The effect of hydrogen ion bombardment on the capping layers of extreme ultraviolet mirrors

Abstract Extreme ultraviolet (EUV) mirrors demand exceptionally high reflectivity, but contamination from carbon, tin, oxide layers, and etching debris during use can degrade their performance and reduce production yields. Hydrogen ion bombardment is commonly employed for surface cleaning; however, prolonged bombardment may induce surface blistering, damaging the mirrors and decreasing reflectivity. This study investigates the behaviour of EUV mirror coatings under hydrogen ion bombardment. Multilayer B 4 C/Mo/B 4 C/Si coatings were deposited on Si substrates using ion beam sputtering with fixed ion beam voltage and current. Surface roughness was measured using atomic force microscopy (AFM), initial thickness was assessed via x-ray reflectometry (XRR), and the structure was analyzed through high-resolution transmission electron microscopy (HR-TEM). Optimized deposition parameters yielded a maximum reflectivity of 51.6% at a 19° incidence angle, as measured by an EUV reflectometer. During operation, EUV exposure induces surface oxidation, while cleaning with hydrogen ions may cause blistering. Oxide coatings possess excellent oxidation resistance and hydrogen protection. Therefore, an oxide protective layer was deposited using atomic layer deposition (ALD) after mirror fabrication. The protective coatings were subjected to hydrogen ion bombardment at fixed voltages and currents for varying durations. Surface morphology, roughness, and cross-sectional structure were characterized using scanning electron microscopy (SEM), AFM, and HR-TEM to evaluate the protective layer’s effectiveness in mitigating hydrogen-induced damage.