Development of a Capacitive Accelerometer Using Parylene (Part 2) : —Measurement Principle Using Fringe Electrical Field and Characterization—

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This article proposes an accelerometer, that consists of a dielectric seismic mass and a comb-shaped planar capacitor below it. Because the simple structure of the device allows using polymer Parylene as the proof mass and the support beams, the technology is greatly simplified and only surface micromachining is required. Measurement principle is based on detecting the capacitance change with respect to the dielectric mass movement in the fringe electrical field. This principle is verified by both a theoretical calculation using an approximate model and a FEM simulation. As Parylene has intrinsic tensile stress, comparatively long beams are necessary for lowering the resonant frequency fr of the sensor structure, i.e., increasing the accelerometer sensitivity. A spiral beam is effective for not only realizing a long beam in a limited space, but also realizing the stress relaxation, which leads to the high sensitivity. Parylene accelerometers with straight beams and spiral beams are fabricated. Their sensitivities are characterized with the aid of an off-chip capacitive readout IC, confirming the good linearity of the output voltage to the input acceleration and the effectiveness of utilizing spiral beams.
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