Patent Application
20070019310
The invention relates to a facet mirror having a multiplicity of mirror facets in illuminating devices for projection exposure machines in microlithography using radiation in the extreme ultraviolet region, the mirror facets each having a reflecting optical surface, and the mirror facets being arranged on a mirror support body. The invention also relates to a method for producing mirror facets, and to an apparatus for positioning a mirror facet on a support body.
U.S. 2003/0058555 A1 discloses a facet mirror that has a multiplicity of mirror facets that are mounted, in turn, on a base plate. Each of the mirror facets has a reflective surface and a magnetic layer that is applied to the opposite side of the reflecting layer on the mirror facet. The mirror facets can be accurately positioned on the base plate with the aid of a positioning device. Moreover, the mirror facets are arranged on the base plate in such a way that they adjoin one another. By virtue of the fact that the base plate contains a magnet and that the mirror facets include on their underside a magnetic film or a magnetic layer, there is no need to use adhesives or other connecting means to connect the mirror facets to the base plate.
The production of such a facet mirror consists, firstly, in applying the reflecting layer to a printed circuit board. Thereafter, a multiplicity of mirror facets are cut out of the printed circuit board, the mirror facets of this type thereafter being arranged on the base plate, the mirror facets being connected to the base plate via magnetic forces such that the mirror facets form a prescribed pattern in a mutually adjoining fashion.
Furthermore, JP 2000098114 A discloses a positioning method for a mirror facet on a main plate, use being made, for accurately positioning the mirror facet, of a reference surface that is located on the main plate. Reference surfaces for positioning in a horizontal direction and a vertical direction are formed on the rear side of the. mirror facet. A block element with the associated corresponding reference surfaces is mounted on the main plate as main base for the mirror facet. The block element is of L-shaped design in this case. In this way, it is possible for a plurality of mirror facets to be joined, in combination with the block element on the main plate, to form a facet mirror.
Production and applications of mirror facets are further described in the following patent documents:
The production of small mirror optics with, for example, a rectangularly edged optical surface can be carried out in general using the conventional standard methods of optical fabrication. If, however, the rectangular optical surface of this type should be very narrow, for example <5 mm, and if there is a tilting to be recessed into the optical surface (meaning, that the optical surface should be tilted regarding a reference surface), the limits of classical optical fabrication quickly become clearer. Such mirror facets are typically a constituent of illuminating systems for EUV lithography.
In particular, the conditions of such mirror facets for EUV lithography need to be observed (considered) in order for the facet mirror to be of very high quality. The prescribed roughnesses are to be observed here, in particular.
Consequently, the object of the invention is to create a method for producing mirror facets for a facet mirror, the mirror facets having a very narrow optical surface and having a tilted optical surface upon completion of the facet mirror.
The object is achieved by means of a method for producing mirror facets for facet mirrors as claimed in claim 1, a facet mirror as claimed in claim 19 and apparatuses for positioning mirror facets on a support body as defined in claims 23 and 26.
According to the invention, the production of facet mirrors with tilted optical surfaces is implemented by virtue of the fact that instead of rotating or tilting the mirror facet or the mirror body, the tilting angles are recessed into the optical surface of the mirror facets, meaning that the tilting angles of the optical surface of the facet mirror relative to a reference surface of said mirror is formed by the machining of the mirror without a tilt of the mirror. Consequently, the optical surface can be produced with an edge that is as sharp as possible at less than 50 μm. Furthermore, the advantage consists in that the individual mirror facets for an ensemble are or can be tightly packed, and possible light losses can thereby be minimized.
Consequently, the tilting angles are firstly recessed into the later optical surface of the mirror facet, a requirement being in this method of production to ensure, in particular, that the optical surface has a very high aspect ratio. Thereafter, the mirror facets are provided with a reflecting layer on the optical surface, and arranged tightly packed against one another on a mirror support body.
An advantageous refinement of the invention provides that, in order to set a tilting angle φx, the mirror facet is brought between the two bearing bodies with an oblique locating face and held there, a tilting angle φy of the mirror facet being set via a screw device that acts on a surface of the mirror facet that is situated opposite the optical surface.
A particular advantage of this method consists in that two tilting angles can be recessed into the surface of the mirror facet with very high accuracy (meaning that a surface of arbitrary shape can be formed into or on a surface of the mirror facet, whereas the formed surface may be tilted regarding one or two tilting angles relative to a reference surface, preferably relative to a reference surface of the mirror facet), it being possible here, particularly, to produce plane tilted surfaces very effectively. Owing to the bearing bodies, which frame the mirror facet, a large area can thereby be machined, and this leads, in turn, to a very high optical quality and the optical surface can therefore be produced with a sharp edge.
A further advantageous refinement of the invention provides that, in order to set tilting angles φx and φy, the mirror facet is arranged on a support body in a machining region of a machining tool, defined abaxially relative to an axis of the machining tool, a surface of the machining tool that machines the mirror facets being designed as a spherical or aspheric surface.
