Improved reflectance and stability of Mo-Si multilayers

Abstract

Commercial EUV lithographic systems require multilayers with higher reflectance and better stability than those published to date. This work represents our effort to meet these specifications. Interface-engineered Mo-Si multilayers with 70% reflectance and 0.545-nm bandwidth at 13.5-nm wavelength and 71% reflectance with 0.49-nm bandwidth at 12.7-nm wavelength were developed. These results were achieved with 50 bilayers. These new multilayers consist of alternating Mo and Si layers separated by thin boron carbide layers. Depositing boron carbide on the interfaces leads to reduction in molybdenum silicide formation of the Mo-on-Si interfaces. Bilayer contraction is reduced by 30%, implying that there is less intermixing of Mo and Si to form silicide. As a result, the Mo-on-Si interfaces are sharper in interface-engineered multilayers than in standard Mo-Si multilayers. The optimum boron carbide thicknesses have been determined and appear to be different for the Mo-on-Si and Si-on-Mo interfaces. The best results were obtained with 0.4-nm-thick boron carbide layers for the Mo-on-Si interfaces and 0.25-nm-thick boron carbide layers for the Si-on-Mo interfaces. The increase in reflectance is consistent with multilayers having sharper and smoother interfaces. A significant improvement in oxidation resistance of EUV multilayers has been achieved with ruthenium-terminated Mo-Si multilayers. The best capping-layer design consists of a Ru layer separated from the top Si layer by a boron carbide diffusion barrier. This design achieves high reflectance and the best oxidation resistance during EUV exposure in a water-vapor (oxidizing) environment. Electron-beam exposures of 4.5 h (in an effort to simulate EUV exposure perturbation of the top layers) in the presence of 5×10 - 7 -Torr water-vapor partial pressure show no measurable reflectance loss and no increase in the oxide thickness of Ru-terminated multilayers.