WO2008104220A1

WO2008104220A1 - Optical imaging arrangement

Info

Publication number: WO2008104220A1
Application number: PCT/EP2007/051855
Authority: WO
Inventors: Dr. Markus KNÜFERMANN; Thomas Bischoff; Günter DENGEL
Original assignee: Carl Zeiss Smt Ag
Priority date: 2007-02-27
Filing date: 2007-02-27
Publication date: 2008-09-04
Also published as: JP2010519761A; JP5055384B2

Abstract

An optical element arrangement is provided comprising an optical element unit, a first support structure and a reference external to the optical element unit. The optical element unit comprises an optical element group, at least one optical element unit reference associated to the optical element group and a second support structure supporting the optical element group. The optical element group comprises a plurality of optical elements defining an optical axis of the optical element group. The optical element unit reference has a reference position with respect to the external reference. The first support structure supports the optical element unit via a plurality of support elements, the plurality of support elements being arranged to support the optical element unit such that, upon thermal expansion of at least one of the optical element unit and the first support structure, the reference position, at least along the optical axis, remains substantially unchanged.

Description

Optical Imaging Arrangement

BACKGROUND OF THE INVENTION

The invention relates to optical element arrangements used in exposure processes, in particular to optical element arrangements used in microlithography systems. It further relates to a method of supporting an optical element unit of such an optical element arrangement. It also relates to an optical imaging method for transferring an image of a pattern onto a substrate. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.

Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical element units comprising optical elements, such as lenses, mirrors, gratings etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to illuminate a pattern formed on a mask, reticle or the like and to transfer an image of this pattern onto a sub- strate such as a wafer. Said optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical element units. In particular with mainly refractive systems, such optical element units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually comprise an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding one or more optical elements.

Optical element groups comprising at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical exposure units holding such optical element groups often have an elongated substantially tubular design due to which they are typically referred to as lens barrels.

Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor de- vices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system.

Among others, the above leads to very strict requirements with respect to the relative position between the components defining, among others, optical and functional planes of the exposure system and participating in the exposure process. Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the components of the optical system, i.e. the illumination sys- tern, the mask, the projection system and the wafer, for example, cooperating in the exposure process must be supported in a defined manner in order to provide and maintain a predetermined spatial relationship between said optical system components which, in turn, guarantees a high quality exposure process.

Among others due to the absorption of a part of the exposure light by the optical elements cooperating in the exposure process, a considerable amount of energy is introduced into these optical elements leading to a considerable heating of these optical elements and their surrounding components. Given the strict position tolerances between the components of the optical system, the thermal expansion of these components resulting from this heating process usually surpasses the tolerance "budget" of the system. Thus, countermeasures have to be taken in order to maintain the predetermined spatial relationship between the optical system components and the optical as well as functional planes of the optical system throughout the entire exposure process.

Several such countermeasures are well known. For example it is common to have one or several cooling circuits to stabilize the temperature of the components during the exposure process. However, this approach is costly and afflicted with a certain inertia. This is due to the fact that only relatively low flow speeds and a relatively low difference with respect to the temperature of the optical components may be established via the cooling medium in the path of the exposure light (which is the origin of the heating process). Otherwise turbulences in the path of the exposure light and excessive temperature gradients within the op- tical components (leading to a distortion of the components) would deteriorate the optical performance of the system.

Furthermore, compensation for these temperature induced alterations is known by actively adjusting the position of components of the optical system. However, such active position- ing is costly and rather intended to compensate for dynamic, vibration induced position alterations of a different frequency and amplitude range. Thus, devices providing active position adjustment in both frequency and amplitude ranges are rather costly.

Furthermore, it is known from WO 2004/038481 A1 (Weber et al.), the disclosure of which is incorporated herein by reference, to support single optical elements (here: a beam splitter) in such am manner that an optical plane (here: the plane of the beam splitting surface) defined by this optical element maintains its position upon thermal expansion of the optical element. While this may be a feasible approach for single components (such as the beam splitter disclosed), supporting a majority or all of the optical elements would be very costly. This is due to the fact that, at lest for a considerable fraction of the optical elements, the support elements required to provide this kind of support would also have to be adapted to other dynamic and adjustability requirements.

SUMMARY OF THE INVENTION

It is thus an object of the invention to, at least to some extent, overcome the above disad- vantages and to provide good and long term reliable imaging properties of an optical system used in an exposure process.

It is a further object of the invention to reduce the effort necessary for an optical system used in an exposure process while at least maintaining the imaging accuracy of the optical system used in an exposure process.

It is a further object of the invention to reduce the effort necessary for the compensation of thermal expansion effects in an optical system used in an exposure process while at least maintaining the imaging accuracy of the optical system used in an exposure process.

These objects are achieved according to the invention which is based on the teaching that a reduction of the effort necessary for the compensation of thermal expansion effects in an optical system while at least maintaining the imaging accuracy of the optical system may be achieved if an optical element unit comprising an optical element group and at least one optical element unit reference associated to the optical element group is supported on a support structure such that, upon thermal expansion of at least one of the optical element unit and the support structure, a reference position between the optical element unit refer- ence and an external reference external to the optical element unit, at least along an optical axis defined optical elements of the optical element group, remains substantially unchanged.

It has turned out that the spatial positioning of the optical element unit reference can be controlled, and thus, may be selected to be one of the optical planes (such as an object plane, an image plane) or functional planes (such as an aperture plane) of the optical element unit. Keeping the reference position of such an optical or functional plane substantially unchanged at least along the optical axis upon thermal expansion of the optical element unit and its support structure, respectively, has the advantage that at least a considerable part of the adverse effects arising with thermally induced expansion of the optical element unit may be compensated by supporting the optical element unit itself in a suitable manner while leaving the support of the optical elements of the optical element unit unchanged.

With the invention, in other words, a center of thermal expansion formed by the optical element unit reference of the optical element unit may be arbitrarily defined. The support to the optical element unit is selected such that the center of thermal expansion, at least along the optical axis of the optical element unit, does not change substantially its position with respect to a given external reference. This external reference may be a globally fixed reference or a variable reference, e.g. defined by another component of the optical system.

