Method for producing a mirror substrate of an optical element, optical element and projection exposure system

Abstract

The invention relates to a method for producing a mirror substrate (201, 301, (401), (501)) of an optical element for a projection exposure system, in particular an EUV projection exposure system (100), comprising a first (202, (402), (502)) and at least one second component (204, 304, (404), (504)), the first component (202, (402), (502)) and the at least one second component (204, 304, (404), (504)) consisting of silicon at least on a side facing a connection (211, 308, (408), (508)), and the method having the following steps: providing / producing the at least two components (202, (402), (502), (204), 304, (404), (504)) of the mirror substrate (201, 301, (401), (501)), and joining the at least two components (202, (402), (502), (204), 304, (404), (504)) by heating to a joining temperature and applying a joining pressure, preferably perpendicularly to a joining surface (203, (205), (403), (405), (503), (505)).

Description

[0001] Method for producing a mirror substrate of an optical element, optical element and projection exposure apparatus.

[0002] Microlithography is used to manufacture microstructured components, such as integrated circuits. The microlithography process is carried out using a lithography system equipped with an illumination system and a projection system. The image of a mask (reticle) illuminated by the illumination system is projected by the projection system onto a substrate coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, for example, a silicon wafer, in order to transfer the mask structure to the light-sensitive coating of the substrate.

[0003] By using EUV radiation, i.e., radiation at wavelengths of approximately 13.5 nm, for example, very small structures can be created on the substrate using a lithography system. This can, for example, further advance the reduction of structure size in the manufacture of semiconductor components.

[0004] In illumination and projection systems designed for the EUV range, mirrors are used as optical elements for the imaging process due to the lack of suitable light-transmitting materials. These optical elements for EUV systems are made from crystals such as silicon, but also from high-performance glass ceramics or metals. For this purpose, blocks are produced from these materials using various processes, from which the mirror substrates for the optical elements are machined using subtractive contouring techniques. This production of the optical elements is complex and cost-intensive, especially for large mirror sizes. Due to the increasing power of useful light sources from generation to generation, it is also particularly advantageous if the mirror substrates contain fluid channels through which tempered water flows, thus carrying the heat away from the optically active surface.This allows the optical elements to be operated at a favorable operating temperature with a minimized temperature gradient in the mirror substrate. This counteracts deformation of the optically active surface, thereby reducing adverse effects on the imaging quality of the optical element. One possible method for producing the fluid channels is drilling, which typically has the disadvantage that the holes can only be drilled straight through the mirror substrate. For the production of mirror substrates made at least partially of silicon from the prior art, reference is made to the documents DE 102018207759 A1 and DE 102014222952 A1.

[0005] Against this background, it is an object of the present invention to provide an improved method for producing a mirror substrate of an optical element for a projection exposure apparatus.

[0006] The inventive method for producing a mirror substrate of an optical element for a projection exposure apparatus, in particular an EUV projection exposure apparatus, comprises a first and at least one second component, wherein the first component and the at least one second component each consist of silicon at least on a side facing a connection.

[0007] In the method, the at least two components of the mirror substrate are first produced and / or prepared. For this purpose, methods known from the prior art can be used, for example. Furthermore, this step can also include surface treatment such as lapping or polishing of a silicon joining surface of the at least two components in order to improve their joinability. For the purposes of this application, the "connection" refers to an area between the at least two components that is formed by joining the two joining surfaces. For the purposes of this application, the "joining surface" refers to the surface over which a joining process of the at least two components takes place.

[0008] In a further step of the inventive method, the at least two components are joined by heating to a joining temperature and applying a joining pressure. The joining pressure is preferably applied perpendicular to the joining surfaces, so that the joining of the two components is maximally supported. In particular, the two joining surfaces are oriented parallel to each other for this purpose. In one embodiment of the method, the at least two components are joined in an evacuated environment. A vacuum provides particularly clean conditions for the joining process of the components, since in particular gaseous contaminants in the immediate environment are removed. The vacuum also lowers the required joining temperature, so that the joining process can be implemented more effectively. A vacuum of 100 - 10' 7 mbar, preferably from 10' 1 up to 10' 3 mbar set.

