Abstract
An optical grating (8) includes a substrate (9), on the surface (9a) of which a periodic structure (10) is formed that is embodied to diffract incident radiation (11), in particular incident EUV radiation, with a specified wavelength (.sub.) into a predetermined order of diffraction, in particular into the first order of diffraction (m=+1). The optical grating also has a coating (12) applied onto the periodic structure with at least one layer (13, 14) that is embodied to suppress the diffraction of the incident radiation into at least one higher order of diffraction (m=+2, . . . ) than the predetermined order of diffraction.
Claims
1. An optical grating, comprising: a substrate having a surface with a periodic structure, wherein the structure is embodied to diffract incident radiation with a predetermined wavelength (.sub.T) into a predetermined order of diffraction, and a coating applied onto the periodic structure, wherein the coating has at least one layer that is embodied to suppress the diffraction of the incident radiation into at least one higher order of diffraction than the predetermined order of diffraction, wherein at least one layer of the coating is embodied as a total reflection layer with a critical angle (.sub.T), wherein the critical angle is smaller than an angle of incidence () of the incident radiation for the predetermined order of diffraction and is greater than the angle of incidence () of the incident radiation for at least one higher order of diffraction.
2. The optical grating as claimed in claim 1, wherein the structure is embodied to diffract extreme ultraviolet (EUV) radiation with the predetermined wavelength (.sub.T) into the predetermined order of diffraction.
3. The optical grating as claimed in claim 1, wherein the structure is embodied to diffract the incident radiation with the predetermined wavelength (.sub.T) into the first order of diffraction.
4. The optical grating as claimed in claim 1, wherein the total reflection layer is formed from a material that has an absorption length of more than 10 nm at the predetermined wavelength (.sub.T).
5. The optical grating as claimed in claim 4, wherein the total reflection layer is formed from a material that has an absorption length of more than 50 nm at the predetermined wavelength (.sub.T).
6. The optical grating as claimed in claim 4, wherein the material of the total reflection layer is selected from the group consisting of: Zr, Pd, C, Ru, Mo, Nb, Sn, Cd, or alloys, carbides, nitrides, oxides, borides or silicides thereof.
7. The optical grating as claimed in claim 1, wherein at least one layer of the coating is embodied as an absorber layer which has a greater absorption length for the predetermined order of diffraction than for at least one higher order of diffraction.
8. The optical grating as claimed in claim 7, wherein the absorber layer has a critical angle (.sub.T) that is greater than an angle of incidence () of the incident radiation for the predetermined order of diffraction.
9. The optical grating as claimed in claim 7, wherein the material of the absorber layer is selected from the group consisting of: Si, Mo, or carbides, nitrides, oxides, or borides thereof, and MoSi.sub.2.
10. The optical grating as claimed in claim 7, wherein the absorber layer is applied onto the total reflection layer.
11. The optical grating as claimed in claim 1, wherein the coating has at least one layer that is embodied to diffract incident radiation with a first polarization state perpendicular to a plane of incidence onto the optical grating more strongly in the predetermined order of diffraction than incident radiation with a second polarization state that is perpendicular to the incident radiation with the first polarization state.
12. The optical grating as claimed in claim 1, wherein the coating has at least one layer a thickness (d.sub.2) and a material of which are selected such that constructive interference occurs for the incident radiation with the predetermined wavelength (.sub.L) in the predetermined order of diffraction and destructive interference occurs for at least one higher order of diffraction.
13. The optical grating as claimed in claim 1, wherein the predetermined wavelength (.sub.T) lies in a wavelength range between 13 nm and 16 nm.
14. The optical grating as claimed in claim 13, wherein the coating has a total reflection layer made of Ru, Zr, Pd, Nb, Mo, or alloys, carbides, nitrides, oxides, borides, or silicides thereof, or C, and an absorber layer, applied to the layer of total internal reflection, made of Si, SiC, Si.sub.3N.sub.4, SiO, or SiO.sub.2.
15. The optical grating as claimed in claim 1, wherein the predetermined wavelength (.sub.T) lies in a wavelength range between 6 nm and 8 nm.
16. The optical grating as claimed in claim 15, wherein the coating has a total reflection layer made of Cd or Sn and an absorber layer made of Mo.
