MIRROR FOR AN ILLUMINATION OPTICAL UNIT OF A PROJECTION EXPOSURE APPARATUS COMPRISING A SPECTRAL FILTER IN THE FORM OF A GRATING STRUCTURE AND METHOD FOR PRODUCING A SPECTRAL FILTER IN THE FORM OF A GRATING STRUCTURE ON A MIRROR

Abstract

A mirror for an illumination optical unit of a projection exposure apparatus comprises a spectral filter in the form of a grating structure, wherein the grating structure has a maximum edge steepness in the range of 15° to 60°.

Claims

1. A mirror, comprising: a reflection surface comprising a spectral filter which comprises a grating structure, wherein: the grating structure comprises a plurality of grating ridges; for each grating ridge, the grating ridges comprises a front side and sidewalls; for each of at least some pairs of adjacent grating ridges, a groove comprising a bottom is between the adjacent grating ridge; and the grating structure has a maximum edge steepness in the range of 15° to 60°.

2. The mirror of claim 1, further comprising a closed protective layer covering the grating structure.

3. The mirror of claim 2, wherein the protective layer comprises a plie comprising at least one member selected from the group consisting of molybdenum and silicon.

4. The mirror of claim 3, further comprising a substrate, wherein: the grating structure is supported by the substrate; and the substrate comprises at least one member selected from the group consisting of amorphous silicon, nickel-phosphorus, silicon dioxide, titanium, platinum, gold, aluminium, titanium oxide, nickel, copper, silver, tantalum and aluminium oxide.

5. The mirror of claim 4, wherein, for each of at least some of the grating ridges, the grating ridge has a cross section having a trapezoid-shaped smallest convex envelope.

6. The mirror of claim 2, further comprising a substrate, wherein: the grating structure is supported by the substrate; and the substrate comprises at least one member selected from the group consisting of amorphous silicon, nickel-phosphorus, silicon dioxide, titanium, platinum, gold, aluminium, titanium oxide, nickel, copper, silver, tantalum and aluminium oxide.

7. The mirror of claim 6, wherein, for each of at least some of the grating ridges, the grating ridge has a cross section having a trapezoid-shaped smallest convex envelope.

8. The mirror of claim 1, further comprising a substrate, wherein: the grating structure is supported by the substrate; and the substrate comprises at least one member selected from the group consisting of amorphous silicon, nickel-phosphorus, silicon dioxide, titanium, platinum, gold, aluminium, titanium oxide, nickel, copper, silver, tantalum and aluminium oxide.

9. The mirror of claim 1, wherein, for each of at least some of the grating ridges, the grating ridge has a cross section having a trapezoid-shaped smallest convex envelope.

10. An optical unit, comprising: a mirror according to claim 1, wherein the optical unit is a projection lithography illumination optical unit.

11. A system, comprising: a projection lithography illumination optical unit which comprises a mirror according to claim 1; and a radiation source configured to generate illumination radiation.

12. An apparatus, comprising: an illumination optical unit comprising a mirror according to claim 1; and a projection optical unit configured to image an object field into an image field, wherein the apparatus is a microlithographic projection exposure apparatus.

13. A method of using a projection exposure system comprising an illumination optical system and an imaging optical system, the method comprising: using the illumination optical system to illuminate a region of a reticle in an object field of the imaging optical system; and using the imaging optical to project at least a part of the illuminated reticle onto a region of a light-sensitive material in an object field of the imaging optical system, wherein the illumination optical system comprises a mirror according to claim 1.

14. A method, comprising: structuring a structuring layer applied on a substrate of a mirror body; and structuring the substrate of the mirror body, wherein structuring the substrate of the mirror body comprises at least one member selected from the group consisting of: etching with an etching angle in the range of 0° to 60°; etching, wherein the structuring layer and the substrate of the mirror body have different etching rates; etching, wherein the structuring layer is provided with a sidewall steepness in the range of 10° to 90° during the structuring; and applying a closed protective layer on the substrate.

