InSb Focal Plane Array Research Articles

InAs/InAsSb Type-II Strained-Layer Superlattice Infrared Photodetectors.

The InAs/InAsSb (Gallium-free) type-II strained-layer superlattice (T2SLS) has emerged in the last decade as a viable infrared detector material with a continuously adjustable band gap capable of accommodating detector cutoff wavelengths ranging from 4 to 15 µm and beyond. When coupled with the unipolar barrier infrared detector architecture, the InAs/InAsSb T2SLS mid-wavelength infrared (MWIR) focal plane array (FPA) has demonstrated a significantly higher operating temperature than InSb FPA, a major incumbent technology. In this brief review paper, we describe the emergence of the InAs/InAsSb T2SLS infrared photodetector technology, point out its advantages and disadvantages, and survey its recent development.

Mid-wavelength infrared InAsSb/InAs nBn detectors and FPAs with very low dark current density

There was a significant improvement in the performance of infrared nBn detectors utilizing InAs/InAsSb absorbers culminating in the development of infrared focal plane arrays (FPAs) with excellent operability (99.7%) and operating at temperature significantly higher than InSb FPAs. In this work, we demonstrated that these detectors can operate with very low dark current densities enabling their use in applications with a low-to-medium level of background illumination. We showed that single pixel photodetectors with a cut-off wavelength of 4.8 μm and a quantum efficiency of QEmax = 35% under backside-illumination have a dark current density of 1 × 10−10 A/cm2 at an operating bias of −0.1 V and temperature T = 100 K. Additionally, we compared the single pixel dark current results with measurements of the dark current in FPAs. The FPA showed excellent performance with an operability of 99.7% and was able to reach a mode dark current density of 5 × 10−11 A/cm2 at 80 K.

Tomographic reconstruction of a jet engine exhaust plume using an infrared hyperspectral imager

Midwave infrared (MWIR) measurements of a well-characterized JetCat turbine engine were made using a Telops infrared Fourier transform spectrometer (IFTS) hyperspectral imager (HSI), which utilizes a 320 × 256 indium-antimonide (InSb) focal plane array, calibrated to give a spectral distribution of radiance for each pixel within the image. Measurements were also made using a Santa Barbara focal plane IR imager (InSb, 320 × 256 array) that reported integrated radiance values in the MWIR. These measurements were made at varying engine RPM and different nozzle orientations to determine temperature and concentrations of the flow. The different orientations allow for the determination of the degree of asymmetry in the flow in two dimensions. The spectral information from these measurements was used to determine temperature and species concentrations of carbon dioxide and water vapor produced by the combustion of the Jet-A fuel. This was done utilizing the hyperspectral tomography (HT) process, which combines multiple measurement paths and high-resolution spectral data to determine the temperatures and concentrations of CO2 and H2O throughout the volume. Corresponding local temperature and pressure readings were taken in order to corroborate the radiometric data. Solution sets on a grid of 0.15-in. spacing based on measurements made at 2 cm − 1 spectral resolution were generated for six exhaust planes throughout the exhaust flow of a 4 in . × 1 in . rectangular exhaust nozzle. The HT process generated core plume temperature values within ±35 K of measured thermocouple data for exhaust planes near the nozzle exit plane. These results show the successful application of the HT algorithms to IFTS HSI data for temperature and species concentration reconstruction.

Long-Term Stability of a 640 × 512 InSb Focal Plane Array with a Pitch of 15 μm

The long-term stability of a 640x512 InSb focal plane array (FPA) with a pitch of 15 μm combined with a Stirling cooler and an interface block has been investigated.The dependences of the FPA correctability index on the operation time after a two-point correction of the irregularity have been obtained. The FPAs with two different circuits of the readout LSI cells that differ in the integration capacitance and transmission coefficients are considered. It has been found that, for the InSb FPA, the long-term stability is as high as several hours, which ensures continuous operation of the array in thermal imaging systems without additional calibrations.

