Optical element, optical system, and method for manufacturing an optical element

Abstract

The invention relates to an optical element, in particular a lens (1) or a mirror, preferably for a semiconductor technology installation, in particular for an illumination system or for a projection lens of a projection exposure installation, comprising: a substrate (2) and a coating (3) which is applied to a surface (2a) of the substrate (2). The coating (3) has at least one refractive layer (4) with a thickness (d(x,y)) that varies depending on location, wherein a minimum thickness (dMin) of the refractive layer (4) is at least 5 λB, preferably at least 10 λB, particularly preferably at least 15 λB, and wherein a difference (dMax – dMin) between a maximum thickness (dMax) and the minimum thickness (dMin) of the refractive layer (4) is greater than λB, preferably greater than 2 λB, in particular greater than 3 λB, wherein λB denotes the useful wavelength of the optical element (1). The invention also relates to an optical system, preferably a semiconductor technology installation, comprising: a beam source for providing useful radiation at a useful wavelength (λB), preferably in the DUV wavelength range, in particular at 468 nm, 365 nm, 248 nm, 193 nm or 157 nm, and at least one optical element (1), which is designed as described above.

Description

[0001] Optical element, optical system and method for manufacturing an optical element

[0002] Reference to related registration

[0003] This application claims priority over German patent application DE102024204450.4 dated May 14, 2024, the entire disclosure of which is incorporated herein by reference.

[0004] Background of the invention

[0005] The invention relates to an optical element, in particular a lens or a mirror, preferably for a semiconductor technology system, especially for a lighting system or for a projection lens of a projection exposure system, comprising: a substrate and a coating applied to a surface of the substrate. The invention also relates to an optical system, preferably a semiconductor technology system, in particular a lighting system or a projection lens of a projection exposure system, and a method for manufacturing an optical element.

[0006] In addition to the rapid development of semiconductor technology systems in the form of projection exposure systems for the EUV wavelength range, projection exposure systems for the DUV wavelength range, i.e. for useful wavelengths between 150 nm and 470 nm, for example at a useful wavelength of 436 nm (Hg g-line), 365 nm (i-line), 248 nm (KrF laser) or 193 nm (ArF laser), are also continuously increasing their throughput and thus their economic benefit in the production of non-critical semiconductor layers, which make up a large proportion in the production of electronic semiconductor components.

[0007] In the production of high numerical aperture lenses for EUV lithography, processes have been developed that enable the sputtering of layers with highly variable thickness profiles. If the thickness profile exhibits a practically arbitrarily variable spatial dependence, such a layer is also referred to as a freeform layer. German patent DE102012215359A1 proposes using apertures or shielding elements for the production of freeform layers. German patent DE102012205615A1 proposes using an electric or magnetic deflection field to deflect ions during the coating process.

[0008] The layers described above can be used to provide a particularly high reflectivity of the optical element, in this case a reflective optical element such as a mirror, by means of optical interference. Such layers can also be used to provide a particularly low reflectivity of the optical element, in this case a transmissive optical element such as a lens. Another application of such layers is the protection of the optical element, for example the substrate or underlying layers, to extend the service life of the optical element.

[0009] Patent W02009 / 024164A1 describes the use of a mirror element as a correction element. The mirror element can have a non-rotationally symmetric coating and / or a non-rotationally symmetric cover layer. German patent DE102007058862A1 describes an optical system with an optical element that has a layer system embedded in the optical element. Besides the use of layers or coatings on optical elements, it is also known to utilize the optical properties of liquids. US patent US2006 / 0066962A1 describes an arrangement of optical elements comprising two rigid optical elements and a liquid that at least partially fills a space between the two optical elements. US patent US2008 / 0030869A1 describes a projection lens for microlithography with at least one liquid lens formed from a transparent liquid.US2006 / 0012885A1 describes a projection lens which may be designed as an immersion lens in which a working distance between a final optical element and the image plane is filled with an immersion medium having a refractive index greater than one.

[0010] Object of the invention

[0011] The object of the invention is to provide an optical element, an optical system and a method for manufacturing an optical element, which open up further application possibilities for coatings.

[0012] Subject matter of the invention

[0013] This problem is solved by an optical element of the type mentioned above, in which the coating comprises at least one refractive layer with a spatially varying thickness, wherein a minimum thickness of the refractive layer is at least 5 AB, preferably at least 10 AB, and particularly preferably at least 15 AB, and wherein a difference between a maximum thickness and the minimum thickness of the refractive layer is greater than AB, preferably greater than 2 AB, and particularly greater than 3 AB, where AB denotes the useful wavelength of the optical element. The refractive layer is typically transparent at the useful wavelength of the optical element. The optical element according to the invention comprises at least one refractive layer that is used in a conventional optical manner, i.e., without interference, and assumes the role of an optical element.The refractive layer is relatively thick compared to the useful wavelength in order to minimize interference effects. The refractive layer also exhibits a relatively large variation in thickness, which can be used to create a lens effect and / or to individually optimize the fit or surface shape of the optical element. Such individual optimization is particularly possible if the refractive layer has a non-rotationally symmetric thickness, i.e., if the refractive layer is a freeform layer (su).

[0014] The refractive layer described here typically reduces the residual reflectivity at a refractively used surface of a substrate of a transmissive optical element by less than 50%, preferably by less than 10%, compared to the residual reflectivity without the refractive layer. The optical element, more precisely the surface through which the radiation is to pass, exhibits a first residual reflectivity Ri in its completely uncoated state. In a state where only the refractive layer is applied to the surface, this layer, or the optical element, exhibits a second residual reflectivity R2. The ratio of the second residual reflectivity R2 to the first residual reflectivity Ri is: R2 / Ri > 0.5, preferably > 0.9.In contrast, an interference layer applied to the surface to reduce residual reflectivity—without the refractive layer and without any other layers—generates a third residual reflectivity R3, where typically R3 / Ri < 0.5. For example, a quartz-air junction on a lens with an uncoated surface has a residual reflectivity Ri of approximately 4%. An interference layer in the form of an anti-reflective coating regularly reduces this residual reflectivity Ri to a residual reflectivity R3 below 1%, which corresponds to a (relative) change of 75% or a ratio R3 / Ri = 0.25. The refractive layer, by comparison, has a relatively small influence on the residual reflectivity of the transmissive optical element. Therefore, it is advantageous to apply at least one interference layer to the refractive layer to achieve anti-reflective properties (su).

