Spectral filter membrane

Abstract

A spectral filter membrane for an EUV lithographic apparatus, the spectral filter membrane comprising a core layer and a cap layer, wherein the core layer and the cap layer are configured such that undesired radiation re-emission from the membrane is directed with a predetermined % in a main direction, and wherein the undesired radiation is IR radiation with a wavelength of at least one of >4.4 μm, >10 μm, >21.4 μm, in a range of 4.4 to 500 μm, in a range of 10 to 500 μm, and in a range of 21.4 to 500 μm.

Description

SPECTRAL FILTER MEMBRANECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24186777.9 which was filed on 05 July 2024, EP application 24198148.9 which was filed on 03 September 2024 and which are incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a spectral filter membrane. The present invention has particular, but not exclusive, use in connection with EUV lithographic apparatuses and EUV lithographic tools. The present invention also relates to methods of manufacturing spectral filter membranes.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses reflective surfaces instead of lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector, the multilayers may be alternatingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a massproduction process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing thephase defects. Such phase defects may be correctly detected by using the same or similar (13.5 nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks. For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Forth, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0006] In EUV lithography, before light reaches the substrate to write patterns in EUV-sensitive resist, the light should be filtered from 155 - 400 nm deep ultraviolet (DUV) radiation, which contributes not to imaging but only gives an unwanted global background contribution, resulting in contrast loss. To this end a dynamic gas lock membrane (DGEm) may be used to filter out-of-band DUV radiation and to block infrared (IR) radiation such that both do not reach the resist on the substrate. Membranes may be used as filters, such as spectral purity filters. The membrane may be referred to as a spectral filter membrane. Membranes may be used as filters, such as spectral purity filters or as part of a dynamic gas lock of a lithographic apparatus. It is beneficial for a membrane to be highly transmissive, for example having a single-pass EUV transmission (EUVT) of 90% or higher.

[0007] Membranes (which may be referred to as pellicles) may be provided to protect the patterning device, substrate or other optical components from airborne particles and other forms of contamination. Membranes may also be used to provide a passage for lithographic radiation between regions of the lithographic apparatus which are sealed from one another.

[0008] It is desirable to provide a membrane which addresses a problem associated with the prior art.SUMMARY

[0009] According to a first aspect of the invention, there is provided a spectral filter membrane for an EUV lithographic apparatus, the spectral filter membrane comprising a core layer and a cap layer, wherein the core layer and the cap layer are configured such that undesired radiation re-emission from the membrane is directed with a predetermined % in a main direction, and wherein the undesired radiation is IR radiation with a wavelength of at least one of >4.4 pm, >21.4 pm, in a range of 4.4 to 500 pm, in a range of 10 to 500 pm, and in a range of 21.4 to 500 pm.

[0010] Advantageously, the membrane may not only block IR radiation but also may radiate it away from a desired direction, such as away from a resist-covered substrate when it is used at the location of the dynamic gas lock membrane. The membrane may have elevated (e.g., 10 pm) IR emissivity. The membrane may have increased strength and reliability, and an improved cap layer when compared to previous membranes.

[0011] The cap layer may be located on the opposite side of the core layer from incoming radiation, and the main direction may be opposite the direction of incoming radiation in use. This may be if the re-emission is desired in the direction away from where the incident radiation came from (i.e. opposite the direction of the incoming radiation in use). The cap layer may be located on the same side of the core layer as the incoming radiation, and the main direction may be in the direction of the incoming radiation in use. This may be if the re-emission is desired to be in the direction where the light came from (i.e. in the same direction as the direction of the incoming radiation in use).

[0012] The core layer may be a dielectric layer.

[0013] The core layer may comprise at least one of Si, Si iNr. and Si i+xNr. Advantageously, these materials provide high transmission of EUV radiation.

[0014] The undesired radiation that is re-emitted from the membrane may be considered to be directionally emitted.

[0015] The material of the core layer and material and thickness of the cap layer may be selected such that the undesired radiation is directed with the predetermined % in the main direction.

[0016] The core layer and the cap layer may be configured such that undesired radiation re-emission is directed more in the main direction than in another direction.

[0017] The other direction may be opposite to the main direction. The other direction may be in the direction of incoming radiation in use.

[0018] The core layer and the cap layer may be configured such that an amount of undesired radiation re-emission that may be directed in the main direction is at least one of: greater than or equal to 50%, 68%, 74%, 85%, 92%, and 94%. The core layer and the cap layer may be configured such that an amount of undesired radiation re-emission that may be directed in the other direction (i.e. opposite the main direction) is at least one of: greater than or equal to 6%, 8%, 12%, 15%, 50%. The amount of undesired radiation re-emission that is directed in the main direction may be at least one of: greater than or equal to 6%, 8%, 12%, 15%, 50%, 60%, 68%, 70%, 74%, 80%, 85%, 90%, 92%, 94%, and 95%.