Calculation and verification of thermal stress in InSb focal plane arrays detector

Both the InSb chip fracture and the local delamination appearing in the negative electrode region of InSb infrared focal plane arrays (IRFPAs) detectors in liquid nitrogen shock tests are the major failure patterns of the InSb IRFPAs. To analyze the inducements to these typical failure patterns, it is imperative to obtain both the thermal mismatch stresses values and their distributions in the InSb IRFPAs. In this paper, the analytical model developed by C. H. Hsueh is firstly employed to calculate the in-plane normal stresses in the center region of the InSb IRFPAs, the in-plane shear forces per unit width outside the center region of the InSb IRFPA, and the peeling moments per unit width outside the center region of the InSb IRFPA. Secondly, the structural model of the InSb IRFPAs created by us with ANSYS software is also employed to extract the distributions of the in-plane normal stresses, the distributions of the interfacial shear stresses, and the distributions of the out-of-plane normal stresses. After that comparing both the calculated in-plane normal stresses and the interfacial shear forces with the corresponding simulated results, we think that the calculating accuracy of the analytical model is comparable to the simulating accuracy of the ANSYS model of the InSb IRFPAs, and the relative errors are smaller than 10%. All these findings provide the technical means in guiding both the structural design and the structural optimization of the InSb IRFPAs assembly in the future.

Negative electrode structure design in InSb focal plane array detector for deformation reduction

When an indium antimonide (InSb) infrared focal plane array (IRFPA) is subjected to a thermal shock test, most of the cracks originate from the region over the negative electrode, which restricts its final yield. In light of the proposed equivalent modeling, three negative electrode structures are assessed to eliminate the accumulated deformation around the negative electrode. Simulation results show that when a thicker indium bump array is connected directly with negative InSb material, the accumulated thermal deformation is the minimum, the top surface of InSb chip is the smoothest, and the square checkerboard buckling pattern, present clearly in both gold buffer layer and sparse thicker indium bump array structure, seems to be unclear. All these mean that a thicker indium bump array structure is a good choice, which will benefit to reduce fracture probability of InSb IRFPAs under thermal shock.

MTF Issues in Small-Pixel-Pitch Planar Quantum IR Detectors

The current trend in quantum infrared (IR) detector development is the design of very small-pixel-pitch large arrays. From the previous 30 μm pitch, the standard pixel pitch today is 15 μm and is expected to decrease to 12 μm in the next few years. Furthermore, focal-plane arrays (FPAs) with pixel pitch as small as 10 μm have been demonstrated. Such ultrasmall-pixel pitches are very small compared with the typical length ruling the electrical characteristics of the absorbing materials, namely the minority-carrier diffusion length. As an example, for low-doped n-type HgCdTe or InSb material, this diffusion length is on the order of 30 μm to 50 μm, i.e., three to five times the targeted pixel pitches. This has strong consequences for the modulation transfer function (MTF) of planar structures, where the lateral extension of the photodiode is limited by diffusion. For such aspect ratios, the self-confinement of neighboring diodes may not be efficient enough to maintain an optimal MTF. Therefore, this issue has to be addressed to take full advantage of the pixel pitch reduction in terms of image resolution. The aim of this work is to investigate the evolution of the MTF of HgCdTe and InSb FPAs when decreasing the pixel pitch below 15 μm. Both experimental measurements and finite-element simulations are used to discuss this issue. Different scenarios are compared, namely deep mesa etch between pixels, internal drift, surface recombination, and thin absorbing layers.

Investigation of Radiation Collection by InSb Infrared Focal-Plane Arrays with Micro-optic Structures

Three designs of micro-optic structures have been analyzed by two-dimensional simulation. Compared with traditional spherical microlenses, the micro-optic structures have the same ability to collect radiation and do not have the disadvantages of traditional microlenses. In our analysis the micro-optic structures are simple grooved notches above the space between two adjacent mesas. We also investigate the characteristics of InSb focal-plane arrays with both spherical microlenses and micro-optic structures under oblique incident radiation. Empirical formulas were derived to describe the response and crosstalk as a function of incident radiation angle. Our results show that the micro-optic structures can be effectively used in radiation collection for InSb infrared focal-plane arrays.