15. The method of claim 14, wherein structuring the substrate of the mirror body comprises at least one member selected from the group consisting of: inert dry etching with an etching angle in the range of 0° to 60°; inert dry etching, wherein the structuring layer and the substrate of the mirror body have different etching rates; inert dry etching, wherein the structuring layer is provided with a sidewall steepness in the range of 0° to 60° during structuring; reactive dry etching comprising controlling a composition of an etching gas to set a ratio of the etching rates of substrate of the mirror body and structuring layer; and wet chemical etching comprising controlling a composition of an etching medium to set a ratio of the etching rates of substrate of the mirror body and the structuring layer.

16. The method of claim 14, wherein structuring the substrate of the mirror body comprises using a combination of inert etching and reactive etching.

17. The method of claim 14, wherein structuring the substrate of the mirror body comprises etching method with a longitudinal etching angle in the range of 0° to 60°.

18. The method of claim 14, wherein structuring the substrate of the mirror body comprises etching method with a transverse etching angle in the range of 0° to 60°.

19. The method of claim 14, further comprising setting a sidewall steepness of the structuring layer by controlling at least one parameter in a targeted manner, wherein the at least one parameter comprises at least one member selected from the group consisting of a focusing a laser beam for structuring the structuring layer, an intensity of the exposure in a lithography process for structuring the structuring layer, a duration of the development of a lithography process for structuring the structuring layer, a hard bake of the structuring layer after a structuring thereof, a re-flow of the structuring layer after a structuring thereof, and a temperature of the structuring layer after a structuring thereof.

20. The method of claim 14, further comprising setting a sidewall steepness of the structuring layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Features and details of the disclosure are evident from the description of a plurality of exemplary embodiments with reference to the figures. In the figures:

[0058] FIG. 1 schematically shows a meridional section through a projection exposure apparatus for EUV projection lithography;

[0059] FIG. 2 shows a schematic illustration of a mirror comprising a spectral filter in the form of a grating structure;

[0060] FIG. 3 schematically shows an excerpt from a grating structure in the region of the edge of a grating ridge;

[0061] FIG. 4A schematically shows a first variant of a method for structuring a substrate;

[0062] FIG. 4B shows by way of example a schematic illustration of the substrate structured via the method in accordance with FIG. 4A with a structuring layer applied thereon;

[0063] FIGS. 5A to 5C schematically show an alternative variant of a method for structuring a substrate with an initial state (FIG. 5A), an intermediate product (FIG. 5B) and the finished structured substrate (FIG. 5C);

[0064] FIGS. 6A to 6C show a further variant of a method for structuring a substrate with an initial state (FIG. 6A), an intermediate product (FIG. 6B) and the finished structured substrate (FIG. 6C) for the case where the etching rate is lower in the structuring layer than in the substrate;

[0065] FIGS. 7A to 7C show a further variant of a method for structuring a substrate with an initial state (FIG. 7A), an intermediate product (FIG. 7B) and the finished structured substrate (FIG. 7C) for the case where the etching rate is higher in the structuring layer than in the substrate;

[0066] FIGS. 8A and 8B show by way of example schematic illustrations for elucidating the influence of the focusing of a laser beam on the structuring of the structuring layer;

[0067] FIG. 9 schematically shows an illustration for elucidating the effect of a lengthened development duration on the sidewall steepness of a structuring layer;

[0068] FIG. 10A schematically shows an excerpt from a substrate with structuring layer without hard bake;

[0069] FIG. 10B shows an illustration in accordance with FIG. 10A after a hard bake; and

[0070] FIG. 11 schematically shows an illustration of an excerpt from a substrate with applied structuring layer after isotropic, wet-chemical etching.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0071] s Firstly, the general construction of a microlithographic projection exposure apparatus 1 will be described.