[0009] In a further embodiment of the method, when the at least two components are joined together, at least one fluid channel structure is formed in the resulting mirror substrate in the region of the connection. To form the at least one fluid channel structure, at least one of the two components can have a pre-structuring, for example in the form of a groove. In a first variant of the embodiment, only one of the two components includes a pre-structuring, so that when joined together, the second component acts as a type of cover, resulting in the fluid channel structure in the resulting mirror substrate of the optical element. In a second variant of the embodiment, both components have such a pre-structuring, so that the fluid channel structure in the resulting mirror substrate of the optical element is composed of two parts.The pre-structurings have rectangular contours and / or rounded rectangular contours and / or round contours and / or oval contours. In a further variant of the embodiment of the method, the surface structure of the fluid channel structures is processed after assembly using a chemical and / or a physical processing method. In both variants of the embodiment, complex processing of the mirror substrate is omitted in order to introduce fluid channel structures using a subtractive process after the assembly of the at least two components. A mirror substrate produced in this way enables active temperature control of the optical element during later operation of the projection exposure system or during a further processing step of the mirror substrate due to the at least one fluid channel structure. Deformation-related imaging errors, for example caused by a radiation-induced...

[0010] Heat input can thus be minimized.

[0011] In a further embodiment of the method, the first and at least one second component are joined directly to one another. This prevents foreign matter from being introduced into the area where the two components are joined. Rather, by directly joining the two silicon joining surfaces, monolithic structures can be created in the area of ​​the connection, which are characterized by exceptional stability.

[0012] In a further embodiment of the method, in which the first and at least one second component are joined directly to one another, the Rms value of the surface roughness of at least one joining surface of the at least two components is less than five nanometers. This low surface roughness allows the required joining pressure to be reduced. An Rms value of the surface roughness of a joining surface made of silicon can be achieved with a minimal polishing effort of approximately one day.

[0013] In a further embodiment of the method, in which the first and at least one second component are joined directly to one another, the joining temperature is 1100–1250°C. Silicon has a brittle-ductile transition in the range of 700–1100°C. To avoid the risk of fracture, it is therefore particularly advantageous if the joining temperature is above 1100°C. Furthermore, the maximum joining temperature should be approximately 90% of the melting point of silicon, which corresponds to approximately 1250°C.

[0014] In a further embodiment of the method, in which the first and the at least one second component are joined directly to one another, the joining pressure is 0.1 MPa - 15 MPa, preferably 0.3 - 0.8 MPa, particularly preferably 0.5 MPa. The required joining pressure depends on the roughness and waviness of the silicon surfaces. The rougher and wavier the surfaces, the greater the required pressure. For a roughness corresponding to the Rms value of 5 nm, for example, a joining pressure of 0.5 MPa has proven particularly advantageous. In a further embodiment of the method, a mediator layer is provided for joining the at least two components. This step takes place before the actual joining by heating to a joining temperature and applying a joining pressure. The mediator layer is placed between the silicon joining surfaces of the at least two components for the joining process.As a result, the first and at least one second component are not joined directly to each other. The intermediary layer enables the connection of at least two joining surfaces with increased surface roughness. In addition, a lower joining temperature can be used by selecting the appropriate material.

[0015] In a further embodiment of the method, in which a mediator layer is provided for joining the at least two components, at least one joining surface of the at least two components is convex. This convex curvature can be caused by the pre-processing of the joining surface, in particular by polishing. Typical flatness values ​​are in the range of 10 - 50 pm.

[0016] In a further embodiment of the method, in which the mediator layer is provided for joining the at least two components, the RMS value of the surface roughness of at least one joining surface of the at least two components is less than 100 nanometers. This RMS value can be achieved with a simple cloth polish.

