Optical system, beam splitter and use of a beam splitter in an optical system

Abstract

The invention relates to an optical system, in particular for characterizing a microlithographic mask, comprising a light source (101, 201, 251) for generating light having a wavelength of less than 30 nm, an illumination light path leading from the light source to an object plane, an imaging light path leading from the object plane to an image plane, and a beamsplitter (103, 205, 254) through which both the illumination light path and the imaging light path extend. The invention also relates to a beamsplitter and the use of a beamsplitter in an optical system.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to German patent application DE 10 2022 100 591.7, filed on January 12, 2022. The contents of that application are incorporated herein by reference. Technical Field

[0003] The present invention relates to an optical system specifically used for characterizing microlithography masks. Background Technology

[0004] Microlithography is used to produce microstructured components, such as integrated circuits or liquid crystal displays. The microlithography process is performed in a so-called projection exposure apparatus, which includes an illumination device and a projection lens. In this case, an image of a mask (=mask master) illuminated by the illumination device is projected through the projection lens onto a substrate (e.g., a silicon wafer). This substrate is coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection lens so as to transfer the mask structure onto the photosensitive coating of the substrate.

[0005] In photolithography, undesirable defects on the mask have a particularly detrimental effect because they can be reproduced in each illumination step. Therefore, to minimize mask defects and achieve successful mask repair, direct analysis of the imaging effects on potential defect locations is necessary. Consequently, in principle, there is a need for rapid and simple measurement or identification of the mask. In this context, the practice of recording and evaluating a spatial image of a portion of the mask in a mask inspection apparatus is particularly known, where, to record the spatial image, the structure to be measured on the mask is illuminated by a magnifying illumination optics unit, and light from the mask is projected onto a detector unit via an imaging optics unit and detected by the detector unit. In this process, even relatively large continuous areas, particularly the entire optically effective surface of the mask, can be scanned to detect defects.

[0006] Figure 6 The general conventional structure of a mask inspection device 600 is shown, which includes an illumination optics unit 601 and an imaging optics unit 605, wherein the light source ( Figure 6 Light (not shown) enters the illumination optical unit 601 and guides the illumination beam 602 to a mask 603 arranged in the object plane of the imaging optical unit 605, wherein the area of ​​the mask 603 illuminated is imaged onto the sensor device 606 (e.g., a CCD camera) by means of the imaging beam 604 through / via the imaging optical unit 605.

[0007] Regarding the prior art, purely as an example, see DE 10 2017 221 146 A1, US 2014 / 0240686A1, US 7 982 854 B2, DE 10 2012 202 057 B4, and the publication “Mo / Si multilayer-coated amplitude-division beam splitters for XUV radiation sources” by R. Sobierajski et al. (J. Synchrotron Rad., 2013, Vol. 20, pp. 249–257). Summary of the Invention

[0008] The object of this invention is to provide an optical system specifically for characterizing microlithography masks, which allows for the best possible utilization of the resolution available in the EUV range, thereby improving measurement accuracy.

[0009] This objective is achieved by an optical system with features described below.

[0010] The optical system according to the present invention includes:

[0011] A light source used to generate light with wavelengths less than 30 nm;

[0012] The illumination path from the light source to the object plane;

[0013] The imaging optical path from the object plane to the image plane; and

[0014] The beam splitter extends through both the illumination and imaging optical paths.

[0015] This invention is particularly based on the concept that, through a beam splitter in an imaging optics system designed for EUV operation, the illumination path from the light source to the object plane is separated from the imaging path from the object plane to the image plane, thereby partially guiding both the illumination and imaging paths via the beam splitter. Therefore, according to the invention, substantially perpendicular illumination of the object plane can be achieved even without geometric separation of the illumination and imaging paths, and this is particularly advantageous for a variety of reasons, especially when applied according to the invention to a one-piece apparatus for characterizing microlithographic masks, wherein the mask to be characterized is in the object plane under perpendicular illumination:

[0016] First, the fundamentally vertical mask illumination implemented according to the present invention takes into account the following facts, according to Figure 3The angle-dependent curves depicted in the graph show that for relatively large incident angles (relative to the surface normal), the reflectance of masks designed for the EUV range decreases significantly (according to...). Figure 3 The diagram shows that, within an incident angle range greater than 12°, the illumination and imaging paths ultimately achieve a correspondingly limited incident angle range (corresponding to a numerical aperture of approximately NA = 0.2), because the mask only reflects sufficiently within this incident angle range. According to the invention, both the illumination and imaging paths are guided via a beam splitter, which allows the NA range to be fully utilized for both the illumination and imaging paths, and thus allows for a relatively high resolution (particularly compared to the resolution ultimately achievable within an EUV projection exposure apparatus).

