Description

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a damping device of an optical element of a pro- jection exposure machine. The invention further relates to an optical element for illuminating systems and projection systems of a projection exposure machine or for systems in which an optical element is exactly positioned with a damping device.

Description of the related prior art

Projection exposure machines for semiconductor lithography are used for the photolithographic production of semiconductor components and other finely structured devices. In this case, the pattern of a mask (reti- cle) is projected onto a substrate (wafer) usually coated with a photosensitive layer. To this end, a projection exposure machine comprises a projection objective with a plurality of optical elements such as lenses, mirrors, gratings or plane parallel plates, which are supported via mounts in an objective housing of the projection objective. The optical elements cause a refraction and/or a reflection of an electromagnetic radiation emitted by an illuminating device.

A mounting technology for optical elements of a projection objective is disclosed, for example, in US 6,229,657 B1. US 6,229,657 B1 discloses a mount comprising an inner ring permanently connected to the optical element, and an outer ring, the inner ring and the outer ring being inter- connected via joints, in particular via solid joints, distributed on the periphery.

So-called manipulators are used to compensate for aberrations. Such manipulators comprise an optical element and adjusting elements which are distributed on the circumference of the optical element and by means of which the optical element can be positioned and/or deformed. These manipulations of the optical element cause a change in the imaging which compensates an aberration of the entire system. In one re- finement, mounts for such manipulators comprise an inner ring or holder by means of which the optical element is accommodated, and an outer ring or a support device, it being possible to move the inner ring relative to the outer ring by means of the adjusting elements.

The joints and/or the adjusting elements decouple the optical elements, preferably from external structures, in terms of deformation. The connections between the optical element and the external structures preferably have a low stiffness.

In addition, imaging of ever smaller structures is required in microtech- nology. Larger and therefore also heavier optical elements can be used in order to satisfy these requirements. Both the low stiffness and the greater mass of the optical elements respectively lower the natural frequencies of the system. This increases the risk of exciting the optical elements to vibrations from outside.

It is therefore disclosed, for example, in WO 2006/084657 A1 , to provide a damper element between the outer ring and the inner ring.

Furthermore, it is disclosed in WO 2007/006577 A1 to provide a mass damper with an additional mass or a plurality of additional masses on the optical element, vibrational energy of the optical element being dissi- pated by friction by means of the additional mass(es). Such an additional mass is denoted below as damper mass. WO 2007/006577 A1 discloses fitting an annular damper mass on an optical element or an inner ring by means of a damper element. The damper element is arranged between the annular damper mass and the optical element or the inner ring and is, for example, a fibrous material such as paper, felt or an elastomer. Such mass dampers are also denoted as annular absorbers. A good vibration damping can be attained by means of annular absorbers for systems with one degree of freedom.

Further known are so-called segmented absorbers which are arranged spaced apart from one another distributed over the periphery of the optical element. Each segmented absorber comprises a damping mass and a damper element. Such segmented absorbers enable the vibration damping of a number of degrees of freedom.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a damping device for absorbing or dissipating the vibration energy of an optical element of a projection exposure machine which is excited to vibrations with a number of degrees of freedom. It is a further object of the invention to provide an optical element for a projection objective of a projection exposure machine, and a projection exposure machine with a damping device.

This and further objects are achieved by the subject matter having the features of Claims 1 , 10 and 13. Further advantages of the invention follow from the subclaims. The wording of all the claims is hereby incorporated by reference in the content of the description.

In accordance with a first aspect of the invention, a damping device of an optical element of a projection exposure machine, is provided which comprises at least two mass dampers arranged spaced apart from one another, the mass dampers each having at least one damper mass and at least one damping element, and the damper masses of the mass dampers being interconnected by means of at least one connecting ele- ment.

The element to be damped can have an arbitrary three dimensional restricted geometric form. For example, the element is fashioned in the shape of a lens or disc.

In the context of the invention, a "damping element" is designated as an element which has spring and/or damping properties. A spring element can be provided in a simple embodiment. The damping element preferably comprises a material which dissipates vibrational energy, for exam- pie, paper, felt and/or an elastomer. It has emerged in this case that a fluoroelastomer, in particular a fluororubber, for example Viton®, has good properties for dissipating vibrational energy. At the same time, a fluororubber is suitable for application in a projection objective because of the vacuum resistance and the good thermal and UV resistance. Other elastomers are also advantageous depending on application.

