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

Abstract

We present results from the characterization and optimization of six Skipper CCDs for use in a prototype focal plane for the SOAR Integral Field Spectrograph (SIFS). We tested eight Skipper CCDs and selected six for SIFS based on performance results. The Skipper CCDs are 6k $\times$ 1k, 15 $\mu$m pixels, thick, fully-depleted, $p$-channel devices that have been thinned to $\sim 250\mu$m, backside processed, and treated with an antireflective coating. We optimize readout time to achieve $<4.3$ e${}^{-}$ rms/pixel in a single non-destructive readout and $0.5$ e${}^{-}$ rms/pixel in $5\%$ of the detector. We demonstrate single-photon counting with $N_{\mathrm{samp}}$ = 400 (${\sigma }_{0{\mathrm{e}}^{-}}\sim$ 0.18 e${}^{-}$ rms/pixel) for all 24 amplifiers (four amplifiers per detector). We also perform conventional CCD characterization measurements such as cosmetic defects ($<0.45\%$ ``bad" pixels), dark current ($\sim 2\times 10^{-4}$ e${}^{-}$/pixel/sec.), charge transfer inefficiency ($3.44\times 10^{-7}$ on average), and charge diffusion (PSF $<7.5\mu$m). We report on characterization and optimization measurements that are only enabled by photon-counting. Such results include voltage optimization to achieve full-well capacities $\sim 40,000-63,000$ e${}^{-}$ while maintaining photon-counting capabilities, clock induced charge optimization, non-linearity measurements at low signals (few tens of electrons). Furthermore, we perform measurements of the brighter-fatter effect and absolute quantum efficiency ($\gtrsim 80\%$ between 450 nm and 980 nm; $\gtrsim 90\%$ between 600 nm and 900 nm) using Skipper CCDs.