17. The optical grating as claimed in claim 1, wherein the periodic structure comprises a blaze structure.
18. The optical grating as claimed in claim 1, having a reflectivity of more than 50% for incident radiation with the predetermined wavelength (.sub.T) in the predetermined order of diffraction.
19. An optical arrangement, comprising: a light source configured to produce radiation and at least one optical grating as claimed in claim 1 and arranged to diffract the radiation of the light source with the predetermined wavelength (.sub.T) into the predetermined order of diffraction.
20. The optical arrangement as claimed in claim 19, configured as an EUV lithography system, wherein the light source is configured to produce EUV radiation.
21. The optical arrangement as claimed in claim 19, wherein the incident radiation is incident on the optical grating at at least one angle of incidence () in an angle of incidence range () between 70 and 90.
22. An optical grating, comprising: a substrate having a surface with a periodic structure, wherein the structure is embodied to diffract incident radiation with a predetermined wavelength (.sub.T) into a predetermined order of diffraction, and a coating applied onto the periodic structure, wherein the coating has at least one layer that is embodied to suppress the diffraction of the incident radiation into at least one higher order of diffraction than the predetermined order of diffraction, wherein the coating has at least one layer that is embodied to diffract incident radiation with a first polarization state perpendicular to a plane of incidence onto the optical grating more strongly in the predetermined order of diffraction than incident radiation with a second polarization state that is perpendicular to the incident radiation with the first polarization state.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Exemplary embodiments are represented in the schematic drawing and are explained in the following description. In the drawing:
(2) FIG. 1 shows a schematic illustration of an EUV lithography apparatus having a monochromator in the form of an optical grating,
(3) FIG. 2 shows a schematic illustration of a detail of the optical grating with a coating having a total reflection layer and an absorber layer,
(4) FIGS. 3A and 3B show schematic illustrations of the critical angle and the absorption length of ZrN, as a material of the total reflection layer, as a function of the wavelength,
(5) FIGS. 4A and 4B show schematic illustrations of the reflectivity of an optical grating with a total reflection layer in the form of ZrN as a function of the wavelength, using a logarithmic and a linear scale,
(6) FIGS. 5A and 5B show schematic illustrations of the critical angle and the absorption length of SiC, as a material of the absorber layer, as a function of the wavelength,
(7) FIG. 6 shows a schematic illustration of the reflectivity of an optical grating with a total reflection layer made of Ru and an absorber layer made of SiC as a function of the wavelength,
(8) FIGS. 7A and 7B show schematic illustrations of the reflectivity of an optical grating with a total reflection layer made of ZrN and an absorber layer made of SiC as a function of the wavelength with a logarithmic and with a linear scale,
(9) FIGS. 8A and 8B show schematic illustrations of the damping of the absorber layer made of SiC and of a transmission filter made of SiC, as a function of the wavelength,
(10) FIGS. 9A and 9B show schematic illustrations of the critical angle and the absorption length of Mo, as a material of the absorber layer, as a function of the wavelength,
(11) FIGS. 10A and 10B show schematic illustrations of the reflectivity of an optical grating with a total reflection layer made of Cd and an absorber layer made of Mo as a function of the wavelength in a logarithmic and in a linear scale,
(12) FIGS. 11A and 11B show schematic illustrations of the reflectivity of an optical grating with a total reflection layer made of Sn and an absorber layer made of Mo as a function of the wavelength in a logarithmic and in a linear scale, and
(13) FIG. 12 shows a schematic illustration of the reflectivity of an absorber layer made of SiC in the case of two different layer thicknesses.
DETAILED DESCRIPTION
(14) FIG. 1 shows a schematic view of an EUV lithography system in the form of an EUV lithography apparatus 1 which consists of a beam-shaping system 2, an illumination system 3 and a projection system 4, which are accommodated in separate vacuum housings and are arranged in succession in a beam path 6 proceeding from an EUV light source 5 of the beam-shaping system 2. A plasma source or a synchrotron can serve for example as the EUV light source 5. The radiation emerging from the EUV light source 5 in the wavelength range between about 5 nm and about 20 nm is first focused in a collimator 7. The desired used wavelength .sub.T can be filtered out with the aid of a downstream monochromator in the form of an optical grating 8. Optionally, the angle of incidence range can be varied in the process, as indicated by a double-headed arrow. In the EUV wavelength range, the collimator 7 and the monochromator 8 are usually formed as reflective optical elements.
