Lens element of a microlithographic projection exposure apparatus designed to operate with DUV, and method and apparatus for forming an antireflection layer

Abstract

The present invention relates to a lens element of a microlithography projection exposure apparatus designed to operate with DUV, and a method and apparatus for forming an antireflection layer. According to one aspect, in the case of the lens element (100) according to the present invention, an antireflection layer (102, 302) is formed on the lens substrate, and the antireflection layer (102, 302) includes a first material having a relatively low refractive index and a second material having a relatively high refractive index, and the mixing ratio of the first material and the second material changes in the lateral direction and / or the longitudinal direction.

Description

Technical Field

[0001] This application claims the priority of German Patent Application No. 10 2022 207 068.2 filed on July 11, 2022. The content of this German application is incorporated herein by reference.

[0002] The present invention relates to a lens element of a microlithography projection exposure apparatus designed to operate in DUV, and a method and apparatus for forming an antireflection layer.

Background Art

[0003] Microlithography is used in the manufacture of microstructured electronic devices. The microlithography process is performed in a so-called projection exposure apparatus including an illumination device and a projection lens. By projecting an image of a mask (reticle) illuminated by the illumination device onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and disposed on the image plane of the projection lens by the projection lens, the mask structure is transferred to the photosensitive coating of the substrate.

[0004] In the case of an antireflection layer used for a lens element in the DUV region (i.e., for example, about 365 nm, about 248 nm, or about 193 nm), its realization poses a severe challenge considering the increasing requirements regarding the lithography system (e.g., regarding minimizing wavefront aberration while providing the best antireflection effect over a wide wavelength range). FIGS. 5a and 5b show schematic diagrams for explaining a conventional method of forming an antireflection layer on a lens element substrate 501, in which sublayers 502a, 502b of materials having different refractive indices from separate evaporation sources 511, 512 are alternately coated. The actual problem is that the increase in the number of sublayers in the layer configuration of the antireflection layer is limited due to the undesirable effects of an accompanying increase in thickness (in particular, an increase in radiation absorption and the resulting refractive index variation or wavefront aberration, and an increase in the deformation effect of the antireflection layer due to an increase in line tension).

[0005] Yet another problem that actually occurs in terms of manufacturing when forming an antireflection layer on a curved surface (e.g., a spherical surface) lens element is merely schematically shown in FIG. 6. During the formation of the antireflection layer 602 on the lens element substrate 601 (which rotates about the spin axis during the coating process as shown in FIG. 6), the material applied by the evaporation source 610 hits the lens element edge at a deposition angle that is considerably larger with respect to each surface normal than the center of the lens element. Thus, the formation of the antireflection layer at the lens element edge is carried out with a relatively high porosity. This situation further affects the profile of the layer thickness (which is usually set based on adding a desired mass in the coating process). In the absence of an aperture effect, the layer thickness at the lens element edge is smaller than that at the center of the lens element, but the porosity is high. Thus, the layer thickness does not become as small as geometrically expected because the layer thickness depends on the cosine of the deposition angle when the material density at the center and the edge is the same. This undesirable effect can be canceled by redistributing the material across the lens element surface using a shielding aperture as shown in FIGS. 7a - 7d, but the optical performance remains impaired, especially because the refractive index decreases towards the lens element edge due to the above-mentioned porosity. By increasing the layer thickness at the lens element edge compared to the center of the lens element, the values of reflection loss and polarization separation occurring at the lens element edge can be approximated to the respective values at the center of the lens element. However, since the refractive index at the lens element edge is lower, the optical performance of the antireflection layer at the center of the lens element cannot be fully achieved. For example, according to FIGS. 7a - 7d, a 7% thick antireflection layer having a more porous and thus lower refractive sublayer exhibits a higher transmission loss due to reflection and a higher polarization separation ability at a large incident angle compared to the antireflection layer at the center of the lens element.

