Materials, architectures, and methods for use with extreme ultraviolet radiation in lithography and other components

Abstract

New classes of materials and associated components for use in devices and systems operating at ultraviolet (UV), extreme ultraviolet (EUV), and / or soft X-ray wavelengths are described. This invention relates to increasing the the Fresnel number of EUV reflective coating. Such a material design, its architecture and its combinations may be used to make components such as mirrors, lenses or other optics, panels, light sources, photomasks, photoresists, or other components for use in applications such as lithography, wafer patterning, astronomical and space applications, biomedical applications, or other applications.

Description

MATERIALS, ARCHITECTURES, AND METHODS FOR USE WITH EXTREME ULTRAVIOLET RADIATION IN LITHOGRAPHY AND OTHER COMPONENTSFIELD OF THE INVENTION

[0001] The field of the invention is the design and fabrication of optical materials, EUV materials, and more particularly reflective materials for wavelength ranges strongly absorbed by many traditional optical materials.BACKGROUND

[0002] Optical lithography systems are commonly used for fabrication, for example, devices. The resolving power of such systems is proportional to the exposure wavelength. Thus, shorter wavelengths can improve resolution in fabrication. Extreme ultraviolet lithography (EUVL) uses electromagnetic radiation at extreme ultraviolet (EUV) wavelengths (approximately 120 nanometers to 0.1 nanometers). Accordingly, photons at these wavelengths have energies in the range of approximately 10 electron volts (eV) to 12.4 keV (corresponding to 124 nm and 0.1 nm, respectively).

[0003] Extreme ultraviolet wavelengths may be generated artificially by devices such as plasma and synchrotron light sources. Using EUV wavelengths for lithography has potential advantages of reducing feature sizes in devices such as semiconductor chips as well as in other applications such as polymer electronics, solar cells, biotech, medical technologies and space technologies.

[0004] In Lithography a system of mirrors, lenses, filters, films, masks and detectors are used for metrology, transmission, reflection and other components. In a single system the components have a given spectral and angular bandwidth. The radiation transmitted through the system or reflected from component to component is called the in-band radiation.

[0005] The selection of reflective materials, or coatings used in elements of lithography are often severely limited. Traditional material combinations consist of a Molybdenum-Silicon (Mo-Si) multilayers which produce up to 72% theoretical reflectivity. Each coating has a reflective spectral curve, and a reflective angular curve. A reflective spectralcurve specifies reflectivity as a function of the radiation’s wavelength. A reflective angular curve specifies reflectivity as a function of the radiation’s angle of incidence. The FWHM bandwidth is the spectral range when the reflectivity is 50% of its value. The Mo-Si layer is used on the mirrors, EUV collector and photomasks in EUV lithography systems. Other traditional multilayer combinations include Mo-Si and Boron Carbide, collectively referred to as the state of the art.

[0006] Since maximizing peak reflectivity is the primary concern, materials are often selected in combination to produce the highest absolute spectral reflectance. However, if maximizing the peak reflectivity is combined with a narrow spectral bandwidth, then transmitted radiation known as the in-band radiation in a consecutive sequence of components is limited by matching the peak spectral wavelength from component to component, and the bandwidth of the reflected coating.

[0007] The energy transmitted to the wafer in a lithography system is also limited by the bandwidth of the reflective coating, its total reflective spectral range and energy transmitted by each component. The out of band radiation is absorbed by the component and results in adverse heating of the component.SUMMARY OF THE INVENTION

[0008] This disclosure relates generally to materials, devices, apparatus, and methods for use with ultraviolet (UV), deep ultraviolet (DUV), extreme ultraviolet (EUV) and soft X-ray radiation, such as in lithography (EUVL) or other applications. More specifically, but not exclusively, the disclosure relates to materials and components for use in UV, EUV and soft X-ray applications, as well as methods of fabrication and use of such materials and components in apparatus, devices, and systems using EUV radiation.

