METHOD FOR DETERMINING THE THICKNESS OF A CONTAMINATING LAYER AND/OR THE TYPE OF CONTAMINATING MATERIAL, OPTICAL ELEMENT AND EUV-LITHOGRAPHY SYSTEM

Abstract

The invention relates to a method for determining the thickness of a contaminating layer and/or the type of a contaminating material on a surface (7) in an optical system, in particular on a surface (7) in an EUV lithography system, comprising: irradiating the surface (7) on which plasmonic nanoparticles (8a,b) are formed with measurement radiation (10), detecting the measurement radiation (10a) scattered at the plasmonic nanoparticles (8a,b), and determining the thickness of the contaminating layer and/or the type of the contaminating material on the basis of the detected measurement radiation (10a). The invention also relates to an optical element (1) for reflecting EUV radiation (4), and to an EUV lithography system.

Claims

1. Method for determining the thickness (d1, d2) of a contaminating layer (13) and/or the type of a contaminating material on a surface (7) in an optical system, in particular on a surface (7) in an EUV lithography system (101), comprising: irradiating the surface (7) on which plasmonic nanoparticles (8a,b) are formed with measurement radiation (10), detecting the measurement radiation (10a) scattered at the plasmonic nanoparticles (8a,b), and determining the thickness (d1, d2) of the contaminating layer (13) and/or the type of the contaminating material on the basis of the detected measurement radiation (10a).

2. Method according to claim 1, wherein a thickness-dependent wavelength shift (Δλ.sub.max,d1, Δλ.sub.max,d2) of a spectral distribution (15a) of the measurement radiation (10a) scattered at the plasmonic nanoparticles (8a,b) is determined for the purpose of determining the thickness (d1, d2) of the contaminating layer (13).

3. Method according to claim 1 or 2, wherein wavelength shifts (Δλ.sub.max,a, Δλ.sub.max,b) of a plurality of spectral distributions (15a, 15b) of the measurement radiation (10a) scattered at plasmonic nanoparticles (8a, 8b) having in each case at least one different property are determined for the purpose of determining the type of the contaminating material.

4. Method according to claim 3, wherein the at least one different property is selected from the group comprising: size of the plasmonic nanoparticles (8a,b), geometry of the plasmonic nanoparticles (8a,b) and material of the plasmonic nanoparticles (8a,b).

5. Method according to any of claims 2 to 4, wherein the wavelength shift (Δλ.sub.max) of the spectral distribution (15a) is determined on the basis of a shift in a maximum wavelength (λ.sub.max) of the spectral distribution (15a) or on the basis of a change in the scattered light intensity (I.sub.s1, I.sub.s2) at at least two measurement wavelengths (λ.sub.1, λ.sub.2) of the spectral distribution (15a).

6. Method according to any of the preceding claims, wherein the measurement radiation (10a) scattered at the plasmonic nanoparticles (8a,b) is detected in a spatially resolved manner.

7. Method according to any of the preceding claims, wherein the plasmonic nanoparticles (8a,b) are formed on a surface (7) of a capping layer (6) of a coating (3) of an optical element (1), which coating reflects EUV radiation (4).

8. Method according to claim 7, wherein the plasmonic nanoparticles (8a,b) are applied to the surface (7) of the capping layer (6) or are at least partly embedded into the capping layer (6).

9. Method according to any of the preceding claims, wherein at least the steps of irradiating and detecting are carried out during the operation of the optical system, in particular during the operation of the EUV lithography system (101).

10. Optical element (1) for reflecting EUV radiation (4), comprising: a substrate (2), a coating (3) for reflecting EUV radiation (4), said coating being formed on the substrate (2), wherein the coating (3) comprises a capping layer (6) having a surface (7) facing away from the substrate (2), and wherein plasmonic nanoparticles (8a,b) are formed on the surface (7) of the capping layer (6).

11. Optical element according to claim 10, wherein the plasmonic nanoparticles (8a,b) are formed from a metallic material, in particular from Au, Ag or Cu.

12. Optical element according to either of claims 10 and 11, wherein the plasmonic nanoparticles (8a,b) on the surface (7) of the capping layer (6) have a width (b) that is smaller than the height (h) of the plasmonic nanoparticles (8a,b), or vice versa.

13. Optical element according to any of claims 10 to 12, wherein the plasmonic nanoparticles (8a,b) are arranged on the surface (7) in a grid (14).

14. Optical element according to any of claims 10 to 13, wherein plasmonic nanoparticles (8a,b) having at least one different property are arranged on the surface (7).