[0006] For the state of the art, see, for example, Patent Document 1, Patent Document 2, Non-Patent Document 1, and Non-Patent Document 2.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Non-Patent Document

[0008]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] An object of the present invention is to provide a lens element of a microlithography projection exposure apparatus designed to operate in DUV, a method and an apparatus for forming an antireflection layer, while at least partially avoiding the above problems and enabling improvement in optical performance.

Means for Solving the Problems

[0010] This object is achieved by the features described in the independent patent claims.

[0011] According to one aspect, the present invention relates to a lens element of a microlithography projection exposure apparatus designed to operate with DUV, an antireflection layer is formed on a lens element substrate of the lens element, the antireflection layer has a first material with a relatively low refractive index and a second material with a relatively high refractive index, and relates to a lens element in which the mixing ratio of the first material and the second material changes in the lateral direction and / or the longitudinal direction.

[0012] The present invention is particularly based on the concept of designing an antireflection layer on a lens element intended for use in a microlithography projection exposure apparatus designed to operate with DUV, the antireflection layer being composed of a material with a relatively low refractive index and a material with a relatively high refractive index, such that the mixing ratio of these materials changes in the lateral direction and / or the longitudinal direction.

[0013] Here and in the following text, it is understood that the "lateral direction" means the direction parallel to the layer (i.e., along the surface of the antireflection layer), and the "longitudinal direction" means the direction perpendicular to the plane of the layer or the lateral plane (i.e., the direction of the stacking direction).

[0014] The present invention, firstly, includes embodiments in which the change in the mixing ratio is achieved by co-evaporation (simultaneous evaporation) from separate evaporation sources with different evaporation rates, and this change in the evaporation rate of each of the two materials is accompanied by a change in the lens element substrate area coated from the center of the lens element towards the edge of the lens element. According to this method, as will be explained in more detail below, the undesirable effect that the porosity of the antireflection layer becomes higher in relation to manufacturing towards the edge of the lens element can be compensated for in that the undesirable refractive index decrease near the edge of the lens element associated with the increase in porosity is offset by increasing the corresponding evaporation rate of the high refractive index material.

[0015] Furthermore, the present invention includes embodiments in which the above change in the mixing ratio is made in the vertical direction (i.e., perpendicular to the horizontal direction), achieving the object of obtaining the best antireflection effect over a relatively large incident angle range with a relatively small total thickness of the antireflection layer (thus avoiding the problems associated with the large total layer thickness described above). For this setting of the mixing ratio that changes in the vertical direction, furthermore, the present invention sets the opening periods of the shutters respectively assigned to the evaporation sources as intended while realizing the evaporation of each material at a constant evaporation rate, so that the change in the mixing ratio of the materials according to the present invention is achieved, thereby avoiding the difficulties arising in the variable closed-loop control of the evaporation rate.

[0016] In other words, according to the present invention, by using appropriately synchronized switching between the open and closed states of the shutters for each material (i.e., on the one hand, a high-refractive-index material and on the other hand, a low-refractive-index material), although the evaporation rate of each of the separate evaporation sources itself remains constant over time, an effective evaporation rate that changes with time or with the gradual formation of the antireflection layer is set.

[0017] In this case, it is also possible to advantageously combine the above two concepts, i.e., on the one hand, compensating for the effect that the porosity increases toward the lens element edge during the formation of the antireflection layer, and on the other hand, producing the best antireflection effect over a wide incident angle range with a relatively small total thickness of the antireflection layer. For example, in an antireflection layer designed for a wavelength of 365 nm, the refractive index decrease of the high-refractive-index Al2O3 sublayer from the value n0 = 1.72 at the center of the lens element to the value n1 = 1.52 at the lens element edge (see also FIG. 7a) can be compensated by the co-evaporation of HfO2 with a refractive index of 2.1, and thus the same optical performance as at the center of the lens element can be achieved (FIGS. 7b and 7c). Considering that the refractive index of HfO2 also decreases by 12% from the value 2.1 to the value n2 = 1.85 at the lens element edge in the same ratio as Al2O3 due to the increasing porosity toward the lens element edge, a mixing ratio of 60% HfO2 to 40% Al2O3 must be selected at the lens element edge (refractive index obtained according to the mixing ratio x = 0.6 (1 - x) * n1 + x *(due to n2 substantially corresponding to the value n0 = 1.72 of Al2O3 at the center of the lens element).

