4B2.12PL Force-Balanced Accelerometer with mG Resolution,

Fabricated using Silicon Fusion Bonding and Deep Reactive Ion Etching B.P. van DrieEnhuizen’, N.I. Maluf”’, I.E. Opris’ and G.T.A. Kovacs2

Lucas NovaSensor, 1055 Mission Court, Fremont, CA 94539, USA Center for Integrated Systems, Stanford, CA 94305, USA

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SUMMARY A single crystal silicon accelerometer with mG resolution (1 G = 9.81 m/s’) has been fabricated using Silicon Fusion Bonding (SFB) and Deep Reactive Ion Etching (DRIE). This allows thick devices (up to several hundred pm) to be defined with high aspect ratios (up to 25), resulting in high sensitivity and low cross-axis sensitivity. Prototypes use a hybrid approach, with an 1.0 x 1.5 mm2 mechanical element and a capacitive sensor interface providing closed-loop force- balancing to minimize non-linearity. The bandwidth is 1 kHz and the sensitivity is 700 mV/G. The dynamic range is 44 dB, corresponding to a resolution of 35 mG for a 5 G (full scale) device and 7 mG for a 1 G device. The resolution is currently limited by lif noise in the electronic interface, but will be reduced with an improved design of the capacitive sensing interface (currently in fabrication), thus resulting in sub-mG resolution.

Keywords: Accelerometer, SFB, DRIE, force balancing.

INTRODUCTION For low-G applications lateral surface micromachined accelerometers can be used, which combine low cost with small non-linearities when closed-loop force-balancing is used. However, their limited mass, results in a high thermal- mechanical noise. The small aspect ratio (- 1) limits the out-of- plane stiffness and therefore cross-axis sensitivity. These limitations can be overcome by combining Silicon Fusion Bonding (SFB) and Deep Reactive Ion Etching (DRIE) to fabricate high aspect ratio (up to 25), single crystal silicon accelerometers [ 11. A CMOS capacitive sensor interface provides force balancing in a closed loop configuration.

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Figure 1: Cross-section of the SFB/DRIE based accelerometer.

SFB/DRIE FABRICATION The SFBiDRIE process starts with an etched cavity in a handle wafer, buried under a SFB wafer. This wafer is polished down to a desired thickness and determines the device thickness (typically between 20 pm and 200 pm). At this point circuitry can be integrated, using standard IC fabrication processes. After

circuit processing, the mechanical structures are defined and released using DFUE of trenches with standard photoresist as a masking layer (Fig. 1). The DRIE is performed using a tool available from Surface Technology Systems (Abercarn, Wales, UK) . The sensor (Fig. 2) uses two electrodes (or two connected pairs) for differential measurement of capacitance changes and two other electrodes for electrostatic actuation, used in a feedback configuration as well as self test.

Figure 2: A SEM of an accelerometer with the inset showing an enlarged image of the high aspect ratio suspension and electrodes. The four corner electrodes are used for sensing, the two middle for actuation.

ELECTRONIC SENSOR INTERFACE A schematic block diagram of the CMOS electronic interface circuit is shown in Fig. 3. and consists of five basic blocks: (1) An oscillator, which converts a difference in sense capacitance into a phase difference between two high frequency oscillating outputs; ( 2 ) a phase detector, which converts the phase difference into a DC output voltage; (3) an integrator, which ensures stability; (4) a phase shifter, which corrects for mechanical poles within the closed-loop bandwidth and ( 5 ) output amplifiers, which drive the actuation electrodes. In the oscillator a constant current (- 100 pA) charges the two sense capacitors, whose charging voltages trigger Schmitt- triggers. When a Schmitt-trigger is activated, a reset then discharges the sense capacitors (after a short delay to ensure a minimum output pulse length) and resets the Schmitt-triggers. This results in an oscillator running at -5 MHz for 5 pF sense capacitors and a threshold voltage of 3.4 V for the Schmitt triggers. Capacitance changes result in modulation of the pulse width (-34 ps i s ) and of the relative phase between oscillator outputs. The phase detector (Fig. 4) converts the phase difference into a DC output signal by integrating a constant

