Dual-band Megapixel QWIP FPAs
A dual-band FPA camera would provide the absolute temperature of a target with unknown emissivity, which is extremely important to the process of identifying a temperature difference between missile targets, warheads, and decoys. Dual-band infrared FPAs can also play important roles in Earth and planetary remote sensing, astronomy, and other applications. Furthermore, monolithically integrated pixel-co-located, simultaneously readable, dual-band FPAs eliminate the beam splitters, filters, moving filter wheels, and rigorous optical alignment requirements imposed on dual-band systems based on two separate single-band FPAs or a broad-band FPA system with filters. Dual-band FPAs also reduce the mass, volume, and power requirements of dual-band systems. Due to the inherent properties such as narrow-band response, wavelength tailorability, and stability (i.e., low 1/f noise) associated with the GaAs-based QWIP, it is an ideal candidate for large-format dual-band infrared FPAs.
Demonstrated the first megapixel, dual-band, pixel-co-located, simultaneously readable QWIP FPA. This dual-band QWIP device is based on 4–5-µm mid-wavelength infrared (MWIR) and 7.5–9-µm long-wavelength infrared (LWIR) QWIP devices separated by a 0.5-micron-thick, heavily doped, n-type GaAs layer. Both device structures and heavily doped contact layers were grown in situ during a single growth run using MBE. The photosensitive MQW region of each QWIP device is transparent at other wavelengths, which is an important advantage over conventional interband detectors. This spectral transparency makes QWIPs ideally suited for dual-band FPAs with negligible spectral cross-talk. Megapixel dual-band QWIP detector arrays were fabricated using stepper-based photolithography, ICP dry etching, and e-beam metal evaporation processes developed at the MDL. These detector arrays were hybridized with silicon complementary metal-oxide-semiconductor (CMOS)–based, direct injection, megapixel readout integrated circuits using an indium bump bonding technique. A selected dual-band megapixel QWIP FPA has been mounted onto the cold finger of a liquid pore-fill dewar, and the FPA was cooled to 70 K. The FPA was back-illuminated through the flat thinned substrate membrane (thickness 500 Å). This initial array gave good images, with 98% of the pixels working, which is excellent given the difficulty of the fabrication process. Video images were taken at a frame rate of 30 Hz at temperatures as high as 70K, using two readout integrated circuit (ROIC) capacitors having charge capacities of 3.4×106 and 13.6×106 electrons for the MWIR and LWIR bands, respectively. The pixel pitch of the array is 30 µm and dimensions of the FPA are 34×39 mm2. The initial GaAs substrate of these dual-band FPAs is completely removed, leaving only a 50-nm–thick GaAs membrane. Thus, these dualband QWIP FPAs are not vulnerable to delamination and indium bump breakage during thermal recycling process, and they have negligible pixel-to-pixel optical cross-talk.