Air bubble-induced light-scattering effect on image quality in 193 nm immersion lithography

As an emerging technique, water immersion lithography, offers the capability of reducing critical dimensions by increasing the numerical aperture that is due to the higher refractive indices of immersion liquids than that of air. However, in the process of forming a water fluid layer between the resist and the lens surfaces, air bubbles are often created because of the high surface tension of water. The presence of air bubbles in the immersion layer will degrade the image quality because of the inhomogeneity-induced light scattering in the optical path. Analysis by geometrical optics indicates that the total reflection of light causes the enhancement of scattering in the region in which the scattering angle is less than the critical scattering angle, which is 92 deg at 193 nm. Based on Mie theory, numerical evaluation of scattering that is due to air bubbles, polystyrene spheres, and poly(methyl methacrylate) spheres was conducted for TE, TM, or unpolarized incident light. Comparison of the scattering patterns shows that the polystyrene spheres and air bubbles resemble each other with respect to scattering properties. In this paper, polystyrene spheres are used to mimic air bubbles in studies of lithographic imaging of bubbles in immersion water. In an interferometric lithography system, the distance beyond which bubbles will not print can be estimated by direct counting of defect sites.