Coherence of defect interactions with features in optical imaging

Abstract

For optical projection printing the variation of image intensity due to a defect in proximity to a feature is shown to be far greater than the image intensity produced by the same defect when it is isolated. This large impact is due to a nearly coherent interaction of electric fields of the defect with those of the feature. The effective mutual coherence μeff between a feature and a defect is defined and shown to be closely related to the degree of spatial coherence in the illumination across the mask. An algebraic model for feature variation as a function of defect size and location is derived from knowledge of defect intensity, feature line edge intensity slope, and μeff. It is predicted and confirmed by simulation that a 0.25λ/NA defect which has only a 3% isolated image intensity will create a 12% linewidth variation when adjacent to a 0.8λ/NA minimum feature size line. The linewidth variation perpendicular to the feature edge is directly proportional to the area of the defect. The length of the bulge produced parallel to the feature edge is given by the point spread function for the optical system and is thus independent of feature size. This makes defects appear larger than their actual size in both projection printing and inspection systems.