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An All-Silicon Single-Wafer Micro-g Accelerometer

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An All-Silicon Single-Wafer Micro-g Accelerometer Powered By Docstoc
					        A Low-VoltageTunneling-Based Silicon Microaccelerometer

                                     Chingwen Yeh

        Some high resolution physical sensors, including microaccelerometers, use a
constant tunneling current between one tunneling tip (attached to a movable
microstructure) and its counter-electrode to sense displacement. Figure 1 shows the
general operating principal of a micromachined tunneling accelerometer. As the tip is
brought sufficiently close to its counter-electrode (within a few Å) using electrostatic
force generated by the bottom deflection electrode, a tunneling current (Itun) is established
and remains constant if the tunneling voltage (Vtun) and distance between the tip and
counter-electrode are unchanged. Once the proof-mass is displaced due to acceleration,
the readout circuit responds to the change of current and adjusts the bottom deflection
voltage (Vo) to move the proof-mass back to its original position, thus maintaining a
constant tunneling current. Acceleration can be measured by reading out the bottom
deflection voltage in this closed-loop system. Tunneling accelerometers can achieve very
high sensitivity with a small size since the tunneling current is highly-sensitive to
displacement, typically changing by a factor of two for each Å of displacement.

        Low-voltage operation (≈10V) is a very important feature if micromachined
tunneling sensors are to find widespread use in commercial applications. In particular,
this allows the readout electronics of tunneling devices to be CMOS-compatible, thus
enabling them to be incorporated into a portable, battery-operated multi-sensor
microsystems. The objective of this project was the design and development of a low-
voltage tunneling-accelerometer with its CMOS interface circuitry.

        The device is fabricated using bulk silicon micromachining technology and the
boron etch-stop dissolved wafer process. An SEM picture of the fabricated devices is
presented in Figure 2. This accelerometer takes about 400x400µm2 area including the
proof mass and support beams, with a noise spectral density of the sensor-circuit module
4mg/√Hz (at 0.5Hz) to and 0.1mg/√Hz (at 2.5kHz), and minimum detectable acceleration
of 8mg in a 2.5kHz bandwidth. After continuous operation over 720 hours, the
accelerometer shows an offset and sensitivity variation of less than 0.5% without abrupt
interruption of the power supply.
Figure 1: The general structure of the low-voltage tunneling-based accelerometer.




Figure 2: The SEM photograph of a tunneling microaccelerometer fabricated using
silicon-wafer-dissolved process and glass bonding. The picture shows the top electrode,
and the perforated proof mass partially visible under this electrode.
For more information please refer to:

1) C. Yeh and K. Najafi, "A Low-Voltage Bulk-Silicon Tunneling-Based
Microaccelerometer," Technical Digest, IEEE Int. Electron Devices Meeting (IEDM),
Washington, D.C., pp. 593-596, December 1995.

2) C. Yeh and K. Najafi, "A Low-Voltage Tunneliing-Based Silicon
Microaccelerometer," IEEE Trans. Electron Devices, vol. 44, no. 11, pp. 1875-1882,
November 1997.

3) C. Yeh and K. Najafi, "Micromachined Tunneling Accelerometer with a Low-Voltage
CMOS Interface Circuit," Proc. Int. Conf. on Solid-State Sensors and Actuators,
Transducers '97, Chicago, pp. 1213-1216, June 1997.

				
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