Thus, according to a first aspect of the invention there is provided an optical element ar- rangement comprising an optical element unit, a first support structure and an external reference being external to the optical element unit. The optical element unit comprises an optical element group, at least one optical element unit reference associated to the optical element group and a second support structure supporting the optical element group. The optical element group comprises a plurality of optical elements defining an optical axis of the optical element group. The optical element unit reference has a reference position with respect to the external reference. The first support structure supports the optical element unit via a plurality of support elements, the plurality of support elements being arranged to support the optical element unit such that, upon thermal expansion of at least one of the optical element unit and the first support structure, the reference position, at least along the optical axis, remains substantially unchanged.

According to a second aspect of the invention there is provided an optical imaging arrangement comprising a mask unit adapted to receive a pattern, a substrate unit adapted to receive a substrate, an optical projection unit adapted to transfer an image of the pattern onto the substrate and the optical element arrangement according to the first aspect of the invention forming part of an illumination unit adapted to illuminate the pattern received in the mask unit.

According to a third aspect of the invention there is provided a method of supporting an optical element unit, the method comprising providing an optical element unit and an external reference being external to the optical element unit, the optical element unit comprising an optical element group and at least one optical element unit reference associated to the optical element group, the optical element group comprising a plurality of optical elements defining an optical axis of the optical element group, the optical element unit reference having a reference position with respect to the external reference, and supporting the optical element unit on a support structure such that, upon thermal expansion of at least one of the optical element unit and the first support structure, a reference position between the optical element unit reference and the external reference, at least along the optical axis, remains substantially unchanged.

According to a fourth aspect of the invention there is provided an optical imaging method comprising providing a pattern, a substrate, an optical projection unit adapted to transfer an image of the pattern onto the substrate and an illumination unit adapted to illuminate the pattern, the illumination unit comprising unit an optical element unit, supporting the optical element unit using the method according to the third aspect of the invention, and using the illumination unit to illuminate the pattern to transfer the image of the pattern onto the substrate.

Further aspects and embodiments of the invention will become apparent from the dependent claims and the following description of preferred embodiments which refers to the appended figures. All combinations of the features disclosed, whether explicitly recited in the claims or not, are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation of a preferred embodiment of an optical imaging arrangement according to the invention which comprises an optical element arrangement according to the invention and with which preferred embodiments of methods according to the invention may be executed; Figure 2 is a schematic sectional representation of a detail of the optical imaging arrangement along line M-Il of Figure 1 ;

Figure 3 is a schematic sectional representation of a further detail of the optical imaging arrangement along line Ill-Ill of Figure 2;

Figure 4 is a block diagram of a preferred embodiment of an optical imaging method according to the invention comprising a method of supporting an optical element unit which may be executed with the optical imaging arrangement of Figure 1 ;

Figure 5 is a schematic representation of a further preferred embodiment of an optical element arrangement according to the invention;

Figure 6 is a schematic representation of a further preferred embodiment of an optical element arrangement according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

First embodiment

In the following, a first preferred embodiment of an optical imaging arrangement 101 ac- cording to the invention which comprises an optical element arrangement in the form of an illumination system 102 according to the invention and with which preferred embodiments of methods according to the invention may be executed will be described with reference to Figures 1 to 3.

Figure 1 is a schematic and not-to-scale representation of the optical imaging arrangement in the form of an optical exposure apparatus 101. The optical exposure apparatus 101 comprises an optical projection unit 103 adapted to transfer an image of a pattern formed on a mask 104.1 of a mask unit 104 onto a substrate 105.1 of a substrate unit 105. To this end, the illumination system 106 illuminates the mask 104. The optical projection unit 104 receives the light coming from the mask 104.1 and projects the image of the pattern formed on the mask 104.1 onto the substrate 105.1 , e.g. a wafer or the like.

The illumination system 102 comprises a light source 106, an optical element unit 107, an optical element unit 108 and an optical element 109. The light source 106 emits light to- wards the optical element unit 107 which are both supported on a ground structure 1 10. The optical element unit 107 guides the light towards the optical element unit 108 which is supported on a first support structure in the form of a so called metrology frame 11 1 which, in turn, is supported on the ground structure 1 10 via vibration isolating means 112. The op- tical element unit 108 guides the light towards the optical element 109 supported by the metrology frame 1 1 1. The optical element 109, in turn, guides the light towards the mask 104.1.

The optical element unit 108 forms a first optical element unit in the sense of the invention and comprises a plurality of first optical elements 108.1 forming a first optical element group 108.2 in the sense of the invention. The first optical elements 108.1 define a first optical axis 108.3 and an object plane 108.4 of the first optical element group 108.2 and, thus, of the first optical element unit 8. The object plane 108.4 of the first optical element group 108.2 hereby forms a first optical element unit reference of the first optical element unit 108.

The first optical elements 108.1 are received within a housing 108.5 of the first optical ele- ment unit 108. The housing 108.5 forms a second support structure supporting the first optical elements 108.1. As will be explained in further detail below, the first optical element unit 108 is supported on the metrology frame 1 10 via three support elements, a first support element 108.6, a second support element 108.7 and a third support element 108.8 (see Figure 2). It will be appreciated that the support elements 108.6, 108.7, 108.8 are only shown in a symbolical representation in Figure 1 and 2 while Figure 3 shows a schematic representation of a real embodiment of such a support element 108.6.

The optical element unit 107 forms a second optical element unit in the sense of the invention and comprises a plurality of second optical elements 107.1 forming a second optical element group 107.2 in the sense of the invention. The second optical elements 107.1 de- fine a second optical axis 107.3 and a image plane 107.4 of the second optical element group 107.2 and, thus, of the second optical element unit 8. The image plane 107.4 of the second optical element group 107.2 hereby forms a second optical element unit reference of the second optical element unit 8.

The second optical elements 107.1 are received within a housing 107.5 of the second opti- cal element unit 107. The housing 107.5 forms a fourth support structure supporting the second optical elements 107.1. As will be explained in further detail below, the second optical element unit 107 is supported on the metrology frame 1 10 via three support elements The optical projection unit 103 holds an optical element group 103.1 held within a housing 103.2 of the optical projection unit 103, often also referred to as the projection optics box (POB). The optical element group 103.1 comprises a number of optical elements 103.3 in the form of lenses, mirrors gratings etc. These optical elements 103.3 are positioned with respect to one another along an axis 103.4 of the optical projection unit 103 in up to all six degrees of freedom.