In particular, it is thereby possible for defined tilting angles to be recessed into the surface of the mirror facets using a spherical or an aspheric machining method, the mirror facet being arranged abaxially on a support body. Furthermore, given the abaxial positioning, arbitrarily edged mirror facet bodies can be used to set defined tilting angles. A further advantage exists in this case, specifically that a plurality of mirror facets can be processed simultaneously, and that several radii differing arbitrarily can now be used.
Advantageous refinements and developments of the invention emerge from the further subclaims and the following exemplary embodiments described in principle in the drawing, in which:
Illustrated schematically in
A first method for machining rectangularly edged optical surfaces 2 of the mirror facet 1 with the requirements already named is shown below.
A tilting angle φy about a short mirror facet side (y axis) can be set highly accurately by two micrometer screws 4, as is illustrated in
The setting of the two tilting angles φx and φy is performed simultaneously according to the invention. Consequently, it is possible in this way during the fabrication process, for example using standard methods in optics such as grinding and polishing, for the two tilting angles φx and φy to be recessed simultaneously into the optical surface 2 by a machining tool, machining (milling, grinding, lapping, polishing) the optical surface 2 enlarged by the bearing bodies 3. This means that the fabrication process offers the possibility to form an arbitrary optical surface 2 (like plane or curved surfaces of any curvature e.g. spherical or aspherical surfaces), being tilted relative to the base surface of the mirror tilting angles φx and φy have been introduced into the optical surface 2 and after the high-accuracy quality for the optical surface 2 has been achieved, a reflecting layer can be applied to the optical surface 2. Only thereafter are the mirror facets 1 arranged and permanently mounted on a basic body for the purpose of fabricating a facet mirror.
This method can be used, in particular, to produce plane optical surfaces 2 with high accuracy. It is, however, also conceivable to use this method for spherical or aspheric surfaces, in which case, when use is made of a spherical or an aspheric tool, the latter should work on the optical surface 2 provided only in a centered fashion, since otherwise the tilting angles introduced are, or can be, affected by error. Thus, however, it is possible for the mirror facets 1 clamped into the bearing bodies 3 to be machined one after another. However, it would also be possible to set the mirror facets 1 via special computer programs in such a way that the spherical or aspheric tool can simultaneously machine a plurality of mirror facets 1.
This method is likewise suitable for machining metal mirrors, and also for machining glass, glass ceramic or silicon mirrors or mirrors comprising semiconductor material. It would also be possible with the aid of this method to provide arbitrarily edged mirror facets 1 (mirror facets 1 with arbitrary shape of the optical surface 2), with tilting angles φx and φy, it being necessary, however, to bear in mind that the bearing bodies 3 should be provided with locating faces that correspond, in turn, to the outer surfaces of the mirror facets 1, in order thus to achieve a very high accuracy.
Furthermore,
Illustrated schematically in
Moreover, the mirror facets 1 in
Δh=√{square root over (R2−r2)}+R, or Δh=R−√{square root over (R2−r2)}
R being the radius of the sphere, and r being the normal distance of the centre of the mirror facet 1 to the rotation axis 7.
Analogously, it is also possible to set two tilting angles (rotation about x and y), as is illustrated in
XO=R sin φy and
YO=R sin φx cos φy,
R being the radius of the spherical surface 2.
Let the spherical radius be R=100 mm, and let φx=2° and φy−3.5° hold for the tilting angles φx and φy. The positions pertaining to the angles φx and φy are thus x=61.05 mm and y=−34.83 mm. If the tilting angles are small, which means <10°, the contribution to the angular error that comes about owing to the positioning of the mirror facet 1 can be estimated as follows:
Δφx=Δy/R and
Δφy=Δx/R,
the angles φx and φy being given in rad. Given a positional uncertainty of, for example, Δx=5 μm, the sharp reduction in the relatively large radius R results in an angle error of Δφy=5 μrad, which corresponds approximately to 1″.
Positional uncertainties of approximately 1 μm can be set using microscopic observation, for example with the aid of portal microscopes or of suitable aids such as, for example, high-accuracy end measures (or gauge blocks), and tilting angles can thereby be achieved with an accuracy of 1 μrad.
However, it is possible thereby for this method of the abaxial positioning to be carried out without any problem to set defined tilting angles with the aid of arbitrarily edged mirror facet bodies 1, and this method is likewise not restricted to spherical surfaces. It is also possible in this way to produce mirror facets 1 with tilted aspheric surfaces 2.
A further possibility is shown according to the invention in
The angle β is very small in the case of flat radii, for example R˜1000 mm, and then constitutes only a correction to the wedge angle α that essentially sets the tilt. The aim in
The method according to the invention therefore permits the mirror facets 1 to be positioned at virtually any desired positions on the support body 6 in order to produce a surface 2, tilted in a spherically or an aspherically defined fashion, with arbitrary angles.
If the optical surface 2 is machined with the aid of a spherical or aspheric tool 5, the two tilting angles φx and φy can be recessed into the optical surface 2 in a fashion defined via the distance between the mirror facet 1 and the spherical axis 7. The angular error is examined in this case via the positional uncertainty of the mirror facet 1, and is particularly small whenever the radius R of the tool 5 or the radius of the spherical or aspheric surface 2 becomes large.
