Patent ID: 8076655

Claim:
Method of cleaning optical surfaces in an irradiation unit for EUV-radiation in order to remove contaminations deposited on said optical surfaces, the method including a cleaning step in which a first gas or gas mixture is brought into contact with said optical surfaces thereby forming a volatile compound with a first portion of said contaminations, wherein in an operation pause of the irradiation unit for EUV-radiation prior to the cleaning step, a pretreatment step is performed, in which a second gas or gas mixture is brought into contact with said optical surfaces, wherein said second gas or gas mixture is selected to react with a second portion of said contaminations different from said first portion to form a reaction product, which is able to form a volatile compound with said first gas or gas mixture, wherein after the cleaning step residues still remaining on the optical surfaces are removed from said optical surfaces or converted into compounds less absorbing radiation of the irradiation unit by a treatment with a third gas or gas mixture.