Optical system, in particular for characterizing a microlithography mask

Abstract

The present invention relates to an optical system, particularly for characterizing a microlithography mask, comprising a light source (101, 201, 251) for generating light of a wavelength of less than 30 nm, an illumination beam path extending from the light source to an object plane, an imaging beam path extending from the object plane to an image plane, and a beam splitter (103, 205, 254) through which both the illumination beam path and the imaging beam path proceed.

Description

Technology Field

[0001] Cross-reference regarding 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 this application are incorporated herein by reference.

[0003] The present invention relates particularly to an optical system for characterizing a microlithography mask. Background Technology

[0004] Microlithography is used to produce microstructured components, such as integrated circuits or LCDs. The microlithography process is performed in what is known as a projection exposure device, which includes an illumination device and a projection lens. An image of a mask (= reticle) illuminated by the illumination device is projected onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) by the projection lens in this case, and is aligned with the image plane of the projection lens to transfer the mask structure onto the photosensitive coating of the substrate.

[0005] In the lithography process, unwanted defects in the mask have a particularly adverse effect because they can be reproduced at every illumination stage. Therefore, to minimize mask defects and achieve successful mask repair, it is desirable to directly analyze the imaging effect of possible defect locations. Consequently, there is a demand for rapid and easy measurement or verification of the mask in principle. In this case, a known practice involves recording and evaluating aerial images of mask sections, particularly in mask inspection devices. To record the aerial image, the structure to be measured on the mask is illuminated by a magnifying illumination optical unit, and light emanating from the mask is projected through an imaging optical unit to a detector unit and detected by said detector unit. In this process, a relatively large continuous area, specifically the entire optically effective surface of the mask, can be scanned to locate defects.

[0006] In a general conventional structure, FIG. 6 illustrates a mask inspection device (600) comprising an illumination optical unit (601) and an imaging optical unit (605), wherein light from a light source (not shown in FIG. 6) enters the illumination optical unit (601) and directs an illumination beam (602) onto a mask (603) arranged on an object plane of the imaging optical unit (605), wherein the illumination area of ​​the mask (603) is imaged onto a sensor array (606), such as a CCD camera, by means of the imaging optical unit (605) by means of the imaging beam (604).

[0007] With regard to prior art, purely by example, DE 10 2017 221 146 A1, US 2014 / 0240686 A1, US 7,982,854 B2, DE 10 2012 202 057 B4 and publication R. Sobierajski et al: " Beam splitter for Mo / Si multilayer coated amplitude split XUV radiation source ", J. Synchrotron Rad. (2013) 20, pages 249-257 are referenced.

[0008] The object of the present invention is to provide an optical system, in particular an optical system for characterizing a microlithography mask, which makes full use of the resolution obtainable in the EUV range and enables the associated increased measurement accuracy.

[0009] This objective is achieved by an optical system according to the features of independent claim 1.

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

[0011] - A light source that generates light with a wavelength of less than 30 nm;

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

[0013] - Imaging beam path leading from the object plane to the image plane; and

[0014] - Includes a beam splitter through which the illumination beam path and the imaging beam path pass.

[0015] The present invention is based on the concept that an illumination beam path extending from a light source to an object plane can be separated from an imaging beam path extending from the object plane to an image plane by a beam splitter of an imaging optical system designed for EUV operation in such a way that both the illumination beam path and the imaging beam path are partially guided through the beam splitter. Consequently, illumination substantially perpendicular to the object plane is possible according to the present invention without geometric separation of the illumination beam path and the imaging beam path, which is particularly advantageous for various reasons to equipment for characterizing a microlithography mask in which the mask to be characterized is located in the object plane under perpendicular illumination, particularly during an application according to the present invention.

[0016] First, considering the fact that the substantially perpendicular mask illumination possible according to the present invention is significantly offset from the reflection of the mask designed for the EUV range according to the angle-dependent curve illustrated in the diagram of FIG. 3 at a relatively large angle of incidence (relative to the surface normal)—in the angle of incidence range of 12° or more according to the diagram of FIG. 3—and that the mask reflects sufficiently only within this angle of incidence range, the corresponding limited angle of incidence range (corresponding to an numerical aperture of approximately NA = 0.2) can ultimately be used for the illumination beam path and the imaging beam path. Guiding both the illumination beam path and the imaging beam path according to the present invention through a single beam splitter now allows the NA range to be fully utilized for both the illumination beam path and the imaging beam path, and consequently enables a relatively high resolution (particularly compared to the resolution ultimately obtainable within an EUV projection exposure device).

