WO2018054795A1

WO2018054795A1 - Reflective optical element

Info

Publication number: WO2018054795A1
Application number: PCT/EP2017/073377
Authority: WO
Inventors: Rolf Freimann; Hans Willy Becker; Christian GRASSE
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2016-09-20
Filing date: 2017-09-17
Publication date: 2018-03-29
Also published as: DE102016218028A1

Abstract

In order to increase reflectivity, a reflective optical element (50) for the extreme ultraviolet wavelength range is proposed, said element comprising a multilayer system (51) on a substrate (52). The multilayer system (51) has first and second layers (54, 55) made of respective materials with different real components of the refractive index at a specific wavelength in the extreme ultraviolet wavelength range, and the layers are arranged in an alternating manner, wherein a transition layer (58, 59) with a refractive index which is variable over the thickness (D) of the transition layer at the specific wavelength is arranged between a first (54) and a second layer (55) or a second (55) and a first layer (54).

Description

Reflective optical element

The present invention relates to an ultraviolet wavelength reflective optical element having a multi-layer system on a substrate, the multi-layer system having first and second layers of respective refractive index different refractive index materials at a particular wavelength in the extreme ultraviolet wavelength range arranged alternately and wherein between at least a first and a second layer or a second and a first layer, a transition layer is arranged, whose refractive index in the determined

Wavelength across the thickness of the transition layer is variable. Furthermore, the invention relates to an optical system for EUV lithography and to an EUV lithography apparatus with such a reflective optical element. The present application takes priority of German application 10 2016 218 028.2 of 20.

September 2017, the contents of which are fully incorporated by reference.

In EUV lithography devices, for the lithography of semiconductor devices, reflective optical elements for the extreme ultraviolet (EUV) wavelength range (e.g., wavelengths between about 5 nm and 20 nm) such as photomasks or mirrors based on multilayer systems are employed. Since EUV lithography devices generally have a plurality of reflective optical elements, they must have the highest possible reflectivity in order to ensure a sufficiently high overall reflectivity.

Reflective optical elements for the EUV wavelength range generally have multilayer systems. These are alternately applied layers of a material with a higher real part of the refractive index at the working wavelength (also

Spacer) and a lower refractive index material at the working wavelength (also called an absorber), with an absorber-spacer pair forming a stack or a period. This somehow simulates a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection occurs. The thicknesses of the individual layers as well as the repeating stacks may be constant over the entire multi-layer system or even vary depending on which

Reflection profile to be achieved.

For real reflective optical elements, time may vary between the two

Absorber and Spaceriagen or the spacer and absorber layers mixed layers of both Form materials, which means that the actual reflectivity is less than the theoretically possible reflectivity. Thus, the reflectivity losses are not too large, additional layers between the absorber and Spaceriagen or the spacer and absorber layers can be provided, which act as diffusion barriers.

However, these barrier layers can in turn lead to reflectivity losses. In Juan I. Larruquert, J. Opt. Soc. At the. A, Vol. 21, No. 9, 1750-1760, September 2004

Reflectivities for reflective optical elements with in particular molybdenum as

Absorber material and silicon calculated as a spacer material. Constant thick mixed layers of molybdenum and silicon or constantly thick barrier layers of boron carbide are taken into account in the calculation of the reflectivity. To increase the reflectivity is trying to optimize the thicknesses of the molybdenum and silicon layers. In a second approach, the reflectivity is calculated with barrier layers of materials other than boron carbide. In Juan I. Larruquert, J. Opt. Soc. At the. A, Vol. 19, no. 2, 391-397, February 2002 is proposed in particular for the wavelengths of 50 nm and 49.3 nm, for

Reflectivity enhancement Multi-layer systems consisting of stacks of up to seven different materials with different complex indices of refraction at each

Use wavelengths. All the stacks of a multi-layer system have the same structure, while the thicknesses of the individual material layers are optimized for maximum reflectivity.

EP 1 065 532 A2 discloses a reflector for reflecting reflectivity for reflecting radiation in the extreme ultraviolet wavelength range, the reflector comprising a stack of alternating layers of a first and a second material, wherein the first material has a lower real refractive index in the desired wavelength range than the second material and wherein at least one layer of a third material inserted into the stack is provided, the third material being selected from the group consisting of Rb, RbCl, RbBr, Sr, Y, Zr, Ru, Rh, Tc, Pd, Nb and Be and alloys and compounds of these materials.

