Optical element having a coating for influencing heating radiation and optical arrangement

Abstract

The disclosure relates to an optical element, including: a substrate, a first coating, which is disposed on a first side of the substrate and is configured for reflecting radiation having a used wavelength (λ.sub.EUV) in the EUV wavelength range, and a second coating, which is disposed on a second side of the substrate, for influencing heating radiation that is incident on the second side of the substrate. The disclosure also relates to an optical arrangement having at least one such optical element.

Claims

1. An optical element, comprising: a substrate having first and second sides; a first coating supported by the first side of the substrate; and a second coating supported by the second side of the substrate, wherein: the substrate comprises a glass; the first coating reflects EUV radiation; the second coating transmits radiation at a first wavelength; the second coating comprises a member selected from the group consisting of an absorbing layer that absorbs radiation having a second wavelength and a transmitting layer that transmits radiation having the second wavelength; the first wavelength is in a range selected from the group consisting of the visible range and the infrared range; the second wavelength is in a range selected from the group consisting of the visible range and the infrared range; the second wavelength is different from the first wavelength; and the optical element is an EUV mirror.

2. The optical element of claim 1, wherein the second coating further comprises an anti-reflecting layer that suppresses reflection of radiation at the second wavelength, and the absorbing layer is between the substrate and the anti-reflection layer.

3. The optical element of claim 2, wherein a maximum absorbance of the absorbing layer is at wavelengths of more than 1500 nm.

4. The optical element of claim 3, wherein a maximum suppression of the anti-reflection layer is at wavelengths of more than 1500 nm.

5. The optical element of claim 2, wherein a maximum suppression of the anti-reflection layer is at wavelengths of more than 1500 nm.

6. The optical element of claim 1, wherein the absorbing layer has a maximum transmission at wavelengths less than 1500 nm.

7. The optical element of claim 1, wherein the substrate comprises a material that is at least partially absorbent for radiation at the second wavelength.

8. The optical element of claim 7, wherein the substrate comprises a material that is at least partially transparent for radiation at the first wavelength.

9. The optical element of claim 1, wherein the substrate comprises a material that is at least partially transparent for radiation at the first wavelength.

10. The optical element of claim 1, wherein the second coating further comprises an anti-reflecting layer that suppresses reflection of radiation at the first wavelength and at the second wavelength, and the transmitting layer is between the substrate and the anti-reflection layer.

11. The optical element of claim 1, wherein the glass comprises a silicate glass.

12. The optical element of claim 1, wherein the glass comprises a quartz glass.

13. The optical element of claim 1, wherein the glass comprises a TiO.sub.2-doped quartz glass.

14. The optical element of claim 1, wherein the glass comprises a glass ceramic.

15. The optical element of claim 1, wherein the second coating comprises an absorbing layer that absorbs radiation having the second wavelength.

16. The optical element of claim 1, wherein the second coating comprises a transmitting layer that transmits radiation having the second wavelength.

17. An arrangement, comprising: an optical element according to claim 1; and a light source configured to generate radiation at a wavelength in a range selected from the group consisting of visible radiation and infrared radiation, wherein the second coating is between the light source and the substrate.

18. The arrangement of claim 17, wherein: the arrangement comprises a plurality of light sources in a grid-type arrangement; and for each light source, the second coating is between the light source and the substrate.

19. The arrangement of claim 18, wherein the arrangement is an EUV lithography apparatus.

20. The arrangement of claim 17, wherein the arrangement is an EUV lithography apparatus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description, in which:

(2) FIG. 1 shows a schematic illustration of an optical element in the form of an EUV mirror and a device for thermally influencing the EUV mirror, in which heating radiation is absorbed at an absorbing layer of a coating which is disposed on the bottom side of the EUV mirror,

(3) FIG. 2A shows an illustration similar to FIG. 1, in which heating radiation at a first heating wavelength is absorbed at the coating, and heating radiation at a second heating wavelength is transmitted by the coating,

(4) FIG. 2B shows an illustration similar to FIG. 1, in which heating radiation at a first heating wavelength is absorbed within the volume of a mirror substrate, and heating radiation at a second heating wavelength is transmitted by the coating,

(5) FIGS. 3A-3C show schematic illustrations similar to FIG. 2A, in which heating radiation at a third heating wavelength is reflected by a reflective layer of a coating which is disposed on the front side of the EUV mirror, and

(6) FIG. 4 shows a schematic illustration of an EUV lithography apparatus.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(7) Identical reference signs are used in the following description of the drawings for components that are the same or functionally the same.

