Method of Bonding Two Components by ..Fusion Bonding" to Form a Bonded Structure

Cross-reference to Related Applications

This application claims priority under 35 U. S. C. §119(a) to German Patent Application No. 10 2008 01 1 354.9, filed on February 27, 2008, the entire contents of which are hereby incorporated by reference. This application also claims the benefit under 35 U. S. C. 119(e)(1) of U.S. Provisional Application No. 61/031 ,751 , filed February 27, 2008, the entire disclosure of which is considered part of and is incorporated by reference in the disclosure of this application.

Background to the Invention

The present invention relates to a method of bonding at least a first and a second component to form a bonded structure by "fusion bonding", a bonded structure, an optical element, and a holding device for a wafer which are made of such a bonded structure, as well as a projection lens and a projection exposure apparatus with such an optical element.

In the manufacture of optical elements, e.g. lenses or phase shifters, as well as in the manufacture of mechanical elements such as holding devices for wafers it is often desired to bond components with different optical or mechanical properties, as appropriate, to form a bonded structure. In this context it is known to use so-called bonding methods, such as e.g. low temperature bonding, which conventionally is performed at room temperature or a slightly higher temperature, and in which a bonding agent which as a rule is liquid is arranged between the components to be bonded which is cured subsequently.

In addition, the so-called direct bonding "(fusion bonding)" is used in which the materials of the components are pressed together and typically heated up to just under the transition temperature in order to bond the said materials at a bonding site without an additional bonding agent being required. Subsequently, the bonded structure normally is processed mechanically to obtain an optical element having the desired shape. At the end of this process an element made of a material which does not occur naturally in this configuration is obtained.

The terminal lens of a projection lens for microlithography is an example for such an optical element, which may be composed of several lens shells of a crystalline material, e.g. lutetium aluminum garnet (LuAG, Lu ₃ AI ₅ Oi ₂ ), which may be arranged twisted to one another at a predetermined angle each to compensate the natural birefringence in the material.

In addition it is known from WO 2006/061225 A1 to use polycristalline materials with a grain size above the wavelength of the radiation used as a terminal element of a projection lens. In some of the materials described there, e.g. such as polycristalline spinel (MgAI ₂ O ₄ ) the intervening problem is that components of a maxim thickness of approx. 40 mm only are producible from these materials. Since the terminal lens in general is much thicker several spinel components of this type have to be arranged one on top of the other and bonded preferably by means of "fusion bonding".

In microlithography mechanical devices for holding a wafer (wafer chuck, wafer stage, wafer table) are required in addition which in part are realizable monolithically, which, however, generally are relatively heavy owing to the required sizes. Thus in such applications a bonded structure is frequently required in which cavities are formed between the bonded components to reduce their weight. Further it may be required to use a material having a high hardness and thus a high density in part of the holding device, the use of which is not required in another part of the bonded structure so that a material having

a lower density may be used there to reduce the weight, provided the holding device is implemented as a bonded structure. Generally, in this case materials are bonded with one another having a similar coefficient of thermal expansion (CTE) as described in greater detail in the applicant's PCT/EP2007/006963 which is incorporated by reference in its entirety in this application.

In the ,,fusion bonding" method the surfaces to be bonded are prepared as a rule by being wrung upon each other. Wringing is the bonding of two materials in which surfaces are held by molecular forces of attraction only, i.e. it is a "detachable" bonding which may be separated in part or completely (under the influence of moisture or due to wedge effects). However, in the wringing process, the components to be joined are required to be free from particles, which is achieved by cleaning, and the surfaces have to be polished until a more or less perfect planarity is achieved. In diameters of approx. 200 mm, equal to an area of approx. 400 cm ² , which are generally needed for the optical element described above as well as for the holding device the problem occurs, however, that though conventional cleaning methods, such as cleansing with acetone or rubbing with buckskin may be performed, still the surfaces do not adhere to each other, since adhesion depends on the absolute number of the contaminant particles which are present on the surfaces, which increases as the dimensions of the surface increases, even though cleaning is performed. Without such a cleaning step the wringing of a large area is not possible generally, which is the reason why large surfaces cannot be "fusion bonded" at all or by achieving insufficient results only.