[0017] Furthermore, since the concept according to the invention allows for a basically vertical mask illumination, it is advantageous, especially when determining placement errors on the mask (so-called "registration"), because the possible focusing errors in mask positioning have less impact on the accuracy of the corresponding position determination compared to tilted mask illumination.

[0018] By using a beam splitter suitable for operation in the EUV range (which must meet relatively stringent requirements in terms of planar geometry and transmission uniformity), the present invention intentionally accepts increased manufacturing costs and increased light loss (considering that the light in the illumination and imaging paths must be guided twice via the beam splitter) in order to obtain the aforementioned advantages, particularly considering the improved resolution and relatively greater insensitivity to focusing errors, and consequently, higher measurement accuracy.

[0019] According to one embodiment, the centroid ray of the illumination beam incident on the object plane in the illumination path is incident on the object plane at an angle not exceeding 6° relative to the surface normal, more particularly, at an angle not exceeding 4°, and even more particularly, at an angle of 0°.

[0020] In the optical system according to the invention, according to one embodiment, light in the illumination path is reflected at the beamsplitter, and light in the imaging path is transmitted through the beamsplitter. This configuration is particularly advantageous because the imaging path, which is more sensitive to optical aberrations, uses the beamsplitter only during transmission, and considering the small thickness of the beamsplitter and the fact that the refractive index in EUV is essentially a value of 1, the possible inhomogeneities of the beamsplitter have no significant impact on the achievable image quality (unlike the case of reflection operation). However, the invention is not limited in this respect, and therefore, in principle, embodiments of beamsplitter operation in both the transmission and reflection operations in the illumination path should also be considered to be included in the invention.

[0021] According to one embodiment, the beam splitter includes a multilayer system on a thin film with a thickness of less than 250 nm, more particularly, less than 100 nm, and even more particularly, less than 40 nm.

[0022] According to one embodiment, the multilayer system includes a plurality of partial layer stacks made of a first thin layer made of a first material and a second thin layer made of a second material, the number of partial layer stacks being less than 20, more particularly, less than 15.

[0023] In various embodiments, the first material may comprise molybdenum (Mo) or ruthenium (Ru). Furthermore, the second material may comprise silicon (Si).

[0024] According to one embodiment, the multilayer system includes at least one thin layer with varying thickness. This is particularly advantageous if the angle of incidence of light on the beamsplitter varies significantly over the area of ​​the beamsplitter. In this case, the layer thickness distribution can be selected such that the layer thickness is optimized for small angles of incidence in regions where such angles are small, and optimized for large angles of incidence in regions where such angles are large.

[0025] According to one embodiment, the thin film is made of silicon (Si), silicon nitride (SiN), or carbon nanotubes.

[0026] To avoid impairing the function of the beam splitter according to the invention due to deformation or ripples, a thin film having a multilayer system thereon can be housed in a suitable support, such as a clamping support.

[0027] According to one embodiment, the beam splitter has a transmission non-uniformity of less than 1% over its optical operating area, more particularly, less than 0.8%, and even more particularly, less than 0.5%.

[0028] According to one embodiment, the beam splitter has an optical operating area whose size is at least 30 mm in one spatial direction, and more particularly, at least 40 mm.

[0029] According to one embodiment, the imaging optical path extends substantially telecentrically on the mask side or the object plane side.

[0030] According to one embodiment, at least one mirror, and more particularly, at least two mirrors, are present along the imaging optical path between the object plane and the beam splitter. By means of one or more such mirrors (each mirror having a planar or curved geometry on its optically effective surface), and through a substantially collimated optical path positioned at the beam splitter location via said mirrors, the angle load of the beam splitter can be reduced. However, the invention is not limited thereto, and embodiments in which no other optical components or mirrors exist between the beam splitter and the object plane or mask should also be considered included in the invention. This configuration, where no additional mirrors exist between the object plane and the beam splitter, can also advantageously reduce the overall number of mirrors used within the optical system, which is desirable in principle.

[0031] According to one embodiment, there are therefore no mirrors along the imaging optical path between the object plane and the beam splitter.

[0032] According to one embodiment, the light from the light source has a wavelength of less than 15 nm, and more particularly, the wavelength is in the range of 13 nm to 14 nm.

[0033] According to one embodiment, the optical system is a one-piece device for characterizing a microlithography mask, wherein the mask to be characterized, arranged in the object plane, is illuminated via an illumination optical path and imaged onto a sensor unit arranged in the image plane via an imaging optical path.

