DE102022203644A1 - Process for producing a substrate and a reflective optical element for EUV lithography - Google Patents

Abstract

The invention relates to a method for producing a substrate (30) for a reflective optical element for EUV lithography, comprising the steps of: providing a metallic base body (31) and depositing a smoothing layer (32) on the metallic base body (31) by means of a A deposition process comprising at least one atomic layer deposition step in which at least a portion of the planarizing layer (32) is deposited by atomic layer deposition. The invention also relates to a method for producing a reflective optical element for EUV lithography, comprising the steps: producing a substrate (30) according to the method described above, and depositing a coating for reflecting EUV radiation on the substrate. The invention also relates to a reflective optical element for EUV lithography, which is produced according to the method, and an EUV lithography system which includes at least one such reflective optical element.

Description

Background of the Invention

The invention relates to a method for producing a substrate for a reflective optical element for EUV lithography. The invention also relates to a method for producing a reflective optical element for EUV lithography, as well as a reflective optical element for EUV lithography and an EUV lithography system.

Optical arrangements in the form of projection exposure systems are used to produce microstructured or nanostructured components in microelectronics or microsystems technology using optical lithography. Such projection exposure systems have an illumination system for illuminating a photomask (reticle) with electromagnetic radiation in a narrow spectral range around a working wavelength. Furthermore, these systems have a projection-optical system in order to use the radiation to project a structure of the reticle onto a radiation-sensitive layer of a wafer.

In order to achieve the smallest possible structure width for the semiconductor components to be produced, newer projection exposure systems, so-called EUV lithography systems, are designed for a working wavelength in the extreme ultraviolet (EUV) wavelength range, i.e. in a range from approx. 5 nm to approx. 30 nm . Since wavelengths in this range are strongly absorbed by almost all materials, typically no transmissive optical elements can be used. A use of reflective optical elements is required. Such optical elements reflecting EUV radiation can be, for example, mirrors, reflective monochromators, collimators or photomasks. Since EUV radiation is also strongly absorbed by air molecules, the beam path of the EUV radiation is arranged inside a vacuum chamber.

Optical elements that reflect EUV radiation can also be used in other optical arrangements (EUV lithography systems) that are used in the context of EUV lithography. Examples of this are metrology systems for examining exposed or to be exposed wafers, for examining reticles, and for examining other components of EUV lithography systems, such as mirrors.

In the production of substrates for reflective optical elements for EUV lithography, the difficulty arises of achieving the necessary low surface roughness. Typical values for the required surface roughness are: Mid Spatial Frequency Roughness (MSFR) for period lengths in the range of 5 µm to 20 µm: RMS < 100 pm; High Spatial Frequency Roughness (HSFR) for period lengths in the range from 15 nm to 0.5 µm: RMS < 150 pm.

If the substrates are made of glass or glass-ceramic, the surface roughness can be brought into the μm range by polishing. However, in some cases, particularly in the case of reflective optical elements in the lighting system, the use of substrates with metallic base bodies is desirable, since these have high thermal conductivity.

In order to achieve the desired low surface roughness here as well, a relatively thick (a few µm) polishing layer is applied to the metallic base body, which can be polished to the desired surface roughness.

For example, in the U.S. 9,541,685 B2 describes a method which includes microstructuring of a starting substrate, application of a polishing layer to the microstructured substrate, polishing of the polishing layer, which results in a surface roughness of less than 0.2 nm, and application of a reflective layer to the polishing layer, wherein the reflective layer has a surface roughness of less than 0.2 nm and is designed to reflect EUV radiation. The starting substrate preferably comprises a material from the group consisting of: metals, metal alloys, semiconductors and compounds thereof. Quartz and amorphous or crystalline silicon are mentioned as materials for the polishing layer.

The polishing layer can be deposited, for example, by magnetron sputtering. A problem when depositing the polishing layer by magnetron sputtering is that shadowing by particles leads to porous structures in the polishing layer. Also, high-energy particles can bombard the surface and lead to point defects. The inclusion of the sputtering gas in the layer can also lead to point defects and in particular to the implantation of foreign atoms and the risk of outgassing with subsequent bubble formation. These problems generally do not directly affect the surface of the reflective optical element, but result in imperfect layer growth, which dominates the growth of thin layers up to a thickness of several micrometers. In particular, the realization of closed and uniform (polishing) layers in this thickness regime cannot currently be achieved with magnetron sputtering.

object of the invention

The object of the invention is to provide a simple and inexpensive method for producing a substrate with high thermal conductivity and low surface roughness for a reflective optical element for EUV lithography and a method for producing a reflective optical element for EUV lithography place.

subject of the invention

According to a first aspect, this object is achieved by a method for producing a substrate for a reflective optical element for EUV lithography, comprising the steps: providing a metallic base body and depositing a smoothing layer on the metallic base body by means of a deposition process that includes at least one atomic layer deposition step comprises in which at least part of the smoothing layer is deposited by means of atomic layer deposition.

The use of a metal body ensures high thermal conductivity. The metallic base body can be formed in particular from copper or aluminum.