From US 2005/0215028 A 1 a method has become known in which an amorphous and non-hydrated intermediate layer is deposited on one of the components to be bonded and both components are arranged in a spaced relationship with the intermediate layer therebetween. One or both components are heated before being brought into contact. Finally, a voltage is applied for obtaining a durable bond between the two components.

In US 2003/0211705 there is described a low temperature bonding method in which the surfaces of the components to be bonded from materials, such as silicon, silicon nitride or silicon dioxide, are activated, i.e. conditioned by cleansing or etching, at room temperature prior to the bonding. In the context so-called very slight etching methods may be used with the micro roughness of the surfaces being preserved to the largest possible extent, or reactive iron etching or plasma etching may be applied.

From the web site ,,www.heise.de/newsticker/meldung/76740" a so-called "silicon Velcro" has become known for the generation of a detachable bond for semiconductor chips in which the surface of a silicon element is roughened, e.g. by ion sputtering and thus a fine structure of silicon needles is generated. The heating of the components in order to produce a tight bond shall be foregone due to the Velcro.

Object of the Invention

It is the object of the invention to provide a method of bonding two components enabling a direct bonding ("fusion bonding") of even large surface areas to be joined, a bonded structure, an optical element as well as a holding device for a wafer made of the said bonded structure, a projection lens and a projection exposure apparatus with such an optical element.

Summary of the Invention

The object of the present invention is solved according to a first aspect of the invention by a method of the type mentioned in the introduction comprising the steps of: a) depositing at least one layer preferably having a porous structure onto a surface of at least the first, preferably of each of the components, b) roughening of the at least one deposited layer, c) bringing together the surface

of the first component with a surface of the second component, and d) bonding of the components to form the bonded structure by ..fusion bonding". The invention is based on the knowledge that the wringing which is conventionally made prior the ..fusion bonding" may be substituted by placing two surfaces on one another, at least one of the said surfaces being roughened, which also leads to the fixing of the two surfaces relative to each other (cf. Velcro), so that the "fusion bonding" may be performed in a successive step.

In this connection an additional layer which generally is a polycristalline layer, is deposited on the material of the first component, having a porous structure so that the latter may be roughened with great ease. As a rule, a layer of this kind is also deposited on the further components to be bonded, too, to enable the surfaces to be affixed well one to the other. As an alternative, however, it is also possible to only provide the first component with such a layer, if appropriate.

Preferably materials having the same chemical composition are selected for the components and the layer deposited thereon. Materials having the same composition are understood to be materials the composition of which (geometric structure) may be different, the chemical structural formula of which is identical, however. Particularly, a layer, e.g. on a component of spinel (MgAIaO ₄ ) may be generated by co-depositing of MgO and AI ₂ O ₃ at a ratio of 1 :1. In this case the geometric composition of the layer is different from the spinel structure, but the material of the layer and of the component exhibits the same chemical structural formula, such that a good adhesion of the layer with the component is still assured.

In an advantageous embodiment the same material is selected for the components to be bonded. This material preferably is identical to the material of the layer(s). In the above example, after the ,,fusion bonding" of the spinel layer with two spinel components, a bonded structure of a single material may thus be obtained, which essentially is homogenous from the optical point of view

since the porous layer transforms into a compact spinel layer during "fusion bonding".

