Coatings for extreme ultraviolet and soft x-ray optics

Abstract

To enhance transmission and reflection in an EUV / SX wavelength range.SOLUTION: Coatings for use in an extreme ultraviolet / soft X-ray spectrum / DUV from 0.1 nm to 250 nm include one or more sub-wavelength "A-layers" alternately aligned with sub-wavelength "B-layers". The A-layers may include Group 1 material, Group 2 material, and Group 18 material. The B-layers may include transition metal, lanthanide, actinide, or one of combinations thereof. The A-layers and / or the B-layers may include nanostructures with features sized or shaped similarly to predicted defects. Additional top layers may include higher-atomic-number A-layer materials, hydrophobic materials, or charged materials.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] This application claims priority to US Prov. Pat. App. Ser. No. 62 / 186,741, filed June 30, 2015, which is incorporated herein by reference in its entirety.

[0001] A related field is the design and fabrication of optical coatings, and more particularly, reflective, transmissive, or wavelength-selective coatings for wavelength ranges that are strongly absorbed by many conventional optical materials. [Background technology]

[0002]

[0002] Far ultraviolet light (EUV, 10-120 nm wavelength), soft x-ray (SX, 0.1-10 nm wavelength) and deep ultraviolet light (DUV, 120 nm-250 nm) are some of the possible approaches to lithography with resolution <22 nm, facilitating further miniaturization of integrated electronic components. Other applications include analytical chemistry (e.g., identification of chemicals by optical resonance), astronomy (e.g., mapping of nebulae, planetary and stellar atmospheres), biology (study of biological material samples), and medicine (imaging and contamination cleaning).

[0003]

[0003] Applications requiring a sharp image or a tightly focused spot with continuous wave power or pulsed energy above a threshold may utilize beam shaping optics (e.g., lenses or curved mirrors), beam patterning optics (e.g., photomasks or diffusers), beam splitting optics (e.g., beam splitters, filters, or diffraction gratings), or beam steering optics, e.g., plane mirrors or prisms, depending on the required optical path length and the size or shape of the system base plate.

[0004]

[0004] Each passive optical element along the optical path from a light source to a target, such as a workpiece or photodetector, introduces light loss due to absorption, scattering, vignetting, and other loss mechanisms. The losses cumulatively reduce the efficiency of the system (the fraction of source light that reaches the workpiece). When low efficiency reduces the light at the target below a practical threshold for an application, a more powerful or more energetic light source may be required to compensate for some of the losses.

[0005]

[0005] Losses can be a much bigger concern in the EUV / SX / DUV wavelength range. Because the atomic resonances of many elements correspond to EUV / SX wavelengths and / or because EUV photon energies exceed the band gap of all materials, virtually all materials exhibit significant absorption at those wavelengths, and the more powerful the EUV / SX / source (e.g., plasma, synchrotron) that is required to deliver above-threshold levels of light to the target, the more the EUV / SX / source will cost and the more waste heat it will dissipate that can degrade the focus or image quality in several ways. A desirable power level for lithography is approximately 200W. The limitations of the EUV / SX source are believed to be the primary factor in the sustained slower speed of EUV / SX lithography compared to immersion lithography.

[0006]

[0006] Excessive absorption of EUV / SX light from powerful sources can damage optics in the beam train. Damaged thin films absorb more light than undamaged thin films, so the damage threshold decreases as the amount of existing damage increases. That is, once damage is initiated, the damage accelerates. Ruthenium capping layers may be used to protect the optics, but the thickness may be limited to 2.5 nm or less to avoid more light loss due to absorption. These thin caps slow down the onset of ablation and other damage, but continued or repeated exposures will wear away the capping layer, leaving the underlying thin film stack unprotected.

[0007]

[0007] Some EUV / SX sources, such as plasmas, emit particles as well as light. These particles can contaminate the workpiece / wafer, optics, masks, and / or walls and other hardware in the processing chamber. Generally, a pellicle may be placed to block contaminant particles from the light path, but pellicles for EUV / SX can be difficult to fabricate because conventional pellicle materials absorb EUV / SX light.

[0008] Coatings for transmission, reflection, and filtering A typical EUV / SX includes alternating layers of boron-silicon (B-Si), tungsten-carbon (WC), and tungsten-boron-carbon (WBC). One EUV / SX thin film stack uses alternating layers of molybdenum and silicon (Mo-Si). This type of reflective coating is approximately 67% efficient at wavelengths near 13.5 nm. Absorption in silicon is often the limiting factor. The maximum number of layer pairs or periods may be limited to approximately 40 or less.

[0009]

[0009] Therefore, science and industry would benefit from rough, low absorption coatings to enhance transmission and reflection in the EUV / SX wavelength range. Summary of the Invention

[0010]

[0010] A coating for an optical substrate is designed for a particular operating wavelength λ and operating angle of incidence θ. The coating may include a first layer ("A layer") consisting essentially of one of an alkali metal, a noble gas, a halogen, an alkaline earth metal excluding beryllium, or a combination thereof. The materials and combinations may be single elements, isotopes, ions, compounds, alloys, mixtures, nanolaminates, non-stoichiometric variants, or ternary materials, or other combinations. In some embodiments, the coating material may be selected from a smaller group including alkali metals, noble gases, and combinations thereof.

[0011]

[0011] The thickness of the first layer may be less than λ. In the EUV / SX / DUV range of 0.1 nm≦λ≦250 nm, some non-classical layer thicknesses may be implemented with sub-wavelength thicknesses, as well as, or even better than, classical interference layers whose thickness is an integer multiple of λ / (4 n1cos(θ)), where λ is the operating wavelength, n1 is the real part of the complex refractive index of the first layer at wavelength λ, and θ is the angle of incidence with respect to the surface normal. Non-classical solutions can be found numerically using finite element calculations.