[0034] However, the invention is not limited thereto, but can also be advantageously used in further applications, such as in optical systems for characterizing photolithographic mask blanks or in microscopes.

[0035] Furthermore, the present invention also relates to a beam splitter for use in an optical system having the above-described features, and the use of the beam splitter in an optical system having the above-described features, wherein the beam splitter comprises a multilayer system on a thin film having a thickness of less than 250 nm, more particularly, less than 100 nm, and even more particularly, less than 40 nm. Regarding the advantages and preferred configurations of the beam splitter, refer to the foregoing explanation in the context of the optical system according to the invention.

[0036] Further configurations of the invention will be apparent from the description.

[0037] The invention will now be explained in more detail with reference to the exemplary embodiments shown in the accompanying drawings. Attached Figure Description

[0038] In the attached diagram:

[0039] Figure 1 A schematic diagram is shown to illustrate a possible structure of an optical system for characterizing a mask according to the present invention;

[0040] Figures 2a-2b A schematic diagram is shown for explaining a further embodiment of an optical system for characterizing a mask according to the present invention;

[0041] Figure 3 A graph showing the angle-dependent curves of reflectivity for masks designed for the EUV range is presented;

[0042] Figure 4 a-4c shows the explanation of the illumination and imaging optical paths ( Figure 4 a) Numerical aperture available in EUV systems under geometric separation conditions ( Figure 4 A schematic diagram illustrating exemplary uses of b) and 4c);

[0043] Figure 5 a-5b illustrates the use of beam splitters commonly employed in illumination and imaging optical paths ( Figure 5 Under case a), the numerical aperture (NAP) available in the EUV system Figure 5 b) A schematic diagram of possible uses; and

[0044] Figure 6 A schematic diagram is shown to explain the conventional overall structure of a one-piece device used to characterize a mask. Detailed Implementation

[0045] Below, refer to Figure 1-5 The schematic diagram in b illustrates exemplary embodiments of the optical system according to the invention. A common feature of these embodiments is the use of a beam splitter in the imaging optical system designed for operation in EUV. This use results in both the illumination and imaging optical paths extending partially through the beam splitter, and thus allows for substantially perpendicular illumination or imaging of an object (e.g., a mask to be characterized), even if the illumination and imaging optical paths are not geometrically separated.

[0046] In purely illustrative representation, Figure 1 A possible structure of the optical system for characterizing an EUV lithography mask according to the invention is shown in the first embodiment.

[0047] according to Figure 1 The (EUV) light emitted by the light source 101 initially strikes the mask 106 to be characterized, located in the object plane, and travels along the illumination path through mirror 102 and the beam splitter 103 according to the invention, as well as additional mirrors 104 and 105. After reflection at the mask 106, the light enters the imaging path, which initially again passes through mirrors 105 and 104 and the beam splitter 103, and then through additional mirrors 107 and 108 to reach the sensor unit 109 (e.g., a CCD camera) arranged in the image plane.

[0048] according to Figure 1 In an exemplary embodiment (however, the invention is not limited thereto), beam splitter 103 performs a reflective operation on the illumination optical path and a transmissive operation on the imaging optical path. Therefore, any potential inhomogeneities in beam splitter 103 do not significantly affect the imaging quality in the imaging optical path, which is more sensitive to aberrations compared to the illumination optical path. In a further embodiment, beam splitter 103 may, conversely, perform a transmissive operation on the illumination optical path and a reflective operation on the imaging optical path.

[0049] To achieve transmission of EUV light, the beam splitter 103 is designed to be coated with a thin film of a multilayer system made of multiple alternating molybdenum (Mo) and silicon (Si) layers, and the thickness of the film is less than 250 nm, more particularly, less than 100 nm, and even more particularly, less than 40 nm (the film can be made of, for example, silicon (Si), silicon nitride (SiN), or carbon nanotubes). To achieve substantially corresponding orders of magnitude of reflectivity and transmittance of the beam splitter 103, in one exemplary embodiment, the multilayer system has a total of 12 partial layer stacks, each made of one molybdenum layer and one silicon layer. However, the invention is not limited thereto; to provide sufficient transmittance for the beam splitter 103, the number of partial layer stacks is preferably less than 20, more particularly, less than 15.

[0050] In addition to the alternating layers described above, other (functional) layers may be provided, such as diffusion-blocking layers, barrier layers, etc. In this case, regarding exemplary layer materials, reference is made only to US 7,982,854 B2, DE 10 2012 202057 B4, DE 10 2017 221 146 A1, DE 10 2014 222 534 A1, and the publication “Mo / Si multilayer-coated amplitude-division beam splitters for XUV radiation sources” by R. Sobierajski et al. (Synchrotron Radiation Impurities, 2013, Vol. 20, pp. 249–257).

