Collector optic cleaning by in-situ hydrogen plasma

Extreme ultraviolet (EUV) lithography sources produce EUV photons by means of a hot, dense, highly-ionized Sn plasma. This plasma expels high-energy Sn ions and neutrals, which deposit on the collector optic used to focus the EUV light. This Sn deposition lowers the reflectivity of the collector optic, necessitating downtime for collector cleaning and replacement. A method is being developed to clean the collector with an in-situ hydrogen plasma, which provides hydrogen radicals that etch the Sn by forming gaseous SnH₄. This method has the potential to significantly reduce collector-related source downtime. EUV reflectivity restoration and Sn cleaning have been demonstrated on multilayer mirror samples attached to a Sn-coated 300mm-diameter steel dummy collector driven at 300W RF power with 500sccm H₂ and a pressure of 260mTorr. Use of the in-situ cleaning method is also being studied at industrially-applicable high pressure (1.3 Torr). Plasma creation across the dummy collector surface has been demonstrated at 1.3 Torr with 1000sccm H₂ flow, and etch rates have been measured. Additionally, etching has been demonstrated at higher flow rates up to 3200sccm. A catalytic probe has been used to measure radical density at various pressures and flows. The results lend further credence to the hypothesis that Sn removal is limited not by radical creation but by the removal of SnH₄ from the plasma. Additionally, further progress has been made in an attempt to model the physical processes behind Sn removal.