WO2005047979A2

WO2005047979A2 - Optical system

Info

Publication number: WO2005047979A2
Application number: PCT/EP2004/012113
Authority: WO
Inventors: Erik Sohmen; Paul Kaufmann; Werner Schwarz
Original assignee: Carl Zeiss Smt Ag
Priority date: 2003-11-06
Filing date: 2004-10-27
Publication date: 2005-05-26
Also published as: WO2005047979A3

Abstract

An optical signal, in particular for projection microlithography, having optical elements (2), in particular mirrors and/or lenses, in a sealed sheath in which there is an inert gas or vacuum, and having a bearing (1) with a bearing gap. The bearing is a fluid bearing and the bearing gap (7) has restriction stages and is pumped differentially in two or more stages.

Description

Optical system

The invention relates to an optical system, in particular for projection microlithography.

Optical systems with bearings are known from the prior art, in which a bearing gap is located between two components which can move with respect to one another, and with air being introduced into the bearing gap. So-called air bearings such as these are used, for example, as accurate and robust rotary bearings for measurement machines.

However, if optical components such as projection microlithography objectives are intended to be operated in a vacuum, that is to say they are intended to be positioned exactly and robustly to an accuracy of a few nanometers and are intended to be rotated very precisely about their optical axis, then the known bearings reach their limits, in particular because the air continuously has the tendency to escape from the air gap owing to the low environmental pressure in the vacuum, and the ultra-hard vacuum can thus not be achieved.

Although sealing elements are known from the prior art which can be used to provide a seal between the ultra-hard vacuum and the atmospheric pressure, bearings which are equipped with such sealing elements can, however, not also ensure the accuracy that is required for optical applications.

The object of the present invention is therefore to provide an optical system by means of which an optical element can be borne very accurately in a vacuum. According to the present invention, this object is achieved by an optical system, in particular for projection microlithography, having optical elements, in particular mirrors and/or lenses, in a sealed sheath in which there is an inert gas or vacuum, and having a bearing with a bearing gap, with the bearing being a fluid bearing and the bearing gap having restriction stages and being pumped differentially in two or more stages.

The embodiment of the bearing as a fluid bearing which is pumped differentially in two or more stages results in a very low leakage rate, which is suitable for use in an ultra-hard vacuum. A bearing such as this thus allows the use of an optical component in an ultra-hard vacuum.

In a more far-reaching embodiment, the bearing has a rigid component and a component which can rotate with respect to the rigid component and is suitable for holding the optical component, with the bearing gap to which at least one fluid supply line leads being located between the rigid component and the component which can rotate, in order to introduce fluid into the bearing gap between the rigid component and the component which can rotate and in order to create a fluid cushion in the bearing gap, and with at least one line leading from the bearing gap to a pumping device in order to pump the fluid out of the bearing gap such that a continuous supporting cushion is produced in the bearing gap.

The combination of a fluid supply line which introduces fluid into the bearing gap with a line which pumps the fluid out of the bearing gap results in a continuous supporting cushion for the component that can rotate, in the bearing gap. This means that the fluid cannot escape in an uncontrolled manner from the bearing gap and that a fluid cushion is formed within the bearing gap such that the component which can rotate is mounted very robustly with respect to the rigid component, thus ensuring exact guidance of the component which can rotate with respect to the rigid component.

The invention will be described in principle in the following text with reference to the attached drawings.

Figure 1 shows a first section through one embodiment of the optical system with the bearing according to the present invention;

Figure 2 shows a further section through the optical system shown in Figure 1;

Figure 3 shows a further section through the optical system shown in Figure 1;

Figure 4 shows a plan view of the optical system shown in Figure 1; and

Figure 5 shows a section through a further embodiment of an optical system according to the present invention.

A bearing 1, various sections through which are illustrated in Figures 1, 2 and 3, for bearing an optical component 2 in an optical system, in particular for projection microlithography, has a rigid, stationary component 3 and a component 4 which can rotate with respect to the rigid component 3. The component 4 which can rotate is suitable for holding the optical component 2, and for this purpose has a hole 5.

