REFLECTIVE OPTICAL ELEMENT AND METHOD FOR REPAIRING AND / OR CONDITIONING A REFLECTIVE OPTICAL ELEMENT - Patent application

Abstract

To make the substrate reusable, a reflective optical element (52) for use in the extreme ultraviolet wavelength range is proposed, comprising a substrate (59) and a reflective coating (54), with a reactive release layer (62) disposed between the substrate and the reflective coating. For repair and / or adjustment of the reflective optical element, it is proposed to carry out the steps of providing a reflective optical element for use in the extreme ultraviolet wavelength range, comprising a substrate and a reflective coating, with a reactive release layer disposed between the substrate and the reflective coating, introducing an energy pulse into the reactive release layer to initiate a reaction in the reactive release layer, thereby changing the volume of the reactive release layer, and peeling off the reflective coating and the reactive release layer.

Description

[Technical field]

[0001] The present invention relates to a reflective optical element for the extreme ultraviolet wavelength range, comprising a substrate and a reflective coating.Furthermore, the present invention relates to a method for repairing and / or adjusting a reflective optical element for the EUV wavelength range.This application claims the priority of German Patent Application No. 10 2021 200 748.1 filed on January 28, 2021, which is incorporated herein by reference in its entirety. [Background technology]

[0002] In order to allow ever finer structures to be produced during the manufacture of semiconductor components by lithography, shorter and shorter wavelengths of light are used. When operating in the extreme ultraviolet (EUV) wavelength range, for example in particular at wavelengths between about 5 nm and 30 nm, the use of elements such as transmissive lens elements becomes impossible and masks consisting of reflective optical elements, i.e. illumination and projection lenses or a reflective coating on a substrate, are used.

[0003] It is often necessary to remove the reflective coating from the substrate of the reflective optical element again, so that the substrate can be recoated if possible for cost reasons. This can happen, for example, if a quality inspection shows that the existing coating does not meet specifications, or if degradation of the layer requires the end user to renew the coating.

[0004] It is desirable to remove the original coating with as little residue as possible. There are various procedures to do this. The original coating can be removed by polishing. However, this involves a lot of time and cost. It can take weeks to months to return the substrate to its uncoated state. Furthermore, substrates, especially aspheric substrates or substrates with freeform surface designs, cannot always be polished back so that they can be used again in the same reflective optical element.

[0005] US Pat. No. 5,399,633 discloses providing a release layer between the substrate and the reflective coating, which has low adhesion so that the reflective coating can be removed virtually without residue during mechanical removal, for example with an adhesive tape. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] DE 10 2012 200 454 A1 Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to provide a reflective optical element for the extreme ultraviolet wavelength range and a method for repairing and / or adjusting the reflective optical element, which allows the optical coating to be removed from the substrate with as little effort as possible so that the substrate can be reused, even in the case of reflective optical elements having a relatively complex surface shape. [Means for solving the problem]

[0008] In a first aspect, this object is achieved by a reflective optical element for extreme ultraviolet wavelengths comprising a substrate and a reflective coating, characterized in that a reactive release layer is disposed between the substrate and the reflective coating.

[0009] According to the inventors' realization, if a reactive release layer is provided between the substrate and the reflective coating, which has different physical properties after the reaction occurs than before the reaction and can consequently influence the peeling behavior of the reflective coating, it is possible to achieve, on the one hand, good adhesion of the reflective coating during use of the reflective optical element and, on the other hand, peeling of the reflective coating for the purpose of repairing or adjusting the reflective optical element.

[0010] Advantageously, the reflective optical element comprises at least one further reactive release layer and one further reflective coating, with at least two reactive release layers and at least two reflective coatings arranged alternately, such that the reflective coating exposed to vacuum can be removed, together with the reactive release layer underneath, if necessary, to make the further reflective coating available without recoating. Two, three, four or more reflective coatings with reactive release layers can be provided.

