EP4222539A1 - Method for producing an optical element, optical element, device for producing an optical element, secondary gas and projection exposure system - Google Patents

Abstract

The invention relates to a method for producing an optical element (2), in particular for a projection exposure system (400), according to which a protective layer (11) consisting of a protective material is applied to a surface of a main body (7) until a protective layer thickness is obtained, wherein the main body (7) has a substrate (17) comprising a reflective layer (18) applied to the substrate (17). According to the invention, the protective layer (11) is at least substantially defect-free.

Description

Process for manufacturing an optical element, optical element, apparatus for manufacturing an optical element, secondary apparatus and projection exposure apparatus

The present application claims the priority of German Patent Application No. 10 2020 212 353.5, the content of which is incorporated herein by reference in its entirety.

The invention relates to a method for producing an optical element, in particular for a projection exposure system, according to which a cover layer formed from a cover material is applied to a surface of a base body until a cover layer thickness is reached, the base body having a substrate with a reflection layer applied to the substrate.

The invention also relates to an optical element, in particular a mirror of a projection exposure system, with a base body, the base body having a substrate with a reflective layer applied to the substrate, and a cover layer applied to a surface of the base body and made of a cover material and having a cover layer thickness having.

The invention also relates to a device for producing an optical element, in particular for a projection exposure system, with a target made of a target material, a coating device which is set up for separating particles of the target material by means of an ionized working gas for coating a base body, the base body having a Having a substrate with a reflective layer applied to the substrate, and a working chamber for receiving the base body and a vacuum device for forming a vacuum in the working chamber.

The invention also relates to the use of a secondary gas.

The invention also relates to a projection exposure system for semiconductor lithography with a radiation source and an optical system which has at least one optical element.

In a known manner, optical elements influence the properties of light waves interacting with them. Precise surface processing of the optical elements is necessary to avoid undesirable structures in the resulting wave fronts. Examples of optical elements that can be mentioned are planar mirrors, concave mirrors, convex mirrors, facet mirrors, convex lenses, concave lenses, convex-concave lenses, plano-convex lenses and plano-concave lenses. Glass and silicon, among others, are known as materials for optical elements, in particular mirrors.

Projection exposure systems have a large number of optical elements. In particular when using the optical elements in a microlithographic EUV (Extreme Ultra Violetj projection exposure system), the nature of the optical elements is of particular importance. The optical elements are exposed to a variety of harmful influences that change their nature and thus impair their functionality, since the light modulated by the optical elements, for example an EUV mirror, has a very small wavelength and the resulting wave fronts do be disturbed by the slightest impairment of the condition of the optical element. On the other hand, the structures shown on the projection surface are very small and therefore also susceptible to the slightest change in the nature of the optical element. The damaging influences that can act on the optical element include, for example, EUV light, which is highly energetic and whose energy can damage the optical element when it is absorbed.

It is known in practice that, in order to generate EUV light, tin droplets are ionized in such a way that a plasma is created which emits EUV radiation in all directions.

The EUV radiation is caught, for example, by collector mirrors which, in addition to the EUV light emitted by the plasma, are also exposed to the damaging effects of tin ions and tin drops. Furthermore, the resulting plasma has a damaging effect on the collector mirrors due to the presence of, for example, hydrogen ions and radicals, oxygen species and oxygen radicals, water or water in the gas phase, nitrogen species and nitrogen radicals, noble gases and noble gas ions and the reaction products of the named gases.

In addition, optical elements of an EUV projection exposure system are exposed to contaminants, such as hydrocarbons. In the area of the tin plasma, there are also very high temperatures which, due to thermal expansion, in particular different thermal expansion of various elements of the optical element, in particular the collector mirror, can lead to warping, warping and, in particular, to damage.

Cleaning media used to clean the optical elements of, for example, tin contamination and/or hydrocarbon contamination are often also detrimental to the optical elements.

The above-mentioned damaging effects can become noticeable on the optical element by blistering of the coatings and/or by the detachment of layers of the coatings and/or by the application of an unwanted layer of tin and an unwanted mixing of the layer materials forming the layers with tin.

In order to prevent the named damaging effects from impairing optically relevant layers of the optical element, it is known from practice to provide the optical element with a cover layer. US Pat. No. 8,501,373 B2 describes a passivation of a multi-layer mirror for EUV lithography.

It is also known from practice that the optical element has a substrate on which a reflection layer system is applied, on which in turn a barrier layer or a plurality of barrier layers are applied. Accordingly, the task of the cover layer is to protect the at least one barrier layer underneath it and the reflective layer system underneath it from external influences.

However, the top layer itself is also exposed to the damaging influences described, which is why damage to the top layer itself can also be observed.

It is known from the prior art that the cover layer is formed by means of sputtering.

A disadvantage of the prior art is that the cover layers produced according to the prior art only have a low durability under the damaging influences mentioned. In particular, covering layers produced according to the state of the art can show damage patterns, such as blistering, delamination, tin coating and mixing with tin, even after a short time.

Further disadvantages of methods known from the prior art for producing a cover layer are oxidation and/or mixing of layers lying underneath the cover layer during a deposition process of the cover layer. In this case, the oxidation and/or the mixing can lead to a reduction in the reflectivity of the optical element, in particular of a mirror.

A further disadvantage is, for example, a defective change in the target, in English target poisoning, if the top layer is formed by means of reactive sputtering.

The object of the present invention is to provide a method for producing an optical element which avoids the disadvantages of the prior art, in particular providing long-lasting and functional cover layers.

According to the invention, this object is achieved by a method having the features specified in claim 1.

The present invention is also based on the object of creating an optical element, in particular a mirror of a projection exposure system, which avoids the disadvantages of the prior art and is durable and functional, in particular with regard to its optical properties. According to the invention, this object is achieved by an optical element having the features specified in claim 23.

The present invention is also based on the object of creating a device for producing an optical element which avoids the disadvantages of the prior art and in particular enables long-lasting and functional cover layers to be formed.

According to the invention, this object is achieved by a device having the features specified in claim 30.

The present invention is also based on the object of making available a secondary gas which avoids the disadvantages of the prior art, with the formation of a long-lasting and functional top layer being made possible in particular.

According to the invention, this object is achieved by a secondary gas having the features specified in claim 42 .

The present invention is also based on the object of creating a projection exposure system for semiconductor lithography which avoids the disadvantages of the prior art and in particular enables long-lasting and reliable operation.

According to the invention, this object is achieved by a projection exposure system having the features specified in claim 43.

In the method according to the invention for producing an optical element, in particular for a projection exposure system, a cover layer formed from a cover material is applied to a surface of a base body until a cover layer thickness is reached, the base body having a substrate with a reflection layer applied to the substrate. According to the invention, it is provided that the cover layer is formed at least approximately free of defects.

The inventors have recognized that cover layers of an optical element which are designed to be at least approximately defect-free have an advantageously long service life and an advantageously high performance.

Defects can be understood to mean, for example, structural defects, such as pinholes, pores, grain boundaries and/or dislocations, as well as particle and/or contamination deposits.

One can say that there are almost no defects, for example, if the number of defects within an area of 100 μm ² is less than 11. In the context of the invention, the base body is to be understood as being formed from a substrate with a reflection layer applied to the substrate and optionally one or more barrier layers applied to the reflection layer, optionally only in partial areas. That is, according to the invention, the top layer is applied to a reflective layer—located below the top layer after the top layer has been applied—if the surface of the base body is formed by a reflective layer or the top layer is applied to one or more—after the top layer has been applied, underneath the top layer lying - applied barrier layers, provided that the surface of the base body is formed by one or more barrier layers.

The formation of the base body from a substrate with a reflection layer applied to the substrate is to be understood within the scope of the invention in a broad interpretation such that the base body has a substrate with a reflection layer applied to the substrate. In particular, the base body can also have a barrier layer, as illustrated above.

In a narrow interpretation, the base body is to be understood within the scope of the invention as merely being formed from a substrate with a reflection layer applied to the substrate and optionally one or more barrier layers applied to the reflection layer, optionally only in partial areas.

