The contents of German patent application DE 10 2012 202 167.1 is incorporated by reference.

The invention relates to a device for the magnetic-field-compensated positioning of a component, to an optical assembly comprising such a device, and to a method for the magnetic-field-compensated positioning of an optical component. The invention furthermore relates to an optical unit for a projection exposure apparatus, to an illumination system for a projection exposure apparatus, and to a projection exposure apparatus. Finally, the invention relates to a method for producing a microstructured component, and to a component produced according to the method. A projection exposure apparatus for microlithography is known from

EP 1 884 831 A2, for example. In order to achieve the required accuracies in such a projection exposure apparatus, it is necessary to arrange the optical components very precisely. There is a constant need for improvement in this regard.

The invention is therefore based on the object of improving a device for mounting a component, in particular an optical component.

This object is achieved by means of the features of Claim 1. The heart of the invention consists in providing a device for mounting a component with means for compensating for an external magnetic field. The means are designed, in particular, in such a way that at least one component of a predefined external magnetic field can be compensated for at least partly, in particular to the extent of at least 50%, in particular to the extent of at least 70%, in particular to the extent of at least 90%, in particular to the extent of at least 95%, in particular to the extent of at least 99%. Preferably, the means are designed in such a way that a plurality of, in particular all, components of a predefined external magnetic field can be compensated for at least partly, in particular to the extent of at least 50%, in particular to the extent of at least 70%, in particular to the extent of at least 90%, in particular to the extent of at least 95%, in particular to the extent of at least 99%.

The external magnetic field can be static or temporally variable. In particu- lar in the case of temporally variable external magnetic fields, so-called alternating fields, a compensation thereof is particularly advantageous in order to reduce, in particular to completely avoid, undesirable inductive effects on the components of a projection optical unit. Accordingly, the compensation of the external magnetic field can also be static or dynamic, in particular controllable, preferably controllable by closed-loop control. The means for compensating for the external magnetic field can also comprise either static means, for example permanent magnets, or dynamic means, for example electromagnets. The external magnetic field can have various causes and/or sources. It can be caused for example by magnets in the wafer mount. Since the wafer mount can be displaceable relative to the lens, this can lead to a temporally variable magnetic field. By virtue of the compensatability of an external magnetic field in the region of the holding unit, the effects of said external magnetic field on the holding unit can be at least reduced, in particular completely prevented. In particular a resultant force on the holding unit can be reduced, in particular completely avoided. Furthermore, magnetic-field-induced deformations of the holding unit can be reduced, in particular completely avoided. In particular, this makes it possible to improve the precision of the holding device for positioning the optical component. This is advantageous, in particular, if the holding unit at least partly consists of a magneto strictive, an electrically conductive and/or a magnetizable material.

Preferably, the means for compensating for the external magnetic field comprise means for generating an opposing field. The opposing field is oriented in particular at least in sections in an antiparallel fashion, i.e. oppositely, with respect to the external magnetic field.

The opposing field that can be generated by means of the compensation means has, in particular, at least one controllable amplitude distribution. In principle, it can also have a controllable directional distribution. The amplitude and/or directional distribution of the opposing field can thus be flexibly adapted to those/that of the external magnetic field. Such an opposing field can be generated in a particularly simple manner by means of coils. Generally, the means for generating a suitable opposing field comprise at least one coil. They can comprise in particular a plurality of coils, in particular at least three coils, in particular at least six coils. A larger number of coils improves the flexibility of the generation of an op- posing field having a predefined amplitude and directional distribution.

In the case of a coil, the amplitude of the opposing field that can be generated by it can be influenced in a simple manner by control of the coil cur- rent. The coils are electrically connected in particular via suitable amplifiers to a control unit for controlling the coil current.

The coils have, in particular, at least one turn which is arranged around a holding strut of the holding unit. What is thereby achieved is that the opposing field that can be generated by means of the coils is oriented at least in sections parallel to the holding struts of the holding unit. As a result, it is possible to effectively prevent a change in length of the holding struts along their longitudinal direction on account of the effects of the external magnetic field.

The coils for generating the opposing field can advantageously be arranged in a contact-free fashion with respect to the holding struts. They can advantageously also be arranged in a contact-free fashion with respect to the op- tical component. They are arranged, in particular, in a manner mechanically decoupled from the optical component.