[0017] In a further embodiment of the method, in which the mediator layer is provided for joining the at least two components, the mediator layer has a layer thickness of 5 μm to 1.5 mm. This window of possible layer thicknesses allows different wavinesses of the joining surfaces of the at least two components to be addressed. The waviness of the joining surfaces should not exceed the layer thickness of the glass film and is preferably less than 10% of the layer thickness. At the same time, a layer thickness of 5 μm to 1.5 mm for the mediator layer ensures good handling thereof so that it can be provided with as little damage as possible. In a further embodiment of the method, in which the mediator layer is provided for joining the at least two components, the joining temperature is above, preferably 10% above, a glass transition temperature of the mediator layer.This ensures the deformability of the mediator layer, which ensures the most complete contact possible between the mediator layer and the joining surfaces, even with significant waviness of the joining surfaces. This minimizes the amount of inclusions that arise in the joint area due to incomplete contact between a joining surface and the mediator layer. Furthermore, the deformability can limit the required joining pressure.

[0018] In a further embodiment of the method in which the mediator layer is provided for joining the at least two components, the mediator layer consists of borosilicate glass, alkali-free glass or silicon.

[0019] In a further embodiment of the method, in which the first and at least one second component are joined directly to one another, the joining pressure is 0.1 MPa - 15 MPa, preferably 0.3 - 0.8 MPa, particularly preferably 0.5 MPa. The required joining pressure depends on the roughness and waviness of the silicon surfaces. The rougher and wavier the surfaces, the greater the required pressure. For a roughness corresponding to the Rms value of 5 nm, for example, a joining pressure of 0.5 MPa has proven particularly advantageous.

[0020] Further aspects of the invention initially relate to an optical element with a mirror substrate, which is produced by a method according to one of claims 1 to 13.

[0021] In a further aspect, the invention also relates to an optical element with a mirror substrate, in which at least one chemical and / or physical property of the mirror substrate changes abruptly along at least one spatial direction.

[0022] Furthermore, the invention relates to a projection exposure system for semiconductor lithography which has at least one optical element, wherein at least one of the optical elements is at least partially produced by a method according to one of claims 1 to 13 and / or at least one of the optical elements is an optical element according to claim 14.

[0023] This results in the aforementioned advantages for the optical element according to the invention and for the projection exposure system according to the invention. Further advantages and preferred features emerge from the above description and from the claims.

[0024] The invention will be explained in more detail below with reference to the drawings.

[0025] Figure 1 shows an EUV projection exposure system,

[0026] Figures 2a and 2b show a device for the inventive method for producing a mirror substrate by directly connecting two components,

[0027] Figures 3a and 3b show a device for an embodiment of the inventive method for producing a mirror substrate with fluid channels,

[0028] Figures 4a and 4b show a device for an embodiment of the inventive method for producing a mirror substrate by connecting two components by means of an intermediary layer

[0029] Figures 5a and 5b show a device for an embodiment of the inventive method for producing a mirror substrate by joining two components by means of a mediator layer, wherein the surfaces to be joined are convex,

[0030] Figure 6 Flowchart of an embodiment of the inventive method

[0031] Figure 1 shows an example of the basic structure of an EUV projection exposure system (100) for semiconductor lithography. An illumination system (101) of the projection exposure system (100) comprises, in addition to a radiation source (102), illumination optics (103) for illuminating an object field (104) in an object plane (105). A reticle (106) arranged in the object field (104) is illuminated and held by a reticle holder (107), a section of which is schematically shown. Projection optics (108) serve to image the object field (104) into an image field (109) in an image plane (110). A structure on the reticle (106) is imaged onto a light-sensitive layer of a wafer (111) arranged in the image plane (110) in the region of the image field (109), which wafer is held by a wafer holder (112), also shown in detail.