[0011] Preferably, the reflective coating is configured as a multilayer system including alternating layers of at least two different materials with different real parts of the refractive index in the extreme ultraviolet wavelength range. Such reflective optical elements are particularly suitable for use at quasi-normal incidence of EUV radiation, but can also be designed for broader band reflection.

[0012] In a preferred embodiment, the reactive release layer comprises at least two substances that can react with each other after the introduction of an energy pulse such that the volume of the reactive release layer is different from that before the introduction of the energy pulse. This volume change generates stresses that facilitate release from the substrate, preferably releasing the reactive release layer from the substrate as completely as possible together with the reflective coating. This can occur, for example, when the reactive release layer is peeled from the substrate together with the reflective coating.

[0013] Preferably, at least two substances are exothermically reactive with each other, which has the advantage that the energy pulse only needs to be introduced locally into the reactive release layer as activation energy, and the energy generated during the reaction locally in the release layer can further act as activation energy to initiate reactions in adjacent regions, so that in the best case the reaction can propagate throughout the reactive release layer.

[0014] Advantageously, the reactive release layer includes multiple alternating layers of at least two materials, such that the number of interfaces between the at least two materials can be increased, thereby allowing reactions between the materials to proceed more efficiently overall across the reactive release layer.

[0015] Preferably, the at least two materials are two metals or one metal and one semiconductor, which allows targeted selection of materials that, upon reaction with each other, result in layers with a sufficiently large difference in physical parameters, such as a difference in volume, to generate stresses in the reflective optical element that may be sufficient to allow the release layer and the reflective coating to be released.

[0016] Particularly preferably, one of the at least two materials is one from the group consisting of titanium, zirconium, and niobium, and the other of the at least two materials is one from the group consisting of silicon and carbon, or one of the at least two materials is palladium and the other of the at least two materials is aluminum. During the course of the reaction, these combinations of materials result in a large change in the volume of the reactive release layer, and the reaction proceeds further exothermically.

[0017] In yet another aspect, the object is a method for repairing and / or conditioning a reflective optical element for the EUV wavelength range, comprising the steps of: Providing a reflective optical element for extreme ultraviolet wavelengths, in particular a reflective optical element as described above, comprising a substrate and a reflective coating, with a reactive release layer disposed between the substrate and the reflective coating; introducing an energy pulse into the reactive release layer to initiate a reaction in the reactive release layer, thereby changing the volume of the reactive release layer; stripping the reflective coating and the reactive release layer; This is achieved by a method comprising:

[0018] It has been found that it is possible to adjust and / or repair the reflective optical elements even in the case of a substrate with a freeform surface, if a reactive release layer is provided between the substrate and the reflective coating that may need to be replaced, and this reactive release layer is subjected to an energy pulse that acts as an activation energy to initiate a reaction in the reactive release layer when an adjustment or repair is to be performed. The volume change caused by the reaction generates stresses between the release layer and the substrate and between the release layer and the reflective coating, which causes the reflective coating and the reactive release layer to peel off. After the peeling process, the remaining reflective optical elements are adjusted in that they are available for coating renewal.

[0019] Preferably, the energy pulse is introduced to the reactive release layer as an electric spark, a laser beam, an electron beam, or an ion beam, or a combination thereof. Laser beams, electron beams, and ion beams can also be pulsed and used to introduce energy to the reactive release layer through a reflective coating. Exposed areas of the release layer can also be provided for introducing an energy pulse to the release layer, particularly by electric spark or application of a voltage to the reactive release layer, but also by irradiation.

[0020] Preferably, the reactive release layer has a volume change of ±5% or more upon the introduction of an energy pulse, which can generate high stress in the reflective optical element, thereby peeling off the reflective coating and the reactive release layer.

[0021] In a preferred embodiment, after stripping of the reflective coating and reactive release layer, a new reflective coating is applied to obtain a repaired reflective optical element.

[0022] Advantageously, in this case a new reactive release layer is applied between the substrate and the new reflective coating so that the reflective optical element thus obtained can at a given point in time be further adjusted and / or repaired once more as described above.

