Reflective optical element, and optical system of a microlithographic projection exposure apparatus

Abstract

A reflective optical element (50) having a substrate (52) and a multilayer system (51) that has a plurality of partial stacks (53), each with a first layer (54) of a first material and a second layer (55) of a second material. The first material and the second material differ from one another in refractive index at an operating wavelength of the optical element. Each of the partial stacks has a thickness (D.sub.i) and a layer thickness ratio (.sub.i), wherein the layer thickness ratio is the quotient of the thickness of the respective first layer and the partial stack thickness (D.sub.i). In a first section of the multilayer system, for at least one of the two variables of partial stack thickness (D.sub.i) and layer thickness ratio (.sub.i), the mean square deviation from the respective mean values therefor is at least 10% less than in a second section of the multilayer system.

Claims

1. A reflective optical element, comprising a substrate and a multilayer system arranged on the substrate, wherein the multilayer system has a plurality of partial stacks each comprising a first layer of a first material having a first thickness and at least one second layer of a second material having a second thickness, wherein the first material and the second material differ from one another in respective values of the real part of the refractive index at an operating wavelength of the reflective optical element, wherein each of the partial stacks has a respective partial stack thickness (D.sub.i) and a respective layer thickness ratio (.sub.i), wherein the respective layer thickness ratio (.sub.i) is defined as a quotient of the thickness of the respective first layer and the respective partial stack thickness (D.sub.i); wherein in a first section of the multilayer system, for at least one of: the respective partial stack thickness (D.sub.i) and the respective layer thickness ratio (.sub.i), a mean square deviation from respective mean values therefor is nonzero and at least 10% less than in a second section of the multilayer system; wherein for the first section of the multilayer system, the thicknesses a are such that for the first section |(D.sub.iD.sub.i+1)/D.sub.i|0.1; wherein for the second section of the multilayer system, the respective partial stack thickness (D.sub.i) and the respective layer thickness ratio (.sub.i) are such that the reflective optical element has a reflectivity R, a wavelength dependence of which in a wavelength interval of =0.5 nm has a PV value of less than 0.25, wherein the PV value is defined as PV=(R.sub.max.sub._.sub.relR.sub.min.sub._.sub.rel)/R.sub.max.sub._.sub.abs, wherein R.sub.max.sub._.sub.rel denotes a maximum reflectivity value in the wavelength interval , R.sub.min.sub._.sub.rel denotes a minimum reflectivity value in the wavelength interval , and R.sub.max.sub._.sub.abs denotes an absolute maximum reflectivity value; and wherein the respective partial stack thickness (D.sub.i) and the respective layer thickness ratio (.sub.i) in the multilayer system are such that a wavelength dependence of the reflectivity R of the reflective optical element in a wavelength interval of =0.5 nm has at least two local reflectivity extrema which differ from one another in reflectivity by at least 0.1%, relative to a larger value of the two local reflectivity extrema.

2. The reflective optical element as claimed in claim 1, wherein, in the first section of the multilayer system, for at least one of: the respective partial stack thickness (D.sub.i) and the respective layer thickness ratio (.sub.i), the mean square deviation from the respective mean values therefor is at least 20% less than in the second section of the multilayer system.

3. The reflective optical element as claimed in claim 2, wherein, in the first section of the multilayer system, for at least one of: the respective partial stack thickness (D.sub.i) and the respective layer thickness ratio (.sub.i), the mean square deviation from the respective mean values therefor is at least 50% less than in the second section of the multilayer system.

4. The reflective optical element as claimed in claim 1, wherein the wavelength dependence of the reflectivity R in a wavelength interval of =0.5 nm has a PV value of less than 0.20.

5. The reflective optical element as claimed in claim 4, wherein the wavelength dependence of the reflectivity R in a wavelength interval of =0.5 nm has a PV value of less than 0.15.

6. The reflective optical element as claimed in claim 1, wherein the second section is arranged closer to the substrate than is the first section.

7. The reflective optical element as claimed in claim 1, wherein the first section and the second section jointly form an entire multilayer system.

8. The reflective optical element as claimed in claim 1, wherein the local reflectivity extrema differ from one another in reflectivity by at most 5%, relative to the larger value.

