Method for manufacturing an optical element for a lithographic apparatus

Abstract

To provide a method of producing an optical element for a lithography apparatus.SOLUTION: A method of producing an optical element (200) for a lithography apparatus (100), comprising: a step (S2) of a) detecting a height profile (216) of a surface (208) of a crystal substrate (202) of the optical element (200); and a step (S3) of b) ascertaining, using the height profile detected (216), an installed orientation (δ2, δ4, δ6) of the optical element (200) in an optical system (102, 104) of the lithography apparatus (100) in relation to stress-induced birefringence on incidence of polarized radiation (204), wherein the installed orientation (δ2, δ4, δ6) is an orientation in relation to rotation of the optical element (200) about a center axis (214) of the optical element (200) that runs through the surface (208).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing an optical element for a lithographic apparatus.

[0002] The content of priority application DE 10 2022 118 146.4 is incorporated by reference in its entirety. [Background technology]

[0003] Microlithography is used in the manufacture of microstructured component parts, e.g., integrated circuits. The microlithography process is carried out by a lithography apparatus having an illumination system and a projection system. An image of a mask (reticle), illuminated by the illumination system, is projected by the projection system onto a substrate, e.g., a silicon wafer, which is coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection system, in order to transfer the mask structure into the photosensitive coating of the substrate.

[0004] To ensure a sufficient lifetime, optical elements of lithography devices can be fabricated from crystalline substrates, such as calcium fluoride (CaF). Crystals with cubic symmetry, such as CaF, are free of symmetry-breaking irregularities and are optically isotropic. However, the crystal growth process can induce stresses, for example, due to material processing or temperature gradients. These stresses can result in stress-induced birefringence, for example, under mechanical stress. This can affect the polarization properties of radiation transmitted by the optical element. This limits the resolution of the lithography device.

[0005] It is known that the distortion of polarization properties is minimal when radiation propagates in the 0111 crystal direction of the crystal lattice of the crystalline substrate. Furthermore, for example, U.S. Pat. No. 6,904,073 discloses that the distortion of polarization properties varies with rotation (clocking) of the optical element about its central axis, and in particular has six minima in the distribution of rotation angles. Therefore, by appropriate rotation orientation of the optical element, it is possible to minimize the effects of stress-induced birefringence in the case of transmitted polarized radiation. However, known methods for determining the optimal rotation orientation are inconvenient and require complex test setups. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] U.S. Patent No. 6,904,073 Summary of the Invention

[0007] Against this background, it is an object of the present invention to provide an improved method for producing optical elements for lithographic apparatus.

[0008] Accordingly, a method for manufacturing an optical element for a lithographic apparatus is proposed, said method comprising the following steps: a) detecting a height profile of a surface of a crystalline substrate of an optical element; b) using the detected height profile to ascertain an installed orientation of an optical element of an optical system of the lithographic apparatus in relation to stress-induced birefringence for incident polarized radiation, the installed orientation having an orientation related to a rotation of the optical element about a central axis of the optical element passing through the surface.

[0009] In this way, it is possible to determine the optimal mounting orientation of an optical element in relation to stress-induced birefringence in a simpler way. In particular, the proposed determination of the optimal mounting orientation can be continuously and without great extra difficulty inserted into the conventional method for manufacturing the crystal substrate of the optical element. This replaces the process step that is required anyway to detect the height profile (surface fit), so that information about the optimal mounting orientation, which is of great value to users of the optical element, is generated in a simpler way.

[0010] The effect of stress-induced birefringence is more particularly that the polarization properties of radiation transmitted by the optical element are altered and distorted, for example in the case of stress-induced birefringence radiation passing through the optical element suffers a loss of contrast due to a change in polarization direction.

[0011] Stress-induced birefringence, and the resulting change in the polarization properties of transmitted radiation, is affected by rotation ("clocking") of the optical element about its central axis. Thus, for transmitted polarized radiation, the effects of stress-induced birefringence can be minimized by appropriate rotational orientation of the optical element.

[0012] The mounting orientation is determined in particular with reference to (e.g., as a function of) the determined height profile. The mounting orientation thus determined is in particular a mounting orientation related to stress-induced birefringence. The mounting orientation of the optical element of the optical system determined using the detected height profile in particular results in low and / or minimal stress-induced birefringence for incident polarized radiation, resulting in low distortion of the polarization properties of the radiation transmitted through the optical element.

[0013] This method is particularly useful for producing a crystalline substrate for an optical element. The surface of the crystalline substrate is in particular an end face of the optical element. The surface of the crystalline substrate can be flat or curved.

[0014] The surface height profile describes, in particular, the surface structure of the crystalline substrate and can be used to derive, in particular, a favorable mounting orientation of optical elements in the optical system of a lithographic apparatus with respect to stress-induced birefringence.

[0015] Detecting the height profile of a surface includes, for example, detecting the surface fit (of a surface fit image), i.e., detecting the variation of the actual surface shape from the intended surface shape. The height profile can be detected, for example, using interferometry.