In a preferred variant the material is selected from the group comprising: spinel (MgAI ₂ O ₄ ), aluminum oxide (AI ₂ O ₃ ), LuAG (Lu ₃ AI ₅ Oi ₂ ), cordierite (Mg ₂ AI ₄ Si ₅ O ₁₈ ), silicon carbide (SiC), (quartz) glass and glass-ceramics, in particular Zerodur, ULE or Clearceram. Besides LuAG, spinel and aluminum oxide are considered appropriate materials for a terminal element of a projection lens for microlithography due to their high refractive index. Owing to the dimensions of the terminal element, as the case may be, the latter may not be made from a single component, depending on the selected material. In bonding the surfaces to be bonded in such a terminal element, where as a rule large areas have to be bonded, the problem occurs that said surfaces cannot be cleaned sufficiently well to enable wringing. It will be understood that for the making of an optical element, besides the materials having the UV radiation transparency mentioned above, other materials having UV radiation transparency may be used as well, which in particular when used as a terminal element of a projection lens for microlithography, should have a refractive index greater than quartz glass at a wavelength of 193 nm. However, UV radiation intransparent materials such as cordierite or silicon carbide are suitable materials for the components of the bonded structure, in particular, if said structure is to be used as a holding device for a wafer.

In a further variant the layer is deposited by means of a method selected from the group comprising: physical vapour deposition (PVD, physical gas phase deposition), in particular thermal vaporization, electron beam vaporization, sputtering (cathodic sputtering), ionized cluster beam deposition (ICVD, cluster beam technology), and chemical vapour deposition (CVD chemical gas phase deposition). The methods mentioned hereinbefore concern technologies for the deposition of substrates by vapour deposition, wherein a differentiation is made between physical vapour deposition in which a chemical reaction in the

vaporization process does not take place, and chemical vapour deposition wherein such a chemical reaction takes place. It will be understood that also variants of the methods mentioned may be employed, e.g. magnetron sputtering wherein a low-temperature plasma in a noble gas (usually argon) is used to strip a target material and to deposit said material on an opposite substrate, or ion beam sputtering, wherein an ion beam is employed for this purpose. The deposition in this case must not be necessarily performed under vacuum conditions as usually, since it is preferred that the layer is porous. CVD at atmospheric pressure is particularly suited to make such a porous layer.

In a particularly advantageous variant the deposition of the layer is performed at a deposition temperature of less than 1000 ⁰ C, preferably of less than 300 ⁰ C. In doing so the coming into existence of porous polycristalline layers may be achieved, a favourable precondition for the roughening to be made later on.

In a particularly advantageous variant, the surfaces of the components for depositing the layer are provided having a surface roughness of 1.0 nm rms or less and preferably a surface planarity of less than λ, in particular of less than λ / 2, with λ = 632 nm. In doing so, the deposition of the layer to be made thereafter may be facilitated. If the bonding structure is to be used as an optical element, it is deemed particularly advantageous if the surface roughness is at λ or below, with λ designating the measurement wavelength of 632 nm (He-Ne laser).

In a further preferred variant the surface is cleaned and degreased prior to the deposition, whereby the deposition of a layer having particularly advantageous bonding characteristics may be facilitated.

It is preferred to deposit the layer by co-depositing at least two constituents of the material. As already shown hereinabove in the spinel example, the layer(s)

may be deposited by vapour deposition of two or more constituents at a ratio such that the chemical composition of the desired layer material is achieved.

In an advantageous variant the layers are deposited with a thickness of 500 nm or less, preferably with 100 nm or less. By depositing a thin layer, said layer may be transformed in a relatively efficient manner into a compact material layer in the subsequent tempering during the ,,fusion bonding" such that the method may be performed with little processing time being consumed.

Preferably, the roughening is performed by means of a method selected from the group comprising: dry etching, wet etching and ion beam sputtering. All of the methods provide for the roughening of the surfaces to such an extent only that a displacement of the said surfaces in the subsequent process of ,,fusion bonding" relative to each other does not occur. Due to the roughening a layer with a column-type structure akin to the Velcro mentioned hereinabove is generated.

In a particularly advantageous variant the surfaces are fixed at a static pressure of more than 1 bar after the joining, which is favourable for the ensuing ..fusion bonding".

