JP2009156863A - Optical element for x-ray - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce film thickness unevenness that occurs in association with the provision of a stress buffer layer to suppress film stress that causes the deformation of a substrate. <P>SOLUTION: An optical element for X-ray includes the substrate, a first multilayer film having a reflection property with respect to light in a soft X-ray wavelength range, and a second multilayer film, disposed between the substrate and the first multilayer film, for reducing film stress of the first multilayer film. The second multilayer film includes a periodic structure having a unit period film thickness which is 90% or more and less than 110% of a two or more integral multiple of 7 nm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical element for X-rays used as a multilayer mirror in an optical system such as an exposure apparatus.

  In general, for light in an X-ray wavelength range of 40 nm or less, the refractive index of a substance is expressed as n = 1−δ−iβ (δ, β: positive real number), and both δ and β are 1 (The imaginary part β of the refractive index represents X-ray absorption). Therefore, the refractive index is close to 1, and X-rays are hardly refracted.

  Therefore, a lens using refraction like light in the visible light region cannot be used for guiding light in the X-ray wavelength region. Therefore, an optical element using reflection is used for guiding light. However, since the refractive index is close to 1, the reflectance is very low, and most of the X-rays are transmitted or absorbed. Therefore, by laminating many layers of materials with a large difference between the refractive index in the wavelength range of the X-rays used and the refractive index of the vacuum and a material with a small difference, a large number of reflective surfaces serving as interfaces between them are formed. Multi-layer reflectors with different film thicknesses based on optical interference theory have been developed.

  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, vacuum evaporation, and CVD. Recently, as the development of multilayer mirrors for X-rays progresses, multilayer films have been evaluated, and the practicality of some combinations of materials is being clarified. For example, a multilayer film of a combination of Mo / Si exhibits a high reflectance on the long wavelength side of the silicon absorption edge of 123 mm, and is thus excellent as a multilayer film reflector used in the reflection optical system of a soft X-ray reduction projection exposure apparatus. Yes. This soft X-ray reduction projection exposure apparatus is regarded as an effective means for meeting the demand for miniaturization of semiconductor circuits, and research and development are actively conducted.

  On the other hand, for the practical application of multilayer reflectors, the reduction of reflectance, light durability, film thickness distribution control to obtain a desired film thickness distribution within the optical element surface, and the cause of deformation of the optical element surface Many problems remain, such as suppression of film stress. In particular, when a manufacturing error occurs in the film thickness distribution, the surface shape of the optical element whose surface shape is precisely polished is changed according to the optical design. This is an important issue because it significantly reduces the optical aberration performance of the exposure apparatus. In addition, film stress is also an important issue because it changes the surface shape of the optical element and causes a reduction in optical aberration performance. That is, in order to ensure the optical aberration performance of the exposure apparatus, it can be said that it is necessary to achieve both precise film thickness distribution control and suppression of film stress.

  Among these problems, techniques disclosed in Japanese Patent Application Publication No. 2002-504715 and Japanese Patent Application Publication No. 2002-525698 are known as countermeasures for film stress.

  Japanese Patent Laid-Open No. 2002-504715 proposes to offset the deformation of the optical element by forming a single or multi-layer stress buffer layer having a different stress direction between the substrate and the reflective layer formed of the multi-layer film. In particular, in the stress buffer layer composed of Mo / Si, the number of pairs of Mo fraction and stress buffer layer (5.8 nm) having a unit period film thickness comparable to the unit period film thickness (6.9 nm) of the reflective layer. A method for minimizing stress by optimizing the above has been proposed.

  Japanese Patent Laid-Open No. 2002-525698 proposes a method in which the stress buffer layer is made of Mo / Si or Mo / Be so as to have an effect as a reflective layer and the total film thickness is reduced.

  Specifically, a configuration having stresses in opposite directions at the upper and lower portions of the reflective layer has been proposed.

  However, the conventional method for preventing the deformation of the substrate due to the film stress of the multilayer film in the multilayer film reflector has the following unsolved problems.

  As disclosed in JP-T-2002-504715 (Patent Document 1), a method of offsetting deformation by forming a stress buffer layer of a multilayer film having different stress directions between a substrate and the multilayer film is as follows. There was a serious problem.

