Lithography system having separable lenses

Abstract

The invention relates to a projection system, comprising at least one optical unit, with the optical unit comprising at least one optical element and the optical element having at least one plane surface. The problem of providing a projection system that is flexible and cost-effective is solved by virtue of the fact that formula (I) applies to the optical unit, where Dmax is the maximum optically clear diameter of the optical unit and dmin is the minimum optically clear thickness of the optical unit. Further, the invention relates to a projection exposure apparatus for microlithography, comprising a projection system according to the invention, an optical design for a projection system, in particular a projection system according to the invention, and a method for producing a projection system, in particular a projection system according to the invention.

Description

[0001] Lithography system with detachable lenses

[0002] Technical area

[0003] The invention relates to a projection system comprising at least one optical unit, wherein the optical unit comprises at least one optical element, wherein the optical element has at least one planar surface. Furthermore, the invention relates to a projection exposure system comprising a projection system according to the invention, an optical design for producing a projection system, in particular a projection system according to the invention, and a method for producing a projection system, in particular a projection system according to the invention.

[0004] The subject matter of German patent application 102023204235.5 is hereby incorporated by reference.

[0005] background

[0006] Projection systems for microlithography are subject to high accuracy requirements. They are also exposed to high levels of stress. During operation, the optical elements and their mounts heat up due to the high radiation power. Lens heating can change the mechanical and optical properties of the optical elements of the projection system. Thermal expansion can alter the shape of the optical elements, for example, the yoke and thickness of a lens. The refractive index of the optical material can also change with temperature. This, in turn, leads to aberrations that can significantly impair the imaging performance of the projection system.

[0007] The degradation of imaging performance can be described using wave aberration, in particular by an expansion of the wave aberration according to Zernike polynomials. Lens heating is particularly noticeable through rotationally symmetric and astigmatic distortions of the wavefront. While rotationally symmetric distortions can be corrected by rigid-body movements of the optical elements, astigmatic distortions lead to aberrations that can no longer be corrected by rigid-body movements. The leading effect of lens heating is lowest-order field-constant astigmatism, which is described in particular by the Zernike polynomial Z5. Higher-order field-constant astigmatism, Z12, and possibly anamorphism, which can be described by a field-linear Z2 and Z3, can also develop.

[0008] For the correction of astigmatic aberrations, for example, a projection lens for microlithography is known from document WO 2006 / 125617 A2, which comprises lenses that can be deformed to correct image defects. However, the optical and mechanical implementation of a deformable lens is associated with significant costs and also reduces the transmission of the projection system by two additional optical surfaces. If two or three independent aberrations are present due to lens heating, two or three deformable lens elements are required for the corresponding correction. This leads to high costs and transmission losses due to the additional optical surfaces. In addition, at the time of manufacture of the optical design underlying the projection system, it is often not clear how exactly the projection system will be operated, i.e. which image defects will actually arise due to lens heating.This applies especially to higher-order aberrations, especially higher-order astigmatism, Z12, and anamorphism. The use of deformable lenses is therefore only advisable when a clear need is foreseeable.

[0009] The object of the present invention is to provide a projection system that overcomes the disadvantages of the prior art. In particular, a projection system is to be provided that is flexible and cost-effective. A further object of the invention is to provide a projection exposure apparatus with a projection system, an optical design for producing a projection system, and a method for producing a projection system.

[0010] Description of the invention

[0011] According to a first teaching of the present invention, the object is achieved by a projection system comprising at least one optical unit, wherein the optical unit comprises at least one optical element, wherein the optical element has at least one planar surface, in that the following applies to the optical unit: where Dmax is the maximum optically free diameter of the optical

[0012] Unit and dmm is the minimum optical free thickness of the optical unit.

[0013] The projection system is, for example, a projection system for microlithography. In particular, it is a projection system for DUV (deep ultraviolet light) lithography. The projection system can comprise a projection lens, in particular a projection lens for microlithography. The image quality in microlithography is determined on the one hand by the projection lens and on the other hand by the illumination system. The illumination system should provide the most uniform possible illumination of the field plane in which the structure-bearing mask, the so-called reticle, is arranged. The projection lens images the field plane in an image plane, the so-called wafer plane, in which a light-sensitive object, typically a wafer with a suitable photoresist, is arranged.

[0014] The projection system comprises at least one optical unit. The optical unit comprises at least one optical element, wherein the optical element has at least one planar surface. A planar surface can be understood as a flat, level surface. In particular, a planar surface is characterized by the fact that it has no curvature. The planar surface is arranged in particular perpendicular to the optical axis. For example, the optical element is a plano lens or a plano plate. A plano lens is to be understood as a lens with a planar surface and a curved, for example spherical or aspherical, surface. In particular, the optical element is a plano-convex or a plano-concave lens. A plano plate comprises two opposite, parallel planar surfaces. The optical element is preferably transparent.In particular, the optical element may comprise a substrate, wherein the substrate may comprise, for example, quartz glass and / or any other type of glass, for example FK5, LLF1 or LF5.

