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

Due to the well established GaAs material and processing technology QWIPs sire viable candidates for high resolution (>128x128 pixels), low cost LWIR (8-12 mu m) 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. 320x240 pixels QWIP arrays have been fabricated and evaluated. The Fabrication involves hybridizing GaAs chips consisting of defector 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: i) reduction of thermal mismatch between materials and thus permitting large array sizes, ii) enhancement of absorptance, and iii) elimination of optical cross-talk between pixels, The intended operating temperature range is 70-73 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) = 40 mK is achieved. Finally, the presence of fixed-pattern noise and its influence on the image performance are discussed.