Lithography system with detachable lenses

Abstract

We provide a lithography system equipped with separable lenses. [Solution] The present invention relates to a projection system comprising at least one optical unit, wherein the optical unit comprises at least one optical element, and the optical element has at least one plane. The problem of providing a flexible and cost-effective projection system is solved by the fact that equation (1) applies to the optical unit, where D max d is the maximum optical effective diameter of the optical unit. min is the minimum optically effective thickness of the optical unit. The present invention further relates to a projection exposure apparatus for microlithography comprising a projection system according to the present invention, a projection system, an optical design for a projection system according to the present invention, and a method for fabricating a projection system, in particular a projection system according to the present invention.

Description

[Technical Field]

[0001] The present invention relates to a projection system comprising at least one optical unit, wherein the optical unit comprises at least one optical element, and the optical element has at least one plane. The present invention further relates to a projection exposure apparatus comprising the projection system according to the present invention, a projection system, an optical design for fabricating the projection system according to the present invention, and a method for fabricating the projection system according to the present invention.

[0002] The subject matter of German Patent Application No. 102023204235.5 is incorporated herein by reference. [Background technology]

[0003] Microlithography projection systems are subject to stringent requirements regarding precision. Furthermore, microlithography projection systems are subjected to heavy loads. During the operation of the projection system, the optical elements and their mounts are heated by high radiant power. The mechanical and optical properties of the optical elements of the projection system can change due to so-called lens heating. This can cause thermal expansion, leading to changes in the shape of the optical elements, such as the form and thickness of the lens elements. The refractive index of the optical material may also change with temperature. This, in turn, can cause aberrations, which can lead to a significant decrease in the imaging performance of the projection system.

[0004] The degradation of imaging performance can be explained using wavefront aberrations, particularly those described by the Zernike polynomial. Lens heating is particularly pronounced by disturbances of rotational symmetry and astigmatism at the wavefront. While rotational symmetry disturbances may be corrected by the rigid body motion of the optical element, astigmatism disturbances cause aberrations that can no longer be corrected by rigid body motion. Lens heating primarily causes lowest-order field-constant astigmatism, which is described particularly by the Zernike polynomial Z5. Higher-order field-constant astigmatism Z12, and anamorphisms that can be described by field-linear Z2 and Z3 (optionally), may also occur as secondary effects.

[0005] To correct astigmatic aberration, for example, the document, International Publication No. 2006 / 125617, discloses a microlithographic projection lens equipped with a deformable lens element for the purpose of correcting aberrations. However, the optical and mechanical implementation of a deformable lens element leads to considerable cost, and the two additional optical surfaces further reduce the transmittance of the projection system. If two or three distinct aberrations exist due to lens heating, the corresponding corrections also require two or three deformable lens elements. This results in high cost and reduced transmittance due to the additional optical surfaces. Furthermore, when manufacturing the optical designs that form the basis of the projection system, it is often not precisely clear how the projection system will operate, that is, what aberrations may occur due to lens heating. This mainly applies to higher-order aberrations, especially higher-order astigmatic aberration Z12, and anamorphism. For this reason, the use of a deformable lens element is only proposed when the need is clearly evident beforehand.

[0006] The problem addressed by this invention is to provide a projection system that solves the shortcomings of the prior art. In particular, the objective is to provide a flexible and cost-effective projection system. Another problem addressed by this invention is to clarify a projection exposure apparatus equipped with a projection system, an optical design for fabricating the projection system, and a method for fabricating the projection system. [Overview of the project]

[0007] According to the first teaching of the present invention, the problem is solved by a projection system comprising at least one optical unit, the optical unit comprising at least one optical element, the optical element having at least one plane, and the optical unit comprising the following formula

[0008]

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[0009] The projection system is, for example, a microlithography projection system. A microlithography projection system is particularly a projection system for DUV (deep ultraviolet light) lithography. The projection system may include projection lenses, especially microlithography projection lenses. In microlithography, image quality is determined first by the projection lens and second by the illumination system. The field plane on which the mask supporting the structure, the so-called reticle, is placed should be illuminated as uniformly as possible by the illumination system. The projection lens projects the field plane onto the image plane, the so-called wafer plane, on which a photosensitive object, typically a wafer having a suitable photoresist, is placed.

