Method for spatially limiting material removal, optical element and semiconductor technology system
Patent Information
- Application Number
- PCT/EP2025/053500
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-03-07
- Filing Date
- 2025-02-11
- Publication Date
- 2025-10-02
AI Technical Summary
Existing methods fail to spatially limit material removal of etchable material beneath reflective coatings of EUV radiation-reflecting optical elements, leading to under-etching and large damaged areas due to reactive species penetrating through coating defects.
Form an etching front on the etchable material adjacent to damaged areas and supply an oxygen-containing gas to oxidize the material, forming a protective wall that limits material removal by reacting with reactive species, thus preventing further etching.
The protective wall significantly reduces the extent of material removal, extending the service life of the optical element by a factor of approximately 10 in radius and 100 in area without altering the coating, and can be repeatedly formed to enhance protection.
Smart Images
Figure EP2025053500_02102025_PF_FP_ABST
Abstract
Description
[0001] Method for spatially limiting material removal, optical element and semiconductor technology system
[0002] Reference to related application
[0003] This application claims priority from German patent application DE 102024202159.8 filed on March 7, 2024, the entire disclosure of which is incorporated by reference into this application.
[0004] Background of the invention
[0005] The invention relates to a method for spatially limiting material removal during undercutting of an EUV radiation-reflecting coating of an optical element covering an etchable material. The invention also relates to an optical element comprising a substrate, a reflective coating for reflecting EUV radiation, and covering an etchable material, as well as a semiconductor technology system, in particular an EUV lithography system.
[0006] For the purposes of this application, a semiconductor technology system is understood to mean an optical system or an optical arrangement for lithography, i.e., an optical system that can be used in the field of lithography. In addition to a lithography system used to manufacture semiconductor components, the system can, for example, be an inspection system for inspecting a photomask used in a lithography system (hereinafter also referred to as a reticle), for inspecting a semiconductor substrate to be structured (hereinafter also referred to as a wafer), or a metrology system used to measure a lithography system or parts thereof, for example, for measuring a projection system.
[0007] Semiconductor technology equipment can be operated, in particular, with useful radiation in the form of EUV radiation. EUV radiation is defined as radiation in a wavelength range between approximately 5 nm and approximately 30 nm, for example, at 13.5 nm. Since EUV radiation is strongly absorbed by most known materials, it is typically guided through the semiconductor lithography system using reflective optical elements.
[0008] Optical elements for reflecting EUV radiation, also referred to as EUV mirrors, are exposed to harsh conditions during operation in semiconductor technology systems, particularly in EUV lithography systems. For example, the reflective coating is hit by EUV radiation with a high radiant power. The vacuum environment in which EUV mirrors are typically operated contains residual gases, such as hydrogen, water, and other residual gases commonly found in ultra-high vacuums.
[0009] US 10,073,361 B2 discloses an EUV lithography system comprising at least one optical element with an optical surface arranged in a vacuum environment, as well as a supply device for supplying hydrogen into the vacuum environment. At least one silicon-containing surface is also arranged in the vacuum environment. The supply device is designed for the additional supply of an oxygen-containing gas into the vacuum environment and has a metering device for adjusting an oxygen partial pressure at the at least one silicon-containing surface and / or at the optical surface. The supply of oxygen is intended to counteract the formation of volatile hydrides. The oxygen partial pressure should be less than 5 x 10' 5 mbar to avoid oxidation of the materials present on the surfaces.
[0010] DE 10 2022 212 167 A1 describes an EUV source module for an EUV projection exposure system, which comprises an EUV source for generating useful EUV radiation, a vacuum chamber, and a gas source fluidly connected to the vacuum chamber via at least one valve. The gas source provides nitrogen and / or hydrogen, and optionally oxygen and / or water vapor.
[0011] US 2011 / 0109892 A1 describes a source module for a lithography system designed to generate EUV radiation and secondary radiation. The source module comprises a buffer gas, which can be, for example, C2H4, NH3, O3, CH3OH, CO2, CP, C2H6, C2H2, NH2D, SiHsCl, SiHsF, or SiHsBr. Mixtures with H2, He, Ne, and Ar are also possible.
[0012] DE 10 2016 217 633 A1 describes an optical arrangement in a projection exposure system for EUV lithography, which comprises a housing in which a plurality of optical elements are arranged. Arranged within the housing are a plurality of sub-housings, each of which is assigned a separate gas supply, via which the interior of the corresponding sub-housing can be individually exposed to its own gas atmosphere. At least one of these sub-housings can be exposed to oxygen, nitrogen, carbon dioxide, xenon, argon, krypton, chlorine, bromine, or mixtures thereof. One of the sub-housings can surround a collector mirror of a light source.
[0013] The residual gases present in the vicinity of the optical element can be converted into reactive species such as ions or radicals by the action of EUV radiation. For example, molecular hydrogen present in the environment can be converted into a hydrogen-containing plasma, which is also referred to as hydrogen plasma.
