The Atmospheric Infrared Sounder (AIRS) is a key facility instrument in the NASA Earth Observing System (EOS) program, being implemented to obtain comprehensive long-term measurements of earth processes affecting global change. The instrument performs passive IR remote sensing using a high resolution grating spectrometer with a wide spectral coverage (3.7 15.4 μm) directing radiation onto a hybrid HgCdTe IRFPA operating at 58K in a vacuum package cooled to 155K.

AIRS is a key facility instrument in the NASA Earth Observing System (EOS) program, a spaceborne, global observation system being implemented to obtain comprehensive long-term measurements of earth processes affecting global change. Designed to provide new and more accurate data about the atmosphere, land, and oceans for application in climate studies and weather prediction, AIRS performs passive IR remote sensing using a high resolution grating spectrometer with a wide spectral coverage focal plane assembly (FPA) operating at 58 K in a unique vacuum dewar package cooled to 155 K. The hybrid HgCdTe focal plane consists of 12 modules, 10 photovoltaic (PV) utilizing silicon readout integrated circuits (ROICs) in both direct and indirect hybrid configurations, and 2 photoconductive (PC) led out to warm electronics. This complex focal plane has a large optical footprint, 53 mm X 66 mm, and receives energy dispersed from the grating through a precision filter assembly containing 17 narrowband filters. Designed to prevent any interaction between the PV and PC devices, the FPA incorporates extensive shielding and lead routing in the multilayer carriers and flex cables, as well as features in the ROIC design. The 526 lines necessary to operate the FPA are led out of the vacuum dewar, which is cooled via the spectrometer. The focal plane is cooled to 58 K through a sapphire rod interfaced to a pulse tube cooler. The Engineering Model (EM) and Protoflight Model (PFM) detector/dewar assemblies have been fabricated, assembled, tested, and delivered for system integration, and the EM instrument has been assembled and tested. The key design features of the FPA and dewar assembly have been presented in previous SPIE symposiums and will be briefly reviewed. In this paper the emphasis will be on performance results such as sensitivity, linearity, assembly tolerances, environmental test results, and other parameters of interest, as well as a detailed review of the actual flight hardware assembly.

The Atmospheric Infrared Sounder (AIRS) is a high resolution IR spectrometer (lambda/Delta lambda congruent to 1200) which will map global temperatures and identify atmospheric aerosols from orbit by monitoring key atmospheric absorption lines. The focal plane consists of ten bilinear photovoltaic (PV) and two photoconductive (PC) HgCdTe detector arrays (modules) sampling a 3.7 to 15.4 mu m spectral window in 15 bands. To attain the desired temperature accuracy, tight constraints on focal plane performance parameters such as linearity better than 0.1%, quantum efficiency (QE) on the order of 70%, low noise or noise equivalent quantum flux density (NEQFD), and no outages at key spectral lines have been imposed. Assessment of focal plane performance begins at the detector and readout levels where flight candidate detector arrays and CMOS readouts are selected. PV detector arrays and their readouts are hybridized (PC modules are wire-bonded directly) into modules which are then individually tested under simulated flight conditions. Five of the twelve module types are incorporated into an engineering-level (EM) focal plane upon which the module level tests are repeated as a prelude to the fabrication and testing of a separate, fully populated, flight-level (PFM) focal plane. Module testing has demonstrated that many difficult system requirements have been met, and work continues to optimize module performance. Lockheed Martin IR Imaging Systems'(LMIRIS) overall design of the infrared (IR) Detector/Dewar assembly and focal plane development program is given, followed by a summary of PV and PC module data.

