The Effects Of Boron Doping On The Performance Of Microaccelerometers

As one of the core component of the micro inertial measurement unit(MIMU), micro accelerometer has advantages of miniaturization, low power consumption, high precision and high integration. Comb accelerometer studied in this paper is the representative of the accelerometers. Driven by strong market demand and high-tech, the design and fabrication of the comb accelerometer has become one of the hot spots in MEMS field.In order to improve the electrical properties of silicon wafer and provide a high selectivity to etchant solution, the micro accelerometer is manufactured through selfstopped etch technique based on boron doping process. The silicon atom substituted by boron atom during the diffusion process will induce the lattice mismatch between Si and B, which will deform the fabricated structure, change the gap and facing areas of each pair of the comb fingers. However, under ideal conditions, the differential capacitance of completely symmetrical micro accelerometer is approximately linear to the external acceleration input. Therefore, buckling caused by boron doping affects the output of sensor by influencing the differential capacitance values. Additionally, the actual width of the folded beam will be smaller than the designed width because of the fabrication error during the deep reactive ion etching process(DRIE), the stiffness of the folded beam will be changed and then affect the output of the accelerometer. When considering the doping effect as well as the etching error, the analysis of the electrostatic stiffness, sensitivity and zero output of the accelerometer will be more complex. The main contents are:1. The equivalent multilayer structure model of the accelerometer was established, and the displacements in both the sensitive and thickness direction for each pair of the comb fingers were calculated. According to the calculated results of the deformed structure, the capacitance was calculated with different diffusion depth, and the effect of diffusion depth on the sensitivity of the accelerometer was investigated.2. Considering the doping effect as well as the etching error, the mechanical model of the sensitive mass was established. The whole displacement of the mass, the electrostatic stiffness, sensitivity and zero output of the accelerometer were theoretically calculated. The results indicates displacement of the mass increase with the increased error of fold beam, and when the maximum error of the folded beam e is about 0.3 μm, the displacement of the mass reaches a maximum value of 7.93 nm.3. The finite element analysis software COMSOL MUTIPHYSICS is used to simulate the capacitance and sensitivity of the accelerometer with the effect of boron doping. Furthermore, the model with considering boron doping as well as the etching error was established in COMSOL, the differential capacitance, sensitivity and zero bias of the accelerometer were obtained, and the simulated results were compared with the theoretical results.