An arrangement of one or a plurality of coils around the projection lens is also possible.

Preferably, the means for compensating for the external magnetic field are part of a regulating unit.

The regulating unit comprises at least one sensor besides the coils and the control unit for controlling the coil current. The sensor is designed in such a way that it can detect a change in position of the optical component in at least one direction. In particular a change in position of the optical component in a plurality of, in particular at least two, in particular at least three, linearly independent directions can be detected by means of the sensor. In principle, it is also possible to provide sensors which can detect the absolute position of the optical component. Preferably, each of the coils for generating the opposing field is assigned a respective sensor. In the case of a pairwise arrangement of the holding struts and the associated coils, it is possible in this case to provide a respective common sensor for two pair- wise associated coils.

The sensors are arranged, in particular, in each case adjacent to one of the holding struts of the holding unit. In the case of a pairwise arrangement of the holding struts, the sensors are arranged in each case adjacent to one of the bipods formed by two associated holding struts.

The sensors are, in particular, contactless sensors. In particular optical, ca- pacitive or Hall sensors can be provided.

In particular, at least three sensors are provided. The sensors are arranged, in particular, in such a way that they span a plane. They are preferably arranged in a manner spaced apart uniformly with respect to one another. In the case of three sensors, they are arranged, in particular, in an equilateral triangle. The plane spanned by the sensors is preferably parallel to a reflection surface of the optical component designed as a mirror.

The sensors are preferably connected to the control unit in a data- transmitting manner. Provision is made, in particular, for designing the regulating unit as a closed loop. The regulating unit can have in particular a plurality of, in particular at least three, such closed loops. Each closed loop comprises at least one of the sensors and at least one, in particular at least two, of the coils. It additionally preferably comprises in each case a sepa- rate amplifier, which is electrically connected to the coils and via which the coil current is controllable.

Provision can be made for using at least one, in particular a plurality of permanent magnets as means for compensating for the external magnetic field. Said magnets are useful, in particular, for compensating for a temporally constant external magnetic field and/or for at least partly compensating for an external magnetic field having a constant average value that differs from zero.

A combination of one or a plurality of permanent magnets with the coils described above can be particularly advantageous.

What can be achieved, in particular, by means of the compensation means is that the positioning of the optical component is independent of the external magnetic field. According to the invention, it has been recognized that an external magnetic field, in particular a temporally variable external magnetic field, can have an influence on the positioning of the optical component. This influence is compensated for at least partly, in particular to the greatest possible extent, in particular to the extent of at least 50%, in particular to the extent of at least 70%, in particular to the extent of at least 90%, in particular to the extent of at least 95%, in particular to the extent of at least 99%, by the means according to the invention for at least partly compensating for the external magnetic field. The absolute precision of the positioning of the optical component is significantly improved as a result. In particular a positioning of the optical component with a precision of better than 10 nm, in particular better than 3 nm, in particular better than 1 nm, is possible by means of the device according to the invention. This is advantageous in particular in the positioning of optical components in an EUV projection exposure apparatus.

A further object of the invention consists in improving an optical assembly. This object is achieved by means of the features of Claim 9. The advantages correspond to those described above.

The optical component is, for example, a mirror, in particular a mirror for electromagnetic radiation in the EUV range.

It goes without saying that other components, for example a reticle holder or a wafer holder, or other optical components can also be positioned precisely by means of the holding device according to the invention. A further object of the invention consists in improving a method for positioning an optical component. This object is achieved by means of the features of Claim 10. The heart of the invention consists in generating an opposing field having a predefined directional and amplitude distribution for the compensation of an external magnetic field. The precision of the posi- tioning of the optical component is significantly improved as a result. The parameters of the opposing field, in particular the amplitude and/or directional distribution thereof, can be temporally variable. They can be in particular controllable, in particular controllable by closed-loop control. They can preferably be flexibly adapted to variations of the external magnetic field. Preferably, at least one closed loop is provided for adapting at least one parameter of the opposing field to a temporal variation of the external magnetic field. The adaptation of the opposing field can thus preferably be effected automatically. It is preferably effected substantially without any delay with respect to a variation of the external magnetic field. A further object of the invention consists in improving an optical unit for a projection exposure apparatus. This object is achieved by means of the features of Claim 1 1. The advantages correspond to those described above.