[0019] The core layer may have a predetermined refractive index squared n2for wavelengths of the undesired radiation and the cap layer may have a predetermined parameter f, where f is a product of thickness and conductivity of the cap layer, and wherein the predetermined refractive index squared may be in a range of 5.6-11.7. The predetermined parameter f may be in a range of 2-13.

[0020] A directionality of 50% may be provided by n2= 1 and f = 2. The directionality rises to 92% for n2= 12 and f = 13. The predetermined refractive index squared may be in a range of 1-12.

[0021] For 21.4 pm radiation, the core layer and the cap layer respectively, may have substantially at least one of: n2=11.7 and f=13.

[0022] This combination of n2and f provides a good combination of absorption, directionality and EUV transmission. Other combinations of values may be used. For 4.4 pm radiation, the core layer and the cap layer respectively, may have substantially at least one of: n2=12 and f=5; n2=5.6 and f=2.5.

[0023] This combination of n2and f provides a good combination of absorption, directionality and EUV transmission. Other combinations of values may be used.

[0024] The core layer may comprise a separation layer located between two sub-core layers.

[0025] The separation layer may be at least one of: a grain sintering inhibition layer (GSIL), MoSii. Mo Si ;. RuSii and a B4C layer. The separation layer may be between two Si layers.

[0026] The cap layer may comprise at least one of: a metal, Ru, RU43M057, and Re (Rhenium).

[0027] The membrane may further comprise a further cap layer located on the opposite side of the core layer from the cap layer.

[0028] The core layer may be positioned between the cap layer and the further cap layer.

[0029] The further cap layer may comprise at least one of: a hydrogen plasma-resistant material, a metal oxide, Y2O3, YiSiCU RU43M057, HfCF, and RuSiOr.

[0030] The further cap layer refractive index may substantially match to the core layer refractive index for wavelengths of the undesired radiation.

[0031] The further cap layer and the core layer may be, optically, a single layer without interface. This may mean that no propagating wave reflections occur at specific wavelengths in the IR. The combined further cap layer and core layer may be considered to be a single dielectric backing layer (DBL).

[0032] For 21.4 pm radiation, the further cap layer refractive index squared n2may be 1.7.

[0033] For 4.4 pm radiation, the further cap layer refractive index squared n2may be 3.4 or 5.3.

[0034] The further cap layer refractive index may substantially match to the environment refractive index in use for wavelengths of the undesired radiation.

[0035] The environment refractive index may be 1.

[0036] The environment may be a hydrogen plasma environment. The environment may be at a pressure below atmospheric pressure.

[0037] The membrane may further comprise an intermediate layer positioned between the cap layer and the core layer for separating the cap layer and the core layer.

[0038] The intermediate layer may comprise at least one of: a metal, RU43M057, and Mo.

[0039] According to a second aspect of the invention, there is provided a lithographic apparatus comprising the membrane as described above.

[0040] The lithographic apparatus may be an EUV lithographic apparatus.

[0041] According to a third aspect of the invention, there is provided a method of fabricating a spectral filter membrane for an EUV lithographic apparatus, the method comprising providing a substrate, forming a membrane comprising a core layer and a cap layer on the substrate, then etching away at least part of the substrate; wherein the core layer and the cap layer are configured such that undesired radiation re-emission from the membrane is directed with a predetermined % in a main direction, and wherein the undesired radiation is IR radiation with a wavelength of at least one of >4.4 pm, >10 pm, >21.4 pm, in a range of 4.4 to 500 pm, in a range of 10 to 500 pm, and in a range of 21.4 to 500 pm.

[0042] The method may further comprise providing a further cap layer on the core layer such that the core layer is arranged between the cap layer and the further cap layer.BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 A depicts a known membrane;Figure 2B depicts a membrane;Figure 3A-3B depict graphs showing TDUV for various materials and thicknesses for different wavelengths;Figures 4A-4B depict membranes.DETAILED DESCRIPTION

[0044] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0045] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0046] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projectionsystem PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0047] A membrane 20 is depicted in the path of the radiation to protect the substrate W. The membrane 20 may form part of a membrane assembly and may be supported by a frame (not shown). The membrane may be referred to as a pellicle or a pellicle membrane. The membrane 20 may be relatively thin and may be substantially transparent to EUV radiation (although it will absorb a small amount of EUV radiation). The membrane 20 forms part of a dynamic gas lock of the lithographic apparatus LA. The membrane 20 may be referred to as a dynamic gas lock membrane (DGLm). The membrane 20 may be used to filter out-of-band DUV radiation and may block infrared (IR) radiation such that both do not reach the resist on the substrate W. The membrane 20 may in addition act to protect the substrate W from particle contamination. It will be appreciated that the membrane 20 may be located in any required position and may be used to protect any elements of the lithographic apparatus e.g. the patterning device MA in the lithographic apparatus.