[0012]

[0012] The noble gas component may be included in the first layer as a noble gas compound, e.g., XeF6. If the noble gas compound is a strong oxidizer, an oxidation barrier on either or both sides of the noble gas compound may prevent the noble gas compound from oxidizing adjacent materials. In embodiments where the outer layers of the thin film stack are at risk of exposure to oxygen (e.g., when the processing chamber or the like is opened to the atmosphere to clean or replace optics or other hardware), an oxygen barrier may be selectively formed in those outer layers. Preferably, the oxidation barrier, if present, is included in the design formula so as not to impair the performance of the coating.

[0013]

[0013] Optionally, a capping layer having a higher damage threshold than the first layer may be placed between the first layer and the ambient environment. The capping material is selected from the higher atomic number members of the material set of the first layer. The capping layer may protect the first layer from particles or EUV / SX damage. In some embodiments, the capping layer is electrically charged, allowing the layer to repel or deflect incoming particles of similar charge before they can reach the optical surface and become defects. For example, plasmas based on spraying molten tin tend to emit positively charged particles. Preferably, the capping layer, if present, is included in the electromagnetic equations so as not to impair the performance of the coating.

[0014]

[0014] Optionally, a hydrophobic layer may be formed between the first or top layer and a source of liquid, such as the external environment or a hygroscopic substrate. Known hydrophobic layers, such as polymers, monolayers (self-assembled and otherwise), or nanostructured thin films, may be used. A hydrophobic layer with high surface energy prevents liquid absorption that may otherwise accelerate EUV / SX absorption and damage, for example, plasma tin droplet systems. Preferably, the hydrophobic layer, if present, is included in the design formula so as not to impair the performance of the coating. In some embodiments where the coated optical element is expected to remain in use through the ablation of one or more of the outer layers of the coating, multiple hydrophobic layers may be placed through some portion of the stack so that if one hydrophobic layer is ablated, another hydrophobic layer is readily visible.

[0015]

[0015] The second layer ("B layer") may be formed above or below the first layer such that the two layers together constitute a period or layer pair. The composition of the second layer may consist essentially of one of transition metals, lanthanides, actinides, or combinations thereof. The second layer may be a single element, isotope, ion, compound, alloy, mixture, nanolaminate, non-stoichiometric variant, or ternary material, or other combination. In some embodiments, the second layer is selected from the fifth period of groups 3-9 (Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd). Like the first layer, the thickness of the second layer is less than λ. In the EUV / SX / DUV range of 0.1 nm≦λ≦120 nm, for sub-wavelength thicknesses, some non-classical layer thicknesses may be implemented, as well as, or even better, classical interference layers, where the thickness of the second layer is an integer multiple of λ / (4 n2cos(θ)), where λ is the wavelength, n2 is the real part of the complex refractive index of the second layer at wavelength λ for the incident medium, and θ is the angle of incidence with respect to the surface normal. These solutions can be found numerically using finite element calculations. The first layer may have a lower absorption than Si or the second layer. The second layer may have a real part of its refractive index that is different from that of the surrounding environment (e.g., air, gas, vacuum) than the first layer.

[0016] In some embodiments, the second layer may be non-porous and the first layer may be porous such that the pores are filled with a low absorbing substance such as a gas, vacuum, or filler replacement of the light path through the first layer. The pores may be open to the surrounding environment or may be sealed. The open pores may allow the injected noble gas to flow through the layer. The sealed pores may contain gas trapped during the formation of the layer, for example, by bubble nucleation. The pores may be etched holes or channels, may constitute a void structure, or may be spaces within a crystal lattice. Optionally, one or more pores may be used to contain or contain the noble gas component of the composition of the first layer. The population of pores may be uniformly distributed throughout the second layer, serving to reduce the overall bulk density of the material and exhibiting a layer with an isotropic reduced density material.

[0017]

[0017] Multiple periods of the first and second layers may be stacked to further increase or decrease the reflectivity of the optical element. The low absorption of the first layer compared to conventional Si makes stacks of 40-400 layers practical as a means to enhance reflectivity or extend the lifetime of the optical element once successive layers are ablated. In some embodiments, the stack may include only periods with the same first layer and the same second layer. Alternatively, the stack may use two or more composition options for the first and second layers. For example, the outermost layer may be formulated for high damage threshold and the inner layer may be formulated for low absorption. In some embodiments, the combined thickness of the first and second layers may be less than λ. The layers may also be graded with a range of periods from top to bottom of the multi-stacked layers. In some embodiments, the order of layers A and B as the first and second layers (ABABAB) may be reversed (BABABA). Optionally, any layer in the stack may be stoichiometric or non-stoichiometric.

[0018]

[0018] Optionally, the capping layer or one or more other layers may be charged to repel charged particles coming from the plasma or other EUV / SX source. The charge may be provided by ions trapped in the layer or may be imposed by connecting the capping layer or an adjacent layer to an ungrounded electric field, for example through a contact. The capping layer may also be made of a material that has a higher atomic number than ruthenium, creating a higher interatomic repulsive potential. This reduces the ion stopping distance of incoming bombarding particles into the coating.

[0019]

[0019] The optical reflector may include at least one porous low absorption layer and one non-porous high reflection layer, each having a sub-wavelength thickness. Optionally, the sum of the thicknesses of the first and second layers is also less than the operating wavelength. Optionally, the holes in the porous layer may be spaces or voids in the nanostructure.

[0020]

[0020] Defects are a significant problem in EUV light source systems, especially when plasma sources are present. Plasma sources produce a large number of ions that embed into other components in the system, resulting in the destruction of coatings, capping layers, lenses, mirrors, filters, and photomasks. When defects are present or partially embedded in the multilayer, they impair the reflectivity of the coating. In some embodiments, the first layer, the second layer, or both may include nanostructures with features that optically hide the visibility of defects.

[0021]

[0021] A method of making an optical element may include providing a substrate and forming a first layer over the substrate. The first layer may consist essentially of one of an alkali metal, a noble gas, a halogen, an alkaline earth metal excluding beryllium, or a combination thereof. The first layer may have a subwavelength thickness for an operating wavelength between 0.1 nm and 250 nm. A second layer of subwavelength thickness may be formed above or below the first layer. The second layer may consist essentially of one of a transition metal, a lanthanide, an actinide, or a combination thereof.

