JP2011141129A - Multilayer film optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce a multilayer film optical element usable in a wide wavelength band by a simple manufacturing process. <P>SOLUTION: The multilayer film reflecting mirror 10 includes a multilayer film structure 20 on a flat substrate 11, and an upper stacked structure 30 on the multilayer film structure 20. The multilayer film structure 20 includes first high density substance layers 21 and first low density substance layers 22, which are periodically stacked. The upper stacked structure 30 includes a second high density substance layer 31 and a second low density substance layer 32, which are stacked. The first low density substance layer 22 and the second low density substance layer 32 are stacked in the order in which they are directly in contact with each other at an interface between the multilayer film structure 20 and the upper stacked structure 30. Therefore, when the material of the second high density substance layer 31 is the same as that of the first high density substance layer 21, and the material of the second low density substance layer 32 is the same as that of the first low density substance layer 22, a stacked structure including the order of ABAB...ABABBA is formed from a substrate 11 side. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a structure of a multilayer optical element having high reflection efficiency and diffraction efficiency in a wide wavelength range in the X-ray region.

  It is known that the reflectivity of a substance (bulk material) in the X-ray region is extremely low, and there is almost no refractive optical element such as a lens used in the visible light region. However, since the refractive index (real part of the complex refractive index) is slightly smaller than 1, a total reflection mirror in which a thin single layer film such as Pt or Au is laminated on a smooth substrate is effective, and a high reflectance is obtained. It is possible. In the case of a single-layer total reflection mirror, the reflectance is significantly reduced when the incident angle deviates from the total reflection region. As a means to overcome this, it is known that a multilayer film in which layers made of heavy elements and layers made of light elements are alternately laminated can obtain a high reflectance even at a higher angle than the total reflection region. If this multilayer film is formed on a flat substrate having a high height, a flat mirror having a high reflectivity for X-rays is obtained. Even when the substrate is curved, a curved mirror can be obtained in the same manner, and an imaging optical system can be configured using this even in an X-ray region without a lens.

  The high X-ray reflectivity of such a multilayer film is due to Bragg reflection (or diffraction) due to the periodic structure of the layer made of heavy elements and the layer made of light elements. That is, a particularly high reflectance can be obtained when the incident angle, wavelength, and layer thickness are set so that the reflected waves from the respective layers are intensified. An X-ray optical element configured using such a multilayer film is extremely useful in various X-ray optical instruments (such as a microscope and a telescope).

  In particular, it is possible to increase the diffraction efficiency by forming the multilayer film on the diffraction surface of the diffraction grating and matching the diffraction conditions in the diffraction grating with the Bragg diffraction conditions in the multilayer film. For example, as shown in Patent Document 1, a multilayer laminar diffraction grating having such a configuration is known.

  However, in the multilayer mirror, when the incident angle is fixed, a high reflectance can be obtained only in the vicinity of a wavelength that satisfies the Bragg condition. Therefore, there is a problem that the wavelength band (reflection width) in which this mirror can be used is extremely narrow. The narrowing of the reflection width becomes more pronounced as the X-ray energy is higher (shorter wavelength).

In order to solve this problem, for example, a technique called a multilayer supermirror described in Non-Patent Document 1 is known. In the multilayer supermirror, the periodic length D is increased toward the upper part (surface side) of the multilayer film, and the D is decreased toward the lower part (substrate side) so that long wavelength (low energy) X-rays are mainly generated. In the upper part of the multilayer film, short wavelength (high energy) X-rays are mainly reflected in the lower part of the multilayer film. As a result, X-rays in a wide wavelength band from short wavelengths to long wavelengths can be reflected.

A. Erco et al., “Graded X-ray Optics for Synchrotron Radiation Application”, Journal of Synchrotron Radiation, vol. 5, p239, 1998

JP 2006-133280 A

  However, in any of the techniques described above, the production of these optical elements is complicated.

  A conventional multilayer film is generally formed by alternately and periodically laminating two types of target materials corresponding to a heavy element layer and a light element layer on a substrate by a method such as sputtering. The layer of the element and the layer of the light element have a constant periodic length that does not change between the upper part and the lower part. Therefore, when the stack thickness (deposition rate) per unit time of each substance during film formation is constant, the stack thickness can be controlled by the film formation time. In contrast, when the multilayer supermirror described in Non-Patent Document 1 is manufactured, it is necessary to change the film formation time for each film formation. For this reason, the control of the laminated thickness becomes more complicated than in the case of a conventional multilayer film having a constant period length, and its production becomes difficult.