[0048] Whilst efforts may be made to maintain a clean environment inside the lithographic apparatus LA, particles may still be present inside the lithographic apparatus LA. In the absence of a membrane 20, particles may be deposited onto the substrate W or patterning device MA.

[0049] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0050] A relative vacuum, i.e. a small amount of gas (e.g., hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0051] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0052] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1 , which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator 3. Although tin is referred to in the following description,any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0053] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0054] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0055] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0056] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a free electron laser (FEL) or a discharge produced plasma (DPP) source may be used to generate EUV radiation.

[0057] Figure 2B depicts a membrane 20B, which may be referred to as a spectral filter membrane. The membrane 20B comprises a core layer 22B, a cap layer 24B and a further cap layer 26B. The core layer 22B is positioned between the cap layer 24B and the further cap layer 26B. The cap layer 24B is located on the opposite side of the core layer from incoming radiation. In other words, the cap layer 24B is located on the side of the membrane 20B closest to the substrate W (not shown) in use. The cap layer 24B may be considered to be on the bottom of the membrane 20B. The further cap layer 26B may be considered to be on the top of the membrane 20B.

[0058] In addition, there is an intermediate layer 28B positioned between the core layer 22B and the cap layer 24B. It will be appreciated that the intermediate layer 28B is located adjacent to the core layer 22B and adjacent to the cap layer 24B. The intermediate layer 28B may be considered to form part of acombined cap layer (in combination with the cap layer 24B). The intermediate layer 28B prevents Silicide formation and thermal dewetting of the cap layer 24B.

[0059] The core layer 22B is a dielectric. The core layer 22B includes a separation layer 29B located between two sub-core layers 23B. The separation layer 29B may be referred to as a grain sintering inhibition layer (GSIL). The GSIL prevents internal grains in the core layer 22B to sinter throughout the whole thickness of the core layer when the material is heated during membrane production or afterwards during operation in the lithographic apparatus LA. This retains the strength of the membrane 20B and raises its reliability, which is beneficial as current DGLms (example in Figure 2A) greatly suffer from breakages. The thickness of the core layer 22B may be divided into two sections of reduced grain sizes (i.e., the two sub-core layers 23B) separated by the separation layer 29B. Grain size reduction can be brought about by making a layer deposition with a division overlayer, and next a second deposition layer, such that the first and second deposition layers are separated by the division layer.

[0060] The two sub-core layers 23B comprise Si, one (e.g. the upper one) having a thickness of 10.5 nm and the other (the lower one) having a thickness of 10 nm, and the separation layer 29B comprises B4C with a thickness of 2 nm. The cap layer 24B comprises Ru with a thickness of 2.4 nm and the intermediate layer 28B comprises RU43M057 with a thickness of 0.6 nm. The further cap layer 26B comprises HfOz with a thickness of 1.5 nm. Using HfOi provides increased plasma resistance at least when compared with the ZrOz-SiOzs of the current DGLm (see Figure 2A) - this is placed at the top of the membrane 20B instead of the bottom as in the current DGLm. It will be appreciated that, in other examples, the thicknesses of the layers may be different. One or more of the thicknesses may be chosen to be different (e.g. slightly deviate from the thicknesses mentioned) to make a similar membrane of a bit less directionality (for instance, choosing different Ru and / or RU43M057 thicknesses changes the value of f away from f=13, lowering directionality). Example ranges may be: separation layer 29B (B4C): 1 - 5 nm, preferably 2 nm; cap layer 24B (Ru): 2 - 3 nm, preferably 2.4 nm; intermediate layer 28B (RU43M057): 0.4 - 1.5 nm, preferably 0.6 nm; further cap layer 26B (HfOz): 1 - 3 nm, preferably 1.5 nm; and sub-core layers 23B (Si): 7 - 15 nm, preferably 10 or 10.5 nm (for each sub-core layer).

[0061] The HfOi cap 26B provides resistance to oxidation, Hydrogen plasma, and EUV. In addition, HfOi provides refractive index matching at 21.4 microns (see further below). The HfOi cap may have a thickness of at least 1.5 nm to ensure that it is a closed layer. The HfOi layer may be thin to minimise EUV transmission losses.