(8) FIG. 1 schematically shows an optical element 1 in the form of an EUV mirror which has a substrate 2 made of ULE®, a first coating 3, applied on a first side (upper side) 2a of the substrate 2, in the form of an EUV coating, and a second coating 4, applied on a second side (bottom side) 2b of the substrate 2 which is located opposite the first side.

(9) The EUV coating 3 has a coating 3b (what is known as an HR coating) which reflects EUV radiation 5 at a used wavelength λ.sub.EUV. Applied on the reflective coating 3b is additionally a cover layer or a cover layer system (what is known as a cap coating 3c), which is intended to protect the entire EUV coating 3 against oxidation or corrosion, for example if the EUV mirror 1 is cleaned by way of a hydrogen plasma. The cap coating 3c is arranged adjacently to an optical surface 6 of the EUV mirror 1, which forms the boundary surface of the EUV mirror 1 with the environment.

(10) The reflective coating 3b has a plurality of individual layers (not illustrated in FIG. 1), which typically consist of layer pairs of two materials having different refractive indices. If EUV radiation 5 at a used wavelength in the range of λ.sub.EUV=13.5 nm is used, the individual layers are typically made of molybdenum and silicon. In dependence on the used wavelength λ.sub.EUV, other material combinations such as for example molybdenum and beryllium, ruthenium and beryllium, or lanthanum and B.sub.4C are likewise possible. In addition to the individual layers, the reflective coating 3b typically has intermediate layers to prevent diffusion (what are known as barrier layers).

(11) The EUV coating 3 of FIG. 1 has, below the reflective coating 3b, what is known as an SPL (substrate protection layer) coating 3a to protect the substrate 2 against damaging EUV radiation 5. In addition or alternatively to an SPL coating 3a, what is known as an ASL (anti-stress layer) coating can also be provided below the reflective coating 3b on the EUV mirror 1 in order to avoid undesired deformations due to layer stresses.

(12) FIG. 1 likewise shows a device 20 for thermally influencing the EUV mirror 1, having a plurality of heating light sources 8, two of which are illustrated by way of example in FIG. 1, which can be attached to a heat sink 21. The heating light sources 8 are configured for generating heating radiation 9 at a first heating wavelength λ.sub.1H, with which the second side 2b of the substrate 2, more specifically the second coating 4, is irradiated.

(13) The second coating 4 has an absorbing layer 4a, which is applied in the example shown directly to the bottom side 2b of the substrate 2 and which has absorbing properties for the heating radiation 9 at the first heating wavelength λ.sub.1H. The material of the absorbing layer 4a can be, for example, a layer of germanium (Ge). Germanium is sufficiently transparent up to a wavelength of approximately 1.5 μm, in particular between 400 nm and 1000 nm. An anti-reflection layer 4b which serves for suppressing the reflection of the heating radiation 9 at the first heating wavelength λ.sub.1H is applied onto the absorbing layer 4a. The anti-reflection layer 4b can be, for example, a multilayer coating or a layer stack, for example the following layer stack: (1Si 4.981Si.sub.3N.sub.4){circumflex over ( )}5. Details relating to this layer stack can be gathered from the patent application DE 102014204171.6, which is incorporated in the content of this application with respect to this aspect.

(14) In the shown example, the first heating wavelength λ.sub.1H is within the IR range at approximately 2000 nm, with typical values for the first heating wavelength λ.sub.1H being between approximately 2000 nm and 2100 nm or between 2300 nm and 2500 nm. The material of the absorbing layer 4a is selected such that the absorptance has a maximum in the above-stated wavelength range of more than 1.5 μm. However, since materials exist which have strongly absorbing properties for electromagnetic radiation over a wide wavelength range, it is not absolutely necessary for the absorbing layer 4a to have a maximum of its absorbance A.sub.1H within the above-stated wavelength range.

