JP2000089010A - Multi-layer reflector - Google Patents

Abstract

(57) [Summary] [Problem] An emission line spectrum of He at a wavelength of 30.4 nm (He
Provided is a multilayer reflector for a soft X-ray region, which can suppress the reflection of II) and can accurately observe the solar spectrum when used in an X-ray telescope. SOLUTION: On a substrate 1, a first layer 2 of a substance having a small difference between a refractive index in a soft X-ray region and a refractive index of a vacuum and a second layer 3 of a large substance are alternately laminated a plurality of times to form a multilayer. In a multilayer reflector provided with a film, a wavelength of 3
A multilayer mirror comprising an antireflection layer 4 for reducing the reflectance of 0.4 nm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multilayer reflector used in the soft X-ray region, and more particularly to a multilayer reflector suitable for use in an X-ray telescope for observing the sun. .

[0002]

2. Description of the Related Art The complex refractive index n of a substance in the X-ray region
Is represented by n = 1−δ−ik (δ, k: real number, k represents X-ray absorption), and both δ and k are much smaller than 1, so that in the X-ray region, A lens utilizing such refraction cannot be used. Therefore, in the X-ray region, an optical system using reflection is used.

However, the reflecting optical system in the X-ray region is
The reflectance is very small at an incident angle closer to the perpendicular than the total reflection critical angle θc (about 20 ° or less at a wavelength of 10 nm). Therefore, by laminating a number of layers of a combination of substances having the highest possible amplitude reflectance at the interface on the substrate, a large number of reflection surfaces (for example, several hundred layers) are provided, and the phases of the respective reflected waves are matched. A multilayer mirror in which the thickness of each layer is adjusted based on the theory of optical interference is used.

More specifically, the multilayer film mirror has a refractive index at an X-ray wavelength to be used and a vacuum refractive index (= 1).
This is obtained by alternately laminating a material layer (first layer) having a small difference between the material layer (first layer) and a material layer (second layer) having a large difference many times on a substrate to provide an alternating multilayer film. As a typical example of the combination of each layer constituting the alternating multilayer film, W (tungsten) /
C (carbon), Mo (molybdenum) / Si (silicon) and the like are conventionally known, and include sputtering, vacuum deposition, and CVD (Chemical Vapor Depos).
It is formed by a thin film forming technique such as an ition.

[0005] Since this multilayer film reflecting mirror can also reflect X-rays vertically, an optical system having less aberration than an oblique incidence optical system using total reflection can be constructed. In addition, the multilayer mirror is represented by the Bragg equation: 2 dsin θ = mλ
X-rays are strongly reflected only when satisfying (d: cycle length of multilayer film, θ: oblique incident angle, λ: wavelength of X-ray, m: positive integer), and thus has wavelength selectivity. Here, d corresponds to the layer thickness (film thickness) of a laminate in which a material layer having a small difference in refractive index and a material layer having a large refractive index are laminated one by one.

In recent years, research on the so-called X-ray astronomy field, in which an X-ray telescope is installed in outer space to observe an X-ray image of a celestial body, has been actively performed. In Japan, X-ray satellites named “Yoko” and “Asuka” have
An X-ray telescope is mounted to observe X-rays from celestial bodies such as the sun. However, the X-ray telescope mounted on these artificial satellites is based on an oblique incidence optical system using total reflection, and thus has the problem that there is no wavelength selectivity, and the resolution is poor due to large aberration. I was

For example, in the observation of high-temperature plasma and corona generated by solar flares, it is desired to obtain an image in a specific temperature range. For this purpose, an X-ray telescope using a multilayer reflector has been developed. Is being done. Since the temperature of the observation target can be known from the wavelength of X-rays generated therefrom, if an image of only X-rays of a specific wavelength is observed by the multilayer optical system, only a specific temperature distribution is extracted and observed. You can do it.