The optical elements 103.3 cooperate to transfer the image of the pattern formed on the mask 104.1 onto the substrate 105.1. The mask 104.1 is received on a mask table 104.2 of the mask unit 104, the mask table 104.2 being supported by the metrology frame 1 1 1. In a similar way, the substrate 105.1 is received on a substrate table 105.2 of the substrate unit 105, the substrate table 105.2 as well being supported by the metrology frame 1 1 1.

The image of the pattern formed on the mask 104.1 is usually reduced in size and transferred to several target areas of the substrate 105.1. The image of the pattern formed on the mask 104.1 may be transferred to the respective target area on the substrate 105.1 in two different ways depending on the design of the optical exposure apparatus 101. If the optical exposure apparatus 101 is designed as a so called wafer stepper apparatus, the entire image of the pattern is transferred to the respective target area on the substrate 105.1 in one single step by irradiating the entire pattern formed on the mask 104.1. If the optical exposure apparatus 101 is designed as a so called step-and-scan apparatus, the image of the pattern is transferred to the respective target area on the substrate 105.1 by progressively scanning the mask table 104.2 and thus the pattern formed on the mask 104.1 under the projection beam while performing a corresponding scanning movement of the substrate table 105.2 and, thus, of the substrate 105.1 at the same time.

In both cases, the relative position of the optical elements 103.3, 107.1 , 108.1 and 109 with respect to each other as well as with respect to the mask 104.1 and with respect to the substrate 105.1 has to be maintained within predetermined limits to obtain a high quality imaging result. Hereby it is of particular importance to keep certain optical or functional references, such as the image plane 107.4 and the object plane 108.4 or an aperture plane, in a given reference position with respect to other given references.

For the first optical element unit reference 108.4, i.e. the object plane 108.4, it has turned out to be beneficial that its reference position with respect to an external reference defined by the metrology frame 1 1 1 be kept substantially constant upon thermal expansion of the first optical element unit 108. Here, the external reference may be any suitable point on the metrology frame 1 1 1 external to the first optical element unit 108.

To achieve this, the first support element 108.6, the second support element 108.7 and the third support element 108.8 are designed and located such that - within certain limits -each of them provides for substantially unrestricted movement along one translational direction. Thus, the first support element 108.6 provides for substantially unrestricted movement along a first translational direction 108.9. The second support element 108.7 provides for substantially unrestricted movement along a second translational direction 108.10. The third support element 108.8 provides for substantially unrestricted movement along a third translational direction 108.1 1.

At any point in time, the respective support element 108.6, 108.7, 108.8 provides for restriction of movement along the other two translational directions forming an orthogonal system with the unconstrained translational direction 108.9, 108.10, 108.1 1 , respectively. In other words, the respective support element 108.6, 108.7, 108.8 provides for one unconstrained translational degree of freedom and two constrained translational degrees of freedom.

It will be appreciated that the respective support element 108.6, 108.7, 108.8 preferably provides one or more unconstrained rotational degrees of freedom in order to allow substantially kinematic support of the first optical element unit 108.

One embodiment of the support elements 108.6, 108.7, 108.8 can be seen in a schematic way from the first support element 108.6 shown in Figure 3. Here, the first support element 108.6 comprises a leaf spring element 108.12 that is compliant in the first translational direction 108.9 and substantially rigid in the plane 108.13 perpendicular to this first translational direction 108.9.

Depending on the desired displacement, the leaf spring element 108.12 exerts an increas- ing counterforce counteracting the displacement along the first translational direction 108.9. However, it will be appreciated that, in the sense of the invention, this is negligible as long as this counterforce is considerably smaller than the counterforce counteracting excursions within the plane 108.13 perpendicular to this first translational direction 108.9. In other words, it is sufficient that the rigidity of the first support element 108.6 along the first trans- lational direction 108.9 is considerably smaller than the rigidity within the plane 108.13 perpendicular to this first translational direction 108.9. It will be further appreciated that, with other embodiments of the invention, other types of support elements may be chosen, such as a ball or pin running in a (preferably V-shaped) groove, a pin movable back and forth in a mating hole or other linear guide mechanisms such as dovetail guides etc.

As can be seen from Figure 2, the first optical axis 108.3 and the first translational direction 108.9 are parallel and define an axial plane 108.14. The first translational direction 108.9, the second translational direction 108.10 and the third translational direction 108.1 1 are coplanar and all intersect at an intersection location 108.15. This intersection location 108.15 is located at the object plane 108.4 of the first optical element group 108.2 received within the first optical element unit 108 (both indicated by dashed contours in Figure 2). The second translational direction 108.10 and the third translational direction 108.1 1 are both inclined with respect to the axial plane 108.14 by an angle Y₁ = γ₂ = γ.

This symmetric arrangement of the second translational direction 108.10 and the third translational direction 108.11 and the common intersection location 108.15 have the ad- vantage that, upon uniform thermally induced expansion of the first optical element unit 108 - i.e. upon a uniform temperature and temperature rise within the first optical element unit 108 and upon substantially identical coefficients of thermal expansion α_x = oc_y = α along and transverse to the first optical axis 108.3 - the location of the intersection location 108.15 does not change substantially with respect to the external reference defined by the metrol- ogy frame 11 1. As a consequence, at least in a good approximation, also the object plane 108.4 of the first optical element group 108.2, along the first optical axis 108.3, substantially maintains its position with respect to the external reference, i.e. the metrology frame 1 1 1.

In other words, the support elements 108.6, 108.7, 108.8, via the intersection location 108.15 of their unconstrained degrees of freedom, define a center of thermal expansion of the first optical element unit 108 that maintains its position with respect to the metrology frame 1 1 1.

With the arrangement of the support elements 108.6, 108.7, 108.8 shown in Figure 1 and 2, the first optical axis 108.3, upon thermally induced expansion of the optical element unit 108, undergoes a lateral shift in the z direction (see Figure 1 ). However, it will be appreci- ated that, with other embodiments of the invention, the support elements may be positioned such that the first, second and third translational direction are located close to the first optical axis, in some cases even coplanar with the first optical axis, such that this lateral shift may be at least reduced. Furthermore, additionally or alternatively, the support elements may be designed to compensate for this lateral shift of the first optical axis.

