Study of the imaging properties in high numerical aperture projection optics

Properties of the high numerical aperture (NA) imaging including the immersion lens (NA greater than or equal to 1) have been studied by using the in-house simulator based on the vector diffraction theory. It has been shown that the resolution does not improve in proportion to the NA because the apodization effect which arises at the focal region reduces the resolution as the NA increases. Resolution reduction has been estimated from the analysis of the point images as 4% (3%) at NA 0.9 and 8% (6%) at NA 1.2 under unpolarized illumination with the 1.5 (1.7) of refractive index of the resist. From the investigation of the numerical accuracy, it has been shown that in high NA (>0.9) and large illumination sigma, the shadowing effect of the mask pattern affects the image properties. This effect can be treated by considering the diffraction of the oblique illumination and the normal incidence approximation used in most commercially available simulators is not sufficiently accurate.