Optical illumination and critical dimension analysis using the through-focus focus metric method

In this paper we present recent developments in optical microscope image analysis using both, best focus optical image as well as those images conventionally considered out of focus for metrology applications. Depending on the type of analysis, considerable information can be deduced with the additional use of the out of focus optical images. One method for analyzing the complete set of images is to calculate the total "edge slope" from an image, as the target is moved through-focus. A plot of the sum of the mean square slope is defined as the through-focus focus metric. We present a unique method for evaluating the angular illumination homogeneity in an optical microscope (with Koehler illumination configuration), based on the through-focus focus metric approach. Both theoretical simulations and experimental results are presented to demonstrate this approach. We present a second application based on the through-focus focus metric method for evaluating critical dimensions (CD) with demonstrated nanometer sensitivity for both experimental and optical simulations. An additional approach to analyzing the complete set of images is to assemble or align the through focus image intensity profiles such that the x-axis represents the position on the target, the y-axis represents the focus (or defocus) position of the target with respect to the lens and the z-axis represents the image intensity. This two-dimensional image is referred to as the through focus image map. Using recent simulation results we apply the through focus image map to CD and overlay analysis and demonstrate nanometer sensitivity in the theoretical results.