The optical unit is, in particular, an illumination optical unit for illuminating an object field or a projection optical unit for imaging the object field into an image field. Further objects of the invention consist in improving an illumination system for a projection exposure apparatus and a projection exposure apparatus.

These objects are achieved by means of the features of Claims 12 and 13. The advantages correspond to those described above.

Finally, one object of the invention consists in improving a method for producing a microstmctured component and a component produced in this way.

These objects are achieved by means of the features of Claims 14 and 15. For the advantages, reference should be made to those described above.

Further advantages and details of the invention are evident from the de- scription of exemplary embodiments with reference to the drawings, in which:

Figure 1 schematically shows a meridional section through a projection exposure apparatus for EUV projection lithography, Figure 2 shows a schematic illustration of an optical component with a device according to the invention for the magnetic-field- compensated positioning thereof,

Figure 3 shows a schematic illustration of an excerpt from an optical component with a device according to the invention in accordance with a further exemplary embodiment, Figure 4 shows a schematic illustration of an excerpt from an optical component with a device according to the invention in accordance with a further exemplary embodiment,

Figure 5 shows a schematic illustration of an excerpt from an optical com- ponent with a device according to the invention in accordance with a further exemplary embodiment, and

Figure 6 shows a schematic illustration of a further arrangement of a device according to the invention for compensating for an external magnetic field.

Figure 1 schematically shows a projection exposure apparatus 1 for micro- lithography in a meridional section. An illumination system 2 of the projection exposure apparatus 1 has, alongside a radiation source 3, an illumination optical unit 4 for exposing an object field 5 in an object plane 6. In this case, a reticle 7 arranged in the object field 5 is exposed, said reticle being held by a reticle holder 8 (only partially illustrated). A projection optical unit 9 serves for imaging the object field 5 into an image field 10 in an image plane 1 1. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 12 arranged in the region of the image field 10 in the image plane 1 1, said wafer being held by a wafer holder 13 (likewise illustrated schematically). The radiation source 3 is an EUV radiation source having an emitted used radiation in the range of between 5 nm and 30 nm. The radiation source 3 can also be a radiation source having an emitted used radiation in a different wavelength range. However, the high-precision positioning according to the invention of an optical component is advantageous in particular for use in an EUV projection exposure apparatus. In this case, a plasma source can be involved, for example a GDPP (gas discharge-produced plasma) source or an LPP (laser-produced plasma) source. A radiation source based on a synchrotron can also be used for the radiation source 3. Information concerning such a radiation source can be found by the person skilled in the art for example from US 6,859,515 B2. EUV radiation 14 emerging from the radiation source 3 is concentrated by a collector 15. Downstream of the collector 15, the EUV radiation 14 propagates through an intermediate focal plane 16 before it impinges on a field facet mirror 17 having a multiplicity of field facets 23. The field facet mirror 17 is arranged in a plane of the illumination optical unit 4 which is optically conjugate with respect to the object plane 6.

The EUV radiation 14 is also designated hereinafter as illumination light or as imaging light.

Downstream of the field facet mirror 17, the EUV radiation 14 is reflected by a pupil facet mirror 18 having a multiplicity of pupil facets 24. The pupil facet mirror 18 is arranged in a pupil plane of the illumination optical unit 4 which is optically conjugate with respect to a pupil plane of the pro- jection optical unit 9. With the aid of the pupil facet mirror 18 and an imaging optical assembly in the form of a transfer optical unit 19 having mirrors 20, 21 and 22 designated in the order of the beam path, field facets of the field facet mirror 17 are imaged into the object field 5. The last mirror 22 of the transfer optical unit 19 is a mirror for grazing incidence ("grazing incidence mirror"). The pupil facet mirror 18 and the transfer optical unit 19 form a sequential optical unit for transferring the illumination light 14 into the object field 5. The transfer optical unit 19 can be dispensed with, in particular, when the pupil facet mirror 18 is arranged in an entrance pupil of the projection optical unit 9.

For simpler description of positional relationships, a Cartesian xyz coordinate system is depicted in Figure 1. In Figure 1 , the x-axis runs perpendicularly to the plane of the drawing into the latter. The y-axis runs towards the right. The z-axis runs downward. The object plane 6 and the image plane 1 1 both run parallel to the xy plane.