[0032] The radiation source (102) can emit EUV radiation (113), in particular in the range between 5 nm and 30 nm, in particular 13.5 nm. Optical elements of various optical configurations and mechanically adjustable optical elements are used to control the radiation path of the EUV radiation (113). In the EUV projection exposure system (100) shown in Figure 1, the optical elements are designed as adjustable mirrors in suitable embodiments mentioned below only as examples. Individual optical elements designed as mirrors can consist of several segments with separate optical partial surfaces.

[0033] The EUV radiation (113) generated by the radiation source (102) is aligned by a collector mirror integrated into the radiation source (102) such that the EUV radiation (113) passes through an intermediate focus in the region of an intermediate focal plane (114) before impinging on a field facet mirror (115). After the field facet mirror (115), the EUV radiation (113) is reflected by a pupil facet mirror (116). With the aid of the pupil facet mirror (116) and further mirrors (117, 118, 119), field facets of the field facet mirror (115) are imaged into the object field (104). See corresponding US9411241 B2.

[0034] The reticle (106) arranged in the object field (104) can, for example, be a reflective photomask having reflective and non-reflective, or at least less highly reflective, regions for generating at least one structure on the reticle (106). Alternatively, the reticle (106) can be a plurality of micromirrors arranged in a one-dimensional or multi-dimensional array and optionally movable about at least one axis to adjust the angle of incidence of the EUV radiation on the respective mirror.

[0035] The reticle (106) reflects a portion of the beam path of the illumination optics (103) and forms a beam path in the projection optics (108). This beam path radiates the information about the structure of the reticle into the projection optics (108), which generates an image of the reticle or a respective subregion thereof on the wafer (111) arranged in the image plane (110). The wafer comprises a semiconductor material, e.g., silicon, and is arranged on a wafer holder (112), which is also referred to as a wafer stage.

[0036] In the present example, the projection lens (108) has six reflective optical elements (120) to (125) designed as mirrors to generate an image of the reticle (106) on the wafer (111). Typically, the number of mirrors in a projection lens such as (108) is between four and eight, but if necessary, only two or even ten mirrors can be used. Projection lenses are known from US2016 / 0327868A1 and DE102018207277A1.

[0037] Figures 2a and 2b each show a device (200) for the inventive method for producing a mirror substrate (201) of an optical element for a projection exposure apparatus, in particular an EUV projection exposure apparatus (100) according to Figure 1.

[0038] For this purpose, a first component (202) with a first joining surface (203) and a second component (204) with a second joining surface (205) are provided in a chamber (206) in Figure 2a. The components (202), (204) shown in Figures 2a and 2b are made of silicon. However, it is also possible for the components (202), (204) to be made of silicon only in one area of ​​the joining surfaces (203), (205).

[0039] The two joining surfaces (203) and (205) are advantageously oriented parallel to each other and preferably have an Rms value of the surface roughness that is at least less than five nanometers. The chamber (206) comprises a first means (207) for heating the two components (202), (204) to a joining temperature and a second means (208) for applying a joining pressure, preferably perpendicular to the joining surfaces (203), (205). The first means (207) for heating the two components (202), (204) can, for example, be a media-based

[0040] The device (207) can be a temperature control device for integral temperature control of the chamber (206). Furthermore, the device (207) can be designed, for example, as a radiant heater for targeted temperature control of the two components (202), (204). With the device (207) for heating the first component (202) and the second component (204), a joining temperature of 1100-1250°C is typically achieved.

