SUBSTRATE FOR AN EUV-LITHOGRAPHY MIRROR

Abstract

Substrates suitable for mirrors used at wavelengths in the EUV wavelength range have substrates (1) including a base body (2) made of a precipitation-hardened alloy, of an intermetallic phase of an alloy system, of a particulate composite or of an alloy having a composition which, in the phase diagram of the corresponding alloy system, lies in a region which is bounded by phase stability lines. Preferably, the base body (2) is made of a precipitation-hardened copper or aluminum alloy. A highly reflective layer (6) is preferably provided on a polishing layer (3) of the substrate (1) of the EUV mirror (5).

Claims

1. (canceled)

2. (canceled)

3. A substrate for a mirror for EUV lithography comprising: a base body consisting essentially of an alloy having a composition which, in the phase diagram of the corresponding alloy system, lies in a region which is bounded by phase stability lines.

4. The substrate according to claim 3, wherein the alloy is an alloy with a substitution lattice.

5. The substrate according to claim 3, wherein the alloy is precipitation-hardened.

6. The substrate according to claim 3, wherein the alloy is a copper or aluminum alloy.

7-13. (canceled)

14. A substrate for a mirror for EUV lithography comprising: a base body consisting essentially of an intermetallic phase of an alloy system.

15. The substrate according to claim 14, wherein the base body consists essentially of an intermetallic phase in which the stoichiometric standard composition is observed.

16. The substrate according to claim 14, wherein the base body consists essentially of an intermetallic phase having a composition which corresponds to a phase stability line in the phase diagram of the alloy system.

17. The substrate according to claim 14, wherein the alloy has a composition which, in the phase diagram of the corresponding alloy system, lies in a region which is bounded by phase stability lines.

18. The substrate according to claim 14, wherein the base body consists essentially of an intermetallic phase having the same Bravais lattice as components of the base body in crystalline form.

19. The substrate according to claim 14, wherein the alloy system is a binary system, one component of which is copper.

20. The substrate according to claim 14, wherein the alloy system is a binary aluminum-copper system.

21-26. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention will be explained in more detail with reference to exemplary embodiments. In this respect,

[0033] FIGS. 1A,B schematically show two variants of a substrate in section;

[0034] FIGS. 2A,B schematically show two variants of a mirror in section; and

[0035] FIG. 3 shows a phase diagram for a binary aluminum-copper system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0036] FIG. 1a schematically shows a first variant of an embodiment of a substrate 1 comprising a base body 2 and a polishing layer 3 applied thereto. The base body 2 and the polishing layer 3 perform different functions. Whereas a good dimensional stability is a priority for the base body 2, good machining and polishing properties are of primary importance for the polishing layer 3.

[0037] The polishing layer can be applied by conventional vacuum coating processes, for example sputtering processes, electron beam evaporation, molecular beam epitaxy or ion beam-assisted coating. If the polishing layer is a metallic material, for example copper, nickel-phosphorus or nickel-boron, it is preferably applied without external current. Nickel-phosphorus or nickel-boron polishing layers, in particular, can also be applied as dispersion layers, in which case polytetrafluoroethylene can serve as the dispersant, for example.

[0038] Nickel-phosphorus or nickel-boron polishing layers, in particular, are preferably applied with relatively high concentrations of phosphorus or boron, such that they are present predominantly or even completely in amorphous form and thereby have better polishing properties. They can then be hardened by, for example, heat treatment, plasma treatment or ion bombardment. Silicon as polishing layer material can also be deposited in amorphous or crystalline form in a manner controlled by the coating process. Amorphous silicon can be polished more effectively than crystalline silicon and, if required, can likewise be hardened by heat treatment, plasma treatment or ion bombardment. Polishing layers made of silicon or silicon dioxide can also be smoothed through use of ion beams. The polishing layer can also be made of silicon carbide or of indium-tin oxide.

[0039] Preferred thicknesses of the polishing layer 3 can be about 5 μm to 10 μm for metal-based, polished polishing layers. In the case of non-metallic polishing layers 3, preferred layer thicknesses are about 1.5 μm to 3 μm. Using conventional polishing processes, metallic polishing layers can be polished to root mean squared roughnesses of less than 0.3 nm in the spatial frequency range of 1 μm to 200 μm and to root mean squared roughnesses of less than 0.25 nm in the spatial frequency range of 0.01 μm to 1 μm. Using conventional polishing processes, non-metallic polishing layers can be polished to root mean squared roughnesses of less than 0.3 nm over the entire spatial frequency range of 0.01 μm to 200 μm.

[0040] FIG. 1b schematically shows a variant of the substrate 1 shown in FIG. 1a, in which an adhesion-promoter layer 4 is arranged between the base body 2 and the polishing layer 3. It is preferable that the adhesion-promoter layer 4 can have a thickness of up to 1 μm, preferably of between 100 nm and 500 nm. By way of example, it can be applied using CVD (chemical vapor deposition) or PVD (physical vapor deposition) processes.

