Spectral purity filter for a multi-layer mirror, lithographic apparatus including such multi-layer mirror, method for enlarging the ratio of desired radiation and undesired radiation, and device manufacturing method
A multi-layer mirror includes on top of the multi-layer mirror a spectral purity enhancement layer, for example for application in an EUV lithographic apparatus. This spectral purity enhancement layer includes a first spectral purity enhancement layer, but between the multi-layer mirror and first spectral purity enhancement layer there may optionally be an intermediate layer or a second spectral purity enhancement layer and intermediate layer. Hence, multi-layer mirrors with the following configurations are possible: multi-layer mirror/first spectral purity enhancement layer; multi-layer mirror/intermediate layer/first spectral purity enhancement layer; and multi-layer mirror/second spectral purity enhancement layer/intermediate layer/first spectral purity enhancement layer. The spectral purity of normal incidence radiation may be enhanced, such that DUV radiation is diminished relatively stronger than EUV radiation.
1. A multi-layer mirror, comprising a multi-layer stack, the multi-layer stack comprising a plurality of alternating layers with a multi-layer stack top layer and a spectral filter top layer arranged on the multi-layer stack, the spectral filter top layer comprising a first spectral purity enhancement layer comprising a first material and having a layer thickness d 1 and being arranged on the multi-layer stack top layer, wherein the first material is selected from SiN, Si 3 N 4 , SiO 2 , ZnS, Te, diamond, CsI, Se, SiC, amorphous carbon, MgF 2 , CaF 2 , TiO 2 , Ge, PbF 2 , ZrO 2 , BaTiO 3 , LiF or NaF, wherein the first material and the layer thickness d 1 are selected to enlarge the ratio of radiation having a wavelength selected from a first wavelength range of 5-20 nm and radiation having a wavelength selected from a second wavelength range of 100-400 nm in a beam of radiation of a source emitting radiation with a wavelength in each wavelength range.
2. A multi-layer mirror according to claim 1 , wherein the spectral filter top layer further comprises a cap layer on top of the first spectral purity enhancement layer, the cap layer comprising a second material selected from Ru, BN, B 4 C, B, C, TiN, Pd, Rh, Au, C 2 F 4 , SiN, Si 3 N 4 , SiC, MgF 2 or LiF.
3. A multi-layer mirror according to claim 1 , wherein the spectral filter top layer further comprises a cap layer on top of the first spectral purity enhancement layer, comprising Ru and having a second layer thickness between 0.5 and 2.5 nm.
4. A multi-layer mirror according to claim 1 , wherein the first spectral purity enhancement layer has an imaginary part of the complex index of refraction k≦0.25*n+1.07, wherein n is the real part of the complex index of refraction.
5. A multi-layer mirror according to claim 1 , wherein the first spectral purity enhancement layer has a real part of the complex index of refraction equal or larger than 2 and an imaginary part of the complex index of refraction equal or smaller than 1.6.
6. A multi-layer mirror according to claim 1 , wherein the first material and the layer thickness d 1 are configured to minimize absorption and/or destructive interference of radiation having a wavelength selected from a first wavelength range of 5-20 nm, and maximize absorption and/or destructive interference of radiation having a wavelength selected from a second wavelength range of 100-400 nm.
7. A multi-layer mirror according to claim 1 , wherein the first material and the layer thickness d 1 of the layer comprised in the spectral filter top layer are designed such as to fulfill the following criteria:
r
1
=
t
12
·
t
21
·
r
23
·
exp
(
i
·
2
·
K
2
·
t
)
Q
=
t
12
·
t
21
·
r
23
·
exp
(
i
·
2
·
K
2
·
t
)
r
12
destructive
interference
:
arg
(
Q
)
=
π
(
3
)
wherein:
r 12 is the Fresnel amplitude reflection coefficient for the reflection of a normal incident plane wave entering from a first layer 1 and reflecting at the interface between the first layer 1 and a second layer 2 , wherein the first layer 1 and the second layer 2 are the atmosphere above the multi-layer mirror and the spectral filter top layer, respectively;
r 23 is the Fresnel amplitude reflection coefficient for the reflection of a normal incident plane wave entering from the second layer 2 and reflecting at the interface between the second layer 2 and a third layer 3 , wherein the second layer 2 and the third layer 3 are the spectral filter top layer and the multi-layer stack top layer, respectively;
t pq is the Fresnel amplitude transmission coefficient for transmission of a plane wave from a layer p into a layer q;
λ is the wavelength of the radiation;
t is the thickness of the spectral filter top layer;
K 2 is the wave number in medium p; and
N p =n p +j*k p , the complex index of refraction of medium p.
8. A multi-layer mirror according to claim 1 , wherein the multi-layer mirror is a normal incidence mirror.
9. A multi-layer mirror according to claim 1 , wherein the multi-layer mirror is a normal incidence mirror configured to reflect radiation having a wavelength selected from a first wavelength range of 5-20 nm.
10. A multi-layer mirror according to claim 1 , wherein the multi-layer mirror is a normal incidence Si/Mo multi-layer mirror configured to reflect radiation having a wavelength selected from the wavelength range of 12-15 nm.
11. A multi-layer mirror according to claim 1 , wherein the first spectral purity enhancement layers comprises Si 3 N 4 And has a layer thickness between 4 and 11 nm.
