JP4352977B2 - Multilayer reflector and EUV exposure apparatus - Google Patents

Description

  The present invention relates to a multilayer mirror configured by laminating thin films having different refractive indexes on a substrate, and an EUV (Extreme UltraViolet) exposure apparatus using the multilayer mirror.

  At present, reduction projection exposure that provides a high processing speed is widely used as a method of manufacturing a semiconductor integrated circuit. In recent years, with the progress of miniaturization of semiconductor integrated circuit elements, in order to improve the resolving power of the optical system limited by the diffraction limit of light, in place of conventional ultraviolet rays, a wavelength shorter than this is about 11 to 14 nm. Projection lithography technology using ultra-short ultraviolet rays (in the present specification and claims, refers to light having a wavelength of 100 nm or less and X-rays and may be abbreviated as EUV) has been developed (for example, D Tichenor, et a1, SPIE Proc. 2437 (1995) 292). This technique is also recently called EUV lithography.

  EUV lithography is expected as a future lithography technology having a resolution of 50 nm or less, which cannot be realized by conventional optical lithography (wavelength of about 190 nm or more). In a reduction projection exposure optical system using visible or ultraviolet light, a lens that is a transmissive optical element can be used, and a reduction projection optical system that requires high resolution is composed of a number of lenses. On the other hand, since all substances are absorbed in an EUV exposure apparatus, the optical system needs to be constituted by a reflecting mirror.

  In a projection optical system using a lens, light travels in one direction along the optical axis. However, when the projection optical system is configured with a reflecting mirror, the optical axis is folded back and forth many times. In order to prevent spatial interference between the light beam and the reflector substrate, there is a restriction on the numerical aperture (NA) of the optical system. Currently, a projection optical system consisting of four or six reflectors has been proposed, but in order to obtain sufficient resolution, six reflectors that can obtain a larger numerical aperture (NA) are used. The optical system that has been used is promising.

In this wavelength range, since the refractive index of the substance is very close to 1, conventional optical elements utilizing refraction and reflection cannot be used. Therefore, an oblique incidence mirror using total reflection due to the refractive index being slightly smaller than 1, and a multilayer mirror that obtains a high reflectivity as a whole by superimposing a number of phases of weak reflected light at the interface. Etc. are used. In the wavelength range near 13.4 nm, using a Mo / Si multilayer film in which molybdenum (Mo) layers and silicon (Si) layers are alternately stacked, a reflectivity of 67.5% can be obtained at normal incidence, and the wavelength is near 11.3 nm. In this wavelength region, when a Mo / Be multilayer film in which Mo layers and beryllium (Be) layers are alternately stacked is used, a reflectivity of 70.2% can be obtained at normal incidence. (For example, see C. Montcalm, SPIE Proc., Vol. 3331 (1998) P42.)
JP 2000-88999 A D. Tichenor, et a1, SPIE Proc., 2437 (1995) 292 C.Montcalm, SPIE Proc., Vol.3331 (1998) P42 LE Klebanoff et al, "First Environmental data from the Engineering Test Stand" 2nd Annual International Workshop on EUV lithography, International SEMATECH (2000)

In a reflective optical system used for actual EUV lithography, contamination of the surface when used for a long time becomes a problem. In order to transmit EUV light, the exposure atmosphere including the optical system is evacuated to a vacuum, but it is impossible to achieve a complete vacuum, and exposure is performed at a pressure of about 1 × 10 ⁻⁴ Pa.

  At this time, consideration is given so that hydrocarbon gas is not included in the residual gas as much as possible, but it cannot be completely eliminated, and hydrocarbon gas is always present although it is in a small amount. When EUV light having photon energy close to 100 eV is irradiated on the reflecting mirror surface in this atmosphere, hydrocarbon gas is decomposed by the EUV light, and carbon adheres to the reflecting mirror surface. This is called carbon contamination (carbon contamination), and if carbon contamination adheres to the surface of the reflecting mirror, it causes deterioration of optical characteristics such as a decrease in reflectance.

In order to solve this problem, a proposal has been made to remove carbon contamination by flowing oxygen into an atmosphere and irradiating EUV light in the atmosphere to react oxygen and carbon to CO ₂ (see FIG. 2). No. 2000-88999). However, this method has a strong oxidizing action on the reflecting mirror surface, and surface oxidation becomes a problem. In order to overcome this problem, a method for removing carbon contamination while suppressing oxidation under the condition of introducing a gas containing ethanol has been reported (LE Klebanoff et al, "First Environmental data from the Engineering Test Stand"). 2nd Annual International Workshop on EUV lithography, International SEMATECH (2000)) is not able to completely suppress oxidation.

