Exploring the capabilities of immersion lithography through simulation

Immersion lithography has recently emerged as the leading candidate for extending 193nm lithography to the 45nm lithography node and beyond. By immersing the wafer in a high index fluid, lens designs with numerical apertures (NAs) approaching the refractive index of the fluid are possible. While such a high numerical aperture is normally accompanied by an extreme decrease in the depth of focus at the resolution limit, an advantage of the immersion approach to increasing the numerical aperture is that the depth of focus is increased by at least a factor of the refractive index, mitigating some of the DOF loss due to the higher NA and smaller feature. Though this technique for resolution enhancement is receiving significant attention, useful experimental data on the subtle effects of such high NA imaging is one to two years away. Thus, simulation is expected to bridge the gap in immersion lithography research. In this paper, the fundamental imaging physics of immersion lithography will be described. The impact of resolution and depth of focus will be explored, as well as the subtle though significant influence of hyper NAs on polarization related thin film effects and the definition of intensity. With a rigorous model in place, the use of immersion lithography for extending 193nm towards its ultimate limits will be explored.