Method for depositing an outer layer, reflective optical element for the EUV wavelength range, and EUV lithography system

Abstract

The invention relates to a method for depositing an outer layer (35) on a surface (36) of a reflective optical element (30) for use in the EUV wavelength range, the deposition being carried out in at least one macrocycle (37), the macrocycle (37) comprising the steps of at least partially depositing the outer layer (35) by an atomic layer deposition (ALD) process in at least one ALD cycle, and partially etching back the outer layer (35). The invention further relates to a reflective optical element (30) for use in the EUV wavelength range, having a surface (36) with an outer layer (35) deposited by the method, and to an EUV lithography system comprising at least one such reflective optical element (30).

Description

[Technical field]

[0001] [Reference to Related Applications] This application claims priority from German Patent Application No. 10 2021 206 168.0, filed on June 16, 2021, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a method for depositing an outer layer on a surface of a reflective optical element for use in the EUV wavelength range. The present invention also relates to a reflective optical element for use in the EUV wavelength range having a surface with an outer layer deposited by said method, and to an EUV lithography system comprising at least one such reflective optical element. [Background technology]

[0003] Microstructured or nanostructured components for microelectronics or microsystems technology are produced by optical lithography using optical devices in the form of projection exposure apparatus. Such projection exposure apparatuses contain an illumination system for illuminating a photomask (reticle) with electromagnetic radiation in a narrow spectral range around an operating wavelength. The device also contains projection optics for projecting the structure of the reticle onto a radiation-sensitive layer of a wafer using radiation.

[0004] In order to minimize the structure widths of the semiconductor components to be manufactured, state-of-the-art projection exposure apparatus, so-called EUV lithography apparatus, are designed for operating wavelengths in the extreme ultraviolet (EUV) wavelength range, i.e. within the range of approximately 5 nm to approximately 30 nm. Since wavelengths in this range are strongly absorbed by almost all materials, the use of transmissive optical elements is usually not possible. The use of reflective optical elements is necessary. Such optical elements that reflect 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 in a vacuum chamber.

[0005] Optical elements that reflect EUV radiation can also be used in other optical devices used in connection with EUV lithography (EUV lithography systems), examples of which are metrology systems for inspecting exposed or to-be-exposed wafers, e.g. reticles, and metrology systems for inspecting further components of the EUV lithography device, such as mirrors.

[0006] During operation of an EUV lithography apparatus or system, the residual gas remaining in the vacuum chamber contains hydrocarbons. One source of hydrocarbons is outgassing from components arranged in the vacuum chamber. These components can be, for example, sensors, cables, masks or photoresist of the wafer to be structured. Another source of hydrocarbons can be vapors from vacuum pump oil diffusing into the vacuum chamber. Dissociation of the hydrocarbons caused by EUV radiation then leads to carbon contamination on the optical surfaces of the reflecting optical elements.

[0007] In addition to carbon contamination, oxidation of the optical surfaces may also occur. Oxidation is mainly caused by free oxygen radicals generated by the action of EUV radiation on water or oxygen molecules. Other contaminants, such as tin or silicon, may also be deposited on the optical surfaces. The use of reactive hydrogen has been proposed to clean such contamination from optical surfaces, see for example US Pat. No. 5,399,633.

[0008] However, reactive hydrogen causes etching attack on exposed, generally uncoated surfaces of materials or components of optical devices for EUV lithography. As a result, etching products are formed, which are converted into the gas phase and released into the vacuum environment. In particular, some elements form volatile hydrides in the presence of hydrogen ions and / or hydrogen radicals. Examples of such elements include tin, zinc, phosphorus, silicon, lead, and fluorine. In general, it is not possible to completely avoid positioning components containing at least one of these elements in a vacuum environment. Etching products can then deposit on the surfaces of reflective optical elements, especially in optically utilized areas. These deposits reduce the cumulative reflectivity of the optical device, lowering throughput and increasing costs.

[0009] Patent document 2 proposes protecting the material of the body of an optical element (or possibly the material of a functional coating applied to the body) in at least one surface area outside the optically useful surface area by at least one shield from etching attack and thus from partial material removal by hydrogen plasma.

[0010] Patent Document 3 describes that at least a portion of a cleaning device designed to supply a flow of hydrogen radicals may include a material having a surface recombination coefficient of hydrogen radicals of 0.02 or less. Patent Document 4 discloses an optical device for EUV lithography having an open channel with an inner wall coated with a coating containing a material having a hydrogen recombination coefficient of 0.08 or more to reduce the intrusion level of active hydrogen.

[0011] Patent document 5 describes an EUV projection exposure apparatus with several components which at least partially contain layers of noble metals, for example Rh, Ru, Ir, Pd, Pt, the minimum layer thickness of which is selected such that hydrogen ions and / or hydrogen radicals cannot penetrate the layer.

[0012] Reactive hydrogen can lead to the formation of bubbles and even to the delamination of the reflective coating of the optical element. The suspected mechanism is the inward diffusion of reactive (atomic) hydrogen into the reflective coating and the recombination of the diffused reactive hydrogen into molecular hydrogen. To solve this problem, Patent Document 6 proposes to place a functional layer between the reflective coating and the substrate, which reduces the hydrogen concentration on the side of the substrate of the reflective optical element facing the reflective coating by half or less.

[0013] Patent document 7 describes a mirror having a reflective layer and a barrier layer system. The barrier layer system is arranged between the reflective layer and the mirror substrate. The barrier layer system reduces the penetration of hydrogen atoms into the mirror substrate by a factor of 10 or more compared to a similar arrangement without the barrier layer system.

[0014] Patent Document 8 describes a method for forming a hydrogen protective layer, which is capable of withstanding high mechanical stress, particularly when a substrate having a hydrogen protective layer formed on its surface is stretched.

[0015] A method for providing a mirror for the EUV wavelength range with a dynamic protection layer that protects the mirror from etching attack by ions formed as a result of irradiation in the EUV wavelength range is also described in US Pat. No. 5,399,363.

