Tilting mirror
The invention relates to a tilting mirror, in particular as mirror facet for a facet mirror comprising a plurality of these tilting mirrors, having a mirror surface and a bearing device, a method for adjusting such a tilting mirror, and the use of such a tilting mirror.
Tilting mirrors, in particular as mirror facet for facet mirrors comprising a plurality of these mirror facets, are known from the prior art.
Thus, for example, GB 2 255 195 A describes such facet mir- rors having individual tilting mirrors and appropriate bearing elements for the facet mirrors, whose intended use is to be looked for, in particular, in the field of solar energy technology.
Each individual one of the tilting mirrors is constructed in this case such that it comprises a mirror surface which is connected via a rod to a sphere which is fastened in appropriate bearing devices. The accuracy of such arrangements with regard to the possibility of adjusting them or the like is extremely limited in this case, since the individual tilting mirrors are held in a comparatively loose fashion and maladjustment can come about very easily and quickly.
In the case of such a design, it is therefore certainly im- possible to implement the accuracies which are required in the adjustment and which are necessary for the preferred case of application for the abovenamed invention in a lens for semiconductor lithography, in particular for use with radiation in the region of the extreme ultraviolet. Further- more, it is possible during adjustment to access the individual mirror facets only from the side of their mirror sur-
face, and so it is comparatively complicated and difficult to align the individual mirrors when they are illuminated.
Moreover, EP 0 72β 479 A2 describes a tilting mirror ar- rangement which has at least one tilting mirror, a basic body and at least one mirror bearing with an at least virtually fixed fulcrum between the tilting mirror and the base. Moreover, in the case of mirror surfaces with a characteristic length of less than 40 mm, the overall size of the total arrangement of tilting mirror bearing and tilting mirror housing is arranged below the mirror surface in such a way that, when projected onto the mirror plane, it does not project beyond it, or does so only slightly when the tilting mirror is deflected. Such tilting mirrors are used, for ex- ample, in the field of laser technology.
Such mirrors can also be adjusted when illuminated, owing to the possibility of mounting the basic body correspondingly via the mirror bearing and readjusting it, if appropriate. However, the design is very complicated, and so a very high outlay on overall space, adjusting elements, costs and the like is to be expected in the case of facet mirrors which could be formed from these tilting mirrors.
It is therefore the object of the invention to avoid the abovenamed disadvantages of the prior art, and to provide a tilting mirror, in particular as a mirror facet for a facet mirror comprising a plurality of these tilting mirrors, which is of very simple design.
This object is achieved according to the invention by virtue of the fact that the mirror surface is constructed as the surface of a recess in a spherical member, the spherical member being mounted in the bearing device.
The mirror surface is introduced into the spherical member,
which simultaneously serves as bearing member for mounting the tilting mirror in a bearing device. This provides a design which permits the tilting mirror to be adjusted freely and, in the case where it is used as a unipartite mirror facet in a facet mirror, independently of the other mirror facets. The design from one sphere as mirror member and bearing member is very simple and cost-effective here.
An alternative embodiment is to arrange the mirror surface in a recess in a spherical member, the spherical member being mounted in the bearing device.
Further advantages stem from the possibility of fabricating the spherical member separately from the actual mirror sur- face, and then connecting the latter via joining methods known per se such as, for example, bonding, soldering, wringing together, pinning, screwing or the like. Particularly when used in the field of EUV lithography, that is to say at wavelengths in the region of approximately 13 nm, the mirror surface must satisfy extremely strict requirements placed on surface quality. In the case of an embodiment in which the mirror surface is introduced indirectly into the spherical member, the possibility thereby arises of using a different material for the region of the mirror surface, or of correspondingly processing differently the material which may also be the same. It is therefore possible to achieve the required surface quality in the region of the mirror surface without the need for the entire spherical member to be processed in a correspondingly expensive way, or for it to consist of a material processed in this way.
In a particularly advantageous refinement of the invention, the bearing device is constructed as a conical bore in a plate, the spherical member being arranged in the bore. This yields a very simple design with a very simple mounting of the mirrors, it being possible for the latter to be adjusted
very easily by means of rotating the spheres in the bearing device. In particular, a surface normal of the mirror surface is tilted appropriately in the desired way in the case of such an adjustment.
A method for adjusting such a tilting mirror follows from the characterizing part of Claim 13, the spherical member then being introduced, in accordance with some advantageous developments of the invention, between two plates which in each case have a conical bore for the spherical members. Moreover, for the adjustment the spherical member has a lever element which is located on the side of the spherical member averted from the mirror surface.
In the case of such a method, the adjustment of the mirror surface of the tilting mirror can be carried out via force actions on the lever element, after which the position of the tilting mirror is fixed by pressing the first and the second plates against one another.
This yields a tilting mirror, or in the case of the use of a plurality of spherical members, a facet mirror which permits a highly precise adjustment and a secure fixing of the position of the mirror surface after the adjustment. Pressing together the two plates between which the respective spherical members are mounted results in a design which keeps the spherical members reliably in their previously adjusted position even in the case of vibrations, jolts or the like.
In a particularly advantageous development of the method, the adjustment can be performed with illumination, such that the conditions, for example, of a thermal type, actually obtaining are present for the mirror facets as early as during adjustment. Owing to the possibility of adjustment from the side of the mirror facets averted from the mirror surface, in this case there is no disturbance of the illumination and
thus of the conditions by the operation of adjustment. Moreover, the adjustment can be performed with the aid of the result of the illumination itself, and so the accuracy to be achieved can be improved in the real later conditions of use .
