The present invention relates to a reflective optical element for the extreme ultraviolet and soft X-ray wavelength region having a reflective surface with a multilayer system. Moreover, the present invention relates to an optical system for EUV lithography with such a reflective optical element.

For the lithography of semiconductor components, in EUV lithography devices use is made of reflective optical elements for the extreme ultraviolet (EUV) and soft X-ray (SX) wavelength region (for example wavelengths of between approximately 5 nm and 20 nm) such as, for example, photomasks or mirrors on the basis of multilayer systems. Since EUV lithography devices generally have a plurality of reflective optical elements, the latter must have as high a reflectivity as possible in order to ensure a sufficiently high total reflectivity. The reflectivity and the service life of the reflective optical elements can be reduced by contamination of the optically used reflective surface of the reflective optical elements, which results from the shortwave irradiation together with residual gases in the operating atmosphere. Since a plurality of reflective optical elements are usually arranged one behind another in an EUV lithography device, even relatively small amounts of contaminations on each individual reflective optical element affect the total reflectivity to a relatively great extent.

Contamination can occur, for example, because of moisture residues. In this case, water molecules are dissociated by the EUV and/or SX radiation, and the resulting free oxygen radicals oxidize the optically active surfaces of the reflective optical elements. A further source of contamination is polymers that can, for example, originate from the vacuum pumps used in EUV lithography devices, or from residues of photoresists that are used on the semiconductor substrates to be patterned, and which lead, under the influence of the operating radiation, to carbon contaminations on the reflective optical elements. Whereas oxidative contaminations are generally irreversible, it is possible, in particular, to remove carbon contaminations by, inter alia, treatment with reactive hydrogen by having the reactive hydrogen react with the carbon containing residues to form volatile compounds. Reactive hydrogen can be hydrogen radicals or else ionized hydrogen atoms or molecules.

However, it has been observed that under the influence of reactive hydrogen that is used for cleaning, or because of the interaction of the EUV and/or SX radiation with hydrogen present in the residual atmosphere it is possible that individual layers, in particular those right at the surface of the multilayer system, can become detached. It is thought that the hydrogen, for example in silicon layers, reacts to form silane compounds.

It is therefore an object of the present invention to provide a reflective optical element that can be used in EUV lithography even in the presence of reactive hydrogen.

This object is achieved by a reflective optical element for the extreme ultraviolet and soft X- ray wavelength region having a reflective surface with a multilayer system in the case of which the reflective surface has a protective layer system with an uppermost layer composed of silicon carbide or ruthenium, the protective layer system having a thickness of between 5 nm and 25 nm.

It has emerged that the provision of such a protective layer system protects the multilayer system arranged thereunder, in particular against reactive hydrogen with a kinetic energy of a few hundred electron volts that would otherwise penetrate a plurality of nanometers into the multilayer system and could lead there to a detachment of the uppermost layers. What is meant by uppermost layers are the layers adjacent to the vacuum or the surroundings of the reflective optical element. It is thought that, in particular, this high energy hydrogen is decelerated in the protective layer system. In particular, starting from a protective layer system thickness of 5 nm, the quantity of the high energy reactive hydrogen that penetrates into the uppermost layers of the multilayer system lying thereunder is reduced in such a way that the detachment of the uppermost layers of the multilayer system is observed only seldom even in the case of lengthy use in an installation for EUV lithography. The thicker the protective layer system, the better is the protection against reactive hydrogen, since the diffusion of low energy reactive hydrogen, with kinetic energies that can even be only a few electron volts, as far as into the uppermost layers of the multilayer system is then rendered difficult in addition. At the same time, it is to be borne in mind that the maximum reflectivity of the respective reflective optical element is still sufficient for use in EUV lithography, and so protective layer layer thicknesses of more than 25 nm are rather disadvantageous.

