FIELD OF THE INVENTION

The present invention relates to a process for cleaning optical elements for the ultraviolet wavelength range having at least one metal-containing layer on a surface. Furthermore, the invention relates to an optical system comprising an optical element for the ultraviolet wavelength range having at least one metal-containing layer on a surface. The present application claims the priority of German Patent Application 10 2017 207 030.7 of Apr. 26, 2017, the disclosure of which is hereby incorporated in to the present application in its entirety.

BACKGROUND

In particular, in the relatively short-wave ultraviolet wavelength range up to 120 nm, also called vacuum ultraviolet radiation (VUV radiation), it is not possible just to employ transmissive optical elements, rather it is often necessary also to have recourse to reflective optical elements. Optical elements comprising, on a substrate, a metallic layer and overlying that a protective layer or a highly reflective layer system have proven to be particularly worthwhile in this case. The protective layer or the layer system can comprise one or more fluorides. Such optical elements are known for example from U.S. Pat. No. 5,850,309.

The protective layer and the layer system, respectively, have the function, inter alia, of protecting the metallic layer against contamination, in particular as a result of oxidation. However, it has been observed that even low oxygen or water concentrations in the parts per million range can result in a degradation of the coating on the metallic layer and subsequently of the metallic layer as well. DE 10 2009 001 488 A1 discloses, for optical elements for the extreme ultraviolet wavelength range, approximately 1 nm to 20 nm, cleaning their surface with activated hydrogen and simultaneously providing a monomer, in particular of a hydrocarbon, which polymerizes under the influence of the activated hydrogen at the surface of the optical element to be cleaned and thereby forms a protective layer having low adsorption for metal hydrides which may form during the cleaning and in particular during operation with plasma radiation sources.

SUMMARY

It is an object of the present invention to propose a cleaning process for optical elements for the ultraviolet wavelength range, in particular up to 120 nm.

This object is achieved, according to one formulation, through a process for cleaning optical elements for the ultraviolet wavelength range having at least one metal-containing layer on a surface, comprising:

supplying activated hydrogen to the surface having a metal-containing layer;

subsequently supplying inert gas with an H2O volume fraction of less than 5 ppm.

Supplying activated hydrogen, i.e. hydrogen radicals or ionized hydrogen, results in contaminations being cleaned from the surface of the optical element. In particular, the oxidation of the metal-containing layer can be reversed. In order in this case to reduce the risk of renewed oxidation of the cleaned surface as a result of a concentration of water, oxygen, hydroxide ions or hydroxyl radicals, which is potentially increased as a result of the treatment with activated hydrogen, the atmosphere in the region of the surface of the optical element having a metal-containing layer is replaced by dry inert gas, namely inert gas having an H2O volume fraction of less than 5 ppm, preferably less than 1 ppm, particularly preferably less than 0.2 ppm. The proposed process is particularly well suited to optical elements having a metallic layer, in particular without an additional protective coating or with one which has already been damaged during operation of the optical element such that the metallic layer is no longer completely covered or through which a finite proportion of oxygen can diffuse.

Advantageously, the activated hydrogen is supplied by molecular hydrogen being added to inert gas with a proportion of 1-20 vol. % and this mixture being guided past a heating wire arrangement or being exposed to dissociating radiation. By way of the action of heat or the dissociating radiation, the molecular hydrogen can be split into hydrogen radicals or else ionized. The activated hydrogen has higher reaction rates than molecular hydrogen, with the result that better cleaning rates can also be achieved with activated hydrogen.

Preferably, the inert gas with an H2O volume fraction of less than 5 ppm, preferably less than 1 ppm, particularly preferably less than 0.2 ppm, is supplied for between 1 min and 100 min in order to sufficiently exchange and dry the atmosphere in the region of the optical element to be cleaned.

Advantageously, the activated hydrogen is supplied for between 1 min and 100 min in order to sufficiently remove contaminations present such as, for instance, hydrocarbon-containing deposits or oxidation of the surface, in particular of the metal-containing layer.

In preferred embodiments, the optical element is heated. This can be done while supplying activated hydrogen, while supplying dry inert gas or during both. During the process of supplying activated hydrogen, the heat input makes it possible to accelerate the reduction of the contamination through hydrogen on the surface of the optical element. During the process of supplying dry inert gas, the heat input makes it possible to accelerate the drying of the surface of the optical element.

Preferably, the optical element is heated to a temperature of up to 120° C., particularly preferably up to 100° C. In this regard, the cleaning can be accelerated without the optical element being adversely affected by excessively high heat input. If the cleaning takes place during ongoing operation, the optical element is preferably heated to approximately 40° C. to 50° C.

In order to support the drying process, advantageously, a cold trap and/or a drying agent and/or an area comprising a platinum metal are/is arranged in the vicinity of the surface having a metal-containing layer. It is thereby possible to further reduce the water and oxygen concentration in the inert gas.

In preferred embodiments, the sequence of supplying activated hydrogen and supplying inert gas is repeated at least once. In this way, the cleaning can be made more gentle and/or faster, for example by adapting the duration of the individual operations. In this way, the cleaning can be carried out particularly well during ongoing operation since, as necessary, activated hydrogen can be supplied again and again and in between the probability of oxidation can be reduced by purging with the dry inert gas.

Particularly preferably, the water concentration and/or oxide concentration are/is measured at the surface having a metal-containing layer and one or a plurality of operations for cleaning the optical element is or are begun or ended depending on one or a plurality of defined threshold values. In this regard, for instance, it is possible to begin the supply of activated hydrogen in the event of an oxide concentration being exceeded and to switch to the supply of dry inert gas in the event of said oxide concentration being undershot. It is also possible for only the water concentration or both parameters to be monitored and used for starting or ending the supply of activated hydrogen or dry inert gas.

Optionally, a concluding operation can involve adding inert gas having a fluorine or fluoride fraction or a hydrocarbon fraction. The hydrocarbon fraction can result in the formation of a polymeric protective layer affording protection against oxidation of the surface of the optical element having a metal-containing layer. The fluorine or fluoride fraction can result in the formation of a fluoride protective layer on the surface for protection against oxidation.

Furthermore, the object is achieved with an optical system comprising an optical element for the ultraviolet wavelength range having at least one metal-containing layer on a surface in a housing and a supply device for supplying activated hydrogen into the housing, a supply device for supplying inert gas having an H2O volume fraction of less than 5 ppm, preferably less than 1 ppm, particularly preferably less than 0.2 ppm, into the housing, and a discharge device for pumping gas out of the housing. Such an optical system can be used for cleaning the optical element having at least one metal-containing layer on a surface as just described.

Advantageously, for supporting the reduction operation and/or the drying operation, said optical system comprises a heating device and/or a radiation source.

Preferably, for reducing further the water and oxygen content in the inert gas, a cold trap and/or a drying agent and/or an area comprising a platinum metal are/is arranged in the vicinity of the surface having a metal-containing layer.

In particularly preferred embodiments, said optical system comprises a measuring unit for measuring the water and/or oxide concentration at the surface having a metal-containing layer. The cleaning process explained above can be controlled depending on the measured water and/or oxide concentration.