[0022]

[0022] The multilayer or components thereof may be produced by a deposition process including one or more of sputtering, evaporation, thermal or e-beam evaporation, pulsed laser deposition, atomic layer deposition, molecular layer deposition, atomic layer epitaxy, ion beam deposition, e-beam deposition, electrodeposition, electroformation, chemical vapor deposition, plasma assisted deposition, physical vapor deposition, chemical vapor deposition, pulsed chemical vapor deposition, laser excitation, epitaxy, pulsed laser deposition, spin coating, droplet coating, spray deposition, pyrolysis. Smoothing of the multilayer thin film may be achieved by chemical mechanical polishing, template stripping, or AFM / SEM, electron beam or ion beam radiation, vapor annealing, atomic layer etching, nanoparticle slurry etching, or other planarization steps.

[0023]

[0023] The multilayer combination consisting of a combination of layers A and B as alternating first and second layers represents a better alternative to Mo-Si multilayers. The combination has more resistance and tolerance to defects due to the larger interatomic potential, robustness, and tensile strength. Defects are a significant problem in EUV light source systems, especially when plasma sources are present. Plasma sources generate a large number of ions that embed into other components in the system, resulting in the destruction of coatings, capping layers, lenses, mirrors, filters, and photomasks. When defects are present or partially embedded in the multilayer, they impair the reflectivity of the coating. The reflectivity tradeoff per destroyed layer can be calculated for different material combinations by simulation and experiment. The reflectivity tradeoff calculated as the reduction in peak reflectivity per destroyed layer as a percentage of the peak reflectivity is as follows:

[0024] Reflectance Tradeoff=100×(Peak Reflectance(Maximum Period)−Peak Reflectance(Maximum Period−1) / (Peak Reflectance(Maximum Period))

[0025] Here, maximum period is the maximum number of periods of alternating layers that produces the maximum peak reflectance.

[0025]

[0026] In a typical Mo-Si multilayer, the reflectance tradeoff per destroyed layer is approximately 0.4%. If a layer A layer B combination is used, the reflectance tradeoff may be less, for example 0.006%. Defects also occur in multilayer deposition processes.

[0026]

[0027] In one embodiment, the second layer containing group B is the top layer and is closest to the EUV radiation. The first layer containing group A elements.

[0027]

[0028] The multilayer may be used in combination with a hydrophobic layer such as parylene or a nanostructured hydrophobic material placed between or on the metal layer. The hydrophobic layer protects the metal layer from exposure or decomposition in air or during the fabrication process. For example, when the multilayer is used in a photomask, an absorber layer is patterned on top of the multilayer. Patterning requires a series of processing steps including deposition and etching that can introduce defects. Sometimes the mask is subjected to a cleaning process that exposes the multilayer to moisture and air. The hydrophobic material may be made from an inorganic base, such as titanium nitride or titanium dioxide, or may be a self-assembled monolayer or a passivation layer.

[0028]

[0029] The multilayer or components thereof may be produced by deposition processes including sputtering, evaporation, thermal or e-beam evaporation, pulsed laser deposition, atomic layer deposition, molecular layer deposition, atomic layer epitaxy, ion beam deposition, e-beam deposition, electrodeposition, electroforming, chemical vapor deposition, plasma assisted deposition, physical vapor deposition, chemical vapor deposition, pulsed chemical vapor deposition, laser excitation, epitaxy, pulsed laser deposition, spin coating, droplet coating, spray deposition, pyrolysis.

[0029]

[0030] A Layer A Layer B multilayer may be used with the capping layer if the capping layer is thicker than 3 nm. Typically, on EUV photomasks, the capping layer is made of ruthenium and is 2.5 nm thick since the total reflectance is substantially reduced with greater thickness. With Group A-Group B multilayers, the capping layer may be thicker than 2.5 nm and provide substantially more protection from defects.

[0030]

[0031] Smoothing of the multilayer thin film may be accomplished by chemical mechanical polishing, template stripping, or AFM / SEM, electron beam or ion beam irradiation, vapor annealing, atomic layer etching, nanoparticle slurry etching, or other planarization steps.

[0031]

[0032] The defects in the Group A-Group B multilayer may then be removed by a cleaning process, for example a mask cleaning process.

[0032]

[0033] Multilayers may be fabricated on a substrate, where the substrate is curved, convex, or concave, thus achieving a two-dimensional or three-dimensional architecture.

[0033]

[0034] In some cases, the Group A or Group B materials may differ from the standard stoichiometry.

[0034]

[0035] In another embodiment, the Group A and Group B materials may be used on two-dimensional, three-dimensional, or periodic structures. The periodic structure may be on a lens, mask, mirror, filter, substrate, or other component. The combined structure may incorporate nano-sized elements therein. The nanostructured elements may reduce the visibility of defects. The nanostructure itself may provide a topology that prevents the defect from entering, or may electromagnetically hide or mask some or all of the defect. The nanostructured elements may be combined with reflective, transmissive, or absorbing elements. The defect is typically obscured within a distance equal to the period of the periodic or nanostructure, or the integral distance of the wavelength.

[0035]

[0036] The multilayer configuration may be characterized by SEM, AFM, EUV light source, AIMS or actinic radiation, FIB, beamline, reflectometry, profilometry. In another embodiment, the material may be used in a characterization setup. The material may act as a reference in the setup or may be measured in the characterization setup. The characterization setup may measure the transmittance, reflectance, absorption, refractive index, scattering, roughness, resistivity, uniformity, bandwidth, angular range, depth of focus, electromagnetic intensity, wavelength sensitivity, amplitude, or phase of the material. The characterization setup may be an ellipsometer, reflectometer, spectrophotometer, X-ray diffraction tool (XRD), X-ray photoelectron spectroscopy (XPS), or TEM. The characterization setup may use a light source or laser source or tabletop x-ray source, detector, camera, translation stage or rotation stage with one or more degrees of freedom. The characterization setup may perform electrical measurements to determine conductance or resistance.

