U.S. patent application number 11/695626 was filed with the patent office on 2007-09-06 for faceted mirror apparatus.
Invention is credited to Stefan Dornheim, Andreas Heisler, Andreas Seifert, Markus Weiss.
Application Number | 20070206301 11/695626 |
Document ID | / |
Family ID | 34593333 |
Filed Date | 2007-09-06 |
United States Patent
Application |
20070206301 |
Kind Code |
A1 |
Seifert; Andreas ; et
al. |
September 6, 2007 |
FACETED MIRROR APPARATUS
Abstract
In a method for the production of a facetted mirror 24 having a
plurality of mirror facets 12 and 12', which have mirror surfaces
15 and are fitted into reception bores 22, 22' of a support plate
16, the mirror facets 12, 12' are made in a first method step. At
least one of the mirror facets is fitted into the associated
reception bore of the support plate in a second method step, after
which the ACTUAL position of the optical axis of at least one
mirror surface of an associated mirror facet 12, 12' fitted into
the support plate is respectively determined in a third step and
compared with a SET position of a predetermined optical axis.
Knowing the measured values determined for the at least one mirror
facet 12, 12' in the third method step, reprocessing of the mirror
facet and/or of the reception bore is subsequently carried out in a
further method step if there is an angular deviation between the
ACTUAL position and the SET position.
Inventors: |
Seifert; Andreas; (Aalen,
DE) ; Weiss; Markus; (Aalen, DE) ; Heisler;
Andreas; (Steinheim, DE) ; Dornheim; Stefan;
(Heidenheim, DE) |
Correspondence
Address: |
GRAY ROBINSON, P.A.
P.O. Box 2328
FT. LAUDERDALE
FL
33303-9998
US
|
Family ID: |
34593333 |
Appl. No.: |
11/695626 |
Filed: |
April 3, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10936317 |
Sep 8, 2004 |
7246909 |
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11695626 |
Apr 3, 2007 |
|
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PCT/EP04/00331 |
Jan 17, 2004 |
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10936317 |
Sep 8, 2004 |
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Current U.S.
Class: |
359/855 |
Current CPC
Class: |
G03F 7/70825 20130101;
G03F 7/70166 20130101; G02B 7/1824 20130101; B82Y 10/00 20130101;
G03F 7/702 20130101 |
Class at
Publication: |
359/855 |
International
Class: |
G02B 5/08 20060101
G02B005/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2003 |
DE |
103 02 664.9 |
Claims
1-42. (canceled)
43. A facetted mirror apparatus, comprising: a facetted mirror
having a plurality of mirror facets which generate a beam of rays,
wherein each of the mirror facets has a mirror surface with an
optical axis, and the mirror facets are each fitted into reception
bores of a support plate so that the optical axes of the mirror
surfaces are respectively aligned in order to generate the beam of
rays, each of the facets having an outer surface which bears
directly on the inner wall of the reception bore to provide a
stable alignment of the facet within the reception bore.
44. The apparatus of claim 43, wherein the mirror facets each have
a cylindrical mirror facet head with a mirror surface and a
cylindrical mirror facet base, and they are fitted into stepped
reception bores of the support plate with a step ledge.
45. The apparatus of claim 44, wherein the step ledge of the
reception bore is respectively intended as a reprocessing face.
46. The apparatus of claim 43, wherein the mirror facets are
reprocessed on their back faces, remote from the mirror surfaces,
with which they respectively bear on the step ledge of the
reception bore.
47. The apparatus of claim 43, wherein the mirror facets each have
a conical mirror facet head, with a mirror surface, and a
cylindrical mirror facet base, and they are received in reception
bores of the support plate with correspondingly matched conical
cylindrical shapes.
48. The apparatus of claim 47, wherein the conical region of the
mirror facet head or the conical region of the reception bore is
respectively intended as a reprocessing face.
49. The apparatus of claim 43, wherein the mirror facets are made
of silicon.
50. The apparatus of claim 43, wherein the mirror facets are made
of a stainless steel alloy.
