Projection exposure apparatus for semiconductor lithography with improved component adjustment, and adjustment method

Abstract

The invention relates to a projection printing system (1) for semiconductor lithography, comprising at least one component (21) and a structural member (22) of the projection printing system (1), wherein the component (21) is attached to the structural member, and the component (21) and / or the structural member (22) comprises at least one stopper (28, 40) for applying to a reference surface (25) on the structural member and / or the component (21). The stopper (28, 40) is designed so that it is movable relative to the component (21) and / or the structural member (22) attached to the structural member (22), so that the stopper can be moved away from the reference surface (25). Furthermore, the present invention relates to a method for adjusting a component (21) on a structural member (22) of a projection printing system (1), comprising the following steps: attaching at least one stop (28, 40) to the component (21) or the structural member (22); positioning the component (21) so that the at least one stop (28, 40) is in mechanical contact with a reference surface (25) on the component (21) or the structural member (22); attaching the component (21) to the structural member (22); and moving the stop (28, 40) away from the reference surface (25).

Description

[0001] This application claims the priority of German patent application DE 10 2018 200 524.9, the contents of which are incorporated herein in their entirety by reference.

[0002] The invention relates to a projection exposure apparatus for semiconductor lithography and to a method for adjusting components of a projection exposure apparatus.

[0003] Extremely stringent requirements are placed on projection exposure tools with regard to imaging accuracy, which depends primarily on the positioning of the optical elements of the projection exposure tool.

[0004] In the past, only a few optical elements were steerable, that is to say adjustable in terms of their position. All optical components are arranged on a structural part or frame, and further components such as actuators and sensors for manipulating the optical elements have also been fixed to the frame. The increasingly stringent demands placed on projection exposure apparatuses generation after generation with regard to the positioning of the optical elements have led to the use of a second frame which serves as a reference for the sensors and whose purpose is to reduce or avoid actuation of the sensors by the actuators, which would impair the positioning of the optical components.

[0005] In addition to the excitation decoupled in this way, the alignment of the actuator and the sensor during mounting on the frame becomes increasingly important. Even extremely accurate alignment of the actuator and the sensor during mounting needs to be achieved in up to 6 degrees of freedom, which, according to the prior art, is achieved by using exchangeable spacers between the screwing point and the stops of the actuator and the sensor, which are exchanged until the sensor and the actuator are correctly positioned. In this case, the structural part has a reference surface against which the stop is pushed and then tightened.

[0006] A disadvantage of this method of aligning and tightening the sensor and actuator is that the stresses can remain constant during the tightening of the sensor and actuator to the structural part due to the contact between the stop and the reference surface, and the stresses only relax again over time. Such relaxation leads to changes in the position of the components, which should be avoided.

[0007] The object of the invention is to specify a projection exposure apparatus and a method in which the positioning and screwing of a component to a structural part can be achieved without residual stresses.

[0008] This object is achieved by an apparatus having the features of the independent patent claim 1 and by a method having the features of the independent method claim. The dependent claims relate to advantageous developments and variants of the invention.

[0009] The projection exposure apparatus for semiconductor lithography according to the present invention comprises at least one component and a structural part of the projection exposure apparatus, wherein the component is fixed or can be fixed to the structural part. In addition, the component and / or the structural part has at least one stopper for abutting against a reference surface at the structural part and / or the component. According to the present invention, the stopper is embodied to be movable relative to the component and / or the structural part fixed to the structural part, so that the stopper can be moved away from the reference surface.

[0010] In other words, the stop is designed such that, after adjustment of the component, it can be moved away from the component at least to the extent that the stop is no longer in mechanical contact with the component and furthermore no forces are transmitted between the stop and the component. In particular, this avoids situations in which undesired changes in the position of the component occur in the event of slackening of the component on the structural part, i.e. mechanical stresses between the component and the reference surface due to mechanical creep are reduced.

[0011] In an advantageous embodiment of the invention, the stop element comprises a mounting element and a stop screw arranged in the mounting element, wherein the stop screw can comprise a contact surface, which can preferably be embodied in a convex manner.

[0012] By virtue of the fact that the stop comprises a stop screw, a desired separation between the stop and the reference surface can be achieved in a particularly simple manner by simply turning the stop screw.

[0013] If the stop screw is designed to be adjustable in length, this leads to further options for adjusting the component.