[0010] The projection system comprises at least one optical unit. The optical unit comprises at least one optical element, and the optical element has at least one plane. The plane may be understood as a planar and flat surface. In particular, the plane is distinguished by having no curvature. In particular, the plane is arranged perpendicular to the optical axis. The optical element is, for example, a plano lens element or a plane plate. The plano lens element should be understood to mean a lens element having a plane and a curved surface, such as a spherical or aspherical surface. The optical element is in particular a plano-convex lens element or a plano-concave lens element. The plane plate has two opposing parallel planes. The optical element is preferably transparent. The optical element may in particular comprise a substrate, and the substrate may comprise, for example, quartz glass and / or any other type of glass, such as FK5, LLF1, or LF5.

[0011] The following formula applies to at least one optical unit of the projection system

[0012]

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[0013]

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[0014] It comprises at least one optical element having at least one plane, and

[0015]

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[0016]

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[0017]

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[0018] A particular advantage in this regard is that only a single optical design is needed to create the alternative projection system. The optical design comprises an optical unit consisting of optical elements, and the optical elements

[0019]

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[0020]

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[0021] According to a first advantageous configuration of the projection system, the optical unit comprises optical elements. Each optical element has at least one plane. The optical element is, for example, a planar lens element or a planar plate. The optical element has the following formula

[0022]

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[0023] The projection system may further include, in addition to the optical unit consisting of optical elements, at least one deformable optical element, particularly a deformable lens element. The deformable optical element acts in particular to correct the lowest-order constant field astigmatism Z5. Since the lowest-order constant field astigmatism is the main effect during lens heating, it may be useful in this simplest embodiment of a projection system comprising optical elements as an optical unit, and also in the underlying optical design, to already have at least one deformable optical element. The deformable optical element may be, for example, a deformable lens element with little refractive power. The deformable optical element is preferably located in the divergent beam path near the pupil of the system.

[0024] In a more advantageous configuration of the projection system, the optical unit comprises at least one plane plate and an optical element having at least one plane, positioned adjacent to the plane plate, wherein at least one plane of the optical element adjacent to the plane plate faces the plane plate. The optical unit, for example, comprises a plane plate and a plane lens element adjacent to the plane plate, having a plane lens element surface facing the plane plate. It is also conceivable that the optical unit comprises two plane plates positioned adjacent to each other. The fact that the plane plate and the optical element having at least one plane are positioned adjacent to each other may be understood as meaning that no other optical element is positioned between the plane plate and the optical element having at least one plane. However, the plane plate and the optical element having at least one plane do not need to be positioned directly in contact with each other. In particular, the plane plates may be positioned offset along the optical axis.

[0025] At least one planar plate and the optical elements of the optical unit adjacent to at least one planar plate are in common, under the following conditions regarding the ratio

[0026]

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[0027] A more advantageous configuration of the projection system is that at least one plane plate is deformable. The plane plate has two opposing planes. It is preferable that the plane plate of the optical unit is deformable. The plane plate may be controlled in particular by using a mechanism that allows deformation of the plane plate, in particular astigmatic deformation. The effect of lens heating is thus advantageous as it may be compensated for. For example, to set the lowest-order astigmatic aberration Z5 using only one deformable plane plate, the deformation of the deformable plane plate produces a shape deformation that can be described mainly by Z5, which includes a small component of the higher-order astigmatic aberration Z12. Therefore, in order to allow the lowest-order astigmatic aberration Z5 and the higher-order astigmatic aberration Z12 to be changed 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 element, so that the contribution of one to Z5 or Z12 can be compensated for by the contribution of the other.

[0028] According to a more advantageous configuration of the projection system, the following equation applies to the plane plate:

[0029]

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[0030] According to a more advantageous configuration of the projection system, the following equation applies to the plane plate:

[0031]

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[0032] According to the second teaching, the aforementioned problems relating to microlithography projection lithography apparatuses are solved by projection lithography apparatuses equipped with the projection system according to the first teaching. The projection lithography apparatus may be configured to generate an image of an object placed on an object surface onto the image plane using a light source that emits projection light. Microlithography projection lithography apparatuses operate to fabricate microstructures or nanostructures for microelectronics or microsystems technology. The projection lithography apparatus is used to reduce and image a structure formed on a photomask onto a wafer or the like in order to generate a corresponding structure on a wafer by a microlithography process. The projection lithography apparatus may be a DUV projection lithography apparatus with an exposure wavelength of approximately 365 nm, approximately 248 nm, approximately 193 nm, or approximately 157 nm. The projection lithography apparatus operates 24 hours a day, and any failure will affect the performance of the projection lithography apparatus.

