WO2008071269A1

WO2008071269A1 - Apparatus and method for mask metrology

Info

Publication number: WO2008071269A1
Application number: PCT/EP2007/009601
Authority: WO
Inventors: Yim-Bun-Patrick Kwan; Stefan Xalter
Original assignee: Carl-Zeiss Sms Gmbh
Priority date: 2006-12-15
Filing date: 2007-11-06
Publication date: 2008-06-19
Also published as: TW200839465A; WO2008071269A8

Abstract

There is provided a mask metrology apparatus (1) comprising: a mask holder (2) for holding a mask (3) having an object plane with a plurality of marks (4), a mask positioning device (5) for positioning the mask holder in a predetermined position, measurement optics (6) for measuring the position of the marks (4) of the mask (3) held by the mask holder (2), wherein the mask holder (2) holds the mask (3) with the object plane substantially parallel to the direction of gravity and supports the mask (3) along a horizontal edge (13, 35) of the mask (3) with a uniformly distributed force against gravity.

Description

APPARATUS AMD METHOD FOR MASK METROLOGY

The invention relates to a mask metrology apparatus as well as to a measuring method for a mask.

In current generation of photomask metrology apparatuses one of the major factors contributing to the limit of absolute accuracy is the deformation of the mask itself under gravity. It is common practice to support the mask kinematically at three points and to correct then the elastic distortion of the mask in the object plane due to gravity by using results from Finite Element Analysis or by calibration using multiple measurements with different reticle orientations. The amount of distortion to be corrected amounts to some 20 mm in the scan direction of the metrology apparatus, some 80 nm in the transferred direction and nearly 1 μm out-of-plane. While these amounts can be subtracted numerically from the measurement results without problems, it is questionable how reliable the final results may be when one is looking for accuracy which is substantially less than 5% of the amount of correction being applied.

A further contributing factor to inaccuracy is the variation in reflected index of air to pressure, temperature and humidity. This in turn affects the accuracies of the interferometers commonly used to measure the position of the photomask / photomask holder in relation to the measurement optics in multiple degrees of freedom. Experience has shown that the resulting measurement on certainty is on the order of > 2 nm.

It is therefore an object of the invention to provide a mask metrology apparatus and a measuring method for a mask, in which the elastic distortion of the mask object plane due to gravity is minimized. The object is achieved by a mask metrology apparatus comprising a mask holder for holding a mask having an object plane with a plurality of marks, a mask positioning device for positioning the mask holder in a predetermined position, measurement optics for measuring the position of the marks of the mask held by the mask holder, wherein the mask holder holds the mask with the object plane substantially parallel to the direction of gravity and supports the mask along a horizontal edge of the mask with a uniformly distributed force against gravity.

The distortion due to gravity is minimized since the mask is held with the object planes substantially parallel to the direction of gravity and is supported along a horizontal edge with a uniformly distributed force. A numerical analysis has shown that the in-plane distortion can be < 4 nm. Furthermore, the distortion pattern is very close to a uniform strip pattern in the horizontal direction and can be analyzed both numerically and analytically to a high degree of accuracy. Therefore, not only the absolute amount of in-plane distortion is considerably lower than for a horizontally mounted mask than is usual for mask metrology apparatuses, but also their correction pattern is much simpler and can be applied to a high degree of accuracy.

The mask holder can comprise a plurality of mechanical springs, which are spaced from each other along the horizontal edge for supporting the mask. The springs can be for example helical or leaf springs. Therefore, supporting the mask along a horizontal edge of the mask with a uniformly distributed force against gravity can be realized in a simple way.

The mask holder can comprise an inflated tube closed at both ends for supporting the mask. In particular, the tube can be inflatable and supplied with a constant pressure. The pressure can be adapted to the mask to be held, so that the uniformly distributed force against gravity along the horizontal edge of the mask can be realized.

The mask holder can comprise an air bearing for supporting the mask. In such an air bearing the force applied to the horizontal edge is substantially evenly distributed.

