Optical component, inparticular collector for use in EUV lithography

The invention relates to an optical component, in particular to a collector for use in EUV lithography, comprising at least one optically effective element which heats up when irradiated with light.

An optical collector of the afore-mentioned kind is known from WO 02/065482 A2. This known collector comprises a plurality of mirror shells which are nested into one another, wherein cooling devices are arranged in an unused area between adjacent shells and which are spaced apart from these adjacent shells.

The invention relates in particular to minimizing thermal gradients in a collector for EUV lithography. This type of lithography uses light in the extreme ultraviolet spectral range, in particular light of a wavelength of 13 nm, to form the image of a reticle on a wafer. Nevertheless, the invention also relates to general optical components.

The simulation results for the collector of the EUV illumination system show that the collector heats up extremely during operation and is deformed as a result. This deformation changes increasingly during operation and changes the optical behavior

of the system. This changing optical behavior during operation is also referred to as the so-called "transient effect".

Although the present invention is described in particular with reference to a collector for use in EUV lithography, the present invention is not restricted to this. It goes without saying that the invention can also be applied generally to other optical components.

With reference to Figure 1 of the accompanying drawings, the heating up of a general optical component is briefly explained. Figure 1 shows an incident ray of light 1, which impinges on an optical component. The optical component is, for example, a mirror, which has an optically effective element, which has a substrate 4, for example a substrate body, and an optically effective layer 3, for example a reflective layer, at which the ray of light is deflected or reflected, as indicated by the reference numeral 2. Substrate 4 and layer 3 form the body of the optically effective element. Other optical components, such as lenses, prisms, gratings, beam splitters etc., are conceivable as other application cases.

Some of the incident energy 1 is absorbed on the optical layer 3 or in the substrate 4 of the optically effective element. This produces heat 5, which spreads out in the substrate 4. This heat is in most cases poorly dissipated, since the mounting elements 6 with respect to the holding structure 7 and the material of the substrate 4 of the optically effective element have a poor thermal conduction. The body expands. This expansion is impeded by the mounting elements 6 and the holding structure 7, which leads to local deformations of the optically effective element, and consequently to impairments of the optical performance of the device in which the optical component is used.

To sum up, the following problems occur. On the one hand, the optical component becomes too hot. As a result, the substrate material of the optically effective element and the optical layers, such as the layer 3, could be destroyed. On the other hand,

the optical component; in particular the optically effective element, is deformed so severely that the optical performance of the system does not conform to the required specification. Furthermore, the deformation of the optically effective element can change during operation ("transient effects"). A one-off (static) correction of the resultant error in the optical system, for example with the aid of other optical components, is consequently inadequate.

The document WO 2005/054547 A2 discloses a method for fabricating metallic components which, according to that document, also include optical components, for example mirrors, which are used in EUV systems. With the methods described there, a first layer is electroplated on a substrate, on which in turn a mask layer of a fusable material is applied. Channels are made in the mask layer, and another layer is subsequently electroplated over the mask layer including the previously formed channels. After fusing the mask layer channels come into exist which serve as cooling conduits.

The disadvantage of this method is that the fabrication of the cooling system is done simultaneously with the fabrication of the optically effective layer, which, on the one hand, can affect the optically effective layer, and, on the other hand, increases the expenditure of the fabrication method as well. Furthermore, the tightness of the cooling conduits is not easily controllable with this fabrication process.

The document EP 1 387 054 A2 discloses a cooling device and a cooling method for cooling an optical component which is operated in a vacuum atmosphere. The cooling device comprises a cooling part which absorbs heat from the optical element and dissipates it by radiation. The cooling part is arranged away from the optical component. A control device for controlling the temperature of the radiant cooling part is provided.

This cooling device is costly, since it requires a considerable number of additional parts, whereby the installation space of the optical component is also disadvanta- geously increased. Furthermore, cooling based on radiation is not very effective.

The document US 2004/0051984 Al discloses devices and methods for cooling optical components which are used in vacuum atmospheres. The cooling device has a heat-absorbing plate, which is arranged in the vicinity of an optical component along the surface of the latter facing away from the surface of the optical component that is exposed to light. In the case of this cooling device too, the heat from the optical component is dissipated by radiation to the cooling device. In addition, a method for cooling the optical component by means of a cooling medium by convection is described there.

In the case of an optical component that undergoes high thermal loading, as it is the case for the collector of a EUV lithography system operated with a high-power source, cooling of this type is in most cases inadequate, because excessively high temperatures occur on the optical component, leading to severe deformation or damaging the optical layers and the optical materials, in particular under thermome- chanical stress.

Further optical components having a cooling device are disclosed in JP 2002-005586, DE 41 11 554 Al, US 3,637,296, US 4,443,059, US 5,073,831, US 4,264,146, DE 33 39 076 Al, US 3,731,992, US 3,781,094, WO 2005/054547 A2.

One aim of the present invention is to provide a different cooling concept for an optical collector which on the one hand is less costly and on the other hand is nevertheless effective in that deteriorations in the performance behavior of the optical collector, and with it of the optical system in which the optical collector is used, are to be avoided or at least reduced.

Another aim of the present invention is to provide a different cooling concept for an optical component which on the one hand is less costly and on the other hand is nevertheless effective in that deteriorations in the performance behavior of the optical component, and with it of the optical system in which the optical component is used, are to be avoided or at least reduced.

It is therefore an object of the present invention to provide an optical collector on which a cooling concept of this type is realized.

It is another object of the present invention to provide an optical component on which a cooling concept of this type is realized.