[0033] According to the third instruction, the aforementioned problem is solved by a projection system, in particular an optical design for the projection system according to the first instruction, the optical design comprising at least one optical unit, the optical unit consisting of optical elements, the optical elements having at least one plane, and the optical elements having the following equation

[0034]

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[0035]

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[0036] The optical design comprises an optical unit consisting of optical elements, and the optical elements

[0037]

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[0038]

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[0039] The optical unit of an optical design may have, for example, the following formula

[0040]

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[0041]

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[0042] According to the fourth instruction, the aforementioned problem is solved by a method for fabricating a projection system, particularly a projection system according to the first instruction, which includes the steps of preparing an optical design according to the third instruction and fabricating a projection system based on the optical design. - An optical element having at least one plane, or - An optical element having at least one plane and at least one plane, arranged adjacent to the plane, wherein at least one plane of the optical element adjacent to the plane faces the plane. The optical unit of the projection system is selected.

[0043] The optical units of the projection system may be selected, and in particular may be replaced, using this method, for example, depending on the aberrations to be corrected. The configuration of the optical units may also be selected according to cost and / or transmittance requirements. Thus, for example, a user who wants to avoid correction options for cost reasons may decide that the projection system will consist of only planar lens elements, according to the underlying optical design. It is particularly advantageous that at least one additional optical element may be introduced into the beam path of the projection system by past illumination without affecting imaging performance, depending on requirements or needs.

[0044] This disclosure also includes the subject matter of the following clauses. 1) A projection system comprising at least one optical unit, wherein the optical unit comprises at least one optical element, and the optical element has at least one plane, The optical unit has the following formula

[0045]

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[0046]

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[0047]

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[0048]

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[0049] In this specification, the exemplary configurations of the present invention described above should be understood to be disclosed in any combination of each other. Any individual feature of each embodiment may be combined with any desired feature or all of the features of other embodiments. Further configurations and advantages of the present invention will be described below in detail with reference to the drawings, in relation to some exemplary embodiments of the present invention.

[0050] Exemplary embodiments and variations of the present invention will be described in detail below with reference to the drawings. The teachings of this disclosure can be best understood from the detailed description below, together with the accompanying drawings. The drawings are schematic and simplified, showing only the details necessary to understand the claims, while other details are omitted. The same reference numerals are used throughout for identical or corresponding parts. Any individual feature of each teaching may be combined with any desired or all features of other teachings. These and other embodiments, features, and / or technical effects are evident and clarified by the drawings described below. [Brief explanation of the drawing]

[0051] [Figure 1] This is a schematic diagram of an embodiment of an optical design according to the third teaching. [Figure 2] This is a schematic diagram of an embodiment of the first teaching-based projection system, fabricated based on the optical design shown in Figure 1. [Figure 3] Figure 2 is a schematic diagram of residual aberrations in an embodiment of the projection system. [Figure 4]This is a schematic diagram of another embodiment of the projection system based on the optical design shown in Figure 1, which was fabricated using the first teaching method as a basis. [Figure 5] This is a schematic diagram of another embodiment of the projection system based on the optical design shown in Figure 1, which was fabricated using the first teaching method as a basis. [Figure 6] Figure 5 is a schematic diagram of residual aberrations in an embodiment of the projection system. [Figure 7] This is a schematic diagram of another embodiment of the projection system based on the optical design shown in Figure 1, which was fabricated using the first teaching method as a basis. [Figure 8] This is a schematic diagram of another embodiment of the projection system according to the first teaching. [Modes for carrying out the invention]

[0052] Figure 1 shows an optical design 10 for a projection system. The optical design 10 comprises a number of optical elements capable of forming a projection system. The optical design 10 is provided with an object plane and an image plane. The optical design 10 comprises, for example, a reticle 11 on the object plane and a wafer 12 on the image plane. The optical design 10 may be an optical design for a projection lens. In particular, the optical design 10 is an optical design for a microlithography projection system. The optical design 10 may function, for example, as an optical design for projection systems 1, 1', 1'', and 1''' shown in Figures 2, 4, 5, and 6. The optical design 10 preferably has the specifications shown in Table 1.

