Nanometer interface and materials control for multilayer EUV-optical applications

An overview is given of the progress in thin film and surface physics involved in multilayered systems with nanometer scale periodicity. When properly engineered, these enable the synthesis of reflective optics for the Extreme UV wavelength range. Design, deposition, and analysis of these structures have been driven by the demanding application of Extreme UV photolithography. This review addresses the selection of the wavelength in relation to the optical constants of materials, the layer growth mechanisms and ways to reduce layer roughness and interlayer formation. Special attention is given to the development of thin diffusion barrier layers between the materials in the multilayers to enhance the optical contrast and to reduce the interdiffusion. Practical issues like reduction of multilayer induced stress and enlargement of the reflectance bandwidth are also discussed, as well as the development of capping layers to control surface physics processes occurring under EUV irradiation. A description of the multilayer deposition techniques is given and the deposition of multilayers on large, heavily curved optics for real lithography systems is discussed.