Optical system with an EUV mirror and method for operating an optical system

Abstract

Optical system with an EUV mirror (17, 37, 46, 53, 54), wherein the EUV mirror (17, 37, 46, 53, 54) is provided with an optical surface (30) designed for the reflection of EUV radiation. An EUV beam path (27) generated by an EUV radiation source (14, 44, 51) is directed onto the optical surface (30) of the EUV mirror (17, 37, 46, 53, 54) such that the EUV beam path (27) strikes the EUV mirror at an angle (34, 59, 60) of no more than 12°. The optical surface (30) is coated with a carbon layer (33), wherein the carbon layer (33) has a thickness of at least 8 nm. The invention also relates to a method for operating an optical system.

Description

[0001] The invention relates to an optical system with an EUV mirror and a method for operating an optical system.

[0002] Microlithographic projection exposure systems are used for the production of integrated circuits with extremely small structures. A photomask illuminated with very short-wavelength, extreme ultraviolet (EUV) radiation is projected onto a lithography object to transfer the mask structure onto the object.

[0003] The projection exposure system comprises several EUV mirrors, each with an optical surface from which the EUV radiation is reflected. To minimize EUV radiation losses within the projection exposure system, the EUV mirrors have an optical surface with high reflectivity for EUV radiation.

[0004] In addition to the projection exposure system itself, there are other optical systems that are operated in conjunction with microlithographic projection exposure systems. These include, for example, measuring devices used to examine the reflectivity of EUV mirrors, devices for examining modules and subsystems, and measuring devices used to examine the properties or condition of photomasks.

[0005] In all cases, the problem can arise that the optical surface of an EUV mirror used in the optical system becomes contaminated by the deposition of carbon-containing species, causing the reflectivity of the EUV mirror to decrease progressively during operation. Generally, an optical system becomes unusable if even a single EUV mirror in the system no longer has sufficient reflectivity. Therefore, the deposition of a carbon layer on an EUV mirror during operation can significantly shorten the lifespan of an optical system.

[0006] Document US 2006 / 0145094 A1 relates to an optical element with an antireflection coating designed to reflect only extreme ultraviolet (EUV) radiation. Document US 2014 / 0168758 A1 discloses a mirror for reflecting ultraviolet (EUV) light. Document US 2005 / 0279951 A1 discloses a grazing-light mirror comprising a mirror substrate and a mirror surface layer. Document US 2017 / 0248851 A1 relates to illumination optics for EUV projection lithography. Document US 2010 / 0182710 A1 relates to a method for manufacturing an optical element or part of an optical element. Document US 2020 / 0174378 A1 discloses a method for adjusting the geometry of at least one surface region of a component of an optical arrangement.Document US 2022 / 0107570 A1 concerns an optical projection unit for microlithography, comprising a plurality of mirrors and a numerical aperture with a value greater than 0.5.

[0007] The invention is based on the objective of presenting an optical system with an EUV mirror and a method for operating an optical system, with which these disadvantages are avoided. This objective is achieved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0008] The optical system according to the invention comprises an EUV mirror. The EUV mirror is provided with an optical surface designed for the reflection of EUV radiation. An EUV beam path generated by an EUV radiation source is directed onto the optical surface of the EUV mirror, such that the EUV radiation strikes the EUV mirror at an angle of no more than 12°. The EUV mirror is thus operated at grazing incidence. The optical surface is coated with a carbon layer, the carbon layer having a thickness of at least 8 nm. The carbon layer is a carbon layer deposited during operation of the optical system. The optical system comprises a vacuum housing in the interior of which the EUV mirror is arranged. The vacuum housing contains a vacuum atmosphere without hydrogen purge gas.

[0009] If there is a range of propagation directions of the EUV radiation within the EUV beam path, as is the case, for example, with a convergent or divergent beam path, then the angle specifications refer to the central ray of the EUV beam path.