[0053] [Table 1] The "SURF" column identifies the number of the surface that is distinguished by its refractive surface or any other arbitrary viewpoint. The "RADIUS" column identifies the radius r (in mm) of each surface, and the "THICKNESS" column identifies the distance d (in mm) between each surface and the subsequent surface, called the thickness. The "MATERIAL" column identifies the material of the optical element, and the "INDEX" column identifies the refractive index of the material at the operating wavelength of 248.413 nm for the optical design 10. The "SEMIDIAM" column identifies the usable free radius or free optical radius (semidiameter) (in mm) of the optical element. A radius r=0 (in the "RADIUS" column) corresponds to a plane. Some optical surfaces are aspherical.

[0054] [Table 2] The corresponding aspherical data may be obtained from Table 2, and the aspherical surface is calculated according to equation (1).

[0055]

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[0056]

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[0057] When the cone constant K=0, equation (1) can be simplified to equation (2).

[0058]

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[0059] Figure 2 shows a microlithography projection system 1 fabricated based on the optical design 10. In a microlithography projection system, optical elements such as lens elements may be heated due to high radiant power. The main effect caused by so-called lens heating is the lowest-order wavefront field constant astigmatism deformation Z5. When lens heating occurs, this wavefront shift should be corrected.

[0060] For this purpose, the projection system 1 or optical design 10 includes a deformable optical element 3''' (hatched). The deformable optical element 3''' is a meniscus-shaped lens element with little refractive power. The deformable optical element 3''' is located in the divergent beam path near the pupil 13. Using the deformable optical element 3''', it is possible to correct the lowest-order field constant astigmatism deformation Z5. For this purpose, the deformable optical element 3''' is given the following equation

[0061]

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[0062] Numerical calculations show that the astigmatic deformation of the deformable optical element 3'' mainly produces a constant field astigmatic aberration Z5, with approximately 10% of the remaining constant field astigmatic aberration Z12 being residual aberration. Figure 3 shows the residual aberration when a constant field astigmatic aberration Z5 of exactly 15 nm is set on a stationary rectangular image plane using the deformable optical element 3''. In this case, - Z4 (focus), Z9, and Z16 (spherical aberration) are shown in the left column. - Z2 / 3 (tilt), Z7 / 8, and Z14 / 15 (comb) are located in the center column. - Z5 / 6, Z12 / 13 (astigmatism), and Z10 / 11 (trefoil) are visible in the right-hand column.

[0063] For example, rotational symmetric aberrations may be corrected using the conventional manipulator concept, which employs an optical component movable along the optical axis, a so-called Z-manipulator. This effectively prevents anamorphism or higher-order field curvature. It is clear that a constant field astigmatism Z12 of approximately 1.8 nm remains as a limiting residual aberration.

[0064] The optical design 10 shown in Figure 1 or the projection system 1 shown in Figure 2 does not have a thin lens element suitable as a second deformable optical element near the deformable optical element 3''', but the optical units 2, 2', which have optical elements 3, 3', are designed so that the planar plates can be separated from their respective optical elements 3, 3' and moved a further small amount into the space in front of the rest of their respective optical elements 3, 3'. In this case, each separation and axial movement does not affect the aberration of the optical design.

[0065] In order for the plane plate to be separated from the optical element, the optical element must have at least one plane and a certain minimum thickness, otherwise the resulting optical element would no longer have sufficient thickness. Therefore, each of the optical units 2, 2' included in the optical design 10 or projection system 1 is given by the following equation

[0066]

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[0067] If, for example, lens heating during operation reveals that the projection system also produces higher-order field constant astigmatism Z12 in addition to the lowest-order field constant astigmatism Z5, then the higher-order field constant astigmatism may also be corrected. For this purpose, an additional deformable optical element may be activated at a similar position in the beam path, i.e., in the non-collimated region of the beam path near the pupil. If deformation occurs, this deformable optical element will again produce mainly field constant Z5, along with a certain amount of additional Z12. However, by manipulating both deformable lens elements, it may be possible to find a linear combination of deformations that allows for substantially pure correction of Z5 or Z12 of the wavefront.

