Method for in situ protection of an aluminum layer and optical arrangement for the VUV wavelength range
A method for in situ protection of a surface ( 7 a ) of an aluminum layer ( 7 ) of a VUV radiation reflecting coating ( 6 ) of an optical element ( 4 ), arranged in an interior of an optical arrangement, against the growth of an aluminum oxide layer ( 8 ), including carrying out an atomic layer etching process for layer-by-layer removal of the aluminum oxide layer from the surface. The etching process includes a surface modification step and a material detachment step. At least one boron halide is supplied as a surface modifying reactant to the interior in pulsed fashion during the surface modification step. A plasma is generated at a surface ( 8 a ) of the aluminum oxide layer, at least during the material detachment step. The atomic layer etching process is performed until the aluminum oxide layer reaches a given thickness (D), or the aluminum oxide layer is kept below that thickness (D) by the process.
1. A method for in situ dynamic protection of a surface of an aluminum layer of a vacuum ultraviolet (VUV) radiation reflecting coating of an optical element, which layer is arranged in an interior of an optical arrangement for a VUV wavelength range, against growth of an aluminum oxide layer, said method comprising:
carrying out an atomic layer etching process for layer-by-layer removal of the aluminum oxide layer from the surface of the aluminum layer, wherein the atomic layer etching process comprises:
a surface modification step and
a material detachment step,
wherein at least one boron halide is supplied as a surface modifying reactant to the interior in pulsed fashion during the surface modification step, and wherein a plasma is generated at a surface of the aluminum oxide layer in the interior, at least during the material detachment step,
wherein the atomic layer etching process is carried out until the aluminum oxide layer reaches a thickness of less than 5 nm, or wherein the aluminum oxide layer is kept at a thickness of less than 5 nm by the atomic layer etching process.
2. The method as claimed in claim 1 , wherein the plasma is generated in pulsed fashion.
3. The method as claimed in claim 1 , wherein the plasma is generated as remote plasma.
4. The method as claimed in claim 3 , wherein the remote plasma is selected from the group consisting essentially of: inductively coupled plasma, surface wave plasma, microwave plasma, and helicon wave plasma.
5. The method as claimed in claim 1 , wherein the surface modification step and/or the material detachment step are carried out during an operating pause of the optical arrangement.
6. The method as claimed in claim 1 , wherein at least the material detachment step is carried out while the optical element is irradiated by VUV radiation.
7. The method as claimed in claim 1 , wherein at least one material-detaching reactant is supplied in pulsed fashion to the interior during the material detachment step.
8. The method as claimed in claim 7 , wherein the at least one material-detaching reactant contains chlorine (Cl 2 ).
9. The method as claimed in claim 1 , further comprising:
monitoring at least one property of the reflective optical element that differs in accordance with the thickness of the aluminum oxide layer, and
adjusting the atomic layer etching process based on the at least one monitored property.
10. The method as claimed in claim 9 , wherein the at least one property of the reflective optical element is reflectivity of the optical element.
11. The method as claimed in claim 1 , wherein BBr 3 and/or BCl 3 is supplied as a surface modifying reactant to the interior in pulsed fashion during the surface modification step.