15. Optical element according to claim 14, wherein the property is selected from the group comprising: size of the plasmonic nanoparticles (8a,b), geometry of the plasmonic nanoparticles (8a,b) and material of the plasmonic nanoparticles (8a,b).

16. EUV lithography system (101), comprising: at least one surface (7) on which plasmonic nanoparticles (8a,b) are formed, at least one measurement radiation source (9) for irradiating the surface (7) on which the plasmonic nanoparticles (8) are formed with measurement radiation (10), at least one detector (11) for detecting the measurement radiation (10a) scattered at the plasmonic nanoparticles (8a,b) and an evaluation device (12) for determining the thickness (d1, d2) of a contaminating layer (13) and/or the type of the contaminating material on the surface (7) on the basis of the detected measurement radiation (10a).

17. EUV lithography system according to claim 16, wherein the surface (7) on which the plasmonic nanoparticles (8a,b) are formed forms the surface (7) of the capping layer (6) of an optical element (1) according to any of claims 10 to 13.

18. EUV lithography system according to either of claims 16 and 17, wherein the detector (12) is designed for the preferably spatially resolved detection of a spectral distribution of the measurement radiation (10a).

19. EUV lithography system according to any of claims 16 to 18, wherein the measurement radiation source (9) is designed for generating measurement radiation (10) in the visible wavelength range.

Description

DRAWING

[0044] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:

[0045] FIG. 1 shows a schematic illustration of an optical element for reflecting EUV radiation having a surface on which plasmonic nanoparticles are formed,

[0046] FIGS. 2a,b show schematic illustrations of spectral distributions of measurement radiation scattered at plasmonic nanoparticles,

[0047] FIG. 3 shows a schematic illustration of a map of contaminations on the surface of the optical element, and

[0048] FIG. 4 shows a schematic illustration of an EUV lithography apparatus comprising an optical element embodied as in FIG. 1.

[0049] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

[0050] FIG. 1 schematically shows an optical element 1 in the form of an EUV mirror. The optical element 1 comprises a substrate 2, to which a coating 3 is applied, which is designed for reflecting EUV radiation 4 in the example shown. The optical element 1 shown in FIG. 1 is designed for reflecting EUV radiation 4 that is incident on the surface 7 of the capping layer 6 with normal incidence. The angles α of incidence at which the EUV radiation 4 impinges on the optical element 1 are in this case typically less than approximately 45°. In order to optimize the optical element 1 for normal incidence, in the case of the coating 3 shown in FIG. 1, a multilayer coating 5 having a high reflectivity for the EUV radiation 4 is applied below the capping layer 6.

[0051] For this purpose, the multilayer coating 5 typically comprises alternating layers of a high refractive index material and a low refractive index material. The materials of the high and low refractive index layers of the multilayer coating 5 depend on the wavelength of the EUV radiation 4 that is intended to be reflected at the optical element 1. At a wavelength of approximately 13.5 nm, the multilayer coating 5 typically comprises alternating layers of silicon and molybdenum. The capping layer 6 may be formed for example from rhodium, ruthenium or some other material which protects the underlying multilayer coating 5 against influences from the environment.

[0052] As an alternative to the reflective coating 3 optimized for normal incidence as shown in FIG. 1, the optical element 1 may also be optimized for reflecting EUV radiation 4 incident with grazing incidence, i.e. for EUV radiation 4 that is incident on the surface 7 at angles α of incidence of more than approximately 60° with respect to the surface normal. In this case, instead of the multilayer coating 5 shown in FIG. 1, if appropriate just one individual reflective layer may be present in the coating 3, which is typically formed from a material having a low refractive index and a low absorption for the EUV radiation 4 incident with grazing incidence, in particular from a metallic material, e.g. Ru, Mo or Nb. The design of the optical element 1 for reflecting EUV radiation 4 incident with normal incidence or with grazing incidence is of secondary importance for the contamination effects under consideration here.

[0053] In the case of the optical element 1 shown in FIG. 1, plasmonic nanoparticles 8a, 8b are formed on the surface 7 of the capping layer 6. The nanoparticles 8a,b consist of a material which, upon irradiation with measurement radiation 10 from a measurement radiation source 9, for example in the form of a white light source, results in said nanoparticles being excited to carry out collective oscillations in the form of (localized) plasmons. The plasmonic nanoparticles 8a,b may be formed for example from a metallic material, such as Au, Ag or Cu, the absorption bands of which lie in the visible wavelength range, such that these can be excited with the aid of the measurement radiation 10 that likewise lies in the visible wavelength range.

[0054] In the example shown in FIG. 1, the measurement radiation 10a scattered at the plasmonic nanoparticles 8a,b is detected with the aid of a detector 11. An evaluation device 12 is connected to the detector 11 in order to evaluate the detected measurement radiation 10a scattered at the plasmonic nanoparticles 8a,b.