  Since oxygen absorbs EUV light strongly, there is a problem that the reflectivity of the reflecting mirror decreases due to oxidation of the surface layer.

  The present invention has been made in view of such circumstances, and is a multilayer film reflecting mirror whose reflection characteristics are not easily deteriorated by oxidation, and a multilayer in which a decrease in reflectance is not increased by adhesion of a small amount of carbon contamination. It is an object of the present invention to provide a film reflecting mirror and an EUV exposure apparatus using the multilayer film reflecting mirror.

  The first means for solving the above problem is to use interference of reflected light generated at the boundary of each thin film by forming a multilayer film by superposing thin films made of materials having different refractive indexes on a base material. A multilayer film reflecting mirror having a reflective film having a practical reflectance, and an anti-oxidation film made of a material that prevents the multilayer film from being oxidized is provided on the surface of the reflective film. The multilayer film reflector (Claim 1) is characterized in that the thickness of the antioxidant film is made thinner than the thickness capable of obtaining the highest reflectivity of the multilayer film reflector as a whole.

  As described above, the multilayer mirror is configured by superposing thin films made of substances having different refractive indexes on a base material (the overlapping thin films only need to have different refractive indexes, and typically two types of refraction are used. It is constructed by alternately laminating substances with different rates), and by aligning a number of weak reflected light at the interface of the thin film (using interference), a large number of them are superimposed to obtain a high reflectivity as a whole. is there.

  Since carbon contamination adheres to the surface of such a multilayer film reflector as it is used, it is known to oxidize and remove with oxygen. However, at this time, the reflection film itself is oxidized and the reflectance is increased. There is a problem that it decreases. Accordingly, the inventor has come up with the idea of preventing oxidation of the reflective film by forming an antioxidant film on the surface of the reflective film.

  However, these methods make it difficult to optimize the conditions. If the oxidizing action is weak, carbon contamination remains slightly, and if the oxidizing action is stronger than necessary, the oxidation of the antioxidant film proceeds. This reduces the reflectivity in either case.

  However, since it is generally difficult to recover the reflectivity of the antireflection film whose oxidation has progressed on the surface, it is preferable to oxidize the carbon contamination under the condition that a slight amount of carbon contamination remains. This means has been devised in consideration of such practical conditions.

  That is, the thickness of the antioxidant film is set to be thinner than the thickness at which the highest reflectivity is obtained as the entire multilayer film reflecting mirror. This effect will be described below with reference to FIG. FIG. 1 shows a multilayer film formed by forming Ru as an antioxidant film on a reflective film in which 50 layers of 2.8 nm thick Mo and 4.2 nm thick Si are alternately formed on a substrate. This is a graph showing the relationship between the thickness of carbon attached to the Ru surface and the reflectivity of the multilayer mirror (target wavelength is about 13.5 nm) using the Ru thickness as a parameter. Is shown.

  As can be seen from this result, when the C layer (carbon contamination layer) is not attached to the surface (C thickness 0 nm), the Ru layer thickness is about 2 nm (in the figure, □ is tilted 45 degrees) The maximum reflectivity is indicated by (). However, in this case, when the C layer is formed on the surface, the reflectance rapidly decreases, and when the thickness of the C layer is 1 nm, the reflectance decreases by about 1%. That is, if the thickness of the outermost antioxidant film is optimized without considering the carbon contamination adhering to the surface, the reflectivity rapidly decreases due to the adhesion of the carbon contamination.

  On the other hand, when the thickness of the Ru layer is 1 nm (indicated by a black triangle in the figure), even if the C layer is formed with a thickness of 0 to 1.5 nm, the decrease in reflectance remains within 0.5%. Comparing these two conditions, when the thickness of the C layer is 0.5 nm or more, the reflectance is higher when the thickness of the Ru layer is 1 nm.

  Thus, by making the thickness of the Ru layer thinner than the thickness at which the reflectivity of the multilayer mirror is maximized when the C layer is not present, the reflection when carbon contamination adheres to the surface is achieved. The rate can be kept high. In addition, even when the amount of carbon contamination deposited on the reflecting surface is not uniform, the in-plane reflectance distribution can be kept more uniform.

