METHOD FOR MANUFACTURING A REFLECTIVE OPTICAL ELEMENT AND REFLECTIVE OPTICAL ELEMENT

Description

[0001] The present invention relates to a method for manufacturing a reflective optical element for the extreme ultraviolet wavelength range (about 1 nm to 20 nm) composed of at least two upper units, wherein each upper unit comprises a plurality of subunits, and to a reflective optical element for the extreme ultraviolet wavelength range composed of at least two upper units, wherein each upper unit comprises a plurality of subunits. It further relates to an optical element and an EUV lithography device comprising such a reflective optical element.

[0002] From DE 10 2012 213 937 A1, a mirror array for use in the illumination system of an EUV lithography device is known. This mirror array is composed of several subarrays, each of which in turn has a plurality of individual mirrors. To enable simplified maintenance of the illumination optics, it is provided that all individual mirrors of all subarrays are provided with a reflective coating that is designed to be broadband enough to cover all angles of incidence that can occur at the various subarrays during operation of the mirror array. In particular, it is a micromirror array designed as a microelectromechanical system.

[0003] DE 10 2011 077 234 A1 describes an EUV mirror arrangement with a multitude of mirror elements arranged next to each other, in which the reflectivity can be influenced by changing the thickness of a piezoelectric layer.

[0004] From US patent 2008 / 0165415 A1, it is known to provide mirrors with different reflective mirror coatings in an optical system, especially for EUV lithography, namely with multilayer systems with different varying layer thicknesses.

[0005] It is an object of the present invention to improve mirror arrays, especially for EUV lithography, in such a way as to increase their reflectivity.

[0006] In a first aspect, this task is solved by a method for manufacturing a reflective optical element for the extreme ultraviolet wavelength range composed of at least two upper units, wherein each upper unit has a plurality of subunits, with the steps: Dividing the upper units into bandwidth classes with a maximum bandwidth determined for each bandwidth class; applying a reflective coating to each upper unit, adapted to the angle of incidence and angle of incidence bandwidths determined over its surface, wherein a multilayer system with period length is applied to all upper units as a reflective coating, the layer sequence and / or layer thicknesses of which are adapted to the maximum bandwidth determined for the respective bandwidth class, wherein base multilayer systems are selected differently for each bandwidth class and adapted to the required bandwidth, and wherein the desired

[0007] The period length for each upper unit is optimized individually depending on the angle of incidence occurring during operation by modifying the period length of the base multilayer system by a factor for each upper unit within a bandwidth class.

[0008] This manufacturing process has the advantage that, on the one hand, reflectivity can be increased by adapting the reflective coating on each upper unit to the incidence angles and incidence angle ranges occurring there during operation. On the other hand, the effort involved in manufacturing the reflective optical element and subsequently maintaining it remains relatively limited, since the optimization, particularly regarding incidence angle ranges, can be restricted to the upper units. Especially in EUV lithography devices, several reflective optical elements are arranged in series. Therefore, the overall reflectivity within EUV lithography devices is not very high, and any gains in reflectivity are advantageous.

[0009] The maximum incidence angle bandwidth is determined for each upper unit. The upper units are then classified into bandwidth classes, and a multilayer system is applied to each upper unit as a reflective coating. The layer sequence and / or layer thicknesses of this system are adapted to the maximum bandwidth determined for the respective bandwidth class. This allows for higher reflectivity of the reflective optical element. This approach is particularly preferred for more than two upper units. A base multilayer system is defined for each class. The angular bandwidth of a multilayer system can be influenced, for example, by layer sequences exhibiting aperiodicity or by layer thickness gradients perpendicular to the substrate. Advantageously, the respective base multilayer system is designed for the bandwidth of the upper unit with the largest incidence angle bandwidth within a class.