[0014] In particular, adjustability of the length of the stop screw can be achieved by virtue of the fact that the stop screw comprises an adjusting screw and a contact screw; in this case, the contact screw can be screwed into an internal thread in the shaft of the adjusting screw.

[0015] A change in the length of the stop screw can then be achieved by rotating and securing the contact screw in the adjustment screw.

[0016] By virtue of the fact that the contact screw can be embodied so as to be fixable in the adjusting screw by means of a locking screw, a protection against rotation of the contact screw relative to the adjusting screw can be achieved.

[0017] Furthermore, there may also be an anti-rotation guard for fixing the stop screw after it has been moved away from the reference surface; in this case, the anti-rotation guard may in particular comprise an elastically movable element and a cutout for receiving the elastically movable element.

[0018] The method according to the invention for adjusting a component on a structural part of a projection exposure apparatus comprises the following method steps:

[0019] - fixing at least one stop to the component or structural part,

[0020] - positioning the component so that at least one stop is in mechanical contact with the reference surface at the component or structural part,

[0021] - fastening components to structural parts,

[0022] - Move the stop away from the reference surface.

[0023] In this case, moving the stop away from the reference surface can be performed by turning back the stop screw.

[0024] If appropriate, after the component has been first fixed to the structural part, the position of the component on the structural part can be monitored. If during monitoring it is determined that a correction is required, the stop should first be adapted, and then an updated positioning, fixing and monitoring of the position of the component can be carried out.

[0025] Exemplary embodiments and variants of the present invention are explained in more detail below with reference to the accompanying drawings. In the drawings:

[0026] Figure 1 shows the basic construction of an EUV projection exposure apparatus in which the present invention can be implemented,

[0027] FIG. 2 shows structural parts and components with a stopper according to the prior art,

[0028] Figure 3 A first embodiment of the present invention is shown.

[0029] Figure 4 A second embodiment of the present invention is shown.

[0030] Figure 5 shows a detailed diagram of the present invention, and

[0031] Figure 6 A flow chart of the regulating method according to the present invention is shown.

[0032] Figure 1 An example of a basic construction of a microlithography EUV projection exposure apparatus 1 in which the invention can be used is shown. The illumination system of the projection exposure apparatus 1 has, in addition to the light source 3, an illumination optical unit 4 which illuminates an object field 5 in an object plane 6. EUV radiation 14 in the form of optical use radiation generated by the light source 3 is collimated by a collector which is integrated in the light source 3 so that the radiation passes through an intermediate focus in the region of an intermediate focal plane 15 and is then incident on the field facet mirror 2. Downstream of the field facet mirror 2, the EUV radiation 14 is reflected by a pupil facet mirror 16. By means of the pupil facet mirror 16 and an optical assembly 17 with mirrors 18, 19 and 20, the field facets of the field facet mirror 2 are imaged into the object field 5.

[0033] A reticle 7 is illuminated, which is arranged in the object field 5 and is held by a schematically illustrated reticle holder 8. A projection optical unit 9, which is only schematically illustrated, serves to image the object field 5 into an image field 10 in an image plane 11. The structures on the reticle 7 are imaged onto a photosensitive layer of a wafer 12, which is arranged in the region of the image field 10 in the image plane 11 and is held by a likewise partially illustrated wafer holder 13. The light source 3 can emit the radiation used, in particular in the range between 5 nm and 30 nm.

[0034] The invention can likewise be used in a DUV device, which is not explicitly shown here. A DUV device is in principle arranged similarly to the above-described EUV device 1, wherein mirrors and lens elements can be used as optical elements in the DUV device, and the light source of the DUV device emits the radiation used in the wavelength range of 100 nm to 300 nm.

[0035] FIG2 shows a sensor 21 which, in the example shown, is arranged on a sensor frame 22 as a structural part of a lithographic apparatus according to the prior art. The sensor frame 22 serves as a positioning Figure 1 The sensor 21 is used to determine the position and orientation of the mirror relative to the sensor frame 22 and therefore also relative to other mirrors, the position and orientation of the sensor being likewise determined relative to the same sensor frame 22. The alignment of such a sensor is a decisive factor for the accuracy with which the position and orientation of the mirror is determined and must therefore itself be sufficiently accurate and as reproducible as possible.