The reticle holder 8 is displaceable in a controlled manner such that during the projection exposure the reticle 7 can be displaced in a displacement direction in the object plane 6 parallel to the y-direction. Correspondingly, the wafer holder 13 is displaceable in a controlled manner such that the wafer 12 is displaceable in a displacement direction in the image plane 1 1 parallel to the y-direction. As a result, the reticle 7 and the wafer 12 can be scanned firstly by the object field 5 and secondly by the image field 10. The displacement direction is also designated hereinafter as the scanning direction. The displacement of the reticle 7 and of the wafer 12 in the scanning direction can preferably be effected synchronously relative to one another. The projection optical unit 9 comprises at least one optical component for imaging the object field 5 into the image field 10. The optical component is, in particular, a mirror. The latter preferably bears a multilayer coating for optimizing the reflectivity of the wavelength of the used radiation 14.

The projection optical unit 9 comprises, in particular, at least four mirrors. It can have five, six, seven, eight or more mirrors. In this case, one or a plurality of the mirrors can have a through-opening for the used radiation 14. In particular the mirror which is arranged closest to the image field 10 and which forms the penultimate mirror in the beam path of the projection optical unit 9 can have a through-opening for the used radiation 14.

Further aspects of a device 26 for the magnetic-field-compensated positioning of a component 25, in particular of an optical component, in particular of a mirror 30, are described below with reference to Figure 2.

The device 26 for the magnetic-field-compensated positioning of the optical component 25 comprises a holding unit 27 for mounting the optical component 25. The holding unit 27 comprises a baseplate 28 and six hold- ing struts 29. The baseplate 28 is also designated as a mounting block. It is composed of a non-magnetic material. It can be composed, in particular, of a glass ceramic. It is preferably composed of a material having a low coefficient of thermal expansion. The coefficient of thermal expansion of the material of the baseplate 28 is, in particular, <10 ^"6 /K. The baseplate 28 can, in principle, also be composed of a magnetic material.

The holding struts 29 are arranged on the baseplate 28 and mechanically connected thereto. They are arranged respectively in pairs. Two associated holding struts 29 are in each case also designated as a bipod. The six hold- ing struts 29 together form a so-called hexapod. The three bipods of the hexapod are arranged in an equilateral triangle on the baseplate 28.

An alternative number and/or arrangement of the holding struts 29 on the baseplate 28 are/is likewise possible. Generally, the holding unit 27 comprises at least one holding strut 29.

The holding struts 29 extend in each case in a longitudinal direction 32, which is depicted only for one of the holding struts 29 in Figure 2. They have in particular an extent in the range of 1 cm to 100 cm, in particular in the range of 2 cm to 60 cm, in the longitudinal direction 32. Perpendicularly to the longitudinal direction 32, the holding struts 29 have for example a round, in particular circular, or polygonal, in particular square or hexagonal, cross section. Alternative embodiments of the holding struts 29 are likewise possible.

The holding struts 29 are connected to the mirror 30 in each case at their opposite end relative to the baseplate 28. The holding struts 29 have a flexure 31. The flexure 31 is arranged, in particular, in the region of an end of the holding struts 29 which faces the mirror 30. It can be designed as a circumferentially extending, bead-shaped constriction. Such a constriction facilitates bending of the holding strut 29 in a direction transversely with respect to the longitudinal direction 32 thereof.

The flexure 31 forms a decoupling element for mechanically decoupling the mirror 30 from the holding struts 29. By means of the decoupling ele- ment, the mirror 30 is decoupled from the holding struts 29 in particular in at least one direction.

Instead of a circumferentially extending constriction, the holding strut 29 can also have a constriction which reduces the cross section of the holding strut 29 in a first direction, while it leaves said cross section substantially unchanged in a second direction perpendicular to said first direction. This is also designated hereinafter as a one-dimensional or linear constriction, since it facilitates bending of the holding strut 29 exclusively in one direc- tion, namely in the direction of the reduced cross section. The flexure 31 has one degree of freedom in this case.

Preferably, in the case of a linear constriction, two constrictions of this type i.e. two flexures 31, are provided per holding strut 29, which are arranged in a manner rotated relative to one another. The two constrictions are preferably arranged perpendicularly to one another and thus form a type of Cardan joint.

