DE102024202159A1 - Method for spatially limiting material removal, optical element and semiconductor technology system - Google Patents

Abstract

The invention relates to a method for spatially limiting material removal during the under-etching of a coating (27) of an optical element (25) which covers an etchable material (29) and reflects EUV radiation, 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, during the formation of the etching front (35), and supplying an oxygen-containing gas (O ₂ , ...) to the etching front (35) to oxidize the etchable material (39) to form a protective wall (36) for spatially limiting the material removal, wherein the semiconductor lithography system, in particular the EUV lithography system, during the supply of the oxygen-containing gas (O ₂ , ...) to the etching front (35) is not operated in normal operation, in particular not in exposure operation. The invention also relates to an optical element (25), comprising: a substrate and a reflective coating (27) for reflecting EUV radiation, which coating covers an etchable material (29). The etchable material (29) is removed in an under-etched region (33) adjacent to a damaged area (30) of the reflective coating (27), and the under-etched region (33) has at least one protective wall (36, 36') made of oxidized etchable material (29). The invention also relates to a semiconductor technology system, in particular an EUV lithography system, which comprises at least one such optical element (25).

Description

Background of the invention

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.

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 be, for example, an inspection system for inspecting a photomask (hereinafter also referred to as a reticle) used in a lithography system, 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, to measure a projection system.

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.

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. In the vacuum environment in which EUV mirrors are typically operated, residual gases are present, such as hydrogen, water, and other residual gases commonly found in ultra-high vacuums.

From the US 10,073,361 B2 An EUV lithography system has become known which has at least one optical element with an optical surface, which is arranged in a vacuum environment, and 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 to additionally supply an oxygen-containing gas into the vacuum environment and has a dosing device for setting 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 × 10 ^-5 mbar in order to avoid oxidation of the materials present on the surfaces.

In the DE 10 2022 212 167 A1 An EUV source module for an EUV projection exposure system is described, comprising 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.

The US 2011/0109892 A1 describes a source module for a lithography system designed to generate EUV radiation and secondary radiation. The source module has a buffer gas, which can be, for example, C ₂ H ₄ , NH ₃ , O ₃ , CH ₃ OH, CO ₂ , CH ₄ , C ₂ H ₆ , C ₂ H ₂ , NH ₂ D, SiH ₃ Cl, SiH ₃ F, or SiH ₃ Br. Mixtures with H ₂ , He, Ne, and Ar are also possible.

In the DE 10 2016 217 633 A1 An optical arrangement in a projection exposure system for EUV lithography is described, comprising 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.

The residual gases present in the environment of the optical element can be converted into reactive species such as ions or radicals by the effect 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.

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 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. Under-etching can create very large damaged regions on the optical surface of the reflective optical element. Undercutting can also occur when the surface of the reflective coating is cleaned using a plasma or wet-chemical process, e.g., using a corrosive cleaning agent, e.g., in the form of an acid. In this case, too, the corrosive cleaning agent may penetrate through holes or other damage in the reflective coating and partially remove the etchable material.

Object of the invention

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.

Subject of the invention

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.

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.

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.

In the second step, oxygen in the form of an oxygen-containing gas is supplied to the etch front that was formed in the under-etched area in order to form a protective wall made of oxidized etchable material. The oxygen-containing gas can be molecular oxygen _O2 or another type of gas that contains oxygen, for example _H2O , _O3 , _NO2 , _N2O , mixtures thereof, etc. In contrast to the non-oxidized etchable material, the oxidized etchable material of the protective wall is not eroded by reactive species, for example 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).

To effect oxidation, a significantly higher concentration of oxygen is typically required in the environment of the optical element than is favorable for the regular operation of the semiconductor lithography system, in particular for the exposure operation of the EUV lithography system. Therefore, the semiconductor technology system is not operated in regular operation when the oxygen-containing gas is supplied to the etching front, and in the case of a semiconductor technology system in the form of an EUV lithography system, not in the exposure mode. operation, in the case of a wafer or mask inspection system not in inspection or measuring mode, but in an oxidation mode specifically designed for this purpose.