[0036] To this end, the sensor frame 22 shown in FIG2 exhibits an L-shaped protrusion 24 extending beyond the plane to which the sensor 21 is fixed (hereinafter referred to as the fixing plane 23). The two sides of the L-shaped protrusion 24, which are perpendicular to the fixing plane 23 and point to the sensor 21, together with the fixing plane 23 of the sensor 21, form a cube corner with which the sensor 21 is in contact (that is, mechanical contact) with three sides, and the three sides of the cube corner serve as reference surfaces 25 for aligning the sensor 21 in space. These three planes determine the orientation of the sensor 21 on the sensor frame 23 in all six degrees of freedom.

[0037] The first side surface of the sensor 21 (which is the side surface of the sensor 21 contacting the fixing plane 23) has three screwing points 26, by which the sensor 21 is fixed to the fixing plane 23. Compared with the first side surface, the second side surface of the sensor 21 (which points to the short side of the L-shaped protrusion 24) has a stopper not shown in the figure, through which the sensor 21 contacts the reference surface 25. The third adjacent side surface (which points to the long side of the L-shaped protrusion 24) has two stops, which are also not shown in the figure, and both contact the reference surface 25 at the stopper points 27. In this exemplary embodiment, the sensor 21 has six contact points with the sensor frame 22.

[0038] The above-mentioned reference surface 25 and the fixing plane 23 of the sensor frame 22 as well as the stops of the sensor 21 are subject to manufacturing fluctuations which have the following effect: after the sensor 21 is fixed, wherein all stops of the sensor 21 contact the reference surface 25 and the three screwing points 26 contact the fixing plane 23, that is to say all six contact points of the sensor 21 against the sensor frame 22, the position and alignment of the sensor 21 are not accurate enough. Therefore, in order to compensate for the manufacturing tolerances, all six contact points of the sensor 21 must be adjustable. The adjustment of the contact points is usually achieved by means of spacers. The spacers are very accurately manufactured washers, also called spacers. For each pair of sensor 21 and sensor frame 22, the thickness of the spacers for the six contact points is calculated individually.

[0039] The contact between the sensor 21 and the sensor frame 22 at six points may cause stresses during the fixing of the sensor 21. The stresses may vary over time, and the position of the sensor 21 relative to the sensor frame 22 may then vary. This variation over time, which is also called drift, is disadvantageous because its presence means that the increasing requirements of the latest generation systems regarding the drift stability of the sensor 21 can no longer be met.

[0040] In lithographic apparatuses, linear scales, capacitive sensors or interferometers can be used as sensors, by way of example. In the case of interferometers, which often have to measure relatively large distances between a reference surface of the sensor 21 and a reference surface of the object to be measured, this has led to very stringent requirements with regard to the accuracy of the alignment of the interferometer relative to the measured object, and in particular also with regard to the temporal stability of the orientation of the interferometer on the sensor frame 22. Therefore, any change over time in the orientation of the interferometer on the sensor frame 22 in the direction of light leads to a corruption of the measured values ​​and thus of the measured position of the measured component. In this context, a disadvantage of the arrangement shown in FIG. 2 is that, due to the fact that the interferometer is screwed to the sensor frame 22, initially mechanical stresses occur which relax over time and which mean that over time the increasing requirements with regard to the temporal stability of the distance measurement can no longer be met.

[0041] Figure 3 A first embodiment of the invention is shown. In the embodiment shown, the stresses remaining in the sensor due to the contact between the sensor and the sensor frame at all six contact points are not released until after the sensor has been tightened by means of the fact that the three stops 28, as shown in FIG. 2 , for positioning the sensor in the cube corner after the sensor has been tightened onto the fixing plane 23 of the sensor frame are adjusted so that they are no longer in contact with the reference surface 25 of the L-shaped protuberance 24 of the sensor frame, i.e. no longer in contact with the reference surface 25. Stress that may have been introduced into the sensor due to the tightening is thus released. Changes in the position of the sensor and / or changes in the alignment of the sensor relative to the sensor frame, i.e. changes due to the stress relief, can be compensated by calibration of the system. This calibration can be carried out in particular outside the manufacturing operation of the semiconductor lithography apparatus.

