HgCdTe Focal Plane Arrays Research Articles

Imaging polarimetry in the LWIR with microgrid polarimeters

Microgrid polarimeters have emerged over the past decade as a viable tool for performing real-time, highly accurate polarimetric imagery. A microgrid polarimeter operates by integrating a focal plane array (FPA) with an array of micropolarizing optics. Mircrogrids have the advantage of being relatively compact, rugged, and inherently spatiotemporally aligned. However, they have the single disadvantage that the various polarization measurements that go into estimating the Stokes parameters at a particular pixel are actually coming from separate locations in the field. Hence, a microgrid polarimeter performs best where there is no image information, obviating the need for an imaging polarimeter! Recently we have been working with a LWIR microgrid polarimeter at the College of Optical Sciences. Our instrument is a DRS Sensors & Targeting Systems 640 x 480 HgCdTe FPA with linear polarizers at 0°, 45°, 90°, and 135° [1]. In this paper we will review our recent results that derive methods for artifact-free reconstruction of band limited imagery.

LONG-WAVE INFRARED 2048-ELEMENTS LINEAR HgCdTe FOCAL PLANE ARRAY

The technology and performance of long-wave 2048-elements linear HgCdTe focal plane array were reported.The n-on-p photodiodes were fabricated by planar ion implantation and hybridized with readout circuit by indirect flip chip technique.The 2048-elements linear array consists of 8 segments of 256-element linear arrays that are "butted" on a single substrate in a staggered format.The cut off wavelength and R0A of the photodiode are 9.9μm and 10Ωcm2,respectively.The average detectivity of the 2048-elements linear array is 9.3×1010 cmHz1/2W-1.The ununiformity of response is 8%.The array peak operability is larger than 99.5%.

High throughput operando studies using Fourier transform infrared imaging and Raman spectroscopy

A prototype high throughput operando (HTO) reactor designed and built for catalyst screening and characterization combines Fourier transform infrared (FT-IR) imaging and Raman spectroscopy in operando conditions. Using a focal plane array detector (HgCdTe focal plane array, 128x128 pixels, and 1610 Hz frame rate) for the FT-IR imaging system, the catalyst activity and selectivity of all parallel reaction channels can be simultaneously followed. Each image data set possesses 16 384 IR spectra with a spectral range of 800-4000 cm(-1) and with an 8 cm(-1) resolution. Depending on the signal-to-noise ratio, 2-20 s are needed to generate a full image of all reaction channels for a data set. Results on reactant conversion and product selectivity are obtained from FT-IR spectral analysis. Six novel Raman probes, one for each reaction channel, were specially designed and house built at Pacific Northwest National Laboratory, to simultaneously collect Raman spectra of the catalysts and possible reaction intermediates on the catalyst surface under operando conditions. As a model system, methanol partial oxidation reaction on silica-supported molybdenum oxide (MoO3SiO2) catalysts has been studied under different reaction conditions to demonstrate the performance of the HTO reactor.

Improved Defect and Fourier Transform Infrared Spectroscopy Analysis for Prediction of Yield for HgCdTe Multilayer Heterostructures

The ability to accurately predict HgCdTe focal plane array (FPA) performance using nondestructive, postgrowth wafer analysis is of great importance. These predictions, if accurate, reduce costs by screening the wafers prior to processing, and selecting only those wafers that are most likely to yield FPAs that meet program specifications. In this paper, we examine the use of a macrodefect inspection tool, the NSX 1255, from August Technology. This inspection tool has the ability to measure defects 0.5 μm and larger and store the location and size data to a file. We have then, through the use of custom written software, been able to analyze these data on a wafer by wafer basis. We have also incorporated the use of a thin film transmission matrix model to analyze room-temperature Fourier transform infrared spectroscopy (FTIR) transmission spectra. This technique, which is applied to the entire wafer surface, can be used to determine the individual layer thicknesses as well as their compositions. Then, using analytical expressions for bandgap, absorption, and index of refraction, we can predict responsivity and quantum efficiency. Through the use of these two inspection tools and our analysis software, we are able to overlay FPA die information and perform statistics on a die-per-die basis. This allows us to effectively “pass” or “fail” each FPA based on the program specifications. We are then able to set a minimum criterion for the number of FPAs that pass on any given wafer. That wafer is then sent off to processing if it meets this criterion. Furthermore, knowing why a wafer fails before it reaches processing allows for real time feedback to the epilayer growth process. This allows for run-to-run adjustments in order to keep as many wafers within specifications as possible and increases yield overall.