[0036]

[0037] The combination of materials, either the multilayers or the nanostructures, may be designed to be spectrally reflective for one range of wavelengths and spectrally transmissive for another range of wavelengths, absorptive, or reflective in different directions, for example, when used in a pellicle, the material may be configured to be transmissive in the EUV wavelength range and in the DUV wavelength range, and when used on a coating, the material may be reflective in different directions in the DUV and EUV wavelength ranges.

[0037]

[0038] The materials of Layer A and Layer B may be used in one embodiment to form part of a mask defect compensation configuration in which the absorber layer pattern is adapted to compensate for phase changes caused by defects.

[0038]

[0039] The capping or protective layer may be formed by any charged material, such as a positively charged ionic material. The charged capping layer deflects any overlying charged particles, such as defects, that may affect the structure.

[0039]

[0040] The capping layer may be formed by any material with an atomic number greater than that of ruthenium. With more reflective multilayers, the capping layer may be chosen for a greater atomic number with a greater associated ion stopping distance. This protects the underlying reflective structure. A higher atomic number means a greater stopping distance, but also increased absorption. However, the higher the reflectivity of the multilayer, the more absorbing the capping layer may be tolerated. [Brief description of the drawings]

[0040] [Figure 1]

[0041] 1A and 1B are schematic diagrams of a thin film stack. [Diagram 2]

[0042] FIG. 1 is a reproduction of a periodic table highlighting candidate materials for the disclosed thin film stacks. [Diagram 3]

[0043] Graph of numerically modeled reflectance spectrum for 12-14 nm wavelengths. [Figure 4]

[0044] Diagram showing a technique for incorporating noble gases into the solid A layer. [Diagram 5]

[0045] FIG. 1 illustrates an example of a noble gas being incorporated into an A-layer by flowing through the open nanostructure of one or more other A-layer materials. [Figure 6]

[0046] Simplified diagram of absorption in non-porous and porous absorbing media. The physics underlying these effects is much more complex for the EUV / SX features and sub-wavelength features for the depicted macroscopic primary beam optics, but the end result is at least qualitatively similar. [Figure 7]

[0047] FIG. 13 illustrates the effect of a porous layer on the penetration depth of light in a thin film stack. [Figure 8]

[0048] Illustrated ablation of an optical coating by an EUV / SX light source. [Figure 9]

[0049] FIG. 1 illustrates a thin film stack having an extra layer to mitigate the effects of ablation. [Figure 10]

[0050] FIG. 1 shows the effect of nanostructure on defect visibility. [Figure 11]

[0051] 1 is a process flow chart for forming an AB thin film stack on a substrate. Optical component fabrication may have multiple steps, not all of which are affected by the disclosed subject matter. Thus, a fabrication method may include other processes before and after the processes shown, or intermediate steps between the processes shown, and still fall within the scope of the disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041]

[0052] The following description provides some specific details of the embodiments to further the reader's understanding of the presented concepts. However, alternative embodiments of the presented concepts may be practiced without some or all of these specific details. In other instances, well-known processing operations have not been described in detail so as not to unnecessarily obscure the described concepts. While some concepts are described in connection with specific embodiments, it will be understood that these embodiments are not intended to be limiting. definition

[0053] As used herein, the following terms shall have the following meanings.

[0042]

[0054] Approximately: ±10% unless otherwise stated.

[0043]

[0055] Atoms, molecules: including isotopes and ions

[0056] Above (a layer): May be directly above that layer, or may be above that layer with an intervening structure or layer between them.

[0044]

[0057] Combinations (of chemical elements): may include, but are not limited to, elemental compounds, alloys, mixtures, microlaminates or nanolaminates, isotopes, ions, ternary materials, non-stoichiometric materials.

[0045]

[0058] Essentially: An active ingredient that has been intentionally added. Inactive ingredients or trace impurities that do not affect the function of the coating may also be present in the formulation within the scope of this disclosure.

[0046]

[0059] Including: Without limitation, including unless otherwise stated.

[0047]

[0060] EUV / SX / DUV: Any range of wavelengths from 0.1nm to 250nm.

[0048]

[0061] Layer: A layer of a thin film. It may encompass all or a portion of a substrate. It may include sublayers, gradients, interface regions, or structures. It may be applied by atomic or molecular layer deposition, chemical vapor deposition (including plasma-assisted and pulsed), dip coating, droplet coating, electroforming (e.g., electrodeposition, electroplating), epitaxy, evaporation (e.g., thermal, e-beam), laser deposition (including laser excitation of one or more precursors), particle beam deposition (e.g., electron, ion), physical vapor deposition, pyrolysis, spin coating, spray deposition sputtering, or any other known method appropriate for the layer material and substrate.

[0049]

[0062] Nanostructure, Nanoscale: Having a size or feature size between about 1 nm and 150 nm.

[0050]

[0063] Substrate: A solid object that has been or will be coated with the disclosed EUV / SX interference coating. A "substrate" need not be completely bare, but may include previously formed layers or structures.

[0051]

[0064] Workpiece: An object that is coated or otherwise processed with EUV / SX radiation transmitted or reflected by the disclosed EUV / SX coatings on one or more optical elements, e.g., a wafer. For example, it may be a generalized substrate or superstrate, but need not be the "substrate" of an EUV / SX optical element per se.

[0052]

[0065] 1A and 1B show schematic diagrams of a thin film stack of multiple A / B layer periods.

[0053]

[0066] The substrate 101 may be flat as shown, or non-flat (e.g., curved, microstructured, or nanostructured). The thin film stack includes a first A layer 102.1, a first B layer 104.1, a second A layer 102.2, a second B layer 104.2, a top (Nth) A layer 102.N, a top (Nth) B layer 104.N, and (not shown) third through (N-1) A and B layers between the B layer 104.2 and the A layer 102.N. N may be 4 to 100, depending on the application. The A layers essentially include at least one of an alkali metal, a noble gas, a halogen, or an alkaline earth metal having an atomic number greater than beryllium. The B layers essentially include at least one of a transition metal, a lanthanide, or an actinide. The interface 103 between the A and B layers may include other materials, for example a moisture or oxygen barrier. Additional layers or structures may be formed above or below the stack.