[0015] On a reflectively used surface of a substrate of a reflective optical element, e.g., a mirror, the refractive layer typically increases the reflectivity by less than 3%, usually by less than 1% compared to the reflectivity without the refractive layer. The refractive layer also has a small effect on the reflectivity of a reflectively used surface of a reflective optical element, e.g., a mirror. Due to the difference in the relative effectiveness of an interference layer that reduces reflections on a refracting surface and an interference layer that increases the reflectivity on a reflective surface, different ratios apply to the three reflectivities Ri, R2, and R3: The ratio of the first reflectivity Ri (i.e., without a refractive layer) to the second reflectivity R2 (i.e., with a refractive layer) is typically: R2 / Ri < 1.03, preferably < 1.01 (so).In contrast, the ratio between the third reflectivity R3, i.e. with an interference layer increasing the reflectivity, and the first reflectivity Ri is R3 / Ri > 1.05, usually R3 / Ri > 1.1.

[0016] In one embodiment, the usable wavelength AB of the optical element lies in the DUV wavelength range, preferably at 436 nm, 365 nm, 248 nm, 193 nm, or 157 nm. Freeform coating technology, as used for EUV optics, enables the application of layers with a comparatively large thickness and a spatially varying thickness profile with high precision, thus opening up new application possibilities for layers or coatings in the DUV wavelength range. To generate the refractive effect, applying a refractive layer to a substrate may be more advantageous than removing material from the substrate, since the latter requires extensive polishing to achieve the desired roughness values, whereas the coating—depending on the chosen coating process—can be applied without any post-processing.

[0017] In this embodiment, the optical element is designed for the DUV wavelength range and is configured to transmit or reflect radiation at the useful wavelength AB. The following values ​​for the thickness or thickness profile of the refractive layer typically refer to optical elements for the DUV wavelength range.

[0018] In one embodiment, the minimum thickness of the refractive layer is at least 3 pm, preferably at least 5 pm, particularly preferably at least 10 pm, and especially at least 20 pm. In principle, a large thickness of the refractive layer is desirable for the refractive effect; however, real-world process limitations such as manufacturing time and layer stresses restrict this approach.

[0019] In another embodiment, the refractive index of the substrate material differs from the refractive index of the refractive layer material by more than 0.1, preferably by more than 0.2. Since the thickness of the refractive layer and its spatially dependent thickness variation are comparatively small compared to a conventional lens, a relatively large difference between the refractive index of the refractive layer material and the refractive index of the substrate is advantageous for the refractive effect of the layer. If the refractive effect of the refractive layer is caused solely by the spatially dependent variation of the surface of the refractive layer, as can be the case, for example, with a mirror (su), the refractive index of the refractive layer material may not be relevant.

[0020] In another embodiment, the difference between the maximum and minimum thickness of the refractive layer is greater than 500 nm, preferably greater than 1 pm, and particularly greater than 2 pm. A comparatively large thickness variation of the refractive layer is advantageous in order to achieve the desired refractive effect. In particular, the difference between the thickness of the refractive layer at the center of an optically used area of ​​the optical element and the thickness of the refractive layer at the lateral edge of the refractive layer can be greater than 500 nm, preferably greater than 1 pm, and particularly greater than 2 pm. The optical axis of the optical element, or the beam path of the useful radiation, typically runs through the center of the optically used area of ​​the optical element.

[0021] Preferably, the difference between the maximum thickness and the minimum thickness of the refractive layer is between 10% and 50% of the minimum thickness of the refractive layer, i.e., 0.1 < (dMax - dMin) / dMin < 0.5, where dMax denotes the maximum thickness and dMin the minimum thickness of the refractive layer.

[0022] In another embodiment, the refractive layer has its minimum thickness at the center of an optically effective area of ​​the optical element and its maximum thickness at the lateral edge of the refractive layer, or vice versa. For an optical element in the form of a lens, the center of the optically effective area lies on the optical axis, and the edge of the refractive layer typically extends to the lateral edge of the lens. In this case, the refractive layer exhibits a pronounced thickness gradient or variation between the center and the edge of the optically effective area and can, for example, act like a diverging or converging lens. The thickness of the refractive layer can vary rotationally symmetrically around the center of the optically effective area, but this is not mandatory.In the case that the optical element is a mirror, the center of the optically effective area is located in the center of the useful beam incident on the mirror.

[0023] One application of such an optical element is to replicate the optical effect of a cemented lens. In a cemented lens, two lenses made of different materials are joined together with a transparent cement to form a single optical component. This is often used for the chromatic correction of an optical system. However, over years of operation, the cement, which is initially transparent at the operating wavelength, can degrade and absorb light, change its refractive power contribution, and / or reduce its bond strength. This has negative consequences for the imaging performance of the optical system, either through light loss, which may vary depending on the application and across the field and is therefore difficult to compensate for, or through aberrations caused either by changes in the refractive power of the cement itself and / or by a relative positional drift of the bonded lenses.

[0024] If a relatively thick refractive layer with a strong lateral gradient in thickness is applied to the surface of the substrate, e.g., a lens substrate, where the minimum layer thickness is, for example, 10 pm and the difference in layer thickness between the center and the edge of the optically used area or the coating is 1 pm, this refractive layer has a refractive index that differs from that of the substrate and also contributes its own, albeit small, refractive power in this example. By using suitable materials or, if necessary, an adhesion promoter layer, it can be ensured that no lifetime degradation of the optical element occurs that is comparable to that seen with a cemented element. The optical effect of the cemented element can be achieved in the optical element described here without the use of cement, while the manufacturing costs can be significantly lower than those of a cemented element.

[0025] Even if the optical effect of the refractive layer is potentially less than that of a cemented element, a new application opens up, since the refractive layer does not aim for path differences smaller than the optical wavelength with mutual interference of partial waves, but rather a classic spatially variable refractive power achieved through the lateral thickness gradients of the layer. The refractive layer thus differs from the conventional application of coatings.

[0026] Applying a refractive layer to both sides of a transmitting optical element enhances its refractive power. Alternatively, multiple refractive layers can be applied on top of each other to achieve a greater overall refractive power. As described above, real-world process limitations such as manufacturing time and layer stresses restrict this approach.

