Determination of the Total X-Ray Dose in the SiO2 Layer of Backside Electron Irradiated ICCD Imagers

The voltage and electron fluence dependent total X-ray dose absorbed in the SiO2 layer of intensified charge coupled devices (ICCDs) operated in the backside electron irradiated mode has been calculated. The calculations were based on spectral intensity distribution and absolute dose measurements at what would be the Si-SiO2 interface of an actual device. It is shown that the fraction of the incident electron energy converted into X-rays that is produced and transmitted through the ~10 ¿m Si substrate is ~1×10-4 at 10 keV and ~3×10-4 at 20 keV. Approximately 1.5 percent of this total transmitted X-ray energy is absorbed in 100 nm SiO2 and between 85-97 percent of that X-ray energy is in the characteristic K¿ line at 1.732 keV. The dose deposited in the SiO2 per electron incident on backside of a 10 ¿m Si substrate is D=9.7×10-14E(keV)2.2 rad(SiO2)/e-cm-2, and of a 13 ¿m Si substrate it is D = 3.5×10-14E(keV)2.44 rad(SiO2)/e-cm-2. With overcast starlight illuminance levels (~-10-4 Lux) the rate of dose deposition in the oxide of a typical ICCD operated at 10, 15, and 20 kV is 2.3, 6.3, and 12.7 rad(SiO2)/hr., respectively. At these dose rates, and particularly with reduced operating voltages, megarad hardened CCDs in low light level ICCD-TV camera tubes are expected to have useful operating lifetimes up to a hundred thousand hours.