A portable long-wavelength infrared electronic camera having a cutoff wavelength of 9 μm has been built around an image sensor in the form of a 640 × 512- pixel array of AlxGa1–xAs/GaAs quantum well infrared photodetectors (QWIPs). This camera is an intermediate product of a continuing program to develop high-resolution, high-sensitivity infrared cameras.

This Image Was Generated From One Frame of video readout, at frame rate of 30 Hz, from the camera described in the text.

Major features of the design and fabrication of the camera are the following:

  • The QWIPs are of the bound-to-quasibound type, for which the thermionic component of dark current is less than for other types. [This concept was discussed in more detail in “Bound-to- Quasi-Bound Quantum-Well Infrared Photodetectors” (NPO-19633), NASA Tech Briefs, Vol. 22, No. 9 (September 1998), page 54.]
  • The basic multiple-quantum-well (MQW) structure of the QWIP array in the present camera is a stack of about 50 identical quantum-well bilayers. Each bilayer comprises (1) a 45-Å-thick well layer of GaAs n-doped at a density ≈5 × 1017 cm3 and (2) a 500-Å-thick barrier layer of Al0.3Ga0.7As.
  • The MQW structure is sandwiched between 0.5-μm-thick top and bottom contact layers of GaAs doped similarly to the well layers.
  • All of the aforementioned layers were fabricated on a semi-insulating GaAs substrate by molecular-beam epitaxy. A 300-Å-thick Al0.3Ga0.7As stop-etch layer was grown on top of the top contact layer. A 0.7-μm-thick GaAs cap layer was grown on top of the stop-etch layer. A cross-grating structure for coupling light into the QWIPs was fabricated in the cap layer by photolithography and dry chemical etching. [The cross-grating-coupler concept was described in “Cross-Grating Coupling for Focal-Plane Arrays of QWIPs” (NPO-19657), NASA Tech Briefs, Vol. 22, No. 1 (January 1998), page 6a.]
  • The array of 640 × 512 photodetectors, with a pitch of 25 μm and a pixel size of 23 × 23 μm2, was then formed by wet chemical etching through the MQW layers into the bottom contact layer. The cross gratings on the tops of the detectors thus formed were covered with Au/Ge and Au for ohmic contact and reflection.
  • Indium bumps were evaporated onto the top (Au/Ge)/Au layers, then the bumps were used to bond (hybridize) the array to a silicon-based complementary metal oxide semiconductor (CMOS) integrated-circuit 640 × 512 readout multiplexer.

As described thus far, with the exception of the sizes and numbers of pixels, the QWIP-array/readout-multiplexer is nearly identical to that of the prior camera. An important difference is that in the present camera, the readout multiplexer is part of a commercial infrared-camera body that includes two “back-end” video signal processing circuits and a germanium lens of 100-mm focal length and 5.5° field of view. The lens is designed to be transparent in the wavelength range of 7 to 14 μm (compatible with a nominal QWIP operational wavelength of 8.5 μm). The digital acquisition resolution of the camera circuitry is 14 bits, so that the instantaneous dynamic range of the camera is 16,384. However, the dynamic range of the QWIPs is 85 dB.

The camera has been demonstrated to produce excellent video imagery (see figure). Whereas prior infrared cameras based on detectors of different types have been limited to thermal resolutions in excess of 30 mK, this camera is expected to exhibit significantly finer thermal resolution: On the basis of single-pixel test data, a noise equivalent differential temperature of 8 mK is expected in operation at a temperature at 65 K with f/2 (focal length ÷ aperture diameter = 2) optics and a background temperature of 300 K. The array of photodetectors has exhibited background-limited performance at an operating temperature of 72 K using the same optics and background conditions. Optimization of operating conditions (including frame rate, integration time, and QWIP bias voltage) is expected to lead to even better performance.

This work was done by Sarath Gunapala, Sumith Bandara, John Liu, and Sir B. Rafol of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category.

NPO-30624

Topics:
Body structures Chemicals Electronics & Computers Fabrication Frames Imaging and visualization Manufacturing processes Optics Optimization Semiconductors Sensors and actuators
This Brief includes a Technical Support Package (TSP).
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A 640 x 512-Pixel Portable Long-Wavelength Infrared Camera

(reference NPO-30624) is currently available for download from the TSP library.

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This article first appeared in the January, 2007 issue of NASA Tech Briefs Magazine (Vol. 31 No. 1).

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Overview

The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing a significant innovation: a 640 × 512-Pixel Portable Long-Wavelength Infrared (LWIR) Camera, identified by NTR Number 30624. This camera utilizes advanced technology, specifically a 9 µm cutoff GaAs/AlxGa1-xAs quantum well infrared photodetector, which allows for high-resolution imaging in the long-wavelength infrared spectrum.

The document emphasizes the camera's potential applications in various fields, including aerospace, scientific research, and commercial sectors. It is part of NASA's Commercial Technology Program, which aims to disseminate aerospace-related developments that have broader technological implications. The camera's design and functionality are intended to support a range of uses, from environmental monitoring to military applications, showcasing its versatility.

The document also references a paper presented at the SPIE AeroSense conference in April 2002, authored by a team of researchers including S. D. Gunapala and others, which provides further technical insights into the camera's capabilities and underlying technology.

Additionally, the Technical Support Package includes contact information for the Innovative Technology Assets Management at JPL, offering further assistance and information regarding research and technology in this area. The document serves as a resource for those interested in the technological advancements represented by the LWIR camera and highlights NASA's commitment to innovation and collaboration with commercial partners.

It is important to note that the document contains proprietary information and is subject to U.S. export control regulations. As such, users are advised to comply with all applicable laws when utilizing the information contained within.

In summary, the Technical Support Package provides a comprehensive overview of the 640 × 512-Pixel Portable LWIR Camera, its technological foundation, potential applications, and avenues for further inquiry, reflecting NASA's ongoing efforts to advance aerospace technology and its applications in various fields.