Monitoring reticle molecular contamination in ASML EUV Alpha Demo Tool

Molecular contamination risk to an EUV reticle exposed to up to 1600J/cm2 of 13.5 nm EUV radiation in ASML Alpha Demo Tool (ADT) is negligible. Carbon film (< 0.5 nm) deposition and oxidation (surface oxides ~1 nm) are the two main molecular contaminants observed on this EUV reticle. These results run counter to recent empirical results obtained from EUV micro-exposure tools (MET) which suggest that molecular contamination of EUV reticles, even at the very low partial pressures expected in the exposure chamber of EUV exposure tools, poses challenges in the implementation of EUV lithography in large-scale volume manufacturing of devices. To assess the molecular contamination risk to the use and lifetime of a given EUV reticle, we monitored the contamination buildup on a specially designed reticle during one year as it was exposed on ASML ADT located in Albany, New York. The reticle was analyzed with a suite of complementary surface analytical technique (such as Auger Electron Spectroscopy, AES) and chemical analytical techniques (such as Grazing Incidence Reflection Fourier Transform Infra-red Spectroscopy, GIR-FTIR), as well as imaging technique (such as Scanning Electron Microscopy). The influence of molecular contamination on the reflectivity of this reticle was measured at the Lawrence Berkeley Advanced Light Source EUV reflectometry. The differences in the contamination outcome of EUV reticles exposed in ASML ADT and MET may be related to the implementation of active contamination mitigation schemes in ADT and the lack of similar schemes in METs.