[0016] US Pat. No. 5,399,633 describes a method for at least partially removing a contamination layer from an optical surface of an EUV radiation reflective optical element by means of an atomic layer etching process, which may be carried out as a spatial atomic layer etching process.

[0017] A method for atomic layer machining of optical surfaces of optical elements designed to reflect EUV radiation is also described in US Pat. No. 5,399,433, in which an atomic layer etching process is performed to remove contaminants from the curved optical surface. The atomic layer machining may include an atomic layer deposition process to deposit material on the optical surface.

[0018] US Patent No. 5,999,633 describes a self-cleaning reflective optical element having an outer metal layer that protects the reflective surface of the optical element from oxidation and transmits more than 90% of EUV radiation. The outer metal layer is a ruthenium layer. An intermediate metal layer made of chromium, molybdenum, or titanium can be provided.

[0019] The use of a ruthenium layer on a reflective coating of a photomask for the EUV wavelength range is also described in Non-Patent Document 1. The reflective coating is a stack of a silicon layer and a molybdenum layer. The top layer of the stack is a silicon layer. The ruthenium layer serves to protect the silicon from oxidation. It is described in the document that during cleaning of such a photomask with a chemical cleaning solution, diffusion of oxygen through the ruthenium layer and oxidation of the silicon below the ruthenium layer can occur. As a result, damage and peeling of the ruthenium layer is possible (see also FIG. 3 of the document).

[0020] Patent document 13 describes a method for applying an outer layer to a reflective coating of an optical element that reflects EUV radiation by atomic layer deposition (ALD), preferably by spatial atomic layer deposition. In one variant, before applying the outer layer, at least one protective layer, which may contain at least one noble metal, for example ruthenium, is applied to the reflective coating. The outer layer, which may be, for example, an oxide outer layer, is intended to allow a reduction in the deposition of contamination on the reflective optical element.

[0021] Atomic layer deposition is a type of deposition method characterized by two or more cyclically performed self-terminating surface reactions. Typically, an ALD cycle includes two surface reactions, a first partial reaction with a precursor, e.g., a metal precursor, followed by a second partial reaction with a co-reactant, e.g., water. In conventional ALD methods carried out in a reaction chamber, the precursor and the co-reactant are not present simultaneously in the reaction chamber, with an inert gas purge between the partial reactions. In contrast, in spatial atomic layer deposition, the partial reactions are carried out in different volume regions. To carry out the partial reactions, the substrate to be coated is moved relative to these volume regions. Typically, a large number of ALD cycles are carried out. Unique features of ALD methods are the excellent layer thickness control and the high conformality of the layers thus deposited.

[0022] The use of atomic layer deposition for the deposition of thin layers on optical elements has been discussed many times in the prior art. For example, US Pat. No. 5,399,436 describes the deposition of conformal layers on micro-optical elements by atomic layer deposition and related methods. US Pat. No. 5,499,436 also describes atomic layer deposition as a conformal coating process for the deposition of layers of a multilayer stack of coatings on optical elements that reflect EUV radiation. US Pat. No. 5,499,436 further discloses the manufacture of Fresnel zone plates by atomic layer deposition. US Pat. No. 5,499,436 also describes a special gas injector that injects gases into the process chamber of an apparatus for atomic layer deposition. US Pat. No. 5,499,436 and US Pat. No. 5,499,436. Designs of spatial atomic layer deposition can be found, for example, in US Pat. No. 5,399,436, US Pat. No. 5,499,436, US Pat. No. 5,499,436 and US Pat. No. 5,503,133.

[0023] A typical problem with the outer layer of a reflective optical element for the EUV wavelength range is the loss of reflectivity caused by the outer layer, which in the case of a projection exposure apparatus for the EUV wavelength range translates directly into a reduced throughput and therefore increased costs. [Prior art documents] [Patent documents]

[0024] [Patent Document 1] International Publication No. 2008 / 034582 [Patent Document 2] International Publication No. 2019 / 025162 [Patent Document 3] US Patent Application Publication No. 2007 / 0125964 [Patent Document 4] DE 10 2015 203 160 A1 [Patent Document 5] DE 10 2015 215 014 A1 [Patent Document 6] US Patent Application Publication No. 2019 / 0171108 [Patent Document 7] US Patent Application Publication No. 2019 / 0339428 [Patent Document 8] German Patent Application No. 10 2020 212869.3 [Patent Document 9] European Patent No. 1522895 [Patent Document 10] International Publication No. 2019 / 007927 [Patent Document 11] European Patent Application No. 20183384.5 [Patent Document 12] European Patent No. 1364231 [Patent Document 13] DE 10 2017 213 172 A1 [Patent Document 14] International Publication No. 2004 / 095086 [Patent Document 15] International Publication No. 2013 / 113537 [Patent Document 16] US Patent Application Publication No. 2016 / 008668 [Patent Document 17] U.S. Pat. No. 9,410,248 [Patent Document 18] U.S. Pat. No. 4,058,430 [Patent Document 19] U.S. Pat. No. 7,413,982 [Patent Document 20] International Publication No. 2010 / 024671 [Non-patent literature]

[0025] [Non-Patent Document 1] “Ruthenium capping layer preservation for 100X clean through pH driven effects” by D. Dattilo et al., Proc. SPIE 9635, Photomask Technology 2015, 96351B [Non-Patent Document 2] “Spatial atomic layer deposition: A route towards further industrialization of atomic layer deposition” by P. Poodt et al., J. Vac. Sci. Technol. A 30, 010802-1 (2012) Summary of the Invention [Problem to be solved by the invention]

[0026] It is therefore an object of the present invention to provide a method for depositing an outer layer on a reflective optical element for the EUV wavelength range that effectively protects the reflective optical element while at the same time minimizing loss of reflectivity. [Means for solving the problem]

[0027] This object is achieved in a first aspect by a method for depositing an outer layer on a surface of a reflective optical element for EUV wavelength range, the deposition being carried out in at least one macrocycle comprising the steps of at least partially depositing the outer layer by an atomic layer deposition (ALD) process in at least one ALD cycle and partially etching back the outer layer.

