G. Destefanis

Infrared focal plane array with a built-in stationary Fourier-transform spectrometer: basic concepts.

A novel configuration of stationary Fourier transform infrared (FTIR) spectrometer is presented. Contrary to classic configurations, the interferometer is directly integrated in the focal plane array (FPA) during its process of fabrication. A first, to the best of our knowledge, demonstration of the spectrometric function has been achieved departing from a well-known structure of an HgCdTe photodetector. We show that the obtained FTIR-FPA can be described by intrinsic parameters such as an optical path difference and a so-called spectrometric efficiency. First experimental results are presented.

Single- and two-color infrared focal plane arrays made by MBE in HgCdTe

We present here recent developments obtained at LETI infrared laboratory in the field of infrared detectors made in HgCdTe material and using the molecular beam epitaxial growth technique (MBE). We discuss the metallurgical points (growth temperature and flux control) that lead to achieve excellent quality epitaxial layers grown by MBE. We show a run-to-run reproducibility measured on growth run of more than 15 layers. The crystalline quality, surface morphology, and composition uniformity are excellent. The etch pits density (EPD) are in the low 105.cm-2 when HgCdTe grows on a CdZnTe substrate. Transport properties reveal a low n-type carrier concentration in the 1014 to 1015.cm-3 range with a carrier mobility in excess of 105 cm2/V/sec at 77K for epilayers grown with 10 micrometers cutoff wavelength. We describe the performances of several kinds of our HgCdTe- MBE devices: single color MWIR and LWIR detectors on HgCdTe/CdZnTe operating at 77K in respectively (3-5 micrometers ) and (8-12 micrometers ) wavelength range; single color MWIR detectors on HgCdTe grown on germanium heterosubstrate operating at 77K in the (3-5 micrometers ) wavelength range; two color HgCdTe detectors operating within the MWIR (3-5 micrometers ) band.

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.

Single-scan extraction of two-dimensional parameters of infrared focal plane arrays utilizing a Fourier-transform spectrometer

We present what is believed to be a novel experimental method to measure the technological parameters (spectral response and quantum yield) of an infrared focal plane array. This method makes original use of a Fourier transform spectrometer, which allows us to simultaneously extract the spectral performances of all pixels from one single set of measurements. The methodology used and the principle of the experimental setup are detailed. A Fourier analysis is shown to provide various optogeometrical information on the detector microstructure. A demonstrator based on the HgCdTe technology was designed, and satisfactory experimental results were obtained.

High-performance LWIR 256 x 256 HgCdTe focal plane array operating at 88 K

A 256 by 256 IRCMOS array with a 35 micron pitch operating at 88 K and above 10 microns has been developed at LETI/LIR. High performances have been obtained owing on one hand to a reduced dark current detector technology and on the other hand to a new readout circuit architecture which maximizes both charge handling capacity and responsivity. We have measured a NEDT of 13 mK at 88 K for a cutoff wavelength of 10.1 micrometer. A description of the array is given and the main electro-optical characteristics of the component are presented.

From visible to infrared: a new detector approach

Sofradir infrared detectors manufacturing is based on the use of a Mercury Cadmium Telluride (MCT) technology hybridized with silicon readout circuit covering a bandwidth from 0.8 to 14 μm thanks to the ability of MCT material to be tuned in terms of cut-off wavelength. Most of the time, infrared detectors are used to answer applications operating between 0.8 μm and 15 μm. New emerging applications express a need for detectors covering a larger waveband and in particular detectors with waveband sensitivity from the visible spectrum up to infrared spectrum. Some of these applications are for example hyperspectral applications where a panchromatic channel is generally associated to an infrared channel. For these applications, the availability of a detector covering these two channels can greatly simplify the instrument architecture. Other potential applications can be spectroscopic applications in visible range needing an extension of the sensitivity of the sensor in near infrared spectrum which cannot be answered with high performances by classical silicon sensors because of the loss of sensitivity between 0.8 μm and 1 μm. Physically, MCT material is able to operate in the visible range and has a potential to offer a high quantum efficiency and large field factor thanks to the hybrid structure. In addition, Sofradir N on P ion implantation process as well as Sofradir hybridization process offer specific advantages to develop high performances detectors sensitive both in visible and infrared spectra. This kind of detector can be an interesting alternative to answer applications needing a large waveband detector. In this paper, Sofradir approach to develop a new kind of detectors sensitive from visible to infrared spectra is presented. Potential applications and the interest of these new Sofradir detectors are discussed versus these needs. Finally, the last results and performances of these detectors are presented.