The reflection layer can have a multilayer coating.

The multilayers of the multilayer can alternately have a material with a higher real part of a refractive index at a working wavelength of the optical element and a material with a lower real part of the refractive index at the working wavelength of the optical element.

It can be provided that the substrate has a material with a low coefficient of thermal expansion, e.g. Zerodur®, ULE® or Clearceram®. The optical element can be designed to reflect EUV radiation which strikes the optical element under normal incidence, i.e. at angles of incidence α typically less than approximately 45° to the surface normal. A reflective multilayer system, i.e. a reflective multilayer coating, can be applied to the substrate for the reflection of the EUV radiation. The multi-layer system can have alternately applied layers of a material with a higher real part of the refractive index at the working wavelength (also called spacer) and a material with a lower real part of the refractive index at the working wavelength (also called absorber), with an absorber-spacer pair forming a stack. This structure of the multilayer system simulates a crystal in a certain way, the lattice planes of which correspond to the absorber layers at which Bragg reflection takes place. In order to ensure sufficient reflectivity, the multilayer system can have a number of generally more than fifty alternating layers.

The thicknesses of the individual layers as well as the repeating stacks can be constant over the entire multi-layer system or also vary, depending on which spectral or angle-dependent reflection profile is to be achieved. The reflection profile can also be influenced in a targeted manner by adding more or less absorbing materials to the basic structure of absorber and spacer in order to increase the possible maximum reflectivity at the respective working wavelength. For this purpose, absorber and/or spacer materials can be exchanged for one another in some stacks, or the stacks can be constructed from more than one absorber and/or spacer material. The absorber and spacer materials can have constant or varying thicknesses across all stacks in order to optimize the reflectivity. Furthermore, additional layers can also be provided, for example as diffusion barriers between spacer and absorber layers.

In a possible exemplary embodiment, in which the optical element is optimized for a working wavelength of 13.5 nm, i.e. in an optical element which has the maximum reflectivity at a wavelength of 13.5 nm with essentially normal incidence of EUV radiation, the stacks of the multi-layer system can have alternating silicon layers and molybdenum layers. The silicon layers correspond to the layers with the higher real part of the refractive index at 13.5 nm and the molybdenum layers to the layers with the lower real part of the refractive index at 13.5 nm. Depending on the exact value of the working wavelength, other material combinations such as molybdenum and Beryllium, ruthenium and beryllium or lanthanum and B4C also possible.

The reflective layer can in particular be a reflective layer system, for example formed by a multi-layer coating made of molybdenum-silicon (MoSi).

The one or more barrier layers lying optionally on the reflection layer can be advantageous in particular in the case of mirrors with normal incidence (normal incidence, NI).

The reflection layer can also be a, preferably comparatively thick, coating for mirrors with grazing incidence (Gl).

If there are complex layer systems between the substrate and the cover layer, such as a reflection layer system, and possibly one or more barrier layers, these are particularly susceptible to harmful effects, for example from a plasma when generating an EUV light.

Layer systems of this type therefore particularly benefit from a cover layer formed without defects using the method according to the invention.

Furthermore, the production of such a base body precoated by a reflective layer and possibly one or more barrier layers is particularly complex and therefore also cost-intensive, so that a longevity of a cover layer is of particular importance. Furthermore, a cover layer should be applied to such a base body using a particularly reliable method, since an incorrectly applied cover layer can in this case mean that the base body can no longer be used.

In an advantageous development of the method according to the invention, it can be provided that the cover layer is formed with sharp boundaries.

The inventors have found that a cover layer of an optical element that has sharp boundary surfaces with the base body is particularly durable and has high performance. In particular, it can be provided that the cover material forming the cover layer and the surface material forming the surface of the base body penetrate each other by less than 10 nm, preferably less than 1 nm, preferably less than 0.1 nm.

In an advantageous development of the method according to the invention, it can be provided that particles of a target material are continuously isolated on at least one target by bombardment with ions of a working gas, with a discharge voltage being applied for at least indirect ionization of the working gas and with the cover layer in connection with a defect-avoiding method is trained.

By expanding a sputtering process with a defect-avoiding process, the cover layer can be formed by sputtering, as is known from the prior art, whereby the use of sputtering systems known from the prior art, for example, allows a particularly efficient production of the cover layer. To avoid defects, the sputtering process is expanded to include a defect-avoiding process in order to advantageously reduce the density of defects per unit area of the cover layer in such a way that a cover layer that is almost defect-free can be formed.

In an advantageous development of the method according to the invention, it can be provided that the cover layer is formed from a cover material which has a stoichiometric composition.

It can be advantageous if the cover material is in a chemically pure form. The cover layer is thus formed from a chemically pure cover material, as a result of which defects are minimized, for example by avoiding lattice defects which can be caused by the incorporation of chemically foreign particles. A stoichiometric composition has the elements forming the cap material in the stoichiometric ratio of those compounds which are desired to form the cap material. A stoichiometric composition of the cover material can thus indicate a chemical purity of the cover material.

In particular, the use of oxides as cover material can be advantageous. An advantageous development of the method according to the invention can consist in zirconium oxide, ZrOx and/or titanium oxide, TiO _x and/or niobium oxide, NbO _x and/or yttrium oxide, YO _x and/or hafnium oxide, HfO _x and/or cerium oxide, CeO _x and/or lanthanum oxide, LaO _x and/or tantalum oxide, TaO _x and/or aluminum oxide, AlO _x and/or erbium oxide, ErO _x and/or tungsten oxide, WO _x and/or chromium oxide, CrO _x and/or scandium oxide, ScO _x and/or vanadium oxide, VO _X are provided in pure form and/or as a mixture as cover material.

In particular, it can also be provided that multi-component mixtures of at least two or more of the oxides mentioned form the covering material.

In particular, it can be provided that the cover material consists of one of the aforementioned oxides or a mixture of these, as well as possibly unavoidable impurities. The cover layer can preferably consist exclusively of the cover material.

The index x as the stoichiometric coefficient in the aforementioned oxides describes the stoichiometric composition between the element forming the oxide and the oxygen. The index x can be an integer index but also a rational number resulting from the chemistry of the respective oxides.

Furthermore, in an advantageous development of the method according to the invention, it can be provided that the cover layer thickness is 0.1 nm to 20 nm, preferably 0.3 nm to 10 nm, particularly preferably 0.5 to 3 nm.

In the context of the invention, such a layer thickness has shown particularly advantageous properties with regard to service life, performance and optical properties.

Furthermore, it can be provided that the cover layer is formed from an amorphous and/or a crystalline cover material. In particular, it can be provided that the covering material has both an amorphous and a crystalline structure, with zones having an amorphous structure and zones having a crystalline structure preferably being arranged along the surface normal of the surface.

In particular, it can be provided that the optical element is a mirror with grazing incidence (English: grazing incidence, GI) and/or that the optical element is a mirror for perpendicular incidence (English: normal incidence, NI).

In an advantageous development it can be provided that the particles of the target material form a cover material, move to the base body and are deposited on the base body and form the cover layer. If the covering material is formed from particles of a target material, the covering material can be made available in a particularly simple manner since it is identical to the target material. Further treatment of the target material is therefore not necessary and the application of the cover layer is therefore particularly efficient.

Such a method is called sputtering or cathode sputtering.

A comparison of gentle sputtering methods without a connection to optical elements is described in the publication Comparison of low damage sputter deposition techniques to enable the application of very thin a-Si passivation films, AIP Conference Proceedings 2147, 040009 (2019). Reference is made here to use in silicon solar cells.

In a further advantageous development of the method according to the invention, it can be provided that a reaction gas reacts with the particles of the target material and forms particles of the cover material and the particles of the cover material move to the base body and are deposited on the base body and form the cover layer.

Representing the covering material by reacting the particles of the target material with a reaction gas has the advantage that the surface enlargement of the target material associated with the separation of the particles of the target material means that a reaction of the particles of the target material with the reaction gas is particularly complete. If the cover material thus formed settles on the raw material, a particularly advantageously chemically pure and thus stoichiometric cover layer is formed.

