JP2006173497A - Optical element and projection aligner using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element which shows a good reflection property over a long period of time in usage under extreme ultraviolet rays. <P>SOLUTION: By forming a protection layer 30 of a material having a threshold energy of photoelectron emission higher than Si, a degree at which the optical element 40 produces photoelectrons according to the incident light can be reduced than the case that a multilayer film 20 is exposed. The "electron emission capacity" means a degree of producing photoelectrons according to the incident light, and in particular it can be evaluated by an inverse number of the threshold energy of photoelectron emission occurring by photoexcitation of the surface of a material. In other words, an inverse number of a work function is an indicator of electron emission capacity for a material such as a metal, an inverse number of ionization potential is an indicator of electron emission capacity for a material such as a semiconductor, while an inverse number of a difference in energy between the upper end of the valence electron band and the vacuum level is an indicator of electron emission capacity for a material such as an insulator. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a reflective optical element used for extreme ultraviolet rays and the like and a projection exposure apparatus using the same.

In recent years, with the miniaturization of semiconductor integrated circuits, in order to improve the resolution of an optical system achieved by the light diffraction limit, extreme ultraviolet rays having a shorter wavelength (11 to 14 nm) are used instead of conventional ultraviolet rays. The exposure technology that has been developed has been developed. This is expected to enable exposure with a pattern size of about 5 to 70 nm. However, since the refractive index of the material in this region is close to 1, it is impossible to use a transmission refraction type optical element as in the past. A reflective optical element is used. The mask used in the projection exposure apparatus is also a normal reflection type optical element from the viewpoint of securing transmittance. At this time, in order to achieve a high reflectance in the optical element, it is common to form a reflective surface by alternately laminating a substance having a high refractive index and a substance having a low refractive index on the substrate in the wavelength range of use. (See Patent Document 1).
Japanese Patent Laid-Open No. 2003-14893

  In the projection exposure apparatus, when the optical element as described above is used under extreme ultraviolet rays, the environment is a vacuum, but organic substances and the like cannot be completely excluded from the periphery of the optical element. On the other hand, extreme ultraviolet light has very large energy. At this time, the organic substance and the substance on the surface of the optical element are irradiated with extreme ultraviolet rays to cause photochemical vapor deposition (photo CVD), and a carbon film is generated on the surface of the optical element. Due to these phenomena, the reflection characteristics of the optical element are deteriorated, resulting in a problem that the life of the projection exposure apparatus is shortened.

  Therefore, an object of the present invention is to provide an optical element that exhibits good reflection characteristics over a long period of time when used in extreme ultraviolet light or the like.

  Another object of the present invention is to provide a projection exposure apparatus in which the above optical element is incorporated as a projection optical system for extreme ultraviolet rays.

  In order to solve the above problems, an optical element according to the first invention includes (a) a supporting substrate, (b) a multilayer film that is supported on the substrate and reflects extreme ultraviolet rays, and (c). And a protective layer provided on the outermost layer of the multilayer film and containing a photoelectron suppressing material. In the above optical element, the multilayer film is formed by, for example, laminating a Mo layer made of a material having a large refractive index difference with respect to the refractive index of vacuum in an extreme ultraviolet region and a Si layer made of a material having a small refractive index difference on a substrate. Can be.

  In the present invention, the optical element is a reflective element having a multilayer film, and has good reflection characteristics against extreme ultraviolet rays and the like. Moreover, in this invention, the protective layer containing a photoelectron suppression material is provided on the outermost layer of a multilayer film. In this case, since generation of photoelectrons is suppressed as compared with the case where the multilayer film is exposed, a phenomenon in which a carbon film is generated on the surface of the optical element by optical CVD can be suppressed. That is, since the possibility that organic substances existing in and near the surface of the optical element are activated by photoelectrons emitted from the optical element is reduced, the active organic substance is polymerized on the surface of the optical element and adhesion is enhanced and extreme. It is possible to suppress a phenomenon that a carbon film is formed under the influence of ultraviolet rays.

  In the optical element according to the second invention, in the optical element according to the first invention, the outermost layer of the multilayer film contains Si as a main component. In this case, the outermost layer of the multilayer film has a surface in which Si is predominantly present, and at least absorption at the surface layer of the multilayer film can be set relatively low.

