Abstract:
The disclosure relates to an optical assembly for a projection exposure apparatus for semiconductor lithography. The optical assembly includes at least one optical element and a mounting body for mechanically fixing the element in a supporting structure. The optical assembly also includes at least one cooling body for dissipating heat from the element. The mounting body and the cooling body are separate from one another. The optical element is connected to the cooling body via at least one heat-conducting element. The disclosure also relates to a projection exposure apparatus including an optical assembly according to the disclosure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit under 35 U.S.C. §119 of German patent application No. 10 2013 220 710.7, filed Oct. 14, 2013, the entire contents of which are incorporated by reference herein. 
     FIELD 
     The disclosure relates to an optical assembly for a projection exposure apparatus for semiconductor lithography and to a projection exposure apparatus for semiconductor lithography. 
     BACKGROUND 
     Optical assemblies are often installed in a larger unit, such as for example an EUV illumination system. The optical element is in this case a three-dimensional geometrical body that is generally delimited by multiple surface areas and is used for performing one or more optical functions, such as for example the deflection or other manipulation of electromagnetic radiation. According to the optical beam path, the electromagnetic radiation is incident at least on one surface of the optical element. This surface is referred to hereinafter as the optically active surface. The optically active surface may be divided into multiple sub-surfaces, may be convexly or concavely curved in sub-portions and also include abrupt changes in topography, as are usual for example in the case of a diffractive optical structure. The optically active surface may include, among other things, a multilayer layer and/or other coatings, such as for example also an absorber layer. The multilayer layer may be formed via a contiguous coating vapour-deposited onto the optical element. The optically active surface may also be arranged on an MEM that is acting as an optical element. The individual optical elements may be constructed from different materials and/or generally also have different component geometries. During the operation of the illumination system, different illumination settings, such as for example an annular setting, a dipole, quadrupole or other multipole setting, can also be set by corresponding alignment of the optically active surface. This has the consequence that, depending on the illumination setting, electromagnetic radiation is incident on the optical elements with locally differing intensity. Furthermore, in the case of some of the optical elements considered here, locally differing absorber layers are applied. These absorber layers are used to achieve defined spatially resolved intensity distribution of the reflected radiation after the reflection of the EUV radiation at these optical elements. Depending on the degree of absorption of EUV radiation, the energy input at the optical elements may differ locally. During the operation of the illumination system, IR radiation from other optical elements or from mechanical components may also be incident on the optical element considered and be completely or partially absorbed there. The IR radiation may for example also originate from a heated component—such as for example a so-called sigma diaphragm—which has previously absorbed EUV radiation. Consequently, there may altogether be a locally differing heat distribution on the optically active surface of the optical elements. In particular, even with a homogeneous or symmetrical energy input, asymmetric local heat distributions may occur, if for example the absorption properties for electromagnetic radiation on the component differ in a spatially resolved manner. In the case where the optical elements are uncooled, sometimes temperatures of 200° C. and above can occur during the operation of the illumination systems. The spatially resolved heat distribution may result in undesired deformations of the optical elements or mounting bodies thereof. In particular when changing an illumination setting and when there is constant thermal loading over a certain period of time, deformations of the optical elements or the mounting bodies thereof may occur when considered over time in comparison with the basic state without thermal loading at ambient temperature, or some other previously set steady state or quasi steady state. 
     SUMMARY 
     The disclosure seeks to comparatively quickly dissipate heat from an optical element in an optical assembly, wherein the heat is produced in the optical assembly during operation. The disclose also seeks to provide the desired imaging performance of the optical system during operation and to provide a stable mounting of the optical elements. 
     According to the disclosure, an optical assembly may be for example a deflection mirror in an EUV illumination system for semiconductor lithography. Similarly, an optical assembly may also include multiple optical elements. For example, a facet mirror made up of a mirror supporting body and many individual mirror facets as optical elements is likewise referred to hereinafter as an optical assembly. 
     An optical assembly according to the disclosure, in particular for a projection exposure apparatus for semiconductor lithography, includes at least one optical element with a mounting body for mechanically fixing the optical element in a supporting structure and at least one cooling body for dissipating heat from the optical element. The mounting body and the cooling body are configured as components that are separate from one another. The optical element is connected to the cooling body via at least one heat-conducting element. An advantage of this measure is that of breaking down the complexity involved in the design, production and installation by providing the mounting body and the cooling body as separate structural units. It is likewise advantageous that a simple integration or arrangement of electrical or electronic components in or on the mounting body is made possible, since the mounting body is not flowed through directly by coolant. There is consequently altogether a lower integration density. 
