Modeling mask scattered field at oblique incidence

Abstract

In optical lithography light diffracted from the mask has been customary assumed to have constant amplitude with the angle of incidence of the light illuminating the mask. This approximation, known as constant scattering coefficient approximation, has been successfully used at small NA. As the NA increases to unity and beyond, to cope with the continuous demand for shrinking integrated circuits device dimensions and densities, the validity of this approximation becomes questionable. In this paper, we study diffracted field variation with the angle of incidence using physical theory of diffraction. An asymptotic theory like the physical theory of diffraction allows us to better understand, quantify, and model using analytical formulae, induced effects of light diffraction from mask at oblique incidence. This paper presents a semi analytical model that describes diffracted field variation with angle of incidence. The model accuracy is validated by comparison with rigorous field simulations using Panoramic software.