In particular, the smoothing layer can be deposited directly on the metallic base body. In particular, the deposition process includes exactly one atomic layer deposition step, in which the required surface roughness can possibly already be achieved. Alternatively, the deposition process may also include two or more than two atomic layer deposition steps. The atomic layer deposition step or each of the atomic layer deposition steps preferably comprises a plurality of ALD (atomic layer deposition) cycles. The deposition rate, ie the thickness of the layer deposited per ALD cycle, is typically 0.25 nm/cycle for SiN, for example, but can be higher or lower, see the article "Atomic Layer Deposition of Silicon Nitride Thin Films: A Review of Recent Progress, Challenges, and Outlooks" by X. Meng et al., Materials, 9, 1007 (2016) . The number of ALD cycles per atomic layer deposition step determines the thickness of the layer deposited in an atomic layer deposition step. The atomic layer deposition is preferably carried out with plasma support.

There is extensive literature on so-called atomic layer deposition (ALD) processes. This is a class of deposition processes that are characterized by two or more self-terminating surface reactions that are carried out cyclically. Typically, an ALD cycle involves two surface reactions, a first partial reaction with a so-called precursor, e.g. a metal precursor, and then a second partial reaction with a co-reactant, e.g. water. In conventional ALD processes, which are carried out in a reaction chamber, an inert gas is flushed between the partial reactions so that precursor and co-reactant are never present in the reaction chamber at the same time. In the case of spatial atomic layer deposition, on the other hand, the partial reactions take place in different volume areas. To carry out the partial reactions, the substrate to be coated is moved relative to these volume areas. A large number of ALD cycles are usually carried out. ALD processes are characterized by excellent layer thickness control and high conformity of the layers deposited with it.

The use of atomic layer deposition to deposit thin layers on optical elements is discussed in different contexts. For example, describes the WO 2004/095086 A2 the deposition of conformal layers on micro-optical elements using atomic layer deposition and related methods. The US 2016/0086681 A1 further discloses the fabrication of Fresnel zone plates by atomic layer deposition.

In the field of EUV lithography describes the WO 2013/113537 A2 atomic layer deposition as a conformal coating process for depositing the layers of a multilayer stack of a coating of an optical element reflecting EUV radiation.

Furthermore, in the DE 10 2020 207 807 A1 describes a method for producing an optical element for an EUV projection exposure system. The method includes the application of at least one shaping layer (which is also referred to as an etching layer) according to a predetermined layer thickness profile on a substrate with a basic topography. At least two shaping layers are preferably applied, which are each separated from one another by an etch stop layer. The intermediate product obtained in this way is provided with a single-stage or multi-stage lattice structure in structuring steps in which the shaping layers are etched. The etch stop layers limit the etching depth, which significantly reduces the demands on the etching process. The lattice structure serves in particular to suppress infrared radiation. The shaping layers can consist, among other things, of a material which can be applied by an atomic layer deposition process.

Atomic layer deposition achieves a defect-free, continuous smooth layer even at low thicknesses. Furthermore, due to the conformal nature of the atomic layer deposition, the surface roughness of the smoothing layer is significantly reduced compared to the surface roughness of the metallic base body. Typically, this eliminates the need for polishing. Due to the lower thickness of the smoothing layer and the fact that polishing is not required, the process is simpler, faster and cheaper than the processes described above.

In a variant of this method, the deposition process comprises, after at least one of the atomic layer deposition steps, an etching back step in which the smoothing layer is partially etched back. Detailed information on etching back thin layers deposited using an ALD process can be found in the article "Deposit and etchback approach for ultrathin Al ₂ O ₃ films with low pinhole density using atomic layer deposition and atomic layer etching" by JC Gertsch et al., Journal of Vacuum Science & Technology A 39, 062602 (2021), the thin layers discussed there being Al ₂ O ₃ layers.

In the simplest case, the deposition process includes exactly one atomic layer deposition step and exactly one etch-back step. First, the smoothing layer is deposited by means of atomic layer deposition, with the thickness of the initially deposited layer being greater than the final thickness of the smoothing layer. The smoothing layer is then etched back to the final thickness. As a result, a smoother, more continuous smoothing layer is achieved than if one dispenses with etching back and deposits the smoothing layer in the final thickness from the outset using atomic layer deposition.

If the deposition process comprises a plurality of atomic layer deposition steps, an etching back step can be carried out in particular after each atomic layer deposition step. Alternatively, for example, the deposition process may also include an etch-back step after each atomic layer deposition step except for the last atomic layer deposition step. The cyclic atomic layer deposition and partial etching back leads to closed layers even with very small thicknesses (typically less than 2 nm).

The etch-back step or steps have, in particular, a further smoothing effect. The surface roughness remaining after an atomic layer deposition step can thus be further reduced.

In a further variant of this method, SiO _x , SiN _x or SiN _x O _y C _z is deposited in at least one of the atomic layer deposition steps by means of atomic layer deposition. In particular, in at least one of the atomic layer deposition steps, SiO ₂ is deposited in each case by means of atomic layer deposition.

Suitable Si precursors are, for example, monosilane (SiH ₄ ), disilane (Si ₂ H ₆ ), their derivatives, for example Si ₂ H ₂ Cl ₂ , aminosilanes, and trisilylamine (TSA, CAS 13862-16-3) and its derivatives, in particular TTMSA, DTDN1-H1, DTDN2-H2 and DTDN3-H3, see the article "Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride Using a Novel Silylamine Precursor" by J.-M. Park et al., ACS Appl. Mater. Interfaces 8, 20665 (2016) , the content of which is hereby fully included in this application.

Aminosilanes such as tris(dimethylamino)silane (3DMAS, CAS 15112-89-7) and bis(diethylamino)silane (BDEAS, CAS 27804-64-4), preferably with plasma-activated oxygen, are particularly suitable as precursors for the effective deposition of SiO ₂ as a co-reactant, as well as the precursor AP-LTO 330 with ozone as a co-reactant, see the article "Thermal and plasma enhanced atomic layer deposition of SiO2 using commercial silicon precursors" by M. Putkonen et al., Thin Solid Films 558, 93 (2014) .

