JP2006228840A - Soft x-ray optical device and instrument - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a higher-quality soft X-ray optical device and soft X-ray optical instrument by suppressing carbon contamination or oxidation of the surface. <P>SOLUTION: A multilayered film reflector is made by stacking on a substrate a plurality of paired layers, each consisting of a first layer and a second layer which have a different refractive index in a soft X-ray wavelength band (the second layer has a refractive index smaller than that of the first layer in vacuum). A protection layer consisting of Y3A15012 is formed on the uppermost layer of the multilayered film reflector to form the soft X-ray multilayered film optical device. The film thickness of the surface protection layer is smaller than that of the first layer and/or the second layer which constitute the multilayered film reflector and have a different refractive index. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a multilayer reflector used in a semiconductor exposure apparatus and other optical elements, and an optical apparatus using the same.

  Along with miniaturization of semiconductor integrated circuits, a projection lithography technique using X-rays having a shorter wavelength (11 to 14 nm) instead of conventional ultraviolet rays has been developed. This technique is also recently called EUV (Extreme UltraViolet) lithography, and is expected as a technique capable of obtaining a resolution of 70 nm or less that cannot be realized by conventional optical lithography using light beams having a wavelength of 190 nm or 157 nm.

When using soft X-rays, the refractive index of a substance is
n = 1−δ−iβ (δ, β: positive real number)
Both δ and β are very small compared to 1 (the imaginary part β of the refractive index represents X-ray absorption). Accordingly, the refractive index is close to 1, and X-rays are hardly refracted, and X-rays are always absorbed. Therefore, a lens system using refraction like light in the visible light region cannot be used for light in the X-ray wavelength region. Therefore, an optical system using reflection is used.

  However, since the refractive index is close to 1, the reflectivity is very low, and most X-rays are transmitted or absorbed. In order to solve this problem, a material having a large difference between the refractive index in the wavelength region of the X-ray to be used and the refractive index of the vacuum (= 1) and a material having a small difference are stacked alternately. Thus, a multilayer reflector has been developed in which a large number of reflecting surfaces as the interfaces are provided, and the thicknesses of the respective layers are adjusted based on the optical interference theory so that the phases of the reflected waves from the interfaces coincide with each other.

  As typical examples of such multilayer mirrors, combinations of W (tungsten) / C (carbon), Mo (molybdenum) / Si (silicon), and the like are known. And these multilayer films were produced by thin film formation techniques, such as sputtering and vacuum evaporation. In addition, since the multilayer mirror can also reflect X-rays vertically, an optical system with less aberration than the oblique incidence optical system using total reflection can be configured.

  The multilayer mirror reflects X-rays strongly when Bragg's formula: 2dsin θ = nλ (d: period length of multilayer film, θ: oblique incident angle, π / 2−incident angle, λ: X-ray wavelength) is satisfied. Since it has wavelength dependency to reflect, it is necessary to select each factor to satisfy this equation.

  When molybdenum (Mo) / silicon (Si) is used as the multilayer film, it is known that a high reflectance is exhibited on the long wavelength side of the L absorption edge of silicon having a wavelength of 12.6 nm. Therefore, a multilayer film reflecting mirror having a high reflectance of 67% or more at a direct incidence (incident angle of 0 °) around a wavelength of 13 nm can be produced relatively easily.

  The soft X-ray reflector is generally used in a vacuum in order to prevent absorption by air. In addition, when a soft X-ray reflecting mirror is used in reduction projection lithography, contamination of the optical element with carbon is a problem. Below, the mechanism of generation of optical element contamination by carbon will be briefly described.

  The vacuum used in soft X-ray optical equipment often contains residual gas containing hydrocarbons. Examples of the residual gas containing hydrocarbons include those caused by oil used in an evacuation system (vacuum pump) and those caused by a lubricant in a movable part inside the apparatus.

  In the case of an EUV exposure apparatus, an organic substance (for example, a photoresist) is introduced into the apparatus, and by placing the organic substance in a vacuum, the organic substance is decomposed, desorbed or separated, and a gas containing hydrocarbons is generated in the apparatus. Will be released inside.

  The residual gas molecules containing the hydrocarbon are physically adsorbed on the surface of the multilayer mirror in the apparatus.

