Patent ID: 8097092

Claim:
A method of cleaning and after treatment of optical surfaces in an irradiation unit, the irradiation unit comprising a plasma radiation source emitting EUV-radiation and/or soft X-rays, a first volume following the plasma radiation source and containing first optical components with the optical surfaces, and a second volume following the first volume and containing second optical components, the method comprising at least one cleaning step in which a first gas or gas mixture is brought into contact with the optical surfaces, thereby forming volatile compounds with contaminations released from a plasma radiation source deposited on the optical surfaces, wherein: the compounds are pumped out of the first volume together with the first gas or gas mixture, in an after treatment step following the cleaning step the radiation source is operated once or several times in order to release residues of the cleaning step from the optical surfaces by irradiating the optical surfaces with the EUV-radiation or soft X-rays while the first volume is separated from the second volume, and the released residues are pumped out of the first volume.