51. The apparatus of claim 43, wherein the mirror facets each have
a mirror facet head with a mirror surface, on which a marking is
provided for azimuthal orientation.
52. The apparatus of claim 43, wherein a combination of spring
elements and fastening elements is provided as fixing means for the
mirror facets in the reception bores of the support plate.
53. The apparatus of claim 43, wherein the support plate is made of
silicon.
54. The apparatus of claim 43, further, comprising: an illumination
instrument optically coupled to the facetted mirror to provide
illumination incident upon the facetted mirror, and a
microlithography projection objective optically coupled to the
facetted mirror to project an image formed using the beam of rays
generated by the facetted mirror.
55. The apparatus of claim 54, wherein the mirror facets of the
support body have reprocessing faces for alignment of at least one
mirror surface.
56. The apparatus of claim 54, wherein the mirror facets each have
a cylindrical mirror facet head, with a mirror surface, and a
cylindrical mirror facet base, and they are fitted into stepped
reception bores of the support plate with a step ledge.
57. The apparatus of claim 56, wherein the step ledge of the
stepped reception bore is respectively intended for alignment of
the optical axis.
58. The apparatus of claim 56, wherein the mirror facets are
intended for alignment of the optical axis on back faces, remote
from the mirror surfaces, with which they respectively bear on the
step ledge of the reception bores.
59. The apparatus of claim 54, wherein the mirror facets each have
a conical mirror facet head with a mirror surface and a cylindrical
mirror facet base, and they are received in reception bores of the
support plate with correspondingly matched conical and cylindrical
shapes.
60. The apparatus of claim 59, wherein the conical region of the
mirror facet head or the conical region of the reception bore is
respectively intended for alignment of the optical axis.
61. The apparatus of claim 54, wherein the illumination instrument
provides illumination within a wavelength range of the extreme
ultraviolet.
Description
[0001] This is a divisional of U.S. Ser. No. 10/936,317 which in
turn is a continuation of International Application No.
PCT/EP2004/00031 filed on Jan. 17, 2004 which designates the U.S.
and claims priority to German Application No. 103 02 664.9 filed
Jan. 24, 2003.
[0002] The invention relates to a method for the production of a
facetted mirror having a plurality of mirror facets, in particular
for an illumination instrument in a projection exposure system for
microlithography, and here in particular for use with illumination
in the extreme ultraviolet range. The invention also relates to a
method for processing reception bores and to a facetted mirror
having a plurality of mirror facets.
[0003] Facetted mirrors comprise a plurality of mirror facets and
are already known in the prior art.
[0004] Prior citation WO 03/067304 is based on manipulators which
allow adjustment of the mirror facets. For example, it is known
that the mirror facets have a spherical body, a mirror surface
being arranged in a recess of the spherical body and the side of
the spherical body remote from the mirror surface being mounted in
a bearing instrument. At each of the mirror facets, a lever element
is arranged on the side of the spherical body remote from the
mirror surface. Adjusting means, by which the spherical body can be
moved about its mid-point, engage on the lever element in a region
remote from the spherical body.
[0005] With a structure of this type, it is not readily possible to
achieve the requisite accuracies for relatively small mirror facets
when used with radiation in the extreme ultraviolet range.
[0006] Mirror facets whose mirror surfaces are arranged on a
support element are furthermore known from prior citation WO
03/067288. The support element has adjusting means by which the
angular position of the mirror surface can be adjusted in at least
one space direction in a plane at least approximately perpendicular
to the optical axis of the mirror surface.
[0007] For further prior art, reference is made to WO 03/050586 A2,
DE 100 30 495 A1, EP 0 145 243 A2 and XP-002281181 "Ion Beam and
Plasma Jet Etching for Optical Component Fabrication" in
Lithographic and Micromachining Techniques for Optical Component
Fabrication, Ernst-Bernhard Kley, Hans Peter Herzig, editors,
Proceedings of SPIE Vol. 4440 (2001).