Specifically, 13.28n of the emission line spectrum of iron
m (Fe XIV), 17.11 nm (FeIX), 18.04 nm
(Fe XI), 19.24 nm to 19.51 nm (Fe XII),
20.20 nm (Fe XIII), 21.13 nm (Fe XI)
V), a wavelength of 28.42 nm (Fe XV) is important. These wavelengths are 0.9 MK, 1.3 MK, 1.4, respectively.
This corresponds to a temperature of MK, 1.6 MK, 1.8 MK, 2.0 MK.

In general, a multilayer reflector has a reflectance of several to several tens of percent at a peak wavelength, and a reflectance of about 0.1 to 1% other than near the peak wavelength. At this time, the contrast between the peak wavelength and other wavelengths is at least about 1/10 to 1/100. The light source used when using a multilayer reflector is synchrotron radiation (SR
Light) and a laser plasma X-ray source (LPX).

Although the SR light is a white light source, a spectroscope may be provided upstream of the multilayer mirror to extract and use only a desired wavelength. A light source using characteristic X-rays of elements such as LPX can obtain only a desired wavelength. As described above, if these light sources are used, only the desired wavelength can be extracted, so that light having a wavelength other than the peak wavelength is not mixed into the reflected light.

[0011]

However, when observing the sun with an X-ray telescope, a large number of bright line spectra in the solar spectrum poses a problem. That is, the observation wavelength is
It is selected from emission line spectra having relatively high intensities. However, since there is a bright line spectrum having high intensity other than the observation wavelength, it becomes an observation noise.

In order to prevent wavelengths other than the observation wavelength from being mixed into the reflected light, a multilayer film having a high wavelength resolution is desirable. However, the improvement in wavelength resolution is limited by the refractive index and absorption of the material constituting the multilayer film.
Mixing of other wavelengths cannot be prevented only by the multilayer film. When these bright line spectra are on the shorter wavelength side than the observed bright line spectrum, it is easy to remove by a filter, but when they are on the longer wavelength side, it is difficult.

In particular, the emission line spectrum (30.4 nm) of He II has a very strong spectral intensity.
It has about 50 times the intensity of 1.13 nm (Fe XIV) and is on the longer wavelength side than the observation wavelength, so no filter can be used. Therefore, 30.4nm in reflected light
, And there is a problem that observation of the solar spectrum cannot be performed accurately.

The present invention has been made in view of such a problem, and has a bright line spectrum of He (wavelength: 30.4 nm).
It is an object of the present invention to provide a multilayer reflector for a soft X-ray region, which can suppress the reflection of II) and can accurately observe the solar spectrum when used in an X-ray telescope. I do.

[0015]

Accordingly, the present invention firstly provides "a first layer of a substance having a small difference between a refractive index in a soft X-ray region and a refractive index in a vacuum, and a second layer of a substance having a large refractive index in a soft X-ray region. In a multilayer mirror in which a multilayer film is provided by laminating two layers alternately a plurality of times, an antireflection layer for lowering the reflectance to soft X-rays having a wavelength of 30.4 nm is further provided on the multilayer film. And a multilayer reflector (claim 1).

Further, the present invention secondly provides that "the thickness of the antireflection layer is at least 10 nm.
The present invention provides a multilayer reflector (claim 2). Also,
The third aspect of the present invention is that “the multilayer film is formed of the first layer and the second layer.
A multilayer reflector (claim 3) according to claim 1 or 2, wherein 40 or more pairs of layers are laminated.

Further, the present invention provides a fourth aspect of the present invention: "a laminated body in which the first layer and the second layer are laminated one by one so that the center wavelength of the reflectance is a combination of wavelengths in the range of 13 nm to 30 nm. The multilayer mirror according to any one of claims 1 to 3, wherein a period length of the stack corresponding to a film thickness and a ratio of a thickness of the second layer to the period length are set. Claim 4) "is provided.

Fifth, according to the present invention, the "reflection center wavelength is 13.28 nm (or approximately 13.28 nm), 17.11 nm (or approximately 1
7.11 nm), 18.04 nm (or approximately 18.04 nm), 19.24-19.51
nm (or approximately 19.24 to 19.51 nm), 20.20 nm (or approximately 20.
20. A multi-layer reflector according to claim 4, wherein the combination is a combination of 21.13 nm (or approximately 21.13 nm) and 28.42 nm (or approximately 28.42 nm). .

