Analytical Modeling and Simulation of Capacitive Micromachined Ultrasonic Transducer
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Ultrasound technology has been studied, developed and widely used in medical imaging. Ultrasonic imaging was invented to substitute the high cost ionizing radiation in magnetic resonance imaging (MRI) system, because it is portable, has low production cost and can be embedded with external circuitry in making sensing or actuating devices. However, many conventional ultrasonic imaging systems nowadays use bulky and expensive piezoelectric Micromachined Ultrasonic Transducers (pMUTs). These utilize the piezoelectric effect of membrane material for sensing modalities, which can limit their applications and meet restrictions such as low image resolution, weak sensitivity and impedance mismatching.
Recently, capacitive Micromachined Ultrasonic Transducers (cMUTs) emerge as an alternative choice to traditional PZT ceramic based transducers in ultrasonic imaging. They not only have better bandwidth and sensitivity in producing better imaging resolution, low signal-to-noise ratio, but they allow the fabrication and integration with other electrical circuits much easier. Possessing many advantages over pMUTs, cMUTs attract interests from MEMS researchers and become more popular in medical imaging, non-destructive testing, ultrasound ranging, and flow metering applications.
This thesis presents the design characterization, modeling and microfabrication of cMUTs. Four cMUTs arrays are designed with different array geometries, membranes’ dimensions as well as number of cMUT cells in an array. cMUTs are first analytically modeled and then simulated using finite element method (FEM) analysis to compare the numerical analysis results. The analytical cMUTs modeling is characterized for spring constant, resonant frequency and pull-in voltage. COMSOL Multiphysics 4.3b is use for simulation the profound insight of cMUTs’ behavior such as: resonant frequencies, pull-in voltage, membrane’s deflection and implied spring constant.
This thesis also lays out the detail fabrication process flow of cMUTs. They are promised to be fabricated using mature of semiconductor processes which include photolithography, metal deposition and selective etching. In this thesis, in order to define the cMUT’s membrane, we will use direct wafer bonding technique with both chemical and O₂ plasma surface activation processes to decrease the process temperature to as low as 400⁰C. In this thesis, the fabrication of cMUTs is proposed at low process temperature, providing compatibility with CMOS integration.