[0016] The determined mounting orientation may, for example, have one or more values ​​of a rotation angle (azimuthal angle) for rotation about a central axis.

[0017] The central axis may be, for example, a plane normal to the surface, or may be, for example, perpendicular to the main plane of extent of the optical element.

[0018] The central axis or axis of rotation is, for example, an axis that passes through the center of mass of the optical element. The central axis or axis of rotation is, for example, an axis that runs parallel to the surface normal of the outer face of the surface of the crystalline substrate that is closest to the center of mass. The above characteristics of the central axis or axis of rotation may not relate to the final shape of the finished optical element, but may also relate to the preceding process steps that give the optical element its final shape.

[0019] The central axis or axis of rotation is for example the intensity-weighted sum of all normal vectors of the illuminated area of ​​the optical element at the respective position relative to the incidence of polarized radiation.

[0020] "Incident of polarized radiation" in the operation of an optical element of an optical system includes the incident of linearly polarized radiation, vertically polarized radiation, and / or horizontally polarized radiation on the optical element. Polarized radiation is, for example, polarized DUV radiation. The propagation direction / emission direction of the radiation towards the optical element during operation of the optical element is, for example, inclined by the angle of incidence relative to the central axis. The angle of incidence has, for example, a value in the range of 30° to 60° and / or is, for example, 45°. However, the angle of incidence may also have a different value.

[0021] Locating a mounting orientation of an optical element using the determined height profile includes, for example, using the determined height profile to locate a mounting orientation of an optical element of an optical system that results in low and / or minimal stress-induced birefringence for incident polarized radiation compared to other mounting orientations with respect to rotation of the optical element about a central axis.

[0022] In one embodiment, the method further comprises the following step preceding step a): polishing the surface of the crystalline substrate to create a surface height profile. Includes.

[0023] In particular, polishing the surface creates a height profile of the surface that visualizes the preferred mounting orientation associated with stress-induced birefringence for incident polarized radiation. Furthermore, polishing the surface can be described as visualizing structures present in the crystalline substrate that exhibit and / or cause stress-induced birefringence on the surface as surface structures with a height profile.

[0024] Polishing the surface may include polishing the entire surface of one side (e.g., end face) of the crystalline substrate. However, polishing the surface may further include polishing only a portion of the surface of one side (e.g., end face) of the crystalline substrate. This may have the advantage of eliminating a further polishing step to remove the height profile.

[0025] In a further embodiment, the polishing comprises magneto-rheological polishing of the surface.

[0026] Magnetorheological polishing or finishing (MRF) is performed using a magnetorheological fluid consisting of magnetic particles, abrasive media, and water. For example, the fluid is continuously applied to a rotating wheel through a nozzle. The rotating wheel has, for example, magnets beneath the wheel surface to generate a magnetic field that alters the viscosity of the fluid. For example, the magnetic particles (e.g., iron particles) in the fluid become aligned within the magnetic field, forming a rigid structure that adheres to the wheel, and the water and abrasive particles become concentrated on the surface as a solidified thin abrasive layer.

[0027] The crystalline substrate is, for example, clamped to a movable holder and immersed in the polishing layer up to the surface to be treated, the movable holder further comprising, for example, drive means, a control unit, etc., for positioning the crystalline substrate (e.g., (fully) automatically).

[0028] In a further embodiment, polishing of the surface is performed by sweeping in a spiral across the surface, the spiral sweeping proceeding from an outer region of the surface toward the center in a spiral centered on the center of the surface defined by a central axis.

[0029] The surface is polished in particular by the so-called round method, also called the R-phi method, in that the surface is swept in azimuth with decreasing radius, similar to the way a pickup sweeps across a phonographic record.

[0030] Applicant has experimentally established that polishing by the round method removes surface material to create surface structures with height profiles from which preferred mounting orientations associated with stress-induced birefringence can be inferred. In other words, it has been found that polishing the surface visualizes structures present in the crystalline substrate that exhibit and / or cause stress-induced birefringence on the surface as surface structures with height profiles.

[0031] The spiral sweep of the surface can be performed, for example, by moving the crystalline substrate and / or by moving the polishing tool / polishing head of the polishing device.

[0032] In a further embodiment, the surface is polished by rotating the optical element about a central axis and simultaneously moving a polishing tool radially towards the center of the surface defined by the central axis.

[0033] In a further embodiment, the method further comprises the following step after step b): Marking the determined mounting orientation on the optical element. Includes.

[0034] This allows the ascertained mounting orientation to be read at a later point in time on the optical element itself and / or confirmed by a measuring instrument, for example a commercially available interferometer.

[0035] In the marking operation, in particular, the marking is applied to the optical element, for example to the crystal substrate, the outer surface of the crystal substrate, the surface of the crystal substrate, and / or an edge region of the surface of the crystal substrate.