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.

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, thick. The thicker the protective wall, the more resistant it is to etching. However, the thickness of the protective wall cannot be increased indefinitely by adding or exposing it to oxygen. The etchable material can be silicon, for example, which is converted into _SiO2 or _SiOx during oxidation.

In one variant, the etching front, before the oxygen-containing gas is supplied, has a lateral distance of more than 5 µm, preferably more than 10 µm, in particular more than 20 µm, 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 impacted 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.

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, i.e., the optical element remains in the semiconductor technology system during both steps.

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, in an environment of the optical element to form the etching front. The plasma can, for example, be a hydrogen plasma containing 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 environment of 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., XCDA ("extreme clean dry air"), or with other gases, such as oxygen or water. The oxygen or oxygen concentration in the vicinity of the optical element is present at a concentration during the formation of the etch front 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 range from, 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 the exposure mode of the EUV lithography system. During exposure mode, a residual gas or a residual gas atmosphere containing hydrogen is typically present in the vicinity of the optical element. Under the influence of the EUV radiation, a plasma is formed from this plasma, which removes the etchable material if it is exposed to the environment in the area of the damaged area.

There are various options for supplying the oxygen-containing gas to the etching front.

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.

In another 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.

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.

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.

In a further variant, when the oxygen-containing gas is supplied to the etching front, the optical element is in a vacuum environment to which the oxygen-containing gas is supplied. In principle, it is possible for the vacuum in the environment of the optical element to not be broken for the oxidation of the etchable material (see above). In this case, the concentration of oxygen 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, N ₂ O, NO ₂ , O ₃ , 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 absolutely necessary.

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 total pressure in the vacuum environment of the optical element is typically less than 10 ^-1 mbar. As described above, the partial pressure of the oxygen-containing gas is generally well above the partial pressure of oxygen usually used during exposure so that the protective wall can be formed by the oxidation of the etchable material. If the oxygen-containing gas forms a mixture of several oxygen-containing gas types, e.g. a mixture of H ₂ O and O ₂ , the partial pressure of the oxygen-containing gas is understood to be the sum of the partial pressures of these gas types.

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 the initial commissioning of the optical element. As described above, when cleaning the optical element, for example, using CO ₂ , e.g. in the form of CO ₂ snow, using During the use of a plasma or during wet chemical cleaning (manual or mechanical), e.g. using an acid, damaged areas may form in the reflective coating where 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 has been formed, the semiconductor lithography system can be operated for a long time in normal mode, or in the case of an EUV lithography system, in exposure mode, because the protective wall protects the etchable material from further under-etching.

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 may therefore 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 several times to form one or more further protective walls to increase the blocking effect. The distance of the further protective wall from the (first) protective wall can be comparatively small and on the order of a few micrometers. As a rule, the number of further protective walls formed in the manner described above should not exceed approximately five to ten.

Typically, a protective wall, once formed, remains intact throughout 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 process described above can be repeated, i.e., oxygen or an oxygen-containing gas is again supplied to the etching front in the under-etched area to form a new protective wall.

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.

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 with a hydrogen plasma. When silicon comes into contact with oxygen, silicon oxide SiO _x or SiO ₂ is formed, which generally does not form highly 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., for example.

In a further embodiment, the etchable material is contained in an intermediate layer between the reflective coating and the substrate and/or in the substrate. The intermediate layer is generally a functional layer that typically has no optical function. The intermediate layer can be, for example, an adhesion promoter layer or a structurable layer through 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 aid of the reflective coating. In this case, the reflective coating is typically applied directly to the substrate. However, it is also possible in principle for both the intermediate layer and the substrate to contain an etchable material.

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.

Normal incidence of EUV radiation is typically understood to mean an incidence of EUV radiation at an angle of incidence of typically less than approximately 45° to the surface normal of the surface of the reflective optical element. The reflective multilayer coating is typically The coating is optimized for the reflection of EUV radiation at a given wavelength, which usually 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 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 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 or be formed from a metallic material, for example from Mo, Ru or Nb.