[0028] The reflective optical element for the EUV wavelength range is for example a mirror, for example a collector mirror of a projection exposure apparatus, or a photomask. To reflect radiation in the EUV wavelength range, the reflective optical element may have a reflective coating applied to the substrate. The reflective coating may for example comprise a stack of silicon and molybdenum layers. The reflection of EUV radiation is in this case based on interference effects. Alternatively, the reflective coating may serve to reflect EUV radiation at grazing incidence.

[0029] A macrocycle refers here to two sequentially performed method steps of the above method, namely at least partial deposition by atomic layer deposition in a first step and a subsequent partial etch-back in a second step. The atomic layer deposition can also be plasma assisted.

[0030] The outer layer is deposited in one or more macrocycles, during which the deposited material of the outer layer is partially etched back. This procedure results in a continuous and at the same time relatively thin outer layer. Since the outer layer is continuous, damage to the reflective optical elements, for example the reflective coating in the form of a stack arranged under the outer layer, is effectively suppressed. Undesirable degradation effects are significantly suppressed. As a result, premature and costly cleaning or replacement of the reflective optical elements is not necessary. As a result of the small final thickness of the outer layer, the reflectivity losses are also small. The final thickness of the outer layer in this application is understood to mean the thickness of the outer layer after depositing it in at least one macrocycle, i.e. the thickness after the end of the method.

[0031] In the simplest case, the outer layer is deposited by performing only one macrocycle. First, the outer layer is deposited by atomic layer deposition, with the thickness of the initial deposited layer being greater than the final thickness of the outer layer. The outer layer is then etched back to its final thickness. As a result, a smoother, more continuous and thinner outer layer is achieved than if the etch-back were omitted and the outer layer were deposited to its final thickness from scratch by atomic layer deposition.

[0032] Alternatively, it is possible to carry out several macrocycles. In the ALD step of the first macrocycle, growth nuclei are first formed, then several islands are formed. These are partially etched back in the second step of the first macrocycle. During the ALD step of the second macrocycle, new randomly distributed growth nuclei are formed. At the same time, the islands remaining after the etch back grow again. This description also applies to the other macrocycles. This procedure results in a much more uniform overall layer growth. A continuous outer layer is formed even for relatively small layer thicknesses, typically less than 2 nm.

[0033] Two or more macrocycles are usually preferred over deposition with only one macrocycle, because the ALD step does not necessarily form a continuous outer layer on the surface of the reflective optical element, even if the outer layer initially grown in the ALD step is relatively thick, and in some circumstances may leave small holes in the surface that can further grow as a result of the etch-back.

[0034] For at least partial deposition, it is possible to use thermal or plasma-assisted atomic layer deposition processes. The co-reactant(s) can therefore be thermal co-reactants or co-reactants in plasma form. It is also possible to use gas injectors, as described for example in US Pat. No. 5,399,993, for atomic layer deposition in ALD reactors, to inject gases into the process chamber. The partial etch-back can also be plasma-assisted and / or thermal.

[0035] One of the advantages of atomic layer deposition over conventional deposition methods such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) is the relatively low process temperature. While temperatures below 500 °C are obtained for CVD and PVD methods, atomic layer deposition can often be carried out even at room temperature. Thus, the use of atomic layer deposition makes it possible to avoid thermal damage of reflective optical elements. Furthermore, atomic layer deposition is much more suitable for the production of thin, continuous, defect-free layers. In contrast to conventional deposition methods, atomic layer deposition, due to its self-terminating nature and the continuous growth in the ALD cycle, also leads to a smoothing of the surface as the different growth fronts merge during deposition. More details on this can be found for example in the paper "Spatial ALD Challenges and Opportunities in Advanced Integrated Circuit Manufacturing" by D. O'Meara (PRiME 2020, Paper G02-1655), see especially page 13.

[0036] In one variant of this method, the final thickness of the outer layer at the end of the method is less than 4 nm, preferably less than 2 nm, more preferably between 2 nm and 1 nm.

[0037] In yet another variant of this method, the surface of the reflective optical element has a protective layer on which the outer layer is deposited, the protective layer being at least partially made of metal, preferably a noble metal. The protective layer is usually thicker than the outer layer and is generally deposited by, for example, a vapor deposition method, in particular by sputtering, rather than atomic layer deposition. In principle, the protective layer can alternatively be deposited by atomic layer deposition. The noble metal can be, for example, rhodium, ruthenium, palladium or zirconium. Since the outer layer is continuous, damage to the outer layer itself, the protective layer and the reflective coating, especially due to diffusion of O2 and H2, is suppressed or reduced.

[0038] In yet another variant of this method, the number of macrocycles is 2 or more, preferably 5 or more, more preferably 10 or more. As mentioned above, repeated etch-back of the outer layer can produce a continuous outer layer even with a small layer thickness. The final thickness of the outer layer depends on the number of macrocycles, the number of ALD cycles per macrocycle, the growth per ALD cycle, and the layer thickness removed with the etch-back of each macrocycle, in particular the number of etch-back steps per macrocycle. Thus, a given final thickness of the outer layer can usually be achieved by several different combinations of the mentioned parameters.

[0039] In yet another variation of this method, the number of ALD cycles per macrocycle is 1-100, preferably 10-100.

[0040] In yet another variant of the method, the outer layer consists at least partially of at least one oxide. It has been found that an outer layer consisting at least partially of an oxide significantly reduces the deposition of contamination, in particular of hydrides formed as a result of outgassing by hydrogen, making the removal of contamination easier. However, the oxide outer layer also entails challenges. In particular when depositing an oxide outer layer on a metallic protective layer, it is difficult to achieve a completely continuous outer layer, since the surface energies of metal and oxide usually differ greatly from one another. As a result of these different surface energies, so-called Stranski-Krastanov (island) growth is usually observed. In contrast, in an ideal layer-by-layer growth, individual islands formed from growth nuclei first grow and gradually merge, eventually forming a continuous, gap-free layer of a relatively large thickness. As a result, the method of the invention is particularly advantageous for the deposition of an oxide outer layer, in particular on a metallic protective layer.