  In the optical element according to the third invention, in the optical elements according to the first and second inventions, the electron emission ability of the photoelectron suppressing material is lower than the electron emission ability of Si. In this case, the amount of photoelectrons emitted from the optical element can be reduced as compared with the case where a Si film or a layer made of a substance having a higher electron emission capacity is present on the outermost layer of the multilayer film. Film formation can be reliably suppressed. Here, the “electron emission ability” means the degree to which photoelectrons are generated according to incident light, and for example, the threshold energy of photoelectron emission can be indexed.

Further, in the optical element according to the fourth invention, in the optical element according to the third invention, the photoelectron suppressing material is As, I, Re, W, NiO, Cu ₂ O, CuO, Co, Ge, Se, AlSb. , GaP, GaAs, InP, InAs, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe are included as a main component. These substances are suitable for suppressing carbon film formation under extreme ultraviolet rays. When the protective layer is an oxide, it tends to be chemically and structurally stable.

  A projection exposure apparatus according to a fifth aspect of the invention includes (a) a light source that generates extreme ultraviolet light, (b) an illumination optical system that guides the extreme ultraviolet light from the light source to a transfer mask, and (c) a mask. A projection optical system for forming the pattern image on the sensitive substrate. In the projection exposure apparatus, at least one of the mask, the illumination optical system, and the projection optical system includes the optical elements according to the first to fourth inventions.

  In the projection exposure apparatus, by using any of the optical elements described above, it is possible to suppress a decrease in the reflectance of the optical element due to the formation of a carbon film on the surface of the optical element. Reflective characteristics can be maintained over a long period of time, and the optical element and thus the projection exposure apparatus have a long life.

[First Embodiment]
FIG. 1 is a cross-sectional view showing the structure of the optical element according to the first embodiment. The optical element 40 of the present embodiment is, for example, a concave reflecting mirror, and includes a substrate 10 that supports a multilayer film structure, a reflective multilayer film 20, and a protective layer 30 that is a surface layer.

  The substrate 10 is formed by processing, for example, synthetic quartz glass or low expansion glass, and its upper surface 10a is polished to a mirror surface with a predetermined accuracy. The upper surface 10a can be a concave surface as shown in the figure, but can be a convex surface, a flat surface, a multi-surface, or other shapes depending on the application of the optical element 40.

  The multilayer film 20 is a reflective film composed of several to several hundred thin films formed by, for example, alternately laminating two kinds of substances having different refractive indexes on the substrate 10. The multilayer film 20 is formed by laminating a large number of materials with low absorption in order to increase the reflectance of the optical element 40 that is a reflecting mirror, and based on the optical interference theory so that the phases of the reflected waves are matched. The thickness of each layer is adjusted. That is, the thin film layer L1 having a relatively low refractive index and the thin film layer L2 having a relatively high refractive index are reflected on the substrate 10 with respect to the wavelength region of extreme ultraviolet rays used in the projection exposure apparatus. The multilayer film 20 is formed by stacking alternately or in an arbitrary order with a predetermined film thickness so that the phases are matched. The two types of thin film layers L1 and L2 constituting the multilayer film 20 can be a molybdenum layer and a silicon layer, respectively.

In the multilayer film 20, a diffusion preventing film (not shown) can be further provided between the thin film layer L1 and the thin film layer L2. When Mo, Si, or the like is used as the thin film layers L1 and L2 forming the multilayer film 20, the materials forming each of them are mixed near the boundary between the thin film layer L1 and the thin film layer L2, and the interface is ambiguous. Prone. Thereby, the reflection characteristic is affected, and the reflectance of the optical element 40 may be lowered. Therefore, in order to clarify the interface, a diffusion prevention film is further provided between the thin film layer L1 and the thin film layer L2 in forming the multilayer film 20. As the material, for example, B ₄ C, C, MoC, MoO ₂ or the like is used. By clarifying the interface in this way, the reflection characteristics of the optical element 40 are improved.