The apparatus can comprise a kinematic fixation unit for holding the mask kinematically against movements in the optical plane to the mask holder. Such a kinematic fixation unit can prevent a movement of the mask as a result of the applied acceleration force due to the movement of the mask for positioning the mask with respect to the measurement optics.

The kinematic fixation unit can provide a three-point kinematic support with three fixation holders in the mask object plane. Each fixation holder can comprise a vacuum pad and a leaf spring connected thereto. The vacuum pad can be attached to the mask and the leaf spring is connected to the mask holder. The vacuum pads allow quick exchange of the masks to be tested and are designed such that the load path of the suction force is as short as possible. Further, the vacuum pads can only be stiff over a small area in line with the leaf spring. Therefore, distortions on the mask due to the suction force of the vacuum pads can be minimized.

A mask position measuring unit for measuring the position of the mask holder with respect to the measurement optics can be provided and can comprise at least one grating and at least two sensor heads. Further, the mask measuring unit can comprise three measuring groups, each of which comprises one line grating and one sensor head, wherein each measuring group is provided for measuring one degree of freedom of movement of the mask holder with respect to the measuring optics. The sensor heads can be fixed to the mask holder and the line gratings can be fixed to the measurement optics.

In this way the position of the mask holder can be measured with high accuracy.

The horizontal edge supported by the mask holder can be the lower edge of the mask in the mask holder. However, it is also possible that the horizontal edge is the upper edge of the mask. In addition, it is possible that both the upper and the lower edge of the mask are supported with a uniformly distributed force against gravity.

Further, there is provided a mask metrology apparatus comprising a mask holder for holding a mask having an object plane with a plurality of marks, a mask positioning device for positioning the mask holder in a predetermined position, measurement optics for measuring the position of the marks of the mask held by the mask holder, and a mask position measuring unit for measuring the position of the mask holder, said mask position measuring unit comprising three measuring groups, each of which includes a line grating and a sensor head, wherein each measuring group is provided for measuring one degree of freedom of movement of the mask holder with respect to the measuring optics.

With such a metrology apparatus the position of the mask holder with respect to the measuring optics can be measured with extremely high accuracy.

The sensor heads can be fixed to the mask holder and the line gratings can be fixed to the measuring optics. This leads to a further improvement in accuracy.

In the above described mask metrology apparatuses they can be provided a position module which measures the position of the gratings of the mask position measuring unit with respect to each other. Further, the position module can measure the position of the gratings of the mask position measuring unit with respect to the measurement optics. These measured position (position information) can be used for enhancing the accuracy of the measurement results of the mask metrology apparatus. In particular, the gratings can be linked mathematically to a stable "monolithic" virtual reference with which the position of the marks of the mask can be determined. Further, the position of the virtual reference with respect to the measurement optics can also be considered when determining the position of the marks.

There is further provided a measuring method for a mask comprising the steps of: providing a mask having an object plane with a plurality of marks in a mask holder, positioning the mask held in the mask holder in a predetermined position with respect to measurement optics, measuring the position of the marks of the mask held by the mask holder with the measurement optics, wherein the mask holder holds the mask with the object plane substantially parallel to the direction of gravity and supports the mask along a horizontal edge of the mask with a uniformly distributed force against gravity.

In this method the elastic distortion of the mask due to gravity is minimized, so that the accuracy of the measuring method is increased.

The mask holder can comprise a plurality of mechanical springs, which are spaced from each other along the horizontal edge for supporting the mask. This is an easy way of realizing the support of the mask along a horizontal edge of the mask with uniformly distributed force against gravity.

Further, the mask holder con comprise an inflated tube closed at both ends for supporting the mask, in particular the tube can be inflatable and can be supplied with a constant pressure. This makes it possible to adapt the uniformly distributed force to the mask to be held.