[0041] In one development of this variant, the at least one oxide is SiO2, TiO x , and ZrO2. xrefers here to various titanium oxides. Suitable precursors for the various oxides are detailed below. The silicon precursors used for the deposition of SiO2 can be, for example, aminosilanes such as bis(diethylamino)silane (CAS 27804-64-4) or bis(tert-butylamino)silane (CAS 186598-40-3) or tris(dimethylamino)silane (CAS 15112-89-7), or chlorosilanes such as SiCl4 or SiH2Cl2. TiO x For the deposition of ZiO2, it is possible to use alkoxy-based titanium precursors, such as titanium(IV) ethoxide (CAS 3087-36-3) or titanium(IV) isopropoxide (CAS 546-68-9), or chlorine-based titanium precursors, such as TiCl4. The zirconium precursors used for the deposition of ZiO2 can be amino compounds, such as tetrakis(ethylmethylamino)zirconium (CAS 175923-04-3), or amidinates, such as tetrakis(N,N'-dimethylacetamidinato)zirconium. The oxidizing co-reactant used in all cases can be O2, for example in the form of O2 plasma, ozone, H2O, or H2O2.

[0042] In yet another variant of this method, the etch-back is performed by a dry etching process, preferably a reactive ion etching process and / or an atomic layer etching process (ALE). Similar to the analogous atomic layer deposition, atomic layer etching describes an etching process in which two or more cyclically performed self-terminating surface reactions are performed. The etch-back can alternatively be performed by a reactive ion etching process or a plasma-assisted dry etching process.

[0043] In yet another variation of this method, the at least partial deposition by an atomic layer deposition process is performed in at least one ALD region, and the partial etch-back is performed in at least one etch region spatially separated from the at least one ALD region.

[0044] The at least one ALD region and the at least one etching region are each at least one volume region in a spatially separated arrangement, i.e., spaced apart from one another. Furthermore, an inert gas flow between the volume regions forming a gas curtain can be used for the spatial separation. The inert gas can be, for example, Ar or N2. The precursors and co-reactants of the ALD regions can also be spatially separated from one another by a gas curtain, for example, an inert gas flow.

[0045] The at least one ALD region and the at least one etch region are further preferably isolated from the ambient atmosphere, e.g., by a similar inert gas flow, so that gases in the ambient atmosphere that may adversely affect deposition, e.g., O2 and CO2, do not enter either the ALD region or the etch region.

[0046] In the case of multiple ALD regions and / or multiple etching regions, these are also spatially separated from each other. The at least partial deposition by atomic layer deposition and the partial etch-back are performed by a relative movement between the ALD region and the etching region on the one hand and between the ALD region and the reflective optical element on the other hand, so that at least a partial region of the surface to be coated of the reflective optical element is sequentially exposed to at least one ALD region and at least one etching region. For example, if the etch-back is performed by reactive ion etching, there may be a pressure difference between the at least one ALD region and the at least one etching region. The method can also be performed by a reactor having multiple reaction chambers, in which case at least one of the reaction chambers serves as at least one ALD region and at least one other reaction chamber serves as at least one etching region. In order to be able to perform the atomic layer deposition and the partial etch-back sequentially, the reflective optical element can be moved between the reaction chambers. For (deep) reactive ion etching, such reactors are discussed in the article "Cyclic etch / passivation-deposition as an all-spatial concept towards high-rate room temperature Atomic Layer Etching" by F. Roozeboom et al., ECS J. Solid State Sc. Technol., 4, N5067 (2015), see also U.S. Pat. No. 9,761,458, in which the method for (deep) reactive ion etching described there replaces the conventional passivation step with the deposition of a passivation layer by atomic layer deposition.

[0047] In one development of the above mentioned variant, the etch back is performed by a spatial atomic layer etching process, in which atomic layer etching is performed in at least two spatially separated etching regions, one of the self-limiting partial reactions now taking place in each of the etching regions.

[0048] In yet another variation of this method, the atomic layer deposition process is carried out as a spatial atomic layer deposition process. In a spatial atomic layer deposition process, atomic layer deposition is carried out in at least two spatially separated ALD regions. A first partial reaction, i.e., a reaction with a precursor, occurs in at least one of the ALD regions. A second partial reaction, i.e., a reaction with a co-reactant, occurs in at least one other ALD region.

[0049] The advantage of using spatially separated ALD and etch regions, especially spatial atomic layer processing, i.e. spatial atomic layer etching and / or spatial atomic layer deposition processes, is the high throughput, ease of scalability, and high deposition and / or etch rates achieved thereby. Spatial atomic layer processing can be performed at atmospheric pressure, which allows particularly low process temperatures, thereby avoiding thermal damage to reflective optical elements. Other etching methods can also be performed at atmospheric pressure. In the case of plasma-assisted deposition, the process temperature can be even lower. In the case of partial etch-back, plasma can potentially accelerate the etching reaction.

[0050] Atmospheric pressure processing is particularly advantageous for reflective optics for EUV wavelengths, as they are typically relatively large, which makes processing under high or low vacuum conditions difficult to achieve as with conventional dry etching methods, including conventional variations of reactive ion etching and atomic layer etching.

[0051] In one development of this variant, deposition is performed by a processing head having a processing surface, supply channels for supplying the process medium and the inert gas to the processing surface, and exhaust channels for exhausting the reaction products, the process medium, and the inert gas from the processing surface, and the ALD region and at least one etching region are provided spatially separated along the processing surface by the inert gas.

