In EUV lithography, carbon deposition on the optics seriously degrades performance. A nice experiment at the University of Hyogo has shown that bleeding oxygen or ozone into the optics while irradiating with EUV can remove the deposits. [J. Vac. Sci. Technol. B 29, 011030 (2011); doi:10.1116/1.3533945]
An old invention by Somekh comes to mind; he discovered the same trick for electron beam systems in 1999. [US 6394109]