The position of the optical axis or of the tool axis 7 must be known ih this case with sufficient accuracy.
When producing mirror facets 1 with an aspheric optical surface 2, it can be advantageous to recess three tilting angles, specifically φx, φy and φz, into the optical surface 2.
The fabrication of the U-shaped body element 10, and the position of the centering bores 12 need not necessarily be machined very precisely. The position of the finally mounted U-shaped body element 10 can be determined, for example, with the aid of a coordinate measuring machine, and subsequently the mirror position can be fixed relatively to the axis of symmetry 7 of the tool 5 via the high-accuracy end measures 11.
The mirror facet 1 can now be pressed against the end measures 11 via suitable clamping elements 13, it being possible, for the purpose of clamping the long facet side, to press the corresponding clamping element against the U limbs of the body element 10 with the aid of screw elements 14′ and fasten it there. Through holes can be present for this purpose in the corresponding clamping element 13, and threads can be present in the U-shaped body element 10 or U limbs. Suitable spring elements for clamping could also be used here. A clamping element 13′ that is mounted on the short facet side of the mirror facet 1 can be pressed against the mirror facet 1 via two screw elements 14 that have a spherical end in this exemplary embodiment. Threaded bores are likewise required for this purpose in the U-shaped body element 10. Here, as well, clamping can be implemented via suitable spring elements.
Since the level in the spherical surface of the tool 5 varies as a function of the mirror position, the differences in level can be balanced out, if appropriate, with the aid of a defined base plate, for example an end measure that can be mounted below the mirror facet 1. The correction of the level is performed via the circle or sphere formula already stated:
Δh=√{square root over (R2−r2)}+R, or Δh=R−√{square root over (R2−r2)}
R once again representing the radius of the sphere of the tool 5, r being the distance of the mirror midpoint or of the point on the mirror facet 1 at which the tilting angles are specified from the axis of rotation of the tool 5. In order to be able to machine the edges of the mirror facets 1 as sharply as possible, they can be surrounded with accurately fabricated and accurately measured auxiliary elements 15 of the same height and the same material as illustrated in
The methods of abaxial positioning for setting defined tilting angles can therefore be carried out with arbitrarily edged mirror facet bodies 1 and is not restricted to aspherical, spherical or plane surfaces. Mirror facets 1 with tilted aspheric surfaces can also be fabricated or produced in the same way. If, for example, the mirror surface 1 are not rectangularly edged, use can be made, as shown in
The module 16 can be fixed in further ways on the carrier plate 6 for the machining process, for example via magnetic holders, use being made of magnets that can be switched on and off. Furthermore, the fixing can also be performed by vacuum clamping, bonding or cementing, in which case a defined bonding area should be present when use is made of adhesive or cementing means, in order to comply with the tilting angle tolerances.
The fixing of the mirror facet 1 and the module 16 on the carrier body 6 should take place under observation in all instances when no fixed position is prescribed, for example by bores on the carrier body 6. It is also possible to operate with defined stops that uniquely define the position of the mirror facet 1 on the support body 6, and thus in relation to the axis of symmetry (tool axis) 7.
Using mirror facets 33, 34 according to the present invention allow the formation of a compact facet mirror 30 with the advantage that the geometrical projection of the optical surfaces of two adjacent mirror facets like 32, 33 or 34, 35 or 33, 34 onto the support body 31 cover at least an area of the same size as the geometrical projection of the respective mirror facets onto said support body 31. This feature holds especially for adjacent mirror facets with at least one tilted optical surface, meaning that at least one mirror facet of adjacent mirror facets has at least one tilted optical surface as it is the case for the mirror facets 33, 34 with their respective tilted surfaces 37 and 38. The tilted optical surfaces can be plane, spherical or aspherical or can have a curved structure, such that a normal or normal plane differs from the one's of the reference surface. Of course the optical surfaces can be concave or convex in one or two directions, or can be both concave in one and convex in another direction. Advantageously the reference surface is the surface essential opposite to the optical surface of the mirror facet of this invention.
Due to the advantage regarding the mentioned projections with the inventive facet mirror an area or surface of the support body 31 can be covered with optical surfaces like mirrors without getting leaks of optical surfaces on said area or surface of the support body. To show this advantage more clearly it is referred to
Such due to the special arrangement the mirror facets 42 and 43 correspond to the mirror facets 32 and 33 of the facet mirror 30 of
Preventing or minimising leaks or gaps 45 in the optical surface of the facet mirror 30 has the advantage that the efficiency for reflection is optimized, even for mirrors with a complex reflection pattern.
The present invention should not be limited to the described embodiments. Additional embodiments of the present invention may be achieved by combining and/or exchanging features of the various described embodiments.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP04/07478 | 7/8/2004 | WO | 4/3/2006 |
| Number | Date | Country | |
|---|---|---|---|
| 60485759 | Jul 2003 | US |