Such a tilting mirror can be used particularly favourably as a mirror facet for a facet mirror in a lens for semiconductor lithography, and here, in particular, for lithography operating with the extreme ultraviolet (EϋV) as radiation.
Facet mirrors which use a multiplicity of individually freely adjustable facets to guide the EUV radiation in the lens have proved to be particularly favourable in the case of such lenses, and so this must surely be seen as one of the preferred fields of use of the tilting mirror according to the invention.
Further advantageous refinements of the invention emerge from the remaining subclaims and from the exemplary embodiment represented below with the aid of the drawing.
The sole figure attached shows a cross section through a possible design, shown in principle, of a facet mirror with the use of tilting mirrors in accordance with the invention.
To be seen in the sole figure attached is a part of a facet mirror 1 which has three tilting mirrors or three mirror facets 2 in the exemplary embodiment illustrated here. Each of the mirror facets 2 is constructed in this case as a spherical member 3. Located in each of the spherical members 3 is a recess 4 which is provided here with a reference numeral only at one of the spherical members 3, which has been supplemented by a dashed line to form the cross-sectional shape of a sphere. The remaining surface of the spherical member 3 in the region of this recess 4 then forms the mir-
ror surface 5 which is additionally symbolized in each case by its surface normal n in the sole figure attached.
Each of the mirror facets 2 is mounted in a bearing device. In this case, the bearing device comprises a conical bore 6 which is introduced into a plate 7 which is denoted below as lower carrier plate 7. The spherical member 3 lies in this conical bore 6, whose larger opening diameter is arranged in this case such that the spherical member 3 lies in the coni- cal bore 6 but cannot fall through the latter.
Moreover, the bearing device has devices for reliably retaining the spherical member 3. In accordance with the exemplary embodiment illustrated here, these devices are con- structed as a further conical bore 8 which is provided in a second plate 9. This second plate 9 is denoted in this case below as upper carrier plate 9, for the sake of better understanding. The larger opening angle of the further conical bore 8 is arranged such that it faces the lower carrier plate 7.
In addition to the construction of the devices for reliable retention by the upper carrier plate 9, a different type of construction of these devices, for example, by means of springs, magnetic forces or the like, would also be conceivable.
It is particularly favourable for the ideal mode of operation when an adjusting device is fitted at each of the mir- ror facets 2 on its side averted from the mirror surface 5. This adjusting device can be constructed, for example, as a lever element 10 connected to the spherical member 3. Via such a lever element 10, which projects through the conical bore 6 in the lower carrier plate 7, the facet mirror 1 can thus be adjusted, under illumination, from behind, that is to say from its side averted from the illumination, that is
to say under the conditions provided for correct operation. The transmission ratio between the movement of the mirror surface 5 or its surface normal n and the deflection of the lever element 10 can be set in this case by the length of the lever element 10. It is particularly rational in this case when the lever element 10 is constructed to be aligned with the surface normal n of the mirror surface 5.
It is particularly favourable when the position of the mir- ror surface 5 is adjusted via appropriate forces on the side of the lever element 10 averted from the spherical member 3. These forces can be applied, to the lever element 10, for example, via actuators which are indicated in principle here by the arrows A. Conceivable here as actuators are all known forms of actuators which, for example, use pneumatic, hydraulic, piezoelectric, magnetic or mechanical forces.
The procedure in adjusting such a facet mirror 1, which can be used, for example, as facet mirror 1 in a lens for EUV lithography, is such that the mirror facets 2 are inserted into the conical bores 6 of the lower carrier plate 7. Thereafter, the upper carrier plate 9 with its conical bores 8 is positioned above the mirror facets 2 and lowered. The two plates 7, 9 then lie loosely on one another such that the positions of the mirror facets 2 can still be varied via the lever elements 10. With the entire facet mirror 1 illuminated, each individual mirror surface 5 of the individual mirror facets 2 is then adjusted via appropriate force actions on the lever element 10. As soon as the position of all the mirror facets 2 is adjusted in the way desired, this position is fixed by pressing the first and second plates against one another. It is particularly favourable in this case when the lower carrier plate 7 is constructed from a material which is much softer than the material of the spherical members 3. Conceivable here, for example, would be a material combination of a ceramic or crystalline material
for the spherical members 3 and a soft metal, such as brass, copper or aluminium, for the lower carrier plate 7. By comparison therewith, the upper carrier plate 9 should be constructed from a material which is somewhat harder than the material of the lower carrier plate 7 but which is also much softer than the material of the spherical members 3. This ensures that upon the two plates 7 and 9 being pressed together, the spherical members 3 are easily pressed into the lower carrier plate 7, and that their position is ensured by friction forces (even in the case of vibrations, shock or the like) .
Should the final adjustment have experienced a maladjustment as the plates 7, 9 were pressed together, something which can happen, for example, by screwing the two plates 7, 9 together, it is then possible, by loosening this screwed connection and by pressing the two plates 7, 9 apart by means of compressed air introduced therebetween to achieve a state in which the friction between the lower carrier plate 7 and the spherical members 3 is reduced to such an extent that renewed adjustment is possible before the two plates 7, 9 are then pressed against one another again after adjustment has been performed.
In order to ensure the finally set position of the individual mirror facets, the latter can also still be bonded or soldered to at least one of the plates 7, 9 as well.