Method for producing a capping layer composed of silicon oxide on an EUV mirror, EUV mirror, and EUV lithography apparatus

Abstract

A method for producing a capping layer (18) composed of silicon oxide SiO.sub.x on a coating (16) of a mirror (13), the coating reflecting EUV radiation (6) e.g. for use in an EUV lithography apparatus or in an EUV mask metrology system. The method includes irradiating a capping layer (18) composed of silicon nitride SiN.sub.x or composed of silicon oxynitride SiN.sub.xO.sub.y for converting the silicon nitride SiN.sub.x or the silicon oxynitride SiN.sub.xO.sub.y of the capping layer (18) into silicon oxide SiO.sub.x. An associated mirror (13) includes a capping layer comprised of silicon oxide SiO.sub.X, and can be provided in an associated EUV lithography apparatus.

Claims

1. A method for producing a capping layer composed of silicon oxide SiO.sub.x on a mirror, wherein the mirror comprises a substrate and a reflective coating applied to the substrate, and wherein the reflective coating reflects extreme-ultraviolet (EUV) radiation and comprises the capping layer as a topmost layer of the mirror, said method comprising: irradiating the capping layer, which is composed of silicon nitride SiN.sub.x or of silicon oxynitride SiN.sub.xO.sub.y prior to said irradiating, for converting the silicon nitride SiN.sub.x or the silicon oxynitride SiN.sub.xO.sub.y of the capping layer into the silicon oxide SiO.sub.x, wherein said irradiating of the capping layer is effected in a residual gas atmosphere having at least one of: (i) an oxygen partial pressure (p(O.sub.2)) of between 10.sup.7 mbar and 10.sup.11 mbar and (ii) a water partial pressure (p(H.sub.2O)) of between 10.sup.5 mbar and 10.sup.9 mbar.

2. Method according to claim 1, wherein the capping layer is irradiated with EUV radiation having a power density of at least 200 mW/mm.sup.2.

3. Method according to claim 1, wherein the mirror is mounted in an EUV lithography apparatus, and said irradiating of the capping layer is carried out in the EUV lithography apparatus.

4. Method according to claim 1, further comprising: prior to said irradiating, applying the capping layer composed of silicon nitride SiN.sub.x or composed of silicon oxynitride SiN.sub.xO.sub.y by vapor deposition onto a layer of the reflective coating.

5. Method according to claim 4, wherein the capping layer is deposited by physical vapor deposition onto the layer of the reflective coating.

6. Method according to claim 4, wherein a nitrogen proportion x in the SiN.sub.x or in the SiN.sub.xO.sub.Y of between 0.4 and 1.4 is set during said applying of the capping layer.

7. Method according to claim 4, wherein an oxygen proportion y<0.9 in the SiN.sub.xO.sub.y is set during said applying of the capping layer.

8. Method according to claim 4, wherein an oxygen proportion y<0.4 in the SiN.sub.xO.sub.y is set during said applying of the capping layer.

9. Method according to claim 1, further comprising: prior to said irradiating, applying the reflective coating to a substrate such that the reflective coating has a reflection maximum at an operating wavelength .sub.B in the EUV wavelength range, and wherein a maximum (I.sub.max) or a minimum (I.sub.min) of the field intensity (I) of a standing wave that forms upon the reflection of radiation at the operating wavelength .sub.B at the reflective coating is arranged at a distance of at most 0.1 .sub.B from a surface of the capping layer of the reflective coating.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Exemplary embodiments are illustrated in the schematic drawing and are explained in the description below. In the figures:

(2) FIG. 1 shows a schematic illustration of an embodiment of an EUV lithography apparatus according to the invention,

(3) FIG. 2 shows a schematic illustration of an EUV mirror for such an EUV lithography apparatus during irradiation for converting a capping layer composed of silicon oxynitride or silicon nitride into a capping layer composed of silicon oxide,

(4) FIGS. 3a,b show two spectra obtained during an XPS analysis of a capping layer composed of silicon oxynitride,

(5) FIGS. 4a,b show a comparison of the reflectivity and respectively of the field intensity with regard to the vacuum interface of a standing wave of a reflective coating with a capping layer composed of silicon oxynitride and respectively with a capping layer composed of silicon oxide as a function of the wavelength, and

(6) FIGS. 5a,b show an illustration analogous to FIGS. 4a,b wherein the standing wave has a maximum of the field intensity at the surface of the capping layer.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(7) FIG. 1 schematically shows a projection exposure apparatus for EUV lithography, which is designated hereinafter as EUV lithography apparatus 1.

