Study Of MEMS Capacitive Ultrasonic Sensor Based On The Silicon Wafer Bonding Technology

Ultrasonic imaging has been used in a broad range of applications covering from themarine geology, nondestructive testing, medical diagnosis and treatment to industrial andagricultural production.Seeing the great potential of huge economic benefits and defectsexisting in the traditional standard piezoelectric transducers, MEMS capacitive ultrasonicsensor has emerged as an alternative sensor with great advantages in great matching withliquid and acoustic medium, higher sensitivity and lower cost with the benefit of batchmanufacturing. In combination with the advantages of MEMS technology, the main contentand results of this paper are as follows:(1)Derived and established the theoretical model of the sensor, including lumpedmechanical model and equivalent circuit model.Introduced the working principle in detail anddid research into the field of the sensor’s performance indicators, such as operating frequency,collapse voltage, sensitivity, output sound pressure and electromechanical couplingcoefficient.(2)Studied the relationship between sensor’s structure parameters and performance,andcarried out the influence law via considering the radius, thickness, distance between the gapand residual stress.Thus building the foundation for the optimization design of the sensor.(3)Optimized the MEMS capacitive ultrasonic sensor array. By introducing the beamscanning,bcam forming,focusing and steering,the array imaging principle was stated.Evaluated the array performance in the range of grating lobe, side lobes,main lobe，sharpnessangle and beam width.Proposed the design target of ultrasonic array and the final geometricalsize of the array was carried out by the simulation and optimization.(4)Verified the performance of the designed sensor via FEM simulation.Make sure that the designed sensor has robust ability in operation,and the modal operating frequency andmode has been ensured.Other performance including the collapse voltage, sensitivity outputsound pressure and frequency response has been simulated.(5)Designed the fabrication process and mask used in fabricating of the sensor andmanufactured the actual sensor.Considering the characteristics of the bonding wafertechnology, appropriated raw material was chosen.Finally, encapsulation of the sensor wasdesigned and the sensor was packaged.(6)Characterized the morphology and performance of the sensor product.Theperformance of the sensor was verified with SEM, Veeco optical proiflometer, LEXT lasermicroscope, Polytec micro-system analyser and Aglient precision impedance analyzer.