Higher resolution and wider IR spectral coverage is needed to improved infrared sounding instruments. The Atmospheric Infrared Sounder (AIRS), chosen by NASA to fly on the Earth Observing System, addresses these needs with advanced PV HgCdTe detector arrays designed to cover the spectral range from 3.7 micrometers to 13.6 micrometers with an average resolution of (lambda) /(Delta) (lambda) equals 1200. High performance detectors and advanced readout integrated circuit electronics make it possible to meet mission requirements. For convenience, the AIRS focal plane has been partitioned into four MWIR modules spanning the spectral range from 3.7 micrometers to 8.22 micrometers , and six LWIR modules for wavelengths above 8.8 micrometers . This paper focuses on the AIRS readout device and recent developments in p-on-n heterojunction detector technology at Loral. The detector arrays, operating at 60 K, readily satisfies the requirements of the AIRS instrument. Detector arrays with 4.7 micrometers cutoff wavelength at 60 K and 20 mV reverse bias have RdAs typically greater than 1010 (Omega) (DOT) cm2, with dark signals less than 0.6 fA and detector capacitances less than 0.6 pf for a 50 micrometers by 10 micrometers detector. AR coated MW arrays exhibit quantum efficiencies of greater than 80 percent. Reverse breakdowns are more than -150 mV. Module data for 15.1 micrometers detectors with anti-reflection coating exhibit quantum efficiencies greater than 70 percent and dark currents less than 8 nanoamps at 20 mV reverse bias. Also, excellent module linearity meeting the AIRS stringent requirements is achieved. Of course, measurements of MW detectors require extremely high gain transimpedance amplifiers. The AIRS MWIR readout structures prove to be exceptional in their ability to characterize these high impedance detectors. The charge sensitive input amplifiers on these readout devices utilize an equivalent input integration capacitor of less than 10 fFd to achieve ultrahigh transimpedance gain, and reset noise is suppressed with on focal plane correlated double sampling. LWIR readouts use ultralow noise buffered direct injection preamplifiers. The readouts have a robust architectures with differential input and outputs to minimize EMI and built in redundancy for survivability. Description of the readout device is presented, as well as linearity measurements of both the readout and complete modules.

AIRS is a key facility instrument on the first post meridian platform as part of NASA'a Earth Observing System (EOS) program. The Atmospheric Infrared Sounder measurement technique is based on passive IR remote sensing using a high spectral resolution grating spectrometer. The structure of the infrared focal plane for the AIRS instrument has been defined and is presented in this paper. The optical footprint of 8.1 mm by 36.3 mm along with the necessary support and interface components leads to a focal plane assembly of 53 mm by 66 mm, the largest ever built at LIRIS. With 4208 diodes and 274 photoconductors in the same focal plane to achieve the wide spectral coverage from 3.7 to 15.4 micrometers , a modular approach is required. Ten PV modules utilize silicon readout integrated circuits (ROICs) joined to the detector arrays as either direct or indirect hybrids while two PC modules cover the 13.7 to 15.4 mm range, optically chopped and led out to uncooled preamplifiers. The simultaneous operation of PV and PC devices in the same focal plane has required unique approaches to shielding, ROIC output design and lead routing. High D*'s of 7E14 and 3E11 cm- Hz1/2/W are needed to meet the sensitivity requirements of the 4.2 and 15.0 micrometers regions respectively. The 35 micrometers by 800 micrometers PC detectors on a 50 micrometers pitch have necessitated modifications to standard delineation techniques, while the MW performance is nearly D* BLIP for PV devices. Dispersed energy is presented to the modules through 17 narrow band filters packaged into a single precision assembly mounted within 0.18-0.25 mm of the focal plane surface. The more than 50 components comprising the focal plane in conjunction with the tightly spaced optical pattern presented by the grating add a high degree of complexity to the assembly process. This paper focuses on the architectural constraints derived from performance, interface, and reliability requirements. Key aspects of these requirements are presented and their impact on the partitioning of the 12 modules is discussed. The rationale for the spectral range assigned to each module is reviewed relative to PV and PC performance capabilites. ROIC design guidelines and physical contraints due to manufacturability and assembly. Results of structural and thermal analyses for the various module configurations and assembled focal plane to determine compliance with the stringent stability and positional requirements are presneted. Specific features of the module carrier/interface boards and the multilayered focal plane carrier/interface board are included as well as a review of the overall assembly sequence of the focal plane as influenced by repairability and reliability considerations. The comprehensive redundance strategy applied to the design of the FPA/dewar assembly will be reviewed, and the approach for operation/survival in the radiation environment is discussed. Key features of the ROIC, PV, and PC array designs will be presented along with results of analyses performed.

A class AB CMOS output buffer has been designed for use on an IR focal plane array. Given the requirements for power dissipation and load capacitance a class A output, such as a source follower, would be unsuitable. The approach taken uses a class AB amplifier configured as a charge integrator. Thus it converts a charge packet in the focal plane multiplexer to a voltage which is then the output of the focal plane. With a quiescent current of 18 micro-a and a load capacitance of 100 pf, the amplifier has an open loop unity gain bandwidth of 900 khz. Integral nonlinearity is better than .03 percent over 5.5 volts when run with VDD-VSS = 6v.

A direct gate coupled input circuit between a 1.0 to 2.5 micrometer photovoltaic mercury-cadmium telluride detector and a CCD multiplexer is proposed for low background applications. Theory predicts a D*λpeak on the order of 1012 cm Hz1/2/W when thephotodiode is at 200 K. Laboratory measurements support the theoretical analysis.