Michael L. Winn

Detection of buried land mines using a dual-band LWIR/LWIR QWIP focal plane array

Abstract We report on the development and testing of a new dual-band infrared focal plane array (FPA) specifically designed to detect buried land mines. The detector response spectra were tailored to take advantage of the sharp spectral features associated with disturbed soils. The goal was to have a “blue” channel with peak response near 9.2 μm and a “red” channel with maximum response at 10.5 μm. The quantum well infrared photodetector is particularly suited for this application because of the flexibility available in designing the peak wavelength of the detector and the relatively narrow width of the response spectrum. FPAs were produced and tested under the US Army Research Laboratory’s Advanced Sensors Collaborative Research Alliance in co-operation with the Night Vision and Electronic Sensors Directorate. We report on laboratory measurements of the response spectra, the dark current as a function of operating temperature, and the conversion efficiency in both the blue and red channels. Imagery was taken in the field of buried anti-tank mines. The images were analyzed by combining the data from the two channels into single fused images.

Comparison of HgCdTe and QWIP dual-band focal plane arrays

We report on results of laboratory and field tests of dual- band MWIR/LWIR focal plane arrays (FPAs) produced under the Army Research Laboratorys Multidomain Smart Sensor Federated Laboratory program. The FPAs were made by DRS Infrared Technologies using the HgCdTe material system and by BAE Systems using QWIP technology. The HgCdTe array used the DRS HDVIPTM process to bond two single-color detector structures to a 640 X 480-pixel single-color read-out integrated circuit (ROIC) to produce a dual-band 320 X 240 pixel array. The MWIR and LWIR pixels are co-located and have a high fill factor. The images from each band may be read out either sequentially (alternating frames) or simultaneously. The alternating frame approach must be used to produce optimal imagery in both bands under normal background conditions. The QWIP FPA was produced using MBE-grown III-V materials. The LWIR section consisted of GaAs quantum wells and AlGaAs barriers and the MWIR section used InGaAs quantum wells with AlGaAs barriers. The detector arrays were processed with three ohmic contacts for each pixel allowing for independent bias control over both the MWIR and LWIR sections. The arrays were indium bump-bonded to an ROIC (specifically designed for two color operation) which puts out the imagery from both bands simultaneously. The ROIC has variable gain and windowing capabilities. Both FPAs were tested under similar ambient conditions with similar optical components. The FPAs were subjected to a standard series of laboratory performance tests. The relative advantages and disadvantages of the two material systems for producing medium-format dual-band FPAs are discussed.

Dual band QWIP MWIR/LWIR focal plane array test results

We report on the results of laboratory and field tests on a pixel-registered, 2-color MWIR/LWIR 256 X 256 QWIP FPA with simultaneous integrating capability. The FPA studied contained stacked QWIP structures with spectral peaks at 5.1 micrometer and 9.0 micrometer. Normally incident radiation was coupled into the devices using a diffraction grating designed to operate in both spectral bands. Each pixel is connected to the read-out integrated circuit by three bumps to permit the application of separate bias levels to each QWIP stack and allow simultaneous integration of the signal current in each band. We found the FPA to have high pixel operability, well balanced response, good imaging performance, high optical fill factor, and low spectral crosstalk. We present data on measurements of the noise-equivalent temperature difference of the FPA in both bands as functions of temperature and bias. The FPA data are compared to single-pixel data taken on devices from the same wafer. We also present data on the sensitivity of this FPA to polarized light. It is found that the LWIR portion of the device is very sensitive to the direction of polarization of the incident light. The MWIR part of the device is relatively insensitive to the polarization. In addition, imagery was taken with this FPA of military targets in the field. Image fusion techniques were applied to the resulting images.

Thin active region, type II superlattice photodiode arrays: Single-pixel and focal plane array characterization

We have measured the radiometric properties of two midwave infrared photodiode arrays (320×256pixel2 format) fabricated from the same wafer comprising a thin (0.24μm), not intentionally doped InAs∕GaSb superlattice between a p-doped GaSb layer and a n-doped InAs layer. One of the arrays was indium bump bonded to a silicon fanout chip to allow for the measurement of properties of individual pixels, and one was bonded to a readout integrated circuit to enable array-scale measurements and infrared imaging. The superlattice layer is thin enough that it is fully depleted at zero bias, and the collection efficiency of photogenerated carriers in the intrinsic region is close to unity. This simplifies the interpretation of photocurrent data as compared with previous measurements made on thick superlattices with complex doping profiles. Superlattice absorption coefficient curves, obtained from measurements of the external quantum efficiency using two different assumptions for optical coupling into the chip, bracke...

Comparison of HgCdTe and quantum-well infrared photodetector dual-band focal plane arrays

We report on results of laboratory and field tests of dual-band focal plane arrays (FPAs) in the medium-wave infrared (MWIR) and long-wave infrared (LWIR), produced under the Army Research Laboratorys Multidomain Smart Sensor Federated Laboratory program. The FPAs were made by DRS Infrared Technologies using the HgCdTe material system, and by BAE Systems using quantum-well infrared photodetector (QWIP) technology. The HgCdTe array used the DRS HDVIPTM process to bond two single-color detector structures to a 640×480-pixel single-color readout integrated circuit (ROIC) to produce a dual-band 320×240 pixel array. The MWIR and LWIR pixels are co-located and have a large fill factor. The images from each band may be read out either sequentially (alternating frames) or simultaneously. The alternating-frame approach must be used to produce optimal imagery in both bands under normal background conditions. The QWIP FPA was produced using III-V materials grown by molecular-beam epitaxy (MBE). The LWIR section consisted of GaAs quantum wells and AlGaAs barriers, and the MWIR section used InGaAs quantum wells with AlGaAs barriers. The detector arrays were processed with three ohmic contacts for each pixel, allowing for independent bias control over both the MWIR and LWIR sections. The arrays were indium bump-bonded to an ROIC (specifically designed for two-color operation), which puts out the imagery from both bands simultaneously. The ROIC has variable gain and windowing capabilities. Both FPAs were tested under similar ambient conditions with similar optical components. The FPAs were subjected to a standard series of laboratory performance tests. The advantages and disadvantages of the two material systems for producing medium-format dual-band FPAs are discussed.