[0027] In another embodiment, the thickness of the refractive layer does not vary rotationally symmetrically about the center of an optically used area of ​​the optical element. In this case, the refractive layer is typically a freeform layer. This can be used, for example, to individually improve the properties of the optical element, such as the pass, as described in more detail below.

[0028] In another embodiment, the refractive layer is doped with foreign atoms, particularly ionized foreign atoms, at least in a partial volume. These foreign atoms are preferably selected from the group consisting of silicon, nitrogen, and hydrogen. Doping makes it possible to change the refractive index of the refractive layer. This can be advantageous for increasing the difference in refractive index compared to the substrate. Doping can also be used when several refractive layers are deposited on top of each other on the substrate. For example, in this case, a first refractive layer made of a first material can first be deposited on the substrate and then its refractive index modified by doping. A second refractive layer of the same material can then be deposited, either differently or without doping, and thus its refractive index differs from that of the first refractive layer.Conversely, it is also possible not to dope the first refractive layer, but to dope one or all subsequent refractive layers. It goes without saying that the refractive layer as a whole can be doped with foreign atoms or with ionized foreign atoms.

[0029] Two or more refractive layers made of different materials can be applied to the substrate. The advantage of such an arrangement is that the refractive indices of the successive refractive layers, while different, are sufficiently similar to minimize reflection at any of the internal material boundaries or interfaces. For example, the individual refractive index differences between the refractive layers can be less than 0.2, or preferably less than 0.1. By using several superimposed refractive layers, a significant refractive index difference between the substrate and the surroundings can still be achieved. In this case, the optical effect of the entire set of refractive layers approaches that of a gradient-index medium (GRIN medium) with a continuous refractive index profile.

[0030] In another embodiment, the substrate is formed from amorphous or crystalline quartz material, and the refractive layer is formed from a porous quartz material with a density that is at least 0.1% lower than the density of the quartz material of the substrate. In this embodiment, a porous SiO2 layer is vapor-deposited onto the quartz substrate to generate the desired refractive index difference of more than 0.1 compared to the substrate.

[0031] In another embodiment, the refractive layer comprises at least one material selected from the group consisting of: Si2, Ti2, Al2O3, HfO2, MgF2, AIF3, NaF, LaFs, amorphous silicon, or a lithium compound. Si2 is suitable as a refractive layer—even without density changes—if the substrate material is not also quartz or fused silica, but rather a high- or low-refractive-index optical glass, for example, FK5 glass or LLF1 glass from Schott. With such substrates, the desired refractive index difference of more than 0.1 can be achieved with a refractive layer made of Si2.

[0032] In principle, the material of the refractive layer in a transmissive optical element is chosen depending on the optical element's useful wavelength and the minimum thickness of the refractive layer: With a refractive layer thickness of approximately 1 pm or more, fluoride layers and SiO2 are sufficiently transparent for a useful wavelength of 193 nm, while HfO2 and TiO2 are only sufficiently transparent at a useful wavelength of 365 nm. AlO2O3 is sufficiently transparent at both 248 nm and 365 nm with a layer thickness > 1 pm.

[0033] In another embodiment, the substrate is made of glass, preferably fused silica, crystalline quartz, or calcium fluoride. The glass need not necessarily be fused silica; rather, another optical glass can be used, for example, a high- or low-refractive-index glass, e.g., Schott FK5 or LLF1. In a further embodiment, the refractive layer has a maximum lateral gradient of thickness of at least 0.01% / mm, preferably at least 0.025% / mm, particularly preferably at least 0.05% / mm, and especially at least 0.1% / mm. The percentage of the lateral gradient refers to the minimum thickness of the refractive layer. The maximum lateral gradient of the refractive layer thickness is the lateral gradient measured between the location of maximum thickness and the location of minimum thickness of the refractive layer.The lateral gradient is calculated by taking into account the distance between these two locations in the lateral direction.

[0034] In a further embodiment, the coating comprises at least one interference layer, which preferably has a thickness of less than 50%, more preferably less than 20%, and particularly less than 10% of the minimum thickness of the refractive layer. The refractive, classically optically active layer can be combined in various ways with interference layers known per se. The interference layer(s) can be used, for example, to enhance the reflectivity of an optical element in the form of a mirror, or, in the case of an optical element in the form of a lens, to reduce the reflectivity of the optical element, i.e., as anti-reflective layers.

[0035] In a further development of this embodiment, the coating has at least one first interference layer on a side of the refractive layer facing away from the substrate and / or at least one second interference layer between the refractive layer and the substrate. In the case of a reflective optical element, the first interference layer typically serves to enhance reflectivity, and in the case of a transmissive optical element, it typically serves as an anti-reflective layer or for anti-reflection (so). The second interference layer, which is arranged between the refractive layer and the substrate, serves for anti-reflection at the interface between the refractive layer and the substrate and is generally only required in the case of a transmissive optical element. The second interference layer is particularly advantageous if the refractive index of the refractive layer differs significantly from the refractive index of the substrate.The second interference layer can, for example, be a gradient-index layer whose refractive index varies in the thickness direction of the gradient-index layer. The refractive index of the gradient-index layer typically matches the refractive index of the substrate on the side facing the substrate. On the side facing the refractive layer, the refractive index typically matches the refractive index of the refractive layer. Between these two sides, the refractive index of the gradient-index layer typically varies continuously.

[0036] A further aspect of the invention relates to an optical system, preferably a semiconductor technology system, in particular an illumination system or a projection lens for microlithography, comprising: a beam source for providing useful radiation at a useful wavelength, preferably in the DUV wavelength range, in particular at 468 nm, 365 nm, 248 nm, 193 nm or 157 nm, and at least one optical element configured as described above. The semiconductor lithography system may be a projection exposure system for exposing a wafer or a part of such a projection exposure system, for example, an illumination system or a projection lens. The semiconductor lithography system may also be an inspection system, e.g., for inspecting masks, wafers or the like used in semiconductor lithography.The invention also relates to a method for manufacturing an optical element, configured as described above, comprising: applying the refractive layer to the substrate, preferably by dipping, PECVD, sputtering, electron beam evaporation, or pulsed laser deposition. The refractive layer can be applied directly to the substrate. However, it is also possible for one or more intermediate layers to be present between the refractive layer and the substrate, e.g., in the form of adhesion promoter layers or the like. As described above, a sequence of two or more refractive layers can also be applied to the substrate.