[0010] In this and the following embodiments, the multilayer systems are preferably multilayer systems consisting of alternating layers of a material with a lower real part of the refractive index in the extreme ultraviolet wavelength range and a material with a higher real part of the refractive index in the extreme ultraviolet wavelength range, particularly at the operating wavelength at which the lithographic process is carried out, on a substrate. The alternating layers can be grouped into periods of a certain length. These multilayer systems are particularly suitable for the extreme ultraviolet wavelength range and, as is known, can be very flexibly designed by selecting the materials, the layer sequence, and the thickness ratios for desired mean angles of incidence and incidence angle bandwidths at a chosen wavelength.In particular, for a given layer sequence, changing the layer thicknesses by a constant factor can shift the angle of incidence at which the highest reflectivity is achieved for a selected wavelength.

[0011] Preferably, each upper unit is divided into area units, and the angles of incidence occurring during operation are determined for each upper unit across all its area units. Subsequently, a desired period length of a multilayer system is determined based on the determined angles of incidence, and the variation of the desired period length across the area units for each upper unit is approximated by a polynomial of degree n, where n is a non-negative integer. Finally, the corresponding multilayer system is applied to each upper unit as a reflective coating.

[0012] In the simplest versions, a constant desired period length is determined for each upper unit, corresponding to a zero-degree polynomial. Approaches for determining desired period lengths based on specific angles of incidence encountered during operation are widely known. For example, DE 2013 203 364 A1 discloses various approaches for determining a desired period length from a maximum and a minimum angle of incidence. In this way, among others, the reflective coatings for all upper units can be produced in a single coating batch, whereby different layer thicknesses for each upper unit to be coated can be set using a period length profile across the coating holder in the coating system.

[0013] In further variations, the desired period length profile across the respective superunit can be approximated by first-degree polynomials, i.e., by linear functions. Superunits designed in this way can also be grouped into batches and coated in a single process. The higher the degree of the polynomial used to approximate the desired period length, the smaller the deviation of the applied period profile from the ideal period profile. This increases the reflectivity of the superunits and the reflective optical element composed from them. Sufficient thickness control across the surface can be achieved during coating, for example, by using honeycomb masks.For more complex thickness profiles, a coating process as described in DE 10 2012 205 615 A1 can also be used, in which layer-forming particles are ionized and applied to the surface to be coated in a targeted manner using electric and / or magnetic fields. Alternatively or additionally, the subunits can be oriented differently relative to the coating source in order to influence the applied layer thicknesses. This embodiment allows the production of reflective optical elements with particularly high overall reflectivity.

[0014] It should be noted that the desired period length can be approximated one-dimensionally in one direction across the surface or two-dimensionally across the surface of the upper units.

[0015] It is particularly advantageous if the area units correspond to the subunits of the higher-level units. For this purpose, an area unit can be identical to the area of ​​a subunit. Depending on the number of subunits, however, it can also be useful to group several adjacent subunits into area units, especially if the distribution of the angles of incidence or the range of angles of incidence across these subunits is comparable.

[0016] Advantageously, a reflective coating is applied to the upper units by mounting them on a coating holder rotating about an axis, with areas of constant layer thickness arranged concentrically around the axis. Depending on the required layer thicknesses or period lengths, the upper units can be arranged at different distances from the axis of rotation of the coating holder in order to coat upper units with different reflective coatings adapted to the incidence angles and incidence angle ranges occurring in operation, all within the same batch.

[0017] In another aspect, the problem is solved by a reflective optical element for the extreme ultraviolet wavelength range, manufactured as previously described, which comprises upper units from at least two bandwidth classes, where one bandwidth class comprises at least two upper units.

[0018] Such composite reflective optical elements exhibit a higher reflectivity than those known from the prior art, but can still be manufactured with relatively little effort.

[0019] Here, each upper unit of the reflective optical element has a multilayer system adapted to the angle of incidence occurring during operation at precisely that upper unit. With respect to the angle of incidence range, at least two upper units have multilayer systems adapted to the same angle of incidence range. In particular, the multilayer systems can exhibit aperiodicities or thickness gradients perpendicular to the multilayer system surface.