The holding struts 29 are composed of a magneto strictive material, for ex- ample Invar, nickel or a samarium- iron compound. The material of the holding struts 29 preferably has a low coefficient of thermal expansion. The coefficient of linear thermal expansion of the material of the holding struts is in particular at most 10 ^"5 /K, in particular at most 5· 10 ^"6 /K, preferably at most 3 · 10 ^"6 /K.

The holding struts 29 have an expansion which can be varied by the action of an external magnetic field. The external magnetic field can be caused, for example, by magnets of the wafer holder 13. Coils 33 are provided as means for compensating for the external magnetic field. The coils 33 form, in particular, means for generating a magnetic field in the region of the holding unit 27. They are part of the device 26. An opposing magnetic field can be generated by means of the coils 33. The maximum field strength of the opposing field that can be generated by means of the coils 33 in the region of the holding struts 29 is in the range of approximately 1 μΤ to approximately 1 T. It is dependent, inter alia, on the number of turns of the coils 33 and can be flexibly adapted to the respective requirements.

By means of the opposing field that can be generated by the coils 33, a change in length of the holding struts 29, in particular in the longitudinal direction 32, on account of the external magnetic field and/or a corresponding magnetic-field-induced force can be prevented at least partly, in particular to the greatest possible extent. What can be achieved, in particular by means of the opposing field is that the maximum change in length of the holding struts 29 that is caused by the external magnetic field in the longitudinal direction 32 is less than 10 nm, in particular less than 3 nm, in particular less than 1 nm. The maximum relative change is at most 10 ^"5 , in particular at most 10 ^"6 , in particular at most 10 ^"7 . In particular an undesirable tilting of the mirror 30 can be prevented by the generation of a suitable opposing field for compensating for the external magnetic field. With the aid of the device 26 according to the invention, the mirror 30 can be positioned in particular in such a way that its optical axis 39 runs in a predefined direction with an accuracy of better than 0.1 mrad, in particular better than 0.01 mrad, in particular better than 0.001 mrad.

The coils 33 each have at least one turn which is arranged around one of the holding struts 29. Preferably, one of the coils 33 is arranged around each of the holding struts 29. The number of coils 33 thus corresponds pre- cisely to the number of holding struts 29. The coils 33 extend over a length of at least 50%, in particular at least 75%, in particular at least 90%, of the respectively assigned holding struts 29. The coils 33 are arranged, in particular, in such a way that the magnetic field that can be generated by them, in the region of the associated holding strut 29, is oriented in each case parallel to the longitudinal direction 32 thereof. The coils 33 are mechanically decoupled from the mirror 30. They are arranged, in particular, in a contact- free manner with respect to the mir- ror 30. They can furthermore be arranged in a contact- free manner with respect to the holding struts 29.

The coils 33 are preferably controllable independently of one another. In accordance with the embodiment illustrated in Figure 2, the two coils 33 arranged around the holding struts 29 of a bipod are electrically connected to an amplifier 34 in each case by means of supply leads 41. For driving the six coils 33 of the three bipods of the holding units 27, the device thus comprises six amplifiers 34. For their part, the amplifiers 34 are connected to a common control unit 35 in a signal-transmitting manner. By means of the control unit 35, the electric current fed to the coils 33 via the amplifiers 34 is controllable in such a way that the opposing field that can be generated by means of the coils 33 in the region of the holding unit 27, in particular in the region of the holding struts 29, has a predetermined directional and amplitude distribution. A magnetic-field-compensated position- ing of the mirrors 30 is achieved as a result. For the compensation of the external magnetic field, that is to say for the precise positioning of the mirror 30, the coils 33 generate an opposing field having a predefined directional and amplitude distribution in the region of the holding unit 27, in particular in the region of the holding struts 29. This leads to an at least partial compensation of the external magnetic field in the region of the holding struts 29 and thus counteracts a change in length thereof. The precision of the positioning of the mirror 30 is improved as a result. The external magnetic field can be compensated for by the opposing field in particular to the extent of at least 50%, in particular to the extent of at least 70%, in particular to the extent of at least 90%, in particular to the extent of at least 95%, in particular to the extent of at least 99%. In one particularly advantageous embodiment, each of the bipods is assigned a sensor 36. The sensor 36 can be incorporated, i.e. integrated, in particular into the holding strut 29. A change in position and/or the position of the mirror 30 in the region of the associated bipod can be detected by means of the sensor 36. In particular a change in length and/or the position of the mirror 30 in at least one direction, in particular in the plane spanned by the holding struts 29 of the bipod associated with the respective sensor 36, can be detected by means of the sensor 36.