  Thus, it has been difficult to obtain an X-ray optical element that can be used in a wide wavelength band by a simple manufacturing process.

  The present invention has been made in view of such problems, and an object thereof is to provide an invention that solves the above problems.

In order to solve the above problems, the present invention has the following configurations.
The multilayer optical element of the present invention has a structure in which a low density material layer and a high density material layer having a higher density than the low density material layer are alternately and periodically stacked on a substrate. A multilayer optical element, wherein a multilayer film structure in which a first low-density material layer and a first high-density material layer are alternately and periodically stacked is formed on the substrate, and a second low-density material An upper laminated structure comprising a layer and a second high-density material layer is formed on the multilayer structure so that the first low-density material layer and the second low-density material layer are in direct contact with each other. It is characterized by that.
In the multilayer optical element of the present invention, the material of the first high-density substance layer and the material of the second high-density substance layer are W or Ni.
In the multilayer film optical element of the present invention, the multilayer film _{D 1} the period length of the structure, when _{D 2} a thickness of the upper laminated structure, the ratio of _{D 2} for _{D 1,} D 2 / _{D 1} is 1.0 It is characterized by being within a range of ± 0.05.
In the multilayer optical element of the present invention, the material of the first low-density substance layer or the second low-density substance layer is any of B ₄ C, B, C, and SiO ₂ .
In the multilayer optical element of the present invention, the substrate is any one of a mirror substrate, a diffraction grating substrate, and a zone plate substrate.

  Since the present invention is configured as described above, an X-ray optical element that can be used in a wide wavelength band can be obtained by a simple manufacturing process.

It is a figure which shows the cross-section of the multilayer-film reflective mirror used as embodiment of this invention. It is a figure which shows the structure at the time of using only a multilayer film structure in the multilayer film mirror used as embodiment of this invention. It is the result of having calculated the reflectance spectrum of 2-4 keV at the time of using only a multilayer film structure. It is a figure which shows a structure when only the upper laminated structure vicinity exists in the multilayer-film reflective mirror used as embodiment of this invention. It is the result of calculating the reflectance spectrum of 2-4 keV when only the upper laminated structure vicinity exists. It is the result of having calculated the reflectance spectrum of 2-4 keV in the multilayer-film reflective mirror used as embodiment of this invention. Reflection at 2 to 4 keV calculated in the case where the first and second low-density material layers are the same in the multilayer reflector according to the embodiment of the present invention, and four types of materials are selected for the low-density material layers. It is a result of a rate spectrum. In the multilayer reflector according to the embodiment of the present invention, the first and second high-density material layers are the same, and three kinds of materials are selected as the high-density material layer, and the reflectance spectrum is 2 to 4 keV. Is the result of calculating. Reflectivity of 2 to 4 keV in the case where the material of the first high-density substance layer is fixed and three kinds of materials are selected as the second high-density substance layer in the multilayer reflector according to the embodiment of the present invention It is the result of calculating the spectrum. Reflectivity of 2 to 4 keV when the material of the second high-density substance layer is fixed and three kinds of materials are selected as the first high-density substance layer in the multilayer reflector according to the embodiment of the present invention It is the result of calculating the spectrum. The reflectance spectrum of 2~4keV in multilayer reflector serving as an embodiment of the present invention is the result of calculation of the stacked total number N ₂ dependence of upper laminated structure. The reflectance spectrum of 2~4keV in multilayer reflector serving as an embodiment of the present invention is the result of calculation of the thickness D ₂ dependent upper laminated structure. Maximum reflectance at 2~4keV multilayer-film reflective mirror as the embodiment of the present invention, the minimum reflectance is a result of calculation of the D ₂ dependence of the integrated reflectance. In the multilayer-film reflective mirror used as embodiment of this invention, it is the result of having calculated the reflectance spectrum at the time of using Ni as a material of a high-density substance layer and making an energy band respond | correspond to 1.5-3 keV. When the total number N ₂ of the upper laminated structure is 2 and 3 when W and B ₄ C are respectively used as the material of the high-density material layer and the low-density material layer in the multilayer reflector according to the embodiment of the present invention. It is the result of having calculated the reflectance spectrum of 2-4 keV. When the total number N ₂ of the upper laminated structure is 2 and 3 when Ni and B ₄ C are used as the materials of the high-density material layer and the low-density material layer in the multilayer reflector according to the embodiment of the present invention. It is the result of having calculated the reflectance spectrum of 1-3 keV.