     The mounting body may be produced by using a material with a comparatively low coefficient of thermal expansion, which may have a value in a range from 0.1×10 −6  m/m/K to 25×10 −6  m/m/K, and/or a comparatively high modulus of elasticity in a range from 70×10 9  N/m 2  to 600×10 9  N/m 2 . The use of materials with the stated parameters has the effect that the mounting body is mechanically stable, and consequently undergoes little deformation. The high heat absorption in the optical elements consequently has virtually no influence on the mounting body in terms of deformation. 
     The material for the mounting body may contain Invar or Zerodur. It is advantageous that a low-expansion, stable mounting body can be achieved with these materials. Alternatively or in combination, the material for the mounting body may contain silicon carbide, molybdenum alloys, tungsten alloys, copper alloys, aluminium alloys or high-grade steels. 
     The optical element may be connected to the heat-conducting element and/or be arranged on it in direct physical contact. It is advantageous that no great installation forces are involved for the heat transfer within the mounting body, since installation takes place at or on the heat-conducting element. This avoids deformations of the mounting. The physical contact between the optical element and the heat-conducting element can be established by an adhesive bond. 
     It is ensured by the use of a heat-conducting element that the predominant part of the heat to be dissipated from the optical element is not dissipated by way of the supporting structure and/or the mounting body, but by way of the heat-conducting element itself. The supporting structure and/or the mounting body can consequently be optimized from purely mechanical aspects, in particular also with regard to material cross sections. This gives rise to extended possibilities in the construction and design of the optical arrangement with regard to mechanical securement. 
     The heat-conducting element may comprise a heat-conducting pipe, also known as a heat pipe. The heat-conducting element may contain copper, aluminium, gold or silver, possibly in corresponding alloys. 
     The heat pipe may contain water as the working liquid. An advantage of the use of a heat pipe is that the mechanical disadvantages of a pipeline or a tube through which water is actively pumped—in particular vibrations caused by the flow of the medium—occur very rarely in the case of the heat-conducting element. Other working liquids are also conceivable, depending on the working area and the temperature of the cooler. 
     In an advantageous implementation, the heat-conducting element may have a head part to be arranged on the optical element and/or an end part to be arranged on the cooling body. It is possible for the heat pipe to be arranged between the head part and the end part. The material for the head part may contain copper, high-grade steel, silicon, silicon carbide, molybdenum alloys or tungsten alloys. An advantage of these materials is that the head part can be designed with good thermal conductivity and adapted, preferably lower, thermal expansion. The material for the end part may contain copper, copper alloys, aluminium alloys, silicon carbide, molybdenum alloys or tungsten alloys. Good thermal conductivity is important for the end part and the head part, and is obtained from the aforementioned materials. 
     In an advantageous implementation, the heat-conducting element may be designed as one part. In this case, the one-part heat-conducting element may be produced at low cost as a soldering assembly. 
     The thermal resistance of the heat-conducting element may have a value in a range from 0.5 K/W to 5.0 K/W, preferably in a range from 0.5 K/W to 2.0 K/W. Great amounts of heat can be advantageously dissipated by way of the heat-conducting elements with low temperature gradients. For example, a heat pipe with a diameter of 4 mm that is operated with water can, depending on the working temperature range, transport 20 W to 35 W of heat output. 
     In an advantageous implementation, an additional part for fastening the head part may be arranged on the mounting body. 
     The heat pipe may have a flexible outer sheath. An advantage of the flexible heat-conducting elements is that they only transfer small forces to the mounting body when there are positional changes due to mechanical and/or thermal influences on their connection locations in the cooling body, these small forces in turn only having a small influence on the deformation of the mounting body. The heat-conducting element can consequently be configured as mechanically largely neutral, that is to say comparatively soft, so that no additional possibly disturbing forces are introduced into the mounting body and the optical element through the heat-conducting element. 
     In an advantageous implementation, the outer sheath may comprise a material with a modulus of elasticity with a value in the range from 15×10 9  N/m 2  to 220×10 9  N/m 2 . 