The protective layer 30 protects the multilayer film 20 from adhesion of the carbon film by covering the entire surface of the multilayer film 20. The protective layer 30 is formed of a photoelectron suppressing material having an electron emission ability lower than that of Si or a material containing this as a main component. More specifically, the protective layer 30 is made of a substance such as arsenic, iodine, rhenium, tungsten, cobalt, germanium, selenium, or a material containing these as a main component. Further, the protective layer 30 is, nickel oxide (NiO), cuprous oxide _(Cu 2 O), may also be formed of a material that an oxide or major component thereof, such as cupric oxide (CuO). The protective layer 30 includes aluminum antimony (AlSb), gallium phosphide (GaP), gallium arsenide (GaAs), indium phosphide (InP), indium arsenide (InAs), zinc sulfide (ZnS), and zinc selenide (ZnSe). , Zinc telluride (ZnTe), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe), or a compound containing these as a main component.

  Here, the protective layer 30 formed of the photoelectron suppressing material has a role of suppressing generation of photoelectrons emitted from the surface of the optical element 40 as compared with the case where the multilayer film 20 is exposed. Here, the amount of photoelectrons generated in response to incident light is reduced by producing the photoelectron suppressing material constituting the protective layer 30 from a material having a threshold energy for photoelectron emission higher than that of Si of the thin film layer L2. Is considered to be relatively less than when exposed to the surface. Therefore, when the protective layer 30 is made of a material whose photoelectron emission threshold energy is higher than that of Si, the multilayer film 20 has an “electron emission ability” that is the degree that the optical element 40 generates photoelectrons according to incident light. It can be made smaller than when exposed. In this case, “electron emission ability” means the degree to which photoelectrons are generated in response to incident light, and specifically, it can be evaluated by the reciprocal of the threshold energy of photoelectron emission generated by photoexcitation of the surface of each material. it can. That is, for materials such as metals, the reciprocal of the work function is an indicator of electron emission ability, and for materials such as semiconductors, the reciprocal of the ionization potential (energy difference between the bottom of the conductive band and the vacuum level) is usually the electron emission ability. For materials such as insulators, the reciprocal of the ionization potential (energy difference between the valence band upper end and the vacuum level) is usually used as an index of electron emission ability.

From the above consideration, in the case of the present embodiment, the material for forming the protective layer 30 is a material having a threshold energy for photoelectron emission higher than that of Si, specifically, As, I, Re, W, Co described above. , Ge, Se, NiO, Cu ₂ O, CuO, AlSb, GaP, GaAs, InP, InAs, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, etc. It was decided to form with a low material. As described above, since the protective layer 30 is made of a material whose electron emission ability is relatively lower than that of the multilayer film 20, the generation of photoelectrons is suppressed as compared with the case where the multilayer film 20 is exposed. The phenomenon that a carbon film is generated on the surface of the optical element 40 by CVD can be suppressed. That is, since the possibility that organic substances existing in and near the surface of the optical element 40 are activated by photoelectrons emitted from the surface of the optical element 40 is reduced, the active organic substance is polymerized on the surface of the optical element 40 to enhance adhesion. In addition, the phenomenon of carbon film formation due to the influence of extreme ultraviolet rays can be suppressed. Thereby, it can suppress that the reflectance of the optical element 40 falls gradually under extreme ultraviolet irradiation, and the reflective characteristic of the optical element 40 can be maintained over a long period of time.

  The protective layer 30 may be formed by any method such as vapor deposition or sputtering as long as a dense film can be formed without deteriorating the surface roughness. In addition, another material may be formed between the film, that is, the protective layer 30 and the surface layer of the multilayer film 20 in order to help form a dense film without deteriorating the surface roughness.

  In order to effectively exhibit the photoelectron suppression function of the protective layer 30 in the formation of the protective layer 30, a film thickness that is usually required for forming a dense film, that is, a large film thickness is desired. However, extreme ultraviolet rays may be absorbed by the protective layer 30 in some cases, and the thickness of the protective layer 30 is, for example, about 4 nm or less. The specific thickness is appropriately determined according to the reflection characteristics desired for the optical element 40.