[0041] The second means (208) for applying a joining pressure to the components (202), (204) to be joined is designed in this embodiment as a stamp that is placed in the upper chamber region of the chamber (206). Furthermore, the chamber (206) comprises a third means (209) for evacuating an interior space (210) of the chamber (206). This allows the two components (202), (204) to be joined optionally in an evacuated environment. The means (209) can, for example, be designed as a vacuum pump, with which a vacuum of 100 - 10' can be created in the interior space (210). 7 mbar, preferably from 10' 1 up to 10' 3mbar. The vacuum provides particularly clean conditions for the joining process of the two components (202), (204), since in particular gaseous contaminants in the immediate vicinity of the interior (210) are removed. The vacuum also lowers the required joining temperature so that the joining process can be implemented more effectively. In Figure 2b, a targeted movement (symbolized by black arrows) of the second means (208) in the direction of the second component (204) applies a joining pressure to both components (202), (204) perpendicular to the two joining surfaces. The joining pressure is advantageously in the range 0.1 MPa - 15 MPa, preferably 0.3 - 0.8 MPa. As a result, a connection (211) is formed between the two components (202), (204) in an area of ​​the previously existing joining surfaces (203), (205), whereby the mirror substrate (201) is formed by the two components (202), (204).In this embodiment, the two components (202), (204) are joined directly to one another. By directly joining the two silicon joining surfaces (203), (205), monolithic structures are preferably constructed in the region of the connection (211), which are characterized by particular stability.

[0042] Figures 3a and 3b also show the device (200) for a further embodiment of the inventive method for producing a mirror substrate (301) of an optical element for a projection exposure apparatus, in particular an EUV projection exposure apparatus (100) according to Figure 1.

[0043] In this case, the mirror substrate (301) in Figure 3b has a plurality of fluid channel structures (311) in the region of a connection 308, which are created by joining the first component (202) to a second component (304). The second component (304) is also made of silicon, but can also only consist of silicon in regions around a joining surface (305). In the variant of the embodiment shown, the second component (304) has a pre-structuring in the form of a plurality of grooves (306) for forming the fluid channel structures, so that during joining the first component (202) acts as a type of cover, resulting in the fluid channel structures (311) in the resulting mirror substrate (301) of the optical element. In a further variant of the embodiment, the component (202) also additionally has such a pre-structuring.The pre-structuring of the component and / or the component has rectangular contours and / or round contours and / or oval contours. In a further variant of the embodiment of the method, the surface structure of the fluid channel structures is processed after assembly using a chemical and / or physical processing method. In both variants of the embodiment, complex processing of the mirror substrate is eliminated in order to introduce fluid channel structures using a subtractive process following the assembly of the at least two components. A mirror substrate (301) produced in this way enables active temperature control of the optical element during subsequent operation of the projection exposure system due to the fluid channel structures (311). Deformation-related imaging errors, for example caused by radiation-induced heat input, can thus be minimized.Figures 4a and 4b also show the device (200) for a further embodiment of the inventive method for producing a mirror substrate (401) of an optical element for a projection exposure apparatus, in particular an EUV projection exposure apparatus (100) according to Figure 1.

[0044] For this purpose, a first component (402) with a first joining surface (403) and a second component (404) with a second joining surface (405) are provided in the chamber (206) in Figure 4a. The components (402), (404) shown in Figures 4a and 4b are made of silicon. However, it is also possible for the components (402), (404) to consist of silicon only in an area of ​​the joining surfaces (403), (405). Characteristic of this embodiment is an intermediary layer (406) which is provided for joining the first component (402) and the second component (404). The intermediary layer (406) typically has a thickness of 5 pm to 1.5 mm and is typically made of borosilicate glass, alkali-free glass or silicon. The two joining surfaces (403) and (405) are advantageously oriented parallel to each other and preferably have an Rms value of the surface roughness that is at least less than 100 nanometers.This Rms value can be achieved with a simple cloth polish. Thus, the provision of the mediator layer (406) generally improves the joinability of components with a tendency toward increased roughness.

[0045] For the joining process, the first means (207) provides a joining temperature that is above, preferably 10% above, a glass transition temperature of the mediator layer (406). This ensures deformability of the mediator layer (406), whereby the most complete contact possible between the mediator layer and the joining surfaces (403), (405) is achieved even with greater waviness of the joining surfaces (403), (405). This minimizes the proportion of inclusions that arise in the region of the connection (408) due to incomplete contact between at least one joining surface (403), (405) and the mediator layer (406). In addition, the deformability can limit the necessary joining pressure.

