Modeling and characterization of thin film coatings for high frequency CMUT annular arrays

Sub-mm CMUT annular arrays are very attractive for generating high quality imaging beams with a small channel count for intravascular imaging. In this work, we analyzed the performance of 840 μm diameter, 40 MHz CMUT annular arrays using analytical and FEA methods. We experimentally characterized several types of RTV films at high frequencies as protective coatings and evaluated their effect on the annular array performance through FEA. The FEA of the CMUT array was conducted using ANSYS and the analytical calculations were based on the Rayleigh integral. To accurately model the effects of RTV layers in ANSYS with the FLUID79 element, the frequency dependent attenuation of ~1 mm thick RTV-60 and RTV-615 samples were measured using two high frequency CMUT arrays in water. The attenuation of the RTV samples was fit to the relation α~Bfⁿ (Np/m) over the frequency range of 5 MHz to 30 MHz with excellent agreement (B= 2.46E-9, n=1.67 for RTV-60, B=2.13E-4, n= 0.956 for RTV-615). With these measurements, attenuation was included in the FEA for 5-15 μm thick RTV layers and the resulting fields were subsequently propagated into the fluid via post processing in MATLAB. The frequency domain and transient pressure fields for both focused and unfocused operation were obtained using axisymmetric models. An ideal equal area annular piston array, CMUT piston array, CMUT array, and an RTV coated CMUT array were considered. Initial hydrophone results with annular arrays are presented showing a loss in bandwidth when coated with RTV-615, in agreement with simulations.