The various signal read-out techniques available for monolithic and hybrid integrated focal plane arrays are described. These include MOS switch, charge coupled and charge injection devices. Indications of performance limitations are given.
Focal plane arrays technologies are now ready for use in infrared equipment such as thermal imagers, surveillance systems, missiles seekers and optoelectronic countermeasures. In this paper we review the differents technologies developped during the next past years and we compare them to define their range of application. These technologies involve to modify the system design to optimise the trade off between caracteristics of the detector and the other parts : optics, cryogeny, optomechanical subassembly, electronic and signal processing. We discuss this impact on other technologies and techniques (eg. Silicon Technology, IR image processing,) to realise the new generation of IR systems.
The liquid phase epitaxial technique is used to grow Hgl_x Cdx Te (x = .23) from a Te - rich solution onto a Cdl_y ZnyTe (y = .04) buffer layer grown from a Te-rich solution onto a Cdi_yZnyTe bulk substrate in an open tube multibin horizontal slider apparatus.Growth conditions and physical characterizations of both the buffer layer and the CdHgTe layer are given ; electrical properties of the CdHgTe layer are also presen-ted. PV detectors were successfully obtained on such a structure using an ion-implanted technology and their characteristics at 77 K for a 10.1 ,um cut-off wavelength are given.
The new (HgZn)Te alloys were found to present numerous advantages over (HgCd)Te, such as increased stability of the crystal lattice and reduced dislocation density. A suitable growth process is already in place. Adaptation of the classical (HgCd)Te planar process technology (ion implantation instead of meicury diffusion) permits the duplication of the performance characteristics found in (HgCd)Te. A particular attribute of this material is its exceptional performance at very long wavelengths and its stability at elevated temperatures. With growth method and process technology already in existence, series fabrication of these devices can be implemented without major problems. In view of the numerous technical advantages offered by (HgZn)Te over (HgCd)Te detectors, these may be considered to be serious candidates for future infrared systems.
Integrated receivers associating an amplifier with a photodiode are studied. The field effect transistor used for the amplifier has been designed and characterized. The FET tran-sistor has been fabricated on a GaAs semi-insulating substrate using Liquid Phase Epitaxy. The structure consists of two epitaxial layers, i.e. one N-GaAs layer for the channel and a P-GaAlAs layer for the gate. The design, technological process and current-voltage characteristics are described. Transconductance values over 10 mA/V have been achieved and the transistor have shown a threshold voltage of -4.6 volts. Integrated receivers whose field effect transistor is associated with a photodiode, enhance the detector sensitivity. The results measured on the device show current gain of 15 with a photodiode load resistance set to 2 kSt... Optical sensitivity and current amplification better than 15 are found on transistors with a high doping level ND. The results shown by the FET will be applied to a type of sensor presenting static optical responsivity.
The polyvinylidene fluoride, PVCV, has demonstrated, since a few years, its better feasibility than that exhibited by ceramics in pyroelectric detection, and performances of discrete PVDF IR detectors have already been presented . A new step toward the development of a focal plane matrix array is analyzed in this paper, in which technology and properties of a PVDF linear array are detailed. A first, advantages and drawbacks of the photolithography technique applied to PVDF are widely discussed and compared with the metal deposition using direct evaporation or sputtering through a metallic mask. Then, the use of the R.T. poling procedure developed by F. Bauer on a biaxially stretched PVDF is recalled. Finally, the technique used to elaborate a 10 pixels linear array, accounting for a proper selection of the polymer thickness is presented. Influence of this choice on the thermal cut off frequency, on the electrical cut off frequency and on the response of the elementary unit is determined and modelized. Characterization of tie whole structure, concerning response dispersion and cross talk between units and sensitivity of the linear array is analyzed. Results are shown to be most encouraging to prepare the next step dealing with the F.P. matrix array.
The circuit presented in this paper associates the classical direct injection and integration for each pixel with a random access scanning for the array. This focal plane implementation has two major advantages : i - the design of the pixel allows a larger integration capacitance in the 70 m pitch. ii - in the large arrays, the random access addressing mode allows the selection of sub-pictures for signal processing. The device is described. Performances of 32*32 pixels 70 m pitch circuits hybridized to CMT diodes arrays (cut-off wavelength : 5.5 and 10.1 m) are presented.
InSb has served as an important mid-wave IR (X=3-5pm) detector material for several decades. In this presentation, we will briefly review General Electric's InSb Charge Injection Device technology. Emphasis will be placed on device performance as a function of material parameters. A new InSb materials technology utilizing liquid phase epitaxy will be described. This epitaxial growth technology improves InSb material parameters and increases minority carrier lifetimes by more than two orders of magnitude to near the Auger limit. Comparisons will be made between available bulk material parameters and that of the epitaxial material.