[0055] FIG. 2a shows the scattered light intensity Is, which is detected in a spatially resolved manner by the detector 11 in the example shown, as a function of the wavelength λ. For the wavelength-dependent detection of the scattered measurement radiation 10a, the detector 11 typically comprises a spectrometer. In the example shown, the detector 11 has a spatial resolution having a magnitude such that the measurement radiation 10a scattered only by one individual nanoparticle of the plasmonic nanoparticles 8a shown in FIG. 1 can be detected. The spectral distribution 15a of the measurement radiation 10a scattered at said plasmonic nanoparticle 8a is illustrated for three different cases in FIG. 2a:

[0056] The spectral distribution 15a illustrated on the left in FIG. 2a corresponds to the case shown in FIG. 1 in which the surface 7 of the capping layer 6 is free of contaminations. As can be discerned in FIG. 2a, the spectral distribution 15a and thus also the maximum wavelength λ.sub.Max thereof shifts to higher wavelengths λ if a contaminating layer 13 shown in FIG. 2a forms on the surface 7.

[0057] As can likewise be discerned in FIG. 2a, the contaminating layer 13 produces a first wavelength shift Δλ.sub.Max,d1 in the case of a first thickness d.sub.1 and a second wavelength shift Δλ.sub.Max,d2, which is greater than the first wavelength shift Δλ.sub.Max,d1, in the case of a second, larger thickness d.sub.2. The absolute value of the wavelength shift Δλ.sub.Max is thus a measure of the respective thickness d1, d2 of the contaminating layer 13. On the basis of a calibration measurement carried out beforehand it is possible—given known material of the contaminating layer 13—to assign a specific thickness d1, d2 of the contaminating layer 13 to a wavelength shift Δλ.sub.Max.

[0058] As is shown in FIG. 2a, the wavelength shift Δλ.sub.Max can also be determined not by recording the entire spectral distribution 15a of the scattered measurement radiation 10a depending on the wavelength λ by means of the detector 11, but rather by determining a (first and second) scattered light intensity I.sub.s1, I.sub.s2 only for two fixedly predefined measurement wavelengths λ.sub.1, λ.sub.2. The first measurement wavelength λ.sub.1 is formed on the left-hand edge and the second measurement wavelength λ.sub.2 on the right-hand edge of the spectral distribution 15a. Given a sufficiently large spacing of the measurement wavelengths λ.sub.1, λ.sub.2 from the centre of the spectral distribution 15a and given a sufficiently small wavelength shift Δλ.sub.Max to greater wavelengths λ, the first scattered light intensity I.sub.s1 decreases, while the second scattered light intensity I.sub.s2 increases, such that on the basis of the difference between the two scattered light intensities I.sub.s1, I.sub.s2, for example on the basis of the difference I.sub.s1−I.sub.s2 or on the basis of the quotient I.sub.s1/I.sub.s2 it is possible to deduce the absolute value of the wavelength shift Δλ.sub.Max and thus the thickness d1, d2 of the contaminating layer 13.

[0059] The nanoparticles 8a,b formed on the surface 7 typically have a geometry or a suitable aspect ratio adapted for generating plasmons. As can be discerned in FIG. 2a, the (maximum) lateral width b of the nanoparticles 8a,b is typically greater than the height h of the nanoparticles 8a,b, wherein a typical order of magnitude for the height h is between approximately 5 nm and approximately 30 nm and wherein a typical order of magnitude for the width b is between approximately 5 nm and approximately 100 nm.

[0060] As can likewise be discerned in FIG. 1, the plasmonic nanoparticles 8a,b are arranged on the surface 7 in a regular arrangement or in a grid 14, that is to say that the midpoints of adjacent plasmonic nanoparticles 8a,b are at regular distances A from one another, which may be for example of the order of magnitude of a number of millimetres. The distance A between adjacent nanostructures 8a,b should not be chosen to be too small, in order that it is possible to comply with the specification of the optical element 1 with regard to reflectivity and, if appropriate, with regard to imaging aberrations. In order to avoid a parasitic crosstalk between adjacent plasmonic nanoparticles 8a,b, the distance A should typically be greater than approximately 20 nm.

[0061] The plasmonic nanoparticles 8a,b may be formed on the surface 7 for example with the aid of so-called “nanosphere lithography” (NSL), which fosters the production of periodic arrays or grids 14 of plasmonic nanoparticles 8a,b. In this case, as is shown in FIG. 1, the nanoparticles 8a,b may be applied to the surface 7 of the capping layer 6 or be wholly or partly embedded into the capping layer 6, as is indicated by way of example in FIG. 2a. The deeper the plasmonic nanoparticles 8a,b are buried into the capping layer 6, the lower, however, typically the sensitivity of the scattered light measurement described further above, that is to say that the resolution in determining the thickness d1, d2 of the contaminating layer 13 typically decreases.