  In the case of a stress buffer layer composed of a single layer, the film thickness is measured by a contact-type step meter or spectroscopic measurement using light having the same wavelength as the single layer film thickness. However, with these methods, it has been difficult to measure the film thickness with an accuracy of 1 nm or less, which is particularly required for an X-ray exposure apparatus. For this reason, even if the stress buffer layer formed on the substrate has unevenness in film thickness depending on the location, the film thickness cannot be controlled below the accuracy of film thickness measurement. That is, when a stress buffer layer composed of a single layer is used for an optical element such as an exposure apparatus, measurement cannot be performed even if film thickness unevenness occurs in the stress buffer layer, and film thickness unevenness exists in the stress buffer layer. Since the surface shape of the reflective layer formed thereon also changes, a surface shape error of the optical element occurs.

  On the other hand, with a stress buffer layer having a multilayer structure, it is possible to measure the film thickness with high accuracy by using diffraction by light having the same wavelength as the wavelength used. However, as disclosed in JP-T-2002-504715 (Patent Document 1), in a method for minimizing stress by optimizing Mo fraction and the like in a stress buffer layer composed of Mo / Si, Since the unit periodic film thickness is as small as the reflective layer (approximately 7 nm), it is necessary to increase the number of pairs of stress buffer layers. Therefore, even if each layer has only a slight film thickness unevenness, the total film thickness is large, resulting in an increase in the film thickness unevenness of the stress buffer layer, resulting in a surface shape error of the optical element, and the exposure performance of the exposure apparatus. It was a factor to make it worse.

In addition, in the configuration disclosed in Japanese Translation of PCT International Publication No. 2002-525698 (Patent Document 2), since there is a minimum necessary Mo layer thickness and Si layer thickness, the Mo / Si thickness ratio is extremely reduced. Since it cannot be increased or decreased, the stress range that can be controlled by adjusting each fraction of the combination of Mo / Si and Mo / Be is narrowed. Therefore, sufficient removal of the film stress cannot be achieved and the film stress remains. After all, it is necessary to increase the number of pairs of stress buffer layers to such an extent that the stress buffer layer can remove a sufficient film stress, which may cause a surface shape error of the optical element (Japanese Patent Laid-Open No. 2002-504715) ) Have similar problems.
JP-T-2002-504715 Special Table 2002-525698

  It is an object of the present invention to provide an optical element for X-ray which has an improved optical performance by reducing a surface shape error due to uneven thickness of the stress buffer layer by reducing the total thickness of the stress buffer layer. To do.

The X-ray optical element of the present invention that solves the above problems is
Provided between the substrate, the first multilayer film having reflection characteristics with respect to light in the soft X-ray wavelength region, and between the first multilayer film and the substrate, to reduce the film stress of the first multilayer film An X-ray optical element having a second multilayer film,
The second multilayer film is an optical element for X-rays having a periodic structure, and a unit periodic film thickness of the periodic structure is 90% or more and less than 110%, which is an integer multiple of 2 or more of 7 nm.

  In optimizing the periodic structure of the second multilayer film, the number of necessary pairs is suppressed by increasing the unit periodic film thickness, and the total film thickness is suppressed. In addition, by setting the unit periodic film thickness to an integral multiple of approximately 7 nm, the film thickness can be measured with a spectroscopic measuring instrument, and the film thickness control during film formation can be performed with high accuracy. As a result, it is possible to provide an optical element for X-rays having high optical performance with suppressed film thickness unevenness and suppressed surface shape error.

  The X-ray optical element of the present invention is an optical element having high reflection characteristics particularly for soft X-rays having a wavelength of 12 nm to 15 nm. In the soft X-ray wavelength region, a multilayer film close to a multilayer structure having a unit periodic film thickness of 7 nm has a clear reflectance peak for light with a high incident angle that is about 30 ° from the vertical direction (0 °) to the film surface. It is known to show. Since the reflectance peak position depends on the unit periodic film thickness of the multilayer film having a periodic structure, the film thickness of the multilayer film can be measured with high accuracy using this peak. Spectrometers (see FIG. 4) using soft X-rays with a wavelength of 12 nm to 15 nm are achieving very good wavelength accuracy, and are almost the only ones that achieve the required film thickness measurement accuracy of optical elements for semiconductor exposure apparatuses. It is a method.

