Exploring the challenges of pushing the resolution of DUV immersion lithography

With the advent of ArF immersion lithography, printing gratings down to size of 38nm became possible using dipole illumination settings. Further reduction towards a k1 of 0.25 results in a very small pupil pole size which brings increased imaging complexity. Effects to consider include decrease in contrast, optics lifetime impact and increased impact of lens aberrations and heating. Next to that, metrology markers, with a significantly larger pitch compared to the product feature, have a different imaging response to these extreme pupil settings adding to the complexity. In this work, using a combination of scanner exposures as well as simulations, we demonstrate these challenges in detail. Furthermore, we propose how these effects can be mitigated by modifying the illumination pupil using a new Source Mask Optimization (SMO) flow for an improved imaging, overlay and productivity performance.