[0072] FIG. 1 schematically shows a microlithographic projection exposure apparatus 1 in a meridional section. An illumination system 2 of the projection exposure apparatus 1 has, besides a radiation source 3, an illumination optical unit 4 for the exposure of an object field 5 in an object plane 6. In this case, a reticle arranged in the object field 5 and not illustrated in the drawing, the reticle being held by a reticle holder (likewise not illustrated), is exposed. A projection optical unit 7 serves for imaging the object field 5 into an image field 8 in an image plane 9. A structure on the reticle is imaged onto a light-sensitive layer of a wafer arranged in the region of the image field 8 in the image plane 9, the wafer likewise not being illustrated in the drawing and being held by a wafer holder (likewise not illustrated).

[0073] The radiation source 3 is an EUV radiation source having an emitted used radiation in the range of between 5 nm and 30 nm. This may be a plasma source, for example a GDPP (gas discharge-produced plasma) source or an LPP (laser-produced plasma) source. By way of example, tin can be excited to form a plasma using a carbon dioxide laser operating at a wavelength of 10.6 μm, that is to say in the infrared range. A radiation source based on a synchrotron can also be used for the radiation source 3. Information about such a radiation source can be found by the person skilled in the art for example in U.S. Pat. No. 6,859,515 B2. EUV radiation 10 emerging from the radiation source 3 is focused by a collector 11. A corresponding collector is known from EP 1 225 481 A. Downstream of the collector 11, the EUV radiation 10 propagates through an intermediate focal plane 12 before being incident on a field facet mirror 13 with a multiplicity of field facets 13a. The field facet mirror 13 is arranged in a plane of the illumination optical unit 4 which is optically conjugate with respect to the object plane 6.

[0074] The EUV radiation 10 is also referred to hereinafter as illumination light or as imaging light.

[0075] Downstream of the field facet mirror 13, the EUV radiation 10 is reflected by a pupil facet mirror 14 with a multiplicity of pupil facets 14a. The pupil facet mirror 14 is arranged in a pupil plane of the illumination optical unit 4, which is optically conjugate with respect to a pupil plane of the projection optical unit 7. With the aid of the pupil facet mirror 14 and an imaging optical assembly in the form of a transfer optical unit 15 comprising mirrors 16, 17 and 18 designated in the order of the beam path, field individual facets 19 of the field facet mirror 13, which are also referred to as subfields or as individual-mirror groups and are described in even greater detail below, are imaged into the object field 5. The last mirror 18 of the transfer optical unit 15 is a mirror for grazing incidence (“grazing incidence mirror”).

[0076] FIG. 2 illustrates by way of example and schematically the reflection surface of a mirror comprising a spectral filter in the form of a grating structure 30. The grating structure 30 serves as a spectral filter for masking out radiation having wavelengths in a predefined range, in particular for masking out wavelengths in the infrared range.

[0077] The grating structure 30 comprises a plurality of grating ridges 31. The grating ridges 31 each have a front side 32 and sidewalls 33. Grooves 34 are in each case formed between the grating ridges 31. The grooves 34 each have a bottom 35.

[0078] The grating ridges 31 each have in particular a trapezoid-shaped cross section. The cross section can correspond to an isosceles trapezoid or a non-isosceles trapezoid. It is non-rectangular, in particular. It is non-triangular, in particular.

[0079] That area proportion of the total reflection surface area of the mirror, in particular of the total area of the grating structure 30, which is constituted by the sidewalls 33, in plan view, in particular in perpendicular projection, is at most 10%, in particular at most 5%, in particular at most 3%, in particular at most 2%, in particular at most 1%, in particular at most 0.5%, in particular at most 0.3%.

[0080] Instead of a trapezoid-shaped cross section, the grating ridges 31 can generally also have a cross section having a trapezoid-shaped smallest convex envelope. In principle, the front side 32 of the grating ridges 31 need not be oriented parallel to the bottom 35 of the adjacent grooves 34.