[0008] The structure of these mirror facets is relatively
elaborate, so that such facetted mirror arrangements entail an
increased adjustment outlay and are relatively expensive.
[0009] Facetted mirrors need to withstand heavy thermal loads due
to the radiation, especially in the extreme ultraviolet range, and
the known arrangements meet the stringent requirements concerning
thermal loads only to a small extent.
[0010] In respect of this, it is an object of the invention to
optimize a facetted mirror, especially for ultrahigh vacuum
requirements, and to provide a structure which makes do with the
fewest possible parts and guarantees a reliable, long-term stable
and simple structure.
[0011] According to the invention, in a method for the production
of a facetted mirror having a plurality of mirror facets which
generate a beam of rays, the mirror facets having mirror surfaces
each with an optical axis, and they are each fitted into reception
bores of a support plate, this object is achieved in that the
mirror facets are made in a first method step, after which at least
one of the mirror facets is fitted into the associated reception
bore of the support plate in a second method step, after which the
ACTUAL position of the optical axis of at least one mirror surface
of an associated mirror facet fitted into the support plate is
determined in a third step and compared with a SET position of a
predetermined optical axis, after which, knowing the measured
values determined for the at least one mirror facet in the third
method step, reprocessing of the mirror facet and/or of the
reception bore is carried out in a further method step if there is
an angular deviation between the ACTUAL position and the SET
position.
[0012] The respectively selected optical axis can be determined via
a reference face according to the invention. For example, an
arbitrarily picked mirror surface of a mirror facet fitted into the
support plate may be used as a reference face. Further mirror
facets, and preferably all of them, are then aligned with their
mirror surfaces with respect to the mirror surface thus acting as a
reference face in order to achieve the required beam of rays, which
likewise has a principal optical axis.
[0013] Instead of a mirror surface as the reference face, a
reference face may also be provided on the support plate, with
respect to which further mirror surfaces, and preferably all of
them, are aligned with their respective optical axes in order to
generate the intended beam of rays. A plurality of reference faces
may furthermore be used for alignment of the mirror facets. This is
particularly advantageous when the mirror facets are aligned in
groups. Method steps two, three and the further method step may be
carried out either individually, successively in groups or even all
together for the mirror facets to be fitted into the reception
bores of the support plate.
[0014] Very advantageous reprocessing can be carried out by
so-called ion beam figuring (IBF). This is controlled, highly
accurate surface smoothing by ion beams in order to reduce the
roughness and for extremely exact surfaces. Any angular deviation
from the intended angle can thereby be reduced further.
[0015] As an alternative, it is also possible to carry out the
reprocessing by evaporation of metal interlayers, for example gold
layers. The metal layers are then correspondingly evaporated onto
the mirror facet and/or support plate, so that any angular
deviations are reduced further.
[0016] In another method according to the invention for the
production of a facetted mirror having a plurality of mirror facets
which generate a beam of rays, the mirror facets having mirror
surfaces each with an optical axis, and they are each fitted into
reception bores of a support plate, the mirror facets may be made
in a first method step, after which at least one of the mirror
facets is measured by a measuring device in a second method step,
after which, knowing the measurement result, the associated
reception bore is formed in the support plate, after which the at
least one mirror facet is fitted into the associated reception bore
of the support plate in a third method step, after which the ACTUAL
position of the optical axis of the at least one mirror surface of
the mirror facet fitted into the support plate is determined in a
fourth step and compared with a SET position of a predetermined
optical axis, after which, knowing the measured values determined
for the at least one mirror facet in the fourth method step,
reprocessing of the mirror facet and/or of the reception bore is
carried out in a further method step if there is an angular
deviation between the ACTUAL position and the SET position.
[0017] In this method according to the invention, knowing these
values, the associated reception bore is formed in the support
plate by using the measured values obtained in the second method
step for the at least one measured mirror facet with respect to its
optical axis, after which the fine tuning and optional reprocessing
in order to achieve a best-possible fabrication accuracy are
carried out in the further method step.
