CZT sensors for Computed Tomography: from crystal growth to image quality

Abstract

Recent advances in Traveling Heater Method (THM) growth and device fabrication that require additional processing steps have enabled to dramatically improve hole transport properties and reduce polarization effects in Cadmium Zinc Telluride (CZT) material. As a result high flux operation of CZT sensors at rates in excess of 200 Mcps/mm2 is now possible and has enabled multiple medical imaging companies to start building prototype Computed Tomography (CT) scanners. CZT sensors are also finding new commercial applications in non-destructive testing (NDT) and baggage scanning. In order to prepare for high volume commercial production we are moving from individual tile processing to whole wafer processing using silicon methodologies, such as waxless processing, cassette based/touchless wafer handling. We have been developing parametric level screening at the wafer stage to ensure high wafer quality before detector fabrication in order to maximize production yields. These process improvements enable us, and other CZT manufacturers who pursue similar developments, to provide high volume production for photon counting applications in an economically feasible manner. CZT sensors are capable of delivering both high count rates and high-resolution spectroscopic performance, although it is challenging to achieve both of these attributes simultaneously. The paper discusses material challenges, detector design trade-offs and ASIC architectures required to build cost-effective CZT based detection systems. Photon counting ASICs are essential part of the integrated module platforms as charge-sensitive electronics needs to deal with charge-sharing and pile-up effects.