From CdZnTe bulk growth to HgCdTe infra-red detectors: mastering the chain for high-performance and reliable imaging

High-performance infrared detectors based on HgCdTe technology require high quality epilayers, for which bulk CdZnTe is considered as the ideal substrate, thanks to its ability to perfectly match its lattice constant. Reaching very high crystal quality of the material in terms of subgrain boundary absence, low dislocation density, homogeneous zinc distribution, and low micro-defect density is paramount to obtaining excellent image quality. Sofradir takes advantage of growing its own CdZnTe crystals for producing substrates, and thus controlling the quality of HgCdTe epilayers, which allows reaching high-performance imaging. Indeed, mastering the whole manufacturing chain from raw material to Focal Plane Array and throughout all the front-end and back-end steps delivers a unique opportunity for process improvements. This paper shows how the latest process improvements do translate into detector image quality and reliability improvements, focusing on Front End process (substrates and epilayers), showing for the first time correlation between substrate microscopic defects and FPA image quality. This was achieved thanks to the research collaboration between Sofradir and CEA-LETI. This global process optimization is done thanks to a large set of characterizations performed at each process step, such as IR-microscopy for the substrate inspection, chemical revelation of dislocations and x-ray double-crystal rocking curve mappings for the epitaxial layer. Image quality is examined in terms of operability, and excess noise. Finally, in addition to process improvements, knowing how each critical process step impacts the following one and translates into the final image quality allows sorting units at the right process step, which serves yield and product quality. These benefits of the Sofradir’s vertical integration model are illustrated on MWIR and LWIR technologies.