The natural frequencies of the mass dampers can be designed for a frequency to be eliminated. Given this frequency, the mass dampers draw vibrational energy from the structure, i.e. the optical element excited to vibration. The mass dampers act in each case as segmented absorbers, and so it is possible to damp a plurality of degrees of freedom. The damper masses of the mass dampers are interconnected such that the damper masses provide mutual support. In each case, mutually adjacent damper masses are preferably interconnected via a connecting element or a plurality of connecting elements. The support of the damper masses eliminates parasitic solid-state modes such as can occur with conventional segmented absorbers. In one refinement, the individual mass dampers are operationally connected to the element at attachment points distributed discretely over the periphery of the element to be damped. This renders it possible to target the placement of the mass dampers at vibration excitation points and/or points of maximum vibra- tion amplitudes. In one refinement, a plurality of discrete, i.e. spatially distributed, attachment points are provided for a damper mass.

In one advantageous refinement, a mass of a connecting element is less than the mass of a damper mass. As a result of this, the connecting elements have a high natural frequency and are not excited to flexural vibrations or are excited at least only in a high frequency range which is of little relevance to the associated system, for example, in a range of greater than 1000 Hz. In addition, or as an alternative, the natural frequency of the connecting element can be influenced by the stiffness thereof, in order to displace parasitic vibrational modes into an uncritical range, for example, a range of greater than 1000 Hz.

In a further refinement, at least two damper masses and a connecting element are designed as a conjoint component, in particular as a unipar- tite or integral component. The component is preferably fashioned as an annular element, in particular as a closed annular element. Such an annular element can be arranged in a simple way on the rotationally symmetrical element. In other refinements, other configurations of the component are provided which are matched to the shape of the element to be damped. In one advantageous refinement, a closed annular element is provided which has three damper masses and three connecting elements which are arranged in an alternating fashion. It is usual for three translational and three rotational vibrational modes to occur. These can be effectively absorbed by three damper masses in conjunction with a simple design of the system. The component preferably has at least one joint, in particular a solid joint, by means of which a damper mass and a connecting element adjacent thereto are decoupled for a vibration transmission. In the context of the invention a spot on a unipartite component which has reduced flexural stiffness at least as compared with the damper mass is denoted as "solid joint". A solid joint can be implemented in a simple manner in a unipartite component. It is preferred to provide a reduction in the cross section as solid joint, in particular a discontinuous or sudden reduction in the cross section. The reduction in the cross section is performed in one refinement in such a way that the connecting element and the solid joint are fashioned as segments of the component with the same cross section. In other refinements, the solid joint has a smaller cross section than the damper mass and than the connecting element. A reduction in the cross section is preferably implemented by a variation in the height of the annular element. In other refinements, a variation in the radius of the annulus is provided. In yet other refinements, the component comprises a plurality of discrete damper masses which are interconnected by means of spring elements. The spring elements function in this case, for example, as connecting masses and as joints. Alternatively or in addi- tion, a hinge is provided.

In a further refinement, the component is coupled directly or indirectly to the element to be damped at at least one bearing point. A further stabilization in the x- and y-directions is attained owing to the bearing point. The bearing point is preferably fashioned and/or arranged in such a way as not to impair a vibrational response in the z-direction of the damping device. In one refinement, the bearing point is arranged to this end in a vibration node. In other refinements, a bearing is designed as a movable bearing in the z-direction. In one refinement, in the case of an optical element the bearing point is provided on an inner ring accommodating the optical element such that an indirect attachment results. In another refinement of the invention, it is provided that at least one of the vibration absorbers has at least two or more damping elements, the damper mass in which case is coupled to the element to be damped by means of the damping elements. The damper mass and the damping elements are preferably tuned to one another in this case in such a way that the damper mass constitutes a vibration form whose maximum vibration amplitude lies in the region of the damping elements.

In accordance with a further aspect of the invention, an optical element for a projection objective of a projection exposure machine is provided with an inventive damping device. The mass dampers, in particular the damper masses, the damping elements and/or the attachment points can be optimally adapted in terms of their arrangement, size and/or number to vibration forms of the element to be damped. In other words, the vibration form of the damping device matches in an adaptive fashion to the optical element, for example a manipulator.