(8) The EUV lithography apparatus 1 comprises a ray generating system 2, an illumination system 3 and a projection system 4, which are accommodated in separate vacuum housings and arranged successively in a beam path 6 proceeding from an EUV light source 5 of the ray shaping system 2. By way of example, a plasma source or a synchrotron can serve as EUV light source 5. The radiation in the wavelength range of between approximately 5 nm and approximately 20 nm that emerges from the light source 5 is firstly concentrated in a collimator 7. With the aid of a downstream monochromator 8, the desired operating wavelength .sub.B, which is approximately 13.5 nm in the present example is filtered out by variation of the angle of incidence, as indicated by a double-headed arrow. The collimator 7 and the monochromator 8 are embodied as reflective optical elements.

(9) The radiation treated with regard to wavelength and spatial distribution in the ray generating system 2 is introduced into the illumination system 3, which has a first and second reflective optical element 9, 10. The two reflective optical elements 9, 10 direct the radiation onto a photomask 11 as further reflective optical element, which has a structure that is imaged onto a wafer 12 on a reduced scale by the projection system 4. For this purpose, a third and fourth reflective optical element 13, 14 are provided in the projection system 4.

(10) The reflective optical elements 9, 10, 11, 13, 14 each have an optical surface that is subjected to the EUV radiation 6 from the light source 5. In this case, the optical elements 9, 10, 11, 13, 14 are operated under vacuum conditions in a residual gas atmosphere 19. Since the interior of the projection exposure apparatus 1 cannot be baked out, the presence of residual gas constituents in the vacuum environment cannot be completely avoided.

(11) The EUV lithography apparatus 1 has a gas feed device 20 with a feed channel 21, which is connected to a gas reservoir (not shown) and serves for feeding and for discharging different gas constituents into and from the residual gas atmosphere 19 of the EUV lithography apparatus 1. As is shown in FIG. 1, nitrogen (N.sub.2), water (H.sub.2O), oxygen (O.sub.2) and hydrogen (H.sub.2) are present, inter alia, as gas constituents in the residual gas atmosphere 19. Corresponding feed channels can also be provided in the illumination system 3 and/or in the ray generating system 2 or else a central feed channel can be provided for the entire EUV lithography apparatus 1. A control device (not shown) serves to control the gas feed device 20 and to control further functions of the EUV lithography apparatus 1.

(12) The construction of one of the reflective optical elements 13 in the EUV lithography apparatus 1, said element also being designated as EUV mirror hereinafter, is described by way of example hereinafter with reference to FIG. 2. The EUV mirror 13 has a substrate 15 composed of a material having a low coefficient of thermal expansion, which is typically less than 100 ppb/K at 22 C. or over a temperature range of approximately 5 C. to approximately 35 C. One material which has these properties is silicate or quartz glass doped with titanium dioxide, which typically has a silicate glass proportion of more than 90%. One such silicate glass that is commercially available is sold by Corning Inc. under the trade name ULE (Ultra Low Expansion glass). A further group of materials having a very low coefficient of thermal expansion is glass ceramics, in which the ratio of the crystal phase to the glass phase is set such that the coefficients of thermal expansion of the different phases virtually cancel one another out. Such glass ceramics are offered e.g. by Schott AG under the trade names Zerodur, and by Ohara Inc. under the trade name Clearceram. For the reflective optical elements 9, 10 arranged in the illumination system 3, an e.g. metallic substrate material can also be used, if appropriate, instead of a zero expansion material.

