320 x 240 pixels quantum well infrared photodetector (QWIP) array for thermal imaging: fabrication and evaluation

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Abstract
Due to the well established GaAs material and processing technology QWIPs are viable candidates for high resolution (greater than 128 by 128 pixels), low cost LWIR (8 - 12 micrometer) focal plane arrays (FPAs). Usually n-doped AlGaAs/GaAs QWIPs are used since, at least to date, these have been shown to provide the highest performance. Fabrication and evaluation of 320 by 240 pixels QWIP arrays have also been done. The fabrication involves hybridizing GaAs chips consisting of detector mesas to specially designed CMOS readout chips. The hybridization is effected by indium bump flip-chip bonding. Optical coupling into the detectors is performed by using optimized, etched, two-dimensional gratings combined with GaAs substrate thinning down. The advantages of substrate removal are: (1) reduction of thermal mismatch between materials and thus permitting large array sizes, (2) enhancement of absorptance, and (3) elimination of optical cross-talk between pixels. The intended operating temperature range is 70 to 73 K, achievable by a miniature Stirling cooler. Excellent wafer uniformities resulting in responsivity uniformities of 3.3% across an array are found, and a temperature resolution NETD (noise equivalent temperature difference) equals 40 mK is achieved. Finally, the presence of fixed-pattern noise and its influence on the image performance are discussed.

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