TEST DEVICE AND METHOD FOR TESTING A MIRROR

Abstract

A test appliance and a method for testing a mirror, e.g., a mirror of a microlithographic projection exposure apparatus. The test appliance has a computer-generated hologram (CGH), and a test can be carried out on at least a portion of the mirror by way of an interferometric superposition of a test wave that is directed onto the mirror by this computer-generated hologram and a reference wave. Here, the computer-generated hologram (CGH) (120, 320) is designed in such a way that, during operation of the appliance, it provides a first test wave for testing a first portion of the mirror (101, 301) by interferometric superposition with a reference wave in a first position of the mirror (101, 301) and at least a second test wave for testing a second portion of the mirror (101, 301) by interferometric superposition with a reference wave in a second position of the mirror (101, 301).

Claims

1. A test appliance for testing a mirror, comprising: a computer-generated hologram (CGH) configured to carry out a test on at least a portion of the mirror by interferometrically superposing a test wave that is directed onto the mirror by the computer-generated hologram and a reference wave, wherein the computer-generated hologram is configured to provide, during operation of the appliance, a first test wave for testing a first portion of the mirror by interferometric superposition with a reference wave in a first position of the mirror and at least a second test wave for testing a second portion of the mirror by interferometric superposition with a reference wave in a second position of the mirror.

2. The test appliance as claimed in claim 1, wherein the computer-generated hologram configured to provide the first test wave and the second test wave has a complex encoding of CGH structures that differ from one another.

3. The test appliance as claimed in claim 1, wherein the computer-generated hologram is further configured to weight respective intensities of the first test wave and of the second test wave differently from one another by no more than 20%.

4. The test appliance as claimed in claim 1, wherein the computer-generated hologram is further configured to provide, during operation of the appliance, at least one calibration wave for the interferometric superposition with a reference wave after the reflection of the calibration wave at a calibration mirror.

5. The test appliance as claimed in claim 4, wherein the computer-generated hologram is further configured to provide, during operation of the appliance, at least two calibration waves, in particular at least three calibration waves, for the interferometric superposition with a reference wave after the reflection of the calibration waves at calibration mirrors that differ from one another.

6. A method for testing a mirror, comprising: recording a first interferogram between a first test wave that is reflected at the mirror and a reference wave; recording a second interferogram between a second test wave that is reflected at the mirror and a reference wave; directing the first test wave and the second test wave onto the mirror with a same computer-generated hologram (CGH); and modifying a position of the mirror between the recording of the first interferogram and the recording of the second interferogram.

7. The method as claimed in claim 6, wherein the first test wave is reflected at a first portion of the mirror and the second test wave is reflected at a second portion of the mirror that is different from the first portion.

8. The method as claimed in claim 6, wherein the mirror is displaced and/or twisted between the recording of the first interferogram and the recording of the second interferogram.

9. The method as claimed in claim 6, wherein the computer-generated hologram remains in a same position during the recording of the first interferogram and the recording of the second interferogram.

10. The test appliance as claimed in claim 1, wherein the mirror is a mirror of a microlithographic projection exposure apparatus,

11. The test appliance as claimed in claim 3, wherein the computer-generated hologram is further configured to weight respective intensities of the first test wave and of the second test wave differently from one another by no more than 10%.

12. The test appliance as claimed in claim 5, wherein the computer-generated hologram is further configured to provide, during operation of the appliance, at least three calibration waves for the interferometric superposition with a reference wave after the reflection of the calibration waves at calibration mirrors that differ from one another.

13. The method for testing a mirror as claimed in claim 6, wherein the mirror is a component of a microlithographic projection exposure apparatus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In the drawings:

[0039] FIGS. 1-4 show schematic illustrations for explaining exemplary embodiments of the present invention;

[0040] FIGS. 5-6 show schematic illustrations for explaining a conventional functional principle of an interferometric test appliance for testing a mirror; and

[0041] FIG. 7 shows a schematic illustration of a projection exposure apparatus designed for operation in EUV.

DETAILED DESCRIPTION

[0042] FIG. 7 shows a schematic illustration of an exemplary projection exposure apparatus which is designed for operation in extreme ultraviolet (EUV) light and which comprises mirrors which are testable by a method according to the invention.