Further, the mask holder can comprise an air bearing for supporting the mask. With such an air bearing an absolutely evenly distributed force can be applied to the horizontal edge.

The mask can be kinematically fixed against movements in the object plane to the mask holder. This leads to a high accuracy for the measurement.

Further, the position of the mask holder can be measured. In particular, the position of the mask holder can be measured by using three measuring groups, each comprising one line grating and one sensor head, wherein each group is provided for measuring one degree of freedom of movement. Further, the step of measuring the position of the marks on the mask can be carried out at least twice with rotated orientation of the mask. In particular, it is possible to repeat the measurement of the mask rotated by 0°, 90°, 180° and 270° so that uncertainties due to different boundary conditions for the uniformly distributed force for supporting the mask can be minimized.

In the measuring method for a mask there can be used a masked position measuring unit for measuring the position of the mask holder. The mask position measuring unit can comprise three measuring groups, each of which comprises one line grating and one sensor head, wherein each measuring group is provided for measuring one degree of freedom of movement of the mask holder with the respect to the measuring optics.

Further, the position of the gratings with respect to each other can be measured. In addition the position of the gratings with respect to the measurement optics can be measured.

With this position information of the gratings the accuracy of the measuring method when measuring the position of the marks of the mask can be enhanced.

The invention is explained in more detail below, essentially by way of example with reference to the drawing wherein:

Fig. 1 schematically shows a mask metrology apparatus;

Fig. 2 schematically shows a top view of a mask 3;

Fig. 3 schematically shows an embodiment of the holder in a perspective view;

Fig. 4 schematically shows another embodiment of the holder in a perspective view; Fig. 5 schematically shows a side view of a further embodiment of the holder 2;

Fig. 6 schematically shows a kinematic fixation unit in a perspective view;

Fig. 7 schematically shows an enlarged sectional and perspective view of the vacuum pad 23 of the fixation holder 19 of Fig. 6;

Fig. 8 shows a perspective view of the mask position measuring unit 11 ; Fig. 9 shows a side view of the mask position measuring unit 11 of Fig. 8;

Fig. 10 shows a rear perspective view of the sub-gratings of the mask position measuring unit

11 ;

Fig. 11 shows a front view of the mask position measuring unit 11;

Fig. 12 shows a perspective view of the mask positioning device 5; Fig. 13 shows a further perspective view of the mask positioning device 5. Figure 1 schematically depicts a mask metrology apparatus according to an embodiment of the invention. The apparatus 1 comprises a mask holder 2 which holds a mask 3 to be tested.

As can be seen in Figure 2 the mask 3 comprises a plurality of marks 4 in the form of crosses. The crosses are between the pattern sections (not shown) used for manufacturing integrated circuits when the mask is used within a lithographic projection apparatus. In Figure 2 the marks 4 are not shown to scale. In reality, the marks 4 are approximately 10 to 20 μm and the mask has a rectangular shape with lengths of approximately 100 to 150 mm. On the mask 3 there can be about 200 to 300 marks 4.

For testing the mask 3 the distance between the marks 4 has to be measured with high precision. For this purpose the mask holder 2 together with the marks 4 can be moved by means of a mask positioning device 5 so that each mark 4 is positioned in a predetermined position with respect to the measuring optics 6. The measuring optics 6 is formed as a microscope having the necessary lenses 7, a beam splitter 8 for the illumination radiation coming from the illumination source 9 and an image detector 10 for detecting the enlarged image of the respective mark 4.

The apparatus 1 further comprises a mask position measuring unit for measuring the position of the mask holder 2 with respect to the measuring optics 6. In Figure 1 there is only schematically shown the arrow 11 for the mask position measuring unit.

Further, the apparatus 1 comprises a control unit 12 receiving signals from the mask positioning device 5, the image detector 10 and the mask position measuring unit 11 and controlling the mask positioning device 5, the image detector 10 as well as the mask position measuring unit 11.

