WO2023016681A1

WO2023016681A1 - Mirror for a microlithographic projection exposure apparatus

Info

Publication number: WO2023016681A1
Application number: PCT/EP2022/066006
Authority: WO
Inventors: Stefan Xalter; Soeren KNORR
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2021-08-10
Filing date: 2022-06-13
Publication date: 2023-02-16
Also published as: TW202319784A; DE102021208664A1

Abstract

The invention relates to a mirror for a microlithographic projection exposure apparatus (1), wherein the mirror has a mirror body (26), which comprises a first mirror part (28), a second mirror part (29) and an optical surface (27) for the reflection of light and a plurality of cooling channels (32). The optical surface (27) is arranged on the first mirror part (28) and has a curved design. The first mirror part (28) and the second mirror part (29) are rigidly connected to one another in the region of a first connecting surface (30) of the first mirror part (28) and a second connecting surface (31) of the second mirror part (29). The first connecting surface (30) and the second connecting surface (31) have a plane design. The first connecting surface (30) is arranged at a distance from the optical surface (27), and the cooling channels (32) are arranged at a clear distance from the optical surface (27). The clear distance between the cooling channels (32) and the optical surface (27) has, in terms of percentage, a lower variation than the distance between the first connecting surface (30) and the optical surface (27).

Description

Mirror for a microlithographic projection exposure apparatus

This application claims priority to German Patent Application 10 2021 208 664.0, filed August 10, 2021, the entire disclosure of which is considered part of and is incorporated by reference in the disclosure of this application.

The invention relates to a mirror for a microlithographic projection exposure apparatus. The invention furthermore relates to an illumination system, to a projection optical unit and to a projection exposure apparatus having such a mirror.

Projection exposure apparatuses are used in particular in the production of semiconductors and generally have an illumination system and a projection optical unit. The illumination system generates from the light of a light source a desired light distribution for the illumination of a reticle, which is also referred to as a mask. The projection optical unit is used to image the reticle onto a light-sensitive material, which has been applied, for example, on a wafer or on another substrate, in particular made from a semiconductor material. In this way, the light-sensitive material is exposed in a structured manner to a pattern predefined by the reticle. Since the reticle has tiny structure elements, which are intended to be transferred to the substrate with high precision, it is required that the illumination system generates a desired light distribution precisely and reproducibly and the imaging by the projection optical unit takes place precisely and reproducibly.

In addition to further optical elements, the illumination system and the projection optical unit may have in the light path at least one mirror which deflects the light in a predefined way by reflection at its optical surface. How the light deflection specifically takes place depends on the shape of the optical surface. Since the mirror does not reflect the light completely but absorbs a portion of the light and converts it into heat, the mirror heats up during operation. This temperature increase leads to a thermal expansion of the mirror material and an associated deformation of the optical surface of the mirror and influencing of the light deflection at the optical surface. If the mirror is an integral part of the illumination system, the light distribution generated by the illumination system deviates from the specification. If the mirror is an integral part of the projection optical unit, imaging aberrations will occur during the imaging using the projection optical unit. By using materials that have an extremely low coefficient of thermal expansion, the deformation of the optical surface has so far been able to be kept within acceptable limits. However, the more powerful the available light sources are becoming, the greater will be the temperature increase and the resulting problems. In addition, the illumination and the imaging of the reticle need to be carried out with ever higher precision as the miniaturization in semiconductor production increases. The result is that an increasing number of influencing factors must be taken into account, which it has so far been possible to tolerate, or that already existing measures for compensating the influencing factors must be improved or be substituted by better measures.

It is already known to counteract the heating of a mirror of a microlithographic projection exposure apparatus by way of cooling the mirror. For example, US 7591561 B2 discloses an internally cooled mirror having a concavely curved optical surface. The known mirror has a top part and a bottom part each with a connecting surface in whose region the top part and the bottom part are connected to each other. Cooling channels that extend from the respective connecting surface into the material are formed in the top part or in the bottom part. In one variant, the connecting surfaces are formed curved analogously to the optical surface, and the top part or the bottom part consequently has curved cooling channels. This variant has the disadvantage that the production of the connection between the top part and the bottom part is a very difficult procedure. In a further variant, the connecting surfaces have a plane design, and the top part or the bottom part has planar cooling channels. The problem with this further variant is that the central region of the concave optical surface is cooled more than the peripheral region, and as a result an inhomogeneous temperature distribution forms in the region of the optical surface of the mirror in the operating state of the projection exposure apparatus. It is therefore not possible to maintain an optimum operating temperature for the entire optical surface, and consequently undesirable thermal expansions occur, which negatively influence the optical properties of the mirror.

The invention is based on the object of forming a mirror for a microlithographic projection exposure apparatus such that said mirror can be produced with acceptable outlay and manifests as little thermally induced influencing of its optical properties as possible during the operation of the projection exposure apparatus.

This object is achieved by means of the combinations of features of the coordinate claims. The mirror according to the invention for a microlithographic projection exposure apparatus has a mirror body, which comprises a first mirror part, a second mirror part, an optical surface for the reflection of light and a plurality of cooling channels. The optical surface is arranged on the first mirror part and has a curved design. The first mirror part and the second mirror part are rigidly connected to one another in the region of a first connecting surface of the first mirror part and a second connecting surface of the second mirror part. The first connecting surface and the second connecting surface have a plane design. The first connecting surface is arranged at a distance from the optical surface, and the cooling channels are arranged at a clear distance from the optical surface. The clear distance between the cooling channels and the optical surface has, in terms of percentage, a lower location-dependent variation than the distance between the first connecting surface and the optical surface.

The invention has the advantage that the mirror body can be produced with acceptable outlay, since the connecting surfaces of the first mirror part and of the second mirror part have a plane design and therefore can be connected to each other with the aid of known bonding techniques. Despite the plane design of the connecting surfaces, it is possible to ensure by way of the low location-dependent variation of the clear distance between the cooling channels and the optical surface that a comparatively homogeneous cooling of the mirror body in the region of the optical surface takes place and that this region can be maintained at a desirable temperature that has only minor spatial fluctuations. In this way, thermally induced influencing of the optical properties of the mirror during the operation of the projection exposure apparatus can be kept low. It is here particularly advantageous if the material of the first mirror part is adapted to the desired temperature in a manner such that the thermal expansion of the material at the desired temperature is minimal.

The cooling channels can extend from the first connecting surface to a first depth into the first mirror part. A small distance between the cooling channels and the optical surface and thus a good cooling effect are able to be realized thereby.