[0037] The refractive layer can be applied to the substrate in various ways. For example, it can be created by dip coating. Dip coating is suitable for producing refractive layers with a thickness between approximately 5 pm and 200 pm. To achieve varying thicknesses depending on the location or for smoothing, the refractive layer can be finished after application by polishing or ion beam figuring. A layer with a thickness on the order of approximately 0.5 pm to 20 pm can be applied using a PECVD (plasma-enhanced chemical vapor deposition) process. For example, a refractive layer of Si2 can be applied using an HMDSO (hexamethyldisiloxane) / O2 process, or a refractive layer of Al2O3 can be applied using a TMA (trimethylaluminium) / O2 process.In this case too, smoothing can be achieved by subsequent pass reworking or by polishing and / or ion blasting.

[0038] Magnetron sputtering processes enable the deposition of a refractive

[0039] A layer with a thickness on the order of approximately 0.1 pm to approximately 20 m. For example, SiCh can be deposited using reactive Si sputtering, and similarly TiCh, Al₂O₃, HfCh, etc. Regarding the thickness profile of the refractive layer, a maximum lateral gradient on the order of 0.2% / mm can be achieved using apertures, a suitably chosen path profile, and rate modulation (e.g., a 1 pm thickness difference between the center and the edge of an optical element in the form of a lens with a diameter of 100 mm and a center thickness of 10 pm). Using honeycomb masks, gradients on the order of 1% / mm or higher can potentially be achieved.

[0040] Electron beam evaporation enables the deposition of refractive layers with thicknesses ranging from approximately 0.1 pm to approximately 10 pm. In addition to oxide layers, fluoride layers such as MgF₂, AIF₃, NaF, or LaF₆ can also be deposited by electron beam evaporation. Here, too, radially symmetrical thickness gradients on the order of approximately 0.1% / mm can be achieved using aperture technology. Honeycomb masks are generally required for the fabrication of freeform layers. The refractive layer can also be applied to the substrate using other methods, such as pulsed laser deposition.

[0041] In one variant, after the refractive layer has been applied, it undergoes a shape-modifying treatment, preferably by a subtractive process, in particular by ion beam treatment, or by a compacting process, in particular by electron bombardment. As described above, depending on the application method used, e.g., in dip coating, this can smooth the refractive layer and, if necessary, introduce a lateral thickness gradient.In another variant, the procedure includes: measuring the optical element or another optical element to determine aberrations that are generated by a deviation of at least one optical property of the optical element or the other optical element from a target specification, defining a deviation of the location-dependent thickness of the refractive layer from a design target specification such that the aberrations are at least partially compensated by the deviation, and applying the refractive layer with the deviation of the location-dependent thickness from the design target specification.

[0042] In this variant of the method, the optical element, or another optical element used in the same optical system, is measured with respect to at least one optical property, for example, its passivity and / or the homogeneity of the refractive index of the substrate material during the transmission of useful radiation. Based on the optical aberrations caused by deviations of the passivity and / or transmission from the target value, an objective function can be defined, and a thickness profile individually tailored to the optical element can be determined and implemented. The deviation of the location-dependent thickness profile from the design target allows for at least partial compensation of the aberrations or image errors caused by the deviation of the optical property from the target value; that is, the objective function determined based on the aberrations is improved.

[0043] If the measured optical element matches the optical element onto which the refractive layer is applied, individual element measurement can, for example, compensate for deviations of the pass from an optimal target value. Alternatively, data obtained from measurements of the other optical element and / or a measurement of the optical system regarding deviations of the optical path length from a target value can be compensated for by an individually determined pass or thickness profile of the refractive layer; that is, an individual freeform layer profile of the refractive layer serves to correct the individual element or the optical system (su).

[0044] An alternative approach involves: measuring an optical system in which the optical element or an identical optical element is located to determine aberrations of the optical system; defining a deviation of the spatially dependent thickness of the refractive layer from a design target such that the aberrations are at least partially compensated by the deviation; and applying the refractive layer with this deviation of the spatially dependent thickness from the design target to the substrate of the optical element. In this case as well, an objective function can be determined based on the aberrations of the optical element, and an individual thickness profile of the refractive layer can be defined and implemented such that the deviation of the spatially dependent thickness of the refractive layer from the design target compensates for aberrations of the optical element in such a way that the objective function is improved.

[0045] In the variant described here, the optical element—without the refractive layer—is measured within the optical system. After measurement, it is removed from the optical system to apply the refractive coating. Following application of the refractive layer, the optical element is reinserted into the optical system. Since the optical design effect of the refractive layer—applied later—is not considered during the optical measurement in this case, it is necessary to account for this effect computationally during the measurement and / or to use dedicated compensation optics that largely replicate the optical effect of the refractive layer and are removed from the optical system before or during commissioning.

[0046] Alternatively, in the variant described above, the optical elements can be physically different from each other during the measurement and operation of the optical system. In this case, an optical element identical in construction to the optical element onto which the refractive coating is applied is used for the measurement. This identical optical element has a refractive coating whose optical properties and thickness profile are known with sufficient accuracy, for example, through the single-component measurement described above. After the optical system has been measured, the optical element with the refractive coating is manufactured, with the individualized thickness profile, i.e.,The deviation from the design specification, relative to the known thickness profile of the refractive layer of the identical optical element, is designed in such a way that, on the one hand, the aberration correction of the entire optical system is improved and, on the other hand, optical differences, e.g., due to material inhomogeneities of the substrate surface under the refractive layer and / or the surface on the back of the optical element, which exist between the identical optical element with the refractive layer and the optical element onto which the refractive layer is applied, are compensated.

[0047] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures in the drawing, which show details essential to the invention, and from the claims. The individual features can be implemented individually or in any combination in a variant of the invention. [Drawing]

[0048] Examples of implementation are shown in the schematic drawing and are explained in the following description. It shows

[0049] Fig. 1 shows a schematic representation of a DUV lithography system with a lighting system and a projection lens.

[0050] Fig. 2 shows a schematic representation of a lens with a coating that has a refractive layer,

[0051] Fig. 3 is a schematic representation analogous to Fig. 2, in which the coating has an additional refractive layer.

[0052] Fig. 4 is a schematic representation analogous to Fig. 2, in which the refractive layer forms a freeform layer.