It is an object of the present invention to propose an alternative reflective optical element with the highest possible reflectivity. This object is achieved by a reflective optical element for the extreme

ultraviolet wavelength region having a multilayer system on a substrate, wherein the multilayer system comprises first and second layers of respective different refractive index real refractive index materials at a particular wavelength in the extreme ultraviolet wavelength region, which are alternately arranged, and between at least a first and a second layer or layers a second layer and a first layer, a transition layer is arranged whose refractive index in the determined

Wavelength is variable across the thickness of the transition layer, and wherein the course of the refractive index across the thickness of an exponential curve, a logarithmic curve, a sinusoidal or a course of a polynomial is approximated.

It has been proven that by deliberately arranging one or more

Transitional layers which have a refractive index which is variable over their thickness and in which the course of the refractive index is approximated by the thickness to an exponential curve, a logarithmic curve, a sinusoidal curve or a curve of a polynomial, higher reflectivities can be achieved than with reflective optical elements instead of the transition layer (s) each have barrier layers.

In particular, these courses are suitable for manageable expenditure in the

To provide reflective optical elements with good reflectivity.

Preferably, a transition layer is arranged between each first and second layer and / or each second and first layer whose refractive index is variable over the thickness of the transition layer at the particular wavelength. Particularly preferred is between all first and second or second and first layers such

Transition layer provided. As a result, the reflectivity can be increased particularly well compared with reflective optical elements with conventional barrier layers.

In preferred embodiments, the at least one transition layer consists of four or more individual layers which are different at the particular wavelength

Have refractive indices. The discretization of the transition layer in single layers simplifies both simulation calculations to calculate the expected reflectivity and the actual production by conventional physical and / or chemical vapor deposition methods from the gas phase. Preferably, the materials for the first layer and for the second layer each have one of the elements of the group consisting of Mo, Nb, Ru, Rh, Pd, Ag, Pt, Au, Ir, Os, Re, Cr, W, Ta, Fe, Co, Sn, Ni, Si, Cu, Zn, Lu, Er , Tm, Ho, Hf, Tb, T, Zr, C, Sc, B, Y, Nd, Pm, Eu, Sr, Be, La, Ce, Al, Ge, Zb, Pr. Advantageously, two elements for each of the first and second layers are combined with each other, both of which have the smallest possible imaginary part and at the same time as different as possible real parts of the refractive index.

The reflective optical element is preferably designed in such a way that the at least one transition layer consists of further material which has material of neither the first nor the second layer. In this way, more degrees of freedom in the optimization of the transition layer can be exploited.

In preferred embodiments, the at least one transition layer comprises material of the first and / or the second layer with a variable proportion over the thickness of the transition layer. The provision of material of the first and / or second layer in the

Transition layer allowed to keep the cost of manufacturing less than at

To provide transition layers of any materials and yet reflective optical elements with relatively high reflectivity.

In preferred embodiments, at least one transition layer consists of material of the first and material of the second layer. Such transition layers can be considered as artificial mixed layers, which, however, in view of the

Reflectivity were optimized by the material proportions across the thickness of

Transition layer were selectively varied. In further preferred embodiments, at least one transition layer consists of material of the first layer, of material of the second layer and a third material. In still other preferred embodiments, there is at least one

Transition layer of material of the first layer, of material of the second layer, a third material and a fourth material. These variants also make it possible to keep the outlay in terms of production lower than in the case of transition layers made of any desired materials and yet to provide reflective optical elements with a relatively high reflectivity. In addition, the third material and possibly the fourth material can be selected with regard to further functions, for example, whether they can act as a diffusion barrier between the materials of the first and second layers. Particularly advantageously, the further material or the third material and optionally the fourth material have one or more of carbon, boron carbide, boron, boron nitride, silicon carbide, silicon nitride, silicon boride, molybdenum carbide, molybdenum boride, molybdenum nitride, molybdenum silicide, niobium, niobium carbide, niobium nitride, Niobium boride, niobium silicide, yttrium, yttrium carbide, yttrium boride, yttrium nitride, yttrium silicide, zircon, zirconium boride, zirconium nitride, zirconium carbide, zirconium silicide, tungsten, tantalum. In the case of the third and possibly fourth material, their proportion of the transition layer is lower than, for example, in pure barrier layers, so that possible negative influences on the reflectivity are less. The further material can be selected with regard to the highest possible reflectivity and the lowest possible diffusion.