[0009] In certain embodiments, the disclosure relates to an element that can be used in a light exposure system, wherein the system or subsystem includes a light source to transmit light having a wavelength of approximately 13.5 nm

[0010] In another embodiment, the disclosure relates to an element that can be used in a light exposure system containing a photomask, mirror or lens, substrate element, or optical surface, or a lithography system. The system or subsystem can include a light source to transmit light having a wavelength. The element can include a material having plurality of structuralfeatures or one or more material combinations. The system can include a wafer to be patterned by exposure to light reflected from an EUV photomask.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIGURE 1. The reflectivity response as a function of (a) wavelength and (b) angle of a multilayer Mo-Si. The FWHM spectral bandwidth is 0.63 nm. Normal incidence is 90 degrees. 70 degrees represents a difference of 20 degrees from normal incidence.

[0012] FIGURE 2 illustrates the underlying design principles of (Mo-Si) left, where absorption (k) is minimized and maximized fresnel architecture (right), where the fresnel number and reflectivity per layer is maximized.

[0013] FIGURE 3 shows a table exemplifying typical Fresnel numbers of multilayer architectures which conform to the new design principle of maximizing reflectivity per layer, compared to the state of the Art Mo-Si

[0014] FIGURE 4 Reflectivity of the architecture combination F as described in the table in Figure 3. The number of bilayer pairs have been reduced to 29 and the maximized fresnel number produces higher peak reflectivity (left) at 13.5 nm wavelength and angular bandwidth.

[0015] FIGURE 5 A schematic illustrating Zeff, (left) the depth of effective plane from which the effective reflective wave emanates. It is desirable to minimize Zeff. For Mo-Si Z off is 50. 68 nm but for high fresnel number coatings, masks or architectures, Zeff may be as low as 19.7 nm. The graph on the right shows the correlation between Zeff and NILS. The lower the Zeff value, the higher the NILS value. NILS is defined as normalized image log slope. NILS represents the patterning contrast on the photoresist on the wafer, highlighting dark and bright regions forming the semiconductor integrated circuit pattern.DETAILED DESCRIPTION

[0016] This invention relates to architectures for EUV multilayer coatings, design principles, and efficiency of reflectivity within the depth of the multilayer architecture. The coating may be used in optics, mirrors, photomasks, reticles or other surfaces.

[0017] A typical EUV Bragg mirror a consists of a stack of bilayer pairs, each pair containing two alternating materials, each of approximately quarter wavelength thickness. Each material of each layer has an associated refractive index, consisting of a real part (n) and an imaginary part (k), also representing the absorption. In a typical state of the art EUV mirror, materials for the layers are selected to minimize the k value, and maximize the number of bilayer pairs. In this way when light is incident on the surface of the EUV Bragg mirror, some proportion is transmitted and some proportion is reflected. When the transmitted light reaches the next interface, of a bilayer pair, it is equally transmitted and reflected. This happens over multiple surfaces until the light is eventually reflected. (Figure 5) The more the number of interfaces the more the total reflection. When light is transmitted at each interface some proportion is absorbed. Therefore the total amount of light reflected is never truly equal to the incident light. In the state of the art mirror, designers try to maximize the number of interfaces for reflection and minimize the absorption through each interface. This is the design principle of a state of the art mirror such a Molybdenum-Silicon (Mo-Si) which uses 40 bilayer pairs of alternating molybdenum silicon materials, each of approximately quarter wavelength thickness to form an EUV reflector of light at 13.5 nm. This primary design principle has been used to design EUV mirrors for the last 40 years, since 1974.

[0018] The invention described herein is better understood with reference to the Figures. Figure 1 shows the reflectivity of Mo-Si multilayer architecture, with Refl as a function of Wavelength and angle.

[0019] Figure 2 shows the underlying design principles of (Mo-Si) left, where absorption (k) is minimized and the number of bilayer pairs are maximized, and the maximized fresnel architecture (right), where the fresnel number (Fp)and reflectivity per layer is maximized, and the number of bilayer pairs are minimized.

[0020] Figure 3 is a table exemplifying typical Fresnel numbers of multilayer architectures which conform to the new design principle of maximizing reflectivity per layer, compared to the state of the Art Mo- Si.

[0021] Figure 4 Shows the reflectivity of the architecture combination F as described in the table in Figure 3. The number of bilayer pairs have been reduced to 29 and the maximized fresnel number produces higher peak reflectivity (left) at 13.5 nm wavelength and angular bandwidth.