Silicon carbide and ruthenium have proved to be particularly suitable as uppermost layer, that is to say as layer that closes off from the surroundings or the vacuum, of the protective layer system, since they have proved to be inert to reactive hydrogen. Moreover, it has been found with silicon carbide, in particular, that the likelihood of a deposition of tin and tin compounds as well as metal hydride compounds, in general, is particularly low, something which is particularly advantageous given the use of the reflective optical element in the surroundings of an EUV radiation source on the basis of a tin plasma, since fewer tin impurities of the reflective surface occur. It has, moreover, proved that both in the case of an uppermost layer composed of silicon carbide, and of an uppermost layer composed of ruthenium, the radiation in the wavelength region between 300 nm and 100 nm which is emitted, in particular, in addition to the EUV and SX radiation by plasma radiation sources, is more effectively suppressed than in the case of conventional reflective optical elements without the protective layer system proposed here. This is advantageous, in particular, for the use of the reflective optical elements in EUV lithography, where this radiation in the ultraviolet wavelength region could otherwise lead to incorrect exposures of a wafer coated with photoresist, since conventional photoresists react not only to EUV, but also to UV radiation. In a preferred embodiment, the protective layer system consists of an uppermost layer composed of silicon carbide or ruthenium of thickness from 5 nm to 25 nm. For this case of a protective layer system composed of a single layer, it is particularly preferred for the uppermost layer to have a thickness between 1 1 nm and 19 nm, in order to ensure particularly effective protection against reactive hydrogen in conjunction with good reflectivity. Reflective optical elements with only an uppermost layer as protective layer system can be produced with particular ease.

In a further preferred embodiment, the protective layer system under the uppermost layer has at least two alternatingly arranged layers composed of, on the one hand, molybdenum or ruthenium and of, on the other hand, carbon, a carbide or a nitride, preferably from a material from the group of silicon carbide, silicon nitride, boron carbide and carbon. What is meant here by alternatingly arranged is that two layers of the same material are not arranged directly adjacent to one another. For this case of a protective layer system composed of a plurality of layers, it is particularly preferred that the protective layer system has a total thickness of between 10 nm and 22 nm in order to ensure a particularly effective protection against reactive hydrogen in conjunction with good reflectivity. The advantage of multilayer protective layer systems with the abovementioned materials resides in the fact that the losses in reflectivity are less by comparison with single layer protective layer systems. In particularly simple variants, the protective layer system has only one additional layer between uppermost layer and multilayer system. Given an uppermost layer composed of silicon carbide, the additional layer is composed of molybdenum or ruthenium. Given an uppermost layer composed of ruthenium, the additional layer is carbon, a carbide, preferably silicon carbide or boron carbide, or a nitride, preferably silicon nitride. In further simple variants, the protective layer system has exactly two additional layers between uppermost layer and the multilayer system.

In a particularly preferred embodiment, the protective layer system under the uppermost layer composed of silicon carbide has at least two alternatingly arranged layers composed of molybdenum and silicon carbide. In this case, the protective layer system particularly preferably has a total thickness of between 1 1 nm and 22 nm. A reflective optical element with such a protective layer system composed of just two materials can be produced with acceptable effort. At the same time, an optimum protection against, in particular, high energy reactive hydrogen can be achieved at the same time as lowest possible losses concerning the maximum reflectivity. Moreover, this reflective optical element is particularly suitable for use in EUV lithography with metal plasma radiation sources, in particular tin plasma radiation sources, since the likelihood of the contamination of the reflective surface by tin or tin compounds or metal hydrides is particularly low and, moreover, radiation in the ultraviolet wavelength region is particularly effectively suppressed.

The multilayer system advantageously has alternatingly arranged layers of a material with a smaller real part of the refractive index in the extreme ultraviolet and soft X-ray wavelength region and of a material with a higher real part of the refractive index in the extreme ultraviolet and soft X-ray wavelength region, the material with higher real part of the refractive index being silicon. It has been found that layers composed of pure silicon, in particular, are particularly strongly attacked by penetrating reactive hydrogen, even when further layers composed of other material are arranged thereabove. Silicon layers, too, are effectively protected against high energy reactive hydrogen, in particular, by the protective layer system proposed here. In the case of multilayer systems with silicon as material with a higher real part of the refractive index, the material with lower real part of the refractive index is advantageously molybdenum in order to obtain high reflectivities in the wavelength region around 12.5 nm to 14.5 nm, in particular.