According to an aspect of the invention, this object is achieved by an optical collector for use in EUV lithography, comprising at least one optically effective element in form of a mirror shell having a substantially cup-like structure, which heats up when irradiated with light, further comprising at least one mounting element for fastening the at least one optically effective element on a holding structure, the at least one optically effective element having a body, and further comprising an active cooling system which has at least one cooling conduit to which a cooling medium can be admitted, wherein the at least one cooling conduit is provided on a cooling body which is directly connected with the body of the optically effective element or is integrated in same.

According to another aspect of the invention, this object is achieved by an optical component, comprising at least one optically effective element which heats up when irradiated with light, further comprising at least one mounting element for fastening the at least one optically effective element on a holding structure, the at least one optically effective element having a body, and further comprising an active cooling system which has at least one cooling conduit to which a cooling medium can be admitted, wherein the at least one cooling conduit is provided on a cooling body

which is directly connected with the body of the optically effective element or is integrated in same.

The cooling concept according to the invention is based on the idea of cooling the optically effective element, i.e. the mirror shell, of the optical component itself, to be precise by means of an active cooling system which has at least one cooling conduit to which a cooling medium can be admitted and is arranged directly on the optically effective element or is integrated in it. Direct cooling of the optically effective element of the optical collector itself represents more effective cooling, to a temperature of the optical collector which is as low as possible and constant, since the cooling now acts directly on that element which heats up first and the most extremely, that is to say the optically effective element itself.

The formation of the at least one cooling conduit on a separate cooling body which can be preferably pre-fabricated accordingly, has the advantage that the fabrication of the cooling body can be done independently from the fabrication of the optically effective element until the cooling body is finished, and only in a later process, the cooling body is connected to the body of the optically effective element. Impairments of the optically effective region of the optically effective element can thus be avoided or at least be reduced, and it is also possible to easierly control the tightness of the cooling system in case that the cooling body is already fabricated in cool medium proof fashion. The method for fabricating the optical collector is thus facilitated.

Preferably, the cooling body comprises the at least one cooling conduit already in cool medium proof fashion, or the cooling body is made cooling medium proof just by the connection to the body of the optically effective element.

In a further preferred configuration, the body has a base layer, which forms, or at least has, the optically effective region of the at least one optically effective element,

and at least one cover layer connected to the base layer, the cooling body being arranged between the base layer and the at least one cover layer.

Such a multilayered structure of the optically effective element represents a measure which makes the integration of the cooling body into the body of the optically effective element possible in an advantageously low-cost way.

As provided in a further preferred configuration, the base layer and the at least one cover layer are produced by means of electroforming, i.e. the base layer and the at least one cover layer are successively produced, and for example, after production of the base layer, the parts intended for the cooling system can first be provided on the base layer, before the outer layer is electrodeposited or applied in some other way.

As an alternative to the afore-mentioned embodiment, the body comprises a base layer which forms the optically effective region of the at least one optically effective element, and at least one cover layer, wherein the cooling body is formed by the at least one cover layer which is preformed such that the at least one cooling conduit is formed as a cavity between the base layer and the cover layer by locally keeping the at least one cover layer away from the base layer.

In this case, the cover layer itself forms the cooling body, and the connection to the body substantially serves as a sealing of the cooling device having the at least one cooling conduit. The cover layer is preformed according to the formation of the cavities. This configuration of the optical collector or component has the advantage of still further simplified, and consequently lower-cost, production of the optical collector, since the at least one cooling conduit does not have to be formed as a separate cooling conduit, but instead the cooling medium can be conducted directly through the cavity between the at least one outer layer and the base layer. Furthermore, heat absorption over a particularly large area can be achieved in this way.

The cavity can extend in circumferential and/or in axial direction.

As an alternative to this, the at least one cooling conduit of the cooling body may also be formed as a separate cooling conduit and be arranged on a rear side of the body and connected to the latter in a heat-conducting manner.

The advantage of this measure is that, in particular in the case of the provision of a plurality of cooling conduits, they can already be prefabricated to form the cooling body, which then only has to be connected subsequently to the body of the optically effective element.

In the case of the configuration of the body of the optically effective element with a base layer and at least one cover layer, it is similarly preferred if the at least one cooling conduit of the cooling body is formed as a separate cooling conduit which is arranged between the base layer and the at least one cover layer.

In this case, the at least one separate cooling conduit may be formed as a thin tube with a fully circumferentially closed wall, or the at least one separate cooling conduit may be formed as a partly circumferentially open channel element which lies with its open side flush against the base layer or the at least one outer layer.

In the case of the last configuration, the sealing of the at least one separate cooling conduit by the base layer or the at least one outer layer is accomplished in this way.

As a further alternative, the at least one separate cooling conduit may be formed as a partly circumferentially open channel element of which the open side is arranged facing away from the body or the base layer, and the open side being closed by a covering element.

Furthermore, it is preferred if a filler for filling gaps is present between the at least one cooling conduit and the body of the at least one optically effective element.

The advantage of the provision of a filler is that the filler closes inner bridges between the body and the cooling conduit caused by dimensional inaccuracies of the body of the optically effective element and/or of the cooling conduit, so that the optical collector or component according to the invention is suitable in particular for vacuum applications, since cavities causing outgassing are avoided.

For further minimizing air bridges between the cooling conduit and the body of the optically effective element, the cooling conduit is preferably adapted to the shape of the body of the optically effective element as exactly as possible.

For this purpose, it is possible for example for the at least one cooling conduit to be pre-bent, for example if it is formed as a tube, in order for it to be adapted to a bending of the body of the optically effective element. As an alternative or in addition, the body of the optically effective element may also be produced such that it is rather thicker at first, and subsequently machined down to the desired size while maintaining the correct dimensions. This also improves the dimensional accuracy, and with it the dimensionally tiue adaptation of the cooling conduit to the body of the optically effective element.