(15) The material of the anti-reflection layer 4b and the layer thickness thereof are selected such that an anti-reflective effect sets in within the above-stated wavelength range, i.e. for the anti-reflection layer 4b, the suppression of the reflection R.sub.1H of the heating radiation 9 is at a maximum at the first heating wavelength λ.sub.1H. In place of an individual anti-reflection layer 4b, an anti-reflection coating can also be formed on the second coating 4, i.e. a plurality of anti-reflection layers 4b which together have an anti-reflective effect.

(16) The heating radiation 9 serves for thermally influencing the EUV mirror 1, more specifically for generating a targeted location-dependent heat introduction into the absorbing layer 4a in order to produce a desired temperature profile in the proximity of the bottom side 2b of the substrate 2 or within the substrate volume which adjoins it. The desired temperature profile typically corresponds to a thermal profile which runs counter to the thermal profile produced in the region of the bottom side 2b of the substrate 2 due to the presence of the heat sink 21, with the result that, in the ideal case, in total, a temperature is established in the substrate 2 which is constant over the entire bottom side 2b.

(17) Accordingly, homogenization of the temperature distribution can also be effected at the upper side 2a of the substrate 2 by irradiating the upper side 2a of the substrate 2 with additional heating radiation 14 which is typically generated by a plurality of further heating light sources 15. In the example shown, the heating wavelength λ.sub.1H of the further heating light sources 15 corresponds to the first heating wavelength λ.sub.1H, although this is not absolutely necessary.

(18) During operation, EUV radiation 5 is incident on the EUV mirror 1, the intensity distribution of which varies in a location-dependent manner over the optical surface 6 and is generally not constant over time. The intensity distribution of the EUV radiation 5 which varies in a location-dependent manner results in a locally differing heat introduction at the upper side 2a of the EUV mirror 1, and thus in a temperature distribution which is not spatially or temporally constant. The further heating radiation 14 serves for counter-heating, that is to say those regions in which the substrate 2 or the EUV coating 3 has a comparatively low temperature are additionally heated to homogenize the temperature distribution and to obtain, in the ideal case, a constant temperature on the optical surface 6 overall.

(19) In the example illustrated in FIG. 2A, and in contrast with FIG. 1, the upper side 2a of the substrate 2 is heated from the bottom side 2b of the substrate 2, i.e. through the substrate 2. For this purpose, a second heating light source 10 is arranged on the heat sink 21, which generates heating radiation 11 at a second heating wavelength λ.sub.2H, which in the illustrated example is approximately 400 nm, with typical values being between approximately 400 nm and approximately 1500 nm.

(20) The second coating 4, more specifically the layer 4a which absorbs the heating radiation 9 at the first heating wavelength λ.sub.1H, is transparent for the second heating wavelength λ.sub.2H. The heating radiation 11 at the second heating wavelength λ.sub.2H is transmitted by the substrate 2 and absorbed at the EUV coating 3. If the absorption by the EUV coating 3 is not sufficient, an additional absorbing layer or coating, for example a metallic layer, can be provided, if appropriate, on the side thereof which faces the substrate 2.

(21) In the ideal case, the anti-reflection layer 4b is configured such that the suppression of the reflection is maximum both for heating radiation 9 of the first heating wavelength λ.sub.1H and for heating radiation 11 of the second heating wavelength λ.sub.2H. If appropriate, the layer 4a, which transmits both the heating radiation 11 at the second heating wavelength λ.sub.2H, can also serve as an anti-reflection layer for the second heating wavelength λ.sub.2H and possibly for the first heating wavelength λ.sub.1H, such that the provision of an additional anti-reflection layer can be dispensed with.

(22) In one alternative exemplary embodiment illustrated in FIG. 2B, the second coating 4 has a layer 4a′ which transmits both heating radiation 9 at the first heating wavelength λ.sub.1H and also heating radiation 11 at the second heating wavelength λ.sub.2H. The transmissive layer 4a′ in the example shown is made of germanium (Ge) and has a high transmittance T.sub.1H, T.sub.2H for both heating wavelengths λ.sub.1H, λ.sub.2H. The heating radiation 11 at the second heating wavelength λ.sub.2H, which is generated by the second heating light source 10, passes through the substrate 2, as in FIG. 2A, and is absorbed at the EUV coating 3, more specifically at the SPL coating 3c, in order to produce heat introduction here. The heating radiation 9 at the first heating wavelength λ.sub.1H, which is generated by the first heating light source 8 and is in the IR range, is strongly absorbed within the volume of the substrate 2 made of ULE® and therefore produces a heat introduction in the proximity of the bottom side 2b of the substrate 2.