[0017] Furthermore, consequently, the mask-side telecentric beam path can be realized through substantially vertical mask illumination, which is permitted by the concept according to the present invention. This is particularly advantageous when determining placement (so-called "registration") errors on the mask, because—compared to inclined mask illumination—focus errors that may occur in mask positioning have a less severe impact on the accuracy of each positioning.

[0018] In order to obtain the aforementioned advantages in return, particularly in terms of increased resolution and relatively greater insensitivity to focusing errors and the consequently greater measurement accuracy, by using a beam splitter suitable for operation in the EUV range (which must meet relatively strict requirements in terms of planar geometry and transmission homogeneity), the present invention intentionally accommodates increased manufacturing costs and increased optical loss (in terms of light in the illumination and imaging beam paths that must be guided a total of two times through the beam splitter).

[0019] According to one embodiment, the center ray of the illumination beam incident on the object plane in the illumination beam path is incident on the object plane at an angle of 6° or less with respect to the surface normal, particularly an angle of 4° or less, and even more particularly an angle of 0°.

[0020] In the optical system according to the present invention, light in the illumination beam path is reflected by the beam splitter, and light in the imaging beam path is transmitted through the beam splitter according to the embodiment. This configuration is particularly advantageous in that the imaging beam path, which is relatively sensitive to optical aberrations, uses only the beam splitter during transmission, and consequently, considering the thin thickness of the beam splitter and the refractive index in EUV having a value of nearly 1 (unlike in the case of reflection), the possible non-uniformity of the beam splitter does not significantly affect the image quality that can be obtained. However, the present invention is not limited thereto, and therefore, in principle, embodiments in which the beam splitter operates in reverse—transmission for the illumination beam path and reflection for the imaging beam path—should also be considered to be included in the present invention.

[0021] According to one embodiment, the beam splitter comprises a multilayer system on a membrane, the thickness of which is less than 250 nm, particularly less than 100 nm, more particularly less than 40 nm.

[0022] According to an embodiment, the multilayer system comprises a plurality of partial layer stacks, each consisting of a first laminar made of a first material and a second laminar made of a second material, and the number of partial layer stacks is less than 20, particularly less than 15.

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

[0024] According to an embodiment, the multilayer system comprises at least one laminar having various thicknesses. This is particularly advantageous when the angle of incidence of light on the beam splitter varies significantly across the beam splitter region. In this case, a layer thickness profile can be selected such that the layer thickness is optimized for the small angle of incidence in the region where the small angle of incidence occurs, and the layer thickness is optimized for the large angle of incidence in the region where the large angle of incidence occurs.

[0025] According to the examples, the membrane is produced from silicon (Si), silicon nitride (SiN), or carbon nanotubes.

[0026] To prevent functional impairment of the beam splitter according to the present invention as a result of deformation or bending, a membrane having a multilayer system positioned thereon can be accommodated in a suitable mount, for example, a clamping mount.

[0027] According to the embodiment, the beam splitter has a transmission non-homogeneity of less than 1%, particularly less than 0.8%, and more particularly less than 0.5% over the optically used area.

[0028] According to one embodiment, the beam splitter has an optically used area, and its dimensions are at least 30 mm in one spatial direction, more specifically at least 40 mm.

[0029] According to the embodiment, the image beam path proceeds substantially telecentrically on the mask side or the object plane side.

[0030] According to an embodiment, there is at least one mirror, more specifically at least two mirrors, along the imaging beam path between the object plane and the beam splitter. By one or more of these mirrors (each having a planar or curved shape on an optically effective surface), it is possible to reduce the angular load of the beam splitter due to the substantially collimated beam path established at the position of the beam splitter by said mirror(s). However, the invention is not limited thereto, and therefore embodiments in which there are no further optical components or mirrors between the beam splitter and the object plane or mask should also be considered to be included in the invention. Such a configuration in which there are no additional mirrors between the object plane and the beam splitter may likewise be advantageous in terms of a reduction, which is, in principle, desirable, of the total number of mirrors used within the optical system.