[0053] Fig. 5 shows a schematic representation of a mirror with a refractive layer onto which a reflective layer is applied.

[0054] Fig. 6 shows a schematic representation of the refractive index of an interference layer positioned between the refractive layer and a substrate of the lens of Fig. 4 for antireflection.

[0055] Fig. 7 shows a schematic representation of the residual reflectivity of the lens from Fig. 4 with and without an antireflective coating as a function of wavelength, as well as

[0056] Fig. 8 shows a schematic representation of the residual reflectivity of the lens from Fig. 4 with and without an antireflective coating as a function of the angle of incidence. In the following description of the drawings, identical reference numerals are used for identical or functionally equivalent components.

[0057] Fig. 1 shows a schematic view of a DUV projection exposure system 100, which comprises a beam shaping and illumination system 102 and a projection lens 104. DUV stands for "deep ultraviolet" and refers to a wavelength of the working light between 150 nm and 470 nm. The DUV projection exposure system 100 has a beam source in the form of a DUV light source 106. For example, an ArF excimer laser can be used as the DUV light source 106, emitting useful radiation 108 at a useful wavelength AB of 193 nm. The DUV light source 106 can also be configured to generate useful radiation 108 at a different wavelength within the DUV wavelength range, for example, at a useful wavelength AB of 436 nm, 365 nm, 248 nm, or 157 nm.

[0058] The beam shaping and illumination system 102 shown in Fig. 1 directs the useful radiation 108 onto a photomask 120. The photomask 120 is designed as a transmissive optical element and can be arranged outside the beam shaping and illumination system 102 and the projection lens 104. The photomask 120 has a structure which is reduced in size by means of the projection lens 104 and imaged onto a wafer 124 or the like.

[0059] The projection lens 104 comprises several lenses 128, 140 and / or mirrors 130 for imaging the photomask 120 onto the wafer 124. Individual lenses 128, 140 and / or mirrors 130 of the projection lens 104 can be arranged symmetrically with respect to the optical axis 126 of the projection lens 104. It should be noted that the number of lenses and mirrors of the DUV projection exposure system 100 is not limited to the number shown. More or fewer lenses and / or mirrors may also be provided. Furthermore, the mirrors are generally curved on their front surface for beam shaping.

[0060] An air gap between the last lens 140 and the wafer 124 can be replaced by a liquid medium 132 with a refractive index > 1. The liquid medium 132 can be, for example, highly purified water. Such a setup is also known as immersion lithography and exhibits increased photolithographic resolution.

[0061] Fig. 2 shows an example of an optical element in the form of a lens 1, which can be used, for example, as one of the lenses 128, 140 of the projection system 104 of Fig. 1. The lens 1 has a substrate 2 that forms a lens body, which in the example shown is biconvex. A coating 3 is applied to a surface 2a of the substrate 2 provided for the transmission of the useful radiation 108. In the example shown, this coating consists of a refractive layer 4 and an interference layer 5. The lens 1 has a diameter D of approximately 100 mm and is made of glass, more precisely, fused silica. Depending on the useful wavelength AB, the substrate 2 can also be made of another material that is transparent for the respective useful wavelength AB, for example, crystalline quartz, a fluoride crystal, e.g., calcium fluoride, or a high- or low-refractive-index optical glass, e.g., FK5 glass or LLF1 glass.

[0062] The refractive layer 4 has a thickness d(x,y) that varies depending on the location x, y on the surface 2a of the substrate 2. The position x, y refers here to an xyz coordinate system whose z-direction is parallel to an optical axis 6 of the lens 1. As can be seen in Fig. 2, the refractive layer 4 has a minimum thickness dMin at the center Z of an optically used area of ​​the lens 1, which in the example shown corresponds essentially to the entire surface 2a of the lens 1. The optical axis 6 passes through the center Z of the optically used area 2a.

[0063] The minimum thickness dMin of the refractive layer 4 in the example shown is approximately 10 pm, i.e., more than 50 times the useful wavelength AB of 193 nm. To reduce interference effects and enhance the refractive effect (su), the minimum thickness dMin of the refractive layer 4 should be at least 5 AB, at least 10 AB, and especially at least 15 AB.

[0064] The refractive layer 4 has a maximum thickness dMax at its lateral edge R. The difference dMax - dMin between the maximum thickness dMax and the minimum thickness dMin of the refractive layer 2 is approximately [value missing] in the example shown.

[0065] 1 pm. The thickness profile or the spherical curvature of the surface 4a of the refractive layer 4 is rotationally symmetric to the optical axis 6 of the lens 1. The refractive layer 4 thus acts like a convex-concave lens, whereby the refractive power of the refractive layer 4 is lower than that of a conventional lens due to its comparatively small radius of curvature.

[0066] In principle, for the refractive effect of refractive layer 4, it is advantageous if the difference dMax - dMin between the maximum thickness dMax and the minimum thickness dMin is as large as possible. The difference dMax - dMin should be greater than AB, greater than 2 AB, and especially greater than 3 AB. In absolute terms, it is advantageous if the difference dMax - dMin is greater than 500 nm, greater than 1 pm, and especially greater than 2 pm.

[0067] The thickness gradient, or the difference dMax - dMin between the maximum thickness dMax and the minimum thickness dMin of the refractive layer 4, which can be generated during the deposition of the refractive layer 4, increases with increasing minimum thickness dMin of the refractive layer 4. For the function of the refractive layer 4 as lens 1, it is therefore advantageous if it has a comparatively large minimum thickness dMin, which is at least 3 pm, at least 5 pm, at least 10 pm, or at least 20 pm. In principle, a thickness dMin of the refractive layer 4 on the order of millimeters would be desirable; however, the thickness dMin with which the refractive layer 4 can be applied is limited by process constraints such as the production time of the refractive layer 4 or by layer stresses.

[0068] In addition to the difference dMax - dMin between the maximum thickness dMax and the minimum thickness dMin of refractive layer 4, the maximum lateral gradient of the thickness d(x,y) is also essential for the refractive effect of refractive layer 4. The maximum lateral gradient of the thickness d(x,y) is defined as the difference dMax - dMin between the maximum thickness dMax and the minimum thickness dMin, relative to the minimum thickness dMin of refractive layer 4, divided by the lateral distance between the location with maximum thickness dMax and the location with minimum thickness dMin. In the example shown: (dMax - dMin) / (dMax D / 2) = 1 pm / (10 pm ≤ 50 mm) = 0.2% / mm. In general, it is advantageous if the maximum lateral gradient is at least 0.01% / mm, at least 0.025% / mm, at least 0.05% / mm, or at least 0.1% / mm.