Material considerations for third generation infrared photon detectors

In the paper, issues associated with the development and exploitation of materials used in fabrication of third generation infrared photon detectors are discussed. In this class of detectors two main competitors, HgCdTe photodiodes and quantum well photoconductors are considered. The performance figures of merit of state-of-the-art HgCdTe and QWIP focal plane arrays (FPAs) are similar because the main limitations come from the readout circuits. The metallurgical issues of the epitaxial layers such as uniformity and number of defected elements are the serious problems in the case of long wavelength infrared (LWIR) and very LWIR (VLWIR) HgCdTe FPAs. It is predicted that superlattice based InAs/GaInSb system grown on GaSb substrate seems to be an alternative to HgCdTe with good spatial uniformity and an ability to span cutoff wavelength from 3 to 25μm. In this context the material properties of type II superlattices are considered more in detail.

Correlated Noise and Gain in Unfilled and Epoxy‐Underfilled Hybridized HgCdTe Detectors

ABSTRACTSpatially correlated noise in focal plane arrays (FPAs) affects the photometric signal‐to‐noise ratio and can lead to overestimation of conversion gain (detector nodal capacitance). Capacitive coupling leads to charge sharing between neighboring pixels and correlates the measured pixel signal. This correlation can lead to image degradation, in particular for undersampled large survey telescopes. We present new comparative measurements of the interpixel coupling of 1.7 and 2.5 μm cutoff HgCdTe FPAs. Rockwell Scientific FPAs with epoxy underfill exhibit correlations that result in a 16%–20% systematic overestimation of the conversion gain. Results for a Raytheon Vision Systems 1.7 μm cutoff HgCdTe detector tested both before and after epoxy underfill show increased coupling in the presence of the epoxy, but the coupling is not spatially symmetric. We speculate that the difference in the observed correlation for each vendor is related to multiplexer geometry and pixel nodal capacitance.

Compact high-resolution spaceborne echelle grating spectrometer with acousto-optical tunable filter based order sorting for the infrared domain from 22 to 43 μm

A new compact spaceborne high-resolution spectrometer developed for the European Space Agency's Venus Express spacecraft is described. It operates in the IR wavelength range of 2.2 to 4.3 microm and measures absorption spectra of minor constituents in the Venusian atmosphere. It uses a novel echelle grating with a groove density of 4 lines/mm in a Littrow configuration in combination with an IR acousto-optic tunable filter for order sorting and an actively cooled HgCdTe focal plane array of 256 by 320 pixels. It is designed to obtain an instrument line profile of 0.2 cm(-1). First results on optical and spectral properties are reported.

Competitive technologies of third generation infrared photon detectors

AbstractHitherto, two families of multielement infrared (IR) detectors are used for principal military and civilian infrared applications; one is used for scanning systems (first generation) and the other is used for staring systems (second generation). Third generation systems are being developed nowadays. In the common understanding, third generation IR systems provide enhanced capabilities like larger number of pixels, higher frame rates, better thermal resolution as well as multicolour functionality and other on-chip functions.In the paper, issues associated with the development and exploitation of materials used in fabrication of third generation infrared photon detectors are discussed. In this class of detectors two main competitors, HgCdTe photodiodes and quantum well IR photoconductors (QWIPs) are considered. The performance figures of merit of state-of-the-art HgCdTe and QWIP focal plane arrays (FPAs) are similar because the main limitations come from the readout circuits. However, the metallurgical issues of the epitaxial layers such as uniformity and number of defected elements are the serious problems in the case of long wavelength infrared (LWIR) and very LWIR (VLWIR) HgCdTe FPAs. It is predicted that superlattice based InAs/GaInSb system grown on GaSb substrate seems to be an attractive to HgCdTe with good spatial uniformity and an ability to span cutoff wavelength from 3 to 25 μm.

