Alain Manissadjian

Progress in HgCdTe homojunction infrared detectors

Abstract SOFRADIR/LIR HgCdTe homojunction infrared (IR) detector technology has already demonstrated its high maturity level by delivering more than 1000 s and third generation detection dewar assemblies. This specific HgCdTe photovoltaic technology proves high performance including low fixed pattern noise and high yield. This is due to the high impedance photodiode (including the longwave band) obtained thanks to the high-quality HgCdTe material used and the planar homojunction technology based on a very efficient passivation and ion implantation. Furthermore, much progress has been made in reducing IR detector cost and proposing new detector dewar assemblies. The efforts are mostly dedicated to the increase in HgCdTe wafer dimensions and yield for large staring arrays, and to the increase in IR detector operating temperature in order to reduce dewar and cooler assembly cost as well as cooler input power. Thus, Sofradir offers HgCdTe staring arrays operating at full performance at over 90 K for 8–10 μm wave band and at around 130 K for 3–5 μm wave band. Based on existing detector results, the main technological progress is presented. In particular, a 320 × 240 InfRared Focal Plane Array (IRFPA) is presented in 3–5 μm at 130 K and in 8–10 μm with the same performance at 80 and 90 K. Those new staring arrays provide very high thermal sensitivity (less than 10 mK) and low fixed pattern noise. A 5 cm long linear array using a special butting hybridization technique and allowing zero defect at the joints is introduced. It consists of a one 1500 × 1 linear array sensitive in the 3–5 μm wave band. The same format, sensitive in the 8–12 μm wave band, is also presented. Finally, future trends are discussed.

Quantum well infrared photodetectors: present and future

A review of the III-V Lab activities in the field of quantum well infrared photodetectors (QWIPs) is presented. We discuss the specific advantages of this type of detector and present the production facilities and status. A large section is dedicated to broadband QWIPs for space applications and to QWIPs on InP for mid-wavelength infrared detection. We review the progress of QWIP technology for the next generation (dual band, polarimetric, and multispectral) of thermal imagers. Finally, the state-of-the-art of very long wavelength QWIPs is discussed.

Single color and dual band QWIP production results

Since 1997, Sofradir has been working with Thales Research and Technologies (TRT) to develop and implement Quantum Well Infrared Photodetectors (QWIP) as an alternative and complementary offer with Mercury Cadmium Telluride (MCT) Long Wave (LW) detectors, to provide large LW staring arrays. Thanks to the low dark current technology developed by TRT, the QWIP detectors can be worked at FPA temperature above 73K, enabling the development of new compact IR cameras thanks to the use of compact microcoolers, and today, Sofradir is entering production with these highly compact QWIP components. For the Long Wave applications, SOFRADIR offers the European TV/4 format with the VEGA-LW detector (25μm pitch 384×288 IDDCA) and the full TV format with the SIRIUS-LW detector (20μm pitch 640×512 IDDCA). The first one is under production for several hundreds of units, to equip the Catherine-XP thermal imager from Thales. The second one has been initially developed for the Catherine-MP high resolution (SXGA) thermal imager and is ready for production. Both detectors present highly uniform performances and sharp images with NETD in the 50mK range when working around 75K at video frame rate. The TV/4 VEGA detector is also offered as a demonstrator for the Mid Wave applications, with a QWIP array adapted to this waveband. In the same time, a dual band MW-LW similar array is developed with spatial coherence, and is currently under demonstration. The performances of these four QWIP detectors are reviewed in this paper.

Improved IR detectors to swap heavy systems for SWaP

Cooled IR technologies are challenged for answering new system needs like the compactness and the reduction of cryopower which is a key feature for the SWaP (Size, Weight and Power) requirements. Over the last years, SOFRADIR has improved its HgCdTe technology, with effect on dark current reduction, opening the way for High Operating Temperature (HOT) systems that can get rid of the 80K temperature constraint, and therefore releases the Stirling cooler engine power consumption. Performances of the 640×512 15μm pitch LW detector working above 100K will be presented. A compact 640×512 15μm pitch MW detector presenting high EO performance above 130K with cut-off wavelength above 5.0μm has been developed. Its different performances with respect to the market requirements for SWaP will be discussed. High performance compact systems will make no compromise on detector resolution. The pixel pitch reduction is the answer for resolution enhancement with size reduction. We will therefore also discuss the ongoing developments and market needs for SWaP systems.

SOFRADIR infrared detector products: the past and the future

Sofradir has developed second and third generation InfraRed (IR) detectors sensitive in different wavebands covering the 1 to 16 micrometers spectral range. The main material used for cooled IR detector is HgCdTe and Sofradir extends its product range using QWIP for 8-9 micrometers large staring arrays and microbolometers based on amorphous silicon (Si:(alpha) ) thermometer material for uncooled technology. Array characteristics and performances are presented (including new results) and the maturity of technologies and products are discussed for present as well as for short term production activities.