(23) The second coating 4 has an anti-reflection coating or an anti-reflection layer 4b, which serves both for suppressing the reflection of the heating radiation 9 at the first heating wavelength λ.sub.1H and for suppressing the reflection of the heating radiation 11 at the second heating wavelength λ.sub.2H. If appropriate, it is also possible to dispense with the provision of the transparent layer 4a′. The second heating wavelength λ.sub.2H can be selected to be between approximately 2650 nm and approximately 2800 nm, or between approximately 4000 nm and approximately 10 000 nm, in particular between 4500 nm and 5500 nm. According to the preceding example, the first heating wavelength λ.sub.1H can be between approximately 2000 nm and approximately 2100 nm, and between approximately 2300 nm and approximately 2500 nm.

(24) FIGS. 3A-3C show examples of EUV mirrors 1, in which the EUV coating 3 has an additional, bottommost layer 3d, which is configured for reflecting heating radiation 13 at a third heating wavelength λ.sub.3H, which is generated by a third heating light source 12. In place of an additional reflective layer 3d, as is shown in FIGS. 3A-3C, it is also possible, if appropriate, for the SPL coating 3a to serve as the layer which reflects heating radiation 13 at the third heating wavelength λ.sub.3H, with the result that the additional reflective layer 3d can be dispensed with. The heating radiation 13 at the third heating wavelength λ.sub.3H serves for generating heat introduction within the volume of the substrate 2, which likewise serves for homogenizing the temperature distribution.

(25) FIG. 3A shows an example of an EUV mirror 1, in which the heating radiation 9 at the first heating wavelength λ.sub.1H and the heating radiation 11 at the second heating wavelength λ.sub.2H are supplied analogously to FIG. 2A. In addition, heating radiation 13, which has a third heating wavelength λ.sub.3H and is generated by a third heating light source 12, is transmitted by the second coating 4, passes through the substrate 2, is incident on the reflective layer 3d, and is reflected at the latter back into the substrate 2. The absorptance of the ULE® material of the substrate 2 with respect to the heating radiation 13 at the third heating wavelength λ.sub.3H in this case is medium strong, such that the heating radiation 13, which is reflected at the reflective layer 3d, does not propagate all the way to the bottom side 2b of the substrate 2 and cannot exit at the bottom side 2b.

(26) In the example shown in FIG. 3A, in which the heating radiation 13 is absorbed by the substrate 2 with medium strength, the third heating wavelength λ.sub.3H is approximately 3600 nm, with typical values for the third heating wavelength λ.sub.3H in this case being, in dependence on the thickness of the substrate 2, between approximately 3500 nm and approximately 3700 nm. With a specified thickness of the substrate 2, it is possible to ascertain the optimum heating wavelength on the basis of a wavelength-dependent transmission curve for the material of the substrate 2, in the present case ULE®. Similar relationships apply to a substrate 2 made of a different material, such as for example Zerodur®. The reflectance R.sub.3H of the reflective layer 3d is maximum, or has a maximum, within the above-stated wavelength range.

(27) In the example shown in FIG. 3A, the second coating 4 has an anti-reflection layer 4b, which, in addition to suppressing the reflection of the heating radiation 9 at the first heating wavelength λ.sub.1H and to suppressing the reflection of the heating radiation 11 at the second heating wavelength λ.sub.2H, is also configured for suppressing the reflection of the heating radiation 13 at the third heating wavelength λ.sub.3H. The anti-reflection layer 4b typically has a local maximum of the suppression of the reflection or a minimum reflectance at the respective heating wavelength λ.sub.1H, λ.sub.2H, λ.sub.3H.