The optical component 2 could, of course, also be connected to the component 4 which can rotate in some manner other than by means of the hole 5, for example by means of a conventional flanged connection. In order to allow assembly, the component 4 which can rotate is in the present case formed in three parts.

The optical component 2 is, for example, an EUV lithography objective, which is illustrated only in the form of a black box in Figures 1 and 2, is held robustly and in position to an accuracy of a few nanometers, and, in addition, has to be able to rotate very accurately about its optical axis, which is annotated by the reference symbol 6. The bearing 1 which is described in more detail in the following text could be used for widely differing purposes and for completely different optical components 2.

The bearing 1 and the optical component 2 are arranged in an ultra-hard vacuum, which is produced by a vacuum chamber, that is to say a sealed sheath, that is indicated by means of a dashed line. The devices that are required for this purpose, such as an appropriate vacuum pump, are not illustrated in the figures.

A bearing gap 7, which is subdivided into two or more areas by means of a projection 8 on the component 4 which can rotate and extends into a recess 9 in the rigid component 3, is located between the rigid component 3 and the component 4 which can rotate. In this way, the projection 8 results in two envelope surfaces 10 and 11 as well as a total of four planar surfaces 12, 13, 14 and 15 between the rigid component 3 and the component 4 which can rotate. Of these, the planar surfaces 14 and 15 are associated with the projection 8. The bearing gap 7 thus has an area 16 which is associated with the envelope surface 10 and runs in the axial direction, an area 17 which is associated with the envelope surface 11 and runs in the axial direction, an area 18 which is associated with the planar surface 14 and runs in the radial direction, and an area 19 which is associated with the planar surface 15 and likewise runs in the radial direction. In this case, the lower area 19, which is associated with the planar surface 15 and runs in the radial direction, absorbs the axial load, and the upper area 18, which is associated with the planar surface 14 and runs in the radial direction, absorbs the opposing force, which acts upward. The two planar surfaces 12 and 13 in this case do not form a part, or form only a small part of the bearing gap 7. In an embodiment of the bearing 1, which is not illustrated, the projection 8 could also be associated with the rigid component 3 and the recess 9 could be associated with the component 4 which can rotate. As will be described in the following text, the bearing 1 is a fluid bearing and the bearing gap 7 has restriction stages and is differentially pumped in two or more stages.

Pressurized fluid is introduced into the bearing gap 7 via two fluid supply lines 20 and 21 which are provided with separate pumping devices 22 and 23. By way of example, the fluid pressure may be 5 bar and, for example, it is possible to use air or an inert or noble gas such as argon, or else a gas mixture, as the fluid that is introduced. In this case, the fluid supply line 20 leads into an annular channel 24 which is arranged in the interior of the rigid component 3 and from where it is passed via supply holes 25, which can be seen in Figure 1, to the area 16 and via supply holes 26, which can be seen in Figure 2, to the area 18 of the bearing gap 7. The fluid supply line 21 also leads in a similar manner into an annular channel 27, which is connected via supply holes 28 to the area 17 and via supply holes 29 to the area 19 of the bearing gap 7, thus distributing the fluid around the circumference of the bearing gap 7. It should be noted that a greater number of supply holes 25, 26, 28, 29, for example 12, are provided around the circumference of the bearing gap 7, although only one of the supply holes 25, 26, 28 and 29 can be seen in each of the figures 1 and 2, with the supply holes 25 and 28 being visible in Figure 1, and the supply holes 26 and 29 being visible in Figure 2. The large number of supply holes 25, 26, 28 and 29 results in better distribution of the fluid which is supplied to the bearing gap 7.