[0051] exist Figure 1 In an exemplary embodiment, due to the deflection effect of mirrors 105 and 104, EUV light is incident on beam splitter 103 in the imaging optical path in a substantially collimated manner (i.e., the light rays are approximately parallel). As a result, the incident angle is substantially constant over the transmission optics usage area of ​​beam splitter 103, that is, the angle load on beam splitter 103 is low.

[0052] However, the present invention is not limited thereto. Figure 2a -b illustrates possible further embodiments of the optical system according to the invention under different conditions in schematic form.

[0053] according to Figures 2a-2b There are no mirrors in the imaging optical path between the mask and the beam splitter. Therefore, in order to compensate for the angular load that occurs on the beam splitter in the imaging optical path, the multilayer system of the beam splitter can have a varying thickness (i.e., at least one thin layer with varying thickness).

[0054] according to Figure 2a EUV light from light source 201 travels along the illumination path, passes through three mirrors 202, 203, and 204, reaches the beam splitter 205 according to the invention, and then is incident on the mask 206 to be characterized, located in the object plane. After reflection at the mask 206, the light enters the imaging path, which initially passes through the beam splitter 205 again, and then through additional mirrors 207-210 to reach the sensor unit 211 arranged in the image plane.

[0055] according to Figure 2b EUV light from light source 251 travels along the illumination path, passes through two mirrors 252 and 253, reaches the beam splitter 254 according to the invention, and then is incident on the mask 255 to be characterized, located in the object plane. After reflection at the mask 255, the light enters the imaging path, which initially passes through the beam splitter 254 again, and then through additional mirrors 256-259 to reach the sensor unit 260 arranged in the image plane.

[0056] Based on Figure 1 and Figures 2a-2b In each of the above embodiments, substantially perpendicular mask illumination is achieved in both the illumination and imaging optical paths by using the beam splitter according to the invention. In this respect, the geometric separation of the illumination and imaging optical paths can be eliminated (which would be necessary without using the beam splitter according to the invention). Referring below... Figure 4 a-4c and Figure 5 The schematic diagrams in a-5b illustrate the advantages of this situation, particularly the resolution ultimately achievable using the optical system according to the invention.

[0057] Figure 4 Initially, a purely schematic and very simplified representation shows the possible conventional geometric beam splitting in the case of oblique illumination of mask 400, where "401" represents the illumination beam incident on mask 401 along the illumination path, and "402" represents the imaging beam traveling in the imaging path after reflection at mask 400. Figure 4 b and Figure 4In c, the circular regions under different conditions represent the maximum usable numerical aperture (NA = 0.2) that takes into account the reflectivity of the mask 400. Figure 3 (Maximum angle of incidence of 12°). Figure 4 In b, this is based on an optical system with a mask-side numerical aperture (NA) of 0.0825 in the x and y directions (relative to the plotted coordinate system), while... Figure 4 In c, this is based on an optical system with a mask-side numerical aperture (NA) of 0.06875 in the y-direction and 0.1375 in the x-direction. Clearly, based on... Figure 4 b and Figure 4 c. The maximum available numerical aperture (NA) is used with different efficiencies depending on the mask reflectivity, and in any case, the maximum available numerical aperture (NA) is only partially used.

[0058] Figure 5 a is similar to Figure 4 The representation of 'a' illustrates a substantially vertical mask illumination that can be achieved according to the invention, where geometric beam splitting is omitted due to the use of beam splitter 503. In this case, the illumination beam 501 traveling in the illumination optical path is reflected towards the mask 500 by beam splitter 503, and then the reflected light in the imaging optical path is transmitted through beam splitter 503 as the imaging beam. Figure 5 b. A relevant optical system can be designed to take into account the reflectivity of the mask 500 (according to...). Figure 3 The maximum available numerical aperture (NA = 0.2, corresponding to a maximum incident angle of 12°) is used entirely for both the illumination and imaging optical paths. Therefore, the achievable resolution is significantly increased.

[0059] Another advantage of the concept of achieving substantially vertical mask illumination according to the invention is that a telecentric optical path can be realized, which is advantageous, especially when determining placement errors on the mask (so-called "registration"), because the possible focusing errors in mask positioning have less of an impact on the accuracy of the corresponding position determination compared to tilted mask illumination.