[0010] The invention is based on the understanding that the influence of a carbon layer deposited on the optical surface of an EUV mirror on its reflectivity depends strongly on the angle at which the EUV radiation strikes the mirror. For EUV mirrors operating at grazing incidence, the angle between the EUV radiation and the optical surface is often on the order of 20°. Significantly smaller angles are considered less practical. While it is possible to further increase the reflectivity of a single EUV mirror by using smaller angles, a greater number of EUV mirrors are required to achieve a specific degree of beam deflection.

[0011] For an EUV mirror onto which EUV radiation is incident at an angle of 20°, the reflectivity drops to approximately 50% when the carbon layer reaches a thickness of 10 nm. This means that only 50% of the EUV radiation is reflected compared to an initial state in which the optical surface of the EUV mirror is not yet contaminated. Generally, such a drop in reflectivity renders an optical system unusable.

[0012] The invention proposes to significantly reduce the angle at which the EUV radiation strikes the EUV mirror compared to previously used values, in order to achieve an optical system that still functions even when the optical surface is covered with a carbon layer 8 nm thick or more. For example, if an EUV mirror is operated at a 10° angle, it still has a reflectivity of more than 80% when the carbon layer is 10 nm thick.

[0013] If the incident EUV radiation has an incidence angle bandwidth, then components of the EUV radiation lying in different regions of the incidence angle bandwidth are reflected to varying degrees. In one embodiment, the system according to the invention is operated such that those components of the incident EUV radiation which, within the incidence angle bandwidth, form the largest angle with the optical surface of the EUV mirror, strike the optical surface of the EUV mirror at an angle of no more than 12°. The angle at which the central ray of the EUV radiation strikes the optical surface is then correspondingly smaller. The incidence angle bandwidth is referred to as the FWHM bandwidth.

[0014] During the operation of an optical system according to the invention, carbon-containing species are formed through interaction between the EUV radiation and residual gas remaining in the vacuum atmosphere of the system. These species are deposited on the optical surface of EUV mirrors and, after prolonged operation, form a layer on the optical surface. A carbon layer within the meaning of the invention is a layer with a composition such as is typically formed during the operation of an EUV system. In terms of the number of particles in the carbon layer, carbon generally constitutes at least 60%, preferably at least 70%. The carbon layer can contain between 5% and 30% oxygen. The proportion of the remaining components in the carbon layer can be less than 20%.

[0015] The carbon layer is a carbon layer deposited during operation of the optical system. This means that the optical surface of the EUV mirror is not coated with a carbon layer during manufacturing. The carbon layer only forms through the use of the EUV mirror under the EUV conditions of the optical system. The invention makes it possible to continue operating the optical system even when the carbon layer has grown to a thickness exceeding 8 nm. For example, the carbon layer can have a thickness of at least 10 nm, preferably at least 15 nm, and more preferably at least 30 nm.

[0016] For EUV mirrors operating at grazing incidence, where the EUV radiation strikes the mirror at an angle of 25° or less, high reflectivity for EUV radiation can be achieved if the optical surface is formed by a single layer. This reflective layer can be made of ruthenium (Ru), for example. A reflective layer consisting of more than one layer is also possible. The reflective layer can be made of one or more other materials. In particular, the reflective layer can include one or more of the following materials: ruthenium (Ru), rhodium (Rh), palladium (Pd), gold (Au), platinum (Pt), niobium (Nb), molybdenum (Mo), and zirconium (Zr). The thickness of the reflective layer can be, for example, between 20 nm and 200 nm, preferably between 30 nm and 80 nm.

[0017] In contrast, for an EUV mirror operated under normal incidence, where the angle between the incident EUV radiation and the optical surface of the EUV mirror is between 60° and 90°, sufficient reflectivity for EUV radiation can only be achieved with a reflective coating system consisting of several layers.

