Modeling fluid thermomechanical response for immersion lithography scanning

Abstract

Immersion lithography has been proposed as a method of improving optical lithography resolution to 50 nm and below, by increasing the index of refraction in the space between the lens and wafer by substituting a high refractive index liquid for the low refractive index air that currently fills the gap. During exposure, energy is deposited onto the wafer; there is a temperature rise in the liquid due both to absorption of light in the fluid and to heat transfer from the heated portions of the wafer. This temperature increase can change the liquid's index of refraction, which may lead to optical degradation. In addition, the increased viscosity of the liquid when compared with air results in potentially significant stresses induced on both the lens and the wafer surface. These stresses may, in turn, cause birefringence of the lens elements. Thermal and flow numerical simulations have been used to analyze the liquid heating and predict the normal and shear forces on the lens. Parametric studies have been performed in order to investigate how the fluid properties affect liquid heating and the resultant effects on imaging. Detailed studies of the forces induced on the lens element have been performed, taking water as the immersion fluid.