(13) A reflective coating 16 is applied to the substrate 15, said reflective coating having a plurality of individual layers 17a, 17b consisting alternately of materials having different refractive indices, specifically in the present case, silicon and molybdenum in the present case. In addition to the individual layers shown in FIG. 2, the reflective coating 16 can also comprise intermediate layers for preventing diffusion or the like. The illustration of such auxiliary layers in the figures has been omitted.

(14) The reflective coating 16 has a capping layer 18 in order to prevent oxidation of the underlying individual layers 17a, 17b and in order to simplify cleaning of contaminating substances attached to the surface 18a of the capping layer 18. The capping layer 18 of the mirror 13 was applied to the topmost silicon layer 17a of the reflecting coating 16 by vapor deposition. The capping layer 18 has a thickness d1 of e.g. approximately 1.5 nm and absorbs the impinging EUV radiation 6 only to a small extent also on account of the small layer thickness. Depending on the application, the layer thickness can be between approximately 1 nm and approximately 20 nm, comparatively large layer thicknesses being used, in particular, in the vicinity of the collector or collimator 7.

(15) In the exemplary embodiment illustrated, the EUV mirror 13 has a planar surface 18a. This was chosen thus merely in order to simplify the illustration, that is to say that the EUV mirror 13 can also have a curved surface form, wherein e.g. concave surface forms or convex surface forms are possible, which can be embodied spherically and also aspherically.

(16) FIG. 2 shows the capping layer 18 during homogeneous irradiation with EUV radiation 6 in order to convert the material of the capping layer 18, which material was applied during the coating of the EUV mirror in the form of silicon oxynitride SiO.sub.xN.sub.Y or of silicon nitride SiN.sub.x, into silicon oxide SiO.sub.x. The irradiation of the EUV mirror 13 can be effected in an apparatus which is specially designed for this purpose and which has an EUV light source. It is more advantageous if the irradiation is performed directly in the EUV lithography apparatus 1 itself, in particular in-situ, i.e. if the EUV mirror 13 is arranged, during irradiation, at the same location at which it is also used during the exposure operation of the EUV lithography apparatus 1.

(17) In order that the EUV mirror 13 as shown in FIG. 2 is subjected to EUV radiation 6 as homogeneously as possible during irradiation, if appropriate additional reflective optical elements can be provided in the EUV lithography apparatus, which are introduced into the beam path of the EUV light source 5 only during the pre-irradiation, in order to collimate the EUV radiation 6 before it impinges on the EUV mirror 13.

(18) Alternatively, it is possible to arrange the EUV mirror 13 for the conversion of the material of the capping layer 18 at a different location than in the EUV lithography apparatus 1, for example in an irradiation apparatus which is provided separately for this purpose and which enables the capping layer 18 to be prepared in a well-defined manner. In such an irradiation apparatus (not illustrated in the figures), in particular EUV radiation 6 with a collimated beam path can be generated, such that the surface 18a, as shown in FIG. 2, can be subjected homogeneously to EUV radiation. As an alternative to the irradiation of the surface 18a with EUV light, it is also possible to use radiation at other wavelengths, e.g. UV radiation (having wavelengths of <390 nm) or X-ray radiation.

(19) The use of an irradiation apparatus has also proved to be advantageous for producing a high power density (e.g. of more than 200 mW/mm.sup.2 or of more than 1000 mW/mm.sup.2) at the surface 18a of the EUV mirror 13, since said surface 18a can be subjected to the radiation of the EUV light source 5 directly, i.e. without reflections that reduce the power density at a plurality of optical elements.

(20) While the EUV mirror 13 is subjected to EUV radiation 6 in the EUV lithography apparatus 1 or in an irradiation apparatus in order to achieve the conversion of the material of the capping layer 18, it has proved to be advantageous to provide there a residual gas atmosphere having a high oxygen proportion, wherein suitable oxygen partial pressures are typically in the range of between approximately 10.sup.7 mbar and 10.sup.11 mbar, in particular around approximately 10.sup.9 mbar. Alternatively or additionally, during the irradiation it is also possible to use a water partial pressure of between approximately 10.sup.5 mbar and 10.sup.9 mbar, preferably around approximately 10.sup.7 mbar, in order to promote the conversion of the capping layer 18. A lower oxygen partial pressure and/or water partial pressure is typically used for exposure operation in an EUV lithography apparatus 1 after the conversion of the capping layer 18.