The reflective optical element presented here is suitable as a mirror for normal or quasi-normal incidence. Advantageously, it can also be designed as a mirror for grazing incidence. The mirror can enter over the entire optically used area

A multi-layer system or it may be that on at least a partial surface of the substrate of the mirror for grazing incidence no multilayer system is provided. In this case, the distribution with or without a multilayer system is advantageously adapted to the angle of incidence distribution or the angle of incidence bandwidth distribution over the mirror surface. By suitable choice of the thicknesses of the first and second layers as well as of the transition layers and their configuration, it is possible to provide reflective optical elements which, in the range of angles of incidence of significantly higher than 20 ° to

Have surface normal significant reflectivities. For partial surfaces with

Incidence angles of over 70 °, for example, metallic coatings

be provided, in which the reflectivity is based on total reflection and with which higher reflectivities than with multilayer systems can be achieved in this range of angles of incidence.

In a further aspect, the object is achieved by an optical system for EUV lithography with a reflective optical element as described above. In a further aspect, the object is also achieved by an EUV

A lithographic apparatus having a reflective optical element as described or an optical system as mentioned above.

The present invention will be explained in more detail with reference to preferred embodiments. Show this schematically an embodiment of an EUV lithography device with optical systems; schematically an embodiment of a reflective optical element; a first course of the refractive index across the thickness; a second course of the refractive index across the thickness; a third course of the refractive index across the thickness; a fourth course of the refractive index across the thickness; a fifth course of the refractive index across the thickness; a sixth course of the refractive index across the thickness; a seventh course of the refractive index across the thickness; an eighth course of the refractive index across the thickness;

Reflectivity gradients as a function of the wavelength for

various embodiments of a reflective optical

element; the Reflektivitätsverläufe from Figure 1 1 for a narrower

Wavelength range; an embodiment of a reflective optical element as a mirror for grazing incidence; and schematically the reflectivity as a function of the angle of incidence for the mirror from FIG. 13. FIG. 1 schematically shows an EUV lithography device 10. Essential components are the illumination system 14, the photomask 17 and the

Projection System 20. The EUV lithography apparatus 10 is operated under vacuum conditions so that the EUV radiation is absorbed as little as possible in its interior.

For example, a plasma source or a synchrotron can serve as the radiation source 12. The example shown here is a plasma source. The emitted radiation in the wavelength range of about 5 nm to 20 nm is first from

Collector mirror 13 bundled. In the example shown here, the radiation source 12 and the collector mirror 13 are integrated into the illumination system 14. In variants, only the collector mirror 13 or neither the collector mirror 13 nor the

Radiation source 12 may be integrated into the illumination system 14. Im in Figure 1

illustrated example, the illumination system 14 in the beam path behind the

Collector mirror 13 two mirrors 15, 16, to which the beam from the collector mirror 13 is passed. The mirrors 15, 16, in turn, direct the beam onto the photomask 17, which has the structure to be imaged onto the wafer 21. The photomask 17 is also a reflective optical element for the EUV wavelength range, which is changed depending on the manufacturing process. With the aid of the projection system 20, the beam reflected by the photomask 17 is projected onto the wafer 21, thereby imaging the structure of the photomask 17 onto it. The

Projection system 20 has two mirrors 18, 19 in the example shown. It should be noted that both the projection system 20 and the illumination system 14 may each have only one or even three, four, five or more mirrors.

