Optical element for reflection of UV radiation, method for manufacturing the same and projection exposure apparatus comprising the same
View Patent ↗An optical element ( 1 a, 1 b ) for reflecting UV radiation at an operating wavelength below 250 nm, preferably at 193 nm, which has a substrate ( 2 a, 2 b ), a reflective layer ( 3 a, 3 b ) made of aluminum superimposed on the substrate ( 2 a, 2 b ). The reflective aluminum layer ( 3 a, 3 b ) is not transparent to UV radiation and is (111)-plane oriented. The reflective optical element ( 1 a, 1 b ) has a reflectivity of more than 85%, preferably of more than 88%, and even more preferably of more than 92%, in a range of incident angles of at least 10°, preferably of at least 15°, at the operating wavelength. Also disclosed is an optical element having a reflective layer made from a material having a melting point higher than that of aluminum, as well as methods for producing such optical elements, and optical arrangements incorporating such optical elements.
1. An optical element configured to reflect ultraviolet radiation at an operating wavelength (λ0) below 250 nm, comprising:
a substrate,
a reflective aluminum layer superimposed on the substrate, the reflective aluminum layer not being transparent to the ultraviolet radiation,
wherein:
the reflective aluminum layer is (111)-plane oriented,
the reflective optical element has a reflectivity of more than 85% in a range of incident angles of at least 10° at the operating wavelength, and
the reflective aluminum layer is superimposed on a base material having a hexagonal lattice structure with a (001)-plane or a (002)-plane forming a hexagonal surface structure.
2. The optical element according to claim 1 , wherein a ratio between two integer multiples of the lattice constants (d111A1; d111Si, d111CaF2, d002LaF3) of the reflective aluminum layer in the (111)-plane and of the hexagonal surface structure of the base material differs by less than 10% from unity, the integer multiples of the lattice constants (d111A1; d111Si, d111CaF2, d002LaF3) being selected from a range of values between one and ten.
3. The optical element according to claim 1 , wherein the base material is selected from the group consisting of: calcium fluoride (CaF2), silicon (Si), and lanthanum fluoride (LaF3).
4. The optical element according to claim 1 , wherein the substrate is made of the base material.
5. The optical element according to claim 1 , wherein an interlayer between the substrate and the reflective aluminum layer is made of the base material.
6. The optical element according to claim 1 , wherein a dielectric multi-layer system is superimposed on the reflective aluminum layer.
7. The optical element according to claim 1 , wherein a protective layer of chiolite (Na5Al3F14) is superimposed on the reflective aluminum layer.
8. The optical element according to claim 1 , wherein the substrate has a thermal conductivity of at least 12 W/(K m).
9. A microlithographic arrangement comprising the optical element according to claim 1 and configured as at least one of: a projection exposure apparatus, an optical beam delivery system, an illumination system, and a projection lens.