12. A multi-layer mirror according to claim 1 , wherein the multi-layer stack top layer comprises a cap layer, the multi-layer stack top layer comprising a second material selected from Ru, BN, B 4 C, B, C, TiN, Pd, Rh, Au, C 2 F 4 , SiN, Si 3 N 4 , SiC, MgF 2 or LiF.
13. A multi-layer mirror according to claim 1 , wherein the multi-layer stack top layer comprises a cap layer, the multi-layer stack top layer comprising Ru and having a second layer thickness between 0.5 and 2.5 nm.
14. A multi-layer mirror according to claim 1 , wherein the multi-layer stack top layer comprises a cap layer, the multi-layer stack top layer comprising Ru having a second layer thickness between 0.5 and 2.5 nm, and the first spectral filter top layer comprises SiN, Si 3 N 4 , having the layer thickness d 1 between 4 and 11 nm.
15. A multi-layer mirror according to claim 1 , wherein the layer thickness d 1 is between 0.5 and 30 nm.
16. A lithographic apparatus comprising one or more multi-layer mirrors comprising a multi-layer stack, the multi-layer stack comprising a plurality of alternating layers with a multi-layer stack top layer and a spectral filter top layer arranged on the multi-layer stack, the spectral filter top layer comprising a first spectral purity enhancement layer comprising a first material and having a layer thickness d 1 and being arranged on the multi-layer stack top layer, wherein the first material is selected from SiN, Si 3 N 4 , SiO 2 , ZnS, Te, diamond, CsI, Se, SiC, amorphous carbon, MgF 2 , CaF 2 , TiO 2 , Ge, PbF 2 , ZrO 2 , BaTiO 3 , LiF or NaF, and the thickness d 1 is between 0.5 and 30 nm, wherein the first material and the layer thickness d 1 are selected to enlarge the ratio of radiation having a wavelength selected from a first wavelength range of 5-20 nm and radiation having a wavelength selected from a second wavelength range of 100-400 nm in a beam of radiation of a source emitting radiation with a wavelength in each wavelength range.
17. A lithographic apparatus according to claim 16 , further comprising a plurality of multi-layer mirrors, wherein the materials of the layers and the layer thicknesses of the layers in the spectral filter top layers of the different multi-layer mirrors are configured to minimize absorption and/or destructive interference of radiation having a wavelength selected from a first wavelength range of 5-20 nm, and maximize absorption and/or destructive interference of radiation having a wavelength selected from a substantial part of the wavelength range of 100-400 nm.
18. A method, comprising reflecting a beam of radiation with a multi-layer mirror, the multi-layer mirror comprising a multi-layer stack, the multi-layer stack comprising a plurality of alternating layers with a multi-layer stack top layer and a spectral filter top layer arranged on the multi-layer stack, the spectral filter top layer comprising a first spectral purity enhancement layer comprising a first material and having a layer thickness d 1 and being arranged on the multi-layer stack top layer, wherein the first material is selected from SiN, Si 3 N 4 , SiO 2 , ZnS, Te, diamond, CsI, Se, SiC, amorphous carbon, MgF 2 , CaF 2 , TiO 2 , Ge, PbF 2 , ZrO 2 , BaTiO 3 , LiF or NaF, and the thickness d 1 is between 0.5 and 30 nm, wherein the first material and the layer thickness d 1 are selected to enlarge the ratio of radiation having a wavelength selected from a first wavelength range of 5-20 nm and radiation having a wavelength selected from a second wavelength range of 100-400 nm in the beam of radiation, the beam having radiation with a wavelength in each wavelength range.
19. A method according to claim 18 , further comprising reflecting the beam of radiation with a plurality of multi-layer mirrors.
20. A method according to claim 18 , wherein the beam of radiation is reflected by a plurality of multi-layer mirrors, the materials of the layers and the layer thicknesses of the layers in the spectral filter top layers of the plurality of multi-layer mirrors are configured to minimize absorption and/or destructive interference of radiation having a wavelength selected from the first wavelength range of 5-20 nm, and maximize absorption and/or destructive interference of radiation having a wavelength selected from a second wavelength range of 100-400 nm.
21. A device manufacturing method, comprising:
providing a beam of radiation;
patterning the beam of radiation;
projecting the patterned beam of radiation onto a target portion of a substrate; and
reflecting at least part of the beam of radiation on a multi-layer mirror, the multi-layer mirror comprising a multi-layer stack, the multi-layer stack comprising a plurality of alternating layers with a multi-layer stack top layer and a spectral filter top layer arranged on the multi-layer stack, the spectral filter top layer comprising a first spectral purity enhancement layer comprising a first material and having a layer thickness d 1 and being arranged on the multi-layer stack top layer, wherein the first material is selected from SiN, Si 3 N 4 , SiO 2 , ZnS, Te, diamond, CsI, Se, SiC, amorphous carbon, MgF 2 , CaF 2 , TiO 2 , Ge, PbF 2 , ZrO 2 , BaTiO 3 , LiF or NaF, and the layer thickness d 1 is between 0.5 and 30 nm, wherein the first material and the layer thickness d 1 are selected to enlarge the ratio of radiation having a wavelength selected from a first wavelength range of 5-20 nm and radiation having a wavelength selected from a second wavelength range of 100-400 nm in the beam of radiation, the beam having radiation with a wavelength in each wavelength range.