  The present invention utilizes a spectrophotometer using soft X-rays with a wavelength of 12 nm to 15 nm that enables such high-accuracy measurement, and is a multilayer film whose unit periodic film thickness is close to an integer multiple of 2 or more of 7 nm. The thickness was made paying attention to the point that it is possible to measure with high accuracy.

  The best mode for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 shows a film configuration of an optical element for X-rays according to an embodiment. A first multilayer film 11 having a high reflection characteristic that acts as a reflection mirror in a soft X-ray wavelength region on a substrate 10; And a second multilayer film 12 having a periodic structure interposed between the multilayer film 11 and the substrate 10.

  The multilayer film 11 has a structure composed of alternating layers of a plurality of materials having different refractive indexes with respect to the X-ray wavelength used.

  The multilayer film 12 is composed of alternating layers of a plurality of materials having different refractive indexes with respect to the X-ray wavelength to be used as a stress buffer layer, and the alternating layers have periodicity with respect to the film order and film thickness. The unit periodic film thickness is 90% or more and less than 110%, which is an integer multiple of 2 or more with respect to 7 nm. By setting the unit periodic film thickness in this way, the required number of pairs can be greatly reduced and the total film thickness can be suppressed as compared with the stress buffer layer having a unit periodic film thickness of approximately 7 nm. In addition, film thickness measurement using a spectrophotometer utilizing high incident angle measurement with soft X-rays having a wavelength of 12 nm to 15 nm is also possible. The spectrophotometer using soft X-rays can be measured with an accuracy of 0.02% or less because the wavelength used is short and, depending on the material, a clear reflectance peak can be measured at a high incident angle. Is possible.

  The surface of the substrate 10 is optically polished.

  The first multilayer film 11 has a film configuration in which materials and film thicknesses are selected so that desired spectral characteristics can be obtained with respect to the wavelength used. Particularly in the soft X-ray region, Mo / Si, Ru / Si, W / Si, Ru / Be, Ru / Mo / Si, Ru / Mo / Be, Mo / Be, Mo2 C / Si, Mo2 C / Be, Mo / B4 C / Si / B4 C, Mo / C / Si / C, Ru / B4 C / Si / B4 C, Ru / C / Si / C, Ru / B4 C / Be / B4 C, Ru / C / Be Any of / C, W / C / Si / C, and W / C / Si may be used. Further, a layer made of a material different from that inside the multilayer film may be added to the outermost surface portion of the multilayer film 11.

  The stress possessed by the multilayer film is determined by the unit film thickness ratio of the multilayer film, the total number of multilayer films, the degree of vacuum in the vacuum chamber during film formation, and the film formation conditions. Therefore, the stress of the multilayer film 11 can be known in advance. Alternatively, a multilayer film may be prototyped on the test piece and the stress may be measured.

  A stress buffering effect can be obtained by providing a stress buffer layer between the multilayer film 11 and the substrate so as to have a stress opposite in sign to the stress of the multilayer film 11 thus obtained.

  The X-ray optical element of the present invention is characterized in that the stress buffer layer having a periodic structure is obtained after the film forming conditions of the stress buffer layer are determined by some method so as to exhibit the stress buffer effect as described above. The multilayer film 12 has a unit period of 90% or more and less than 110%, which is an integer multiple of 2 or more with respect to 7 nm, so as to have a clear reflectance peak at a high incident angle in a soft X-ray region having a wavelength of 12 to 15 nm It has a film thickness. The material constituting the unit period is Mo / Si, Ru / Si, W / Si, Ru / Be, Ru / Mo / Si, Ru / Mo / Be, Mo / Be, Mo2 C / Si, Mo2 C / Be, Mo / B4 C / Si / B4 C, Mo / C / Si / C, Ru / B4 C / Si / B4 C, Ru / C / Si / C, Ru / B4 C / Be / B4 C, Ru / It is selected from C / Be / C, W / C / Si / C, and W / C / Si. In particular, the use of Mo / Si is effective as a stress buffer layer because the stress buffer effect is enhanced. In that case, it is more preferable that the film thickness of Mo is 11.6 nm or more and less than 14.4 nm and the film thickness of Si is 1 nm or more and less than 2 nm. In this case, the stress buffering effect is particularly strong, and since a clear reflection peak is shown for soft X-rays at a high incident angle, it is possible to measure the film thickness with high accuracy. Can be formed.