[0018] In this exemplary method as well, the mirror facets may be
measured individually or successively in groups or even all
together. The same applies to the reception bores determined with
the aid of the measured values being formed in the support
plate.
[0019] In order to achieve the requisite accuracy, the fabrication
is thus performed in steps. The individual facets are first made to
within the best-possible angular error, for example 200''. This
deviation is then determined by a measuring device, for example an
angle measuring instrument, preferably with an autocollimation
telescope with a positioning stage. The reception bores for the
mirror facets are then formed in the support plate with the
best-possible fabrication accuracy, which is from 20'' to 100'',
preferably from 30'' to 50''. It is then possible to establish
directly which of the mirror facets will be fitted with which of
the previously measured angular errors into which of the reception
bores. If need be, the reception bores may therefore be corrected
directly according to the measured angular errors of the mirror
facets. The residual error after the mirror facets have been fitted
into the respective reception bores is in turn determined via an
angle measuring instrument. Owing to the fabrication accuracies to
be achieved, this error lies in a range that makes it possible to
correct the residual error via a precision surface processing
method, for example the IBF method or the evaporation of metal
interlayers.
[0020] It is thus possible to achieve the requisite quality
concerning alignment of the individual mirror facets. The mirror
facets are subsequently fixed in the support plate so that a stable
alignment is guaranteed even over a long period of time.
Furthermore, the direct contact between the mirror facet and the
support plate leads to a structure which ideally dissipates the
heat absorbed by the mirror facets. This allows a simpler, less
expensive, very stable, less shock-sensitive, material-reducing,
adhesive-free and thermally unproblematic structure of a facetted
mirror in the ultrahigh vacuum range, especially for use in EUV
lithography.
[0021] Compared to solutions with individual manipulators for the
mirror facets, the method for the production of facetted mirrors
not only has advantages concerning a low assembly and adjustment
outlay and inexpensive production, but this method also makes it
possible to produce substantially smaller facetted mirrors and to
arrange a very large number of them in a facetted mirror, a
diameter range of from 3 mm to 50 mm being readily achievable for
the mirror facets.
[0022] FIG. 1 shows a structure of an EUV illumination instrument
with a light source, an illumination instrument and a projection
objective;
[0023] FIG. 2 shows a longitudinal section of a cylindrically
designed mirror facet, the mirror facet being mounted in a support
plate;
[0024] FIG. 3 shows a brief representation relating to a production
method for a facetted mirror;
[0025] FIGS. 4a and 4b show excerpts of a longitudinal section
corresponding to FIG. 2 with examples of reprocessing faces;
[0026] FIG. 5 shows a longitudinal section of a conically designed
mirror facet, the mirror facet being mounted in a support plate;
and
[0027] FIG. 6 shows a plan view of a facetted mirror, which
contains both embodiments of the mirror facets represented in FIG.
2 and FIG. 3.
[0028] FIG. 1 shows a summary representation of an EUV projection
exposure system with a complete EUV illumination instrument 1 with
a light source 1a and a projection objective 2 (only briefly
represented). The illumination instrument 1 also contains a plane
mirror 3, a first facetted mirror 4 as a first optical element with
a multiplicity of facetted mirrors, a subsequently arranged second
facetted mirror 5 as a second optical element with a multiplicity
of facetted mirrors, and two imaging mirrors 6a and 6b. The imaging
mirrors 6a and 6b are used to image the mirror facets of the
facetted mirror 5 into an entry pupil of the projection objective
2. A reticle 7 can be displaced as a scanning system in the y
direction. A reticle plane 8 also simultaneously constitutes the
object plane.
[0029] In order to bring different light channels into the optical
path of the illumination instrument for the setting adjustment, for
example, there are a greater number of mirror facets on the second
facetted mirror 5 than correspond to the number of mirror facets on
the first facetted mirror 4. The mirror facets are not represented
in FIG. 1 for the sake of clarity. As the object to be exposed,
there is a wafer 11 on a support unit 9.