The present invention is also directed to a sixth aspect, wherein at least one of the first layer, the second layer, and the antireflection layer is
A multilayer reflector (claim 6) according to any one of claims 1 to 5, characterized by being composed of 1, Al compound, Si or Si compound. The present invention seventhly provides a method according to any one of claims 1 to 6, wherein the antireflection layer is made of a material having low absorption among the materials constituting the multilayer film. A multilayer reflector (Claim 7) is provided.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, a multilayer film reflecting mirror has a multilayer film provided on a substrate by alternately laminating a layer made of a high refractive material and a layer made of a low refractive material a plurality of times. ,
By making the phases of the reflected light at these interfaces uniform, a high reflectance can be obtained at a desired wavelength.

The following four forms can be cited as laminating forms of the multilayer film. (1) substrate / [first layer / second layer] ^N (2) substrate / [second layer / first layer] ^N (3) substrate / [first layer / second layer] ^N / first layer ( 4) Substrate / [second layer / first layer] ^N / second layer Here, for example, [first layer / second layer] ^N is the first layer and the second layer.
A laminate in which layers are alternately laminated N times (N: an integer of 2 or more) is shown. For example, [first layer / second layer] ^N / first layer is
A laminate in which only the first layer is further provided on the laminate [first layer / second layer] ^N is shown.

When a single-layer film is further added as an anti-reflection layer on the multilayer film, the spectral reflectance changes due to interference between light reflected by the anti-reflection layer and light reflected by the multilayer film thereunder. At this time, if the refractive index and the layer thickness of the antireflection layer to be added are appropriately selected, the reflectance at a wavelength at which the reflectance is desired to be suppressed (hereinafter referred to as a reverse peak wavelength) can be reduced.

Therefore, a multilayer film is formed by alternately laminating a first layer of a substance having a small difference between a refractive index in a soft X-ray region and a refractive index of a vacuum and a second layer of a large substance on a substrate plural times. In the multi-layer reflecting mirror according to the present invention (claims 1 to 7), an anti-reflection layer for lowering the reflectivity for soft X-rays having a wavelength of 30.4 nm is further provided on the multi-layer film.

That is, according to the multilayer mirror according to the present invention (claims 1 to 7), it is possible to suppress the reflection of the bright line spectrum (He II) of He having a wavelength of 30.4 nm.
When used in an X-ray telescope, observation of the solar spectrum can be performed accurately. When the material of the antireflection layer is different from the material of the first layer and / or the second layer, the following four forms are available as the form of the laminate according to the present invention in which the antireflection layer is provided on the multilayer film. A form can be mentioned.

(1) substrate / [first layer / second layer] ^N / anti-reflection layer (2) substrate / [second layer / first layer] ^N / anti-reflection layer (3) substrate / [first layer / Second layer] ^N / first layer / anti-reflection layer (4) substrate / [second layer / first layer] ^N / second layer / anti-reflection layer Here, the material of the anti-reflection layer is the first layer When they are the same, (1) substrate / [first layer / second layer] ^N / anti-reflection layer (4) substrate / [second layer / first layer] ^N / second layer / anti-reflection layer In the case of a laminated form and the material of the antireflection layer is the same as that of the second layer, (2) substrate / [second layer / first layer] ^N / antireflection layer (3) substrate / [first layer / Second layer] ^N / first layer / anti-reflection layer.

When the thickness of the antireflection layer according to the present invention is larger than 7.6 nm which is λ / 4 of 30.4 nm to be antireflection, the reflectance at 30.4 nm can be suppressed. It is preferable to set the film thickness as described above because the effect is large (claim 2). In order to increase the reflectance and wavelength resolution of the multilayer mirror, it is important to select a material forming the multilayer.