[0042] like Figure 3The depicted first embodiment of the stop 28 according to the invention comprises a stop mounting 29 or mounting and a stop screw 30. In this case, the stop 28 is arranged in a recess 34 in a base plate 35 of the sensor 21, which is not shown in its entirety in the figure. The sensor 21 and the stop 28 are in mechanical contact with each other via a sensor-side interface surface 36 and a stop-side interface surface 37. The stop screw 30 has a screw head 31 at a first end of its shaft and a contact surface 32 at a second end of the shaft, which contact surface is embodied in a convex manner in the example shown. The convex embodiment of the contact surface 32 leads to an advantageous point contact between the contact surface 32 and the reference surface 25 of the sensor frame. The solid line shows the position of the stop screw 30 in the mounting 29 before the sensor is screwed to the sensor frame, that is, the contact surface 32 abuts against the reference surface 25. The stop screw 30 is screwed into the mounting part 29 to such an extent that the underside of the screw head 31 bears on the side of the mounting part 29 facing the screw head and is pressed there, that is, the screw head 31 contacts the mounting part in a flat manner. This side of the mounting part is therefore the reference surface for the head 31 of the stop screw 30 in the fully screwed-in position.

[0043] Figure 3 The dotted lines in the figure (which are not covered by the solid lines) show the position of the stop screw 30 after the sensor 21 has been tightened to the sensor frame and the stop screw 30 has been screwed out of the mounting 29 to such an extent that the contact surface 32 of the stop screw no longer contacts the reference surface 25 of the sensor frame.

[0044] Figure 3 The stop screw 30 in the exemplary embodiment shown includes a hexagonal piece 33 formed on the periphery of the shaft at an area adjacent to the convex contact surface. Other shaped elements can also be matched to the shaft of the stop screw, and elements like the hexagonal piece 33 can be used as appropriate critical engagement areas.

[0045] After tightening by the arrangement shown in the figures, the stresses which may have remained due to the tightening of the sensor 21 on the sensor frame 22 can be directly released. Relaxation of the stresses over time, which would change the reference of the sensor 21, is thus advantageously avoided and the need for recalibration of the sensor 21 is substantially eliminated.

[0046] If the position and orientation of the sensor 21 relative to the sensor frame is not yet optimal after the initial tightening and a stop screw of a different length is required, this can be done. Figure 3 The stop screw 30 shown in FIG. 1 is unscrewed from the mount and replaced with a different stop screw having the desired length.

[0047] Figure 4 Other embodiments of the present invention are described in detail. Figure 4The stopper 40 shown in FIG. 4 comprises a stopper mounting 41 and a stopper screw 42, which in turn comprises an adjustment screw 43 and a contact screw 44. In the example shown, the stopper mounting is fixed to the sensor 21 by means of a screw 60.

[0048] exist Figure 4 In the illustrated mounting, the stop screw 42 is shown in a tightened state, that is to say the underside of the head of the stop screw 42 is fixedly pressed against the interface surface 45 of the mounting. As already mentioned, in the illustrated example, the stop screw 42 comprises a first part, the adjustment screw 43, and a second part, the contact screw 44, which is screwed into an internal thread formed in the shaft of the adjustment screw 43. For its part, the adjustment screw 43 is screwed into an internal thread formed in the mounting 41 via an external thread formed on its shaft.

[0049] The adjusting screw 43 can thus be moved relative to the mounting 41, on the one hand in order to be screwed into the mounting 41 until the bottom side of the screw head comes into contact with the interface surface 45 of the mounting 41 and is pressed onto the latter, and on the other hand in order to screw the adjusting screw 43 out of the mounting 41 in order to replace the latter or, after the sensor has been screwed to the sensor frame, to release the protruding contact surface 46 from the reference surface of the sensor frame, thereby creating a distance between the contact surface 46 and the reference surface in order to reduce stresses that may be generated by screwing the sensor to the sensor frame.

[0050] Advantageously, due to Figure 4 With the two-part construction of the stop screw shown, the length of the stop screw 42 can vary as the contact screw 44 is screwed out or in. In cases where the length of the stop screw 42 may need to be adapted to the optimal positioning of the sensor in the sensor frame, a single type of stop screw can be used, which greatly reduces the necessary inventory.