It is also possible to use other plasmas, such as NH ₃ for the formation of nitrides, N ₂ O for the formation of oxynitrides and CO ₂ for the formation of oxycarbides. After deposition, the smoothing layer is preferably compressed by heat and/or a plasma, which can serve as a chemically passive or chemically active co-reactant for further conversion, for example to oxynitrides or oxycarbides. When a plasma is formed solely from inert gases such as noble gases, such as Ar and/or He, these co-reactants are chemically passive (inactive). However, the energetically excited gas species formed by or in the plasma are raised to energetically higher energy levels and make these gas species physically active. The energy accumulated in these excited gas species is applied to the surface of the layer that has been deposited up to that point and leads to compression, for example. In this case, the co-reactants are not chemically but physically active. On the other hand, if a plasma contains chemically reactive gases such as NH ₃ , NO ₂ , CH ₄ and/or H ₂ /N ₂ as co-reactants, these co-reactants are referred to as (thermo-)chemically active. In addition to this chemical effect The physical effect described above also occurs in this case.

The atomic layer deposition of silicon nitride is preferably plasma-assisted and at temperatures between 250° C. and 400° C., with monosilane and an N ₂ plasma being used as a precursor or co-reactant. The plasma is preferably excited at a frequency of 13.56 MHz.

Alternatively, monosilane and a H ₂ /N ₂ plasma can be used as a precursor or co-reactant at temperatures between 300°C and 400°C.

Furthermore, silylamine derivatives such as bis(dimethylaminomethylsilyl)trimethylsilylamine (C ₉ H ₂₉ N ₃ Si ₃ , DTDN2-H 2 ) can be used as precursors for the plasma-assisted atomic layer deposition of silicon nitride. This connection differs from the methods described in the article by J.-M. Park et al. mentioned silylamine derivatives the lowest dissociation energy, as well as a sufficient vapor pressure, and when used with an N ₂ plasma as a co-reactant at temperatures of about 250 ° C leads to a growth rate of 0.36 Å per ALD cycle.

For the purposes of the present application, SiN _x O _y C _z is (intentionally) non-stoichiometric SiN _x with proportions of oxygen and carbon. SiN _x O _y C _z can be deposited, for example, by means of plasma-assisted atomic layer deposition, with the process parameters being selected such that a non-stoichiometric composition results. Suitable parameter ranges can be found, for example, in the article by J.-M. Park et al. So is in the 10b of the article mentioned that with an RF plasma power of 400 W, the deposited SiN _x layer has a proportion of approx. 5% carbon and approx. 10% oxygen and thus deviates significantly from a stoichiometric composition.

In a development of this variant, the smoothing layer is formed at least partially, in particular completely, from SiO _x , SiN _x or SiN _x O _y C _z . The smoothing layer is preferably formed at least partially, in particular completely, from SiO ₂ .

In a further variant of this method, the deposition process includes a reduction step after each atomic layer deposition step, in which the deposited SiO _x , SiN _x or SiN _x O _y C _z is at least partially reduced to Si and possibly crystallized. A strongly reducing atmosphere, for example a hydrogen radical plasma, is preferably used for this purpose.

In a development of this variant, the etching back step, in which the smoothing layer is partially etched back, is carried out immediately after at least one of the reduction steps, or the reduction step forms the etching back step, in which the smoothing layer is partially etched back. In particular, a (soft) etching back step can be carried out after each of the reduction steps. In this case, no further step, in particular no reduction step, is carried out between the reduction step and the etching-back step. Alternatively, the reduction step may form the etch back step. In this case, the smoothing layer is partially etched back and at the same time the SiO _x , SiN _x or SiN _x O _y C _z is at least partially reduced to Si. In the event that the smoothing layer is not to have elemental silicon, but rather a Si compound, the reduction step can be dispensed with. As a result, the smoothing layer can be formed from Si, in particular at least partially, in particular completely. The silicon is in particular amorphous silicon.

Each reduction step preferably simultaneously represents one of the back-etching steps. In particular, SiN _x O _y C _z can be reduced and etched simultaneously with a reducing gas mixture such as CF ₄ /H ₂ or SF ₆ /CH ₄ /N ₂ . As a result of the reduction step, a layer with a high proportion of silicon and low proportions of nitrogen, oxygen and carbon typically forms from the SiN _x O _y C _z .

In the article by J.-M. Park et al. described plasma-assisted deposition of SiN _x O _y C _z with an RF plasma power of 400 W, the high RF plasma power causes the layer to break up (ie the layer becomes amorphous) and traces of nitrogen, carbon and hydrogen of the Activated precursor and reactive contaminants such as oxygen or water from the environment are incorporated into the layer. As a result, the layer can be reduced and/or etched back relatively easily. For a wet etching process, etching rates increased by a factor of up to 10 are required for a layer deposited with such an RF plasma power 10d of the article by J.-M. Park et al. shown. Correspondingly high etching rates can also be achieved with a dry etching method using a reducing gas mixture that contains, for example, CF ₄ /H ₂ or SF ₆ /CH ₄ /N ₂ (see above).

