3D Monte Carlo modeling of the SEM: Are there applications to photomask metrology?
John S. Villarrubia, Andras Vladar, Michael T. Postek
The ability to model the effect of fields due to charges trapped in insulators with floating conductors has been added to JMONSEL (Java Monte Carlo simulator for Secondary Electrons) and applied to a simple photomask metal on glass geometry. These capabilities are prerequisites if model-based scanning electron microscope (SEM) metrology is to be extended beyond wafer to photomask applications, where such samples must frequently be measured. Model-based SEM is an alternative to the customary threshold- or gradient-based approach. It is more demanding inasmuch as it requires a model of the physics of image formation, but the reward is greater accuracy, lower sensitivity to secondary sample characteristics (e.g., edge shape) that affect the intensity, and information about 3D geometrical shape (not simply the width) of the measured features. The prerequisites are ability to measure a signal, such as the SEM image, that is sensitive to changes in the parameters one wishes to measure and a model that describes the relationship between the signal and the parameters. The simulation shows the development of the potential energy barrier to electron escape during an initial transient charging-up phase, accompanied by increasing electron recapture and stabilization of the average yield.