[0046] As shown in Figure 4b, this is applied to both components (402), (404) as well as the mediator layer (406) preventing the components, perpendicular to the two joining surfaces (403), (405), by a targeted movement (symbolized by black arrows) of the second means (208) in the direction of the second component (404). The joining pressure is advantageously in the range of 0.1 MPa - 15 MPa, preferably 0.3 - 0.8 MPa. As a result, a connection (408) is formed between the two components (402), (404) in a region of the previously existing joining surfaces (403), (405) connected by the mediator layer (406), whereby the mirror substrate (401) is formed by the two components (402), (404). Furthermore, in this embodiment, the chamber (206) also comprises the third means (209) for evacuating the interior (210) of the chamber (206).This allows the two components (402), (404) and the mediator layer to optionally be joined in an evacuated environment.

[0047] Figures 5a and 5b also show the device (200) for a further embodiment of the inventive method for producing a mirror substrate (501) of an optical element for a projection exposure apparatus, in particular an EUV projection exposure apparatus (100) according to Figure 1.

[0048] For this purpose, a first component (502) with a first joining surface (503) and a second component (504) with a second joining surface (505) as well as the mediator layer (406) are provided in the chamber (206) in Figure 5a. The two joining surfaces (503) and (505) have a convex shape. This convex shape can be due to the pre-processing of the joining surfaces (503), (505), in particular a polish. Typical flatness values ​​are in the range of 10 - 50 pm. In principle, it is also possible for only one of the two joining surfaces (503), (505) to be convex.

[0049] According to Figure 5b, a connection (508) is formed between the two components (502), (504) in a region of the previously existing convex joining surfaces (503), (505) connected by the mediating layer (406), whereby the mirror substrate (501) is formed by the two components (502), (504). The mediating layer (406), heated above its glass transition temperature by the first agent (207), adapts to the convex surface shape of the joining surfaces (503), (505).

[0050] Figure 6 shows a flow diagram of an embodiment of the inventive method for producing a mirror substrate of an optical element for a projection exposure apparatus, in particular an EUV projection exposure apparatus according to Figure 1. The method in Figure 6 is applicable to the substrates of the optical elements (215-219) and (220-225) shown in Figure 1 within a projection exposure apparatus. In a first step (S1), a first and at least one second component are provided for producing a mirror substrate. The first component and the at least one second component consist of silicon at least on a side facing a connection. Thus, the first and / or the at least one second component can consist entirely of silicon. It is also possible for the first and / or the at least one second component to consist of silicon only in the region of their respective joining surfaces.

[0051] Preferably, in a second step (S2), a mediator layer is provided for joining the at least two components. The use of a mediator layer is particularly helpful for components with increased surface roughness of the joining surface.

[0052] In a third step (S3), the at least two components are joined together by heating to a joining temperature and applying a joining pressure, preferably perpendicular to the joining surfaces.

[0053] List of reference symbols

[0054] 100 projection exposure system

[0055] 101 Lighting system

[0056] 102 Radiation source

[0057] 103 Lighting optics

[0058] 104 Object field

[0059] 105 Object level

[0060] 106 reticles

[0061] 107 reticle holders

[0062] 108 Projection optics

[0063] 109 image field

[0064] 110 Image plane

[0065] 111 wafers

[0066] 112 wafer holders

[0067] 113 EUV radiation

[0068] 114 Intermediate focal plane

[0069] 115 field facet mirrors

[0070] 116 Pupillary facet mirror

[0071] 117 — 119 additional mirrors of the lighting optics

[0072] 120 - 125 additional optical elements of the projection optics

[0073] 200 Device for the inventive method

[0074] 201 Mirror substrate according to an embodiment of the inventive method

[0075] 202 first component

[0076] 203 first joining surface

[0077] 204 second component

[0078] 205 second joining surface

[0079] 206 Chamber

[0080] 207 First means of heating

[0081] 208 Second means for applying joining pressure

[0082] 209 third means of evacuation

[0083] 210 Chamber interior

[0084] 211 connection

[0085] 301 Mirror substrate according to an embodiment of the inventive method second component

