Development of a high-fill-factor thermopile array for space-borne far-infrared radiometry

Thermal radiometers are crucial for space-based infrared observations. Thermopile detectors are uncooled thermal detectors that offer advantages over bolometers, such as broadband capability, diffraction-limited pixels, low 1/f noise, and insensitivity to temperature drifts. However, thermopiles do not currently have the format size and fill factor to enable sufficient spectral and spatial resolution for demanding future missions, such as for far-infrared remote sensing of the Earth, Moon, and outer planets of the Solar System. The fill factor needs to be increased to minimize the space between pixels where infrared radiation is not absorbed to enable an instrument with true imaging capability that does not require multiple passes to scan a scene. Compared to the state-of-the-art detector used in Polar Radiant Energy in the Far-Infrared (PREFIRE), the detector we present in this work has an 8x larger format size with diffraction-limited pixels. The two main challenges in increasing the fill factor in the current detector architecture are to decrease the width of the wiring bus between pixel rows and to increase the length of the absorber structure compared to what has been previously done at JPL. We demonstrate the successful fabrication of a multi-tiered wiring bus that effectively halves the spacing between pixel rows compared to using a wiring bus on a single plane. In future work, we plan to develop a process to fabricate larger absorber structures and integrate the multi-tiered wiring bus for an expected near unity fill factor, where the entire image plane is tiled with diffraction-limited pixels.