[0020] In preferred embodiments, the reflective coatings are designed as multilayer systems whose layer thicknesses vary according to a function that corresponds to a polynomial of degree n, where n is a non-negative integer. This can be a one-dimensional or a two-dimensional polynomial over the surface of the multilayer system of degree 0, 1, 2, 3, 4, or any higher degree.

[0021] Preferably, each subunit is configured as an individually actuated mirror. Most preferably, each upper unit is configured as a microelectronic system of micromirrors. This makes the reflective optical element particularly suitable for use in optical systems or EUV lithography devices in locations where larger-area elements are required that must be aligned differently and precisely across their entire surface simultaneously.

[0022] Preferably, the reflective optical element is designed as a field faceted mirror.

[0023] In particular, reflective optical elements based on microelectromechanical systems of micromirrors, such as mirror arrays, are suitable for use as field facet mirrors. The tilt of the individual actuated micromirrors is adjusted during operation to emulate both the tilt and curvature of a field facet. Each upper unit, for example, contributes to several field facets as a subarray, and each field facet is formed from sections of several upper units. Each field facet has an individual mean angle of incidence and a unique range of angles of incidence. The reflective optical element described here is particularly well-suited for providing good reflectivity by considering the distribution of angles of incidence across the field facet mirror. The reflective optical element presented here can also be configured as a pupil facet mirror.

[0024] In other aspects, the task is solved by an optical system, especially for EUV lithography, or by an EUV lithography device with a reflective optical element as described above.

[0025] The present invention will be explained in more detail with reference to preferred embodiments. To this end, we will show... Figure 1: A schematic view of an embodiment of an EUV lithography device; Figure 2: A schematic view of an embodiment of an illumination system; Figure 3: A schematic view of an embodiment of a reflective optical element with seven upper units; Figure 4: A schematic view of an upper unit; Figure 5: A schematic diagram of a multilayer system; Figure 6: The distribution of angles of incidence over the surface of a reflective optical element with five upper units; Figure 7: The reflective coating of a conventional reflective optical element with five upper units; Figure 8: The reflective coating of a first embodiment of a reflective optical element according to the invention with five upper units; Figure 9: The reflectivity as a function of the angle of incidence for two different multilayer systems;Figure 10 shows the reflective coating of a second embodiment of a reflective optical element according to the invention with five upper units; Figures 11 and 12 show two further schematic views of the reflective optical element made of [material not specified]. Figure 3 Figure 13 shows a possible arrangement of upper units to be coated in a coating system; Figure 14 shows the reflective coating of a third embodiment of a reflective optical element according to the invention with five upper units; Figure 15 shows the reflective coating of a fourth embodiment of a reflective optical element according to the invention with five upper units; Figure 16 shows the average reflectivity at average angles of incidence for the optical reflective elements from the Figures 10 to 14 Figure 17 shows the mean reflectivity at extreme angles of incidence for the optical reflective elements from the Figures 10 to 14Figure 18 shows the mean PV value at mean angles of incidence for the optical reflective elements from the Figures 10 to 14 ; and Figure 19, the mean PV value at extreme angles of incidence for the optical reflective elements from the Figures 10 to 14 .

[0026] In Figure 1 A schematic diagram shows a projection exposure system 100 for the production of, for example, microelectronic components, which is operated in a scan mode along a scan direction 126 with a working wavelength in the EUV range and which can have one or more optical elements with an additional coating. The in Figure 1The projection exposure system 100 shown has a point-like plasma radiation source. The radiation from the laser source 102 is directed via a condenser lens 104 onto suitable material, which is introduced via the feeder 108 and excited to a plasma 106. The radiation emitted by the plasma 106 is focused by the collector mirror 110 onto the intermediate focus Z. Appropriate apertures 111 at the intermediate focus Z ensure that no unwanted scatter radiation reaches the subsequent mirrors 112, 114, 116, 118, 120 of the illumination system of the projection exposure system 100. The plane mirror 122 serves to fold the system in order to provide installation spaces for mechanical and electronic components in the object plane, in which the holder for the reticle 124 is arranged. In the lighting system, in the present example, a field faceted mirror 114 and a pupil faceted mirror 116 follow the mirror 112.The field facet mirror 114 serves to project a multitude of images from the radiation source of the projection exposure system into a pupil plane. A second facet mirror, acting as a pupil facet mirror 116, is located in this plane and superimposes the images of the facets of the field facet mirror 114 onto the object plane to achieve the most homogeneous illumination possible. The mirrors 118 and 120, arranged downstream of the facet mirrors 114 and 116, primarily serve to shape the field in the object plane. A structured reticulum 124 is arranged in the object plane, and its structure is projected onto the object 130 to be exposed, for example, a wafer, by means of a projection lens 128, which in this example has six mirrors.The reticulum 124 can be moved in the direction 126 shown in the projection exposure system 100, which is designed here as a scanning system, and is successively illuminated section by section in order to project the respective structures of the reticulum 124 onto, for example, a wafer 130 with the projection lens.

