In situ collector cleaning and extreme ultraviolet reflectivity restoration by hydrogen plasma for extreme ultraviolet sources

Laser-produced Sn plasmas used to generate extreme ultraviolet (EUV) light for lithography cause the release of Sn ions and neutrals in the EUV source chamber. These Sn atoms condense and deposit on the multilayer collector optic, which reduces its ability to reflect EUV light. This lowers the source throughput and eventually necessitates downtime for collector cleaning. In this paper, an in situ plasma-based collector cleaning technique is presented and experimentally demonstrated. First, the technique is shown to completely clean a 300 mm diameter stainless steel dummy collector. Second, simulations and secondary ion mass spectroscopy depth profiles show that the technique does not Erode the real multilayer mirrors. Finally, EUV reflectivity measurements demonstrate the ability of the technique to restore EUV reflectivity to Sn-coated multilayer mirrors. This technique has the potential to be used in conjunction with source operation, eliminating cleaning-related source downtime.