[0018] Accordingly, the present invention particularly relates to, on the one hand, embodiments in which the mixing ratio of the high-refractive-index material and the low-refractive-index material is changed in the lateral direction by compensating for the porosity that increases towards the edge of the lens element, so that substantially constant optical performance (particularly, a constant average refractive index) is achieved over the entire surface of the lens element from the center of the lens element to the edge of the lens element, and further, on the other hand, embodiments in which the mixing ratio of the high-refractive-index material and the low-refractive-index material is changed in the longitudinal direction (by using the above-described synchronous opening and closing of the shutters at a constant evaporation rate of separate evaporation sources), so that a good antireflection effect is achieved over a wide range of incident angles with a relatively small total thickness of the antireflection layer.

[0019] According to one embodiment, the lens element has at least one curved lens element surface. In particular, the lens element can also be a cylindrical lens element.

[0020] The present invention also relates to a microlithography projection exposure apparatus having at least one lens element having the above-described characteristics.

[0021] The present invention further relates to a method for forming an antireflection layer on a lens element substrate of a lens element of a microlithography projection exposure apparatus designed to operate in DUV, wherein the antireflection layer is formed from a first material having a relatively low refractive index and at least one second material having a relatively high refractive index, and relates to a method in which the mixing ratio of the first material and the second material changes in the lateral direction and / or the longitudinal direction.

[0022] According to one embodiment, the antireflection layer is formed using separate evaporation sources for the first material and the second material that supply each material through shutters that open intermittently.

[0023] According to one embodiment, the evaporation sources are operated at a constant evaporation rate, and the change in the mixing ratio is achieved by setting the opening period of the shutters as desired.

[0024] According to one embodiment, co-evaporation from an evaporation source is performed, and an increase in the evaporation rate of a second material compared to the evaporation rate of a first material is synchronized with a change in a region in each coating of a lens element substrate. With this synchronization, in particular, the evaporation rate of the second material can be increased from the central region of the lens element toward the edge region of the lens element compared to the evaporation rate of the first material.

[0025] According to one embodiment, a change in each coating region of a lens element substrate is performed by a change in the relative position of an aperture stop positioned with respect to a lens element and shielded by a diaphragm.

[0026] The present invention further relates to an apparatus for forming an antireflection layer on a lens element substrate of a lens element of a microlithography projection exposure apparatus designed to operate in DUV, comprising a first evaporation source for a first material having a relatively low refractive index and a second evaporation source for a second material having a relatively high refractive index, wherein separate shutters are respectively assigned to the first evaporation source and the second evaporation source, and an apparatus provided with a device for intermittently opening these shutters.

[0027] According to one embodiment, the apparatus further comprises a diaphragm and a device for changing the relative position of an aperture stop positioned with respect to a lens element and shielded by the diaphragm.

[0028] According to one embodiment, the apparatus is configured to change the position of an aperture stop with respect to a lens element in synchronization with an increase in the evaporation rate of a second material compared to the evaporation rate of a first material.

[0029] Still other embodiments of the present invention will be apparent from the description and the dependent claims.

[0030] The present invention will be described in more detail below with reference to exemplary embodiments shown in the accompanying drawings.

Brief Description of the Drawings

[0031]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7a

Figure 7b

Figure 7c

Figure 7d

Figure 8a

Figure 8b

DETAILED DESCRIPTION OF THE INVENTION

[0032] In the following text, various embodiments regarding the implementation of an antireflection layer on a lens element substrate of a lens element designed to operate with DUV in a microlithographic projection exposure apparatus will be described. These embodiments are common in that the mixing ratio of a first material with a relatively low refractive index and a second material with a relatively high refractive index changes, and this change occurs in the lateral and / or longitudinal directions according to exemplary embodiments during the formation of the antireflection layer.