[0068] Figure 4 shows a microlithography projection system 1' similarly fabricated based on the optical design 10. The projection system 1' shown in Figure 4 comprises two optical units 2, 2', each optical unit 2, 2' having the following equation

[0069]

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[0070] In projection system 1', the optical unit 2' thus consists of two optical elements 3a and 3b, rather than one optical element 3. Optical element 3a is a planar lens element 5', which in this case is a plano-convex lens element. The other optical element 3b is a planar plate 4'. The planar lens element 5' is positioned adjacent to the planar plate 4', and the plane 6' of the planar lens element 5' faces the planar plate 4'. The planar plate 4' may move in the space in front of the plane 6' of the planar lens element 5'. In this case, separation and axial movement do not affect the aberrations of projection system 1'. However, the planar plate 4 may be controlled using a mechanism that allows astigmatic deformation. The planar plate 4' of projection system 1' has,

[0071]

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[0072] Calculations of the deformation profile of a plane plate under an astigmatic force applied via the mount at the edge of the lens element show that the deformable plane plate 4', namely the deformable optical element 3'' and the resulting deformable optical element 3b, makes it possible to set the lowest-order field constant astigmatism Z5 and the higher-order field constant astigmatism Z12 almost completely and independently of each other.

[0073] If, due to lens heating during operation, the projection system reveals that it also has higher-order field constant astigmatism Z12 as well as the lowest-order field constant astigmatism Z5, the higher-order field constant astigmatism may also be corrected using the same optical design 10 as a starting point. On the other hand, if, for reasons such as cost or transmittance, it is necessary to avoid the possibility of correcting the higher-order field constant astigmatism Z12, it is possible to select a modified form of the prototype by the optical design 10, i.e., a projection system 1 in which the optical unit 2' consists only of optical elements 3', i.e., the planar plate is not separated from the optical elements 3', as shown in Figure 2.

[0074] Figure 5 shows a microlithography projection system 1'' similarly fabricated based on the optical design 10. In the projection system 1'' according to Figure 5, the plane plate 4 is separated from the optical element 3 of the optical unit 2 according to the optical design 10. Here, in the optical unit 2, which includes the plane plate 4 and the plane lens element 5, the parameter C has a value of C ≈ 0.096. The condition of the plane plate 4 is that it can be moved to a suitable position in the beam path along the optical axis 14. Here again, the separation and movement do not affect the optical aberration of the image, and the correction state remains the same. However, the plane plate 4 may be controlled using a mechanism that allows for astigmatic deformation.

[0075] The flat plate 4 may be used in particular for correcting anamorphism. Anamorphism can be corrected mainly in the optical beam path where the peripheral rays extend parallel to the axis, thus the beam is collimated, and the principal rays are at a considerable height relative to the optical axis 14. When H / OBH = 0.755, u / NA = 0.056, and v = 0.347, the parameter B of the flat plate 4 is given by the following equation

[0076]

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[0077] Figure 7 shows a microlithography projection system 1''' similarly fabricated based on the optical design 10. In projection system 1''' of Figure 7, both the modification forms known for projection system 1' and the modification forms known for projection system 1'' were implemented. Projection system 1''' thus comprises three deformable optical elements, namely deformable optical element 3''', a planar plate 4, and a planar plate 4', capable of correcting the main aberrations caused by lens heating, namely constant field astigmatism Z5, constant field astigmatism Z12, and anamorphism. With respect to parameters A and B of planar plate 4 and planar plate 4', there are no changes here compared to the parameters for projection system 1' and fabrication system 1''. In planar plate 4', parameter A is as

[0078]

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[0079]

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[0080] The projection system 1'''' in Figure 8 is another exemplary embodiment of the projection system. In projection system 1'''', the deformable lens element functions as a deformable optical element 3'''', which may primarily correct the lowest-order field constant astigmatism Z5. In the deformable optical element 3'''', parameter A is as

[0081]

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[0082] Furthermore, an optical unit 2'' is provided, comprising a flat plate 4'' and a flat lens element 5'' embodied here as a plano-concave lens element. In the original optical design (not shown), the flat plate 4'' was provided as part of the flat lens element 5'', but in this case, the flat plate 4'' is separated so that the optical unit 2'' of the projection system 1'''' comprises the flat plate 4'' and the flat lens element 5''. The optical unit 2'' of the projection system 1'''' comprising the flat plate 4'' and the flat lens element 5'' can, for example, have a maximum effective aperture D max , 144.14, and minimum effective optical thickness d min , has 48.40. In optical unit 2'', parameter C therefore has the value C = 0.3358.