[0069] To enhance the optical effect of the refractive layer 4, it is advantageous for the refractive index ns of the substrate 2 material to deviate as much as possible from the refractive index nR of the refractive layer 4 material. The deviation, or refractive index difference |ns - HR|, should be greater than 0.1, ideally greater than 0.2. A material is selected for the refractive layer 4 that, on the one hand, meets the requirement for the deviation of the refractive index difference |ns - HR| and, on the other hand, exhibits sufficient transparency for the useful wavelength AB. The refractive layer 4 can, for example, consist of at least one material selected from the following group: SiC>2, TiCh, Al₂O₃, HfCh, MgF₂, AlF₃, NaF, LaFs, amorphous silicon, or a lithium compound.

[0070] In the example described here, where the useful wavelength AB is 193 nm, fluoride layers or SiÜ2 can be used as material for the refractive layer 4.

[0071] Since substrate 2 in the example shown is made of quartz glass, applying a refractive layer 4 made of quartz material generally does not result in a sufficient refractive index difference |ns - HR|. If the refractive layer 4 is to be made of quartz material in this case as well, a porous quartz material is applied to substrate 2 whose density PR is at least 0.1% lower than the density ps of the quartz glass material of substrate 2.

[0072] To reduce the reflection of the lens 1, the coating 3 has an interference layer 5, the thickness of which di is constant and significantly less than the minimum thickness dMin of the refractive layer 4. In the example shown, the thickness di is approximately 100 nm and, like the thickness d(x, y) of the refractive layer 4, is not shown to scale in Fig. 2. The thickness di of the interference layer 5 is typically less than 50%, less than 20%, or less than 10% of the minimum thickness dMin of the refractive layer 4. Instead of the single interference layer 5 shown in Fig. 2, two or more interference layers can also be applied to one another to reduce the reflection of the lens 1.

[0073] It is understood that one or more interference layers can also be applied to a surface 2b of the substrate 2, which faces away from the refractive layer 4, for the purpose of reducing reflections on the lens 1; these are not shown in Fig. 2. To enhance the refractive effect, a further refractive layer can be applied to the surface 2b of the substrate 2, which faces away from the refractive layer 4; this is not shown in Fig. 2.

[0074] Fig. 3 shows a lens 1 that differs from the lens 1 shown in Fig. 2 in that a further refractive layer 4a is applied to enhance the lens effect on the refractive layer 4. The further refractive layer 4a also has a location-dependent varying thickness d(x,y) and, with respect to its maximum thickness dMax and its minimum thickness dMin, essentially corresponds to the refractive layer 4 of Fig. 2. To minimize reflections at the interface between the refractive layer 4 and the further refractive layer 4a, the further refractive layer 4a has a refractive index nR. a on, whose difference |nR a - HR| to the refractive index nR of refractive layer 4 is less than 0.2, in the example shown less than 0.1. The further refractive layer 4a can be made of a different material than refractive layer 4.

[0075] To generate a comparatively small refractive index difference |nR a - HR| it is also possible that the refractive layer 4 and the further refractive layer 4a are formed from the same material and the refractive indices nR aThe refractive index nR of the refractive layer 4 and the further refractive layer 4a can differ from each other due to different doping. For this purpose, the refractive layer 4 can, for example, be doped with foreign atoms, more precisely with ionized foreign atoms 8, at least in a partial volume 7, as shown in Fig. 3. In the example shown, the partial volume 7 is located adjacent to the surface 2a of the substrate 2, but it can also be located adjacent to the interface between the refractive layer 4 and the further refractive layer 4a, or elsewhere. It is also possible for the entire refractive layer 4 to be doped with foreign atoms 8. These foreign atoms or ions 8 can be, for example, silicon, nitrogen, or hydrogen. The further refractive layer 4a is not doped with foreign atoms 8 and therefore differs from the refractive index nR of the refractive layer 4.The optical effect of the two refractive layers 4, 4a resembles a gradient-index medium and approximates a continuous refractive index profile. It is understood that, alternatively or additionally, the further refractive layer 4a can be doped with foreign atoms or ions 8, at least in a partial volume 7.

[0076] In the fabrication of the lenses 1 shown in Figures 2 and 3, the refractive layer 4 and the further refractive layer 4a are typically applied to the substrate 2 by a dipping process, PECVD, a sputtering process, electron beam evaporation, or pulsed laser deposition. The type of process is determined depending on the material of the refractive layer and on the minimum thickness dMin or the desired thickness gradient of the refractive layer 4. Depending on the chosen process, the refractive layer 4 can be modified after application, for example, by a subtractive process, in particular by ion beam processing, or by a compacting process, in particular by electron bombardment, to create the desired variation in the thickness d(x,y) of the refractive layer 4 or to smooth the refractive layer 4.The application of the refractive layer 4 and the interference layer 5, as well as further layers of the coating 3, can be carried out in particular in one and the same coating system.

[0077] Fig. 4 shows a lens 1 that differs from the lens 1 shown in Fig. 2 in that the thickness d(x,y) of the refractive layer 4 does not vary rotationally symmetrically about the center Z of the optically used area 2a of the lens 1. In the example shown, the refractive layer 4 forms a freeform layer. The spatially varying thickness d(x,y) of the refractive layer 4 is composed of a design target specification in the form of a target layer thickness d. sThe design parameters (x,y) and a deviation Ad(x,y) from the target layer thickness ds(x,y) are combined, i.e., d(x,y) = ds(x,y) + Ad(x,y). The design target value ds(x,y) is identical for all lenses that are to be arranged at a specific position in a respective projection exposure system 100. The deviation Ad(x,y) is individually defined for exactly one lens 1 and serves to reduce aberrations of the lens 1 and / or of other optical elements of the projection exposure system 100 as a whole.

