Optical element for UV or EUV lithography with coatings having optimized stress and thickness
View Patent ↗To reduce deformations which may be caused by a functional coating on a substrate in an optical element for UV or EUV lithography, an optical element is suggested comprising a functional coating ( 46 ) on a first surface ( 42 ) of a substrate ( 41 ), wherein the substrate ( 41 ) comprises a second surface ( 43 ) having a common edge ( 45 ) with the first surface ( 42 ), and the second surface ( 43 ) also has a coating ( 47 ) and the thickness t 2 and the stress σ 2 of the coating ( 47 ) on the second surface ( 33, 43 ) are chosen such that, in combination with the thickness t 1 and the stress σ 1 of the functional coating ( 36, 46 ) on the first surface ( 32, 42 ), the condition t 1 · σ 1 t 2 · σ 2 = X is fulfilled, wherein X has a value between 0.8 and 5.0.
1. An optical element configured for at least one of ultraviolet and extreme-ultraviolet lithography, comprising:
a substrate having a first surface and a second surface that has a common edge with the first surface,
a functional coating on the first surface, and
a coating on the second surface,
wherein the thickness t 2 and the stress σ 2 of the coating on the second surface in combination with the thickness t 1 and the stress σ 1 of the functional coating on the first surface fulfill the condition
t
1
·
σ
1
t
2
·
σ
2
=
X
,
wherein X has a value between 0.8 and 5.0.
2. The optical element according to claim 1 , wherein the functional coating is configured as at least one of an optically effective coating and a polishable coating.
3. The optical element according to claim 1 , wherein at least one of the functional coating on the first surface and the coating on the second surface extend to the common edge.
4. The optical element according to claim 1 , wherein tensile stresses or compressive stresses stemming from at least one of the material and the thickness of the coating on the second surface are present also in the functional coating on the first surface.
5. The optical element according to claim 1 , wherein X has a value between 1.2 and 3.0.
6. The optical element according to claim 5 , wherein X has a value between 1.4 and 1.8.
7. The optical element according to claim 1 , wherein the stress σ 2 of the coating on the second surface is at least approximately equal to the stress σ 1 of the functional coating on the first surface, in order to fulfill the condition
t
1
t
2
=
X
,
and wherein X has a value between 0.8 and 5.0.
8. The optical element according to claim 7 , wherein X has a value between 1.2 and 3.0.
9. The optical element according to claim 8 , wherein X has a value between 1.4 and 1.8.
10. The optical element according to claim 1 , wherein the substrate has a third surface opposite the first surface, and further comprising a further coating on the third surface.
11. The optical element according to claim 10 , wherein the product of the thickness t 3 and the stress σ 3 of the further coating on the third surface is at least approximately equal to the product of the thickness t 1 and the stress σ 1 of the functional coating on the first surface.
12. The optical element according to claim 1 , wherein the coefficient of thermal expansion of both the functional coating and the coating on the second surface is greater than the coefficient of thermal expansion of the substrate.
13. The optical element according to claim 1 , wherein the coefficient of thermal expansion of both the functional coating and the coating on the second surface is less than the coefficient of thermal expansion of the substrate.
14. The optical element according to claim 1 , wherein the coefficient of thermal expansion of the coating on the second surface is at least approximately equal to the coefficient of thermal expansion of the substrate.
15. The optical element according to claim 1 , wherein the coefficient of thermal expansion of the coating on the second surface has the same sign as the coefficient of thermal expansion of the substrate.
16. The optical element according to claim 1 , wherein the coating on the second surface comprises at least one material from the group consisting of amorphous silicon, nickel-phosphorus alloy, diamond-like carbon, molybdenum, and silicon dioxide.
17. The optical element according to claim 1 , wherein the substrate consists essentially of a material from the group consisting of copper, copper alloy, aluminum, aluminum alloy, aluminum-silicon alloy, fused silica, doped fused silica, titanium-doped fused silica, glass ceramic, calcium fluoride, silicon carbide, silicon-silicon carbide, and silicon.
18. The optical element according to claim 1 , formed as a facet of a facet mirror or as a mirror segment of a mirror segment array.
19. An illumination system configured for at least one of ultraviolet and extreme-ultraviolet lithography, comprising:
at least one optical element comprising
a substrate having a first surface and a second surface that has a common edge with the first surface,
a functional coating on the first surface, and
a coating on the second surface,
wherein the thickness t 2 and the stress σ 2 of the coating on the second surface in combination with the thickness t 1 and the stress σ 1 of the functional coating on the first surface fulfill the condition
t
1
·
σ
1
t
2
·
σ
2
=
X
,
wherein X has a value between 0.8 and 5.0.
20. A lithography apparatus configured for at least one of the ultraviolet or extreme ultraviolet wavelength ranges, comprising:
at least one optical element comprising
a substrate having a first surface and a second surface that has a common edge with the first surface,
a functional coating on the first surface, and
a coating on the second surface,
wherein the thickness t 2 and the stress σ 2 of the coating on the second surface in combination with the thickness t 1 and the stress σ 1 of the functional coating on the first surface fulfill the condition
t
1
·
σ
1
t
2
·
σ
2
=
X
,
wherein X has a value between 0.8 and 5.0.