In a further refinement of the invention, the damping device is fastened on the periphery of the optical element. In another refinement, the damp- ing device is fastened on the periphery of a holder accommodating the optical element. The holder is, for example, the inner ring of a mount. In this case, the arrangement can be suitably selected depending on the application.

In accordance with yet another aspect of the invention a projection exposure machine with an inventive optical element is provided. It is possible in this case to provide an optimum damping of vibration to avoid and/or reduce aberrations. Vibrations are preferably eliminated by the damping device, and parasitic natural frequencies of the mass damper are shifted into ranges which are assumed to be harmless for the projection exposure machine. Such harmless frequency ranges lie, for exam- pie, above approximately 1000 Hz for known projection exposure machines.

As well as emerging from the claims, the above and further features also emerge from the description and the drawings, it being possible for the individual features to be implemented in each case for themselves alone or severally in the form of sub-combinations for embodiments of the invention and in other fields, and for them to constitute advantageous designs also capable of protection for themselves. Uniform reference sym- bols are used in the drawings for identical or similar components.

BRIEF DESCRIPTION OF THE DRAWINGS

In diagrammatic form, in the drawings:

Figure 1 : shows a top view of an optical element with a damping device;

Figure 2: shows a side view of the device in accordance with Figure 1 ;

Figure 3: shows a damping mass for a damping device in accordance with Figure 1 ;

Figure 4: shows a damping element for a damping device; and

Figure 5: shows a projection exposure machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figures 1 and 2 show diagrammatically an optical element 1 with a mount 2 and a damping device. Here, Figure 1 shows a top view, and Figure 2 a side view of the system. The optical element 1 is, for exam- pie, a lens that is supported in a conventional way in a mount 2 comprising an inner ring 20 and an outer ring 22. Various mounting technologies are disclosed, for example, in US 6,229,657 B1 , the content of which is incorporated hereby by reference as a constituent of the description.

The inner ring 20 is rigidly connected to the optical element 1. The inner ring 20 is mounted in the outer ring 22 via bearings 24. The bearings 24 illustrated diagrammatically in Figures 1 and 2 are, for example, fashioned as solid joints with a stiffness c and a damping d. It is possible to provide between the outer ring 22 and the inner ring 20 further adjusting elements with a sensor system and/or actuator system (not illustrated) which can be used to displace and/or deform the optical element 1 actively by comparison with the outer ring 22.

Provided on the optical element 1 is a damping device with a plurality of mass dampers 3 for the purpose of dissipating vibrational energy. The damping device comprises at least two mass dampers 3 distributed over the periphery of the optical element 1. In the exemplary embodiment illustrated in Figures 1 and 2, four mass dampers 3 are provided which are distributed uniformly over the periphery of the optical element 1. In other refinements, more or less than four mass dampers 3 are provided, it being possible for the mass dampers 3 to be distributed uniformly or non-uniformly over the periphery of the optical element 1.

The mass dampers 3 comprise a damper mass 30 each. The damper masses 30 are connected to the inner ring 20 and/or the optical element 1 via at least one damping element 32 or a plurality of damping elements 32, each. The mass dampers 3 are arranged on the inner ring 20 in the exemplary embodiment illustrated diagrammatically in Figures 1 and 2. In other refinements, the mass dampers 3 are fitted directly on the optical element 1 , for example, being bonded thereto. As may be seen from Figure 2, the damping elements 32 are designed as spring damper ele- ments in the exemplary embodiment illustrated. The damping elements 32 of different damper masses 30 are spatially separated from one another such that the damper masses 30 are connected to the structure to be damped, for example the optical element, at spatially separated at- tachment points.

As may be seen from Figure 1 , the individual damper masses 30 are connected via connecting elements 4 whose mass is less than the mass of the damper masses 30. The connecting elements 4 are illustrated diagrammatically as stiff spring elements in Figure 1. The stiffness of the connecting elements 4 depends on a material used for this purpose, and on the geometry, especially the cross-sectional area, of the connecting elements 4. Because of the low mass of the connecting elements 4, the connecting elements 4 are not excited to flexural vibrations or are ex- cited to flexural vibrations only in a very high frequency range. The damper masses 30 are supported against one another in x- and y- directions by the connecting elements 4. As a result, parasitic solid-state modes such as can occur, for example, in the case of a damping device with segmented absorbers known from the prior art do not arise.