HgCdTe performance for high operating temperatures

Sofradir/Lir HgCdTe homojunction IR detector technology has already demonstrated its high maturity level by delivering more than 1000 second and third generation detector dewar assemblies adapted to LWIR and MWIR waveband applications. More recently, Sofradir and Lir started to work on HgCdTe detectors for SWIR applications. One of the main advantage of HgCdTe material is its ability to operate at high temperatures with high performance, and therefore to reduce the cooling constraints (size, cost...) by using small cryocoolers or by using thermoelectric coolers. As a matter of fact, high performance HgCdTe IRFPAs operate at temperatures up to 100 Kelvin for LWIR, up to 130 Kelvin for MWIR and up to more than 200 Kelvin for SWIR. However tradeoffs between performance and operating temperature are possible for many applications and therefore MWIR IRFPA can be proposed at 150 Kelvin or 200 Kelvin for example. This paper presents the advantages of the use of the Sofradir/Lir HgCdTe technology for high operating temperatures, based on the high performance demonstrated, and the several tradeoffs which are possible for various applications. Performance measured on HgCdTe photodiodes are presented, for several combinations of cut-off wavelengths and operating temperatures. The results are compared to potential applications and examples of IRFPA results are given.

Status of MCT focal plane arrays in France

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 epitaxy layer 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-on-p MCT detectors was increase of several tens of Kelvin. In addition the development of the p-on-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 epitaxy layer. 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 Photodiode (APD) are described.

Two color QWIP and extended wavebands

Since 2002, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on GaAs and related III-V compounds, at THALES Research and Technology Laboratory. The QWIP technology allows the realization of large staring arrays for Thermal Imagers (TI) working in the long-wave infrared (LWIR) band (8-12 μm). In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and has been the key parameter for the production to start. The 640x512 LWIR focal plane arrays (FPAs) with 20μm pitch was the demonstration that state of the art performances can be achieved even with small pixels. This opened the field for the realization of usable and affordable megapixel FPAs. Thales Research & Technology (TRT) has been developing third generation GaAs LWIR QWIP arrays for volume manufacture of high performance low cost thermal imaging cameras. In the past, another widely claimed advantage for QWIPs was the so-called band-gap engineering and versatility of the III-V processing allowing the custom design of quantum structures to fulfil the requirements of specific applications such as very long wavelength (VLWIR) or multispectral detection. In this presentation, we present the performances of both our first 384x288, 25 μm pitch, MWIR (3-5μm) / LWIR (8-9 μm) dual-band FPAs, and the current status of QWIPs for MWIR (< 5μm) and VLWIR (>15μm) arrays.

Reliability optimization for IR detectors with compact cryo-coolers

IR applications are more and more demanding regarding reliability. It is the case of handheld and lightweight UAV applications. To answer these needs, Sofradir and Thales Cryogenics developed a new product family in order to minimize system weight, cost and to increase detector and cooler reliability. Thales Cryogenics has been working on RM integral Stirling cryocoolers since 1995. Then, as a result of several design improvements, it has been possible to increase significantly the efficiency and the reliability of the RM2-Xi cooler over the last 2 years. The RM2-Xi reliability is measured through life time tests which are run continuously on samples taken out from the mass production. Several new tests profiles have been implemented with different climatic and cooler operation conditions. The results gathered enable an accurate evaluation of the cryocooler reliability in the various mission profiles of the customers applications. Another important performance of an integral cryocooler is efficiency. New mechanical and thermal designs have been implemented. The resulting improvements will be presented and compared with the characteristics of the cryocooler previous version. Based on this new design, Sofradir offers new IR detector products well adapted to handheld and high reliability systems. These new product designs are discussed as well as reliability analysis results.

Infrared SWAP detectors: pushing the limits

The growing demand for compact and low consumption infrared cooled detectors is driven by different products segments. Hand Held Thermal Imagers, UAV, small gimbals are some of them. End users are requiring devices easy to use with fast cool down time, excellent portability, low acoustic noise with no trade-offs in reliability and performance. These requirements are pushing the technology developments toward constant innovations on detectors, coolers, read out circuits and proximity electronic boards. In this paper we are discussing the different figures of merit and highlighting the challenges for the different components. An update on the developments of HOT technology for most advanced pixel pitch will be presented. Very compact products are driving the developments for innovative coolers and cryogenic solutions. A low power compact architecture is a must for electronic boards to optimize the overall system power consumption. Finally a look to the future requirements for further shrink will be addressed.