Use of molecular oxygen to reduce EUV-induced carbon contamination of optics

Carbon deposition and removal experiments on Mo/Si multilayer mirror (MLM) samples were performed using extreme ultraviolet (EUV) light on Beamline 12.0.1.2 of the Advanced Light Source, Lawrence Berkeley National Laboratory (LBNL). Carbon (C) was deposited onto Mo/Si multilayer mirror (MLM) samples when hydrocarbon vapors were intentionally introduced into the MLM test chamber in the presence of EUV at 13.44 nm (92.3eV). The carbon deposits so formed were removed by molecular oxygen + EUV. The MLM reflectivities and photoemission were measured in-situ during these carbon deposition and cleaning procedures. Auger Electron Spectroscopy (AES) sputter-through profiling of the samples was performed after experimental runs to help determine C layer thickness and the near-surface compositional-depth profiles of all samples studied. EUV powers were varied from similar to0.2 mW/mm(2) to 3 MW/mm(2) (at 13.44 nm) during both deposition and cleaning experiments and the oxygen pressure ranged from similar to5 x 10(-5) to 5 x 10(-4) Torr during the cleaning experiments. C deposition rates as high as similar to8 nm/hr were observed, while cleaning rates as high as similar to5 nm/hr could be achieved when the highest oxygen pressures were used. A limited set of experiments involving intentional oxygen-only exposure of the MLM samples showed that slow oxidation of the MLM surface could occur.