[0022] Figure 5 shows schematic illustrating Zeff, (left) the depth of effective plane from which the effective reflective wave emanates. It is desirable to minimize Zeff. For Mo-Si Z off is 50. 68 nm but for high fresnel number coatings, masks or architectures, Zeff may be as low as 19.7 nm. The graph on the right shows the correlation between Zeff and NILS. The lower the Zeff value, the higher the NILS value. NILS is defined as normalized image log slope. NILS represents the patterning contrast on the photoresist on the wafer, highlighting dark and bright regions forming the semiconductor integrated circuit pattern.

[0023] This invention specifically relates to a different design principle and architecture for an EUV mirror. This new design principle has not been used before as the primary design principle in the EUV, soft X-ray wavelength range, as absorption was always considered to dominate. The new design principle implemented through the architecture of a multilayer mirror e.g. a stack of bilayer pairs, does not seek to minimize the absorption, nor the k value, nor increase the number of bilayer pairs. It seeks to maximize the reflectivity efficiency of each interface and reduce the number of bilayer pairs. To maximize the reflectivity efficiency of each interface, this embodiment seeks to maximize the Fresnel number defined aswhere and n2 are the real refractive index parts of materials 1 and 2 and ki and k2 are the imaginary refractive index parts of materials 1 and 2 forming each consecutive bilayer pair. If the Fresnel number has a numerical value greater than 0.00155 it will outperform the state of the art mirror of Mo-Si. Specifically the embodiment and design principle serves to maximize the difference between the real part of the refractive index of each adjacent layer, ensuring that light is more efficiently reflected from the architecture with each consecutive interface. There is a better energy confinement per bilayer pair and more resonant energy oscillation in this design of this architecture. However, the overall Fresnel number must bemaximized per bilayer pair. In a typical embodiment, the Fresnel number may be 0.00170, depending on the materials selected for layers 1 and 2 and the architecture may have only 29 bilayer pairs. In another embodiment the architecture may have 40 layers and the angular bandwidth may be 17 degrees and the Fresnel number may be 0.00362. Figure 2 illustrates a comparison of this new design principle compared to the state of the art.

[0024] When the emerging reflectivity as a function of depth from the top of the multilayer mirror is maximized, with as few as possible interface reflections, the reflectivity efficiency is maximized, and the reflection per layer or bilayer is maximized.

[0025] The angle of incidence of light on a Fresnel number maximized mirror may range from 2 degrees to 20 degrees from normal incidence (where normal incidence is shown as 90 degrees in the figures). The full width half max of the angular bandwidth may range from 5 degrees to 18 degrees.

[0026] Typical examples of Fresnel numbers that can produce a EUV mirrors are 0.00170, 0.00231, 0.00596, 0.00128. The number of bilayer pairs can range from 10 upwards. For example it could be 10, or 20, or 29, or 40 or 60. Figure 3 illustrates embodiments of Fresnel numbers formed from different architectures and different material combinations. Figure 4 illustrates the reflectivity as a function of wavelength and angle from a high fresnel number coating of combination F, as seen in figure 3. The peak reflectivity and angular bandwidth (FWHM) exceed the equivalent values of Mo-Si when compared to Figure 1.

[0027] Each multilayer architecture has a depth below the top surface called the Z effective or Zeff. Even though reflectivity emerging from the architecture is actually distributed over a range of depths, Zeff defines the effective plane from which the reflective wave emanates. Architectures which have a high reflectivity efficiency, or Fresnel number > 0.00155, equally have a lower Zeff than Mo-Si. The Zeff plane for Mo-Si is typically at a depth of 50 nm. The Zeff values for higher Fresnel number architectures range from 10 nm to 50 nm. In some cases they may be 15 nm, 20 nm, 30 nm, 35, nm, 40 nm or 35 nm or 45 nm, or values in between.

[0028] Reducing the Zeff value has the additional benefit of increasing NILS, (normalized image slope), a value that represents the patterning fidelity and contrast of the image on the wafer plane. Figure 5 shows a schematic illustrating values of Zeff and its correlation to NILS. A high value of NILS is desired. Ideally NILS should be > 1.5.