  Residual gas molecules physically adsorbed on the surface of the multilayer mirror are repeatedly desorbed and adsorbed on the surface and do not grow thick as they are. However, when the multilayer mirror is irradiated with EUV light, secondary electrons are generated, and the secondary electrons decompose the residual gas molecules containing the hydrocarbons adsorbed on the surface to deposit carbon. In this way, the adsorbed gas molecules are gradually decomposed and deposited, so that a carbon layer is formed on the surface of the multilayer mirror, and the thickness of the carbon layer increases in proportion to the irradiation amount of EUV light. (See Non-Patent Document 1).

  As described above, when a carbon layer is formed on the surface of the multilayer-film reflective mirror, there is a problem that the reflectance of the reflective mirror decreases. FIG. 1 and FIG. 2 show graphs showing the influence of reflectance when a carbon layer is formed on the surface of a multilayer mirror (calculated values). Specifically, it is a graph showing the change in reflectance (calculated value) due to the formation of a carbon layer on a Mo / Si multilayer mirror (number of layers: 40 pairs, period length: 6.94 nm, top layer is Si). . In FIG. 2, the horizontal axis represents the thickness (nm) of the carbon layer, and the vertical axis represents the reflectance (%: 13.5 nm). As shown in FIG. 2, the reflectivity hardly decreases when the thickness of the carbon layer is 2 nm or less. However, when the thickness exceeds 2 nm, the reflectivity gradually decreases, and at 5 nm, the reflectivity decreases by about 4.4%.

  In the EUV exposure apparatus, even if the reflectivity of the multilayer mirror is slightly decreased, the throughput of the exposure apparatus is greatly affected.

  As described above, as a method for preventing the contamination of the optical element due to the carbon layer deposition on the surface of the multilayer reflector, a technique for introducing oxygen or water vapor into the use atmosphere as in Non-Patent Document 2 has been developed. Yes.

  According to the above document, oxygen or water vapor is decomposed by EUV light irradiation to generate radicals. The generated oxygen radicals react with gas molecules containing hydrocarbons physically adsorbed on the surface of the optical element and a carbon layer deposited on the surface to form carbon dioxide gas. Since the carbon dioxide gas formed is a gas, it is exhausted by a vacuum pump to remove carbon contamination.

K. Boller etal., Nucl. Instr. And Meth. 208 (1983273) M. Malinowskiet al., Proc.SPIE 4343 (2001) 347

  However, the method described in the above document utilizes an oxidation reaction by oxygen radicals and oxidizes not only the carbon deposited on the surface but also the multilayer film surface. When the surface of the multilayer film is oxidized, there arises a problem that the reflectance is lowered due to the surface oxidation.

  The present invention has been made in view of the above-described conventional problems, and an object thereof is to provide a soft X-ray optical element and a soft X-ray optical instrument with higher quality by suppressing carbon contamination or oxidation on the surface. There is.

  In the present invention, a plurality of layer pairs composed of a first layer and a second layer (having a lower refractive index in vacuum than the first layer) having different refractive indexes in the soft X-ray wavelength region are stacked on a substrate. A multilayer X-ray mirror comprising: a soft X-ray multilayer optical element comprising a protective layer made of Y3A15012 provided on the uppermost layer of the multi-layer reflector; and an optical apparatus including the same. The configuration is also included.

  The film thickness of the surface protective layer is characterized by being smaller than at least one of the first layer and the second layer having different refractive indexes forming the multilayer film reflecting mirror.

  A multilayer film reflecting mirror and means for introducing a gas composed of molecules containing oxygen into the use atmosphere of the multilayer film reflecting mirror are provided.

  By providing the surface protective layer of the present invention, it is possible to produce a multilayer mirror having high quality and resistance to contamination.

  Examples of the present invention will be described below.

  FIG. 3 is a schematic sectional view showing a multilayer-film reflective mirror according to an embodiment of the present invention. However, for the sake of simplicity, the number of layers is less than the actual number of multilayer films. Furthermore, although the multilayer mirror shape used in an exposure machine or the like generally has a curvature, the present invention will be described using a planar shape.

  As shown in FIG. 3, the multilayer-film reflective mirror according to the embodiment of the present invention includes a first layer (heavy atom layer) made of a material having a large difference between the refractive index in the soft X-ray region and the refractive index in vacuum. 3 and second layers (light atomic layers) 2 made of a material having a small difference between the refractive index and the refractive index of the vacuum are alternately stacked on the substrate 4. The substrate 4 is made of low thermal expansion glass and polished to a surface roughness of about 0.3 nm RMS or less.