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.

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.

drawing

• 1 schematisch im Meridionalschnitt eine Projektionsbelich-tungsanlage für die EUV-Projektionslithografie,
• 2a,b schematische Darstellungen eines EUV-Spiegels der Projektionsbelichtungsanlage von 1 mit einem Substrat und mit einer reflektierenden Beschichtung, die benachbart zu einer Schadstelle unterätzt ist,
• 3a-d schematische Darstellungen des Unterätzens der reflektierenden Beschichtung von 2a,b zu unterschiedlichen Zeitpunkten,
• 4a-c schematische Darstellungen des Bildens einer Ätzfront in einem unterätzten Bereich sowie des Zuführens eines Sauerstoff enthaltenden Gases zu der Ätzfront zum Ausbilden einer Schutzwand, sowie
• 4d,e schematische Darstellungen des Bildens einer weiteren Ätzfront in dem unterätzten Bereich sowie des Zuführens eines Sauerstoff enthaltenden Gases zu der weiteren Ätzfront zum Ausbilden einer weiteren Schutzwand.

Examples of embodiments are shown in the schematic drawing and are explained in the following description.

• 1 schematic meridional section of a projection exposure system for EUV projection lithography,
• 2a ,b schematic representations of an EUV mirror of the projection exposure system of 1 with a substrate and with a reflective coating which is undercut adjacent to a damaged area,
• 3a -d schematic representations of the undercutting of the reflective coating of 2a ,b at different times,
• 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
• 4d ,e schematic representations of the formation of a further etching front in the under-etched region and of the supply of an oxygen-containing gas to the further etching front to form a further protective wall.

In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

In the following, with reference to 1 The essential components of an optical arrangement for EUV lithography in the form of a projection exposure system 1 for microlithography are described by way of example. The description of the basic structure of the projection exposure system 1 and its components is not to be understood as limiting.

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.

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.

In 1 For explanation, a Cartesian xyz coordinate system is shown. The x-direction is perpendicular to the drawing plane. The y-direction is horizontal and the z-direction is vertical. The scanning direction is in the 1 along the y-direction. The z-direction runs perpendicular to the object plane 6.

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 each other.

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, in particular, a wavelength in the range between 5 nm and 30 nm. The radiation source 3 can be a plasma source, for example, an LPP source (laser produced plasma) or a DPP source (gas discharged produced plasma). It can also be a synchrotron-based radiation source. The radiation source 3 can be a free-electron laser (FEL).

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 (GI), 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.

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 comprises a deflecting mirror 19 and, downstream of this in the beam path, a first facet mirror 20. The deflecting mirror 19 can be a planar 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. Of these facets 21, 1 Only a few are shown as examples. In the beam path of the illumination optics 4, a second facet mirror 22 is arranged downstream of the first facet mirror 20. The second facet mirror 22 comprises a plurality of second facets 23.

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.

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.

In the 1 In the example shown, 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.

The mirrors Mi, just like the mirrors of the illumination optics 4, can have a highly reflective coating for the illumination radiation 16.

2a ,b show a highly simplified detail of a reflective optical element in the form of an EUV mirror 25 of the projection exposure apparatus 1 of 1 , which may 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 is an intermediate layer 28, which is covered by the reflective coating 27.

In the example shown, the reflective coating 27 is a multi-layer coating for reflecting EUV radiation 16 (cf. 1 ) under normal incidence, which has alternating layers of silicon and molybdenum, which for simplicity are 2a ,b are not illustrated. 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, for example, 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, for example, from Ru.

The intermediate layer 28 can serve as a structurable layer, e.g., for creating a lattice structure not shown, 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, SiO ₂ , Ge, B, Sn, Sn alloys, bismuth, antimony, etc.

As shown in the top view of the reflective coating 27 in 2a As can be seen, the reflective coating 27 has a locally limited damage area 30 in the form of a hole, which extends over the entire thickness of the reflective coating 27, as in 2b can be seen. 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.