[0062] Different types of plasmonic nanoparticles 8a,b may be arranged on the surface 7. By way of example, FIG. 1 shows a (second) plasmonic nanoparticle 8b, which differs by its irregular geometry from the substantially parallelepipedal geometry of the other (first) plasmonic nanoparticle 8a on the surface 7 of the optical element 1. It goes without saying that more than just one plasmonic nanoparticle 8b having an irregular geometry may be arranged on the surface 7 shown in FIG. 1; by way of example, every second one of the nanoparticles 8a,b may have an irregular geometry.

[0063] As in FIG. 2a, FIG. 2b also shows the spectral distribution 15a of the scattered light intensity I.sub.s of the measurement radiation 10a scattered at the parallelepipedal nanoparticle 8a from FIG. 1. In addition, FIG. 2b also illustrates the spectral distribution 15b of the measurement radiation 10a scattered at the plasmonic nanoparticle 8b having the irregular geometry. As can be discerned in FIG. 2b, the maximum wavelength λ.sub.Max,a of the spectral distribution 15a of the measurement radiation 10a scattered at the first parallelepipedal nanoparticle 8a differs from the maximum wavelength λ.sub.Max,b of the measurement radiation 10a scattered at the second parallelepipedal nanoparticle 8b.

[0064] Particularly if the scattered measurement radiation 10a that is detected by the detector 11 does not originate from an individual nanoparticle 8a, 8b but rather from a plurality of (first and second) nanoparticles 8a, 8b, it is advantageous if the maximum wavelengths λ.sub.Max,a, λ.sub.Max,b of the two spectral distributions 15a, 15b are sufficiently far apart that these can be separated spectrally well during the evaluation in the evaluation device 12, since in this case the sum of the two spectral distributions 15a, 15b shown in FIG. 2b is detected as scattered light intensity I.sub.s.

[0065] In addition to the different maximum wavelengths λ.sub.Max,a, λ.sub.Max,b of the two spectral distributions 15a, 15b, the absolute value of the wavelength shift Δλ.sub.Max,a, Δλ.sub.Max,b (given an identical or constant thickness of the contaminating layer 13) of the two spectral distributions 15a, 15b also differs depending on the type of the material of the contaminating layer 13, to put it more precisely depending on the refractive index of said material. If, for a plurality of different thicknesses d1, d2 of a contaminating layer 13 composed of a known material, the respective wavelength shift Δλ.sub.Max,a, Δλ.sub.Max,b is determined and stored in a list or table, these values form a type of signature for a respective contaminating material of the layer 13.

[0066] If the respective wavelength shift Δλ.sub.Max,a, Δλ.sub.Max,b is determined during a measurement with the aid of the evaluation device 12, in addition to the thickness d1, d2 of the contaminating layer 13 by means of a comparison with the known values of the wavelength shifts Δλ.sub.Max,a, Δλ.sub.Max,b stored for different materials in the table, it is possible to determine the material of the contaminating layer 13.

[0067] It goes without saying that for distinguishing a multiplicity of different materials of the contaminating layer 13, it is possible to arrange on the surface 7 not just the plasmonic nanoparticles 8a,b having two different geometries as shown in FIG. 1, but also plasmonic nanoparticles having a third, fourth, . . . geometry, which differ respectively from the first, second, . . . geometry. Generally it is advantageous to arrange on the surface 7 plasmonic nanoparticles 8a,b which differ from one another in at least one property which enables or fosters the distinguishing of different materials of the contaminating layer 13. Besides a different geometry of the plasmonic nanoparticles 8a,b, the property may be for example the size of the nanoparticles 8a,b, that is to say that the size of the plasmonic nanoparticles 8a,b on the surface 7 may vary. The material of the plasmonic nanoparticles 8a,b on the surface 7 may also be chosen differently in order to be able to determine the type of the contaminating material of the layer 13 or identify or distinguish a multiplicity of different materials.