9. The reflective optical element as claimed in claim 1, wherein the first material is selected from the group consisting of molybdenum (Mo), ruthenium (Ru) and rhodium (Rh).

10. The reflective optical element as claimed in claim 1, wherein the second material is silicon (Si).

11. The reflective optical element as claimed in claim 1, and configured for an operating wavelength of less than 30 nm.

12. The reflective optical element as claimed in claim 11, configured for an operating wavelength of less than 15 nm.

13. An optical system of a microlithographic projection exposure apparatus, comprising a reflective optical element as claimed in claim 1.

14. A microlithographic projection exposure apparatus, comprising an optical system as claimed in claim 13.

15. A reflective optical element, comprising a substrate and a multilayer system arranged on the substrate, wherein the multilayer system has a plurality of partial stacks each comprising a first layer of a first material and at least one second layer of a second material, wherein the first material and the second material differ from one another in respective values of the real part of the refractive index at an operating wavelength of the reflective optical element; wherein the reflective optical element has a reflectivity R, the wavelength dependence of which in a wavelength interval of =0.5 nm has a PV value of less than 0.25, wherein the PV value is defined as PV=(R.sub.max.sub._.sub.relR.sub.min.sub._.sub.rel)/R.sub.max.sub._.sub.abs, wherein R.sub.max.sub._.sub.rel denotes a maximum reflectivity value in the wavelength interval , R.sub.min.sub._.sub.rel denotes a minimum reflectivity value in the wavelength interval , and R.sub.max.sub._.sub.abs denotes an absolute maximum reflectivity; and wherein a wavelength dependence of the reflectivity R of the reflective optical element in a wavelength interval of =0.5 nm has at least two local extrema which differ from one another in reflectivity by at least 0.1% and by at most 5%, respectively relative to a larger value of the two local extrema.

16. The reflective optical element as claimed in claim 15, wherein two local extrema differ from one another in reflectivity by at least 0.5%, relative to the larger value.

17. The reflective optical element as claimed in claim 15, wherein the local extrema differ from one another in reflectivity by at most 2.5%, relative to the larger value.

18. The reflective optical element as claimed in claim 17, wherein the local extrema differ from one another in reflectivity by at most 1%, relative to the larger value.

19. The reflective optical element as claimed in claim 15, wherein the first material is selected from the group consisting of molybdenum (Mo), ruthenium (Ru) and rhodium (Rh).

20. The reflective optical element as claimed in claim 15, wherein the second material is silicon (Si).

21. The reflective optical element as claimed in claim 15, and configured for an operating wavelength of less than 30 nm.

22. The reflective optical element as claimed in claim 21, configured for an operating wavelength of less than 15 nm.

23. An optical system of a microlithographic projection exposure apparatus, comprising a reflective optical element as claimed in claim 15.

24. A microlithographic projection exposure apparatus, comprising an optical system as claimed in claim 23.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 a schematic illustration of a microlithographic projection exposure apparatus designed for operation in the EUV;

(2) FIG. 2 a schematic illustration of a reflective optical element;

(3) FIGS. 3A-3B diagrams for elucidating the reflectivity profile R vs. for reflective optical elements comprising different multilayer systems;

(4) FIGS. 4A-4C diagrams of layer thickness profiles for a multilayer system having two different periodic sections;

(5) FIGS. 5A-5C diagrams of layer thickness profiles for a multilayer system having two sections of differently pronounced aperiodicity;

(6) FIG. 6 a diffractogram measured for the multilayer system from FIG. 4;

(7) FIG. 7 a CuK.sub.-diffractogram measured for the multilayer system from FIG. 5;

(8) FIG. 8 a diagram with a reflectivity curve R vs. for elucidating a further aspect of the present invention;

(9) FIG. 9 a CuK.sub.-diffractogram measured for a purely periodic multilayer system; and

(10) FIGS. 10A-10C diagrams of layer thickness profiles for the multilayer system from FIG. 9.

