AIRS

Focal Plane Assembly

The AIRS IR Focal Plane Assembly (FPA) represents a major advance in infrared technology, incorporating a number of state of the art features in a space qualified, high reliability configuration. Key to its operation is an advanced hybrid PV/PC HgCdTe focal plane consisting of 10 PV modules and 2 PC modules each individually optimized for a particular grating order/wavelength range (see diagram).

The PV modules consist of 1, 2 or 4 bi-linear arrays of back-side-illuminated HgCdTe detectors, each bump mounted to a low power, 1.2 m CMOS read-out integrated circuit (ROIC) that provides the first stage of signal integration and multiplexing. The 10 PV modules contain a total of 4208 detectors multiplexed down to 26 outputs, with each PV module having a unique set of requirements and constraints in terms of wavelength coverage, signal and background flux, and sensitivity (D*). Technologically, the most stressing are the long wavelength modules, particularly M10 which covers the band from 12.7 m to 13.7 m and required a detector material cutoff wavelength of 15.6 m. Considerable development in the areas of LWIR detector material growth, radiation tolerant ROIC design and fabrication, and detector/ROIC interconnect was required to satisfy system requirements. To mitigate risk in the longest wavelength region, 13.7 m to 15.4 m, PC HgCdTe detector technology was used. This spectral region is covered in two modules with individual leads for each of the 274 low impedance PC detectors, a distinct disadvantage attendant with this technology.

The set of 12 modules is mounted to a common ceramic motherboard. The motherboard contains a complex arrangement of power, command and control, and signal interconnects to all PV modules as well as individual lead-outs for the PC detectors. The AIRS FPA is unique in its hybrid PV/PC approach and required special care in the routing, shielding and grounding of very low noise (nV) PC signals in the presence of high level (V) PV signals. A total of 526 leads interconnect to the motherboard assembly using a series of 10 high-density, thin-film flex cables specifically designed for cryogenic operation. Modules are individually assembled, tested and positioned onto the motherboard using computer controlled stages to maintain the requisite optical alignment tolerances of +/-15 m. Overlaying the focal plane module is an IR bandpass filter assembly containing 17 individual filters used for grating order selection as well as background suppression. The filter set is precisely registered to the focal plane using a dark mirror coated frame for stray light control. The assembly also includes a cold shield and a field flattener lens, which is part of the Schmidt exit optics.