[0015] For at least one optical unit of the projection system g

[0016] 0.075, where Dmax is the maximum optical free diameter of the optical unit and dmin is the minimum optical free thickness of the optical unit. The maximum optical free diameter Dmax of the optical unit can be understood as the maximum diameter of the region of the optical unit through which radiation passes. The maximum optical free diameter Dmax is limited in particular by the outer marginal rays. The minimum optical free thickness dmin can be understood as the minimum thickness of the optical unit parallel to the direction of the optical axis in the optically free region. For plano-concave lenses, for example, the minimum optical free thickness dmin is usually the thickness of the lens at the height of the optical axis. For plano-convex lenses, it can in particular be the thickness of the lens at the outermost edge of the optically free region. This means in particular the minimum of the axial thickness and the marginal thickness.C defines the ratio between the maximum optical free diameter Dmax and the minimum optical free thickness dmin of the optical unit. C

[0017] Because the projection system comprises at least one optical unit, which comprises at least one optical element with at least one planar surface and for which the following applies, a flexible projection system is advantageously provided provided. In particular, the optical unit of the projection system can be designed as one optical element, or the optical unit can also comprise at least two optical elements. For example, it is conceivable that the optical unit of the projection system consists of one optical element, the following applies to the optical element: C where Dmaxder maximum optical free diameter and dmin is the minimum optical free thickness. It is also conceivable that the optical unit consists of at least two optical elements, where the optical elements together meet the condition that the ratio , where Dmax is the maximum optical free

[0018] Diameter and dmin is the minimum optical thickness. The minimum optical thickness dmin of the optical unit consisting of at least two optical elements is the total thickness dmin, which is the minimum optical thickness dmtni, .... dminn of the at least two optical elements according to dmin =dminl + ... + dminn. In the projection system, the optical unit can thus be configured as one optical element or at least two optical elements. Preferably, no plane plate is arranged near the optical unit, in particular, no plane plate is arranged adjacent to the optical unit.

[0019] What is particularly advantageous here is that only a single optical design is required for the production of the alternative projection systems. If the optical design comprises an optical unit consisting of an optical element, the optical element can

[0020] Projection system must correspond to the optical design and as an optical unit also comprise only one optical element, for which However, an appropriately designed optical element in the underlying optical design that satisfies the condition C > 0.075 also allows at least one further optical element to be split off from this optical element. Separation means that instead of one optical element, at least one further optical element is provided during manufacture of the projection system. Based on the optical design, a projection system can also be manufactured in which the optical unit consists of at least two optical elements instead of one optical element. In order to split off another optical element, such as a plane plate, from an optical element, such as a lens, the element must have a certain minimum thickness, since otherwise the resulting optical elements will no longer have sufficient thickness.The parameter C must therefore have a minimum value of 0.075 to ensure mechanical stability in the separation of an optical element. For example, for the optical unit of the projection system, C > 0.080, preferably C > 0.090, applies. If these values ​​are exceeded, for example, sharp-edged converging lenses can result.

[0021] According to a first advantageous embodiment of the projection system, the optical unit consists of the optical element. The optical element comprises at least one planar surface. The optical element is, for example, a planar lens or a planar plate. The following applies to the optical element: C = >

[0022] Dmax 0.075, where Dmax is the maximum optical free diameter of the optical element and dmin is the minimum optical free thickness of the optical element. This makes it possible to provide a particularly cost-effective projection system. The design of the projection system, in particular of the optical unit, can correspond in particular to the design provided for in the underlying optical design.

[0023] It is conceivable that the projection system, in addition to the optical unit consisting of an optical element, also comprises at least one deformable optical element, in particular a deformable lens. The deformable optical element serves in particular to correct field-constant astigmatism of the lowest order, Z5. Since this is the leading effect in lens heating, it can be useful if at least one deformable optical element is provided even in this simplest embodiment of the projection system with an optical element as the optical unit and also in the underlying optical design. For example, the deformable optical element can be a deformable, low-refractive-power lens. The deformable optical element is preferably located in the divergent beam path near the system pupil.

[0024] According to a further advantageous embodiment of the projection system, the optical unit consists of at least one plane plate and an optical element adjacent to the plane plate with at least one plane surface, wherein the at least one plane surface of the optical element adjacent to the plane plate faces the plane plate. The optical unit consists, for example, of a plane plate and an adjacent plane lens with one of the

[0025] Flat lens surface facing the plane plate. It is also conceivable for the optical unit to consist of two plane plates that are arranged adjacent to one another. The fact that the plane plate and the optical element with the at least one plane surface are arranged adjacent to one another can be understood to mean that no further optical element is arranged between the plane plate and the optical element with the at least one plane surface. However, the plane plate and the optical element with the at least one plane surface do not have to be arranged directly next to one another. It is particularly conceivable for the plane plate to be arranged offset on the optical axis.