[0036] The markings can be permanent or non-permanent, for example, painted (e.g., with a pen, a marker pen, and / or a silver marker pen) or engraved (e.g., by laser engraving and / or sandblast engraving).

[0037] In a further embodiment, the method includes, after marking the optical element with the determined mounting orientation, polishing the surface to remove a height profile of the surface such that the marking identifying the determined mounting orientation on the optical element is preserved.

[0038] This allows the surface structure introduced in the first polishing step to be removed again in a second polishing step, for example by rounding, thereby achieving a lower surface roughness without losing the markings for optimal mounting orientation.

[0039] The second polishing step is performed, for example, by serpentine magnetorheological polishing.

[0040] In further embodiments, the crystalline substrate comprises a crystal with cubic symmetry, a monocrystal, a fluoride crystal, calcium fluoride, magnesium fluoride, barium fluoride, and / or lutetium aluminum garnet.

[0041] Crystals with cubic symmetry, such as calcium fluoride (CaF2), have a high degree of crystal symmetry. A single crystal (also called a monocrystalline) is a macroscopic crystal whose units (atoms, ions, or molecules) form a continuous, uniform, and homogeneous crystal lattice. The empirical formula of magnesium fluoride is MgF2, the empirical formula of barium fluoride is BaF2, and the empirical formula of lutetium aluminum garnet is LuAG.

[0042] In a further embodiment, the surface of the crystalline substrate is formed by a

[0111] crystallographic plane of the crystalline substrate.

[0043] In further embodiments, the surface of the crystalline substrate is formed by a

[0100] crystal face, a

[0010] crystal face, or a

[0001] crystal face of the crystalline substrate.

[0044] The nomenclature of the crystallographic faces

[0111] ,

[0100] ,

[0010] , and

[0001] corresponds to the nomenclature of the faces of a crystal lattice based on the Miller indices a, b, c, which is conventional in crystallography.

[0045] The surface of the crystalline substrate may alternatively be formed in any other plane with respect to the crystalline order of the crystalline substrate.

[0046] In further embodiments, the optical element of the lithographic apparatus comprises a transmitting optical element, a partially transmitting optical element, a beam splitter, a beam splitter of an optical pulse extender, a lens element, and / or a chamber window.

[0047] The optical pulse extender is also called an optical pulse stretcher.The chamber window of the lithographic apparatus is for example a chamber window of a gas chamber of a light source of the lithographic apparatus.

[0048] In a further embodiment, determining the mounting orientation of the optical element determines a rotation angle of the optical element relative to a rotation of the optical element about a central axis that results in less and / or the smallest stress-induced birefringence for incident polarized radiation compared to other rotation angles for rotation of the optical element about the central axis.

[0049] Furthermore, multiple values ​​of the rotation angle of the optical element, for example, relative to the rotation of the optical element about the central axis, at which the stress-induced birefringence for incident polarized radiation has a minimum (eg, a local minimum) are identified.

[0050] In a further embodiment, determining the mounting orientation of the optical element determines the rotation angle of the optical element relative to the plane of polarization of the incident polarized radiation.

[0051] The optimum rotation angle of the optical element relative to the plane of polarization of the incident polarized radiation has, for example, a value between 0° and 90°.

[0052] The plane of polarization of incident polarized electromagnetic radiation is formed by the vector of the electric field of, for example, linearly polarized incident radiation.

[0053] In a further embodiment, determining the mounting orientation of the optical element determines an angular distribution of height values ​​of the determined height profile of the surface, the angle of the angular distribution corresponding to a respective rotation angle of the optical element relative to a rotation of the optical element about a central axis.

[0054] For example, the height profile of a surface is detected by ascertaining a surface-fitted image, and height values, or intensity values ​​corresponding to height values, are integrated and / or averaged within a predetermined azimuthal range of the surface-fitted image (e.g., a circular segment of the surface in the case of a circular surface).

[0055] "A" or "an" in this context should not be understood as necessarily limiting to exactly one element. Instead, a plurality of elements, e.g., two, three, or more elements, may also be provided. Also, any other numbers used herein should not be understood as strictly limiting to the recited number of elements. Instead, unless otherwise specified, upward and downward numerical deviations are possible.

[0056] Further possible implementations of the present invention further include not explicitly mentioned combinations of features or embodiments described above or below with respect to the exemplary embodiments, in which case the skilled person will further add individual aspects as improvements or supplements to the respective basic form of the invention.

