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18th of April 2017

APSOLUTE

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Complementary metal oxide semiconductor (CMOS) active pixel sensor (APS) prototypes made using 0.18 μm technology process have been developed for the Extreme Ultraviolet Imager (EUI) of the Solar Orbiter mission. For EUI instrument, a UV CMOS Active Pixel Sensors (APS) prototype called APSOLUTE (APS Optimized for Low-noise and Ultraviolet Tests and Experiments) was developed with CMOSIS (Be), the Centre Spatial de Liège and the STCE/Royal Observatory of Belgium.

APSOLUTE is a back-side illuminated (BSI) CMOS image sensor made in 0.18 μm CMOS process with 10 µm pixel pitch pinned photodiode. The EUV sensitivity is achieved with backside illumination on a Silicon-on-Insulator material based solution. The epitaxial silicon layer is thinned down to less than 3 μm.

Two types of image sensors have been built: a 256x256 pixel sensor containing 16 pixel variants, organized in blocks of 64x64 pixels, and a 1024x1024 pixel sensor that implements the ‘best guess pixel’ out of the 16 variants.

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Apsolute prototype (256x256) contains 16 pixel variants organized in blocks of 64x64 pixels

The sensor applied a so-call “dual-transfer” scheme to achieve high dynamic range and to minimize the noise contribution from the analog chaine. Two gain (low and high) paths are designed for all pixels in each row. The signal with lower conversion gain can be amplified and being added to the high gain signal off-chip to reconstruct the final image.

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Images of the Apsolute prototypes (1024x1024) showing the dual gain operation (Low and High gain) and the final image after reconstruction

A set of measurement calibration campaigns and radiation tests leaded by the STCE/ROB researchers have been carried out to characterize and compare the pixel variants, and reach a technology readiness level (TRL5) to assess the maturity of evolving technologiescompatible with the overall EUI project development schedule.

The first results of the XUV-to-VIS sensitivity calibration, the noise level measurement and the radiation hardness were performed in 2011 by the STCE/ROB researchers. BSI imagers have been characterized before, during and after the following radiation tests:

  • Total Ionization Dose (TID), using Co60 gamma-ray sources up to 150 kRad(SiO_2),
  • Single Event Effect (SEE) tests with ions cocktails up to a LET of 67.7 MeV.cm^2/mg,
  • Displacement Damages (DD), performed with proton up to 15MeV and a cumulated fluence of 4 x 10^{11}protons/cm^2.

This includes also EUV calibration campaign performed at the PTB/BESSY II synchrotronin Berlin (Germany) by the STCE/ROB team.

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EUV calibration at PTB/BESSY II

The APSOLUTE characterization tests were mainly performed at DeMeLab (STCE/ROB) in the visible range on the back side illuminated (BSI) Apsolute prototype(s) in the frame of the pixel design selection. All measurements have been repeated several times to check the stability and the reproducibility of the prototypes. Sequential acquistion software and data processing tools have been developed by the STCE/ROB researchersto estimate the detector parameters.

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APSOLUTE 1024x1024 pixels format back side illuminated