Radiation Tolerance of Fully-Depleted P-Channel CCDs Designed for the SNAP Satellite

Abstract

Abstract-Thick, fully depleted p-channel charge-coupled devices (CCDs) have been developed at the Lawrence Berkeley National Laboratory (LBNL). These CCDs have several advantages over conventional thin, n-channel CCDs, including enhanced quantum efficiency and reduced fringing at nearinfrared wavelengths and improved radiation tolerance. Here we report results from the irradiation of CCDs with 12.5 and 55 MeV protons at the LBNL 88-Inch Cyclotron and with 0.1 -1 MeV electrons at the LBNL 60 Co source. These studies indicate that the LBNL CCDs perform well after irradiation, even in the parameters in which significant degradation is observed in other CCDs: charge transfer efficiency, dark current, and isolated hot pixels. Modeling the radiation exposure over a sixyear mission lifetime with no annealing, we expect an increase in dark current of 20 e − /pixel/hr, and a degradation of charge transfer efficiency in the parallel direction of 3 × 10 −6 and 1 × 10 −6 in the serial direction. The dark current is observed to improve with an annealing cycle, while the parallel CTE is relatively unaffected and the serial CTE is somewhat degraded. As expected, the radiation tolerance of the p-channel LBNL CCDs is significantly improved over the conventional n-channel CCDs that are currently employed in space-based telescopes such as the Hubble Space Telescope.

Postirradiation behavior of p-channel charge-coupled devices irradiated at 153 K

“…The observed trend in dark current is similar to previously reported behaviour 6 , including the second dip after the first period at room temperature. The other irradiated region follows a similar trend.…”

“…The increased tolerance to displacement damage of a p-channel CCD was first demonstrated in 1997 2 and since then a number of other studies have demonstrated an improved hardness to radiation induced charge transfer inefficiency (CTI) when compared to n-channel CCDs [3][4][5][6][7] . The improvement has been linked to the number and type of defects formed within the buried channel, 3 and how these defects impact charge transfer, i.e.…”

“…The radiation induced defects which increase CTI in a p-channel CCD have been linked to the divancancy and other traps related to carbon and oxygen interstitials [3][4][5][6][7][8] . As there are less radiation induced defects in a p-channel CCD which have been shown to increase CTI and because they are formed in smaller quantities (the divacancy is a second order defect and boron defects have not 4 been shown to impact CTE) when compared to an n-channel equivalent, p-channel CCDs are more hard to radiation induced CTI.…”

“…The aforementioned studies have shown that performing an irradiation at only room temperature can give misleading impressions of both the evolution of dark current, bright defects 6. 9-11, 15-17 and CTE 6,9,[12][13][14][15][16][17] . These factors can have a significant impact on the selection of the optimal operating conditions.…”

“…The CTE performance is complicated by the changes in dark current and trap evolution and will be discussed in a future publication. 6 …”