Low-stress and high-reflectivity molybdenum/silicon multilayers deposited by low-pressure rotary magnet cathode sputtering for EUV lithography

Abstract

We fabricated molybdenum (Mo)/silicon (Si) multilayers with low-pressure rotary magnet cathode (RMC) sputtering, which can be performed at a lower gas pressure (about 0.1Pa) than conventional magnetron sputtering. We obtained high reflectivity at a wavelength of extreme ultraviolet (EUV) in Mo/Si multilayers sputtered with low-pressure xenon (Xe) gas. The measured maximum reflectivity was approximately 71%. We confirmed that the multilayers sputtered at a lower pressure exhibited higher reflectivity. From the transmission electron microscope (TEM) cross-sectional images we observed thinner interdiffusion layers between Mo and Si layers in multilayers sputtered by RMC sputtering than in those sputtered by ion-beam sputtering. The Mo single layer deposited by RMC sputtering had tensile stress and the Si single layer had compressive stress. By changing the Γ ratio (the fractional thickness ratio of a Mo layer to the total thickness of a Mo layer and a Si layer) of the multilayer coatings, the film stress was controlled from tensile to compressive. However, for large Γ ratios, the interface roughness was increased and the EUV reflectivity was reduced. We have developed a doubly stacked multilayer structure that can provide low stress and high EUV reflectivity simultaneously. By using this technique, the stress of multilayer coatings was reduced to -6MPa while maintaining the high EUV reflectivity of 69%.