[0030] FIG. 2 shows an individual mirror facet 12 with a mirror
surface 15, which is formed by a cylindrical mirror facet head 13
and a cylindrical mirror facet base 14. In the exemplary
embodiment, the cylindrical mirror facet head 13 has a diameter of
for example 20 mm, and the cylindrical mirror facet base 14 has a
diameter of about 8 mm. The mirror facet 12 has an overall length
of 60 mm, for example. For reasons of processing and thermal
loading, silicon is selected as the material of the mirror facet
12. The mirror facet 12 may of course be made of a stainless steel
alloy or other materials which meet the requirements for
polishability, mechanical, thermal and long-term stability and
ultrahigh vacuum compatibility (UHV compatibility). What is
important when selecting the materials for the mirror facets 12, in
particular, is that the materials being used should have a high
thermal conductivity so that the heat generated because of the beam
power can be dissipated.
[0031] The mirror surface 15 of the mirror facet head 13 may be
spherical or concave with a radius of, for example, 2000 mm. The
mirror surfaces 15 may also be designed to be plane, spherical,
aspherical, convex or concave. A marking 23 (see FIG. 6) is also
applied to the mirror surface 15 for correct azimuthal orientation
of the mirror facet 12. The marking 23 needs to be aligned with a
corresponding marking on a plane support plate 16, which has
reception bores 22 for the mirror facets 12. The support plate 16
is used to receive all the mirror facets 12, which together form
the facetted mirror 4 or 5. The mirror facets 12 are in this case
received individually in a particular reception bore 22. The
longitudinal axes of the individual reception bores 22 are
generally at different angles with respect to the optical axis, so
that an intended beam of rays (see reference numeral "27" in FIG.
1) is obtained overall from the multiplicity of mirror facets 12
fitted into the support plate 16.
[0032] The support plate 16 may also be designed to be aspherical,
if the mirror facets 12 are not intended to be arranged in a plane.
For example, the support plate 16 is formed by steel with a
thickness of, for example, 50 mm. The support plate 16 may also be
made of other materials, for example silicon, so that good heat
dissipation is achieved.
[0033] The cylindrical mirror facet base 14 is provided with a
screw thread 17 in order to hold the mirror facet 12 in its
position with a defined force, for example produced by a threaded
nut 18 and a spring 19, after it has been reprocessed. The spring
19 may be formed as a cylindrical spring or as a spring washer made
of stainless steel. This is particularly advantageous when
materials with different thermal expansion coefficients are being
used for the support plate 16 and for the mirror facet 12, as in
the present exemplary embodiment.
[0034] Owing to the different longitudinal axes of the reception
bores 22, the mirror facets 12 are arranged at different
inclination angles on the support plate 16, so that the impinging
rays are reflected in a predetermined direction in order to
generate the beam of rays 27. The optical axis 20 of each mirror
facet 12 therefore needs to lie in a particular set direction. For
this reason, the mirror surface 15 must be aligned very
accurately.
[0035] After the mirror facets 12 have been made, for example, the
relation between its mirror surface 15 and a mirror back face 15'
remote from the mirror surface 15 is measured for each mirror facet
12 by a measuring device 28 (not shown, see FIG. 3). Also, the
relation between the lateral face 21 of the mirror facet head 13
and the mirror surface 15 may advantageously be measured so that an
error to be corrected can be determined. This means that a set
angle direction and an actual angle direction are determined, the
difference between them is taken and the angular deviation then
needs to be eliminated.
[0036] This may be done in a plurality of steps. The first step
deals with the reception bores 22. Since the mirror surfaces 15 can
only be made with an accuracy of about 200'' with respect to the
support plate 16, the reception bores 22 are respectively processed
at the place of the position-determined lateral faces 21 of the
mirror facet 12 until the two relational faces are tilted by the
predetermined angular correction (in most cases 200''). The finely
processed lateral face 21 with a measured error can then
additionally achieve the requisite accuracy of about 10'' by
evaporating a wedge-shaped metal interlayer.