The clear distance between the cooling channels and the optical surface can be constant at least for some cooling channels in each case over the greatest region of the longitudinal extent of the cooling channels. The clear distance between the cooling channels and the optical surface can be constant in each case in particular over the entire longitudinal extent of the cooling channels. This makes a particularly uniform cooling effect and thus a particularly homogeneous temperature distribution possible in the region of the optical surface.

Furthermore, the clear distance between the cooling channels and the optical surface can be constant at least for some cooling channels over the greatest region of the longitudinal extent of the cooling channels and have the same value for said cooling channels. The clear distance between the cooling channels and the optical surface can be constant in particular over the entire longitudinal extent of the cooling channels and have the same value for said cooling channels. This makes further improved homogenization of the temperature distribution possible.

In addition, the invention relates to a mirror for a microlithographic projection exposure apparatus, wherein the mirror has a mirror body comprising a first mirror part, a second mirror part, an optical surface for reflecting light and a plurality of cooling channels. The optical surface is arranged on the first mirror part and has a curved design. The first mirror part and the second mirror part are rigidly connected to one another in the region of a first connecting surface of the first mirror part and a second connecting surface of the second mirror part. The first connecting surface and the second connecting surface have a plane design. The cooling channels extend from the first connecting surface to a first depth into the first mirror part. The first depth varies depending on the location at least for some cooling channels.

At least for some cooling channels, the first depth can vary along the longitudinal extent of the cooling channels. In addition, at least for some cooling channels, the first depth can have different values along a line extending perpendicular to the longitudinal extent of the cooling channels.

The cooling channels can each have a minimum value and a maximum value for the first depth, and the minimum values can differ for at least some cooling channels and/or the maximum values can differ for at least some cooling channels.

The cooling channels can extend from the second connecting surface to a second depth into the second mirror part. The second depth can vary depending on the location at least for some cooling channels. In particular, the depth can vary at least for some cooling channels along the longitudinal extent of the cooling channels. In addition or alternatively, at least for some cooling channels, the second depth can have different values along a line extending perpendicular to the longitudinal extent of the cooling channels. These measures offer the possibility of compensating for the variation in the first depth. In particular at least some cooling channels can have a constant cross section over the greatest region of their longitudinal extent or even over their entire longitudinal extent.

At least some cooling channels can extend parallel to the optical surface over the greatest region of their longitudinal extent. In particular, these cooling channels can extend parallel to the optical surface over their entire longitudinal extent. This allows very homogeneous cooling.

The cooling channels can be designed and/or connected to a fluid distributor for supplying fluid into the cooling channels and to a fluid collector for removing the fluid from the cooling channels in a manner such that a fluid can continuously flow through each of them. The fluid distributor and the fluid collector can be formed to be spatially separate from one another. In particular, the cooling channels can be designed and/or connected to the fluid distributor and to the fluid collector in a manner such that the fluid continuously flows through them in each case over the greatest part of their longitudinal extent, preferably at least 90% of their longitudinal extent, more preferably over their entire longitudinal extent. Furthermore, the cooling channels can be designed and/or connected to the fluid distributor and to the fluid collector in a manner such that the fluid can in each case continuously flow through the greatest part of their volume, preferably at least 90% of their volume, more preferably through their entire volume. Alternatively or in addition, the cooling channels can be designed and/or connected to the fluid distributor and to the fluid collector in a manner such that the fluid can in each case continuously flow through the greatest part of their cross section, preferably at least 90% of their cross section, more preferably through their entire cross section. Furthermore, the cooling channels can be designed and/or connected to the fluid distributor and to the fluid collector in a manner such that in each case the greatest part of a boundary surface, which forms the clear distance of the respective cooling channel from the optical surface, preferably at least 90% of the boundary surface, more preferably the entire boundary surface, can be continuously wetted by the flowing fluid. The cooling channels can be fluidically connected to the fluid distributor and to the fluid collector. In particular, the cooling channels can be connected to the fluid distributor via a first fluidic connection and to the fluid collector via a second fluidic connection. Here, the first fluidic connection and the second fluidic connection can be formed to be spatially separate from one another. The cooling channels can be fluidically connected to the fluid distributor and to the fluid collector in, with respect to their longitudinal extent, opposite end regions.

One or more of the previously mentioned conditions can be met for in each case the greatest part of the cooling channels, preferably for at least 90% of the cooling channels, more preferably for all cooling channels.

The invention furthermore relates to a microlithographic illumination system having a mirror according to the invention. Such an illumination system has the advantage that the specifications for the generated illumination light can be observed precisely even in the case of a high light output.

The invention additionally relates to a microlithographic projection optical unit having a mirror according to the invention. Such a projection optical unit has very low thermally induced imaging aberrations.

Finally, the invention relates to a microlithographic projection exposure apparatus comprising an illumination system according to the invention and/or a projection optical unit according to the invention.

In such a projection exposure apparatus, there is also the possibility of designing the cooling channels such that regions of the optical surface that are provided for irradiation with a higher light intensity during the operation of the projection exposure apparatus are cooled more than regions of the optical surface that are provided for irradiation with a lower light intensity during the operation of the projection exposure apparatus. This has the advantage that appropriate cooling is possible and an approximately homogeneous temperature distribution can be achieved despite an inhomogeneous light intensity distribution. The inhomogeneous cooling of the mirror can also be realized independently of one or more of the other aforementioned features of the mirror according to the invention. For example, greater cooling can be achieved due to smaller clear distances between the cooling channels and the optical surface and/or a greater number of cooling channels per unit area of the first connecting surface and/or a greater cross section of the cooling channels.

The invention will be explained in more detail below on the basis of the exemplary embodiments that are illustrated in the drawing,

In the figures,

Fig. 1 schematically shows an exemplary embodiment of a projection exposure apparatus for EUV projection lithography in a meridional section,

Fig. 2 shows an exemplary embodiment of a projection exposure apparatus for DUV projection lithography in a schematic illustration,

Fig. 3 shows a schematic sectional illustration of a first exemplary embodiment of a mirror body according to the invention,

Fig. 4 shows a further schematic sectional illustration of the first exemplary embodiment of the mirror body illustrated in Figure 3,

Fig. 5 shows a schematic plan view of the first mirror part of the first exemplary embodiment of the mirror body,

Fig. 6 shows a schematic plan view of the second mirror part of the first exemplary embodiment of the mirror body,

Fig. 7 shows a second exemplary embodiment of the mirror body in an illustration corresponding to Figure 3, and

Fig. 8 shows the second exemplary embodiment of the mirror body in an illustration corresponding to Figure 4. Figure 1 schematically shows an exemplary embodiment of a projection exposure apparatus 1 for EUV projection lithography in a meridional section.