[0051] In order to fix the contact screw 44 after the desired length has been set, the thread can be supported; this can be achieved by a force acting on the shaft of the contact screw 44 and causing the side walls of the external thread of the contact screw 44 to be supported against the side walls of the internal thread of the adjustment screw 43, thus creating a frictional engagement between the two thread side walls, which in turn prevents undesired rotation. For the purpose of such support of the contact screw 44 and the adjustment screw 43, a so-called locking screw 47 can be used, such as Figure 4 The locking screw is guided through a through hole in the head of the adjusting screw 43 and is screwed into an internal thread introduced in the shaft of the contact screw 44. The external thread of the contact screw 44 can thus be supported by the internal thread of the adjusting screw 43, regardless of the screw-in depth, and rotation can thus be prevented. The force for supporting the thread can also be achieved by an arrangement which is not explicitly shown in the present case and which has a spring pressed onto the shaft of the contact screw 44.

[0052] Alternatively or additionally, the length of the stop screw 42 can also be changed by means of a washer 48 between the contact screw 44 and the adjustment screw 43, as also shown in FIG. Figure 4 Depending on the requirements in terms of the length of the stop screw 42 , a washer 48 (also referred to as a spacer) is milled to a specified thickness and then positioned between the head of the contact screw 44 and the adjustment screw 43 .

[0053] Alternatively or additionally, also as Figure 4 As shown, it is also conceivable to provide a washer 49 between the adjustment screw 43 and the interface surface 45 of the mounting member 41, so that although the length of the stop screw 42 cannot be adjusted, the relative position of the contact surface 46 relative to the interface surface 45 of the mounting member 41 can actually be adjusted.

[0054] In the first case, the necessary thread support between the contact screw 44 and the adjusting screw 43 is achieved by screwing the contact screw 44 into the adjusting screw 43 until the stop and the head of the contact screw 44 are pressed onto the shaft or the washer 48 of the adjusting screw 43 .

[0055] When the adjusting screw 43 is fully screwed in, the adjusting screw is tightened onto the washer 49 or the mounting 41 until the stop of the screw head, thereby ensuring the support of the thread. In this case, a locking screw is not necessary.

[0056] After the adjusting screw 43 has been turned back, it does not need to be protected from undesired rotation due to vibrations occurring during operation of the device.

[0057] Advantageously, an anti-rotation protection member may also be provided at the screw head of the adjusting screw 43, such as Figure 4 The illustrated embodiment may be implemented, for example, as a spring-loaded pin 50 as an elastically movable element passing through the screw head, which latches into a cutout 51 provided in the interface surface 45 of the mounting 41, or as a spring 52 fixed in the interface surface 45 of the mounting, which in turn latches as an elastically movable element into a cutout 53 provided on the periphery of the screw head. Both illustrated variants prevent further rotation of the stop screw 42 and thus prevent contact between the contact surface 46 of the stop 40 and the reference surface, or prevent the stop screw 42 from being completely screwed out of the mounting 41. This solution can also be used for Figure 3 An integral stop screw 30 is shown.

[0058] For other implementations of the present invention, for example, Figure 4As shown and described further above, this makes it possible, for example, to change the length of the stop screw 42 without removing the stop from the component or structural part, that is to say from the outside. If, in addition to the contact screw head, which is implemented, for example, as a hexagonal piece, a possibility is provided for fixing the adjusting screw in order to enable a movement, in particular a rotation, of the contact screw and adjusting screw relative to each other, a solution can be achieved with a washer 48 or a spacer between the contact screw 44 and the adjusting screw 43 without having to unscrew the stop from the sensor.

[0059] Figure 5 A top view of a sensor 21 and a stop 40 fixed to the sensor 21 is shown. In the example shown, the mounting is fixed to the sensor 21 by means of four screws 60. The stop 40 can also be fixed to a part of a structural part, such as a sensor frame, and a corresponding reference surface can be formed on the sensor. The arrangement is functionally identical and is influenced by other criteria, such as the construction space or accessibility of the component and / or structural part. The form of the mounting 41 and the type of tightening, that is to say the number and arrangement of the screws 60, are provided as examples and can also be implemented in any way.

[0060] Figure 6 It shows that according to the present invention Figure 3-5 The device is arranged as a method for adjusting and stress-free mounting of components on structural parts.

[0061] Prior to the method step shown in the figure, the length of the individual stop pieces is defined and the stop screws are installed into the mounting pieces of the stop pieces in such a way that the interface surface of the mounting piece carries the screw head over the entire area.

[0062] In a first method step, the stop previously mounted in this way is fastened to the component at the screwing points provided for this purpose.

[0063] In a second method step, the component is pushed into the reference stop provided for assembly, that is to say the cube corner.