[0037] In addition or as an alternative, if the necessary accuracy
has not yet been achieved, it is possible for the mirror surface 15
to be reprocessed by ion beam figuring (IBF).
[0038] Optionally, the necessary accuracy may also be achieved only
with ion beam figuring, ablation of the face to be treated being
carried out in the final processing by ions. The ablation of
typically 1 to 2 .mu.m necessary for the IBF processing can be
carried out in this processing step without impairing the surface
roughness. A final accuracy of about 1'' can thus be achieved.
[0039] In the exemplary embodiment according to FIG. 2, the
reception bore 22 is respectively designed as a stepped bore with a
step ledge 22a for matching to the cylindrical mirror facet head 13
and the cylindrical mirror facet base 14. Instead of relational
faces exactly assigned to each other between the lateral face 21
and the inner wall of the reception bore 22, a mirror back face
15', that is to say a face on the side of the mirror facet head 13
remote from the mirror surface 15, may respectively be provided as
an alternative in this region as an angle-defining guide face in
conjunction with the step ledge 22a. Instead of reprocessing the
lateral face 21 or the reception bore 22 in the region of the
lateral face 21, in this case either the mirror back face 15' or
the step ledge 22a is reprocessed. The reprocessing may again be
carried out, for example, by ion beam figuring or by application of
a metal interlayer on one of the two parts. Since the reprocessing
involves angular corrections, during the reprocessing either
wedge-shaped IBF ablation or wedge-shaped application of a metal
interlayer is carried out in respect of the face to be reprocessed,
in order to achieve an angular correction. In other words: On the
face to be processed, a layer is applied or ablated which has a
different thickness or is ablated obliquely to its surface, in
order to generate the intended angular correction.
[0040] The alignment of a mirror facet 12 and the reprocessing
method will briefly be described below with reference to FIGS. 3,
4a and 4b. In order to be able to determine the exact place and the
scope of the reprocessing, one or more mirror facets 12 are fitted,
for example provisionally, into the associated reception bores 22
after having been measured by the measuring device 28, and the
reflectivity is measured (see FIG. 3 top). In order to use the
multiplicity of mirror facets 12 fitted into the support plate 16
to generate the beam of rays 27, which is for example guided to the
facetted mirror 5, the mirror facets 12 respectively need to be
aligned with their optical axis 20 in a predetermined way. To that
end, a reference face should be provided. This may be done, for
example, by a mirror surface 15 of a mirror facet 12, as
represented for example in FIG. 3 left bottom. In relation to the
optical axis 20 of this mirror facet 12, all the other optical axes
of the remaining mirror facets 12 are then measured with regard to
their respective position and in each case correspondingly aligned
so that the mirror facets 12 together lead to the intended beam of
rays 27. To that end, as required, processing faces of the mirror
facets 12, or of the bores 22 in which they are received, will be
reprocessed accordingly.
[0041] Examples of possible reprocessing faces are represented in
FIGS. 4a and 4b. For clearer representation, the angular changes
indicated in FIGS. 4a and 4b and the wedge-shaped ablations or
applications are indicated greatly enlarged. In practice, the
corrections generally take place in the range of up to 100''.
[0042] According to FIG. 4a, there is an angular deviation
.epsilon. as the correction angle between the actual position or
the actual value of the optical axis 20' and the set position or
the set value of the optical axis 20. In order to correct the
angular error .epsilon., according to FIG. 4a, the mirror surface
15 is therefore ablated accordingly in a wedge shape with the
correction angle .epsilon..
[0043] As an alternative to this, of course, it is also possible to
ablate the mirror facet head 13 with the correction angle .epsilon.
on its back face 15' remote from the mirror surface 15. Since the
mirror facet head 13 bears with its back face 15' on the step ledge
22a of the stepped bore 22, the position of the longitudinal axis
of the mirror facet 12 is in this way corrected accordingly in the
reception bore 22. Of course, a correspondingly large play between
the diameter of the stepped bore 22 and the diameter of the
cylindrical mirror facet base 14 therefore needs to be provided in
the region of the mirror facet base 14, so that there is enough
lateral free space for alignment of the mirror facet 12.