[0027] In Figure 2Figure 1 shows a radiation source in conjunction with an illumination system 11, which is part of a projection exposure device for EUV lithography. A collector 1 is arranged around a light source formed by a plasma droplet 2, which is excited by an infrared laser 3. To obtain wavelengths in the EUV wavelength range, for example around 13.5 nm, tin can be excited to a plasma using a carbon dioxide laser operating at a wavelength of 10.6 µm. Solid-state lasers can also be used instead of a carbon dioxide laser. Following the collector 1, after the aperture 5 at the intermediate focus 4, are a field faceted mirror 16 with individual facets 18 and a pupil faceted mirror 17 with individual facets 19. Before the radiation hits the reticle 13, which is to be scanned in the y-direction and has the structure to be projected onto a wafer, it is deflected by a folding mirror 12.The folding mirror 12 has less of an optical function; rather, it serves to optimize the space requirements of the lighting system 11.

[0028] It should be noted that a wide variety of radiation sources can be used in UV or EUV lithography, including plasma sources based on laser excitation (LPP sources) or gas discharge (DPP sources), synchrotron radiation sources, or free electron lasers (FELs). Furthermore, the collectors can be designed in various ways, including Wolter collectors or ellipsoidal collectors, preferably adapted to the specific radiation source used.

[0029] The faceted mirrors, in particular the field faceted mirror, are designed in the present example as a reflective optical element for the extreme ultraviolet wavelength range, composed of at least two upper units, each upper unit comprising a plurality of subunits and each upper unit having a reflective coating adapted to the angles of incidence and angle-of-incidence bandwidths occurring over the surface of the upper unit during operation. The field faceted mirror was manufactured by first determining the angles of incidence and angle-of-incidence bandwidths occurring over the surface of each upper unit during operation and then applying a reflective coating to each upper unit adapted to the angles of incidence and angle-of-incidence bandwidths determined over its surface.

[0030] In Figure 3A reflective optical element 30 according to the invention is shown, which has five upper units 31 and is designed to replace a conventional field faceted mirror 40 with five field facets 41 that is not designed as a micromirror array. As shown in Figure 4 As an example, a superunit 31 comprises a plurality of subunits 32, which in this example are configured as individually actuated micromirrors. Together they form a microelectromechanical system.

[0031] As in Figure 3As shown, each field facet 41 is formed by several superunits 31, and each superunit 31 contributes to more than one field facet 41. Due to its different positions and curvature, each field facet 41 has an individual angle of incidence and an individual angle of incidence range. Thus, different angles of incidence with different angles of incidence ranges occur at different locations within each superunit 31.