[0018] The EUV mirror can comprise a mirror body on which the optical surface is formed. The mirror body can be made of a material whose coefficient of thermal expansion has a zero crossing temperature (ZCT). An example of such a material is titanium silicate glass known as ULE (Ultra Low Expansion Glass), as offered, for example, by Corning Incorporated.

[0019] The consideration that an excessively small angle between the incident EUV radiation and the optical surface is not advantageous also applies to the EUV mirror according to the invention. With a smaller angle, a larger number of EUV mirrors are required to achieve a specific beam deflection, resulting in more disadvantages than advantages overall. Therefore, the angle between the incident EUV beam path and the optical surface can be at least 4°, preferably at least 6°, and preferably at least 8°.

[0020] The optical system can comprise a first EUV mirror and a second EUV mirror, each of the two EUV mirrors having one or more features described in relation to the first EUV mirror. The difference between the angles at which the EUV radiation strikes the first EUV mirror and the second EUV mirror can be less than 6°, preferably less than 4°, and more preferably less than 2°.

[0021] The first and second EUV mirrors can have the same plane of incidence. The plane of incidence is defined as the plane defined by the central ray of the incident EUV beam and the central ray of the reflected EUV beam at an EUV mirror. The beam deflection caused by the two EUV mirrors then occurs within this single plane.

[0022] The first and second EUV mirrors can jointly cause a deflection of the EUV beam path, which lies between 24° and 48°, preferably between 30° and 40°. Beam deflection is defined as the angle between the EUV radiation incident on the first EUV mirror and the EUV radiation reflected by the second EUV mirror. Viewed in the direction of propagation, the EUV radiation first strikes the first EUV mirror and then the second EUV mirror. The beam deflection can occur within a single plane.

[0023] The optical system can be designed such that no further optical element is arranged between the first EUV mirror and the second EUV mirror to modify the direction of the EUV beam path. The optical system can comprise more than two EUV mirrors, each of which has one or more features of an EUV mirror according to the invention.

[0024] The optical system can comprise one or more EUV mirrors onto which the EUV beam path strikes at an angle greater than 12°. These can include one or more EUV mirrors operating at grazing incidence and / or one or more EUV mirrors operating at normal incidence, where the angle between the incident EUV radiation and the optical surface of the EUV mirror is between 60° and 90°. It is an insight of the invention that the reflectivity of an optical surface is affected to a lesser extent by a carbon layer deposited during operation at normal incidence than at grazing incidence. In many cases, an EUV mirror operating at normal incidence can be used effectively up to a carbon layer thickness of, for example, 20 nm.Assuming that the carbon layer builds up on all EUV mirrors of the optical system at the same rate, a range exceeding 10 nm for the layer thickness of the carbon layer is thus opened up, within which an optical system can be used effectively that includes EUV mirrors according to the invention and EUV mirrors operated under normal incidence.

[0025] Since the rate at which carbon layers build up on the optical surfaces of the EUV mirrors of the optical system also depends on the local intensity of the EUV radiation, there are optical systems in which the carbon layer builds up at different rates on different EUV mirrors. In such an optical system, EUV mirrors according to the invention can be arranged in the sections of the EUV beam path where the carbon layer builds up quickly, and EUV mirrors not according to the invention can be arranged in those sections of the EUV beam path where the carbon layer builds up more slowly.

[0026] Since gases generally have low transmission of EUV radiation, it is advantageous for the optical system to operate under vacuum. The optical system comprises a vacuum housing in which the EUV mirrors are arranged. A vacuum can be maintained inside the housing during operation of the optical system.