Modeling and stress analysis of large format InSb focal plane arrays detector under thermal shock

Higher fracture probability, appearing in large format InSb infrared focal plane arrays detector under thermal shock loadings, limits its applicability and suitability for large format equipment, and has been an urgent problem to be solved. In order to understand the fracture mechanism and improve the reliability, three dimensional modeling and stress analysis of large format InSb detector is necessary. However, there are few reports on three dimensional modeling and simulation of large format InSb detector, due to huge meshing numbers and time-consuming operation to solve. To solve the problems, basing on the thermal mismatch displacement formula, an equivalent modeling method is proposed in this paper. With the proposed equivalent modeling method, employing the ANSYS software, three dimensional large format InSb detector is modeled, and the maximum Von Mises stress appearing in InSb chip dependent on array format is researched. According to the maximum Von Mises stress location shift and stress increasing tendency, the adaptability range of the proposed equivalent method is also derived, that is, for 16×16, 32×32 and 64×64 format, its adaptability ranges are not larger than 64×64, 256×256 and 1024×1024 format, respectively. Taking 1024×1024 InSb detector as an example, the Von Mises stress distribution appearing in InSb chip, Si readout integrated circuits and indium bump arrays are described, and the causes are discussed in detail. All these will provide a feasible research plan to identify the fracture origins of InSb chip and reduce fracture probability for large format InSb detector.

Finite Element Analysis on Structural Stress of 32x32 InSb Focal Plane Arrays Integrated with Microlens Arrays

Finite Element Analysis on Structural Stress of 32x32 InSb Focal Plane Arrays Integrated with Microlens Arrays

Megapixel digital InSb detector for midwave infrared imaging

Since the late 1990s Semiconductor devices (SCDs) has developed and manufactured a variety of InSb two-dimensional (2D) focal plane arrays (FPAs) that were implemented in many infrared (IR) systems and applications. SCD routinely manufactures both analog and digital InSb FPAs with array formats of 320×256, 480×384, and 640×512 elements, and pitch size in the range 15 to 30 μm. These FPAs are available in many packaging configurations, including fully integrated detector-Dewarcooler-assembly, with either closed-cycle Stirling or open-loop Joule-Thomson coolers. In response to a need for very high resolution midwave IR (MWIR) detectors and systems, SCD has developed a large format 2D InSb detector with 1280×1024 elements and pixel size of 15 μm. The ROIC is fabricated in CMOS 0.18-μm technology, that enables the small pixel circuitry and relatively low power generation at the focal plane. The digital ROIC has an analog to digital (A/D) converter per-channel and allows for full frame readout at a rate of 100 Hz. Such on-chip A/D conversion eliminates the need for several A/D converters with fairly high power consumption at the system level. The digital readout, together with the InSb detector technology, lead to a wide linear dynamic range and low residual nonuniformity, which is stable over a long period of time following a nonuniformity correction procedure. A special Dewar was designed to withstand harsh environmental conditions while minimizing the contribution to the heat load of the detector. The Dewar together with the low power ROIC, enable a megapixel detector with overall low size, weight, and power with respect to comparable large format detectors. A variety of applications with this detector make use of different cold shields with different f-number and spectral filters. In this paper we present actual performance characteristics of the megapixel InSb detector and demonstrate its high manufacturability.

InSb Focal Plane Array Chemical Imaging Enables Assessment of Unit Process Efficiency for Milling Operation

In the dry milling of wheat flour, each unit process (roller mill, purifier, sifter, etc.) produces a mixture with varying amounts of wheat endosperm and non-endosperm byproducts. Chemical images with 82 000 pixels of each intermediate product stream issuing from an individual processing machine are readily analyzed in terms of the relative amount of endosperm and non-endosperm. Approximately three minutes is required to produce an image of each intermediate product stream. Applying partial least squares (PLS) chemometric software to identify individual pixels, which enables calculation of the relative amount of endosperm and non-endosperm, is not a time-limiting factor. When relative flow rates are known for each stream, mass balance can be calculated from each intermediate stream in terms of the product (endosperm content) and the lower value non-endosperm byproduct. Data is presented from a purifier in a commercial flour mill. Intermediate streams collected from a run with optimized operational parameters were compared to those of another run before adjustment. The endosperm (product) mass balance profile for each run enabled assessment of operational efficiency. The devised chemical imaging analysis system would be particularly useful in commissioning of a new mill or to optimize existing wheat milling systems. Also, when raw material differs from that for which previous optimization was established, a new optimization may be in order. The ability to acquire a large number of spectra from a specimen and apply multivariate statistics to identify each pixel and subsequently count pixels accommodates heterogeneity and reports the results from averaging a very large number of individual spectra. A second illustration of the utility of the imaging method is presented centering on streams from the first and second break unit operations at the beginning of the roller mill process.