It will be further appreciated that, with other embodiments of the invention, the first, second and third translational directions do not have to be coplanar but may be arbitrarily located in space as long as the common intersection location of all these translational directions is maintained. In particular, it will be appreciated that the angles γi and γ₂ do not necessarily have to be the same. It will be further appreciated that, with other embodiments of the invention, more support elements than only the first, second and third support element with their first, second and third translational direction may be provided as long as the common intersection location of all these unconstrained translational directions of all these support elements is maintained.

It will be further appreciated that the above explanations have, first of all, been given under the assumption that the first optical element unit 108 has substantially identical coefficients of thermal expansion α_x = oc_y = α along and transverse to the first optical axis 108.3. How- ever, if this is not true, i.e. if there is a difference between the coefficient of thermal expansion Oc_x along the first optical axis 108.3 and the coefficient of thermal expansion oc_y transverse to the first optical axis 108.3, the support elements 108.6, 108.7, 108.8 may simply account for this by modifying the angle Y₁ = γ₂ = γ along their thermally induced displacement as it is indicated in Figure 2 by the dashed contours 1 13, 1 14. In other words, in this case, the angles γi = γ₂ = γ are a function of the respective coefficient of thermal expansion Oc_x, OC_y and the desired reference position.

Furthermore, it will be appreciated that such a directional difference between the coefficient of thermal expansion oc_x along the first optical axis 108.3 and the coefficient of thermal expansion OC_y transverse to the first optical axis 108.3 may be used to introduce a defined shift into the reference position between the object plane 108.4 of the first optical element unit 108 (or any other desired optical or functional reference of the first optical element unit 108) and the metrology frame 1 1 1. This may for example be used in case the metrology frame 1 1 1 itself undergoes a temperature change and, thus, thermally induced expansion.

This defined shift within the reference position between the object plane 108.4 of the first optical element unit 108 (or any other desired optical or functional reference of the first optical element unit 108) and the metrology frame 1 1 1 may be selected such that it substantially compensates for the thermally induced expansion of the metrology frame 1 1 1. It will be appreciated that, in this case, the shift with respect to the metrology frame 1 1 1 depends on the respective coefficient of thermal expansion α_x, oc_y and the angle Y₁ = γ₂ = γ. The angles Y₁ and γ₂ may thus be selected such that the desired shift with respect to the metrology frame 11 1 is achieved. Here as well directional differences in the coefficient of thermal ex- pansion of the metrology frame 1 11 may be taken into account and compensated for.

Thus, the reference position of the object plane 108.4 of the first optical element unit 108 (or any other desired optical or functional reference of the first optical element unit 108) with respect to a global external reference, e.g. the ground structure 1 10, may be kept unchanged upon thermally induced expansion of both the first optical element unit 108 and the metrology frame 1 1 1.

It will be appreciated that the support elements 107.6 of the second optical element unit 107 are designed in a manner similar to the support elements 108.6, 108.7, 108.8 of the first optical element unit 108. In particular, they are designed such that, at least in a good approximation, also the image plane 107.4 of the second optical element group 107.2, along the exit part of the folded second optical axis 107.3, substantially maintains its position with respect to an external reference in the form of the ground structure 1 10.

In other words, here as well, the support elements 107.6 via the intersection location of their unconstrained degrees of freedom, define a center of thermal expansion of the second optical element unit 107 that keeps its position with respect to the ground structure 1 10.

With the present example it is assumed that the metrology frame 1 11 has a fixed relation to the ground structure 1 10 and has a cooling system preventing considerable thermally induced expansion. Thus, given the above support of the first optical element unit 108 and the second optical element unit 107, the position of the image plane 107.4 of the second optical element unit 107 substantially coincides with the object plane 108.4 of the first opti- cal element unit 108 at any temperature expected during normal operation of the apparatus 101 ,. This has a beneficial effect on the illumination of the mask 104.1 and, thus, on the result of the exposure process.

With the optical exposure apparatus 101 of Figure 1 a preferred embodiment of an optical imaging method according to the invention comprising a method of supporting an optical element unit according to the invention may be executed as it will be described below with reference to Figures 1 to 4. In a step 1 15, the components of the optical exposure apparatus 101 including the mask 104.1 with a pattern, the substrate 105.1 , the optical projection unit 103 adapted to transfer an image of the pattern of the mask 104.1 onto the substrate 105.1 and the illumination system 106, 107, 108, 109 adapted to illuminate the pattern of the mask 104.1 and com- prising the first optical element unit 108 are provided.

In a step 116, the components of the optical exposure apparatus 101 are put into a spatial relation to provide the configuration as it has been described in the context of Figures 1 to 3. In particular, in step 1 16, the first optical element unit 108 is supported on the metrology frame 1 11 such that, upon thermally induced expansion of the first optical element unit 108, the reference position of the object plane 108.4 with respect to the metrology frame 1 1 1 remains essentially unchanged as it has been described above.

In a step 1 16, the illumination system 106, 107, 108, 109 is used to illuminate the pattern of the mask 104.1 , such that the optical projection unit 103 transfers an image of the pattern of the mask 104.1 onto the substrate 105.1 as it has been described above.

Second embodiment

In the following, a second preferred embodiment of an optical imaging arrangement 201 according to the invention with which preferred embodiments of methods according to the invention may be executed will be described with reference to Figure 5.

Figure 5 is a schematic and not-to-scale representation of a first optical element unit 208 and a second optical element unit 207 of the illumination system of an optical imaging arrangement in the form of an optical exposure apparatus 201.

The embodiment of Figure 5, in its design and functionality, largely corresponds to the embodiment of Figure 1. In particular, in Figure 5, similar or identical parts have been given the same reference numeral increased by 100. Thus, it is here mainly referred to the explana- tions given above and, primarily, only the differences will be discussed.

The main difference with respect to the first embodiment lies within the fact that the first optical element unit 208 and the second optical element unit 207 are both supported on the metrology frame 21 1. Again, the first optical element unit 208 comprises a plurality of first optical elements 208.1 forming a first optical element group 208.2. The first optical elements 208.1 define a first optical axis 208.3 and an object plane 208.4 of the first optical element group 208.2 and, thus, of the first optical element unit 8. The object plane 208.4 of the first optical element group 208.2 hereby forms a first optical element unit reference of the first optical element unit 208.

The first optical elements 208.1 are received within a housing 208.5 of the first optical element unit 208. The housing 208.5 forms a second support structure supporting the first optical elements 208.1. The first optical element unit 208.1 is supported on the metrology frame 210 via a first support element 208.6, a second support element 208.7 and a third support element 208.8 (all shown in a symbolical representation in Figure 5).