(28) Whereas in the examples shown in FIGS. 2A, 2B and FIG. 3A the heating radiation 9, 11, 13 is aligned substantially perpendicular to the bottom side 2b of the substrate 2, in the example shown in FIG. 3B, the heating radiation 13 at the third heating wavelength λ.sub.3H is aligned with an angle a with respect to the surface normal of the bottom side 2b of the substrate 2, which is typically no more than approximately 10°. In order to align the heating radiation 13 under the angle α, the third heating light source 12 can be positioned on the heat sink 21 such that it is, if appropriate, suitably tilted, and/or the emission characteristic thereof can be appropriately set. As is likewise seen in FIG. 3B, the third heating light source 12, which can be configured, for example, as a laser diode, generates linearly polarized heating radiation 13 which, in the example shown in FIG. 3B, has a first polarization state (s-polarization) with respect to an XZ plane, which corresponds to the drawing plane, of an XYZ coordinate system.

(29) In the example shown in FIG. 3B, the second coating 4 has a polarization-selective layer 4a″, which transmits the s-polarized heating radiation 13 at the third heating wavelength λ.sub.3H, with the result that it passes through the substrate 2 and is reflected back at the layer 3d, which reflects the heating radiation 13, of the first coating 3. Arranged between the upper side 2a of the substrate 2 and the layer 3d, which reflects the heating radiation 13, a polarization-changing layer 3e is arranged in FIG. 3B, through which the heating radiation 13 at the third heating wavelength λ.sub.3H passes twice, and which effects a rotation of the polarization direction of the heating radiation 13 by 90° such that the reflected heating radiation 13 is p-polarized. The p-polarized heating radiation 13 is incident on the polarization-selective layer 4a″ of the second coating 4, and is reflected thereby back into the volume of the substrate 2.

(30) The EUV mirror 1, shown in FIG. 3C, differs from the EUV mirror 1, shown in FIG. 3B, merely in that the second coating 4 rather than the first coating 3 is provided with a polarization-changing layer 4c. The s-polarized heating radiation 13 passes through the polarization-changing layer 4c and is circularly polarized thereby before the heating radiation 13 enters the substrate 2. The circularly polarized heating radiation 13, which is reflected back at the reflective layer 3d, is again incident on the polarization-changing layer 4c and is converted into p-polarized heating radiation 13. The p-polarized heating radiation 13 is incident on the polarization-selective layer 4a″, and is reflected thereby back into the substrate 2.

(31) In the examples shown in FIG. 3B and FIG. 3C, the third heating wavelength λ.sub.3H is selected such that it is absorbed only slightly by the substrate 2, i.e. it is typically between approximately 400 nm and approximately 2300 nm. As can be seen in FIGS. 3A-3C, the entire second coating 4 is configured to be transmissive for the heating radiation 13 at the third heating wavelength λ.sub.3H and for the heating radiation 11 at the second heating wavelength λ.sub.2H. The polarization-selective layer 4a″ is here configured to absorb the heating radiation 9 at the first heating wavelength λ.sub.1H in order to heat the substrate 2 in the region of the bottom side 2b thereof.

(32) In the devices 20, which are described further above in connection with FIGS. 2A, 2B and FIGS. 3A-3C, in each case only a first, second or third heating light source 8, 10, 12 for generating heating radiation 9, 11, 13 at a respective heating wavelength λ.sub.1H, λ.sub.2H, λ.sub.3H is shown. However, typically a plurality of first, second or third heating light sources 8, 10, 12 are arranged on the heat sink 21 in a grid-type arrangement (matrix) so as to achieve thermal influencing of the EUV mirror with a desired spatial resolution. The heating radiation 8, 10, 12, generated by a heating light source, can be substantially monochromatic, i.e. the radiation intensity is concentrated around the maximum at the heating wavelength, as is the case for example in laser diodes or LEDs. Alternatively, it is also possible to use heating light sources which emit heating radiation in a comparatively broadband wavelength range, wherein the desired heating wavelength or a narrow-band heating wavelength range is selected by suitable wavelength-selective filters.

(33) As an alternative to the devices 20, which are shown further above in connection with FIG. 1 to FIGS. 3A-3C, the first, second and/or third heating light sources 8, 10, 12 can be arranged at a distance from the heat sink 21 and be supplied to the EUV mirror 1 by way of beam guide devices, for example in the form of fibre-optic cables. In order to align the heating radiation 9, 11, 13 in this case with the substrate 2, it is possible for deflection devices to be attached to the heat sink 21, for example, which deflect the heating radiation 9, 11, 13, which exits the fibre-optic cables, in the direction of the bottom side 2b of the substrate 2.