[0081] Between the front side 32 of the grating ridges 31 and the bottom 35 of the grooves 34, there is an offset V in the direction of a surface normal 36 with respect to the substrate 37. The offset V is, in particular, in the region of one quarter wavelength in the infrared range. The offset V is, in particular, in the range of 1 micrometer to 10 micrometers. Other values are likewise possible.

[0082] The offset V is also referred to as the groove depth of the grating structure 30. For further details, reference should be made to DE 10 2012 010 093 A1.

[0083] According to the disclosure, it has been recognized that an embodiment of the grating ridges 31 with steep or even negative sidewalls has the effect that after a protective layer 38 has been applied, the substrate is not completely enclosed by the protective layer 38. This can have the effect that the substrate is attacked in an aggressive atmosphere, in particular in an atmosphere comprising ionized hydrogen. The hydrogen can lead, in particular, to the substrate 37 breaking up or to stresses that can cause layer detachments.

[0084] According to the disclosure, it has therefore been recognized that it is advantageous if the sidewalls 33 each have a sidewall steepness b in the range of 15° to 60°. In this case, the sidewall steepness b, also referred to as edge steepness, is measured in relation to a local tangential plane 39 in the region of the bottom 35 of the groove 34 adjacent to the sidewall 33 (see FIG. 3).

[0085] Such a defined sidewall steepness b has the effect that it is possible to ensure that the protective layer 38 is closed, in particular covers the substrate 37 completely and without gaps.

[0086] s A description is given below of various variants regarding how sidewalls 33 having a defined sidewall steepness b can be produced using suitable process implementation, in particular during the structuring of a structuring layer 40, in particular with the aid of a lithography process, and/or during the etching of the structuring layer 40 and of the substrate 37, in particular.

[0087] In particular, a layer composed of photoresist (PR) serves as the structuring layer 40. The layer can be structured flexibly and precisely via a structuring step, for example via a lithographic method.

[0088] FIG. 4A schematically illustrates an inert dry etching method for structuring the substrate 37. In a method of this type, etching is carried out in a vacuum using accelerated directional ions, wherein the material removal is produced purely physically via corrosions. In this case, there are in principle two possibilities for setting the sidewall steepness b. In the variant illustrated in FIG. 4A, the sidewall steepness b is influenced by an angle ew of incidence of the ions. The angle ew of incidence is also referred to as the etching angle. In this case, the ion beam 41 can be tilted parallel to the orientation of the grooves 34. This is also referred to as longitudinal tilting. The ion beam 41 can also be tilted transversely with respect to the orientation of the grooves 34. This is referred to as transverse tilting. It is also possible to tilt the ion beam 41 in relation to a surface normal 42 with respect to the substrate 37 and to rotate the tilted ion beam 41 about the surface normal 42.

[0089] On account of the shading by the structuring layer 40, a sidewall 33 having a sidewall steepness b of less than 90° is formed (see FIG. 4B). The sidewall steepness b can be influenced, in particular set, by the choice of the angle ew of incidence.

[0090] In the case of inert dry etching, the sidewall steepness b can also be influenced by virtue of the structuring layer 40 and the substrate 37 having different etching rates. This is illustrated by way of example in FIGS. 5A to 5C. As is evident in particular from the intermediate product (see FIG. 5B), differences in the etching rates lead to an influencing of the sidewall steepness b. In this figure and the subsequent figures, the etched region is illustrated in a hatched manner in each case.

[0091] Different etching rates can be achieved, in particular, by the selection of different resists for the structuring layer 40.

[0092] In addition to the influence of different etching rates, the sidewall steepness b can be influenced, in particular set, here by the steepness c of a sidewall 43 of the structuring layer 40. The steepness c is also referred to as the resist steepness. It can be chosen flexibly and precisely in the lithography step for structuring the structuring layer 40.

[0093] Moreover, in this variant, too, the sidewall steepness b can be influenced, in particular set, by the choice of etching angle.