Advantageously, the ..fusion bonding" is performed at a temperature of 2100°C maximum, preferably at 1500°C, particularly preferred at 1300 ⁰ C maximum. If the temperature in the bonding process is not too high, it may be achieved that the deformations due to the increase of the viscosity near the transition temperature will be as low as possible. The bonding temperature may be selected as high as to induce surface-fusing, i.e. a temperature of approx. 2100°C for spinel, e.g., or a temperature of approx. 2000°C for aluminum oxide may be achieved. Preferably, the ..fusion bonding" is performed with the aid of a process gas, such as e.g. argon or oxygen.

In an advantageous variant the ,,fusion bonding" is performed at a temperature of 70 % or less, preferably of 60 % or less of the melting temperature of the layer. In this case, the ..fusion bonding" is performed by a hidden diffusion or migration, such that the fusing of the surfaces to be bonded is not required. To arrive at the hidden diffusion, however, a temperature must be achieved as a rule which is at least at 50 % of the layer's melting temperature. It will be understood that upon the bonding of two layers made of different layer materials, the melting temperature of the layer having the higher melting point is the relevant melting temperature.

A second aspect of the invention is realized in a bonded structure, particularly made in accordance with the method described hereinabove, having at least a first and a second component which are bonded with one another at two surfaces, wherein an intermediate layer is arranged between the surfaces which is formed by ..fusion bonding" of the components with at least one roughened layer on a surface of at least the first, preferably of each of the components, the at least one roughened layer preferably having a porous structure. Herein the component and the layer deposited thereon preferably comprise, in particular consist of materials having the same chemical composition. In particular, the components as well may comprise or consist of the same material, so that in the case of components having UV radiation transparency an optically homogenous bonded structure may be obtained.

A bonded structure in which several components may be arranged in the manner described above one upon each other may be employed for forming an optical element, e.g. a terminal element of a projection lens for micro- lithography. Alternatively, a holding device for a wafer, in particular a wafer chuck or a wafer table, may be made of a bonded structure of this type.

In a particularly advantageous embodiment the intermediate layer extends over a surface of at least 100 cm ² , preferably of at least 400 cm ² . It is very difficult to

bond such large surfaces by wringing, so that the bonding in the fashion described above is the only way to generate such a bonded structure.

In an advantageous variant the material of the components is polycristalline. Polycristalline materials may be used for making optical elements in microlithography as detailed in WO 2006/061225 A1 cited in the introduction, which is incorporated by reference in this application. In particular, polycristalline spinel or polycristalline aluminum oxide may be used for this purpose.

A further aspect of the invention is realized in an optical element which is made of a bonded structure as described above. Typically, the optical element, e.g. is cut out of the cylinder-shaped bonded structure by means of mechanical processing in order to obtain the desired shape for the respective application.

One aspect of the invention is realized in a holding device for a wafer, in particular a wafer chuck or wafer table which is made of the above described bonded structure. In holding devices of this type which are joined by using several components cavities may be formed with particular ease. Further, different regions of the holding devices may be made of different types of material which are matched to the requirements of the pertinent region.

Yet another aspect of the invention is realized in a projection lens for microlithography for imaging a structure onto a light-sensitive substrate having at least one optical element of this type preferably representing a terminal element of the projection lens. In particular in projection lenses for immersion lithography such terminal elements show a large thickness, in order to couple the radiation of the projection lens into the immersion liquid. Terminal elements of materials which are inobtainable in the required thickness, such as e.g. spinel or aluminum oxide, may be generated from a bonded structure in the manner described above.

One aspect of the invention is realized in a projection exposure apparatus for immersion lithography having a projection lens as described above, in which the optical element is disposed opposite of the light-sensitive substrate, wherein an immersion liquid is arranged between the light-sensitive substrate and the optical element. As mentioned before, in this case the optical element shows a particularly large thickness such that said optical element advantageously is made of a bonded structure.

Further features and advantages are stated in the following description of exemplary embodiments, with reference to the figures of the drawing which shows significant details, and are defined by the claims. Individual features can each be used singly, or several of them can be taken together in any desired combination, in order to implement desired variations.