(21) The application of the reflective coating 16 to the substrate 15 is described in greater detail below. Firstly, the layers 17a, 17b composed of silicon and composed of molybdenum, respectively, are applied to the substrate 15, for which purpose a PVD method is typically used. The capping layer 18 is then applied to the topmost silicon layer 17a, for which purpose a PVD method is likewise used, in the present example preferably a sputtering process, as a result of which the deposited silicon oxynitride SiN.sub.XO.sub.y or silicon nitride SiN.sub.X has an amorphous structure. In the coating method, both the nitrogen proportion x of the capping layer 18 and the oxygen proportion y (in the case of a capping layer composed of SiN.sub.XO.sub.Y) can be set through a suitable choice of the coating parameters, wherein advantageous values for the nitrogen proportion x are between approximately 0.4 and 1.4, preferably between 0.7 and 1.4, and in particular between approximately 1.0 and approximately 1.4. In the case of a capping layer composed of SiN.sub.XO.sub.Y, the oxygen proportion y is generally y=0.9, and if appropriate y=0.4 or less, wherein a variation of the respective proportions x and y within the capping layer 18 may be provided in a process-governed manner. A capping layer 18 having a spatially homogeneous composition in which x=1.0 or x>1.0 and y=0.4 or y<0.4 is particularly advantageous.

(22) As described above, the exact composition of the silicon oxynitride SiN.sub.xO.sub.y or of the silicon nitride SiNx is dependent on the coating parameters. During sputtering, a magnetic field can additionally be used (magnetron sputtering) and, if appropriate, pulsed discharges can be used for the coating, as in the case of so-called High Power Impulse Magnetron Sputtering (HiPIMS) or in the case of High Power Pulsed Magnetron Sputtering (HPPMS), which are described for example in WO 2010/127845 A1 cited in the introduction. Instead of a PVD method, it is also possible, if appropriate, to apply the capping layer 18 using a CVD method, in particular a PE-CVD or LP-CVD method, cf. the website of Crystec at http://www.crystec.com/trinitre.htm or U.S. Pat. No. 5,773,100 cited in the introduction.

(23) The coating parameters which influence the nitrogen proportion of the capping layer 18 also include the composition of the gas atmosphere during deposition, as described below on the basis of an XPS spectral analysis (cf. FIGS. 3a,b) of a silicon oxynitride capping layer 18 applied to a silicon layer 17a via a sputtering process. The thicknesses of the layers 17a, 17b of the reflective coating 16 were chosen in the present example such that they have a reflection maximum at the operating wavelength .sub.B of 13.5 nm.

(24) The graphs in FIGS. 3a,b illustrate the Si 2p bond of the XPS spectrum (as a function of the bond energy E.sub.B), wherein the intensity I of the photoelectrons which emerge at a shallow angle from the surface 18a of the capping layer 18 is represented by a dashed curve, while the intensity I of photoelectrons which emerge at a steep angle (virtually perpendicularly) with respect to the surface 18a is represented as a solid line. The solid intensity curve thus includes information about the composition of the capping layer 18 at a greater depth, whereas the dashed intensity curve substantially shows information about a near-surface depth range of approximately 1-2 nm of the capping layer 18.

(25) In FIGS. 3a,b it can be discerned that the Si 2p spectrum substantially has three peaks at different bond energies E.sub.B. A first bond energy E.sub.B at approximately 103.5 eV is characteristic of the oxide bond, a second bond energy E.sub.B at approximately 102 eV corresponds to the silicon nitride bond, while a third bond energy E.sub.B of approximately 99 eV corresponds to the unbound semiconductor state, i.e. unbound, elemental silicon.

(26) The two graphs in FIG. 3a and FIG. 3b differ in that additional nitrogen in gaseous form was added during the coating process in FIG. 3b, while this was not the case in FIG. 3a. Comparison between FIG. 3a and FIG. 3b clearly reveals that the oxygen peak at approximately 103.5 eV in FIG. 3b is significantly smaller than in FIG. 3a, and that conversely the peak of the nitride bond at approximately 102 eV has risen significantly, i.e. the composition or the stoichiometry of the silicon oxynitride material essentially depends on the composition of the gas atmosphere during coating.