[0057] Further advantageous configurations and aspects of the invention are the subject of the dependent claims and also of the examples of the invention described below.The invention is explained in detail below on the basis of preferred embodiments with reference to the attached drawings. [Brief explanation of the drawings]

[0058] [Figure 1]1 is a schematic diagram of one embodiment of a DUV lithography apparatus; [Figure 2] 2 shows optical elements of the lithographic apparatus of FIG. 1; [Figure 3] FIG. 3 is a top view of the optical element of FIG. 2. [Figure 4] FIG. 3 is a front view of the optical element of FIG. 2. [Figure 5] FIG. 1 is a diagram showing the

[0111] plane of a crystal lattice. [Figure 6] FIG. 1 shows the

[0100] plane of a crystal lattice. [Figure 7] FIG. 1 is a diagram showing the

[0010] plane of a crystal lattice. [Figure 8] FIG. 1 is a diagram showing the

[0001] plane of a crystal lattice. [Figure 9] 3 shows the crystalline substrate of the optical element of FIG. 2 during a polishing operation of the surface of the optical element. [Figure 10] FIG. 10 is a view similar to FIG. 9 showing the polishing pattern. [Figure 11] FIG. 10 shows a grayscale image of the height profile of the surface of FIG. 9 after polishing. [Figure 12] FIG. 12 is a diagram showing the angular distribution of image height values ​​in the height profile of FIG. 11. [Figure 13] 1 is a flow chart illustrating a method for manufacturing an optical element for a lithographic apparatus. DETAILED DESCRIPTION OF THE INVENTION

[0059] Unless otherwise indicated, identical or functionally identical elements are given the same reference numerals in the figures. It should also be noted that the illustrations in the figures are not necessarily to scale.

[0060] FIG. 1 shows a schematic diagram of a DUV lithography apparatus 100 including a beam-shaping and illumination system 102 and a projection system 104 (hereinafter also referred to as a "projection lens"). In this case, DUV stands for "deep ultraviolet" and refers to a working light wavelength of 30 to 250 nm. The beam-shaping and illumination system 102 and the projection system 104 are preferably each disposed within a vacuum housing (not shown). Each vacuum housing is evacuated using an evacuation device (not shown). The vacuum housing is surrounded by a machine chamber (not shown), which may include a drive unit for mechanically moving or adjusting optical elements. Additionally, an electrical controller or the like may be disposed within the machine chamber.

[0061] The DUV lithography apparatus 100 comprises a light source 106. For example, an ArF excimer laser emitting radiation 108 in the DUV range, for example at 193 nm, may be provided as the light source 106. In the beam shaping and illumination system 102, the radiation 108 is focused and a desired operating wavelength (working light) is selected from the radiation 108. The beam shaping and illumination system 102 may comprise optical elements (not shown), for example mirrors, lens elements.

[0062] After passing through the beam shaping and illumination system 102, the radiation 108 is directed onto a photomask (reticle) 110. The photomask 110 takes the form of a transmissive optical element and may be located outside the systems 102, 104. The photomask 110 has structures that are imaged onto a wafer 112 in reduced form by the projection system 104.

[0063] The projection system 104 has a plurality of lens elements 114, 116, 118 and / or mirrors 120, 122 for projecting an image of the photomask 110 onto the wafer 112. In this case, the individual lens elements 114, 116, 118 and / or mirrors 120, 122 of the projection system 104 may be arranged symmetrically with respect to an optical axis 124 of the projection system 104. It should be noted that the number of lens elements and mirrors shown here is merely exemplary and is not limited to the number shown. A greater or lesser number of lens elements 114, 116, 118 and / or mirrors 120, 122 may also be provided.

[0064] The gap between the last lens element (not shown) and the wafer 112 may be replaced with a liquid medium 126 having a refractive index greater than 1. For example, the liquid medium 126 may be high-purity water. Such a setup is also called immersion lithography and improves the resolution of photolithography. The medium 126 may also be called an immersion liquid.

[0065] The ArF excimer laser used as the light source 106 in the DUV lithography apparatus 100 by way of example emits radiation in the form of short optical pulses with a duration of approximately 20 ns. For typical pulse energies of 10 mJ or more, the high power peak of the laser poses a significant degradation risk to downstream optical elements in the beam shaping and illumination system 102 and the projection system 104. To avoid degradation of the downstream optical system, an optical pulse extender (OPuS) 128 can be used. The optical pulse stretcher 128 includes one or more beam splitters 130 (e.g., 45° beam splitters) that outcouple a portion of the radiation 108. The outcoupled portion of the radiation 108 then experiences a time delay relative to the portion of the radiation 108 transmitted by the beam splitter 130 using multiple reflections at highly reflective mirrors (not shown), before following the portion of the radiation 108 transmitted by the beam splitter 130 after being reflected again by the beam splitter 130. The highly reflective mirror is adjustably mounted on, for example, a holder 132 .

[0066] The beam splitter 130 used in the optical pulse stretcher 128 is here made in particular from a crystalline material with cubic symmetry, for example calcium fluoride (CaF2).

[0067] Lens elements for the DUV lithography apparatus 100, such as lens elements 114, 116, 118, or the chamber window 134 of the gas chamber of the light source 106, can also be fabricated from a crystalline material with cubic symmetry, such as CaF2.