  Hereinafter, a multilayer reflector will be described as a multilayer optical element according to an embodiment of the present invention. This reflecting mirror has a configuration in which a multilayer film is formed on a substrate, and can be used as an X-ray reflecting mirror in the region of several keV. FIG. 1 shows a cross section of the multilayer-film reflective mirror 10. In the multilayer mirror 10, a multilayer structure 20 is formed on a flat substrate 11, and an upper laminated structure 30 is formed thereon.

Here, as the substrate 11, a mirror substrate formed of quartz (SiO ₂ ) or the like can be used. The reflectance of the multilayer mirror 10 greatly depends on the flatness of the surface (the upper surface in FIG. 1), and this flatness is the surface of the substrate 11 (the surface on which the multilayer structure 20 is formed). Depends on the flatness of As the flatness, for example, the surface roughness is preferably smaller than the wavelength of X-rays to be reflected.

In the multilayer film structure 20, the first high-density material layer 21 and the first low-density material layer 22 are periodically stacked as described in Patent Document 1. Both the material constituting the first high-density substance layer 21 and the material constituting the first low-density substance layer 22 may be a single element or a compound. However, the difference in the refractive index (real part of the complex refractive index) between the first high-density material layer 21 and the first low-density material layer 22 is large, and the extinction coefficient (imaginary part of the complex refractive index) However, a small size is necessary to increase the reflectance. In this multilayer film structure 20, a first high-density material layer 21 (hereinafter referred to as A) and a first low-density material layer 22 (hereinafter referred to as B) are sequentially formed from the substrate 11 side. These are periodically stacked in this order. This configuration is the same as the multilayer film structure used in the X-ray optical element (X-ray reflecting mirror or the like). Therefore, by using the multilayer structure 20, the reflectance in a specific wavelength region can be increased using Bragg reflection. The stacking Total (first dense material layer 21, respectively, and one layer of the first low-density material layer 22) and N _1. In order to increase the efficiency of Bragg reflection as in the case of a conventional multilayer mirror, N ₁ >> 2 is set.

Similarly, in the upper laminated structure 30, a second high-density material layer 31 and a second low-density material layer 32 are laminated. The materials constituting the second high-density substance layer 31 and the second low-density substance layer 32 are the same as those of the first high-density substance layer 21 and the first low-density substance layer 22, respectively. When the stacked total number defined in the same manner as the multilayer film structure 20 and N _2, is set to N 2 _{= 2.} Further, the stacking order is such that the first low-density material layer 22 and the second low-density material layer 32 are in direct contact with each other at the interface between the multilayer film structure 20 and the upper stacked structure 30. Therefore, when the material of the second high-density substance layer 31 is the same as that of the first high-density substance layer 21 and the material of the second low-density substance layer 32 is the same as that of the first low-density substance layer 22. In the structure of FIG. 1, a laminated structure in the order of ABAB... ABABBA is formed from the lower side. That is, the order of only the uppermost two layers is different from that of the lower layers.

As a material constituting the first high-density material layer 21 and the second high-density material layer 31, for example, tungsten (W), cobalt (Co), molybdenum (Mo), nickel (Ni), or the like is used. As materials constituting the first low-density material layer 22 and the second low-density material layer 32, for example, carbon (C), silicon (Si), boron (B), boron carbide (B ₄ C), or the like is used. It is done. These materials may be simple elements or compounds. Further, the first high-density material layer 21 and the second high-density material layer 31, or the first low-density material layer 22 and the second low-density material layer 32 do not need to be made of the same material. Furthermore, the thicknesses of the first high-density material layer 21 and the second high-density material layer 31, or the first low-density material layer 22 and the second low-density material layer 32 need not be the same.