In order to minimize in-plane distortions in the mask 3 in the apparatus 1 the mask 3 is vertically held by the mask holder 2 such that the lower edge 13 of the mask 3 is supported against gravity by means of a uniformly distributed load.

This leads to the advantage that the in-plane distortion is considerably reduced compared with the in-plane distortion of the mask when being held in a horizontal plane. Furthermore, the distortion pattern of the mask 3 in the apparatus of Figure 1 is very close to a uniform strip pattern in the horizontal direction and can be analyzed both numerically and analytically to a high degree of accuracy. Thus, not only the absolute amount of in-plane distortion is much lower than for a horizontally mounted mask, but also the correction pattern used when processing the image data of the image detector 10 is much simpler and can be applied to a high degree of accuracy.

As shown in Figure 3 of the mask holder 2 it can comprise a plurality of closely spaced, low- stiffness mechanical springs 14 (e.g. helical or leaf springs) along the bottom edge 13 of the mask 3. Properly dimensioned, this provides a substantially vertical force uniformly distributed (at a large number of discrete points) along the bottom edge 13 and largely independent on the mask position (vertically and horizontally) with respect to the measurement optics 6.

In Figure 4 there is shown another embodiment of the mask holder 2. This mask holder comprises an inflatable tube having at least the length of the bottom edge 13. Both ends of the tube 15 are closed and the tube 15 is supplied with a constant pressure P just sufficient to compensate for the gravitational force of the mask 3. The tube 15 can be formed as tubing.

Another embodiment of the mask holder 2 is shown in Figure 5, in which the mask 3 is supported by an air cushion with uniform pressure which is in effect a low-stiffness air bearing 16. The low-stiffness (constant force independent of the gap between the mask holder 2 and the mask 3) can be achieved by effecting a large pressure drop across the flow restrictor 17 which is supplied with aerostatic pressure P from the lower side in Figure 5. The pressure between the air bearing 16 and the mask 3 is substantially constant over the length of the lower edge 13 as the flow restriction across the mask thickness is low and the flow is essentially 2-dimensional (in the plane of the drawing in Figure 5). The required average pressure is on the order of 3 kPa for a 6-inch mask 3 (reticle in quartz). The air bearing 16 comprises an entirely non-contact support for the mask 3 and the applied force on the mask 3 is truly evenly distributed.

When the mask positioning device 5 moves the mask holder 2 and therefore the mask 3 relative to the measuring optics 6, an acceleration force is applied to the mask 3. Since the movement is in the object plane of the mask and probably also in the three degrees of freedom out-of-plane (e.g. for achieving the best focus for the measuring optics 6) a kinematic fixation unit 16 can be provided for holding the mask 3 kinematically against movement in the object plane to the mask holder 2.

In Figures 6 and 7 an embodiment of kinematic fixation unit 18 is schematically shown. In this embodiment the kinematic fixation unit 18 comprises three fixation holders 19, 20, 21 all having the same structure. Therefore, in the following only the fixation holder 19 is described in further detail. The fixation holder 19 comprises a leaf spring 22. One end of the leaf spring 22 is fixed (not shown) to the mask holder 2. The other end of the leaf spring is connected to a vacuum pad 23 as shown in Figure 6 and 7. The leaf spring 22 can be integrally formed with the vacuum pad 23. Further, it is also possible that the vacuum pad 23 and the leaf spring 22 are separate elements which are connected together.

The leaf spring 22 is designed to be stiff in the plane of the spring, but compliant out-of-plane, thus allowing compensation for differential thermal expansion between the mask 3 and the mask holder 2 without introducing significant in-plane distortion on the mask 3. The vacuum pads 23 allow a quick exchange of the masks 3 and are designed such that the load path of the suction force is as short as possible. Further, the vacuum pad 23 is stiff only over a small area in line with the leaf spring. This way the distortion on the mask due to the suction force can also be minimized.

The mask position measuring unit 11 can be embodied as a laser interferometer.

