Reflective optical systems typically combine precise aligned mirrors, housings, interface structures et cetera. For MICADO (Multi-AO Imaging Camera for Deep Observations), a first-light instrument for ELT, Fraunhofer IOF realize several optical sub-systems, including ten freeform shaped metal optics for the Collimator, the high-resolution Imager, the low-resolution Imager and the Camera.
In this paper, the challenges of freeform manufacturing and metrology will be described. For manufacturing of those mirror substrates suitable technologies, as slow tool servo (STS) and fast tool servo (FTS) diamond turning and for further correction (e.g., magnetorheological finishing) and smoothing steps (e.g., chemical-mechanical polishing), sub-aperture tools are required. For interferometry of freeform shaped optical surfaces, computer generated holograms including reference fiducials are realized. After manufacturing the mirror substrates, the optical surface will be coated with a high-reflective gold coating.
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.
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