Extending critical dimension measurement for optical microlithography with robust SEM image contours

Dimensions of fine and complex structures printed by microlithography inside a photoresist are inspected with the help of Scanning Electron Microscope generating high resolution top down greyscale digital images. The edges of the photoresist can be extracted from the images to produce contours that best exploit the relevant image information. Unfortunately, such contours are usually of bad quality and subject to edge detection errors due to the noise of the SEM image and the roughness of the photoresist. In this work, we introduce a new method to deal with contours to easily complete complex operations like smoothing of contours or robust averaging of multiple contours without the help of any reference contour layer. Our approach is to use level set to represent any 2D contour as a 3D surface. We demonstrate that we can easily smooth complex 2D contours of critical structures of around 50nm width. We also managed to generate very reliable contours by averaging multiple contours of low quality. Finally, the level set method was used to locally derive confidence about the determination of the average contours.