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4B2.12PL positive current (- 100 pA) on a reference capacitor (1 nF) during the time the phase difference is positive. During the time the phase difference is negative, a negative current is integrated. For a bandwidth of 1 kHz and an oscillator frequency of 5 MHz, this results in an integration over 5000 pulses, which yields a phase detector sensitivity of 17 mV per fF of capacitive imbalance. The integrator ensures the stability of the feedback system and determines the bandwidth of the system, by defining one dominant pole at zero. The phase shifting network introduces two zeroes to compensate for the negative phase shift resulting from the two complex poles of the (almost undamped) mechanical system. These poles are located at the resonance frequency, approximately 3 kHz. The phase shifter can be used to fine tune the phase margin of the closed loop, to increase the bandwidth of the system beyond the resonance frequency. The integrator pole and the phase shifter zeroes are defined by external capacitors, which allow easy adjustment. Finally the differential output signal is fed back to the actuation electrodes to provide the counteracting electrostatic force to null the displacement. Since the displacement of the force balanced system is negligible, the systems non-linearities are minimized.

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Oscillator ......................................

Schmidt Triggers 1 ,

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4 Feedback Gain

Figure 3: A schematic block diagram of the electronic sensor interface. The two sense capacitors integrate a constant current in an oscillator circuit. The phase difference between the two pulse train is detected, jiltered then amplijied and fed back to the differential actuator electrodes.

In2

Figure 4: Phase detector circuit. In1 and In2 are connected to the two outputs of the oscillator.

MEASUREMENTS The tested prototypes use a 2y-CMOS capacitive sense interface fabricated via the foundry service MOSIS. The measurements shown here result from a 1.0 x 1.5 mm2 sensor design, which consists of a set of 40 capacitive sense electrodes

as well as another of 40 actuation electrodes for force- balancing. The electrodes are 6 pm wide, 50 pm thick, with a spacing of 4 pm. The weight of the proof mass is 43 pg. The resonant frequency is 3.1 kHz, which is larger than the bandwidth (1 kHz) of the closed loop system. The nominal capacitance of the sense electrodes is - 3 pF which readily allows using a hybrid approach instead of on-chip integration. The sensitivity of the open loop device is 1.6 fF/G (for small G’s). The integrated mechanical noise over a 1 kHz bandwidth is 300 yGms for a quality factor Q = 1. The system is expected to have a much higher quality factor, resulting in even lower mechanical noise. The sensitivity was measured by rotating of the sensitive axis with respect to the gravitational field of the earth, using a 1200-step stepper motor. The results show a sensitivity of 700 mV/G with a noise floor of 35 mG (over a 1 kHz bandwidth) for a 5 G device (-5 to +5 V differential output). The measured noise spectrum shows a l/f dependence (Fig. 5) and is inherent to foundry CMOS processes. Preliminary shock tests indicate that the devices are immune to out-of-plane shocks in excess of 5000G. Further testing is underway to fully characterize the device’s immunity to shock.

1000 10000 Hz

Figure 5: Measured noise spectrum of the closed-loop system, showing the l/f nature The integrated noise over the 1 kHz system bandwidth is 35 mG,,,.

CONCLUSIONS The use of a SFBiDRIE based sensor in a force balancing system allows the fabrication of low cost, robust, high performance lateral capacitive accelerometers. The l/f noise of the current ASIC design limits the dynamic range to 44 dB due to electronic l/f noise A new design circumventing this noise limitation increases the dynamic range to > 60 dB thus reducing the noise floor of a 1 G device to the sub-mG range.

REFERENCES [ l ] E. Klaassen, K. Petersen, J.M. Noworolski, J. Logan,

N. Maluf, J. Brown, C. Storment, W. McCulley and G. Kovacs, “ Silicon Fusion Bonding and Deep Reactive Ion Etching; A New Technology for Microstructures”, Proceedings of the 8th International Conference on Solid- State Sensors and Actuators, Transducers ‘95, Stockholm, Sweden, June 1995, Vol. 1, pp. 556-559.

ACKNOWLEDGMENTS The work was funded under DARPA contract DAALO 1-94-C- 3411.

1230 TRANSDUCERS ’97 1997 lnternational Conference on Solid-state Sensors and Actuators Chicago, June 1619, 1997