Liquid immersion lithography: microscopic polarizabilities and the role of orientation contributions to light scattering

Abstract

When investigating fluids for liquid immersion lithography scattering of light is more crucial than absorption. The reason is that pure absorption can be compensated by an increase of exposure time or light intensity, while scattered light decreases the imaging contrast on the photoresist. We therefore carefully investigate the scattering of light in a molecular liquid. Light can be scattered due to a number of mechanism. These are inelastic mechanism like Raman scattering, elastic scattering on micro- and nano bubbles but also quasielastic scattering on density fluctuations. In addition to the quasielastic scattering on density fluctuations a molecular fluid shows scattering on orientation degrees of freedom. Based on the known anisotropy of the polarizability of the water molecule, we calculate the scattering components due to orientation fluctuations. Among these the polarization ratio under 90° scattering is calculated and the relation between orientation and center-off-mass contributions for different polarization directions is evaluated. While the amount of scattering due to molecular orientations seems to be still moderate for water it is expected to be larger for most fluids, like fluorinated polymers. For three different fluor--organic molecules, which are in discussion for immersion fluids, the molecular polarizability is calculated using an abinitio method. The resulting polarizabilities are used to estimate the scattering due to orientation motion of these molecules. As a result the scattering due to orientation motions has the potential to increase the scattering level remarkably.