[0026] The at least one plane plate and the optical element of the optical unit adjacent to the at least one plane plate together satisfy the condition that the ratio C = > 0.075, where Dmax is the maximum optical

[0027] Dmax is the free diameter and dmm is the minimum optical free thickness. Instead of a single optical element with at least one planar surface, the optical unit of the projection system can also consist of two or more optical elements. The projection system can therefore be designed or modified as required. Preferably, each of the optical elements of the optical unit comprises at least one planar surface.

[0028] According to a further advantageous embodiment of the projection system, the at least one plane plate is deformable. A plane plate comprises two opposing plane surfaces. The plane plate of the optical unit is preferably deformable. In particular, the plane plate can be controlled by a mechanism that allows deformation, in particular an astigmatic deformation, of the plane plate. In this way, lens heating effects can advantageously be corrected. For example, to adjust a lowest-order astigmatism, Z5, with only one deformable plane plate, the deformation of the deformable plane plate leads to a fitting deformation that can be described predominantly by a Z5 with small amounts of higher-order astigmatism, Z12.In order to be able to vary lowest order astigmatism, Z5, and higher order astigmatism, Z12, independently of each other, two deformable optical elements are required, in particular two deformable plane plates or, for example, a deformable plane plate and a deformable lens, whose contributions to Z5 or to Z12 can compensate each other.

[0029] According to a further advantageous embodiment of the projection system, the following applies to the plane plate: A = < 0.5, where H is the height of the main ray at the location of the plane plate, h is the height of the marginal ray at the location of the plane plate, u is the angle of the marginal ray at the location of the plane plate, and NA is the image-side numerical aperture. In order to be able to adjust for wave aberrations with field-constant astigmatism, for example low-order field-constant astigmatism and / or higher-order field-constant astigmatism, in particular Z5 and / or Z12, the plane plate is preferably arranged in the divergent or convergent beam path of the projection system. The plane plate has a finite thickness, so that the size of the sub-aperture differs on both sides of the plane plate. The plane plate is preferably arranged in or near the system pupil. In this way, the sub-apertures of the beams from different field points overlap as well as possible, thus avoiding field dependencies.A particularly favorably positioned plane plate is close to the pupil (H / h small) with a large marginal ray angle (u / NA large), thus having a small value of A.

[0030] According to a further advantageous embodiment of the projection system, the following applies to the where H is the height of the main ray at the location of the Flat plate, OBH is the object circle radius, u is the angle of the marginal ray at the location of the flat plate, NA is the image-side numerical aperture, and v is the angle of the chief ray of the flat plate. In order to correct anamorphism with an optical element, the optical element is preferably arranged at a point in the beam path where the marginal ray angle is small, e.g., compared to the image-side numerical aperture. The chief ray angle is preferably large, so that the subaperture on both sides of the flat plate has as different heights as possible. This also means that the marginal ray height is small. The chief ray height is preferably large compared to the radius of the object circle.Anamorphism can be corrected primarily at points in the optical beam path where the marginal ray is parallel to the axis, i.e. the bundles are collimated, and where the principal ray has a significant height to the optical axis and encloses a significant angle with the optical axis.

[0031] According to a second teaching, the above-mentioned object for a projection exposure system for microlithography is achieved in that the projection exposure system comprises a projection system according to the first teaching. A projection exposure system can be provided for generating an image of an object arranged in an object plane in an image plane using a light source emitting projection light. Projection exposure systems for microlithography serve to manufacture microstructured or nanostructured components of microelectronics or microsystems technology. With the aid of projection exposure systems, structures formed on a photomask are imaged in a reduced manner onto wafers or the like in order to generate the corresponding structures on the wafer using microlithographic processes.In particular, a projection exposure apparatus may be a DUV projection exposure apparatus having an exposure wavelength of approximately 365 nm or approximately 248 nm or approximately 193 nm or approximately 157 nm.

[0032] Projection exposure systems operate around the clock and any malfunction affects the performance of the projection exposure system.