As in 2a ,b, the removal of the etchable material 29 is not limited to a volume region of the intermediate layer 28 lying directly beneath the damaged area 30, but rather the etchable material 29 is also removed in the vicinity of the damaged area 30, the lateral extent of which 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 formed adjacent to the damaged area 30 of the reflective coating 27, which is covered by the reflective coating 27. The substrate 26 is in the 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, for example, be 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 , AlO _x , TiO _x , TaO _x , one or more carbides, nitrides, or borides.

3a -d illustrate the time sequence until the in 2a ,b, in which the reflective coating 27 was undercut in a comparatively large area 33. As in 3a As can be seen, the mirror 25 is in the exposure mode of the EUV lithography system 1 in a vacuum environment 34 in which a hydrogen plasma 31 is generated by the influence of the EUV radiation 16, which, in addition to the 2a ,b also contains hydrogen ions 31b. Since the EUV lithography system 1 was ventilated before the exposure operation, a temporary protective wall 37 consisting of oxidized etchable material 29, in the example shown, SiO ₂ , has formed on an etching front 35 of the etchable material 29. The temporary protective wall 37 is removed during the exposure operation of the EUV lithography system 1 by the impinging hydrogen ions 31b.

As in 3b As shown, 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 the lateral direction and remove the etchable material 29 also 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 the lateral direction further into the etchable material 29 and enlarges the under-etched area 33, as shown in 3c ,d is shown.

In order to reduce the lateral extent of the undercut area 33, a process is carried out which is described below using 4a -e. In the process, in a first step, 4a ,b, the hydrogen plasma 31 is generated in the environment 34 of the mirror 25 in order to create an etching front 35 in the under-etched area 33 adjacent to the damaged area 30 on the etchable material 29, as shown in 4b is shown. To generate the hydrogen plasma 31, the EUV lithography system 1 is operated in exposure mode. The exposure mode is continued until the etching front 35 has a desired lateral distance A from the damaged area 30. The distance A can be more than 5 µm, more than 10 µm, or more than 20 µm and is selected such that the etching front 35 is sufficiently protected from the hydrogen ions 31b by a projecting section of the reflective coating 27, which is located above the under-etched area 33 and forms a roof or shield, but the roof does not yet become unstable.

Once the desired lateral distance A is reached, as shown in 4c As shown, in an oxidation operation of the EUV lithography system 1, an oxygen-containing gas, in the example shown in the form of molecular oxygen O ₂ , is supplied to the etching front 35 in the under-etched region 33. The molecular oxygen O ₂ 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 considerably slows down 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 shown in 4d can be seen.

In order to increase the blocking effect for the hydrogen radicals 31a, after supplying molecular oxygen O ₂ 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. The further etching front 35' is again supplied with an oxygen-containing gas in the form of molecular oxygen O ₂ to form a further protective wall 36', as shown in 4e can be seen. It is understood that more than two protective walls 36, 36', ... can be formed in this way in order to increase the blocking effect for the hydrogen radicals 31a.

The method described above, which comprises the steps of forming the etching front 35 and supplying an oxygen-containing gas, e.g. O ₂ , H ₂ O, O ₃ , NO ₂ , N ₂ O, mixtures thereof, ... to the etching front 35 in order to form the protective wall 36 - or optionally a plurality of protective walls 36, 36', ... - can be carried out in particular after cleaning the mirror 25, e.g. using CO ₂ , or when the mirror 25 is first put into operation.

There are various possibilities for supplying oxygen or the oxygen-containing gas O ₂ , ... 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 O ₂ , ... is supplied to the etching front 35. The EUV lithography system 1 can be ventilated when the oxygen O ₂ 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.

