Analyses Of A Novel Grid Strip Biaxial MEMS Inertial Sensor

Micro-Electro-Mechanical-System is a rapidly-developing and ever-growing area, and with the representative of accelerometer, MEMS sensors are the most widely-used MEMS devices at present. With the advantage of high-sensitivity, low-temperature coefficient, as well as low power consumption and low cost, capacitive accelerometer has been widely used in the area of auto industry, consumer commodities, oil exploration and navigation etc.Basic principle, structure and detecting principle of capacitive accelerometer are analyzed in this thesis, and major detecting methods of capacitive MEMS devices are compared, and then the method of area-change with the structure of grid strip capacitors were adopted in the structure design. Air damping of the accelerometer is analyzed which proves that the grid strip structure which works on the condition of slide-film damping has low air damping and it can effectively improve the resolution of the device; effects of electrostatic force is also simulated which indicating that the resolution of the designed accelerometer can be improved through voltage increase of detecting signal.A biaxial capacitive MEMS accelerometer based on grid strip structure is designed according to analyses above, which consists of supporting beam, connecting beam, proof mass with grid strip detecting electrodes, glass substrate and aluminum detecting electrodes deposited on glass substrate. Damping adjustment combs were designed to adjust damping property of the designed accelerometer; anti-collision structure is designed on the sensitive axes to protect the accelerometer from damage induced by large acceleration.Finite element analysis software ANSYS was employed to analyze the performances of the designed accelerometer, including modal analyses which proves that the sensitive moving direction is the same as the requirement of design; the static sensitivity analyses which prove that the accelerometer has nearly the same sensitivity on x- and y- axes; strength analyses which proves that the accelerometer can sustain 1000g acceleration; transverse analyses which prove that the transverse effect of the accelerometer is near 0.Because of the small-scale character of capacitive MEMS devices, capacitor fringe effects are proved to have significant influence on capacitance of capacitive MEMS devices. Fringe effect of grid strip structure and parallel structure is analyzed and compared. Electro-Magnetic field analyzing software ANSOFT-Maxwell was employed to simulate the impact of fringe effect on grid strip capacitors, including the capacitance change of grid strip capacitors with the change of electrode length, width, overlap width, plate thickness, plates distance and grid strip density etc, and the analyzed results shows that the fringe effect would cause nonlinear change of grid strip capacitors with the change of overlap length and width; and that the capacitance would increase with the increase of grid strip width and plate thickness; and that the sensitivity can be improved by increase of grid strip density under the same area, however the linearity of the devices would decrease correspondently. Otherwise, influence of capacitor fringe effect on the designed accelerometer is analyzed combined with the size of the accelerometer.Layout of the designed biaxial MEMS accelerometer is designed. Then the main manufacturing processes are introduced and then the manufacturing flow of the accelerometer was designed based on bulk-silicon micromachining; the main processes of the fabrication are also introduced. Micro-loading effect induced by DRIE process was analyzed and the accelerometer structure was modified to cope with this effect. The designed accelerometer was manufactured which proves the feasibility of the designed process and then primary static test was issued which show that the capacitance sensitivities of the manufactured accelerometer are 0.53pF/g and 0.49pF/g on x- and y- axis respectively, which indicates that the designed accelerometer can fulfill the needs of biaxial acceleration detection; at last the damping performance of grid strip structure was analyzed by using a resonator with grid strip structure, which proves that devices with grid strip structure has low air damping and high Q factor and thus good dynamic performance.