(34) FIG. 4 shows, in a highly schematic fashion, an optical arrangement in the form of an EUV lithography apparatus 101, in which the EUV mirrors 1 of FIG. 1, FIGS. 2A, 2B or of FIGS. 3A-3C can be integrated. The EUV lithography apparatus 101 has an EUV light source 102 for generating EUV radiation, which has a high energy density in an EUV wavelength range below 50 nm, in particular between about 5 nm and about 15 nm. The EUV light source 102 may for example take the form of a plasma light source for generating a laser-induced plasma or be formed as a synchrotron radiation source. In particular in the former case, a collector mirror 103 may be used, as shown in FIG. 4, in order to focus the EUV radiation of the EUV light source 102 into an illumination beam 104 and in this way increase the energy density further. The illumination beam 104 serves for the illumination of a structured object M via an illumination system 110, which in the present example has five reflective optical elements 112 to 116 (mirrors).

(35) The structured object M may be for example a reflective mask, which has reflective and non-reflective, or at least much less reflective, regions for producing at least one structure on the object M. Alternatively, the structured object M may be a plurality of micro-mirrors, which are arranged in a one-dimensional or multi-dimensional arrangement and which are possibly movable about at least one axis, in order to set the angle of incidence of the EUV radiation 104 on the respective mirror.

(36) The structured object M reflects part of the illumination beam 104 and forms a projection beam path 105, which carries the information about the structure of the structured object M and is radiated into a projection lens 120, which produces a projected image of the structured object M or of a respective partial region thereof on a substrate W. The substrate W, for example a wafer, includes a semiconductor material, for example silicon, and is arranged on a mounting, which is also referred to as a wafer stage WS.

(37) In the present example, the projection lens 120 has six reflective optical elements 121 to 126 (mirrors) in order to produce an image of the structure that is present on the structured object M on the wafer W. The number of mirrors in a projection lens 120 typically lies between four and eight; however, only two mirrors may also possibly be used.

(38) In order to achieve a high imaging quality in the imaging of a respective object point OP of the structured object M onto a respective image point IP on the wafer W, highest desired properties are imposed on the surface form of the mirrors 121 to 126; and the position or the alignment of the mirrors 121 to 126 in relation to one another and in relation to the object M and the substrate W also involves precision in the nanometre range. Each of the EUV mirrors 121 to 126 can be configured as described further above in connection with FIG. 1, FIGS. 2A, 2B and FIGS. 3A-3C, and a dedicated device 20 for thermal manipulation, which can be configured as described above, for example, can be assigned thereto.

(39) In the projection lens 120, illustrated in FIG. 4, the sixth mirror 126 is configured in the form of a thermally influenceable EUV mirror 1 according to FIG. 3A, and a device 21 for thermal manipulation is assigned thereto, which is configured to individually drive the heating light sources 8, 10, 12 (not shown in FIG. 4) to set a desired, typically homogeneous temperature distribution in the EUV mirror 126 and to thus avoid undesired deformations and resulting undesired aberrations on the optical surface 6 (cf. FIG. 3A) of the sixth EUV mirror 126.

(40) It is additionally possible for one or more sensors for capturing the temperature of the EUV mirror 126 or of the optical surface 6 and/or the temperature of the substrate 2 of the EUV mirror 126 to be arranged in the EUV lithography apparatus 101, so that the device 20 for thermal influencing can effect regulation of a location-dependent heat introduction into the EUV mirror 126 in order to produce in a targeted fashion a desired location- and time-dependent heat introduction in the EUV mirror 126, with the result that the temperature distribution of the EUV mirror 126 is homogenized.

(41) Additionally or alternatively, it is also possible for the EUV mask 130 to be thermally influenced by way of a device 20, as is illustrated in FIGS. 2A, 2B or in FIGS. 3A-3C. The EUV mask 130 in this case is constructed like the EUV mirrors 1, which are described further above, wherein partial regions in the form of an absorber material, which do not or only slightly reflect the incident EUV radiation 5, are formed additionally on the upper side of the EUV coating 3. The absorbing partial regions together with the reflective partial regions form the structure of the EUV mask 130 to be imaged.

(42) It is to be understood that the EUV mirrors 1, described further above, or the devices 20 for thermal influencing, can also be advantageously used in other optical systems for the EUV wavelength range, for example in inspection systems for EUV masks.