DETAILED DESCRIPTION

(11) FIG. 1 schematically illustrates a microlithographic projection exposure apparatus 10 designed for operation in the EUV. Integral components are an illumination system 14, a mask 17 and a projection lens 20. The microlithographic projection exposure apparatus 10 is operated under vacuum conditions in order to minimize absorption losses of the EUV radiation. In the example illustrated, a plasma source is used as a radiation source 12. A synchrotron can also be used as a radiation source. The emitted radiation in the wavelength range of approximately 5 nm to 20 nm is firstly fixed by a collector mirror 13 and then introduced into the illumination system 14. In the example illustrated in FIG. 1, the illumination system 14 has two mirrors 15, 16. The mirrors 15, 16 guide the beam onto the mask 17 having the structure which is intended to be imaged onto the wafer 21. The mask 17 is likewise a reflective optical element for the EUV and soft X-ray wavelength range, which is exchanged depending on the production process. With the aid of the projection system 20, the beam reflected from the mask 17 is projected onto the wafer 21 and the structure of the mask is thereby imaged onto the wafer. In the example illustrated, the projection system 20 has two mirrors 18, 19. However, both the projection system 20 and the illumination system 14 can have respectively simply one, three, four, five or even more mirrors.

(12) FIG. 2 schematically illustrates an exemplary, basic structure of a reflective optical element 50 based on a multilayer system 51. The multilayer system 51 has alternate layers of a material having a comparatively higher real part of the refractive index at the operating wavelength (also called spacer) and of a material having a comparatively lower real part of the refractive index at the operating wavelength (also called absorber), wherein an absorber-spacer pair forms a partial stack 53. A crystal is thereby simulated whose lattice planes correspond to the absorber layers at which Bragg reflection takes place. The thicknesses of the individual layers 54, 55 and also of the repeating partial stacks 53 can be constant over the entire multilayer system 51 or else vary, depending on what spectral or angle-dependent reflection profile is intended to be achieved. The reflection profile can also be influenced in a targeted manner by the basic structure composed of absorber and spacer being supplemented by further more and less absorbent materials in order to increase the possible maximum reflectivity at the respective operating wavelength. For this purpose, in some partial stacks absorber and/or spacer material can be interchanged, or the partial stacks can be constructed from more than one absorber and/or spacer material or have additional layers composed of further materials. The absorber and spacer materials can have constant or else varying thicknesses over all the partial stacks in order to optimize the reflectivity. Furthermore, it is also possible to provide additional layers for example as diffusion barriers between spacer and absorber layers.

(13) The multilayer system 51 is applied on a substrate 52 and forms a reflective surface 60. Materials having a low coefficient of thermal expansion are preferably chosen as substrate materials. A protective layer 56 can be provided on the multilayer system 51 in order to protect the reflective optical element 50 against contamination, inter alia.

(14) The concept according to the invention is explained in greater detail below with reference to FIGS. 3A-7.

(15) FIG. 3A illustrates the reflectivity for three reflective optical elements (explained in greater detail below) comprising different multilayer systems as a function of the wavelength for an angle of incidence of 10 with respect to the surface normal, wherein the multilayer systems are based respectively on molybdenum (Mo) as absorber material and silicon (Si) as spacer material. By way of example, ruthenium (Ru) and/or rhodium (Rh) can also be used as an alternative or in addition to molybdenum as absorber material.

(16) One of the reflective optical elements (dotted line in FIG. 3A) has a standard multilayer system having the construction illustrated in FIGS. 10A-10C. This involves a purely periodic multilayer system having 40 molybdenum-silicon partial stacks or periods. In this multilayer system, both the partial stack thickness D and the layer thickness ratio within a partial stack are constant over the entire multilayer system in the vertical layer construction. In FIGS. 10A-10C, the thickness d of the respective (molybdenum and silicon) individual layers (FIG. 10A) and the partial stack thickness D (FIG. 10B) and the layer thickness ratio (FIG. 10C) are plotted as a function of the number or index of the partial stack. The layer thickness ratio is defined here as the quotient of the thickness of the low refractive index layer (e.g. molybdenum) and the partial stack thickness (i.e. the layer thickness ratio has the value 0.5 given identical thicknesses of molybdenum layer and silicon layer). Layer is understood here to mean an individual layer (having uniform or homogeneous optical properties). Each low refractive index (e.g. molybdenum) layer together with a high refractive index (e.g. silicon) layer in each case forms a partial stack, wherein the total thickness of these two layers corresponds to the partial stack thickness.