[0094] In the case of a reactive dry etching method, etching is carried out in a vacuum using accelerated directional ions, wherein the material removal takes place to the greatest possible extent by way of chemical reactions of the ions with the materials of the surface. In this case, the sidewall steepness b can be set by targeted selection of the chemical components. Possible etching gases here are O.sub.2 (C.sub.12, Sf.sub.6, CF.sub.4, CHF.sub.3, O.sub.2, C.sub.2F.sub.6, CF.sub.6, SICl.sub.4, BCl.sub.3) and a mixture thereof.

[0095] By way of example, a defined etching of the structuring layer 40 is settable using a targeted selection of the composition of the etching gas, for example by changing the oxygen content thereof. The etching rate of the structuring layer 40 can be lower (FIGS. 6A to 6C), or higher (FIGS. 7A to 7C), than the etching rate of the substrate 37.

[0096] A given sidewall steepness c of the sidewalls 43 of the structuring layer 40 can thus lead to a flatter or steeper sidewall steepness in the substrate 37.

[0097] Combined etching is also possible. In the case of combined etching, the etching removal is achieved simultaneously with chemical and physical removal. This can be achieved for example by using reactive etching gases and applying them directionally and in an accelerated manner onto the surface of the substrate 37 with the structuring layer 40 applied thereon. It is thereby possible to combine the variants described above, in particular the influencing possibilities for setting the sidewall steepness b.

[0098] The sidewall steepness c of the sidewalls 43 of the structuring layer 40 can be influenced by various factors in the lithography process. It can be influenced, in particular, by the intensity of the exposure in the lithography process. It can be influenced by targeted focusing of a laser beam 44 (see FIG. 8A).

[0099] By using a collimated laser beam 44, it is possible to achieve a higher sidewall steepness c in the structuring layer 40 (see FIG. 8B).

[0100] The development operation of the lithography process for structuring the structuring layer 40 also influences the sidewall steepness c of the resist structure. A dark removal of the resist also always takes place during the development of the exposed structuring layer 40. The dark removal results in edge rounding. FIG. 9 illustrates by way of example pronounced edge rounding as a consequence of a lengthened development duration.

[0101] Hard bake and reflow can also be used in a targeted manner for influencing the sidewall steepness c of the structuring layer 40. Thermal reflow of developed photoresist structures can be used in a targeted manner for structuring the structuring layer 40 and thus for influencing the sidewall steepness b of the grating ridges 31 that is produced via a dry etching method, for example. A hard bake results, in particular, in spherical or cylindrical rounding of the resist edges. FIG. 10A illustrates by way of example a substrate 37 with a structured structuring layer 40 applied thereon without or before a hard bake. FIG. 10B illustrates the corresponding structure having been thermally rounded after a hard bake.

[0102] Different etching rates of the structuring layer 40 and of the substrate 37 can be achieved in the case of a wet-chemical etching method as well. A corresponding influencing of the sidewall steepness b as in the case of the inert dry etching method in accordance with FIG. 5B is thus customary.

[0103] In the case, too, of isotropic, diffusion-limited etching of the substrate 37, the sidewall steepness b can be influenced in a targeted manner. In the case of a method of this type, the extent of an undercut region 45 is dependent, in particular, on the intermixing of the etching solution.

[0104] After the structuring of the substrate 37, the grating structure 30 is provided with the closed protective layer 38. The protective layer 38 is applied in particular on the substrate 37. It can be deposited in particular on the substrate 37. It is also possible to allow the protective layer 38 to grow on the substrate 37.

[0105] In particular, a molybdenum-silicon double-ply structure can serve as the protective layer. Details of such a layer stack are known from the prior art.

[0106] With the aid of the projection exposure apparatus 1, at least one part of the reticle in the object field 5 is imaged onto a region of a light-sensitive layer on the wafer in the image field 8 for the lithographic production of a microstructured or nanostructured component, in particular of a semiconductor component, for example of a microchip. Depending on the embodiment of the projection exposure apparatus 1 as a scanner or as a stepper, the reticle and the wafer are moved in a temporally synchronized manner in the y-direction continuously in scanner operation or step by step in stepper operation.