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Temporal fluctuations have recently been discovered in the dark current of protondisplacementdamaged CCDs. These fluctuations take the form of random telegraph signals with well defined amplitudes and time constants (for the high and low dark current states). Temperature and annealing behaviour have been studied, as has the dependence on proton fluence. A bistable defect mechanism is proposed.

A new automated method able to detect multi level random telegraph signals in pixel arrays and to extract their main characteristics is presented. The proposed method is applied to several proton irradiated pixel arrays manufactured using a 0.18 μm CMOS process dedicated to imaging. Despite the large proton energy range and the large fluence range used, similar exponential RTS amplitude distributions are observed. A universal mean amplitude is extracted from these distributions and the number of RTS defects appears to scale well with total NIEL. These conclusions allow the prediction of RTS amplitude distributions. The effect of electric field on RTS amplitude is also studied and no significant relation between electric field and RTS amplitude is observed. Index Terms-Random telegraph signal, RTS, proton irradiation, CMOS image sensors (CIS), active pixel sensors (APS).

An experimental batch of p-buried channel CCDs has been fabricated and characterised for proton-induced radiation damage. Dark current effects were similar to conventional nchannel CCDs, but radiation-induced changes in charge transfer inefficiency were reduced by approximately a factor 3 for -30°C operation and background signal -2,000 electrons/pixel; though this is a lower limit and further reduction may be possible in future CCD batches.