[0068] It is known that optically isotropic crystals can exhibit stress-induced birefringence of an incident light beam, for example as a result of mechanical stress. Birefringence means that the refractive index depends on the polarization direction. Also, cubic crystals such as CaF2, which are essentially optically isotropic, can become birefringent (stress-induced birefringence), for example, under mechanical stress. Sources of such irregularities and stresses can arise from the crystal growth process, material processing, mechanical stress, mechanical contact with the mount, temperature gradients resulting from non-uniform heating during operation, and / or as a result of material degradation (possibly related to the development of slip planes).

[0069] Stress-induced birefringence in the beam splitter 130, one of the lens elements 114, 116, 118, the chamber window 134, or other optical elements of the DUV lithography apparatus 100 can perturb the polarization properties of the transmitted radiation 108. In particular, different refractions of the two polarization components of the radiation 108 can occur at the surfaces of the optical elements 114, 116, 118, 130, 134 in question, resulting in different polarizations and therefore splitting of the polarization components. In addition, a phase difference can occur between the polarization components of the transmitted radiation as it passes through each optical element 130, 114, 116, 118, 134. The result is a blurred image, which limits the achievable resolution of the DUV lithography apparatus 100.

[0070] 2 shows, by way of example, an optical element 200 of the DUV lithography apparatus 100. The optical element 200 is, for example, the beam splitter 130 of the optical pulse stretcher 128. However, in other examples, it can be another optical element (e.g., 114, 116, 118, 134) of the DUV lithography apparatus 100. The optical element 200 has a crystalline substrate 202. The crystalline substrate 202 includes, for example, a CaF2 crystal. In its ready-made state, the optical element 200 further has a coating or the like, which is not shown in the figure and will not be further described below, since the coating is related to the process for manufacturing the crystalline substrate 202, in particular the process for treating the crystalline substrate 202.

[0071] The crystalline substrate 202 has an end face 206, which has a surface 208 and faces the incident radiation 204. It should be noted that with a coating (not shown) applied to the end face 206, the surface 208 of the crystalline substrate 202 is not visible, contrary to representations in figures (e.g., FIG. 2).

[0072] The surface 208 of the crystalline substrate 202 can be formed, for example, by a

[0111] crystal plane 302 ( FIG. 5 ) of the crystal lattice 300 of the crystalline substrate 202. Alternatively, the surface 208 of the crystalline substrate 202 may also be formed, for example, by a

[0100] crystal plane 304, a

[0010] crystal plane 306, or a

[0001] crystal plane 308 of the crystal lattice 300 of the crystalline substrate 202. In other examples, the surface 208 of the crystalline substrate 202 may also be formed by any other plane of the crystal lattice 300 of the crystalline substrate 202.

[0073] 5-8, these crystal faces 302, 304, 306, and 308 of a cubic crystal 300, e.g., a CaF2 single crystal, are identified. The nomenclature of the crystal faces 302, 304, 306, and 308 corresponds to the nomenclature of crystal faces of a crystal lattice based on Miller indices a, b, and c, as is customary in crystallography. In FIGS. 5-8, crystal face 302 is indicated by [abc]=

[0111] , crystal face 304 is indicated by [abc]=

[0100] , crystal face 306 is indicated by [abc]=

[0010] , and crystal face 308 is indicated by [abc]=

[0001] .

[0074] The radiation 204 incident on the optical element 200 as shown in FIG. 2 is, for example, linearly polarized DUV radiation (similar to radiation 108 in FIG. 1 ) having a propagation direction 210. The vector of the electric field of the radiation 204 is labeled with the reference character E. The radiation 204 shown in FIG. 2 is specifically vertically polarized. The polarization plane 212 of the radiation 204 is formed by the electric field vector E and the propagation direction 210. In other examples, the radiation 204 may be, for example, horizontally polarized. As will be explained in more detail below, by appropriate rotation of the optical element 200 about the central axis 214, i.e., by appropriate adjustment of the rotation angle α (azimuthal angle α) of the optical element 200, it is possible to minimize disturbance of the polarization properties of the radiation 204 as it passes through the optical element 200.

[0075] Figure 3 shows the optical element 200 of Figure 2 in a top view. As can be seen in Figure 3, the propagation direction 210 of incident radiation 204 is tilted at an angle β with respect to the surface 208. The angle β is, for example, 45°. The radiation transmitted through the optical element 200 is identified by the reference numeral 204'. If the optical element 200 is a beam splitter, a reflected component of the radiation 204 will also be present, but is not shown in the figure for clarity.

[0076] 4 shows the optical element of FIG. 2 in a front view looking at the surface 208 of the crystalline substrate 202 (it is further noted here that, when one or more coatings are applied to the surface 208 of the crystalline substrate 202, the surface 208 is no longer visible, contrary to the representation in the figure). FIG. 4 again shows the rotation of the optical element 200 about the central axis 214 for the adjustment of the angle α. This adjustment of the angle α is also called "clocking." Furthermore, values ​​of the angle α of 0°, 90°, 180°, 270°, and 360° are shown in FIG. 4.