In this respect it is of advantage that the cooling system, in particular with a number of cooling conduits, can be initially prefabricated in a separate working step, and then the finished cooling body with the at least one cooling conduit only has to be firmly connected to or integrated in the body of the optically effective element.

Here, too, in a case where the body of the optically effective element has a base layer and at least one cover layer, it may be envisaged to arrange the cooling body between the base layer and the at least one cover layer.

As already mentioned, the cooling body can have the at least one cooling conduit with the cooling medium already confined in a sealed manner.

In the case of this configuration, the cooling body is consequently already prefabricated as a system which is cooling medium proof and is then connected to the body of the optically effective element, whereby further sealing measures are no longer required during the production of the optical component.

As likewise already mentioned above, it is preferred if the active cooling system has a plurality of cooling conduits which are arranged in or on the body of the optically effective element.

In this case, the cooling conduits may preferably be connected in parallel via a common distributing channel and/or a common collecting channel.

The provision of a common distributing channel and/or a common collecting channel has the advantage that only one feed line or one discharge line has to be provided for the feeding and/or discharge of the cooling medium, as is the case in a further preferred configuration. A further advantage of this measure is that the drop in pressure in the cooling conduits is kept small, or a higher volumetric flow of the cooling medium, and with it a better cooling effect, can be achieved with the same drop in pressure.

In order to cool as large a surface area of the body as possible effectively, and consequently to avoid or at least reduce deformations caused by thermal gradients within the body of the optically effective element, the at least one cooling conduit is preferably formed along a meandering, spiral or straight line, in the latter case a plurality of cooling conduits being arranged next to one another on or in the body of the optically effective element, in order to cool a large surface area of the optically effective element.

The body of the at least one optically effective element is preferably produced by electroforming, i.e. by depositing or growing a galvanic layer on a core, which is separated from the body after the body is completed.

In particular in a case where the body of the optically effective element has a base layer and at least one outer layer, these two layers are also preferably produced by electroforming.

In the case of electroforming, one major advantage is that the cooling system can be sealed without the use of adhesives or the like, since the use of adhesives has the disadvantage that they can age, in particular on account of the repeated heat exposure, and the adhesive bond can become detached during operation. In the case of electroforming it is possible to dispense with any kind of adhesive bond.

In a further preferred configuration, at least one temperature sensor is arranged on or in the body of the at least one optically effective element.

By means of the temperature sensor, the temperature of the optically effective element of the optical collector or component can be monitored during operation and, if appropriate, controlled by a control system which acts on the cooling system.

In the case where the body of the optically effective element has a base layer and at least one cover layer, the temperature sensor is embedded between the base layer and the at least one cover layer.

In particular in connection with one of the previously mentioned configurations according to which the base layer and the at least one cover layer are produced as galvanic layers, the temperature sensor can in this way be integrated in the body of the optically effective element without the use of adhesives.

In a comparable way, the at least one mounting element by which the optically effective element is connected to the holding structure is also firmly connected to the body of the at least one optically effective element, in that a portion of the mounting element is embedded between the base layer and the at least one cover layer if the body has a structure with a base layer and at least one cover layer.

In this case too, a particular advantage is that, if the base layer and at least one cover layer are galvanically produced, the at least one mounting element can be firmly connected to the body of the at least one optically effective element without the use of adhesives.

Furthermore, at least one heating element may also be arranged preferably on or in the body of the at least one optically effective element.

This may be, for example, a heating element based on electrical resistance heating, as is provided in a further preferred configuration.

The provision of at least one heating element for the optically effective element has the advantage that the optically effective element can be rapidly brought to operating temperature by means of the heating element, and, in conjunction with the active cooling system, the operating temperature can be kept as constant as possible during the operation of the collector or component, in order to avoid or at least reduce transient effects in the optically effective element, and consequently in the collector or component.

Instead of the use of a heating element based on electrical resistance heating, a heating medium may also preferably be admitted to the at least one cooling conduit instead of the cooling medium or in addition to it, for example a warm medium may be conducted through the at least one cooling conduit at the beginning of the operation of the collector or component, until the optical component is at operating temperature, and subsequently the temperature of the warming medium is reduced in the sense of cooling.

The afore-mentioned embodiments can be provided in a general optical component as well as in a collector for EUV lithography.

The collector preferably has a plurality of mirror shells nested one in the other, wherein more than one of the mirror shells has a cooling body connected thereto or integrated therein according to one or more of the above embodiments.

In the case of the provision of an active cooling system according to one or more of the aforementioned configurations, this can lead to an increase in thickness of the individual mirror shells. However, in order to avoid part of the light being cut out in the case of grazing light incidence, in a further preferred configuration it is provided that an end of the mirror shell that is facing a light source during operation of the component is formed such that it is tapered or thinner than the remaining portion of the mirror shell. The cooling body on the rear side (unused side) of the mirror shell is tapered or thinner accordingly than a remaining portion of the cooling body.

Consequently, cutting out of part of the light is avoided or at least reduced on the ray entry side of the respective mirror shell by the tapering or reduction in thickness, whereby the optical properties of the collector are not impaired in spite of the provision of an active cooling system of the mirror shells.

The end of the body of the mirror shell may in this case preferably be formed, for example machined, such that it runs to a point or be produced separately as a wedge. Furthermore, it is preferred if the end of the body of the mirror shell is mirrored or polished, in order to reduce the introduction of heat into the mirror shell at the end of the body.

Further advantages and features emerge from the description which follows and the accompanying drawings.

It goes without saying that the features mentioned above and those still to be explained below can be used not only in the respectively specified combination but also in other combinations or on their own without departing from the scope of the present invention.

