive undercut of the silicon substrate at the post (see Figure 5) causes the interface oxide layer between the substrate and structural poly to be exposed during the HF release and get etched away, which in turn will result in a soft anchor that dissipates energy [10].

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Figure 5: SEM view of the anchor of a poly ring gyroscope. Excessive undercut of the post can

result in dissipation of energy.

Another possible source of dissipation of energy in trench-refilled polysilicon beams are voids and keyholes that can be generated during the polysilicon refill process, as shown in Figure 6. In order to avoid this problem, trenches with completely vertical (or slightly slanted inward) sidewall profile are needed and enough polysilicon needs to be deposited to completely refill trenches at the wide intersection points [10].

Figure 6: Voids and keyholes generated during the

polysilicon refill process.

Finally, an important limitation

worth mentioning in the HARPSS process is the RIE lag effect. The height of high aspect-ratio narrow trenches is usually less

than the height of medium and wide trenches due this RIE lag. When etching 12 µm wide trenches to a depth of 220 µm, 5 µm wide trenches would only etch to a depth of 175 µm, lagging about 45 µm or 20% (Figure 7). This reduces to 9% for 70 µm deep trenches [11].

Figure 7: RIE lag effect.

V. DRAPER LABORATORY TUNING FORK GYROSCOPE

Tuning forks are a classical example of vibratory gyroscopes. The tuning fork, as illustrated in Figure 8, consists of two tines that are connected to a junction bar. In operation, the tines are differentially resonated to a fixed amplitude, and when rotated, Coriolis force causes a differential sinusoidal force to develop on the individual tines, orthogonal to the main vibration. This force is detected either as differential bending of the tuning fork tines or as a torsional vibration of the tuning fork system. The actuation mechanisms used for driving the vibrating structure into resonance are primarily electrostatic, electromagnetic, or piezoelectric. To sense the Coriolis-induced vibrations, capacitive, piezoresistive, or piezoelectric detection mechanisms are

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used. Optical detection is feasible, but generally too expensive to implement [1].

Figure 8: Tuning Fork Vibratory Gyroscope [1].

The Charles Stark Draper Laboratory

has developed a vibratory tuning-fork rate gyroscope that is fabricated on silicon chips. Draper has had a strong history with MEMS inertial sensors and has made great strides in simplifying the fabrication process as well as improving the performance of their gyroscopes. Draper has been inventing inertial guidance systems for earth and space applications for over 50 years [12]. In 1993, they were the first to demonstrate rate sensing with a micromachined silicon sensor.

The gyroscope’s operation is shown in the schematic of Figure 9. Electrostatic forces are generated by exciting the combs and do not depend on the lateral position of the masses. This improves the sensitivity of the gyroscope due to the large amplitude vibrations. The gyroscope’s design is a silicon structure suspended above a glass substrate containing metallization deposited for sensor interfacing. The silicon structure contains two masses suspended by a sequence of beams anchored to the glass substrate [13]. When voltages are applied to

the outer motor drives, the two masses are electrostatically forced to generate lateral, in-plane oscillatory motion. This in-plane vibration results in the drive velocity V [4].

Figure 9: Schematic drawing of the comb-drive

tuning fork gyroscope, showing input rate Ω, Coriolis forces F1 and F2, and horizontal drive

velocity V [4]. An angular rate is then applied about

the input axis, perpendicular to the velocity vector of the masses. This causes one mass to go up and the other to go down through Coriolis forces. The two masses move in equal and opposite directions so that antiparallel motion is induced. Capacitors under each of the masses are used to measure the resultant motion, providing a signal proportional to the rate input [4].

A. FABRICATION PROCESS

Tuning fork gyroscopes are fabricated using a dissolved wafer process using reactive ion etching and boron diffusion to define the final structure. This results in a sensor die size of approximately 1 mm [13]. Figure 10 shows the steps of this process, which involves both silicon and

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glass processing. In the Mask 1 step, a p-type (100) silicon wafer of moderate doping (>1Ω-cm) is used [4]. Recesses are etched into the silicon using KOH to define the height of the silicon above the glass as well as gap spacing for the capacitive sensing plates. High temperature (1175°C) boron diffusion of 5 to 10 µm is next. This defines the thickness of the structure. Mask 2 defines the structure’s pattern features and then micromachined using a reactive ion etch process [4]. By etching past the p++ etch stop, the structures are released. Straight sidewalls and high aspect ratios are obtained by using a CF3Br chemistry for etching in a parallel plate reactor.