[0041] Such substrates 1 can be further processed to form EUV mirrors 5, as is shown schematically in FIG. 2a in a first variant of an embodiment, by applying a highly reflective layer 6 to the polishing layer 3. For use in the case of EUV radiation in the wavelength range of about 5 nm to 20 nm and with normal incidence of radiation, the highly reflective layer 6 is particularly preferably a multilayer system of alternating layers of material with a differing real part of the complex refractive index via which a crystal with network planes at which Bragg diffraction takes place is simulated to some extent. A multilayer system of alternating layers of silicon and molybdenum can be applied, for example, for use at 13 nm to 14 nm. Particularly if the highly reflective layer 6 is configured as a multilayer system, it is preferably applied using conventional vacuum coating processes such as, for example, sputtering processes, electron beam evaporation, molecular beam epitaxy or ion-beam-assisted coating. For use in the case of EUV radiation in the wavelength range of about 5 nm to 20 nm and with grazing incidence of radiation, preference is given to mirrors with an uppermost layer of metal, for example of ruthenium.

[0042] FIG. 2b schematically shows a further variant of the mirror 5 shown in FIG. 2a, in which an adhesion-promoter layer 4 is arranged between the base body 2 and the polishing layer 3 of the substrate 1 of the mirror 5.

[0043] In a first example, the base body 2 of the mirror 5 or of the substrate 1 can be made of a particulate composite. In particular, the base body 2 can be made of a particulate composite having a metallic matrix. By way of example, the latter can be a 2000 to 7000 series aluminum alloy, preferably a 5000 to 7000 series aluminum alloy, copper, a low-alloy copper alloy or copper niobate. The preferably spheroidal dispersoids of an extent in the range of 1 nm to 20 nm are advantageously titanium carbide, titanium oxide, aluminum oxide, silicon carbide, silicon oxide, graphite or diamond-like carbon, it also being possible for dispersoids of differing materials to be provided in the matrix. These materials can be produced by powder metallurgy, for example. The base body 2 can also be made of a particulate composite having a ceramic matrix. By way of example, particulate composites having a silicon or carbon matrix and silicon carbide dispersoids are particularly suitable. As a result of their covalent bond, they have a particularly high lattice rigidity. It is particularly preferable for the dispersoids to be distributed as homogeneously as possible in the matrix, for said dispersoids to be as small as possible and for the composite to have the smallest possible dispersoid spacings.

[0044] In a second example, the base body 2 can be made of an alloy having components which have similar atomic radii and have a structure with a substitution lattice. By way of example, this may be the alloy system copper-nickel or silicon-aluminum.

[0045] In a third example, the base body 2 can be made of a precipitation-hardened alloy. By way of example, it can be made of precipitation-hardened copper or aluminum alloys such as AlCu4Mg1, CuCr, CuNi1Si, CuCr1Zr, CuZr, CuCoBe, CuNiSi. In specific embodiments, the alloys were subjected to a further heat treatment after the precipitation hardening, this having the effect that the precipitations assume a spheroidal form in order to reduce stress or distortion energies in the material so as to further increase the high-temperature strength. To this end, the material is held for one to two hours at a temperature at which the basic phase of the matrix of the particulate composite is stable, whereas other phases in solutions go indeed into solution. Then, the temperature of the material is fluctuated repeatedly around this temperature range, and subsequently the material is slowly cooled at about 10° C. to 20° C. per hour.

[0046] In a fourth example, the base body 2 can be made of an intermetallic phase. FIG. 3 shows the phase diagram of a binary aluminum-copper system, the intermetallic phases of which are particularly suitable as the material of the base body 2. At 300° C., sixteen intermetallic phases of Al.sub.xCu.sub.y, where x, y are integers, are stable. Of these, ten intermetallic phases stay stable upon cooling to room temperature (not shown here). The most important phases are indicated in FIG. 3 with the stoichiometric composition thereof. All of them lie at phase boundary lines which run parallel to the temperature axis over a certain temperature range. As a result, the microstructure thereof remains completely unchanged in these respective temperature ranges. Particular preference is given to Al.sub.2Cu, Al.sub.2Cu.sub.3 or Al.sub.3Cu.sub.5, inter alia, as the material of a base body of a mirror substrate for EUV lithography. In variations, it is also possible to use other binary alloy systems, one component of which is copper, for example binary systems of copper and zinc, tin, lanthanum, cerium, silicon or titanium.

[0047] In a fifth example, the base body 2 can also be made of an alloy having a composition which lies between two phase stability lines. These regions are shaded gray in FIG. 3. Since the precipitation processes have been stopped by heat treatments, these alloys are present in a thermally stable phase. In this respect, preference is given to compositions from particularly wide ranges, for instance between Al.sub.2Cu and AlCu.

[0048] The substrates of the examples mentioned here have particularly high strengths of 300 MPa or more, even at temperatures of up to 150° C., and also have a good long-term stability. The substrates, which comprise copper in the base body thereof, additionally have high thermal conductivities, and therefore they can be readily cooled. On account of their special base body, the substrates do not experience any changes in microstructure in temperature ranges which arise in long-term operation of mirrors in EUV projection exposure apparatuses. As a result, EUV mirrors having such a substrate have the advantage that the roughness values thereof remain substantially constant over their service life, in particular in the spatial frequency range of 0.1 μm to 200 μm. The EUV mirrors described here are suitable both for use in the illumination system, with which a mask or a reticle is illuminated with EUV radiation, and in the projection system, with which the structure of the mask or of the reticle is projected onto an object to be exposed, for example a semiconductor wafer, of an EUV projection exposure apparatus. Owing to their high-temperature strength and resilience, they are particularly suitable for mirrors arranged further forward in the beam path, where the thermal loading is higher, for instance in the illumination system. They are particularly suitable for use as facets of pupil facet mirrors and particularly of field facet mirrors.

[0049] The above description of the specific embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures disclosed. The applicant seeks, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.