OPTICAL SYSTEM, AND METHOD FOR OPERATING AN OPTICAL SYSTEM

Abstract

An optical system, for example in a microlithographic projection exposure apparatus, comprises a mirror and a temperature-regulating device. The mirror has an optical effective surface and a mirror substrate. A plurality of temperature-regulating zones are arranged in the mirror substrate. The temperature-regulating device is used to adjust the temperatures present in each of the temperature-regulating zones independently of one another. The temperature-regulating zones are arranged in at least two planes at different distances from the optical effective surface. The temperature-regulating zones in the at least two planes are configured as cooling channels through which, independently of one another, a cooling fluid at a variably adjustable cooling fluid temperature is able to flow. A method for operating such an optical system is provided.

Claims

1. An optical system, comprising: a mirror comprising an optically effective surface and a mirror substrate, the mirror substrate comprising a plurality of temperature control zones; a temperature control device configured to set temperatures in the temperature control zones independently of one another; and a device, wherein: the temperature control zones are in two planes at different distances from the optically effective surface; the temperature control zones in the two planes comprise cooling channels through which, independently of one another, a cooling fluid with a variably settable cooling fluid temperature is flowable; and the device is configured to ascertain a cooling power that is output when the cooling fluid flows through the cooling channels.

2. The optical system of claim 1, further comprising a regulation unit configured to temporally variably regulate the temperatures set in the temperature control zones by the temperature control device.

3. The optical system of claim 2, wherein the regulation unit is configured so that a determination, underlying the regulation, of a current heating state of the mirror is performed based on the cooling power that is output by the cooling fluid when it flows through the cooling channels.

4. (canceled)

5. (canceled)

6. The optical system of claim 1, wherein: the mirror substrate comprises a first mirror substrate part comprising a first mirror substrate material; the mirror substrate comprises a second mirror substrate part on a side of the first mirror substrate part facing away from the optically effective surface; and the second mirror substrate part comprises a second mirror substrate material different from the first mirror substrate material.

7. The optical system of claim 6, wherein the two planes are assigned to different mirror substrate parts.

8. The optical system of claim 6, wherein the first mirror substrate material has a lower average coefficient of thermal expansion than the second mirror substrate material.

9. The optical system of claim 1, wherein: a first plurality of temperature control zones are in a first plane of the two planes; and the temperature control zones located in the first plane are settable independently of one another.

10. The optical system of claim 1, wherein the temperature control device comprises a plurality of Peltier elements, and each Peltier element is assigned to a respective temperature control zone.

11. The optical system of claim 1, wherein the temperature control device comprises a plurality of radiant heaters, and each radiant heater is assigned to a respective temperature control zone.

12. The optical system of claim 1, wherein the optically effective surface is configured to reflect electromagnetic radiation at a wavelength of less than 30 nm.

13. The optical system of claim 1, wherein the optical system is a projection lens a microlithographic projection exposure apparatus, or the optical system is an illumination device of a microlithographic projection exposure apparatus.

14. A method, comprising: providing the optical system of claim 1; and setting the temperatures in the temperature control zones independently of one another so that a deformation, due to different thermal expansion of the temperature control zones belonging to different of the two planes, corresponds to a desired deformation.

15. The method of claim 14, further comprising temporally variably regulating the temperatures respectively set in the temperature control zones.

16. The method of claim 14, further comprising flowing the cooling fluid through the cooling channels while independently variably setting the temperature of the cooling-fluid temperature in the cooling channels.

17. The method of claim 14, further comprising: flowing the cooling fluid through the cooling channels; ascertaining a cooling power that is output when the cooling fluid flows through the cooling channels; and performing a determination, underlying the regulation, of a current heating state of the mirror based on the ascertained cooling power.

18. An optical system, comprising: a mirror comprising an optically effective surface and a mirror substrate, the mirror substrate comprising a plurality of cooling channels therein, the cooling channels configured so that, independently of one another, a cooling fluid with a variable cooling-fluid temperature is flowable therethrough; a device configured to ascertain a cooling power output when the cooling fluid flows through the cooling channels; and a regulating unit configured to temporally variably regulate temperatures set in the temperature control zones via the temperature control device, wherein a determination, underlying this regulation, of a current heating state of the mirror is performable based on the ascertained cooling power output when the cooling fluid flows through the cooling channels.

19. A method, comprising: providing the optical system of claim 18; and setting the temperatures in the temperature control zones independently of one another so that a deformation, due to different thermal expansion of the temperature control zones belonging to different of the two planes, corresponds to a desired deformation.