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.

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.

Advances in detector technologies for visible and infrared wavefront sensing

The purpose of this paper is to give an overview of the state of the art wavefront sensor detectors developments held in Europe for the last decade. The success of the next generation of instruments for 8 to 40-m class telescopes will depend on the ability of Adaptive Optics (AO) systems to provide excellent image quality and stability. This will be achieved by increasing the sampling, wavelength range and correction quality of the wave front error in both spatial and time domains. The modern generation of AO wavefront sensor detectors development started in the late nineties with the CCD50 detector fabricated by e2v technologies under ESO contract for the ESO NACO AO system. With a 128x128 pixels format, this 8 outputs CCD offered a 500 Hz frame rate with a readout noise of 7e-. A major breakthrough has been achieved with the recent development by e2v technologies of the CCD220. This 240x240 pixels 8 outputs EMCCD (CCD with internal multiplication) has been jointly funded by ESO and Europe under the FP6 programme. The CCD220 and the OCAM2 camera that operates the detector are now the most sensitive system in the world for advanced adaptive optics systems, offering less than 0.2 e readout noise at a frame rate of 1500 Hz with negligible dark current. Extremely easy to operate, OCAM2 only needs a 24 V power supply and a modest water cooling circuit. This system, commercialized by First Light Imaging, is extensively described in this paper. An upgrade of OCAM2 is foreseen to boost its frame rate to 2 kHz, opening the window of XAO wavefront sensing for the ELT using 4 synchronized cameras and pyramid wavefront sensing. Since this major success, new developments started in Europe. One is fully dedicated to Natural and Laser Guide Star AO for the E-ELT with ESO involvement. The spot elongation from a LGS Shack Hartman wavefront sensor necessitates an increase of the pixel format. Two detectors are currently developed by e2v. The NGSD will be a 880x840 pixels CMOS detector with a readout noise of 3 e (goal 1e) at 700 Hz frame rate. The LGSD is a scaling of the NGSD with 1760x1680 pixels and 3 e readout noise (goal 1e) at 700 Hz (goal 1000 Hz) frame rate. New technologies will be developed for that purpose: advanced CMOS pixel architecture, CMOS back thinned and back illuminated device for very high QE, full digital outputs with signal digital conversion on chip. In addition, the CMOS technology is extremely robust in a telescope environment. Both detectors will be used on the European ELT but also interest potentially all giant telescopes under development. Additional developments also started for wavefront sensing in the infrared based on a new technological breakthrough using ultra low noise Avalanche Photodiode (APD) arrays within the RAPID project. Developed by the SOFRADIR and CEA/LETI manufacturers, the latter will offer a 320x240 8 outputs 30 microns IR array, sensitive from 0.4 to 3.2 microns, with 2 e readout noise at 1500 Hz frame rate. The high QE response is almost flat over this wavelength range. Advanced packaging with miniature cryostat using liquid nitrogen free pulse tube cryocoolers is currently developed for this programme in order to allow use on this detector in any type of environment. First results of this project are detailed here. These programs are held with several partners, among them are the French astronomical laboratories (LAM, OHP, IPAG), the detector manufacturers (e2v technologies, Sofradir, CEA/LETI) and other partners (ESO, ONERA, IAC, GTC). Funding is: Opticon FP6 and FP7 from European Commission, ESO, CNRS and Université de Provence, Sofradir, ONERA, CEA/LETI and the French FUI (DGCIS).