  Note that a layer having oxidation resistance and catalytic activity may be provided between the protective layer 30 and the multilayer film 20. Due to the presence of the oxidation resistant layer, moisture and oxygen supplied from the periphery of the optical element 40 and causing oxidation are prevented from entering the inside of the protective layer 30 and the multilayer film 20, and the acid resistance of the optical element 40 under extreme ultraviolet irradiation is prevented. Can be improved. Further, due to the presence of the catalytic action layer, carbon in the organic substance supplied to the surface of the optical element 40 or the vicinity thereof can be converted into carbon dioxide. Therefore, it is possible to suppress the occurrence of a photo CVD phenomenon in which a carbon film is gradually deposited on the surface of the optical element 40 in combination with the protective layer 30, and that the reflectance of the multilayer film 20 is reduced as the carbon film is deposited. Can be prevented.

[Second Embodiment]
FIG. 2 is a view for explaining the structure of the projection exposure apparatus according to the second embodiment, in which the optical element 40 of the first embodiment is incorporated as an optical component.

  As shown in FIG. 2, the projection exposure apparatus 200 includes, as an optical system, a light source device 50 that generates extreme ultraviolet rays (wavelength 11 to 14 nm), and an illumination optical system 60 that illuminates the mask MA with extreme ultraviolet illumination light. And a projection optical system 70 for transferring the pattern image of the mask MA to the wafer WA which is a sensitive substrate, and as a mechanical mechanism, a mask stage 81 for supporting the mask MA and a wafer stage 82 for supporting the wafer WA are provided.

  The light source device 50 includes a laser light source 51 that generates laser light for plasma excitation, and a tube 52 that supplies a gas such as xenon that is a target material into the housing SC. Further, the light source device 50 is provided with a capacitor 54 and a collimator mirror 55. By condensing the laser light from the laser light source 51 with respect to xenon emitted from the tip of the tube 52, the target material in that portion is turned into plasma to generate extreme ultraviolet rays. The condenser 54 collects extreme ultraviolet light generated at the tip S of the tube 52. The extreme ultraviolet rays that have passed through the condenser 54 are emitted outside the casing SC while being converged, and enter the collimator mirror 55. Note that, instead of the light source light from the laser plasma type light source device 50 as described above, light emitted from a discharge plasma light source, a SOR light source, or the like can be used.

  The illumination optical system 60 includes reflection type optical integrators 61 and 62, a condenser mirror 63, a deflection mirror 64, and the like. Light source light from the light source device 50 is condensed by the condenser mirror 63 while being made uniform as illumination light by the optical integrators 61 and 62, and is incident on a predetermined region (for example, a belt-like region) on the mask MA through the deflection mirror 64. As a result, a predetermined area on the mask MA can be uniformly illuminated by extreme ultraviolet rays having an appropriate wavelength.

  Note that there is no substance having sufficient transmittance in the wavelength range of extreme ultraviolet rays, and a reflective mask is used as the mask MA instead of a transmissive mask.

  The projection optical system 70 is a reduction projection system including a large number of mirrors 71, 72, 73 and 74. A circuit pattern, which is a pattern image formed on the mask MA, forms an image on the wafer WA coated with a resist by the projection optical system 70 and is transferred to the resist. In this case, the area where the circuit pattern is projected at once is a linear or arc-shaped slit area, and a rectangular pattern formed on the mask MA, for example, by scanning exposure that moves the mask MA and the wafer WA synchronously. This circuit pattern can be transferred to a rectangular area on the wafer WA without waste.

Of the light source device 50 described above, the portion disposed on the optical path of extreme ultraviolet rays, the illumination optical system 60, and the projection optical system 70 are disposed in the vacuum vessel 84, and attenuation of exposure light is prevented. Yes. That is, the extreme ultraviolet rays are absorbed and attenuated by the atmosphere, but the entire apparatus is blocked from the outside by the vacuum vessel 84 and the optical path of the extreme ultraviolet rays is set to a predetermined degree of vacuum (for example, 1.3 × 10 ⁻³ Pa or less). By maintaining it, the attenuation of extreme ultraviolet rays, that is, the decrease in brightness and contrast of the transferred image is prevented.

  In the above projection exposure apparatus, the optical elements 54, 55, 61, 62, 63, 64, 71, 72, 73, 74 and the mask MA that are arranged on the optical path of extreme ultraviolet rays and the optical elements exemplified in FIG. 40 is used. At this time, the shape of the optical surface of the optical element 40 is not limited to the concave surface, and is appropriately adjusted depending on the place of incorporation such as a flat surface, a convex surface, or a multi-surface.