(15) The EUV radiation at the used wavelength .sub.T, which is manipulated in the beam-shaping system 2 with regard to wavelength and spatial distribution, is introduced into the illumination system 3, which has a first and a second reflective optical element M1, M2. The two reflective optical elements M1, M2 guide the radiation onto a photomask M as a further reflective optical element, which has a structure that is imaged with the projection system 4 onto a wafer W on a reduced scale. To this end, a third and a fourth reflective optical element M3, M4 are provided in the projection system 4. The reflective optical elements M1 to M4, M respectively have an optical surface, which are arranged in the beam path 6 of the EUV lithography apparatus 1.
(16) The monochromator of the EUV lithography system 1 of FIG. 1 is embodied as an optical grating 8, which is described in more detail below on the basis of FIG. 2.
(17) As may be seenseen in FIG. 2, the optical grating 8 has a substrate 9 made of quartz glass in the shown example. It is understood that the substrate 9 can also be formed from a different material, e.g., a glass ceramic or doped quartz glass. A periodic structure in the form of a blaze structure 10 with steps that are triangular in cross section is formed on the surface 9a of the substrate 9, said blaze structure having a period length or a grating spacing d and a blaze angle .sub.B at which the steps are inclined in relation to the horizontal, illustrated using dashed lines in FIG. 2, along which the substrate 9 or the optical grating 8 extends.
(18) The general grating equation, as set forth below, applies to the blaze structure:
d(sin()+sin((.sub.m))=m .sub.T,
where denotes the angle of incidence and .sub.m denotes the angle of emergence of incident radiation 11 and m denotes the order of diffraction, and .sub.T denotes the used wavelength or a predetermined wavelength, for which the blaze structure 10 is optimized. The angle of emergence , shown in FIG. 2, in this case denotes the angle of emergence of the first order of diffraction (actually .sub.1, i.e., m=+1). For higher orders of diffraction, m=+2, m=+3, etc., the corresponding angles of emergence, .sub.2, .sub.3, etc., not shown in FIG. 2, increase.
(19) The blaze structure 10, i.e., in particular, the grating constant d and the blaze angle .sub.B, are optimized for the predetermined wavelength kr and a predetermined angle of incidence range in such a way that the intensity of the incident radiation 11 is concentrated in exactly one order of diffraction, the first order of diffraction (m=+1) in the shown example, while the diffraction into the higher orders of diffraction (m=+2, m=+3, . . . ), at which the diffracted radiation has wavelengths that lie at integer fractions 1/m of the predetermined wavelength .sub.T, should be minimized.
(20) However, the diffraction of radiation 11 incident on the optical grating 8 cannot be suppressed completely for all higher orders of diffraction m=+2, m=+3, . . . with the aid of the blaze structure 10. Therefore, there may be unwanted wavelength components in the radiation 11a that is diffracted or reflected at the optical grating 8, said unwanted wavelength components deviating from the predetermined wavelength kr, and so the radiation 11a that is reflected or diffracted at the optical grating 8 is not completely monochromatized.
(21) In the example shown in FIG. 2, a coating 12 has been applied onto the substrate 9 of the optical grating 8, more precisely onto the blaze structure 10, said coating consisting of a total reflection layer 13 and an absorber layer 14 applied to the total reflection layer 13, both layers serving to suppress the diffraction of incident radiation 11 at the optical grating 8 into orders of diffraction that are higher than the 1st order of diffraction (m=+1), as will be described in more detail below. The type of material of the total reflection layer 13 and of the absorber layer 14 depends on the predetermined wavelength .sub.T, for which the optical grating 8 is optimized.
(22) As mentioned further above, the used wavelength .sub.T of the EUV lithography apparatus 1, which corresponds to the predetermined wavelength .sub.T, is 13.5 nm in the shown example. As described below on the basis of FIGS. 3A and 3B and FIGS. 4A and 4B, ZrN can be used as material for the total reflection layer 13 for this predetermined wavelength .sub.T, more precisely for a predetermined wavelength .sub.T that lies in an operating wavelength range .sub.1 between approximately 13 nm and 16 nm.