Such a process is often called reactive sputtering.

An advantageous development of the invention can consist in the fact that the reaction gas is oxygen.

The oxides described above can be prepared particularly advantageously in situ by using oxygen as the reaction gas.

A further advantageous development of the invention can consist in the defect-avoiding method being designed in such a way that a damage potential of particles of the target material after separation and/or particles of the cover material and/or ions and/or atoms and/or electrons of the working gas and /or particles of the reaction gas are reduced in terms of at least one damage parameter before they strike the base body and/or a covering layer that forms.

An embodiment of the defect-avoiding method in such a way that a damage potential of particles with regard to at least one damage parameter is reduced has the advantage that harmful Influences that can lead to defects in the top layer and are mediated by particles are reduced in a targeted manner. In particular, at the microscopic level of particles, a damage potential can be assigned particularly efficiently to one or more damage parameters. Influencing the properties of the particles that contribute to the damage parameter can therefore lead to a reduction in the damage potential in a particularly targeted manner. The microscopic analysis of the damage potential for the damage parameters leading to the damage potential enables a targeted reduction of the damage potential by influencing the damage parameters.

An advantageous development of the invention can consist in the at least one damage parameter being a kinetic energy which is preferably above a threshold value and is reduced.

If a kinetic energy that is greater than a threshold value is identified as the damage parameter, the kinetic energy of particles that is greater than this threshold value can advantageously be reduced. Particles, in particular ions of the working gas, which have a particularly high energy, in particular a kinetic energy which is above the threshold value, can have a particularly damaging effect on the base body or the covering layer which is being formed. For example, ions of the working gas, which are reflected at the target with high kinetic energy and hit the base body and/or the covering layer that is being formed, can shoot so-called pinholes into the layer that is being formed. Furthermore, the chemical composition of the cover layer and/or the base body can be changed by the ions penetrating into the cover layer and/or the base body. This can lead to the occurrence of defects. It is therefore advantageous either to reduce a kinetic energy that is above a damage-causing threshold value and/or to prevent particles that have such a high kinetic energy from striking the base body and/or the covering layer that is being formed. Both measures can reduce the damage parameter of the kinetic energy of particles that hit the base body and/or the covering layer that forms.

An advantageous development of the invention can consist in the cover layer being formed by sputtering, with charged particles being caught in a magnetic trap in the defect-avoiding method.

It can be advantageous to keep the charged particles, in particular ions of the working gas, trapped in a magnetic trap by the effect of a magnetic field. Since a Lorentz force acts particularly on particles with high speed and thus also high kinetic energy, a magnetic trap can advantageously prevent particularly fast particles from reaching the base body and/or the covering layer that is being formed, by having these particularly fast particles, for example, fall on a be forced away from the base body and/or the covering layer that is being formed. In this way, the damage potential of these very fast particles can be reduced. In an advantageous development of the method according to the invention, it can be provided that the defect-avoiding method is designed as facing-targets sputtering.

By supplementing the sputtering process with a defect-avoiding process in the form of an arrangement of two targets in such a way that the targets face each other, it can be achieved that, for example, particles with a high kinetic energy, which are reflected on one target, do not fall on the base body and/or or the forming cover layer, but on the second target opposite the first target. This increases the probability that the fast-moving particle will separate a particle of the target material and thus not have a damaging effect on the base body and/or the covering layer that is being formed.

A combination of the configuration of opposing targets and a magnetic trap can be used to ensure that charged particles, which have a high kinetic energy, are trapped in particular in the space between the opposing targets.

Defects are also avoided by reducing the damage potential of the particles.

Without having a connection to optical elements, facing targets sputtering is known from the documents EP 1 505 170 B1, DE 11 2008 000 252 T5, WO 2018/069091 A1 and EP 3 438 322 A1.

The publication EP 1 505 170 B1 describes a use of facing target sputtering in connection with organic electroluminescent devices.

The publication DE 11 2008 000 252 T5 describes the use of facing target sputtering in connection with the formation of a thin film on a resin substrate.

The publication WO 2018/069091 A1 describes a use of facing target sputtering in connection with the production of light-emitting diodes (LED).

The publication EP 3 438 322 A1 mentions the use of facing target sputtering in connection with the production of liquid crystal displays and solar batteries.

In an advantageous development of the method according to the invention, it can be provided that the ions of the working gas are formed by a remote plasma source in the defect-avoiding method.

If a plasma source, which forms ions of the working gas from the working gas, is at a spatial distance from the base body and/or the covering layer that is being formed, very rapid impacts are possible Ions of the working gas on the base body and/or the covering layer being formed are less likely than when the plasma is formed in the immediate vicinity of the target and/or the base body and/or the covering layer being formed. In particular, high-energy particles can be filtered out by transporting the ionized working gas to the target.

It can be advantageous if the discharge voltage is reduced when using a secondary gas in the defect-avoiding method.

As a result, an acceleration and thus also a kinetic energy and thus also a damage potential of charged particles can advantageously be reduced.

It can be advantageous if the cover layer is formed by sputtering, with the at least one target being formed as a dual-cathode magnetron with an active and/or passive anode in the defect-avoiding method.

By using a dual-cathode magnetron, high-energy, charged particles can advantageously be trapped by the formation of a magnetic field. In this case, when an anode of the magnetron is designed as an active anode, the anode causes both the ionization of the working gas and the capture of the high-energy particles. If the anode is designed as a passive anode, on the other hand, only the magnetic field of the magnetron is formed by the anode.

A dual-cathode magnetron for coating processes is known from publication EP 2 186 108 B1 without having a connection to optical elements.

It can be advantageous if the cover layer is formed by sputtering, with the working gas being ionized by Penning ionization with a secondary gas in the defect-avoiding method.

If the working gas is ionized by Penning ionization with a secondary gas, the secondary gas first changes to an electronically excited state and transfers its excitation energy to the working gas, causing it to be ionized. As a result, ions of the working gas can be made available without the working gas being ionized directly by the discharge voltage. This can lead to an advantageously lower occurrence of particularly fast ions in the working gas.

Without having a connection to optical elements, a Penning ionization in the sputtering process is known from the publication Influence of Unbalanced Magnetron and Penning ionization for RF Reactive Magetron Sputtering, Jpn. J. Appl. physics Vol 38 (1999) pp.186-191, Part 1 No. 1A, January 1999. Without having a connection to optical elements, Penning ionization in the sputtering process is also known from the publication Are the Argon metastables important in high power impulse magnetron sputtering discharges?, PHYSICS OF PLASMA 22, 113508 (2015).

Without showing a connection to optical elements, a Penning ionization in the sputtering process is also known from the publication Niobiom films produced by magnetron sputtering using an AR-HE mixture as discharge gas Proceedings of he 1995 Workshop on RF Superconductivity, Gif-sur Yvette, France (SRF95C22), pp 479 - 483 known. A use for the production of superconducting radio frequency acceleration resonators is described here.

It can be advantageous if the discharge voltage is reduced when using a secondary gas in the defect-avoiding method.

Advantageously, the discharge voltage can be reduced as a result of the Penning ionization, as a result of which ionized particles in the working gas are accelerated to a lesser extent by the electric field caused by the discharge voltage and thus have a lower kinetic energy. This reduces the damage potential of charged particles in a manner that avoids defects.

In an advantageous development of the invention, it can be provided that an electronic activation energy of the secondary gas is greater than an ionization energy of the working gas.

In order to ensure a particularly efficient process of the Penning ionization, it can be advantageous if the electronic activation energy of the secondary gas is greater than the ionization energy of the working gas, which means that there is a particularly high probability of a particle of the working gas ionizing when it encounters an electronically activated particle Particles of the secondary gas can be effected.

In particular, it can advantageously be provided that the secondary gas is helium and the working gas is argon.