[0014] An etchable material may be located beneath the reflective coating of the optical element, which is etched away or removed upon contact with a reactive species, for example in the form of a plasma, e.g. in the form of hydrogen radicals or hydrogen ions. This is generally unproblematic if the reflective coating completely covers the etchable material. However, if the reflective coating is damaged and has localized defects, e.g. in the form of holes or the like, the reactive species can penetrate through to the etchable material beneath the reflective coating and remove it. In this way, during operation of the optical element, the etchable material is removed at the location of the respective defect, and an under-etched region forms beneath the reflective coating in the vicinity of the defect.Underetching can create very large damaged areas on the optical surface of the reflective optical element. Underetching can also occur when the surface of the reflective coating is cleaned using a plasma or wet-chemical process, e.g., using an etching cleaning medium, e.g., in the form of an acid. In this case, too, the etching cleaning medium can potentially penetrate through holes or other damage in the reflective coating and partially remove the etchable material.
[0015] Object of the invention
[0016] The object of the invention is to provide a method, an optical element, and a system in semiconductor technology in which material removal of the etchable material is spatially limited.
[0017] This object is achieved by a method of the type mentioned at the outset, comprising: forming an etching front on the etchable material in an under-etched region of the reflective coating adjacent to a damaged area of the reflective coating, wherein the optical element is arranged in a semiconductor lithography system, in particular in an EUV lithography system, during the formation of the etching front, and supplying an oxygen-containing gas to the etching front to oxidize the etchable material to form a protective wall for spatially limiting the material removal, wherein the semiconductor lithography system, in particular the EUV lithography system, is not operated in normal operation, in particular not in exposure operation, when the oxygen-containing gas is supplied to the etching front.
[0018] In the method, an etch front is formed in the etchable material in a first step, specifically in an under-etched region of the reflective coating. For this purpose, the etchable material is brought into contact with reactive, usually gaseous species at the damaged area of the reflective coating, which remove the etchable material to form an etch front. The material removal in the first step is continued until an under-etched region with the etch front forms laterally adjacent to the damaged area beneath the reflective coating. In this case, the optical element is arranged in a semiconductor lithography system, in particular in an EUV lithography system.
[0019] The formation of the etch front in the under-etched area is advantageous because the reflective coating in the under-etched area acts as a roof, protecting the etch front from direct attack by ions or other reactive species that could pass through the damaged area in the reflective coating, oriented substantially perpendicular to the reflective coating, and erode the protective wall formed in the second step.
[0020] In the second step, oxygen in the form of an oxygen-containing gas is supplied to the etch front formed in the under-etched area to form a protective wall of oxidized etchable material. The oxygen-containing gas can be molecular oxygen O2 or another type of gas containing oxygen, for example H2O, O3, NO2, N2O, mixtures thereof, etc. The oxidized etchable material of the protective wall, in contrast to the non-oxidized etchable material, is not eroded by reactive species, e.g., in the form of radicals, that pass from the damaged area into the under-etched area. The protective wall is protected from reactive species in the form of ions or the like by the reflective coating, which acts like a roof or shield (see above).
[0021] To effect oxidation, a significantly higher concentration of oxygen is typically required in the vicinity of the optical element than is optimal for regular operation of the semiconductor lithography system, particularly for the exposure mode of the EUV lithography system. Therefore, when the oxygen-containing gas is supplied to the etching front, the semiconductor technology system is not operated in regular operation. In the case of a semiconductor technology system in the form of an EUV lithography system, it is not operated in exposure mode. In the case of a wafer or mask inspection system, it is not operated in inspection or measurement mode, but rather in an oxidation mode specifically designed for this purpose.
[0022] The protective wall can be used to limit the material removal of the etchable material. It is possible that the protective wall completely prevents material removal, but it is also possible that the material removal is limited by significantly slowing it down compared to the case where no protective wall is present. For example, it has been observed that the radius of the under-etched region around the damaged area can be reduced by a factor of approximately 10 and the area by a factor of approximately 100 with the help of the protective wall, without having to change the etchable material and / or the reflective coating. In this way, the service life of the optical element during operation in a semiconductor technology system can be significantly increased.
[0023] The protective wall made of the oxidized, etchable material is usually very thin, typically on the order of several nanometers, or even just a few nanometers, in some cases. The thicker the protective wall, the more stable it is against etching. However, the thickness of the protective wall cannot be increased indefinitely by the addition of or contact with oxygen. The etchable material can be, for example, silicon, which upon oxidation converts to SiO2 or SiO x is converted.
