Charge-coupled devices (CCDs) currently constitute the standard detector for precision astronomical telescopes such as the Large Synoptic Survey Telescope (LSST) due to their high linearity, sensitivity and dynamic range. Charge transfer properties can however be degraded by the presence of defect levels in the silicon band-gap, which can act as trapping centers for signal charge. The technique of single trap-pumping can be used as a tool to probe the underlying properties of relevant defect levels and potentially mitigate against their effects. In this paper we present a single trap-pumping study of the LSST E2V CCD250 across a temperature range of -30◦ to -110◦, a much larger range than previously studied using this approach. The predominant defect level of relevance for CCDs appearing at these temperatures is shown to be the single-acceptor level of the silicon divacancy. Using experimental data and a basic Monte-Carlo model of the trap-pumping process we examine the defect level properties, with an attempt made to account for both the capture and emission of signal charge.
The brighter-fatter effect is a well known phenomenon in thick, back illuminated CCDs, causing asymmetric increase in the observed size of point sources via correlated charge collection with higher signal levels. Over recent years, the effect of various operating parameters (such as the back bias applied) on the size of measured correlations has been well established. Less well studied is the consequence of changing the effective collection gate width of the CCD, which is of limited accessibility to experiment though several values are available in 3 or 4 phase devices. In this proceeding we present collection gate width experiments using both flat field and spot projection illuminations on a thick back illuminated device as used for the LSST project. We report on the size of the variation of measured correlations with gate width as compared with backside bias voltage, and find that gate width constitutes a small but significant contribution. In light of these results, we give comment on device optimisation when minimising correlated charge collection effects is desired.
KEYWORDS: Cadmium sulfide, Video, Signal to noise ratio, Signal processing, Charge-coupled devices, Video processing, Interference (communication), Analog electronics, Clocks, Capacitance
Correlated double sampling (CDS) is a process used in many charge-coupled device readout systems to cancel the reset noise component that would otherwise dominate. CDS processing typically consists of subtracting the integrated video signal during a “signal” period from that during a “reset” period. The response of this processing depends, therefore, on the shape of the video signal with respect to the integration bounds. In particular, the amount of noise appearing in the final image and the linearity of the pixel value with signal charge are affected by the choice of the CDS timing intervals. We use a digital CDS readout system which highly oversamples the video signal (as compared with the pixel rate) to reconstruct pixel values for different CDS timings using identical raw video signal data. We use this technique to develop insights into optimal strategy for selecting CDS timings both in the digital case (where the raw video signal may be available) and in the general case (where it is not). In particular, we show that the linearity of the CDS operation allows subtraction of the raw video signals of pixels in bias images from those in illuminated images to directly show the effects of CDS processing on the final (subtracted) pixel values.
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