[0023] The invention will now be explained in more detail with reference to preferred exemplary embodiments. [Brief description of the drawings]

[0024] [Figure 1] 1 shows a schematic diagram of a first embodiment of a reflective optical element according to the invention; [Diagram 2] 2 shows a schematic diagram of a second embodiment of a reflective optical element according to the invention; [Diagram 3] Figure 3a shows a schematic representation of a third embodiment of a reflective optical element according to the invention during introduction of an energy pulse, Figure 3b shows a schematic representation of a third embodiment of a reflective optical element according to the invention during stripping of the reflective coating, and Figure 3c shows a schematic representation of a third embodiment of a reflective optical element according to the invention after stripping of the reflective coating. [Figure 4] 4 shows a schematic diagram of a fourth embodiment of a reflective optical element according to the invention; [Diagram 5] 5 shows a schematic diagram of a fifth embodiment of a reflective optical element according to the invention; [Figure 6] 2 illustrates diagrammatically an exemplary sequence of a method according to the present invention for repairing and / or adjusting a reflective optical element; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] FIG. 1 shows a schematic configuration of a reflective optical element 50 with a reactive release layer 60 on a substrate 59 and a reflective coating 54 thereon, which in this example is configured as a multilayer system including alternating layers of at least two different materials with different real parts of the refractive index in the extreme ultraviolet wavelength range. This example includes layers of a material with a high real part of the refractive index at the operating wavelength, for example at which the lithographic exposure is performed (also called spacer 56) and a material with a low real part of the refractive index at the operating wavelength (also called absorber 57), applied to the substrate 59 in an alternating manner, with the absorber-spacer pair forming a stack 55. In a way, this mimics a crystal with lattice planes corresponding to the absorbing layers in which the Bragg reflection occurs. Typically, reflective optical elements for EUV lithography devices or optical systems are designed such that each wavelength of maximum reflectivity substantially corresponds to an operating wavelength of the lithography process or other application of the optical system.

[0026] The thickness of the individual layers 56, 57, as well as the thickness of the repeating stack 55, may be constant throughout the multilayer system 54 or may vary laterally over the area or total thickness of the multilayer system 54, depending on the spectral or angle-dependent reflection profile or maximum reflectance at the operating wavelength to be achieved. If the layer thickness is substantially constant throughout the multilayer system 54, it is also referred to as a period 55 rather than a stack 55. To increase the maximum reflectance at each operating wavelength, the basic structure of absorbers 57 and spacers 56 can also be supplemented with other more and less absorbing materials to affect the reflection profile in a targeted way. To this end, the absorber and / or spacer materials may be interchanged in some stacks, or the stack may consist of two or more absorber and / or spacer materials. Furthermore, it is possible to provide additional layers between the spacer and absorber layers 56, 57 as diffusion barriers. For example, a common material combination at an operating wavelength of 13.4 nm is molybdenum as absorber material and silicon as spacer material. The periods 55 often have a thickness of about 6.7 nm here, the spacer layer 56 being usually thicker than the absorber layer 57. Further typical material combinations are in particular silicon-ruthenium or molybdenum-beryllium. Furthermore, a protective layer 53, which may also be of multilayer design, can be provided on the multilayer system 54.

[0027] Typical substrate materials of reflective optical elements for EUV lithography are silicon, silicon carbide, silicon-impregnated silicon carbide, fused silica, titanium-doped fused silica, glass, and glass-ceramics. In particular for such substrate materials, it is further possible to provide a layer of a material between the reflective coating 54 and the substrate 59 that is highly absorbing for radiation in the EUV wavelength range used in the operation of the reflective optical element 50, in order to protect the substrate 59 from radiation damage, for example from unwanted compression. Furthermore, the substrate can also be made of copper, aluminum, a copper alloy, an aluminum alloy, or a copper-aluminum alloy.

[0028] The reactive release layer may comprise at least two layers of different materials. It may preferably comprise a plurality of alternating layers of at least two materials. In particular, the reactive release layer comprises at least two materials that can react with each other after the introduction of an energy pulse such that the volume of the release layer is different from that before the introduction of the energy pulse.