However, the mask position measuring unit 11 can also be embodied as shown in Figure 8. In this embodiment three sub-gratings 24, 25, 26 are provided on a plane front side 27 of a reference structure 28. The reference structure 28 also holds the measurement optics 6 (as shown in Figure 1 ).

For each of the sub-gratings 24 - 26 there is provided a sensor head 29, 30, 31 attached to the mask holder 2 (not shown in Figure 8).

As can be seen in Figure 8 all gratings 25 - 27 are line gratings, wherein the lines of the sub- gratings 25 and 26 extend in the horizontal direction and the lines of the sub-grating 27 extend in the vertical direction. Therefore, the sub-gratings 25 and 26 are used to detect movements in the vertical direction and the sub-grating 27 is used to detect motions in the horizontal direction. Beside the movements in the vertical and horizontal direction the mask position measuring unit 11 of Figure 8 is also able to detect the yaw angle of the mask 3.

Since the sensor heads 29 - 31 can be conveniently placed at close proximity to the vacuum pad 23 of the fixation holders 19 - 21 , very short mechanical coupling of the sensor heads 29 - 31 to the mask 3 can be ensured. Thereby, any errors due to thermal drifts, elastic deformation of the mask holder 2, etc. can be minimized.

The grating surface of the sub-gratings 24 - 26 should also be placed as close as possible to the object plane of the mask 3, to minimize Abbe errors resulting from pitch and roll of the mask. For best thermal stability the sub-gratings should be manufactured from a material with low or zero coefficient of thermal expansion (CTE), such as Invar, Quartz, or Zerodur. In particular, glass ceramics can be used. Further, an active thermal stabilization to mK levels can be used alone or in connection with the material as mentioned above.

The three sub-gratings 24 — 26 are mounted kinematically, e.g. via three leaf springs 32 (Figures 9 and 10) arranged 120° apart, onto the reference structure 28. In principle, the sub- gratings 24 - 26 have to be calibrated in situ and therefore in-plane distortion is arguably less significant than for the mask 3. However, one can apply the same mounting strategy to the sub- gratings 24 - 26 as that applied to the mask 3 as described above, i.e. a 3-point kinematic support normal to the grating plane and a uniformly distributed support along the bottom edge against gravity.

In order to be able to monitor the relative stability of the three sub-gratings 24 - 26 to each other, multiple distance sensors 33, 34 (Figure 11), e.g. capacitive sensors, can be incorporated in zerodur blocks between the three sub-gratings 24 - 26. The reference structure 28 itself can also be formed of a material of low or zero coefficient of thermal expansion. The signals of the multiple distance sensors 33 and 34 are fed to the control unit 12. Knowing the displacements between the sub-gratings 24 - 26 during measurement, the three sub-gratings 24 - 26 can be linked mathematically to a stable 'monolithic' virtual reference, with which the position of the mask 3 is determined.

To ensure that any drift of the measurement optics 6 relative to the reference structure 28 is also taken into account, the multiple distance sensors 33 and 34 can also be used for measuring the distance between the measurement optics 6 and the reference structure 28.

In Figures 12 and 13 there is shown an embodiment of the mask positioning device 5 comprising a separate force-frame 40 through which the reaction forces and moments of the mask position device 5 are coupled to ground and hence decoupled from the vibration-isolated mask holder 2. To achieve high positioning accuracy with ease and robustness, the mask positioning device 5 is cascaded such that it incorporates a coarse, long-range positioning stage 41 and a fine, short-range positioning stage 42.

Here, the mask positioning device 5 is placed on the opposite side of the object plane of the mask 3. As such, any reaction forces from the mask positioning device 5 are coupled directly to ground without direct influence on the sub-gratings 24 - 26. The coarse stage 41 , which only needs to provide long-range (150-400 mm) with accuracies on the order of 10 μm can be a simple gantry system with rolling element guideways and driven by rotary servo motors via ballscrews. Many alternatives exist, e.g. aerostatic guideways, friction belt drives, linear direct- drive motors, etc. Some form of gravity compensation 44 may be incorporated for the vertical axis, either using a pneumatic cylinder or a counter-balance mass. Figure 12 illustrates such a gantry device.

