Comparison of deposition techniques for Mo/Si reflective multilayers for EUV mask blanks

We present a comparison of the properties of the Mo/Si multilayers deposited by three techniques: secondary ion beam deposition (IBD), DC magnetron sputtering (MS), and biased-target ion beam deposition (BTIBD). Based on sputter simulations, we demonstrate that the energetics of the depositing adatoms and the intermixing depth depend strongly on the target/beam voltage, process pressure and target-to-substrate distance. XRR measurements indicate that the multilayers deposited by MS are much rougher than those deposited by IBD and BTIBD. SIMS and RBS measurements suggest that the Mo layers have low contamination of Fe and Al indicating insignificant beam overspray onto the target mounting material, as well as minimal Ar implantation. High-resolution TEM analysis shows intermixed layer thickness in the range of 0.3 to 1.9 nm and interfacial roughness in the range of 100 to 500 pm. Optical calculations suggest that the observed level of contamination is expected to have minimal impact on the EUV peak reflectivity. The intermixed layers could result in a 2-5% decrease in peak reflectivity. Further, the optical simulations predict that the 100-200pm range interfacial roughness in IBD and BTIBD cause a <0.5% reduction in peak reflectivity, whereas the 200-500pm range interfacial roughness in MS films is expected to compromise the peak reflectivity by up to 5%. This comparison of a variety of thin film techniques for the deposition of Mo and Si has allowed us to gain a deeper insight into the challenges faced during the fabrication of EUV mask blanks.