  The multilayer structure 20 and the upper laminated structure 30 can be formed by, for example, a sputtering method. In this case, the thickness of each layer can be controlled by appropriately switching the target made of the material constituting each layer and setting each film formation time.

  The reflectance of the multilayer-film reflective mirror 10 having this structure is high in a wide wavelength band (energy band). Below, the reflectance spectrum (wavelength dependence of reflectance) of this multilayer-film reflective mirror will be described.

  First, the reflectance when there is no upper laminated structure 30 will be described. As shown in FIG. 2, the structure in this case is the same as that of a conventionally known multilayer mirror. In this case, the condition for maximizing the reflectance with respect to the incident light having the wavelength λ is expressed by the following formula, where the incident angle θ is an angle formed with the normal of the reflecting surface as shown in FIG.

Here, D ₁ is the periodic length of the multilayer film structure 20 and is equal to the sum of the thickness of the first high-density material layer 21 and the thickness of the first low-density material layer 22. m is an integer that is the order of Bragg diffraction (usually 1), and δ is the average refractive index of the multilayer film structure 20 (first high-density material layer 21 and first low-density material layer 22 in consideration of the film thickness ratio). Δ = 1−n where n is the weighted average of the real part of the complex refractive index. The refractive index n in the soft X-ray region is a value slightly smaller than 1. Further, the thickness ratio of the first low-density material layer 22 for the cycle length D ₁ and gamma _1.

In FIG. 2, the incident angle θ is set to 88 °, and the energy of the reflected X-ray is set to 3.6 keV. The first high-density material layer 21 is W (difference from 1 of the real part of the complex refractive index: 2.30965 × 10 ⁻⁴ ), and the first low-density material layer 22 is B ₄ C (same as 3.82424). X10 ⁻⁵ ) and γ ₁ = 0.5, δ = 1.34604 × 10 ⁻⁵ , and the period length D ₁ that maximizes the reflectance from the equation (1) is 5.6 nm. The laminated total number _{N 1} 40, results of calculating the reflectance spectra to incident energy 2~4keV when the substrate 11 was set to SiO ₂ is 3. Here, it is assumed that the substrate 11, the first high-density material layer 21, and the first low-density material layer 22 are all flat (the influence of the unevenness on the reflectance can be ignored).

  In this reflectance spectrum, there is a peak at about 3.5 keV, and a reflectance of 20% or more is obtained over the full width at half maximum (about 0.5 keV) of this peak. However, on the low energy side (around 2.5 keV), the reflectance is reduced to about 5%, which is about 1/5 of the peak reflectance. That is, the energy band in which high reflectivity can be obtained in this structure is extremely narrow. This is a general characteristic of multilayer reflectors conventionally known. In addition, in the location of 2.3 keV and 2.6 keV in FIG. 3, the reflectance is reduced due to the influence of the inner shell absorption edge of W used as the first high-density material layer 21.

Next, the reflectance in the vicinity of the upper part when the upper laminated structure 30 is formed on the multilayer film structure 20 will be described. Here, for simplification, the material and thickness of the second high-density material layer 31 are equal to those of the first high-density material layer 21, and the material and thickness of the second low-density material layer 32 are the same as those of the first high-density material layer 32. And the low density material layer 22. Therefore, the thickness _D 2 = 5.6 nm, the ratio gamma ₂ = 0.5 in the thickness of the second low-density material layer 32 for _{D 2} of the upper laminated structure 30.

As the structure near this upper part, the reflectance of a structure consisting of only the uppermost four layers (the upper laminated structure 30 and the two layers below it) was calculated in the same manner as described above. That is, this structure is specifically formed from the lower side by a W layer having a thickness of 2.8 nm (first high-density material layer 21) and a B ₄ C layer having a thickness of 2.8 nm (first low-density material layer). 22) A B ₄ C layer (second low-density material layer 32) having a thickness of 2.8 nm and a W layer (second high-density material layer 31) having a thickness of 2.8 nm are formed as shown in FIG. ing. Here, since the two intermediate low-density material layers (B ₄ C layer) are integrated, this structure is substantially W (2.8 nm) / B ₄ C (5.6 nm) / W (2 .8 nm).