A new SWIR focal plane has been designed for the French Space Agency's SPOT IV satellite. It consists of a linear array of InGaAs photodiodes, operating at ambient temperature, associated to Si CCD multiplexers. The CCDs are of the two phase n-buried channel type, operating up to 2 MHz. Their dynamic range exceeds 104, with a noise level of 1100 electrons. Any kind of current source sensors may be coupled to this readout circuit which has 150 lateral inputs.
The topics of this paper deal with the IR-CCD detectors manufactured by SOFRADIR the new French joint venture. Description of the IRCCD technology and the advantages of this approach are given. In conclusion, some IR-CCD typical results are given.
A Schottky barrier IR-CCD focal plane array with 128 x 64 picture elements was developed. The device utilizes n-channel CCD shift registers with an interline transfer format. To achieve high photoresponse the Schottky barrier detectors are constructed with a thin layer of PtSi separated from an aluminum mirror by a layer of Si3N4. The thickness of the Si3N4 layer is adapted to the wavelength of about 4/μm. The device is illuminated from the backside. The focal plane arrays show an excellent homogeneity of the responsivity. Quantum efficiencies of several percent are achieved. The black body (500K) detectivity of the devices is in the order of 1-2.1011 cm W-1 sec-1/2 measured with a cooled filter in the wavelength range 3.3 - 3.5/μm. With this device a camera system has been built to demonstrate the possibilities for thermal imaging with PtSi-Schottky barrier detectors. The output signal from the on-chip amplifier is processed with the double correlated sampling method and subsequent 12 bit A/D-conversion. A pixel-by-pixel offset correction is applied. Examples of thermal images taken with this camera system will be presented.
A thermal imager employing a two-dimensional hybrid array of pyroelectric detectors with MOSFET readout has been built. In this paper the design and theoretical performance of the detector is discussed and the results of performance measurements are presented.
Short wavelength (1-2.5 μm) 128 by 128 focal plane arrays have been fabricated and demonstrated with high pixel yields and dark current limited performance. Smaller arrays (32 by 32 and 64 by 64) have already been used successfully in the Airborne Imaging Spectrometer (AIS) instrument in the United States, Australia, and Western Europe. Ground-track swath width is enhanced by the development of 128 by 128 arrays. The long-term goal is to develop a 150 by 1000 mosaicked focal plane for use in the High Resolution Imaging Spectrometer (HIRIS) instrument, a facility instrument on the Earth Observing System (Eos). This instrument will map the earth with from 5 to 10 nm spectral resolution and 30 m spatial resolution for flux levels of 1010 to 1012 ph/cm2-s with An f/3.8 optical system. The detector material is HgCdTe grown by liquid phase epitaxy on a sapphire substrate which yields high-performance uniform detectors. The detectors are ion implanted photodiodes with ZnS passivation. They are mated to a silicon multiplexer through indium columns to fabricate the hybrid. Operational pixel yields of greater than 99 percent have been obtained. Reliability tests on 128 by 128 hybrids show that the device is mechanically durable to more than 50 thermal cycles. The detector arrays have been characterized at temperatures of 80-150 K. Mean RoA values of 4 x 106 ohms-cm2 and leakage currents below 5 x 10-13 A have been measured at 150 K. The peak quantum efficiency at 2.4 μm is 80 percent. These results indicate that the detectors have sufficient performance for dark current limited operation at low scene albedos. The multiplexer is an EG&G Reticon FET switch imager with two output amplifiers. It is a standard visible imager that has been modified for indium column growth and detector substrate bias control. The internal circuitry has not been changed. The charge is integrated in the detector capacitance under reverse bias and then transferred through common video lines into odd and even bucket brigades and clocked out through the output amplifiers. The read noise of the multiplexer is 950 electrons, and the maximum readout rate is 10 MHz. The maximum charge storage capacity is 5 x 106 electrons.
Photovoltaic CMT-Si CCD hybrid focal plane arrays for the Infrared Array Camera IRAC currently developed at ESO have been tested. The arrays have a format of 32 x 32 and 64 x 64 pixels. The cutoff wavelength of the detector is 3.9 microns but can be custom designed between 2.5 and 5 microns. The detectors consist of cylindrically shaped p-n junctions which are connected to a buried channel CCD by a special loophole interconnect technology. The input stage of the CCD is a direct injection circuit. Test results of the arrays relevant to ground based astronomy will be discussed and data on efficiency, fill factor, dark current, limiting noise performance and uniformity are presented. The tests were performed on reject engineering arrays. Scientific arrays are under development. Yet, test results obtained so far are promissing and demonstrate the potential of these devices for IR astronomy.
ISOCAM is one of the science instruments in the Infrared Space Observatory. It is a 2-channel I.R. Astro-nomical Imager intented to observe at very low flux levels, thanks to the use of a liquid helium cooled telescope. This paper describes the Focal Plane Assembly design of the short wavelength channel. The operation of a 32x32 1nSb CID-SAT arra detector has been demonstrated. The problems encountered in the design of the cooled electronics and theyomponent selectionprocess are discussed in the light of specific ISO constraints, such as thermal control and radiation shielding.