[0027] Due to the low pressure to which the components in the vacuum housing are exposed, outgassing can occur from components located within the vacuum housing. Through interaction between these outgassings and the EUV radiation, contaminants can form, from which the carbon layer develops. To slow down the formation of the carbon layer, a hydrogen purge gas can be introduced into the vacuum housing. Through interaction between the EUV radiation and the hydrogen, a plasma is generated, including ionic plasma species (H₂). +) or radical plasma species (H) are formed. The hydrogen plasma has the effect of removing contaminants from the surfaces of the components in the vacuum housing. The optical system according to the invention can be operated under such a hydrogen atmosphere. The optical system is operated in a vacuum atmosphere without hydrogen purge gas. In this way, the equipment complexity can be reduced. The disadvantage that the carbon layers on the EUV mirrors of the optical system build up more quickly is compensated for according to the invention by allowing carbon layers of greater thickness before the optical system becomes unusable.

[0028] The optical system according to the invention can be configured to illuminate a photomask or a section of a photomask with EUV radiation. Illumination means that an area is illuminated with substantially uniform brightness. Alternatively or additionally, the optical system according to the invention can be configured to image a photomask onto an object plane. A wafer can be arranged in the object plane, which is exposed based on a structure formed on the photomask. It is also possible that an EUV image sensor is arranged in the object plane, so that an image of the photomask can be captured with the EUV image sensor. The photomask can have an aspect ratio between 1:1 and 1:3, preferably between 1:1 and 1:2, and particularly preferably 1:1 or 1:2. The photomask can be substantially rectangular.The photomask can be 12.70 cm (5 in) to 17.78 cm (7 in) long and wide, preferably 15.24 cm (6 in) long and wide. Alternatively, the photomask can be 12.70 cm (5 in) to 17.78 cm (7 in) long and 25.40 cm (10 in) to 35.56 cm (14 in) wide, preferably 15.24 cm (6 in) long and 30.48 cm (12 in) wide.

[0029] In an alternative embodiment, the optical system according to the invention is designed to direct the EUV radiation onto a measurement object. Additionally or alternatively, the optical system can be designed to direct EUV radiation reflected from a measurement object onto a detector. The optical system can, for example, be used to obtain a measurement of the reflectivity of the measurement object. The detector can be designed to determine the intensity of the incident EUV radiation. In one variant, the detector is an EUV image sensor.

[0030] Extreme ultraviolet (EUV) radiation refers to electromagnetic radiation in the extreme ultraviolet spectral range with wavelengths between 5 nm and 100 nm, particularly between 5 nm and 30 nm. Specifically, EUV radiation can have a wavelength of 13.5 nm.

[0031] The invention also relates to a method for operating an optical system in which an EUV beam path generated by an EUV radiation source is directed onto an optical surface of an EUV mirror. The EUV beam path is directed onto the EUV mirror at an angle of no more than 12°. The optical surface is coated with a carbon layer. The carbon layer has a thickness of at least 8 nm. The carbon layer is a carbon layer deposited during operation of the optical system. The optical system comprises a vacuum housing in the interior of which the EUV mirror is arranged. The vacuum housing contains a vacuum atmosphere without hydrogen purge gas.

[0032] The disclosure includes further developments of the method that are described in connection with the optical system according to the invention.

[0033] The invention is described below by way of example with reference to the accompanying drawings and advantageous embodiments. The drawings show: Fig. 1: a microlithographic projection exposure system; Fig. 2: an EUV mirror of the projection exposure system Fig. 1; Fig. 3: a detail of the EUV mirror from Fig. 2 in enlarged view; Fig. 4, Fig. 5: the view according to Fig. 3 in other states of the EUV mirror; Fig. 6: a device for examining photomasks; Fig. 7: two EUV mirrors of the device made of Fig. 6; Fig. 8: a device for examining the reflection properties of a measurement object; Fig. 9: a representation of the reflectivity of an EUV mirror as a function of the angle of incidence; Fig. 10: a representation of the reflectivity of a single EUV mirror as a function of the thickness of the carbon layer; Fig. 11: a representation of the reflectivity of a combination of several EUV mirrors as a function of the thickness of the carbon layer.