[0062] B4C layer 29B provides refractive index matching at 21.4 microns relative to the adjacent Si core sub-layers 23B, and in addition provides chemical stability.

[0063] The intermediate layer 28B is formed from RU43M057. The intermediate layer 28B prevents silicidation and thermal dewetting. The thickness of the intermediate layer 28B is chosen such that together with the 2.4 nm Ru cap layer 24B the condition f = 13 arises (see further below).

[0064] In general, all the layers (i.e., the further cap 26B, the core layer 23B, the separation layer 29B, the cap layer 24B, and the intermediate layer 28B) have a specifically chosen combination ofoptical properties, thickness and composition, such that special boundary conditions arise for transmission, reflection and absorption of >21.4 pm IR radiation.

[0065] The cap layer 24B is specifically placed on the bottom of the membrane 20B, such that it meets the light last. These provisions result in the membrane 20B being highly directional in >21.4 pm IR absorption and emissivity, such that it radiates this emission strongly in a single (main) direction (i.e. up, away from the substrate W) instead of a more or less equal distribution between up and down. The IR radiation may be considered to be undesired radiation (e.g., not desired to be incident on the substrate W).

[0066] It has been found that a membrane 20B with these combinations of materials and thickness provide TEUV=88%, TDUV=2%, and [for IR>21.4pm] TIR=7%, Adown=92% and Aup=8%. For clarity AdownIR>21jim= 92% & A"piR>2ipm= 8% means IR radiation of >21.4 pm is 92% absorbed if it travels downward, and 8% absorbed if it travels upward. After the absorption, IR of >21.4 pm is re-emitted for -92% upwards, and -8% downwards for that situation. When TEUV is used here, the wavelength of light is 13.5 nm.

[0067] More generally, the membrane 20B is a directional emitter that radiates 92% of the emitted power for wavelengths >21.4 pm IR in a single main direction (i.e., the main direction may be opposite the direction of incoming radiation in use) and 8% in the opposite direction (i.e., towards the substrate W in use). In other words, the membrane 20B radiates 92% of its >21.4 pm IR radiation upward when the light travels downward. The membrane 20B effectively absorbs (filters) IR radiation and then reradiates this IR radiation almost exclusively in the single main direction. It will be appreciated that the direction of emission (i.e. the main direction) can be chosen to be in the opposite direction (i.e. to upward) by placing the cap layer 24B on the bottom of the membrane 20B and the further cap layer 26B on the top of the membrane 20B.

[0068] The directionality generally holds for most metals for all IR wavelengths >10 pm, e.g. for 10 - 500 pm. The reason that analytical solutions do not apply for <10 pm is mathematical with physical fundament of electron relaxation time as key physical property. This follows from the Hagen-Rubens relation.

[0069] EUV membranes are used in a low-pressure H2 environment and are exposed to EUV and out-of-band (OOB) radiation. The core layer 22B provides membrane strength and comprises Si. To protect the material of the core layer 22B from aggressive lithographic apparatus LA conditions, capping layers are used.

[0070] HfCh used as the further cap layer here is good H-plasma etch resistant (more generally, HfCL is a good H-plasma etch resistant metal oxide), and in addition has a desirable n2value (see further below). The further cap layer 26B has a refractive index that is close to that of the environment, such that strong propagating wave reflections that occur at specific wavelengths in the IR are avoided. The separation layer 29B is chosen to match its refractive index to the two sub-core layers 23B. This is done to make these three layers optically a single layer without interface, such that no propagating wavereflections occur at specific wavelengths in the IR. The combined separation layer 29B and the subcore layers 23B (i.e. the core layer 22B) can then be considered as a so called single dielectric backing layer (DBL).

[0071] When a DBL is placed adjacent to an electrically conductive absorber (for membrane thermal emissivity) of specific thickness then conditions can arise in which radiation absorption is highly directional. Specific optical property combinations of DBLs and metal absorbers provide conditions for specific IR radiation absorption. Wien’s displacement law may be used to estimate wavelengths of maximum absorption for particular materials and the Hagen-Rubens relation may be used to calculate for what range of wavelengths those applied formulas are valid.

