Patricia Costa

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.

Bi-color and dual-band HgCdTe infrared focal plane arrays at DEFIR

The purpose of this paper is to present the latest developments in Defir (LETI / Sofradir joint laboratory) in the field of bi-color and dual band infrared focal plane arrays (FPA) made with HgCdTe. The npn structure is achieved using the Molecular Beam Epitaxy (MBE) technique, planar ion implantation, and both dry and wet etching steps. This back to back diode architecture that allows a perfect spatial coherence with a high field factor and large quantum efficiencies needs only one indium bump connection per pixel. This makes it possible to achieve small pitches (below 25μm) and opens the way to the fabrication of large FPAs (TV/4 to TV) with reasonable wafer sizes. In this paper we present electro optical characterizations of 256x256 prototypes fabricated in Defir operating in two MWIR bands (3.1 and 5μm) with a pitch of 25μm that exhibit background limited performances together with a very high operability (above 99.9%) and NEDT below 22mK for integration time of only 0.5ms. In parallel an industrial product soon available from Sofradir has been developed with a 320x256 format and with a 30μm pitch operating in the same bands. This product exhibits the same operability and NETD as low as 15mK for an integration time as short as 1 ms. Finally, last results regarding 256x256 prototypes operating in MWIR/LWIR bands are presented, together with preliminary APD operating mode for the MWIR photodiodes of this last dual band detector.

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.

MCT technology challenges for mass production

Second generation infrared (IR) detectors are now mature at the production level. These detectors are mostly based on HgCdTe (MCT) materials technology. The main second generation detectors at the mass production level are the 288 4 long wave length for most of the European forward-looking infrared (FLIR) and the 480 6 long wave length for the U.S. Army SADA applications. As far as the 288 4 is concerned, SOFRADIR has delivered more than 3000 units already and the market is estimated to be 15,000 units at least! The market is also very large for SADA II units and SOFRADIR has produced them since the end of 1999. Thus, SOFRADIR produces large quantities of mercury cadmium telluride (MCT) detectors and has a unique experience for MCT detectors in mass production. MCT materials technology challenges for mass production concern the main following issues: Quality and reproducibility, MCT wafer size increase, array yield level increase, and the collective manufacturing approach. These issues are discussed in detail in this paper as well as future trends.

High-performance infrared detectors at Sofradir

The performance of an InfraRed (IR) system is based on a high spatial resolution and on a high thermal resolution. An increase in spatial resolution means an increase in number of pixels, a decrease in detector pitch and an increase in the detector pixel MTF. Regarding thermal resolution increase, it will be achieved mainly by an increase in the maximum quantity of charges which can be stored in the silicon read-out circuits for 2D staring arrays. At present, only cooled detectors answer this need of high performance detectors, such as 2D arrays with TV format resolution and high NETD. In this paper these trends regarding high performance are discussed and recent IRFPA results at Sofradir are presented. Finally, a comparison with uncooled detectors, also processed at Sofradir, is presented, to outline the remaining gap between both types of detectors.

Large staring arrays at SOFRADIR

Sofradir HgCdTe homojunction IR detector technology has already demonstrated its high maturity level by enabling the delivery of more than 7000 second and third generation IR detectors mainly adapted to LWIR and MWIR waveband applications. The candidates for these detectors are the high performance military applications as well as the space applications, and the commercial application. The new challenge for cooled detectors is the cost-effective production of very large staring arrays. The best approach is the pitch reduciotn keeping constant the performance, which allow to reduce the production cost and to offer larger formats using the same cryogenics. Sofradir MCT technology is now mature at production level for 20μm pitch and even less and has demonstrated its ability to operate a thigher temperatures than other competition materials. This reduces the cooling constraints by using smaller and less costly coolers. The new results of 640×512 MWIR arrays with 20μm pitch are presented as well as the tradeoffs for larger aray formats. QWIPs are also a solution for LWIR detectos offered by Sofradir and Thales Research Technology and this unique technology allows Sofradie to offer large staring ararys with reduced cooling constraints thanks to the high operating temperature achieved keeping constant the performance. Thus QWIP detector performances and production are discussed.

Cooled large IR staring arrays: toward third generation

SOFRADIR has moved to large quantity production for 2.5 generation IR detectors (320x256 format) since 2001. The move from 2nd generation to 2.5-generation IR detectors, mainly for MWIR applications, has been successfully achieved at SOFRADIR thanks to improvements of the technologies fully dedicated to performance improvements as well as production capacity increase for staring arrays. Then, in order to prepare future military and industrial needs, SOFRADIR has been working in close relationship with CEA-LETI/LIR on third generation development based on HgCdTe materials. This effective approach has been one of the keys to success in preparing third generation IR detectors. Three main areas are investigated for the third generation IR detectors: large IRFPA manufacturing following a cost effective approach, development of new IR detector structures and design of new silicon readout circuits. Developments in progress are presented regarding these main areas.

General analysis of infrared focal plane array performance versus focal plane array operating temperature, number of TDI elements, diode area and cut-off wavelength

IR systems require more and more performance (high sensitivity, resolution,...) to be adapted to specific system applications (such as surveillance and tracking systems...). To achieve such requirements, IRFPA manufacturers have to perform tradeoffs involving many parameters such as FPA operating temperature, number of TDI elements, cutoff wavelength, and diode area. IRFPA technologies and system limitations must be taken into account for these analyses. Thus, the authors present the general analysis of effects of these parameters on Sofradir IRFPA performance mainly utilizing 8 to 12 micron spectral band mercury cadmium telluride detector arrays. Impacts on electro-optical performance parameters and on thermal characteristics are presented. For example TDI linear scanning arrays are analyzed with emphasis on high IRFPA performance based on existing IRFPA technologies. Advantages of choices of different IRFPA configurations are presented.