a novel silicon bulk gyroscope - CiteSeerX

A NOVEL SILICON BULK GYROSCOPE Gert I.Andersson*, Nils Hedenstierna**, Per Svensson*** and Håkan Pettersson**** *The IMEGO Institute, Aschebergsgatan 46, Building 11, SE-411 33 Göteborg, Sweden, phone +46 31 7721915, fax +46 31 7723622, e-mail: [email protected] **SensoNor asa., phone +46 31 290366, e-mail: [email protected] ***Monolitsystem AB, phone +46 31 274890, e-mail: [email protected] ****Autoliv AB, Research, phone +46 322 626338, e-mail: [email protected]

ABSTRACT This paper reports on a novel silicon bulk gyroscope for automotive applications. The novel gyroscope structure that we have called the “Butterfly-gyro” has a gyroscopic scale factor comparable to that of tuning fork gyros. The gyro is simple to manufacture with single sided electrostatic exitation and capacitive detection. As the two masses vibrate in anti-phase the offset is smaller and the gyro is less sensitive to linear and angular vibrations. The manufactured prototypes operate at atmospheric pressure and show excellent linearity. The best samples have a resolution of approximately 0.1 °/sec at 50 Hz bandwidth.

in the subatomic range. In a vehicle this must be performed in a vibration rich environment. These and other demanding requirements have caused great challenges, regarding for instance packaging and manufacturing, to the angular rate sensor programmes, often resulting in delays.

INTRODUCTION There is an increasing need for reliable, low cost angular rate sensors for various automotive applications such as vehicle chassis stabilisation, safety systems and navigation systems. Some luxury cars already have systems based on gyroscopes but for standard cars present sensor prices are too high and a broader market introduction is not anticipated before the beginning of the next millennium. Several research and development projects have therefore been initiated and a variety of silicon and quartz angular rate sensors employing different excitation and detection mechanisms have been presented. The piezeoelectric effect is utilised for excitation and detection in the quartz tuning fork gyroscope [1]. The bulk micromachined double gimbal sensor in silicon [2] makes use of electrostatic excitation and capacitive detection. Some of the proposed gyroscopes manufactured by surface micromachining also utilise electrostatic excitation and capacitive detection [3,4,5]. Sensors having electromagnetic excitation [6] and others with piezeoelectric drive and piezoresistive read out [7] have also been presented. Furthermore, a complete gyroscope function comprising sensor element and ASIC on a single chip has been demonstrated [8]. The signal levels to be detected are caused by Coriolis force induced deflections on the sensor element, typically

Figure 1. Photograph showing a prototype of the “Butterfly-gyro”. The sensor presented here, shown in Figure 1, is primarily intended for rollover applications with a full scale range of typically +/- 250 °/s. In this paper we present the theory of operation, electronics design, manufacturing process and experimental results on the new sensor concept.

THEORY OF OPERATION In a bulk silicon process where the silicon is anodically bonded to glass it is possible to generate vertical electrostatic forces (perpendicular to the surface) efficiently. Correspondingly, vertical vibrations can easily be capacitively detected with high accuracy. However, to generate horisontal forces or detect horisontal vibrations (parallel to the surface) it has previously been necessary to use comb structures, which are not easily incorporated into a bulk process. Hence, a type of vibrating gyroscope

Extended abstract Transducers ‘99, Paper no. 3D1.1

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where the excitation forces and the detection are vertical is preferred in a bulk process. Unfortunately, this easily leads to low sensitivity gyroscopes [2]. The purpose of the excitation is to give the vibrating masses as high velocity as possible since the Coriolis force is proportional to the velocity (and the angular rate around the sensitivity axis). The Coriolis force then excites a secondary vibration which is measured capacitively. A vertical excitation force gives a vertical velocity. The Coriolis force is perpendicular to the velocity of each mass point. Hence, the Coriolis forces are horisontal and this leads to horisontal movements that can not be detected efficiently. In surface micromachined gyroscopes this problem is avoided using comb drives which excite a horisontal vibration. Electrodes under the mass are used to detect the vertical movement. Essentially, this is the major problem in the design of bulk gyroscopes. Can vertical forces be used to drive a horisontal vibration? One solution to this problem is to use a thick mass and excite an angular vibration giving a horisontal velocity in the upper and lower part of the mass. This leads to perpendicular and horisontal Coriolis forces that in turn drive angular vibrations perpendicular to the former. The sensitivity of the double-gimbal gyroscope [2,3] depends critically on the height/width ratio but as most micromachining process give “flat” structures the sensitivity is low. The “four-leaf clover” structure [9] with a standing bar gives a high sensitivity but at the cost of a complicated assembly of the bar to the silicon.

angular gain is typically 1.6-1.9 in this type of structure. Many different structures can be designed using this principle. We choose to use a structure with two masses vibrating in anti-phase which gives smaller offsets and lower sensitivity to external vibration. In the excited vibration mode the beams bend in their weakest direction. The centre of the beam connecting each mass to the frame is placed at the centre-of-gravity of the mass. When the beam bends each mass rotates around an axis directed 35.27° from the vertical axis and going through the centre-of-gravity. The inner beam connects the two masses and makes the wanted anti-phase bending mode lower in frequency than the in-phase bending mode. There is also a mode where the beams bend in the stiffest direction but the resonance frequency is much higher because of the higher stiffness and lower moment of inertia. The detected mode is a torsion of the beams and an anti-phase rotation of the masses around the detection axis. There is also an in-phase torsion mode which has a lower frequency because there is no torsion of the inner beam.

