MULTILAYER OPTICAL ELEMENT

摘要

The stress free multilayer systems contain multilayer systems with two different material combinations of opposite stress, where both systems give good reflectivity at the design wavelengths. The main advantage of the multilayer system design is that stress compensation does not require the deposition of any additional layers, thereby reducing the total film thickness compared to the earlier buffer layer approach. If the optical performance of the two systems at the design wavelength differs, the system with the poorer performance is deposited first, and then the system with better performance last, thus forming the top of the multilayer system. For a wavelength of about 13.4 nm, the wavelength presently used for extreme ultraviolet (EUV) lithography, stress free multilayers can be formed on a substrate using Mo/Be multilayers in the bottom of the stack and Mo/Si multilayers at the top of the stack with the switch-over point selected to obtain approximately zero stress. In this multilayer system, the switch-over point is at about the half point of the total thickness of the stack, and for the Mo/Be-Mo/Si system, there may be about 25 deposition periods Mo/Be to about 20 deposition periods Mo/Si.