(17) The element in accordance with FIGS. 10A-10C has a comparatively high maximum reflectivity of approximately 65% at a wavelength of 13.7 nm and an angle of incidence of 10 with respect to the surface normal, wherein the reflectivity curve in accordance with FIG. 3A is comparatively narrowband for this purely periodic layer construction. FIG. 9 shows the associated CuK.sub.-diffractogram for the purely periodic layer construction having the layer thickness profile illustrated in FIGS. 10A-10C, in which the thickness d of the (molybdenum and silicon) individual layers (FIG. 10A) and the partial stack thickness D (FIG. 10B) and the layer thickness ratio (FIG. 10C) are constant over the entire layer construction. Accordingly, the corresponding peaks are comparatively narrow in the CuK.sub.-diffractogram.

(18) FIGS. 4A-4C show for comparison the construction of a multilayer system which is constructed as a conventional broadband multilayer system made from two sections having different periodicities. In the case of these sections, the partial stack thickness D has a constant, relatively small value in a first or upper section and a constant, relatively large value in a second or lower section (starting from the partial stack having the number 13). The first section, arranged further away from the substrate, comprises 12 partial stacks or periods of a partial stack thickness D of approximately 6.4 nm and with a layer thickness ratio of approximately 0.45. Arranged underneath on the substrate is the second section, which for its part comprises 10 partial stacks or periods with a partial stack thickness D of approximately 7.0 nm and a layer thickness ratio of approximately 0.5. This bipartite structure of the multilayer system leads, in accordance with FIG. 3A, to a reflectivity curve (dashed curve) having a lower maximum reflectivity of less than 55% in comparison with the standard multilayer system, wherein the full width half maximum (FWHM) of the curve is approximately 0.9 nm (in comparison with the value 0.6 nm in the case of the standard multilayer system from FIGS. 9-10C).

(19) FIG. 6 illustrates the CuK.sub.-diffractogram of this reflective optical element. The abovementioned construction has the effect that over an angle-of-incidence range of 0 to 6 also in the case of the reflective optical element comprising a bipartite periodic multilayer system a multiplicity of sharp peaks still exist which can be used for control and optimization of the lateral layer thickness profile of the multilayer system during production.

(20) In order to obtain a reflectivity curve having even less dependence on the wavelength or on the angle of incidence, the invention proposes a multilayer system which, although it is inherently aperiodic or stochastic, has at least one section that deviates comparatively little from the periodicity.

(21) In FIGS. 5A-5C, for a reflective optical element according to the invention, the thickness d of the respective (molybdenum and silicon) individual layers (FIG. 5A) and the partial stack thickness D (FIG. 5B) and the layer thickness ratio (FIG. 5C) are plotted as a function of the number or index of the partial stack. A multilayer system composed of two different sections is involved in the example from FIGS. 5A-5C. Firstly a totally stochastic section of the multilayer system is applied on a substrate, which comprises twenty-five partial stacks (having the index 18 to 42). Arranged thereabove is a comparatively periodic section which comprises seventeen stacks and in which the partial stack thicknesses D.sub.i of adjacent partial stacks fluctuate by less than 10%, while the layer thickness ratio or the proportion of molybdenum in the respective partial stack tends to increase toward the substrate.

(22) As is evident from the reflectivity curve in FIG. 3A, the structure of the multilayer system in accordance with FIGS. 5A-5C leads to a particularly broadband reflectivity profile in which the full width half maximum is at approximately 1.1 nm of the wavelength. At the same time, the multilayer system of this reflective optical element in accordance with FIGS. 5A-5C has the advantage that, in accordance with FIG. 7, the associated CuK.sub.-diffractogram (e.g. at a wavelength of 0.154 nm) likewise still has a plurality of sufficiently sharp peaks. Over an angle-of-incidence range of 0 to 6, a multiplicity of even more greatly pronounced peaks are found, of which, in the example illustrated here, four peaks identified by four respective vertical lines were selected for the purpose of controlling or optimizing the lateral layer thickness profile.