[0054]

[0067] The A layers may or may not all have the same composition or thickness. Similarly, the B layers may or may not all have the same composition or thickness. Transmissive optics for the EUV / SX spectrum have traditionally been very difficult to fabricate because all materials absorb at these wavelengths. The goal may be advanced by using these AB coatings, which can be more transmissive than historical coating materials, on moderately non-absorbing substrates such as thin pellicles.

[0055]

[0068] Typically, A layers are chosen for low absorption and B layers for high reflectivity. Classical interference coating dimensions do not necessarily perform best in EUV / SX, where reflection is dominated by interface scattering. Numerical finite element analysis using Maxwell's equations can more reliably yield the optimal set of materials and dimensions.

[0056]

[0069] FIG. 1B shows a schematic of a thin film stack of multiple B / A layer periods. The substrate 201 may include layers or structures below the layers or structures shown, with a B layer 204.1 closest to the substrate rather than the A layer 202.1 of FIG. 1A. The B / A pattern is repeated with a second B layer 204.2, a second A layer 202.2, and any number of additional periods up to a total number N (e.g., 10-400), with an Nth A layer 202.N on top and an Nth B layer 204.N immediately below. The stack may have either a B layer or an A layer on top, and the number of layers does not necessarily have to be an even number.

[0057]

[0070] FIG. 2 is a reproduction of the periodic table highlighting candidate materials for the disclosed thin film stack. The A layer materials occupy areas 210 and 220 outlined by a black background: Group 1, i.e., alkali metals; Group 2, i.e., alkaline earth metals (except beryllium); Group 7, i.e., halogens; and Group 8, i.e., noble gases. The A layer may include only one of these materials or may include combinations of these materials. These elements and combinations thereof are poor absorbers in the EUV / SX spectrum because their outer electron shells are filled (noble gases), mostly filled (halogens), or mostly empty (alkali and alkaline earth metals). At 13.5 nm, the least absorbing may be the Group 1 and Group 18 elements, and the most reflective may be the 5th period of Groups 3-9 (Y, Zr, Nb, Mo, Tc, Ru, Rh).

[0058]

[0071] In general, the higher atomic numbers in these groups are least likely to absorb EUV / SX because their outer shell electrons are shielded and therefore less tightly bound than the inner shell electrons, and more easily bind. Exceptions are noted. For example, krypton and xenon form more compounds more readily than helium or neon, but in this document stable radon compounds may not form. However, it may be possible to capture or inject radon as a non-bonded atom in a structure made from one or more elements of other groups. The B layer materials are in area 230 with hatched background, i.e., transition metals, lanthanides, and actinides from groups 3 to 12.

[0059]

[0072] FIG. 3 is a graph of the numerically modeled reflectance spectrum for wavelengths from 12 to 14 nm.

[0060]

[0073] Curve 310 results from a finite element electromagnetic model of a conventional Mo-Si thin film stack and shows a peak at about 67%, which is in reasonable agreement with the reported measurements. The peak is higher at about 80%, narrower at about 5 nm, and may have some low amplitude ringing 24, but no sidebands.

[0061]

[0074] For use in the A layer, compounds that are gaseous in this temperature range can sometimes be incorporated in the same manner as unbound gas atoms, but noble gas compounds are preferably solid and stable at typical ambient processing temperatures, since the A layer is intended to provide a low EUV / SX absorbing segment of the optical path. Halides and hydrates have low absorption.

[0062]

[0075] As shown in FIG. 4A, potentially usable xenon compounds 407 include fluorides XeF2, XeF4, XeF6, hydrates (e.g., hydrates made by compressing Xe in water), as well as other halides and complex ions. FIG. 4B shows a substrate 401 with an A-layer 412 above the substrate (some very simple embodiments may use a single layer of A-layer material and no B-layer) and an oxygen barrier 413 between the A-layer and the substrate. Some noble gas compounds, such as XeF6, are strong oxidizers that can attack even oxide-glass substrates. Additionally or alternatively, if the noble gas compound layer is exposed to ambient air (including but not limited to during manufacture, storage, installation, any type of use, cleaning, or repair), another oxygen source may be used. In some embodiments, the oxygen barrier 413 may be placed above the A-layer, below it, or both.

[0063]

[0076] FIG. 4C illustrates, without limitation, a clathrate or cage compound that includes free noble gas atoms 413 trapped in a crystal lattice 417. The noble gas atoms in the cage compound are not precisely bonded, but are quasi-mechanically trapped within the structural intervals. Some lattices have been observed to trap Xe, Kr, and Ar, but Ne and He are often small enough to escape. FIG. 4D illustrates a carbon fullerene cage compound with noble gas atoms 413 trapped within a fullerene shell 427. 60 Fullerenes are known to trap, for example, He, Ne, Ar, Kr, and Xe, however, an ideal fullerene for use as the A layer would have a low density of carbon atoms to limit EUV / SX absorption.

[0064]

[0077] 5 shows an example of a noble gas incorporated into an A layer by flowing through an open nanostructure of one or more other A layer materials. Nanopillars 531 are organized in an array 537 with interstitial openings. The noble gas may be passively in the nanostructure openings as a result of immersion or may be actively driven through and into the openings by a gas flow system. The nanostructures may be open on top as shown or may have a smooth cover layer on top similar to base layer 536 shown herein below.

[0065]

[0078] A simplified diagram of absorption in non-porous and porous absorbing media is shown in Figure 6. The physics underlying these effects is much more complex for the EUV / SX features and sub-wavelength features for the depicted macroscopic primary beam optics, but the end result is at least qualitatively similar.

[0066]

[0079] Plane-parallel windows 602 and 612 are made from the same bulk material (e.g., silicon or A-layer material) with absorption coefficient α1. Both are immersed in the same surrounding medium (e.g., vacuum or air) with absorption coefficient α0. Window 602 is solid, while window 612 has holes 611 filled with the α0 medium.

[0067]

[0080] Ideal light pencils or rays 603.1 and 603.2 have an initial intensity I0 at their respective x=0 locations at α0. By the Beer-Lambert law, the intensity at any x is: When light travels through media with different absorption coefficients α, its intensity always decreases exponentially, but the parameters of the exponential curve change as the rays enter and exit different media.