FIG. 5 shows the result of calculating the reflectance spectrum under the same conditions as in FIG. 3 for this three-layer structure. This reflectivity is significantly different from the result of FIG. 3, and no sharp peak is seen. In particular, the reflectivity is 20% or more on the low energy side (around 2.5 keV) where high reflectivity was not obtained in FIG. This is because this three-layer structure can be substantially regarded as a multilayer film having a period length of 8.4 nm. In this case, when the energy at which the effect of Bragg reflection appears is estimated from the equation (1), it is about 2 It can be understood from .6 keV. In other words, the general feature of the multilayer-film reflective mirror in which the peak reflectance is low but the reflection width is wide as compared with the case where N ₂ >> 2 is reduced by reducing the number of stacked layers N ₂ is used. On the other hand, the reflectance at 3.6 keV at which high reflectance was obtained in FIG. 3 is extremely low. This means that this three-layer structure transmits X-rays of this energy well without reflecting them. These features greatly contribute to widening the bandwidth.

  As shown in FIG. 1, when the upper laminated structure 30 is formed on the multilayer film structure 20, a band where a high reflectance can be obtained can be expanded by these synergistic effects. FIG. 6 shows the result of calculation of the reflectance spectrum when the multilayer mirror 10 having the structure of FIG. 1 is set as described above. In the same figure, the results of FIGS. 3 and 5 are also shown. In this reflectance spectrum, the peak reflectance is not as high as in the case of FIG. 3, but the reflectance is 10 to 17% (average reflectance 14.3%) in a wide band of 2 to 4 keV. That is, it is possible to widen the band where high reflectance can be obtained.

  In manufacturing the structure of FIG. 1, in particular, the material and thickness of the second high-density substance layer 31 are equal to those of the first high-density substance layer 21, and the material and thickness of the second low-density substance layer 32 are set to be the same. When the first low density material layer 22 is made equal, the film forming conditions are the same. Therefore, the structure shown in FIG. 1 (a structure in which the upper laminated structure 30 is formed on the multilayer film structure 20) can be obtained only by changing the two kinds of film forming sequences. Therefore, this structure can be manufactured very easily and with high controllability. On the other hand, in the case of a conventionally known multilayer supermirror, it is necessary to form the film while changing the periodic length sequentially from the upper part to the lower part. It becomes difficult.

In addition, the above results do not greatly depend on the materials of the first high-density material layer 21, the first low-density material layer 22, the second high-density material layer 31, and the second low-density material layer 32. FIG. 7 shows that the materials of the first low-density material layer 22 and the second low-density material layer 32 are B ₄ C (in the above case), B, C, and SiO ₂ , and the others are the same as described above. It is the result of having calculated the reflectance spectrum of the multilayer film reflective mirror 10 in the case of doing. In this figure, the vertical axis (reflectance) is shown larger than that in FIG. Since the influence of absorption mainly depends on the material of the high-density material layer, the qualitative tendency of the reflectance spectrum is the same regardless of the material of the low-density material layer, and the effect of broadening the bandwidth is also clear. That is, the above effect can be obtained regardless of the material of the low density material layer.

  On the other hand, in FIG. 8, the materials of the first high-density material layer 21 and the second high-density material layer 31 are three types of W (in the above case), Pt, and Co, and the others are the same as in the case of FIG. It is the result of having calculated the reflectance spectrum of the multilayer-film reflective mirror 10 in the case of doing. In the case of using W and Pt, it can be confirmed that the broadband has been achieved. However, in the case of Pt, a decrease in reflectance is particularly observed at 2.1 to 2.2 keV. This is an influence of the M absorption edge of Pt.

On the other hand, in the case of Co, the peak in the vicinity 3.4keV is remarkable, the other reflectance for small laminated total N ₁ at an energy range of is vibrating. This vibration disappears when N ₁ > 200, and as a result, the reflectance spectrum becomes an envelope. That is, N ₁ is preferably large in order to obtain a smooth reflectance spectrum without such vibration. Although the effect of widening the band is small compared with the case where W and Pt are used, the reflectance of 7% or more is obtained on the low energy side even when N ₁ = 40, and the effect of widening the band is obtained. That is, in the case of Co, the reflectance of 2 to 3 keV when only the multilayer film structure 20 is 3% or less, and lower than the case of W shown in FIG. Is the main factor. Thus, in this multilayer-film reflective mirror 10, the influence of the material of the high-density substance layer is relatively greater than that of the material of the low-density substance layer.