[0024] In one variant, the etching front, before the oxygen-containing gas is supplied, has a lateral distance of more than 5 pm, preferably more than 10 pm, in particular more than 20 pm, from the damaged area of the reflective coating. As described above, the etching front at which the protective wall is formed should be formed far enough away from the damaged area that it cannot be directly hit by reactive species, e.g. in the form of ions, which could result in the protective wall being removed. The lateral distance is understood to be the maximum lateral distance between the etching front and the edge of the damaged area. In the event that the etchable material forms an intermediate layer (see below), the lateral distance can approximately correspond to the thickness of the intermediate layer or be greater than the thickness of the intermediate layer.It is understood that the lateral distance of the protective wall from the damaged area before the oxygen is supplied should not be too large in order to limit the material removal around the damaged area in the lateral direction.
[0025] In another variant, the optical element is positioned in the semiconductor technology system, particularly in the EUV lithography system, during the supply of the oxygen-containing gas. Typically, both the formation of the etch front and the supply of oxygen occur in situ, meaning the optical element remains in the semiconductor technology system during both steps.
[0026] In a further variant, a plasma, in particular a hydrogen plasma, is generated in the semiconductor lithography system, preferably in an exposure operation of the EUV lithography system, to form the etching front in an area surrounding the optical element. The plasma can, for example, be a hydrogen plasma that contains both hydrogen radials and hydrogen ions. The hydrogen radicals can reach the etchable material even in the under-etched region laterally adjacent to the damaged area and remove it, provided the etchable material is not protected by the protective wall. To generate the plasma, a semiconductor technology system in the form of an EUV lithography system typically uses an EUV light source that converts a residual gas present in the area surrounding the optical element, e.g. in the form of molecular hydrogen, into a plasma.During plasma formation, only molecular hydrogen may be present in the vicinity of the optical element, but it is also possible for the hydrogen to be mixed with clean air, e.g. with XCDA (“extreme clean dry air”), or with other gases, such as oxygen, or with water. When the etch front is formed, the oxygen or the oxygen concentration in the vicinity of the optical element is present at a concentration that does not lead to oxidation of the etchable material. The time required to form the etch front at a sufficient distance from the damaged area depends on the power of the EUV light source and can be on the order of magnitude of, for example, 50 to 500 hours. Since the time required to form the etch front is comparatively long, it is advantageous if the step of forming the etch front is carried out during exposure operation of the EUV lithography system.During exposure, a residual gas or a residual gas atmosphere containing hydrogen is typically present in the vicinity of the optical element, from which a plasma is formed under the influence of EUV radiation, which removes the etchable material if it is exposed to the environment in the area of the damaged area.
[0027] There are various options for supplying the oxygen-containing gas to the etching front.
[0028] In one variant, the semiconductor lithography system, particularly the EUV lithography system, is ventilated when the oxygen-containing gas is supplied to the etching front. In this variant, the vacuum surrounding the optical element is broken when the oxygen-containing gas is supplied, and the optical element is exposed to ambient air. The oxygen present in the ambient air causes the oxidation of the etchable material and the formation of the protective wall. To form the protective wall, it is typically sufficient if the optical element or the vacuum chamber in which the optical element is arranged is ventilated for a few seconds.
[0029] In a further variant, an oxygen-containing plasma is generated in the vicinity of the optical element when the oxygen-containing gas is supplied to the etching front. In this variant, the optical element is arranged in a semiconductor technology system, for example in an EUV lithography system, and a vacuum present in the vicinity of the optical element is generally not broken. In the simplest case, oxygen or an oxygen-containing gas with a high oxygen partial pressure is briefly introduced into the vacuum chamber or into the vacuum environment of the optical element while the EUV light source of the semiconductor technology system is activated. In this case, an oxygen plasma or activated oxygen is formed solely by the operation of the EUV light source. The EUV light source is typically switched on outside of exposure mode to generate the oxygen-containing plasma (see above).Alternatively, a small plasma source can be attached to the vacuum chamber, e.g., screwed on, to generate an oxygen-containing plasma within the chamber and thus in the vicinity of the optical element. In this case, too, it is not necessary to break the vacuum in the vicinity of the optical element.
[0030] In the event that the chamber or the semiconductor lithography system is at risk of being damaged when generating an oxygen-containing plasma in the vicinity of the optical element, it is generally possible to supply the oxygen or the oxygen-containing gas to the etching front or to generate the oxygen plasma ex-situ in a separate plasma treatment system and then reintroduce the optical element into the semiconductor technology system.
[0031] The partial pressure of the oxygen-containing gas during oxygen plasma generation can range from 0.01 mbar to approximately 50 mbar, for example. The time required to form a protective wall with a thickness on the order of several nanometers is on the order of a few minutes in a conventional plasma oxidation setup.
[0032] In another variant, when the oxygen-containing gas is supplied to the etching front, the optical element is located in a vacuum environment to which the oxygen-containing gas is supplied. In principle, it is possible that the vacuum in the environment of the optical element is not broken for the oxidation of the etchable material (see above). In this case, the oxygen concentration in the environment of the optical element can be increased via a gas inlet that is usually already present or, if necessary, an additional gas inlet, for example, by increasing the proportion of clean air supplied during exposure and present in the environment by one order of magnitude or, if necessary, by several orders of magnitude. The oxygen-containing gas can also be molecular oxygen, water, N2O, NO2, O3, etc. In this variant, an oxygen-containing plasma can be generated in the vacuum environment of the optical element (see above).), but this is not mandatory.