[0029] In the example shown in FIG. 1, the reactive release layer 60 comprises, without limiting generality, a number of alternating layers 61, 66 of two different materials. These two materials can react with each other, particularly to bring about a volume change, when a certain amount of energy sufficient to act as an activation energy is introduced into the reactive release layer 60. Preferably, the volume change is more than ±5%, in order to generate a high probability of stress between the reactive release layer 60 and firstly the substrate 59 and secondly the reflective coating 54, so that the reactive release layer 60 and the reflective coating 54 can be peeled off. It has been found to be particularly suitable if the at least two materials are two metals or one metal and one semiconductor.

[0030] The more layers 61, 66 that are provided, the more interfaces at which the reaction can take place. Advantageously, at least one of the selected materials has a high absorption or effective cross section for the radiation used to introduce the activation energy in the form of an energy pulse, firstly to be able to convert the radiation energy sufficiently into activation energy, and secondly to be able to protect the substrate 59 from damage caused by this radiation, and / or a high absorption for the EUV radiation used in the operation of the reflective optical element 50, in order to protect the substrate 59 from corresponding radiation damage. Additional layers can also be provided between the reactive release layer 60 and the substrate 59 to protect the substrate 59 from the activation radiation and, if necessary, from EUV radiation during operation.

[0031] FIG. 2 shows a second exemplary embodiment of a reflective optical element 52 for the EUV wavelength range at the start of local irradiation through the reflective coating 54. In the case of the reflective optical element 52, a reactive release layer 62 is arranged between the substrate 59 and the reflective coating 54, similar to the example shown in FIG. 1. In the example shown here, similar to the example from FIG. 1, the reactive release layer 62 provided is a plurality of alternating layers 68, 69 of two different substances. Unlike the example shown in FIG. 1, the two different substances are selected to react exothermically with each other under the effect of an activation energy, for example as a result of irradiation with electromagnetic radiation, electrons or ions. As a result, the reactive release layer 62 is initially formed in a metastable state.

[0032] To introduce the activation energy required to initiate the reaction into the reactive release layer 62, in the example shown, electrons (represented by wavy arrows) are irradiated, specifically through the reflective coating 54. This can also be done by irradiation with ions or electromagnetic radiation. The irradiation can be carried out as individual pulses or a series of pulses, and can also be locally incident at different locations over the area of ​​the reflective coating 54 or the reactive release layer 62. The substances of the reactive release layer 62 are selected to react exothermically with each other, so that a locally limited introduction of an energy pulse is sufficient to cause a reaction of these substances throughout the entire release layer 62.

[0033] In the exemplary embodiment shown in Figures 3a-3c, the materials of the reactive release layer 63 are selected to react with each other after the introduction of an energy pulse, resulting in a subsequent change in the volume of the reactive release layer 63, as in the example shown in Figure 2, with a volume change of ±5% or more being sufficient to completely peel the reactive release layer 63 together with the reflective coating 54 from the substrate 59, as shown diagrammatically in Figures 3b, c. Furthermore, since at least two materials react with each other exothermically, the introduction of one or more energy pulses laterally into the reactive release layer 63 may be sufficient to initiate a reaction throughout the reactive release layer 63. As in the above example, the reactive release layer includes multiple alternating layers of at least two materials. The energy pulse(s) may be introduced laterally as a radiation pulse of electrons, ions, or electromagnetic radiation (represented by wavy arrows) or by an electric spark, which is introduced laterally directly into the reactive release layer 63. It is also possible to provide one or more areas of the reactive release layer 63 that are not covered by the reflective coating 54 and where the reactive release layer 63 is exposed in order to introduce one or more electrical sparks. The exposed areas can also be contacted with an electrode in order to introduce an electrical energy pulse to the reactive release layer 63.