[0044] FIG. 4b represents another alternative for correction of the
deviation of the actual position of the optical axis 20'' from the
set position of the optical axis 20. Instead of the back face 15',
as can be seen, the step ledge 22a is reprocessed here in a wedge
shape with the correction angle .epsilon. by corresponding
ablation. Here again, the position of the longitudinal axis of the
mirror facet 12 is in this way corrected accordingly so that the
angular deviation is minimized as far as possible--within
predetermined tolerance limits. The optimum case would, of course,
be when the angular deviation of the correction angle .epsilon.
becomes 0.
[0045] When all the mirror facets have been aligned with their
respective optical axes in relation to the reference face, so that
they generate the intended beam of rays 27, then it may sometimes
still be necessary merely to correct the position of the support
plate 16 overall, if the position of the principal axis of the beam
of rays still needs to be aligned more accurately with the
subsequent facetted mirror.
[0046] Instead of the reference face of the mirror surface 15 of
the mirror facet 12 according to the representation in FIG. 3 left
bottom, of course, any selected measuring face of the support plate
16 may be used as a reference face, relative to which the
individual optical axes 20 of the mirror facets 12 will then be
aligned. In FIG. 4b, the measuring face is merely represented
briefly by the reference numeral 28. The represented measuring or
plane face 28 may, for example, be scanned with a coordinate
measuring machine and recorded by measuring techniques, after which
a relationship with the individual optical axes 20 of the mirror
facets 12 is established.
[0047] For alignment of the individual optical axes with the
reference face, owing to the spatial dimension of the beam of rays,
two tilt axes accordingly need to be taken into account when
processing the respective reprocessing face. In the example
represented in FIGS. 4a and 4b with the correction angle .epsilon.,
the second correction angle is in a plane lying at 90.degree. to
the correction angle .epsilon..
[0048] Instead of the ablation method as represented in FIGS. 4a
and 4b, of course, it is also possible to carry out an application
method, for example by evaporating metal interlayers which are
likewise to be applied in a wedge shape according to the correction
angle .epsilon.. This means, for example, that instead of the
wedge-shaped ablation of the step ledge 22a represented by dashes
in FIG. 4b, a metal layer is accordingly raised by evaporation in a
wedge shape as represented by dots and dashes. In the same way, of
course, a wedge-shaped metal interlayer may also be applied to the
mirror surface 15 instead of ablation of the mirror surface 15. The
same applies to the back face 15'.
[0049] When a metal interlayer is to be evaporated, for example,
gold may be used for this since it is very highly suitable with
respect to a good thermal junction as well as processing and
softness, together with adaptation to the shape in question. Other
metals may naturally also be used, for example noble metals,
gallium, platinum, silver or indium. It is advantageous to pick a
metal which can be deposited very easily but nevertheless produces
a good thermal contact.
[0050] Besides the evaporation of metal interlayers or IBF
processing, another option is to rotate the tilted mirror facets 12
and 12' about their longitudinal axes. The effect of rotation is
that the correction may then only need to be carried out in one
angular direction, so that it is possible to simplify the other
process steps. If a higher accuracy can be achieved only in one
direction, but not in another direction, then the effect of
rotating the mirror facets 12 may be that the accuracy is equally
high in all directions. If still necessary, the requisite accuracy
may be achieved after this by either ion beam figuring or
evaporation of metal interlayers.