Inductively coupled plasma etching for large format HgCdTe focal plane array fabrication

Inductively coupled plasma (ICP) using hydrogen-based gas chemistry has been developed to meet requirements for deep HgCdTe mesa etching and shallow CdTe passivation etching in large format HgCdTe infrared focal plane array (FPA) fabrication. Large format 2048×2048, 20-µm unit-cell short wavelength infrared (SWIR) and 2560×512, 25-µm unit-cell midwavelength infrared (MWIR) double-layer heterojunction (DLHJ) p-on-n HgCdTe FPAs fabricated using ICP processing exhibit >99% pixel operability. The HgCdTe FPAs are grown by molecular beam epitaxy (MBE) on Si substrates with suitable buffer layers. Midwavelength infrared detectors fabricated from 4-in. MBE-grown HgCdTe/Si substrates using ICP for mesa delineation and CdTe passivation etching demonstrate measured spectral characteristics, RoA product, and quantum efficiency comparable to detectors fabricated using wet chemical processes. Mechanical samples prepared to examine physical characteristics of ICP reveal plasma with high energy and low ion angle distribution, which is necessary for fine definition, high-aspect ratio mesa etching with accurate replication of photolithographic mask dimensions.

Current voltage modeling of current limiting mechanisms in HgCdTe focal plane array photodetectors

An automated iterative nonlinear fitting program has been developed to model current-voltage (I–V) data measured on HgCdTe infrared (IR) detector diodes. This model includes the ideal diode diffusion, generation-recombination, band-to-band tunneling, trap-assisted tunneling (TAT), and avalanche breakdown as potential current limiting mechanisms in an IR detector diode. The modeling presented herein allows one to easily distinguish, and more importantly to quantitatively compare, the amount of influence each current limiting mechanism has on various detectors’ I–V characteristics. Longer cutoff wavelength detectors often exhibit significant current limitations due to tunneling processes. The temperature dependence of these tunneling characteristics is thoroughly investigated for two diodes.

Mid- and Far-IR Focal Plane Arrays Based on Hg[sub 1 – ][sub x]Cd[sub x]Te Photodiodes

This review presents the results of the research and development work carried out at the ORION RD&P Center (Federal State Unitary Enterprise) on HgCdTe photodiodes and focal plane arrays based on solid solutions of mercury and cadmium tellurides for the 3–5 and 8–12 μm spectral ranges. The structure, topology, and parameters of the photodiodes and staring and time-delay-integration focal plane arrays, as well as the structure and circuitry of silicon multiplexers developed for the focal plane arrays, are discussed. The parameters of photodiode arrays of various formats and of a prototype thermal imager based on a 128 × 128 staring focal plane array are presented.

HgCdTe focal plane arrays for dual-color mid- and long-wavelength infrared detection

Raytheon Vision Systems (RVS, Goleta, CA) in collaboration with HRL Laboratories (Malibu, CA) is contributing to the maturation and manufacturing readiness of third-generation, dual-color, HgCdTe infrared staring focal plane arrays (FPAs). This paper will highlight data from the routine growth and fabrication of 256×256 30-µm unit-cell staring FPAs that provide dual-color detection in the mid-wavelength infrared (MWIR) and long wavelength infrared (LWIR) spectral regions. The FPAs configured for MWIR/MWIR, MWIR/LWIR, and LWIR/LWIR detection are used for target identification, signature recognition, and clutter rejection in a wide variety of space and ground-based applications. Optimized triple-layer heterojunction (TLHJ) device designs and molecular beam epitaxy (MBE) growth using in-situ controls has contributed to individual bands in all dual-color FPA configurations exhibiting high operability (>99%) and both performance and FPA functionality comparable to state-of-the-art, single-color technology. The measured spectral cross talk from out-of-band radiation for either band is also typically less than 10%. An FPA architecture based on a single-mesa, single-indium bump, and sequential-mode operation leverages current single-color processes in production while also providing compatibility with existing second-generation technologies.

HgCdTe/Si materials for long wavelength infrared detectors

The heteroepitaxial growth of HgCdTe on large-area Si substrates is an enabling technology leading to the production of low-cost, large-format infrared focal plane arrays (FPAs). This approach will allow HgCdTe FPA technology to be scaled beyond the limitations of bulk CdZnTe substrates. We have already achieved excellent mid-wavelength infrared (MWIR) and short wavelength infrared (SWIR) detector and FPA results using HgCdTe grown on 4-in. Si substrates using molecular beam epitaxy (MBE), and this work was focused on extending these results into the long wavelength infrared (LWIR) spectral regime. A series of nine p-on-n LWIR HgCdTe double-layer heterojunction (DLHJ) detector structures were grown on 4-in. Si substrates. The HgCdTe composition uniformity was very good over the entire 4-in. wafer with a typical maximum nonuniformity of 2.2% at the very edge of the wafer; run-to-run composition reproducibility, realized with real-time feedback control using spectroscopic ellipsometry, was also very good. Both secondary ion mass spectrometry (SIMS) and Hall-effect measurements showed well-behaved doping and majority carrier properties, respectively. Preliminary detector results were promising for this initial work and good broad-band spectral response was demonstrated; 61% quantum efficiency was measured, which is very good compared to a maximum allowed value of 70% for a non-antireflection-coated Si surface. The R0A products for HgCdTe/Si detectors in the 9.6-µm and 12-µm cutoff range were at least one order of magnitude below typical results for detectors fabricated on bulk CdZnTe substrates. This lower performance was attributed to an elevated dislocation density, which is in the mid-106 cm−2 range. The dislocation density in HgCdTe/Si needs to be reduced to <106 cm−2 to make high-performance LWIR detectors, and multiple approaches are being tried across the infrared community to achieve this result because the technological payoff is significant.

