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The present study reports the fabrication and characterization of a high-sensitivity surface micromachinable accelerometer using a ferroelectric material having a large dielectric constant, such as bulk PZT (ε r = 2600). The measurement principle is the detection of capacitance change with respect to the dielectric mass movement in the fringe electrical field. Considering the practical fabrication, a ferroelectric material is used for the substrate instead of the suspended proof mass. Since capacitance is increased not mechanically but electrically, high sensitivity can be expected while maintaining mechanical stiffness, which has the advantage of preventing touch down of the proof mass and widening the allowable measuring range of acceleration. A fabricated sensor is characterized. A high sensitivity of several sub-pF/g is confirmed, which is hundreds of times higher compared to that of a sensor fabricated using the Parylene polymer (ε r = 3.15). This sensitivity is an order higher compared to those of other reported capacitive microaccelerometers, which have comparable proof mass sizes and comparable detectable acceleration ranges to the newly fabricated sensor. 

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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. The Parylene accelerometer being developed by authors, in which a proof mass is supported by several beams, is focused on. The stiffness k and the resonant frequency r f of this sensor structure under tensile stress are investigated. The FEM simulation is employed for the structure with straight beams, and it is proven that the relationship of The structure with spiral beams is proposed to obtain long beams with good space efficiency. Figure 1 shows the FEM simulation results of the relationship between tensile stress int σ and r f , confirming the r f using spiral beams is lower than that using straight beams. Parylene suspended microstructures are practically fabricated as shown in Fig. 2, and their experimental r f are obtained using a laser Doppler vibrometer (LDV). As for structures using straight beams, the experimental data of r f corresponding to l is shown in Fig. 3, with simulated data under int σ =10 MPa, and fitting curve of 0 35. Although slight error of index number exists, this trend coincides well with the simulated one, supporting the validity of the simulation.As for structures with spiral beams, Table 1 shows the comparison between experimental r f and simulated ones. The simulation assumes that conformal tensile stress exists everywhere in the structure. The experimental r f are much lowered compared to those using straight beams (see Fig. 3). Moreover, the tensile stress is less than 5 MPa (near no stress) according to this result, whereas actual tensile stress of Parylene film is approximately 10 MPa. This means the assumption of conformal tensile of everywhere is not effective, i.e., the actual tensile stress is much relaxed by using spiral beams. It is proven that using spiral beams is effective for lowering r f , in the viewpoint of not only realizing a long beam in a limited space but also realizing stress relaxation. Non-memberParylene 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 r f . 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 1/ r f . To cope with this problem, a structure with spiral shaped beams is proposed, and its r f is simulated. Parylene suspended microstructures are practically fabricated, and their experimental r f 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.