In a further variant of this method, the partial etching back is carried out using a dry etching process, preferably using a reactive ion etching process and/or using atomic layer etching. In particular when using atomic layer etching, a smoothing effect is achieved with the etching back step. As etching gases, for example, hydrogen, oxygen, nitrogen and / or CF ₄ can be used. Fluorine-containing gas mixtures, such as CF ₄ /H ₂ , CF ₄ /O 2 /N 2 , SF ₆ /O ₂ /N ₂ , SF ₆ /CH _{4 /} N ₂ , SF, are particularly suitable as _etching gases for etching back SiN _{x 6} /CH ₄ /N ₂ /O ₂ , NF ₃ /O ₂ /N ₂ and NF ₃ /O ₂ /Ar. It should be noted that gas mixtures with O ₂ always lead to a higher proportion of oxygen in the resulting layer compared to gas mixtures that do not contain any O ₂ .

In addition to atomic layer deposition, so-called atomic layer etching is one of the methods for atomic layer processing. In atomic layer etching, individual atomic layers are removed in a controlled manner using two separate, self-terminating partial reactions that are carried out cyclically.

In the field of EUV lithography, atomic layer etching is used, for example, in WO 2019/007927 A1 discussed. This describes a method for at least partially removing a contamination layer from an optical surface of an optical element reflecting EUV radiation by means of an atomic layer etching process, which can be carried out as a three-dimensional atomic layer etching process.

A method for atomic layer processing of an optical surface of an optical element, which is designed to reflect EUV radiation, is also in EP 3 933 882 A1 described. There, an atomic layer etch process is performed to remove contaminants from the curved optical surface. Atomic layer processing may further include an atomic layer deposition process to deposit material on the optical surface.

In a further variant of this method, the smoothing layer is at least partially deposited by means of atomic layer deposition in at least one atomic layer deposition area and the etching-back step takes place in at least one etching area (hereinafter also: ALD area), which is spatially separated from the at least one atomic layer deposition area.

The at least one ALD region and the at least one etching region are each at least one volume region that are spatially separated, that is to say arranged at a distance from one another. In addition, an inert gas stream can be used between the volume areas for spatial separation, which forms a gas curtain. The inert gas can be Ar or N ₂ , for example. The precursor and the co-reactant in the ALD region can also be spatially separated from one another by a gas curtain, for example an inert gas stream.

The at least one ALD area and the at least one etching area are also preferably sealed off from the surrounding atmosphere, for example also by means of an inert gas flow. Gases in the surrounding atmosphere that potentially have a detrimental effect on the deposition, such as O ₂ and CO ₂ , therefore do not reach the ALD region or the etch region.

In the case of a plurality of ALD regions and/or a plurality of etching regions, these are also spatially separated from one another. The at least partial deposition by means of atomic layer deposition and the partial etching back takes place by means of a relative movement between the ALD area and the etched area on the one hand and the metallic base body on the other hand, whereby at least partial areas of the metallic base body are successively exposed to the at least one ALD area and the at least one etched area. There can also be a pressure difference between the at least one ALD region and the at least one etching region, for example if the etching back is performed by means of reactive ion etching. The method can also be carried out using a reactor which has a plurality of reaction chambers, with at least one of the reaction chambers serving as at least one ALD region and at least one other of the reaction chambers serving as at least one etching region. In order to be able to carry out the atomic layer deposition and the partial etching back one after the other, the metallic base body can be moved between the reaction chambers.

In a further variant of this method, the atomic layer etching is carried out as spatial atomic layer etching. Spatial atomic layer etching is characterized by being carried out in at least two spatially separated etching areas. One of the self-terminating partial reactions takes place in each of the etching areas.

In a further variant of this method, the atomic layer deposition is carried out as spatial atomic layer deposition. In a spatial atomic layer deposition process, the atomic layer deposition occurs in at least two spatially separated ALD regions. The first partial reaction, ie the reaction with the precursor, takes place in at least one of the ALD areas, and the second partial reaction, ie the reaction with the co-reactant, takes place in at least one other of the ALD areas. A particular advantage of spatial atomic layer deposition lies in the short process times.

Advantages of the spatially separated ALD areas and etching areas, in particular the spatial ones Atomic layer processing, ie the use of spatial atomic layer etching and/or spatial atomic layer deposition, lies in the high throughput achieved thereby, the simple scalability and the high deposition and/or etching rates. The spatial atomic layer processing can take place at atmospheric pressure and allows particularly low process temperatures.

Preferably, the planarizing layer is deposited by atmospheric pressure spatial atomic layer deposition in combination with a dry etching process, and preferably using the same apparatus, to avoid exposing the planarizing layer to unwanted contaminants during the process.

In a further variant of this method, the smoothing layer is polished after it has been deposited. Typically, the surface roughness after the smoothing layer has been deposited is so low that the substrate can be used without polishing for the production of a reflective optical element for EUV lithography. If the desired surface roughness has not yet been achieved after deposition or in order to further reduce the surface roughness, the smoothing layer can be additionally polished. In this case, the polishing is in any case less complicated than in the case of the methods known from the prior art. The method of the present invention is compatible with the polishing processes typically used to polish polished layers on metallic substrates; there is no need for a complex new development or adaptation of the polishing processes.

In a further variant, the final thickness of the smoothing layer is less than 100 μm, preferably less than 10 μm, particularly preferably less than 1 μm. The final thickness of the smoothing layer refers to the thickness of the smoothing layer after the completion of the deposition process. As has been described above, when carrying out the deposition process described above, in which at least part of the smoothing layer is deposited by means of atomic layer deposition, a smoothing layer with a comparatively small (final) thickness is sufficient.