The formula below gives the amplitude of the detected vibration, ψ0, as a function of the excited amplitude, θ 0, the Q-factor of the detected vibration, Q det, the excitation frequency, ωexc, and the angular rate, Ωin . The expression is general for all types of vibrating gyros but the angular gain, Ag , varies for different type of gyroscopes. The constant describes how well the Coriolis forces couples between two modes. For a tuning fork gyro the angular gain is 2 but a double-gimbal gyro can have an angular gain as low as 0.05.

ψ 0 = Ag

Q detθ 0 Ω in ω exc

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The basic idea behind the new “Butterfly” structure [10], shown in Figure 2, is to use a beam with an asymmetric cross section so that vertical electrostatic forces bend the beams both vertically and horisontally. The vibrating masses are shaped so that the velocity vectors are essentially horisontal which gives vertical Coriolis forces and vertical movements that are capacitively detected. The

Figure 2. A sketch showing the “Butterfly”-gyroscope The frequencies of the excited and the detected mode must be matched. The excitation frequency is determined by the ratio of the moment of inertia around the excitation axis to the bending stiffness. The detection frequency is determined by the moment of inertia around the detection axis and the torsion stiffness. The bending stiffness and the torsion stiffness have approximately the same


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dependence on length, width and height of the beams so the matching is controlled by the width/length ratio of the masses. An increase in width increases the inertia around the excitation axis more than around the detection axis. Thus, the frequency matching has a slight dependence on the beam geometry and this is a disadvantage because it constricts the design but it is also an important advantage as frequency matching becomes less sensitive to process variations. The excitation electrodes are placed centered on the detection axis and away from the excitation axis. The electrostatic force from the electrodes gives the masses a torque around the sensitivity axis, which translates to a lower effective torque around the excitation axis. A vertical movement over the excitation electrodes gives a larger horisontal movement over the detection electrodes. Hence, the excitation electrodes have a short arm that gives a lower excitation torque. However, due to the small vertical movement the electrode gap can be reduced and the electrostatic force increased quadratically. The net result is a larger excitation torque and a larger excitation amplitude. Conversely, the detection electrodes are placed centered on the excitation axis and away from the detection axis. They are connected cross-wise to make the total capacitance sensitive to the anti-phase rotation of the masses around the detection axis. The electrodes are also used to adjust the resonance frequency of the detected vibration mode. Due to the nonlinearity of the electrostatic force, an increase in DC-bias and HF-voltage leads to a reduced resonance frequency. The prototype of the sensor is operated at atmospheric pressure which makes squeezed-film damping very important. The electrode gap is approximately 5 _m. To reduce the damping ditches are etched. Furthermore, the areas on the glass which are not covered by electrodes are etched deep. Still, the detected mode has a Q-factor of only 5-10. Surprisingly, the Q-factor of the excitated mode is typically two orders of magnitude larger. This is explained by the fact that a majority of the mode energy is in the horisontal movement that is not subjected to squeezed-film damping.

detection mode reduces the sensitivity of the “Butterfly” gyroscope to most types of process variations. For example, a constant gradient in the electrode gap does not give any offset signal. More important, the structure is not sensitive to misalignment between glass and silicon. However, rotational misalignment gives an offset signal but this is a minor problem. Unfortunately, the structure is sensitive to the type of asymmetry that is created by rotational misalignment between the anisotropic etch mask and the crystal. In the prototypes produced so far, this is the largest source of offset signal. With improved alignment this problem will be reduced drastically. For an ideal “butterfly” gyro external linear or angular vibrations do not influence either the excited or the detected mode. This is in contrast to a tuning fork gyroscope where the detected mode is directly excited by an external angular vibration. Still, process imperfections can make the “Butterfly” gyroscope sensitive to vibrations.

ELECTRONICS DESIGN The DC-bias, the voltages for electrostatic excitation (≈2kHz) and the HF-voltages for capacitive detection (≈1MHz) are all applied to the electrodes on the glass. The silicon is one electrical node and connected to a lownoise amplifier. Using different frequencies, capacitive detection monitors both the excited vibration and the secondary vibration. A sinusoidal voltage at the mechanical resonance frequency is applied to the excitation electrodes and as a part of the feedback also to the detection electrode. The HF-signal from the amplifier is synchronously demodulated in two steps giving four channels as shown in Figure 3. The channel carrying information on the excitated vibration in phase with the VCO-signal is fed back to the VCO itself in such a way that the oscillator is locked to the mechanical resonance frequency. The angular rate information is in the secondary vibration in quadrature with the VCO-signal. To remove the offset signal and avoid saturation of the amplifier both secondary vibration channels are integrated and used in a feedback. This gives the sensor a high-pass transfer function with a breakpoint of approximately 0.1 Hz.

