Development of an in-situ Sn cleaning method for extreme ultraviolet light lithography

The development of a successful extreme ultraviolet light source for lithography relies on the ability to maintain collector optic cleanliness. Cleanliness is required to maintain the reflectivity of the collector optic, thus maintaining the light power output at the intermediate focus. In this paper, an in-situ method is explored to remove Sn from a contaminated collector optic. Hydrogen plasma is used to promote Sn etching while maintaining the integrity of the collector optic's multi-layer structure. The removal rate of Sn is investigated as a function of various operational parameters including chamber pressure, plasma electron density, as well as plasma electron temperature. Initial results are presented using an external RF-plasma source. The use of the collector optic as a RF-antenna is also investigated to optimize the etching rate of the hydrogen plasma. Initial plasma parameter measurements reveal electron densities on the order of 10¹¹-10¹² cm^-3, with electron temperatures on the order of 1-3 eV. An optimized etch rate of ~125 nm/min off of Si was observed using 1000 W, 80 mTorr, and a flow rate of 50 sccm of H2. These initial measurements are used as a basis for optimizing the etching rate off of the collector optic. Such results are important in allowing the long-term usage of a single collector optic to minimize operating costs involved with replacing the optic as well as tool downtime.