[0033] FIGS. 1 and 2 first show schematic diagrams for explaining a method of forming an antireflection layer on a lens element substrate indicated by "101", and a first evaporation source 111 having a first material with a relatively low refractive index and a second evaporation source 112 having a second material with a relatively high refractive index are used. The formation of the antireflection layer indicated by "102" is carried out by forming a changing mixing ratio in the lateral direction. Materials with a relatively low refractive index are MgF2, AlF3, SiO2, thiolite (Na5Al3F 14 ) or cryolite (Na3AlF6). Materials with a relatively high refractive index that can be used are, depending on the wavelength of use of the optical system or projection exposure apparatus, LaF3, or oxide materials such as Al2O3, HfO2, TiO2, or ZrO2. Preferably, a high refractive index fluoride material should be mixed with a low refractive index fluoride material, and a high refractive index oxide material should be mixed with a low refractive index oxide material.

[0034] The formation of a changing mixing ratio in the lateral direction is first carried out by shifting the shielding aperture 120 during the coating process (the lens element substrate 101 rotates about the spin axis as shown in FIG. 2) so that the relative position of the aperture diaphragm 120a positioned with respect to the shielding aperture 120 with respect to the lens element substrate 101 changes. At the same time (i.e., synchronized with the above change in relative position and the change in the region during each coating from the center of the lens element to the edge of the lens element), the evaporation rates of the first and second materials change. In an embodiment, the lens element 100 including the lens element substrate 101 and the antireflection layer 102 can also be a cylindrical lens element (extending in the plane of the drawing from FIG. 2), in which case the aforementioned rotation of the lens element substrate 101 is omitted.

[0035] Typical evaporation rates in suitable thermal evaporation processes such as electron beam evaporation and heating boat evaporation are in the range of 0.05 nm / s to 2 nm / s, preferably in the range of 0.2 nm / s to 0.5 nm / s. For wavelengths in the range of 193 nm to 365 nm, the single layer thickness of the antireflection coating is in the range of 2 nm to 200 nm, preferably in the range of 30 nm to 60 nm. This means that the normal coating time ranges from 60 seconds to 300 seconds, and in the case of an aperture stop of 120a with a diameter of 10 mm, the shift at the center of the lens element is performed at a rate of 10 mm per minute to 2 mm per minute, and correspondingly, it must become slower toward the edge of the lens element.

[0036] Assuming that the evaporation rate of the first material is represented by α, the refractive index of the first material is represented by n1, the evaporation rate of the second material is represented by β, and the refractive index of the second material is represented by n2, the following applies to the refractive index of the resulting antireflection layer.

[0037]

Equation

[0038] The changes in the evaporation rates α and β are made such that the evaporation rate of the second material with a relatively high refractive index increases toward the edge of the lens element with respect to the evaporation rate of the low refractive index material. As a result, the undesirable effect described above, that the porosity increases as the deposition angle increases toward the edge of the lens element, can be compensated for with respect to the resulting average refractive index of the lens element and thus the optical performance.

[0039] At the same time, as a result of the porosity increasing towards the lens element edge, the desired target mass for the end of the coating process is usually only achieved after a delay, and thus only after the thickness has increased. This causes undesirable thickness variations during the formation of the antireflection layer, which can be reduced. When the high refractive index material used according to the present invention is selected to be denser than the low refractive index material, the target mass suitable for the end of the sealing process is also achieved earlier towards the lens element edge accordingly, so that the above-mentioned thickness profile is compensated for.

[0040] As shown in FIGS. 8a to 8c, FIG. 3 shows a schematic diagram for explaining still another exemplary embodiment in which, for the purpose of achieving a maximum antireflection effect over a wide range of incident angles with a relatively small total layer thickness of the antireflection layer, the change in the mixing ratio according to the present invention is performed in the vertical direction here. In the case of a microlithography projection exposure apparatus, the normal range of incident angles can extend from 0° to 60°, and the reflectance should not exceed 0.1% up to an incident angle of 30°. In order to avoid lens element heating due to layer absorption and lens element deformation and crack formation in the layer due to layer stress, the total layer thickness should be less than 200 nm, preferably less than 100 nm.