[0052] Both spatial atomic layer deposition and partial etch-back, e.g. spatial atomic layer etching, are performed in this case by a single combined device. The process medium is one or more precursors, co-reactant(s), and / or one or more etching gases. The etching gas(es) can be, for example, CF4, SF6, NF3, CHCl3, Cl2, or a mixture of these gases, or a mixture of one or more of these gases with O2. The etching gas can be used for plasma-assisted and / or thermal etching. The precursors are usually supplied in gaseous form (possibly in heated form) or as plasma. Suitable inert gases are, for example, Ar or N2. The reaction products are compounds that form volatile by-products in the atomic layer deposition or partial etch-back. The relative movement between the ALD region or the etching region and the reflective optical element results in the relative movement between the processing head and the reflective optical element. The supply and exhaust channels are, for example, in a parallel arrangement. Alternatively, the supply and exhaust channels can also be in a circular or radial arrangement. In this case, the relative movement between the processing head and the reflecting optical element is a rotational movement.

[0053] The design of the processing head and the selection of the process parameters are made such that the individual process steps and partial reactions are carried out separately from each other. In particular, for the spatial separation between the individual ALD regions and between the ALD region and at least one etching region, an inert gas is supplied and removed again. If the gas flows of all process media and inert gas, i.e. the gas amounts flowing per unit time, are appropriately selected, the distance between the processing face of the processing head and the surface to be coated, as well as the distance between the supply channel and the discharge channel, are selected here in such a way that, as a result of the relative movement between the processing head and the reflecting optical element, the surface to be coated of the reflecting optical element comes into contact with the individual process media individually. In particular, there is no mixing of different process media and thus no reaction of the process media in the gas phase.

[0054] SiO2, TiO xIn the spatial atomic layer deposition of SiO2, the time period during which each partial area of ​​the surface of the reflective optical element to be coated is in contact with the ALD area (also called exposure time) is typically 100 ms to 500 ms. In the spatial atomic layer deposition of SiO2, the flow rate of the inert gas flow (e.g., Ar gas flow) acting as a carrier gas for the precursor and flowing through the bubbler is preferably 50 sccm to 500 sccm. x In the case of spatial atomic layer deposition of ZrO2, the flow rate of the inert gas flow is preferably 50 sccm to 700 sccm. Furthermore, in these cases, the vapor pressure of the precursor can be increased by heating the bubbler to which the precursor is supplied.

[0055] In one development of this variant, the partial etch-back is performed by a plasma source based on a dielectric barrier discharge. Preferably, in this case, a mixture of CF4 and N2 or a mixture of CF4, O2 and N2 serves as the process medium for the etch-back, which is usually performed at atmospheric pressure. Only free radicals contribute to the etching reaction. As a result, since there are no ions, no ion damage, in particular ion implantation, occurs during the etch-back. The flow rate of the CF4 gas flow is preferably between 100 sccm and 500 sccm, the flow rate of the total gas flow is preferably between 5 slm and 10 slm. The volume fraction of O2 is preferably between 5% and 20%. The applied AC voltage is preferably between 100 V and 170 V, the frequency of the AC voltage is preferably between 50 kHz and 100 kHz.

[0056] In yet another variant of this method, the surface of the reflective optical element is curved and the processing surface of the processing head has a shape that matches the curved surface of the reflective optical element. The shape of the processing surface preferably corresponds to the shape of the reflective optical element. The distance between the processing surface and the curved surface of the reflective optical element during atomic layer deposition and partial etch back is typically 20 μm to 100 μm in this case.

[0057] In yet another variant of the method, the outer layer is deposited on a partial region, in particular a damaged partial region, of a surface of a reflective optical element, in particular a collector mirror of an EUV lithography system, where the outer layer has been completely or partially removed. Prior to deposition of the outer layer, the damaged partial region can be identified by suitable metrology, for example by EUV radiometry.

[0058] Yet another aspect of the invention relates to a reflective optical element for the EUV wavelength range having a surface with an outer layer deposited by one of the methods described above or variations thereof.

[0059] Yet another aspect of the invention relates to an EUV lithography system comprising at least one reflective optical element as described above.

[0060] Further features and advantages of the invention will become apparent from the following description of an exemplary embodiment of the invention, with reference to the figures of the drawing which show the details essential to the invention, and from the claims. Each of the individual features can be implemented separately and individually or collectively in any combination in one variant of the invention.

[0061] Examples are shown in the schematic drawings and explained in the following description. [Brief description of the drawings]

[0062] [Figure 1] 1 is a schematic illustration of a meridian section of a projection exposure apparatus for EUV projection lithography; [Diagram 2] FIG. 2 is a schematic diagram of a cross section of a top layer of a reflective optical element for EUV wavelengths, including an outer layer deposited in one or more macrocycles. [Diagram 3] FIG. 1 is a schematic diagram of the deposition of an outer layer on the surface of a reflective optical element for the EUV wavelength range in one macrocycle. [Figure 4] FIG. 1 is a schematic diagram of deposition of an outer layer on a surface of a reflective optical element for the EUV wavelength range over two or more macrocycles. [Diagram 5]FIG. 1 is a schematic diagram of a processing head for depositing an outer layer on a planar surface of a reflective optical element for the EUV wavelength range. [Figure 6] FIG. 1 is a schematic diagram of a processing head for depositing an outer layer on a curved surface of a reflective optical element for the EUV wavelength range. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0063] In the following description of the drawings, the same reference numbers are used for components that are the same or have the same function.

[0064] The essential components of an EUV lithography system in the form of a projection exposure apparatus 1 for microlithography are exemplarily described below with reference to Figure 1. The description of the basic structure of the projection exposure apparatus 1 and its components should not be considered here as having a limiting effect.

[0065] One design of the illumination system 2 of an EUV projection exposure apparatus 1 comprises, in addition to the light source or radiation source 3, an illumination optical unit 4 for illumination of an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system. In this case, the illumination system does not include the light source 3.

[0066] 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 is displaceable in particular in the scanning direction by a reticle displacement drive 9.