[0032] In Figure 5The structure of a subunit 50 of the reflective optical element is shown schematically. The example shown is a micromirror element based on a multilayer system 51 deposited on a substrate 52. Materials with a low coefficient of thermal expansion are preferably chosen as substrate materials. The multilayer system 51 essentially consists of alternating layers of a material with a higher real part of the refractive index at the operating wavelength, at which, for example, lithographic exposure is carried out (also called spacer 55), and a material with a lower real part of the refractive index at the operating wavelength (also called absorber 54). In the example shown here, an absorber-spacer pair forms a stack 53, which corresponds to one period in periodic multilayer systems.This simulates, in a sense, a crystal whose lattice planes correspond to the absorber layers where Bragg reflection occurs. The thicknesses of the individual layers 54, 55, as well as the repeating stacks 53, can be constant or vary across the entire multilayer system 51, depending on the desired spectral or angle-dependent reflection profile. The reflection profile can also be specifically influenced by supplementing the basic structure of absorber 54 and spacer 55 with additional materials of varying absorbance to increase the maximum possible reflectivity at the respective operating wavelength. For this purpose, absorber and / or spacer materials can be exchanged in some stacks, or additional layers of other materials can be provided. The absorber and spacer materials can have constant or varying thicknesses across all stacks to optimize reflectivity.Furthermore, additional layers, for example as diffusion barriers between spacer and absorber layers 55, 54, can be provided in individual or all stacks to increase thermal stability. The first layer adjacent to the substrate 52 can be an absorber, a spacer, or an additional layer. To protect the reflective coating from external influences, a protective layer 56 can be provided at the vacuum level; this layer can also consist of more than one layer. Preferred materials for the EUV wavelength range include molybdenum as an absorber material and silicon as a spacer material.

[0033] Broadband reflective coatings can be produced in the usual way using multilayer systems that are, for example, periodic with a small number of periods, have two or more periodic subsystems with different period lengths, or are completely aperiodic. They can also exhibit a layer thickness gradient perpendicular to the substrate. The angle of incidence with highest reflectivity for a given multilayer system can be shifted, for example, by changing the period length of that multilayer system. Various approaches for determining a desired period length for the multilayer system of a reflective optical element as a reflective coating, based on, for example, a maximum and a minimum angle of incidence observed during operation, are described, for example, in DE 2013 203 364 A1.A disadvantage of choosing an identical reflective coating for all upper units is the comparatively low overall reflectivity.

[0034] In Figure 6The angles of incidence across the surface of a reflective optical element configured as a micromirror array are shown as an example. It features five upper units, B1 to B5, configured as subarrays, each comprising a plurality of actuable micromirrors as subunits. In this example, the reflective optical element serves as a faceted mirror, specifically a field faceted mirror, with each upper unit corresponding to one of five conventional field facets. During operation, for example in the illumination system of an EUV lithography device, different angles of incidence distributions affect each area unit of an upper unit B1 to B5, which can be assigned to a field facet. Of these, the following are shown for each upper unit B1 to B2: Figure 6The maximum angle of incidence (short dashed line), the mean angle of incidence (solid line), and the minimum angle of incidence (long dashed line) are plotted in degrees across various positions, each corresponding to a unit area or field facet. The difference between the minimum and maximum angles of incidence allows the respective angle of incidence range to be determined.

[0035] In the Figure 6In the example shown, the maximum angles of incidence in the first upper unit B1 fluctuate between approximately 7.5° and approximately 15.5°, the minimum angles of incidence between approximately 4.5° and approximately 11.5°, and the angle of incidence range between approximately 2° and approximately 7°. In the second upper unit B2, the maximum angles of incidence fluctuate between approximately 11° and approximately 18°, the minimum angles of incidence between approximately 4° and approximately 12.5°, and the angle of incidence range between approximately 2° and approximately 8°. In the third upper unit B3, the maximum angles of incidence fluctuate between approximately 8.5° and approximately 18°, the minimum angles of incidence between approximately 3.5° and approximately 15.5°, and the angle of incidence range between approximately 2° and approximately 7.5°. For the fourth upper unit B4, the maximum angles of incidence vary between approximately 5.5° and approximately 17.5°, the minimum angles of incidence between approximately 3.5° and approximately 13°, and the angle of incidence range between approximately 2° and approximately 8°. For the fifth upper unit B5, the maximum angles of incidence vary between approximately...9.5° and approximately 16.5°, the minimum angles of incidence between approximately 5° and approximately 13° and the angle of incidence range between approximately 2° and approximately 5°.

