Research On Resonant Accelerometers Based On Microresonators Electrothermally Excitated And Piezoresistively Detected Using Polysilicon Resistors

MEMS accelerometer has small volume, quasi-digital nature of the outputsignal, anti-interference ability and other characteristics, being widely used inaerospace, missile guidance, general aviation, vehicle control, industrial automation,prospecting, health care, toys, smart phones, tablets, earthquake monitoring andothers.This paper puts forward a new type of silicon resonant accelerometer whichadopted the electrothermal excitation and piezoresistive detection using polysiliconresistors. The polysilicon resistors were planted between the insulating layers ofsilicon oxide and silicon nitride, avoided PN junction leakage problems ofMonocrystalline silicon resistors and the coupling interference between excitationand the picking signal and reducing the difficulty of the test circuit.Though theoretical calculation and finite element analysis, the paper analyzedthe accelerometer with thermodynamics, statics and dynamics, then obtained therelational expression between the resonance frequency and acceleration, got theanalytical expression between micro-beam resonance frequency and structuraldimensions, material properties, thermal power, Wheatstone bridge static Joule heat,acceleration. At last the paper optimized the structure parameters.As the resonant element of the accelerometer, the micro resonant beam structurewas composed with SiO2thin film with the thermal oxidation method and siliconnitride thin films deposited by LPCVD and PECVD. The initial stress of the resonantbeam was tensile by optimizing the thickness of the three films to compensatethermal stress caused by static excitation power and joule heat on detection of bridgethat avoided buckling deformation of the resonant beam.The accelerometer structure was produced with the combination of KOHsolution wet etching and ICP dry etching that saved the cost and production cycle.<110> stick with two-different-length branches was designed as the compensation forthe convex corner, reducing the cutting Angle and the whole chip size; Fixture and AR-PC504corrosion resistant glue was designed to protect Al line, reducing therepeated process. Though optimizing the etching gas and flow parameters, the ICPoptimal etching parameters were obtained for different materials. Finally, this paperstudied the cause of the fracture in the release process for the structure and improvedthe release process, improving the yield of the device.The temperature coefficient of polysilicon resistors was tested as33.2ppm/℃with multimeter Agilent34401A and the resonant frequency216.20KHz with sweepfrequency technique. The effect of the static excitation power and the joule heat onWheatstone bridge on the resonant frequency was also tested and fit the relationalexpression between them. Finally, the free fall experiment showed that theacceleration of1g led to the resonant frequency offset of25Hz.