Challenges of SEM metrology at sub-10 nm

The uncertainty associated with scanning electron microscopy (SEM) metrology is significant because SEM image brightness is complexly related to the size and shape of the feature, its material, the geometry of the pattern, as well as SEM setup. While regularly used methods of extracting critical dimensions (CD) rely on image brightness analysis, the myCD software uses a physical model of the SEM in order to improve the accuracy of measurements. Metrology below 10 nm was studied in this paper. Patterns were fabricated using electron beam lithography and nanoimprint; they were imaged by SEM and examined using myCD. Factors that are important for metrology at the sub-10 nm size range were studied using advanced Monte Carlo software; the beam size, voltage, detector and linewidth were varied. SEM images were processed using myCD, which utilizes an analytic model of the SEM and so does not require any libraries. The top and bottom sizes, as well as wall angles and line width roughness were analyzed. The CD and profile results from top down SEM images were compared to the vertical crossections. The challenges of sub-10 nm metrology are discussed, mainly regarding the quality of SEM images and the physics of image formation.