The second optical element unit 207 comprises a plurality of second optical elements 207.1 forming a second optical element group 207.2. The second optical elements 207.1 define a second optical axis 207.3 and an image plane 207.4 of the second optical element group 207.2 and, thus, of the second optical element unit 8. The image plane 207.4 of the second optical element group 207.2 hereby forms a second optical element unit reference of the second optical element unit 8.

The second optical elements 207.1 are received within a housing 207.5 of the second optical element unit 207. The housing 207.5 forms a second support structure supporting the second optical elements 207.1. The second optical element unit 207.1 is supported on the metrology frame 210 via a fourth support element 207.6, a fifth support element 207.7 and a sixth support element 207.8 (all shown in a symbolical representation in Figure 5).

To achieve that the reference position of the object plane 208.4 with respect to the metrology frame 21 1 remains substantially unchanged upon thermally induced expansion of the first optical element unit 208, the first support element 208.6, the second support element 208.7 and the third support element 208.8 are designed and located such that - within certain limits - each of them provides for substantially unrestricted movement along one translational direction. Thus, the first support element 208.6 provides for substantially unrestricted movement along a first translational direction 208.9. The second support element 208.7 provides for substantially unrestricted movement along a second translational direction 208.10. The third support element 208.8 provides for substantially unrestricted movement along a third translational direction 208.1 1. To achieve that the reference position of the image plane 207.4 with respect to the metrology frame 21 1 remains substantially unchanged upon thermally induced expansion of the second optical element unit 207, the fourth support element 207.6, the fifth support element

207.7 and the sixth support element 207.8 are designed and located such that - within certain limits - each of them provides for substantially unrestricted movement along one translational direction. Thus, the fourth support element 207.6 provides for substantially unrestricted movement along a fourth translational direction 207.9. The fifth support element 207.7 provides for substantially unrestricted movement along a fifth translational direction 207.10. The sixth support element 207.8 provides for substantially unrestricted movement along a sixth translational direction 207.1 1.

At any point in time, the respective support element 207.6, 207.7, 207.8, 208.6, 208.7,

208.8 provides for restriction of movement along the other two translational directions forming an orthogonal system with the unconstrained translational direction 207.9, 207.10, 207.1 1 , 208.9, 208.10, 208.1 1 , respectively. In other words, the respective support element 207.6, 207.7, 207.8, 208.6, 208.7, 208.8 provides for one unconstrained translational degree of freedom and two constrained translational degrees of freedom.

It will be appreciated that the respective support element 207.6, 207.7, 207.8, 208.6, 208.7, 208.8 preferably provides one or more unconstrained rotational degrees of freedom in order to allow substantially statically determined support of the first optical element unit 208 and the second optical element unit 207, respectively.

The real design of the support elements 207.6, 207.7, 207.8, 208.6, 208.7, 208.8 can be similar to the one of the first support element 108.6 shown in Figure 3. However, with other embodiments of the invention, other types of support elements may be chosen, such as a ball or pin running in a (preferably V-shaped) groove, a pin movable back and forth in a mating hole or other linear guide mechanisms such as dovetail guides etc.

In the embodiment shown in Figure 5, the first optical axis 208.3 and the first translational direction 208.9 coincide. The first translational direction 208.9, the second translational direction 208.10 and the third translational direction 208.1 1 are coplanar and all intersect at an intersection location 208.15. This intersection location 208.15 is located at the object plane 208.4 of the first optical element group 208.2 received within the first optical element unit 208 (both only indicated by dashed contours in Figure 5). The second translational direction 208.10 and the third translational direction 208.11 are both inclined with respect to the first optical axis 208.3 by an angle Y₁ = J₂. This symmetric arrangement of the second translational direction 208.10 and the third translational direction 208.11 and the common intersection location 208.15 have the advantage that, upon uniform thermally induced expansion of the first optical element unit 208 - i.e. upon a uniform temperature and temperature rise within the first optical element unit 208 and upon substantially identical coefficients of thermal expansion α_x = oc_y = α along and transverse to the first optical axis 208.3 - the location of the intersection location 208.15 does not change substantially with respect to the external reference defined by the metrology frame 21 1. As a consequence, at least in a good approximation, also the object plane 208.4 of the first optical element group 208.2, along the first optical axis 208.3, substantially maintains its position with respect to the external reference, i.e. the metrology frame 21 1.

In other words, the support elements 208.6, 208.7, 208.8, via the intersection location 208.15 of their unconstrained degrees of freedom, define a center of thermal expansion of the first optical element unit 208 that maintains its position with respect to the metrology frame 21 1.

Due to the coplanar arrangement of the support elements 208.6, 208.7, 208.8 and the first optical axis 208.3, upon thermally induced expansion of the optical element unit 208, no lateral shift of the first optical axis 208.3 in the z direction, i.e. perpendicular to the plane of the drawing occurs.

Furthermore, the second optical axis 207.3 and the fourth translational direction 207.9 coin- cide. The fourth translational direction 207.9, the fifth translational direction 207.10 and the sixth translational direction 207.1 1 are coplanar and all intersect at an intersection location 207.15. This intersection location 207.15 is located at the image plane 207.4 of the second optical element group 207.2 received within the second optical element unit 207 (both only indicated by dashed contours in Figure 5). The fifth translational direction 207.10 and the sixth translational direction 207.1 1 are both inclined with respect to the second optical axis 207.3 by an angle γ₃ = γ₄.

This symmetric arrangement of the fifth translational direction 207.10 and the sixth translational direction 207.11 and the common intersection location 207.15 have the advantage that, upon uniform thermally induced expansion of the second optical element unit 207 - i.e. upon a uniform temperature and temperature rise within the second optical element unit 207 and upon substantially identical coefficients of thermal expansion α_x = oc_y = α along and transverse to the second optical axis 207.3 - the location of the intersection location 207.15 does not change substantially with respect to the external reference defined by the metrol- ogy frame 21 1. As a consequence, at least in a good approximation, also the image plane 207.4 of the second optical element group 207.2, along the second optical axis 207.3, substantially maintains its position with respect to the external reference, i.e. the metrology frame 21 1.

In other words, the support elements 207.6, 207.7, 207.8, via the intersection location

207.15 of their unconstrained degrees of freedom, define a center of thermal expansion of the second optical element unit 207 that keeps its position with respect to the metrology frame 21 1.

