Patent ID: 7462842

Claim:
The method for removing, cleaning or preventing contamination or combinations thereof on a surface of an optical element of an EUV-lithographic device during their exposure to EUV radiation in an EUV-lithographic system having a residual gas atmosphere, comprising the step of: supplying gas to the EUV-lithographic device, wherein said gas is a least one of the group consisting of carbon monoxide, carbon dioxide, hydrogen, oxygen, nitrogen, SF 6 , Ne, Kr, Xe, alkane, alkene, alkyne, ketone, aldehyde, formic acid, acetic acid, propionic acid, hydrogen peroxide, hydrazine, SO 2 , F, Cl, Br, I, chloromethane, dicholoromethane, trichloromethane, carbon tetrachioride, carbon tetrafluoride, fluoromethane, difluoromethane, ammonia, phospine, antimony hydride, hydrogen fluoride, hydrogen chloride, hydrogen bromide, hydrogen iodide, boron fluoride, diborane, nitrogen trifluoride, hydrogen sulfide, hydrogen selenide, hydrogen telluride, and other halogen/hydrogen containing gas, and wherein a photocurrent generated during radiation by photo emission from the surface is measured and, if at least two predefined threshold values of the photocurrent are exceeded or fallen short of, at least one gas is supplied or the supply of at least one gas is interrupted.