Discrimination of isogenic wheat by InSb focal plane array chemical imaging

Waxy wheat lines differ from their non-waxy wild-type counterparts in their function, end use, and their amylopectin and amylose contents. In certain uses, including breadmaking, blending of some waxy (low amylose) wheat flour with that of normal (wild) wheat has been used to advantage. This fact has brought about interest in developing new lines that incorporate desirable traits from both genetic sources. Such prospects are the motivation for this analytical work. Chemical imaging is a convenient way to objectively discriminant between normal wild parent and triple null waxy and potentially to recognize the contribution of the gene of one parent incorporated into a new cultivar. Wavelength-dependent images of kernel cross-sections highlight either of the pure types. PLS and select PCA factor images also provide discrimination. Single null cross products are not discriminated. Spectra from individual endosperm pixels are selected from images to examine details of the chemical microstructure.

Discrimination between electronic and optical blooming in an InSb focal-plane array under high-intensity excitation

Infrared (IR) focal-plane arrays (FPA’s) with high spatial resolution play an important role in military imaging. Image degradation through blooming of the FPA by either thermal or monochromatic sources seriously limits their effectiveness. However, little has been published discerning the phenomena responsible for this blooming. We studied the blooming effects in a 640 × 480 indium-antimonide (InSb) FPA with 20-μm pixel-to-pixel spacing and operated at 77 K. Electronic and optical blooming from both a high-intensity blackbody source and a 4.6-μm narrow-line-width laser source was studied. A 30-μm pinhole mask was fastened directly to the FPA to achieve spatial isolation of individual pixels in order to separate the optical and electronic effects. Both sources were used to bloom the FPA with and without the mask, and images were captured and analyzed. We provide conclusive evidence that optical effects of ghosting, diffraction, and lens and housing scatter dominate, resulting in global ( i.e., large area of the FPA) loss of image quality, while effects due to electronic phenomena, such as carrier-diffusion, are minimal and locally confined to on-the-order-of-one pixel. A simple model of carrier transport was constructed to provide the theoretical basis for our conclusions.

Mid-infrared imaging with a solid immersion lens and broadband laser source

The application of hemispherical solid immersion lenses (SILs) is an approach to increasing the spatial resolution attainable with infrared (IR) microscopy. A hemispherical SIL forms an image for which the effective numerical aperture and magnification are increased by a factor of the SIL material index of refraction. A microscope designed for exploration of IR SIL imaging based on a hemispherical ZnSe SIL, an InSb focal plane array detector, and a broadband IR laser source is described. The imaging characteristics of this system are reported, including the spatial resolution improvement achieved in the imaging of organic test samples.

Molecular beam epitaxy grown In1−xAlxSb∕InSb structures for infrared detectors

The core of SCD’s present day focal plane array (FPA) technology is a two-dimensional array of InSb photodiodes. It is produced by ion implantation and is attached to a silicon focal plane processor by indium bumps. We have extended this technology with In1−xAlxSb (0<x<0.03) alloys grown by molecular beam epitaxy on InSb substrates, utilizing a p-i-n structure. The advantages of the grown structures over those fabricated by implantation are already demonstrated in 320×256 element FPAs based on epitaxial InSb mesa diodes. These FPAs have dark currents at 100K comparable to those of implanted FPAs at 77K, with an operability (fraction of working diodes) in excess of 99.5%, and with a residual non-uniformity (RNU) at 100K of less than 0.03% (standard deviation/dynamic range) after a two-point non-uniformity correction (NUC). The RNU remains very low (<0.1%) even when the FPA temperature is shifted by as much as 10K from the NUC temperature. The average dark current at 100K and also the width of its distribution are further reduced by increasing the Al concentration in the FPAs by up to 3%, in accordance with the increase in the alloy energy gap. This has enabled us to achieve high quality thermal images at even higher temperatures than in the binary InSb FPAs. A key issue in achieving highly uniform FPAs is the degree of control of the InAlSb composition, misfit strain relaxation, and defect density. A series of In1−xAlxSb layers with x up to 6% was grown in order to study the effect of the growth conditions on these features. We discuss our results in terms of the misfit strain relaxation mechanisms in the In1−xAlxSb structures including dislocations, tilt, microcracks, and morphological crosshatch patterns.

