Investigating SEM metrology effects using a detailed SEM simulation and stochastic resist model

A Monte Carlo electron scattering simulation tool that can create SEM images of 3D features with arbitrary geometry has been developed. This is combined with both a stochastic resist model and synthetic 3D features to probe the effect of the effect of roughness on SEM measurements. Sidewall roughness makes it difficult to precisely identify the true feature width of a line because the roughness increases the SEM signal non-proportionally to the amount of material with which it is interacting. LER generally under predicts sidewall surface roughness because the SEM has an averaging effect as the electron beam interacts with a volume of material. LER becomes a better measure of surface roughness as the correlation length of the surface roughness increases. Decreasing film thickness causes a decrease in the linewidth and increase in LER measured by SEM, especially for features 35 nm thick and below. This occurs even if the true 3D feature width and roughness is approximately constant, meaning that the apparent change in linewidth and LER is a metrology effect. Threshold based estimations of line edges are difficult because the threshold choice that best matches the true feature width changes with the feature geometry. Model based library fits of linescans do not appear to provide a solution because sidewall roughness and sidewall angle have similar effects on the linescan meaning no unique linescan likely exists.