[0033] In a further development of this variant, the partial pressure of the oxygen-containing gas in the vacuum environment is more than 10' 3 mbar, preferably more than 1 mbar. The vacuum environment of the optical element typically has a total pressure of less than 10' 1 mbar. As described above, the partial pressure of the oxygen-containing gas is generally significantly higher than the partial pressure of oxygen typically used during exposure, so that the protective wall can be formed through oxidation of the etchable material. In the case where the oxygen-containing gas forms a mixture of several oxygen-containing gases, e.g., a mixture of H2O and O2, the partial pressure of the oxygen-containing gas is understood to be the sum of the partial pressures of these gases.
[0034] In a further variant, the formation of the etching front and the supply of the oxygen-containing gas to the etching front takes place after cleaning the optical element or during initial commissioning of the optical element. As described above, when cleaning the optical element, e.g. using CO2, e.g. in the form of CO2 snow, using a plasma or during wet-chemical cleaning (manual or machine-based), e.g. using an acid, damaged areas can form in the reflective coating at which the etchable material is exposed to the environment. By forming the protective wall, the etching removal during subsequent operation of the optical element in the semiconductor technology system can be limited.The same applies to the initial commissioning of the optical element: In this case, too, there may be damaged areas on the reflective coating where the etchable material would be removed during operation of the optical element in the semiconductor technology system. After the protective wall is formed, the semiconductor lithography system can be operated for a long time in normal operation, or in the case of an EUV lithography system, in exposure mode, since the protective wall protects the etchable material from further undercutting.
[0035] In a further variant, the method comprises: removing etchable material adjacent to the protective wall to form a further etching front in the under-etched area, and supplying an oxygen-containing gas to the further etching front to oxidize the etchable material and form a further protective wall to spatially limit the material removal. As described above, the protective wall is comparatively thin and can therefore possibly be permeable to reactive species in the form of radicals in some places. In this case, it is useful to repeat the steps described above once or possibly several times to form one or more further protective walls in order to increase the blocking effect. The distance of the further protective wall from the (first) protective wall can be comparatively small and be on the order of a few micrometers.As a rule, the number of additional protective walls formed in the manner described above should not exceed approximately five to ten. Typically, a protective wall, once formed, remains intact for the entire service life of the optical element, i.e., it is generally not removed. If the protective wall is removed during the service life of the optical element, the second step of the method described above can be repeated, i.e., oxygen or an oxygen-containing gas is again supplied to the etching front in the under-etched region to form a new protective wall.
[0036] A further aspect of the invention relates to an optical element of the type mentioned above, in which the etchable material is removed in an under-etched region adjacent to a damaged area of the reflective coating, and in which the under-etched region has at least one protective wall made of oxidized etchable material. Such an optical element can be manufactured using the method described above. The protective wall(s) limit the material removal of the etchable material.
[0037] In one embodiment, the etchable material is selected from the group comprising: amorphous silicon, silicon, Ge, B, Sn, Sn alloys, bismuth, and antimony. Silicon forms highly volatile reaction products in the form of silanes, i.e., silicon-hydrogen compounds, with activated hydrogen or a hydrogen plasma. When silicon comes into contact with oxygen, silicon oxide (SiO) is formed. xor SiO2, which generally does not form volatile hydrides upon contact with hydrogen radicals. In addition to an etchable material containing silicon, protective walls can also be formed on other etchable materials, provided their oxides are resistant to attack by hydrogen radicals, as is the case with Ge, B, etc.
[0038] In a further embodiment, the etchable material is in a
[0039] Intermediate layer between the reflective coating and the substrate and / or contained in the substrate. The intermediate layer is usually a functional layer that typically has no optical function. The intermediate layer can be, for example, an adhesion promoter layer or a structurable layer by means of which, for example, a grating structure or the like can be realized. Alternatively or additionally, the substrate itself can comprise the etchable material, which is protected from etching with the help of the reflective coating. In this case, the reflective coating is typically applied directly to the substrate. In principle, however, it is also possible for both the intermediate layer and the substrate to contain an etchable material.
[0040] The reflective coating may be a multilayer coating for reflecting EUV radiation incident on the reflective optical element under normal incidence, wherein the multilayer coating comprises alternating layers of a first material and a second material with different refractive indices.