[0036] In Figure 7 A reflective optical element composed of five upper units B1 to B5 is shown, according to the aforementioned prior art. It has an identical reflective coating on all five upper units B1 to B5, which is designated "const". Furthermore, the ideal desired period length is plotted across the individual positions and designated "ideal". In this example, the reflective coating is a periodic multilayer system. To optimize the reflectivity, the period length was globally increased by slightly more than 3% across all upper units B1 to B5.

[0037] To increase the overall reflectivity of the reflective optical element, particularly when used as a field facet mirror in the illumination system of an EUV lithography device, it is proposed here to consider the angles of incidence and angle-of-incidence distribution occurring during operation separately for each upper unit when applying the reflective coating. Advantageously, the reflective coatings are designed as multilayer systems whose layer thicknesses and / or layer sequences are adapted to the angles of incidence and angle-of-incidence bandwidths occurring across the surface of the upper unit during operation. In particular, it is exploited that, especially when using multilayer systems as the basis of the reflective coating suitable for a specific angle-of-incidence bandwidth, the angle of incidence with the highest reflectivity at a specific wavelength can be changed by altering the period.

[0038] In a first embodiment, which is not covered by the claimed subject matter, the reflective coating of the reflective optical element is designed as a multilayer system with a specific layer sequence based on a desired broadband for all upper units. Preferably, in this embodiment, the desired broadband is determined by the upper unit with the largest incidence angle bandwidth. The desired period length is optimized for each upper unit individually, depending on the incidence angles occurring during operation. In this way, the variation of the desired period length across the area units for each upper unit is approximated by a zero-degree polynomial, and a corresponding reflective coating is applied.

[0039] In Figure 8 is shown such an embodiment, which is analogous to the representation in Figure 7also has five upper units B1 to B5 and is designed for use as a field facet mirror with the in Figure 6 The depicted angles of incidence are designed for each upper unit B1 to B5. For each area unit, the desired period length was determined based on the angles of incidence occurring there. Based on the angles of incidence and angle distribution determined for each upper unit B1 to B5, the factor by which the desired period length of the multilayer system underlying the reflective coating with the desired broadband was modified for each upper unit was calculated. Figure 8 In the example shown, the factor fluctuates between slightly below 1.03 and almost 1.04. The period length for the respective upper unit is shown as a thick solid line labeled "mean".

[0040] Since the reflective coating for all upper units B1 to B5 is based on the same multilayer system with the desired broadband coverage, and thus the materials, their sequence as layers, and the layer thickness ratios are identical for all upper units, all five upper units can be coated as a single batch, with a different layer thickness distribution being set via the coating holder. Depending on which upper unit is to have which period length, they are arranged accordingly on the coating holder for coating. For example, a coating holder rotating around an axis can be used for this purpose. Regions of constant layer thickness lie on concentric circles around the axis of rotation.

[0041] According to the claimed subject matter, the upper units are classified into different classes depending on the determined angle of incidence range. In the example shown here, two classes can be used. The first class contains upper units B1 and B5, which have an angle of incidence range of approximately 12°. The second class contains upper units B2, B3, and B4, which have an angle of incidence range of approximately 14° (see Figure 6 The base multilayer systems are selected differently for both classes and adapted to the required bandwidth. The reflectivity as a function of the angle of incidence at a wavelength of 13.5 nm is shown in Figure 9The two base multilayer systems are shown, with the first class represented by a dotted line labeled "standard" and the second by a solid line labeled "broadband." The two classes can be coated in two different batches. By varying the rotationally symmetrical thickness profile of the individual layers, resulting in a thickness gradient perpendicular to the substrate, coatings with different broadband coverage can be obtained on different radii of the coating holder. Additionally, the desired period length is taken into account during the coating of each individual top layer, as explained.