  Further, although the manufacturing process is more complicated than the above case, the material system of the multilayer film structure 20 and the upper laminated structure 30 can be changed. Even in this case, it is the material of the high-density substance layer that has a relatively large influence. 9 shows that the material of the second high-density material layer 31 is W (in the above case), Pt, and Co, and the material of the first high-density material layer 21 is W. It is the result of having calculated the reflectance spectrum at the same time. When Pt and Co are used, only the high-density material layer in the multilayer structure 20 and the upper laminated structure 30 is different. Although any material is used for the second high-density material layer 31, the band is widened, but the effect is highest when W is used. However, even when Co is used, the region where the reflectance is 10% or more is sufficiently wide.

  Conversely, FIG. 10 shows that the first high-density material layer 21 in the multilayer structure 20 is made of three types of W (in the above case), Pt, and Co, the second high-density material layer 31 is W, and the others are shown in FIG. 6 is a result of calculating a reflectance spectrum in the same manner as in the case of FIG. Again, in any material, the bandwidth is widened, but W is the most effective and Co is the least effective. However, even when Co is used, the region where the reflectance is 10% or more is sufficiently wide.

  From the above, in the band of 2 to 4 keV, where it has been difficult to obtain an effective reflective optical element, W is used as the material for the first high-density material layer 21 and the second high-density material layer 31. It is particularly preferred.

Next, a description will be given effect of stacking the total number N ₂ in the upper laminated structure 30. In FIG. 11, the first high-density material layer 21, the first low-density material layer 22, the second high-density material layer 31, and the second low-density material layer 32 are the same as those in FIG. (for the) stacked total number _{N 2} of the structure 30 2, the results of the reflectance spectrum was calculated in the case of the 4, 6, 8, 10. Here, in the case of N ₂ = 4, 6, 8, 10, the second low-density material layer 32 and the second high-density material layer 31 are stacked in the same order as in the case of FIG. Therefore, in these cases, it can be considered that two multilayer film structures are substantially laminated.

From this result, it can be seen that the bandwidth can be particularly remarkably increased when N ₂ = 2. As described above, this is due to the fact that the non-periodicity of the upper layer contributes to the broadening of the bandwidth of the multilayer reflector 10. That is, when N ₂ becomes large, the effect of Bragg reflection due to the periodicity of the upper laminated structure 30 becomes high, and thus a broadband cannot be achieved.

This is because, in the multilayer mirror 10, an upper laminated structure 30 that contributes to an improvement in reflectance particularly in a low energy region, and a thick low-density material layer (a first layer at the boundary between the multilayer film structure 20 and the first multilayer structure 30). This shows that the optical path difference between the low-density material layer 22 and the second low-density material layer 32) contributes to a wider band. Therefore, it is preferable that N ₂ = 2, that is, the second high-density material layer 31 and the second low-density material layer 32 are formed one by one.

Next, the case where the thickness _{D 2} of the cycle length _{D 1} and the upper laminated structure 30 in the multi-layer film structure 20 _(N 2 = 2) are different. FIG. 12 shows the result of calculating the reflectance spectrum when D ₁ = 5.6 nm (same as above) and D ₂ is changed. Here, D ₂ / D ₁ is shown as a parameter, and other parameters (γ ₂ etc.) are the same as those in the case of FIG. From this result, it is clear that the shape (broadband) of the reflectance spectrum has D ₂ / D ₁ dependency, and in order to obtain a uniform reflectance, the vicinity of D ₂ / D ₁ = 1 is preferable.

In order to further clarify this point, FIG. 13 shows the D ₂ dependency (or D ₂ / D ₁ dependency) of the maximum and minimum values of reflectance and the integrated reflectance when the energy range is 2 to 4 keV. From this result, the difference between the maximum value and the minimum value of the reflectance becomes the smallest when D ₂ / D ₁ = 1.0 (D ₂ = D ₁ ), that is, the multilayer structure 20 and the upper laminated structure. It can be seen that the period lengths of 30 are equal. On the other hand, the maximum value of the integrated reflectance is around D ₂ = 6.5 nm (around D ₂ / D ₁ = 1.16). In the case of widening the band so that the difference between the maximum value and the minimum value of the reflectance is 0.1 or less, it is preferable that D ₂ / D ₁ is in the range of 1.0 ± 0.05 from the result of FIG. . In this region, the integrated reflectance is also sufficiently high, being 95% or more of the peak.