[0041] According to FIG. 3, the intermittent shielding of each evaporation source (indicated by "311" in FIG. 3) is performed to change the mixing ratio, and each opening period of the shutter used for this purpose can be set as desired. As a result, even when the evaporation source 311 is operated at a constant evaporation rate, the "effective rate" at which the material is supplied to the lens element substrate 301 for the formation of the antireflection layer 302 changes.

[0042] When the shutter opens for a period t1 and closes for a period t2, the following effective rate is obtained at a constant evaporation rate β of the evaporation source 311.

[0043]

Equation

[0044] By appropriately setting the ratio of the exposure period t1 (during which vapor deposition is performed on the lens element substrate 301) to the shielding period t2, it is possible to set an arbitrary effective speed from 0 to a certain value β according to the speed of the evaporation source 311. In this case, since the switching of the shutter opening and closing is executed very quickly, it is preferable that about one atomic monolayer of the material is applied in one cycle. In this case, a substantially continuous gradient mixed layer can be produced from different materials. At a normal evaporation rate of 0.05 nm per second to 0.2 nm per second, the switching of the shutter opening and closing must be performed every 1 second to 4 seconds.

[0045] Using separate evaporation sources at a constant evaporation rate respectively is advantageous because it can avoid the difficulties usually associated with continuous closed-loop evaporation rate control.

[0046] As described above, even in the case of a lens element that may have a strong curvature, in order to achieve substantially constant optical performance across the lens element surface from the center of the lens element to the edge of the lens element (as a "lateral" result), and also to achieve a good antireflection effect over a wide incident angle range with a relatively small total thickness of the antireflection layer (as a "longitudinal" result), the lateral (from the center of the lens element to the edge of the lens element) change in the mixing ratio of the high refractive index material and the low refractive index material according to the present invention can be combined with the longitudinal change in the mixing ratio of the high refractive index material and the low refractive index material.

[0047] Here, starting from the embodiment described with reference to FIG. 2, as described above with reference to FIG. 3, it is possible to produce a gradient mixed layer (see FIG. 8a) of different materials with a changing mixing ratio in the longitudinal direction while the shielding aperture 120 is temporarily fixed or the position of the aperture stop 120a positioned in this shielding aperture 120 is temporarily in a constant state.

[0048] FIG. 4 shows the configuration of a microlithography projection exposure apparatus 400 designed to operate in the DUV that is theoretically possible.

[0049] The projection exposure apparatus 400 shown in FIG. 4 includes an illumination device 410 and a projection lens 420. The illumination device 410 is used to illuminate a structure-carrying mask (reticle) 415 with light from a light source unit 405. The light source unit 405 includes, for example, a laser light source in the form of an ArF excimer laser for a use wavelength of 193 nm (or in the form of a KrF excimer laser for a use wavelength of 248 nm or a mercury lamp for a use wavelength of 365 nm), and a beam shaping optical unit that generates a parallel light beam. The laser light source can be designed according to the present invention.

[0050] In the illustrated example, the illumination device 410 has an optical unit 411 that particularly includes a deflection mirror 412. The optical unit 411 includes, for example, a diffractive optical element (DOE) and a zoom axicon system in order to generate different illumination settings (i.e., the intensity distribution on the pupil plane of the illumination device 410). In the beam path downstream of the optical unit 411 in the light propagation direction, there are arranged an optical mixing device (not shown) that may have, for example, in a known manner, an arrangement of micro-optical elements suitable for achieving optical mixing, and a lens element group 413. Downstream of that, a field plane including a reticle masking system (REMA) is positioned. The field plane is imaged by a REMA lens 414 following in the light propagation direction onto a structure-carrying mask (reticle) 415 arranged on another field plane, thus defining an illumination region on the reticle. The structure-carrying mask 415 is imaged onto a lens element substrate or wafer 430 provided with a photosensitive layer (photoresist) using the projection lens 420. The projection lens 420 may be particularly designed for immersion operation. In that case, the immersion medium is positioned upstream of the wafer or its photosensitive layer with respect to the light propagation direction. Further, the projection lens may have, for example, a numerical aperture NA greater than 0.85, particularly greater than 1.1.