[0034] Regardless of the method of introducing the energy pulse or pulses, it has been found to be advantageous for the at least two substances of the reactive release layer 63 to be two metals or one metal and one semiconductor, on the one hand in order to have a stable reactive release layer 63 during storage and normal operation of the reflective optical element and, on the other hand, in order to enable a reaction resulting in a volume change.

[0035] It has been found to be particularly suitable if one of the at least two substances is one from the group consisting of titanium, zirconium and niobium and the other of the at least two substances is one from the group consisting of silicon and carbon, or if one of the at least two substances is palladium and the other of the at least two substances is aluminum, which makes it possible to achieve a volume change of more than ±5% in the course of the reaction and thus ensure that the release layer and the reflective coating are released as completely as possible.

[0036] The following is a non-exhaustive list of exemplary possible material combinations.

[0037] Starting materials Reaction products Volume change Heat of reaction Ti+C TiC -23.4% -93kJ / Mol 5 Ti+3 Si Ti5Si3 -16.2% -72kJ / Mol 5 Zr+3 Si Zr5Si3 -15.12% -72kJ / Mol 5 Nb+3 Si Nb5Si3 -15.15% -57kJ / Mol Pd+Al PdAl -7.85% -92kJ / Mol Zr+Al ZrAl +2.5% -45kJ / Mol Ti+Al TiAl +3.61% -36kJ / Mol

[0038] Generally, the progress of the reaction in the reactive release layer can be influenced not only by the selection of suitable substances and their stoichiometry, but also by the configuration of the reactive release layer, for example the number of individual layers and their thickness. In particular, when the release layer is composed of at least two periods with layers of different materials and the layer material sequence remains constant from period to period, it is advantageous if the corresponding layer thickness ratio is also kept constant so that the stoichiometry does not change. This makes it possible to ensure that the reaction progresses as completely as possible. Furthermore, material combinations with more than two materials are also possible.

[0039] FIG. 4 shows a fourth exemplary embodiment of the proposed reflective optical element, in which the release layer arranged between the reflective coating 54 and the substrate 59 is embodied structured in that the release layer has partial regions 64a, 64b, at least one of the partial regions, preferably both of which are embodied as a reactive release layer. In yet another variant, three, four, five, six or more partial regions can be provided. The partial regions here comprise different material combinations. In this respect, for example, one of the partial regions can exhibit a volume expansion and another one a volume contraction after the reaction in order to introduce layer stresses over the entire area of ​​the release layer. Furthermore, the individual partial regions can also be optimized in design in such a way that layer or coating residues that may still be present after the reaction has progressed can be satisfactorily removed, for example by wet chemical methods.

[0040] 5 shows a fifth exemplary embodiment of the reflective optical element proposed herein, which comprises at least one further reactive release layer and at least one further reflective coating, where at least two reactive release layers and at least two reflective coatings are arranged alternately. For the sake of clarity, in the example shown here, exactly two reflective coatings 54, 54' and two reactive release layers 65, 65' are arranged alternately on the substrate 59. In further variations, three, four, five, six or more reflective coatings and / or reactive release layers can be involved. This allows the reflective coating exposed to vacuum to be removed together with the reactive release layer underneath, if necessary, to make available the further reflective coating without recoating.

[0041] FIG. 6 shows a schematic diagram of an exemplary sequence of the proposed method for repairing and / or conditioning a reflective optical element for the EUV wavelength range. In a first step 601 “Preparation of mirror with reactive release layer”, in the described example, a reflective optical element for the extreme ultraviolet wavelength range is prepared, which is configured as a mirror for an EUV lithography device, with a reactive release layer arranged between the substrate and the reflective coating. For possible configurations of the reactive release layer in particular, see the above description. If, for the purpose of conditioning, the reflective coating does not meet the requirements for normal operation of the EUV lithography device, a reaction is induced in the reactive release layer in step 603 “Introduction of an energy pulse into the reactive release layer”. The reaction proceeding in the release layer results in step 605 “Volume change of the reactive release layer”. As a result, stresses are introduced in the EUV mirror, firstly between the substrate and secondly between the reactive release layer or the reflective coating. These stresses allow the reactive release layer and the reflective coating to be peeled off, which corresponds to step 607 “Removal of the reactive release layer and the reflective coating”. In the best case, the reactive release layer is dimensioned such that the stresses generated are high enough to expose the substrate virtually free of residues. This can be achieved in particular by a particularly thick reactive release layer having several individual layers. Possible residues can be removed in known manner, for example by wet chemical methods, or by reactive etching, or mechanically by polishing. Thus, for a mirror similar to the example shown in FIG. 5, we have a conditioned and repaired mirror with the reflective coating close to the substrate (reference number 54' in FIG. 5) exposed and ready for use again. For a mirror similar to the example shown in FIGS. 3a-3c, we have a conditioned mirror in that step 609 "application of a new reflective coating" can be used to repair the mirror to form a mirror ready for use again for an EUV lithography device.