     The heat-conducting element may be exchangeable. It is advantageous that, after the removal of the heat-conducting element, an in-situ repair or adaptation of the optical element is possible if a component or optical element is defective, or for exchanging it without dismantling the entire optical assembly. The components may be extractably arranged on the mounting body, so that the corresponding component could be extracted from the side of the optically active surface or from the side of the optical assembly that is facing away from the optically active surface, and then be exchanged or repaired in situ. 
     The heat transfer between the head part and the optical element or the mounting body and/or between the end part and the cooling body may take place by mechanical contact. In an advantageous way, the mechanical contact between these components may be established by a mechanical, magnetic or adhesive connection. 
     In an advantageous implementation of the disclosure, the head part may be designed such that it can be screwed into and unscrewed from or inserted into and withdrawn from or pushed into and extracted from the mounting body and/or the end part may be designed such that it can be screwed into and unscrewed from or inserted into and withdrawn from or pushed into and extracted from the cooling body. 
     The cooling body may be operated with liquid cooling, preferably water cooling, and/or the cooling body may be configured as a plate or strip cooler. The advantage of water is that it has a very low thermal resistance and transports the heat away very effectively, and consequently a lower temperature can be achieved at the optical element by water cooling. The cooling body may also be operated in the range below 0° C., preferably with glycol water cooling. In this way, a lower temperature can be achieved at the optical element. 
     In an advantageous implementation, the cooling body may be mountable on the supporting structure via vibration dampers and/or the cooling body may be arranged in such a way that it is isolated from the mounting body in terms of vibration. 
     The optical element may be a MEMS module or a mirror facet. 
     The advantage that is achieved by separating the mounting body and the cooling body is that of avoiding disturbing effects on the optical properties of the optical element considered. The thermal loading and in particular the heat distribution in the components can also change over time, for example if a change of the illumination setting is performed. The cooling used here, via heat-conducting elements, may be configured such that, apart from the local adaptation, it is additionally adaptive over time. 
     Furthermore, the optical elements of a module may be cooled to differing degrees. Furthermore, it is also possible to attach controllable cooling bodies to the heat-conducting elements, the cooling bodies having single or multiple locally adapted cooling zones, so that a locally adaptable and/or settable cooling of the optical elements is made possible. 
     The optical assembly may for example be a multilayer mirror for EUV lithography with a mirror support and a multilayer layer arranged thereon as an optical element. The multilayer mirror may be provided with at least one heat-conducting element. Various sub-regions of the mirror support may be connected to one or more heat-conducting elements and be cooled to varying degrees there. 
     The heat-conducting elements may also be formed as cooling lines connected to the cooling body. The heat-conducting elements configured as cooling lines are consequently separate components that are attached to the cooling body. 
     The optical element may also be cooled via a cooling body with the latter being in thermal contact with the optical element by way of heat-conducting elements, and a greater degree of cooling being achieved in a first sub-region by the contact surface between the heat-conducting element and the optical element being greater than in a further sub-region. The forming of heat-conducting elements that are adapted to the desired local cooling rate consequently represents a further alternative for the cooling of the optical element in various sub-regions. In this case, the entire configuration of the system can be adapted to new circumstances in an easy way by exchanging the heat-conducting elements. 
     The optical element may also be provided on the optically active surface with an EUV absorber layer, which has different thicknesses in different regions of the optically active surface. Application of the EUV absorber layer particularly allows locally adapted intensity modulation of the electromagnetic radiation that is incident on the optical element. A different degree of cooling is provided at the sub-regions of great thickness of the absorber layer in comparison with the sub-regions with a smaller thickness of the EUV absorber layer, on account of the greater energy input there. 
     The optical assembly may also be a facet mirror with a plurality of single mirror facets. A heat-conducting element may be arranged on at least one single mirror facet. The desired spatially resolved cooling is in this case achieved by individual or groups of single mirror facets being cooled to differing degrees. 
