Extending a GTD-based image formation technique to EUV lithography

Abstract

An image formation technique based on the Geometrical Theory of Diffraction was presented last conference. The technique is a scalar technique and is applicable to infinitely thin and perfectly conducting mask. We explore in this paper the extension of the technique to 1D Extreme Ultra-Violet(EUV) Lithography mask, taking into consideration both the material property and the topography of the mask. Vectorial nature of light is incorporated in the treatment. Results obtained are promising and encouraging. Computation time is relatively much shorter and the technique could simulate irradiance profile for any illumination angle. The technique is simple and elegant and lends understanding to image formation. We conclude that the asymmetry-through-focus characteristic usually found in EUV and Phase Mask imaging is an imaging phenomenon. We also conclude that corrections for proximity effect and pattern infidelity will be needed when EUV Lithography is introduced at the 32 nm node, assuming a system NA of 0.25. Lastly, for a partially coherent illumination, it appears necessary to compute the irradiance corresponding to each illumination point individually.