[0031] According to the embodiment, consequently, there is no mirror along the imaging beam path between the object plane and the beam splitter.

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

[0033] According to one embodiment, the optical system is part of an apparatus for featuring a microlithography mask having a mask to be characterized arranged on an object plane that is illuminated through an illumination beam path and imaged on a sensor unit arranged on an image plane through an imaging beam path.

[0034] However, the present invention is not limited thereto and can be advantageously used in additional applications, such as optical systems or microscopes for characterizing mask blanks of lithography masks, for example.

[0035] In addition, the present invention relates to a beam splitter for use in an optical system having the aforementioned features, and to the use of the beam splitter in an optical system having the aforementioned features, wherein the beam splitter comprises a multilayer system on a membrane, the thickness of which is less than 250 nm, more particularly less than 100 nm, and even more particularly less than 40 nm. With regard to the advantages and preferred configurations of the beam splitter, reference is made to the foregoing description in relation to the optical system according to the present invention.

[0036] Further configurations of the present invention are apparent from the detailed description and dependent claims.

[0037] The present invention is described in more detail below based on exemplary embodiments illustrated in the attached drawings. Brief explanation of the drawing

[0038] FIG. 1 illustrates a schematic diagram for explaining a possible structure of an optical system according to the present invention for characterizing a mask; FIGS. 2a-2b illustrates schematic diagrams for explaining additional embodiments of an optical system according to the present invention for characterizing a mask; Figure 3 shows a diagram regarding the angle-dependent curve of the reflectance of a mask designed for the EUV range; FIGS. 4a-4c illustrate schematic diagrams to explain exemplary use of numerical apertures available in EUV systems in the case of geometric separation of illumination and imaging beam paths (Fig. 4a) (Fig. 4b and Fig. 4c); FIGS. 5a-5b illustrate schematic diagrams to explain the possible utilization of available numerical apertures (Fig. 5b) in an EUV system when using a beam splitter commonly used for the illumination and imaging beam path (Fig. 5a); and Figure 6 is a schematic diagram illustrating the general structure of equipment for characterizing a conventional mask. Specific details for implementing the invention

[0039] Below, exemplary embodiments of an optical system according to the present invention are described with reference to the schematic diagram of FIG. 1-5b. Common to these embodiments is the use of a beam splitter in an imaging optical system designed to operate in EUV, the use of which allows both the illumination beam path and the imaging beam path to partially pass through the beam splitter and consequently allows for substantially perpendicular illumination or imaging of an object, e.g., a mask to be characterized, without geometric separation of the illumination beam path and the imaging beam path.

[0040] As a purely schematic diagram, FIG. 1 illustrates a possible structure of an optical system according to the present invention for characterizing an EUV lithography mask in a first embodiment.

[0041] According to FIG. 1, light (EUV) emitted by a light source (101) is initially incident on a mask (106) to be characterized located on an object plane, which passes along an illumination beam path through a mirror (102), a beam splitter (103), and additional mirrors (104 and 105) according to the present invention. Following reflection from the mask (106), the light initially proceeds again through the mirrors (105, 104) and the beam splitter (103) and then enters an image beam path that proceeds through additional mirrors (107 and 108) to a sensor unit (109) (e.g., a CCD camera) arranged on an image plane.

[0042] According to the exemplary embodiment of FIG. 1—but the invention is not limited thereto—the beam splitter (103) operates in a reflective manner with respect to the illumination beam path and, in contrast, in a transmissive manner with respect to the imaging beam path. Consequently, possible non-uniformity of the beam splitter (103) is maintained without significantly affecting the imaging quality of the imaging beam path, which is substantially more sensitive to aberrations compared to the illumination beam path. In a further embodiment, the beam splitter (103) may also operate in reverse, in a transmissive manner with respect to the illumination beam path and in a reflective manner with respect to the imaging beam path.

[0043] To enable transmission operation for EUV light, the beam splitter (103) is coated with a multilayer system consisting of a plurality of alternating molybdenum (Mo) and silicon (Si) layers and designed with a membrane having a thickness of less than 250 nm, particularly less than 100 nm, more particularly less than 40 nm (the membrane may be made from, for example, silicon (Si), silicon nitride (SiN), or carbon nanotubes). To achieve substantially corresponding magnitudes of reflectance and transmittance of the beam splitter (103), the multilayer system has a total of 12 partial layer stacks, each consisting of one molybdenum layer and one silicon layer in one exemplary embodiment. However, the invention is not limited thereto, and to provide sufficient transmittance of the beam splitter (103), the number of partial layer stacks is preferably less than 20, particularly less than 15.