Both the collector mirror 13 and the mirrors 15, 16, 17, 18 and the photomask 17 can be used as a reflective optical element for the extreme ultraviolet

Wavelength range may be formed with a multilayer system on a substrate, wherein the multi-layer system has first and second layers of respective materials with different real part of the refractive index at a certain wavelength in the extreme ultraviolet wavelength range, which are arranged alternately, and wherein between at least a first and a second layer or a second and a first layer, a transition layer is arranged whose refractive index at the certain

Wavelength across the thickness of the transition layer is variable. FIG. 2 schematically shows the structure of a reflective optical element 50. The illustrated example is a reflective optical element based on a multi-layer system 51. These are, in essence, alternately applied first layers of higher refractive index refractive index material at the operating wavelength at which, for example, the lithographic exposure is performed (also called spacer 54) and second layers of a lower refractive index real material at the operating wavelength (Also called absorber 55), wherein in the example shown here, an absorber-spacer pair forms a stack 53, which corresponds in periodic multi-layer systems of a period. This somehow simulates a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection occurs. The thicknesses of the individual layers 54, 55 as well as the repeating stack 53 may be constant over the entire multi-layer system 51 or even vary, depending on which spectral or angle-dependent reflection profile is to be achieved. The reflection profile can also be selectively influenced by the basic structure

Absorber 55 and spacer 54 is supplemented by further absorber or spacer materials in order to increase the maximum possible reflectivity at the respective operating wavelength. For this purpose, in some stacks absorber and / or spacer materials can be exchanged for each other or the stack of more than one absorber and / or

Spacer material are constructed or have additional layers of other materials. The absorber and spacer materials can have constant or varying thicknesses over all stacks in order to optimize the reflectivity.

In addition, transition layers 59, 58 are provided in the present example between all absorber and Spaceriagen 55, 54 and all Spacer and Absoberoberlagen 54, 55. In further variants, only between the absorber and Spaceriagen 55, 54 or only between the spacer and absorber layers 54, 55, a transition layer 59 and 58 may be provided. In other variants are also more or less absorber and

Spaceriagenpaare or spacer and Absorberlagenpaare provided between which no transition layer 59 and 58 are arranged.

The multi-layer system 51 is applied to a substrate 52. As substrate materials, materials with a low thermal expansion coefficient are preferably selected. In the example shown here, the first layer adjoining the substrate 52 is a spacer layer 54. Other variants, not shown, may be an absorber layer 55 or a transition layer 59. On the multi-layer system 51 For example, a protective layer 56 may be provided which protects the reflective optical element 50 against contamination and may be formed in multiple layers.

In the figures 3 to 10 are different exemplary embodiments of

Transition layer whose refractive index is variable at a certain wavelength across the thickness of the transition layer, shown schematically. All of the exemplary embodiments shown here are based on transition layers comprising spacer material and absorber layer material having a portion variable over the thickness D of the transition layer. For the sake of clarity, only the proportion C _{A of} the absorber material is plotted over the thickness d. The transition layer consists in the embodiments according to Figures 3 to 8 of absorber and spacer material. In the embodiments according to FIGS. 3 to 8, the proportion of the spacer material corresponds to 1-C _A. The thickness d is considered starting on the surface of a spacer layer in the direction of the adjacent absorber layer. With the material composition of the transition layer, the refractive index at a constant wavelength changes accordingly.

In FIG. 3, the absorber material component C _A increases exponentially from 0 to 1 over the thickness D of the transition layer. In the course shown in FIG. 4, the exponential increase in the proportion of absorber material C _{A was} discretized, specifically in five individual layers of constant thickness in the example shown here. In other variants, four or six or seven or more individual layers can be applied. The thickness of the individual layers does not necessarily have to be constant. Both also apply to the following examples. Furthermore, in other variants, not shown here, the increase in the proportion of absorber material C _{A can be} logarithmic or approximated by discretization to a logarithmic progression.

In the embodiment shown in FIG. 5, the course of the absorber material component C _{A is} sinusoidal or cosinusoidal. This is based on the so-called rugate design, which was studied for reflective optical elements for the EUV wavelength range in Ronald R. Willey in his presentation at the Conference on Design and Technology of Coatings, 24 September 2003 in Bonassola, Italy the contents of which are referred to in their entirety. Sinusoidally variable refractive index filters reflect exactly one wavelength. The zero position of the sine curve is the base refractive index, from which the layer thickness can be derived. The amplitude determines the bandwidth, the period determines the wavelength and the period number determines the notch depth of the filter. Im in Figure 6 shown course of Absorbermaterialanteils C _A , the sinusoidal course was discretized according to Figure 5 and approximated by six individual layers. In further variants, the course of the absorber material component C _A or of the refractive index can be completely free over the thickness d.