[0034] In Fig. Figure 1 shows a schematic representation of a microlithographic EUV projection exposure system. The projection exposure system comprises an exposure beam source 14, an illumination system 10, and a projection lens 22, which are operated together in a vacuum chamber 23. A negative pressure is maintained in the vacuum chamber 23 during operation of the EUV projection exposure system.

[0035] The exposure beam source 14 generates electromagnetic radiation in the EUV range with a wavelength of 13.5 nm. The exposure radiation emitted from the exposure beam source 14 is focused by a collector 15 into an intermediate focal plane 16. The EUV beam path 27 exiting the intermediate focal plane 16, which has a central beam 26, is directed by the illumination system 10 into an object plane 12, so that an object field in the object plane 12 is illuminated with uniform radiation intensity.

[0036] The illumination system 10 comprises a deflecting mirror 17, which deflects the EUV beam path 27 onto a first faceted mirror 18. A second faceted mirror 19 is arranged downstream of the first faceted mirror 18. The facets of the first faceted mirror 18 are imaged onto the object plane 12 by the second faceted mirror 19.

[0037] In the object plane 12, a photomask 13 is arranged, which is imaged onto an image plane 21 via a plurality of mirrors M1-M6 of the projection lens 22. A structure formed on the photomask 13 is transferred to a radiation-sensitive layer of a wafer 20 arranged in the image plane 21. The photomask 13 is suspended from a first scanning device 24, and the wafer 20 rests on a second scanning device 25, so that the wafer 20 can be exposed in a single scanning operation in which the photomask 13 and the wafer 20 are moved synchronously.

[0038] The optical elements of the projection exposure system, with which the EUV beam path 27 is shaped and deflected, form an optical system within the meaning of the invention. The optical system comprises an EUV mirror in the form of the deflecting mirror 17, at whose optical surface 30 the EUV beam path 27 is reflected, see Fig. 2. The central ray 26 of the EUV beam path 27 incident on the optical surface 30 forms an angle 34 of 10° with the optical surface 30.

[0039] According to the enlarged illustration in Fig. In the deflecting mirror 17, a mirror body 31 is formed from a titanium silicate glass with ultra-low thermal expansion (“ultra-low expansion material”). The mirror body 31 is coated with a reflective layer 32, which in this embodiment consists of ruthenium. The reflective layer 32 can have a thickness on the order of, for example, 50 nm.

[0040] The ruthenium reflective layer 32 has a high reflectivity for EUV radiation incident at a small angle, i.e., at grazing incidence. Since the other EUV mirrors of the projection exposure system also have a high reflectivity for EUV radiation, the wafer 20 can be exposed with a sufficiently high intensity of EUV radiation.

[0041] During operation of the projection exposure system, impurities accumulate on the reflective layer 32, eventually forming a carbon layer 33. Fig. Figure 4 shows the deflecting mirror 17 in a state where the carbon layer 33 has built up to a thickness of 10 nm. Compared to the one in Fig. In the state shown in Figure 3, the reflectivity of the deflecting mirror 17 decreases by approximately 20% due to the 10 nm thick carbon layer 33. This loss of reflectivity is significantly less than in EUV mirrors operated conventionally at grazing incidence, where the EUV radiation occurs at an angle of, for example, 20°. In that case, the loss of reflectivity would already be on the order of 50%, and thus so high that practical use of the optical system would no longer be possible.

[0042] This results from Fig. Figure 9 shows the reflectivity of an EUV mirror as a function of the angle at which the EUV radiation strikes the optical surface of the EUV mirror. A first curve 62 shows the reflectivity curve in the angular range between 0° and 30° for an EUV mirror whose optical surface is free of a carbon layer. A second curve 63 shows the curve for a 5 nm thick carbon layer, a third curve 64 the curve for a 10 nm thick carbon layer, a fourth curve 65 the curve for a 20 nm thick carbon layer, a fifth curve 66 the curve for a 30 nm thick carbon layer, and a sixth curve 75 the curve for a 40 nm thick carbon layer.