[0067] For the purposes of illustration, Fig. 1 shows a Cartesian xyz coordinate system. The x direction extends perpendicular to the plane of the drawing. The y direction extends horizontally and the z direction extends vertically. The scanning direction extends in Fig. 1 in the y direction. The z direction extends perpendicular to the object plane 6.

[0068] The projection exposure apparatus 1 comprises a projection system 10. By means of the projection system 10, an object field 5 is imaged into an image field 11 in an image plane 12. A structure on a reticle 7 is imaged onto a photosensitive layer of a wafer 13, which is arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable, in particular in the y-direction, by a wafer displacement drive 15. The displacement of the reticle 7 firstly by the reticle displacement drive 9 and the displacement of the wafer 13 secondly by the wafer displacement drive 15 can be synchronized with respect to one another.

[0069] The radiation source 3 is an EUV radiation source. The radiation source 3 in particular emits EUV radiation 16, also referred to in the following as working radiation, illumination radiation or illumination light. In particular, the working radiation has a wavelength in the range of 5 nm to 30 nm. The radiation source 3 can be a plasma source, for example an LPP source (Laser Produced Plasma) or a GDPP source (Gas Discharge Plasma). It can also be a synchrotron-based radiation source. The radiation source 3 can be a Free Electron Laser (FEL).

[0070] The illumination radiation 16 leaving the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 may be a collector mirror with one or more ellipsoidal and / or hyperbolic reflecting surfaces. The illumination radiation 16 may be incident on at least one reflecting surface of the collector mirror 17 at grazing incidence (GI), i.e. at an angle of incidence greater than 45°, or at normal incidence (NI), i.e. at an angle of incidence smaller than 45°. The collector mirror 17 may be structured and / or coated, firstly to optimize its reflectivity for the radiation used and secondly to suppress extraneous light.

[0071] The illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18 downstream of the collector 17. The intermediate focal plane 18 may provide a separation between the source module comprising the radiation source 3 and the collector mirror 17 and the illumination optical unit 4.

[0072] The illumination optical unit 4 comprises a deflection mirror 19 and a first facet mirror 20 arranged downstream thereof in the beam path. The deflection mirror 19 may be a plane deflection mirror or a mirror with a beam-influencing effect beyond a pure deflection effect. Alternatively or additionally, the deflection mirror 19 may be designed as a spectral filter which separates the used optical wavelength of the illumination radiation 16 from extraneous light of wavelengths deviating therefrom. The first facet mirror 20 comprises a number of individual first facets 21, also referred to as field facets in the following. FIG. 1 shows only some of said facets 21 by way of example. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optical unit 4. The second facet mirror 22 comprises a number of second facets 23.

[0073] The illumination optical unit 4 therefore forms a double-facet system. This basic principle is also called a fly's eye integrator. By means of a second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-shaping mirror in the beam path upstream of the object field 5 or indeed the final mirror for the illumination radiation 16.

[0074] The projection system 10 comprises a number of mirrors Mi, which are numbered consecutively according to their location in the beam path of the projection exposure apparatus 1 .

[0075] In the example shown in Fig. 1, the projection system 10 comprises six mirrors M1-M6. Four, eight, ten, twelve or any other number of mirrors Mi are equally possible. The penultimate mirror M5 and the final mirror M6 each have an aperture for the passage of the illumination radiation 16. The projection system 10 is a double-shielded optical unit. The projection optical unit 10 has an image-side numerical aperture which may be greater than 0.4 or 0.5, may be greater than 0.6, and may be, for example, 0.7 or 0.75.

[0076] Like the mirrors of the illumination optical unit 4 , the mirror Mi may have a highly reflective coating for the illumination radiation 16 .

[0077] 2 shows a cross-section of the top layer of a reflective optical element 30 that reflects radiation in the EUV wavelength range. The reflective optical element 30 shown is a mirror, but could also be another reflective optical element, such as a photomask.

[0078] To reflect radiation in the EUV wavelength range, the reflective optical element 30 has a reflective coating 31 in the form of a stack applied to a substrate, not shown in FIG. 2. The stack typically comprises 50 to 100 bilayers 32, each bilayer 32 comprising a first layer 33 of a first layer material and a second layer 33' of a second layer material. In the illustrated case, the first layer material is silicon and the second layer material is molybdenum, but other materials can also be used as layer materials. The reflection of EUV radiation in the illustrated case of the reflective coating 31 in the form of a stack is based on interference effects. Alternatively, the reflective coating 31 may have only a few layers and serve to reflect EUV radiation at grazing incidence.

[0079] The reflective optical element 30 also has a protective layer 34, at least partly made of ruthenium, deposited by sputtering and serving to protect the reflective coating 31, in particular against oxidation. Alternatively, the protective layer 34 may also consist of another noble metal or another metal, or its oxide, nitride or boride. The protective layer 34 may also be deposited using methods other than sputtering. The reflective optical element 30 does not necessarily have to have a protective layer 34.

[0080] An outer layer 35 is deposited on the protective layer 34, the deposition being carried out in at least one macrocycle including at least one ALD cycle at least partially depositing the outer layer 35 by an atomic layer deposition (ALD) process and partially etching back the outer layer 35. The combination of atomic layer deposition and partial etch back in one or more macrocycles results in a robust method of depositing a thin and at the same time continuous layer. The continuous outer layer 35 prevents or reduces damage to the reflective coating 31 and the protective layer 34, in particular damage resulting from the diffusion of O2 and H2. The outer layer 35 shown in FIG. 2 has a final thickness d of about 2 nm at the end of the method. The final thickness d of the outer layer 35 may alternatively be less than 4 nm, less than 2 nm, in particular between 2 nm and 1 nm. As a result of the small final thickness d, only a small loss of reflectivity due to the outer layer 35 occurs. The protective layer 34 shown has a thickness greater than the outer layer 35. Alternatively, the protective layer 34 may be thinner than the outer layer 35. The illustrated outer layer 35 is at least partially made of SiO2, but may also be at least partially made of another oxide, such as TiO x Or ZrO2, or another material, such as a stack or mixture of oxides.