(23) FIG. 3A shows the critical angle .sub.T of ZrN as a function of the wavelength for a wavelength range between approximately 2 nm and approximately 18 nm. As may be seenseen in FIG. 3A, the critical angle .sub.T increases to shorter wavelengths, wherein the critical angle .sub.T in the operating wavelength range .sub.1 is less than the angle of incidence of the incident radiation 11, and so ZrN acts as a total reflection layer 13 for the 1st order of diffraction or for a predetermined wavelength .sub.T, which lies in the operating wavelength range .sub.1. For the purposes of suppressing the 2nd order of diffraction, which corresponds to a wavelength range .sub.2 between 6.5 nm and 8 nm, the critical angle .sub.T should be greater than the angle of incidence so that no total internal reflection of the 2nd order of diffraction occurs at the total reflection layer 13. As may be seen in FIG. 3A, this condition is satisfied for an operating angle of incidence range between a minimum angle of incidence of approximately 72 and a maximum angle of incidence of approximately 80. Consequently, the total reflection layer 13 made of ZrN suppresses the diffraction of the incident radiation 11 into the 2nd order diffraction for incident radiation 11 with an angle of incidence that lies between the minimum angle of incidence .sub.MIN and the maximum angle of incidence .sub.MAX of the operating angle of incidence range and which has a predetermined wavelength .sub.T that lies in the operating wavelength range .sub.1.
(24) FIG. 3B shows the absorption length L in nm, i.e., the length at which the intensity of radiation penetrating into the ZrN material has dropped to 1/e, i.e., to approximately 63% of the intensity at the surface of the ZrN material. As may be seen in FIG. 3B, the absorption length for the predetermined wavelength .sub.T or for the entire operating wavelength range .sub.1 between approximately 13 nm and approximately 16 nm lies at more than 50 nm, i.e., the absorption of the total reflection layer 13 is comparatively low. Accordingly, the reflectivity R as a function of the wavelength for the wavelengths, relevant here, between 2 nm and 18 nm, shown in FIGS. 4A and 4B, emerges for three angles of incidence .sub.1=76, .sub.2=74, .sub.3=78 from the operating angle of incidence range. As emerges from FIG. 4B in particular, the reflectivity R of the total reflection layer 13 is more than approximately 50%, even more than approximately 70% for the third angle of incidence .sub.3 of approximately 78, in the case of wavelengths in the operating wavelength range between 13 nm and 16 nm, in particular at 13.5 nm.
(25) In the example described here, in which the operating wavelength range lies between 13 nm and 16 nm, the absorber layer 14 is formed from SiC. The absorber layer 14 made out of this material likewise renders it possible to suppress higher orders of diffraction m=+2, m=+3, . . . , as will be described below on the basis of FIGS. 5A and 5B, which, in a manner analogous to FIGS. 3A, 3B, show the critical angle .sub.T and the absorption length L of SiC. As may be seen from FIG. 5A, the critical angle .sub.T lies at more than approximately 80 for a predetermined wavelength of approximately 13 nm or .sub.T=13.5 nm, and consequently it is greater than the angle of incidence in an operating angle of incidence range of the optical grating 8 between approximately 72 and approximately 78. This also applies to half the predetermined wavelength .sub.T of approximately 6.5 nm or 6.75 nm, i.e., the absorber layer 14 is not totally reflective for the incident radiation 11 in the case of both the 1st order of diffraction and the 2nd order of diffraction.
(26) As may be seen in FIG. 5B, the absorption length L is long (absorption length between approximately 240 nm and approximately 270 nm) for the operating wavelength range .sub.1 between approximately 13 nm and approximately 16 nm, while the absorption length L is smaller by approximately a factor of 4 (absorption length between approximately 40 nm and approximately 60 nm) in the wavelength range .sub.2 with the half wavelengths of the operating wavelength range .sub.2, between approximately 6.5 nm and approximately 8 nm. Consequently, the absorption of the absorber layer 14 for incident radiation 11 diffracted into the 2nd order of diffraction is approximately four times as large as for incident radiation 11 diffracted into the 1st order of diffraction.
(27) Despite the presence of the absorber layer 14, the reflectivity R of the optical grating 8, or the diffraction efficiency thereof, is not reduced significantly, as may be seen on the basis of FIG. 6, which shows the reflectivity R of an absorber layer made of SiC with a thickness d.sub.2 (see FIG. 2) of approximately 7.5 nm on a total reflection layer 13 made of ruthenium with a thickness d.sub.1 of approximately 30 nm, which likewise satisfies the above-described requirements on the total reflection layer 13 for the operating wavelength range between approximately 13 nm and approximately 16 nm.