Brief Description of the Drawings

Exemplary embodiments are represented in the schematic drawings and are explained in the subsequent description in which

Figs. 1a-d show a schematic illustration of multiple process steps in a variant of the inventive method for generating a bonded structure,

Fig. 2 shows a schematic illustration of a projection exposure apparatus with an optical element made of a bonded structure of this type,

Fig. 3 shows a schematic illustration of a bonded structure designed as a wafer chuck (wafer clamping device) as well as a vacuum source for generating an underpressure; and

Fig. 4 shows a schematic illustration of a wafer table configured as a bonded structure having several components which are interconnected.

Detailed Description of Preferred Embodiments

Figs. 1a-d schematically show a method of bonding two components 1 , 2 of polycristalline material, e.g. of spinel (MgAI ₂ O ₄ ), having a thickness of approx. 40 mm each. When the process is started, the roughness of surfaces 1a, 2a of components 1 , 2 is brought to less than 1.0 nm rms and the surface planarity to less than λ by using an expedient smoothing method, e.g. polishing, and the said surfaces 1a, 2a are degreased and cleaned. In a first step (cf. Fig. 1a) a layer 3 with a thickness of approx. 100 nm, consisting of spinel as well, is deposited onto surface 1 a of component 1 , said surface being preconditioned in such a way, by thermal vaporization under vacuum. It will be understood that other thin-layer methods may be employed for deposition which are based in particular on the physical or chemical gas deposition principle. Further, instead of spinel, magnesium oxide MgO and aluminum oxide AI ₂ O ₃ may also be used as a deposition material which is deposited by co-depositing at a 1 : 1 ratio onto surface 1a. In both cases a layer 3 is forming on the surface 1a, the chemical composition of which matches that of component 1. Here, a deposition temperature TB is selected which is less than 300°C to achieve a porous, polycristalline structure of layer 3.

In a successive step (cf. Fig. 1 b) the porous structure of layer 3 is exploited to roughen the surface 3a thereof by a simple means by using an ion beam 8 for sputtering said surface, such that essentially a column or needle-like surface 3a is formed on layer 3.

The two process steps shown in Figs. 1 a, b are performed analogously for the second component 2, whereby a layer 4, shown in Fig. 1c, which is roughened

as well is formed. Thereafter, the two surfaces 1a, 2a of components 1 , 2 are then placed one on top of the other as shown in Fig. 1c with the two layers 3, 4 adhering to each other such that a polycristalline intermediate layer 5 is formed. The two components 1 , 2 are subsequently subjected to a static pressure of more than 1 bar before said components 1 , 2 are bonded by ,,fusion bonding" in a subsequent step shown in Fig. 1d, i.e. they are tempered at temperatures of up to 2100°C maximum with the aid of a process gas such as e.g. argon or oxygen. During the tempering the embedded porous spinel intermediate layer 5 transforms into a compact spinel layer bonding the two surfaces 1a, 2a of the components 1 , 2 such that a bonded structure 6 is generated. After the bonded structure 6 has cooled down to room temperature, the bonding process is finalized. As an alternative to ..fusion bonding" by surface-fusing, ..fusion bonding" may also be performed at lower temperatures, typically at approx. 60 % to 70 % of the melting temperature of the layers 3, 4 or the components 1 , 2, respectively, wherein, in this case, the bonding is achieved by hidden diffusion.

It will be understood that due to the repetition of the process steps described above several components may be superimposed such that bonded structures of an almost arbitrary height may be generated. Further, the bonding site, i.e. the interim layer 5 may be designed extending over a large area, and in particular, a surface of more than 100 cm ² or 400 cm ² may be covered.

Besides the example illustrated hereinabove, in which spinel was selected as a material of the bonded structure 6 or the components 1 , 2, this method may also be applied, e.g. in aluminum oxide (AI2O3) or LuAG (Lu ₃ AI ₅ Oi ₂ ), wherein in this case aluminum oxide (AI ₂ O ₃ ) or LuAG (Lu ₃ AI ₅ Oi ₂ ) rnay be selected as well as a layer material to obtain an optically homogeneous bonded structure.

