Performance evaluation of an SOI pixel sensor with in-pixel binary counters

Abstract

Pixel detectors fabricated with the silicon-on-insulator (SOI) technology suffered from the digital pickup, due to the capacitive coupling between the sensing electrode and the in-pixel circuit. In order to tackle this issue, an advanced process called double SOI has been developed. A prototype chip CPIXTEG3b adopting this new process was designed and characterized. While optimization concerning the double-SOI design and testing of the single pixel were already presented in a separate publication, this paper focuses on its noise performance of the full matrix and X-ray detection utilizing a synchrotron photon beam. Equivalent noise charge (ENC) of the full pixel matrix was measured with electrical pulse tests. The threshold dispersion was minimized by the DAC tuning in each individual pixel. As a photon-counting X-ray imager, noise count accumulated up to 1 h was measured. Detection efficiency was measured with a micro-focused beam as well as a flat field generated by the X-ray scattering on a glassy carbon at the KEK PF beam line 14A. The typical ENC is $$52~ \hbox {e}^{-}$$ and the sigma of threshold dispersion is $$10 \,\hbox {e}^{-}$$ over the full matrix. A merit of “zero” noise count is also demonstrated, which is consistent with the low ENC. The prototype chip has been tested with microbeam and used to measure the beam profile to be with a full width of $$50~\upmu \hbox {m}$$ at 2.4% of the maximum height. Both sensor depletion and charge sharing between neighboring pixels have been carefully characterized, providing insights for further development. The homogeneity of response to X-ray photons has been demonstrated in the flat field test. This work has drawn a final conclusion to the solution of digital pickup issue and opened a promising prospect in low-noise and high-resolution X-ray imaging.