A further aspect of the invention relates to a method for producing a reflective optical element for EUV lithography, comprising the steps: producing a substrate according to the method as described above, and depositing a coating for reflecting EUV radiation on the substrate.

The reflective optical element for EUV lithography can be, for example, a facet mirror or a deflection mirror of an illumination optics of a projection exposure system or a collector mirror.

In a variant of this method, before the coating for reflecting EUV radiation is deposited, a structuring layer is deposited on the substrate by means of atomic layer deposition. The structuring layer preferably has a thickness of more than 1 μm, in particular several micrometers. The structuring layer can be structured in particular by means of mechanical processing, e.g. milling, or by means of etching processes in combination with optical lithography.

In a variant of this method, the structuring layer is formed at least partially, in particular completely, from at least one material selected from the group consisting of: silicon, germanium, carbon, boron, titanium, zirconium, niobium, tantalum, tungsten, vanadium, and their alloys Compounds, in particular their oxides, carbides, borides, nitrides and silicides, and mixed compounds, noble metals, in particular ruthenium, rhodium, palladium, platinum, iridium, osmium, rhenium and their alloys, see also those cited at the outset DE 10 2020 207 807 A1 .

In a further variant of this method, the method also includes the step of structuring the metal base body, the smoothing layer and/or the structuring layer to form a lattice structure. The grating structure is in particular a diffraction grating. The diffraction grating can be used, for example, to filter undesired spectral components from the radiation reflected at the optical element (so-called "spectral purity filter"). A method for producing a spectral filter in the form of a lattice structure on a mirror for an illumination optics of a projection exposure system is in DE 10 2018 220 629 A1 described.

A further aspect of the invention relates to a reflective optical element for EUV lithography, produced by the method described above. As described above, the optical element can be, for example, an optical element in the illumination system of a projection exposure system, for example a facet mirror or deflection mirror, or a collector mirror that is arranged in a radiation source module of the projection exposure system.

A further aspect of the invention relates to an EUV lithography system comprising at least one reflective optical element as above described is formed. The EUV lithography system can in particular be a projection exposure system for EUV lithography.

Further features and advantages of the invention result from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can each be implemented individually or together in any combination in a variant of the invention.

character list

• 1 eine schematische Darstellung einer Projektionsbelichtungsanlage für die EUV-Projektionslithographie im Meridionalschnitt,
• 2 eine schematische Darstellung der Herstellung eines Substrats für ein reflektives optisches Element für die EUV-Lithographie mittels eines Verfahrens, bei dem mittels eines Abscheidungsprozesses, der genau einen Atomlagenabscheidungsschritt und genau einen Rückätzschritt umfasst, eine Glättschicht auf einem metallischen Grundkörper abgeschieden wird,
• 3 eine schematische Darstellung einer Variante des in der 2 gezeigten Verfahrens, wobei der Rückätzschritt zu einer Verringerung der Oberflächenrauheit der Glättschicht führt,
• 4 eine schematische Darstellung einer Variante des in der 2 gezeigten Verfahrens, wobei der Abscheidungsprozess eine Vielzahl von Atomlagenabscheidungs- und Rückätzschritten umfasst,
• 5 eine schematische Darstellung eines Bearbeitungskopfs zur Abscheidung einer Glättschicht für die Herstellung eines Substrats mit dem erfindungsgemäßen Verfahren,
• 6 eine schematische Darstellung eines oberflächennahen Bereichs eines reflektiven optischen Elements für die EUV-Lithographie, das aus einem mittels dem erfindungsgenäßen Verfahren hergestellten Substrates gefertigt wurde, sowie
• 7 eine schematische Darstellung einer Variante des in der 6 gezeigten reflektiven optischen Elements, wobei das reflektive optische Element zusätzlich eine Strukturierungsschicht aufweist.

Exemplary embodiments are shown in the schematic drawing and are explained in the following description. It shows

• 1 a schematic representation of a projection exposure system for EUV projection lithography in meridional section,
• 2 a schematic representation of the production of a substrate for a reflective optical element for EUV lithography by means of a method in which a smoothing layer is deposited on a metallic base body by means of a deposition process which comprises precisely one atomic layer deposition step and precisely one etching-back step,
• 3 a schematic representation of a variant of in the 2 the method shown, the etching-back step leading to a reduction in the surface roughness of the smoothing layer,
• 4 a schematic representation of a variant of in the 2 method shown, wherein the deposition process comprises a plurality of atomic layer deposition and etch-back steps,
• 5 a schematic representation of a processing head for depositing a smoothing layer for the production of a substrate with the method according to the invention,
• 6 a schematic representation of a region close to the surface of a reflective optical element for EUV lithography, which was manufactured from a substrate produced by means of the method according to the invention, and
• 7 a schematic representation of a variant of in the 6 reflective optical element shown, wherein the reflective optical element additionally has a structuring layer.

The following are referring to 1 exemplarily describes the essential components of an EUV lithography system in the form of a projection exposure system 1 for microlithography. The description of the basic structure of the projection exposure system 1 and of its components is not to be understood as limiting here.

One embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, 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 lighting 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 a Cartesian xyz coordinate system is drawn in for explanation. The x-direction points perpendicularly out of the plane of the drawing. 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 in an image field 11 in an image plane 12. A structure on the reticle 7 is imaged on a light-sensitive layer of a wafer arranged in the region of the image field 11 in the image plane 12 13. 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 via the reticle displacement drive 9 on the one hand and the wafer 13 on the other hand via the wafer displacement drive 15 can be synchronized with one another.

