Multilayer Extreme UV Optics

Ultrathin layered structures are known to act well as Bragg reflectors for light with wavelengths down to a few nanometers. However, the application of such structures in multi-element optics for photolithography at the wavelength of 13.5 nm has imposed tremendous challenges to the underlying thin film physics. Required is full physical and chemical stability, 10 W/cm2 radiation damage resistance, atomically sharp refractive index profiles, and dimension-controlled thicknesses down to the sub-nanometer range. Layer interdiffusion in the multilayers, for instance, needs to be controlled over pm distances during the typically 30,000 hrs lifetime of the optics. At the top surface of the multilayers, photo-induced chemical phenomena need to be mitigated to avoid even monolayers of contamination during usage of the optics. Pursuing such properties has been the goal of several thin film and optical research programmes, executed in collaboration with end users of the optics. This work has enabled an improved understanding of the basic physics with results immediately being used in state-of-the-art photolithography. An overview will be given of the relevant physics processes as well as the required optics fabrication techniques.