[0044] Additional (functional) layers, e.g., diffusion barrier layers, barrier layers, etc., may be provided in addition to the aforementioned alternating layers. In this context and with respect to exemplary layer materials, purely by example, US 7,982,854 B2, DE 10 2012 202 057 B4, DE 10 2017 221 146 A1, DE 10 2014 222 534 A1 and publication R. Sobierajski et al: " Beam splitter for Mo / Si multilayer coated amplitude split XUV radiation source See ", J. Synchrotron Rad. (2013) 20, pages 249-257.

[0045] In the exemplary embodiment of FIG. 1, EUV light is incident on the beam splitter (103) in a substantially collimated manner (i.e., the light rays are approximately parallel) in the imaging beam path as a result of the deflection effect of the mirrors (105, 104), and as a result, the angle of incidence is substantially constant across the area of ​​the beam splitter (103) optically used for transmission, i.e., the angular load on the beam splitter (103) is low.

[0046] However, the present invention is not limited thereto. FIGS. 2a-b schematically illustrates possible additional embodiments of the optical system according to the present invention in each case.

[0047] According to FIG. 2a-2b, there is no mirror in the imaging beam path between the mask and the beam splitter. Consequently, to compensate for the angular load occurring on the beam splitter in the imaging beam path, the multilayer system of the beam splitter may have various thicknesses (i.e., at least one laminar having various thicknesses).

[0048] According to FIG. 2a, EUV light from a light source (201) travels along an illumination beam path through three mirrors (202, 203, 204) to reach a beam splitter (205) according to the present invention and is incident on a mask (206) to be characterized located on an object plane. Following reflection from the mask (206), the light enters an imaging beam path that initially travels again through the beam splitter (205) and then through additional mirrors (207-210) to a sensor unit (211) arranged on an image plane.

[0049] According to FIG. 2b, EUV light from a light source (251) travels along an illumination beam path through two mirrors (252, 253) to reach a beam splitter (254) according to the present invention, and then is incident on a mask (255) to be characterized located on an object plane. Following reflection from the mask (255), the light enters an image beam path that initially travels again through the beam splitter (254) and then through additional mirrors (256-259) to proceed to a sensor unit (260) arranged on an image plane.

[0050] In each of the embodiments described above based on FIGS. 1 and FIGS. 2a-2b, mask illumination that is substantially perpendicular to both the illumination beam path and the imaging beam path is achieved by using the beam splitter according to the present invention, and in this regard, it is possible to omit the geometric separation of the illumination beam path and the imaging beam path (as required without using the beam splitter according to the present invention). In particular, the advantages arising from this environment regarding the resolution ultimately obtainable using the optical system according to the present invention are described below with reference to the schematic diagrams in FIGS. 4a-4c and FIGS. 5a-5b.

[0051] FIG. 4a initially illustrates, in a purely schematic and highly simplified representation, a conventional geometric beam split possible for oblique illumination of the mask (400), where “401” represents the illumination beam incident on the mask (401) along the illumination beam path and “402” represents the imaging beam proceeding in the imaging beam path after reflection from the mask (400). In FIG. 4b and FIG. 4c, the circular area in each case represents, in principle, the maximum available numerical aperture (a value of NA = 0.2 corresponding to a maximum angle of incidence of 12° according to FIG. 3) taking into account the reflectivity of the mask (400). In FIG. 4b, this is based on an optical system having a mask-side numerical aperture (NA) of 0.0825 in the x and y directions (relative to the plotted coordinate system), whereas in FIG. 4c, this is based on an optical system having a mask-side numerical aperture (NA) of 0.06875 in the y direction and 0.1375 in the x direction. According to FIGS. 4b and 4c, the maximum available numerical aperture (NA) according to the reflectance of the mask is used with different efficiencies, and in any case, it is evident that the maximum available numerical aperture (NA) is used only partially.