[0043] An alternative representation shows Fig. 10. The reflectivity of an EUV mirror as a function of the thickness of the carbon layer. A first curve 67 shows the curve for carbon layers between 0 nm and 50 nm at an angle of 16°, at which the EUV radiation is incident on the optical surface of the EUV mirror. A second curve 68 shows the curve for an angle of 8°, a third curve 69 the curve for an angle of 4°, and a fourth curve 40 the curve for an angle of 2°. The reflectivity is higher the smaller the angle.

[0044] The invention makes it possible to extend the service life of the optical system by allowing the system to continue operating even when the carbon layer 33 has grown beyond 10 nm. Fig. Figure 5 shows a state of the deflecting mirror 17 in which the carbon layer 33 has reached a thickness of 20 nm. Given the small angle at which the EUV radiation strikes the deflecting mirror 17, the growth of the carbon layer 33 does not result in a significant loss of reflectivity. The reflectivity is still on the order of 75% compared to that in Fig. The condition shown in section 3. With conventional grazing incidence at an angle of 20°, however, the reflectivity would already have dropped to a value significantly below 20%.

[0045] The remaining EUV mirrors 18, 19, and M1-M6 of the projection exposure system are operated under normal incidence, so that the incident EUV radiation forms an angle of approximately 90° with the respective optical surface. Under normal incidence, the carbon layer that builds up during operation has a significantly smaller effect on reflectivity than under grazing incidence, so the optical system can still be operated effectively even if it is assumed that a carbon layer 20 nm thick has also been deposited on the other EUV mirrors 18, 19, and M1-M6.

[0046] In Fig. Figure 6 shows a measuring device with which microlithographic photomasks 50 can be examined.

[0047] In the measuring device, the photomask 50 is arranged such that the EUV beam path 27 emanating from an EUV radiation source 51, from which in Fig. Figure 6 shows only the central beam 26, which is directed onto the photomask 50 via an illumination system 52. The illumination system 52 comprises a first illumination mirror 53 and a second illumination mirror 54, at which the EUV beam path 26 is reflected and formed into a beam that illuminates an examination field on the surface of the photomask 50 with uniform brightness. An XY positioner 55 can be used to move the photomask in the XY plane to bring different examination fields into the area of ​​the EUV beam path 26.

[0048] The EUV beam path 27, reflected at the photomask 50, continues via a projection lens 56 to an EUV camera 57, which is equipped with an image sensor 58. The projection lens 56 can include one or more EUV mirrors, off which the EUV radiation is reflected and via which the field of view of the photomask 50 is projected onto the image sensor 58 of the EUV camera 57. The EUV radiation source 51, the illumination system 52, the photomask 50, the projection lens 56, and the EUV camera 58 are arranged in a vacuum housing 61, in which a negative pressure is maintained during operation of the measuring device.

[0049] The EUV radiation source 51 is a plasma radiation source in which EUV radiation with a wavelength of 13.5 nm is emitted from a plasma. Tin is a medium suitable for generating a plasma for emitting such EUV radiation. To generate the plasma, a droplet of the medium can be exposed to a laser beam.

[0050] The components of the measuring device, which deflect and shape the EUV beam path 27 between the EUV radiation source 51 and the EUV camera 57, form an optical system according to the invention. The first illumination mirror 53 and the second illumination mirror 54 are EUV mirrors of the optical system.

[0051] According to Fig. The EUV beam path 27 strikes the first illumination mirror 53 at an angle 59 of 8°. The EUV beam path 27 reflected from the first illumination mirror 53 strikes the second illumination mirror 54 at an angle 60 of 8°. Since each of the two illumination mirrors 53, 54 deflects the EUV beam path 27 by 16°, a total beam deflection of 32° within one plane is achieved.