[0033] According to a third teaching, the above-mentioned object for an optical design for a projection system, in particular a projection system according to the first teaching, wherein the optical design comprises at least one optical unit, wherein the optical unit consists of an optical element, wherein the optical element has at least one planar surface, is achieved in that for the optical element, where D max is the maximum optically free diameter of the optical element and dmm is the minimum optical free thickness of the optical element. The optical design can be used for the manufacture of a projection system. For example, the optical design is used for the manufacture of a projection system, which also has an optical unit consisting of an optical element with at least one planar surface, for which the following applies: The optical design is particularly suitable for a projection system in which at least one optical element, in particular at least one plane plate, is subsequently separated from the optical element of the optical design. The advantage here is that only one optical design is required for the production of the alternative projection systems. If the optical design comprises an optical unit consisting of an optical

[0034] element, whereby for the optical element, the

[0035] Projection system must therefore correspond to the optical design and as an optical unit also comprise only one optical element, for which

[0036] Based on the same optical design, however, a projection system can also be produced in which the optical unit comprises at least two optical elements instead of one. It is conceivable, for example, that at the time of the final optical design it is not yet completely clear which aberrations need to be corrected. This depends, for example, on the actual user behavior of the user of a projection system. The design can thus provide at least one optical element which can be optionally retrofitted without affecting the optical aberrations of the design. This is preferably a flat plate, in particular a deformable flat plate which can be subsequently separated from the optical element of the optical unit of the design. In particular, for correcting lens heating effects, it can be advantageous to provide at least one deformable optical element.The split-off plane plate can be moved away from the remaining plane lens along the optical axis without introducing aberrations, in particular to provide sufficient space for the mounting technology of the two elements.

[0037] For example, the optical unit of the optical design is: where H is the height of the principal ray at the location of the optical unit, h is the height of the marginal ray at the location of the optical unit, u is the angle of the marginal ray at the location of the optical unit and NA is the image-side numerical aperture. It is also conceivable, where H is the height of the Chief ray at the location of the optical unit, OBH is the object circle radius, u is the angle of the marginal ray at the location of the plane plate of the optical unit, NA is the image-side numerical aperture and v is the angle of the chief ray at the location of the optical unit.

[0038] According to a fourth teaching, the above-mentioned object is achieved by a method for producing a projection system, in particular a projection system according to the first teaching, the method comprising the steps of: providing an optical design according to the third teaching, and producing a projection system based on the optical design, the optical unit of the projection system being selected as: an optical element having at least one planar surface, or at least one planar plate and an optical element adjacent to the planar plate having at least one planar surface, the at least one planar surface of the optical element adjacent to the planar plate facing the planar plate.

[0039] Using the method, the optical unit of the projection system can be selected, in particular replaced, depending on the imaging errors to be corrected. It is also conceivable that the design of the optical unit is selected depending on cost and / or transmission requirements. For example, users who wish to forgo the option of correction for cost reasons can opt for a projection system in which the optical unit consists only of a plano lens in accordance with the underlying optical design. A particularly advantageous feature is that, depending on requirements or needs, at least one further optical element can be subsequently introduced into the beam path of the projection system without affecting the imaging performance.

[0040] The present disclosure also includes the subject matter of the following clauses: 1. Projection system comprising at least one optical unit, wherein the optical unit comprises at least one optical element, wherein the optical element has at least one planar surface, characterized in that the following applies to the optical unit: C = > 0.075, where Dmax is the maximum optical

[0041] Dmax is the free diameter of the optical unit and dmm is the minimum optical free thickness of the optical unit.

[0042] 2. Projection system according to clause 1, characterized in that the optical unit consists of the optical element.

[0043] 3. Projection system according to clause 1, characterized in that the optical unit consists of at least one plane plate and an optical element adjacent to the plane plate with at least one plane surface, wherein the at least one plane surface of the optical element adjacent to the plane plate faces the plane plate.

[0044] 4. Projection system according to clause 3, characterized in that the at least one plane plate is deformable.

[0045] 5. Projection system according to clause 3 or 4, characterized in that for the plane plate: A = < 0.5, where H is the height of the chief ray at

[0046] Location of the plane plate, h is the height of the marginal ray at the location of the plane plate, u is the angle of the marginal ray at the location of the plane plate, and NA is the image-side numerical aperture. Projection system according to clause 3 or 4, characterized in that H is the height of the chief ray at the location of the plane plate, OBH is the object circle radius, u is the angle of the marginal ray at the location of the plane plate, NA is the image-side numerical aperture, and v is the angle of the chief ray at the location of the plane plate. Projection exposure system for microlithography, characterized in that the projection exposure system comprises a projection system according to one of the preceding clauses. Optical design for a projection system, in particular for a projection system according to one of clauses 1 to 6, wherein the optical design comprises at least one optical unit, wherein the optical unit consists of an optical element, wherein the optical element has at least one planar surface, characterized in that the following applies to the optical element: where Dmax is the maximum optically free diameter of the optical element and dmm is the minimum optically free thickness of the optical element. A method for producing a projection system, in particular a projection system according to one of clauses 1 to 6, the method comprising the steps:

[0047] Providing an optical design according to clause 8, and producing a projection system based on the optical design, wherein the optical unit of the projection system is selected as: o an optical element having at least one planar surface, or o at least one planar plate and an optical element adjacent to the planar plate having at least one planar surface, wherein the at least one planar surface of the optical element adjacent to the planar plate faces the planar plate.

