Highly selective etching of SnO ₂ absorber in binary mask structure for extreme ultra-violet lithography

Among the core EUVL (extreme ultra-violet lithography) technologies for nanoscale patterning below the 30 nm node for Si chip manufacturing, new materials and fabrication processes for highperformance EUVL masks are of considerable importance due to the use of new reflective optics. In this work, the selective etching of SnO ₂ (tin oxide) as a new absorber material, with high EUV absorbance due to its large extinction coefficient, for the binary mask structure of SnO ₂ (absorber layer)/Ru (capping/etch stop layer)/Mo-Si multilayer (reflective layer)/Si (substrate), was investigated. Because infinitely high selectivity of the SnO ₂ layer to the Ru ESL is required due to the ultrathin nature of the Ru layer, various etch parameters were assessed in the inductively coupled Cl ₂/Ar plasmas in order to find the process window required for infinitely high etch selectivity of the SnO ₂ layer. The results showed that the gas flow ratio and V _dc value play an important role in determining the process window for the infinitely high etch selectivity of SnO ₂ to Ru ESL. The high EUV-absorbance SnO ₂ layer, patternable by a dry process, allows a smaller absorber thickness, which can mitigate the geometric shadowing effects observed for high-performance binary EUVL masks.