The radiation source 3 is an EUV radiation source. The radiation source 3 emits in particular EUV radiation 16, which is also referred to below as useful radiation, illumination radiation or illumination light. In particular, the useful radiation has 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, plasma generated with the aid of a laser) or a DPP Source (Gas Discharged Produced Plasma, by gas discharge generated plasma). It can also be a synchrotron-based radiation source. The radiation source 3 can be a free-electron laser (free-electron laser, FEL).

The illumination radiation 16 emanating from the radiation source 3 is bundled 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 in grazing incidence (Grazing Incidence, GI), i.e. with angles of incidence greater than 45°, or in normal incidence (Normal Incidence, NI), i.e. with angles of incidence less than 45° become. 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 focus plane 18. The intermediate focus 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 deflection mirror 19 and a first facet mirror 20 downstream of this in the beam path. The deflection mirror 19 can be a plane deflection mirror or alternatively a mirror with an effect that influences the bundle beyond the pure deflection effect. Alternatively or additionally, the deflection mirror 19 can be designed as a spectral filter, which separates a useful light wavelength of the illumination radiation 16 from stray light of a different wavelength. The first facet mirror 20 includes a multiplicity of individual first facets 21, which are also referred to below as field facets. Of these facets 21 are in the 1 only a few shown as examples. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optics 4. The second facet mirror 22 comprises a plurality of second facets 23. The first facet mirror 20 was also manufactured from a substrate which was manufactured using a method described further below. Alternatively or additionally, for example, the second facet mirror 22 and/or the deflection mirror 19 could also have been manufactured from a substrate that was manufactured using a method described further below.

The illumination optics 4 thus forms a double-faceted system. This basic principle is also known as a honeycomb condenser (Fly's Eye Integrator). The individual first facets 21 are imaged in the object field 5 with the aid of the second facet mirror 22 . The second facet mirror 22 is the last beam-forming mirror or actually the last mirror for the illumination radiation 16 in the beam path in front of the object field 5.

The projection system 10 includes a plurality of mirrors Mi, which are numbered consecutively according to their arrangement in the beam path of the projection exposure system 1 .

At the in the 1 illustrated example, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or another number of mirrors Mi 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 involves doubly obscured optics. The projection optics 10 has an image-side numerical aperture which is greater than 0.4 or 0.5 and which can also be greater than 0.6 and which can be 0.7 or 0.75, for example.

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

In the 2 until 4 shows the production of a substrate 30 for a reflective optical element, not shown here, for EUV lithography using a method that includes the following steps: providing a metal base body 31 and depositing a smoothing layer 32 on the metal base body 31 by means of a deposition process that comprises at least one atomic layer deposition step. A cross section through the smoothing layer 32 and the uppermost areas of the metal base body 31 is shown.

In 2 and 3 the smoothing layer 32 is deposited by means of a deposition process which comprises precisely one atomic layer deposition step and precisely one etching-back step. First, as in 2 and 3 shown on the left side, the smoothing layer 32 is deposited by means of atomic layer deposition. Subsequently, as in 2 and 3 shown on the right side, the smoothing layer 32 partially etched back.

In 2 are three snapshots respectively at the beginning 33, middle 33` and after completion 33" of the atomic layer deposition step and at the beginning 34, Shown in the middle 34' and after completion 34'' of the etching-back step. The time course is indicated by two arrows 35.

The surface roughness of the surface 36" of the smoothing layer 32 after the end 33" of the atomic layer deposition step is less than the surface roughness of the surface 36' of the smoothing layer 32 in the middle 33' of the atomic layer deposition step, which in turn is less than the surface roughness of the surface 36 of the metallic base body 31.

In 3 two snapshots are shown at the start 37 and after completion 37` of the atomic layer deposition step and at the start 38 and after completion 38` of the etch-back step. The timeline is as in 2 indicated by two arrows 39.

On the one hand, it is shown that the thickness d _A of the layer 32 initially deposited in the atomic layer deposition step is greater than the final thickness d to which the smoothing layer 32 is etched back in the etching-back step (cf. also 2 ). The final thickness d of the smoothing layer 32 is typically less than 100 μm, less than 10 μm or less than 1 μm. In principle, however, the final thickness d of the smoothing layer 32 can also be greater than 100 μm. The final thickness d of the smoothing layer 32 is measured at the position at which the smoothing layer 32 has its maximum thickness.

On the other hand is in 3 shown that the surface roughness can be further reduced by the etch back step. The surface roughness of the surface 40 of the smoothing layer 32 after completion 38` of the etching-back step is therefore less than the surface roughness of the surface 36" of the smoothing layer 32 after completion 37` of the atomic layer deposition step, which in turn is less than the surface roughness of the surface 36 of the metallic base body 31. This is applies in particular when the partial etching back is carried out by means of atomic layer etching.

The one described below 4 is based on 5 of the article "Prospects for Thermal Atomic Layer Etching Using Sequential, Self-Limiting Fluorination and Ligand-Exchange Reactions" by S. George and Y. Lee, ACS Nano 10, 4889 (2016). In the representation of 4 the deposition process includes more than one atomic layer deposition step, and an etch back step after each atomic layer deposition step. A total of four snapshots 41, 41', 41'', 41''' are shown. A first snapshot 41 after the first atomic layer deposition step, a second snapshot 41' after the first etch-back step, a third snapshot 41'' after the second atomic layer deposition step, and one fourth snapshot 41''' after a large number of atomic layer deposition steps and etch back steps.

