On-area wire bonding of HgCdTe photoconductive (PC) infrared detectors usually causes material damage underneath the bonding pads. Such damage may propagate into the active area, potentially degrading performance and posing long-term reliability problems. Some off-area bonding fabrication techniques can also induce some degree of semiconductor material damage. In this paper, we report a relatively straightforward off-area bonding scheme that solves these problems. The process uses multiple photolithography and chemical etching steps to create a continuous slope or staircase in the HgCdTe leading down to the epoxy surface. The staircase ensures smooth step coverage for the subsequent metalization. Tri-layer photolithography (resist /metal/resist) and reactive ion etching (RIE) is then used to remove the epoxy in the bonding pad area. Since all areas other than the pad region are protected by the metal film, no RIE radiation damage is induced to the active area. The contact metalization is achieved by using standard liftoff techniques. Our so called staircase off-area bonding fabrication technique can be used to fabricate highly reliable, high density, small-size, detector arrays.
ABSTRACFThe Composite Infrared Spectrometer ((iRS) instrument, an important component of the Cassini Mission, consists of 3 focal plane arrays (FPAs) for sensing JR radiation of the Satumian planetary system. Goddard Space Flight Center has fabricated, tested, and delivered high performance, 10.element HgCdTe photoconductive (PC) arrays for use on CIRS FP3, the focal plane responsible for detection of radiation in the 9.1 to 16.7 jim spectral band. The delivered flight array has peak responsivity 100% above CIRS specification, detectivity 30% or more above specification, and a cutoff wavelength of 17.3 .tm at the operating temperature of 80 K. In order to achieve high performance at low frequency (25 Hz to 46 Hz) while maintaining limited power dissipation (< 0.5 mWattlelement), we adopted a splitgeometry detector structure. This design also ensured the buttability of the PC arrays to Photovoltaic (PV) arrays supplied by CE-Saclay-France for detection of radiation in the 7.1 to 9.1 .tm range. The detector structure is also noteworthy for its use of 0.05 pun Alumina powder-loaded epoxy to minimize reflection at the epoxylHgCdTe interface, thus spoiling undesired optical resonance. This was done in order to meet the CIRS spectral uniformity requirement, which would have been difficult at these long wavelengths without this feature.
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