[0081] As an alternative or in addition to the deposition of the outer layer 35, protection of the reflective optical element 30, including avoidance of peeling of the protective layer 34, in particular as a result of oxidation of the underlying first layer 33 of the first bilayer 32, can also be achieved by means of one or more bonding layers (not shown). A drawback of such a solution is that such bonding layers usually lead to additional absorption of EUV radiation and thus to a loss of reflectivity.

[0082] 3 and 4 show the deposition of an outer layer 35 on a surface 36 of a reflective optical element 30 for the EUV wavelength range in at least one macrocycle 37. In each case a cross section of the outer layer 35 and the topmost region of the reflective optical element 30 is shown. A possibly previously deposited protective layer 34 and a reflective coating 31 of the reflective optical element 30 are not shown here for simplicity.

[0083] In Fig. 3, the deposition is carried out in a single macrocycle 37. First, as shown on the left side of Fig. 3, an outer layer 35 is deposited by atomic layer deposition, with the first deposited layer having a thickness d A is greater than the final thickness d. Then, as shown on the right side of FIG.

[0084] Three snapshots are shown at the beginning 38', during 38', and after 38'' of atomic layer deposition, and at the beginning 39, during 39', and after 39'' of etch back. The time progression is indicated by two arrows 40.

[0085] During atomic layer deposition, at the start 38, growth nuclei are first formed from which individual islands 41 arise. The islands 41 continue to grow and eventually coalesce as shown in the second snapshot 38'. After the end of atomic layer deposition 38'', a relatively large pre-thickness d A There is a continuous outer layer 35 having

[0086] During the etch back, the outer layer 35 is gradually removed. After the etch back is completed, 39'' is a continuous and relatively smooth layer with a preliminary thickness d A There is an outer layer 35 having a final thickness d less than

[0087] FIG. 4, described below, is based on FIG. 5 from the paper “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 FIG. 4, deposition is performed in two or more macrocycles 37. A total of four snapshots 42, 42′, 42″, 42′″ are shown: a first snapshot 42 after the ALD step of the first macrocycle 37, a second snapshot 42′ after the etch-back of the first macrocycle 37, a third snapshot 42″ after the ALD step of the second macrocycle 37′, and a fourth snapshot 42′″ after multiple macrocycles 37, 37′, ….

[0088] In the ALD step of the first macrocycle 37, growth nuclei are first formed, from which several islands 41 arise. These are subsequently partially etched back, as shown in the second snapshot 42'. During the ALD step of the second macrocycle 37', new growth nuclei are formed, this time with a random distribution, from which new islands 41' arise. Furthermore, the islands 41 remaining after the etch-back grow again. Finally, after a number of macrocycles 37, 37', ... a continuous but at the same time thin outer layer 35 is formed, as shown in the fourth snapshot 42'''.

[0089] Figures 5 and 6 show a cross section of a processing head 43 for depositing an outer layer 35 on a reflective optical element 30 for EUV wavelengths in the manner as described in relation to Figures 3 and 4. For simplicity of explanation, Figures 5 and 6 show a Cartesian coordinate system x, y, z.

[0090] The processing head 43 has a processing surface 44 and a supply channel 45 and an exhaust channel 46. The supply channel 45 supplies the process media P, C, A and the inert gas I to the processing surface 44. The exhaust channel 46 exhausts the reaction products R, the process media P, C, A and the inert gas I from the processing surface 44. The outer layer 35 (not shown in Figures 5 and 6) deposited by the processing head 43 consists at least partially of SiO2, but may also consist at least partially of another oxide or another material. The process media are a Si precursor P in the form of SiCl4, a co-reactant C in the form of an O2 plasma and an etching gas A in the form of a mixture of CF4 and N2. However, it is also possible to use other precursors P, co-reactants C, for example H2O and / or H2O2, and etching gas A. The inert gas I is Ar, but may also be another inert gas I, for example N2.

[0091] Two ALD regions 47, 47' and one etching region 48 are provided along the work surface 44. The ALD regions 47, 47' are separated from each other by an inert gas I to prevent a reaction between the precursor P and the co-reactant C in the gas phase. Furthermore, the ALD regions 47, 47' are separated from the etching region 48 by an inert gas I. A first partial reaction, i.e. with the precursor P, occurs in the first ALD region 47, and a second partial reaction, i.e. with the co-reactant C, occurs in the second ALD region 47'. Alternatively, it is also possible to provide more than two ALD regions 47, 47' and / or more than two etching regions 48. If more than one etching region 48 is provided, the partial etch-back can also be performed by a partial atomic layer etching process. In the case of multiple etching regions 48, they are also spatially separated from each other, as in the case of the ALD regions 47, 47'.

[0092] At least partial deposition by atomic layer deposition and partial etch-back is performed by a relative movement 49 between the processing head 43 and the reflective optical element 30, and thus between the ALD zones 47, 47' and the etching zone 48 on the one hand and the reflective optical element 30 on the other hand, so that at least partial areas of the surface 36 to be coated of the reflective optical element 30 are successively exposed to the first ALD zone 47, the second ALD zone 47' and at least one etching zone 48. The distance between the processing surface 44 and the surface 36 of the reflective optical element 30 is between 20 μm and 100 μm. The deposition by the processing head 43 is performed at atmospheric pressure, but can also be performed under other conditions.