[0072] The further cap layer 26B refractive index substantially matches to the environment refractive index (i.e., the low pressure H2 environment) in use for wavelengths >21.4 pm, e.g. for 21.4 - 500 pm. That is, the refractive index squared n2of the cap layer is 1.7 (which is close to value of 1 for the low- pressure H2 environment). This is for directional emission. The separation layer 29B refractive index substantially matches to the sub-core layers 23B refractive index for 21.4 pm radiation. That is, the refractive index squared n2of the separation layer 29B is 10.2 and the refractive index squared n2of the sub-core layers 23B is 11.7. This is approximately n2=12 for 21.4 pm radiation. Matching (squared) refractive indexes reduces reflections that would arise at material boundaries. This is advantageous because such reflections could lower the directionality of emissions from the membrane.

[0073] Metal absorber layers (such as Ru which is used as the cap layer 24B in membrane 20B) form silicides when adjacent to Si-containing materials and when heated together. They also exhibit thermal grain boundary grooving (GBG) and dewetting at elevated temperatures depending on the free-surface energies of layer, substrate and interface. For instance, for Ru absorber layers, Mo layers may have been previously used between the Ru and the Si to prevent dewetting, as in the current DGLm version (see Figure 2A).

[0074] However, in the membrane 20B, an amorphous RU47M057 metal bi-layer is used as the intermediate layer 28B. As this material has no grains, GBG and dewetting can only occur after partial crystallization of the material, which does not occur, nor does oxidation occur, for temperatures of at least higher than 500 °C. The means that dewetting and oxidation only occurs above 500 °C. This a- RU47M057 metal bi-layer being used as the intermediate layer 28B provides good dewetting resistance and oxidation resistance and very low surface roughness, which prevents dewetting of the cap layer 24B. Conductivity and EUV transmission of metal absorbers (i.e., the cap layer 24B) is optimized by introducing bi-layers (i.e., intermediate layer 28B) and tuning the composition ratio.

[0075] A thickness combination of the cap layer 24B and the intermediate layer 28B provide f=l 3, where f is a product of thickness and conductivity.

[0076] The refractive index of the core layer 22B (i.e. the sub-core layers 23B (e.g. n2=l 1.7) and the separation layer 29B (e.g. n2=10.2)) may be set to be such that the dielectric may be considered to be a single optical layer. For membrane 20B, the further cap layer has n2=1.7. Furthermore, arranging thecap layer 24B (and the intermediate layer 28B) to have a parameter f which is tuned to the refractive index of the dielectric allows the directionality of the IR radiation re-emission with a wavelength >21.4 pm. In general, the core layer and the cap layer (and other components) may be configured such that IR radiation re-emission has a predetermined % upwards (e.g., the main direction) for a wavelength >10 pm. The 10 pm holds generally for metals in the IR, specifically for Ru the radiation re-emission occurs directionally for wavelengths of >21.4 pm and for Re for >4.4 pm. For membrane 20B, the IR radiation is directed more in the main direction (upwards - away from the substrate W in use) than in the other direction (downwards - towards the substrate in use). The further cap layer 26B may have n2= 1.7 or at least approximately equal to 1.7. The sub-core layers 23B and the separation layer 29B may have n2= 12 or at least approximately equal to 12. In general, the cap layer, the further cap layer and the separation layer (where used) may be tuned to have the same or similar n2, e.g. such that those may all be considered to be one single optical layer.

[0077] The membrane 20B may be fabricated by depositing the layers of the membrane stack onto a substrate, and then etching the underneath the membrane. This provides a membrane that is freestanding in the center area and supported by the remainder of the substrate on its rim, which is called the border.

[0078] Figure 2A shows a current DGL membrane 20A which is known. The DGLm 20A comprises a core layer 22A, a cap layer 24A and a further cap layer 26A.

[0079] The core layer 22A comprises Si having a thickness of < 25 nm for strength. The cap layer 24A comprises SiOz + ZrOz with a thickness of < 3 nm anti-outgassing and H-plasma etch-retardant bottom layer, which is manufactured by deposition of Zr that oxidizes in ambient to ZrOz and intermixes with its adjacent SiOz layer (native SiOz layer with a thickness of 1.5 nm). The further cap layer 26A comprises Ru+Mo bi-layer for low TIR <18%, with a total thickness of < 6 nm.

[0080] During lithographic apparatus operation the SiOz + ZrOz cap layer is un-stable (i.e., locally densifies) and becomes inhomogeneous, which may provide insufficient membrane protection. In another known version, a Ru / Mo bi-layer (as present on the top - as further cap layer 26A) replaces the ZrOz layer. This results in high EUV absorption and raised membrane temperature, which gives rise to high IR radiation emission towards the substrate W.

[0081] Membrane 20A with these combinations of materials and thickness provides TEUV=86% and TDUV=2%.