[0077] A method for manufacturing an optical element for a lithography apparatus will be described with reference to Figures 9-13. For example, the optical element 200 shown in Figures 2-4 is manufactured for the DUV lithography apparatus 100 shown in Figure 1. In particular, the method determines a mounting orientation of the optical element 200 in an optical system (e.g., the beam shaping and illumination system 102 or the projection system 104 of Figure 1) that is favorable with respect to stress-induced birefringence.

[0078] In the first step S1 of the process, the surface 208 of the crystalline substrate 202 of the optical element 200 is polished. The polishing in step S1 is performed in particular to create a height profile 216 (see enlarged detail in FIG. 3 ) of the surface 208, so that structures present in the crystalline substrate 202 that exhibit and / or cause stress-induced birefringence become visible on the surface 208 as superficial structures having the height profile 216.

[0079] For this purpose, the surface 208 is treated, for example, by a magnetorheological polishing method using the so-called round method (R-phi method), in which the surface 208 is treated in a spiral pattern 218 (FIG. 10). The surface 208 can also be described as being swept in a manner similar to a phonographic record, with a pickup moving from one end of the record to the other in a spiral pattern.

[0080] The magnetorheological polishing apparatus 220 (FIG. 9) includes, for example, a rotating wheel 222 as a tool head having a magnet 224 therein. A magnetorheological fluid 226 is continuously applied to the rotating wheel 222 through a nozzle (not shown). The magnetorheological fluid 226 contains, among other things, magnetic particles, a polishing medium, and water. The magnetic field generated by the magnet 224 changes the viscosity of the fluid 226. For example, iron particles in the fluid 226 become aligned and form a rigid structure that adheres to the wheel 222, and the water and abrasive particles become concentrated on the surface of the wheel 222 as a solidified thin polishing layer. The magnetorheological polishing apparatus 220 further includes a holder (not shown) for holding the crystal substrate 202 and rotating it (arrow 228) around the central axis 214. In addition, the magnetorheological polishing apparatus 220 further includes a device (not shown) for moving the wheel 222 in a radial direction 230 of the optical element 200.

[0081] The surface 208 is polished by rotating the optical element 200 about the central axis 214 in a direction of movement 228 ( FIG. 9 ) while simultaneously moving the polishing tool 222 radially toward a center 232 of the surface 208 defined by the central axis 214. This results in a spiral sweep of the surface 208 by the polishing head 222 (spiral pattern 218 in FIG. 10 ). In particular, the surface 208 is treated in an abrasive manner from an outer region 234 ( FIG. 10 ) of the surface 208 spirally toward the center 232.

[0082] Applicant has experimentally established that polishing by the described round method removes material from surface 208 in a manner that creates a surface structure with a height profile 216 ( FIG. 3 ) indicative of stress-induced birefringence. In other words, the described polishing refines the structures present in crystalline substrate 202 that are associated with stress-induced birefringence as a unique height profile 216 on surface 208. Consequently, this means that the height profile 216 created by polishing according to the present invention corresponds to the stress-induced birefringence when optical element 200 is used in an optical system. As a result, by evaluating height profile 216, it is possible to determine the preferred and / or optimal mounting orientation of optical element 200 (in particular, the optimal azimuthal angle α).

[0083] In a second step S2 of the method, the height profile 216 of the surface 208 of the crystalline substrate 202 produced in step S1 is detected.

[0084] FIG. 11 shows, by way of example, a detected surface-fit image 400 of the surface 208 after polishing in step S1. The surface-fit image 400 is detected, for example, by interferometry. In the surface-fit image 400 of FIG. 11, the height profile 216 ( FIG. 3 ) of the surface 208 after polishing in step S1 is shown in grayscale. Thus, the gray shades of the surface-fit image 400 in FIG. 11 correspond to different height values. Because the surface-fit image 400 is located, for example, by interferometry, the gray shades shown in FIG. 11 can also constitute interferometrically detected intensities, which, in turn, correspond to height values ​​of the height profile 216.

[0085] The height profile 216 detected by the surface-fit image 400, for example (e.g., for a CaF2 crystal substrate), has six local minima 402, 404, 406, 408, 410, and 412 (i.e., minimum values ​​of height H or intensity I) at average angular values ​​δ of δ1, δ2, δ3, δ4, δ5, and δ6 as a function of azimuthal angle δ. For clarity, only three major minima 404, 408, and 412 at average angular values ​​δ of δ2, δ4, and δ6 are identified in FIG. 11 . FIG. 12 shows all six minima 402, 404, 406, 408, 410, and 412. It is noted that for a crystal substrate 202 made of a material other than CaF2, such as BaF2 or LuAG, the height profile 216 detected by the surface-fit image 400 may also have different material-specific characteristics.

[0086] Note that the three depressions 414 visible in the surface-fit image 400 of Figure 11 were caused by a holder (not shown). However, to record the surface-fit image 400, the holder was removed, and therefore the depressions 414 are not important for evaluation.

