Angle-Resolved XPS Depth Profiling for Extreme Ultraviolet (EUV) Lithography Optics Research – Monoatomic vs Cluster Ion Source

In the Semicon Equipment Life-time department of TNO‚ X-ray photoelectron spectroscopy (XPS) analysis is utilized for characterization of optical surfaces‚ particularly mirrors and reticles for extreme ultraviolet (EUV) lithography. This study focuses on assessing the sensitivity of XPS depth profiling to multilayer mirrors which are similar to the EUV mirrors. A commercial Ru-capped Mo/Si multilayer mirror was annealed in vacuum at 330 °C for 48 hours‚ to induce thermal degradation‚ i.e. intermixing and oxidation. Pre and post-annealing‚ traditional XPS depth profiling was conducted using a monatomic Ar ion-gun. However‚ monatomic Ar+ depth profiling results in artefacts because preferential sputtering of light elements alters the stoichiometry‚ while ion-induced intermixing compromises layer contrast. Development of sputtering methods that reduce those artefacts is therefore of importance. To address these challenges‚ we explore the potential of a Gas Cluster Ion Source (GCIS)‚ known for minimizing the said artefacts. While GCIS is gaining traction in XPS‚ its applicability to EUV mirror analysis requires validation because the ion-solid interaction is material-dependent. Moreover‚ given the limited thickness of the layers in an EUV-mirror compared to the XPS information depth‚ we also investigated the combination of sputter depth profiling with angle-resolved (AR-)XPS to enhance the surface sensitivity. The results are shown in Figure (a) and (b). This comparative study between traditional XPS depth profiling and GCIS‚ along with the exploration of AR-XPS‚ provides insights into optimizing analysis techniques for EUV mirrors and reticles. These findings inform further development towards metrology and understanding of EUV optics degradation.