Sub-electron readout noise in a Skipper CCD fabricated on high resistivity silicon

The readout noise for Charge-Coupled Devices (CCDs) has been the main limitation when using these detectors for measuring small amplitude signals. A scientific CCD fabricated on a high-resistivity silicon substrate utilizing a floating gate amplifier with the capability of multiple sampling of the charge signal is described in this paper. The Skipper CCD architecture and its advantages for low noise applications are discussed. A technique for obtaining sub-electron readout noise levels is presented, and its noise and signal characteristics are derived. We demonstrate that with this procedure a very low readout noise of 0.2e (-aEuro parts per thousand) RMS can be achieved. The contribution of other noise sources (output stage, vertical and horizontal charge transfer inefficiency, and dark current noise) are also considered. The optimum number of samples for achieving the total lowest possible noise level is obtained. This technique is applied to an X-rays experiment using a Fe-55 source.