     The heat-conducting element may be integrated under an optically active surface, in particular a reflective surface of a single mirror facet or a multilayer layer, and can in this way dissipate particularly efficiently the heat that is produced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the disclosure is explained in more detail below on the basis of the drawings, in which: 
         FIG. 1  shows a schematic representation of a projection exposure apparatus; 
         FIG. 2  shows a schematic cross-sectional representation of an optical assembly; 
         FIG. 3  shows a schematic representation of a partial detail of the optical assembly of  FIG. 2 ; 
         FIG. 4  shows a schematic representation of a heat-conducting element as a section of  FIG. 3 ; and 
         FIG. 5  shows an optical assembly in the overall representation that is shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows by way of example the basic construction of a microlithographic EUV projection exposure apparatus  400 . An illumination system  401  of the projection exposure apparatus  400  has along with a light source  402  an illumination optics  403  for illuminating an object field  404  in an object plane  405 . A reticle  406 , which is arranged in the object field  404  and is held by a schematically represented reticle holder  407 , is illuminated. A projection optics  408 , which is only schematically represented, serves for projecting an image of the object field  404  into an image field  409  in an image plane  410 . An image of a structure on the reticle  406  is projected onto a light-sensitive layer of a wafer  411 , which is arranged in the region of the image field  409  in the image plane  410  and is held by a wafer holder  412 , which is likewise represented in the form of a detail. The light source  402  can emit used radiation, in particular in the range between 5 nm and 30 nm. 
     A EUV radiation  413  generated via the light source is aligned via a collector integrated in the light source  402  in such a way that it passes through an intermediate focus in the region of an intermediate focal plane  414  before it is incident on a field facet mirror  415 . After the field facet mirror  415 , the EUV radiation  413  is reflected by a pupil facet mirror  416 . With the aid of the pupil facet mirror  416  and an optical assembly  417  with mirrors  418 ,  419  and  420 , images of field facets of the field facet mirror  415  are projected into the object field  404 . 
       FIG. 2  shows an optical assembly  1  with an optical element  2 , which has an optically active surface  11 . The optical element  2  may also be designed (not represented) as a single mirror facet or MEMS module. Alternatively, the optical element  2  may—likewise not represented any more specifically—be constructed as a structural component with a plurality of MEMS modules and/or micro-mirrors and from various layers, electrical and/or electronic components being arranged on the various layers and the optically active surface being distributed over mirrors that are square, rectangular or of some other form. 
     For mechanical fixing, the optical element  2  is mounted in a mounting body  3 . The mounting body  3  is supported by a supporting structure  4 . The connection between the mounting body  3  and the supporting structure  4  may preferably be provided by adhesive bonding, clamping or screwing. The supporting structure  4  itself may be configured with the use of a hexapod (see  FIG. 5 ). Suitable as the material for the supporting structure  4  are materials that preferably contain high-grade steel or aluminium. 
     For dissipating heat from the optical element  2  into a cooling body  5 , the optical element  2  is connected to the cooling body  5  by way of at least one heat-conducting element  6 . In the exemplary embodiment shown in the figure, the cooling body  5  is operated with water cooling  9 ; other cooling media, in particular glycol or the like, are also conceivable. The cooling medium flows through the cooling body, which dissipates the thermal energy transferred to it. The cooling medium enters the cooling body  5  at an inlet, the direction of flow being represented by the arrow E, and leaves the cooling body  5  at an outlet, the direction of flow being represented by the arrow A. The cooling body  5  may—not represented any more specifically—be configured as a plate or strip cooler. The cooling body  5  can be produced from a material that preferably may contain copper, aluminium, high-grade steel or alloys thereof. 
     The cooling body  5  is mounted on the supporting structure  4  by way of a holder  12 , which in the example shown comprises a vibration damper  10 . 
       FIG. 3  shows a view of a detail of the heat-conducting element  6  in the fitted state. The heat-conducting element  6  exhibits a head part  61 , a heat-conducting pipe or heat pipe  62  and an end part  63 . The heat pipe  62  is of a wavy configuration and, because of its form, small wall thickness and small cross section, has a residual flexibility. Consequently, it is possible with the heat pipe  62  to compensate for positional or installation-related tolerances and/or differences in thermal expansion between the components. In the example shown, the optical element  2  is arranged on the head part  61  directly, that is to say in particular is also thermally coupled. The optical element  2  may be in particular what is known as a brick, i.e. a MEMS module with micro-mirrors. The head part  61  may be fastened to the mounting body  3  by way of an additional part  8 , for example a flange. The head part  61  is preferably tightened or clamped onto the mounting body  3  by the additional part  8  and a fastening element  13 . The fastening element  13  may be configured as a screw. The fastening element  13  designed as a screw may—not represented any more specifically—be tightened or loosened by a tool that can be led through the cooling body  5  or through openings in the cooling body  5 . After loosening and removing the screws and flange, which fix the head part  61  in the appropriate position on the mounting body  3 , and after loosening the screw on the cooling body  5 , the heat-conducting element  6  as a whole could for example be extracted upwards in the direction of the optically active surface  11 . 