(27) For the case where, during irradiation, undesirably the capping layer 18 is not subjected to EUV radiation 6 at the entire surface 18a or, in partial regions of the surface 18a, the radiation dose is not high enough for complete conversion of the material of the capping layer 18, it has proved to be advantageous for an antinode or a node of a standing wave that forms in the reflective coating 16 during the irradiation with EUV light 6 to be positioned directly at the surface 18a of the capping layer orif this is not possiblefor the distance between the antinode or node of the standing wave and the surface 18a to be chosen such that it is not more than 0.1 .sub.S.

(28) In order to achieve this, in particular the thickness d1 of the capping layer 18 and the thickness d2 of the underlying silicon layer 17a (and also, if appropriate, the thicknesses of further layers 17a, 17b of the reflective coating 16) can be chosen suitably. If the above condition is met, even upon incomplete replacement of the nitrogen by oxygen in the capping layer 18 the change in the reflectivity caused thereby is comparatively small, as can be discerned with reference to FIG. 4a, which shows the reflectivity R when using a silicon nitride capping layer as a function of the wavelength at which a node of the standing wave is formed at the surface 18a. The coating of the EUV mirror 13, i.e. the thicknesses of the layers 17a, 17b, are in this case optimized for a capping layer composed of silicon nitride, i.e. the maximum reflectivity R is assumed when using a capping layer 18 composed of silicon nitride.

(29) The change in the reflectivity R upon the complete replacement of nitrogen by oxygen is only R/R=0.02% in the case shown in FIG. 4a, and so the associated reflectivity curve for the silicon oxide capping layer 18 cannot be discerned in FIG. 4a since, with the scaling chosen, it corresponds to the reflectivity curve for silicon nitride. FIG. 4b shows the field intensity I at the surface 18a of the capping layer 18 as a function of the wavelength , wherein it can clearly be discerned that said field intensity has an intensity minimum I.sub.min (i.e. a node of the standing wave) at the operating wavelength .sub.S of 13.5 nm.

(30) When an antinode is present at the surface 18a of the capping layer 18, the change in the reflectivity when nitrogen is replaced by oxygen is greater and is R/R=2.1%, as is indicated by a dashed reflectivity curve in FIG. 5a. As is shown in FIG. 5b, the standing wave at the surface 18a of the reflective coating 16 in this case has an intensity maximum I.sub.max approximately at the operating wavelength .sub.B of 13.5 nm. In order to minimize a reduction of the reflectivity R upon the replacement of oxygen by nitrogen in the capping layer 18, it is therefore particularly advantageous to position an intensity minimum of the standing wave at the location of the surface 18a.

(31) The use of a stable capping layer 18 composed of silicon oxide has proved to be advantageous in particular for removing impurities or particles, in particular metal compounds containing tin or carbon, from the surface 18a of the EUV mirror 13, for which purpose hydrogen cleaning is advantageously carried out, during which activated hydrogen, in particular in the form of hydrogen radicals or hydrogen ions, is applied to the surface 18a.

(32) The hydrogen cleaning can be carried out by setting a suitable hydrogen partial pressure p(H.sub.2) in the residual gas atmosphere 19. The hydrogen can be activated by the EUV radiation 6 in proximity to the surface 18a and in the process can be converted into hydrogen ions or hydrogen radicals which clean contaminating substances such as tin or carbon away from the surface 18a. For the hydrogen cleaning it is also possible, however, to provide additional devices in the EUV lithography apparatus 1, for example cleaning heads that serve to generate a hydrogen-containing gas flow directed onto the surface 18a. Cleaning heads of this type are described for example in WO 2009/059614 A1 from this applicant, which is incorporated by reference into the disclosure of the present application. The hydrogen in the gas flow can be activated hydrogen, wherein the activation can be effected for example using an electric field, as is described in WO 2009/059614 A1, or by guiding the (molecular) hydrogen along a heating wire for the purpose of activation.