The bonded structure 6 made in the way described above is particularly suited in the case that a highly refractive material with a refractive index higher than

that of quartz glass at a wavelength of 193 nm is selected for the making of an optical element 7 for immersion lithography as shown in Fig. 2 in a projection exposure apparatus 10 for immersion lithography in the form of a wafer scanner for the manufacture of highly-integrated semiconductor components the mode of operation of which is explained as set forth below.

The projection exposure apparatus 10 comprises as a light source an excimer laser 11 with an operating wavelength of 193 nm, with other operating wavelengths, e.g. 248 nm, also being possible. A downstream illumination system 12 generates a large, sharply delimited and very homogeneously illuminated image field in its exit plane which is adapted to the telecentry requirements of a downstream projection lens 13.

Arranged behind the illumination system 12 is a device 14 for holding and manipulating a photo mask (not shown) in such a way that the said mask lies in the object plane 15 of the projection lens 13 and is moveable in this plane for a scanning operation towards a scanning direction as indicated by an arrow 16.

Downstream of the plane 15, also designated as mask plane, is projection lens 13 imaging at a reduced scale, e.g. scale 1 :4 or 1 :5 or 1 :10, an image of the photo mask on a wafer 17 onto which a photo-resist layer has been placed. The wafer 17 serving as a light-sensitive substrate is arranged such that the plane substrate surface 18 with the photo-resist layer substantially coincides with the image plane 19 of projection lens 13. Wafer 17 is moved by means of a device 20 comprising a scanner drive in order to displace the wafer 17 synchronously with the photo mask and generally into the opposite direction of said mask. The device 20 also comprises manipulators, to traverse the wafer both into the z- direction in parallel to an optical axis 21 of the projection lens and into the x- and y-direction perpendicularly to said axis.

The function of the optical element 7 is that of a terminal element in projection lens 13, said optical element 7 being configured as a transparent plano-convex lens, said optical element 7 having a conical lens part, the face side of which forms the terminal optical surface of projection lens 13 and which is arranged in an operating distance above the substrate surface 18. Between the face side and the substrate surface 18, water is arranged as immersion liquid 22, which flows through the region between the optical element 7 and the wafer 17. By means of the immersion liquid the imaging of structures on the photo mask may be performed with a higher resolution and depth of focus than is possible when the intermediate area between optical element 7 and wafer 17 is filled with a medium with a lower refractive index, e.g. air.

Due to its high refractive index, the optical element 7 is particularly well capable of coupling the radiation into the immersion liquid 22. Since the optical element 7 has a radius of up to 100 mm and an almost comparable height, and spinel discs may only be made up to a height of approx. 40 mm, the optical element 7 cannot be made of a single spinel blank but has to be manufactured by being processed mechanically from a bonded structure. Because of the large diameter of the optical element 7 in this case the said bonded structure may not, or at a great expense only, be generated by a conventional "fusion bonding" process in which the components are wrung to one another, so that the method described above in connection with Fig. 1 may be applied advantageously. The optical element 7 may be formed here in analogy to WO 2006/061225 A 1 of the applicant as cited in the introduction, which is incorporated by reference in the present application in its entirety.

Besides the making of bonded structures of materials having UV radiation transparency, the method described above may also be used for the making of bonded structures having no or only partial UV radiation transparency. In particular, this is possible in case of holding devices for a wafer, the components of which typically are made of cordierite (Mg ₂ AI ₄ Si ₅ Oi _S ), silicon

carbide (SiC), glass or glass-ceramics, in particular Zerodur, ULE or Clearceram. Two examples of holding devices of this kind are represented below at Figs. 3 and 4.