     Alternatively, the head part  61  may—not represented any more specifically—be connected directly to the mounting body  3 , with no need for any additional screws  13  or additional parts  8 . The mounting body  3  and the head part  61  are then connected to one another by mechanical contact. Furthermore, the head part  61  may—likewise not represented any more specifically—be fixedly connected to the mounting body  3 , preferably by a welded or soldered connection. 
     The end part  63  is arranged on the cooling body  5 . For this purpose, the end part  63  may be fastened to the cooling body  5  via a fastening element  14 . The fastening element  14  may be configured as a screw. The end part  63  is connected to the cooling body  5  in direct physical contact. For this purpose, the end part  63  may be designed like a cone and the receptacle in the cooling body  5  for the end part  63  may be correspondingly designed as a counter-bearing. Alternatively, the end part  63  may—not represented any more specifically—be fixedly connected to the cooling body  5 , preferably by a welded or soldered connection. 
     In order to obtain optimum thermal coupling between the cooling body  5  and the end part  63 , a pressed surface that is as large as possible is desired between the components. This is obtained by the clamped connection via the cones or cone and counter-bearing, a surface that is as precise as possible and sufficiently large for establishing the mechanical contact between the cooling body  5  and the end part  63  being made possible via the cones. 
     The heat transfer from the optical element  2  into the cooling body  5  takes place by way of the heat-conducting element  6 , these components from the optical element  2  to the cooling body  5  being in direct physical contact, i.e. the components are thermally coupled to one another. Moreover, the components are produced from materials with low thermal conduction resistances, in order to have overall a thermal conduction resistance that is as low as possible. 
       FIG. 4  shows a view of a detail of the heat-conducting element  6  in a sectional representation, which as a difference from the heat-conducting element  6  represented in  FIG. 3  explains more specifically the interior construction and functioning of the heat pipe  62 . 
     The heat pipe  62  is a sealed and/or closed pipe  64  with a working liquid  65  and the vapour  66  thereof. The pipe  64  may have a round or angular cross section and preferably contain as the material a metal that has a low thermal resistance. Materials that can be mentioned in particular as ones with good thermal conduction are copper, aluminium and silver, or else gold. Furthermore, improved heat conduction should also be taken into consideration when designing the cross section of the heat-conducting element. The inner side of the pipe  64  is provided with a capillary structure  69 . 
     Alternatively, instead of the pipe  64 , a flexible bellows could be used. The capillary structure  69  is in this case likewise flexible. 
     If heat is supplied to the heat pipe  62  from the outside, the liquid  65  inside the heat pipe  62  evaporates. A heat pipe  62  uses the working liquid  65  on the basis of the principle of latent heat of vaporization, the liquid  65  inside the heat pipe  62  being vaporized. The vapour  66  flows in the direction of the temperature gradient and condenses at the cooler locations of the heat pipe  62  in the region of the end part  63 , while giving off the latent heat to the end part  63 . 
     If heat is supplied to the heat pipe  62  by way of the head part  61 , as indicated by the arrow HI, the liquid  65  evaporates at this location and a fraction of it then condenses again at a different location, preferably in the region of the end part  63 . By this taking place, the latent heat of the working liquid  65  is used to bring about a very efficient energy transfer from the head part  61  to the end part  63 . The heat has thus been transported to the end part  63  and can be dissipated, as indicated by the arrow HO. 
     The pipe  64  may be flexibly designed via joints  67  or bellows-like protuberances  68 , in order to compensate for positional or installation-related tolerances and/or differences in thermal expansion between the parts. 
       FIG. 5  shows the optical assembly  1 , which as a difference from the optical assembly  1  represented in  FIG. 2  has a supporting structure  4  with a hexapod connection  15 . The supporting structure  4  has a hexagonal form, in order—not represented any more specifically—to position multiple optical elements with optimum utilization of the available installation space and the smallest possible distance from one another, for example in a mirror of a projection exposure apparatus.