In the following text, the essential component parts of a microlithographic projection exposure apparatus 1 are first described by way of example with reference to Figure 1. The description of the basic construction of the projection exposure apparatus 1 and its components should not be understood here as limiting.

One embodiment of an illumination system 2 of the projection exposure apparatus 1 has, in addition to a light or radiation source 3, an illumination optical unit 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the radiation source 3 may also be provided as a separate module from the remaining illumination system. In this case, the illumination system does not comprise the radiation source 3.

A reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable by way of a reticle displacement drive 9, in particular in a scanning direction.

For purposes of explanation, a Cartesian xyz-coordinate system is shown in Figure 1. The x- direction runs perpendicularly into the plane of the drawing. The y-direction runs horizontally and the z-direction runs vertically. The scanning direction runs along the y-direction in Figure 1. The z-direction runs perpendicular to the object plane 6.

The projection exposure apparatus 1 comprises a projection optical unit 10. The projection optical unit 10 serves for imaging the object field 5 into an image field 11 in an image plane 12. The image plane 12 runs parallel to the object plane 6. Alternatively, an angle between the object plane 6 and the image plane 12 that differs from 0° is also possible.

A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12 or of some other substrate. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable by way of a wafer displacement drive 15, in particular along the y-direction. The displacement, on the one hand, of the reticle 7 by way of the reticle displacement drive 9 and, on the other hand, of the wafer 13 by way of the wafer displacement drive 15 can take place in such a way as to be synchronized with one another.

The radiation source 3 is an EUV radiation source. The radiation source 3 emits illumination radiation 16, which is also referred to below as used radiation or illumination light. In the exemplary embodiment shown, the illumination radiation 16 has a wavelength in the EUV range, in particular in the range between 5 nm and 30 nm. The radiation source 3 can be a plasma source, for example an LPP (laser produced plasma) source or a GDPP (gas discharge produced plasma) source. It can also be a synchrotron-based radiation source. Similarly, the radiation source 3 may be a free electron laser (FEL).

The illumination radiation 16 emerging from the radiation source 3 is focused by a collector 17. The collector 17 may be a collector with one or more ellipsoidal and/or hyperboloidal reflection surfaces. The illumination radiation 16 can be incident on the at least one reflection surface of the collector 17 with grazing incidence (GI), that is to say at angles of incidence of greater than 45°, or with normal incidence (NI), that is to say at angles of incidence of less than 45°. The collector 17 may be structured and/or coated, on the one hand for optimizing its reflectivity for the illumination radiation 16 and on the other hand for suppressing extraneous light.

Downstream of the collector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, having the radiation source 3 and the collector 17, and the illumination optical unit 4.

The illumination optical unit 4 comprises a deflection mirror 19 and, arranged downstream thereof in the beam path, a first facet mirror 20. The deflection mirror 19 can be a plane deflection mirror or, alternatively, a mirror with a beam-influencing effect that goes beyond the purely deflecting effect. Alternatively or in addition, the deflection mirror 19 can be in the form of a spectral filter which separates a used light wavelength of the illumination radiation 16 from extraneous light with a wavelength deviating therefrom. If the first facet mirror 20 is arranged in a plane of the illumination optical unit 4 that is optically conjugate to the object plane 6 as a field plane, it is also referred to as a field facet mirror. The first facet mirror 20 comprises a multiplicity of individual first facets 21, which are also referred to below as field facets. Only some of these first facets 21 are shown in Figure 1 by way of example.

The first facets 21 can be in the form of macroscopic facets, in particular as rectangular facets or as facets with an arcuate peripheral contour or a peripheral contour of part of a circle. The first facets 21 may be in the form of plane facets or alternatively as convexly or concavely curved facets.

As known for example from DE 10 2008 009 600 Al, the first facets 21 themselves may also be composed in each case of a multiplicity of individual mirrors, in particular a multiplicity of micromirrors. The first facet mirror 20 can in particular be formed as a microelectromechanical system (MEMS system). For details, reference is made to DE 10 2008 009 600 Al.

Between the collector 17 and the deflection mirror 19, the illumination radiation 16 travels horizontally, that is to say along the y-direction.

In the beam path of the illumination optical unit 4, a second facet mirror 22 is arranged downstream of the first facet mirror 20. If the second facet mirror 22 is arranged in a pupil plane of the illumination optical unit 4, it is also referred to as a pupil facet mirror. The second facet mirror 22 can also be arranged at a distance from a pupil plane of the illumination optical unit 4. In this case, the combination of the first facet mirror 20 and the second facet mirror 22 is also referred to as a specular reflector. Specular reflectors are known from US 2006/0132747 Al, EP 1 614 008 Bl and US 6,573,978.

The second facet mirror 22 comprises a plurality of second facets 23. In the case of a pupil facet mirror, the second facets 23 are also referred to as pupil facets.

The second facets 23 can likewise be macroscopic facets, which can for example have a round, rectangular or hexagonal periphery, or alternatively be facets made up of micromirrors. In this regard, reference is likewise made to DE 10 2008 009 600 Al.

The second facets 23 can have plane or alternatively convexly or concavely curved reflection surfaces. The illumination optical unit 4 consequently forms a doubly faceted system. This fundamental principle is also referred to as a fly's eye condenser (fly's eye integrator).

It can be advantageous to arrange the second facet mirror 22 not exactly in a plane that is optically conjugate to a pupil plane of the projection optical unit 10. In particular, the second facet mirror 22 may be arranged so as to be tilted in relation to a pupil plane of the projection optical unit 10, as is described for example in DE 10 2017 220 586 Al.

With the aid of the second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-shaping mirror or else, in fact, the last mirror for the illumination radiation 16 in the beam path upstream of the object field 5.

In a further embodiment, not shown, of the illumination optical unit 4, a transfer optical unit contributing in particular to the imaging of the first facets 21 into the object field 5 can be arranged in the beam path between the second facet mirror 22 and the object field 5. The transfer optical unit can have exactly one mirror or alternatively have two or more mirrors, which are arranged one behind the other in the beam path of the illumination optical unit 4. The transfer optical unit can in particular comprise one or two normal-incidence mirrors (NI mirrors) and/or one or two grazing-incidence mirrors (GI mirrors).

In the embodiment shown in Figure 1, the illumination optical unit 4 has exactly three mirrors downstream of the collector 17, specifically the deflection mirror 19, the first facet mirror 20 and the second facet mirror 22.

