DEVICE AND METHOD FOR MOIRÉ MEASUREMENT OF AN OPTICAL TEST SPECIMEN

Abstract

An apparatus for the moir measurement of an optical test object includes a grating arrangement made of a first grating (25, . . . ) which is positionable in the optical beam path upstream of the test object and a second grating (11, . . . ) which is positionable in the optical beam path downstream of the test object, an evaluation unit having at least one detector (12, . . . ), for evaluating moire structures produced by superposition of the two gratings in a detection plane situated downstream of the second grating in the optical beam path, and at least one aperture stop (14, . . . ), by way of which the light distribution which was produced after the light exit from the second grating can be shadowed in a region-wise fashion such that only light of a subset of all field points on the second grating reaches the detection plane.

Claims

1. An apparatus for the moir measurement of an optical test object positioned in an optical beam path, comprising a grating arrangement having a first grating positioned in the optical beam path upstream of the test object and a second grating positioned in the optical beam path downstream of the test object; an evaluation unit having at least one detector, for evaluating moir structures produced by superposition of the two gratings in a detection plane positioned downstream of the second grating in the optical beam path; and at least one aperture stop, configured to shadow a light distribution produced after the optical beam exits the second grating in a region-wise fashion such that light of only a subset of all field points on the second grating reaches the detection plane.

2. The apparatus as claimed in claim 1, wherein the aperture stop is configured such that the subset of all the field points on the second grating which reaches the detection plane is variably settable.

3. The apparatus as claimed in claim 1 wherein the aperture stop is variably settable by displacement of the aperture stop transverse to the light propagation direction and/or by rotation of the aperture stop about an axis that is parallel with respect to the light propagation direction.

4. The apparatus as claimed in claim 1, wherein the aperture stop is configured as a plurality of aperture stops that are configured to differ from one another with respect to the shadowing effected in a stationary position.

5. The apparatus as claimed in claim 1, wherein the aperture stop, or the image produced in the optical beam path thereby, is situated away from the second grating by a distance of less than 100 m.

6. The apparatus as claimed in claim 5, wherein the aperture stop, or the image produced in the optical beam path thereby, is situated away from the second grating by a distance of less than 10 m.

7. The apparatus as claimed in claim 1, wherein the aperture stop is arranged between the second grating and the detector.

8. The apparatus as claimed in claim 1, wherein the detection plane has a distance from the second grating of less than 100 m.

9. The apparatus as claimed in claim 8, wherein the distance from the detection plane to the second grating is less than 200 nm.

10. The apparatus as claimed in claim 1, wherein the optical test object is a projection lens of a microlithographic projection exposure apparatus.

11. The apparatus as claimed in claim 1, wherein the optical test object is configured for operation at an operating wavelength of less than 30 nm.

12. The apparatus as claimed in claim 1, wherein the detector comprises an array of light sensors.

13. The apparatus as claimed in claim 1, wherein the detector comprises a sensor arrangement which is fiber-optically coupled to the detection plane.

14. The apparatus as claimed in claim 1, further comprising an auxiliary optical unit configured to image a light distribution obtained in the detection plane onto the detector.

15. The apparatus as claimed in claim 1, further comprising a quantum converter layer, which absorbs light of a first wavelength range that reaches the detection plane as primary light and emits secondary light of a second wavelength range, which differs from the first wavelength range.

16. The apparatus as claimed in claim 15, wherein the quantum converter layer has in the first wavelength range a penetration depth of less than 10 m.

17. The apparatus as claimed in claim 15, further comprising a color filter layer, which at least partially filters out light that has not been absorbed by the quantum converter layer.

18. A method for the moir measurement of an optical test object using an apparatus as claimed in claim 1, comprising, with the at least one aperture stop, shadowing the light distribution which was produced after the light exits the second grating in a region-wise fashion in a plurality of measurement steps such that in each case only light of a subset of all field points on the second grating reaches the detection plane.

19. The method as claimed in claim 18, further comprising capturing all the field points on the second grating in a sequential measurement series by transitioning the aperture stop into different measurement positions and/or by interchanging aperture stops.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In the figures:

[0042] FIGS. 1A-1C, 2A, 2B, 3A, 3B, and 4-13 show schematic illustrations for explaining different embodiments of the present invention;

[0043] FIGS. 14-15 show schematic illustrations for explaining structure and functional principle of a conventional apparatus for the moir measurement of an optical test object; and

[0044] FIG. 16 shows a schematic illustration for elucidating a problem which occurs in a conventional apparatus for the moir measurement.

