Simulation study on EUV multilayer polarization effects

The impact of polarization was observed in the extreme ultraviolet (EUV) imaging simulations for high NA lithography [3] [4] [5]. It is shown that polarized illumination can improve the local contrast of images or NILS (normalized intensity log slope). This work investigates the possibilities to polarize EUV light by optimized multilayers. The characterization and simulation of multilayer structures has been performed using Dr.LiTHO [10]. The most efficient multilayer polarizers operate close to Brewster angle, where the reflectivity for TM polarized light (RTM) is close to zero, according to Fresnel's equations. A multiobjective optimization algorithm was used to identify the suitable multilayer configurations maximizing reflectivity of TE polarized light (RTE) and fraction of polarization. Fraction of polarization (FoP) was calculated as the ratio between (RTE-RTM)/(RTE+RTM) to obtain the suitable multilayer with variable thickness. The multilayer structure is optimized to have the highest reflectivity of TE polarized light and fraction of polarization at the Brewster angle. It was found that MoSi multilayer can achieve 99.9% fraction of polarization by optimizing the thickness of Si and Mo. In reality, a multilayer polarizer has to operate over certain ranges of incident angles and/or wavelength ranges. Multilayer is optimized for different ranges of wavelength (13 nm : 14 nm) and incidence angles (37° : 47°). Additional simulations investigate the impact of different options in the design of the multilayer (e.g., constant vs. variable bilayer thickness) and materials (e.g., RuSi vs. MoSi multilayers) on the achievable performance.