In a further embodiment of the illumination optical unit 4, there is also no need for the deflection mirror 19, and so the illumination optical unit 4 can then have exactly two mirrors downstream of the collector 17, specifically the first facet mirror 20 and the second facet mirror 22.

The imaging of the first facets 21 into the object plane 6 by means of the second facets 23 or using the second facets 23 and a transfer optical unit is often only approximate imaging. The projection optical unit 10 comprises a plurality of mirrors Mi, which are consecutively numbered in accordance with their arrangement in the beam path of the projection exposure apparatus 1.

In the example shown in Figure 1, the projection optical unit 10 comprises six mirrors Ml to M6. Alternatives with four, eight, ten, twelve or any other number of mirrors Mi are similarly possible. The projection optical unit 10 is a twice- obscured optical unit. The last-but-one mirror M5 and the last mirror M6 have in each case a through-opening, through which, during the exposure of the wafer 13, the radiation contributing to the exposure passes on its way from the reticle 7 to the wafer 13. The projection optical unit 10 has an image-side numerical aperture that is greater than 0.5 and may also be greater than 0.6, and may be for example 0.7 or 0.75.

The reflection surfaces of the mirrors Mi may be in the form of freeform surfaces without an axis of rotational symmetry. Alternatively, the reflection surfaces of the mirrors Mi can be designed as aspheric surfaces with exactly one axis of rotational symmetry of the reflection surface shape. Just like the mirrors of the illumination optical unit 4, the mirrors Mi can have highly reflective coatings for the illumination radiation 16. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

The projection optical unit 10 has a large object-image offset in the y-direction between a y- coordinate of a centre of the object field 5 and a y-coordinate of the centre of the image field 11. This object-image offset in the y-direction may be approximately the same size as a z- distance between the object plane 6 and the image plane 12.

In particular, the projection optical unit 10 can have an anamorphic form. In particular, it has different imaging scales Px, Py in the x- and y-directions. The two imaging scales Px, Py of the projection optical unit 10 are preferably (Px, Py) = (+/-0.25, +/-0.125). A positive imaging scale P means imaging without image inversion. A negative sign for the imaging scale P means imaging with image inversion.

The projection optical unit 10 consequently leads to a reduction in size with a ratio of 4: 1 in the x-direction, that is to say in a direction perpendicular to the scanning direction. The projection optical unit 10 leads to a reduction in size of 8: 1 in the y-direction, that is to say in the scanning direction.

Other imaging scales are likewise possible. Imaging scales with the same sign and the same absolute value in the x-direction and y-direction are also possible, for example with absolute values of 0.125 or of 0.25.

The number of intermediate image planes in the x-direction and in the y-direction in the beam path between the object field 5 and the image field 11 can be the same or, depending on the embodiment of the projection optical unit 10, can differ. Examples of projection optical units 10 with different numbers of such intermediate images in the x-direction and y-direction are known from US 2018/0074303 Al.

In each case one of the second facets 23 is assigned to exactly one of the first facets 21 for respectively forming an illumination channel for illuminating the object field 5. This may in particular produce illumination according to the Kohler principle. The far field is decomposed into a multiplicity of object fields 5 with the aid of the first facets 21. The first facets 21 produce a plurality of images of the intermediate focus on the second facets 23 respectively assigned to them.

By way of an assigned second facet 23, the first facets 21 are in each case imaged onto the reticle 7 in a manner superposed on one another for the purposes of illuminating the object field 5. The illumination of the object field 5 is in particular as homogeneous as possible. It preferably has a uniformity error of less than 2%. The field uniformity can be achieved by way of the superposition of different illumination channels.

The full-area illumination of the entrance pupil of the projection optical unit 10 can be defined geometrically by an arrangement of the second facets 23. The intensity distribution in the entrance pupil of the projection optical unit 10 can be set by selecting the illumination channels, in particular the subset of the second facets 23, which guide light. This intensity distribution is also referred to as illumination setting or illumination pupil filling. A likewise preferred pupil uniformity in the region of sections of an illumination pupil of the illumination optical unit 4 which are illuminated in a defined manner can be achieved by a redistribution of the illumination channels.

Further aspects and details of the illumination of the object field 5 and in particular of the entrance pupil of the projection optical unit 10 are described below.

In particular, the projection optical unit 10 can have a homocentric entrance pupil. The latter can be accessible. It can also be inaccessible.

The entrance pupil of the projection optical unit 10 frequently cannot be exactly illuminated with the second facet mirror 22. When imaging the projection optical unit 10, which images the centre of the second facet mirror 22 telecentrically onto the wafer 13, the aperture rays often do not intersect at a single point. However, it is possible to find an area in which the distance of the aperture rays determined in pairs becomes minimal. This area represents the entrance pupil or an area in real space that is conjugate thereto. In particular, this area has a finite curvature.

It may be the case that the projection optical unit 10 has different positions of the entrance pupil for the tangential beam path and for the sagittal beam path. In this case, an imaging element, in particular an optical component element of the transfer optical unit, should be provided between the second facet mirror 22 and the reticle 7. With the aid of this optical element, the different positions of the tangential entrance pupil and the sagittal entrance pupil can be taken into account.

In the arrangement of the components of the illumination optical unit 4 shown in Figure 1, the second facet mirror 22 is arranged in an area conjugate to the entrance pupil of the projection optical unit 10. The first facet mirror 20 is arranged so as to be tilted in relation to the object plane 6. The first facet mirror 20 is arranged so as to be tilted in relation to an arrangement plane defined by the deflection mirror 19.

The first facet mirror 20 is arranged in tilted fashion with respect to an arrangement plane defined by the second facet mirror 22. Figure 2 shows an exemplary embodiment of a projection exposure apparatus 1 for DUV projection lithography in a schematic illustration. DUV denotes here “deep ultraviolet”. In particular, the projection exposure apparatus 1 may be designed for operation at a wavelength of 193 nm.

The projection exposure apparatus 1 has an illumination optical unit 4 and a projection optical unit 10. The internal structure of the illumination optical unit 4 and the internal structure of the projection optical unit 10, which may in each case comprise for example optical components, sensors, manipulators etc., are not shown in detail. In the case of the projection optical unit 10, a mirror M is indicated as representative of its optical components. The mirror M may be cooled with the aid of a cooling medium, which is provided by a cooling device 24. The cooling medium is a fluid, for example water. In addition or alternatively, the illumination optical unit 4 may have a cooled mirror M and an associated cooling device 24. The projection optical unit 10 and/or the illumination optical unit 4 may also have a plurality of cooled mirrors M and cooling devices 24. In the case of the illumination optical unit 4 and in the case of the projection optical unit 10, lenses and further mirrors - cooled or uncooled - may for example be present as further optical components.