In Fig. 3 a wafer chuck 25 configured as a holding device is shown. In the very simplified illustration of Fig. 3 said device consists only of a bonded structure 6 on which the wafer 17 is to be held and of a vacuum pump 29. A negative vacuum is generated by means of the vacuum pump 29 between wafer 17 and the bonded structure 6, said vacuum sucking the wafer 17 towards the bonded structure 6, as indicated by an arrow in Fig. 3. The bonded structure 6 has a first top plate 26 and a second bottom plate 27, which are congruent both in shape and size. A plurality of supporting elements is arranged between the plates 26, 27 which form a ribbed structure 28 (grid), having a honeycomb structure extending perpendicular to the plates 26, 27. The suction under vacuum may be performed through the honeycombs of grid 28 and the apertures provided between plates 26, 27. Further, the bonded structure 6 is particularly light-weighted owing to the honeycomb structure and the weight of said structure 6 may be approx. 30 % less than a massive cordierite component would have when used as a wafer chuck 25. The top and the bottom plate 26, 27 as well as the ribbed structure 28 are each bonded with one another by the ,,fusion bonding" process described above.

It will be understood that the bonded structure 6 may be part of a wafer chuck as well which is operating by means of electrostatic forces of attraction and in which a high-voltage source for generating an electrical field between the bonded structure 6 and the wafer 17 is provided. A wafer chuck of this type is also suitable for vacuum applications.

Fig. 4 shows a wafer table 30, the purpose of which is to support wafer 17 during the exposure process in a projection exposure apparatus (not shown) for microlithography. The wafer table 30 is recessed in a hollow 31 which is

designed such that the top edge thereof is flush with the wafer 21. The wafer table 30 consists of a first bottom component 32a of cordierite which is bonded by means of the ..fusion bonding" process described above with a second top component 32b of cordierite, with both components 32a, 32b jointly forming the bulk of the wafer table 30. Alternatively, the two components 32a, 32b may be made of other materials as well, preferably of Zerodur.

The two components 32a, 32b of the bulk exhibit recesses opposite each other, whereby plural cavities 33 are formed between the bonded components 32a, 32b. The purpose of the cavities 33 is to provide cooling ducts designed to convey a cooling liquid in order to dissipate the heat which occurs to an increased extent due to the absorption of the radiation in components 32a, 32b in view of the high radiation intensity in microlithography. It will be understood that more cavities, e.g. to accommodate parts to be installed and heating elements may be provided in the wafer table 30, and that the said table may also be provided with a grid structure as described above to achieve an additional reduction of weight.

On the top component 32b of the wafer table 30 multiple supporting structures ("pimples") 34 having equidistant spaces to each other are arranged to support the wafer 17. The said supporting structures each have a bottom component 32c of cordierite which are bonded with a top component 32d of silicon carbide by ,,fusion bonding" as well.

A further component 32e of silicon carbide is disposed on the two components 32c, 32d forming a bulk of the support structures which has a reduced diameter to support wafer 17 here and there. The top component 32d and the further component 32e of the support structures 34 are also bonded by ,,fusion bonding" wherein in the latter case a ,,fusion bonding" process with wringing may be employed because of the small surfaces to be bonded. It will be understood that the components 32c-e of the support structures 34 may consist

completely of silicon carbide, too, or another material which preferably is particularly hard.

The accordingly designed wafer table 30 may withstand the high accelerations in microlithography, wherein due to the type of components selected a lightweight structure is realized and, simultaneously, a good heat dissipation as well as a low heat expansion is ensured, with the high bonding strength of the bondings guaranteeing a high mechanical stability. It will be understood that it is not imperative that each bonding between components 32a-e of the wafer table 30 has to be made by ,,fusion bonding", rather, one or multiple bondings may be made in a different way as well, particularly as described in the applicant's PCT/EP2007/006963.

Only by employing the method of ..fusion bonding" illustrated hereinabove is it possible to bond components with one another to form large surfaces of approx. 400 cm ² and more . It will be understood that the method or the bonded structure described above will not only be employed in microlithography, but advantageously also in other domains, e.g. in X-ray telescopy or in laser processing systems.

The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. The applicant seeks, therefore, to cover all such changes and modifications as fall within the scope of the appended claims, and equivalents thereof.