(23) The circumstance that in the exemplary embodiment in FIGS. 5A-5C in a first section of the multilayer system (in the upper region of the layer sequence) the partial stack thickness D is still approximately constant (for instance up to the partial stack or period having the number 18) has the effect thaton account of an approximately periodic portion still presentdiscrete peaks are still discernible in the associated CuK.sub.-diffractogram in FIG. 7 (in contrast to a perfectly aperiodic layer construction, where such peaks would no longer be identifiable). From the two peaks in the CuK.sub.-diffractogram from FIG. 6, in the reflective optical element according to the invention in accordance with FIG. 7 only the peak of the upper section of the multilayer system having a still approximately constant partial stack thickness still respectively remains since, for the associated reflective optical element having the layer thickness profiles in accordance with FIG. 5, the lower portion of the layer stack is constructed comparatively stochastically.

(24) In FIG. 3B, proceeding from the values from FIG. 3A, the plotting is performed in such a way that the relative PV value for the different reflectivity curves can be read. The relative PV value is defined here as PV=(R.sub.max.sub._.sub.relR.sub.min.sub._.sub.rel)/R.sub.max.sub._.sub.abs, wherein R.sub.max.sub._.sub.rel denotes the maximum reflectivity value in this wavelength interval, R.sub.min.sub._.sub.rel denotes the minimum reflectivity value in this wavelength interval and R.sub.max.sub._.sub.abs denotes the absolute maximum reflectivity value. Preferably, the reflective optical element 50 has a reflectivity R, the wavelength dependence of which in a wavelength interval of =0.5 nm has a PV value of less than 0.25.

(25) The construction of the multilayer system according to the invention makes it possible to provide reflective optical elements which firstly allow a high bandwidth of the reflectivity over the wavelength and/or over the angle of incidence and nevertheless exhibit a sufficient number of pronounced peaks in the diffractogram, with the consequence that it is possible to carry out control and, if appropriate, optimization of the lateral layer thickness profile through X-ray diffractometry.

(26) A further aspect of the present invention is explained below with reference to FIG. 8. This aspect is based on the further insight that the reflection curve determined for a multilayer system according to the invention, as a function of the reflectivity on the wavelength or the angle of incidence, can then be used advantageously for an optimization of the individual parameters of the layer sequence if the reflection curve does not have a perfect plateau, but rather a plurality of local extrema (maxima or minima) in the manner of a superimposed, weak oscillation.

(27) The reflection curve shown merely by way of example in FIG. 8 in this respect (in which reflection curve, analogously to FIG. 3a, the reflectivity is plotted for unpolarized light or after averaging over all polarization states) has as an example three local maxima and two local minima having the reflectivity values plotted in table 1.

(28) TABLE-US-00001 TABLE 1 Wavelength [nm] Reflectivity [%] 13.330 52.125 13.377 52.029 13.480 52.689 13.602 51.451 13.674 51.837

(29) It is then advantageous for an evaluation for the purpose of optimizing the individual parameters of the multilayer system if extrema (i.e. a minimum and a maximum) situated alongside one another in the reflectivity curve differ from one another in terms of the reflectivity by at least 0.1%, preferably by at least 0.5%, in each case relative to the larger value. In the example of FIG. 8 and table 1, e.g. the difference between the local extrema Nos. 3 and 4 is (52.68951.451)/52.689=2.35%, as a result of which a characterization or optimization of the layer parameters is made possible.

(30) Furthermore, advantageously, the relevant reflectivity values for the above-described local extrema (maxima and minima) also do not differ to an excessively great extent, as a result of which it is possible to exploit the circumstance that the optical system (e.g. the microlithographic projection exposure apparatus) is typically operated with a certain spectral distribution and thus different wavelengths and a certain averaging of the intensities obtained for different wavelengths thus also takes place. In other words, the local extrema (maxima and minima) in the reflection curve that are suitable for characterizing or optimizing the multilayer system are compensated for again on account of the averaging effect during operation of the optical system, such that no undesired impairment of the imaging result takes place as a result.

(31) Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments are evident to the person skilled in the art, e.g. through combination and/or exchange of features of individual embodiments. Accordingly, such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the appended patent claims and equivalents thereof.