Modeling the field diffracted from photo mask at oblique incidence.

In optical lithography, light diffracted from photo mask structures has been customarily assumed to be constant with the angle of incidence of the light illuminating the photo mask. As numerical aperture increases to unity and beyond, to cope with the continuous demand for shrinking integrated circuits, device dimensions, and densities, this approximation is no longer valid. In this paper we use the physical theory of diffraction to study, understand, and model the variation of light diffracted from photo mask structures of the order of the wavelength, with the angle of light incidence. We present a semianalytical model that is fast, accurate, and compatible with existing professional software in this domain. The accuracy of the model is studied using the finite-difference time-domain technique and is shown to be below 5% at the image plane, within angles of incidence between +/-20 degrees .