[0068]

[0081] Curve 610 represents the intensity of ray 603.1. Initially, ray 603.1 decreases linearly. When ray 603.1 enters window 612 at X1, the coefficient changes and from X1 to Xmax, the intensity decreases linearly with max In I min,1 decreases proportionally until it reaches

[0069]

[0082] Curve 620 represents the intensity of ray 603.2. Initially, ray 603.2 decreases linearly. When ray 603.2 enters window 612 at X1, the coefficient initially changes and the intensity decreases linearly as it travels through the solid bulk material. However, as ray 603.2 traverses hole 611, the intensity decreases linearly, offsetting the curve twice and decreasing at X1. max In that I min,2 I min,1 A hole filled with any less absorbing material (not necessarily the surrounding medium) has a similar effect, reducing the absorption depending on the thickness of the window (or thin film layer).

[0070]

[0083] 7A-7B show the effect of a porous layer on the penetration depth of light within a thin film stack.

[0071]

[0084] When dozens of layers in a reflective stack all absorb the incident light, some of the bottom layers never receive any light of sufficient intensity to measurably contribute to the reflection. The higher the absorption coefficient, the shorter the distance the light penetrates into the stack.

[0072]

[0085] The stack in Figure 7A has non-porous B layers 704.1-704.3 alternating with non-porous "non-B" layers 702.1-702.3 (which may or may not be made from the disclosed A layer materials). In low intensity EUV / SX applications where thin film stack damage is slow to insignificant, layers 704.1, 702.1, and 704.2 are not used.

[0073]

[0086] In Figure 7B, non-porous B layers 704.1-704.3 are identical to the non-porous B layers of Figure 7A. "Non-B" layers 712-1-712.3 are made from the same bulk material as layers 702.1-702.3 of Figure 7A, but are porous rather than solid. The addition of holes allows incident light to penetrate two layers further into 712.1 than in the stack of Figure 712A.

[0074]

[0087] In sub-wavelength EUV / SX thin film stacks, reflections may be treated to arise from interface scattering: by having more interfaces contribute to the reflection, the effect of defects on any one interface may be reduced.

[0075]

[0088] 8A-8B show the ablation or erosion of an optical coating by an EUV / SX light source.

[0076]

[0089] FIG. 8A shows an undamaged coating on a "new" optical element placed in a processing system. The substrate 101 is the base optical element, not a processed workpiece (see definition: substrate, workpiece). In some embodiments, the substrate 101 may include layers or structures below the layers or structures shown. Above the substrate 101 is a thin film stack of 2N layers with sub-wavelength layer thicknesses. A layers 802.1 (bottom) through 802.(N-1) (second from the top) and 802.N (top A layers) alternate with B layers 804.1 (bottom) through 804.(N-1) (second from the top) and 804.N (top B layers). In some embodiments, the A layers are made from at least one material from Groups 1, 18, 17, or 2, Periods 3 through 7 on the periodic table. In some embodiments, the B layers are made from at least one material from Groups 3 through 12 on the periodic table. In some embodiments, one or more of the A layers may be porous. As shown, the A layers are at the bottom of the stack and the B layers are at the top, although the order of the layers may be reversed and still fall within the scope of the disclosure.

[0077]

[0090] EUV / SX radiation 803 from an EUV / SX source is on top layer 804.N. The EUV / SX source may include synchrotron radiation or plasma resulting from a spray of molten metal such as tin (Sn), for example. Particles 805 (by-products of the EUV / SX source) may also be present. In long wavelength systems, one or more pellicles (very thin beam splitters) may intercept the particles before they reach other optics, but the high EUV / SX extinction coefficients of conventional pellicle materials have prevented their use in this spectrum.

[0078]

[0091] Either or both types of source power ablates the A or B layers, causing an ablation plume 807 to separate from the upper stack layer 804.N. Defects 809 (inclusions, voids, lattice distortions, etc.) may be present in the A and / or B layers. The defects 809 may be caused by exposure to radiation and particles from the EUV / SX source, or may be created earlier by fabrication or maintenance processes such as etching, deposition, cleaning, etc.

[0079]

[0092] 8B shows the worn, partially ablated thin film stack after sustained exposure to radiation and particles from an EUV / SX source such as a plasma. As shown, 804.(N-1), the originally second-to-top B layer, is made visible and is now the top layer. Further exposure to EUV / SX radiation 803 and particles 805 produced by the source as by-products 805 converts more of layer 804.(N-1) into ablation plumes 807.

[0080]

[0093] Some coating stacks within the disclosure include extra layers to extend the useful life of the optical element: if some of the upper layers are ablated, the optical element will still function.

[0081]

[0094] 9A-9D show a thin film stack with extra layers to mitigate the effects of ablation.

[0082]

[0095] FIG. 9A shows a thin film stack with a capping layer. The capping layer 906 may be formed on the Nth A layer 902.N or the Nth B layer 904.N, whichever is uppermost. Unlike the commonly used rough but somewhat absorbing ruthenium or carbon capping layers, which may be limited to a thickness of 2.5 nm or less to suppress EUV / SX absorption, the capping layer 906 may be made thicker than 2.5 nm to have lower absorption and therefore protect the underlying thin film stack for a longer period of time. The lower absorption is achieved by making the capping layer 106 from large atomic or molecular A layer materials, including but not limited to one or more of K, Na, Rb, Cs, Kr, Xe, Sr, or combinations. In general, A layer materials with higher atomic numbers resist damage due to higher interatomic potential and / or tensile strength.

[0083]

[0096] FIG. 9B illustrates a thin film stack with a charged capping layer that repels or deflects incident particles of similar charge. For example, most particles emitted by a molten tin spray plasma are positively charged, showing that a charged capping layer 916 with sufficient positive potential can prevent these particles from reaching the thin film stack and creating defects. As shown, the Nth A layer 902.N or the Nth B layer 904.N (whichever is uppermost). The charged capping layer 916 may be charged by being fabricated of an ion-containing material, a non-stoichiometric material, or by connecting an ungrounded electrical contact in situ, over an underlying layer that is ionic or non-stoichiometric. As the charged particles 915 exit the EUV.SX source, an electrostatic field 917 from the charged top layer 916 repels or deflects the charged particles 915 before they reach the underlying thin film stack, potentially damaging it.