For example, the wavelength resolution increases in proportion to the number of laminations of the multilayer film (the number of laminations) until the reflectance is saturated. It is preferable to use a combination of substances having low absorption in the alternating layers so that is not saturated. That is, the complex refractive index n =
The imaginary part k of 1-δ-ik (a quantity representing the magnitude of absorption of a substance)
It is preferable to use a substance having a small value of.

On the other hand, in order to obtain a high reflectance, it is preferable to use a material having a high reflectance at each interface. The amplitude reflectance r at the multilayer interface in the case of normal incidence is
From the Fresnel equation, the refractive indices of the two substances constituting the multilayer film are given by the following equations, where n ₁ and n ₂ are the refractive indices, respectively.

R = (n ₂ −n ₁ ) / (n ₂ + n ₁ ) = {(δ ₁ −δ ₂ + i (k ₁ −k ₂ ))} / ｛(δ ₁ + δ ₂ + i
(K ₁ + k ₂ )｝ Since a substance having a small k (a substance having a small absorption) is used here, δ >> k, and the above equation can be approximated as the following equation.

R = (δ ₁ −δ ₂ ) / (δ ₁ + δ ₂ ) Therefore, to increase the reflectance at the interface of the multilayer film,
It is preferable to use a combination of substances having a large difference between the two. In order to increase both the reflectance and the wavelength resolution of the multilayer mirror, it is preferable to increase the number of laminations until the reflectance value is almost saturated. In the present invention, it is preferable to provide 40 or more alternating layers. Preferred (claim 3).

In the present invention, the center wavelength of the reflectance is 13 nm.
A period length of the stack corresponding to the thickness of the layered body in which the first layer and the second layer are stacked one by one so that a combination of wavelengths in a range of about 30 nm is obtained; It is preferable to set the ratio of the thickness (claim 4). Above all, the central wavelength of the reflectance is 13.28 nm
(Or approximately 13.28 nm), 17.11 nm (or approximately 17.11 nm), 1
8.04 nm (or approximately 18.04 nm), 19.24-19.51 nm (or approximately 19.24-19.51 nm), 20.20 nm (or approximately 20.20 nm), 2
1.13nm (or approximately 21.13nm), 28.42nm (or approximately 28.42n
m) so that a combination of
It is preferable to set a ratio (Γ) of the thickness of the second layer to the period length (claim 5).

Considering that it is better that k (quantity representing the magnitude of absorption of a substance) in the complex refractive index n = 1-δ-ik is better, a number of substances were examined.
Al compounds, Si or Si compounds have low absorption in the soft X-ray region (especially in the wavelength range of 13 to 30 nm), and when at least one of these materials is used for the multilayer mirror, high reflectance and wavelength resolution can be obtained. Was obtained.

Accordingly, at least one of the first layer, the second layer, and the antireflection layer constituting the multilayer film or the like according to the present invention.
One is preferably composed of Al, an Al compound, Si or a Si compound (claim 6). Further, it is preferable that the antireflection layer according to the present invention be made of a substance having low absorption among the substances constituting the multilayer film according to the present invention, since the peak reflectance of the multilayer mirror is hardly reduced. .

Although various embodiments are conceivable as embodiments of the present invention, it has been found that the embodiment according to Examples described below is particularly preferable. Here, the calculated multilayered SiC / Al multilayer film (period length 10.78 nm, Γ = 0.
FIG. 3 shows the spectral reflectance (R) of the sample having 25 layers and 80 pairs. The peak reflectance is 3 with or without anti-reflection layer
7%, and the wavelength resolution is 34.

In a normal multilayer film, the reflectivity is about 0.2% in the vicinity of a wavelength of 30.4 nm, but in the multilayer film according to the present invention in which the antireflection layer is optimized, the reflectance is 0.003% or less. The contrast with the reflectance at the peak wavelength (21.13 nm) (the reflectance at 30.4 nm / the reflectance at 21.13 nm, hereinafter referred to as the rejection ratio) was improved by about three orders of magnitude. In addition to the calculated values, not only in the actual measured values of each multilayer film actually manufactured, but also in the case of a normal multilayer film having no added anti-reflection layer,
The rejection ratio at 0.4 nm was large and was not suitable as a multilayer reflector for X-ray telescopes (peak reflectance 18%, wavelength resolution 30, rejection ratio 0.02).

