All ACS CCDs are buried channel devices which have a shallow n-type layer implanted below the surface to store and transfer the collected signal charge away from the traps associated with the Si-SiO2 interface. Moreover, ACS CCDs are operated in Multi Pinned Phases (MPP) mode so that the silicon surface is inverted and the surface dark current is suppressed. ACS CCDs therefore have very low dark current. The WFC CCDs are operated in MPP mode only during integration, so the total dark current figure for WFC includes a small component of surface dark current accumulated during the readout time.
Like all CCDs operated in a low earth orbit radiation environment, the ACS CCDs are subject to radiation damage by energetic particles trapped in the radiation belts. Ionization damage and displacement damage are two types of damage caused by protons in silicon. The MPP mode is very effective in mitigating the damage due to ionization such as the generation of surface dark current due to the creation of trapping states in the Si-SiO2 interface. Although only a minor fraction of the total energy is lost by a proton via nonionizing energy loss, the displacement damage can cause significant performance degradation in CCDs by decreasing the charge transfer efficiency (CTE), increasing the average dark current, and introducing pixels with very high dark current (hot pixels). Displacement damage to the silicon lattice occurs mostly due to the interaction between low energy protons and silicon atoms. The generation of phosphorous-vacancy centers introduces an extra level of energy between the conduction band and the valence band of the silicon. New energetic levels in the silicon bandgap have the direct effect of increasing the dark current as a result of carrier generation in the bulk depletion region of the pixel. As a consequence, the dark current of CCDs operated in a radiative environment is predicted to increase with time.
Ground testing of the WFC CCDs, radiated with a cumulative fluence equivalent to 2.5 and 5 years of on-orbit exposure, predicted a linear growth of ~1.5 e-/pixel/hour/year. During the ground tests the WFC CCDs were operated at ~81 ºC, about 4º cooler than the operating temperature on orbit.
The dark current in ACS CCDs is monitored four days per week with the acquisition of four 1000 seconds dark frames (totaling 16 images per week). Dark frames are used to create reference files for the calibration of scientific images, and to track and catalog hot pixels as they evolve. The four daily frames are combined together to remove cosmic rays and to extract hot pixel information for any specific day. The dark reference files are generated by combining two weeks of daily darks in order to reduce the statistical noise. The hot pixel information for a specific day is then added to the combined bi-weekly dark. In order to study the evolution of the dark current with time, the modal dark current value in the cosmic-ray free daily darks is calculated. As expected, the dark current increases with time ( Figure 4.5). The observed linear growth rates of dark current are 2.1 and 1.6 e-/pixel/hour/year for WFC1 and WFC2 respectively, and 2.1 e-/pixel/hour/year for the HRC CCD. These rates are in general agreement with the ground test predictions.
Figure 4.5: Dark rate trend with time for the ACS CCDs (HRC, WFC1,WFC2).
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