Due to the coplanar arrangement of the support elements 207.6, 207.7, 207.8 and the sec- ond optical axis 207.3, upon thermally induced expansion of the second optical element unit 207, no lateral shift of the second optical axis 207.3 in the z direction, i.e. perpendicular to the plane of the drawing occurs.

As can be seen from Figure 5, the first intersection location 208.15 and the second intersection location 207.15 substantially coincide such that the image plane 207.4 of the sec- ond optical element unit 207 and the object plane 208.4 of the first optical element unit 207 substantially coincide at any temperature distribution expected during normal operation of the optical exposure apparatus 201. This leads to a good, stable and reliable illumination result of the optical exposure apparatus 201.

It will be appreciated that, with this embodiment as well, the methods according to the in- vention as they have been described above with reference to Figure 1 to 4 may be executed as well. Thus, in this context, it is here only referred to the above explanations.

Third embodiment

In the following, a third preferred embodiment of an optical imaging arrangement 201 according to the invention with which preferred embodiments of methods according to the in- vention may be executed will be described with reference to Figure 4.

Figure 6 is a schematic and not-to-scale representation of a first optical element unit 308 of the illumination system of an optical imaging arrangement in the form of an optical exposure apparatus 301. The embodiment of Figure 6, in its design and functionality, largely corresponds to the embodiment of Figure 1. In particular, in Figure 6, like or identical parts have been given the same reference numeral increased by 200. Thus, it is here mainly referred to the explanations given above and, primarily, only the differences will be discussed.

Again, the first optical element unit 308 comprises a plurality of first optical elements 308.1 forming a first optical element group 308.2. The first optical elements 308.1 define a first optical axis 308.3 of the first optical element group 308.2 and, thus, of the first optical element unit 8. The first optical element unit 8 has a functional plane in the form of an aperture plane 308.4 located between first optical elements 308.1 of the first optical element group 308.2. The aperture plane 308.4 of the first optical element group 308.2 hereby forms a first optical element unit reference of the first optical element unit 308.

The first optical elements 308.1 are received within a housing 308.5 of the first optical element unit 308. The housing 308.5 forms a second support structure supporting the first optical elements 308.1. The first optical element unit 308.1 is supported on the metrology frame 311 via a first support element 308.6, a second support element 308.7 and a third support element 308.8 (all shown in a symbolical representation in Figure 6).

To achieve that the reference position of the aperture plane 308.4 with respect to the metrology frame 31 1 remains substantially unchanged upon thermally induced expansion of the first optical element unit 308, the first support element 308.6, the second support ele- ment 308.7 and the third support element 308.8 are designed and located such that - within certain limits - each of them provides for substantially unrestricted movement along one translational direction. Thus, the first support element 308.6 provides for substantially unrestricted movement along a first translational direction 308.9. The second support element 308.7 provides for substantially unrestricted movement along a second translational direc- tion 308.10. The third support element 308.8 provides for substantially unrestricted movement along a third translational direction 308.1 1.

At any point in time, the respective support element 308.6, 308.7, 308.8 provides for restriction of movement along the other two translational directions forming an orthogonal system with the unconstrained translational direction 308.9, 308.10, 308.1 1 , respectively. In other words, the respective support element 308.6, 308.7, 308.8 provides for one unconstrained translational degree of freedom and two constrained translational degrees of freedom. It will be appreciated that the respective support element 308.6, 308.7, 308.8 preferably provides one or more unconstrained rotational degrees of freedom in order to allow substantially statically determined support of the first optical element unit 308.

The real design of the support elements 308.6, 308.7, 308.8 again can be similar to the one of the first support element 108.6 shown in Figure 3. However, with other embodiments of the invention, other types of support elements may be chosen, such as a ball or pin running in a (preferably V-shaped) groove, a pin movable back and forth in a mating hole or other linear guide mechanisms such as dovetail guides etc.

In the embodiment shown in Figure 6, the first optical axis 308.3 and the first translational direction 308.9 coincide. The first translational direction 308.9, the second translational direction 308.10 and the third translational direction 308.1 1 are coplanar and all intersect at an intersection location 308.15. This intersection location 308.15 is located at the aperture plane 308.4 of the first optical element group 308.2 received within the first optical element unit 308 (both only indicated by dashed contours in Figure 6). The second translational di- rection 308.10 and the third translational direction 308.11 are both inclined with respect to the first optical axis 308.3 by an angle Y₁ = γ₂ = γ.

This symmetric arrangement of the second translational direction 308.10 and the third translational direction 308.11 and the common intersection location 308.15 have the advantage that, upon uniform thermally induced expansion of the first optical element unit 308 - i.e. upon a uniform temperature and temperature rise within the first optical element unit 308 and upon substantially identical coefficients of thermal expansion α_x = oc_y = α along and transverse to the first optical axis 308.3 - the location of the intersection location 308.15 does not change substantially with respect to the external reference defined by the metrology frame 31 1. As a consequence, at least in a good approximation, also the aperture plane 308.4 of the first optical element group 308.2, along the first optical axis 308.3, substantially maintains its position with respect to the external reference, i.e. the metrology frame 31 1.

In other words, the support elements 308.6, 308.7, 308.8, via the intersection location 308.15 of their unconstrained degrees of freedom, define a center of thermal expansion of the first optical element unit 308 that keeps its position with respect to the metrology frame 31 1. Due to the coplanar arrangement of the support elements 308.6, 308.7, 308.8 and the first optical axis 308.3, upon thermally induced expansion of the optical element unit 308, no lateral shift of the first optical axis 308.3 in the z direction, i.e. perpendicular to the plane of the drawing occurs.

Keeping such an aperture plane 308.4 at a constant reference position may also be beneficial for the imaging performance of the optical exposure apparatus 301.

It will be appreciated that, with this embodiment as well, the methods according to the invention as they have been described above with reference to Figure 1 to 4 may be executed as well. Thus, in this context, it is here only referred to the above explanations.