[0052] Similar in expression to FIG. 4a, FIG. 5a illustrates a substantially vertical mask illumination made possible according to the present invention, in which geometric beam splitting is omitted as a result of using a beam splitter (503). In this case, the illumination beam (501) traveling in the illumination beam path is reflected toward the mask (500) by the beam splitter (503), and the light reflected in the imaging beam path is transmitted through the beam splitter (503) as an imaging beam. According to FIG. 5b, the relevant optical system uses the maximum available numerical aperture (a value of NA = 0.2 corresponding to a maximum angle of incidence of 12° according to FIG. 3) in both the illumination beam path and the imaging beam path, taking into account the reflectivity of the mask (500). As a result, the achievable resolution is greatly increased.

[0053] A telecentric beam path can be realized as another advantage of the concept according to the present invention, which realizes substantially vertical mask illumination, and this is particularly advantageous when determining mask placement errors (so-called "alignment") for the following reason: - compared to oblique mask illumination - focusing errors that may occur in mask positioning have a less serious effect on the accuracy of each positioning.

[0054] Although the present invention has been described based on specific embodiments, various modifications and alternative embodiments will be apparent to those skilled in the art, for example, by combinations and / or exchanges of features of individual embodiments. Accordingly, it goes without saying to those skilled in the art that such modifications and alternative embodiments are also included in the present invention, and that the scope of the present invention is limited only to the meaning of the appended claims and their equivalents.

Claims

Claim 1 An optical system comprising: a light source (101, 201, 251) for generating light of a wavelength of less than 30 nm; an illumination beam path extending from the light source (101, 201, 251) to an object plane; an imaging beam path extending from the object plane to an image plane; and a beam splitter (103, 205, 254) through which both the illumination beam path and the imaging beam path proceed, wherein the beam splitter (103, 205, 254) comprises a multilayer system on a membrane, the thickness of which is less than 250 nm. Claim 2 An optical system according to claim 1, characterized in that the center ray of the illumination beam incident on the object plane in the illumination beam path is incident on the object plane at an angle of 6° or less with respect to the surface normal. Claim 3 An optical system according to claim 1 or claim 2, characterized in that light of the illumination beam path is reflected by the beam splitter (103, 205, 254) and light of the imaging beam path is transmitted through the beam splitter (103, 205, 254). Claim 4 delete Claim 5 An optical system according to claim 1 or claim 2, wherein the multilayer system comprises a plurality of partial layer stacks composed of a first laminar of a first material and a second laminar of a second material, and the number of partial layer stacks is less than 20. Claim 6 An optical system according to claim 5, wherein the first material comprises molybdenum (Mo) or ruthenium (Ru). Claim 7 An optical system according to claim 5, wherein the second material comprises silicon (Si). Claim 8 An optical system according to claim 1 or claim 2, characterized in that the multilayer system comprises at least one laminar having various thicknesses. Claim 9 An optical system according to claim 1 or claim 2, characterized in that the membrane is made of silicon (Si), silicon nitride (SiN), or carbon nanotubes. Claim 10 An optical system according to claim 1 or claim 2, wherein the beam splitter (103, 205, 254) has less than 1% transmission non-homogeneity over an optically used area. Claim 11 An optical system according to claim 1 or claim 2, wherein the beam splitter (103, 205, 254) has an optically used area and its dimensions are at least 30 mm in one spatial direction. Claim 12 An optical system according to claim 1 or claim 2, characterized in that the imaging beam path proceeds telecentrically on the object plane side. Claim 13 An optical system according to claim 1 or claim 2, characterized in that there is no mirror along the imaging beam path between the object plane and the beam splitter (205, 254). Claim 14 An optical system according to claim 1 or claim 2, characterized in that there is at least one mirror (105, 104) along the imaging beam path between the object plane and the beam splitter (103). Claim 15 An optical system according to claim 1 or claim 2, characterized in that light from a light source (101, 201, 251) has a wavelength of less than 15 nm. Claim 16 An optical system according to claim 1 or claim 2, wherein the optical system is one device for characterizing a microlithography mask (106, 206, 255), and the mask to be characterized (106, 206, 255) arranged on an object plane is illuminated via an illumination beam path and imaged onto a sensor unit (109, 211, 260) arranged on an image plane via an imaging beam path. Claim 17 A beam splitter used in the optical system described in claim 1 or claim 2.

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