[0048] The exemplary embodiments of the present invention described above in this specification are also to be understood as disclosed in all combinations with one another. The individual features of each aspect can be combined with any or all features of the other aspects. Further embodiments and advantages of the invention are explained in the following detailed description of some exemplary embodiments of the present invention in conjunction with the drawings.

[0049] Short description of the drawing

[0050] In the following, embodiments and variants of the invention are explained in more detail with reference to the drawings. The teachings of the disclosure can best be understood from the following detailed description in conjunction with the accompanying figures. The figures are schematic and simplified, showing only details to improve the understanding of the claims, while other details are omitted. The same reference numerals are used throughout for identical or corresponding parts. The individual features of each teaching can be combined with any or all features of the other teachings. These and other aspects, features and / or technical effects are apparent from and are clarified by the figures described below. It shows

[0051] Fig. 1 is a schematic representation of an embodiment of an optical design according to the third teaching, Fig. 2 is a schematic representation of an embodiment of a projection system according to the first teaching, which was manufactured based on the optical design according to Fig. 1,

[0052] Fig. 3 is a schematic representation of residual aberrations of an embodiment of a projection system according to Fig. 2,

[0053] Fig. 4 is a schematic representation of another embodiment of a projection system according to the first teaching, which was manufactured based on the optical design according to Fig. 1,

[0054] Fig. 5 is a schematic representation of another embodiment of a projection system according to the first teaching, which was manufactured based on the optical design according to Fig. 1,

[0055] Fig. 6 is a schematic representation of residual aberrations of an embodiment of a projection system according to Fig. 5,

[0056] Fig. 7 is a schematic representation of another embodiment of a projection system according to the first teaching, which was manufactured based on the optical design according to Fig. 1, and

[0057] Fig. 8 is a schematic representation of another embodiment of a projection system according to the first teaching.

[0058] Detailed description of the drawing

[0059] Fig. 1 shows an optical design 10 for a projection system. The optical design 10 comprises a plurality of optical elements that can form a projection system. An object plane and an image plane are provided in the optical design 10. For example, the optical design 10 comprises a reticle 11 in the object plane and a wafer 12 in the image plane. The optical design 10 can be an optical design for a projection lens. In particular, the optical design 10 is an optical design for a projection system for microlithography. The optical design 10 can serve, for example, as an optical design for the projection systems 1, 1', 1", 1'" shown in Figs. 2, 4, 5, and 6. The optical design 10 preferably has a specification according to Table 1:

[0060] Table 1 The "SURF" column indicates the number of a refractive or otherwise distinctive surface. The "RADIUS" column indicates the radius r of the respective surface (in mm), and the "THICKNESS" column indicates the distance d, referred to as the thickness, of the respective surface to the next surface (in mm). The "MATERIAL" column indicates the material of the optical elements, and the "INDEX" column indicates the refractive index of the material at the operating wavelength of optical design 10 of 248.413 nm. The "SEM1D1AM" column indicates the usable free radii or half the free optical diameters of the optical elements (in mm). The radius r = 0 (in the "RADIUS" column) corresponds to a plane. Some optical surfaces are aspherical.

[0061] Table 2

[0062] The corresponding asphere data can be found in Table 2, where the aspherical surfaces are calculated according to formula (1): x

[0063] The inverse of the radius indicates the surface curvature, and h indicates the distance of a surface point from the optical axis 14, i.e., the ray height. Thus, p(h) indicates the sagittal height, i.e., the distance of the surface point from the surface vertex in the z-direction (direction of the optical axis). The constants K, Cl, C2, etc. are given in Table 2.

[0064] If the conic constant K= 0, then formula (1) can be simplified to formula (2):

[0065] The projection systems 1, 1', 1", 1"' of the exemplary embodiments are designed for an image-side numerical aperture NA = 0.8. The object height is 54.41 mm in each case.

[0066] Figure 2 shows a projection system 1 for microlithography, which was manufactured based on the optical design 10. In projection systems for microlithography, the optical elements, such as lenses, can heat up due to the high radiation power. The primary effect of lens heating is the field-constant astigmatic deformation of the lowest-order wavefront, Z5. If lens heating occurs, this deviation in the wavefront should be corrected.

[0067] For this purpose, the projection system 1 or the optical design 10 comprises a deformable optical element 3"' (hatched). The deformable optical element 3"' is a low-refractive-power meniscus-shaped lens. The deformable optical element 3"' is located in the divergent beam path near the pupil 13. With the help of the deformable optical element 3"', the field-constant astigmatic deformation of the lowest order, Z5, can be corrected.