The sensor 36 can be, in particular, an optical, a capacitive or a Hall sensor. It is, in particular, a contactless sensor. The sensor 36 is connected to the control unit 35 in each case in a data- transmitting manner.

By means of the sensors 36 it is possible to activate the means for compensating for the external magnetic field in a manner regulated by closed-loop control. The means for compensating for the external magnetic field are, in particular, part of a regulating unit. The regulating unit can be designed, in particular, as a closed loop. Preferably, a separate closed loop is provided for each bipod. Provision can be made, in particular, for regulating each coil 33 with a dedicated closed-loop control circuit. As sensor 36 it is pos- sible to use the position sensor at the optical element or a Hall sensor at the bipods.

The regulating unit 35 comprises the sensors 36 and actuators 37, which are formed by the holding struts 29 together with the coils 33.

Furthermore, the device 26 for positioning the mirror 30 can comprise one or a plurality of permanent magnets 40. In particular it is possible to provide in each case two permanent magnets 40 per holding strut 29. The permanent magnets 40 can be at an adjustable distance from the holding strut 29. They can also be arranged in a pivotable manner with respect to the holding strut 29, in particular in such a way that their pivoting axis runs perpendicularly to the longitudinal direction 32. In particular a constant external magnetic field can be compensated for at least partly, in particular completely, by means of the permanent magnets 40. The permanent magnets 40 can be arranged, in particular, in a manner displaceable relative to the mirror 30. By means of a displacement of the permanent magnets 40, the amplitude and/or directional distribution of the magnetic field generated by them in the region of the holding unit 27 can be varied, in particular suitably adapted to the corresponding parameters of an external magnetic field.

A further exemplary embodiment of the device 26 is described below with reference to Figure 3. Identical parts are given the same reference signs as in the above-described exemplary embodiment, to the description of which reference is hereby made. In accordance with the exemplary embodiment illustrated in Figure 3, the holding struts 29 in each case have two flexures 31. The flexures 31 here are arranged in each case in the region of the mutually opposite ends of the holding struts 29. The flexure 31 provided in the region of the opposite end of the holding strut 29 relative to the component 25 improves the mechanical flexibility of the connection of the holding strut 29 to the baseplate 28. A sufficient stiffness is ensured by the arrangement of two holding struts 29 running obliquely with respect to one another in a bipod. The holding struts 29 are mechanically connected in each case by means of sleeve-shaped fixing elements 42 firstly to the baseplate 28 and secondly to a connecting element 43. For its part, the connecting element 43 is fixedly connected to the optical component 25. It can be adhesively bonded, in particular, to the optical component 25. As illustrated by way of example in the right-hand half of Figure 3, one advantageous embodiment can provide for arranging shielding elements 47, in particular in the form of magnetically permeable plates, around the coils 33. The magnetically permeable plates are composed of a soft-magnetic material. They are composed, in particular, of a material having a relative permeability of at least 10 000, in particular at least 30 000, in particular at least 50 000. Such magnetically permeable plates 47 can be provided in all embodiments.

Figure 4 illustrates a further exemplary embodiment of a device 26 according to the invention. Identical parts are once again given the same reference signs as in the previous exemplary embodiments, to the description of which reference is hereby made.

In the exemplary embodiment in accordance with Figure 4, the coils 33 are in each case arranged in the region of one of the flexures 31. They are ar- ranged, in particular, in the region of the circumferentially extending bead- shaped constriction. Such an arrangement of the coils 33 is particularly space-saving. Moreover, at these geometrically thin webs having low external fields, a high flux density can be established in the flexures 31. In the exemplary embodiment illustrated in Figure 4, in each case two coils 33 are provided per holding strut 29.

The two coils 33 at the holding strut 29 can in each case be connected to the same amplifier 34. They can be controllable jointly or independently of one another.

A further exemplary embodiment of the invention is described below with reference to Figure 5. Identical parts are given the same reference signs as in the above-described exemplary embodiments, to the description of which reference is hereby made.

