High resolution, wide field-of-view, infrared (IR) imagers find use in defense and civilian applications. The most demanding of them desire uniform sensitivity across an image’s field of view, while maintaining a small and light-weight optical design. These attributes can be achieved by curving of the focal plane array to reduce the need for field curvature correction. Using experimental and numerical methods, we investigated the spherical curving of hybridized arrays to demonstrate mechanical feasibility and opto-electronic performance. Each hybridized array comprised a 4k x 4k, 10 μm pixel pitch, midwave IR (MWIR) detector hybridized to a 67 mm diagonal fanout chip. We curve an array to 139.2 mm radius of curvature, resulting in a pixel area coverage of 0.086 sr. Measurements across the curved array revealed minimal variation in bandgap (<0.1 μm) and no appreciable difference in dark current.
High performance infrared focal plane arrays (FPAs) play a critical role in a wide range of imaging applications. However the high cost associated with the required cooling and serially processed die-level hybridization is major barrier that has thwarted Mid-wavelength Infrared (MWIR) detector technology from penetrating largevolume, low-cost markets. Under the Defense Advanced Research Projects Agency (DARPA) WIRED program, the HRL team has developed a wafer level integration schemes to fabricate large format Antimonidebased MWIR FPAs on Si Read Out Integrated Circuit (ROIC) as a means to achieve significant fab cost reduction and enhanced production scalability. The DARPA-hard challenge we are addressing is the thermal and stress management in the integration of two dissimilar materials to avoid detector and ROIC degradation and to maintain structure integrity at the wafer scale. In addition, a digital ROIC with extremely large well capacity was designed and taped-out, in order to increase the operating temperature of the FPAs. In this talk, we discuss our progress under the DARPA WIRED program.
The main driving force for High Operating Temperature (HOT) detectors is the strong need for low cost, compact IR imaging solution capable of supporting a wide range of military and civilian applications. In the HOT regime where imagers can be cooled with multi-stage thermoelectric coolers, the major portion of the cost is due to the die-level back-end process, from the chip hybridization to final packaging. We present here an approach to achieve significant cost reduction of MWIR imagers by monolithically integrating III-V devices directly on Silicon substrates for wafer-scale fabrication and packaging of focal plane arrays (FPAs). High quality InAs films can be grown on a blanket Silicon wafer by metal-organic chemical vapor deposition (MOCVD) in a low growth temperature regime that complies with the thermal budget of the Si-electronics. High Resolution Transmission Electron Microscopy reveals predominantly oriented, single-crystal-like InAs films, with Σ3(111) twin boundaries, which our band structure calculations predict to be electrically benign. More intriguingly, selective-area growth on SiO2-masked ROIC-like templates is demonstrated with single-crystal-like InAs film nucleation at small Si(001) openings, together with the suppression of unwanted deposition on the dielectric mask. High crystallinity lateral epitaxial overgrowth of the InAs islands and film coalescence is achieved, enabling the potential to fully cover the entire patterned substrate. MBE-grown MWIR devices (λcut-off = 4.1 μm) on blanket InAs/Si templates exhibit a dark current of 2x10-5 A/cm2 , a specific detectivity of 6x1011 Jones and a quantum efficiency (QE) above 60% at 100K. The QE remains constant at high temperatures (<200K) where the dark current approaches that of baseline single-crystal HOT devices grown on native substrates At 230K, it is 6x10-2 A/cm2, yielding a specific detectivity of 1010 Jones.
Barrier detectors based on III-V materials have recently been developed to realize substantial improvements in the performance of mid-wave infrared (MWIR) detectors, enabling FPA performance at high operating temperatures. The relative ease of processing the III-V materials into large-format, small-pitch FPAs offers a cost-effective solution for tactical imaging applications in the MWIR band as an attractive alternative to HgCdTe detectors. In addition, small pixel (5-10μm pitch) detector technology enables a reduction in size of the system components, from the detector and ROIC chips to the focal length of the optics and lens size, resulting in an overall compactness of the sensor package, cooling and associated electronics. To exploit the substantial cost advantages, scalability to larger format (2kx2k/10μm) and superior wafer quality of large-area GaAs substrates, we have fabricated antimony based III-V bulk detectors that were metamorphically grown by MBE on GaAs substrates. The electro-optical characterization of fabricated 2kx2k/10μm FPAs shows low median dark current (3 x 10-5 A/cm2 with λco = 5.11μm or 2.2 x 10-6 A/cm2 with λco = 4.6μm) at 150K, high NEdT operability (3x median value) >99.8% and >60% quantum efficiency (non-ARC). In addition, we report our initial result in developing small pixel (5μm pitch), high definition (HD) MWIR detector technology based on superlattice III-V absorbing layers grown by MBE on GaSb substrates. The FPA radiometric result is showing low median dark current (6.3 x 10-6 A/cm2 at 150K with λco = 5.0μm) with ~50% quantum efficiency (non-ARC), and low NEdT of 20mK (with averaging) at 150K. The detector and FPA test results that validate the viability of Sb-based bulk and superlattice high operating temperature MWIR FPA technology will be discussed during the presentation.
We describe our recent results in developing and maturing small pixel (5μm pitch), high definition (HD) mid-wave infrared (MWIR) detector technology as well as focal-plane-array (FPA) hybrids, and prototype 2.4 Megapixel camera development operating at high temperature with low dark current and high operability. Advances in detector performance over the last several years have enabled III-V high operating temperature (T≥150K), unipolar detectors to emerge as an attractive alternative to HgCdTe detectors. The relative ease of processing the materials into large-format, small-pitch FPAs offers a cost-effective solution for tactical imaging applications in the MWIR band. In addition, small pixel detector technology enables a reduction in size of the system components, from the detector and ROIC chips to the focal length of the optics and lens size, resulting in an overall compactness of the sensor package, cooling and associated electronics. An MBE system has been used to grow antimony-based detector structures with 5.1μm cutoff with low total thickness variation (TTV) across a 3” wafer, in order to realize high interconnect yield for small-pitch FPAs. A unique indium bump scheme is proposed to realize 5μm pitch arrays with high connectivity yield. Several 1kx2k /5μm hybrids have been fabricated using Cyan’s CS3 ROICs with proper backend processing and finally packaged into a portable Dewar camera. The FPA radiometric result is showing low median dark current of 2.3x10-5 A/cm2 with > 99.9% operability, and >60% QE (without AR coating).
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