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.
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.