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Skipper CCD was first introduced in 1990 by Janesick et al. Early works on Skipper CCD reported sub-electron noise without reading signal in the pixel array. Here we present the first successful Skipper CCD sensor with discrete deep sub-electron readout noise and single electron counting capabilities.
The Skipper CCD discussed in this work, is a p-channel CCD fabricated on high resistivity, float-zone refined, n-type silicon \cite{holland2003fully}, developed by MicroSystem Labs of Lawrence Berkeley National Laboratory. A substrate bias is applied to fully deplete the substrate, which is 200\,$\rm \mu m$ thick. The high resistivity, $\approx$10\,$\rm k\Omega cm$, allows for fully depleted operation at a substrate voltage over 40V. The detector has 4126 by 866 square pixels of 15$\times$15\,$\rm \mu m^2$. The Skipper CCD vertical and horizontal registers have three-phase clocks that are designed for split readout, through the output stage at each quadrant of the sensor.
Ultralow readout noise and stable linear gain allows the Skipper CCD to measure charge at the accuracy of
individual electrons simultaneously in pixels with single electrons and with thousands of electrons. This makes the Skipper CCD the most sensitive and robust electromagnetic calorimeter that can operate above liquid nitrogen temperatures. The Skipper CCD can also count individual optical and near-infrared photons. Because nondestructive readout is achieved without any major modification to the CCD fabrication process, this new technology can be directly
implemented in existing CCD manufacturing facilities.
An exciting application in the low-SNR regime is direct, space-based imaging and spectroscopy of terrestrial exoplanets in the habitable zones of nearby stars . The photon flux from exo-Earths is expected to be Oð1Þ per several minutes, necessitating ultralow noise detectors. A detector with subelectron readout noise could reduce exposure times by a factor of 2, which is essential given the large exposure times required . Skipper CCDs are easily manufactured with large formats and are stable over a large dynamic range. In addition, thick fully depleted CCDs can achieve high quantum efficiency between 0.87 and 1 μm where several important spectral lines from water reside.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.131802
https://arxiv.org/abs/1706.00028
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