The glass wafer is processed separately by recessing the glass by 1600 Angstroms, then depositing and lifting off a

multi-metal system on a #7740 Corning glass wafer [4]. Mask 3 defines the glass recess and metal electrode pattern. This results in a planar structure with the metal protruding only 500 Angstroms above the surface of the glass. The metal forms the sense and drive plates of the capacitor and the output leads from the transducer.

The silicon wafer is turned upside down and electrostatically bonded to the glass. In order to create strong chemical bonds, the bonding occurs at 375°C with a potential of 1000 volts applied between the glass and silicon [13]. A metal lead on the glass slightly overlaps the silicon rim in order to make electrical contact with the silicon. The silicon and glass are drawn tightly together, ensuring a low-resistance Si/Au contact to the silicon. The final step in

Figure 10: Dissolved wafer, silicon con glass process [4].

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the process is an etch in ethylene-diamine-pyrocatechol-water (EDP) to dissolve the undoped silicon.

The advantages of this fabrication sequence are that it requires only single-sided processing with two masking steps on silicon and one on the glass. The process is also high yield, compatible with batch processing, and has a low stray capacitance limited by the symmetry of the bond-wires. B. PERFORMANCE

The major difficulty in designing a MEMS gyroscope for guidance, navigation, and control applications is in maintaining accuracy over time and across the harsh environments. Significant improvements have been made in the sensor design, fabrication, electronics architecture, and packaging which have resulted in excellent performance of the tested gyroscopes.

The design of this gyroscope has been shown to be very robust with regards to environment, since the gyroscopes can withstand accelerations of 60,000 g’s. Test results show that instrument scale factor performance is greater than 200 ppm stability, and bias uncertainty is less than 50°/hr. Nominal resolution is 100 to 200 °/hr in a 60 Hz bandwidth, with a best performance of 25°/hr. For a 1 mm gyroscope, the drift is 0.5 °/s over the automotive temperature range of -40°C to 85°C. Smaller temperature ranges of 0.5°C result in an improved drift of less than 0.003°/s [13].

C. APPLICATIONS

These tuning fork gyroscopes are of very small size and low cost. They are also extremely rugged, inherently balanced, and easy to fabricate. This makes them ideal for many applications where micromachined gyroscopes are essential.

The major commercial application of micromachined gyroscopes is using a yaw rate sensor for skid control in anti-lock braking applications for automobiles. This technology is currently available in high end, luxury models, but low cost inertial sensors will enable this safety feature to be added to standard automobiles at minimal cost. Future applications include aided navigation, smart cruise control, and detection of rollover conditions.

The military can benefit from MEMS sensors in the use of guiding gun-launched munitions. This is the most immediate need identified for naval surface ships [4]. All platforms would employ GPS-aiding, where the GPS calibrates the inertial measurement unit during the initial portions of the flight. This would allow the last portion of the flight to be fully inertial in order to circumvent jamming or other electronic counter measures.

D. LIMITATIONS

This design is adequate, but could be made more sensitive if an optical sensor was used instead of a piezoresistive or capacitor. This is generally a very expensive option and would only be considered in high precision areas such as in the military for guidance systems.

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VI. CONCLUSIONS

The HARPSS process for fabricating a microgyroscope has many advantages. It produces a device that has excellent mode matching, high resolution, low zero-rate output, and long-term stability. The fabrication process results in a microgyroscope that is desirable in “rate grade” devices due to its relatively low cost yet high performance.

The low cost, small size, and ruggedness are just a few of the advantages of Draper Laboratory’s design for a micromachined tuning fork gyroscope. The fabrication is simpler than many other configurations, the performance is improved due to the relatively large proof mass drive amplitude, and sticking is reduced because of the static balance and stiffer springs. These advantages make this design preferable for many applications that require gyroscopes of high ratios in performance to cost.

