Future missions to outer planets will have stringent limits on payload mass. Thermal imaging instruments to map planetary surfaces will be part of those payloads, and, consequently, will have to be compact and low mass. For thermal instruments, another key requirement will be state-of-the-art, highly sensitive detectors. Thermopiles are prime candidates for high-resolution thermal mapping in the far-infrared (17- to 250-μm wavelength) spectral range. Thermopile detector arrays can be made to be very lightweight and compact. Furthermore, they require very few ancillary components (e.g., readout electronics, optics, amplifiers), which can add to instrument volume and mass. The implementation of thermopiles on these missions is likely because they (1) generate an output voltage that is proportional to the incoming radiation within the spectral range being mapped; (2) do not require an electrical bias or an optical chopper; (3) have negligible 1/f noise; (4) are radiation hard; and (5) have a reported specific detectivity of 1 × 109 cm·Hz1/2/W at room temperature.
The focus of this work is to fabricate thermopiles using unique semi-metallic materials developed in the Detector Development Laboratory (DDL). Novel semimetallic materials, which have been developed for other projects, were evaluated in terms of a thermoelectric figure of merit, as candidate thermopile materials. Intermetallic thermopiles, which have high Seebeck coefficients and, consequently, high detector sensitivity, require highly specialized fabrication techniques and are susceptible to aging. Semiconductor thermopiles have a limited operating range and are susceptible to high radiation environments. An alternative approach can address these issues, but, in the former case, they are not radiation hard, or in the latter case, they are unstable to environmental conditions (temperature, oxygen). The advantage of using semimetallic materials is that they are radiation hard and their thermoelectric properties do not change appreciably in the temperature range of interest.
This work was done by Ari Brown, Emily Barrentine, and Shahid Aslam of Goddard Space Flight Center. GSC-16837-1