Design And Fabrication Of Thermally Excited Si₃N₄ Resonant Beam Pressure Sensor

Resonant devices are used to measure strain, stress, acceleration, pressure, mass, flow, etc. by detecting the change of the resonant frequency or amplitude of the resonant components. Mechanical resonant sensors are characterized by high accuracy, rapid response, excellent stability, resistance to electrical interference and digital output. With the development of microelectronic and micromachining technology, the dimension of the resonators is becoming smaller and smaller, from meter and centimeter level to micron level. The resonators are usually made of semiconductor such as Silicon, Polysilicon, with structures of beam or membrane, fabricated by micromachining technology. Their small size, light weight, excellent performance and low cost bring them broad application prospect.This thesis represents the design and fabrication of a resonant beam pressure sensor. The resonant characteristics are analyzed using elastic mechanics theory. Using FEA method, the model of the resonant component is built up and the effects of different beam materials and chip structures on the performance of the chip are studied. Finally, Si3N4 beam and Si reflecting membrane are selected with an integrated structure, using electrothermal excitation and piezoresistive detection. The chip is fabricated using micromachining technology and porous silicon sacrificial-layer technique is used to realize the integrated structure. The chip is bonded to a stress isolating mechanical structure and sealed into an evacuated package. Both open-loop and close-loop measuring system are used to test the chip.The testing results accord with the theoretic calculations and FEA simulations satisfactorily. In a vacuum of 10-2 Pa, the Q factor of this sensor is 19502; the sensitivity is 54.89Hz/KPa with a linear correlation coefficient of 0.9997 over a range of 0~300KPa. These results suggest that this sensor is suitable to the pressure measuring in the range of 0~300KPa.