[0082] Membranes 20A, 20B are relatively DUV suppressive. SiN could be used in the core layer but DUV suppression for SiN (even for 100 nm SiN) may be insufficient. This is shown in Figure 3B which is a graph showing TDUV for various Si thicknesses and for 2 nm B4C on 16 nm Si for the membrane 20B, which is OK.

[0083] The membrane 20B has 20.5 nm Si with 2 nm B4C and is in spec for DUV (_— 2%). fembrane 20B is an improved DGLm concept with respect to the current version of Figure 2A.

[0084] Figure 4A depicts a membrane 40A which is similar to membrane 20B of Figure 2B but includes different materials and thicknesses. The membrane 40A comprises a core layer 42A, a cap layer 44A, a further cap layer 46A, and intermediate layer 48A and a separation layer 49A. The separation layer 49A is located between two sub-core layers 43A. The separation layer 49A and the two sub-core layers 43A may be considered to comprise the core layer 42A.

[0085] The two sub-core layers 43A comprise Si, each having a thickness of 8 nm, and the separation layer 49A comprises B4C with a thickness of 2 nm. The cap layer 44A comprises Re with a thickness of 2.2 nm and the intermediate layer 48 A comprises RU43M057 with a thickness of 0.8 nm. The further cap layer 46A comprises Y2O3 with a thickness of 1.5 nm. It will be appreciated that, in other examples, the thicknesses of the layers may be different. Example ranges may be: separation layer 49 A (B4C): 1 - 5 nm, preferably 2 nm; cap layer 44A (Re): 2 - 4 nm, preferably 2.2 nm; intermediate layer 48A (RU43M057): 0.4 - 1.5 nm, preferably 0.8 nm; further cap layer 46A (HfCF): 1 - 3 nm, preferably 1.5 nm; and sub-core layers 43 A (Si): 5 - 10 nm, preferably 8 nm (for each sub-core layer).

[0086] It has been found that membrane 40A with these combinations of materials and thickness provide TEUV=88%, TDUV=2%, and [for IR>4.4 pm] TIR=15%, Adown=74% and Aup=6%. For clarity AdownIR>4 4jim= 74% & A"piR>4.4iim = 6% means IR radiation of >4.4 pm is 74% absorbed if it travels downward, and 6% absorbed if it travels upward. After the absorption, IR of >4.4 pm is re-emitted with emissivity roughly 0.74 upwards and with emissivity roughly 0.06 downwards, which entails the radiated power is directed for -92% upwards, and -8% downwards for that situation.

[0087] These alterations to each layer (when compared with current DGLm version of Figure 2A) bring about the directional emissivity, improved core protection, and the improved strength and reliability via a GSIL, and a bottom metal absorber with anti-dewetting underlayer. Membrane 40A is a directional EUV thermal emission membrane radiator for IR of I >4.4 pm.

[0088] Again here an attempt has been made to match the refractive indices of cap and core layers, but now for I > 4.4 pm assuming a membrane is heated to up to 380 °C. If a membrane is heated by absorption to 380 °C, it cools by (directional) radiation and its temperature drops and thus the wavelengths for which maximum radiation occurs lengthens via Wien’ s displacement law. Hence it has been attempted to match refractive indices across I = 4.4 - 7 pm corresponding to a temperature drop from 380 to 140 °C, and radiation of lower temperatures have not been considered as relevant.

[0089] The further cap layer 46A has n24.4nm=3.4 and n27iim=3.2 and the core layer 42A has n24.47Urn= 12 which lowers directional emission a bit. The separation layer 49A refractive index substantially approaches the sub-core layers 43A refractive index for 4.4-7 pm radiation. That is, the refractive index squared n2of the separation layer 49 A is 10 and the refractive index squared n2of the sub-core layers 43A is 12. This is n2=10-12 for 4.4-7 pm radiation.

[0090] A thickness of the cap layer 44A provides f=5, where f is a product of thickness and conductivity.

[0091] Membrane 40A absorbs 74% of its >4.4 pm IR radiation for downward travelling light. As the ratio of down and upward absorption is 74 / 6 =~12, the ratio for the emission is the inverse (downward absorption is upward emission). So the upward emission is 12x that of the downward emission, so 92% of the re-emitted radiation of the membrane 40A is directed upwards, and 8% downwards. This needs to be calculated this way because the downward and upward absorptions do not add up to 100% because the choice for f = 5 of the metal absorber (the cap layer 44 A) is not optimal at n2= 12 of the Si DBL core layer (optimal would be f = 13). This choice was made as otherwise the TEUV would be very low at f = 13.