[0087] In a third step S3 of the process, the determined height profile 216 is used to determine a preferred mounting orientation of the optical element 200 in the optical system 102, 104, i.e., using, for example, the surface-fit image 400 shown in Figure 11. In particular, a mounting orientation of the optical element 200 is determined that minimizes stress-induced birefringence for incident polarized radiation 204 (Figure 2). The mounting orientation is an orientation related to the rotation (azimuthal rotation) of the optical element 200 about the central axis 214.

[0088] In particular, the rotation angle α (FIG. 2) of the optical element 200 with respect to rotation about the central axis 214 is determined at which the stress-induced birefringence for incident polarized radiation 204 is minimal. Here, for example, it is further possible to determine multiple values ​​of the rotation angle α at which the stress-induced birefringence for incident polarized radiation 204 is minimal (e.g., infinitesimal). In addition, the rotation angle α can also be determined relative to the polarization plane 212 (FIG. 2) of the incident polarized radiation 204.

[0089] As shown in FIG. 12 , the mounting orientation of the optical element 200 is determined by determining the angular distribution 416 of height values ​​H or corresponding intensity values ​​of the detected height profile 216 of the surface 208, i.e., for example, the surface-fitted image 400. For this purpose, the height values ​​H or intensity values ​​I are integrated or averaged over a predetermined angular range Δδ of the detected surface-fitted image 400 (e.g., the circle segment 418 in FIG. 11 ). Note that the angular range Δδ and the corresponding circle segment 418 in FIG. 11 are shown at an excessively large size for illustrative purposes. In addition, the integrated and / or averaged values ​​(e.g., the height values ​​H or intensity values ​​I) are plotted as the angular distribution 416 over the entire 360° circular angular range. Accordingly, FIG. 12 shows six minima at δ1, δ2, δ3, δ4, δ5, and δ6 in the angular distribution 416.

[0090] 12 shows a graph 420 adapted from the literature showing the intrinsic birefringence of a

[0111] oriented CaF2 crystal. As applicants have thus been able to demonstrate, a high degree of agreement is evident between the graph 420 describing the birefringence and the angular distribution 416 determined using the detected height profile 216, i.e., from the surface-fitted image 400, particularly with regard to the locations of the six minima 402, 404, 406, 408, 410, and 412. Applicants can therefore present an alternative method for determining a favorable mounting orientation of an optical element with respect to stress-induced birefringence. In particular, the preferred mounting orientation with respect to stress-induced birefringence, i.e., the preferred rotation angle α with respect to stress-induced birefringence, can be directly determined from the surface-fit image 400 of the height profile 216 as one or more of the angles δ1-δ6 of the minima 402-412 (particularly, δ2, δ4, δ6 of the primary minima 404, 408, 412) without any additional test setup or test method. In particular, it is possible to modify the process steps of polishing the surface 208 (step S1) and detecting the height profile 216 (surface-fit image 400), which are required in any case, so that information regarding the optimal mounting angle α can also be obtained without significant extra difficulty. The angles δ2, δ4, δ6 of the primary minima 404, 408, 412 in FIG. 12 can be considered as the preferred mounting angle α (FIG. 2) for the "clocking" of the optical element 200.

[0091] In a fourth step S4 of the method, the ascertained preferred installation orientation, e.g., one of the angles δ2, δ4, δ6, is marked on the optical element 200. This allows the ascertained preferred mounting orientation to be read on the optical element 200 itself, e.g., by a customer, and / or to be confirmed by a measuring device, e.g., a commercially available interferometer.

[0092] In step S4, in particular, markings 424 (FIGS. 2 and 3) are made on optical element 200 for one or more of the preferred rotation angles δ2, δ4, δ6 determined in step S3. In the illustrated example, markings 424 are painted (e.g., with a marker pen) on outer surface 422 of optical element 200. In other examples, markings 424 may also be engraved on optical element 200 and / or fabricated in an edge region of surface 208.

[0093] In an optional fifth step S5 of the method, surface 208 is polished again. This further polishing step serves to remove height profile 216 of surface 208 created in step S1. It does not remove applied markings 424 (FIG. 2) indicating the determined mounting orientation. Use of step S5 makes it possible to achieve less roughness of surface 208. At the same time, markings 424 are preserved, which indicate to the user the azimuthal angle α at which optical element 200 should be mounted in an optical system (e.g., 102, 104 in FIG. 1) to minimize stress-induced birefringence.