Within the framework of the ISO programme (Infrared Space Observatory), we have developed an infrared detector array for the long wavelength channel of the camera. The achievement of a such component and the main experimental results are presented.
ISOPHOT, the photometer for ESA's Infrared Space Observatory ISO, will use extrinsic photo-conductors for astrophysical research at wavelength ranging from 3 to more than 200 μm. Doped silicon detectors will be applied as single pixels, linear and two dimensional arrays for photometry, polarimetry, mapping and in a low resolution spectrometer. Two dimensional arrays of doped germanium are foreseen as far-infrared cameras. During phase B of the project breadboard models of the most critical detectors have been built and tested with cryogenic multiplexing readout electronics.
The use at liquid helium temperature of a InSb-CID detector array differs significantly from opera-tion at conditions usually adopted by the manufacturer (77K). In particular, the dark current behaviour hugely changes between the two temperatures. Only the tunnel current, independant of temperature conditions, is still active at 4.2K while the thermal-family currents vanish. We have studied the tunnel current of one InSb-MIS detector to determine its suitability to the low background conditions that will be met in the space experiment ISO. The search for the maximum integration time and the best quantum efficiency, the constraint about the photonic response linearity (especially at low photon flux), and the reduction of the readout noise constitute the main points of this study. Moreover, laboratory measurements showed secondary effects due to the detector (lag) or to the wiring (crosstalk). The CID array reactions to high energy radiations (Gamma rays) are finally discussed.
We present a complete infrared camera system, based on a high electron capacity detector (HgCdTe/CCD), that we have used under high background conditions to do astronomical images. The performances of the system and some results are presented, and the use of such a detector in astronomy will be discussed.
The ISM instrument of the PHOBOS space mission is a near-infrared imaging spectrometer for mineralogic studies of planet Mars and Phobos surface, with a 128 spectral elements (splited in two bands 0.8 to 1.6 μm and 1.6 to 3 μm). The description of this instrument is focused on the optical and detection system, based on two focal plane assembly of PbS linear arrays. Finally we will describe the first results of the calibration.
On SPOT 4, of the second generation earth observation satellite, will be implemented a SWIR channel in the 1.55/1.70 μm wavelength. A new GaInAs/InP detector has been developed for this purpose. This document describes the new instrument with emphasis on focal plane assembly, then technology and specifications of the new detectors of 3 000 elements.
The technique of Focal Plane Arrays might find a novel application in the field of short haul free space optical communications. This application consists of spatially multiplexing several streams of digital information so that they can be transmitted simultaneously over a single link. Such parallel transmission could, in some situations, advantageously replace the commonly used serial mode. This paper reports the results of a preliminary experimental investigation of such a focal plane array scheme. The limitations of focal plane beam multiplexing and the trade-offs between the number of channels and range are briefly discussed.
The possibility of improved performance of an air-to-air IR-seeker with the use of an IR-CCD-detector is discussed. A trade off of designing a scanning sensor with respect to detector dimensions, detector integration time, sensitivity and optical resolution is given.
Dome heating effects in an IR-CCD missile seeker have been investigated. The background radiation as a function of time after launch and the charge generated in the IR-CCD storage well has been calculated for two dome materials. Demands on the signal processing in order to maintain an optimized seeker sensitivity and dynamic range during the flight envelope are also discussed.
A two spectral wavelength thermal imager, using two IRCCD array sensors, has been made by french S.A.T. and co-financed by french official D.R.E.T.. The purpose of this study was to demonstrate the ability to achieve future thermal imagers and to compare the efficiency of IRCCD detectors in both 3-5 μm and 8-11 μm wavelength ranges. A description of specific problems inherent to the use of IRCCD detector arrays in such a thermal imager will be given, as well as the choosen solutions. Amont others, the high-linearity scanning system and the compensation for Fixed Pattern Noise (FPN) will be described.
The best design of high performance thermal imagers for the 90's will use horizontal quasi-linear arrays, with focal plane processing, associated with a simple vertical mechanical scanner. These imagers will have performance widely increased with respect to today (50 to 100 % range and resolution improvement).
Simple nonuniformity correction algorithms currently in use can be severely limited by nonlinear response characteristics of the individual pixels in an IR focal plane array. Although more complicated multi-point algorithms improve the correction process they too can be limited by nonlinearities. Furthermore, analysis of single pixel noise power spectrums usually show some level of 1 /f noise. This in turn causes pixel outputs to drift independent of each other thus causing the spatial noise (often called fixed pattern noise) of the array to increase as a function of time since the last calibration. Measurements are presented for two arrays (a HgCdTe hybrid and a Pt:Si CCD) describing pixel nonlinearities, 1/f noise, and residual spatial noise (after nonuniforming correction). Of particular emphasis is spatial noise as a function of the lapsed time since the last calibration and the calibration process selected. The resulting spatial noise is examined in terms of its effect on the NEAT performance of each array tested and comparisons are made. Finally, a discussion of implications for array developers is given.