[0051] FIG. 5 shows a mirror facet 12' with a conical mirror facet
head 13' and a cylindrical mirror facet base 14. Since the
structure of the mirror facet and the reception bore correspond
essentially to the exemplary embodiment according to FIG. 2, the
same reference numerals will also be used for the same parts. The
reception bore 22' is accordingly also designed to be conical in
the region of the mirror facet head 13' and cylindrical in the
region of the mirror facet base 14. The processing steps for the
reception bore 22' are carried out in the same steps as when there
is a cylindrical mirror facet head 13. It should be borne in mind,
however, that the lateral faces 21' must have a very high accuracy
since the mirror facet head 13' bears directly on the inner wall of
the reception bore 22' via the lateral faces 21'. The application
or evaporation of a wedge-shaped metal interlayer for the
reprocessing may be carried out on the mirror surface 15 according
to FIG. 4a and/or on the conical lateral face 21' or on the
reception bore 22' in the region of the lateral face 21'. The same
applies to IBF reprocessing.
[0052] The conical guide has an essential advantage. It is not
self-locking but self-centering, so as to provide a greater bearing
region for the mirror facet head 13' on the inner wall of the
reception bore 22' and therefore very good heat dissipation. Very
steep angles of the reception bore 22' and of the conical mirror
facet head 13' are preferred, in order to obtain a very good
position definition. The conical mirror facet head 13' may have a
radius of, for example, 2000 mm and a diameter of about 20 mm, in
each case measured on the mirror surface 15.
[0053] FIG. 6 shows a plan view of a facetted mirror 24. The
facetted mirror 24 is, for example, respectively represented with a
cylindrical mirror facet 12 and a conical mirror facet 12'. For the
sake of clarity, only two mirror facets 12 and 12' are actually
represented. If need be, of course, there will be substantially
more individual mirror facets in a facetted mirror 24. In practice,
moreover, there will generally be either only cylindrical mirror
facets 12 or only conical mirror facets 12' in one facetted
mirror.
[0054] The marking 23 for the azimuthal alignment can respectively
be seen in the--in the drawing--lower region of the mirror facets
12 and 12'. One or more marking holes 25 or reference faces 26 on
the facetted mirror 24 define the relation with a measuring system,
in order to align it according to its position in the projection
exposure system.
[0055] A facetted mirror 24 contains, for example, 200 mirror
facets, for example only with cylindrical mirror facets 12 or only
with conical mirror facets 12'. However, it is also possible to
provide a mixture of cylindrical and conical mirror facets 12 and
12'.
[0056] In a facetted mirror 24 with a support plate 16, the plate
diameter being about 30 cm, there may be angular differences
relating to the respective optical axes 20 of the individual mirror
facets 12 or 12' of, for example, 6.degree. with respect to a
principal direction. The reprocessing according to the invention is
carried out so that the respective optical axis of a mirror facet
is as exact as possible. The optical axis is in this case given by
the optical effect of the mirror surface as a function of its
shape.
[0057] For a corresponding angular accuracy of the optical axis, it
is therefore necessary for there to be a relation between the
mirror surface 15 and the longitudinal axis in the bore 22 of the
mirror facet 12. The measurements and corresponding reprocessing
operations described above need to be carried out for this reason.
In other words: the longitudinal axis 20 of a mirror facet 12
defines the direction, and the mirror surface 15 with its optical
axis defines the optical effect. The two axes need to be correlated
so that, in operation, the optical axis subsequently lies
accurately in the direction that is necessary in order to produce
an intended beam of rays from the multiplicity of individual mirror
facets 12 or 12'.
[0058] In general, the configuration and alignment of the
individual mirror facets 12 will be provided so that the normal of
the mirror surface 15 corresponds as accurately as possible to the
optical axis 20. This may not be necessary in principle, however,
since the crucial point is that the respective individual optical
axes should be aligned so as to obtain an intended beam of rays 27
which has a corresponding accuracy with respect to the principal
optical axis of the beam of rays and the shape and size of the beam
of rays.
[0059] Since the entire system is operated in a vacuum there should
be no blind regions, that is to say no regions which are fully
closed, in the region of the reception bores 22 and 22'.
Optionally, V-shaped groves extending in the longitudinal direction
may be formed in the reception bores 22 and 22' in order to
evacuate the faces 21 and 21'.
* * * * *