[0094] Although the present invention has been described with reference to exemplary embodiments, it can be varied in many ways. [Explanation of symbols]

[0095] 100 DUV lithography equipment 102 Beam shaping and lighting system 104 Projection System 106 Light source 108 Radiation 110 Photomask 112 wafers 114 Lens Elements 116 Lens Elements 118 Lens Elements 120 Mirror 122 Mirror 124 Optical axis 126 Medium 128 Optical Pulse Extender 130 Beam Splitter 132 Holder 134 Chamber window 200 Optical Elements 202 Crystal Substrate 204, 204' radiation 206 End face 208 Surface 210 Propagation direction 212 Polarization plane 214 Center axis 216 Height Profile 218 Spiral Pattern 220 Equipment 222 Wheels 224 Magnet 226 Fluid 228 Rotation direction 230 directions 232 center 234 areas 300 crystals 302 Crystal Faces 304 Crystal Faces 306 Crystal Faces 308 Crystal Faces 310 Crystal Axis 312 Crystal Axis 314 Crystal Axis 316 Crystal Axis 400 Surface conforming image 402 minimum 404 Minimum 406 minimum 408 minimum 410 minimum 412 minimum 414 Notch 416 Angular distribution 418 yen segments 420 graphs 422 External surface 424 Marking α angle β angle δ angle δ1 angle δ2 angle δ3 angle δ4 angle δ5 angle δ6 angle a Miller index b Miller index c Miller index E electric field vector H Height value I Intensity value S1~S5 Method steps X direction Y direction Z direction

Claims

1. A method of manufacturing an optical element (200) for a lithographic apparatus (100), comprising the steps of: a) detecting a height profile (216) of a surface (208) of a crystalline substrate (202) of the optical element (200); b) using the detected height profile (216) to determine a mounting orientation (δ2, δ4, δ6) of the optical element (200) of the optical system (102, 104) of the lithography apparatus (100) in relation to stress-induced birefringence for incident polarized radiation (204), wherein the mounting orientations (δ2, δ4, δ6) have orientations related to a rotation of the optical element (200) about a central axis (214) of the optical element (200) passing through the surface (208), method.

2. The method comprises the following steps preceding step a): polishing (S1) the surface (208) of the crystalline substrate (202) to create the height profile (216) of the surface (208); The method of claim 1 , comprising:

3. The method of claim 2, wherein the polishing step (S1) comprises magnetorheological polishing of the surface (208).

4. 4. The method of claim 2 or 3, wherein the step of polishing the surface (208) is performed by sweeping across the surface (208) in a spiral manner, the spiral sweep proceeding from an outer region (234) of the surface (208) in a spiral manner about a center (232) of the surface (208) defined by the central axis (214) toward the center (232).

5. 4. The method of claim 2 or 3, wherein the surface (208) is polished (S1) by rotating the optical element (200) about the central axis (214) and simultaneously moving a polishing tool (222) radially toward a center (232) of the surface (208) defined by the central axis (214).

6. The method further comprises the following steps after step b): Step (S4) of marking the determined mounting orientations (δ2, δ4, δ6) on the optical element (200). The method according to any one of claims 1 to 3, comprising:

7. 7. The method of claim 6, wherein after the step (S4) of marking the optical element (200) with the determined mounting orientation (δ2, δ4, δ6), the method comprises the step (S5) of polishing the surface (208) to remove the height profile (216) of the surface (208) such that markings (424) identifying the determined mounting orientation (δ2, δ4, δ6) on the optical element (200) are preserved.

8. The method of any of claims 1 to 3, wherein the crystalline substrate (202) comprises a crystal (300) with cubic symmetry, a single crystal, a fluoride crystal, calcium fluoride, magnesium fluoride, barium fluoride, and / or lutetium aluminum garnet.

9. The method according to any one of claims 1 to 3, wherein the surface (208) of the crystalline substrate (202) is formed by a [111] crystallographic plane (302) of the crystalline substrate (202).

10. 4. The method according to claim 1, wherein the surface (208) of the crystalline substrate (202) is formed by a [100] crystal face (304), a [010] crystal face (306), or a [001] crystal face (308) of the crystalline substrate (202).

11. 4. The method of claim 1, wherein the optical element (200) comprises a transmissive optical element (130, 114, 116, 118) of the lithographic apparatus (100), a partially transmissive optical element (130), a beam splitter (130), a beam splitter (130) of an optical pulse extender (128), a lens element (114, 116, 118), and / or a chamber window (134).

12. 4. The method according to claim 1, wherein the step (S3) of determining the mounting orientation (δ2, δ4, δ6) of the optical element (200) determines a rotation angle (α) of the optical element (200) with respect to a rotation of the optical element (200) about the central axis (214) at which stress-induced birefringence for incidence of the polarized radiation (204) is less and / or minimal compared to other rotation angles with respect to a rotation of the optical element about the central axis.

13. The method according to any one of claims 1 to 3, wherein the step (S3) of determining the mounting orientation (δ2, δ4, δ6) of the optical element (200) determines a rotation angle (α) of the optical element (200) with respect to a plane of polarization (212) of the incident polarized radiation (204).

14. 4. The method according to claim 1, wherein the step (S3) of determining the mounting orientations (δ2, δ4, δ6) of the optical element (200) determines an angular distribution (416) of height values ​​(H) of the determined height profile (216) of the surface (208), the angles (δ) of the angular distribution (416) corresponding to respective rotation angles (α) of the optical element (200) with respect to a rotation of the optical element (200) about the central axis (214).

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