Instead of the two fluid supply lines 20 and 21 with the associated pumping devices 22 and 23, it would also be possible to provide only one fluid supply line with one associated pumping device, and in this case one of the two annular channels 24 and 27 could also be omitted. However, the illustrated embodiment of the separate fluid supply lines 20 and 21 for the areas 16 and 18 of the bearing gap 7 which run in the axial direction and radial direction respectively makes it possible to supply these areas 16 and 18 with a different fluid pressure than that supplied to the areas 17 and 19, so that it is possible to achieve a differential supply for the bearing gap 7, and thus providing bracing for the bearing 1. In order to ensure that a supporting cushion is maintained in the bearing gap 7, the fluid is pumped out of the bearing gap 7 by means of a pumping device 30. For this purpose, the pumping device 30 (which has two or more pumping stages that are not illustrated in detail) is connected to two or more pumping-out or venting lines, which each form pressure stages, and which will be described in more detail in the following text. Instead of just one pumping device 30 with two or more pumping stages, it would also be possible to provide two or more individual pumping devices, which would then be connected to the respective pressure stage.

Since there is a raised pressure of a number of bar, for example 5 bar as stated above, in the bearing gap 7, there is no need in the first stage of the pressure-decreasing process to pump out the fluid by means of the pumping device 30. A line 31 therefore leads from all the edges or ends of the areas 16, 17, 18 and 19 of the bearing gap 7 through the rigid component 3 directly to the atmosphere, and thus reduces the pressure in the respective inlet area into the line 31 to the atmospheric pressure of approximately 1 bar. The fluid which flows into the bearing gap 7 via the supply holes 25, 26, 28 and 29 is thus now only at a pressure of about 1 bar once it has reached the line 31.

Owing to the very greatly reduced pressure in the ultra-hard vacuum in which the bearing 1 is located, the fluid which is located in the bearing gap 7 attempts, of course, to pass via the planar surfaces 12 and 13 outward into the vacuum chamber. In order to prevent this and to keep the losses within the bearing gap 7 as low as possible, and in consequence to achieve a high-precision of the bearing 1, there are a total of eight annular channels 32, 33, 34, 35, 36, 37, 38 and 39 in the present case in the two planar surfaces 12 and 13 of the rigid component 3, and these allow the fluid to be conveyed out of the bearing gap 7 in stages. The annular channels 32, 33, 34 and 35 are in this case arranged in the upper planar surface 12, and the annular channels 36, 37, 38 and 39 are arranged in the lower planar surface 13 of the rigid component 3.

As can be seen from Figure 2, the first or innermost annular channel 32 is connected to a line 40 which leads to a first stage of the pumping device 30. The directly opposite annular channel 36 is also connected to a corresponding line, although this is not illustrated. The annular channel 37, which forms a part of the second pumping-out stage and is located in the lower planar surface 13, is connected to a line 41, which is connected to a second stage of the pumping device 30. In this case, the opposite annular channel 33 is likewise connected to a line which is once again not illustrated.

As can be seen from Figure 3, the annular channel 38 which is located in the lower planar surface 13 and forms a part of the third pumping-out stage is connected to a further line 42, which is connected in a corresponding manner to the third stage of the pumping device 30. Once again, the opposite annular channel 34 in the upper planar surface 12 is also connected to a line which is not illustrated. The last or outermost annular channel 35 (within the bearing gap 7 in the flow direction of the fluid) in the upper planar surface 12 as well as the corresponding annular channel 39 in the lower planar surface 13 have a considerably larger cross section than the other annular channels 32, 33 and 34, as well as 36, 37 and 38. In this case, the annular channel 35 is connected to a line 43 which likewise has a considerably larger cross section than the lines 40, 41 and 42. In the present case, the line 43 is connected to a separate pumping device 44, which produces a reduced pressure which corresponds essentially to the reduced pressure in the vacuum environment, so that the fluid in the bearing gap 7 is pumped out completely and this results essentially in there being no flow losses, that is to say fluid emerging between the rigid component 3 and the component 4 which can rotate. In addition, a non-contacting sealing device of a known type, for example in the form of a labyrinth seal, can for this purpose be arranged between the rigid component 3 and the component 4 which can rotate, in the flow direction of the fluid, within the bearing gap 7 downstream from the annular channels 35 and 39. However, this is not illustrated.