[0041] Normal incidence of EUV radiation typically refers to incidence of EUV radiation at an angle of incidence of less than approximately 45° to the surface normal of the reflective optical element. The reflective multilayer coating is typically optimized for the reflection of EUV radiation at a given wavelength, which generally corresponds to the useful wavelength of the semiconductor technology system in which the optical element is used. For this purpose, the multilayer coating typically comprises a plurality of alternating layers made of a material with a high real part of the refractive index at the useful wavelength and a material with a low real part of the refractive index at the useful wavelength. The materials can be, for example, silicon and molybdenum, but other material combinations are also possible depending on the useful wavelength.Alternatively, the reflective coating can be designed to reflect EUV radiation impinging on the reflective optical element at grazing incidence. Grazing incidence of EUV radiation is typically understood to mean incidence of EUV radiation at an angle of incidence of more than approximately 60° to the surface normal of the surface of the reflective optical element. A reflective coating designed for grazing incidence typically has a maximum reflectivity at at least one angle of incidence that is greater than 60°. Such a reflective coating is typically formed from at least one material that has a low refractive index and low absorption for the EUV radiation impinging at grazing incidence. In this case, the reflective coating can contain a metallic material orbe made of a metallic material, for example Mo, Ru or Nb.
[0042] A further aspect of the invention relates to a semiconductor technology system, in particular an EUV lithography system, comprising: at least one optical element as described above. The optical element of the EUV lithography system can be a mirror of an illumination optics, e.g., a collector mirror, or a mirror of a projection system of the EUV lithography system.
[0043] 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 of the drawing, which illustrate details essential to the invention, and from the claims. The individual features can be implemented individually or in combination in a variant of the invention.
[0044] Drawing Examples of embodiments are shown in the schematic drawing and are explained in the following description. It shows
[0045] Fig. 1 shows a meridional section of a projection exposure system for EUV projection lithography,
[0046] Fig. 2a, b schematic representations of an EUV mirror of the projection exposure apparatus of Fig. 1 with a substrate and with a reflective coating which is undercut adjacent to a damaged area,
[0047] Fig. 3a-d schematic representations of the undercutting of the reflective coating of Fig. 2a, b at different times,
[0048] Fig. 4a-c schematic representations of the formation of an etching front in an under-etched area and the supply of an oxygen-containing gas to the etching front to form a protective wall, and
[0049] Fig. 4d,e schematic representations of the formation of a further etching front in the under-etched region and the supply of an oxygen-containing gas to the further etching front to form a further protective wall.
[0050] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.
[0051] The following describes, by way of example, the essential components of an optical arrangement for EUV lithography in the form of a projection exposure system 1 for microlithography with reference to Fig. 1. The description of the basic structure of the projection exposure system 1 and its components is not intended to be limiting.
[0052] One embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optics 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system. In this case, the illumination system does not include the light source 3.
[0053] A reticle 7 arranged in the object field 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be displaced, in particular in a scanning direction, via a reticle displacement drive 9.
[0054] For illustrative purposes, a Cartesian xyz coordinate system is shown in Fig. 1. The x-direction runs perpendicular to the drawing plane. The y-direction runs horizontally, and the z-direction runs vertically. The scanning direction in Fig. 1 runs along the y-direction. The z-direction runs perpendicular to the object plane 6.
[0055] The projection exposure system 1 comprises a projection system 10. The projection system 10 is used to image the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, in particular along the y-direction, via a wafer displacement drive 15. The displacement of the reticle 7, on the one hand, via the reticle displacement drive 9, and the displacement of the wafer 13, on the other hand, via the wafer displacement drive 15, can be synchronized with one another. The radiation source 3 is an EUV radiation source. The radiation source 3 emits in particular EUV radiation 16, which is also referred to below as useful radiation, illumination radiation or illumination light.The useful radiation has a wavelength in the range between 5 nm and 30 nm. Radiation source 3 can be a plasma source, for example, an LPP (Laser Produced Plasma) or a DPP (Gas Discharged Produced Plasma) source. It can also be a synchrotron-based radiation source. Radiation source 3 can be a free-electron laser (FEL).
[0056] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 can be a collector mirror with one or more ellipsoidal and / or hyperboloidal reflection surfaces. The at least one reflection surface of the collector mirror 17 can be exposed to the illumination radiation 16 at grazing incidence (Gl), i.e., at angles of incidence greater than 45°, or at normal incidence (NI), i.e., at angles of incidence less than 45°. The collector mirror 17 can be structured and / or coated, on the one hand, to optimize its reflectivity for the useful radiation and, on the other hand, to suppress stray light.
[0057] After the collector mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector mirror 17, and the illumination optics 4. The illumination optics 4 comprise a deflecting mirror 19 and, downstream of this in the beam path, a first facet mirror 20. The deflecting mirror 19 can be a flat deflecting mirror or, alternatively, a mirror with a beam-influencing effect beyond the pure deflection effect. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful light wavelength of the illumination radiation 16 from stray light of a different wavelength. The first facet mirror 20 comprises a plurality of individual first facets 21, which are also referred to below as field facets.Only a few of these facets 21 are shown in Fig. 1 as examples. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optics 4. The second facet mirror 22 comprises a plurality of second facets 23.
