A robust focusing and astigmatism correction method for the scanning electron microscope - Part II: Autocorrelation-based coarse focusing method

Abstract

This paper describes the development of a new coarse focusing algorithm for the scanning electron microscope (SEM). The algorithm is based on a new focus parameter that has been developed for quantifying the sharpness of an image. This parameter is found to be more robust to noise compared with other conventional focus parameters. A peak detection technique is also incorporated to shorten the number of iterations needed for the coarse focusing. Results show that the coarse focusing algorithm is able to bring a defocused and astigmatic image to either the best focus, the upper line focus, or the lower line focus in fewer iterations compared with the conventional approach in which the full focus range is searched.