[0051] Although the present invention has been described by specific embodiments, those skilled in the art can recognize a plurality of variations and alternative embodiments, for example, by combining and / or exchanging the features of individual embodiments. Therefore, such variations and alternative embodiments are also included in the present invention, and it is understood by those skilled in the art that the scope of the present invention is limited only within the meaning of the appended claims and their equivalents.

Claims

**Claim 1** A lens element of a microlithography projection exposure apparatus designed to operate with DUV, an antireflection layer (102, 302) is formed on a lens element substrate (101, 301) of the lens element (100), the antireflection layer (102, 302) has a first material with a relatively low refractive index and a second material with a relatively high refractive index, and a lens element in which the mixing ratio of the first material and the second material changes in the lateral direction and / or the longitudinal direction. **Claim 2** The lens element according to claim 1, characterized in that it has at least one curved lens element surface. **Claim 3** The lens element according to claim 1 or 2, characterized in that the mixing ratio of the first material and the second material changes in both the lateral direction and the longitudinal direction. **Claim 4** A microlithography projection exposure apparatus comprising at least one lens element according to any one of claims 1 to 3. **Claim 5** A method of forming an antireflection layer (102, 302) on a lens element substrate (101, 301) of a lens element of a microlithography projection exposure apparatus (400) designed to operate with DUV, the antireflection layer (102, 302) is formed from a first material with a relatively low refractive index and at least one second material with a relatively high refractive index, a method in which the mixing ratio of the first material and the second material changes in the lateral direction and / or the longitudinal direction. **Claim 6** The method according to claim 5, characterized in that the lens element (100) has at least one curved lens element surface. **Claim 7** The method according to claim 5 or 6, characterized in that the antireflection layer (102, 302) is formed using separate evaporation sources for the first material and the second material, and the materials from the evaporation sources are respectively supplied through shutters that open intermittently. **Claim 8** The method according to claim 7, characterized in that the evaporation sources are operated at a constant evaporation rate, and the change in the mixing ratio is achieved by setting the respective opening periods of the shutters assigned to the evaporation sources as desired. **Claim 9** In the method according to claim 7, co-evaporation from the evaporation sources (111, 112) is carried out, and an increase in the evaporation rate of the second material compared to the evaporation rate of the first material is synchronized with a change in the area during each coating of the lens element substrate (101, 301).

10. In the method according to claim 9, in the synchronization, the evaporation rate of the second material rises from the central region of the lens element (100) towards the edge region of the lens element (100) compared to the evaporation rate of the first material.

11. In the method according to claim 9 or 10, the change in the area during the coating of the lens element substrate (101, 301) is effected by a change in the relative position of an aperture stop (120a) positioned with respect to a shielding stop (120) for the lens element (100).

12. An apparatus for forming an antireflection layer (102, 302) on a lens element substrate (101, 301) of a lens element of a microlithographic projection exposure apparatus (400) designed to operate with DUV, comprising a first evaporation source for a first material having a relatively low refractive index and a second evaporation source for a second material having a relatively high refractive index, wherein separate shutters are assigned to the first evaporation source and the second evaporation source respectively, and an apparatus provided with a device for intermittently opening the shutters.

13. The apparatus according to claim 12, further comprising a shielding stop (120) and a device for changing the relative position of an aperture stop (120a) positioned with respect to the shielding stop (120) for the lens element (100).

14. The apparatus according to claim 13, configured to change the position of the aperture stop (120a) with respect to the lens element (100) in synchronization with an increase in the evaporation rate of the second material compared to the evaporation rate of the first material.

Images