[0043] According to FIG. 7, an illumination device in a projection exposure apparatus 10 designed for EUV comprises a field facet mirror 3 and a pupil facet mirror 4. The light from a light source unit comprising a plasma light source 1 and a collector mirror 2 is directed onto the field facet mirror 3. A first telescope mirror 5 and a second telescope mirror 6 are arranged in the light path downstream of the pupil facet mirror 4. A deflection mirror 7 is arranged downstream in the light path, said deflection mirror directing the radiation that is incident on it onto an object field in the object plane of a projection lens comprising six mirrors 21-26. At the location of the object field, a reflective structure-bearing mask 31 is arranged on a mask stage 30, said mask being imaged with the aid of the projection lens into an image plane in which a substrate 41 coated with a light-sensitive layer (photoresist) is situated on a wafer stage 40.

[0044] The mirror that is tested within the scope of the invention can be e.g. any mirror of the projection exposure apparatus 10, for example the mirrors 21 or 22 of the projection lens, or else the mirror 7 of the illumination device.

[0045] Next, a principle underlying the invention will be described below with reference to the schematic depictions in FIG. 1 and FIG. 2.

[0046] Proceeding from the Fizeau arrangement that has already been described on the basis of FIG. 5, FIG. 1 firstly shows a schematic illustration for explaining the testing of a mirror 101 using the reference light that is reflected at a reference surface 110 (“Fizeau plate”) and the measurement light that is reflected at the mirror 101 to be tested, wherein the measurement light is formed to make a wavefront with a computer-generated hologram (CGH) 120. The wavefront corresponds mathematically exactly to the “test form” (i.e. the form of the relevant mirror 101) at an intended distance. The wavefronts that are reflected, firstly, by the reference surface 110 and, secondly, by the corresponding mirror 101, or test object, interfere with one another in an interferometer 105 which, for example, can have the overall structure already explained on the basis of FIG. 6.

[0047] According to the invention, the mirror 101 is tested using one and the same CGH 120 for the entire mirror surface of the mirror 101 to be tested, for the purposes of which the mirror 101 is moved in different positions within the test appliance (as indicated in FIG. 1 by the double-headed arrow). FIG. 1 indicates, in a purely schematic manner, two different positions “A” and “B”.

[0048] In each case, (in view of the assumed large mirror surface) only a portion of the entire mirror surface is tested within the test appliance in these different mirror positions “A” and “B”, the CGH 120 being equipped with a plurality of different CGH structures or use functionalities for this test, in accordance with the schematic diagram shown in FIG. 2. As indicated in FIG. 2, these CGH structures or use functionalities can typically be (locally varying) line gratings with different orientations or grating periods.

[0049] Expressed differently, the CGH 120 has both a first CGH structure or use functionality that is suitable for testing a first mirror region (when the mirror 101 is in the position “A”) and a CGH structure or use functionality that is suitable for testing a second mirror region (corresponding to the position “B” of the mirror 101).

[0050] These CGH structures or use functionalities are realized in the CGH 120 by way of the method of complex encoding, which is known per se. Here, the respective CGH structures or line gratings may each be described by a phase function (with amplitude and phase), wherein the relevant terms may be added, optionally with different weightings. A complex function, which may, in turn, be binarized, emerges in this way, whereby e.g. the resultant structure of the CGH 120 that is depicted merely schematically for a specific point on the CGH in the right-hand part of FIG. 2 is obtained.

[0051] Here, in particular, the relevant resultant overall structure of the CGH 120 that is schematically depicted in the right-hand part of FIG. 2 is distinguished by virtue of a multiplicity of test waves being provided for one and the same test object or mirror as a consequence of the complex encoding described above, with these test waves being assigned to portions of the mirror that differ from one another, i.e. facilitating by way of a displacement of the mirror 101 relative to the CGH 120 an interferometric test of even a comparatively large mirror surface that is combined from the respective portions when one and the same CGH 120 is used.

[0052] In tables 1a and 1b below, exemplary embodiments for the above-described possible weighting of the individual CGH structures are specified.

[0053] As can be seen in each case from table 1a and table 1b, the termed (by way of “grating 1” and “grating 2”) line gratings of the use functionalities (i.e. of the CGH structures for testing the individual portions of the mirror 101) are relatively strongly weighted here relative to the termed (by way of “grating 3” to “grating 5”) calibration functionalities, wherein, moreover, the weighting of the two use functionalities, or “grating 1” and “grating 2”, relative to one another is in correspondence (or deviate only slightly from one another, e.g. less than 20%, in particular less than 10% in respect of the respectively stronger weighting in further embodiments). In addition to the aforementioned two use functionalities or CGH structures, further (use or else calibration) functionalities are encoded on the CGH 120 in the exemplary embodiment (grating 3, grating 4 and grating 5 in tables 1a and 1b).