DETAILED DESCRIPTION

[0045] Provided according to the invention as per FIGS. 1A-1C is an aperture stop 14 (or 14 or 14 in FIG. 1B and FIG. 1C, respectively) which is displaceable transversely to the light propagation direction into different measurement positions.

[0046] It is possible via the aperture stop 14 for the light distribution which has come about after the light exit from the moir mask, or the second grating 11, to be shadowed in a region-wise fashion such that in each case only light of a subset of all field points reaches the detector 12. The selection of the field points which are measurable in a position of the aperture stop 14 can here be such that light coming from different field points cannot superpose (wherein e.g. in one setting only every second, every third or every fourth field point is captured). In the course of performing a plurality of measurements successively, it is possible by displacing the aperture stop 14 into different measurement positions to realize capturing of all field points on the moir mask, or the second grating, in a sequential measurement series.

[0047] Said aperture stop can be arranged, as per FIGS. 1A, 1B, between the moir mask, or the second grating 11 (which is formed on a substrate 13), and the detector 12 and have, in particular as per FIG. 1B, a plurality of aperture openings. As per FIG. 1C, the aperture stop 14 can also be arranged immediately upstream of the moir mask, or the second grating 11, with respect to the light propagation direction.

[0048] In addition, as per FIGS. 2A and 2B, the aperture stop 24 or 24 can also be arranged immediately downstream of the first grating (FIG. 2A) or immediately upstream of the first grating (FIG. 2B), with respect to the light propagation direction. In FIGS. 2A and 2B, 26 and 26 denote the substrate of the first grating, 20 and 20 denote the test object, or the projection lens, and 22 and 22 denote the detector.

[0049] Generally speaking, the aperture stop, or the image produced thereof in the optical beam path, is situated away from the second grating, or the moir mask, preferably by a distance of less than 100 m, in particular less than 80 m, more particularly less than 60 m. In embodiments of the invention, this distance criterion can also be realized by arranging the aperture stop in the region of an intermediate image plane, as is schematically illustrated in FIGS. 3A and 3B. FIG. 3A shows the placement of an aperture stop 34 according to the invention in an intermediate image plane within the illumination device (of which merely one lens element 37 is indicated in FIG. 3A) that is located upstream of the first grating 35. Moreover, components which are analogous or have substantially the same function are denoted in FIG. 3A with reference numerals which are increased by 10 in relation to FIG. 2A. FIG. 3B likewise schematically shows, in strongly simplified fashion, the placement of an aperture stop 34 according to the invention in an intermediate image plane within the test object, or projection lens 30, wherein the first grating is here denoted with 35 and the detector with 32.

[0050] Furthermore described are, in each case proceeding from the basic setup for the moire measurement described with reference to FIGS. 14-16, various embodiments of the invention, in which in each case a small distance between detection plane and moir mask is realized in order to realize a high field resolution while avoiding the problems described in the introductory section above (in particular the overlap of the light cones indicated in FIG. 16).

[0051] In the embodiments illustrated in FIGS. 4 and 5, this is done by realizing a correspondingly small distance between the respectively used (e.g. camera-based) detector and the moir mask, while, in the embodiments illustrated in FIGS. 6-10, in each case a suitable optical signal transmission between the detection plane and the detector (farther away from the moir mask in these examples) is realized.

[0052] FIG. 4 shows a first embodiment, in which a second grating 41, or the moir mask, is applied to a substrate sheet 43 which is located directly on a detector 42 such that the distance between the moir mask, or the second grating 41, and the detection plane is adjusted here via the thickness of the substrate sheet 43.

[0053] The substrate sheet 43 can be e.g. a glass membrane having an exemplary thickness of 25 m. The substrate sheet 43 can be embodied here such that a reflection-reducing effect is attained to reduce undesired interference signals.

[0054] FIG. 5 shows a further embodiment, wherein components which are analogous or have substantially the same function are denoted by reference numerals increased by 10. In contrast to FIG. 4, the moir mask, or the second grating 51, as per FIG. 5 is applied directly to the surface of the detector 52 (which is designed as a camera chip in the exemplary embodiment). Applying the structures of the moir mask, or of the second grating 51, can here already be a constituent part of the manufacturing process of the detector 52, or camera chip.

[0055] FIG. 6 shows a further embodiment, in which, in contrast to FIG. 5, the structures of the moir mask, or of the second grating 61, are not applied directly to the detector 62, or camera chip, but to a protective layer 64 that is situated on the detector 62. Consequently, damage to the detector 62 during the manufacturing process (which can comprise a lithography process including etching steps or electron beam writing) can be prevented. The protective layer 64 can here be applied on the light-sensitive surface of the detector 62 only in region-wise fashion or on the entire light-sensitive surface of the detector 62. Furthermore, the protective layer 64 can also possibly enclose the entire detector 62, or camera chip. The thickness of the protective layer 64 can be selected in suitable fashion to adjust the desired distance between the moir mask, or the second grating 61, and the detector 62, or the detection plane, and in addition to achieve a reflection reduction to eliminate undesired interference signals. In the exemplary embodiment, the protective layer 64 can be produced from quartz glass (SiO.sub.2) and have a thickness ranging from 20 nm to 200 nm.