(28) As may be seen from FIG. 6, the reflectivity R for the wavelength range of the 2nd order of diffraction, i.e., between 6.5 nm and 8 nm, is damped by approximately a factor of 60 in relation to the operating wavelength range between 13 nm and 16 nm, in which the reflectivity R is more than approximately 60%.
(29) FIGS. 7A and 7B show the reflectivity R of an optical grating 8, which has a coating 12 made of a total reflection layer 13 made of ZrN with a thickness d.sub.1 of approximately 100 nm and an absorber layer 14, applied thereon, made of SiC with a thickness d.sub.2 of approximately 3.0 nm. As emerges from a comparison with FIG. 6, the optical grating 8 with the coating 12 of FIGS. 7A and 7B has an even better performance, i.e. a stronger suppression of the 2nd order of diffraction at wavelengths between approximately 6.5 nm and approximately 8 nm and also higher reflectivity R in the operating wavelength range between approximately 13 nm and approximately 16 nm, which lies at more than approximately 75% for the third angle .sub.3.
(30) FIG. 8A shows the damping D, i.e., the ratio between the reflectivity of the optical grating 8 with the absorber layer 14 made of SiC with a thickness d.sub.2 of 7 nm in relation to the reflectivity without such an absorber layer 14. As may be seen in FIG. 8A, the absorber layer 14 produces a damping D between a factor of 15 and a factor of 45 in the second wavelength range .sub.2 between approximately 6.5 nm and approximately 8 nm, i.e., the suppression of the second order of diffraction is significantly increased by the absorber layer 14 for the angle of incidence of 77.5 chosen in this case. In contrast thereto, the damping D in the second wavelength range .sub.2 is significantly lower, and does not even reach a factor of 2.0, in the case of a transmission filter, shown in FIG. 8B, in the form of SiC with a thickness of approximately 30 nm. Therefore, the absorber layer 14 is substantially more effective for suppressing the diffraction into the 2nd order of diffraction when compared to a transmissive wavelength filter.
(31) The absorber layer 14 made of SiC is also embodied to diffract incident radiation 11 with a first polarization state (s-polarization) perpendicular to a plane of incidence X, Y (see FIG. 2) onto the optical grating 8 more strongly in the predetermined order of diffraction m=+1 than incident radiation 11 with a second polarization state (p-polarization) that is perpendicular to the first.
(32) The polarizing effect of the absorber layer 14 can be modified by the thickness d.sub.2 thereof, as emerges from the following table 1, which was calculated for the second angle of incidence .sub.2 of 76 in the case of a predetermined wavelength .sub.T of 13.5 nm:
(33) TABLE-US-00001 TABLE 1 SiC thickness d.sub.2 2 nm 3 nm 4 nm 6 nm s/p-ratio 1.033 1.031 1.029 1.025
(34) Consequently, the polarizing effect of the optical grating 8 can be modified by approximately 0.8% for the variation of the thickness d.sub.2 of the absorber layer 14 between 2 nm and 6 nm. It is understood that a polarizing layer, which has no effect, or only negligible effect, on the absorption of the 1st order of diffraction or the 2nd order of diffraction, can be used instead of an absorber layer 14 for the purposes of producing this effect. Optionally, the coating 12 can have only one such polarizing layer or consist of such a polarizing layer. Instead of SiC, the absorber layer 14 may also be formed from another silicon-containing material, for example Si, Si.sub.3N.sub.4, SiO, SiO.sub.2.
(35) In a manner analogous to FIGS. 5A and 5B, FIGS. 9A and 9B show the critical angle .sub.T and the absorption length L of Mo as a function of the wavelength between 2 nm and 10 nm and between 2 nm and 18 nm, respectively. As may be seen from FIG. 9A, the critical angle .sub.T in the case of a predetermined wavelength .sub.T of approximately 6.5 nm lies at more than 80 and it is therefore greater than the angles of incidence of the radiation 11 incident on the optical grating 8, said angles of incidence typically likewise lying in an operating angle of incidence range between approximately 70 and 80, in particular between approximately 74 and approximately 78. As may be seen in FIG. 9B, the absorption length L in the operating wavelength range between approximately 6 nm and approximately 8 nm is greater than the wavelengths of the 2nd order of diffraction between 3 nm and 4 nm by approximately a factor of 4.5.