[0058] The illumination optics 4 thus form a double-faceted system. This basic principle is also referred to as a fly's-eye integrator. With the help of the second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-forming mirror, or indeed the last mirror for the illumination radiation 16 in the beam path before the object field 5.
[0059] The projection system 10 comprises a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.
[0060] In the example shown in Fig. 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or a different number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection system 10 is a doubly obscured optical system. The projection optical system 10 has an image-side numerical aperture that is greater than 0.4 or 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.
[0061] The mirrors Mi, just like the mirrors of the illumination optics 4, can have a highly reflective coating for the illumination radiation 16.
[0062] Fig. 2a, b show, in a highly simplified form, a detail of a reflective optical element in the form of an EUV mirror 25 of the projection exposure apparatus 1 of Fig. 1, which can be, for example, one of the mirrors Mi of the projection system 10, the collector mirror 17 or one of the mirrors 19, 20, 22 of the illumination optics 4. The mirror 25 has a substrate 26 and a highly reflective coating 27 for reflecting the EUV radiation 16. Between the reflective coating 27 and the substrate 26 there is an intermediate layer 28 which is covered by the reflective coating 27.
[0063] In the example shown, the reflective coating 27 is a multi-layer coating for reflecting EUV radiation 16 (cf. Fig. 1 ) under normal incidence, which has alternating layers of silicon and molybdenum, which are not illustrated in Fig. 2a, b for the sake of simplicity. The reflective coating 27 can also have further layers, for example a cover layer, which can be formed, for example, from Ru, or functional layers, e.g. barrier layers, to prevent interdiffusion between a respective layer of Si and an adjacent layer of Mo. The reflective coating 27 can alternatively be designed to reflect EUV radiation 16 under grazing incidence and have only a single layer of a metallic material, e.g. from Ru. The intermediate layer 28 can be a structurable layer, e.g.serve to create a lattice structure not shown in the image or fulfill another function. The intermediate layer 28 is formed from a material that can be easily processed by etching. The material of the intermediate layer 28 can be, for example, amorphous silicon, silicon, SiO2, Ge, B, Sn, Sn alloys, bismuth, antimony, etc.
[0064] As can be seen in the plan view of the reflective coating 27 in Fig. 2a, the reflective coating 27 has a locally limited damaged area 30 in the form of a hole that extends over the entire thickness of the reflective coating 27, as can be seen in Fig. 2b. A reactive species in the form of hydrogen radicals 31a present in the vicinity of the mirror 25 comes into contact with the etchable material 29 of the intermediate layer 28 via the damaged area 30. The etchable material 29 of the intermediate layer 28 is removed upon contact with the hydrogen radicals 31a, i.e. it reacts with the hydrogen radicals 31a and in the process forms a volatile material 32. For the case described here, in which the etchable material 29 contains silicon, or more precisely consists of silicon, the volatile material 32 can be, for example, a volatile hydride in the form of an SiH compound.
[0065] As can be clearly seen in Fig. 2a, b, the removal of the etchable material 29 is not limited to a volume region of the intermediate layer 28 located directly below the damaged area 30, but rather the etchable material 29 is also removed in an area surrounding the damaged area 30 whose lateral extent is significantly greater than the lateral extent of the damaged area 30. In this way, an under-etched area 33 in the form of a cavity is created adjacent to the damaged area 30 of the reflective coating 27, which is covered by the reflective coating 27. In the example shown in Fig. 2a, b, the substrate 26 is made of a material that is not removed upon contact with the hydrogen radicals 31a. The material of the substrate 26 can be, for example, one or more metals, e.g. Ni, Ti, W, Ta, Fe, Mo, Cr, Al, Sc, V, Co, Y, Zr, Nb, Ru, Rh, Hf, Re, Os, Ir, Pt, lanthanides or one or more oxides, e.g. SiO x, GeO x , BO X , AIO X , TiO x , TaO x , one or more carbides, nitrides or borides.
[0066] Fig. 3a-d illustrate the time sequence until the state shown in Fig. 2a, b is reached, in which the reflective coating 27 has been underetched in a comparatively large area 33. As can be seen in Fig. 3a, the mirror 25 is located in a vacuum environment 34 during exposure operation of the EUV lithography system 1, in which a hydrogen plasma 31 is generated by the influence of the EUV radiation 16, which contains hydrogen ions 31b in addition to the hydrogen radicals 31a shown in Fig. 2a, b. Since the EUV lithography system 1 was ventilated before exposure operation, a temporary protective wall 37 consisting of oxidized etchable material 29, in the example shown of SiO2, has formed on an etching front 35 of the etchable material 29. The temporary protective wall 37 is removed during exposure operation of the EUV lithography system 1 by the impinging hydrogen ions 31 b.
[0067] As shown in Fig. 3b, the hydrogen ions 31b pass through the damaged area 30 essentially perpendicular to the reflective coating 27, while the hydrogen radicals 31a are deflected in a lateral direction and also remove the etchable material 29 in the under-etched area 33 adjacent to the damaged area 30. If the exposure operation is continued, the etching front 35 moves from the damaged area 30 in a lateral direction further into the etchable material 29 and enlarges the under-etched area 33, as shown in Fig. 3c, d.