In general, if the thickness of the antioxidant film is not made 1 nm or more thinner than the thickness at which the highest reflectivity of the multilayer mirror as a whole can be obtained, the reflectance may rapidly decrease due to adhesion of carbon contamination. In addition, if the thickness of the antioxidant film is reduced by more than 3 nm from the thickness at which the highest reflectance of the multilayer reflector is obtained, the reflectance of the multilayer reflector may be lowered .

Second means for solving the above problems, a first hand stage, which the multilayer film is formed by laminating a material comprising molybdenum or molybdenum, a material containing silicon or silicon (Claim 2).

  As described above, adhesion of carbon contamination becomes a problem particularly in a multilayer mirror used for EUV light. Therefore, in the first means and the second means, the multilayer film contains molybdenum or molybdenum. This is particularly effective when the material is formed by laminating a material and silicon or a material containing silicon.

The third means for solving the problem is either the first means or the second means, wherein the antioxidant film is made of ruthenium, platinum, gold, or at least one of them. It is characterized by comprising the substance to contain (Claim 3 ).

Although the effect of the first means has been described in the case where the antioxidant film is ruthenium, the effect of the first means is not limited to the case where the antioxidant film is ruthenium. The anti-oxidation film is required to be not oxidized as much as possible when the carbon contamination is removed by CO ₂ by oxidation, and may be Au (gold) or Pt (platinum), but in that case, the C layer does not exist. It is effective to make it thinner than the maximum thickness in some cases.

  When the antioxidant film is Pt and Au, the relationship (simulation result) between the amount of carbon contamination and the reflectance of the multilayer reflector is shown in FIG. 2 with the thickness of the antioxidant film as a parameter. As shown in FIG. The configuration of the reflective film is the same as that shown in FIG.

  Referring to FIG. 2, when the C layer is not present and the Pt layer has a thickness of 0.75 nm, the multilayer reflector exhibits the maximum reflectance, but when the C layer is formed to 2.5 nm, the reflectance is about Reduced by 1.5%. On the other hand, when the thickness of the Pt layer is 0.25 nm, the reflectance reduction when the C layer is formed to 2.5 nm can be suppressed to about 0.5%.

  As can be seen by comparing FIG. 3 and FIG. 2, when the antireflection film is made of Au, almost the same characteristics as in the case of using Pt are obtained. This is because the target wavelength is about 13.5 nm, and the optical constants of Pt and Au are similar in this wavelength range.

A fourth means for solving the above problem is any one of the first to third means, wherein chromium, titanium, MoC, SiC, and the like are provided between the antioxidant film and the reflective film. It has an intermediate layer made of SiO ₂ , B ₄ C, or a substance containing at least one of them (claim 4 ).

  When an anti-oxidation film is formed on the reflection film, diffusion may occur between these layers and the optical characteristics may change. In particular, if the uppermost thin film of the reflective film is Si and the antioxidant film is made of Pt or Au, these are likely to diffuse into the Si layer. In addition, the bondability between the substance constituting the uppermost layer of the reflective film and the substance constituting the antioxidant film may be poor. For example, the bondability between Si and Ru is not very good.

In this means, in order to prevent these problems, an intermediate layer made of chromium, titanium, MoC, SiC, SiO ₂ , B ₄ C, or a material containing at least one of them is formed.

A fifth means for solving the above-mentioned problem is the fourth means, wherein the thickness of the intermediate layer is 1 nm or less (claim 5 ).

  The thickness of the intermediate layer only needs to be thick enough to prevent diffusion between the antioxidant film and the reflective film to such an extent that it does not cause a problem in practice, and this thickness depends on the material of the antioxidant film and the reflective film. It is determined appropriately depending on the material of the upper thin film. However, if the thickness of the intermediate layer exceeds 1 nm, the amount of decrease in the reflectivity of the multilayer reflector increases, so this thickness is preferably 1 nm or less.

A sixth means for solving the above-mentioned problem is an EUV exposure apparatus (Claim 6 ) characterized by including a multilayer film reflecting mirror of any one of the first to fifth means.

  In this means, when the carbon contamination is oxidized and removed, the reflectance can be increased when used in a state where the oxidation of the antioxidant film is minimized and a little carbon contamination remains. Since a decrease in reflectance due to oxidation of the antioxidant film can be minimized, an EUV exposure apparatus having a long life can be obtained.

  According to the present invention, a multilayer film reflecting mirror whose reflection characteristics are not easily deteriorated by oxidation, and a multilayer film reflecting mirror in which a decrease in reflectivity is not increased by adhesion of a small amount of carbon contamination, and further this multilayer film An EUV exposure apparatus using a reflecting mirror can be provided.