[0052] Compared to a state in which the illumination mirrors 53, 54 are new, i.e., no carbon layer 33 has yet formed, the reflectivity is reduced by less than 20% by a 20 nm thick carbon layer 33. With further increasing thickness of the carbon layer 33, there is no further decrease in reflectivity, so that even with a carbon layer 33 that is, for example, 50 nm thick, the loss of reflectivity is still less than 20%. The structure of the carbon layer 33 therefore no longer represents a practically relevant limitation on the time period over which the measuring device with the mirror arrangement can be used. Fig. 7 can be operated.

[0053] This results from Fig. Figure 11 shows the reflectivity curve as a function of carbon layer thickness for a total beam deflection of 32°. A first curve 71 shows the reflectivity curve between 0 nm and 50 nm carbon layer thickness for a single mirror. A second curve 72 shows the corresponding curve for the case where the 32° beam deflection is achieved with two mirrors. A third curve 73 for four mirrors and a fourth curve 74 for eight mirrors are virtually identical.

[0054] If a single EUV mirror were used for beam deflection by 32°, on which the EUV beam path arrives at an angle of 16°, the reflectivity through a 50 nm thick carbon layer would decrease by more than 60%.

[0055] The loss of reflectivity can be further reduced to some extent if the 32° beam deflection is achieved using more than two EUV mirrors. For example, with four EUV mirrors and an angle of 4° for the incident EUV radiation, the loss of reflectivity is just over 16%. With eight EUV mirrors and an angle of 2° for the incident EUV radiation, the loss is just under 16%. This slight improvement in reflectivity comes at the cost of increased equipment complexity.

[0056] In Fig. Figure 8 shows a reflectometer comprising an optical system according to the invention.

[0057] Given the large number of EUV mirrors 17, 18, 19, M1-M6, on which the EUV radiation in the microlithographic projection exposure system is emitted from Fig. Since the EUV radiation is reflected, it is important for the functionality of the microlithographic projection exposure system that each individual EUV mirror has a high reflectivity for EUV radiation. Therefore, before installation or in connection with maintenance or repair, it may be desirable to measure the reflection properties of one or more EUV mirrors. This can be done using a reflectometer, as described in Fig. 8 is shown.

[0058] The reflectometer, designed for examining a sample 40, comprises an EUV radiation source 44, from which the EUV radiation propagates through an aperture 41. The optical system includes a first collecting mirror 46, at which the EUV beam path 27 is shaped into a convergent beam. The EUV beam path 27 continues via a monochromator 47, an aperture 35, a second collecting mirror 37, and a beam splitter 39 to the sample 40.

[0059] The monochromator 34 is designed as a grating monochromator, configured such that only a narrowly defined wavelength range, for example around 13.5 nm, can pass through the aperture 35. The aperture 35 has a slit-shaped opening, which is imaged onto the sample 40 by the second focusing mirror 37. The long dimension of the slit is oriented perpendicular to the image plane.

[0060] The beam splitter 39 deflects part of the EUV radiation out of the image plane in order to obtain a measure of the intensity of the EUV radiation arriving at the sample 40.

[0061] After reflection at sample 40, the EUV beam path 27 continues in a divergent form to a detector 42 designed as an EUV image sensor. The detector 42 registers the intensity of the incident EUV radiation pixel by pixel.

[0062] Sample 40 will be taken at a site in Fig.The sample 40 is held by a schematically indicated positioning system 48, which allows it to be moved within a plane and tilted out of that plane. The positioning system 48 makes it possible to position the sample 40 so that the EUV radiation strikes different surface areas of the sample 40, while the EUV beam path 27 reflected from the sample 40 continues to propagate towards the detector 42, which remains in the same position.