[0068] For the deformable optical element, the following applies: where H is the

[0069] Height of the main ray 15 at the location of the deformable optical element 3"', h is the height of the marginal ray 16 at the location of the deformable optical element 3"', u is the angle of the marginal ray 16 at the location of the deformable optical element 3"' and NA is the image-side numerical aperture. For the deformable optical element 3"' of the projection system 1 or the optical design 10, the ratio H / h = 0.104 and u / NA = 0.782, so that A = 0.134 and thus the condition A < 0.5 is met.

[0070] A numerical calculation shows that upon astigmatic deformation of the deformable optical element 3"', a field-constant astigmatism Z5 with approximately 10% remaining field-constant astigmatism Z12 is primarily created as residual aberration. Fig. 3 shows the residual aberrations when a field-constant astigmatism of 15 nm Z5 is set with the deformable optical element 3"', specifically in the static, rectangular image field. In the left column, Z4 (focus), Z9, Z16 (spherical aberration), in the middle column Z2 / 3 (distortion), Z7 / 8 and Z14 / 15 (coma), in the right column Z5 / 6, Z12 / 13 (astigmatism) and Z10 / 11 (triangularity).

[0071] Rotationally symmetric aberrations can be corrected, for example, using a classic manipulator concept with optical components that can be moved along the optical axis, so-called Z-manipulators. In particular, virtually no anamorphism or higher-order field curvature occurs. It can be seen that approximately 1.8 nm of field-constant astigmatism Z12 remains as the limiting residual aberration.

[0072] While in the optical design 10 according to Fig. 1 or in the projection system 1 according to Fig. 2 no thin lens is provided in the vicinity of the deformable optical element 3''', which would be suitable as a second deformable optical element, the optical units 2, 2' with the optical elements 3, 3' are designed such that a plane plate can be split off from the respective optical element 3, 3' and shifted slightly further into the air space in front of the remaining part of the respective optical element 3, 3'. The respective splitting and axial shift do not affect the aberrations of the optical design.

[0073] In order to split a plane plate from an optical element, the element must have at least one plane surface and a certain minimum thickness, otherwise the resulting optical elements will no longer have sufficient thickness. For each of the optical units 2 comprised by the optical design 10 or the projection system 1, 2' therefore applies: C = > 0.075, where Dmax is the maximum

[0074] Dmax is the optically free diameter of the optical unit 2, 2' and dmin is the minimum optically free thickness of the optical unit 2, 2'. The minimum value for the parameter C is > 0.075 in order to be able to carry out a splitting off of a plane plate in a mechanically stable manner. The optical unit 2, for example, has a maximum optically free diameter Dmax of 235.72 and a minimum optically free thickness dmin of 22.68. For the optical unit 2, the parameter C therefore has the value C = 0.0962. The further optical unit 2', for example, has a maximum optically free diameter Dmax of 215.56 and a minimum optically free thickness dmin of 23.75, so that the parameter C has the value C = 0.1102, for example.

[0075] For example, if it turns out during operation that a projection system, due to lens eating, suffers from field-constant astigmatism of the lowest order, Z5, as well as field-constant astigmatism of the higher order, Z12, this can also be corrected. To do this, another deformable optical element can be activated at a similar location in the beam path, i.e., near the pupil and non-collimated. Upon deformation, this deformable optical element would again lead to a field-constant Z5 with some additional Z12. However, by activating both deformable lenses, linear combinations of the deformations can be found that enable a nearly pure Z5 or Z12 correction in the wavefront.

[0076] Fig. 4 shows a projection system 1' for microlithography, which was also manufactured based on the optical design 10. The projection system 1' shown in Fig. 4 comprises two optical units 2, 2', where the following applies to each optical unit 2, 2': C = > 0.075, where D max is the maximum optically free

[0077] Dmax

[0078] Diameter and dmin is the minimum optical free thickness. The optical unit 2' consists of two optical elements 3a, 3b. A further optical element has been split off from the optical element 3 provided by the optical unit 2' according to the optical design 10 in Fig. 1. The optical element 3a is the remaining optical element from which the optical element 3b was split off. The flat surface 6' of the optical element 3a, from which the optical element 3b was split off, is marked by a thick line. For the optical unit 2', for example, the parameter C has the value C = 0.1102. The minimum optical free thickness dmin of the optical unit 2' is composed of the minimum optical free thickness dmini of the optical element 3a and dmtn2 of the optical element 3b according to dmin = dmini + dmin2.The maximum optical free diameter Dmax is the maximum diameter of the area of ​​the optical unit through which radiation passes (limited at the top by the upper marginal ray 16).