In the embodiment shown in FIG. 7, the course of the absorber material component CA is linear, ie it corresponds to a polynomial of the first degree. In further variants, the course can also correspond to higher-order polynomials. This linear course is discretized in the embodiment of FIG 8 and is different by five individual layers

Composition and thus different refractive index approximated. These

Embodiment is modified in the embodiments according to Figures 9 and 10 to the effect that in the present examples in each case a single layer is not composed of a mixture of spacer and absorber material, but of a third material. In the embodiment shown in FIG. 9, the mixture of spacer and absorber material was replaced by a third material with the spacer material of similar refractive index in the case of the left-hatched single layer; in the embodiment shown in FIG. 10, the mixture of spacer and absorber material was implemented in the right-hatched single layer a third material with the absorber material similar

Refractive index replaced. In the examples shown here, a single layer of a third material was provided in each case. In variants, more than one single layer of third material may be provided, which in the context of the approaching

Refractive index profile can be arranged arbitrarily. In further variants, instead of at least one single layer of third material at least one single layer of a mixture of absorber material with a third material or spacer material with a third material, wherein in the first case, the refractive index of the third material is comparable to that of the spacer material and in the second case the of the absorber material.

As third materials, their refractive index for EUV wavelengths rather a

Spacer material is comparable, u.a. Carbon, boron carbide, boron, boron nitride,

Silicon carbide, silicon nitride, silicon boride, yttrium, yttrium carbide, yttrium boride, yttrium nitride, yttrium silicide, zircon, zirconium carbide, zirconium boride, zirconium nitride, zirconium silicide. As third materials whose refractive index is more comparable to an absorber material for EUV wavelengths, i.a. Molybdenum carbide, molybdenum boride, molybdenum nitride, molybdenum silicide, niobium, niobium carbide, niobium nitride, niobium boride, niobium silicide,

Tungsten, tantalum should be considered. The materials mentioned also have the Advantage that act as a barrier material for molybdenum as absorber material and silicon as a spacer material, which are often used for reflective optical elements in the wavelength range around 13.5 nm and thus preferably used in EUV lithography, as a diffusion barrier. In general, the materials for absorber and Spaceriagen each one of the elements of the group of Mo, Nb, Ru, Rh, Pd, Ag, Pt, Au, Ir, Os, Re, Cr, W, Ta, Fe, Co, Sn, Ni, Si, Cu, Zn, Lu, Er, Tm, Ho, Hf, Tb, T, Zr, C, Sc, B, Y, Nd, Pm, Eu, Sr, Be, La, Ce, Al, Ge, For example, have Pr.

The reflectivities for three embodiments of the reflective optical element proposed here and for comparative examples were calculated and shown in FIGS. 11 and 12, wherein in FIG. 11 the reflectivity was plotted over the wavelength range from 12.5 nm to 14.5 nm and in Figure 12 over the wavelength range of 13.4 nm to 13.75 nm. They are based on multi-layer systems with molybdenum as

Absorber material and silicon as spacer material. Specifically, the reflectivity of an ideal molybdenum-silicon multilayer system was first calculated, the sixty Mo / Si periods with a period thickness of 6.99 nm and a Molybdänlagendickenanteil at the

Period thickness of 0.4. The layer roughness was below 200 μm for all layers. The maximum reflectivity is 0.7098. In FIGS. 11 and 12, the reflectivity is shown with a thick continuous line. This and all the following calculated

Multi-layer systems complete the vacuum with a silicon layer. The substrate used in all multilayer systems was a quartz substrate and all reflectivities were calculated for normal incidence.

As a further comparative example, the reflectivity of a Mo / Si multilayer system with carbon barrier layers was calculated. There were 60 Mo / Si periods of a period thickness of 7.02 nm and a molybdenum pad thickness fraction at the period thickness of 0.4. Between all silicon and molybdenum layers, a carbon barrier layer of 0.8 nm thickness was arranged. The layer roughness was below 200 μm for all layers. The maximum reflectivity is 0.6939. In FIGS. 1 1 and 12, the reflectivity is a thin one

continuous line drawn.