[0056] FIG. 7 shows a further embodiment, in which, in contrast to the previously described embodiments, the moir mask, or the second grating 71, is arranged on that side of a transparent substrate 73 which faces away from the detector 72. As a consequence of this arrangement, the substrate 73 itself can havedespite the implementation, which is also present here, of a small distance between the moir mask, or the second grating 71, and the detection planea relatively great thickness (e.g. of a few hundred micrometers (m)), as a result of which increased stability of the arrangement can be realized.

[0057] FIG. 8 shows a further embodiment, in which the detector 85 has an array of light sensors, such as e.g. photodiodes, wherein otherwise a thin substrate 83 is again provided between the moir mask, or the second grating 81, and the array 85 of light sensors which forms the detector 82. Hereby, the producibility can be improved with respect to a full-area camera chip as a detector, because achieved is a greater flexibility with respect to the respectively admissible manufacturing steps.

[0058] FIG. 9 and FIG. 10 each show embodiments in which the detector, or a sensor arrangement forming said detector, is fiber-optically coupled to the detection plane. As per FIG. 9 and FIG. 10, light is respectively received immediately downstream of the light exit from the moir mask, or the respective second grating 91 or 101, wherein the respective optical signal is then guided via optical fibers 96 (as per FIG. 9) or a monolithic faceplate 106 (as per FIG. 10) to the respective detector 92 or 102, which itself can be located at a greater distance from the moire mask. Since the light which was recorded in the detection planeagain only at a short distance from the moir maskis fiber-optically transported to the respective detector 92 or 102, no mixing or crosstalk of the respective signals occurs.

[0059] FIGS. 11, 12 and 13 show further embodiments in which, in contrast to the previously described examples, in each case one auxiliary optical unit in the form of an additional projection optical unit is used to image the light field which was acquired in the detection planeonce again located immediately downstream of the moir maskonto the detector 112, 122 or 132, which is farther away. The auxiliary optical unit 118, 128 or 138 is here embodied in each case such that it can transmit the full angle spectrum of the light which has been acquired downstream of the moir mask, or the second grating 111, 121 or 131, or at least a representative portion of the respective angle spectrum.

[0060] In contrast to FIG. 11, the moir mask, or the second grating 121, as per FIG. 12 is situated on a quantum converter layer 129, which absorbs light of a first wavelength range that reaches the detection plane as primary light and emits secondary light of a second wavelength range, which differs from the first wavelength range.

[0061] In the exemplary embodiment, the quantum converter layer 129 can be produced, merely by way of example, of lithium glass, which has a penetration depth of less than 5 m for wavelengths below 350 nm, and emits secondary light in a wavelength range between 360 nm and 500 nm. As a result, the angle distribution transmitted by the auxiliary optical unit 128 can be representative for the actual light intensity in the detection plane, and diffraction effects of the structures situated on the moir mask can be left out of consideration. As a result of the low penetration depth of the material of the quantum converter layer 129 for the primary light and the absorption which consequently takes place after an optical path of only a few micrometers (m) in this material, the distance between detection plane and moir mask is effectively kept low even in this embodiment, because, as a result of the low penetration depth, primary light of different, adjacent field points cannot coincide.

[0062] FIG. 13 shows a further embodiment, wherein components which are analogous or have substantially the same function are denoted by reference signs increased by 10 in relation to FIG. 12.

[0063] Compared to FIG. 12, an additional color filter layer 140, which at least partially filters out light that has not been absorbed by the quantum converter layer 139, is provided in the embodiment of FIG. 13. Hereby, account can be taken of the fact that, when using a comparatively thin quantum converter layer 139, the used (primary) light might have a penetration depth that exceeds the thickness of the quantum converter layer 139 (i.e. the effective distance between the detection plane and the moir mask), wherein the unconverted primary light can here be eliminated by way of the color filter layer 140.

[0064] In further embodiments, an additional protective and/or anti-reflective layer for reducing undesired interference signals or for protective purposes can be used between the moir mask and the quantum converter layer, between the quantum converter layer and the color filter layer, and/or in the beam path downstream of the color filter layer.

[0065] 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, for example 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 accompanying patent claims and the equivalents thereof.