Investigation of Uncooled Microbolometer Focal Plane Array Infrared Camera for Quantitative Thermography

Thermal nondestructive evaluation has shown promise as a potential NDE technology for next generation US Army rotorcraft structures because it is rapid, noncontacting, and able to inspect complex geometries. To successfully apply thermal inspection systems for field use, the cost and size must be lowered. The infrared camera is a major factor contributing to the overall cost of commercially available thermal inspection systems. Recent advances in uncooled microbolometer focal plane array detectors have resulted in low cost, small size/weight, and low power consumption cameras. These attributes make this technology well suited for portable low cost thermal inspection systems. The purpose of this paper is to investigate the capabilities of the new microbolometer infrared cameras for quantitative thermal nondestructive evaluation. Quantitative thermal diffusivity and thickness images are obtained by minimizing the squared difference between the data and a thermal model on samples with fabricated defects. Critical infrared camera features such as spatial and temperature resolution, detector response time, and detector stability are studied by comparing results to a conventional thermal imaging camera using a cooled InSb focal plane array detector. Finally several techniques are presented to improve the camera’s performance. These techniques include temporal background subtraction, use of a synchronized electronic shutter system, and cyclic flash heating.

Multichannel and multicolour infrared thermography in Tore Supra

An imaging spectrometer using a sapphire prism as dispersing element has been conceived at Tore Supra for the spectral range of 1–4 μm. It measures simultaneously at various wavelengths the temperature on distributed high heat-flux elements under plasma impact with 36 optical fibers, four of which are ZrF4 fibers. It employs an InSb focal plane array detector (256*320 pixels) behind a silicon filter and a ZnS window yielding a dynamic range of 200–1500 °C with 20 ms temporal resolution. The fiber transmission and the spatial variation of gain and background of the camera are calibrated using a light source with integrating sphere. With a blackbody source one determines the nonlinearity of the average gain and controls its stability during operation. The spectral dispersion of about 24 nm/pixel is determined with interference filters and controlled with a spectral lamp. The measurements at various wavelengths allow to determine the temperature distribution in the field of view.

Developments in the fabrication and performance of high-quality HgCdTe detectors grown on 4-in. Si substrates

We are continuing to develop our growth and processing capabilities for HgCdTe grown on 4-in. Si substrates by molecular beam epitaxy (MBE). Both short-wave and mid-wave infrared (SWIR and MWIR) double-layer hetero-junctions (DLHJs) have been fabricated. In order to improve the producibility of the material, we have implemented an in-situ growth composition-control system. We have explored dry etching the HgCdTe/Si wafers and seen promising results. No induced damage was observed in these samples. Detector results show that the HgCdTe/Si devices are state-of-the-art, following the diffusion-limited trend line established by other HgCdTe technologies. Focal-plane array (FPA) testing has been performed in order to assess the material over large areas. The FPA configurations range from 128 × 128 to 1,024 × 1,024, with unit cells as small as 20 µm. The MWIR responsivity and NEDT values are comparable to those of existing InSb FPAs. Pixel operabilities well in excess of 99% have been measured. We have also explored the role of growth macrodefects on diode performance and related their impact to FPA operability. The SWIR HgCdTe/Si shows similar results to the MWIR material. Short-wave IR FPA, median dark-current values of less than 0.1 e-/sec have been achieved.

Design and Performance of a Planar Array Infrared Spectrograph That Operates in the 3400 to 2000 cm−1 Region

A prototype, no-moving-parts, plane array infrared spectrograph (PA-IR) capable of routine spectral acquisition in the 3400 to 2000 cm−1 region has been constructed. The instrument includes a continuous source, Czerney–Turner type monochromator system and an infrared camera that incorporates a 320 × 256 pixel InSb focal plane array detector cooled with liquid nitrogen. PA-IR spectra (∼3400 to 2550 cm−1) of polystyrene (PS) and poly(ethylene naphthalate) (PEN) films have been obtained at a resolution of ∼8 cm−1 with excellent signal-to-noise ratios (SNR). Peak-to-peak noise levels of ∼1 × 10−3 absorbance units are observed for single acquisition spectra with 1.5 ms integration times and 17 ms total acquisition times. Integration times as low as 10 μs are possible (with good SNR); however, data acquisition is limited by the frame rate (60 frames/s) of the software acquisition package currently used. In this work, an apertured image of the source is displayed over ∼20 rows of the array and the expected square root improvement in SNR is observed when multiple rows and/or frames are averaged. We have also shown that a square root improvement in SNR continues to occur with further signal averaging, providing noise levels as low as 1.5 × 10−5 absorbance units. Several additional advantages and options associated with the PA-IR method are discussed, including time-resolved spectroscopy, real-time monitoring, and spectroscopic mapping.