[0078] For this purpose, lens 1 can be measured before the refractive layer 4 is applied to the substrate 2 of lens 1. This allows the determination of aberrations caused by a deviation of at least one optical property of the optical element 1 from a target value. This property could be, for example, the surface shape of substrate 2, or a material property such as the refractive index or refractive index inhomogeneity of substrate 2. Based on the measurements and the aberrations, an objective function can be defined to determine the spatially varying deviation Ad(x,y) from the design target ds(x,y), which is suitable for reducing the aberrations.Instead of the lens, another optical element 129, 130, 140 of the projection lens 104 can also be measured in order to determine its aberrations and to reduce aberrations of the projection exposure system 100 by setting a suitable deviation Ad(x,y) from the design target specification ds(x,y).

[0079] Due to the diffraction effect at the photomask 120, aberrations in the projection lens 104 cannot be corrected in the illumination system 102. Nevertheless, the refractive layer 4 can be used in the illumination system 102 to improve the function of the illumination system 102 itself. The illumination system 102 is characterized in particular by the quality parameters of uniformity (consistency of the scan-integrated intensity across the illumination field), deviation of the heavy rays at the photomask 120 from a target value, and, if applicable, polarization (correspondence of the delivered polarization state with a previously selected polarization state deemed advantageous for imaging, e.g., tangential polarization).

[0080] It is also possible to measure the aberrations of the projection exposure unit 100, the lighting system 102, or the projection lens 104 if the optical element 1 or an identical optical element 129, 130, 140 is installed there. In this case, the deviation Ad(x,y) is set such that the aberrations of the projection exposure unit 100, the lighting system 102, or the projection lens 104 are at least partially compensated by the deviation Ad(x,y). During the measurement, the optical element 1—without the applied refractive layer 4—can be installed in the projection exposure unit 100 and can be removed from the projection exposure unit 100, processed, and reinstalled for the application of the refractive layer 4 or the coating 3. In this case, it is necessary to take into account the optical design effect of the refractive layer 4 during measurement.This can be achieved computationally or by using suitable compensation optics that replicate the optical effect of the refractive layer 4. The compensation optics are installed in the projection exposure system 100 for measurement and removed again after the measurement.

[0081] Alternatively, the optical elements used in the measurement and operation of the projection exposure system 100 can differ physically from one another. In this case, an optical element identical in construction to optical element 1 is used during the measurement. The refractive layer 4 is to be applied to the substrate 2 of this optical element. This identical optical element has a refractive layer whose optical properties and thickness profile are known with sufficient accuracy, for example, through the individual component measurement described above. After the measurement of the projection exposure system 100, optical element 1 with the refractive layer 4 is manufactured, with the individualized thickness profile, i.e.,The deviation Ad(x,y) from the design specification ds(x,y), relative to the known thickness profile of the refractive layer of the identical optical element, is designed in such a way that, on the one hand, the aberration correction of the entire projection exposure system 100 is improved and, on the other hand, optical differences, e.g. due to material inhomogeneities, the fit of the substrate 4 under the refractive layer 4 and / or the fit on the back 2b of the lens 1 between the identical lens with the refractive layer and the lens 1, onto which the refractive layer 4 is applied, are compensated.

[0082] The lens 1 shown in Fig. 4 differs from the lens 1 shown in Fig. 2 and Fig. 3 in that, in addition to the first anti-reflective interference layer 5, which is applied to a side of the refractive layer 4 facing away from the substrate 2, a second anti-reflective interference layer 5a is applied between the refractive layer 4 and the substrate 2 to reduce the reflectivity at the interface between the substrate 2 and the refractive layer 4. In the example shown, the second interference layer 5a is designed as a gradient-index layer and has a comparatively small thickness of approximately 180 nm. Fig. 7 shows the refractive index n of the gradient-index layer 5a as a function of the distance A to the surface of the substrate 2 for a wavelength of 248 nm. In the example shown, the substrate 2 has a low refractive index n of approximately 1.5 and is, for example,The substrate is made of quartz glass, and the refractive layer 4 has a high refractive index n of approximately 2.35. As can be seen in Fig. 7, the refractive index n of the gradient-index layer 5a lies between these two values ​​and increases from the surface of the substrate 2 at A=0 to the refractive layer 4 at A = 180 nm.

[0083] The residual reflectivity R of lens 1 from Fig. 4 as a function of wavelength A without the gradient-index layer 5a is shown as a dashed line in Fig. 7. The residual reflectivity R of lens 1 with the gradient-index layer 5a is shown as a solid line in Fig. 7. As can be seen in Fig. 7, the gradient-index layer 5a can significantly reduce the residual reflectivity of lens 1. The same applies to the residual reflectivity R shown in Fig. 8 as a function of the angle of incidence α on the surface of substrate 2. An angle of incidence α of 0° corresponds to perpendicular incidence.

[0084] The gradient-index layer 5a with the refractive index profile shown in Fig. 6 can be produced, for example, by electron beam evaporation using two evaporator sources. The first evaporator source is designed to evaporate a high-refractive-index material, and the second evaporator source is designed to evaporate a low-refractive-index material. During the deposition of the gradient-index layer 5a onto the substrate 2, material is deposited simultaneously from both evaporator sources. The evaporation rate of the first evaporator source increases during deposition, while the evaporation rate of the second evaporator source decreases. By specifying theBy controlling the evaporation rates of the two evaporation sources, a desired mixing ratio of the high-refractive-index and low-refractive-index materials can be set during deposition, thus generating a desired refractive index n of the gradient-index layer 5a. It is understood that the gradient-index layer 5a can also be produced by other means.

[0085] Fig. 5 shows an example of an optical element in the form of a mirror 10, which can be used, for example, in the projection exposure system 100 of Fig. 1 or in another optical system, for example, in a synchrotron. The mirror 10 has a substrate 2 made of quartz glass, on the flat surface 2a of which a refractive layer 4 is applied, which is designed as described above. A much thinner reflective layer 11 in the form of an aluminum layer is applied to the refractive layer 4. In the example shown in Fig. 5, the curvature of the surface 4a of the refractive layer 4 is used for its optical effect; the refractive index of the material of the refractive layer 4 is irrelevant in this case, since the useful radiation does not penetrate through the reflective layer 11. The example shown in Fig.The mirror 10 shown, or more precisely its reflective surface 11a, has a comparatively low curvature and therefore a long focal length.

[0086] Correcting the surface shape to reduce aberrations is generally more advantageous on the refractive layer 4 than on the substrate 2 itself. This is the case, for example, when the measurement described above involves integration into a larger unit, such as mounting the mirror 10 in a socket that is difficult or impossible to reverse, but which, for instance, exceeds the overall geometry of the machining tool. Similarly, components such as bonded bushings, optical elements for position measurement ("targets"), mechanical elements for position determination ("encoders"), etc., can hinder machining of the substrate 2 but are compatible with machining the coating or the refractive layer 4.