  Next, a Mo / Si multilayer film was formed on the substrate 4 using a magnetron sputtering apparatus (not shown). Film formation was performed by depositing 40 cycles of a Si layer of 4.03 nm and a Mo layer of 2.92 nm, and a Si layer of 2.65 nm was formed thereon. Further, 2.58 nm of Y3A15012 was deposited as a protective layer 1 on the upper layer part (calculated reflectance is described in FIG. 4). The multilayer film may be formed by any method such as sputtering or vapor deposition.

For comparison, a film without a protective layer formed on the upper layer was also formed with the same film thickness, and the reflectance (incident angle of 5 degrees) of the multilayer mirror described above was measured. Thereafter, the multilayer mirror was placed in a vacuum apparatus, and an irradiation experiment was performed with EUV light under vacuum. Irradiation was performed at an intensity of 1 mW / mm ² for 6 hours. In order to create an environment close to the actual exposure environment, hydrocarbons from the photoresist were introduced into the chamber. The partial pressure of this gas was about 3 × 10 ⁻⁸ Torr.

  As a comparative example of the present invention, the same experiment was performed in the same environment for a sample in which a protective layer was not formed on the surface described above. In this sample, if the reflectance at 13.5 nm before the experiment is performed after the multilayer film is formed is 1, the sample after the experiment is 0.91, and the reflectance is reduced by about 10%. . In the sample in which the surface protective layer of the present invention was formed, when the reflectance at 13.5 nm before this experiment was 1, the reflectance was relatively about 0.93 after this experiment. In order to confirm this cause, each sample was analyzed by Auger electron spectroscopy. As a result, carbon deposition was confirmed on the sample surface, and the cause of the decrease in reflectance was the carbon on the sample surface caused by the decomposition of hydrocarbons. It was confirmed that this was a precipitate.

  However, it has been reported that this carbide has an effect of suppressing carbon layer formation by applying EUV light in an atmosphere containing moisture (M. Malinowski et al., Proc. SPIE 4343 (2001) 347).

  Next, an experiment was conducted to irradiate the sample with the surface protective layer and the sample without the surface protective layer, which were manufactured in the same manner as described above, with the gas containing hydrocarbons and the vaporized water introduced into the chamber and irradiate with EUV light. It was. The partial pressure of the introduced water was 4 × 10 −6 Torr. As a result, in a multilayer mirror without a protective layer, assuming that the reflectance of the multilayer film before this experiment is 1, the reflectance after this experiment is a relative value of about 0.94. It dropped to.

  In the multilayer reflector having the surface protective layer of the present invention, no decrease in reflectance was observed. When the surface of the multilayer film was analyzed by Auger electron spectroscopy, oxidation of the multilayer film surface was observed in the multilayer film mirror that did not have a surface layer used as a comparative example. No surface oxidation was observed. Thus, by manufacturing the multilayer reflector having the surface layer of the present invention, the surface oxidation can be reduced, and a high-quality multilayer reflector can be manufactured.

  In the above embodiment, the Mo / Si multilayer film is used as the multilayer film material, but a multilayer film made of other materials may be used as the multilayer film. For example, when a Mo / Be multilayer film is used, it is possible to obtain a multilayer film with high reflectivity and resistance to contamination near a wavelength of 11 nm. In addition, if the film thickness of the protective layer coated on the surface is too thick, it will cause a loss of reflectance that cannot be ignored due to absorption of the protective layer situation, so it is better to make it less than the film thickness of each layer used in the multilayer film good.

This is a reflection characteristic calculation value when carbon is deposited on the surface. This is a calculated reflectance at 13.5 nm when carbon is deposited on the surface. It is a Mo / Si40 periodic multilayer film reflection characteristic figure. It is a calculated value of a multilayer film having a protective layer used in this experiment.

Explanation of symbols

1 Surface protective layer 2 Heavy element layer 3 Light element layer 4 Lens (substrate)

Claims (3)

  Multi-layer film reflection in which a plurality of layer pairs composed of a first layer and a second layer having different refractive indexes in the soft X-ray wavelength region (a vacuum refractive index is smaller than the first layer) are stacked on a substrate. In the mirror, a soft X-ray multilayer optical element characterized in that a protective layer made of Y3A15012 is provided on the uppermost layer of the multilayer mirror.   The film thickness of the surface protective layer is smaller than the film thickness of at least one of the first layer and the second layer having different refractive indexes forming the multilayer mirror. Soft X-ray multilayer optical element.   3. A soft X-ray optical instrument comprising: the multilayer mirror according to claim 1; and means for introducing a gas containing oxygen into an atmosphere in which the multilayer mirror is used.

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