The effects of X-ray irradiation on the random noises, especially the random telegraph noises (RTN), of a 45-nm on 65-nm stacked CMOS image sensor with 8.3M 1.1 μm pixels are investigated. It is found that before X-ray irradiation the dominant type of RTN among the noisiest pixels is the source follower (SF) MOSFET channel RTN. In contrast, after X-ray irradiation up to a total ionizing dose of 1 Mrad(SiO 2 ), the RTN becomes dominated by the variable transfer-gate-induced sense node (SN) leakage. These two different types of RTN can be distinguished by their dependence on the transfer gate (TG) OFF voltage and the time between the correlated double sampling (CDS). The magnitude of the RTN from the variable SN leakage is proportional to the CDS time and can be suppressed effectively by increasing the TG OFF voltage, whereas the SF RTN is independent of the CDS time or the TG OFF voltage.INDEX TERMS Random noise (RN), random telegraph noise (RTN), random telegraph signal (RTS), CMOS image sensor (CIS), pinned photodiode (PPD), active pixel sensor (APS), correlated double sampling (CDS), gate induced drain leakage (GIDL), variable junction leakage (VJL), variable retention time (VRT), radiation damage, X-ray, total ionizing dose (TID).

A study of the random telegraph noise (RTN) of a 1.1 μm pitch, 8.3 Mpixel CMOS image sensor (CIS) fabricated in a 45 nm backside-illumination (BSI) technology is presented in this paper. A noise decomposition scheme is used to pinpoint the noise source. The long tail of the random noise (RN) distribution is directly linked to the RTN from the pixel source follower (SF). The full 8.3 Mpixels are classified into four categories according to the observed RTN histogram peaks. A theoretical formula describing the RTN as a function of the time difference between the two phases of the correlated double sampling (CDS) is derived and validated by measured data. An on-chip time constant extraction method is developed and applied to the RTN analysis. The effects of readout circuit bandwidth on the settling ratios of the RTN histograms are investigated and successfully accounted for in a simulation using a RTN behavior model.

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In this paper we present a systematic approach to sort out different types of random telegraph noises (RTN) in CMOS image sensors (CIS) by examining their dependencies on the transfer gate off-voltage, the reset gate off-voltage, the photodiode integration time, and the sense node charge retention time. Besides the well-known source follower RTN, we have identified the RTN caused by varying photodiode dark current, transfer-gate and reset-gate induced sense node leakage. These four types of RTN and the dark signal shot noises dominate the noise distribution tails of CIS and non-CIS chips under test, either with or without X-ray irradiation. The effect of correlated multiple sampling (CMS) on noise reduction is studied and a theoretical model is developed to account for the measurement results.

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