Exemplary embodiments of the invention are represented in the drawings and described in more detail hereafter with reference to said drawings, in which:

Figure 1 shows a basic representation of an optical component on the basis of which the technical problem has been explained;

Figure 2 shows an EUV collector in a perspective representation;

Figure 3 shows a general exemplary embodiment to explain a principle of cooling the optical component according to the present invention;

Figures 4a) to c)

show three main steps in the method of producing a mirror shell of an EUV collector by means of electroforming in a schematic half- representation;

Figures 5 a) and b)

show two further method steps which illustrate the implementation of an active cooling system in the mirror shell in a schematic half- representation;

Figures 6a) and b)

show a further method step in the implementation of an active cooling system in the mirror shell in a schematic half-representation, Figure 6a) being a sectioned side view and Figure 6b) being a plan view of the mirror shell;

Figure 7 shows an alternative implementation of an active cooling system in a modification of the representation in Figure 6;

Figures 8a) to e)

show schematic representations of a mirror shell in side view, various lines followed by one or more cooling conduits of the active cooling system being illustrated;

Figure 9 shows a further alternative of an implementation of an active cooling system in a mirror shell represented only in extract form;

Figures 10a) and b)

show yet another alternative of an implementation of an active cooling system in a mirror shell represented in extract form in Figure 10a), Figure 10b) showing the mirror shell in side view;

Figure 11 shows yet another variant of an implementation of an active cooling system in a mirror shell represented in extract form;

Figure 12 shows yet another variant of an implementation of an active cooling system in a mirror shell represented in extract form;

Figure 13 shows an implementation of the temperature sensor in a mirror shell represented in extract form, in a basic representation;

Figure 14 shows a further basic representation, which illustrates how a mounting element can be integrated in a mirror shell;

Figure 15 shows a half-representation of a mounting element for fastening a mirror shell on a holding structure on its own;

Figure 16 shows a basic representation of a further aspect of the present invention for the implementation of an active cooling system in an optical component;

Figure 17 shows the transfer of the principle according to Figure 16 to a mirror shell;

Figures 18a) and b)

show a variant of the implementation of an active cooling system in a mirror shell, Figure 18a) representing the mirror shell in side view in longitudinal section in half-representation and Figure 18b) representing the mirror shell in a full representation and in cross section;

Figure 19 shows an alternative of the implementation of an active cooling system in a mirror shell in a representation analogous to Figure 18b);

Figures 20a) to d)

show a further aspect of the present invention on the basis of the example of mirror shells represented in extract form.

With reference to Figure 3, first a fundamental aspect of the present invention is explained. Represented in Figure 3 is an optical component provided with the general reference numeral 10, which is a mirror for example.

The optical component 10 has an optically effective element 12, which has a body 14, which has an optically effective layer 16, for example a reflective layer, and

otherwise a substrate. The body 14 is connected to a holding structure 20 by means of mounting elements 18.

The optical component 10 has an active cooling system 22, which has at least one cooling conduit 24, to which a cooling medium can be admitted. The cooling medium may be water for example. In the case of the cooling concept according to the invention, the at least one cooling conduit 24 is arranged directly in or on the body 14, in the exemplary embodiment shown in the body 14, of the optically effective element 12. The cooling of the optical component 10 consequently commences directly at the body 14 of the optically effective element 12.

The cooling medium can be introduced into the cooling conduit 24 according to arrow 26 through an inlet opening and be discharged again according to arrow 28 through an outlet opening. The cooling medium consequently flows through the body 14 of the optically effective element 12.

If light 30 impinges on the optically effective layer 16 of the optically effective element 12 during the operation of the optical component 10 and is reflected at this layer according to 32, heat 34 is absorbed in the body 14 of the optically effective element 12, and this heat is immediately discharged to the cooling medium which is flowing through the cooling conduit 24. As a result, the optically effective element 12 heats up only slightly, and only small deformations of the optically effective element 12 occur. Moreover, the direct active cooling of the optically effective element 12 has the effect that the mounting elements 18 experience no or no significant temperature changes that would lead to deformations of the optically effective element 12, and with it to impairment of the optical performance behavior.

With reference to Figure 2, an optical component in which the cooling concept described with reference to Figure 3 can be realized is now described. In Figure 2 and the following figures still to be described, the same reference numerals are used for the same or comparable parts.

The optical component 10 represented in Figure 2 is an EUV collector used in the course of EUV lithography. As the optically effective element 12, the collector has a plurality of mirror shells 36, which are in themselves respectively formed rotationally symmetrically with respect to an axis of symmetry 38 and are nested one in the other concentrically in relation to one another. The individual mirror shells 36 have an extent in the direction of the axis of symmetry 38, the optically effective layer being respectively formed by the surface which is facing the axis of symmetry 38, that is to say in each case their inner surface. During the operation of the collector 10, light from a light source 40, which is arranged on the axis of symmetry 38 and outside the mirror shells 36, enters the collector 10 and is reflected at the surfaces of the individual mirror shells 36 that are facing the axis of symmetry 38. The incidence of the light of the light source 40 on the individual mirror shells 36 is in this case more or less grazing.

As mounting elements 18, the collector 10 has, by way of example, a plurality of spokes 42, in the exemplary embodiment shown a total of six spokes. The six spokes go over into a common spoked wheel 44.

Each individual one of the mirror shells 36 has a substantially cup-shaped structure, which is open at both longitudinal ends.

With reference to Figures 4a) to 4c), the production of an individual optically effective element 12 in the form of a mirror shell 36 is firstly described in general and in principle.

The optically effective elements 12 in the form of the mirror shells 36 are produced by electroforming.

For this purpose, a core or mandrel 46 of an outer circumferential contour corresponding to the desired contour of the optically effective region or the optically effective layer 16 is used.

