Comparison of X-Ray and Electron Radiation Effects on Dark Current Non-Uniformity and Fluctuations in CMOS Image Sensors

Abstract

This article investigates the dark current as well as the dark current random telegraph signal (RTS) after 1-MeV electron, 3-MeV electron, and 10-keV X-ray irradiations in a pinned photodiode CMOS image sensor (CIS). A large range of deposited ionizing dose from 10 to 525 krad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is considered. The displacement damage dose deposited through electron irradiation ranges from 60 to 1200 TeV · g <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Results on dark current distributions highlight the predominance of the ionizing damage in opposition to the displacement damage induced by the electron irradiations. Moreover, the dark current distributions also suggest that if the ionizing dose is high enough [i.e., beyond 50 krad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )], the trapped positive charges in the silicon oxides create high magnitude electric field regions leading to an electric field enhancement (EFE) of the dark current which is neither present at lower doses nor in pristine image sensors. This EFE mechanism also seems to have a strong influence on the RTS leading to a clear discrepancy from the existing dark current nonuniformity model developed for amplitude distributions in CISs as well as from what is reported in the literature in the more studied ionizing dose range. Annealing treatments after electron irradiations have highlighted the existence of specific population of pixels sharing the same well-defined maximum transition amplitudes (i.e., maximum amplitude between two dark current levels). The results suggest the use of maximum transition amplitude spectroscopy applied to dark current RTS to push forward the investigation on radiation-induced defects creation and identification.