A 3D packaging technology for acoustically optimized integration of 2D CMUT arrays and front end circuits

As compared to piezoelectric technology, MEMS technology employed for Capacitive Micromachined Ultrasonic Transducer (CMUT) fabrication provides increased compatibility with 3D packaging methods, enabling the possible development of advanced transducer-electronics multi-chip modules (MCM) for medical imaging applications. In this paper, an acoustically optimized 3D packaging method for the interconnection of Reverse-Fabricated 2D CMUT arrays and front end ICs using a wafer-level compatible process is presented. The developed packaging method uses Cu pillars and Sn-Ag solder reflow for electrical interconnection, and patterned Benzocy-clobutene (BCB) for mechanical bonding. Process parameters were optimized by analyzing the acoustic behavior of a CMUT supported by a BCB film laying on a silicon substrate using Finite Element Modeling (FEM). Dummy CMUT and ASIC wafers were processed and MCMs were assembled following a chip-to-chip bonding approach using the optimized process parameters. Electrical characterization of the MCMs demonstrated successful contact across the entire fabricated devices. Probe head prototypes were assembled and pulse-echo experiments were carried out using the MCM surface as a reflector to verify the effectiveness of the optimization on the acoustic behavior of the device.