In a particularly preferred embodiment, the reflective optical element is designed as a collector mirror. Collector mirrors are often used in EUV lithography as first mirror in the beam direction downstream of the radiation source, in particular plasma radiation sources, in order to collect the radiation emitted by the radiation source in various directions and reflect it in focused fashion to the subsequent mirror. Because of the high radiation intensity in the surroundings of the radiation source, molecular hydrogen present in the residual gas atmosphere can be converted here with particularly high likelihood into atomic hydrogen with high kinetic energy, thus particularly exposing precisely collector mirrors to the risk of exhibiting detachment phenomena at the upper layers of the multilayer system on the basis of penetrating reactive hydrogen. If the collector mirror is a reflective optical element for the extreme ultraviolet and soft X-ray wavelength region having a reflective surface with a multilayer system, the reflective surface has a protective layer system with an uppermost layer composed of silicon carbide or ruthenium, the protective layer system having a thickness of between 5 nm and 25 nm, it is possible to reduce the occurrence of these detachment phenomena in conjunction with adequate reflectivity.

The object is further achieved by an optical system for EUV lithography having at least one reflective optical element as previously described. Advantageous refinements are to be found in the dependent claims.

The present invention is to be explained in more detail with reference to a preferred exemplary embodiment in which: Figure 1 is a schematic of an embodiment of an EUV lithography device having an illumination system with collector mirror with a protective layer system;

Figures 2a-c are schematics of various embodiments of reflective optical elements;

and

Figure 3 shows a comparison of hydrogen concentration in the multilayer system and maximum reflectivity for various embodiments of the reflective optical elements.

Figure 1 is a schematic of an EUV lithography device 10. Fundamental components are the illumination system 14, the photomask 17 and the projection system 20. The EUV lithography device 10 is operated under vacuum conditions so that the EUV radiation in its interior is absorbed as little as possible. By way of example, the radiation source 12 can be a plasma source or else a synchrotron. In the example illustrated here it is a plasma source. The emitted radiation in the wavelength region from approximately 5 nm to 20 nm is firstly focused by the collector mirror 13. The operating beam is then introduced into the illumination system 14. In the example illustrated in Figure 1 , the illumination system 14 has two mirrors 15, 16. The mirrors 15, 16 direct the beam onto the photomask 17, which has the structure that is to be imaged onto the wafer 21 . The photomask 17 is likewise a reflective optical element for the EUV and soft X-ray wavelength region that is replaced depending on the production process. The projection system 20 is used to project the beam reflected by the photomask 17 onto the wafer 21 , and thereby to image the structure of the photomask onto it. The projection system 20 has two mirrors 18, 19 in the example illustrated. It may be pointed out that both the projection system 20 and the illumination system 14 can respectively have only one or else three, four, five and more mirrors. In order also to extend the service life of the collector mirror 13 for the case in which there is present in the residual gas atmosphere of the illumination system 14 and, in particular, in the surroundings of the radiation source 12 molecular hydrogen that could be split into high energy reactive hydrogen by the radiation, or hydrogen that could originate from the cleaning of the mirrors 13, 15 or 16, and could attack the upper layers of the multilayer system of the collector mirror 13, the collector mirror 13 is equipped with a protective layer system which has an uppermost mirror composed of silicon carbide or ruthenium, the protective layer system having a thickness of between 5 nm and 25 nm. As a result, the collector mirror 13 is protected not only against high energy reactive hydrogen, but also against tin deposits if the plasma radiation source 12 is operated on the basis of tin plasma. Furthermore, the collector mirror 13 has the advantage of largely suppressing ultraviolet radiation emitted by the radiation source 12. Consequently, the collector mirror also has the function of a spectral filter. It may be pointed out that it is also possible to equip any other desired mirrors of the EUV lithography device 10 or its optical systems 14, 20 with a protective layer system described in more detail below, as an alternative or in addition to the collector mirror.

Figures 2a-b are schematics of the structure of exemplary embodiments of reflective optical elements 50 having protective layer system 59. The examples illustrated are reflective optical elements based on a multilayer system 51 . What is involved here is alternatingly applied layers of a material with a higher real part of the refractive index for the operating wavelength for which, for example, the lithographic exposure is carried out (also termed spacers 55) and of a material with a lower real part of the refractive index for the operating wavelength (also termed absorbers 54), an absorber-spacer pair forming a stack 53.

Thereby simulated to a certain extent is a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection takes place. The thicknesses of the individual layers 54, 55, as also the repeating stacks 53 can be constant across the entire multilayer system 51 , or else vary, depending on which spectral or angle-dependent reflection profile is to be achieved. The reflection profile can also be intentionally influenced by supplementing the basic structure of absorber 54 and spacer 55 by further, more or less absorbent, materials in order to increase the possible maximum reflectivity for the respective operating wavelength. To this end, it is possible in some stacks to interchange absorber and/or spacer materials with one another, or to construct the stacks from more than one absorber and/or spacer material. The absorber and spacer materials can have thicknesses that are constant or else varied over all the stacks, in order to optimize the reflectivity. Further, it is also possible to provide additional layers as diffusion barriers between spacer and absorber layers 55, 54.

