Radiometric packaging of uncooled microbolometer FPA arrays for space applications

Abstract

INO has extensive experience in the design and fabrication of focal plane arrays (FPAs) of uncooled microbolometers. In particular, the FPA of 512×3 microbolometers, developed in collaboration with the Canadian Space Agency (CSA), has been selected for use in the NIRST (New Infrared Sensor Technology) radiometer of the SAC-D Aquarius mission. The FPA has been designed for pushbroom scanning of the Earth to provide radiometric data in the mid- and long-wave infrared for the monitoring of fires as well as thermal mapping of ocean temperature. Uncooled microbolometer detectors are suited for space applications due to their low power consumption while still exhibiting adequate performance. Furthermore, the spectral range of their response could be tuned from the mid- to the far-infrared to meet different mission requirements. In order to ensure that the detector receives only the thermal contribution from the desired target and to minimize radiometric error due to variation of the temperature of the surrounding during the measurements, a radiometric package is required. In a radiometric package the detector environment is thermally stabilized by means of a temperature controlled radiation shield. The radiation shield should also be designed to prevent stray radiation from reaching the detector. Under the Space Technology Development Program of the CSA, INO has designed, assembled and tested a radiometric package in order to characterize its performance and compatibility with the space environment. The operating spectral band is defined by the spectral characteristics of a bandpass filter placed in front of the FPA. For typical space missions, the package must pass standard environmental tests without degradation of its performance (thermal cycling from -55 to +85 °C according to MIL-STD-810, random acceleration up to 14 G RMS from 20-2000 Hz and shock up to 75 G). In order to ensure reliability in those conditions while maintaining optimum performance, an adequate selection of materials is necessary. In this paper, INO's radiometric packaging technology for uncooled microbolometer FPA's will be presented. The selection of materials will be discussed and the final choices presented based on thermal simulations and experimental data. The effects of different design parameters on the performance, such as material, shape and thickness of radiation shield and choice of adhesive have been studied. An instantaneous noise equivalent temperature difference (NETD) of ~ 20 mK was obtained under the measurement conditions (broadband LWIR, 140 ms integration time, f/1 optics, characterization in flood exposure). The design of the package reduced the contribution of environmental temperature variations on the offset of the sensor. The equivalent response of the package varied less than 0.08 °C per degree of variation of the temperature of the package. The package also showed low sensitivity to stray radiation as a result of the effectiveness of the radiation shield design. The device successfully passed the prescribed environmental tests without degradation of performance.