20. A method of operating an optical system, the optical system comprising a mirror, the mirror comprising an optically effective surface and a mirror substrate, the mirror substrate comprising cooling channels therein and through which, independently of one another, a cooling fluid with a variably settable cooling-fluid temperature is flowable, the method comprising: ascertaining a cooling power that is output when the cooling fluid flows through the cooling channels; performing a determination of a current heating state of the mirror based on the ascertained cooling power; and using the determined current heating state to regulate the a temperature set in the temperature control zones in a temporally variable manner.

21. The optical system of claim 2, wherein the regulation unit is configured so that a determination, underlying this regulation, of a current heating state of the mirror is performable based on the ascertained cooling power output when the cooling fluid flows through the cooling channels.

22. The optical system of claim 18, wherein: the mirror substrate comprises a first mirror substrate part comprising a first mirror substrate material; the mirror substrate comprises a second mirror substrate part on a side of the first mirror substrate part facing away from the optically effective surface; and the second mirror substrate part comprises a second mirror substrate material different from the first mirror substrate material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In the figures:

[0051] FIG. 1 shows a schematic illustration for elucidating the possible structure of a mirror according to one embodiment of the disclosure;

[0052] FIGS. 2-6 show schematic illustrations for elucidating the possible structure of a mirror according to other embodiments of the disclosure; and

[0053] FIG. 7 shows a schematic illustration of the possible structure of a microlithographic projection exposure apparatus designed for operation in the EUV.

DETAILED DESCRIPTION

[0054] FIG. 7 first of all schematically shows a meridional section through the possible structure of a microlithographic projection exposure apparatus designed for operation in the EUV.

[0055] According to FIG. 7, the projection exposure apparatus 1 comprises an illumination device 2 and a projection lens 10. One embodiment of the illumination device 2 of the projection exposure apparatus 1 has, in addition to a light or radiation source 3, an illumination optical unit 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 may also be provided as a module separate from the rest of the illumination device. In this case, the illumination device does not comprise the light source 3.

[0056] Here, a reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable, for example in a scanning direction, by way of a reticle displacement drive 9. For purposes of elucidation, a Cartesian xyz coordinate system is shown in FIG. 7. The x direction runs perpendicularly to the plane of the drawing. The y direction runs horizontally, and the z direction runs vertically. The scanning direction runs along the y direction in FIG. 7. The z direction runs perpendicularly to the object plane 6.

[0057] The projection lens 10 serves for imaging the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable, for example along the y direction, by way of a wafer displacement drive 15. The displacement on the one hand of the reticle 7 by way of the reticle displacement drive 9 and on the other hand of the wafer 13 by way of the wafer displacement drive 15 may take place in such a way as to be synchronized with one another.

[0058] The radiation source 3 is an EUV radiation source. The radiation source 3 for example emits EUV radiation, which is also referred to below as used radiation or illumination radiation. For example, the used radiation has a wavelength in the range between 5 nm and 30 nm. The radiation source 3 can be for example a plasma source, a synchrotron-based radiation source or a free electron laser (FEL). The illumination radiation 16 emanating from the radiation source 3 is focussed by a collector 17 and propagates through an intermediate focus in an intermediate focal plane 18 into the illumination optical unit 4. The illumination optical unit 4 comprises a deflection mirror 19 and, arranged downstream thereof in the beam path, a first facet mirror 20 (having schematically indicated facets 21) and a second facet mirror 22 (having schematically indicated facets 23).

[0059] The projection lens 10 comprises a plurality of mirrors Mi (i= 1, 2, ...), which are consecutively numbered according to their arrangement in the beam path of the projection exposure apparatus 1. In the example illustrated in FIG. 7, the projection lens 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or any other number of mirrors Mi are similarly possible. The penultimate mirror M5 and the last mirror M6 each have a through opening for the illumination radiation 16. The projection lens 10 is a doubly obscured optical unit. The projection lens 10 has an image-side numerical aperture that is greater than 0.5 and may also be greater than 0.6, and may be for example 0.7 or 0.75.

[0060] During operation of the microlithographic projection exposure apparatus 1, the electromagnetic radiation incident on the optically effective surface of the mirrors is partially absorbed and, as explained in the introduction, results in heating and an associated thermal expansion or deformation, which in turn can result in an impairment of the imaging properties of the optical system. The concept according to the disclosure can be applied to any desired mirror of the microlithographic projection exposure apparatus 1 from FIG. 7. This can be effected to avoid or compensate thermally induced deformations of the relevant mirror itself (for example to compensate a spatial distribution of the zero-crossing temperature) or else to provide an additional degree of freedom in terms of setting the wavefront properties of the entire optical system, that is to say without or with a corrective action achieved by the relevant mirror.