[0092] Through use of the Hagen-Rubens relation, it can be shown that, at least one exception absorber material (e.g., Re) allows directionality for >4.4 pm, which is possible to manufacture in thickness that allows feasible membrane fabrication with decent EUV transmission. This is particularly beneficial for DGLm and pellicle since now almost all of the thermal radiation is directionally reemitted. Other absorber exceptions are possible, and potentially materials can be tailored to have this feature.

[0093] The 10 pm restriction comes from the Hagen-Rubens relation. The equation r| = K = [8 / 2coeo]1 / 2can be interpreted in terms of electron relaxation times. The electron relaxation time comes into play for currents generated by incident electromagnetic waves upon a metal. When the relaxation time is small, the incident waves are in phase with electron density currents for higher frequencies and shorter wavelengths. For most metals this entails that the validity of the Hagen-Rubens relation holds for I > 10 pm.

[0094] The relation between electron relaxation time r and electrical conductivity a for a metal is given by: T = , where m=9.1E-31 kg, n (varies) and e=1.6E-19 C are electron mass, density and charge, respectively.

[0095] The transition angular frequency for which the Hagen-Rubens relation holds is given by a> = 1 2TCC 3T and the relation between angular frequency and wavelength is given by: A = — CO with c the speed of light in vacuum.

[0096] Thus,

[0097] This table shows for a selection of metals the relevant properties.Table: Relevant properties for transition wavelength for selected metals.

[0098] Re has two conduction electrons per atoms and it has a very low conductivity, this combination leads to a very low transition wavelength of 4.4 pm. Re has good oxidation resistance but it does absorb EUV strongly.

[0099] Based on these considerations Re has been used in combination with the anti-dewetting a- RU47M053 metal as combined absorber metal (cap layer 44A and intermediate layer 48A) for the 4.4 pm membrane 40A radiator.

[0100] Membrane 40B is similar to membrane 40A but it does not have a separation layer and membrane 40B comprises a SiN core layer 42B, so it has low DUV suppression and may not be suitable as a DGLm. However, it is an EUVm that directs thermal emission with emissivity roughly 0.68 upwards and with emissivity roughly 0.12 downwards, which entails the radiated power is directed for 85% in a single direction and 15% in the other when it is heated up to 380 °C.

[0101] The core layer 42B comprises Si iNr and has a thickness of 9 nm. In other examples, the core layer may comprise Si i+xNr. This means Si can be added to match n2of the core layer 42B to that of the further cap layer 46B placed on it. If a suitable further cap layer has n2higher than 5.6 of the SEN4 core layer 42B then the n2of the core layer can be raised by adding Si to ensure the directionality. The value of x in Si i+xNr may range from 0 to 5 to allow the SiN material to range in composition from SEN4 (stoichiometric) to SiNo.5, which is very Si-rich SiN. The cap layer 44B comprises Re with a thickness of 1.1 nm and the intermediate layer 48B comprises RU43M057 with a thickness of 0.5 nm. The further cap layer 46B comprises RuSiOr with a thickness of 1.5 nm. It will be appreciated that, in other examples, the thicknesses of the layers may be different. Example ranges may be: cap layer 44B (Re): 0.8 - 1.5 nm, preferably 1.1 nm; intermediate layer 48B (RU43M057): 0.4 - 1.5 nm, preferably 0.5 nm; further cap layer 46B (RuSiOr): 1 - 3 nm, preferably 1.5 nm; and core layer 42B (Si iNr): 8 - 15 nm, preferably 9 nm.

[0102] It has been found that membrane 40B with these combinations of materials and thickness provide TEUV=88%, TDUV=20-100%, and [for IR>4.4 pm] TIR=27%, Adown=68% and Aup=12%. For clarity Ad‘,w"iR>4.4pm = 68% & A"piR>4.4pm = 12% means IR radiation of >4.4 pm is 68% absorbed if it travels downward, and 12% absorbed if it travels upward. After the absorption, IR of >4.4 pm is reemitted for -85% upwards, and -15% downwards for that situation.

[0103] The further cap layer 46B has n 4.47pm — 5.3 and the core layer 42B has n 4.47pm — 5.6. The further cap layer 46B refractive index substantially approaches the core layer 42B refractive index for 4.4-7 pm radiation. That is, the refractive index squared n2of the further cap layer 46B is 5.3 and the refractive index squared n2of the core layer 42B is 5.6. This is n2=5.3-5.6 for 4.4-7 pm radiation.

[0104] A thickness of the cap layer 44A provides f=2.5, where f is a product of thickness and conductivity.