By analogy, at least one cooling device 24, which may for example be connected to the mirror M3, may also be provided in the exemplary embodiment of the projection exposure apparatus 1 shown in Figure 1.

The radiation required for the operation of the projection exposure apparatus 1 is generated by a radiation source 3. The radiation source 3 may be in particular an excimer laser, for example an argon fluoride laser, which generates illumination radiation 16 of the wavelength 193 nm.

Arranged between the illumination optical unit 4 and the projection optical unit 10 is a reticle holder 8, fixed on which is a reticle 7, also referred to as a mask. The reticle holder 8 has a reticle displacement drive 9. Arranged downstream of the projection optical unit 10, seen in the direction of radiation, is a wafer holder 14, which carries a wafer 13 or some other substrate and has a wafer displacement drive 15.

Also shown furthermore in Figure 2 is a control device 25, which is connected to the illumination optical unit 4, the projection optical unit 10, the cooling device 24, the radiation source 3, the reticle holder 8 or the reticle displacement drive 9 and the wafer holder 14 or the wafer displacement drive 15. By analogy, the projection exposure apparatus 1 of Figure 1 may likewise have a control device 25 which may be connected to corresponding components.

The projection exposure apparatus 1 serves the purpose of imaging the reticle 7 onto the wafer 13 with high precision. For this purpose, the reticle 7 is illuminated with the aid of the illumination optical unit 4 and the illuminated reticle 7 is imaged onto the wafer 13 with the aid of the projection optical unit 10. Specifically, the following procedure is adopted:

The illumination optical unit 4 transforms the illumination radiation 16 generated by the radiation source 3 in an exactly defined way by means of its optical components and guides it onto the reticle 7. Depending on the embodiment, the illumination optical unit 4 may be formed in such a way that it illuminates the entire reticle 7 or only a partial region of the reticle 7. The illumination optical unit 4 is capable of illuminating the reticle 7 in such a way that there are almost identical illumination conditions at each illuminated point of the reticle 7. In particular, the intensity and the angular distribution of the incident illumination radiation 16 are almost identical for each illuminated point of the reticle 7.

The illumination optical unit 4 is capable of illuminating the reticle 7 optionally with illumination radiation 16 of a multiplicity of different angular distributions. These angular distributions of the illumination radiation 16 are also referred to as illumination settings. The desired illumination setting is generally selected in dependence on the structure elements formed on the reticle 7. Used relatively often for example are dipole or quadrupole illumination settings, in the case of which the illumination radiation 16 is incident on each illuminated point of the reticle 7 from two different directions or from four different directions, respectively. Depending on the form of the illumination optical unit 4, the different illumination settings may be produced for example by means of different diffractive optical elements in combination with a zoom axicon optical unit or by means of mirror arrays, which have in each case a multiplicity of small mirrors that are arranged next to one another and are individually settable with respect to their angular position. The reticle 7 may be formed for example as a glass plate, which is transparent to the illumination radiation 16 supplied by the illumination optical unit 4 and applied to which are opaque structures, for example in the form of a chromium coating.

The projection exposure apparatus 1 may be formed in such a way that the entire reticle 7 is illuminated at the same time by the illumination optical unit 4 and is imaged completely onto the wafer 13 by the projection optical unit 10 in a single exposure step.

Alternatively, the projection exposure apparatus 1 may also be formed in such a way that only a partial region of the reticle 7 is illuminated at the same time by the illumination optical unit 4 and the reticle displacement drive 9 is controlled by the control device 25 in such a way that, during the exposure of the wafer 13, the reticle 7 is moved in relation to the illumination optical unit 4 and, as a result, the illuminated partial region migrates over the reticle 7 as a whole. The wafer 13 is moved synchronously by suitably adapted control of the wafer displacement drive 15, in which the imaging properties of the projection optical unit 10 are also taken into account, and so the respectively illuminated partial region of the reticle 7 is imaged onto a partial region of the wafer 13 provided for it. This movement of the reticle 7 and of the wafer 13 is also referred to as scanning.

In order to be able to transfer the latent image produced by the exposure of the wafer 13 in both embodiments of the projection exposure apparatus 1 into a physical structure, a lightsensitive layer is applied to the wafer 13. The image of the reticle 7 is formed in this lightsensitive layer by exposure and a permanent structure can be produced from it on the wafer 13 with the aid of subsequent chemical processes.

The reticle 7 is generally imaged onto the wafer 13 not only once, but multiple times next to one another. For this purpose, after each imaging of the reticle 7 onto the wafer 13, the wafer holder 14 is displaced laterally in a way corresponding to the size of the image of the reticle 7 on the wafer 13. The imaging of the reticle 7 may be performed here in each case as a whole or sequentially by scanning. The chemical treatment of the wafer 13 is only started when the desired number of imagings of the reticle 7 onto the wafer 13 have been carried out.

Figure 3 shows a schematic sectional illustration of a first exemplary embodiment of a mirror body 26 embodied according to the invention. Figure 4 shows a further schematic sectional illustration of the first exemplary embodiment, wherein the section plane is turned by 90 degrees with respect to Figure 3.

The mirror body 26 can be, for example, an integral part of the mirror M3 according to Figure 1 or of the mirror M according to Figure 2 and can be made from a material having a very low coefficient of thermal expansion. Suitable materials are, for example, special glasses, in particular quartz glass doped with titanium oxide, or special glass ceramics.

The mirror body 26 has a reflective optical surface 27. The optical surface 27 is curved. Depending on the desired optical properties, the optical surface 27 can be convexly or concavely curved. In the first exemplary embodiment, the optical surface 27 is convexly curved. The curvature may be formed spherically, aspherically or according to a freeform surface. The optical surface 27 can be implemented by way of a coating. The formation of the coating depends on the wavelength at which the optical surface 27 is intended to produce its reflective effect. In the event of a desired reflection in the EUV range, i.e. in the case of the mirror M3 of Figure 1, the coating can be formed in particular from layers of silicon and molybdenum in an alternating sequence. By contrast, if a reflection in the DUV range is provided, i.e. in the case of the mirror M of Figure 2, the coating may be formed as an aluminium layer.