In the foregoing, embodiments of the invention have been described where the second translational direction and the third translational direction are both inclined with respect to the axial plane by an angle Y₁ = γ₂ = γ. However, it will be further appreciated and it is to be emphasized that, with other embodiments of the invention, the first, second and third translational directions do not have to have any specific orientation but may be arbitrarily located in space as long as a common intersection location of all these translational directions is maintained. In particular, it will be appreciated that the angles γi and γ₂ do not necessarily have to be the same. It will be further appreciated that, with other embodiments of the invention, more support elements than only the first, second and third support element with their first, second and third translational direction may be provided as long as the common intersection location of all these unconstrained translational directions of all these support elements is maintained.

Although, in the foregoing, embodiments of the invention have been described where the optical elements are exclusively refractive elements, it will be appreciated that, with other embodiments of the invention, reflective, refractive or diffractive elements or any combina- tions thereof may be used for the optical elements of the optical element units.

Furthermore, it will be appreciated that, with other embodiments of the invention, any optical element unit reference other than an image plane, an object plane or an aperture plane may be selected and kept at a defined reference position upon thermally induced expansion of the respective associated optical element unit.

Although, in the foregoing, embodiments of the invention have been described where the optical element units are exclusively part of an illumination system, it will be appreciated that the invention may also be used in the context of supporting other optical element units, e.g. optical element units of a projection system etc. Furthermore, it will be appreciated that the invention may also be used in the context of optical applications other than microlithogra- phy.

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Claims

What is claimed is:

1. An optical element arrangement comprising: an optical element unit, a first support structure and - an external reference being external to said optical element unit; said optical element unit comprising an optical element group, at least one optical element unit reference associated to said optical element group and a second support structure supporting said optical element group; said optical element group comprising a plurality of optical elements defining an optical axis of said optical element group; said optical element unit reference having a reference position with respect to said external reference; said first support structure supporting said optical element unit via a plurality of support elements; - said plurality of support elements being arranged to support said optical element unit such that, upon thermal expansion of at least one of said optical element unit and said first support structure, said reference position, at least along said optical axis, remains substantially unchanged.

2. The optical element arrangement according to claim 1 , wherein said plurality of sup- port elements comprises at least three support elements.

3. The optical element arrangement according to claim 2, wherein each of said at least three support elements provides for substantially unrestricted movement along a translational direction.

4. The optical element arrangement according to claim 3, wherein at least one of said at least three support elements has at least one component that is compliant along said translational direction and substantially rigid orthogonally to said translational direction.

5. The optical element arrangement according to claim 4, wherein said at least one component comprises a leaf spring element.

6. The optical element arrangement according to claim 3, wherein at least one of said at least three support elements provides at least one unconstrained rotational de- gree of freedom.

7. The optical element arrangement according to claim 3, wherein said translational directions of said at least three support elements all intersect at one intersection location.

8. The optical element arrangement according to claim 3, wherein each of said at least three support elements provides for substantially unrestricted movement only along said translational direction.

9. The optical element arrangement according to claim 3, wherein said translational directions of said at least three support elements are arranged to be substantially coplanar.

10. The optical element arrangement according to claim 3, wherein said at least three support elements comprise a first support element, a second support element and a third support element; said first support element providing for substantially unrestricted movement along a first translational direction; - said second support element providing for substantially unrestricted movement along a second translational direction; said third support element providing for substantially unrestricted movement along a third translational direction; said first translational direction being substantially parallel to said optical axis; - said first translational direction and said optical axis defining an axial plane; at least one of said second translational direction and said third translational direction being inclined with respect to said axial plane.

1 1. The optical element arrangement according to claim 7, wherein said second translational direction is inclined with respect to said axial plane by an angle; said optical element unit having a first longitudinal coefficient of thermal expan- sion along said optical axis and a first transverse coefficient of thermal expansion transverse to said optical axis said first support structure having a second longitudinal coefficient of thermal expansion along said optical axis and a second transverse coefficient of thermal expansion transverse to said optical axis; - said first angle being adapted to compensate for a difference between at least two of said first longitudinal coefficient of thermal expansion, said first transverse coefficient of thermal expansion, said second longitudinal coefficient of thermal expansion and said second transverse coefficient of thermal expansion.

12. The optical element arrangement according to claim 1 1 , wherein said angle is a function of said difference between at least two of said first longitudinal coefficient of thermal expansion, said first transverse coefficient of thermal expansion, said second longitudinal coefficient of thermal expansion and said second transverse coefficient of thermal expansion and the desired reference location.

13. The optical element arrangement according to claim 1 1 , wherein - said external reference has a fixed relation to said first support structure and said angle is a function of the ratio between said first longitudinal coefficient of thermal expansion and said first transverse coefficient of thermal expansion.

14. The optical element arrangement according to claim 1 , wherein said optical element unit reference is one of a functional plane and an optical plane of said optical ele- ment unit.

15. The optical element arrangement according to claim 14, wherein said optical element unit reference is one of an object plane of said optical element group, an image plane of said optical element group a focus point of said optical element group and an aperture plane of said optical element unit.

16. The optical element arrangement according to claim 1 , wherein said optical element unit forms at least a part of an illumination system of an optical exposure apparatus.

17. The optical element arrangement according to claim 1 , wherein said optical element unit is a first optical element unit, said optical element group is a first optical element group and said optical element unit reference is a first optical element unit reference, and a second optical element unit is associated to said first optical element unit; said second optical element unit comprising a second optical element group, said second optical element group having a second optical element unit reference forming said external reference.

18. The optical element arrangement according to claim 17, wherein said first optical element unit reference is one of a functional plane and an optical plane of said first optical element unit and said second optical element unit reference is one of a functional plane and an optical plane of said second optical element unit.

19. The optical element arrangement according to claim 17, wherein said second optical element unit is supported by a third support structure; said third support structure supporting said first support structure.

20. The optical element arrangement according to claim 17, wherein - said first optical element unit reference is an object plane of said first optical element group and said second optical element unit reference is an image plane of said second optical element group, or said first optical element unit reference is an image plane of said first optical element group and said second optical element unit reference is an object plane of said second optical element group.

21. The optical element arrangement according to claim 17, wherein said second optical element unit forms at least a part of an illumination system of an optical exposure apparatus.

