Process optimization for performance improvement in Mo/Si multilayers for EUV mask blanks

We investigate the impact of key ion-beam-deposition (IBD) process conditions on the properties of Mo/Si multilayers as reflective coatings for Extreme Ultraviolet (EUV) mask blanks. Dark-field TEM measurements imply interfacial roughness values of 80-90 picometers. Bright-field TEM measurements indicate intermixed layer thicknesses of 0.5 – 1.9nm. We present reflectivity calculations including these two multilayer imperfections and reveal that roughness at this level has insignificant reflectivity impact. However, this level of intermixing could cause a reflectivity drop of ~ 4%. Ion bombardment simulations provide estimates of the atom energy distribution arriving at the mask blank surface during Mo and Si deposition, and of stopping depths of each atom into the underlying layer. Key parameters to modify the deposition energy, and potentially the intermixing depth, are summarized: beam voltage and deposition pressure. Lower ion beam voltage or higher pressure are both predicted to reduce the intermixing depth by 20-30%. Bright-field TEM measurements of multilayers deposited at various deposition conditions confirm the predictions.