Infrared satellite sensors must be recalibrated in orbit using blackbodies small and light enough to fit on spacecraft. Emissivities very close to one can be achieved by observing closed interiors along rays making multiple reflections off surfaces that are both highly absorbing and specularly reflective. We analyze the effect of slightly nonideal specularly reflective surfaces having a small but finite diffuse reflectivity, showing how it limits the performance of a three-reflection specular blackbody.
The National Polar-orbiting Operational Environmental Satellite System (NPOESS), is overseen by the Integrated Program Office (IPO), a joint effort of the Department of Defense, Department of Commerce and NASA. One of the instruments on the NPOESS satellite is the Cross-track Infrared Sounder (CrIS) instrument. CrIS is a Fourier Transform interferometric infrared (FTIR) sensor used to measure earth radiance at high spectral resolution to derive pressure, temperature, and moisture profiles of the atmosphere from the ground on up. Each CrIS instrument contains three different cutoff wavelength (λ<sub>c</sub>)focal plane modules (FPMs): an SWIR FPM [λ<sub>c</sub>(98 K) ~ 5 mm], MWIR FPM [λ<sub>c</sub>(98 K) ~ 9 mm] and a LWIR FPM [λ<sub>c</sub>(81 K) ~ 15.5 mm]. There are nine large (850 mm diameter) photodiodes per FPM, the nine detectors being arranged in a 3 x 3 array. The nine detectors are placed under tight tolerances in the X, Y, and Z dimensions. The steps involved in the transfer of photodiodes as part of a newly fabricated wafer to the mounting of the photodiodes on the FPM involves many processing steps including a significant amount of dicing, cleaning, wire bonding and baking at elevated temperatures.
Quantum efficiency and 1/f noise in Hg<sub>1-x</sub>Cd<sub>x</sub>Te photodiodes are critical parameters that limit the sensitivity of infrared sounders. The ratio α, defined as the noise current in unit bandwidth i<sub>n</sub>(f = 1 Hz, V<sub>d</sub>, Δf = 1 Hz) to the dark current I<sub>d</sub>(V<sub>d</sub>), that is, α = i<sub>n</sub>/I<sub>d</sub> is one of the parameters used to select photodiodes for placement in FPMs. α is equivalent to √α<sub>H</sub>/N that appears in the well-known Hooge expression. For the sixty-one, λ<sub>c</sub> ~ 9 μm photodiodes measured at 60 mV reverse bias and at 98 K, the average value of α<sub>dark</sub> = 1.3 x 10<sup>-4</sup> in the dark and α<sub>PHOTO</sub> = i<sub>n</sub>/I<sub>PHOTO</sub> is ~ 2 x 10<sup>-6</sup> under illuminated conditions. These values of α are a factor of two lower than that reported previously. The λ<sub>c</sub> ~ 15.5 μm photodiodes have average α<sub>dark</sub> = 1.3 x 10<sup>-5</sup> with the highest performance, diffusion current limited photodiodes having values of α<sub>dark</sub> in the mid 10<sup>-6</sup> range. All of the 850 μm diameter, λ<sub>c</sub> ~ 15.5 μm photodiodes measured have excess low frequency noise, with the best performers having i<sub>n</sub>(f = 100 Hz, V<sub>d</sub> =-60 mV , Δf = 1 Hz) ~ 2 x 10<sup>-11</sup> A/Hz<sup>1/2</sup> and the best photodiode α<sub>dark</sub> = 3.92 x 10<sup>-6</sup>.
I-V measurements, noise, and visual inspections are performed at several steps in the photodiodes manufacturing process. It was observed, following FPM fabrication, photodiode dark current and noise had increased from the initial pre-mounting leadless chip carrier (LCC) measurements for some of the nine photodiodes. The performance degradation observed led to an investigation into the cause (baking at elevated temperatures, mechanical handling, electrical stress etc.) of photodiode degradation that occurred between LCC and FPM testing. Correlations between I-V, noise and surface visual defects have been performed on some λ<sub>c</sub> ~ 15.5 mm photodiodes. This paper outlines the results of the study, correlating the electrical performance observed to visual defects on the surface and to defects seen following cross sectioning of degraded photodiodes. In addition, other lessons learned and the corrective actions implemented that led to the successful manufacture of SWIR, MWIR and LWIR large photodiodes from the material growth to insertion into and successful demonstration of flight FPMs for the CrIS program are described.