  As a result, it is possible to realize a high-quality optical element for X-ray with less film thickness unevenness that effectively reduces film stress. Further, as will be described in detail below, the stress buffering effect per unit of the stress buffer layer can be enhanced, so that the total thickness of the stress buffer layer can be reduced.

  In an exposure apparatus in which a plurality of optical elements for X-rays are mounted, in-plane film thickness unevenness due to substrate deformation due to stress and film thickness measurement error is suppressed, so that exposure with small optical aberration is possible.

  FIG. 4 shows a spectrometer used for measuring the film thickness of the multilayer film in the manufacturing process of the X-ray optical element. In general, a spectrophotometer includes a light source that generates light having a wavelength to be used, a spectroscopic unit that splits light, a stage that controls the posture of an object to be measured, and a detection unit that detects light.

  4 includes a light source 101 that generates soft X-rays, a diffraction grating 102 that splits soft X-rays emitted from the light source 101, and a detector 103 that detects the spectrum from an irradiated object. And having. Spectroscopy is performed by changing the angle of the diffraction grating 102 to perform spectroscopic measurement. Moreover, the incident light incident angle to the irradiation object W can be changed by changing the relative angle between the irradiation light, the irradiation object W, and the detector 103.

  FIG. 1 shows an X-ray optical element that acts as a reflecting mirror in an X-ray optical system according to the first embodiment. The first multilayer film 11 having high reflection characteristics in the soft X-ray wavelength region is composed of 40 pairs of Mo and Si alternating layers. The same film configuration as that of the multilayer film 11 is formed on a test piece made of Si wafer or the like, the surface shape of the test piece before and after film formation is measured by an interferometer, and the film is determined from the surface shape change amount and the Young's modulus of the test piece When the stress was calculated, the film stress of this multilayer film alone was -98.3 N / m.

  The second multilayer film 12 occupying the space between the multilayer film 11 and the substrate 10 is formed by alternately stacking Mo and Si, and the unit periodic film thickness as a pair of Mo / Si film thickness is 90% or more, which is twice as large as 7 nm. The periodic structure of 14.7 nm is in the range of less than 110%. The number of Mo / Si pairs is eleven.

  A multilayer film having the same film configuration as that of the multilayer film 12 is formed on the test piece, and the surface shape of the test piece before and after film formation is measured with an interferometer in the same manner as described above. When the film stress was calculated from the rate, the film stress was +89.2 N / m.

  The substrate 10 is a substrate made of a material having a low thermal expansion coefficient. The surface is optically polished and has a roughness of 0.32 nm RMS. In general, since an optical element substrate of an exposure apparatus is required to have a small thermal expansion coefficient, zero-dur and ULE are considered as materials. In particular, in the EUV exposure apparatus, the optical performance of the substrate, such as the absorption coefficient and the refractive index, is not questioned.

  FIG. 2A shows the spectral reflection characteristics of the multilayer film 12 in the soft X-ray (wavelength 12 nm to 15 nm) region. The spectrophotometer shown in FIG. 4 was used for the measurement. Light was incident on the multilayer film at an angle of 5 ° with respect to the normal direction of the multilayer film surface, and the wavelength of the incident light was changed by changing the angle of the diffraction grating to obtain spectral reflection characteristic data. When the multilayer film 12 is formed on the substrate 10 and the reflection spectral characteristic in the soft X-ray region is measured at an incident light angle of 5 °, it is clear that the wavelength is 13.63 nm and the reflectance is 7.02%. A peak was observed.

  As described above, it was confirmed that the film thickness can be measured with high accuracy by the spectrophotometer shown in FIG.

  FIG. 2B shows the spectral reflectance of the multilayer film 11 at 5 ° incidence. When the multilayer film 11 is formed on the multilayer film 12 on the substrate 10 and the reflection spectral characteristic in the soft X-ray region is measured at an incident light angle of 5 °, the wavelength is 13.50 nm and the reflectance is 69.2%. A clear central reflection peak was observed.