As a first embodiment of the refractive index transition layer-type reflective optical element proposed herein, a transition layer type Mo / Si multilayer system having single-layer approximated exponential

Considered refractive index profile. There were 60 Mo / Si periods of a period thickness of 6.99 nm and a molybdenum layer thickness proportion at the period thickness of 0.3 before. Between all silicon and molybdenum layers, a transition layer with a thickness of 0.8 nm was arranged. The transition layer was composed of six individual layers of a thickness of approximately 0.13 nm made of silicon and molybdenum, which had the following silicon proportions: 0.46 for the first single layer, 0.22 for the second single layer, 0.1 for the third single layer, 0.05 for the fourth single layer and 0.01 for the fifth single layer. The layer roughness was below 200 μm for all layers. The maximum reflectivity is 0.7084. In the figures 1 1 and 12 is the

Reflectivity drawn with a dotted line. As a second embodiment of the refractive index transition layer-type reflective optical element proposed here, a monolayer Si / Si multilayer linear refractive index gradient approximation system was considered. There were 60 Mo / Si periods of a period thickness of 6.99 nm and a molybdenum pad thickness fraction at the period thickness of 0.34. Between all silicon and molybdenum layers, a transition layer with a thickness of 0.8 nm was arranged. The transition layer was composed of five individual layers with a thickness of approximately 0.16 nm made of silicon and molybdenum, which had the following silicon components: 0.83 for the first single layer, 0.67 for the second single layer, 0.5 for the third single layer, 0.33 for the fourth single layer and 0.17 for the fifth single layer. The layer roughness was below 200 μm for all layers. The maximum reflectivity is 0.7066. In FIGS. 1 1 and 12, the reflectivity is indicated by a dot-dash line.

The third embodiment is different from the second embodiment

to the effect that the third single layer was made of carbon. The maximum reflectivity is 0.7066. In FIGS. 11 and 12, the reflectivity is indicated by a dashed line. Although it is a slight over the second embodiment

Observe reflectivity loss. However, it is expected that because of the function of the carbon monolayers as a diffusion barrier between the adjacent individual layers of silicon and molybdenum, the long-term stability, especially at elevated thermal load, should be higher than in the second embodiment.

The fourth embodiment is different from the third embodiment

to that effect, which was additionally the second single layer of yttrium. The maximal

Reflectivity is 0.7066. In FIGS. 1 1 and 12, the reflectivity is indicated by a dashed-double-dotted line. Although it is observed with respect to the second and third embodiments, a slight reflectivity loss. It will, however Expects that because of the function of the carbon single cell and the yttrium Einzellage as a diffusion barrier between the adjacent individual layers of silicon and molybdenum, the long-term stability, especially at elevated thermal load should be higher than in the second and the third embodiment.

It should be noted that the figures 3 to 8 corresponding

It is also possible to achieve refractive index profiles completely without material change, in particular if the at least one transition layer consists of a further material which has neither the first nor the second layer of material. For example, when applying the transition layer, such as when the coating process

carried out ion-beam assisted, corresponding density variations over the transition layer thickness are introduced.

FIG. 13 schematically illustrates an exemplary embodiment of the reflective optical element presented here as a grazing incidence mirror 130. This mirror 130 is designed in a first region 131 for a narrowband incident angle distribution, in the present example for the angle of incidence a1 between incident beam E1 and surface normal N1 or surface normal N1 and reflected beam A1. In this first region 131, for example, metallic coatings such as

Rutheniumbeschichtungen be provided, in which the reflectivity is based on total reflection and in which high angles of incidence, in particular higher than 70 °, higher

Reflections can be achieved as with multilayer systems in this range of angles of incidence. The reflectivity as a function of the angle of incidence at a wavelength of 13.5 nm is shown schematically in FIG. 14 for metallic coatings with a solid line.

In a second region 132, the mirror 130 is for a broader band

Incident angle distribution designed, in the present example, for a range between the angle of incidence a2 between the incident beam E2 and surface normal N2 or

Surface normal N2 and reflected beam A2 and between the angle of incidence a3 between incident beam E3 and surface normal N2 or surface normal N2 and reflected beam A3. In this second area 132 is a multi-layer system with

Transition layer arranged whose refractive index is variable at an operating wavelength of the mirror 130 across the thickness of the transition layer. By a suitable choice of the thicknesses of the first and second layers and the transition layers and their Embodiment can provide reflective optical elements that have in the range of angles of incidence of significantly higher than 20 ° to about 70 ° to the surface normal significant reflectivities that are higher than those achievable with a metallic coating. In FIG. 14, the reflectivity as a function of the angle of incidence at a wavelength of 13.5 nm for a multilayer system of the type proposed here is shown schematically with a dashed line.