TABLE-US-00001 TABLE 1a Overall intensity Intensity of Reflectivity of the after CGH (two Grating Weighting the 1st order mirror passes) + mirror 1 40  11% 4.00% 0.05% 2 40  11% 4.00% 0.05% 3 20 2.5% 80.00% 0.05% 4 20 2.5% 80.00% 0.05% 5 20 2.5% 80.00% 0.05%

TABLE-US-00002 TABLE 1b Overall intensity Intensity of Reflectivity of the after CGH (two Grating Weighting the 1st order mirror passes) + mirror 1 50  12% 4.00% 0.06% 2 50  12% 4.00% 0.06% 3 20 1.8% 80.00% 0.03% 4 20 1.8% 80.00% 0.03% 5 20 1.8% 80.00% 0.03%

[0054] In the exemplary embodiment of FIG. 2, the CGH 120 has a total of five different functionalities or CGH structures as a consequence of the complex encoding, wherein, merely in an exemplary manner, two of these CGH structures may serve to test different mirror regions (when positioning the mirror 101 in the mirror positions “A” and “B” that are depicted on the basis of FIG. 1) and wherein the remaining three CGH structures may serve as calibration functionalities.

[0055] To this end, FIG. 3 shows a schematic illustration, wherein components which are analogous or substantially functionally identical to FIG. 1 are denoted by reference signs increased by “200”. The test appliance in accordance with FIG. 3 has a total of three calibration mirrors 302, 303 and 304, to each of which one of the above-described calibration functionalities of the CGH 320 is assigned. As already described above, the mirror 301 is moved to different positions within the test appliance (as indicated by the double-headed arrow in FIG. 3) for the purposes of testing different mirror regions, wherein the use functionality of the CGH 320 that is designed in accordance with the respective mirror region or assigned to the latter is used for testing purposes.

[0056] In accordance with a further aspect of the invention, the testing of a mirror using one and the same CGH (and moving or displacing the mirror position relative to this CGH) can also be used to reduce or largely eliminate the influence of unwanted disturbing reflections on the test result. Such disturbing reflections typically result from the fact that, in addition to the wanted test wave that impinges on the mirror surface by way of diffraction at the respective CGH structures and that is reflected by said mirror surface, further orders of diffraction that are, however, not wanted during the test also return on the same path as the desired test waves likewise after the reflection at the mirror surface, either randomly or on account of unavoidable errors within the CGH structure, and said further orders of diffraction are likewise able to contribute to the produced interferogram.

[0057] Now, in order to overcome or reduce this problem, it is possible, according to the invention, to carry out the interferometric measurement with one and the same CGH for e.g. two mirror positions that are rotated or slightly displaced with respect to one another, wherein, as indicated in FIG. 4, an “or operation” of the two obtained interferograms 450, 460 is subsequently carried out, with the consequence that the distribution 470 that results from this or operation is at least largely freed from the influence of disturbing reflections (“speckled pattern”).

[0058] Hence, in the exemplary embodiment described above, the use of one and the same CGH in conjunction with two different mirror positions relative to the CGH may also be advantageously used in applications in which the CGH facilitates a direct testing of the entire mirror surface in a single step (i.e. without partial steps for individual portions of the mirror) even on account of the size and design.

[0059] A further advantageous aspect of the complex encoding of a CGH with a plurality of use functionalities according to the invention is that such a CGH (as e.g. shown in FIG. 2) also facilitates the testing of freeform mirror surfaces (which have no intrinsic symmetry), with use also being made here of the fact that different use functionalities can be encoded, or different line gratings can be written, onto one and the same position on the CGH.

[0060] The provision according to the invention of at least two use functionalities on one and the same position of the CGH may further—as an alternative to measuring different portions of the mirror—also be used to measure different mirror geometries, in which case the individual use functionalities or CGH structures are not assigned to different portions of the same mirror in this case, but are assigned to different mirrors.

[0061] Moreover, the provision of at least two use functionalities need not be effectuated on the entire CGH surface, and so the use functionalities may also in each case only be written to a portion of the CGH. In this case, the optionally present calibration functionality must comprise the respective portions of the use functionalities.

[0062] Moreover, the invention is not restricted to a specific number of use functionalities or a specific number of calibration functionalities corresponding to the embodiments described above, and so, in particular, more than two use functionalities or CGH structures, which may be assigned to one of a plurality of portions of the mirror in each case, are also possible.

[0063] Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments are apparent to a person skilled in the art, e.g. by combination and/or exchange of features of individual embodiments. Accordingly, such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the appended patent claims and equivalents thereof.