[0063] The optical elements that shape and deflect the EUV beam path 27 between the EUV radiation source 44 and the sample 40 constitute an optical system according to the invention. The optical system comprises a first EUV mirror and a second EUV mirror, respectively, comprising the first collecting mirror 46 and the second collecting mirror 37. The central ray 26 of the EUV beam path 27 strikes both the first collecting mirror 46 and the second collecting mirror 37 at an angle of 10°. The first collecting mirror 46 and the second collecting mirror 37 have different planes of incidence, so that the EUV beam path 27 is deflected out of the plane.

[0064] The optical system can be operated until the carbon layer formed during operation on the first collecting mirror 46 and the second collecting mirror 47 has reached a thickness of significantly more than 10 nm, for example 20 nm or 30 nm.

Claims

[1] Optical system with an EUV mirror (17, 37, 46, 53, 54), wherein the EUV mirror (17, 37, 46, 53, 54) is provided with an optical surface (30) designed for the reflection of EUV radiation, wherein an EUV beam path (27) generated by an EUV radiation source (14, 44, 51) is directed onto the optical surface (30) of the EUV mirror (17, 37, 46, 53, 54), wherein the EUV beam path (27) strikes the EUV mirror at an angle (34, 59, 60) of not more than 12°, wherein the optical surface (30) is coated with a carbon layer (33), wherein the carbon layer (33) has a thickness of at least 8 nm, wherein the carbon layer (33) has a thickness which is System is deposited carbon layer (33), wherein the optical system comprises a vacuum housing (23, 61), in the interior of which the EUV mirror (17, 37, 46, 53, 54) is arranged, wherein a vacuum atmosphere without hydrogen purge gas is present in the vacuum housing (23, 61). [2] Optical system according to claim 1, wherein the optical surface (30) is formed by a reflection layer with a thickness between 20 nm and 200 nm, preferably between 30 nm and 80 nm. [3] Optical system according to one of claims 1 or 2, wherein the optical surface (30) is formed by a reflective layer made of ruthenium. [4] Optical system according to one of claims 1 to 3, wherein the angle between the EUV beam path (27) and the optical surface (30) is at least 4°, preferably at least 6°, more preferably at least 8°. [5] Optical system according to one of claims 1 to 4, comprising a first EUV mirror (53, 46) and a second EUV mirror (54, 37), wherein the difference between the angle (59) at which the EUV beam path (27) strikes the first EUV mirror (53, 46) and the angle (60) at which the EUV beam path (27) strikes the second EUV mirror (54, 37) is less than 6°, preferably less than 4°, more preferably less than 2°. [6] An optical system according to claim 5, wherein the first EUV mirror (53, 46) and the second EUV mirror (54, 37) have the same plane of incidence. [7] Optical system according to claim 5 or 6, wherein the first EUV mirror (53, 46) and the second EUV mirror (54, 37) together cause a beam deflection of the EUV beam path (27) which is between 24° and 48°, preferably between 30° and 40°. [8] Optical system according to one of claims 5 to 7, wherein no further optical element is arranged between the first EUV mirror (53, 46) and the second EUV mirror (54, 37) with which the direction of the EUV beam path (27) is changed. [9] Optical system according to one of claims 1 to 8, comprising one or more EUV mirrors (M1-M6) onto which the EUV beam path (27) impinges at an angle between 60° and 90°. [10] Method for operating an optical system, in which an EUV beam path (27) generated by an EUV radiation source (14, 44, 51) is directed onto an optical surface (30) of an EUV mirror (17, 37, 46, 53, 54), wherein the EUV beam path (27) is directed onto the EUV mirror at an angle (34, 59, 60) of not more than 12°, wherein the optical surface (30) is coated with a carbon layer (33), wherein the carbon layer (33) has a thickness of at least 8 nm, wherein the carbon layer (33) is a carbon layer (33) deposited during operation of the optical system, wherein the optical system comprises a vacuum housing (23, 61), in the interior of which the EUV mirror (17, 37, 46, 53, 54) is arranged, wherein in the vacuum housing (23, 61) a vacuum atmosphere without hydrogen purge gas is present.

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