[0084]

[0097] 9C shows a thin film stack with a hydrophobic layer on either the Nth A layer 902.N or the Nth B layer 904.N, whichever is uppermost. Tin droplets from a tin plasma source 919 incident on an optical component or photomask may be effectively prevented from damaging the multilayer coating by the hydrophobic layer, which modifies the contact angle of the droplet and the surface energy on the coating, allowing it to be easily cleaned.

[0085]

[0098] As shown, the hydrophobic top layer 926.1 prevents absorbed tin 929 from being absorbed by the A and B layers. Possible suitable types of hydrophobic top layer 926.1 include parylene, silane, a hydrocarbon monolayer, an oxide or nitride on the B layer (e.g., TiN or TiO2 on a Ti B layer), a passivation material, a self-assembled monolayer. Alternatively, the hydrophobic quality may be added by nanostructuring rather than by a specific material that is not already part of the stack. Nanostructuring approaches offer the potential added benefit of reducing the visibility of defects 909 (see FIG. 11).

[0086]

[0099] Figure 9D shows multiple hydrophobic layers to maintain protection against moisture as the successive AB layers are ablated. The stack in Figure 9D initially resembles the stack in Figure 9C, but over time, the top hydrophobic coating 926.1 and the immediately underlying B layer 904.N have been ablated by radiation 903 and particles 905. Subsequent ablation, however, reveals the middle hydrophobic coating 926.2, which now protects the new top layer, A layer 902.N.

[0087]

[0100] 10A-10B show the effect of nanostructure on defect visibility.

[0088]

[0101] 10A shows a smooth layer with nanoscale defects. Layer 1001 has a smooth surface 1002 and defects 1003-1006. Line defect 1003, hole defect 1004, granular defect 1005, and particle defect 1006 are all very noticeable on the smooth surface 1002.

[0089]

[0102] Figure 10B shows a nanostructured layer with the same defects: layer 1011 is patterned with raised nanostructures 1012. Line defects 1003, hole defects 1004, and grain defects 1005 are significantly less visible since their reflectivity degradation does not contribute much.

[0090]

[0103] The nanostructure itself may provide a topology that prevents defects from entering, or may electromagnetically hide or mask some or all of the defects. Nanostructured elements may be combined with reflective, transmissive, or absorptive elements. Defects are typically obscured within a distance equal to the period of the periodic structure or nanostructure, or the integral distance of the wavelength.

[0091]

[0104] 11 is a process flow chart for forming an AB thin film stack on a substrate. Optical component fabrication may have multiple steps, not all of which are affected by the disclosed subject matter. Thus, a fabrication method may include other processes before and after the processes shown, or intermediate steps between the processes shown, and still fall within the scope of the disclosure.

[0092]

[0105] Substrate preparation operations 1101 may include cleaning, passivation, formation of underlying layers or structures, or any other requirements for forming an AB stack.

[0093]

[0106] Form layer 1 operation 1102 may result in either an A layer or a B layer, depending on which is intended to be the bottom layer. Any suitable known technique for forming sub-wavelength thick layers from selected A or B layer materials may be used.

[0094]

[0107] Optionally, the just-formed layer may be smoothed or planarized in operation 1107. Optionally, nanostructures may be formed in operation 1109. Optionally, the layer may be cleaned in operation 1111. Optionally, the new layer may be covered with an intermediate hydrophobic layer in operation 1113.

[0095]

[0108] In act 1104, the next layer is formed, ie, layer B if act 1102 forms layer A, or layer B if act 1102 forms layer A.

[0096]

[0109] Optionally, the just-formed layer may be smoothed or planarized in operation 1107. Optionally, nanostructures may be formed in operation 1109. Optionally, the layer may be cleaned in operation 1111. Optionally, the new layer may be covered with an intermediate hydrophobic layer in operation 1113.

[0097]

[0110] In decision 1110, if all of the intended layers in the stack have not yet been formed, return to operation 1102 to form another layer pair. If all of the intended layers in the stack have been formed.

[0098]

[0111] Optionally, operation 1115 may form a capping layer of a large atomic element or combination from Groups 1 and / or 18 on the periodic table. Optionally, operation 1117 may form an ionic or non-stoichiometric capping layer that may hold a charge to repel or deflect similarly charged particles. In some embodiments, operation 1115 and operation 1117 may be combined to form a charged capping layer of a large atomic Group 1 / Group 18 element or combination.

[0099]

[0112] Optionally, operation 1119 may form a top hydrophobic layer. In some embodiments, operation 1119 may precede operation 1115 and / or operation 1117.

[0100]

[0113] At decision 1120, if the product being created does not require a top absorber layer, proceed to characterization operation 1199. If the product being created requires a top absorber layer (e.g., a photomask, reticle, or similar element), proceed to absorber material layer formation operation 1122, followed by absorber material pattern operation 1124. In some embodiments, the absorber layer may be patterned as it is being formed, such that operations 1122 and 1124 are concurrent. Once the patterned absorber layer is in place, proceed to characterization operation 1199. Industrial Applicability

[0114] The A / B subwavelength coatings disclosed herein may be useful for a variety of EUV / XS optical applications, including, but not limited to, high resolution photolithography, analytical chemistry, such as identifying chemicals by resonance, astronomy, such as mapping planets, nebulae, and stellar atmospheres that emit EUV / XS, biology, such as studying and / or imaging biological material samples, or medicine, such as imaging and cleaning contaminants.

[0101]

[0115] The preceding description and the accompanying drawings set forth exemplary embodiments in some detail to aid in understanding, however the claims may encompass equivalents, permutations, and combinations not expressly set forth herein.

