Design and implementation of capacitive micromachined ultrasonic transducers for high intensity focused ultrasound

High intensity focused ultrasound (HIFU) is a medical procedure for noninvasive treatment of cancers. High intensity focused ultrasound is used to heat and destroy the diseased tissue. Piezoelectricity has been the core mechanism for generation of ultrasound waves in the treatment. Focusing can be done by using spherically curved transducers or using a lens or electronically steering sound waves by using phased arrays. Current research in HIFU technology targets the development of MR-guided miniaturized ultrasonic probes for treatment of cancerous tumors. Capacitive micromachined ultrasonic transducer (CMUT) is an alternative technology to generate and detect ultrasound. CMUT consists of a suspended membrane The advances in CMUT technology, enables fabricating tiny transducer arrays with wide bandwidth makes them a strong candidate for the application. In this thesis, a new methodology is proposed to design and operate CMUTs to generate high pressures under continuous wave excitation. An accurate nonlinear circuit model of CMUT is developed and the model is carried into a SPICE (Simulation Program with Integrated Circuit Emphasis) simulator for fast simulations. The model includes the radiation impedance of the array, thus the operation in a fluid environment can be simulated. The model is verified by doing FEM simulations. The circuit model provides a novel optimization tool for CMUT operating in non-collapse mode. The optimized CMUT parameters are presented and a sample fabrication is done using anodic bonding process. With the process, a 100 m thick silicon wafer is bonded to a glass substrate. A new driving scheme is proposed without a need of DC voltage. Thus, the charge trapping problem in CMUT operation is eliminated. The fabricated device provides 1.8 MPa surface pressure with -28dB second harmonic for a maximum 125V drive voltage at 1.44 MHz which is currently a state of art performance of a CMUT under continuous wave excitation