  The operation of the projection exposure apparatus shown in FIG. 2 will be described below. In this projection exposure apparatus, the mask MA is illuminated by illumination light from the illumination optical system 60, and a pattern image of the mask MA is projected onto the wafer WA by the projection optical system 70. As a result, the pattern image of the mask MA is transferred to the wafer WA.

  In the projection exposure apparatus described above, the optical elements 54, 55, 61, 62, 63, 64, 71, 72, 73, 74, MA, which are controlled with high reflectivity and high accuracy, are used. Exposure is possible. Further, in this projection exposure apparatus, the protective layer 30 shown in FIG. 1 is formed on the surfaces of the optical elements 54, 55, 61, 62, 63, 64, 71, 72, 73, 74, MA, and the carbon film Deposition is suppressed. Therefore, it is possible to prevent the reflection characteristics of these optical elements from deteriorating and thereby to prolong the life of the projection exposure apparatus.

Although the present invention has been described based on the above embodiments, the present invention is not limited to the above embodiments. For example, the protective layer 30 is not limited to a material such as As, I, Re, W, Co, Ge, or Se or a material containing these as a main component, but also includes an element such as Ni, Cu, Pt, Mo, Ru, or the like. It can be formed of a material having a main component. The protective layer 30 is not limited to an oxide of NiO, Cu ₂ O, or CuO or a material containing these as a main component, but can be formed of an oxide such as TiO ₂ or a material containing this as a main component. In addition, C, Pd, Ir, Pt, Au, or the like can be used as a main component or an auxiliary additive component of the protective layer 30.

Further, the order in which the thin film layers L1 and L2 are stacked and the condition of which thin film layer is the uppermost layer can be changed as appropriate according to the use of the optical element 40. The material of the thin film layers L1 and L2 is not limited to the combination of Mo and Si. For example, the multilayer film 20 can be formed by appropriately combining materials such as Mo, ruthenium, and rhodium and materials such as Si, beryllium, and carbon tetraboride (B ₄ C). In the above case, the surface of the multilayer film 20 is usually a substance other than Si, but the protective layer 30 may be formed of a photoelectron suppressing material having a lower electron emission capability than such a substance or a material containing this as a main component. desirable.

  In the above embodiment, the projection exposure apparatus using extreme ultraviolet rays as exposure light has been described. However, an optical element 40 as shown in FIG. 1 can also be incorporated in a projection exposure apparatus that uses ultraviolet rays other than extreme ultraviolet rays as exposure light. Deterioration of the reflection characteristics of the optical element can be suppressed.

  In addition to the projection exposure apparatus, for example, an optical element 40 as shown in FIG. 1 is also incorporated in various optical instruments including a soft X-ray optical instrument such as a soft X-ray microscope and a soft X-ray analyzer. Can do.

It is sectional drawing explaining the optical element which concerns on 1st Embodiment. It is a figure explaining the projection exposure apparatus which concerns on 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Substrate 20 ... Multilayer film 30 ... Protective layer 40 ... Optical element 50 ... Light source device 60 ... Illumination optical system 70 ... Projection optical system 81 ... Mask stage 82 ... Wafer stage L1, L2 ... Thin film layer, WA ... wafer

Claims (5)

A supporting substrate;
A multilayer film supported on the substrate and reflecting extreme ultraviolet rays;
A protective layer provided on the outermost layer of the multilayer film and containing a photoelectron suppressing material;
An optical element comprising:   The optical element according to claim 1, wherein the outermost layer of the multilayer film contains Si as a main component.   3. The optical element according to claim 1, wherein the electron emission ability of the photoelectron suppressing material is lower than that of Si. 4. The photoelectron suppression materials are As, I, Re, W, NiO, Cu ₂ O, CuO, Co, Ge, Se, AlSb, GaP, GaAs, InP, InAs, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe. The optical element according to claim 3, comprising at least one selected from the group consisting of as a main component. A light source that generates extreme ultraviolet light;
An illumination optical system that guides extreme ultraviolet light from the light source to a transfer mask;
A projection optical system that forms a pattern image of the mask on a sensitive substrate;
A projection exposure apparatus, wherein at least one of the mask, the illumination optical system, and the projection optical system includes the optical element according to any one of claims 1 to 4.

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