Characterization and Optimization of Skipper CCDs for the SOAR Integral Field Spectrograph

We present results from the characterization and optimization of Skipper charge-coupled devices (CCDs) for use in a focal plane prototype for the Southern Astrophysical Research Integral Field Spectrograph (SIFS). We tested eight Skipper CCDs and selected six for SIFS based on performance results. The Skipper CCDs are 6k × 1k, 15 μm pixels, thick, fully depleted, p-channel devices that have been thinned to ∼250 μm, backside processed, and treated with an anti-reflective coating. We demonstrate a single-sample readout noise of <4.3 e⁻ rms pixel⁻¹ in all amplifiers. We optimize the readout sequence timing to achieve a readout noise of 0.5 e⁻ rms pixel⁻¹ after 74 non-destructive measurements, which can be accomplished in a region covering 5% of the detector area in a readout time of <4 minutes. We demonstrate single-photon-counting in all 24 amplifiers (four amplifiers per detector) with a readnoise of σ _N ∼ 0.18 e⁻ rms pixel⁻¹ after N _samp = 400 samples, and we constrain the degree of nonlinearity to be ≲1% at low signal levels (0 e⁻ to 50 e⁻). Clock-induced charge (CIC) remains an important issue when the Skipper CCD is configured to provide a large full-well capacity. We achieve a CIC rate of <1.45 × 10⁻³ e⁻ pixel⁻¹ frame⁻¹ for a full-well capacity of ∼900 e⁻, which increases to a CIC rate of ∼3 e⁻ pixel⁻¹ frame⁻¹ for full-well capacities ∼40,000–65,000 e⁻. We also perform conventional CCD characterization measurements such as charge transfer inefficiency (3.44 × 10⁻⁷ on average), dark current (∼2 × 10⁻⁴ e⁻ pixel⁻¹ s⁻¹), photon transfer curves, cosmetic defects (<0.45% “bad” pixels), and charge diffusion (point-spread function < 7.5 μm) to verify that these properties are consistent with expectations from conventional p-channel CCDs used for astronomy. Furthermore, we provide the first measurements of the brighter-fatter effect and absolute quantum efficiency (≳80% between 450 and 980 nm; ≳90% between 600 and 900 nm) using Skipper CCDs.