Evaluation of damage induced by electron beam irradiation to MOS gate pattern and method for damage-free inspection

We analyzed the electron-irradiation damage induced in wafers by SEM inspection, which uses SEM images of voltage contrast formed by the charges on the pattern due to the electron irradiation. MOS capacitors were selected as samples because of their characteristic sensitivity. We studied the effects of electron-beam energy and charging on a MOS-capacitor test element group. To determine flat-band voltage, density of created traps, and oxide fixed charges in the MOS capacitors before and after the irradiations of the capacitors by electron beams under various conditions, we measured high-frequency and quasi-static capacitance-voltage characteristics. We found that the higher-energy electron beam, whose electron range was larger than the thickness of the gate electrode, created traps at the interface between the silicon substrate and the gate dielectric. The flat-band voltage of the MOS capacitor was shifted by the created traps. Although these traps were created by the transmission of the electron beam into the dielectric, they were not created only by charging on the gate electrode; neither was an oxide fixed charge created in the MOS capacitor. Accordingly, for damage-free inspection of MOS devices, the electron-beam energy should be low enough that the electron range is smaller than the thickness of the gate electrode. On the other hand, the flat-band voltage did not shift owing to charging on the pattern surface during the electron irradiation. However, the gate dielectric was broken down by charging on the gate electrode at high voltage. Accordingly, for damage-free inspection, the charging voltage should be controlled so as not to break down the gate dielectric.