  And when the film stress of the state which laminated | stacked the multilayer film 11 on the multilayer film 12 was calculated | required by the method similar to the above, it was -8.8 N / m.

  FIG. 3A is a cross-sectional view showing an X-ray concave reflector that is an X-ray optical element according to the second embodiment, and the substrate 20 is formed of a zero-dur, which is a well-known low-expansion optical material. It has a curved surface 20a that has been polished. As shown in FIG. 3B, a first multilayer film 21 that is an upper multilayer film and a second multilayer film 22 that is a lower multilayer film are formed on the curved surface 20a.

  The multilayer film 21 has a unit period film thickness H1, and the constituent elements of the alternating multilayer film are a Si layer 21a and a Mo layer 21b. The multilayer film 22 has a unit periodic film thickness H2, and the constituent elements of the alternating multilayer film are a Si layer 22a and a Mo layer 22b.

  The substrate 20 is a substrate made of zero-dur.

  As film formation conditions when forming the multilayer films 21 and 22, the film formation conditions are optimized by using a test piece made of a Si wafer or the like and a concave curved surface substrate having the same surface shape as FIG. Do.

  First, a multilayer film corresponding to the reflective layer having the same film configuration as that of the upper multilayer film 21 was formed on the test piece. In this multilayer film, the thickness of the Si layer is 4.12 nm, the thickness of the Mo layer is 2.82 nm, and thus the unit period thickness is 6.94 nm. When 40 pairs of these Mo / Si pairs were formed, the film stress was -96.2 N / m.

  Next, a multilayer film corresponding to the stress buffer layer having the same film configuration as that of the lower multilayer film 22 was formed on the test piece. In this multilayer film, the thickness of the Si layer is 1.1 nm, the thickness of the Mo layer is 13.3 nm, and thus the unit period thickness is 14.4 nm. When the number of laminated layers of the lower multilayer film was optimized so as to offset the stress of the upper multilayer film, the film stress was 97.1 N / m for 9 pairs.

  Next, 40 pairs of multilayer films having the same film configuration as the upper multilayer film 21 were formed on a concave curved substrate having the same surface shape as the substrate 20. At that time, when the reflection spectral characteristics of the soft X-ray region were measured at a plurality of locations in the curved surface at an incident light angle of 5 °, the film thickness unevenness was 0.8%. For the measurement, the spectrophotometer shown in FIG. 4 was used, and the incident angle of the incident light at the measurement point was adjusted to 5 ° by changing the posture of the concave curved substrate as the irradiation object.

  Based on this value, fine adjustments were made to the operation pattern of substrate rotation and revolution at the time of vacuum film formation, and when the film was formed again on the concave curved substrate having the same surface shape as the substrate 20, the film thickness unevenness was up to ± 0.02%. Reduced.

  Next, nine pairs of multilayer films having the same film configuration as the lower multilayer film 22 were formed on a concave curved substrate having the same surface shape as the substrate 20. At that time, the reflection spectral characteristic in the soft X-ray region is measured at an incident light angle of 5 °, and the operation pattern conditions of substrate rotation and revolution during vacuum film formation are optimized so that the film thickness unevenness in the curved surface becomes uniform. The film thickness unevenness was reduced to ± 0.03%.

  Next, nine pairs of multilayer films 22 were formed on the substrate 20 of the concave reflecting mirror of FIG. The reflection spectral characteristic in the soft X-ray region was measured at an incident light angle of 5 °, and it was confirmed that the film thickness unevenness in the curved surface 20a was ± 0.03%.

  Next, 40 pairs of multilayer films 21 were laminated on the multilayer film 22 on the substrate 20. The reflection spectral characteristic in the soft X-ray region was measured at an incident light angle of 5 °, and it was confirmed that the film thickness unevenness in the curved surface 20a was ± 0.02%.

  The in-plane average film stress of the optical element in which 9 pairs of lower multilayer films 22 and 40 pairs of upper multilayer films 21 were laminated on the substrate 20 was +1.1 N / m.