It can be advantageous if the cover layer is formed by sputtering and, in the defect-avoiding process, the particles of the cover material after separation and the ions and/or atoms and/or electrons of the working gas are energetically matched with the working gas by thermalization.

If the probability of the particles colliding before they hit the base body and/or the covering layer being formed is increased, then it is likely that very fast particles will transfer part of their kinetic energy to other particles in the event of a collision. If this process is repeated frequently, the kinetic energies of the particles are energetically adjusted. In order to achieve a sufficiently high collision probability, the particles must have a mean free path be reduced. This prevents a very fast particle from hitting the base body and/or the covering layer that is being formed without braking.

It is advantageous if a pressure of the working gas is set in such a way that thermalization occurs in the defect-avoiding method.

In particular, the probability of a fast particle colliding with other less fast particles can be increased by increasing the pressure of the working gas in such a way that the density of the working gas is so high that an unbraked collision of a fast particle with the base body and/or or the forming top layer is reliably prevented.

Alternatively, it can be provided that a pressure of a thermalization gas is set in such a way that a collision of fast particles of the working gas with particles of the thermalization gas is sufficiently likely to prevent the fast particles from striking the base body and/or the covering layer that forms. The thermalization gas can be, for example, the secondary gas and/or another gas and/or a mixture of other gases. In particular, it can be provided that the thermalization gas is chemically inert in order to prevent the formation of any undesirable reaction products.

It is advantageous if the cover layer is formed by sputtering, with the at least one target being heated and/or melted in the defect-avoiding method.

If the target is heated and/or melted, a lower kinetic energy can be sufficient to separate the particles of the target. This also significantly reduces the probability of the occurrence of particles with particularly high kinetic energy, which can have a potential for damage. This is due to the fact that the energy that is necessary to separate a particle of the target has already been made available in part by the thermal energy that has been supplied to the target.

The use of a heated and/or melted target also enables particles of the target material and/or the cover material formed from the target material to have an advantageously high kinetic energy, while the ions of the working gas have an advantageously low kinetic energy. A high kinetic energy of the particles of the target material can lead to an advantageously sharp definition of the boundaries between the surface of the raw material and the covering layer. It can therefore be advantageous if the particles of the covering material hit the surface of the base body and/or the covering layer that is being formed within a certain speed range. It is therefore advantageous if the particles of the covering material are neither too slow nor too fast. When using a heated and/or melted target, the particles of the cover material can be sufficiently fast to form an almost defect-free cover layer and at the same time be sufficiently slow to avoid defects in the cover layer. In particular, it can be provided that the target is melted and, in addition, is heated until the target material is vaporized. Furthermore, heating the target can advantageously contribute to the sublimation of the target material.

Without having a connection to optical elements, a molten target sputtering method for coating methods is known from publication US 2017/0268122 A1.

Without having a connection to optical elements, a sputtering method with evaporation of the target for coating methods is known from the publication Magnetron Deposition of Coatings with Evaporation of the Target, Technical Physics, 2015, Vol. 60, No. 12, pp. 1790-1795.

It is advantageous if the cover layer is formed by sputtering, with the ions of the working gas being decelerated by an electric field of a grid, which has an electric potential, in the defect-avoiding method.

Advantageously, particularly fast charged particles can also be decelerated by an electric field of a grid, as a result of which their kinetic energy is reduced and their damage potential is reduced.

In particular, it is advantageous if the lattice is positioned in a potential trajectory of a very fast particle in the direction of the base body and/or the covering layer being formed and the lattice has a potential which has a sign which is opposite to that of the charge of the charged particle is. As a result, a charged particle flying towards the lattice does work against the electric field and thus reduces its kinetic energy. A particle passing through the lattice is then accelerated away from the lattice, but if the lattice is suitably positioned at a suitable distance from the base body, it can no longer build up sufficient kinetic energy to develop a potential for damage. On the other hand, particles of the target material and/or the cover material, which move to the base body and form the cover layer, are not charged and can pass through the grid.

The invention further relates to an optical element according to claim 25

The optical element according to the invention, in particular a mirror of a projection exposure system, has a base body, the base body having a substrate with a reflection layer applied to the substrate, and a cover layer formed of a cover material applied to a surface of the base body. In this case, the top layer has a certain top layer thickness. According to the invention, it is provided that the cover layer is designed to be at least approximately defect-free. An almost defect-free design of the top layer on the surface of the base body can advantageously extend the service life and performance of the optical element, for example by preventing and preventing the top layer and/or a layer underneath the top layer, which is a reflective layer, for example, from flaking off /or is reduced.

The base body is also to be understood within the scope of the invention as being formed from a substrate with a reflective layer applied to the substrate and optionally one or more barrier layers applied to the reflective layer. That is, according to the invention, the top layer is applied to a reflective layer—located under the top layer after application of the top layer—if the surface of the base body is formed by a reflective layer or the top layer is applied to one or more—after application of the top layer under the top layer lying - barrier layers applied if the surface of the base body is formed by at least one barrier layer.

The formation of the base body from a substrate with a reflection layer applied to the substrate is to be understood within the scope of the invention in a broad interpretation such that the base body has a substrate with a reflection layer applied to the substrate. In particular, the base body can also have a barrier layer, as illustrated above.

In a narrow interpretation, the base body is to be understood within the scope of the invention as merely being formed from a substrate with a reflection layer applied to the substrate and optionally one or more barrier layers applied to the reflection layer, optionally only in partial areas.

The optical element according to the invention is particularly suitable for use in an EUV projection exposure system. Using the optical element as a collector mirror can also be advantageous here.

In an advantageous development of the optical element according to the invention, it can be provided that the cover layer has sharp boundaries.

Particularly sharp boundary surfaces between the cover layer and the surface of the base body, i.e. between the cover layer and underlying reflective layers and/or at least one barrier layer, lead to a particularly advantageously pronounced chemical purity of the cover layer and the reflective layers forming the surface of the base body and/or the at least one barrier layer forming the surface of the base body. In particular, this can prevent particles from the cover layer from penetrating the underlying reflective layers and/or the at least one underlying barrier layer of the base body and/or particles from the reflective layers forming the surface of the base body and/or the at least one barrier layer from penetrating the cover layer penetration. In particular, it can be provided that the particles of the reflective layers forming the surface of the base body and/or at least one barrier layer and the cover material are no deeper than 10 nm, preferably no deeper than 5 nm, preferably no deeper than 0.1 nm, preferably no deeper than penetrate one atomic layer, preferably less than one atomic layer, deep into each other.

In particular, a loss of function of both the cover layer and the reflective layers forming the surface of the base body and/or the at least one barrier layer can be avoided by a sharp definition of the boundary surfaces.

In an advantageous development of the optical element according to the invention, it can be provided that the cover material has a stoichiometric composition.

A cover material with a stoichiometric composition and thus a cover layer with a stoichiometric composition have the advantage that, with a particularly high chemical purity that is pronounced in this way, defects due to foreign atoms and foreign particles in the cover material are advantageously reduced.

In an advantageous development of the optical element according to the invention, it can be provided that the cover layer is formed by facing target sputtering.

If the cover layer is formed by facing target sputtering, the occurrence of undesired defects is reduced and a cover layer that is almost defect-free is thus formed.

In an advantageous development of the optical element according to the invention, it can be provided that the cover layer is formed by sputtering in conjunction with Penning ionization.

If the cover layer is sputtered onto the surface of the base body, with the working gas of the sputtering process being ionized by means of Penning ionization, the occurrence of particularly fast particles can be reduced and thus also the occurrence of undesired defects in the cover layer.

In an advantageous development of the optical element according to the invention, it can be provided that the cover layer is formed by sputtering in conjunction with thermalization.

An advantageous development of the optical element according to the invention can consist in zirconium oxide, ZrO _x and/or titanium oxide, TiO _x and/or niobium oxide, NbO _x and/or yttrium oxide, YO _x and/or hafnium oxide, HfO _x and/or cerium oxide , CeO _x and/or lanthanum oxide, LaO _x and/or tantalum oxide, TaO _x and/or aluminum oxide, AlIO _X and/or erbium oxide, ErO _x and/or tungsten oxide, WO _X and/or chromium oxide, CrO _x and/or scandium oxide, ScO _x and / or vanadium oxide, VO _X are provided in pure form and / or as a mixture as a cover material. In particular, it can also be provided that multi-component mixtures of at least two or more of the oxides mentioned form the covering material.