[0093] For the deposition of the outer layer 35, the reflective optical element 30 is attached to a substrate holder (not shown). The relative movement between the processing head 43 and the reflective optical element 30 is realized by the movement of the substrate holder. The substrate holder may be approximately the same size as the reflective optical element 30. For example, the size of the substrate holder may be 1 m x 1 m. In some cases, the substrate holder may be heated and / or cooled. Alternatively or additionally, the supply channel 45 and / or the inlet to the supply channel 45 (not shown) may also be heated or cooled. The supply and discharge channels 45, 46 are arranged parallel to the x-axis and have a length of just over 1 m in the x-direction in the illustrated example to cover the entire reflective optical element 30. Along the y-axis, the supply and discharge channels 45, 46 are arranged spatially separated. 5 and 6, the supply channels 45 for the supply of the process media P, C, A may have a greater distance t along the z-axis from the surface 36 of the reflective optical element 30 than the supply channels 45 for the supply of the inert gas I, for example about 100 μm (or less). This helps to improve the separation of the process media P, C, A.

[0094] At least one of the supply channels 45 may also have a plasma source based on a dielectric barrier discharge (not shown), which may serve for surface treatment or may supply free radicals, such as O, H and / or N radicals, for at least partial deposition or partial etch-back. Suitable plasma sources are described, for example, in US Patent Application Publication No. 2017 / 0137939. The entire apparatus, including the processing head 43, the reflective optical element 30 and possibly the substrate holder, may also be arranged in an inert gas environment, for example in a glove box. It is also possible to carry out the method as an in situ method in the EUV lithography system, for example in the projection exposure apparatus 1 shown in FIG. 1, without removing the reflective optical element 30 from the EUV lithography system.

[0095] As an alternative to using a processing head 43, deposition can also be performed by a reactor having multiple reaction chambers, where at least one of the reaction chambers serves as at least one ALD region 47, 47' and at least one other reaction chamber serves as at least one etching region 48. There can also be a pressure differential between the reaction chambers.

[0096] Unlike the example shown in Fig. 5, the surface 36 of the reflecting optical element 30 shown in Fig. 6 is a curved surface. The processing surface 44 of the processing head 43 shown in Fig. 6 has a shape that approximately corresponds to the shape of the curved surface 36 of the reflecting optical element 30. The distance between the processing surface 44 and the curved surface 36 is 20 μm to 100 μm. To realize the relative movement 49 between the processing head 43 and the reflecting optical element 30, the reflecting optical element 30 can be moved as shown in Fig. 6, but generally, the reflecting optical element 30 and / or the processing head 43 are rotated around a rotation axis not shown.

[0097] The curved surface 36 shown in Fig. 6 can be, for example, the surface of the collector mirror 17 shown in Fig. 1. For the collector mirror 17, but also for other reflective optical elements for the EUV wavelength range, it is possible to repair damaged partial areas of the surface 36, from which the outer layer 35 has been completely or partially removed, rather than depositing the outer layer 35 over the entire surface 36. In this case, the outer layer 35 is deposited only over the damaged partial area(s) of the surface 36. The damaged partial area(s) in the case of the collector mirror 17 can be, for example, a mirror segment. The damaged partial areas of the outer layer 35 can be identified by means of a suitable metrology, for example by EUV radiometry.

Claims

1. A method for depositing an outer layer (35) on a surface (36) of a reflective optical element (30) for EUV wavelengths, comprising the steps of: depositing said outer layer (35) at least partially by an atomic layer deposition (ALD) process in at least one ALD cycle; partially etching back said outer layer (35); Deposition is performed in at least one macrocycle (37) comprising: A method according to claim 1, characterized in that said etching back step is performed by a dry etching process, preferably by a reactive ion etching process.

2. 2. The method according to claim 1, characterized in that the final thickness (d) of said outer layer (35) is less than 4 nm, preferably less than 2 nm, more preferably between 2 nm and 1 nm.

3. 3. The method according to claim 1 or 2, characterized in that the surface (36) of the reflective optical element (30) has a protective layer (34) on which the outer layer (35) is deposited, the protective layer (34) consisting at least partially of a metal, preferably a noble metal.

4. 3. The method according to claim 1 or 2, characterized in that the number of macrocycles is 2 or more, preferably 5 or more, more preferably 10 or more.

5. The method according to claim 1 or 2, characterized in that the number of ALD cycles per macrocycle (37) is between 1 and 100, preferably between 10 and 100.

6. 3. The method according to claim 1 or 2, characterized in that said outer layer (35) consists at least in part of at least one oxide.

7. 7. The method of claim 6, wherein the at least one oxide is SiO ２ , TiO ｘ , and ZrO ２ The method according to claim 1, wherein the compound is selected from the group consisting of:

8. 3. The method of claim 1 or 2, characterized in that the step of at least partially depositing the outer layer (35) is performed by the atomic layer deposition process in at least one ALD region (47, 47') and the step of partially etching back is performed in at least one etching region (48) spatially separated from the at least one ALD region (47, 47').

9. 10. The method of claim 8, wherein the atomic layer deposition process is carried out as a spatial atomic layer deposition process.

10. 10. The method of claim 9, wherein the depositing comprises: A processing surface (44); a supply channel (45) for supplying process media (P, C, A) and an inert gas (I) to said working surface (44); an exhaust channel (46) for exhausting the reaction products (R), the process media (P, C, A) and the inert gas (I) from said working surface (44); and wherein the ALD region (47, 47') and the at least one etching region (48) are spatially separated along the processing surface (44) by the inert gas (I).

11. 11. The method of claim 10, wherein the partially etch back step is performed by a dielectric barrier discharge based plasma source.

12. 11. The method of claim 10, wherein the surface (36) of the reflecting optical element (30) is curved, and the machining surface (44) of the machining head (43) has a shape that matches the curved surface (36) of the reflecting optical element (30).

13. 3. The method according to claim 1 or 2, characterized in that the outer layer (35) is deposited on a partial region, in particular a damaged partial region, of the surface (36) of the reflective optical element (30), in particular of a collector mirror (17) of an EUV lithography system (1).

14. A reflective optical element (30) for the EUV wavelength range, characterized in that it has a surface (36) with an outer layer (35) deposited by the method according to claim 1 or 2.

15. An EUV lithography system (1), comprising at least one reflective optical element (30) according to claim 14.

Images