[0061] The disclosure is not restricted to use in a projection exposure apparatus designed for operation in the EUV. For example, the disclosure can also be used in a projection exposure apparatus designed for operation in the DUV (i.e. at wavelengths less than 250 nm, for example less than 200 nm) or also in another optical system.

[0062] FIG. 1 shows a merely schematic illustration of one possible embodiment of a mirror 100 according to the disclosure. The mirror 100 has a mirror substrate 110 (e.g. made of ULE™) and a reflection layer system 120 (e.g. in the form of a molybdenum (Mo) - silicon (Si) multilayer coating stack). Within the mirror substrate 110 there are a plurality of temperature control zones 131-136 and 141-146, which are arranged in two planes at different distances from the optically effective surface 101. In the specific exemplary embodiment, the temperature control zones 131-136 and 141-146 are in the form of cooling channels, through which, independently of one another, a cooling fluid with a variably settable cooling-fluid temperature can flow.

[0063] “150” denotes a temperature control device, via which the temperatures respectively present in the temperature control zones 131-136, 141-146 can be set independently of one another. Merely by way of example, the temperature control device 150 may have a plurality of Peltier elements assigned to the respective temperature control zones.

[0064] Although the disclosure in the exemplary embodiment of FIG. 1 is realized by respective cooling channels through which cooling fluid can flow, the disclosure is not restricted to this. Rather, in other embodiments, the targeted selective setting of different temperatures in different temperature control zones can also be carried out in another suitable way, e.g. via radiant heaters with different focal depths or resistance heating elements.

[0065] The setting of different temperatures in the region of the temperature control zones, 131-136 on the one hand and 141-146 on the other hand, that are located at a first different distance from the optically effective surface 101 results in a deformation of the optically effective surface 101 owing to the different thermal expansion of the mirror substrate material in the relevant planes, in a similar way to what is referred to as the bimetal effect. This can in turn be used to provide an additional degree of freedom in terms of setting the wavefront properties of the optical system (e.g. the projection exposure apparatus 1 from FIG. 7) comprising the mirror 100.

[0066] The temperature setting according to the disclosure in the temperature control zones can be carried out for the one part in order to set a desired deformation of the optically effective surface 101 (e.g. in order to compensate a disturbance or aberration present elsewhere in the optical system). As an alternative, the temperature setting can also be carried out in order to compensate thermally induced deformations in the mirror 100 itself. In the latter case, it is therefore possible for example for the temperature control zones 141-146 to be used to avoid bulging or deformation, otherwise associated with (cooling) operation of the temperature control zones 131-136, of the mirror 100. In the case of this approach, therefore, the temperature control zones 131-136 serve to discharge the heat generated by absorption of the electromagnetic radiation incident on the optically effective surface 101, and the temperature control zones 141-146 serve to compensate a deformation caused in other respects by the temperature control zones 131-136 through which cooling fluid flows.

[0067] FIG. 2 shows a further embodiment of an adaptive mirror 200, wherein components that are analogous or substantially functionally identical in comparison with FIG. 1 are designated by reference numerals increased by “100”. The embodiment of FIG. 2 differs from that of FIG. 1 in that the mirror substrate 210 is composed of different mirror substrate parts 210a, 210b, wherein the mirror substrate part 210a arranged closer to the optically effective surface 201 is produced from a mirror substrate material with a comparatively low average coefficient of thermal expansion. In a specific exemplary embodiment, the mirror substrate material of the first mirror substrate part 210a may be ULE™ and the mirror substrate material of the second mirror substrate part 210b may be quartz glass (SiO.sub.2).

[0068] FIG. 3 shows a further embodiment of an adaptive mirror 300 according to the disclosure, wherein, in turn, components that are analogous or substantially functionally identical to FIG. 2 are designated by reference numerals increased by “100”. The exemplary embodiment of FIG. 3 differs from that of FIG. 2 in that the mirror substrate 310 is composed of three different mirror substrate parts 310a, 310b and 310c, wherein the respective walls of the cooling channels serving as temperature control zones 331-336, 341-346 have already been introduced into these mirror substrate parts in a manner that is favourable from a manufacturing technology perspective. Specifically, the first mirror substrate part 310a (which in turn can be manufactured from ULE™) serves as top part for the cooling channels that serve as the temperature control zones 331-336, and the second mirror substrate part 310b (which can be manufactured e.g. from quartz glass (SiO.sub.2)) serves as bottom part for the cooling channels that form the temperature control zones 341-346. The third mirror substrate part 310c is arranged between the first mirror substrate part 310a and the second mirror substrate part 310b and serves at the same time as bottom part for the cooling channels that form the temperature control zones 331-336 and as top part for the cooling channels that form the temperature control zones 341-346 and may be manufactured either from ULE™ or from quartz glass (SiO.sub.2) depending on the specific conditions.