[0068] In order to reduce the lateral extent of the under-etched region 33, a method is carried out which is described below with reference to Figs. 4a-e. In the method, in a first step shown in Figs. 4a, b, the hydrogen plasma 31 is generated in the surroundings 34 of the mirror 25 in order to form an etching front 35 on the etchable material 29 in the under-etched region 33 adjacent to the damaged area 30, as shown in Fig. 4b. To generate the hydrogen plasma 31, the EUV lithography system 1 is operated in exposure mode. The exposure mode is carried out until the etching front 35 has a desired lateral distance A from the damaged area 30. The distance A can be more than 5 pm, more than 10 pm or more than 20 pm and is selected such that the etching front 35 is formed by a projecting section of the reflective coating 27, which is located above the under-etched region 33 and forms a roof orforms a shield, is sufficiently protected from the hydrogen ions 31 b, but the roof does not yet become unstable.
[0069] Once the desired lateral distance A has been reached, as shown in Fig. 4c, an oxygen-containing gas, in the example shown in the form of molecular oxygen O2, is supplied to the etching front 35 in the under-etched region 33 during an oxidation operation of the EUV lithography system 1. The molecular oxygen O2 oxidizes the etchable material 29 and forms a protective wall 36 made of oxidized etchable material 29, which spatially limits the material removal of the etchable material 29. When the exposure operation is resumed, in the example shown, the protective wall 36 prevents the passage of most of the hydrogen radicals 31a to the etchable material 29 and significantly slows the material removal.However, in the example shown, the protective wall 36 cannot prevent the passage of all hydrogen radicals 31a, so that during the exposure operation, etchable material 29 adjacent to the protective wall 36 is removed and a further etching front 35' is formed on the etchable material, as can be seen in Fig. 4d.
[0070] To increase the blocking effect for the hydrogen radicals 31a, after supplying molecular oxygen O2 to the etching front 35 to form the protective wall 36, a plasma 31 can be generated again in the environment 34 of the mirror 25 in order to selectively remove the etchable material 29 until the further etching front 35' is located at a desired distance A' from the protective wall 36. An oxygen-containing gas in the form of molecular oxygen O2 is again supplied to the further etching front 35' to form a further protective wall 36', as can be seen in Fig. 4e. It is understood that more than two protective walls 36, 36', ... can be formed in this way to increase the blocking effect for the hydrogen radicals 31a.
[0071] The method described above, which comprises the steps of forming the etching front 35 and supplying an oxygen-containing gas, e.g., O2, H2O, O3, NO2, N2O, mixtures thereof, ... to the etching front 35 in order to form the protective wall 36 - or possibly a plurality of protective walls 36, 36', ... - can be carried out in particular after cleaning the mirror 25, e.g., using CO2, or when the mirror 25 is first put into operation.
[0072] There are various options for supplying oxygen or the oxygen-containing gas O2, ... to the etching front 35 for oxidizing the etchable material 29. The EUV lithography system 1 is not operated in exposure mode when the oxygen-containing gas O2, ... is supplied to the etching front 35. The EUV lithography system 1 can be ventilated when the oxygen O2 is supplied to the etching front 35, i.e., the vacuum in the environment 34 of the mirror 25 is broken, and ambient air is supplied to the mirror 25.
[0073] It is also possible that the vacuum in the environment of the mirror 25 is not broken when oxygen O2 is supplied to the etching front 35, i.e. that the mirror 25 is located in a vacuum environment 34 when the oxygen O2 is supplied, as shown in Fig. 4c. In this case, at least one oxygen-containing gas O2, ... can be supplied to the environment 34 of the mirror 25. In the example shown, the supplied gas is molecular oxygen O2, but the gas can also be, for example, water or pure air. It is essential that the concentration or partial pressure pO2 of the oxygen O2 in the vacuum environment 34 is sufficient to cause the oxidation of the etchable material 29 so that the protective wall 36 can form. For this purpose, the concentration or partial pressure of the oxygen O2 in the environment 34 of the mirror 25 should be more than 10' 3mbar, preferably more than 1 mbar. The concentration of oxygen O2, which may be introduced into the residual gas atmosphere in the environment 34 of the mirror 25 during exposure operation, is significantly lower and cannot cause oxidation of the etchable material, especially since hydrogen is typically also present as a reducing gas in the environment 34 of the mirror 25. The supply of oxygen O2 is continued until a protective wall 36 of sufficient thickness has formed, which is typically on the order of a few nanometers.