Mounted on the carriage 43 of the coarse stage 41 are the short-range actuators of the fine stage 42 for the fine positioning of the mask holder 2 to nanometer accuracies. The short-range actuators typically have a high bandwidth (> 150 Hz) but short travel range (< 2 mm). Commonly used actuators include piezoelectric actuators or voice coil (Lorentz) motors. The latter has the advantage that it is a pure force actuator and the actuation force is substantially independent on the relative position between the magnets and the coils. In case voice coil motors are employed, it is advantageous to have the magnets on the mask holder 2 and the coils (with associated electrical and cooling connections) attached to the coarse stage 41.

To minimize static load and hence heating for the vertical actuator, as well as deformation of the mask holder 2 due to gravity, gravity compensators 44, for example magnetostatic or pneumatic, can also be incorporated.

Since the mask 3 / mask holder 2 is mechanically coupled to the force frame 40 and decoupled from the reference structure 28 on which the measurement optics 6 is mounted, it is likely that positioning normal to the mask object plane is also required, if only due to relative movement

(e.g. a drift) between the reference structure 28 and the force frame 40 in that direction. This can be achieved by simply incorporating three additional voice coil actuators 45 between the mask holder 2 and the coarse stage 41 (cf. Figure 13). The top surface of the sub-gratings 24 - 27 can be used as a reference and the distance between the former and the mask holder 2 can be measured with suitable distance sensors, such as capacitive sensors.

In order to avoid such uncertainties due to different boundary conditions it is possible to carry out multiple measurements with the apparatus 1 of Figure 1 , for example, repeated measurements with the mask 3 rotated in the mask holder 2 by 0°, 90°, 180° and 270°.

Further, the mask holder 2 can be embodied such that it supports the upper edge 35 (Fig. 2) of the mask 3. Of course, it is also possible that the mask holder 2 supports both the lower edge 13 and the upper edge 35.

Whilst we have described above specific embodiments of the invention it will be appreciated that the invention may be practiced otherwise than described. The description is not intended to limit the invention.

Claims

1. A mask metrology apparatus (1 ) comprising: a mask holder (2) for holding a mask (3) having an object plane with a plurality of marks (4), a mask positioning device (5) for positioning the mask holder in a predetermined position, measurement optics (6) for measuring the position of the marks (4) of the mask (3) held by the mask holder (2), wherein the mask holder (2) holds the mask (3) with the object plane substantially parallel to the direction of gravity and supports the mask (3) along a horizontal edge (13, 35) of the mask (3) with a uniformly distributed force against gravity.

2. The apparatus according to claim 1 , wherein the mask holder (2) comprises a plurality of mechanical springs (14), which are spaced from each other along the horizontal edge (13, 35), for supporting the mask (3).

3. The apparatus according to any one of the preceding claims, wherein the mask holder (2) comprises an inflated tube (15) closed at both ends for supporting the mask (3).

4. The apparatus according to claim 3, wherein the tube (15) is inflatable and supplied with a constant pressure.

5. The apparatus according to any of the preceding claims, wherein the mask holder (2) comprises an air bearing (16) for supporting the mask (3).

6. The apparatus according to any one of the preceding claims, wherein a kinematic fixation unit (18) is provided for holding the mask (3) kinematically against movement in the object plane to the mask holder (2).

7. The apparatus according to any one of the preceding claims, wherein a mask position measuring unit (11 ) is provided for measuring the position of the mask holder (2).

8. The apparatus according to claim 7, wherein the mask position measuring unit (11 ) comprises at least one grating (25, 26, 27) and at least two sensor heads (29, 30, 31 ).

