THE EFFECT OF BULK TRAPS IN PROTON IRRADIATED EEV CCDS

MOS CCDs are being used as the focal plane detectors for several X-ray astrophysics missions. The intrinsic X-ray spectral resolution in optimised detectors is Fano-limited. The presence of trapping sites in the bulk of the silicon limits the charge transfer efficiency of the CCD and hence the achievable X-ray spectral resolution. The space radiation environment subjects the CCDs to gammas, electrons and protons. Irradiation by protons severely degrades the spectral resolution of the CCD by creating bulk traps. In this paper the effect of bulk traps on the CTI of MOS CCDs is modelled and is compared to experimental data. The use of X-ray CTI analysis is shown to be a very sensitive tool in the analysis of bulk traps. It is shown that the initial device CTI performance may be accounted for by bulk traps in the silicon used to make the devices. At least two different traps are shown to present in the silicon with energy levels at 0.12 eV and 0.30 eV and concentrations of 3 x 10(10) cm-3 and 7 x 10(8) cm-3 respectively. After irradiation with protons (3.6 x 10(9) cm-2, 10 MeV), the shallow traps increase in density, and an additional trap is generated, which is believed to be the silicon E-centre with a measured energy level of 0.42 eV and concentration of 3.5 x 10(10) cm-3.