[0079] In the projection system 1', the optical unit 2' consists of two optical elements 3a, 3b instead of a single optical element 3. The optical element 3a is a plano lens 5', in this case a plano-convex lens. The other optical element 3b is a plano plate 4'. The plano lens 5' is arranged adjacent to the plano plate 4', with the plano surface 6' of the plano lens 5' facing the plano plate 4'. The plano plate 4' can be displaced in the air space in front of the plano surface 6' of the plano lens 5'. The separation and the axial displacement do not affect the aberrations of the projection system 1'. However, the plano plate 4 can be controlled using a mechanism that allows astigmatic deformation. For the plano plate 4' in the projection system 1', A = |~~| = 0.112 applies. The plane plate 4' is arranged near the pupil 13, whereby the beam path is strongly divergent, i.e. non-collimated.The plane plate 4' thus meets the requirements for a good correction effect of field-constant astigmatism.

[0080] A calculation of the deformation profile of the plane plate when astigmatic force is applied via the mount at the lens edge shows that the two deformable optical elements, the deformable optical element 3"' and the now created deformable optical element 3b, the deformable plane plate 4', allow the field-constant lowest-order astigmatism, Z5, and the field-constant higher-order astigmatism, Z12, to be adjusted almost perfectly and independently of each other.

[0081] If, during operation, it becomes apparent that a projection system, due to lens heating, suffers from field-constant astigmatism of the lowest order, Z5, as well as field-constant astigmatism of the higher order, Z12, this can also be corrected starting from the same optical design 10. However, if the option to correct field-constant astigmatism of the higher order, Z12, is to be dispensed with, for example, for cost or transmission reasons, it is possible to opt for the original variant according to the optical design 10, for example, the projection system 1 according to Fig. 2, in which the optical unit 2' consists solely of the optical element 3', i.e., no plane plate has been separated from the optical element 3'.

[0082] Fig. 5 shows a projection system 1" for microlithography, which was also manufactured based on the optical design 10. In the projection system 1" according to Fig. 5, a plane plate 4 has been separated from the optical element 3 of the optical unit 2 according to the optical design 10. For the optical unit 2, now comprising the plane plate 4 and the plane lens 5, the parameter C has the value C ~ 0.096. The plane plate 4 can be moved along the optical axis 14 to a suitable position in the beam path. Here, too, the separation and displacement have no influence on the optical aberrations of the image; the correction state remains the same. However, the plane plate 4 can be controlled with a mechanism that allows astigmatic deformation.

[0083] The plane plate 4 can be used, in particular, to correct anamorphism. Anamorphism can be corrected primarily at locations in the optical beam path where the marginal ray is parallel to the axis, i.e., the beams are collimated, and where the principal ray has a significant height from the optical axis 14. For the parameter B of the plane plate 4, with H / OBH = 0.755, u / NA = 0.056, and v = 0.347, the following applies: = 4.68, where H is the height of the principal ray 15 at the location of the plane plate 4, OBH is the object circle radius, u is the angle of the marginal ray 16 at the location of the plane plate 4, NA is the image-side numerical aperture, and v is the angle of the chief ray 15. B therefore has a value > 2. The plane plate 4, together with the deformable optical element 3"' of the initial optical design 10, can be used effectively to correct anamorphism. Fig. 6 shows the residual errors when an anamorphism of 50 nm Z2 / 3 is to be set using the two deformable optical elements, the deformable optical element 3"' and the plane plate 4 of the projection system 1". The leading residual errors that remain are only 1.15 nm Z2 / 3 and 1.78 nm astigmatism Z5.

[0084] Fig. 7 shows a projection system 1''' for microlithography, which was also manufactured based on the optical design 10. In the projection system 1''' in Fig. 7, the modifications known from the projection system 1' and from the projection system 1" are both implemented. The projection system 1''' thus comprises three deformable optical elements, the deformable optical element 3''', the plane plate 4 and the plane plate 4', with which the leading aberrations caused by lens heating, field-constant astigmatism Z5, field-constant astigmatism Z12 and anamorphism, can be corrected. The parameters A and B for the plane plate 4 and the plane plate 4' remain unchanged compared to the parameters according to the projection system 1' and the projection system 1": The parameter A for the plane plate 4' has the value A = the plane plate 4 applies with H / OBH = 0.755, u / NA = 4.68.

[0085] The projection system 1"" in Fig. 8 is another embodiment of a projection system. In the projection system 1"" a deformable lens acts as the deformable optical element 3"', which can primarily correct a field-constant lowest-order astigmatism, Z5. The parameter A for the deformable optical element 3"' has the value 0.112.