The multilayer system 51 is applied to a substrate 52, and forms a reflective surface 60. Materials with a low coefficient of thermal expansion are preferably selected as substrate materials.

In the embodiment of a reflective optical element 50 illustrated in Figure 2a, the protective layer system consists of an uppermost layer composed of silicon carbide or ruthenium of thickness between 5 nm and 25 nm. In preferred variants, the uppermost layer has a thickness of between 1 1 nm and 19 nm.

In further embodiments, the protective layer system has under the uppermost layer alternatingly arranged layers composed, on the one hand, of molybdenum or ruthenium and, on the other hand, of carbon, a carbide or a nitride, preferably of a material from the group of silicon carbide, silicon nitride, boron carbide and carbon. Advantageously, in these cases, the protective layer system has a thickness of between 10 nm and 22 nm. Two variants are illustrated by way of example in Figures 2b and 2c. In the variant illustrated in Figure 2b, there is a further layer 57 composed of molybdenum or ruthenium arranged under the uppermost layers 56 of the protective layer system, ruthenium being provided purely for the case when the uppermost layer 56 is composed of silicon carbide, and thereunder and above the multilayer system 51 a further layer 58 composed of silicon carbide, silicon nitride, boron carbide or carbon. In the case of an uppermost layer 56 composed of silicon carbide the additional layer 58 is advantageously also composed of silicon carbide, in order to keep the outlay of production as low as possible. All methods that are known and are current in the production of reflective optical elements having a reflective surface with a multilayer system are suitable as production methods. It may be pointed out that for the case of an uppermost layers 56 composed of ruthenium there is advantageously firstly arranged thereunder a layer 58 composed of silicon carbide, silicon nitride, boron carbide or carbon, or another nitride or carbide and, thereunder, towards the multilayer system 51 a layer 57 composed of molybdenum or ruthenium, in order to obtain a higher reflectivity of the resulting reflective optical element.

The variant illustrated in Figure 2c differs from the variant illustrated in Figure 2b to the effect that there are provided not only two, but four further layers 57 and 58 composed respectively of ruthenium or molybdenum or silicon carbide, silicon nitride, boron carbide or carbon in order simultaneously to improve the protection against high energy reactive hydrogen and to reduce the losses in maximum reflectivity as against a comparable reflective optical element without a protective layer system. Of course, an odd number of additional layers 57, 58 can also be provided in the protective layer system 59 in further variants. Moreover, it is possible to provide not only one, two, three, four, five, but also six, seven, eight, nine or more additional layers 57, 58 under the uppermost layer 56.

It may be pointed out that it is possible to arrange between these layers of the protective layer system 59 additional layers that counteract a mixing of two adjacent layers 56, 57, 58 by taking over the function of a diffusion barrier, for example. Depending on the selection of the type of the uppermost layer 56, and on the number and type of the further layers 57, 58, the uppermost layer, adjacent to the protective layer system 59, of the multilayer system 51 can be a spacer layer 55 or an absorber layer 54. It is preferred for there to be adjacent to an uppermost absorber layer 54 a layer 58 or 57 of the protective layer system with a higher real part of the refractive index, and for there to be adjacent to an uppermost spacer layer 55 a layer 57 or 58 with a lower real part of the refractive index with respect to the wavelength of which the multilayer system 51 is designed, in order to obtain as high a reflectivity as possible. It is further advantageous when the uppermost layer, adjacent to the protective layer system, of the multilayer system is an absorber layer in order additionally to protect the uppermost spacer layer of the multilayer system against reactive hydrogen, in particular in the case of spacer layers composed of silicon. In particularly preferred embodiments, the protective layer system under the uppermost layer composed of silicon carbide has alternatingly arranged layers composed of molybdenum and silicon carbide, the protective layer system advantageously having a thickness of between 1 1 nm and 22 nm.

