Noise effects on contact-edge roughness and CD uniformity measurement

The aim of this work is to study the effects of noise on the parameters characterizing the size and roughness of contact edges when they are measured by the analysis of top-down SEM images. The applied methodology is based on the modelling of rough contact edges with controlled roughness parameters and the generation of synthesized top down SEM images with several contact edges and a distribution of Critical Dimension (CD) values (CD nonuniformity). The sources of noise can be the shot noise of SEM electron beam (Poisson-type) and the microscope electronics (Gaussiantype). First, we check out the validity of the model and then we apply it to evaluate the effects of noise in synthesized SEM images with smooth and images with rough contact edges. The results show that in all cases, noise lowers CD and correlation length while it increases the rms value. CD variation is increased with noise in images with smooth and identical contacts whereas it remains almost unaltered in images including rough contacts with CD nonuniformity. Furthermore, we find that the application of a noise smoothing filter before image analysis is able to rectify the values of CD (at small filter parameter) and of rms and correlation length (at larger filter parameters), whereas it leads to larger deviations from the true values of CD variation. Quantitative assessment of the model predictions reveals that the noise induced variations of CD and CER values are inferior to those caused by process stochasticity and material inhomogeneities.

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