Development of a Capacitive Accelerometer Using Parylene (Part 1) : —Study on Resonant Frequency of Parylene Suspended Structure—

概要

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Parylene has intrinsic tensile stress on account of mismatch of thermal coefficient of expansion (TCE) between the substrate and the deposited film. Therefore, the stiffness k of a Parylene suspended structure under tensile stress is much higher than that under no stress, which also leads to its higher resonant frequency of fr. The Parylene accelerometer being developed by authors, in which a proof mass is supported by several beams, is focused on. The FEM simulation is employed for the structure with straight beams, and it is proven that fr∝1/l0.45 holds true under tensile stress of 10 MPa, while fr∝1/l1.5 holds true under no tensile stress, where l is the beam length. This fact means a relatively long beam is necessary under tensile stress for the purpose of lowering fr, i.e., increasing the sensitivity of 1/fr2. To cope with this problem, a structure with spiral shaped beams is proposed, and its fr is simulated. Parylene suspended microstructures are practically fabricated, and their experimental are obtained. By comparing the experimental results with the simulated ones, the validity of the simulation is proven, and the effectiveness of spiral beam is confirmed in the viewpoint of not only realizing a long beam in a limited space but also realizing stress relaxation.
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