A pressure distribution that is typical but should be regarded as being purely by way of example in the bearing gap 7 could be as follows: 5 bar in the areas 16, 17, 18 and 19 of the bearing gap 7, 1 bar in the line 31 which leads to the atmosphere, 10 mbar in the first line 40, 1 x 10^~3 mbar in the second line 41, 1 x 10^~5 mbar in the third line 42 and a pressure which corresponds to the pressure in the vacuum chamber in the fourth line 43. In this case, the line 31 which leads directly to the atmosphere is arranged in the direction of the molecular flow of the fluid within the bearing gap 7 upstream of the lines 40, 41 or 42 and 43 which lead to the pumping device 30. The lines for the various pumping stages are, of course, not connected to one another while, in contrast, it is possible to connect the lines for the same pumping stages. Thus, in addition to the line 31, the four further lines 40, 41, 42 and 43 are provided in the present exemplary embodiment, although it would also be possible to provide a different number of lines, depending on the pressure to be achieved in the vacuum chamber.

The height of the bearing gap 7, that is to say the distance between the rigid component 3 and the component 4 which can rotate, may, for example, be 5 to 8 μm. The height of the bearing gap 7 can be adjusted by variation of the pressures in the areas 18 and 19 of the bearing gap 7, thus making it possible to further minimize the amount of fluid which emerges from the bearing gap 7.

The illustration of the bearing 1 shown in Figure 4 shows the various fluid supply lines, annular channels, supply holes, lines and annular channels in the form of a plan view.

The component 4 which can rotate can be driven with respect to the rigid component 3 by means of a rotary motor which is not illustrated, in which case the rotary motor may be connected to the component 4 which can rotate via, for example, a gearwheel or toothed ring connection, or a belt. It is also feasible to use a linear motor, which operates without any contact, for driving the component 4 which can rotate.

The component 4 which can rotate and/or the rigid component 3 may each be provided with emergency running coatings, in order to prevent these components from being destroyed in the event of failure of the fluid supply and output. The material Bilatal may be mentioned as one example of such a coating, this being a UV-resistant anodic coating which will be famil- iar to those skilled in the art and which reflects UV radiation only to a very minor extent.

The supporting load of the bearing 1 may, for example, be in a range from 100 kg to 1000 kg, or in a range from 1000 kg to 5000 kg.

Figure 5 shows a further embodiment of the bearing 1 for the optical system, illustrating the bearing 1 schematically. The optical element 2, in this case a mirror, is once again arranged on the component 4 which can rotate, which is mounted in the rigid component 3, and can be driven by means of a drive device 45. All of the components are located in a sheath 46 which in this case is in the form of a vacuum chamber and in which there is an ultra-hard vacuum. The air supply line 20 to which the pumping device 22 is connected leads to the air gap 7. Lines 40, 41 and 43 likewise lead away from the air gap 7 to the three pumping devices 30, 44 and 47, which in this case are separate from one another. This also thus results in an optical system which is intended in particular for projection microlithography and in which there is an inert gas or a vacuum in a sealed sheath, and which has the bearing 1 with the bearing gap 7, with the bearing 1 being a fluid bearing and the bearing gap 7 having restriction stages and being pumped differentially in two or more stages.

According to the invention, all the claimed features may be combined in any desired way.

Claims

Patent Claims :

1. An optical system, in particular for projection microlithography, having optical elements, in particular mirrors and/or lenses, in a sealed sheath in which there is an inert gas or vacuum, and having a bearing with a bearing gap, with the bearing being a fluid bearing and the bearing gap having restriction stages and being pumped differentially in two or more stages.

2. The optical system as claimed in claim 1, with the fluid bearing being a rotary bearing.

3. The optical system as claimed in claim 1, which is used in an ultra-hard vacuum.

4. The optical system as claimed in claim 1, with the bearing having a rigid component and a component which can rotate with respect to the rigid component and is suitable for holding the optical component, with the bearing gap to which at least one fluid supply line leads being located between the rigid component and the component which can rotate, in order to introduce fluid into the bearing gap between the rigid component and the component which can rotate and in order to create a fluid cushion in the bearing gap, and with at least one line leading from the bearing gap to a pumping device in order to pump the fluid out of the bearing gap such that a continuous supporting cushion is produced in the bearing gap.