The mirror body 26 contains a first mirror part 28 and a second mirror part 29, which adjoin one another in the region of a plane first connecting surface 30 of the first mirror part 28 and of a plane second connecting surface 31 of the second mirror part 29 and are connected rigidly to one another. The connection between the mirror parts 28, 29 can be brought about for example by a bonding technique. For this purpose, the first connecting surface 30 of the first mirror part 28 and the second connecting surface 31 of the second mirror part 29 are formed to be plane with great precision and polished to a low roughness. Subsequently, the first connecting surface 30 of the first mirror part 28 and the second connecting surface 31 of the second mirror part 29 are moved towards each other, possibly with the supply of heat, until mechanical contact is made, and they are possibly additionally pressed against each other. In the process, covalent bonds are formed between the material of the first mirror part 28 and that of the second mirror part 29. The mirror parts 28, 29 then permanently adhere to one another, even once the contact pressure is no longer maintained. In the first exemplary embodiment, the optical surface 27 is arranged on the first mirror part 28, to be precise on its side facing away from the second mirror part 29. The first mirror part 28 has in the region of its first connecting surface 30 a plurality of cooling channels 32, which extend parallel to one another, are open towards the first connecting surface 30 and extend from the first connecting surface 30 into the first mirror part 28. This is also evident from Figure 5.

Figure 5 shows a schematic plan view of the first mirror part 28 of the first exemplary embodiment of the mirror body 26. It is evident from Figure 5 that the cooling channels 32 are formed as separate, parallel, elongate recesses in the region of the first connecting surface 30 of the first mirror part 28. The section plane of Figure 3 runs parallel to the longitudinal extents of the cooling channels 32, and the section plane of Figure 4 runs perpendicular thereto.

The cooling channels 32 are arranged at a clear distance A from the optical surface 27 and can have, for example, a rectangular cross section defined by a first depth T1 and a width B of the cooling channels 32. The first depth T1 in each case indicates the extent of the cooling channels 32 perpendicular to the first connecting surface 30 of the first mirror part 28 into the first mirror part 28. The width B indicates in each case the extent of the cooling channels 32 perpendicular to their longitudinal extent and parallel to the first connecting surface 30 of the first mirror part 28. The clear distance A of the cooling channels 32 from the optical surface 27 is constant for reasons that will be explained in more detail below. In order to achieve this despite the curvature of the optical surface 27 in each case over the greatest region of the longitudinal extent or even over the entire longitudinal extent of the cooling channels 32, the first depth T1 of the cooling channels 32 varies along their longitudinal extent. In addition, the individual cooling channels 32 each have different values for the first depth T1 at corresponding locations of their longitudinal extent, since the optical surface 27 is also curved along a direction perpendicular to the longitudinal extent of the cooling channels 32. In other words, the first depth T1 has different values along a line running perpendicular to the longitudinal extent of the cooling channels 32. In this case, the first depth T1 can vary in each case within the cooling channels 32, i.e. the cooling channels 32 can have, for example, a curved bottom. It is likewise also possible for the individual cooling channels 32 that follow one another along said line to have different values for the first depth T1 in the region of said line. The stated conditions can be met for some cooling channels 32, for the greatest part of the cooling channels 32 or for all cooling channels 32.

As is evident from Figure 3, the second mirror part 29 has, in the region of its second connecting surface 31, a fluid distributor 33 and a fluid collector 34, which are designed to be open in each case in the direction of the second connecting surface 31 and lie opposite one another in a manner such that they overlap with the opposing end regions of the cooling channels 32 of the first mirror part 28. In other words, the fluid distributor 33 overlaps with the end regions of the cooling channels 32 that are illustrated in Figure 3 on the left, and the fluid collector 34 overlaps with the end regions of the cooling channels 32 that are illustrated in Figure 3 on the right. In this way, a first fluidic connection between the cooling channels 32 and the fluid distributor 33 is formed and a second fluidic connection between the cooling channels 32 and the fluid collector 34 is formed. The formations of the fluid distributor 33 and of the fluid collector 34 are evident from Figure 6.

Figure 6 shows a schematic plan view of the second mirror part 29 of the first exemplary embodiment of the mirror body 26. The fluid distributor 33 extends from the periphery of the second connecting surface 31 of the second mirror part 29, which is illustrated in Figure 6 on the left, by a small amount in the direction of the periphery lying opposite it, that is to say in Figure 6 towards the right. The dimensions of the fluid distributor 33 transversely to the direction of said extent here increase fast, that is to say the fluid distributor 33 becomes wider as the distance from the periphery increases. The fluid collector 34 is formed to be mirror- symmetrical to the fluid distributor 33 with respect to a plane that extends centrally between the left periphery and the opposite right periphery of the second connecting surface 31 of the second mirror part 29. Accordingly, the fluid collector 34 extends from the periphery of the second connecting surface 31 of the second mirror part 29, illustrated in Figure 6 on the right, by a small amount in the direction of the periphery that lies opposite it, that is to say in Figure 6 towards the left. The dimensions of the fluid collector 34 transversely to the direction of this extent increase here fast, that is to say the fluid collector 34 becomes wider as the distance from the periphery increases. Specifically, the shapes of the fluid distributor 33 and of the fluid collector 34 can be selected such that the flow behaviour of the fluid is optimized. For example, it is possible to strive for a flow that is as laminar as possible with few turbulences. It is evident from a combination of Figures 3, 4, 5 and 6 that the fluid distributor 33 illustrated in Figures 3 and 6 on the left is fluidically connected to the end regions of the cooling channels 32 that are illustrated in Figures 3 and 5 on the left and fluidically connects said end regions of the cooling channels 32 to one another. It is furthermore evident that the fluid collector 34 illustrated in Figures 3 and 6 on the right is fluidically connected to the end regions of the cooling channels 32 that are illustrated in Figures 3 and 5 on the right and fluidically connects said end regions of the cooling channels 32 to one another. It is thus possible to supply a fluid to the cooling channels 32 through the fluid distributor 33. The fluid flows through the cooling channels 32 and then flows on into the fluid collector 34, via which it can be removed. The fluid distributor 33 and the fluid collector 34 are generally formed spatially separate from one another. The cooling channels 32 are generally designed and/or connected to the fluid distributor and to the fluid collector in a manner such that the fluid can continuously flow through them in each case over the greatest part of their longitudinal extent, preferably at least 90% of their longitudinal extent, with particular preference over their entire longitudinal extent. Furthermore, the cooling channels 32 can be designed and/or connected to the fluid distributor 33 and to the fluid collector 34 in a manner such that the fluid can in each case continuously flow through the greatest part of their volume, preferably at least 90% of their volume, more preferably through their entire volume. Alternatively or in addition, the cooling channels 32 can be designed and/or connected to the fluid distributor 33 and to the fluid collector 34 in a manner such that the fluid can in each case continuously flow through the greatest part of their cross section, preferably at least 90% of their cross section, more preferably through their entire cross section. Furthermore, the cooling channels 32 can be designed and/or connected to the fluid distributor 33 and to the fluid collector 34 in a manner such that in each case the greatest part of a boundary surface, which forms the clear distance A of the respective cooling channel 32 from the optical surface 27, preferably at least 90% of the boundary surface, more preferably the entire boundary surface, can be continuously wetted by the flowing fluid.

