Analysis of film stress in Mo/Si multilayers for EUV mask blanks

The reflective layer of EUV mask blanks consists of an ion-beam-deposited multilayer of 40-50 repeats of (3nm Mo / 4nm Si) bilayers, capped with a Ru-based protective layer. We present a quantitative analysis of Mo/Si multilayer stress, from stacks deposited onto bare Silicon wafers. We demonstrate compressive stress of -700 MPa within the 4nm Silicon layers. Molybdenum single layer stress varies from compressive to tensile depending on film thickness, and at 3nm is slightly tensile. The net multilayer stress is -300 MPa compressive, dominated by the properties of the Silicon layers. Multilayer stress is insensitive to the total number of bilayers. After annealing at 150°C, stress is relieved by about half, down to -150 MPa compressive. Mask bow can be estimated using the Stoney equation, and we deduce bow levels resulting from the multilayer alone of ~ 0.5 -1 μm. Since stress is independent of the number of bilayers, mask bow increases approximately linearly with the bilayer count. We investigate process tuning parameters, and show that stress can be minimized at lower deposition rate or more off-normal deposition angle. We extend the analysis to potential future multilayers for high-NA or shorter wavelength lithography.