A transmissive spectral purity filter for suppressing infrared radiation in extreme ultraviolet lithography source

We propose a transmissive spectral purity filter (SPF) with rectangular fishnet structures for laser-produced plasma extreme ultraviolet (EUV) lithography source. Such a transmissive SPF consists of a graphene-based thin membrane carrying a Molybdenum absorber pattern for the infrared (IR) radiation suppression. By using using the rigorous coupled wave analysis method, we investigate the optical properties of the transmissive SPF and optimize the structure parameters to suppress the out-of-band IR radiation. We analyze the dependence of the transparency at wavelength of 13.5 nm and infrared radiation suppression on the structure parameters. Theoretical analysis reveals that the transmittance at undesirable wavelength of 10.6μm is as low as 0.11%, while maintaining the in-band EUV transmittance as 75.92%, thereby enabling full separation of EUV and IR radiation from the EUV light source for high volume manufacturing lithography. Especially, the light incident angle on SPF has very little influence on both EUV and IR transmittances, thereby relaxing placement requirements significantly.