Furthermore, in an advantageous development of the optical element according to the invention, it can be provided that the cover layer thickness is 0.1 nm to 20 nm, preferably 0.3 nm to 10 nm, particularly preferably 0.5 to 3 nm.

The invention further relates to a device for producing an optical element according to claim 33.

The device according to the invention for producing an optical element, in particular for a projection exposure system, comprises a target made of a target material, a coating device which is set up for separating particles of the target material by means of an ionized working gas for coating a base body, the base body being a substrate with has a reflection layer applied to the substrate, as well as a working chamber for receiving the base body and a vacuum device for forming a vacuum in the working chamber. According to the invention, at least one limiting device is provided in order to limit the energy of the particles after separation and/or the ions and/or electrons and/or atoms of the working gas which strike the base body. Such a device has the advantage that defects caused by high-energy particles hitting the base body are reduced.

The coating device can in particular be a cathode atomization device or sputtering device.

A cover layer of a base body formed in the device according to the invention is thus advantageously at least approximately defect-free or has few defects.

In an advantageous further development of the device according to the invention, it can be provided that the energy is kinetic energy.

In an advantageous further development of the device according to the invention, it can be provided that the limiting device is designed in such a way that a density of the working gas in the vacuum is changed.

A kinetic energy of particles that hit the base body can be limited in particular by increasing the probability of collision of fast-moving particles with other particles before they hit the base body. This can be done by increasing a density of the working gas in the vacuum compared to a device without a restriction device. As a result, there are more particles of the working gas in a unit volume of the vacuum, with which the probability of collision increases and the kinetic energy of fast-moving particles in particular of the working gas is adjusted.

Such an adjustment process is often referred to as thermalization.

In an advantageous further development of the device according to the invention, it can be provided that the limiting device is designed as a Penning ionization device in such a way that a secondary gas is supplied to the working gas.

If a secondary gas is supplied to the working gas, the working gas can be ionized by the electronically excited secondary gas by means of Penning ionization. In order to feed the secondary gas to the working gas and/or the working chamber, a secondary gas feed device can be provided, which feeds a certain quantity of secondary gas to the working chamber and/or the working gas depending on the pressure prevailing in the chamber and/or a quantity of working gas in the chamber .

In particular, it can be provided that the working gas is supplied with between 10 and 1% by volume, preferably between 1 and 5% by volume, preferably 3% by volume, of secondary gas.

In a particularly advantageous manner, it can be provided that the working gas is argon and the secondary gas is helium.

In a further advantageous further development of the device according to the invention, it can be provided that the limiting device is designed in such a way that an electronic activation energy of the secondary gas is greater than an ionization energy of the working gas.

In a further advantageous further development of the device according to the invention, it can be provided that the limiting device is designed as a magnetic trap.

Forming the limiting device as a magnetic trap has the advantage that particularly fast charged particles can be prevented particularly efficiently in a magnetic trap from striking the base body due to the speed dependency of a Lorentz force.

In a further advantageous further development of the device according to the invention, it can be provided that the limiting device is designed as a heating device for heating and/or melting the target.

The limiting device can advantageously also be designed as a heating device in order to heat and melt the target and thus reduce the energy required to separate a particle of the target material on the target. As a result, ions of the working gas with a lower kinetic energy can be used in an advantageous manner, which is a probability impact of high-energy particles on the raw material and/or the covering layer that is formed is reduced. At the same time, the particles of the target material can have a sufficiently large kinetic energy to form an at least approximately defect-free cover layer on the base body.

In an advantageous further development of the device according to the invention, it can be provided that the limiting device is formed by a grid on an electrostatic potential.

If the limiting device is formed by a grid at a potential, fast-flying charged ions of the working gas can be slowed down by repelling forces if the potential is suitably selected before they hit the base body and/or the cover layer that is formed.

In an advantageous development of the device according to the invention, it can be provided that the limiting device is designed as an afterglow device.

The use of an afterglow plasma is described in US Pat. No. 7,338,581 B2 without being connected to optical elements.

The use of an afterglow device can cause afterglow of the plasma to separate the particles of the target material instead of glowing of the plasma.

In this way, a potential for damage from high-energy particles in the working gas and from high-energy particles in the target material can be advantageously reduced.

In particular, the damage potential can be caused by a significantly different plasma chemistry of the working gas in the afterglow of the plasma compared to the plasma chemistry of the working gas in the plasma glow.

The afterglow of the plasma of the working gas is still a plasma and thus retains most of the properties of a plasma. A harmful potential of the working gas can, however, be advantageously reduced when using afterglow.

In an advantageous development of the device according to the invention, it can be provided that the afterglow device is designed as a remote plasma source.

A remote plasma source forms a plasma of the working gas that is spatially separated from external electromagnetic fields that initiate a discharge of the working gas and thus cause the plasma to form. A plasma that is removed from the external electromagnetic fields and is fed to the target, for example, shows an afterglow when it comes into contact with the target, which leads to a reduced formation of high-energy particles of the target material and/or high-energy particles of the working gas.

In an advantageous development of the device according to the invention, it can be provided that the afterglow device is designed as a pulsed plasma source.

In addition to the above-described spatial separation of the discharge of the plasma and the separation of the target material, this separation can also be effected in terms of time, ie in a time domain. The pulse plasma source discharges the working gas for a short time. In the subsequent time range, there is no active discharge from the plasma source, only afterglow. A large part of the separation process of the particles of the target material is therefore caused by an afterglow of the plasma of the working gas. As a result, a potential damage from high-energy particles in the working gas and/or the target material can be reduced.

An important advantage of a temporal separation between discharge and effect on the target compared to a spatial separation is that a temporal separation can be realized in a closed device. As a result, it is not necessary to transport the afterglow plasma from the plasma source to the target.

As a result, the device according to the invention can be implemented in a significantly more compact manner in comparison to the remote plasma source.

In an advantageous development of the device according to the invention, it can be provided that the limiting device is designed as two targets lying opposite one another.

Such a facing-targets-sputtering geometry has the advantage that high-energy particles of the working gas do not hit the coating and/or the optical element directly.

According to claim 45, the invention further relates to a secondary gas for use in a device according to one of claims 33 to 44.

The secondary gas according to the invention is suitable for use in a device according to one of claims 33 to 44 and/or in a method according to one of claims 1 to 24. According to the invention, it is provided that an electronic activation energy of the secondary gas is greater than an ionization energy of the working gas .

This can advantageously guarantee a high probability of ionization of a particle of the working gas when it comes into contact with an electronically activated particle of the secondary gas. It can advantageously be provided that the secondary gas is a helium and/or a neon and/or an argon and/or a krypton and/or a xenon and/or a mercury and/or a radon and/or a frankium and/or is a hassium. The ionization energies are arranged in descending order in the given order of the elements.

The person skilled in the art can select suitable combinations of at least one working gas and at least one secondary gas from the substances mentioned

In particular, provision can be made for helium and/or neon, which have a higher ionization energy than argon, to be provided as the secondary gas, with argon being provided as the working gas.

The invention also relates to a projection exposure system according to claim 46.

Projection exposure systems have a large number of optical elements. In particular when using the optical elements with a microlithographic EUV (Extreme Ultra Violet) projection exposure system, an optical element produced at least partially with a method according to the invention and/or the device according to the invention can advantageously be used.

Furthermore, it can be provided that the projection exposure system according to the invention has at least one optical element according to the invention, in particular in the form of at least one mirror according to the invention.