(36) While the examples of an optical grating 8 and of the coating 12, described further above, are each optimized for an operating wavelength range between 13 nm and 16 nm, e.g., at approximately 13.5 nm, FIGS. 10A and 10B show an example of a coating 12 which is optimized for an operating wavelength range .sub.1 between approximately 6 nm and approximately 8 nm. The coating 12 has a total reflection layer 13 made of Cd and an absorber layer 14 made of Mo with a layer thickness of d.sub.2=1.5 nm. FIGS. 10A and 10B illustrate the reflectivity R of the coating 12 for three angles of incidence .sub.1=82, .sub.2=80, .sub.3=84 from an operating angle of incidence range between 80 and 90, as a function of the wavelength . As may be seen from FIG. 10B in particular, the reflectivity R is more than approximately 50% both in the case of the first angle of incidence .sub.1 and in the case of the third angle of incidence .sub.3 for the operating wavelength range .sub.1 and it is significantly reduced for the wavelength range of the 2nd order of diffraction, i.e., between 3 nm and a 4 nm.
(37) In a manner analogous to FIGS. 10A and 10B, FIGS. 11A and 11B show an example of a coating 12 which is optimized for an operating wavelength range .sub.1 between approximately 6 nm and approximately 8 nm. The coating 12 has a total reflection layer 13 made of Sn with a thickness d.sub.1=100 nm and an absorber layer 14 made of Mo with a thickness of d.sub.2=2.0 nm. As may be seen on the basis of comparison between FIGS. 10A and 10B and FIGS. 11A and 11B, the reflectivity R, and hence the suppression of the 2nd order of diffraction, for the coating 12 described in conjunction with FIGS. 11A and 11B is similar to that of the coating 12 described in conjunction with FIGS. 10A and 10B, i.e., both coatings 12 have a great suppression of the 2nd order of diffraction with, at the same time, a high reflectivity R in the case of the 1st order of diffraction.
(38) The thickness d.sub.2 of the absorber layer 14 and the material thereof can be selected in such a way that constructive interference occurs for the incident radiation 11 with the predetermined wavelength .sub.L in the predetermined 1st order of diffraction m=+1 and destructive interference, i.e., minimum reflectivity, occurs at least for the 2nd order of diffraction m=+2, as a result of which the 2nd order of diffraction can likewise be effectively suppressed.
(39) FIG. 12 shows the reflectivity R for an absorber layer 14 made of SiC in the case of a thickness d.sub.2 of 9 nm (dashed line) and in the case of a thickness d.sub.2 of 13 nm (solid line). In the case where the absorber layer 14 has a thickness d.sub.2 of 9 nm, the minimum of the reflectivity R lies at a wavelength of approximately 8 nm, while the minimum of the reflectivity R lies at a wavelength of approximately 11 nm in the case of the thickness d.sub.2 of 13 nm. Consequently, a suitable choice of the thickness d.sub.2 of the absorber layer 14 allows the position of the minimum of the reflectivity R to be shifted to the wavelength at which the 2nd order of diffraction should be maximally suppressed. Optionally, this effect can be achieved by one or possibly two or more layers with suitable materials and thicknesses, which do not meet the requirements, described further above, in respect of the total reflection layer 13 and in respect of the absorber layer 14.
(40) In place of a total reflection layer 13 made of one of the materials specified further above, the total reflection layer 13 can also be formed by a chemical compound, for example an alloy, a carbide, nitride, oxide, boride or silicide, which meets the requirements described further above. The same applies to the absorber layer 14, which may likewise be formed from a carbide, nitride, oxide, boride of Si or C or, for example, from MoSi.sub.2.
(41) Even though the subject matter of the examples above relates to an optical grating 8 for radiation in the EUV wavelength range, the optical grating 8, and consequently also the materials of the coating 12, can be optimized for predetermined wavelengths .sub.T in other wavelength ranges, for example for shorter wavelengths in the x-ray range or for longer wavelengths in the VUV wavelength range of up to approximately 250 nm. In wavelength ranges with longer wavelengths than in the EUV wavelength range, an optical grating 8 operated in transmission may possibly also be used in place of an optical grating 8 operated in reflection.