[0102]

[0116] Various processing applications, e.g., semiconductor, integrated optics, and other miniature component fabrication, may use the disclosed thin films and thin film stacks on any reflective (or, if available, transmissive) optics that steer light from a light source or image a photomask or other pattern source. For example, a processing chamber may include a workpiece holder that positions a wafer or other type of workpiece, and a port that lets light into the chamber from the light source or a remote source (e.g., remote plasma). A collector may be positioned to capture and redirect a portion of the source output light that would otherwise travel in an unusable direction along a first optical path from the light source to the photomask. In some embodiments, the collector may collimate or focus that output beam. Other optical components may be positioned in the first optical path to steer or reshape the beam. For example, a beam scrambler or diffuser may spatially split or scatter a portion of the light so that the intensity profile across the photomask is flatter than it would otherwise be. A beam splitter or grating may redirect undesired wavelengths so that they do not blur the image on the workpiece.

[0103]

[0117] Many EUV / SX process systems use a reflective photomask with absorbing areas to provide contrast to the pattern. One or more mirrors (or alternatively refractive or diffractive lenses) may be positioned in a second optical path from the photomask to the workpiece to provide an image of the photomask on the workpiece.

[0104]

[0118] Any of the reflective, transmissive, wavelength-selective, diffractive, scattering, or waveguiding optical components within such systems may potentially include the disclosed thin films and / or thin film stacks.

[0105]

[0119] Although the above detailed description has illustrated, described, and pointed out novel features applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the illustrated devices or algorithms may be made without departing from the spirit of the present disclosure. Thus, nothing in the above description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or essential. As will be recognized, some features can be used or practiced separately from other features, and therefore the processes described herein can be embodied in ways that do not provide all of the features and advantages described herein. The scope of protection is defined by the appended claims, and not by the above description.

Claims

1. An optical element having an operating wavelength λ, A substrate; a first layer on the substrate; wherein the thickness of the first layer is less than the wavelength λ; wherein the first layer consists essentially of an alkali metal, a noble gas, a halogen, a non-beryllium alkaline earth metal, or a combination thereof; wherein the first layer has a lower absorption at λ than a non-porous stoichiometric silicon layer of equal thickness; Wherein, 0.1 nm≦λ≦250 nm. An optical element comprising:

2. The optical element of claim 1 , further comprising an oxygen barrier above or below the first layer.

3. The optical element of claim 1 , further comprising a hydrophobic layer above the first layer.

4. The optical element of claim 3 , wherein the hydrophobic layer comprises nanostructures.

5. A second layer above or below the first layer Further equipped with wherein the thickness of the second layer is less than the wavelength λ; wherein the second layer consists essentially of one of a transition metal, a lanthanide, an actinide, or a combination thereof; Wherein, 0.1 nm≦λ≦250 nm. The optical element according to claim 1 .

6. 6. The optical element of claim 5, further comprising a stack of 41 to 400 additional layers having the optical properties of the first layer alternating with additional layers having the optical properties of the second layer.

7. The optical element of claim 5 , wherein at least one of the first layer or the second layer comprises nanostructures that reduce the visibility of defects.

8. A substrate; a first layer of optical material formed above the substrate and compatible with wavelengths between 0.1 nm and 250 nm; a capping layer formed above the first layer; Equipped with wherein the capping layer consists essentially of an alkali metal, a noble gas, a halogen, a non-beryllium alkaline earth metal, or a combination thereof. product.

9. 9. The article of claim 8, wherein the capping layer has an atomic number greater than the atomic number of ruthenium.

10. 9. The article of manufacture of claim 8, wherein the capping layer is charged to the same polarity as particles present in the operating environment.

11. The article of claim 10 , wherein the capping layer comprises ions.

12. The article of manufacture of claim 10 , wherein the capping layer is electrically coupled to an ungrounded voltage source.

13. The article of claim 8 further comprising a hydrophobic layer above the capping layer.

14. A substrate; a first layer above the substrate; a second layer above the substrate and above or below the first layer; wherein the first layer is porous; wherein the first layer has a lower absorption coefficient at an operating wavelength λ than the second layer; wherein the second layer is non-porous; wherein the first layer has a thickness less than λ; wherein the thickness of the second layer is less than λ; An optical reflector comprising:

15. 15. The optical reflector of claim 14, wherein the first layer comprises a 2D or 3D nanostructure that includes spaces that make the first layer porous.

16. Preparing a substrate; forming a first layer above the substrate; wherein the first layer consists essentially of one of an alkali metal, a noble gas, a halogen, an alkaline earth metal excluding beryllium, or a combination thereof; wherein the thickness of the first layer is less than an operating wavelength λ; Wherein, 0.1 nm≦λ≦250 nm. A method for providing the above.

17. forming a second layer above or below the first layer; Further equipped with wherein the second layer consists essentially of one of a transition metal, a lanthanide, an actinide, or a combination thereof; wherein the thickness of the second layer is less than an operating wavelength λ; Wherein, 0.1 nm≦λ≦250 nm.

17. The method of claim 16.

18. 17. The method of claim 16, wherein the first layer is formed by a technique comprising at least one of sputtering, evaporation, wide angle deposition, rotary sputtering deposition, pulsed laser deposition, atomic layer deposition, pulsed CVD, chemical vapor deposition, molecular layer deposition, atomic layer epitaxy, ion beam deposition, e-beam deposition, electrodeposition, electroformation, chemical vapor deposition, plasma assisted chemical vapor deposition, evaporation, laser excitation, or epitaxy.

19. A processing chamber; a workpiece holder within the processing chamber; a light source that emits a first portion of light from the light source into the processing chamber; a photomask positioned within the processing chamber for patterning the light that illuminates a workpiece in the workpiece holder; a collector that redirects a second portion of the light from the light source along a first optical path from the light source to the photomask; Equipped with wherein the light from the light source has a wavelength between 0.1 nm and 250 nm; wherein at least one of the collector, the photomask, or another optical element that intercepts light from the light source comprises a layer consisting essentially of one of an alkali metal, a noble gas, a halogen, an alkaline earth metal excluding beryllium, or a combination thereof. system.

20. 20. The system of claim 19, further comprising a reflective, transmissive, diffractive, or scattering optical element in the first optical path from the light source to the photomask or in a second optical path between the photomask and the workpiece.

Images