The supply of the fluid to the fluid distributor 33 and the removal of the fluid from the fluid collector 34 can take place with the aid of the cooling device 24, which for this purpose can be connected to the fluid distributor 33 and the fluid collector 34.

Adjusting the temperature of the supplied fluid to a temperature below the temperature of the first mirror part 28 can achieve the effect that the fluid extracts heat from the first mirror part 28 as it flows through the cooling channels 32. This extraction of heat is intended in particular to compensate for the input of heat due to the light that is incident on the optical surface 27 during the operation of the projection exposure apparatus 1. Since the optical surface 27 does not completely reflect the incident light, part of the light is absorbed by the optical surface 27 and, depending on the formation of the optical surface 27, also by the first mirror part 28 and converted into heat. Some of this heat is transferred to the cooling channels 32 and can there be absorbed by the fluid and transported away. Ideally, the heat is removed from all locations of the optical surface 27 in the same way so that a temperature distribution that is as homogeneous as possible results for the entire optical surface 27 and the temperature corresponds to a predefined value as precisely as possible. The material of the mirror can be adapted to the predefined value of the temperature in a manner such that the thermal expansion of the mirror body 26 is minimal at the predefined value of the temperature. In this way, a deformation of the optical surface 27 caused by the thermal expansion effects can be reduced compared to a non-cooled mirror. As a consequence, the imaging aberrations caused by the deformation are also reduced.

Since the material of the first mirror part 28 generally does not have very good thermal conductivity, the local cooling of the optical surface 27 is highly dependent on the clear distance A between the cooling channels 32 and the optical surface 27. It is consequently possible to achieve a homogeneous temperature distribution through a constant clear distance A. As already stated, the cooling channels 32 have a profile of their first depth T1 that is adapted to the shape of the optical surface 27 to realize a constant clear distance A. Accordingly, in the case of a convexly curved optical surface 27, the first depth T1 of the cooling channels 32 can initially increase from the region of the fluid distributor 33 along their longitudinal extent up to approximately the centre of their longitudinal extent and then decrease again towards the fluid collector 34. In a concavely formed optical surface 27, the first depth T1 of the cooling channels 32 can have a profile that is opposite thereto. In both cases, i.e. for a convexly curved optical surface 27 and also for a concavely curved optical surface 27, the individual cooling channels 32 can have different depth profiles to achieve an adaptation to the shape of the optical surface 27 perpendicular to the longitudinal extent of the cooling channels 32. In particular, the maximum and/or the minimum value of the first depth T1 of the individual cooling channel 32 can vary. Since the cooling channels 32 each have a constant width B, i.e. a constant dimension perpendicular to their longitudinal extent and parallel to the first connecting surface 30, the cross section of the cooling channels 32 varies in the first exemplary embodiment along the longitudinal extent of the cooling channels 32 and analogously to the variation of the first depth Tl.

As is evident from Figures 3 and 4, the cooling channels 32 have, in the case of the first exemplary embodiment with a convexly curved optical surface 27 that is illustrated there, a maximum and minimum value of the first depth Tl in the first mirror part 28 that is the lower, the further they are arranged laterally offset from the highest point at the centre of the optical surface 27. In the case of a concavely formed optical surface 27, which is not illustrated in the figures, the cooling channels 32 can have a maximum and minimum value of the first depth Tl in the first mirror part 28 that is the greater, the further they are arranged laterally offset from the lowest point at the centre of the optical surface 27.

As already mentioned, it is also possible for the first depth Tl of the cooling channels 32 in the first mirror part 28 to vary within the respective cooling channel 32 perpendicular to its longitudinal extent depending on the shape of the optical surface 27. However, since the width B of the cooling channels 32 is generally small, this variation is of comparatively low importance.

Figure 7 shows a second exemplary embodiment of the mirror body 26 in an illustration corresponding to Figure 3. Figure 8 shows the second exemplary embodiment of the mirror body 26 in an illustration corresponding to Figure 4.

The second exemplary embodiment of the mirror body 26 substantially corresponds to the first exemplary embodiment. One difference lies in the fact that the cooling channels 32 in the second exemplary embodiment are formed not only in the first mirror part 28 but also in the second mirror part 29. The formation of the cooling channels 32 in the first mirror part 28 in the second exemplary embodiment is identical to the first exemplary embodiment. In the second mirror part 29, the cooling channels 32 in the second exemplary embodiment are formed such that their second depth T2 varies depending on the location in a way complementary to the first depth Tl of the cooling channels 32 in the first mirror part 28. Specifically, the cooling channels 32 in the second mirror part 29 are formed such that the sum of the first depth T1 of the cooling channels 32 in the first mirror part 22 and of the respectively associated second depth T2 of the cooling channels 32 in the second mirror part 29 is constant. This applies to all lateral locations within the cooling channels 32, wherein the sum of the first depth T1 and of the second depth T2 is to be formed in each case at the same lateral location.

As a consequence, in the case of a convexly curved optical surface 27, the second depth T2 of the cooling channels 32 in the second mirror part 29 initially decreases from the region of the fluid distributor 33 along the longitudinal extent of the cooling channels 32 in each case initially up to the centre of the longitudinal extent and then increases again towards the fluid collector 34. In a concavely formed optical surface 27, the second depth T2 of the cooling channels 32 can have a profile that is opposite thereto.

As is evident from Figures 7 and 8, the cooling channels 32 have, in the case of the second exemplary embodiment of the mirror body 26 with a convexly curved optical surface 27 that is illustrated there, a maximum and minimum value of the second depth T2 in the second mirror part 29 that is the greater, the further they are arranged laterally offset from the highest point at the centre of the optical surface 27. In the case of a concavely formed optical surface 27, which is not illustrated in the figures, the cooling channels 32 can have a maximum and minimum value of the second depth T2 in the second mirror part 29 that is the lower, the further they are arranged laterally offset from the lowest point at the centre of the optical surface 27.