[0060] Although the invention has been described based on specific embodiments, many variations and alternative embodiments will be apparent to those skilled in the art, for example by combinations and / or exchanges of features of the various embodiments. Therefore, it will be self-evident to those skilled in the art that such variations and alternative embodiments are also included in the invention, and that the scope of the invention is limited only to the meaning of the appended claims and their equivalents.

Claims

1. An optical system, comprising: Light sources (101, 201, 251) are used to generate light with wavelengths less than 30 nm; An illumination optical path extends from the light source (101, 201, 251) to the object plane, wherein at least one reflector is present in the illumination optical path between the light source and the object plane; An imaging optical path extends from the object plane to the image plane, wherein at least one mirror is present in the imaging optical path between the object plane and the image plane; and Beam splitters (103, 205, 254) are used to extend both the illumination and imaging optical paths for light with wavelengths less than 30 nm.

2. The optical system according to claim 1, characterized in that, The centroidal ray of the illumination beam incident on the object plane in the illumination path is incident on the object plane at an angle not exceeding 6° relative to the surface normal.

3. The optical system according to claim 2, characterized in that, The angle shall not exceed 4°.

4. The optical system according to claim 2, characterized in that, The angle does not exceed 0°.

5. The optical system according to any one of claims 1 to 4, characterized in that, The light in the illumination optical path is reflected at the beam splitters (103, 205, 254), and the light in the imaging optical path is transmitted through the beam splitters (103, 205, 254).

6. The optical system according to any one of claims 1 to 4, characterized in that, The beam splitters (103, 205, 254) comprise a multilayer system on a thin film with a thickness of less than 250 nm.

7. The optical system according to claim 6, characterized in that, The thickness of the film is less than 100 nm.

8. The optical system according to claim 6, characterized in that, The thickness of the film is less than 40 nm.

9. The optical system according to claim 6, characterized in that, The multilayer system includes multiple partial layer stacks made of a first thin layer of a first material and a second thin layer of a second material, the number of which is less than 20.

10. The optical system according to claim 9, characterized in that, The number of partially stacked layers is less than 15.

11. The optical system according to claim 9, characterized in that, The first material contains molybdenum (Mo) or ruthenium (Ru).

12. The optical system according to claim 9, characterized in that, The second material contains silicon (Si).

13. The optical system according to claim 6, characterized in that, The multilayer system includes at least one thin layer with varying thickness.

14. The optical system according to claim 6, characterized in that, The film is made of silicon (Si), silicon nitride (SiN), or carbon nanotubes.

15. The optical system according to any one of claims 1 to 4, characterized in that, The beam splitters (103, 205, 254) have a transmission non-uniformity of less than 1% in their optical operating area.

16. The optical system according to claim 15, characterized in that, In the optical application area, the transmission non-uniformity is less than 0.8%.

17. The optical system according to claim 15, characterized in that, In the optical application area, the transmission non-uniformity is less than 0.5%.

18. The optical system according to any one of claims 1 to 4, characterized in that, The beam splitters (103, 205, 254) have an optical operating area, the size of which is at least 30 mm in one spatial direction.

19. The optical system according to claim 18, characterized in that, The size of the optical application area is at least 40 mm in one spatial direction.

20. The optical system according to any one of claims 1 to 4, characterized in that, The imaging optical path extends substantially telecentrically on the object plane side.

21. The optical system according to any one of claims 1 to 4, characterized in that, There are no mirrors along the imaging optical path between the object plane and the beam splitters (205, 254).

22. The optical system according to any one of claims 1 to 4, characterized in that, Along the imaging optical path between the object plane and the beam splitter (103), there is at least one mirror (105, 104).

23. The optical system according to claim 22, characterized in that, There are at least two mirrors (105, 104).

24. The optical system according to any one of claims 1 to 4, characterized in that, The light from the light sources (101, 201, 251) has a wavelength of less than 15 nm.

25. The optical system according to claim 24, characterized in that, The wavelength is in the range of 13 nm to 14 nm.

26. The optical system according to any one of claims 1 to 4, characterized in that, The optical system is a one-piece device for characterizing micro-lithography masks (106, 206, 255). The masks (106, 206, 255) to be characterized are arranged in the object plane, illuminated by the illumination optical path, and imaged onto sensor units (110, 211, 260) arranged in the image plane by the imaging optical path.

27. A beam splitter for use in an optical system according to any one of claims 1 to 26, wherein, The beam splitters (103, 205, 254) comprise a multilayer system on a thin film with a thickness of less than 250 nm.

28. Use of a beam splitter in an optical system according to any one of claims 1 to 26, wherein, The beam splitters (103, 205, 254) comprise a multilayer system on a thin film with a thickness of less than 250 nm.

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