[0086] In addition, an optical unit 2" comprising a plane plate 4" and a plane lens 5", designed here as a plano-concave lens, is provided. The plane plate 4" was provided in the original optical design (not shown) as part of the plane lens 5", whereby the plane plate 4" was split off, so that the optical unit 2" of the projection system 1"" now comprises the plane plate 4" and the plane lens 5". The optical unit 2" of the projection system 1"" comprising the plane plate 4" and the plane lens 5" has, for example, a maximum optically free diameter Dmax of 144.14 and a minimum optically free thickness dmm of 48.40. The parameter C for the optical unit 2" therefore has the value C = 0.3358.

[0087] The plane plate 4" is located near the pupil 13 in the non-collimated beam path near the wafer 12. The marginal ray height h is therefore significantly greater than the main ray height H. The parameter A has the value for the split-off plane plate 4" A combined deformation of the deformable optical Element 3"' and the split-off plane plate 4" leads to a good joint correctability of field-constant astigmatism Z5 and Z12.

[0088] Alternatively or additionally, a plane plate 4 can be split off from the optical element of the optical unit 2, which, together with the deformable optical element 3''', leads to a correctability of anamorphism, analogous to, for example, in the projection system 1'' according to Fig. 5. The optical unit 2 of the projection system 1'''' has, for example, a maximum optically free diameter Dmax of 236.43 and a minimum optically free thickness dmm of 22.69. The parameter C for the optical unit 2 thus has the value C = 0.0960. The 18.6. In

[0089] Fig. 8 shows the projection system 1"" with the two optional plane plates 4, 4" separated from the respective optical elements 3, 3".

[0090] The projection system 1"" may have a specification according to Table 3. The corresponding asphere data are given in Table 4.

[0091] The 1" projection system is designed for an image-side numerical aperture of NA = 0.8. The object height is 54.41 mm.

[0092] Table 3 Table 4

Claims

Patent claims 1. Projection system (1, 1', 1", 1"', 1"") comprising at least one optical unit (2, 2', 2"), wherein the optical unit (2, 2', 2") comprises at least one optical element (3, 3', 3"), wherein the optical element (3, 3', 3") has at least one planar surface (6, 6', 6"), wherein for the optical unit (2, 2', 2") the following applies: C = ^2- > 0.075, where Dmax is the maximum optical free diameter of the Dmax optical unit (2, 2', 2") and dmm is the minimum optical free thickness of the optical unit (2, 2', 2"), wherein the optical unit (2, 2', 2") consists of at least one flat plate (4, 4', 4") and an optical element (5, 5', 5") adjacent to the flat plate (4, 4', 4") with at least one flat surface (6, 6', 6"), wherein the at least one flat surface (6, 6', 6") of the optical element (5, 5', 5") adjacent to the flat plate (4, 4', 4") faces the flat plate (4, 4', 4"), characterized in that the at least one flat plate (4, 4', 4") is deformable.

2. Projection system (1, 1', 1", 1"', 1"") according to claim 1, characterized in that for the plane plate (4', 4") applies: A = < 0.5, where H is the height of the chief ray (15) at the location of the plane plate (4', 4"), h is the height of the marginal ray (16) at the location of the plane plate (4', 4"), u is the angle of the marginal ray (16) at the location of the plane plate (4', 4") and NA is the image-side numerical aperture.

3. Projection system (1", 1"', 1"") according to claim 1, characterized in that > 2, where H is the height of the main ray (15) at the location of the plane plate (4), OBH is the object circle radius, u is the angle of the marginal ray (16) at the location of the plane plate (4), NA is the image-side numerical aperture and v is the angle of the chief ray (15) at the location of the plane plate (4).

4. Projection exposure apparatus for microlithography, characterized in that the projection exposure apparatus comprises a projection system (1, 1', 1", 1"', 1"") according to one of the preceding claims.

5. A method for producing a projection system, in particular a projection system (1, 1', 1", 1"', 1"") according to one of claims 1 to 3, wherein the method comprises the steps: Providing an optical design (10) for a projection system, wherein the optical design (10) comprises at least one optical unit (2, 2', 2"), wherein the optical unit (2, 2', 2") consists of an optical element (3, 3', 3"), wherein the optical element (3, 3', 3") has at least one planar surface (6, 6', 6"), wherein the following applies to the optical element (3, 3', 3"): C = > 0.075, where Dmax Dmax is the maximum optical free diameter of the optical element (3, 3', 3") and dmin is the minimum optical free thickness of the optical element (3, 3', 3"), and Producing a projection system based on the optical design (10), wherein the optical unit of the projection system is selected as: at least one plane plate (4, 4', 4") and an optical element (5, 5', 5") adjacent to the plane plate (4, 4', 4") with at least one plane surface (6, 6', 6"), wherein the at least one plane surface (6, 6', 6") of the optical element (5, 5', 5") adjacent to the plane plate (4, 4', 4") faces the plane plate (4, 4', 4").

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