It should be noted that reflective optical elements as proposed herein are also very well suited for embodiments as mirrors for normal or quasi-normal incidence.

reference numeral

10 EUV lithography device

12 radiation source

13 collector mirror

14 lighting system

15 mirrors

16 mirrors

17 photomask

18 mirrors

19 mirrors

20 projection system

21 wafers

50 reflective optical element 51 multi-layer system

52 substrate

53 stacks

54 spacers

55 absorber

56 protective layer

58 transition layer

59 transitional layer d thickness

D thick transition layer

CA content of absorber material

N1, N2 surface normals

E1. E2, E3 incident beam

A1, A2, A3 reflected beam α1, 2, α3 angle of incidence

Claims

claims

An ultraviolet wavelength reflective optical element having a multi-layer system on a substrate, the multi-layer system comprising first and second layers of different refractive index different refractive index materials at a particular wavelength in the extreme ultraviolet wavelength range, which are alternately arranged, between at least a first and a second layer or a second and a first layer, a transition layer is arranged, whose refractive index at the particular wavelength over the thickness (D) of the transition layer is variable, characterized in that the course of the refractive index over the thickness (d) to an exponential course, a

logarithmic course, a sinusoidal or a course of a polynomial is approximated.

2. Reflective optical element according to claim 1, characterized in that between each first (54) and second layer (55) and / or each second (55) and first layer (54) a transition layer (58, 59) is arranged, whose Refractive index at the particular wavelength over the thickness (D) of the transition layer (58, 59) is variable.

3. Reflective optical element according to claim 1 or 2, characterized in that the at least one transition layer (58, 59) consists of four or more individual layers, which have different refractive indices at the particular wavelength.

4. Reflective optical element according to one of claims 1 to 3, characterized in that the materials for the first layer (54) and for the second layer (55) each one of the elements of the group of Mo, Nb, Ru, Rh, Pd , Ag, Pt, Au, Ir, Os, Re, Cr, W, Ta, Fe, Co, Sn, Ni, Si, Cu, Zn, Lu, Er, Tm, Ho, Hf, Tb, T, Zr, C , Sc, B, Y, Nd, Pm, Eu, Sr, Be, La, Ce, Al, Ge, Zb, Pr.

5. Reflective optical element according to one of claims 1 to 4, characterized in that the at least one transition layer (58, 59) consists of further material, the material has neither the first nor the second layer.

6. Reflective optical element according to one of claims 1 to 4, characterized in that the at least one transition layer (58, 59) material of the first and / or the second layer (54, 55) with over the thickness (D) of the transition layer ( 58, 59) variable portion.

7. Reflective optical element according to claim 6, characterized in that at least one transition layer (58, 59) consists of material of the first and material of the second layer (54, 55).

8. Reflective optical element according to claim 6, characterized in that at least one transition layer (58, 59) consists of material of the first layer (54), of material of the second layer (55) and a third material.

9. Reflective optical element according to claim 6, characterized in that at least one transition layer (58, 59) of material of the first layer (54), from

Material of the second layer (55), a third material and a fourth material.

10. Reflective optical element according to claim 5, 8 or 9, characterized in that the further material or the third material and optionally the fourth material one or more of the group of carbon, boron carbide, boron, boron nitride, silicon carbide,

Silicon nitride, silicon boride, molybdenum carbide, molybdenum boride, molybdenum nitride,

Molybdenum silicide, niobium, niobium carbide, niobium nitride, niobium boride, niobium silicide, yttrium,

Yttrium carbide, yttrium boride, yttrium nitride, yttrium silicide, zircon, zirconium boride, zirconium nitride, zirconium carbide, zirconium silicide, tungsten, tantalum.

1 1. Reflective optical element according to one of claims 1 to 10, characterized in that it is designed as a mirror (130) for grazing incidence.

12. An optical system for EUV lithography with a reflective optical element according to one of claims 1 to 1 1.

13. EUV lithography apparatus with a reflective optical element according to any one of claims 1 to 1 1 or an optical system according to claim 12.