It is also possible that the vacuum in the environment of the mirror 25 is not broken when oxygen O ₂ is supplied to the etching front 35, ie that the mirror 25 is located in a vacuum environment 34 when the oxygen O ₂ is supplied, as shown in 4c is shown. In this case, at least one oxygen-containing gas O ₂ , ... can be supplied to the environment 34 of the mirror 25. In the example shown, the supplied gas is molecular oxygen O ₂ , but the gas can also be, for example, water or clean air. It is important that the concentration or partial pressure p _o2 of the oxygen O ₂ in the vacuum environment 34 is sufficient to bring about 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 O ₂ in the environment 34 of the mirror 25 should be more than 10 ^-3 mbar, preferably more than 1 mbar. The concentration of oxygen O ₂ , which may be introduced into the residual gas atmosphere surrounding the mirror 25 during exposure, is significantly lower and cannot cause oxidation of the etchable material, especially since hydrogen is typically also present as a reducing gas in the mirror 25 surrounding the mirror 25. The oxygen O ₂ is supplied until a protective wall 36 of sufficient thickness has formed, typically on the order of a few nanometers.

When supplying the oxygen-containing gas in the form of molecular oxygen O ₂ to the etching front 35, an oxygen-containing plasma can also be generated in the vacuum environment 34, ie activated oxygen O ₂ * or oxygen radicals can be generated. To generate the oxygen-containing plasma, oxygen O ₂ 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, generate the oxygen-containing plasma in an external plasma treatment system to oxidize the etchable material 39, and then reinsert the mirror 25 into the EUV lithography system 1.

The method described above can also be applied if no intermediate layer 28 is present, but the substrate 26 contains 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. The method can also be performed in semiconductor lithography systems other than an EUV lithography system 1.

QUOTES CONTAINED IN THE DESCRIPTION

This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 10,073,361 B2 [0005]
• DE 10 2022 212 167 A1 [0006]
• US 2011/0109892 A1 [0007]
• DE 10 2016 217 633 A1 [0008]

Claims (14)

Method for spatially limiting material removal during under-etching of a coating (27) of an optical element (25) which covers an etchable material (29) and reflects 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 (O ₂ , ...) to the etching front (35) for oxidizing the etchable material (39) to form a protective wall (36) for spatially limiting the material removal, wherein the semiconductor lithography system, in particular the EUV lithography system (1), during the supply of the oxygen-containing gas (O ₂ , ...) to the etching front (35) is not operated in normal operation, in particular not in exposure operation. Procedure according to Claim 1 , in which the etching front (35) has a lateral distance (A) of more than 5 µm, preferably of more than 10 µm, in particular of more than 20 µm, from the damaged area (30) of the reflective coating ( ₂₇ ) before the supply of the oxygen-containing gas (O 2 , ...). Procedure according to Claim 1 or 2 , in which the optical element (25) is arranged when supplying the oxygen-containing gas (O ₂ , ...) to the etching front (35) in the semiconductor technology system, in particular in the EUV lithography system (1). 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). 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 (O ₂ , ...) is supplied to the etching front (35). 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 (O ₂ , ...) is supplied to the etching front (35). Method according to one of the Claims 1 until 4 or 6 , in which, when the oxygen-containing gas (O ₂ , ...) is supplied to the etching front (35), the optical element (25) is located in a vacuum environment (34) to which the oxygen-containing gas (O ₂ , ...) is supplied. Procedure according to Claim 7 , in which a partial pressure (p _O2 ) of the oxygen-containing gas (O ₂ , ...) in the vacuum environment (34) is more than 10 ^-3 mbar, preferably more than 1 mbar. 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 (O ₂ , ...) to the etching front (35) takes place after cleaning of the optical element (25) or during the first commissioning of the optical element (25). Method according to one of the preceding claims, further comprising: removing etchable material (29) adjacent to the protective wall (36) to form a further etching front (35') in the under-etched region (33), and supplying an oxygen-containing gas (O ₂ , ...) to the further etching front (35') to oxidize the etchable material (29) to form a further protective wall (36') for spatially limiting the material removal. 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). 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. Optical element according to one of the 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). Semiconductor technology plant, in particular EUV lithography plant (1), comprising: at least one optical element (1) according to one of the Claims 11 until 13 .