[0042] In another, schematically in Figure 10In the illustrated embodiment, the reflective coatings of the upper units B1 to B5 of the reflective optical element are designed as multilayer systems whose layer thicknesses correspond to a first-degree polynomial. To manufacture this embodiment, each upper unit is divided into area units, and the angles of incidence occurring during operation are determined for each upper unit across all its area units. Then, depending on the determined angles of incidence, a desired period length of a multilayer system is determined, which approximates the variation of the desired period length across the area units for each upper unit using a first-degree polynomial. Subsequently, the corresponding multilayer system is applied to each upper unit as a reflective coating. In the present illustration, Figure 10In the example shown, each upper unit B1 to B5 was again divided into five area units, as in the other embodiments explained here, such that each area unit belongs to a different field facet and groups subunits, for example individual micromirrors, with similar angles of incidence and angle-of-incidence bandwidths. These are designated with a position from 1 to 25. The resulting period length for each upper unit B1 to B5, approximated by a straight line, is given in Figure 9 drawn as a thick solid line and labelled "grad".

[0043] Approximating the variation of the mean angle of incidence using linear gradients in as few directions as possible allows the coating of all upper units in a single batch. For this purpose, a layer thickness profile is set during the coating process, which oscillates at a short spatial wavelength via the coating holder. Figure 11is a schematic representation of Figure 3 supplemented by the variation in period length indicated by the hatching. Figure 12The field facets have been omitted to improve visibility of the hatching. Thicker lines indicate a higher period length than thinner lines. In area A, a linear gradient from maximum to minimum to maximum period length is required across the diagonal of the top unit; in area B, also across the diagonal, a linear gradient from minimum to maximum to minimum period length; in area C, also across the diagonal, a linear gradient from maximum to minimum period length; and in area D, a linear gradient from minimum to maximum period length along the longitudinal direction. In a coating system where layer thicknesses can be applied in a radially geometric oscillating manner, the top units to be coated can be arranged on the coating holder according to the required gradients in period length, as shown in Figure 13schematically represented, where the circles concentric around the spin axis S of the coating holder are dashed lines of constant layer thickness.

[0044] In the Figure 14 In the example shown, each superunit B1 to B5 was partitioned as in the previous examples. However, the variation of the ideal desired period length across the area units for each superunit was approximated by a higher-degree polynomial. The resulting relative period length of the basis multilayer system for each superunit B1 to B5 is given in Figure 13 drawn as a thick solid line and labeled "spline".

[0045] Preferably, this approximation of the variation of the mean angles of incidence for each upper unit is not only performed in one direction along a linear gradient, but two-dimensionally over the entire surface of the respective upper unit. A corresponding reflective coating with two-dimensional local thickness variations can be produced, for example, using honeycomb masks. For more complex thickness distributions, a time-controlled method without a mask, as disclosed in DE 10 2012 205 615 A1, can also be used.

[0046] The more higher-degree coefficients are taken into account, the better the course of the ideal desired period length can be approximated. Figure 15The diagram schematically illustrates an example of a period length profile that matches the ideal period length. This profile is shown as a thick solid line and labeled "ideal." Such a reflective coating can be produced particularly well when the subunits of each superunit are individually actuated micromirrors, as is the case, for example, in microelectromechanical systems. Since the applied coating thickness also depends on the orientation of the surface to be coated relative to the particle source, each micromirror can be tilted for coating so that the actual deposited thickness corresponds to the target thickness. In particular, this allows the discontinuities in the ideal desired period length to be approximated across the surface of a superunit.

[0047] Based on the following Figures 16 to 19 The effect of the proposed approach will be illustrated. Figure 16The mean reflectivity is calculated over the entire field facet mirror for the embodiments presented here, according to the Figure 8 The values ​​shown are (mean), 10 (grad), 14 (spline), and 15 (ideal). The reflectivity is adjusted to the value of the conventional reflective optical element. Figure 7 normalized. A significant increase in reflectivity can be achieved simply by adjusting the reflective coating to the mean angle of incidence across each entire upper unit; with the linear approximation, the mean reflectivity can be increased by over 5%, with the quadratic approximation by about 12.5%, and by adjusting to the mean angle of incidence of each field facet by over 15%.

