Practical monitor and control of SEM astigmatism in manufacturing

Abstract

Astigmatism in the modem scanning electron microscope (SEM) is a leading cause oftool drift and poor precision. A straightforward and productive way of assessing astigmatism in an SEM involves determining the best focus settings for neighboring sets ofparallel edges ofdifferent orientations, typically: vertical, horizontal, and diagonal. Clearly if best focus is the same for all targets, then the working astigmatism ofthe instrument is zero. When these readings are unequal, the degree ofastigmatism is proportional to the deviation of individual readings from the mean. This then provides a quantitative way to monitor the degree of astigmatism. Alternatively, this provides the basis for a rapid algorithm for correcting astigmatism with a minimum ofbeam writing. This paper presents methods for objectively monitoring and correcting astigmatism in any SEM with automatic focus. This work also discusses the optimal choice of focus and linewidth measurement targets to use in automatic statistical process control practices for best sensitivity to astigmatism.