Reflectance Tuning at Extreme Ultraviolet (EUV) Wavelengths with Active Multilayer Mirrors

At extreme ultraviolet (EUV) wavelengths the refractive power of transmission type optical components is limited, therefore reflective components are used. Reflective optics (multilayer mirrors) usually consist of many bilayers and each bilayer is composed of a high and a low refractive index material to maximize the partial reflection from the interface. For each bilayer composition, the reflectance is dependent on the wavelength and the incidence angle. Here we propose an active multilayer mirror structure which can tune its reflectance with external voltage. The active multilayer mirror is designed to work at EUV lithography wavelengths (around 13.5 nm) in normal incidence configuration by combining Mo/Si multilayer mirrors, a piezoelectric active layer, a buffer layer and electrodes as shown in the figure. Optimization of the structure for maximum reflectance tuning will be explained for different types of light sources by considering the fabrication process.