Capacitive Micromachined Ultrasonic Transducers for Underwater Applications

Abstract: Capacitive micromachined ultrasonic transducers (CMUT:s) are often used in medical imaging and they show some promise as underwater transducers. This thesis collates the available information about how CMUT:s operate, their strengths and weaknesses and investigates their efficiency as an underwater transducer. The accumulated knowledge was channelled into a simulation of a CMUT as a dampened spring system done in MATLAB and Simulink. The simulation investigated the resonance frequency and bandwidth through simulation and compared the results to experimental results from literature.  CMUT:s have good acoustic matching with water making them sensitive, broadband transducers when used under water. Special care must be taken when choosing the CMUT so that materials and designs can fulfil the task for which it is intended, such as the radius of the membrane, the material of the membrane, the insulating layers in or around the CMUT and the height of the air gap inside. CMUT:s are, for the transmission of sound, less capable than existing lead zirconate-titanate-transducers (PZT-transducers). This problem can be somewhat alleviated through operating the CMUT in collapse-mode but care must be taken so that the CMUT is not damaged during this operation. Simulation results and results from literature show that it is possible to simulate CMUT:s with accuracy. By simulating 10 different CMUT:s, using the geometries and material properties of experimentally tested devices and testing for resonance frequency and bandwidth the results were as follows:The average relative error of resonance frequency was found to be -14 %, if outlier results are excluded and the average relative error of bandwidth proved inaccurate at -54 %