[0086] Nut

[0087] Connection

[0088] Fluid channel

[0089] Mirror substrate according to an embodiment of the inventive method first component first joining surface second component second joining surface

[0090] Intermediary layer

[0091] Connection

[0092] Mirror substrate according to an embodiment of the inventive method first component first joining surface second component second joining surface

[0093] Connection first process step second process step third process step

Claims

Patent claims 1. A method for producing a mirror substrate (201, 301, 401, 501) of an optical element for a projection exposure apparatus, in particular an EUV projection exposure apparatus (100), comprising a first (202, 402, 502) and at least one second component (204, 304, 404, 504), wherein the first component (202, 402, 502) and the at least one second component (204, 304, 404, 504) consist of silicon at least on a side facing a connection (211, 308, 408, 508), and the method comprises the steps: Providing / manufacturing the at least two components (202, 402, 502, 204, 304, 404, 504) of the mirror substrate (201, 301, 401, 501), and Joining the at least two components (202, 402, 502, 204, 304, 404, 504) by heating to a joining temperature and applying a joining pressure, preferably perpendicular to a joining surface (203, 205, 403, 405, 503, 505).

2. Method according to claim 1, characterized in that the at least two components (202, 402, 502, 204, 304, 404, 504) are assembled in an evacuated environment (210).

3. Method according to claim 1 or 2, characterized in that when the at least two components (202, 402, 502, 204, 304, 404, 504) are joined together, at least one fluid channel structure (311) is formed in the region of the connection (211, 308, 408, 508).

4. Method according to claim 1 to 3, characterized in that the first (202) and the at least one second component (204, 304) are joined directly to one another.

5. The method according to claim 4, characterized in that an Rms value of the surface roughness of at least one joining surface (203, 205, 305) of the at least two components is less than five nanometers.

6. Method according to claim 4 or 5, characterized in that the joining temperature is 1100 - 1250°C.

7. The method according to claim 1 - 3, characterized in that an intermediary layer (406) is provided for joining the at least two components (402, 404, 502, 504).

8. Method according to claim 7, characterized in that at least one joining surface (503, 505) of the at least two components is convex.

9. Method according to claim 7 or 8, characterized in that an Rms value of the surface roughness of at least one joining surface (403, 405, 503, 505) of the at least two components is less than 100 nanometers.

10. The method according to claim 7 to 9, characterized in that the mediator layer (406) has a layer thickness of 5 pm to 1.5 mm.

11. Method according to claims 7 to 10, characterized in that the joining temperature is above, preferably 10% above, a glass transition temperature of the mediator layer (406).

12. Method according to claims 7 to 11, characterized in that the mediator layer (406) consists of borosilicate glass, silicon, or alkali-free glass.

13. Method according to claims 1 to 12, characterized in that the joining pressure is 0.1 MPa - 15 MPa, preferably 0.3 - 0.8 MPa.

14. Optical element (115 - 125) with a mirror substrate (201, 301, (401), (501)), which is produced by a method according to one of claims 1 to 13.

15. Optical element (115 - 125) with a mirror substrate (201, 301, (401), (501)) according to claim 14, characterized in that at least one chemical and / or physical property of the mirror substrate changes abruptly along at least one spatial direction.

16. Projection exposure system (100) for semiconductor lithography which has at least one optical element (115 - 125), wherein at least one of the optical elements (115 - 125) is at least partially produced using a method according to one of claims 1 to 13 and / or at least one of the optical elements (115 - 125) is an optical element (115 - 125) according to claim 14 and / or claim 15.

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