It has already been mentioned that the described variation in the second depth T2 of the cooling channels 32 in the second mirror part 29 achieves the effect that the total depth of the cooling channels 32, which is made up of the first depth T1 in the first mirror part 28 and the second depth T2 in the second mirror part 29, is the same for all cooling channels 32. For an identical width B of all cooling channels 32, this results in a constant cross section for all cooling channels 32. A constant cross section makes an optimum fluid flow in all cooling channels 32 possible. In particular, the turbulent energy of the fluid can be reduced and the dynamic vibration of the mirror that is excited thereby can be kept low. In addition, very uniform cooling of the mirror body 26 can be achieved. The dynamic and thermal properties that have been improved in this way in turn lead to an improvement of the optical properties of the mirror and result in an improvement of the overall system. The uniform cooling of the mirror body 26, achieved by the above-described exemplary embodiments, brings about particularly good results if the mirror body 26 is homogeneously heated by the incident light in the region of the optical surface 27. This is generally the case for homogeneous full-area illumination of the optical surface 27. If the light distribution that is incident on the optical surface 27 is not homogeneous, the optical properties of the mirror can be further improved by inhomogeneous cooling that is adapted to the inhomogeneous light distribution. This can be achieved by an appropriate embodiment of the cooling channels 32. For example, in the regions of the optical surface 27 on which a particularly high light intensity is incident, the clear distance A of the cooling channels 32 from the optical surface 27 can be reduced compared to the regions with a lower light intensity, and the cooling effect can be increased thereby. In addition or alternatively, it is also possible to provide a greater number of cooling channels 32 per unit area and/or to provide the cooling channels 32 with a greater cross section in the regions of high light intensity and to likewise attain a greater cooling effect thereby.

Using these or similar measures, the cooling effect for an inhomogeneous light distribution or for a few inhomogeneous light distributions that occur during the operation of the projection exposure apparatus 1 in the region of the optical surface 27 can be optimized.

Reference signs

1 Projection exposure apparatus

2 Illumination system

3 Radiation source

4 Illumination optical unit

5 Object field

6 Object plane

7 Reticle

8 Reticle holder

9 Reticle displacement drive

10 Proj ecti on opti cal unit

11 Image field

12 Image plane

13 Wafer

14 Wafer holder

15 Wafer displacement drive

16 Illumination radiation

17 Collector

18 Intermediate focal plane

19 Deflection mirror

20 First facet mirror

21 First facet

22 Second facet mirror

23 Second facet

24 Cooling device

25 Control device

26 Mirror body

27 Optical surface

28 First mirror part

29 Second mirror part

30 First connecting surface 31 Second connecting surface

32 Cooling channel

33 Fluid distributor

34 Fluid collector

M Mirror

Ml Mirror M2 Mirror

M3 Mirror

M4 Mirror

M5 Mirror

M6 Mirror

Claims

28 Patent claims

1. Mirror for a microlithographic projection exposure apparatus (1), wherein the mirror has a mirror body (26), comprising

- a first mirror part (28),

- a second mirror part (29),

- an optical surface (27) for the reflection of light and

- a plurality of cooling channels (32), wherein

- the optical surface (27) is arranged on the first mirror part (28) and has a curved design,

- the first mirror part (28) and the second mirror part (29) are rigidly connected to one another in the region of a first connecting surface (30) of the first mirror part (28) and a second connecting surface (31) of the second mirror part (29) and

- the first connecting surface (30) and the second connecting surface (31) have a plane design,

- the first connecting surface (30) is arranged at a distance from the optical surface (27),

- the cooling channels (32) are arranged at a clear distance from the optical surface (27),

- the clear distance between the cooling channels (32) and the optical surface (27) has, in terms of percentage, a lower location-dependent variation than the distance between the first connecting surface (30) and the optical surface (27).

2. Mirror according to Claim 1, wherein the cooling channels (32) extend from the first connecting surface (30) to a first depth into the first mirror part (28).

3. Mirror according to either of the preceding claims, wherein the clear distance between the cooling channels (32) and the optical surface (27) is constant at least for some cooling channels (32) in each case over the greatest region of the longitudinal extent of the cooling channels (32).

4. Mirror according to any of the preceding claims, wherein the clear distance between the cooling channels (32) and the optical surface (27) is constant at least for some cooling channels (32) over the greatest region of the longitudinal extent of the cooling channels (32) and has the same value for said cooling channels (32).

5. Mirror for a microlithographic projection exposure apparatus (1), wherein the mirror has a mirror body (26), comprising

- a first mirror part (28),

- a second mirror part (29),

- an optical surface (27) for the reflection of light and

- a plurality of cooling channels (32), wherein

- the cooling channels (32) extend from the first connecting surface (30) to a first depth into the first mirror part (28), and

- the first depth varies depending on the location at least for some cooling channels (32).

6. Mirror according to any of Claims 2 to 5, wherein. at least for some cooling channels (32), the first depth varies along the longitudinal extent of the cooling channels (32).

7. Mirror according to any of Claims 2 to 6, wherein, at least for some cooling channels (32), the first depth has different values along a line extending perpendicular to the longitudinal extent of the cooling channels (32).

8. Mirror according to any of Claims 2 to 7, wherein the cooling channels (32) each have a minimum value and a maximum value for the first depth, and the minimum values differ for at least some cooling channels (32) and/or the maximum values differ for at least some cooling channels (32).

9. Mirror according to any of the preceding claims, wherein the cooling channels (32) extend from the second connecting surface (31) to a second depth into the second mirror part (29).

10. Mirror according to Claim 8, wherein the second depth varies depending on the location at least for some cooling channels (32). Mirror according to any of the preceding claims, wherein at least some cooling channels (32) have a constant cross section over the greatest region of their longitudinal extent. Mirror according to any of the preceding claims, wherein at least some cooling channels (32) run parallel to the optical surface (27) over the greatest region of their longitudinal extent. Microlithographic illumination system having a mirror according to any of the preceding claims. Microlithographic projection optical unit having a mirror according to any of Claims 1 to 13. Microlithographic projection exposure apparatus comprising an illumination system (2) according to Claim 13 and/or a projection optical unit (10) according to Claim 14. Projection exposure apparatus according to Claim 15, wherein the cooling channels (32) are designed such that regions of the optical surface (27) that are provided for irradiation with a higher light intensity during the operation of the projection exposure apparatus (1) are cooled more than regions of the optical surface (27) that are provided for irradiation with a lower light intensity during the operation of the projection exposure apparatus (1).