[0042] The reflective optical element for EUV radiation described here may in particular also be a mask for EUV lithography or an EUV mirror used in an optical system used for inspection of wafers, EUV mirrors or EUV masks.

Claims

1. A reflective optical element for extreme ultraviolet wavelengths comprising a substrate and a reflective coating, characterized in that a reactive release layer (60, 62, 63, 64a, 64b, 65, 65') is disposed between said substrate (59) and said reflective coating (54, 54'), said reactive release layer (60, 62, 63, 64a, 64b, 65, 65') comprising at least two substances capable of reacting with each other after the introduction of an energy pulse such that the volume of said reactive release layer (60, 62, 63, 64a, 64b, 65, 65') is different from that before the introduction of the energy pulse.

2. 2. A reflective optical element according to claim 1, comprising at least one other reactive release layer (65') and at least one other reflective coating (54'), wherein at least two of said reactive release layers (65, 65') and at least two of said reflective coatings (54, 54') are arranged alternately.

3. 3. A reflective optical element according to claim 1 or 2, characterized in that the reflective coating (54) is configured as a multilayer system comprising alternating layers (56, 57) of at least two different materials having different real parts of their refractive indices at wavelengths in the extreme ultraviolet wavelength range.

4. 4. The reflecting optical element according to claim 1, wherein the at least two substances are substances that exothermically react with each other.

5. A reflective optical element according to any one of claims 1 to 4, characterized in that the reactive release layer (60, 62, 63, 64a, 64b, 65, 65') comprises a plurality of layers (61, 66, 68, 69) of the at least two substances arranged one on top of the other.

6. 6. A reflective optical element according to claim 1, wherein the at least two substances are two metals or one metal and one semiconductor.

7. A reflective optical element according to any one of claims 1 to 5, characterized in that one of the at least two substances is one from the group consisting of titanium, zirconium, and niobium, and the other of the at least two substances is one from the group consisting of silicon and carbon.

8. 6. A reflecting optical element according to claim 1, wherein one of said at least two substances is palladium, and the other of said at least two substances is aluminum.

9. 1. A method for repairing and / or conditioning a reflective optical element for EUV wavelength range, comprising: providing a reflective optical element for extreme ultraviolet wavelengths, in particular a reflective optical element according to any one of claims 1 to 8, comprising a substrate and a reflective coating, with a reactive release layer arranged between the substrate and the reflective coating; introducing an energy pulse into the reactive release layer to initiate a reaction in the reactive release layer, thereby changing the volume of the reactive release layer; stripping the reflective coating and the reactive release layer; The method includes:

10. 10. The method of claim 9, wherein the energy pulse is introduced to the reactive release layer as an electric spark, a laser beam, an electron beam, or an ion beam, or a combination thereof.

11. 11. The method according to claim 9 or 10, wherein the reactive release layer undergoes a volume change of ±5% or more upon application of the energy pulse.

12. The method according to any one of claims 9 to 11, characterized in that after stripping of the reflective coating and the reactive release layer, a new reflective coating is applied.

13. 13. The method of claim 12 further comprising applying a new reactive release layer between the substrate and the new reflective coating.

Images