Optimizing the directivity of piezoelectric matrix transducer elements mounted on an ASIC

Over the last decade, clinical studies show a strong interest in real-time 3D imaging. This calls for ultrasound probes with high-element-count 2D matrix transducer arrays. These may be interfaced to an imaging system using an in-probe Application Specific Integrated Circuit (ASIC) that takes care of signal amplification, element switching, sub-array beamforming, etc. Since the ASIC is made from silicon and is mounted directly behind the transducer elements, it can acoustically be regarded as a rigid plate that can sustain traveling lateral waves. These waves lead to acoustical cross-talk between the elements, and results in extra peaks in the directivity pattern. We propose two solutions to this problem, based on numerical simulations. One approach is to decrease the phase velocity in the silicon by reducing the silicon thickness and absorbing the energy using a proper backing material. Another solution is to disturb the waves inside the silicon plate by sub-dicing the back-side of the ASIC. We conclude that both solutions can be used to improve the directivity pattern.