Vacuum system, in particular EUV lithography system, and optical element

Abstract

A vacuum system, in particular an EUV lithography system, includes: a vacuum housing (2), in which a vacuum environment (16) is formed. A surface (2a) of the vacuum housing is subjected to contaminating particles (17) in the vacuum environment. A surface structure (18) at the surface reduces adhesion of the contaminating particles and has pore-shaped depressions (24) separated from one another by webs (25).

Claims

1. A vacuum system, comprising: a vacuum housing of a lithography system for extreme ultraviolet (EUV) radiation, in which a vacuum environment is formed, wherein a surface of the vacuum housing is subjected to contaminating particles in the vacuum environment, wherein a surface structure at the surface reduces adhesion of the contaminating particles, and wherein the surface structure has blind pore-shaped depressions separated from one another by webs, wherein the blind pore-shaped depressions comprise a depth of an order of magnitude of micrometres or less.

2. The vacuum system according to claim 1, wherein the pore-shaped depressions have respective diameters (d.sub.V) which are smaller than diameters (d.sub.P) of the contaminating particles in the vacuum environment.

3. The vacuum system according to claim 1, wherein the pore-shaped depressions have a diameter (d.sub.v) of less than 10 nm.

4. The vacuum system according to claim 1, wherein web widths (B) of the surface structure are smaller than the respective diameters (d.sub.v) of the pore-shaped depressions of the surface structure.

5. The vacuum system according to claim 1, wherein a depth (T) of a respective pore-shaped depression is at least as large as half the diameter (d.sub.V/2) of a respective one of the pore-shaped depressions.

6. The vacuum system according to claim 1, wherein the surface structure has at least one periodic pore structure.

7. The vacuum system according to claim 6, wherein the periodic pore structure has a period length (d.sub.S, d.sub.S3) of less than 10 nm.

8. The vacuum system according to claim 6, wherein the surface structure has a first periodic pore structure having a first period length (d.sub.S1) and a second periodic pore structure applied to the first periodic pore structure and having a second period length (d.sub.S2) which is smaller than the period length (d.sub.S1) of the first periodic pore structure.

9. The vacuum system according to claim 8, wherein the first period length (d.sub.S1) is at least five times the second period length (d.sub.S2).

10. The vacuum system according to claim 1, wherein the surface structure is formed on an inner side of the vacuum housing of a beam shaping system, an illumination system, or a projection system of the lithography system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 shows a schematic illustration of an EUV lithography apparatus,

(3) FIG. 2 shows a schematic illustration of a particle on a surface,

(4) FIGS. 3A and 3B show schematic illustrations of a surface structure formed on a multilayer coating of an EUV mirror, in a plan view and in a sectional illustration, and

(5) FIG. 4 shows a schematic illustration of a surface structure having three periodic pore structures having different period lengths.

(6) In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

DETAILED DESCRIPTION

(7) FIG. 1 schematically shows a vacuum system in the form of an EUV lithography apparatus 1 consisting of a beam shaping system 2, an illumination system 3 and a projection system 4, which are accommodated in separate vacuum housings (designated with the same reference signs) and are arranged successively in a beam path 6 proceeding from an EUV light source 5 of the beam shaping system 2. By way of example, a plasma source or a synchrotron can serve as EUV light source 5. The emerging radiation in the wavelength range of between approximately 5 nm and approximately 20 nm is firstly focussed in a collimator 7. With the aid of a downstream monochromator 8, the desired operating wavelength is filtered out by varying the angle of incidence, as indicated by a double-headed arrow. In the stated wavelength range, the collimator 7 and the monochromator 8 are usually embodied as reflective optical elements, wherein at least the monochromator 8 has no multilayer coating at its optical surface, in order to reflect a wavelength range having the highest possible bandwidth.

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

(9) The reflective optical elements 9, 10, 11, 12, 13, 14 respectively have an optical surface 9a, 10a, 11a, 12a, 13a, 14a, which are arranged in the beam path 6 of the EUV lithography apparatus 1. A further, mechanical component 15 is also arranged in the projection system 4, for example in the form of a sensor or of part or, if appropriate, of the entire inner side 2a of a housing wall of the vacuum housing 2 (or an inner surface 3a, 4a of a housing wall of the other vacuum housings 3, 4). The component 15 likewise has a surface 15a arranged in a vacuum environment 16 in the projection system 4. The vacuum environment 16 is generated with the aid of vacuum pumps (not shown). The total pressure in the vacuum environment 16 of the beam shaping system 2, of the illumination system 3 and of the projection system 4 can be different. The total pressure is typically in the range of between approximately 10.sup.9 mbar and approximately 10.sup.1 mbar.