  Therefore, it is desirable to make the materials and thicknesses of the first high-density substance layer 21 and the second high-density substance layer 31 equal from the viewpoint of simplifying the manufacturing process. However, the materials and thicknesses of the first high-density material layer 21 and the second high-density material layer 31 are not necessarily the same. In this case, although the manufacturing process is complicated, it has the role of a bandpass filter that reduces only the reflectance in a specific energy region while appropriately widening the reflectance by selecting the material and thickness as appropriate. The present invention is highly applicable in that a desired reflectance spectrum can be obtained.

In the above example, the case of 2 to 4 keV was considered as the target energy region, but the multilayer reflector can be similarly widened in other energy bands as well. For example, the material of the first high-density substance layer 21 and the second high-density substance layer 31 is Ni, the material of the first low-density substance layer and the second low-density substance layer is B ₄ C, and D ₁ = FIG. 14 shows the same calculation results as FIG. 6 when D ₂ = 9.4 nm, γ ₁ = γ ₂ = 0.55, N ₁ = 60, and the substrate 11 is made of SiO ₂ . Here, the incident angle is 88.2 degrees. In the figure, as in FIG. 6, the reflectance in the case of only the multilayer structure and the reflectance of only the uppermost four layers are also shown. In this case, a uniformly high reflectance is obtained in the band of 1.5 to 3.0 keV, the reflectance exceeds 10% in all the regions, and the average reflectance is 27.9%. In particular, in the region of 1.6 to 2.8 keV (width 1.2 keV), the reflectance exceeds 20% and is extremely high. Also in the manufacturing process of the multilayer reflector 10, the structure of the multilayer reflector 10 can be easily realized by changing the stacking order of the materials without changing the film formation time.

In this energy region, Co, Cu, Fe or the like is used as the material of the first and second high-density substance layers, and B, C, SiO ₂ or the like is used as the material of the first or second low-density substance. You can also. In particular, it is possible to cope with a desired energy range in consideration of the energy at the absorption edge of the material of the first and second high-density substance layers.

Next, the case where N ₂ = 3 in the upper laminated structure 30 will be described. As shown in FIG. 6, the reflectance spectrum when the first and second high-density material layers 21 and 31 are W and the first and second low-density material layers 22 and 32 are B ₄ C is around 2 keV. The reflectance tends to decrease. Similarly, as shown in FIG. 14, the reflectance spectrum when the first and second high-density material layers 21 and 31 are Ni and the first and second low-density material layers 22 and 32 are B ₄ C. The reflectance tends to decrease in the vicinity of 1.5 keV. In order to improve the reflectance of this region, it is known that the number of stacked layers should be increased in the case of a general multilayer-film reflective mirror. That is, it is considered that the number N ₂ of the upper laminated structures 30 may be increased. However, as described above, when N ₂ ≧ 4, the bandwidth cannot be increased. However, N ₂ = 3, that is, in the upper stacked structure 30, after the second low-density material layer 31 is stacked next to the second low-density material layer 32, the second low-density material layer 32 is stacked again. It is effective to do. However, the thickness is approximately the sum of the thickness of the second low-density material layer 32 and the thickness of the second high-density material layer 31 that have been previously laminated. FIG. 15 shows that the first and second high-density material layers 21 and 31 are W, the first and second low-density material layers 22 and 32 are B ₄ C, and the total number of stacked layers N ₂ = 3 is a result of calculating a reflectance spectrum of 2 to 4 keV when 3. For comparison, the spectrum when the total number of stacked layers N ₂ = 2 is also shown. Thus, by setting N ₂ = 3, the reflectance in the vicinity of 2 keV can be improved. However, since the reflectance in the vicinity of 2.6 keV slightly decreases, it is preferable to select N ₂ = 2 or 3 depending on the purpose.

