Fabrication technology and design for CMUTS on CMOS for IVUS catheters
Zahorian, Jaime S.
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The objective of this research is to develop novel capacitive micromachined ultrasonic transducer (CMUT) arrays for intravascular ultrasonic (IVUS) imaging along with the fabrication processes to allow for monolithic integration of CMUTs with custom CMOS electronics for improved performance. The IVUS imaging arrays include dual-ring arrays for forward-looking volumetric imaging in coronary arteries and annular-ring arrays with dynamic focusing capabilities for side-looking cross sectional imaging applications. Both are capable of integration into an IVUS catheter 1-2 mm in diameter. The research aim of monolithic integration of CMUTs with custom CMOS electronics has been realized mainly through the use of sloped sidewall vias less than 5 µm in diameter, with only one additional masking layer as compared to regular CMUT fabrication. Fabrication of CMUTs has been accomplished with a copper sacrificial layer reducing isolation layers by 50%. Modeling techniques for computational efficient analysis of CMUT arrays were developed for arbitrary geometries and further expanded for use with larger signal analysis. Dual-ring CMUT arrays for forward-looking volumetric imaging have been fabricated with diameters of less than 2 mm with center frequencies at 10 MHz and 20 MHz, respectively, for an imaging range from 1 mm to 1 cm. These arrays, successfully integrated with custom CMOS electronics, have generated 3D volumetric images with only 13 cables necessary. Performance from optimized fabrication has reduced the bias required for a dual-ring array element from 80 V to 42 V and in conjunction with a full electrode transmit array, it was shown that the SNR can be improved by 14 dB. Simulations were shown to be in agreement with experimental characterization indicated transmit surface pressure in excess of 8 MPa. For side-looking IVUS, three versions of annular CMUT arrays with dynamic focusing capabilities have been fabricated for imaging 1 mm to 6 mm in tissue. These arrays are 840 µm in diameter membranes linked to form 8 ring elements with areas that deviate by less than 25 %. Through modeling and simulation undesirable acoustic cross between ring elements was reduced from -13 dB to -22 dB.