A plurality of particular embodiments with various protective layer systems were investigated precisely for the intrusion of high energy reactive hydrogen and their reflectivity in the EUV and/or SX wavelength region. The reflective optical elements had a multilayer system based on alternating silicon and molybdenum layers that was optimized for an operating wavelength of 13.5 nm, that is to say they exhibit the maximum reflectivity at substantially normal incidence at 13.5 nm. In this case, the silicon layers correspond to the layers with higher real part of the refractive index at 13.5 nm, and the molybdenum layers correspond to the layers with lower real part of the refractive index at 13.5 nm. In the present example, the multilayer system had a molybdenum layer as uppermost layer.

The maximum reflectivities reached for the individual embodiments were normalized to the maximum reflectivity of the reflective optical element only with a multilayer system without a protective layer system. Moreover, the hydrogen concentration in the respective uppermost silicon layer of the multilayer system was investigated when the respective reflective optical element was exposed to hydrogen with a kinetic energy of 500 eV.

The result for the reflective optical element without a protective layer system is illustrated in Figure 3 by the triangle for a relative maximum reflectivity of 1 and a hydrogen concentration in the uppermost silicon layer of 500 000 particles per cubic centimeter, the result being normalized to a surface dose of 1 particle per square centimeter.

The first step was to investigate two reflective optical elements having a single layer protective layer system composed of ruthenium on the molybdenum silicon multilayer system. In this case, one element had an uppermost layer composed of ruthenium with a thickness of 10 nm, and the other element had an uppermost layer composed of ruthenium with a thickness of 13 nm. The respective results are symbolized in Figure 3 by the squares. For a 10 nm thick ruthenium layer, 100 000 particles of hydrogen were present per cubic centimeter normalized to a surface dose of 1 particle per square centimeter in the uppermost silicon layer of the multilayer system. The maximum reflectivity was, however, only 79% of the maximum reflectivity without the uppermost layer composed of ruthenium. For a 13 nm thick ruthenium layer, the particle concentration was admittedly lowered further to only 40 000 per cubic centimeter normalized to a surface dose of 1 particle per square centimeter. However, the maximum reflectivity was also only 72% of the reflectivity without the protective layer system.

If reflective optical elements with a single layer protective layer system composed of silicon carbide are tested with the aid of an element with a 12 nm thick silicon carbide layer and of an element with a 19 nm thick silicon carbide layer, comparably low hydrogen

concentrations are obtained in conjunction with a clearly higher maximum reflectivity. The results are symbolized by the crosses in Figure 3. For the reflective optical element with an uppermost layers composed of 12 nm silicon carbide on the molybdenum-silicon multilayer system the result was a hydrogen concentration of 100 000 particles per cubic centimeter normalized to a surface dose of 1 particle per square centimeter in the uppermost silicon layer of the multilayer system in conjunction with a relative maximum reflectivity of 91 %, while for a silicon carbide thickness of 19 nm the result was a hydrogen concentration of 20 000 particles per cubic centimeter normalized to a surface dose of 1 particle per square centimeter in conjunction with a relative maximum reflectivity of 86%.

Moreover, reflective optical elements with multilayer protective layer systems were investigated. Under an uppermost layer of silicon carbide with a thickness of 4 nm, a first element had a further layer composed of molybdenum with a thickness of 3 nm

(corresponding to layer 57 from Figure 2b), and thereunder a further layer composed of silicon carbide with a thickness of 4 nm (corresponding to layer 58 from Figure 2b). Under an uppermost layer composed of silicon carbide with a thickness of 4 nm, a second element alternatingly had two further layers composed of molybdenum with a thickness of 3 nm

(corresponding to layers 57 from Figure 2c), and respectively thereunder two further layers composed of silicon carbide with a thickness of 4 nm (corresponding to the layers 58 from Figure 2b). The molybdenum-silicon multilayer system was likewise located thereunder in both cases. The results of the element with a total of two silicon carbide layers, and for the element with a total of three silicon carbide layers are symbolized in figure 3 by the circles. For the element with two silicon carbide layers at a hydrogen concentration of 100 000 per cubic centimeter normalized to a surface dose of 1 particle per square centimeter in the uppermost silicon layer of the multilayer system a relative maximum reflectivity of 94% was reached, while for the element with three silicon carbide layers a hydrogen concentration of 10 000 per cubic centimeter normalized to a surface dose of 1 particle per square centimeter was reached in the case of a relative maximum reflectivity of still 91 %.