[0069] The use of a material with a comparatively higher coefficient of thermal expansion relative to ULE™ on sides of the mirror substrate part 210b in the embodiment of FIG. 2 or the mirror substrate part 310b or 310c in the embodiment of FIG. 3 is desirable insofar as, in the region of these mirror substrate parts, a greater thermal expansion (relative to the first mirror substrate part 210a and 310a, respectively, that is closer to the optically effective surface) is definitely desired in order to achieve a significant deformation effect in the adaptive mirror according to the disclosure. At the same time, in certain regions the adaptive mirror can be manufactured from a comparatively less expensive material (relative to ULE™) in this way.

[0070] FIG. 4 shows a further embodiment of an adaptive mirror 400, wherein components that are analogous or substantially functionally identical in comparison with FIG. 1 are designated by reference numerals increased by “300”. The embodiment of FIG. 4 differs from that of FIG. 1 in that, instead of the temperature control zones 131-136, only a single temperature control zone 431 (which is continuous, or not segmented into multiple individual temperature control zones) is present in the relevant plane within the mirror substrate 410. In the configuration according to FIG. 4, in terms of this non-segmented temperature control zone 431, a spatial resolution, achievable with the segmentation, in a lateral direction is deliberately dispensed with, in order to in return reduce the total number of cooling fluid ports used and thus firstly decrease the structural outlay and secondly also prevent the risk of tightness defects, or leaks, arising in the region of the cooling channels.

[0071] FIG. 5 shows a further embodiment of an adaptive mirror 500, wherein components that are analogous or substantially functionally identical in comparison with FIG. 1 are designated by reference numerals increased by “400”. The embodiment of FIG. 5 differs from that of FIG. 1 in that, instead of the temperature control zones 141-146 arranged in the plane at a comparatively great distance from the optically effective surface 101 according to the embodiment of FIG. 1, only a single temperature control zone 541 (which is continuous, or not segmented into multiple individual temperature control zones) is provided. In other words, in the embodiment according to FIG. 5, by contrast to that of FIG. 4, segmentation or lateral spatial resolution is dispensed with not within the plane that is closer to the optically effective surface, but rather for the plane that is at a greater distance from the optically effective surface, or the temperature control zone there. It is also the case in this configuration that reducing the total number of cooling fluid ports has the effect of simplifying the structure from a design perspective and of reducing the risk of leaks.

[0072] FIG. 6 shows a further embodiment of an adaptive mirror 600, wherein components that are analogous or substantially functionally identical in comparison with FIG. 1 are designated by reference numerals increased by “500”. The embodiment of FIG. 6 differs from that of FIG. 1 in that the temperature control zones 131-136 and 141-146 that are in the two planes according to FIG. 1 are each replaced by a single temperature control zone 631 and 641 (which is continuous and not segmented into multiple individual temperature control zones), respectively. In this embodiment, segmentation or lateral resolution in the two planes in the region of the temperature control zones 631, 641 is dispensed with, resulting in the number of cooling fluid ports being minimized.

[0073] In all of the embodiments described above, water or any desired other suitable cooling fluid can be used as cooling fluid.

[0074] In further embodiments, the concept according to the disclosure of the cooling channels, or temperature control zones, through which, independently of one another, a cooling fluid flows can also be used in combination with local heating of the optically effective surface of the relevant mirror (e.g. by radiant heaters).

[0075] In all of the embodiments described above on the basis of FIGS. 1-6, the temperature respectively set in the temperature control zones can be regulated in a temporally variable manner. In this respect, a determination, underlying this regulation, of the current heating state of the respective mirror can for example also be carried out on the basis of an ascertainment of the cooling power that is output when the cooling fluid flows through the cooling channels. To that end, for example, the change in temperature of the cooling fluid flowing through a cooling channel can be measured using temperature sensors at the inlet and outlet, and then, given a known flow rate and a known heat transfer coefficient at the respective cooling channel wall, conclusions can be drawn as to the temperature gradient present in the mirror substrate material.

[0076] Even though the disclosure has been described on the basis of specific embodiments, numerous variations and alternative embodiments will be apparent to the person skilled in the art, for example through combination and/or exchange of features of individual embodiments. Accordingly, it goes without saying for a person skilled in the art that such variations and alternative embodiments are also encompassed by the present disclosure, and the scope of the disclosure is restricted only within the meaning of the appended patent claims and the equivalents thereof.