Molecular contamination phenomena in EUVL and mitigation methods with hydrogen

Abstract

In accordance with Gordon Moore’s law, the number of transistors that can be placed on an integrated chip of a certain size is increasing exponentially. This can only be realized with an increase in the spatial resolution of the lithography systems, allowing smaller feature sizes to be printed. Today, Extreme ultraviolet lithography (EUVL), employing a wavelength of 13.5 nm, is the prospective method that fits in this evolution. To focus the generated EUV light, specifically tailored Mo/Si multilayer mirrors are employed, which must keep their reflectivity for 30,000 hours of operation. Therefore contamination on the mirror surfaces (e.g., oxidation, carbon contamination, metal contamination) needs to be prevented. To this end, a thin capping layer (e.g., Ru, Rh, TiO2, ZrO2) can be applied on top of the mirror surfaces. Nevertheless even capped mirrors are prone to contamination upon prolonged use in EUVL. With all the active contamination mechanisms evident in the EUV chamber, the development of effective mitigation methods is required. In this study, the contamination phenomena on the EUVL mirrors and some prospective mitigation methods were studied. It was observed that an oxide film, growing on top of a Ru capped surface, can be removed effectively by exposing it to atomic or molecular hydrogen. It was found that the atomic hydrogen cleaning process proceeded more efficiently, compared with the molecular hydrogen cleaning process. Another type of contamination was observed to arise from metal hydride generation and decomposition phenomena in EUVL. Dedicated pair of quartz crystal microbalances were used to determine the kinetics of generation and decomposition of Sn hydrides. It was shown that atomic hydrogen species are prone to form volatile metal hydrides from Sn deposits, which can subsequently decompose on vital optical surfaces. Nevertheless, capping layers made of a stable metal oxide display significant resistance to this metallic contamination compared to metallic capping layers. To determine quantitatively the flux of atomic hydrogen incident on the exposed surfaces, a highly accurate and sensitive sensor was developed.