On the other hand, in the multilayer film according to the present invention in which the antireflection layer is optimized, not only the calculated values but also the measured values are sufficient for the multilayer film reflecting mirror for the X-ray telescope to be 30.4 nm.
Was obtained. Further, the peak reflectance and the wavelength resolution did not decrease due to optimization of the antireflection layer (peak reflectance 17%, wavelength resolution 30, removal ratio 0.00).
1).

As described above, according to the multilayer mirror according to the present invention, the bright line spectrum (He) of He at a wavelength of 30.4 nm is obtained.
It is possible to suppress the reflection of II), and it is possible to accurately observe the solar spectrum when used in an X-ray telescope. Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

[0038]

Embodiment 1 In this embodiment, SiC was used as a heavy atomic layer and Al was used as a light atomic layer for forming alternating layers. For the substrate 1, mirror-polished synthetic quartz was used.
First, an Al layer 2 and a Si layer were formed on a substrate 1 by ion beam sputtering using SiC and Al targets.
C layers 3 were alternately laminated to produce a multilayer film.

Here, the first layer on the substrate is made of Al and the second layer is made of SiC, and the cycle length of the multilayer film is 10.78 nm, Γ = 0.25,
The number of stacked layers was 80 pairs. Next, a 12.3 nm Al layer 4 was formed as an anti-reflection layer on the multilayer film. FIG. 1 shows a cross-sectional view (in the figure, the number of layers of the multilayer film is smaller than the actual number). The SiC / Al multilayer film (period length 10.78 nm, Γ = 0.25, the number of laminations 80 pairs, the thickness of the antireflection Al layer 12.3 nm) according to the present embodiment and the ordinary SiC / Al multilayer film (period length 10.78 nm) , Γ = 0.25, the number of laminations is 80 pairs, and there is no antireflection layer) (FIG. 2).

The reflectivity of the SiC / Al multilayer film according to this embodiment at a wavelength of 21.13 nm is about 38%, the reflectivity at a wavelength of 30.4 nm is about 0.005%, and the removal ratio is about 0.0
0013. Also, when the soft X-ray reflectivity of the manufactured multilayer film at normal incidence was measured using synchrotron radiation,
Sufficient reflection characteristics were obtained as a multilayer reflector used in an X-ray telescope (peak reflectance: 17%, wavelength resolution: 30, rejection ratio: 0.001).

According to the multilayer mirror of this embodiment, the wavelength 3
It is possible to suppress the reflection of the emission line spectrum of He (HeII) of 0.4 nm, and it is possible to accurately observe the solar spectrum when used in an X-ray telescope.

[0042]

Embodiment 2 In this embodiment, SiC was used as the heavy atomic layer and Al was used as the light atomic layer for forming the alternating layers. For the substrate 1, mirror-polished synthetic quartz was used.
First, an Al layer 2 and a Si layer were formed on a substrate 1 by ion beam sputtering using SiC and Al targets.
C layers 3 were alternately laminated to produce a multilayer film.

Here, the first layer on the substrate is made of Al and the second layer is made of SiC, and the cycle length of the multilayer film is 10.78 nm, Γ = 0.25,
The number of stacked layers was 80 pairs. Next, a 13.7 nm-thick Si layer 4 was formed as an anti-reflection layer on the multilayer film. FIG. 1 shows a cross-sectional view (in the figure, the number of layers of the multilayer film is smaller than the actual number). The SiC / Al multilayer film (period length 10.78 nm, Γ = 0.25, number of laminations 80 pairs, antireflection Si layer thickness 13.7 nm) according to the present embodiment and a normal SiC / Al multilayer film (period length 10.78 nm) , Γ = 0.25, the number of laminations is 80 pairs, and there is no anti-reflection layer) (FIG. 3).