First, the optically effective layer 16 is directly applied to the core 46, as is represented in Figure 4a). For the further formation of the body 14 of the optically effective element 12, a substrate layer 48 is galvanically applied to the optically effective layer 16 according to Figure 4b). The substrate layer 48 is usually a metallic substrate, for example nickel, which is electrodeposited onto the optically effective layer 16. Apart from nickel, other metals, such as copper, silver, tungsten etc., can also be used. As soon as the desired thickness of the substrate layer 48 is achieved, the entire layer comprising the optically effective layer 16 and the substrate layer 48 is separated from the core 46, as is represented in Figure 4c). The optically effective layer 16 has then been transferred to the substrate layer 48.

Starting on the basis of Figure 4b), it is described hereafter with reference to Figures 5 and 6 how an active cooling system, which has already been described in principle with reference to Figure 3, can be implemented in the optically effective element 12 in the form of the mirror shell 36.

According to Figure 5a), at least one channel 50 is machined into the substrate layer 48 produced according to Figure 4b), for example by milling, erosion, grinding or other suitable methods, the channel 50 being filled with an electrically conductive material 52. It goes without saying that a number of such channels 50 can be machined into the substrate layer 48 and then correspondingly filled with the conductive material 52.

Following that, a further layer 54 is optionally applied according to Figure 5a).

Subsequently, the regions of the substrate layer 48 at the ends of the channel 50, that is its end faces, are removed, so that the end faces of the channel 50 lie open and the conductive material, i.e. the filler 52, can be removed again. This takes place for example with solvents or by heating, so that the filler 52 becomes liquid and can run out. Electrically conductive wax, metals or metal alloys with a low melting point come into consideration as fillers. Alternatively, rubber-like materials which are

provided with a release layer and are mechanically drawn out after the electroplating and opening of the channels may also be used.

As an alternative to the use of a filler 52, the channel 50 may be covered with a thin metal foil, which is, for example, self-adhesive or fastened in some other way, before the further layer 54 is electrodeposited on top.

After the removal of the filler 52, the state that is represented in Figure 5b) is reached. The channel 50 then forms the at least one cooling conduit 24, which has already been described in principle with reference to Figure 3, in the mirror shell 36. In the next step according to Figures 6a) and 6b), feed and discharge lines are provided for the at least one cooling conduit 24. For this purpose, annular parts 56 and 58 are arranged in the ends exposed according to figure 5b) of the channel 50, and the resultant cooling conduit 24, forming a distributing channel 60 and a connecting channel 62 which distribute the cooling medium into the cooling conduits 24 running in the longitudinal direction or collect it again from them. Pipe connecting pieces 64 foi the distributing channel 60 and 66 for the connecting channel 62 are introduced into the annular parts 56 and 58, for example welded or soldered in, and serve as a feed line and discharge line for the cooling medium.

In this way, the mirror shell 36 produced in such a manner has a plurality of cooling conduits 24 distributed circumferentially, which run in the longitudinal direction of the mirror shell 36 and are connected in parallel at their one end via the distributing channel 60 and at the other end via the connecting channel 62, as Figure 6b) reveals.

In conclusion, a further galvanic layer 68 is applied over all the parts, covering all the gaps between the parts, and consequently providing the impermeability of the cooling system.

In the case of the previously described exemplary embodiment, the optically effective layer 16 and the substrate layer 48 form a base layer of the body 14 of the optically

effective element 12 in the form of the mirror shell 36, and the layers 54, 68 form at least one cover layer, so that the at least one cooling conduit 24 is arranged as a cavity between the base layer and the cover layer.

Represented in Figure 7 is an alternative to the procedure in Figure 6, in which the at least one cooling conduit 24 is in turn provided at its ends with pipe connecting pieces 64 and 66, the pipe connecting pieces 64 and 66 here having a flange 70 and 72, respectively, which serves for the better sealing of the cooling conduit 24. The variant according to Figure 7 is again preferably completely sealed by electroplating with a galvanic layer 68 as in Figure 6a), alternatively by soldering or other methods.

With reference to Figures 8a) to 8e), various possible, but not exclusive, examples of the routing of one or more cooling conduits 24 on the mirror shell 36 are described in more detail.

With reference to Figure 6, a parallel routing of the cooling medium in a plurality of cooling conduits 24 was described, in the case of which the cooling medium is distributed into the various cooling conduits 24 through a distributing channel 60 and collected again via a collecting channel 62. This variant is schematically represented in Figure 8a). Arrow 26 indicates the direction of the feed and arrow 28 the direction of the discharge of the cooling medium from the cooling conduits 24.

Figure 8b) shows a meandering routing of the at least one cooling conduits 24 fully circumferentially around the mirror shell 36, the one cooling conduit 24 having no branches.

Figure 8c) shows a variant with a plurality of meandering cooling conduits 24, which are connected in parallel via branches or T-pieces, as is indicated at 74.

Figure 8d) shows a variant in which the one cooling conduit 24 is arranged spirally and without branching on or in the body 14 of the mirror shell 36, while Figure 8e)

shows a number of spiral cooling conduits 24, which are again connected in parallel via a distributing channel 60 and a connecting channel 62.

The choice of suitable layout of the at least one cooling conduit 24 is taken for example on the basis of aspects relating to the installation space. A parallel connection is favorable in particular in the case of a confined installation space, a series arrangement is appropriate whenever the at least one cooling conduit may have a large cross section.

The choice of suitable layout of the at least one cooling conduit 24 is also taken, however, with allowance for the drop in pressure and the associated cooling output.

In all variants of the configuration of the cooling conduits as previously described it is preferred if the cooling conduits are formed on a cooling body which can be prefabricated independently from the body or the base layer of the body of the optical component, and can then be connected to the body as will be described below in more detail.

With reference to Figure 9 et seq., configurational variants of the cooling system are likewise described on the basis of a mirror shell 36, in the case of which the at least one cooling conduit is formed as a separate cooling conduit on or in the body of the mirror shell, in particular by fabricating a separate cooling body.