  By manufacturing a concave reflecting mirror having a multilayer film structure by such a procedure, it is possible to realize an optical element for X-rays that has a sufficient stress suppressing effect by the stress buffer layer and has little film thickness unevenness.

  Although the concave substrate is used in the present embodiment, the same effect can be obtained by following the same procedure for the convex substrate.

  The multilayer film 22 which is a stress buffer layer has a unit periodic film thickness of 14.4 nm and is laminated in 9 pairs, so that the total film thickness is 129.6 nm. Since the film thickness unevenness in the substrate surface is ± 0.03%, the substrate surface shape has a shape error of 0.039 nm after the lower multilayer film 22 is formed.

  For comparison, the unit periodic film thickness of the lower multilayer film is set to 7 nm, which is about 1 times λ / 2, and the Mo fraction is changed to have a stress of +91.2 N / m, which is equivalent to the present embodiment. The optical element which obtains the stress buffering effect was manufactured. In this case, 38 pairs of layers are required (total film thickness 266.0 nm), and the total film thickness of the stress buffer layer is 266 nm. Since the film thickness unevenness in the substrate surface is ± 0.03%, if the total film thickness is 266 nm, the optical element surface has a shape error of 0.080 nm after the formation of the lower multilayer film. As described above, this is because the film thickness of Si or Mo has a certain lower limit in order to exhibit the stress buffering effect. Therefore, the conventional method of adjusting the film thickness ratio of Mo / Si per unit has a limit in the stress buffering effect, and therefore the stress buffering effect obtained per 7 nm is smaller than that of the present invention.

  Thereby, in the optical element for X-rays of this invention, it has confirmed that the total film thickness in the state which suppressed film | membrane stress can be reduced, and, thereby, the shape error of the optical element surface by film thickness nonuniformity can be suppressed.

In an exposure apparatus equipped with such an X-ray optical element having a small surface shape error, an optical system having a small residual optical aberration can be obtained.
The conventional stress buffer layer and the stress buffer layer according to the present invention are shown in Table 1 below. It can be seen that the stress buffering effect equivalent to the conventional one is exhibited with a much smaller number of pairs than the conventional stress buffer layer.

1 is a cross-sectional view showing an optical element for X-ray according to Example 1. FIG. The spectral reflection characteristics at 5 ° incidence of the second multilayer film and the spectral reflection characteristics at 5 ° incidence of the first multilayer film are shown. FIGS. 2A and 2B show an X-ray concave reflecting mirror according to Example 2, in which FIG. 3A is a cross-sectional view thereof, and FIG. It is a figure which shows the outline of the spectrometer used in the manufacturing process of the optical element for X-rays by this invention.

Explanation of symbols

10,20 Substrate 11,21 First multilayer film 21,22 Second multilayer film

Claims (5)

A substrate, a first multilayer film having a reflection characteristic with respect to light in a soft X-ray wavelength region, and provided between the first multilayer film and the substrate, reduce film stress of the first multilayer film. An X-ray optical element having a second multilayer film,
The optical element for X-rays, wherein the second multilayer film has a periodic structure, and a unit periodic film thickness of the periodic structure is 90% or more and less than 110%, which is an integer multiple of 2 or more of 7 nm.   The optical element for X-rays according to claim 1, wherein the second multilayer film is made of Mo / Si. The first multilayer film is made of Mo / Si, Ru / Si, W / Si, Ru / Be, Ru / Mo / Si, Ru / Mo / Be, Mo / Be, Mo2 C / Si, Mo2 C / Be, Mo / B4 C / Si / B4 C, Mo / C / Si / C, Ru / B4 C / Si / B4 C, Ru / C / Si / C, Ru / B4 C / Be / B4 C, Ru / C / It has a film configuration of any one of Be / C, W / C / Si / C, and W / C / Si,
The optical element for X-rays according to claim 2, wherein the second multilayer film has a Mo / Si film configuration. In the Mo / Si film configuration of the second multilayer film,
The optical element for X-rays according to claim 2, wherein the film thickness of Mo is 11.6 nm or more and less than 14.4 nm, and the film thickness of Si is 1 nm or more and less than 2 nm.   An exposure apparatus comprising the X-ray optical element according to claim 1.

Images