The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Cross-track Infrared Sounder (CrIS) is a Fourier Transform interferometric sensor that measures earth radiances at high spectral resolution. Algorithms use the data to provide pressure, temperature, and moisture profiles of the atmosphere. The CrIS instrument contains photovoltaic detectors with spectral cut-offs denoted by SWIR, MWIR and LWIR. The CrIS instrument requires large-area, photovoltaic detectors with state-of-art detector performance at temperatures attainable with passive cooling. For example, detectors as large as 1 mm in diameter are required. To address these needs, Molecular Beam Epitaxy (MBE) is used to grow the appropriate bandgap n-type Hg<sub>1-x</sub>Cd<sub>x</sub>Te on lattice matched CdZnTe. The p-side is obtained via arsenic implantation followed by appropriate annealing steps.
The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Cross-track Infrared Sounder (CrIS) is a Fourier Transform interferometer-based sensor used to measure earth radiance at high spectral resolution and low spatial resolution. Measured radiance data are analyzed by end users to provide pressure, temperature and moisture profiles of the atmosphere. The CrIS instrument contains Mercury-Cadmium-Telluride (MCT) photovoltaic (PV) detectors with spectral response in the SWIR, MWIR and LWIR ranges. The CrIS instrument requires large area detectors with state-of-the-art detector performance at temperatures attainable with passive cooling. In the case of the LWIR bands noise associated with the detectors limit the instrument performance. In this paper we describe a study of the noise characteristics of a sample of CrIS MCT PV detectors, emphasizing acquisition and validation of 1/f noise measurements for these devices. Interesting aspects of the 1/f noise dependence on bias-voltage and bias-current are noted. The results are analyzed further in a companion paper1 that emphasizes the relationship between leakage current mechanisms in the diodes and 1/f noise observed.
For many terrestrial IR imaging applications, speed and sensitivity requirements can be met by several sensor technologies, including PtSi staring arrays, second generation scanned MCT arrays, InSb, and MCT staring arrays. However, for high quantum efficiency staring arrays, MCT and InSb in particular, much of the available signal remains unused because of well-fill restrictions. In narrow spectral band imaging applications the available photon flux is dramatically lower and high quantum efficiency becomes critical. One such application is detection and imaging of gases. High quantum efficiency arrays allow the use of narrow band filters centered on the absorption band of the gas, thus enhancing the ability of the imager to discriminate between background and gas emission. Results of imaging studies performed on a camera with a narrow band filter for gas detection are presented and analyzed.
As the emphasis in infrared detector research shifts toward larger and more complicated arrays the amount of time spent on simple single-element and small arrays is decreasing. One set of applications where discrete detectors and arrays are still finding use is in satellites. In addition, scanned imaging arrays based on single element detectors and small arrays are still being manufactured. Discussion here is for small arrays and single element detectors. One of the aspects of detector operation that always needs to be addressed is amplification. Often detectors are attached to amplifiers through rather long leads. Such systems are subject to unwanted microphonic response as a result of the motion of the leads relative to each other or to the ground plane. This sort of microphonic response can many times be eliminated through careful wiring and routing techniques, however, in some severe environments it is not possible to eliminate all microphonic response. A commonly used solution to this problem is to hybridize the detector with a JFET front end to reduce the effective output resistance of the detector circuit relative to the amplifier input. The TIA in such configurations is completed off the focal plane at room temperature. This means that half the circuit is operating at cryogenic temperatures while the other part is operating at room temperature some distance away. Ideally it would be more convenient, if not better, to include the amplifier on the focal plane with the detector. (Of course this hybridization is necessary for large two-dimensional arrays.) Data have been acquired to show some of the limitations and opportunities for such an approach. Typical bipolar operational amplifiers (OP-27, OP-37, LM108) will not operate well at cryogenic temperatures. CMOS operational amplifiers generally will operate at cryogenic temperatures but suffer from high front-end voltage noise. The TLC2201 from Texas Instruments is a CMOS op-amp manufactured for low voltage noise. A discussion of its applicability to IR detector operation is presented herein.