The starting point for the description which follows may again be, in particular, Figure 4b), according to which the optically effective layer 16 and the substrate layer 48 have been galvanically formed, i.e. electroformed, on the core 46.

The optically effective layer 16 and the substrate layer 48 here in turn form the base layer of the body 14 of the optically effective element 12 in the form of the mirror shell 36.

According to Figure 9, fully circumferentially closed thin tubes 76, which represent a corresponding number of cooling conduits 24, are applied to the base layer 16, 48. The tubes 76 may be pre-bent, in order to lay themselves against the base layer 16, 48 better over a large surface area, which is important for the good heat transfer from the base layer 16, 48 to the cooling medium flowing through the tubes 76. Instead of a pre-bending of the tubes 76, they may also be constructed in a multipart manner in the longitudinal direction, i.e. comprise a number of part-tubes which are welded or soldered to one another. As a further alternative, the entire system comprising the thin tubes 76 may be prefabricated as a cooling body, the cooling body produced in this manner then being simply slipped over the base layer 16, 48. This is described in more detail later.

The thin tubes 76 are subsequently connected to the base layer 16, 48 by a filling compound 78 according to Figure 9. The filling compound 78 must be electrically conductive or, if it is not, it may subsequently be provided with an electrically conductive layer, for example conductive silver lacquer.

The filling compound 78 is intended to even out gaps between the thin tubes 76 and the base layer 16, 48 and establish thermal and electrical contact. The filling compound 78 also provides the bond for a galvanic outer layer 80 still to be applied subsequently. The shadow regions in which usually only little material can be brought by electroforming are filled by the filling compound 78.

Instead of applying the cover layer 80, the thin tubes 76 may also be soldered to the base layer 16, 48, the cooling conduits 24 in this case not being arranged in the body 14 but on the body 14 of the optically effective element 12.

Figure 10 shows a further alternative for the implementation of an active cooling system in the form of separate cooling conduits which, in the case of this exemplary embodiment, are not formed as circumferentially closed tubes but as partly circumferentially open channel elements.

The individual channel elements 76 forming the cooling conduits 24 are initially open on a circumferential side facing the body 14 of the optically effective element 12 in the form of the mirror shell 36, as is represented in Figure 10a). The channel elements 76 are already preassembled on the cooling body 82, on the inner side of which the channel elements 76 are arranged. After slipping over the cooling body 82 according to Figure 10b), a galvanic cover layer 80 is again applied according to Figure 10a). The cover layer 80 serves in particular for sealing the cooling system. As an alternative to the electroplating of the cooling body 82 with the coyer layer 80, the cooling body 82 may also be soldered to the base layer 16, 48.

The advantage of the configuration of the cooling system for the optically effective element 12 in the form of a separate cooling body 82 is that the cooling body 82 can be produced in a dimensionally very accurate way. All the classic machining methods, for example turning, milling and erosion, come into consideration as production methods for the cooling body 82. To allow the same materials to be realized in the case of the cooling body 82 and the base layer 16, 48, electroforming also comes into consideration as a method for producing the cooling body 82. On account of the same coefficient of expansion of cooling body 82, base layer 16, 48 and cover layer 80, no thermal stresses and resultant deformations occur as a result of heating of the system. The dimensionally accurate production of the cooling body 82 can be supplemented by the galvanic base layer 16, 48 being subjected to dimensionally accurate finishing, preferably by turning or some other production method, such as for example milling or grinding.

In order to establish good thermal contact between the base layer 16, 48 and the cooling body 82, the cooling body 82 may be coated with a thermally conducting filling material, for example an adhesive, wax or the like. Ceramic adhesives filled with metal powder, which do not outgas if they reach the surface, and are therefore not disadvantageous for vacuum applications, are well suited.

Figure 11 shows a further variant of an implementation of a cooling system in the body 14 of the optically effective element 12 of an optical collector as a modification

of the exemplary embodiment in Figure 10. While in the case of the exemplary embodiment in Figure 10 the individual channel elements 76 are open on a circumferential side facing the base layer 16, 48 and are sealed by bearing against the base layer 16, 48, the channel elements 76 in the case of the exemplary embodiment in Figure 11 are closed on the side facing the base layer 16, 48 and initially open on their circumferential side facing away from the base layer 16, 48, but are respectively closed there by covering elements 84. This reduces the risk of leakage of the cooling system. As in the case of the exemplary embodiment in Figure 10, the channel elements 76 with the covering elements 48 may be formed as a unitary cooling body 82, which then, in a way similar to that described with reference to Figure 10, is slipped over the base layer 16, 48 and fastened by means of a galvanic cover layer 80. The connection of the covering elements 84 to the channel elements 76 may be established for example by means of vacuum soldering, soft soldering or a welding method, for example laser or electron-beam welding. Similarly, the connection of the cooling body 82 produced in this manner to the base layer 16, 48 may be established for example by soldering instead of by electroplating, i.e. by the cover layer 80.

A further alternative is represented in Figure 12. Here, the tubes 76, which in turn are fully circumferentially closed, have been introduced into a support structure 86 and, for example, soldered, adhesively bonded or welded to the latter. The connecting points are located to the sides of the tubes 76 in grooves 88 of the support structure. This allows radial compensation for production tolerances, for example by the bending of the tubes 76, and the tubes 76 are less stressed. Consequently, the risk of deformation of the optically effective element 12 (mirror shell 36) is reduced. Here, too, the tubes 76 and the base structure 86 again together form a cooling body 82, which is slipped over the base layer 16, 48 and electroplated by means of a galvanic outer layer 80. As Figure 12 reveals, the galvanic outer layer 80 only has to be present at the flanks of the support structure 86 and may be left open at the tubes 76.

