Yann Reibel

Infrared dual-band detectors for next generation

The development of DB (Dual-Band) infrared detectors has been the core of research and technological improvements for the last ten years at CEA-LETI and Sofradir: the semi planar structure uses a proven standard process with robust reproducibility, leading to low-risk and a facilitated ramp-up to production. This makes it the natural choice for the third generation detectors proposed by Sofradir. The fabrication of DB MCT detectors is reaching maturity: ALTAIR with 24μm-pixel pitch arrays in TV format are available, showing median NETD around 18mK with operability over 99.5%. A second structure, based on two back to back diodes, with a single contact per pixel translates the DB pixel into smaller cell therefore being more efficient in terms of pitch reduction. These new technologies widen perspectives and open new horizons of applications such as large DB FPA, dual mode capability providing both SAL (Semi Active Laser) and IR operations for more robust target engagement or compact dual color detection with wide-angle integrated optics for missile warning system.

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.

Towards infrared DDCA with an imaging function

Today, both military and civilian applications require miniaturized optical systems in order to give an imagery function to vehicles with small payload capacity. After the development of megapixel focal plane array (FPA) with micro-sized pixels, this miniaturisation will become feasible with the integration of optical functions in the detector area. In the field of cooled infrared imaging system, the detector area is the Detector-Dewar-Cooler Assembly (DDCA). A dewar is a sealed environment where the detector is cooled on a cold plate. We show in this paper that an imagery function can be added to the dewar by simply integrating a single meniscus inside the cold shield. An infrared system with a wide field of view and high throughput is thus obtained without adding optics outside the dewar. The additional mass of the optic is sufficiently small to be compatible with the cryogenic environment of the DDCA. The temperature stabilization of the optic and the reduction of the background radiation are the main advantages of this system. The performance of this camera will be discussed and several evolutions of this camera will be introduced too.

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.

Update on 10μm pixel pitch MCT-based focal plane array with enhanced functionalities

Sofradir was first to show a 10μm focal plane array (FPA) in DSS 2012, and announced the DAPHNIS 10μm product family back in 2014. This pixel pitch is key for enabling more compact sensors and increased resolution. SOFRADIR recently achieved outstanding MTF demonstration at this pixel pitch, which clearly demonstrate the benefit to users of adopting 10μm pixel pitch focal plane array based detectors. The last results, and associated gain in detection performance, are discussed in this paper. Concurrently to pitch downsizing, SOFRADIR also works on a global offer using digital interfaces and smart pixel functionalities. This opens the road to enhanced functionalities such as improved image quality, higher frame rate, lower power consumption and optimum operation for wide thermal conditions scenes. This paper also discusses these enhanced features and strategies allowing easier integration of the detector in the system.

SWIR InGaAs focal plane arrays in France

SWIR detection band benefits from natural (sun, night glow, thermal radiation) or artificial (eye safe lasers) photons sources combined to low atmospheric absorption and specific contrast compared to visible wavelengths. It gives the opportunity to address a large spectrum of applications such as defense and security (night vision, active imaging), space (earth observation), transport (automotive safety) or industry (non destructive process control). InGaAs material appears as a good candidate to satisfy SWIR detection needs. The lattice matching with InP constitutes a double advantage to this material: attractive production capacity and uncooled operation thanks to low dark current level induced by high quality material. The study of InGaAs FPA has begun few years ago with III-VLab, gathering expertise in InGaAs material growth and imaging technology respectively from Alcatel-Lucent and Thales, its two mother companies. This work has led to put quickly on the market a 320x256 InGaAs module. The recent transfer of imagery activities from III-VLab to Sofradir allows developing new high performances products, satisfying customers’ new requirements. Especially, a 640x512 InGaAs module with a pitch of 15µm is actually under development to fill the needs of low light level imaging.

Latest developments in the p-on-n HgCdTe architecture at DEFIR

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.

New developments on InGaAs focal plane array

SWIR detection band benefits from natural (sun, night glow, thermal radiation) or artificial (eye safe lasers) photons sources combined to low atmospheric absorption and specific contrast compared to visible wavelengths. It gives the opportunity to address a large spectrum of applications such as defense and security (night vision, active imaging), space (earth observation), transport (automotive safety) or industry (non destructive process control). InGaAs material appears as a good candidate to satisfy SWIR detection needs. The lattice matching with InP constitutes a double advantage to this material: attractive production capacity and uncooled operation thanks to low dark current level induced by high quality material. The recent transfer of imagery activities from III-VLab to Sofradir provides a framework for the production activity with the manufacturing of high performances products: CACTUS320 SW and CACTUS640 SW. The developments, begun at III-Vlab towards VGA format with 15μm pixel pitch, lead today to the industrialization of a new product: SNAKE SW. On one side, the InGaAs detection array presents high performances in terms of dark current and quantum efficiency. On the other side, the low noise ROIC has different additional functionalities. Then this 640×512 @ 15μm module appears as well suited to answer the needs of a wide range of applications. In this paper, we will present the Sofradir InGaAs technology, some performances optimization and the last developments leading to SNAKE SW.

Latest developments in advanced MCT infrared cooled detectors

MCT technologies under development in France address strategic operational needs. This includes better identification range as well as lighter weight requirement, operation at higher detector temperature and cost reduction issues. This paper describes the status of MCT IR technology in France at Leti and Sofradir. A focus will be made on hot detector technology for SWAP applications. Solutions for high performance detectors such as dual bands or megapixels will be discussed. In the meantime, the development of avalanche photodiodes, integrated optics, or TV format with digital interface is key to bring customers cutting-edge functionalities.

HgCdTe APDS for space applications

HgCdTe avalanche photodiode focal plane arrays (FPAs) and single element detectors have been developed for a large scope of photon starved applications. The present communication present the characteristics of our most recent detector developments that opens the horizon for low infrared (IR) photon number detection with high information conservation for imaging, atmospheric lidar and free space telecommunications. In particular, we report on the performance of TEC cooled large area detectors with sensitive diameters ranging from 30- 200 μm, characterised by detector gains of 2- 20 V/μW and noise equivalent input power of 0.1-1 nW for bandwidths ranging from 20 to 400 MHz.