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.