[0048] Furthermore, examples were also examined in which, additionally, in accordance with the claimed subject matter, the upper units were divided into two bandwidth classes and the upper units B1 and B5 were combined with the one in conjunction with Figure 9 The basic multilayer system "standard" was explained, and the upper units B2 to B4 were provided with the basic multilayer system "broadband". The corresponding values ​​are in Figure 16 with the suffix "+BB". Particularly in the "mean" and "grad" versions, taking into account the incidence angle bandwidths allows for an additional increase in overall reflectivity of several percent.

[0049] These increases in reflectivity can be observed to an even greater extent for the mean reflectivity at the extreme angles of incidence, i.e., the minimum and maximum angles of incidence (see Figure 17 ).

[0050] The mean peak valley values ​​for the mean ( Figure 18 ) as well as the extreme angles of incidence ( Figure 19 The peak valley value is considered, where the mean peak valley value is a measure of the deviation of the actual coating from the ideal reflective coating, which is optimized at each point on the surface for the respective angles of incidence occurring there. The peak valley values ​​are again normalized to the value for Example 7 according to the state of the art. The ideal coating can be closely approximated, especially for the extreme angles of incidence, using the procedure described here.

Claims

1. Method for producing a reflective optical element (30), which is composed of at least two main units (31), for the extreme ultraviolet wavelength range, wherein each main unit (31) has a multiplicity of sub-units (32), having the steps of: - determining angles of incidence and angle of incidence bandwidths occurring over the surface of each main unit (31) during operation, and also the maximum bandwidth of the angles of incidence; - dividing the main units (31) into bandwidth classes with maximum bandwidth determined for the respective bandwidth class; - applying a reflective coating (51) on each main unit (31), which is adapted to the angles of incidence and angle of incidence bandwidths which are respectively determined over the surface thereof, wherein a multilayer system (51) with period length is applied to all main units (31) as a reflective coating, the layer sequence and / or layer thicknesses of said multilayer system being adapted to the maximum bandwidth determined for the respective bandwidth class, wherein basic multilayer systems are selected differently for each bandwidth class and to the required bandwidth, and wherein the desired period length is optimized for each main unit (31) individually in dependence on the angles of incidence occurring during operation, by means of the period length of the basic multilayer system being modified by a factor for each main unit within a bandwidth class.

2. Method according to Claim 1, characterized in that each main unit (31) is divided into surface units, and for each main unit (31) the angles of incidence occurring during operation over all the surface units thereof are determined, in that a desired period length of a multilayer system (51) is ascertained in dependence on the determined angles of incidence, which multilayer system approximates the variation of the desired period length over the surface units for each main unit (31) by an nth-degree polynomial, with n being a non-negative integer, and in that the corresponding multilayer system (51) is applied to each main unit (31) as a reflective coating.

3. Method according to Claim 2, characterized in that the surface units correspond to the sub-units (32).

4. Method according to one of Claims 1 to 3, characterized in that a reflective coating (51) is applied to the main units (31) by locating them on a coating holder which rotates about an axis, wherein regions of constant layer thicknesses are arranged concentrically around the axis.

5. Reflective optical element for the extreme ultraviolet wavelength range, produced according to one of Claims 1 to 4, which comprises main units (31) from at least two bandwidth classes, wherein each bandwidth class comprises in each case at least two main units (31).

6. Reflective optical element according to Claim 5, characterized in that the reflective coatings are embodied as multilayer systems (51), the layer thicknesses of which vary with a function that corresponds in each case to an nth-degree polynomial, with n corresponding to a non-negative integer.

7. Reflective optical element according to Claim 5 or 6, characterized in that each sub-unit (32) is embodied as an individually actuable mirror.

8. Reflective optical element according to one of Claims 5 to 7, characterized in that it is embodied as a field facet mirror (16, 114).

9. Optical system, in particular for EUV lithography, having a reflective optical element (16) according to one of Claims 5 to 8.

10. EUV lithography apparatus having a reflective optical element (114) according to one of Claims 5 to 8.