(10) As can likewise be seen in FIG. 1, the vacuum environment 16 of the projection system 4 contains contaminating particles 17 to which the surfaces 9a to 14a of the optical elements 9 to 14 and the surface 15a of the mechanical component 15 are subjected. FIG. 2 shows by way of example a detail from the surface 15a of the mechanical component 15, which is a vacuum component composed of aluminium in the example shown. A particle 17 illustrated as spherical in an idealized way has deposited on the planar, polished surface 15a of the component 15. The area of contact, assumed to be circular likewise in an idealized way, between the particle 17 and the surface 15a has a contact radius r.sub.k that is relatively large in comparison with the diameter d.sub.P of the particle 17.

(11) In order to reduce the area of contact between the particle 17 and the surface 15a, a surface structure 18 can be applied to the surface 15a, said surface structure reducing the area of contact of the particle 17 with the surface 15a and thus the adhesion of the particle 17 to the surface 15a. Such a surface structure 18, which can also be provided on the surface 15a of the component 15, is shown in FIGS. 3A and 3B on the basis of the example of the last optical element 14 in the beam path of the EUV lithography apparatus 1 from FIG. 1.

(12) The optical element 14 shown in a sectional view in FIG. 3B comprises a substrate 19 and a multilayer coating 20 applied to the substrate 19. The multilayer coating 20 comprises alternately applied layers of a material having a higher real part of the refractive index at the operating wavelength .sub.B (also called spacers 21) and of a material having a lower real part of the refractive index at the operating wavelength .sub.B (also called absorbers 22), wherein an absorber-spacer pair forms a stack. This construction of the multilayer coating 20 simulates in a way a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection takes place. The thicknesses of the individual layers 21, 22 and of the repeating stacks can be constant or else vary over the entire multilayer coating 20, depending on what spectral or angle-dependent reflection profile is intended to be achieved. The absorber and spacer materials can have constant or else varying thicknesses over all the 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 21, 22.

(13) In the present example, in which the optical element 14 was optimized for an operating wavelength .sub.B of 13.5 nm, i.e. in the case of an optical element 14 which has the maximum reflectivity for substantially normal incidence of radiation at a wavelength of 13.5 nm, the stacks of the multilayer coating 20 have alternate silicon and molybdenum layers. In this case, the silicon layers correspond to the layers 21 having a higher real part of the refractive index at 13.5 nm, and the molybdenum layers correspond to the layers 22 having a lower real part of the refractive index at 13.5 nm. Other material combinations such as e.g. molybdenum and beryllium, ruthenium and beryllium or lanthanum and B.sub.4C are likewise possible, depending on the operating wavelength.

(14) As shown in FIG. 3B, a surface structure 18 is formed at the surface 14a of the multilayer coating 20, said surface structure having a periodic pore structure 23 having pore-shaped depressions 24 separated from one another by webs 25, wherein the surface structure 18 has a substantially hexagonal structure, as illustrated in FIG. 3A (each pore-shaped depression 24 is surrounded by six further pore-shaped depressions 24). The pore-shaped depressions 24 have a substantially circular geometry having a diameter d.sub.V which, in the example shown, is smaller than the diameter d.sub.P of a particle 17 arranged above the pore-shaped depression. In the sectional view through the optical element 14 as shown in FIG. 3b, the pore-shaped depressions 24 and the webs 25 form a binary or rectangular surface profile, i.e. the flanks of the webs 25 run approximately vertically and the bottom of the pore-shaped depressions 24 runs substantially parallel to the planar surface of the substrate 19 to which the multilayer coating 20 is applied.

(15) As shown in FIG. 3B, the period length d.sub.s of the periodic pore structure 23, which corresponds to the sum of the diameter d.sub.V of the pore-shaped depression 24 and the width B of the web 25, is slightly larger than the diameter d.sub.P of the particle 17. By contrast, the diameter d.sub.v of the pore-shaped depression 24 is slightly smaller than the diameter d.sub.P of the contaminating particle 17. The area of contact between the contaminating particle 17 and the surface 14a of the optical element 14 is therefore formed exclusively by the circularly circumferential edge 25a of the web 25, which is significantly smaller than the area of contact between the particle 17 and the planar surface 14a from FIG. 2.

(16) In the example shown, the depth T of a respective pore-shaped depression 24 is somewhat more than half the magnitude of the diameter d.sub.P of the pore-shaped depression 24. In this way, it is ensured that a spherical particle 17 that is slightly larger than the diameter d.sub.P of the pore-shaped depression 24, if it contacts the circumferential edge 25a of the web 25 delimiting the depression 24, does not rest on the bottom of the depression 24 and the area of contact with the surface 14a is increased in this way.

(17) In general, the adhesion of particles 17 that are situated above the pore-shaped depressions 24 should be of the order of magnitude of the adhesion of particles 17 that are situated on the webs 25. The ratio between the adhesion at the depressions 24 and the adhesion at the webs 25 can be set by the ratio between the diameter d.sub.v of the pore-shaped depressions 24 and the width B of the webs 25. In principle, in the case of a surface structure 18 having exactly one periodic pore structure 23, it has proved to be advantageous if the widths B of the webs 25 of the surface structure 18 are smaller than the diameters d.sub.V of the pore-shaped depressions 24 of the surface structure 18. Fulfilling such a condition imposed on the widths B of the webs 25 is generally not necessary, however, if the surface structure 18 has two or more, for example three, periodic pore structures 23a-c, as is shown in FIG. 4.

