Super-polished metallic freeform optics for EUV-Application

Super-polished optics are crucial components for extremely short wavelength applications. One of the main applications are illumination optics. Short operational wavelengths of ∼13,5 nm lead to high demands on substrate's surface quality, especially for microroughness features in the high spatial frequency range (HSFR). The excellent surface quality is essential for high-performant reflective mirrors. Typically, mirror substrates for EUV applications are made of glass, glass-ceramics and silicon. This paper describes an alternative approach by using metallic substrates made from electroless Nickel-Phosphorous (NiP) plated Aluminum alloys. The X-Ray amorphous NiP alloy enables several polishing and correction techniques to reduce the surface roughness down to the addressed sub-nm range. The process chain of metal optics includes ultra-precise diamond turning (DT) and chemical mechanical polishing (CMP) steps. Microroughness (HSFR) of < 0.2 nm RMS (AFM, 1 × 1 μm2) by CMP could be generated on flat and curved surfaces as well as on freeform optics. The process chain will be verified by surface characterization techniques, e.g. atomic force microscopy and white light interferometry. The data is evaluated using the established PSD (power spectral density) analysis method. Conclusions about the surface errors concerning the different processing technologies can be verified. This paper shows that super-polished metallic freeform optics are suitable for curved EUV illumination optics.