22. The optical element arrangement according to claim 1 , wherein - said optical element unit is a first optical element unit, said optical element group is a first optical element group, said optical axis is a first optical axis, said optical element unit reference is a first optical element unit reference, said reference position is a first reference position and said plurality of support elements is a first plurality of first support elements, and - a second optical element unit and a third support structure is provided; said optical element unit comprising a second optical element group, at least one second optical element unit reference associated to said second optical element group and a fourth support structure supporting said optical element group; said second optical element group comprising at least one second optical ele- ment defining an optical axis of said optical element group; said second optical element unit reference having a second optical reference position with respect to said external reference; said third support structure supporting said optical element unit via a second plurality of second support elements; - said second plurality of second support elements being arranged to support said second optical element unit such that, upon thermal expansion of at least one of said second optical element unit and said third support structure, said second optical reference position, at least along said second optical axis, remains substantially unchanged.

23. The optical element arrangement according to claim 22, wherein said second optical element unit forms at least a part of an illumination system of an optical exposure apparatus.

24. An optical imaging arrangement comprising a mask unit adapted to receive a pattern, - a substrate unit adapted to receive a substrate; an optical projection unit adapted to transfer an image of said pattern onto said substrate; the optical element arrangement according to claim 1 forming part of an illumination unit adapted to illuminate said pattern received in said mask unit.

25. A method of supporting an optical element unit, said method comprising the steps of: providing an optical element unit and an external reference being external to said optical element unit; said optical element unit comprising an optical element group and at least one optical element unit reference associated to said optical element group; said optical element group comprising a plurality of optical elements defining an optical axis of said optical element group; said optical element unit reference having a reference position with respect to said external reference; - supporting said optical element unit on a support structure such that, upon thermal expansion of at least one of said optical element unit and said first support structure, said reference position, at least along said optical axis, remains substantially unchanged.

26. The method according to claim 25, wherein said supporting said optical element unit comprises supporting said optical element unit at at least three support locations.

27. The method according to claim 26, wherein said supporting said optical element unit comprises providing for substantially unconstrained movement along a translational direction at each of said support locations.

28. The method according to claim 27, wherein said supporting said optical element unit comprises providing at least one support element having at least one component that is compliant along said translational direction and substantially rigid transverse to said translational direction.

29. The method according to claim 28, wherein at least one of said components comprises a leaf spring element.

30. The method according to claim 27, wherein said supporting said optical element unit comprises providing at least one unconstrained rotational degree of freedom at at least one of said support locations.

31. The method according to claim 27, wherein said translational directions all intersect at one intersection location.

32. The method according to claim 27, wherein said supporting said optical element unit comprises providing for substantially unrestricted movement only along said translational direction at each of said support locations.

33. The method according to claim 27, wherein said translational directions at said sup- port locations are arranged to be substantially coplanar.

34. The method according to claim 27, wherein said supporting said optical element unit comprises providing for substantially unrestricted movement along a first translational direction, along a second translational direction and along a third translational direction; - said first translational direction is substantially parallel to said optical axis; said first translational direction and said optical axis define an axial plane; at least one of said second translational direction and said third translational direction are inclined with respect to said axial plane.

35. The method according to claim 34, wherein - said second translational direction is inclined with respect to said axial plane by an angle; said optical element unit has a first longitudinal coefficient of thermal expansion along said optical axis and a first transverse coefficient of thermal expansion transverse to said optical axis - said support structure has a second longitudinal coefficient of thermal expansion along said optical axis and a second transverse coefficient of thermal expansion transverse to said optical axis; said angle is adapted to compensate for a difference between at least two of said first longitudinal coefficient of thermal expansion, said first transverse coefficient of thermal expansion, said second longitudinal coefficient of thermal expansion and said second transverse coefficient of thermal expansion.

36. The method according to claim 35, wherein said angle is a function of said difference between at least two of said first longitudinal coefficient of thermal expansion, said first transverse coefficient of thermal expansion, said second longitudinal coefficient of thermal expansion and said second transverse coefficient of thermal expansion.

37. The method according to claim 35, wherein said external reference has a fixed relation to said support structure and said angle is a function of a ratio of said first longitudinal coefficient of thermal expansion and said first transverse coefficient of thermal expansion.

38. The method according to claim 25, wherein said optical element unit reference is one of a functional plane and an optical plane of said optical element unit.

39. The method according to claim 38, wherein said optical element unit reference is one of an object plane of said optical element group, an image plane of said optical element group a focus point of said optical element group and an aperture plane of said optical element unit.

40. The method according to claim 25, wherein said optical element unit forms at least a part of an illumination system of an optical exposure apparatus.

41. The method according to claim 25, wherein said optical element unit is a first optical element unit, said optical element group is a first optical element group and said optical element unit reference is a first optical element unit reference, and a second optical element unit is associated to said first optical element unit; said second optical element unit comprising a second optical element group, said second optical element group having a second optical element unit reference forming said external reference.

42. The method according to claim 41 , wherein - said first optical element unit reference is one of a functional plane and an optical plane of said first optical element unit and said second optical element unit reference is one of a functional plane and an optical plane of said second optical element unit.

43. The method according to claim 41 , wherein - said second optical element unit is supported by a third support structure; said third support structure supports said first support structure.

44. The method according to claim 41 , wherein said first optical element unit reference is an object plane of said first optical element group and said second optical element unit reference is an image plane of said second optical element group, or said first optical element unit reference is an image plane of said first optical element group and said second optical element unit reference is an object plane of said second optical element group.

45. The method according to claim 41 , wherein said second optical element unit forms at least a part of an illumination system of an optical exposure apparatus.

46. The method according to claim 25, wherein said optical element unit is a first optical element unit, said optical element group is a first optical element group, said optical axis is a first optical axis, said optical element unit reference is a first optical element unit reference and said reference position is a first reference position, and a second optical element unit is provided; said optical element unit comprising a second optical element group and at least one second optical element unit reference associated to said second optical element group; said second optical element group comprising at least one second optical element defining an optical axis of said optical element group; said optical element unit is supported via a support structure such that, upon thermal expansion of at least one of said second optical element unit and said support structure, a second optical reference position between said second optical element unit reference and said external reference, at least along said second optical axis, remains substantially unchanged.

47. The method according to claim 46, wherein said second optical element unit forms at least a part of an illumination system of an optical exposure apparatus.

48. An optical imaging method comprising the steps of: providing a pattern, a substrate, an optical projection unit adapted to transfer an image of said pattern onto said substrate and an illumination unit adapted to illuminate said pattern, said illumination unit comprising unit an optical element unit; supporting said optical element unit using the method according to claim 25; using said illumination unit to illuminate said pattern to transfer said image of said pattern onto said substrate.

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