Features that have been described in connection with one of the objects of the invention, namely given by the method according to the invention, the optical element according to the invention, the device according to the invention, the secondary gas according to the invention and the projection exposure system according to the invention, can also be advantageously implemented for the other objects of the invention. Advantages that were mentioned in connection with one of the objects of the invention, namely given by the method according to the invention, the optical element according to the invention, the device according to the invention, the secondary gas according to the invention and the projection exposure system according to the invention, can also be understood in relation to the other objects of the invention .

In addition, it should be pointed out that terms such as "comprising", "having" or "with" do not exclude any other features or steps. Furthermore, terms such as "a" or "that" which indicate a singular number of steps or features do not exclude a plurality of features or steps - and vice versa.

Exemplary embodiments of the invention are described in more detail below with reference to the drawing.

The figures each show preferred exemplary embodiments in which individual features of the present invention are shown in combination with one another. Features of an embodiment are can also be implemented detached from the other features of the same exemplary embodiment and can accordingly easily be connected by a person skilled in the art to further meaningful combinations and sub-combinations with features of other exemplary embodiments.

Elements with the same function are provided with the same reference symbols in the figures.

They show schematically:

FIG. 1 shows an EUV projection exposure system;

FIG. 2 shows a basic representation of an exemplary embodiment of the device;

FIG. 3 shows a further basic representation of an exemplary embodiment of the device;

FIG. 4 shows a further representation of the principle of an exemplary embodiment of the device;

FIG. 5 shows a further representation of the principle of an exemplary embodiment of the device;

FIG. 6 shows a further representation of the principle of an exemplary embodiment of the device;

FIG. 7 shows a further basic representation of an exemplary embodiment of the device; and

FIG. 8 shows a basic representation of an optical element.

FIG. 1 shows an example of the basic structure of an EUV projection exposure system 400 for semiconductor lithography, for which the invention can preferably be used. In particular, the invention can be used in that at least one optical element of the projection exposure system is manufactured in such a way that - as will be explained below - a cover layer made of a cover material is applied to a surface of a base body until a cover layer thickness is reached, the cover layer being at least is formed almost free of defects.

In addition to a radiation source 402 , an illumination system 401 of the projection exposure system 400 has optics 403 for illuminating an object field 404 in an object plane 405 . A reticle 406 which is arranged in the object field 404 and is held by a reticle holder 407 shown schematically is illuminated. Projection optics 408, shown only schematically, are used to image the object field 404 in an image field 409 in an image plane 410. A structure on the reticle 406 is imaged on a light-sensitive layer of a wafer 411 arranged in the area of the image field 409 in the image plane 410, which is wafer holder 412, also shown in part, is held. The radiation source 402 can emit EUV radiation 413, in particular in the range between 5 nanometers and 30 nanometers, in particular 13.5 nm. To control the radiation path of the EUV radiation 413, optically differently designed and mechanically adjustable optical elements are used. In the EUV projection exposure system 400 shown in FIG. 1, the optical elements are in the form of adjustable mirrors in suitable embodiments that are mentioned below only by way of example.

The EUV radiation 413 generated with the radiation source 402 is aligned by means of a collector 402a integrated in the radiation source 402 in such a way that the EUV radiation 413 passes through an intermediate focus in the region of an intermediate focal plane 414 before the EUV radiation 413 impinges on a field facet mirror 415. Downstream of the field facet mirror 415, the EUV radiation 413 is reflected by a pupil facet mirror 416. Field facets of the field facet mirror 415 are imaged in the object field 404 with the aid of the pupil facet mirror 416 and an optical assembly 417 with mirrors 418, 419, 420.

Figure 2 shows, in conjunction with Figure 8, a basic representation of a device 1 for producing an optical element 2, in particular for a projection exposure system 400 with a target 3 made of a target material, a coating device 4, which is used for separating particles 5 of the target material by means of an ionized working gas 6 for coating a base body 7, the base body 7 having a substrate 17 with a reflection layer 18 applied to the substrate 17, as well as a working chamber 8 for accommodating the base body 7 and a vacuum device 9 for forming a vacuum in the working chamber 8. The device also includes a limiting device 10 in order to limit the energy of the particles 5 after the separation and/or the ions and/or electrons and/or atoms of the working gas 6 which strike the base body 7 .

The optical element 2 can in particular be an optical element 2, 402a, 415, 416, 418, 419, 420 of the projection exposure system 400.

In particular, the optical element 2 can also be a collector mirror 402a of the EUV projection exposure system 400 .

In this case, the base body 7 is formed by a substrate 17, on which the reflection layer 18 made of one or more materials is applied, on which in turn a barrier layer 19 is applied (see FIG. 8).

In another embodiment, not shown, it can be provided that the base body 7 is formed by a substrate 17 on which a reflection layer 18 made of one or more materials is applied, on which in turn a plurality of barrier layers 19 are applied. The device 1 shown in Figure 2 is suitable, for example, for carrying out a method for producing an optical element 2, in particular for a projection exposure system 400. In this case, a cover layer 11 made of a cover material is applied to a surface of the base body 7 until a cover layer thickness is reached, wherein the base body 7 has a substrate 17 with a reflection layer 18 applied to the substrate 17 . The cover layer 11 is here formed at least approximately free of defects.

In the exemplary embodiment shown for the device 1, the method for producing the optical element 2 can be implemented in such a way that the cover layer 11 is formed by sputtering. In this case, particles 5 of the target material are continuously isolated on the at least one target 3 by bombardment with ions of the working gas 6 . Furthermore, there is a discharge voltage for at least indirect ionization of the working gas 6 and the cover layer 11 is formed in connection with a defect-preventing method.

In the present exemplary embodiment, the cover layer 11 is formed from a cover material which has a stoichiometric composition. In addition, in the present exemplary embodiment, the cover layer 11 is formed in such a way that sharp boundaries are formed between the cover layer and the base body.

In the method implemented in the exemplary embodiment, the particles 5 of the target material form a cover material, move to the base body 7, are deposited on the base body 7 and thus form the cover layer 11.

It can be provided that the particles 5 of the target material react with a reaction gas and thus form particles 5 of the cover material and the particles 5 of the cover material then move to the base body 7 and are deposited on the base body 7 and thus form the cover layer 11.

In the present exemplary embodiment, it can be provided that the reaction gas is oxygen.

The limiting device 10, which represents part of the device 1 in the exemplary embodiment illustrated in FIG. 2, is suitable for carrying out a method to avoid defects. The defect-avoiding method provides that there is a potential damage from particles 5 of the target material after separation and/or from particles 5 of the cover material and/or from ions and/or atoms and/or electrons in the working gas 6 and/or from particles in the reaction gas an impact on the base body 7 and/or before impact on the covering layer 11 being formed is reduced with regard to at least one damage parameter. In particular, in the present exemplary embodiment, the at least one damage parameter is a kinetic energy that is preferably above a threshold value. The limiting device 10 reduces the maximum kinetic energy that occurs.

The limiting device 10 therefore limits the kinetic energy as the energy.

The limiting device 10, which is part of the device 1 in the exemplary embodiment illustrated in FIG. 2, is designed in such a way that the density of the working gas 6 in the vacuum is changed. Due to an increased collision frequency between the particles of the working gas 6, their kinetic energy is equalized or limited.

In the present exemplary embodiment, the density of the working gas 6 is increased by the limiting device 10 in that the limiting device supplies working gas 6 to the working chamber 8 . In particular, it can be provided that the limiting device 10 is designed as a dosing device and/or valve device. Furthermore, it can also be provided in the present exemplary embodiment that the limiting device 10 supplies the working chamber 8 with a thermalization gas which does not act as a working gas, but with whose particles the particles of the working gas 6 collide and their kinetic energy is thus matched to one another.

With the delimitation device 10 shown as part of the device 1 shown in Figure 2, a method can be carried out in which the cover layer 11 is formed by sputtering and in the defect-avoiding method the particles 5 of the cover material after separation as well as the ions and/or atoms and / or electrons of the working gas 6 are energetically matched by thermalization with the working gas 6. In particular, by means of the limiting device 10 in the form of a dosing device, a pressure of the working gas 6 can be set in such a way that thermalization occurs in the defect-avoiding method.