[0074] When the oxygen-containing gas in the form of molecular oxygen O2 is supplied to the etching front 35, an oxygen-containing plasma can also be generated in the vacuum environment 34, i.e., activated oxygen O2* or oxygen radicals can be generated. To generate the oxygen-containing plasma, oxygen O2 can be supplied to the mirror 25 in the manner described above and, at the same time, the EUV light source 3 can be activated, which leads to the formation of the oxygen-containing plasma. Alternatively, a plasma source can be attached to a vacuum chamber (not shown) in which the mirror 25 is arranged in order to generate the oxygen-containing plasma.It is also possible in principle to remove the mirror 25 from the EUV lithography system 1, to generate the oxygen-containing plasma in an external plasma treatment system in order to oxidize the etchable material 39 and subsequently to reinsert the mirror 25 into the EUV lithography system 1.
[0075] The method described above can also be applied if no intermediate layer 28 is present, but the substrate 26 is an etchable
[0076] Contains material or forms an etchable material. In this case, too, the lateral extent of the under-etched region 33 can be spatially limited. However, the protective wall 36 generally cannot prevent damage to the substrate 26 upon contact with the hydrogen ions 31b directly beneath the damaged area 30 of the reflective coating 27.
[0077] The process can also be carried out in semiconductor lithography systems other than an EUV lithography system 1.
Claims
Patent claims 1 . A method for spatially limiting material removal during under-etching of a coating (27) of an optical element (25) covering an etchable material (29) and reflecting EUV radiation (16), comprising: Forming an etching front (35) on the etchable material (29) in an under-etched region (33) of the reflective coating (27) adjacent to a damaged area (30) of the reflective coating (27), wherein the optical element (25) is arranged in a semiconductor lithography system, in particular in an EUV lithography system (1), during the formation of the etching front (35), and Supplying an oxygen-containing gas (O2, ... ) to the etching front (35) for oxidizing the etchable material (39) while forming a protective wall (36) for spatially limiting the material removal, wherein the semiconductor lithography system, in particular the EUV lithography system (1 ), when supplying the oxygen-containing gas (O2, ... ) to the etching front (35) is not in normal operation, in particular not in Exposure mode.
2. Method according to claim 1, wherein the etching front (35) has a lateral distance (A) of more than 5 pm, preferably of more than 10 pm, in particular of more than 20 pm from the damaged area (30) of the reflective coating (27) before the supply of the oxygen-containing gas (O2, ...).
3. Method according to claim 1 or 2, wherein the optical element (25) is arranged during the supply of the oxygen-containing gas (O2, ...) to the etching front (35) in the semiconductor technology system, in particular in the EUV lithography system (1).
4. Method according to one of the preceding claims, in which a plasma, in particular a hydrogen plasma (31), is generated in the exposure operation of the EUV lithography system (1) for forming the etching front (35) in an environment (34) of the optical element (25).
5. Method according to one of the preceding claims, in which the semiconductor lithography system, in particular the EUV lithography system (1), is ventilated when the oxygen-containing gas (O2, ...) is supplied to the etching front (35).
6. Method according to one of the preceding claims, in which an oxygen-containing plasma is generated in the environment (34) of the optical element (25) when the oxygen-containing gas (O2, ...) is supplied to the etching front (35).
7. Method according to one of claims 1 to 4 or 6, wherein, when supplying the oxygen-containing gas (O2, ...) to the etching front (35), the optical element (25) is located in a vacuum environment (34) to which the oxygen-containing gas (O2, ...) is supplied.
8. The method according to claim 7, wherein a partial pressure (P02) of the oxygen-containing gas (O2, ...) in the vacuum environment (34) is more than 10' 3 mbar, preferably more than 1 mbar.
9. Method according to one of the preceding claims, in which the formation of the etching front (35) and the supply of the oxygen-containing gas (O2, ...) to the etching front (35) takes place after cleaning of the optical element (25) or during the first commissioning of the optical element (25).
10. Method according to one of the preceding claims, further comprising: removing etchable material (29) adjacent to the protective wall (36) for forming a further etching front (35') in the under-etched area (33), and Supplying an oxygen-containing gas (O2, ...) to the further etching front (35') for oxidizing the etchable material (29) while forming a further protective wall (36') for spatially limiting the material removal.
11. An optical element comprising: a substrate (26), a reflective coating (27) for reflecting EUV radiation (16), which coating covers an etchable material (29), characterized in that the etchable material (29) is removed in an under-etched region (33) adjacent to a damaged area (30) of the reflective coating (27), and in that the under-etched region (33) has at least one protective wall (36, 36') made of oxidized etchable material (29).
12. Optical element according to claim 11, wherein the etchable material (29) is selected from the group comprising: amorphous silicon, silicon, Ge, B, Sn, Sn alloys, bismuth and antimony.
13. Optical element according to one of claims 11 or 12, wherein the etchable material (29) is contained in an intermediate layer (28) between the reflective coating (27) and the substrate (26) and / or in the substrate (26).
14. A semiconductor technology system, in particular an EUV lithography system (1), comprising: at least one optical element (1) according to one of claims 11 to 13.