In the first atomic layer deposition step, growth nuclei form first and, starting from them, some islands 42. As shown in the second snapshot 41', these are then partially etched back. During the second atomic layer deposition step (snapshot 41''), randomly distributed new growth nuclei are now formed and new islands 43 from them. In addition, the islands 42 remaining after etching back grow again. Finally, as shown in the fourth snapshot 41''', a closed, smooth and thin smoothing layer 32 is formed.

Alternatively, the deposition process described above may not include an etch back step or may not include an etch back step after each of the atomic layer deposition steps. Furthermore, the deposition process may include a reduction step after each of the atomic layer deposition steps and before any etch back step. In the reduction step, in particular, SiN _x , SiO _x or SiN _x O _y C _z deposited in the atomic layer deposition step can be at least partially reduced to Si. A highly reducing atmosphere, such as a plasma, containing hydrogen radicals is typically used for this purpose. Alternatively, the reduction step can simultaneously represent the etch-back step. In this case, for example, the (non-stoichiometric) SiN _x O _y C _z can be at least partially reduced to Si with a dry etching process using a reducing gas mixture that contains, for example, CF ₄ /H ₂ or SF ₆ /CH ₄ /N ₂ become.

The 5 shows a cross section of a processing head 44, which is used to deposit a smoothing layer 32, not shown here, on a metal base body 31 by means of the in 2 until 4 described deposition process is used. For easier description is in 5 a Cartesian coordinate system x,y,z is shown.

The processing head 44 has a processing surface 45 and feed channels 46 and discharge channels 47 . Process media P, C, A and inert gas I are supplied to the processing surface 45 by means of the supply channels 46 . Reaction products R, process media P,C,A and inert gas I are removed from the processing surface 45 by means of the removal channels 47 . In the 5 The non-illustrated smoothing layer 31, which is deposited by means of the processing head 44, consists at least partially of SiO ₂ , but it can also at least partially consist of another material, for example silicon nitride. Both Process media are a Si precursor P, a co-reactant C, and an etching gas A. Examples of Si precursors that can be used are aminosilanes such as tris(dimethylamino)silane (3DMAS, CAS 15112-89-7) and bis(diethylamino) silane (BDEAS, CAS 27804-64-4), possibly with plasma-activated oxygen as a co-reactant, and the precursor AP-LTO 330 with ozone as a co-reactant. Hydrogen, nitrogen, oxygen and/or CF ₄ , for example, optionally also plasma-enhanced, can be used as the etching gas. Various fluorine-containing gas mixtures can also be used as etching gases.

In the example shown, two ALD areas 48, 48' and an etching area 49 are provided along the processing surface 45. The ALD areas 48,48' are separated from one another by the inert gas I in order to avoid a reaction of precursor P and co-reactant C in the gas phase. In addition, the ALD areas 48, 48' are separated from the etching area 49 by the inert gas I. The first partial reaction, ie the reaction with the precursor P, takes place in the first ALD area 48, while the second partial reaction, ie the reaction with the co-reactant C, takes place in the second ALD area 48`. Deviating from the illustration, more than two ALD areas 48, 48' and/or more than one etching area 49 can also be provided. If two or more etching regions 49 are provided, the partial etching back can also be carried out by means of a three-dimensional atomic layer etching process. In the case of several etching areas 49, these are each also spatially separated from one another, as is the case with the ALD areas 48, 48'.

The atomic layer deposition and the etching back take place by means of a relative movement 50 between the processing head 44 and the metal base body 31 and thus between the ALD areas 48, 48` and the etching area 49 on the one hand and the metal base body 31 on the other hand, whereby at least partial areas of the surface to be coated of the metallic base body 31 are successively exposed to the first ALD region 48, the second ALD region 48' and the etching region 49. The deposition by means of the processing head 44 takes place at atmospheric pressure, but can also take place under other conditions.

As an alternative to using a processing head 44, the deposition can also take place by means of a reactor which has a plurality of reaction chambers, with at least one of the reaction chambers serving as at least one ALD region and at least one further reaction chamber serving as at least one etching region. There can also be a pressure difference between the reaction chambers.

The 6 shows a cross section through a region near the surface of a reflective optical element 51 for EUV lithography. The reflective optical element 51 shown is a mirror, but it can also be another reflective optical element.

The reflective optical element 51 is made from a substrate 30 which was produced using the method described above. In particular, the substrate 30 has a smoothing layer 32 which has been deposited on a metallic base body 31 by means of a deposition process which comprises at least one atomic layer deposition step. For the reflection of EUV radiation, the reflective optical element 51 has a reflective coating 52 applied to the substrate 30 in the form of a layer stack. The layer stack typically comprises between 50 and 100 double layers 53. Two double layers 53 are shown here as representatives. Each double layer 53 comprises a first layer 54 consisting of a first layer material and a second layer 54' consisting of a second layer material, which each have a different real part of their refractive index. In the illustrated case, the first layer material is silicon, while the second layer material is molybdenum, but other materials can also be used as layer materials. In the illustrated case of a coating 52 for reflecting EUV radiation in the form of a layer stack, the reflection of the EUV radiation is based on interference effects. Alternatively, the coating 52 can also have only a few layers and serve to reflect EUV radiation at grazing incidence.