(18) The surface structure 18 shown in FIG. 4 has a first periodic pore structure 23a, which has a first period length d.sub.S1 and which serves for reducing the adhesion of particles 17a having a first (average) particle diameter d.sub.P1. A second periodic pore structure 23b is superimposed on the first periodic pore structure 23a, said second periodic pore structure having a second, smaller period length d.sub.S2 and serving for reducing the adhesion of particles 17b having a second, smaller particle diameter d.sub.P2. A third periodic pore structure 17c is superimposed on the second periodic pore structure 17b, said third periodic pore structure having a third period length d.sub.S3 that is smaller than the second period length d.sub.S2 and serving for reducing the adhesion of particles 17c having a third particle diameter d.sub.p3 that is smaller than the second particle diameter d.sub.p2.

(19) As was described further above, a surface structure 18 having a periodic pore structure having a predefined period length d.sub.S1 to d.sub.S3 can typically only prevent the adhesion of particles 17a-c whose particle diameter d.sub.P1 to d.sub.P3 is of a predefined order of magnitude. The surface structure shown in FIG. 4 serves to prevent the adhesion of particles 17a-c having particle diameters d.sub.P1 to d.sub.P3 which are of different orders of magnitude. For this purpose, it is necessary that the period lengths d.sub.S1 to d.sub.S3 of the periodic pore structures 17a-c are not too close to one another. Therefore, the first period length d.sub.S1 should be of at least five times the magnitude of the second period length d.sub.S2 and the second period length d.sub.S2 should be of at least five times the magnitude of the third period length d.sub.S3. The third, i.e. smallest, period length d.sub.S3 defines the minimum particle diameter d.sub.p3 which can be prevented from adhesion by the surface structure 18. In the example shown, the third period length d.sub.S3 is less than e.g. 10 nm.

(20) The surface structure 18 shown in FIG. 4 can be provided both at the surface 14a of an optical element 14 and at the surface 15a of a non-optical component 15 of the EUV lithography system 1. The surface structure 18 shown in FIGS. 3A and 3B can, of course, also be provided at the surface 15a of a non-optical component 15. The use of periodic or approximately periodic pore structures 23, 23a-c has proved to be advantageous since such structures can be applied with the aid of structuring methods in which the surface structure or the surface structures is or are formed by self-assembly.

(21) By way of example for the case where the surface 15a of the non-optical component 15 is formed from aluminium, the surface structure 18 shown in FIGS. 3a,b can be produced by anodic oxidation in aqueous electrolytes, as is described in the articlecited further aboveby A. P. Li et al. Hexagonal pore arrays with a 50-420 nm interpore distance formed by self-organisation in anodic alumina, J. Appl. Phys, 84 (11), 6023 (1998). In particular, in the case of the method described therein, the period length d.sub.S of the periodic pore structure 23 or the diameter d.sub.V of a respective pore-shaped depression 24 can be varied by variation of the applied voltage within wide limits (from a few nanometres to the micrometres range).

(22) However, the surface structure 18 can also be realized with the aid of lithographic methods, i.e. by applying a light-sensitive coating to the surface 9a, 10a, 13a, 14a, 15a, exposing the light-sensitive layer for the purpose of structuring the light-sensitive layer, removing the coating in the non-structured regions, and etching the surface 9a, 10a, 13a, 14a, 15a for the purpose of producing the pore-shaped depressions in the regions not protected by the structured coating. In a subsequent step, the structured coating serving as an etching mask is removed completely from the surface 9a, 10a, 13a, 14a, 15a having the desired surface structure 18.

(23) In order to produce a surface structure 18 such as is illustrated in FIG. 4, a plurality of such lithographic structuring processes can be performed successively. In order to produce very small structures of a few nanometres, for example the third periodic pore structure 23c having the third period length d.sub.S3 as shown in FIG. 4, a micellar approach can be used for structuring, this approach being based on the self-assembly of block copolymers loaded with metal salts in conjunction with a subsequent lithography process, as is described in the articledescribed further aboveNano-structured micropatterns by combination of block copolymer self-assembly and UV photolithography, in Nanotechnology 17, 5027 (2006).

(24) To summarize, by providing a surface structure 18 at a surface 9a, 10a, 13a, 14a, 15a which is arranged in a vacuum environment 16 and which is therefore subjected to contaminating particles 17 whose particle diameters are generally not in the macroscopic range, it is possible to achieve an effective reduction of the adhesion of these particles 17 to the surface 9a, 10a, 13a, 14a, 15a. The particles 17 that do not adhere to the surface 9a, 10a, 13a, 14a, 15a can be removed from the vacuum system, for example the EUV lithography system 1, through an extraction by suction (vacuum pumps).