Proton-induced transients and charge collection measurements in a LWIR HgCdTe focal plane array

We compare measurements and modeling of 27 and 63 MeV proton-induced transients in a large-format HgCdTe long wavelength infrared (LWIR) focal plane assembly operating at 40 K. Charge collection measurements describe very limited diffusion of carriers to multiple pixels showing significantly reduced particle induced cross-talk for the lateral diffusion structure.

Recent developments of high-complexity HgCdTe focal plane arrays at leti infrared laboratory

In this article, we present recent developments of the research in France at LETI infrared laboratory in the field of complex third-generation HgCdTe IRCMOS focal plane arrays (FPAs). We illustrate this with three prototypes of FPAs made at LETI, which have involved some technological improvements from the standard process today in production at Sofradir. We present, using molecular-beam epitaxy (MBE) growth, a 128 × 128 dual-band infrared (photodetector)-complementary metal oxide semiconductor (IRCMOS) with a pitch of 50 µm operating within 2–5 µm. Using the more conventional liquid-phase epitaxy (LPE) growth, we show a new generation of high-performance long linear arrays (1500 × 2; pitch, 30 µm) operating in medium-wavelength infrared (MWIR) or long-wavelength infrared (LWIR) bands based on a modular architecture of butted HgCdTe detection circuit and SiCMOS multiplexers. Finally, we present for the first time a megapixel (1000 × 1000) FPA with a pitch of 15 µm operating in the MWIR band that exhibits a very high performance and pixel operability.

Quantum well photoconductors in infrared detector technology

The paper compares the achievements of quantum well infrared photodetector (QWIP) technology with those of competitive technologies, with the emphasis on the material properties, device structure, and their impact on focal plane array (FPA) performance. Special attention is paid to two competitive technologies, QWIP and HgCdTe, in the long-wavelength IR (LWIR) and very-long-wavelength IR (VLWIR) spectral ranges. Because so far, the dialogue between the QWIP and HgCdTe communities is limited, the paper attempts to settle the main issues of both technologies. Such an approach, however, requires the presentation of fundamental limits to the different types of detectors, which is made at the beginning. To write the paper more clearly for readers, many details are included in the Appendix. In comparative studies both photon and thermal detectors are considered. Emphasis is placed on photon detectors. In this group one may distinguish HgCdTe photodiodes, InSb photodiodes, and doped silicon detectors. The potential performance of different materials as infrared detectors is examined utilizing the α/G ratio, where α is the absorption coefficient and G is the thermal generation rate. It is demonstrated that LWIR QWIP’s cannot compete with HgCdTe photodiodes as single devices, especially at higher operating temperatures (&amp;gt;70 K). This is due to the fundamental limitations associated with intersubband transitions. The advantage of HgCdTe is, however, less distinct at temperatures lower than 50 K due to problems inherent in the HgCdTe material (p-type doping, Shockley–Read recombination, trap-assisted tunneling, surface and interface instabilities). Even though QWIP is a photoconductor, several of its properties, such as high impedance, fast response time, long integration time, and low power consumption, comply well with the requirements imposed on the fabrication of large FPA’s. Due to a high material quality at low temperatures, QWIP has potential advantages over HgCdTe in the area of VLWIR FPA applications in terms of array size, uniformity, yield, and cost of the systems. The performance figures of merit of state-of-the-art QWIP and HgCdTe FPA’s are similar because the main limitations come from the readout circuits. Performance is, however, achieved with very different integration times. The choice of the best technology is therefore driven by the specific needs of a system. In the case of readout-limited detectors a low photoconductive gain increases the signal-to-noise ratio and a QWIP FPA can have a better noise equivalent difference temperature than an HgCdTe FPA with a charge well of similar size. Both HgCdTe photodiodes and QWIP’s offer multicolor capability in the MWIR and LWIR range. Powerful possibilities offered by QWIP technology are associated with VLWIR FPA applications and with multicolor detection. The intrinsic advantage of QWIP’s in this niche is due to the relative ease of growing multicolor structures with a very low defect density.