9. The apparatus according to claim 7, wherein the mask position measuring unit (11 ) comprises three measuring groups, each of which comprises one line grating (24, 25, 26) and one sensor head (29, 30, 31 ), wherein each measuring group is provided for measuring one degree of freedom of movement of the mask holder (2) with respect to the measuring optics (6).

10. The apparatus according to claim 9, wherein the position of the gratings with respect to each other is measured.

11. The apparatus according to claim 9 or 10, wherein the position of the gratings with respect to the measurement optics is measured.

12. The apparatus according to any of the claims 8 to 11 , wherein the sensor heads (29, 30, 31 ) are fixed to the mask holder (2) and the line gratings (24, 25, 26) are fixed relative to the measurement optics (6).

13. A mask metrology apparatus (1 ) comprising: a mask holder (2) for holding a mask (3) having an object plane with a plurality of marks (4), a mask positioning device (5) for positioning the mask holder (2) in a predetermined position, measurement optics (6) for measuring the position of the marks (4) of the mask (3) held by the mask holder (2), and a mask position measuring unit (11 ) for measuring the position of the mask holder (2), said mask position measuring unit (11 ) comprising three measuring groups, each of which includes a line grating and a sensor head, wherein each measuring group is provided for measuring one degree of freedom of movement of the mask holder (2) with respect to the measuring optics (6).

14. The apparatus according to claim 13, wherein the sensor heads (29, 30, 31 ) are fixed to the mask holder (2) and the line gratings (24, 25, 26) are fixed relative to the measurement optics (6).

15. The apparatus according to claim 13 or 14, wherein the position of the gratings with respect to each other is measured.

16. The apparatus according to any of the claims 13 to 15, wherein the position of the gratings with respect to the measurement optics is measured.

17. A measuring method for a mask comprising the steps of: providing a mask having an object plane with a plurality of marks (4) in a mask holder (2), positioning the mask (3) held in the mask holder (2) in a predetermined position with respect to measurement optics (6), measuring the position of the marks (4) of the mask (3) held by the mask holder (2) with the measurement optics (6), wherein the mask holder (2) holds the mask (3) with the object plane substantially parallel to the direction of gravity and supports the mask (3) along a horizontal edge of the mask (3) with a uniformly distributed force against gravity.

18. The method according to claim 17, wherein the mask holder (2) comprises a plurality of mechanical springs, which are spaced from each other along the horizontal edge for supporting the mask (3).

19. The method according to claim 17 or 18, wherein the mask holder (2) comprises an inflated tube closed at both ends for supporting the mask (3).

20. The method according to claim 19, wherein the tube is inflatable and supplied with a constant pressure.

21. The method according to any one of claims 17 to 20, wherein the mask holder (2) comprises an air bearing for supporting the mask (3).

22. The method according to any one of claims 17 to 21 , wherein the mask (3) is kinematically fixed against movements in the object plane to the mask holder (2).

23. The method according to any one of claims 17 to 22, wherein the position of the mask holder (2) is measured.

24. The method according to any one of claims 17 to 23, wherein the position of the mask holder (2) is measured using three measuring groups, each comprising one line grating and one sensor head, wherein each group is provided for measuring one degree of freedom of movement.

25. The method according to any one of claims 17 to 24, wherein the step of measuring the position of the marks (4) on the mask (3) is carried out at last twice with rotated orientation of the mask (3).

26. The method according to any of claims 17 to 25, wherein a mask position measuring unit (11 ) is provided for measuring the position of the mask holder (2), and wherein the mask position measuring unit (11 ) comprises three measuring groups, each of which comprises one line grating (24, 25, 26) and one sensor head (29, 30, 31 ), wherein each measuring group is provided for measuring one degree of freedom of movement of the mask holder (2) with respect to the measuring optics (6).

27. The method of claim 26, wherein the position of the gratings with respect to each other is measured.

28. The method according to claim 26 or 27, wherein the position of the gratings with respect to the measurement optics is measured.