The offset and the vibration sensitivity of a vibrating gyroscope is just as important as the signal amplitude. The offset signal originates from asymmetric masses, asymmetric beams or asymmetric electrodes. The antiphase vibration of both the excitation mode and the


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Figure 4. Sketch showing the manufacture of the central beam along the cross section line in Fig. 2: a) An etch mask is defined on the front and back. b) A wafer etched half way through showing how the etch stops against the (111)-planes that form the beams. c) A completely etched wafer where in principle all unprotected surfaces constitute (111)-planes.

EXPERIMENTAL RESULTS

Figure 3. Schematic showing the electronics design.

MANUFACTURING PROCESS The silicon part of the sensor structure is formed in one self-stopping anisotropic etch step preceded only by a double-sided patterning of the front and the back of an oxidised silicon wafer using the method presented at Transducers ’95 [11] as illustrated in Figure 4. For the electrostatic excitation and the capacitive detection the silicon itself constitutes one electrode while the counterelectrodes are placed on an etched glass wafer anodically bonded to the silicon. Both excitation and detection electrodes are placed at the same distance from the silicon, but to minimise the squeezed-film damping in the detection direction the glass between the electrodes is etched deeper.

The overall performance of gyroscope dies and electronics has been tested using a programmable servomotor set-up. An example of the measurement results obtained is shown in Figure 5. As can be seen the manufactured prototype and the reference gyroscope measurements are almost identical. The noise from the best sensors is equivalent to 0.07 °/s at 50 Hz bandwidth. 2 0 -2 -4

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QRS14' ISSG;6:16'

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Time [s]

Figure 5. The measured response from simulated rollover event, for which the maximum angular velocity is about 8 rad/sec. The solid line is the output signal from the Butterfly-gyroscope and the dotted line is the output signal from a Systron Donner QRS14-gyroscope.


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CONCLUSIONS AND OUTLOOK The “Butterfly-gyro” concept has been successfully verified in a first batch of prototypes showing a performance well inside the specifications for a roll-over sensor. The first goal in the continued development is a smaller chip size to reduce the costs. Further, an improved electronic system is necessary to use the gyroscope for yaw rate applications.

ACKNOWLEDGEMENT This project was financed by Autoliv Inc. and we are very grateful for their positive and professional support. The success of this project is also due to Göran Pettersson and Johan Andersson, of the Department of Microelectronics at Chalmers University of Technology for skilful processing and process development work.

Applications,” Advanced Microsystems for Automotive Applications 99 (Berlin, Germany, March 18-19, 1999), pp. 261-270. [9] T. K. Tang et al., “A Packaged Silicon MEMS Vibratory Gyroscope for Microspacecraft,” MEMS 97 (Nagoya, Japan, February 26-30, 1997), pp. 500-503. [10] N. Hedenstierna et. al, “Anordning för mätning av vinkelhastighet”, Patent pending (SE9800194-4). [11] G. I. Andersson, “A novel 3-axis monolithic silicon accelerometer”, Digest of Techn. Papers, Transducers ’95 /Eurosensors IX (Stockholm, Sweden, June, 1995), Volume II, pp. 558-561.

REFERENCES [1] J. Söderkvist, “Micromachined Gyroscopes,” Sensors and Actuators, A43 pp. 65-71, 1994. [2] H. Kuisma et al., “A Bulk Micromachined Silicon Angular Rate Sensor,” Transducers ’97 (Chicago, Illinois, USA, June 16-19, 1997), Volume 2, pp. 875-878. [3] B. Boxhorn et. al, “A Vibratory Micromechanical Gyroscope”, Paper 88-4177-CP, AIAA Guidance, Navigation and Control Conference (Minneapolis, Minnesota USA, 1988), pp. 1033-1040. [4] D.R. Sparks, “A CMOS Integrated Surface Micromachined Angular Rate Sensor: It’s Automotive Applications,” Transducers ’97 (Chicago, Illinois, USA, June 16-19, 1997), Volume 2, pp. 851-854. [5] R. Schellin et al., “A Low Cost Angular Rate Sensor for Automotive Applications in Surface Micromachining Technology,” Advanced Microsystems for Automotive Applications 99 (Berlin, Germany, March 18-19, 1999), pp. 239-250. [6] F. Paelotti et al., “A Silicon Micromachined Vibrating Gyroscope with Piezoresistive Detection and Electromagnetic Excitation,” MEMS 96 (San Diego, California, USA, February 11-15, 1996), pp. 162-167. [7] R. Voss et al., “Silicon Angular Rate Sensor for Automotive Applications with Piezoelectric Drive and Piezoresistive Read-out,” Transducers ’97 (Chicago, Illinois, USA, June 16-19, 1997), Volume 2, pp. 879-882. [8] B. Sulouff, “Integrated Surface Micromachined Gyroscope and Accelerometers for Automotive Sensor


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