Development of a dual-band LWIR/LWIR QWIP focal plane array for detection of buried land mines

We report on the development and testing of a new dual-band infrared (IR) focal plane array (FPA) specifically designed to detect buried land mines. The detector response spectra were tailored to take advantage of the sharp spectral features associated with disturbed soils. The goal was to have a blue channel with peak response near 9.2 micrometers and a red channel with maximum response at 10.5 micrometers . The quantum well infrared photodetector (QWIP) is particularly suited for this application because of the flexibility available in designing the peak wavelength of the detector and the relatively narrow width of the response spectrum. FPAs were produced and tested under the U. S. Army Research Laboratorys Advanced Sensors Collaborative Research Alliance in co-operation with the Night Vision and Electronic Sensors Directorate. We report on laboratory measurements of the response spectra, the dark current as a function of operating temperature, and the conversion efficiency in both the blue and red channels. Imagery was taken in the field of buried anti-tank mines. The images were analyzed by combining the data from the two channels into single fused images.

Large-format and multispectral QWIP infrared focal plane arrays

The next generation of infrared (IR) focal plane arrays (FPAs) will need to be a significant improvement in capability over those used in present-day second generation FLIRs. The Armys Future Combat System requires that the range for target identification be greater than the range of detection for an opposing sensor. To accomplish this mission, the number of pixels on the target must be considerably larger than that possible with 2nd generation FLIR. Therefore, the 3rd generation FLIR will need to be a large format staring FPA with more than 1000 pixels on each side. In addition, a multi-spectral capability will be required to allow operability in challenging ambient environments, discriminate targets from decoys, and to take advantage of the smaller diffraction blur in the MWIR for enhanced image resolution. We report on laboratory measurements of a large format (1024 x 1024 pixels) single-color LWIR IR FPA made using the corrugated quantum well infrared photodetector (QWIP) structure by the ARL/NASA team. The pixel pitch is 18 μm and the spectral response peaks at 8.8 μm with a 9.2 μm cutoff. We report on recent results using a MWIR/LWIR QWIP FPA to image the boost phase of a launch vehicle for missile defense applications and a LWIR/LWIR FPA designed specifically for detecting the disturbed soil associated with buried land mines. Finally, we report on the fabrication of a new read-out integrated circuit (ROIC) specifically designed for multi-spectral operation.

6000-element infrared focal plane array for reconnaissance applications

This paper discusses the design, architecture, and performance of a 6000 element Indium Antimonide Infrared focal plane array. The focal plane array architecture allows for any N x 1000 element sized array to be constructed from its base elements. A uniquely constructed bi-staggered detector geometry is utilized to provide 2:1 over-sampling having 10 micron effective pitch in both the across track and along track directions. Additionally, the detector geometry allows for physical pixel sizes up to 25 microns while sampling at a 10 micron effective pitch to provide alias free imaging with the high signal capture capability of a large pixel. The Indium Antimonide detectors are front-side illuminated P-on-N type mesa diodes having no measurable crosstalk. A complimentary CMOS based Multiplexor in a M x 250 segmented design having up to 10 million electrons full-well output with greater than 14 bits instantaneous dynamic range provides a flexible and low noise readout for the focal plane array. Hybridization of the Indium Antimonide detectors and multiplexor is provided via a Lockheed Martin patented beam-lead technology to provide reliable and producible long linear focal plane arrays for reconnaissance applications. Characterization of the 6000 element Infrared focal plane array is presented including dynamic impedance of the diodes, read-noise, linearity, and non-uniformity. Meadured characteristics of the CMOS multiplexor are also presented in addition to data from the hybridized modules making up the Focal Plane Array.

Lead salt room-temperature MWIR FPA

The development of low-cost uncooled thermal LWIR FPAs is resulting in the emergence of a new generation of infrared sensors for applications where affordability is the prerequisite for volume production. Both ferroelectric detector arrays and silicon-based microbolometers are finding numerous applications from gun sights to automotive FLIRs. There would be significant interest in a similar uncooled offering in the MWIR, but to date, thermal detectors have lacked sufficient sensitivity. The existing uncooled MWIR photon detector technology, based on polycrystalline lead salts, has been relegated to single-element detectors and relatively small linear arrays due to the high dark current and the stigma of being a 50-year-old technology.

Signal processing ROIC for long-wave infrared scanning focal plane arrays

This paper describes the design and evaluation of a 128 X 1 signal processing readout integrated circuit for LWIR HgCdTe scanning infrared focal plane array applications. Smart readout functions, such as charge skimming and on-focal plane gain non-uniformity corrections, are presented along with empirical results measured with LWIR HgCdTe linear arrays.