Cd1-yZnyTe single crystal is the current material of choice to be used as substrate for the growth of lattice-matched Hg1-x CdxTe epilayers with cutoffs wavelengths in the SWIR range (~2,9 μm (x=0,4) to 2μm (x=0,54)). For the manufacturing of large 2k² IR focal plane arrays with a 15 μm pitch, large diameter Cd1-yZnyTe ingots with a state-of-the-art material quality are required. Crystal growth method from the melt; like Vertical Gradient Freeze technique; enables us to get close to 5 inches in diameter, high quality single crystals, after decades of developments. As the growth of high-quality Cd1-yZnyTe single crystal ingots remains a big technological challenge, we present some recent technical achievements in this field, got within the frame of the H2020 ASTEROID project. Some requirements regarding material specification, like Cd1-yZnyTe substrate size, geometrical perfection (TTV, faces parallelism), material quality (crystallinity, dislocations) have imposed many new process updates and developments in our elaboration scheme. State-Of-the-Art 72x73 mm² Cd1-yZnyTe substrates with epiready surface preparations are now available for the Front-End-Of-Line of 2k² IR Focal Plane Array Processing at CEA-LETI / LYNRED (previously named SOFRADIR).
SOFRADIR is the worldwide leader on the cooled IR detector market for high-performance space, military and security applications thanks to a well mastered Mercury Cadmium Telluride (MCT) technology, and recently thanks to the acquisition of III-V technology: InSb, InGaAs, and QWIP quantum detectors. This is the result of strong and continuous development efforts to deliver cutting edge products with improved performances in terms of spatial and thermal resolution, dark current, quantum efficiency, low excess noise and high operability. On one hand the advanced performances of Sofradir product rely on a strong partnership with CEA-LETI materialized in a common laboratory named DEFIR.
On the other hand, these cutting edge performances are made possible thanks to Sofradir vertical industrial model. From the CdZnTe (CZT) and HgCdTe (MCT) crystal growth to the last electro-optical characterization recipe before shipping, and all the intermediate steps in between like IDDCA (Integrated Detector Dewar Cooler Assembly) final pumping cycle, all the manufacturing steps are developed, performed and controlled inhouse. This allows direct feedback between IDDCA, system performances and process or material. State of the art relevant performances for IR detection and imaging will be presented, that is to say low excess noise defects, RFPN (Residual Fixed Pattern Noise), NUC (Non Uniformity Correction) table stability for Daphnis product, 10μm pitch XGA extended MW matrix at 110K and HOT (High Operating Temperature) p-on-n technology, VGA format with 15μm pitch MW at 160K.
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.
SOFRADIR is the worldwide leader on the cooled IR detector market for high-performance space, military and security applications thanks to a well mastered Mercury Cadmium Telluride (MCT) technology, and recently thanks to the acquisition of III-V technology: InSb, InGaAs, and QWIP quantum detectors. This is the result of strong and continuous development efforts to deliver cutting edge products with improved performances in terms of spatial and thermal resolution, dark current, quantum efficiency, low excess noise and high operability. On one hand the advanced performances of Sofradir product rely on a strong partnership with CEA-LETI materialized in a common laboratory named DEFIR. On the other hand, these cutting edge performances are made possible thanks to Sofradir vertical industrial model. From the CdZnTe (CZT) and HgCdTe (MCT) crystal growth to the last electro-optical characterization recipe before shipping, and all the intermediate steps in between like IDDCA (Integrated Detector Dewar Cooler Assembly) final pumping cycle, all the manufacturing steps are developed, performed and controlled inhouse. This allows direct feedback between IDDCA, system performances and process or material. State of the art relevant performances for IR detection and imaging will be presented, that is to say low excess noise defects, RFPN (Residual Fixed Pattern Noise), NUC (Non Uniformity Correction) table stability for Daphnis product, 10μm pitch XGA extended MW matrix at 110K and HOT (High Operating Temperature) p-on-n technology, VGA format with 15μm pitch MW at 160K.
Space applications are requiring low dark current in the long wave infrared at low operating temperature for low flux observation. The applications envisioned with this type of specification are namely scientific and planetary missions. Within the framework of the joint laboratory between Sofradir and the CEA-LETI, a specific development of a TV format focal plane array with a cut-off wavelength of 12.5μm at 40K has been carried out. For this application, the p on n technology has been used. It is based on an In doped HgCdTe absorbing material grown by Liquid Phase Epitaxy (LPE) and an As implanted junction area. This architecture allows decreasing both dark current and series resistance compared to the legacy n on p technology based on Hg vacancies. In this paper, the technological improvements are briefly described. These technological tunings led to a 35% decrease of dark current in the diffusion regime. CEA-LETI and Sofradir demonstrated the ability to use the p on n technology with a long cutoff wavelength in the infrared range.
This paper presents recent improvements introduced in production lines of Mid-Wavelength Infra-Red (MWIR) and Long-Wavelength Infra-Red (LWIR) HgCdTe detectors that increase performances, image quality, and reliability. This was achieved thanks to accurate characterization of RMS noise distributions. Based on many MWIR and LWIR devices RMS distributions, a RMS noise distribution model that accounts for both Background Limited diodes and 1/f noise affected isolated diodes is first proposed. Then, a figure of merit for quantifying the defective pixels is introduced. This figure of merit is shown to be easy to use and robust to statistical variability. Moreover, it does also very well correlate with physics : there is high correlation between the total number of calculated defects and other figures of merit that gauge the material quality or the low frequency noise. The ability to accurately and efficiently quantify RMS noise benefits to Sofradir in its development of highly reliable and performant technologies. Such benefits are illustrated on the latest Sofradir MWIR and LWIR technologies that are demonstrated to be very robust regarding thermal stress and thermal cycling. Finally those technologies are shown to reach high image quality and stability.
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