Embodiments of the present invention will be described below with reference to the drawings.
(Example 1)
As shown in FIG. 4, a Mo / Si multilayer film in which a Mo layer 1 having a thickness of 2.8 nm and a Si layer 2 having a thickness of 4.2 nm are periodically laminated in total (50 pairs) on a substrate 3. In the reflection film, the Ru layer 4 was formed on the Si layer 2 on the outermost surface side of the periodic structure. The Ru layer 4 has two thicknesses of 1 nm and 2 nm.

  As a result, when the thickness of the Ru layer 4 was 1 nm, the reflectance was 0.3% lower when no carbon contamination adhered to the surface than when the thickness was 2 nm. However, even if carbon contamination with a thickness of about 1 nm adheres to the surface, the reflectance hardly decreases. Even when carbon contamination with a thickness of 1.5 nm adheres, the decrease in reflectance is 0.5%. Stayed.

  When the carbon contamination is 0.5nm, the reflectivity is almost the same when the Ru thickness is 1nm and 2nm. When the carbon contamination is 1.0nm or more, the Ru thickness is 1nm. In the case of, the reflectance was higher than that in the case where the Ru thickness was 2 nm.

  Therefore, by making the thickness of the Ru layer deposited on the top layer thinner than the optimized thickness without considering the adhesion of carbon contamination, it is a multilayer film that is not easily affected by carbon contamination. I understand that.

In this embodiment, the Ru layer 4 is formed as an antioxidant film on the uppermost layer of the multilayer film, but the material of the antioxidant film formed on the uppermost layer is not limited to this, and Pt, Au, etc. may be used. Good. Pt and Au are substances that do not oxidize easily. When carbon contamination adhering to the surface is oxidized and removed by UV cleaning or the like, it is difficult to oxidize, and the reflectance is hardly lowered.
(Example 2)
As shown in FIG. 5, a Mo / Si multilayer film in which a Mo layer 1 having a thickness of 2.8 nm and a Si layer 2 having a thickness of 4.2 nm are periodically laminated in a total of 100 layers (50 pairs) on a substrate 3. In the reflection film, a Cr layer 5 having a thickness of 0.5 nm was formed on the Si layer 2 on the outermost surface side of the periodic structure, and a Ru layer 4 having a thickness of 0.5 nm was formed thereon.

  When the Ru layer 4 is formed on the surface of the Si layer 2, the adhesion of the film may not be good. However, adhesion is improved by sandwiching the Cr layer 5 between the Ru layer 4 and the Si layer 2, and film peeling can be prevented. Further, the total thickness of the Ru layer 4 and the Cr layer 5 is 1 nm, which reduces the decrease in reflectivity due to carbon contamination attached to the surface, as in the case where the Ru layer is formed to 1 nm. Can be suppressed.

In this embodiment, the Cr layer 5 is used to improve the adhesion of the Ru layer 4 to the Si layer 2, but both the substance forming the uppermost layer of the reflective film and the substance constituting the antioxidant film are used. For example, Ti, MoC, MoSi ₂ , SiO ₂ , and B ₄ C may be used as long as adhesion is good.

In the present embodiment, the outermost surface is the Ru layer 4, but the present invention is not limited to this, and may be, for example, Pt or Au. Pt and Au are substances that do not oxidize easily. When carbon contamination adhered to the surface is removed by oxidation by UV cleaning or the like, it is difficult to oxidize and the reflectance does not decrease. Pt and Au are materials that easily diffuse into the Si layer, but diffusion can be prevented by sandwiching a Cr layer between the Si layer and the Si layer. The intermediate layer for preventing diffusion is not limited to Cr, and may be Ti, MoC, MoSi ₂ , SiO ₂ , or B ₄ C, for example.

  In this embodiment, the thickness of the Cr layer is 0.5 nm, but this thickness is not limited to this. However, it is desirable that the thickness is 1 nm or less in order to suppress a decrease in reflectance.

  Hereinafter, an EUV exposure apparatus which is an example of an embodiment of the present invention will be described with reference to FIG. The EUV exposure apparatus mainly includes an EUV light source 11, an illumination optical system 12, a stage 15 of a mask 14, a projection optical system 13, and a stage 17 of a wafer 16. The mask 14 is formed with the same size as the pattern to be drawn or an enlarged pattern. The projection optical system 13 includes a plurality of reflecting mirrors 13 a to 13 d and the like, and images the pattern on the mask 14 onto the wafer 16. The reflecting mirrors 13a to 13d are reflecting mirrors having the configuration as shown in the first embodiment or the second embodiment.

