Optimization of ion-beam-deposited Mo/Si multilayers for extreme ultraviolet masks

The aim of our work is to investigate deposition conditions to further optimize the reflectivity of Mo/Si multilayers (MLs) for reflective coatings of extreme ultraviolet mask blanks. Dark-field transmission electron microscopy (TEM) measurements imply interfacial roughness values of 80 to 150 pm. Bright-field TEM images indicate intermixed layer thicknesses of 0.4 to 1.8 nm. We present reflectivity calculations including these two ML imperfections and compare against prior empirical results. Both interfacial roughness and intermixing are predicted to lower the maximum reflectivity. For example, interfacial roughness of 400 pm lowers the maximum reflectivity by ∼2 % . Smoother interfaces with sub-100 pm allow recovery of ∼1.5 % of the reflectivity. Mo/Si intermixing is predicted to lower the maximum reflectivity by up to 6% relative to an ideal ML. Reflectivity could be recovered by ∼3 % by reducing the intermixing depth by only 20% to 30%. We demonstrate ways to reduce roughness or intermixing by ion beam deposition (IBD). 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 IBD parameters to reduce the deposition energy, and hence the intermixing depth, are summarized: beam voltage and deposition pressure. Lower ion beam voltage or higher pressure can together reduce the intermixing depth by at least 20% to 30%. Bright-field TEM measurements of MLs deposited at various deposition conditions confirm the intermixing predictions.