A comparative study and performance characteristics of ion-implanted and heterojunction short-wave infrared HgCdTe focal-plane arrays

Short-wave infrared (SWIR) HgCdTe focal-plane arrays (FPAs) with a cutoff wavelength of 2.5 µm have been produced using both planar ion-implanted and heterojunction-mesa device structures. The two-dimensional FPAs are comprised of a 320 × 256 format with 30-µm pixel pitch and are cooled by a multistage thermo-electric (TE) cooler. Measured RoA values of the two types of device structures show similar results below about 130 K because of the performance-limiting effect of the surface passivation of the heterojunction. However, a substantial difference is seen above 130 K and up to 300 K between the two structures types, with the heterojunction-mesa p-on-n device having an order of magnitude higher RoA value than the planar ion-implanted n-on-p configuration. The difference in the RoA values is reflected in the FPA images of the two different device types, where at 200 K, both FPAs display a clear picture with the n-on-p implanted device having a somewhat lesser resolution. However, no image can be seen from the planar-implanted FPA at 300 K, whereas the heterojunction-mesa FPA still exhibits a notable image at this temperature. These differences are examined and are attributed largely to higher diffusion and generation-recombination (g-r) currents that are thought to be prevalent in the ion-implanted n-on-p device structure. Yet, baking studies carried out show the ion-implanted diodes to be slightly more robust, as experiments reveal that they tend to survive a 120°C heat treatment longer than the mesa devices, which tend to degrade after a certain period of time. The nature of n-type donors in ion-implanted diodes is discussed, and a new theory based on Te antisites is proposed to explain recent experimental findings.

A novel simultaneous unipolar multispectral integrated technology approach for HgCdTe IR detectors and focal plane arrays

In the last few years Rockwell has developed a novel simultaneous unipolar multispectral integrated HgCdTe detector and focal plane array technology that is a natural and relatively straightforward derivative of our baseline double layer planar heterostructure (DLPH) molecular beam epitaxial (MBE) technology. Recently this technology was awarded a U.S. patent. This simultaneous unipolar multispectral integrated technology (SUMIT) shares the high performance characteristics of its DLPH antecedent. Two color focal plane arrays with low-1013 cm−2s−1 background limited detectivity performance (BLIP D*) have been obtained for mid-wave infrared (MWIR, 3–5 m) devices at T>130 K and for long-wave infrared (LWIR, 8–10 m) devices at T∼80 K.

Spatial Noise Reduction by Optical Filters in Mercury Cadmium Telluride Hybrid Focal Plane Arrays

This paper discusses the role of optical filters in the reduction of spatial noise in Hgl-xCdxTe hybrid focal plane arrays (FPAs). It is shown that optical filters of appropriate cut-off wavelength can cause considerable reduction in the spatial noise of both mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) arrays. These filters can result in considerable improvement in the NETD performance of a MWIR HgCdTe FPA, whereas in LWJR FPAs, these can also be used to trade-off the requirement of composition uniformity of the HgCdTe substrate, depending upon the relative magnitude of temporal and spatial noise components

Million frames per second infrared imaging system

An infrared imaging system has been developed for measuring the temperature increase during the dynamic deformation of materials. The system consists of an 8×8 HgCdTe focal plane array, each with its own preamplifier. Outputs from the 64 detector/preamplifiers are digitized using a row-parallel scheme. In this approach, all 64 signals are simultaneously acquired and held using a bank of track and hold amplifiers. An array of eight 8:1 multiplexers then routes the signals to eight 10 MHz digitizers, acquiring data from each row of detectors in parallel. The maximum rate is one million frames per second. A fully reflective lens system was developed, consisting of two Schwarszchild objectives operating at infinite conjugation ratio. The ratio of the focal lengths of the objectives determines the lens magnification. The system has been used to image the distribution of temperature rise near the tip of a notch in a high strength steel sample (C-300) subjected to impact loading by a drop weight testing machine. The results show temperature rises at the crack tip up to around 70 K. Localization of temperature, and hence, of deformation into “U” shaped zones emanating from the notch tip is clearly seen, as is the onset of crack propagation.