The design of a UV CMOS sensor for the ULTRASAT space telescope

ULTRASAT is a scientific satellite carrying a near UV telescope with high sensitivity and large field of view. The mission, led by the Weizmann Institute of Science and the Israeli Space Agency in collaboration with Deutsches Elektronen-Synchrotron (DESY), is expected to be launched in 2025 by NASA. ULTRASAT will revolutionize our understanding of the high energy transient universe and will have a broad scientific impact across the fields of gravitational wave (GW) sources, supernovae, variable and flare stars, active galactic nuclei, tidal disruption events, compact objects, and galaxies. The ULTRASAT camera, developed by DESY, is based on a custom imager, developed by Analog Value and Tower Semiconductors, and manufactured by Tower Semiconductor in Israel. The focal plane array is composed of four independent sensors with an effective area of ~45 x 45 mm² each, providing graceful degradation capabilities. Each sensor array has ~22,450M pixels, with pixel size of 9.5 x 9.5 μm². The sensor implements backside illumination and an anti-reflective coating optimized for the wavelength range 230 to 290nm, achieving high quantum efficiency of >60%. Low dark noise of <0.026 e/sec/pixel is needed to meet the required sensitivity. This is achieved by both operating at - 73^oC, and by a special design including advanced pixel architecture, optical trench between the array of pixels and surrounding circuits and a low noise design of the digital electronics. High dynamic range (HDR) capability is achieved by dual gain 5T pixels. The design of the sensor includes one analog to digital converter (ADC) per column architecture and low voltage differential signal (LVDS) output buffers. The operation is controlled by a dedicated microcontroller and configuration registers. The design employs techniques for mitigating space radiation effects, enabling an operation lifetime of 6 years in GEO.