[0029] Equally reducing Zeff serves to reduce the lithographic dose, the sensitivity requirement of the photoresist used to transfer the pattern of the mask into the silicon wafer. A reduction in dose represents an energy saving in the light intensity in lithographic exposure systems.

[0030] Typical optical elements and components which make use of such architectures for multilayer coatings include mirrors, fdters, lenses, detectors, reflectors, pellicles, substrates, facets, covering layer, capping layer, protective layer, inter diffusion layer, barrier layers, membranes, collectors. These components may be used in lithography systems, printing, scanning, telescopes, inspection tools, light sources, lasers, imaging tools.

[0031] Typical materials for layers 1 and 2 which form the bilayer pair, includes, metals, their alloys, compounds or combinations or mixtures thereof, and dielectrics and gases. Examples of suitable metals include molybdenum, zirconium, ruthenium, niobium, copper, platinum, palladium, rhodium, rhenium, osmium, nickel, chrome, rubidium and all metals in rows 4, 5 and 6 of the periodic table. Typically compounds include carbides, silicides etc e.g. boron carbide. Typical dielectrics include silicon, strontium, carbon, calcium, vacuum. Typical gases include argon, helium, nitrogen which are by products of the deposition. Any combination which satisfies the Fresnel condition can be used.

[0032] The exclusive combination of pure Molybdenum for layer 1 and pure Silicon for layer 2 is excluded. The exclusive combination of pure Ruthenium for layer 1 and silicon for layer 2 is also excluded as this is difficult to make do to the high degree of intermixing between the two layers. When a combination of materials are used in a single layer the effective refractive index is considered for nl and kl, or n2 and k2.

[0033] The process for selecting a material combination is an algorithm. Firstly establish the n value and k value for any layer 1 material , and the n value and k value for any layer 2 material for light at a wavelength of 13.5 nm. The values of the refractive index at 13.5 nm for any given material are well known in the state of the art. Next compute the Fresnel value according to the formula, and tabulate it. This is repeated with many selections for layer 1 and layer 2, n and k combinations. A list of the Fresnel values for these combinations are compiled in the table. Then the Fresnel values which meet the condition, numerical value greater than 0.00155 are selected from the table. These material combinations are viable. For a more select approach materials listed in

[0020] are considered first.

[0034] EUV multilayer architectures are typically made by depositing a coating in a substrate. The substrate may be fused silica, glass, silicon or a low thermal expansion material such as a ceramic or Zerodur.

[0035] The method for depositing a coating includes magnetron sputtering, sputtering, ion beam deposition, molecular beam epitaxy, physical vapor deposition, plasma enhanced chemical vapor deposition, evaporation or atomic layer deposition.

[0036] The present disclosure may be more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, wherein:

Claims

CLAIMSWhat is claimed is:

1. An optical element intended for use in EUV and soft X-ray systems comprising: a coating,wherein the coating consists of a stack of one or more bilayer pairs, forming an architecture; andeach bilayer pair containing two alternative materials, with real and imaginary refractive indices, nl,n2, kl,k2, such that the combination of these values produce a Fresnel number as defined bythat has a numerical value that is greater 0.00155.

2. An optical element of claim 1 wherein the stack consists of 20 or more bilayer pairs 3. An optical element of claim 1 wherein the stack is a EUV mirror reflector designed to reflect at approximately 13.5 nm.

4. An optical element of claim 1 wherein the architecture of the coating is designed to maximize the reflectivity efficiency, and maximize the reflectivity as a function of depth.

5. An optical element of claim 1 wherein the Zeff value of the architecture is less than 50 nm and the Fresnel number is > 0.0155.

6. An optical element of claim 1, wherein the materials of the layers comprise of any one of metals, alloys, mixture, dielectrics except for the exclusive combination of molybdenum and silicon, and ruthenium and silicon.

7. An optical element of claim 1 wherein the element is a photomask, mirror, lens, filter, covering layer, capping layer, substrate, film, pellicle, reflector, detector, collector.

8. A method intended to fabricate a coating of claim 1 for use in an optical element for a wavelength in the range 0.1 nm to 250 nm, wherein the material forming the layers of a bilayer pair is deposited on to a substrate by ion beam deposition, sputtering, or other physical vapor deposition method.

9. An algorithm designed to compute the numerical value of the Fresnel number of claim 1.

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