[0105] Membrane 40B has low DUV suppression and may not be suitable as a DGLm. Its thickness is low to keep TEUV high, which may pose a breakage risk. However, it is a directional EUV thermal emission membrane radiator for IR of I >4.4 pm.

[0106] Mo may be used instead of RU43M057 (e.g. for the intermediate layer).

[0107] MoSiz or MosSis, e.g. having 2 nm thickness, may be used as GSILs. The n2value at 4.4 micron for both is in the range of 12.3 - 20.3. It can be tuned to be on the low side so that it matches very well with the core Si. Similarly RuSiz may be used as a GSIL. It has n2= 6.3 - 12.3 so it can be tuned to be on the high side, and it also matches well with Si (n2= 12). Other materials may be used.

[0108] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0109] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0110] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A spectral filter membrane for an EUV lithographic apparatus, the spectral filter membrane comprising a core layer and a cap layer, wherein the core layer and the cap layer are configured such that undesired radiation re-emission from the membrane is directed with a predetermined % in a main direction, and wherein the undesired radiation is IR radiation with a wavelength of at least one of >4.4 pm, >10 pm, >21.4 pm, in a range of 4.4 to 500 pm, in a range of 10 to 500 pm and in a range of 21.4 to 500 pm.

2. The membrane of claim 1 , wherein material of the core layer and material and thickness of the cap layer are selected such that the undesired radiation is directed with the predetermined % in the main direction.

3. The membrane of either of claims 1 or 2, wherein the core layer and the cap layer are configured such that undesired radiation re-emission is directed more in the main direction than in another direction.

4. The membrane of any preceding claim, wherein the core layer and the cap layer are configured such that an amount of undesired radiation re-emission that is directed in the main direction is at least one of: greater than or equal to 50%, 68%, 74%, 85%, 92%, and 94%.

5. The membrane of any preceding claim, wherein the core layer has a predetermined refractive index squared n2for wavelengths of the undesired radiation and the cap layer has a predetermined parameter f, where f is a product of thickness and conductivity of the cap layer, and wherein the predetermined refractive index squared is in a range of 5.6-11.7 and the predetermined parameter f is in a range of 2-13.

6. The membrane of any preceding claim, wherein, for 21.4 pm radiation, the core layer and the cap layer respectively, have substantially: n2=11.7 and f=13.

7. The membrane of any of claims 1-5, wherein, for 4.4 pm radiation, the core layer and the cap layer respectively, have substantially at least one of: n2=12 and f=5; n2=5.6 and f=2.5.

8. The membrane of any preceding claim, wherein the core layer comprises a separation layer located between two sub-core layers.

9. The membrane of claim 8, wherein the separation layer is at least one of: a grain sintering inhibition layer (GSIL), MoSiz, MosSis, RuSiz and a B4C layer.

10. The membrane of any preceding claim, wherein the cap layer comprises at least one of: a metal, Ru, RU43M057, and Re.

11. The membrane of any preceding claim, further comprising a further cap layer located on the opposite side of the core layer from the cap layer.

12. The membrane of claim 11, wherein the further cap layer comprises at least one of: a hydrogen plasma-resistant material, a metal oxide, YzCh. YzSiOs, RU43M057, HfOz, and RuSiOr.

13. The membrane of either of claims 11 or 12, wherein the further cap layer refractive index substantially matches to the core layer refractive index for wavelengths of the undesired radiation.

14. The membrane of any of claims 11-13, wherein the further cap layer refractive index substantially matches to the environment refractive index in use for wavelengths of the undesired radiation.

15. The membrane of any preceding claim, further comprising an intermediate layer positioned between the cap layer and the core layer for separating the cap layer and the core layer.

16. The membrane of claim 15, wherein the intermediate layer comprises at least one of: a metal, RU43M057, and Mo.

17. A lithographic apparatus comprising the membrane of any preceding claim.

18. The lithographic apparatus of claim 17, wherein the lithographic apparatus is an EUV lithographic apparatus.

19. A method of fabricating a spectral filter membrane for an EUV lithographic apparatus, the method comprising providing a substrate, forming a membrane comprising a core layer and a cap layer on the substrate, then etching away at least part of the substrate; wherein the core layer and the cap layer are configured such that undesired radiation re-emission from the membrane is directed with a predetermined % in a main direction, andwherein the undesired radiation is IR radiation with a wavelength of at least one of >4.4 pm, >10 pm, >21.4 pm, in a range of 4.4 to 500 pm, in a range of 10 to 500, and in a range of 21.4 to 500 pm.

20. The method of claim 19, wherein the method comprises providing a further cap layer on the core layer such that the core layer is arranged between the cap layer and the further cap layer.

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