Claims

Patent claims 1. Optical element, in particular lens (1, 130, 140) or mirror (10, 130), preferably for a semiconductor technology system, in particular for an illumination system (102) or for a projection lens (104) of a projection exposure system (100), comprising: a substrate (2), and a coating (3) applied to a surface (2a) of the substrate (2), characterized in that the coating (3) has at least one refractive layer (4) with a spatially varying thickness (d(x,y)) which is transparent at a useful wavelength AB of the optical element, wherein a minimum thickness (dMin) of the refractive layer (4) is at least 5 AB, preferably at least 10 AB, particularly preferably at least 15 AB, and wherein a difference (dMax - dMin) between a maximum thickness (dMax) and the minimum thickness (dMin) of the refractive layer (4) is greater than AB, preferably greater than 2 AB, especially greater than 3 AB.

2. Optical element according to claim 1, wherein the useful wavelength AB of the optical element (1) is in the DUV wavelength range, preferably at 436 nm, 365 nm, 248 nm, 193 nm or 157 nm.

3. Optical element according to claim 1 or 2, wherein the minimum thickness (dMin) of the refractive layer (4) is at least 3 pm, preferably at least 5 pm, particularly preferably at least 10 pm, and especially at least 20 pm.

4. Optical element according to one of the preceding claims, wherein a refractive index (ns) of the material of the substrate (2) is of a The refractive index (HR) of the material of the refractive layer (4) deviates by more than 0.1, preferably by more than 0.

2.

5. Optical element according to one of the preceding claims, wherein the difference (dMax - dMin) between the maximum thickness (dMax) and the minimum thickness (dMin) of the refractive layer (4) is greater than 500 nm, preferably greater than 1 pm, and particularly greater than 2 pm.

6. Optical element according to any of the preceding claims, wherein the refractive layer (4) has the minimum thickness (dMin) at the center (Z) of an optically effective area (2a) of the optical element (1) and the maximum thickness (dMax) at the lateral edge (R) of the refractive layer (4), or vice versa.

7. Optical element according to one of the preceding claims, wherein the thickness (d(x,y)) of the refractive layer (4) does not vary rotationally symmetrically about a center (Z) of an optically used area (2a) of the optical element (1).

8. Optical element according to one of the preceding claims, wherein the refractive layer (4) is doped at least in a partial volume (7) with foreign atoms, in particular with ionized foreign atoms (8), which are preferably selected from the group comprising: silicon, nitrogen and hydrogen.

9. Optical element according to any of the preceding claims, wherein the substrate (2) is formed from amorphous or crystalline quartz material and wherein the refractive layer (4) is formed from a porous quartz material having a density (PR) that is at least 0.1% lower than the density (ps) of the quartz material of the substrate (2).

10. Optical element according to any of the preceding claims, wherein the refractive layer (4) comprises at least one material selected from the group comprising: SiO2, TiÜ2, Al2O3, HfO2, MgF2, AIF3, NaF, LaFs, amorphous silicon or a lithium compound.

11. Optical element according to one of the preceding claims, wherein the substrate (2) is made of glass, preferably fused silica, quartz or calcium fluoride.

12. Optical element according to one of the preceding claims, wherein the refractive layer (4) has a maximum lateral gradient of thickness (d(x,y)) of at least 0.01% / mm, preferably at least 0.025% / mm, particularly preferably at least 0.05% / mm, and especially at least 0.1% / mm.

13. Optical element according to one of the preceding claims, wherein the coating (3) has at least one interference layer (5, 5a) which preferably has a thickness (di) of less than 50%, more preferably of less than 20%, and in particular of less than 10% of the minimum thickness (dMin) of the refractive layer (4).

14. Optical element according to claim 13, wherein the coating (3) has at least a first interference layer (5) on a side of the refractive layer (4) facing away from the substrate (2) and / or at least a second interference layer (5a) between the refractive layer (4) and the substrate (2).

15. Optical system (100), preferably a semiconductor technology system, in particular an illumination system (102) or a projection lens (104) for microlithography, comprising: a beam source (106) for providing useful radiation (108) at a usable wavelength (AB), preferably in the DUV wavelength range, in particular at 468 nm, 365 nm, 248 nm, 193 nm or 157 nm, and at least one optical element (1 , 128, 130, 140) according to one of the preceding claims.

16. Method for manufacturing an optical element (1 , 128, 130, 140) according to any one of claims 1 to 14, comprising: Application of the refractive layer (4) to the substrate (2), wherein the application is preferably carried out by a dipping process, PECVD, a sputtering process, electron beam evaporation or pulsed laser deposition.

17. Method according to claim 16, wherein after the application of the refractive layer (4) a fit-changing processing of the refractive layer (4) is carried out, preferably by a subtractive process, in particular by ion beam processing, or by a compacting process, in particular by electron bombardment.

18. Method according to claim 16 or 17, further comprising: measuring the optical element (1) or a further optical element (129, 130, 140) to determine aberrations that are generated by a deviation of at least one optical property of the optical element (1) or of the further optical element (129, 130, 140) from a target specification, Defining a deviation (Ad(x,y)) of the spatially dependent thickness (d(x,y)) of the refractive layer (4) from a design target specification (d s (x,y)) such that the aberrations are at least partially compensated by the deviation (Ad(x,y)), as well as Application of the refractive layer (4) with the deviation (Ad(x,y)) of the location-dependent thickness (d(x,y)) from the design target specification (d s (x,y)) onto the substrate (2).

19. Method according to one of claims 16 or 17, further comprising: measuring an optical system (100, 102, 104) in which the optical element (1) or an identical optical element (129, 130, 140) is arranged, for determining aberrations of the optical system (100, 102, 104), and determining a deviation (Ad(x,y)) of the spatially dependent thickness (d(x,y)) of the refractive layer (4) from a design target value (d s(x,y)) such that the aberrations are at least partially compensated by the deviation (Ad(x,y)), as well as Application of the refractive layer (4) with the deviation (Ad(x,y)) of the location-dependent thickness (d(x,y)) from the design target specification (d s (x,y)) onto the substrate (2).

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