[0083] The flat plate 4'' is located near the wafer 12 in the non-collimated beam path near the pupil 13. Therefore, the height h of the peripheral rays is considerably greater than the height H of the principal rays. In the separated flat plate 4'', parameter A is as

[0084]

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[0085] Alternatively, or in addition to this, the flat plate 4 can be separated from the optical elements of the optical unit 2, and the flat plate, together with the deformable optical elements 3'', provides the possibility of correcting anamorphism in a manner similar to, for example, the projection system 1'' shown in Figure 5. The optical unit 2 of the projection system 1'''' has, for example, a maximum effective aperture D max , 236.43, and minimum effective optical thickness d min , has 22.69. In optical unit 2, parameter C therefore has the value C = 0.0960. Parameter B of the separated plane plate 4 has the value as

[0086]

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[0087] Projection system 1'''' may have the specifications shown in Table 3. Table 4 identifies the corresponding aspherical data.

[0088] Projection system 1 is designed to have an image-side numerical aperture NA = 0.8. The height of the object is 54.41 mm.

[0089] [Table 3]

[0090] [Table 4]

Claims

1. A 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''), the optical element (3, 3', 3'') has at least one plane (6, 6', 6''), and the optical unit (2, 2', 2'') comprises the following formula 【Number 1】 This applies, and here, D max The maximum effective aperture of the optical unit (2, 2', 2''), d min This is the minimum effective optical thickness of the optical unit (2, 2', 2''), and the optical unit (2, 2', 2'') consists of optical elements (5, 5', 5'') having at least one flat plate (4, 4', 4'') and at least one plane (6, 6', 6''), and the at least one plane (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''), A projection system (1, 1', 1'', 1'''', 1'''') characterized in that at least one of the planar plates (4, 4', 4'') is deformable.

2. The following formula applies to the aforementioned flat plates (4', 4''): [Math 2] The projection system (1, 1', 1'', 1'''', 1'''') according to claim 1, wherein H is the height of the principal ray (15) at the position of the plane plate (4', 4''), h is the height of the peripheral ray (16) at the position of the plane plate (4', 4''), u is the angle of the peripheral ray (16) at the position of the plane plate (4', 4''), and NA is the image-side numerical aperture.

3. The following formula applies to the aforementioned flat plate (4): [Math 3] The projection system (1'', 1''', 1'''') according to claim 1, wherein H is the height of the principal ray (15) at the position of the flat plate (4), OBH is the radius of the field, u is the angle of the peripheral ray (16) at the position of the flat plate (4), NA is the image-side numerical aperture, and v is the angle of the principal ray (15) at the position of the flat plate (4).

4. A microlithography projection exposure apparatus characterized in that the projection exposure apparatus comprises a projection system (1, 1', 1'', 1'''', 1'''') according to any one of claims 1 to 3.

5. A method for producing a projection system, particularly a projection system (1, 1', 1'', 1''', 1'''') according to any one of claims 1 to 3, - A step of preparing an optical design (10) for a projection system, wherein the optical design (10) comprises at least one optical unit (2, 2', 2''), the optical unit (2, 2', 2'') consists of optical elements (3, 3', 3''), the optical elements (3, 3', 3'') have at least one plane (6, 6', 6''), and the optical elements (3, 3', 3'') have the following formula [Math 4] This applies, and here, D max The maximum effective aperture of the optical element (3, 3', 3''), d min Step, where is the minimum effective optical thickness of the optical element (3, 3', 3''), - A step of manufacturing a projection system based on the optical design (10), wherein the optical unit of the projection system is selected as an optical element (5, 5', 5'') having at least one planar plate (4, 4', 4'') and at least one plane (6, 6', 6''), and the at least one plane (6, 6', 6'') of the optical element (5, 5', 5'') adjacent to the planar plate (4, 4', 4'') faces the planar plate (4, 4', 4''), and Methods that include...

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