The 7 shows a variant of the in the 6 illustrated reflective optical element 51 for EUV lithography, the layers of the layer stack which forms the reflective coating 52 not being illustrated to simplify the illustration. Deviating from 6 the reflective optical element 51 shown here has a structuring layer 55 between the smoothing layer 32 and the coating 52 for reflecting EUV radiation, with the structuring layer 55 being deposited by means of atomic layer deposition. The structuring layer 55 can be formed at least partially, in particular completely, from at least one material selected from the group comprising: silicon, germanium, carbon, boron, titanium, zirconium, niobium, tantalum, tungsten, vanadium, their alloys and compounds, in particular their oxides, carbides, borides, nitrides and silicides and mixed compounds, noble metals, in particular ruthenium, rhodium, palladium, platinum, iridium, osmium, rhenium and their alloys gen. As in 7 As can be seen, the structuring layer 55 was structured on its upper side in order to form a lattice structure 56 . The reflective coating 52 is conformally deposited on the upper side of the structuring layer 55 so that the grating structure 56 assumes the function of a diffraction grating, which in the example shown is used to filter undesired spectral components from the radiation reflected at the optical element 51 .

QUOTES INCLUDED IN DESCRIPTION

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

Patent Literature Cited

• US 9541685 B2 [0008]
• WO 2004/095086 A2 [0015]
• US 2016/0086681 A1 [0015]
• WO 2013/113537 A2 [0016]
• DE 102020207807 A1 [0017, 0053]
• WO 2019/007927 A1 [0038]
• EP 3933882 A1 [0039]
• DE 102018220629 A1 [0054]

Non-patent Literature Cited

• "Atomic Layer Deposition of Silicon Nitride Thin Films: A Review of Recent Progress, Challenges, and Outlooks" by X. Meng et al., Materials, 9, 1007 (2016) [0013]
• "Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride Using a Novel Silylamine Precursor" by J.-M. Park et al., ACS Appl. Mater. Interfaces 8, 20665 (2016) [0024]
• "Thermal and plasma enhanced atomic layer deposition of SiO ₂ using commercial silicon precursors" by M. Putkonen et al., Thin Solid Films 558, 93 (2014) [0025]

Claims (18)

Method for producing a substrate (30) for a reflective optical element (51) for EUV lithography, comprising the steps: - Providing a metallic base body (31) and - Deposition of a smoothing layer (32) on the metallic base body (31) by means of a deposition process which comprises at least one atomic layer deposition step in which at least part of the smoothing layer (32) is deposited by means of atomic layer deposition. procedure after claim 1 , characterized in that the deposition process after at least one of the atomic layer deposition steps comprises an etching back step in which the smoothing layer (32) is partially etched back. procedure after claim 1 or 2 , characterized in that in at least one of the atomic layer deposition steps in each case SiO _x , SiN _x or SiN _x O _y C _z is deposited by means of atomic layer deposition. procedure after claim 3 , characterized in that the smoothing layer (32) is formed at least partially, in particular completely, from SiO _x , SiN _x or SiN _x O _y C _z . procedure after claim 3 , characterized in that the deposition process after each atomic layer deposition step comprises a reduction step in which the deposited SiO _x , SiN _x or SiN _x O _y C _z is at least partially reduced to Si. procedure after claim 5 , characterized in that immediately after at least one of the reduction steps, the etching back step is carried out, in which the smoothing layer (32) is partially etched back, or that the reduction step forms the etching back step, in which the smoothing layer (32) is partially etched back. Procedure according to one of claims 2 until 6 , characterized in that the partial etching back is carried out by means of a dry etching process, preferably by means of a reactive ion etching process and/or by means of atomic layer etching. Procedure according to one of claims 2 until 7 , characterized in that the at least partial deposition of the smoothing layer (32) takes place by means of atomic layer deposition in at least one atomic layer deposition area (48, 48`) and the etching back step takes place in at least one etching area (49) which is separated from the at least one atomic layer deposition area (48, 48` ) is spatially separated. procedure after claim 7 or 8th , characterized in that the atomic layer etching is carried out as spatial atomic layer etching. Method according to one of the preceding claims, characterized in that the atomic layer deposition is carried out as spatial atomic layer deposition. Method according to one of the preceding claims, characterized in that the smoothing layer (32) is polished after deposition. Method according to one of the preceding claims, characterized in that a final thickness (d) of the smoothing layer (32) is less than 100 µm, preferably less than 10 µm, particularly preferably less than 1 µm. Method for producing a reflective optical element (51) for EUV lithography, comprising the steps: Manufacturing a substrate (30) according to the method according to any one of the preceding claims, and Depositing a coating (52) for reflecting EUV radiation on the substrate (30). procedure after Claim 13 , characterized in that before the deposition of the coating (52) for reflecting EUV radiation, a structuring layer (55) is deposited on the substrate (30) by means of atomic layer deposition. procedure after Claim 14 , characterized in that the structuring layer (55) is formed at least partially, in particular completely from at least one material that is selected from the group consisting of: silicon, germanium, carbon, boron, titanium, zirconium, niobium, tantalum, tungsten, vanadium, their alloys and compounds, in particular their oxides, carbides, borides, nitrides and silicides and mixed compounds, precious metals, in particular ruthenium, rhodium, palladium, platinum, iridium, osmium, rhenium and their alloys. Procedure according to one of Claims 13 until 15 , additionally comprising the step of: structuring the metallic base body (31), the smoothing layer (32) and / or the structuring layer (55) to form a lattice structure (56). Reflective optical element (51) for EUV lithography, produced by the method according to one of Claims 13 until 16 . EUV lithography system (1), comprising: at least one reflective optical element (51). Claim 17 .

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