Damage to insulated gate FETs by electron beam metrology and X-ray exposure

A comparison of the damage induced by X-rays and electron-beam radiation on IGFETs has been made. It is observed that the ratio of the threshold voltage shift due to fixed positive charge (ΔV _FPC) to the total threshold voltage shift (due to both fixed positive charge and neutral electron traps) does not show any dependence on the radiation dose in the case of an E-beam irradiation, and shows a negative slope for X-ray irradiated samples. This suggests that the amount of neutral electron traps (NETs) and fixed negative charge (FNC) produced by the ionizing radiation is higher in the case of X-ray irradiation and saturates at much higher doses compared to E-beam irradiation. A study of electron beam damage at various energies shows that E-beam energy of 7 keV does not damage the oxides at all whereas at lOkeV maximum damage is observed. For devices exposed to X-rays, the threshold voltage shift ratios due to the fixed positive charge for different gate oxide thicknesses (12·6nm-50·0nm) indicate a shift of the effective charge centre-id which also depends on the filling of neutral electron traps to form fixed negative charges that partially compensate the fixed positive charges. The threshold voltage shift ratios also indicate a shift in the charge centroid which is pronounced in the thinner oxides. A model for the change in effective centroid of charge and also its dose dependence for different oxide thicknesses has been suggested.