Ru/Ta bilayer approach to EUV mask absorbers: Experimental patterning and simulated imaging perspective

The optical properties and geometry of EUV mask absorbers play an essential role in determining the imaging performance of a mask in EUV lithography. Imaging metrics, including Normalized Image Log Slope (NILS), Telecentricity Error (TCE), and Best Focus Variation (BFV) through pitch deteriorate because of Mask 3-Dimensional (M3D) effects in EUV lithography, which limits the production efficiency. Alternative absorbers, including alloys of Ru and Ta, are anticipated to reduce some of the M3D effects; however, patterning these materials is challenging due to their low etch rates and poor etch selectivity against the Ru mask capping layer. Therefore, we propose a Ru/Ta bilayer approach to EUV mask absorbers and investigate it from a patterning and imaging standpoint. The top Ru layer thickness is calculated using the thin film interference phenomena, and we determine the bottom Ta layer that can produce improved NILS by utilizing the total absorber thickness optimization methodology. We demonstrate the patterning of the Ru/Ta bilayer using a two-step etch; the top Ru layer is patterned with Cl₂-O₂ Reactive Ion Etch (RIE), and the bottom Ta layer with Cl₂-N₂ RIE. The geometry and morphology of the patterned bilayer stack are investigated using TEM (Transmission Electron Microscopy), and interdiffusion at the interface of Ru and Ta is studied using EDS-STEM (Energy Dispersive X-ray Spectroscopy-Scanning Transmission Electron Microscopy). The non-ideal traits of the Ru/Ta bilayer stack, determined by experimental characterization techniques, are used to simulate the imaging performance and then compared against an ideal Ru/Ta bilayer stack, along with the reference Ta-based absorber. Even when non-idealities are considered, the simulation findings demonstrate that the Ru/Ta bilayer absorber exhibits improved NILS and reduced BFV compared to the Ta-based absorber. The outcomes encourage further research into the possibilities of multilayer absorbers, to tailor their optical characteristics by varying the thickness of individual layers.