5. The optical system as claimed in claim 4, with the component which can rotate having at least one projection which extends into a recess in the rigid component such that at least two envelope surfaces, which are separated from one another by the projection, and at least two planar surfaces are located between the rigid component and component which can rotate, with the at least two envelope surfaces each having associated areas of the bearing gap which run in the axial direction, and with at least two of the at least four planar surfaces each having associated areas of the bearing gap which run in the radial direction.

6. The optical system as claimed in claim 5, with those areas of the bearing gap which run in the axial direction and those areas of the bearing gap which run in the radial direction being connected to separate fluid supply lines.

7. The optical system as claimed in claim 4, with the rigid component having at least one projection which extends into a recess in the component which can rotate such that at least two envelope surfaces, which are separated from one another by the projection, and at least four planar surfaces are located between the rigid component and the component which can rotate, with the at least two envelope surfaces each having associated areas of the bearing gap which run in the axial direction, and with at least two of the at least four planar surfaces each having associated areas of the bearing gap which run in the radial direction.

8. The optical system as claimed in claim 7, with those areas of the bearing gap which run in the axial direction and those areas of the bearing gap which run in the radial direction being connected to separate fluid supply lines.

9. The optical system as claimed in claim 4, with an additional line leading from the bearing gap directly to the atmosphere.

10. The optical system as claimed in claim 9, with the line which leads directly to the atmosphere being arranged in the flow direction of the fluid within the bearing gap and upstream of the at least one line which leads to the pumping device.

11. The optical system as claimed in claim 4, with two or more lines which are arranged one behind the other, lead to the pumping device and are connected to respective annular channels being provided in the flow direction of the fluid within the bearing gap in order to allow the fluid to be conveyed out of the bearing gap in stages.

12. The optical system as claimed in claim 11, with four lines which are arranged one behind the other and lead to the pumping device being provided in the flow direction of the fluid within the bearing gap.

13. The optical system as claimed in claim 11, with the last line within the bearing gap in the flow direction of the fluid being connected to an annular channel which has a considerably larger cross section than the other annular channels.

14. The optical system as claimed in claim 11, with the annular channels being located in a planar surface of the rigid component .

15. The optical system as claimed in claim 14, with the at least one fluid supply line being connected to at least one annular channel which is arranged in the interior of the rigid component and is connected to the bearing gap via two or more supply holes which are distributed around the circumference of the bearing gap.

16. The optical system as claimed in claim 15, with the supply holes being connected not only to those areas of the bearing gap which run in the axial direction but also to those areas of the bearing gap which run in the radial direction.

17. The optical system as claimed in claim 4, with the component which can rotate having a hole for holding the optical component .

18. The optical system as claimed in claim 1, with the fluid bearing being operated with air.

19. The optical system as claimed in claim 1, with the fluid bearing being operated with a noble gas.

20. The optical system as claimed in claim 4, with a rotary motor being provided in order to drive the component which can rotate.

21. The optical system as claimed in claim 20, with the rotary motor being connected via a gearwheel transmission to the component which can rotate.

22. The optical system as claimed in claim 20, with the rotary motor being connected via a belt to the component which can rotate.

23. The optical system as claimed in claim 4, with a linear motor being provided in order to drive the component which can rotate.

24. The optical system as claimed in claim 4, with the component which can rotate being provided with an emergency running coating.

25. The optical system as claimed in claim 4, with the rigid component being provided with an emergency running coating.

26. The optical system as claimed in claim 1, with the bearing having a supporting load in a range from 100 kg to 1000 kg.

27. The optical system as claimed in claim 1, with the bearing having a supporting load in a range from 1000 kg to 5000 kg.

28. The optical system as claimed in at least one of the preceding claims, with a vacuum pumping system having two or more vacuum stages being provided in order to convey the fluid in stages.