The damper masses 30 can be optimally adapted to vibration forms of the optical element in terms of their arrangement, size and/or number. Alternatively or in addition, it is possible to influence the damping behaviour by optimizing the number and/or arrangement of the damping ele- ments 32 and/or a variable of the damping elements 32.

Figure 3 is a diagram of a component for an optimized damping device, the damping masses 30 and the connecting elements 4 being designed as integral parts of the component. The component is designed as a closed circular ring, three damper masses 30 and three connecting elements 4 being arranged alternating with one another. The connecting elements 4 have a lesser height h by comparison with the height H of the clamper masses 30, and therefore a smaller cross section. The height H of the damper masses 30 is, for example, approximately twice to eight times the height H of the connecting elements 4. Because of the reduced cross section, there is a reduction in stiffness in the transition between the damper masses 30 and the connecting elements 4 such that a solid joint 34 results. Furthermore, constrictions are provided in the region of the solid joints 34 in the exemplary embodiment illustrated. Because of the different cross section, the mass of the damper masses 30 is higher than the mass of the connecting elements 4. It is preferred to select a material with an E-modulus of at least approximately 70 χ 10 ⁹ N/m ² , in particular of approximately 190 χ 10 ⁹ N/m ² . It is thereby possible to provide connecting elements 4 of adequate stiffness by means of a comparatively small cross-sectional area and thus low mass, such that the natural frequency of the connecting elements 4 is very high and lies in a range causing no damage to the optical element. The entire damper mass is preferably approximately 7% to approximately 12% of the mass of the optical element.

Furthermore, bearing points 36 are provided in the exemplary embodi- ment illustrated for the purpose of supporting the damper masses 30. The bearing points 30 are preferably situated in the region of vibration nodes. In other refinements, the bearing points 36 are provided in the region of the connecting elements 4. In the exemplary embodiment illustrated in Figure 3, the damper masses 30 are attached to the inner ring 20 in accordance with Figure 1 by means of cylindrical damping elements 32, which are also denoted as braids, and are illustrated schematically in Figure 4. In the exemplary embodiment illustrated, one damping element 32 is provided per damper mass 30. In other refinements, two or more damping elements are provided per damper mass 30. Figure 4 is a diagram of a clamping element 32. The clamping element 32 comprises a layer 320 made from a material which dissipates vibrational energy, being able to do so by friction. Paper, felt or the like are examples in this case. It is preferred to provide an elastomer, in particu- lar a fluoroelastomer or fluororubber. Such an elastomer has good characteristics which dissipate vibrational energy. At the same time, a fluoroelastomer is suitable for application in a projection objective. A disc, in particular a metal disc 322, 324, is respectively provided on each side of the layer 320. The metal discs 322, 324 are permanently con- nected to the elastomer 320, for example, by vulcanization.

The following parameters, for example, are thus available for setting the damping characteristics of the passive damping device: a number and distribution of the mass dampers 3, a number of the damping elements 32, a distribution of the damping elements 32 for each damping mass 30, a thickness of the layer 320 of the damping elements 32, a distribution of the damping masses 30 on the optical element 1 , a mass of the damping masses 30, a stiffness of the connecting elements 4 and/or a mass of the connecting elements 4. It is thereby possible to match the damping device 3 well to the optical element 1 and/or to an associated manipulator. Consequently, a damping action can be greatly increased and adapted to different vibrational modes.

Figure 5 is a diagram of a projection exposure machine 5. The projection exposure machine comprises an illuminating device 50 by means of which an illuminating radiation is produced, and a projection objective

51. Arranged between the projection objective 51 and the illuminating device 50 is a mask 52 with a pattern, the pattern of the mask 52 being projected onto a substrate 6 arranged beneath the projection objective 51. The projection objective 51 comprises various optical elements 1 such as lenses, mirrors, gratings or the like. It is preferred to provide a manipulator or a plurality of manipulators in order to compensate aberra- tions based on heating or the like. The manipulators respectively comprise an optical element 1 in accordance with Figure 1 , and a sensor system and/or actuator for positioning and/or deforming the optical element 1.

The optical elements 1 are preferably supported in accordance with Figure 1 by means of a mount comprising an inner ring 20 and an outer ring 22. The outer rings 22 are accommodated in this case in a housing of the projection objective 51. A damping device illustrated diagrammati- cally in Figures 1 to 4 is preferably provided on at least one of the optical elements 1 for the purpose of absorbing vibrational energy.