  The projection optical system 13 has a ring-shaped field of view, and transfers the pattern of the ring-shaped area forming a part of the mask 14 onto the wafer 16. The mask 14 is also of a reflective type. At the time of exposure, the EUV light 18a from the EUV light source 11 is converted into the EUV light 18b for illumination by the illumination optical system 12, the illumination EUV light 18b is irradiated onto the mask 14, and the reflected EUV light 18c is irradiated to the projection optical system 13. And is incident on the wafer 16. By scanning the mask 14 and the wafer 16 synchronously, a desired area (for example, an area for one semiconductor chip) is exposed.

  In the vicinity of the projection optical system of the EUV exposure apparatus, an XPS analyzer (not shown) is provided as a detecting means for detecting carbon on the reflecting surface. In addition, an oxygen inlet and an ultraviolet lamp (not shown) are attached to the vacuum chamber that houses the projection optical system, and when the carbon attachment to the reflecting surfaces of the reflecting mirrors 13a to 13d is detected by the XPS analyzer, Irradiation with an ultraviolet lamp is performed in an atmosphere containing oxygen, so that attached carbon can be removed.

  In this carbon removal process, the reflecting mirrors 13a to 13d have the anti-oxidation film on the surface as shown in Example 1 and Example 2, so that the surface oxidation of the reflection film does not occur and the carbon after the carbon removal is performed. Reflectivity is restored. In the carbon removal process, the carbon removal is stopped with a little carbon remaining so that the antioxidant film is not oxidized. Even if it does in this way, since the reflective mirrors 13a-13d have a structure as shown in Example 1 and Example 2, the reflectance of a reflective mirror does not fall significantly.

In a multilayer reflector in which Ru is formed as an antioxidant film on a reflective film in which 50 layers of Mo of 2.8 nm and Si of 4.2 nm are alternately formed on a substrate, It is a graph showing the relationship between the thickness of carbon adhering to the surface and the reflectivity of the multilayer mirror (target wavelength is about 13.5 nm) using the thickness of Ru as a parameter. 6 is a graph showing the relationship (simulation result) between the amount of carbon contamination deposited and the reflectance of a multilayer mirror when the antioxidant film is Pt, with the thickness of the antioxidant film as a parameter. 6 is a graph showing the relationship (simulation result) between the adhesion amount of carbon contamination and the reflectance of a multilayer mirror when the antioxidant film is Au, using the thickness of the antioxidant film as a parameter. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing a configuration of a multilayer film reflecting mirror that is a first embodiment of the present invention. It is a schematic diagram which shows the structure of the multilayer-film reflective mirror which is the 2nd Example of this invention. 1 is a schematic diagram showing a configuration of an EUV exposure apparatus which is an example of an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Mo layer, 2 ... Si layer, 3 ... Substrate, 4 ... Ru layer, 5 ... Cr layer, 11 ... EUV light source, 12 ... Illumination optical system, 13 ... Projection optical system, 13a-13d ... Reflector, 14 ... Mask, 15 ... Mask stage, 16 ... Wafer, 17 ... Wafer stage, 18a ... EUV light, 18b ... EUV light for illumination, 18c ... Reflected EUV light

Claims (6)

  A multilayer film is formed by superimposing thin films made of substances with different refractive indexes on a base material, and a reflective film having a practical reflectance is obtained by using interference of reflected light generated at the boundary between the thin films. An anti-reflection film made of a substance that prevents oxidation of the multi-layer film, which is different from the substance, is provided on the surface of the reflection film, and the thickness of the anti-oxidation film is a multi-layer reflection mirror A multilayer film reflecting mirror characterized in that it is thinner than a thickness capable of obtaining the highest reflectance as a whole film reflecting mirror. 2. The multilayer film reflector according to claim 1, wherein the multilayer film is formed by stacking molybdenum or a material containing molybdenum and silicon or a material containing silicon. 3. The multilayer mirror according to claim 1, wherein the antioxidant film is made of ruthenium, platinum, gold, or a substance containing at least one of them. The intermediate layer made of chromium, titanium, MoC, SiC, SiO ₂ , B ₄ C, or a material containing at least one of them is provided between the antioxidant film and the reflective film. The multilayer-film reflective mirror of any one of Claims 1-3 . The multilayer mirror according to claim 4 , wherein the intermediate layer has a thickness of 1 nm or less. EUV exposure apparatus characterized by having a multilayer reflector according to any one of claims 1 to 5.

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