The reflectivity of the SiC / Al multilayer film according to this embodiment at a wavelength of 21.13 nm is about 37%, the reflectivity at a wavelength of 30.4 nm is about 0.0003%, and the removal ratio is about 0.1%.
00001. Further, when the soft X-ray reflectivity of the manufactured multilayer film at normal incidence was measured using synchrotron radiation, sufficient reflection characteristics were obtained as a multilayer film reflector used for an X-ray telescope (peak reflectance 17%, Wavelength resolution 3
1, removal ratio 0.001).

According to the multilayer mirror of this embodiment, the wavelength 3
It is possible to suppress the reflection of the emission line spectrum of He (HeII) of 0.4 nm, and it is possible to accurately observe the solar spectrum when used in an X-ray telescope.

[0046]

As described above, according to the multilayer mirror of the present invention, the bright line spectrum of He at a wavelength of 30.4 nm (H
e) The reflection of II) can be suppressed, and the observation of the solar spectrum can be performed accurately when used in an X-ray telescope. That is, the multilayer reflector of the present invention, in the soft X-ray region, in particular, for the wavelength of the iron emission line spectrum important in X-ray observation from the sun, without lowering both the reflectance and the wavelength resolution, The removal ratio at a wavelength of 30.4 nm can be reduced.

Therefore, when the multilayer reflector of the present invention is used for an X-ray telescope, its performance can be remarkably improved.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a multilayer reflector (one example) according to the present invention.

FIG. 2 shows a multilayer reflector for a peak wavelength of 21.13 nm (in which an Al layer optimized as an antireflection layer is added to an alternating multilayer of SiC / Al) and a conventional multilayer reflector in Example 1; FIG. 9 is a diagram comparing spectral reflectances of (an example, an alternating multilayer film of SiC / Al).

FIG. 3 shows a multilayer reflector for a peak wavelength of 21.13 nm in Example 2 (an SiC / Al alternating multilayer film with an optimized Si layer added as an antireflection layer) and a conventional multilayer reflector. FIG. 9 is a diagram comparing spectral reflectances of (an example, an alternating multilayer film of SiC / Al).

[Explanation of Signs of Main Parts]

DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Layer (1st layer) comprised of the low refractive index substance among the substances which comprise a multilayer mirror 3 ... High refractive index substance among the substances which comprise a multilayer mirror Constituent layer (second layer) 4 ... Anti-reflective layer

Claims (7)

[Claims] A multilayer film is formed by alternately laminating a first layer of a substance having a small difference between a refractive index in a soft X-ray region and a refractive index of a vacuum and a second layer of a large substance on a substrate a plurality of times. The multilayer reflector according to claim 1, further comprising an anti-reflection layer for reducing the reflectance of the multilayer film against soft X-rays having a wavelength of 30.4 nm. 2. The multilayer mirror according to claim 1, wherein the thickness of the antireflection layer is 10 nm or more. 3. The multilayer mirror according to claim 1, wherein the multilayer film is formed by stacking at least 40 pairs of the first layer and the second layer. 4. The center wavelength of the reflectance is 13 nm to 30 nm.
So that the combination of wavelengths in the region
2. The method according to claim 1, wherein a cycle length of the laminate corresponding to a film thickness of a laminate in which a layer and a second layer are laminated one by one, and a ratio of a thickness of the second layer to the cycle length are set. ~ 3
The multilayer mirror according to any one of the above. 5. The center wavelength of the reflectance is 13.28 nm (or approximately 1).
3.28 nm), 17.11 nm (or approximately 17.11 nm), 18.04 nm (or approximately 18.04 nm), 19.24-19.51 nm (or approximately 19.24-1
5. The multi-layer reflection according to claim 4, wherein the combination is a combination of 9.51 nm), 20.20 nm (or approximately 20.20 nm), 21.13 nm (or approximately 21.13 nm), and 28.42 nm (or approximately 28.42 nm). mirror. 6. The method according to claim 1, wherein at least one of the first layer, the second layer and the antireflection layer is made of Al, an Al compound, Si or a Si compound. A multilayer reflector according to any one of the above. 7. The multilayer reflector according to claim 1, wherein the antireflection layer is made of a material having a small absorption among the materials constituting the multilayer film.