Similarly, FIG. 16 shows that the first and second high-density material layers 21 and 31 are Ni, and the first and second low-density material layers 22 and 32 are B ₄ C. It is the result of calculating a reflectance spectrum of 1 to 3 keV when N ₂ = 2 and 3. Although the reflectance in the vicinity of 1.8 keV slightly decreases, the reflectance on the lower energy side as well as in the vicinity of 1.5 keV can be improved. In the case of this system, the effect on the reflectance on the low energy side when N ₂ = 3 is great. Thus, laminated total number N _{2 =} but 3 is a broadband is possible in the case of the upper laminated structure 30, because some regions the reflectance is reduced compared with the case of N 2 ₌ 2, the N 2 ₌ 2,3 Of these, a value for obtaining a desired reflectance spectrum may be selected. Also in this respect, the present invention has many applications. However, as described above, when N ₂ = 4 or more, a wide reflection width cannot be expected.

In the above example, N ₁ = 40 in the multilayer film structure 20 is set. However, in other cases, if N ₁ >> 2 and Bragg reflection sufficiently occurs in the multilayer film structure 20, It is clear that the same result as above can be obtained. In addition, although SiO ₂ is used as the substrate 11, it is clear from the principle of the multilayer mirror that the same applies to substrates made of other materials. Further, the first high-density material layer 21 is present at the interface between the multilayer structure 20 and the substrate 11. However, N ₁ is sufficiently large, and the first low-density material layer 22 and the upper laminated layer in the multilayer structure 20 are used. As long as the second low density material layer 32 in the structure 30 is in direct contact, the interface with the substrate 11 may be the first low density material layer 22. These orders are appropriately selected from the adhesion to the substrate 11 and the ease of manufacturing.

  Further, in the above example, the reflecting mirror using the substrate 11 with high flatness has been described as a specific example of the multilayer optical element. However, the same multilayer film structure 20 and upper laminated structure 30 are formed on other types of substrates. If it is formed, other optical elements can be obtained. At this time, the upper laminated structure 30 can be formed in the same manner as the multilayer film structure 20 (or a multilayer film structure used in a conventional multilayer film reflector or the like). That is, if the conventional multilayer film structure is used, the upper laminated structure 30 can be formed on the optical element, and an optical element in which the applicable wavelength in the X-ray region is widened can be obtained. It is done.

  For example, similar to Patent Document 1, if these are formed using a diffraction grating substrate (laminar diffraction grating) in which grooves for forming a diffraction grating are formed as the substrate 11, high diffraction efficiency is obtained in a wide wavelength band. It can be used as a multilayer laminar diffraction grating. Similarly, a zone plate substrate having a structure in which a desired pattern is formed on a thin film can be used as the substrate 11, and the applicable wavelength range can be expanded.

10 Multilayer reflector (Multilayer optical element)
11 Substrate 20 Multilayer Structure 21 First High Density Material Layer 22 First Low Density Material Layer 30 Upper Laminated Structure 31 Second High Density Material Layer 32 Second Low Density Material Layer

Claims (5)

A multilayer optical element comprising a structure in which a low-density material layer and a high-density material layer having a higher density than the low-density material layer are alternately and periodically stacked on a substrate,
A multilayer structure in which first low-density material layers and first high-density material layers are alternately and periodically stacked is formed on the substrate;
The multilayer film includes an upper laminated structure including a second low-density material layer and a second high-density material layer so that the first low-density material layer and the second low-density material layer are in direct contact with each other. Formed on the structure,
A multilayer optical element.   2. The multilayer optical element according to claim 1, wherein the material of the first high-density substance layer and the material of the second high-density substance layer are W or Ni. Wherein D ₁ and period length of the multilayer film _structure, the thickness of the upper laminated structure When D _2, the ratio of D ₂ with respect to D _1,
Multilayer optical element according to claim 3, D ₂ / _{D 1} is being in the range of 1.0 ± 0.05. The material of the first low-density substance layer or the second low-density substance layer is any one of B ₄ C, B, C, and SiO _2. 2. The multilayer optical element according to item 1.   The multilayer optical element according to any one of claims 1 to 4, wherein the substrate is any one of a mirror substrate, a diffraction grating substrate, and a zone plate substrate.

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