[0068] In FIG. 1, by way of example, the measurement radiation 10 emerging from the measurement radiation source 9 impinges only on exactly one plasmonic nanoparticle 8a. It goes without saying, however, that the measurement radiation source 9 may be designed to irradiate a larger region or the entire surface 7 of the capping layer with the measurement radiation 10. In this case, the measurement radiation 10a scattered at the irradiated plasmonic nanoparticles 8a,b may be detected in a spatially resolved manner by the detector 11, wherein the spatial resolution of the detector 11 may be set, if appropriate, e.g. with the aid of a zoom optical unit. By means of the zoom optical unit, it is possible, if appropriate, to represent specific partial regions of the surface 7 with higher spatial resolution or magnification, wherein, if appropriate, it is possible to achieve a spatial resolution for which only the measurement radiation 10a scattered at an individual plasmonic nanoparticle 8a is detected by the detector 11, as is illustrated in FIG. 1.

[0069] The detection of the scattered measurement radiation 10a with the aid of a spatially resolving detector 11 makes it possible to create a map of the contaminations on the surface 7, as is shown by way of example in FIG. 3: at the three partial regions illustrated in a dashed manner in FIG. 3, the wavelength shift Δλ.sub.Max determined in the manner described further above and thus the thickness d1 of the contaminating layer 13 lie above a predefined threshold value, while no contaminating material or only contaminating material having a very small thickness is present outside the partial regions shown in FIG. 3. It goes without saying that, by defining a plurality of threshold values of the wavelength shift Δλ.sub.Max it is possible to create a more detailed spatially resolved map of the contaminating layer 13 on the surface 7 of the optical element 1, in which a plurality of thickness intervals can be distinguished.

[0070] Instead of a spatially resolving detector 11, it is also possible to use a non-spatially resolving detector if the intention is to determine only information regarding the thickness of the contaminating layer 13, which information is integrated over the entire surface 7. If appropriate, the spatially resolving or non-spatially resolving detector 11 may also be moved, for example displaced, relative to the surface 7 in order to determine the thickness d1, d2 of the contaminating layer 13 at different locations of the surface 7. Correspondingly, if appropriate, it is also possible to move the measurement radiation source 9 relative to the surface 7 in order to illuminate different partial regions of the surface 7.

[0071] The optical element 1 in accordance with FIG. 1 may be integrated for example in an optical system in the form of an EUV lithography apparatus 101, which is illustrated highly schematically in FIG. 4. The EUV lithography apparatus 101 has an EUV light source 102 for generating EUV radiation, which has a high energy density in an EUV wavelength range below 50 nm, in particular between about 5 nm and about 15 nm. The EUV light source 102 may for example take the form of a plasma light source for generating a laser-induced plasma or be formed as a synchrotron radiation source. Particularly in the former case, as shown in FIG. 4, a collector mirror 103 can be used to focus the EUV radiation of the EUV light source 102 to form an illumination beam 104 and to increase the energy density further in this way. The illumination beam 104 serves for the illumination of a structured object M by means of an illumination system 110, which in the present example has five reflective optical elements 112 to 116 (mirrors).

[0072] The structured object M may be for example a reflective mask, which has reflective and non-reflective, or at least much less reflective, regions for producing at least one structure on the object M. Alternatively, the structured object M may be a plurality of micro-mirrors, which are arranged in a one-dimensional or multi-dimensional arrangement and which are possibly movable about at least one axis, in order to set the angle of incidence of the EUV radiation 104 on the respective mirror.

[0073] The structured object M reflects a part of the illumination beam 104 and forms a projection beam path 105 that carries the information about the structure of the structured object M and that is radiated into a projection system 120, which generates an imaging of the structured object M or of a respective partial region thereof on a substrate W. The substrate W, for example a wafer, comprises a semiconductor material, e.g. silicon, and also a light-sensitive layer, e.g. a photoresist, and is arranged on a mount, which is also referred to as a wafer stage WS.

[0074] In the present example, the projection system 120 comprises six reflective optical elements 121 to 126 (mirrors) in order to generate an image of the structure present on the structured object M on the wafer W. The number of mirrors in a projection system 120 typically lies between four and eight; however, only two mirrors may also possibly be used.

[0075] In the case of the EUV lithography apparatus 101 shown in FIG. 4, the arrangement comprising the measurement radiation source 9, the detector 11 and the evaluation device 12 as shown in FIG. 1 is shown by way of example on an optical element 115 of the illumination system 110. It goes without saying that any other optical element 112 to 114, 116 of the illumination system 110 or an optical element 121 to 126 of the projection system 120 may also be provided with a corresponding arrangement in order to determine the thickness of a contaminating layer and, if appropriate, the type of the contaminating material.

[0076] As an alternative or in addition to a surface 7 of an optical element 1, a surface of a non-optical element in the EUV lithography apparatus 101 may also be measured with regard to contaminations in the manner described further above.

[0077] Instead of a surface arranged in the EUV lithography apparatus 101, a surface arranged in a different optical system may also be measured with regard to contaminations in the manner described further above.