[0064] In a third method step, the component is screwed to the structural part at its screwing points.

[0065] In a fourth method step, the position of the sensor is checked and, if appropriate, steps 1 to 3 are repeated using a different stop screw.

[0066] In a fifth method step, the stop screw is unscrewed from the mounting element, so that the contact existing between the stop element and the reference surface is eliminated and any stresses that may remain due to the tightening are relieved.

[0067] Reference numerals list

[0068] 1 Projection exposure equipment

[0069] 2-Field Facet Mirrors

[0070] 3 Light Source

[0071] 4 Illumination optical unit

[0072] 5. Material Field

[0073] 6 Object Plane

[0074] 7. Mask Master

[0075] 8 Mask master holder

[0076] 9 Projection optics unit

[0077] 10 Image Field

[0078] 11 Image plane

[0079] 12 chips

[0080] 13 Wafer holder

[0081] 14 EUV Radiation

[0082] 15 Intermediate focal plane

[0083] 16-Pupil Faceted Mirror

[0084] 17 Assembling parts

[0085] 18 Reflector

[0086] 19 Reflector

[0087] 20 Reflector

[0088] 21 Components, Sensors

[0089] 22 Structural parts, sensor frames

[0090] 23 Fixed plane

[0091] 24 Bump

[0092] 25 Reference surface

[0093] 26 Screw point

[0094] 27 Stop point

[0095] 28 Stopper

[0096] 29 Mounting parts

[0097] 30 Stop screw

[0098] 31 Screw head

[0099] 32 Contact Surface

[0100] 33 Hexagonal Parts

[0101] 34 Grooves

[0102] 35 Base Plate

[0103] 36 Sensor side interface surface

[0104] 37 Stopper side interface surface

[0105] 40 Stopper

[0106] 41 Mounting parts

[0107] 42 Stop screw

[0108] 43 Adjustment screw

[0109] 44 Contact screw

[0110] 45 Interface surface of mounting parts

[0111] 46 Contact surface

[0112] 47 Locking screw

[0113] 48 Washer

[0114] 49 Washer

[0115] 50 Elastic movable element

[0116] 51 Incision

[0117] 52 Elastic movable element

[0118] 53 Incision

[0119] 60 Screws

Claims

1. A projection exposure device (1) for semiconductor lithography, the projection exposure device (1) comprising: at least one component (21) and a structural part (22) of the projection exposure apparatus (1), - wherein the component (21) is fixed to the structural part (22), - wherein the component (21) and / or the structural element (22) has at least one stop (28, 40) for abutting against a reference surface (25) at the structural element or at the component (21), It is characterized in that The stopper (28, 40) is embodied to be movable relative to a component (21) fixed to the structural part (22) and / or the structural part (22) so that the stopper can be moved away from the reference surface (25), and wherein the stopper (28, 40) comprises a mounting part (29, 41) and a stop screw (30, 42) arranged in the mounting part (29, 41), wherein fixing the component (21) on the structural part (22) comprises maintaining the fixation between the component (21) and the structural part (22) in a state in which the stopper (28, 40) is spaced apart from the reference surface (25).

2. The projection exposure apparatus (1) according to claim 1, It is characterized in that The stop screw (30, 42) comprises a contact surface (32, 46) which is embodied in a convex manner.

3. The projection exposure apparatus (1) according to claim 1 or 2, It is characterized in that The stop screw (30, 42) is designed to be adjustable in length.

4. The projection exposure apparatus (1) according to claim 3, It is characterized in that The stop screw (42) includes an adjustment screw (43) and a contact screw (44).

5. The projection exposure apparatus (1) according to claim 4, It is characterized in that The length of the stop screw (42) is changed by rotating the contact screw (44) and fixing it in the adjustment screw (43).

6. The projection exposure apparatus (1) according to claim 4 or 5, It is characterized in that The contact screw (44) can be fixed in the adjusting screw (43) by means of a locking screw (47).

7. The projection exposure apparatus (1) according to any one of the preceding claims, It is characterized in that There is an anti-rotation guard for securing the stop screw (42) after it has moved away from the reference surface (25).

8. The projection exposure apparatus (1) according to claim 7, It is characterized in that The anti-rotation guard comprises an elastically movable element (50, 52) and a cutout (51, 53) for receiving the elastically movable element (50, 52).

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