It can be provided that the limiting device 10 is designed as an afterglow device. In particular, it can be provided that the afterglow device is designed as a remote plasma source. The working gas 6 is formed by the plasma source at a spatial distance from the target 3 and/or the cover layer 11 and is then brought to the target 3 .

In this case, the particles 5 of the target material are separated by an afterglow of the plasma.

Provision can also be made for the afterglow device to be in the form of a pulsed plasma source. In this case, the working gas 6 is ionized only in temporal pulses. A separation of the particles 5 of the target material is accordingly largely brought about by an afterglow of the plasma. If the limiting device 10 is designed as an afterglow device and in particular as a remote plasma source, the device 1 is suitable for carrying out a method according to which the ions of the working gas 6 are formed by a remote plasma source as part of the defect-avoiding method.

If the afterglow device is designed as a pulsed plasma source, a method can be carried out in which ions of the working gas 6 form a pulsed plasma as part of the defect-avoiding method. In particular, it can be provided that the at least one target is designed as a dual-cathode magnetron as part of the defect-avoiding method.

Figure 3 shows a device 1, in which the limiting device 10 as a Penning ionization device

12 is designed in such a way that the working gas 6 is supplied with a secondary gas 13 . The Penning ionization device 12 is designed in such a way that an electronic activation energy of the secondary gas

13 is greater than an ionization energy of the working gas 6.

A device 1, which is configured according to the exemplary embodiment illustrated in FIG. 3, makes it possible to carry out a method in which the cover layer 11 is formed by sputtering in conjunction with a method to avoid defects. The defect-avoiding method includes that the working gas 6 is ionized by Penning ionization with a secondary gas 13 .

In this case, in order to avoid defects, the discharge voltage can be reduced if a secondary gas 13 is used.

For this purpose, it is provided that an electronic activation energy of the secondary gas 13 is greater than the ionization energy of the working gas 6.

FIG. 4 shows a basic representation of a further exemplary embodiment of the device 1. In this case, the limiting device 10 is designed as a magnetic trap 14. In this case, the magnetic trap 14 forms a magnetic field in such a way that charged particles with high kinetic energy are deflected away from the cover layer 11 .

In particular, it is provided that the magnetic trap 14 is designed in such a way that ions of the working gas 6 in particular, which have a high kinetic energy, are held in a limited area of the working chamber 8 and in particular do not interact with the cover layer 11 .

The device 1 shown in FIG. 4 can be used, for example, to carry out a method in which the cover layer 11 is formed by sputtering and charged particles, in particular ions of the working gas, are caught in a magnetic trap 14 as part of the defect-avoiding method. FIG. 5 shows a basic representation of a device 1, the limiting device 10 being designed as two targets 3 lying opposite one another. Such a device 1 can be used, for example, to carry out a method in which the defect-avoiding method is designed as facing-targets sputtering. In particular, it is provided that the targets 3 are opposite one another in such a way that, in particular, very fast ions of the working gas 6 cannot strike the base body 7 directly under the cover layer 11 .

Instead, there is a high probability that very fast particles, in particular ions of the working gas 6, will hit the respective opposite target 3.

FIG. 6 shows a further exemplary embodiment of the device 1, the limiting device 10 being designed as a heating device 15 for heating and/or melting the target 3. FIG. Such a device 1 makes it possible, for example, to carry out a method, according to which the cover layer 11 is formed by sputtering and the at least one target 3 being heated and/or melted in the defect-avoiding method.

Melting the target 3 means that less energy is required to separate the particles 5 of the target material. As a result, in the exemplary embodiment shown, it is provided that the discharge voltage is reduced when the at least one target 3 is heated and/or melted.

FIG. 7 shows a basic representation of an exemplary embodiment of the device 1, the limiting device 10 being formed by a grid 16 at an electrostatic potential. Such a device can be used, for example, to carry out a method in which the cover layer 11 is formed by sputtering and in which the ions of the working gas 6 are decelerated by an electric field of a grid 16 which has an electric potential in the defect-avoiding method.

A deceleration of charged particles, in particular the ions of the working gas 6, advantageously reduces the damage potential of the ions of the working gas 6.

Figure 8 shows a basic representation of an optical element 2, in particular a mirror of a projection exposure system 400 with a base body 7, the base body 7 having a substrate 17 with a reflective layer 18 applied to the substrate 17, and a layer applied to a surface of the base body 7 a cover layer 11 formed from a cover material and having a cover layer thickness, the cover layer 11 being formed at least approximately free of defects.

The base body 7 is formed by a substrate 17 on which a reflection layer 18 made of one or more materials is applied, on which in turn a barrier layer 19 is applied. In this case, the cover layer 11 has sharp boundaries both on the side facing the barrier layer 19 and on the side facing away from the barrier layer 19 .

Furthermore, the cover material forming the cover layer 11 has a stoichiometric composition in the present exemplary embodiment. In the present exemplary embodiment, the cover layer 11 can be formed by facing target sputtering and/or by sputtering in connection with Penning ionization and/or by sputtering in connection with thermalization and/or by other methods which have been disclosed within the scope of the invention.

The formation of the base body 7 from a substrate 17 with a reflection layer 18 applied to the substrate 17 is to be understood within the scope of the invention in a broad interpretation such that the base body 7 has a substrate 17 with a reflection layer 18 applied to the substrate 17. The base body 7 can then, in particular, as shown in FIG. 8, also have a barrier layer 19, as described above.

In a narrow interpretation, the base body 7 is to be understood within the scope of the invention as merely being formed from a substrate 17 with a reflection layer 18 applied to the substrate 17 and optionally one or more barrier layers 19 applied to the reflection layer 18, optionally only in partial areas.

Reference list:

1 device

2 optical element

3 targets

4 coating device

5 particles of the target material

6 working gas

7 basic bodies

8 working chamber

9 vacuum device

10 limiting device

11 top layer

12 Penning ionizer

13 secondary gas

14 magnetic trap

15 heating device

16 grids

17 substrate

18 reflective layer

19 barrier layer

projection exposure system

lighting system

Radiation source a collector

optics

object field

object level

reticle

reticle holder

projection optics

image field

picture plane

wafers

wafer holder

EUV radiation

intermediate focal plane

field facet mirror

pupil facet mirror

optical assembly

mirror

mirror

mirror

Claims

32 patent claims: 1. A method for producing an optical element (2), in particular for a projection exposure system (400), according to which a cover layer (11) made of a cover material is applied to a surface of a base body (7) until a cover layer thickness is reached, the base body ( 7) a substrate (17) with a reflective layer (18) applied to the substrate (17), characterized in that the cover layer (11) is formed at least approximately free of defects. 2. The method according to claim 1, characterized in that the cover layer (1 1) is formed with sharp boundaries. 3. The method according to claim 1 or 2, characterized in that the cover layer (11) is formed by sputtering, particles of a target material (5) being continuously isolated on at least one target (3) by bombardment with ions of a working gas (6). are applied, wherein a discharge voltage is present for at least indirect ionization of the working gas (6) and wherein the cover layer (11) is formed in connection with a defect-preventing method. 4. The method according to any one of claims 1 to 3, characterized in that the cover layer (11) is formed from a cover material which has a stoichiometric composition. 5. The method according to any one of claims 3 to 4, characterized in that the particles of the target material form a cover material, move to the base body (7) and are deposited on the base body (7) and form the cover layer (11). 6. The method according to any one of claims 3 to 5, characterized in that a reaction gas reacts with the particles of the target material (5) and forms particles of the cover material and the particles of the cover material move to the base body (7) and attach to the base body (7 ) are deposited and form the top layer (11). 7. The method according to claim 6, characterized in that the reaction gas is oxygen. 8. The method according to any one of claims 3 to 7, characterized in that the defect-avoiding method is designed such that a damage potential of particles of the target material (5) after the separation and / or particles of the cover material and / or ions and / or atoms and/or electrons in the working gas (6) and/or particles in the reaction gas are reduced in terms of at least one damage parameter before they strike the base body (7) and/or a covering layer (11) that forms. 9. The method as claimed in claim 8, characterized in that the at least one damage parameter is a kinetic energy which is preferably above a threshold value and which is reduced. 33 10. The method according to any one of claims 3 to 9, characterized in that the cover layer (11) is formed by sputtering, charged particles being caught in a magnetic trap (14) in the defect-avoiding method. 11. The method according to any one of claims 3 to 10, characterized in that the defect-avoiding method is designed as a facing target sputtering. 12. The method according to any one of claims 3 to 11, characterized in that the ions of the working gas are formed in the defect-avoiding method by a remote plasma source. 13. The method according to any one of claims 3 to 12, characterized in that the ions of the working gas form a pulsed plasma in the defect-avoiding method. 14. The method according to any one of claims 3 to 13, characterized in that the cover layer is formed by sputtering, wherein in the defect-avoiding method the at least one target (3) as a dual-cathode magnetron with an active anode and / or a passive anode is trained. 15. The method according to any one of claims 3 to 14, characterized in that the cover layer (11) is formed by sputtering, the working gas (6) in the defect-avoiding method being ionized by Penning ionization with a secondary gas (13). 16. The method according to claim 15, characterized in that the discharge voltage is reduced when using a secondary gas (13) in the defect-avoiding method. 17. The method according to any one of claims 15 to 16, characterized in that an electronic activation energy of the secondary gas (13) is greater than an ionization energy of the working gas (6). 18. The method according to any one of claims 3 to 17, characterized in that the cover layer (11) is formed by sputtering, and in the defect-avoiding method the particles of the cover material after separation and the ions and/or atoms and/or the electrons of the Working gas (6) are energetically adjusted by thermalization with the working gas (6). 19. The method according to claim 18, characterized in that a pressure of the working gas (6) is adjusted in such a way that thermalization occurs in the defect-avoiding method. 20. The method according to any one of claims 3 to 19, characterized in that the cover layer (11) is formed by sputtering, the at least one target (3) being heated and/or melted in the defect-avoiding method. 21 . Method according to one of Claims 3 to 20, characterized in that the discharge voltage is reduced when the at least one target (3) is heated and/or melted. 22. The method according to any one of claims 3 to 21, characterized in that the cover layer (11) is formed by sputtering, wherein in the defect-avoiding method the ions of the working gas (6) are separated by an electric field of a grid (16), which has an electric Has potential to be slowed down. 23. The method according to any one of claims 1 to 22, characterized in that zirconium oxide, ZrO _x and / or titanium oxide, TiO _x and / or niobium oxide, NbO _x and / or yttrium oxide, YO _x and / or hafnium oxide, HfO _x and / or Cerium oxide, CeO _x and/or lanthanum oxide, LaO _x and/or tantalum oxide, TaO _x and/or aluminum oxide, AlIO _X and/or erbium oxide, ErO _x and/or tungsten oxide, WO _X and/or chromium oxide, CrO _x and/or scandium oxide , ScO _x and/or vanadium oxide, VO _X are provided in pure form and/or as a mixture as cover material. 24. The method according to any one of claims 1 to 23, characterized in that the top layer thickness is 0.1 nm to 20 nm, preferably 0.3 nm to 10 nm, particularly preferably 0.5 to 3 nm. 25. Optical element (2), in particular mirror of a projection exposure system (400), with a base body (7), wherein the base body (7) has a substrate (17) with a reflective layer (18) applied to the substrate (17), and a cover layer (11) applied to a surface of the base body (7) and made of a cover material and having a cover layer thickness, characterized in that the cover layer (11) is designed at least approximately free of defects. 26. Optical element (2) according to claim 25, characterized in that the cover layer (11) has sharp boundaries. 27. Optical element (2) according to claim 25 or 26, characterized in that the cover material has a stoichiometric composition. 28. Optical element (2) according to any one of claims 25 to 27, characterized in that the cover layer (11) is formed by facing target sputtering. 29. Optical element (2) according to any one of claims 25 to 28, characterized in that the cover layer (11) is formed by sputtering in conjunction with Penning ionization. 30. Optical element (2) according to any one of claims 25 to 29, characterized in that the cover layer (11) is formed by sputtering in conjunction with thermalization. 31 . Optical element (2) according to one of Claims 25 to 30, characterized in that zirconium oxide, ZrO _x and/or titanium oxide, TiO _x and/or niobium oxide, NbO _x and/or yttrium oxide, YO _x and/or hafnium oxide, HfO _x and /or cerium oxide, CeO _x and/or lanthanum oxide, LaO _x and/or tantalum oxide, TaO _x and/or aluminum oxide, AlIO _X and/or erbium oxide, ErO _x and/or tungsten oxide, WO _X and/or chromium oxide, CrO _x and/or or scandium oxide, ScO _x and/or vanadium oxide, VO _x are provided in pure form and/or as a mixture as cover material. 32. Optical element (2) according to one of claims 25 to 31, characterized in that the cover layer thickness is 0.1 nm to 20 nm, preferably 0.3 nm to 10 nm, particularly preferably 0.5 to 3 nm. 33. Device (1) for producing an optical element (2), in particular for a projection exposure system (400), with a target (3) made of a target material, a coating device (4) which is used for separating particles (5) of the target material by means of an ionized working gas (6) for coating a base body (7), the base body (7) having a substrate (17) with a reflective layer (18) applied to the substrate (17), and a working chamber (8) for Accommodating the base body (7) and a vacuum device (9) for forming a vacuum in the working chamber (8), characterized in that at least one limiting device (10) is provided to limit the energy of the particles (5) after separation and/or of the ions and/or electrons and/or atoms of the working gas (6) which strike the base body (7). 34. Device (1) according to claim 33, characterized in that the energy is kinetic energy. 35. Device (1) according to claim 33 or 34, characterized in that the limiting device (10) is designed in such a way that a density of the working gas (6) in the vacuum is changed. 36. Device (1) according to one of claims 33 to 35, characterized in that the limiting device (10) is designed as a Penning ionization device (12) such that the working gas (6) is supplied with a secondary gas (13). 37. Device (1) according to claim 36, characterized in that the limiting device (10) is designed such that an electronic activation energy of the secondary gas (13) is greater than an ionization energy of the working gas (6). 38. Device (1) according to any one of claims 33 to 37, characterized in that the limiting device (10) is designed as a magnetic trap (14). 39. Device (1) according to one of claims 33 to 38, characterized in that the limiting device (10) is designed as two opposite targets (3). 40. Device (1) according to one of claims 33 to 39, characterized in that the limiting device (10) is designed as a heating device (15) for heating and/or for melting the target (3). 41 . Device (1) according to one of Claims 33 to 40, characterized in that the limiting device (10) is formed by a grid (16) at an electrostatic potential. 36 42. Device (1) according to one of claims 33 to 41, characterized in that the limiting device (10) is designed as an afterglow device. 43. Device (1) according to claim 42, characterized in that the afterglow device is designed as a remote plasma source. 44. Device (1) according to claim 42 or 43, characterized in that the afterglow device is designed as a pulsed plasma source. 45. Secondary gas (13) for use in a method according to any one of claims 3 to 24 and / or in a device (1) according to any one of claims 33 to 44, characterized in that an electronic activation energy of the secondary gas (13) is greater than an ionization energy of the working gas (6). 46. Projection exposure system (400) for semiconductor lithography with an illumination system (401) with a radiation source (402) and an optical system (403, 408) which has at least one optical element (2, 402a, 415, 416, 418, 419, 420) has, wherein at least one of the optical elements (2, 402a, 415, 416, 418, 419, 420) is at least partially produced using a method according to one of Claims 1 to 24 and/or at least one of the optical elements (2, 402a, 415, 416, 418, 419, 420) is manufactured using a device (1) according to any one of claims 33 to 44 and/or at least one of the optical elements (2, 402a, 415, 416, 418, 419, 420) is an optical one An element according to any one of claims 25 to 32.