Both in the case of the exemplary embodiment in Figure 12 and in the case of the previous exemplary embodiments, the cooling body 82 may be of one part or comprise a number of segments.

With reference to Figure 13, a further aspect of the present invention is now described. Usually, temperature sensors are adhesively attached or mechanically clamped on optical parts. Both adhesive bonding and mechanical clamping have disadvantages. The adhesive bond can become detached, and a mechanical clamp can locally deform the optical component.

Accordingly, it is envisaged in the present invention to electroform a temperature sensor into the optically effective element 12, for example between the base layer 16, 48 and the cover layer 54 in the case of the exemplary embodiment in Figure 5 or the cover layer 80 in Figures 9 to 12. Before the cover layer 58 or 80 is applied, the temperature sensor 90 is preferably fixed by means of a filler 92. Subsequently, the galvanic cover layer 54 or 80 is then applied.

Preferably, the temperature sensor 90 may be accommodated in a thin metal tube, which is electrically conductive and is consequently suitable for the electroforming method.

In the same way, in the course of the electroforming, heating elements may be integrated in the body of the optically effective element. In this case, heating elements which have an electrically conducting metallic surface are to be preferred. In a way similar to the temperature sensor 90, the heating element can then be embedded in the body 14 of the optically effective element 12. By selective cooling and/or heating, in conjunction with the corresponding temperature sensing equipment and control, the collector can be set to an optimum operating temperature without harmful thermal gradients occurring on the optical component, in particular the optically effective element (mirror shell(s)).

In order to bring the optical component to its operating temperature, a heating medium may also be conducted through the previously described cooling conduits 24 at the beginning when the optical component 10 is put into operation, until the operating temperature is reached. When the operating temperature is reached, a

cooling medium may be conducted through the cooling conduits instead of a heating medium, in order to keep the temperature constant.

In Figure 14 it is shown that the mounting elements 18, which fasten the body 14 of the optically effective element 12 to a holding structure 20, can also be advantageously electroformed in the body 14 between the base layer 16, 48 and a cover layer 54, 80. For this purpose, the mounting elements 18 are applied to the galvanieally produced base layer 16, 48, in a way similar to that previously described with reference to the temperature sensor 90. Subsequently, the galvanic cover layer 54, 80 is applied, whereby the mounting element or elements 18 is/are then firmly connected to the body 14 of the optically effective element 12. The mounting elements 18 may then subsequently be screwed for example to the holding structure 20.

Instead of individual holding brackets, as represented in Figure 14, the mounting elements 18 may also form a closed ring 94, which according to Figure 14 is electro- formed in the body 14 of the optically effective element 12. The mounting elements 18 ate consequently formed by the ring 94 and preferably by a plurality of spring legs 96, which contribute to the stress decoupling. The mirror shell 36 can then be screwed or connected in some other way together with the mounting elements 18 to a receiving ring (not represented).

The mounting elements 18 according to Figures 14 and 15 may, however, also be soldered directly onto the body 14 of the optically effective element 12, or in conjunction with the exemplary embodiments in Figures 9 to 12 also be soldered to the respective cooling body 82.

In Figure 16, an optical component 10 is shown in a basic representation in a way corresponding to Figure 3, the following cooling concept being envisaged here for the optically effective element 12.

The body 14 of the optically effective element 12 has a base layer, which comprises the optically effective layer 16 and a substrate layer 48. Furthermore, the body 14 of the optically effective element 12 has a cover layer 54, a cavity 53, which forms the at least one cooling conduit of the active cooling system 22 of the optically effective element 12, being formed between the base layer 16, 48 and the cover layer 54. The body 14 of the optically effective element 12 is consequently formed by two halves, that is the base layer 16, 48 and the cover layer 54. The cooling medium is fed into the cavity 53 at one or more points and discharged at one or more points, the cooling medium being fed in through an inlet connecting piece 64 according to an arrow 26 and discharged through the outlet connecting piece 66 according to an arrow 28.

The formation of the cavity 53 has the effect that the absorbed amount of heat 34 is discharged over a large surface area and very directly to the cooling medium.

Since the cooling medium requires a certain pressure to be able to flow through the feeding and discharge lines, the base layer 16, 48 could deform, in particular in the case of large optical components. Therefore, the configuration described here is particularly well suited for rotationally symmetrical optical components, which are quite rigidly formed.

In Figure 17, an actual exemplary embodiment of the basic structure according to Figure 16 is then shown on the basis of the example of a mirror shell 36 for an EUV collector. As already described above, the mirror shell 36 is rotationally symmetrical with respect to the axis of symmetry 38.

The mirror shell 36 is preferably produced by means of electroforming. The production of the mirror shell 36 by electroforming comprises the production of the optically effective layer 16 and the substrate layer 48 as a base layer. Furthermore, a further shell is produced in the form of the galvanic layer 54, which may be formed not only by electroforming but also by other production methods, for example by turning, bending etc.

Possible undercuts can be created during production by electroforming by using so- called lost cores.

Both shells, that is the base layer 16, 48 and the layer 54, which forms the cooling body, are joined together and electroplated, the latter galvanic cover layer 68 serving as sealing for the layer 54 with respect to the base layer 16, 48 and the inflow and outflow connecting pieces 64 and 66, respectively.

The cooling medium flows through the cavity 53, for example in Figure 17 from top to bottom, and can also be fed in and discharged at the circumference via a number of inflow and outflow connecting pieces 64, 66.

Instead of electroplating, the layer 54 may also be soldered, welded or adhesively bonded to the base layer 16, 48.

