Modeling of local dielectric charging induced by line scan during SEM observation

Evolution in emission of secondary electrons and backscattered electrons from thermally oxidized silicon sample during a single line-scan of a focused electron beam was measured. The influence of charging-up of the sample surface on the electron emission was analytically modeled. The detected current Ic of these electrons decreased sharply at the initial scanning stage and turned steady. The ratio of Ic to incident primary-beam current (Ip) at the steady state could be expressed in terms of “scanning line density” (λin), i.e., the number of incident electrons per unit scan distance. It was found that the ratio of Ic/Ip increased monotonously with decreasing λin. Furthermore, no difference in the change of the Ic/Ip at the steady states was observed with interval, i.e., the time between consecutive single-line scans at the same position, ranging from 10 μs to 500 ms. This result indicates that attenuation of the charge at the sample surface during the single line-scan is neglectable. An analytical model was proposed for correlating Ic/Ip, the potential at the irradiated position (Vs) and λin. The calculated Ic/Ip coincided quantitatively with the experimentally determined ratio. Besides, the analytical model showed that higher magnitude and less variation in Ic/Ip during the single line-scan can be expected with lower λin, which was experimentally verified. The parameter λin can be equally controlled through Ip and the line-scan speed v.