In this exemplary embodiment, two Helmholtz coils 44, i.e. a pair of Helmholtz coils, are provided as means for compensating for the external magnetic field in the region of the holding unit 27. In principle, it is also possible to provide more than one pair of Helmholtz coils, in particular three pairs of Helmholtz coils oriented orthogonally to one another.

The Helmholtz coils 44 of the at least one pair of Helmholtz coils are oriented, in particular, in such a way that their coil axis 45 coincides precisely with the optical axis 39 of the optical component 25.

The Helmholtz coils 44 are held by a holding element 46. The holding element 46 is designed in particular in a hollow-cylindrical fashion. The holding element 46 is open at the end in the direction of the optical axis 39. It can be completely closed in the direction perpendicular to the optical axis 39.

The holding element 46 is composed of a non-magnetic material. Conse- quently, it has no influence on the magnetic field that can be generated by the Helmholtz coils 44. The holding element 46 can project beyond the optical component 25 in the direction of the optical axis 39. It is spaced apart from the optical component 25 in the direction perpendicular to the optical axis 39. The Helmholtz coils 44 have a diameter D that is greater than a diameter d of an envelope of the holding unit 27.

An arrangement of Helmholtz coils 44 is also possible in the embodiments described above. Conversely it is also conceivable, in the exemplary embodiment in accordance with Figure 5, additionally to arrange coils 33 around the holding struts 29 and/or in the region of one or both of the flexures 31.

A further arrangement of the coil 33 for compensating for the external magnetic field is described below with reference to Figure 6. In this em- bodiment, the coil 33 is arranged around a lens 48 (merely illustrated schematically in the figure) of the projection optical unit 9. It is arranged, in particular, in the region of an end of the lens that faces the way of the wafer holder 13. With the aid of such an arrangement, in particular the magnetic field generated by permanent magnets 49 of the wafer holder 13 in the region of the lens 48 can be compensated for at least partly, in particular completely. Since the wafer holder 13 is displaceable relative to the lens 48, a temporally variable magnetic field, i.e. a so-called alternating field, is involved here. With the aid of the coil 33, in particular the indue- tion of currents in the lens 48, in particular in a mount of the lens 48, can be reduced, in particular prevented.

Figure 6 schematically illustrates a single coil 33. One, two, three or more coils 33 can be provided as necessary. A larger number of coils 33 improves the flexibility, i.e. the possibility for particularly comprehensive compensation of different external magnetic fields.

As in the exemplary embodiments described above, the coil 33 is con- nected to the control unit 35. It can advantageously be part of a regulating unit comprising a sensor, in particular a Hall sensor. It can also be connected passively via an amplifier circuit such that an undesirable external magnetic field in the region of the coil 33, in particular in the region of the lens 48, is compensated for at least partly, in particular completely, by the opposing field generated by the coil 33.

Generally, the coil 33 is arranged in the region of the optical component 25 in which currents can be induced on account of the external magnetic field, for example on account of a displacement of the wafer holder 13.

The optical component 25 together with the positioning device 26 forms an optical assembly 38. The latter can be part of the illumination optical unit 4 or of the projection optical unit 9 of the projection exposure apparatus 1. It is also possible for a plurality of the mirrors of the illumination optical unit 4 and/or of the projection optical unit 9 to be provided with a corresponding device 26. In principle, it is conceivable to provide all mirrors of the illumination optical unit 4 and/or of the projection optical unit 9 with the positioning device 26 according to the invention. During the use of the projection exposure apparatus 1 with one of the collector variants described above, the reticle 7 and the wafer 12 are provided, said wafer bearing a coating that is light-sensitive to the illumination light 14. Subsequently, at least one section of the reticle 7 is projected onto the wafer 12 with the aid of the projection exposure apparatus 1. During the projection of the reticle 7 onto the wafer 12, the reticle holder 8 and/or the wafer holder 13 can be displaced in a direction parallel to the object plane 6 and/or parallel to the image plane 1 1 , respectively. The reticle 7 and the wafer 12 can preferably be displaced synchronously with respect to one another. Finally, the light-sensitive layer exposed by the illumination light 14 on the wafer 12 is developed. A micro- or nanostructured component, in particular a semiconductor chip, is produced in this way.