While in the case of the exemplary embodiment shown in Figure 17 the cavity 53 extends substantially over the entire length in the direction of the axis of symmetry 38 of the mirror shell 36, the cavity 53 may also be formed as an axially bounded annular space, as is represented in Figure 18. The layer 54 of the body 14 of the mirror shell 36 is correspondingly merely an axially bounded ring, which is slipped over the base layer 16, 48 and electroplated by means of the outer layer 68. The cavity 53 is correspondingly formed as an annular space, as Figures 18a) and 18b) reveal.

In Figure 18b) it is shown that, after entry of the cooling medium through the inlet connecting piece 64, the flow of the cooling medium branches according to arrows 98 and, at the end, is collected again according to arrows 100, in order to be discharged from the outlet connecting piece 66. As a difference from Figure 17, in the case of this configuration the cooling medium consequently flows in the circumferential direction of the mirror shell 36, while in Figure 17 it takes place in the longitudinal direction of the latter (with respect to the axis of symmetry 38).

An embodiment that is modified in comparison with Figure 18b) is represented in Figure 19. In the case of this configuration, the cooling medium enters the cavity 53, forming the cooling conduits 24, through the inlet connecting piece 64 and always flows through the cavity 53 in the same direction according to arrows 98, and leaves again through the outlet connecting piece 66. A barrier part 102 between the base layer 16, 48 and layer 54 serves the purpose of conducting the flow of the cooling medium after entry into the cavity 53 in the intended direction according to the arrows 98.

A further aspect of the present invention is described with reference to Figures 20a) to 2Od).

In Figure 20a), two mirror shells 36 and 36', nested one in the other, of an EUV collector are represented in extract foim and by way of example. Both the mirror shell 36 and the mirror shell 36' have in each case a cooling body having at least one cooling conduit 24 and 24', respectively, according to one or more of the previously described configurations.

As already mentioned above, the mirror shells 36, 36' of an EUV collector (cf. Figure 2) are operated under grazing light incidence, the incident light being provided with the reference numeral 30 in Figure 20a). The light reflected from the optically effective regions of the mirror shells 36 and 36' is provided with the reference numeral 32.

With mirror shells 36, 36' nested one in the other there is the problem that part of the incident light 30 impinges on optically ineffective areas of the mirror shells, as is represented in Figure 20a) for a ray of light 30a, which impinges on an optically ineffective end face of the mirror shell 36. The ray of light 30a is consequently cut out from the incident light 30 and, in addition, generates heat in the mirror shell 36.

This cutout effect is all the greater the thicker the mirror shell 36 is. In particular if cooling conduits 24, 24' are provided in the mirror shells 36, 36' in order to cool the

mirror shells 36, 36' during operation, such cooling conduits 24, 24' lead to an increase in thickness of the mirror shells 36, 36'.

In order to avoid the cutout effect, in Figure 20b) the mirror shell 36 is represented in a modified configuration, in which the end 37 of the mirror shell 36 that is facing the incident light 30 is formed such that it is thinner than the rest of the body 14 of the mirror shell 36. By thinning the end 37 of the mirror shell 36, the part-ray 30a is now no longer cut out, as a difference from Figure 20a).

However, one consequence of the thinning of the end 37 of the mirror shell 36 is that direct cooling by means of a cooling conduit 24 is no longer possible at this end.

However, in order to improve the thermal properties of the mirror shell 36 again nevertheless, while at the same time eliminating the cutout effect, represented in Figure 20c) is a further modification of the mirror shell 36 in which the end 37 is tapered or runs to a point. This achieves the effect that rays are cut out just as little as in the case of the configuration according to Figure 20b), but the heat conduction of the heat 34 absorbed by the part-rays 30b to the next cooling conduit 24 is improved on account of the greater average cross section of the end 37.

The configurations previously described with reference to Figures 20a) to 20c) are not restricted to the application in the case of mirror shells of an EUV collector but can be applied to all optical components in the case of which a shadowing or cutout effect occurs during operation, for example on account of oblique light incidence.

Figure 2Od) then shows a further exemplary embodiment of an application of the principles from Figures 20b) and c) in the case of a mirror shell 36 of an EUV collector. In the case of an EUV collector, the incident light 30 is reflected on the inner surfaces of the mirror shells 36, as already described above with reference to Figure 2. The previously described cutout or shadowing effect occurs on the sides were the shell begins, that is in the region of the ends 37 of the mirror shells 36.

In one of the previously described exemplary embodiments, the shells 36 are preferably produced by electroforming, at least one cooling conduit, preferably a number of cooling conduits 24, preferably in form of a cooling body, being formed, preferably by electroforming, on a base layer 16, 48, as described above.

The production of ends 37 that taper or run to a point could take place for example by removing a thick layer by turning on a lathe or by other machining methods of production. In electroforming terms, any desired shapes such as a contour running to a point could be produced by masking the electric field, or by special shaping of the anodes.

Finally, according to Figure 2Od), filling bodies 104, which consist of a material with good heat conduction such as copper or are made of the same material as the base layer 16, 48, could also be incorporated.

The filling bodies 104, cooling conduits 24 and base layer 16, 48 are electroplated with a cover layer 68 in a subsequent operation.

Furthermore, the heat absorption in the region of the end of the mirror shells 36 or the end faces of the ends 37 can be further minimized by the ends 37 running to a point in such a way that the end faces no longer have any appreciable thickness. However, this is usually not possible in production terms.

Another possibility is to produce the remaining flat end face in a planar manner and to achieve good roughness for example by facing, polishing, grinding, lapping or other production methods. This allows a higher reflectivity of incident optical rays to be achieved. This reflectivity can moreover be increased by the end faces of the ends 37 of the mirror shells 36 being provided with a mirror layer which reflects back the rays impinging there. As a result, absorption of heat at the end faces is reduced.

As a further option, the optically ineffective rear side of the mirror shells 36 could be machined or coated, in order to minimize absorption of heat by stray radiation.