Micromachined gyroscopes are still in its early development phase. Nevertheless, the first commercial devices for low to medium performance applications are already available and their number will grow considerably in the near future. REFERENCES [1] N.Yaxid, F.Ayazi and K.Najafi,

“Micromachined Inertial Sensors”, Proceedings of the IEEE, col.86, no.8 (1998) 1640-1659.

[2] E.J.Siff and C.L.Emmerich, An Engineering Approach to Gyroscopic

Instruments, Robert Speller & Sons, New York (1960).

[3] Silicon Sensing Systems, “Silicon VSG Rate Gyroscope” <http://www.spp.co.jp/sssj/sindoue.html>

[4] J.Bernstein, et al., “A Micromachined Comb-Drive Tuning Fork Rate Gyroscope”, IEEE (1993)

[5] I.Hopkins, “Performance and Design of a Silicon Micromachined Gyro”, Silicon Sensing Systems press release, (2001).

[6] P.L.Bergstrom and G.G.Li, “Inertial Sensors”, MEMS Handbook. Ed. Gad-El-Hak, CRC Press, (2001).

[7] H.Lefevre, The Fiber-Optic Gyroscope, Artech House, Norwood, MA (1993).

[8] M.W.Putty, “A micromachined vibrating ring gyroscope”, Ph.D. dissertation, University of Michigan, (1995).

[9] J.Soderkvist, “Piezoelectric beams and vibrating angular rate sensors,” IEE Trans. Ultrason., Ferroelect., Frequency Contr., vol.38, (1991).

[10] F.Ayazi and K.Najafi, “A HARPSS Polysilicon Vibrating Ring Gyroscope”, Journal of Microelectromechanical Systems, vol.10, no.2, (2001).

[11] F. Ayazi et al., "A high aspect-ratio polysilicon vibrating ring gyroscope," in Proc. Solid-State Sensor and Actuator Workshop, Hilton Head, SC, June 4-8, 2000, pp. 289-292.

[12] G.Greiff, et al., “Silicon Monolitic Gyroscope”, Transducers ’91, Digest of Technical Papers, 1991 International Conference on Solid State Sensors and Actuators, (1991).

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[13] A.Kourepenis, et al., “Performance of MEMS Inertial Sensors”, IEEE Plans, (1998).

BIOGRAPHICAL SKETCHES Kimberly S. Elliott was born in 1979. She graduated with distinction in 2002 with a B.S. degree in Mechanical Engineering from Iowa State University. She is currently pursuing a Ph.D. in Mechanical Engineering at Northwestern University. Her research interests include controls/dynamics and computational mechanics. Parag Gupta was born in 1980. He received the B.E. and M.Eng. degrees in mechanical engineering from Vanderbilt University in 2002 concentrating on Electrothermal Simulations of Power HEXFET Devices. In September 2002, he joined the mechanical engineering doctoral program at Northwestern University with a Murphy Fellowship. His research interest is primarily nanotribology which meshes well with an NSF IGERT program in existance at Northwestern to be begun in June 2003. Kyle Reed was born in Los Alamos, New Mexico, in 1978. He received his B.S. degree in mechanical engineering (with honors) from The University of Tennessee in Knoxville in 2001. He spent four summers working, in between school years, as an intern at Los Alamos National Laboratory working on explosion simulations, written in Java, and developing finite element models of bomb containment

vessels. In 2001, he was awarded the National Science Foundation Fellowship, which he deferred one year to teach English in Shenzhen, China. Currently, he is using the NSF Fellowship to pursue his Ph.D. in mechanical engineering at Northwestern University, working in the Laboratory for Intelligent Machining Systems. Raquel Rodriguez was born in Monterrey, Mexico, in 1979. She received her B.S. degree in electrical engineering (with highest honors) from the University of Illinois at Chicago in 2002. She is currently pursuing the Ph.D. degree in mechanical engineering at Northwestern University, from which she received a Walter P. Murphy fellowship. Her research interests involve micro/nano fabrication and the manipulation of biomolecules.