The development of a new thermometer material at Lynred has opened new opportunities towards a smaller pixel pitch. This pixel pitch reduction leads to several challenges and technological breakthroughs that are necessary both for the bolometer pixel design and the readout circuit architecture. The symposium presentation and the associated article will present a recent development at CEA-leti and Lynred of a 8.5μm bolometer pixel pitch Focal Plane Array (FPA). The drastic reduction of the pixel area leads to important changes of the micro bolometer features, especially a strong decrease of thermal capacitance Cth and increase of the thermal resistance Rth in order to maintain a high detector responsivity. The rise of the noise due to temperature fluctuation or 1/f contributions have also to be considered. The consequences of those modifications and resulting trade-offs, such as pixel fill factor and optical absorption yields in the 8-14 μm band ; pixel self-heating and Read-Out IC (ROIC) dynamic range, will be discussed. Finally, the electro-optical performances of our 640x480 FPAs are presented, and perspectives will be given.
Historically LYNRED (created from the merger of SOFRADIR and ULIS in 2019) has used amorphous silicon materials (“a-Si”) as thermistor materials for its uncooled microbolometer products. If a-Si materials present several advantages that made the success of LYNRED’s products (easy to use and integrate in thermal camera), their intrinsic bolometric performances (i.e. TCR and 1/f noise) are still lower than the commonly used oxides thermistors[1] (i.e. VOx[2] and TiOx[3]). In order to stay in a leading position regarding sensor performances without any trade-off, LYNRED, with the support of its historical R&D partner the CEA-LETI, developed new materials. This strategy has led to new cutting edge products. At the end of 2020 a new 17 µm pixel pitch product (Pico640s[4]), with one of the highest sensor performance reported on the market (typical thermal sensitivity of 25 mK (f/1, 300K, 30Hz)), has been introduced in our product portfolio. We also launched our state of the art 12 µm product range with performances equivalent to our current 17µm product range. More generally, these developments open up new opportunities toward smaller pixel pitch. The symposium presentation and the associated article will present how we have increased the "Signal to Noise Ratio" (SNR) of our products while keeping all the elements which have been our hallmark. Special attention will be paid to NETD, stability of product characteristics during operation and manufacturing excellence. All these features were obtained only by hardware (at the pixel level) improvements without the need to use sophisticated algorithms or specific ROIC functions, in the spirit of LYNRED’s FPA products.
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.
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.
Since 2005, in the scope of “DEFIR”, the joint laboratory between CEA-LETI and SOFRADIR, p-on-n photodiodes and FPAs (Focal Plane Arrays) have been developed and optimised. This p-on-n architecture, obtained by As implantation into an In doped base layer, offered a significant decrease of the dark current compared to our n-on-p standard architecture. Following these developments, this p-on-n technology has been successfully transferred to SOFRADIR for industrial production [1]. Results obtained on TV format, 15μm pitch, showed that this first architecture has reached its maturity with excellent results in LWIR and MWIR. In parallel, further developments and studies are still in progress at CEA-LETI in order to improve the photodiode performance and understanding of the physical mechanisms. In this way, new p-on-n architectures have been studied on LPE (Liquid Phase Epitaxy) in the VLWIR spectral band. Using this new architecture, the transition temperature, where the dark current shifts from diffusion limited regime to another one, has been lowered by more than 10K. Extremely low dark current has been obtained, down to 50 e-/s/pixel. The p-on-n technology also been studied at DEFIR in SWIR range specifically for space applications were 2Kx2K MCT arrays are required with dark current below 0.01e-/s at 18μm pitch in the 80-140 K. Finally in the MWIR and LWIR spectral bands, the reduction of production cost and the increase of resolution call for smaller pixel pitches with larger format. In this way, first results have been obtained on test diodes with pixel pitch as low as 5 μm. The I(V) and R(V) plots illustrate the very good characteristic of our p-on-n diodes. These photodiodes present large reverse breakdown voltage, witnessing the quality of our device fabrication procedure.
This paper describes the recent developments of Mercury Cadmium Telluride (MCT) infrared technologies in France at Sofradir and CEA-LETI made in the frame of the common laboratory named DEFIR. Among these developments, one can find the crystal growth of high quality and large Cadmium Zinc Telluride (CZT) substrates which is one of the fundamental keys for high quality and affordable detectors. These last years, a great effort was done on this topic and also on MCT epilayer process from Short Waves (SW) to Very Long Waves (VLW). These developments about the quality of the material are needed for the challenge of the High Operating Temperature (HOT). Over these lasts years, the operating temperature of n/p MCT detectors was increase of several tens of Kelvin. In addition the development of the p/n MCT technology that reduces dark current by a factor ~100 saves about twenty Kelvin more. The next step for the increase in operating temperature will be the complex photodiodes architectures using molecular beam epilayer. The reduction of the pixel pitches is another challenge for infrared technologies for Small Weight and Power (SWAP) detectors. Moreover, this reduction allows the increase in the resolution and consequently in the detection range of the systems. In addition, last results on 3rd generation detectors such as multicolor focal plan arrays, 2D, 3D, low noise and high images rate focal plane array using Avalanche Photodiose (APD) are described.
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