JPH05346496A - Multilayer film reflecting mirror - Google Patents

Abstract

PURPOSE:To make reflecting X-rays in a wider wavelength region possible by laminating high density layers and low density layers with different refractive indexes in an X-ray wavelength region by turns with specific thicknesses and period lengths. CONSTITUTION:This is a multilayer film of a laminated structure of at least 6 layers of high density layers and low density layers with different refractive indexes by turns. In the third layer 3a and the lower layer 3b, high density layer 3a and low density layer 3b are laminated periodically with a period length (d) as to fulfill the Bragg reflection condition. The second layer 2 is formed with the thickness of 1/5 to 1/2 of the period length (d). The first layer 1 is formed with the thickness of 1/2 to 3/2 of the period length (d). The X-rays of short wave lengths are reflected by the Bragg reflection at period layer 3 below the third layer 3a. In addition, X-rays of long wavelengths are totally reflected by the first layer 1. The second layer 2 is inserted not to damage the Bragg reflection.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray reflecting mirror having a structure using a multilayer film, and more particularly to a multilayer film reflecting mirror suitable for application to an X-ray focusing device.

[0002]

2. Description of the Related Art It is generally known that X-rays are easily absorbed by substances. In addition, since the refractive index of all substances in the wavelength range of X-ray is slightly smaller than 1, the optical system used in the wavelength range of X-ray is greatly restricted. The reason will be described below.

First, when the X-ray is reflected, the refractive index is smaller than 1. Therefore, the X-ray is incident only on the reflecting surface at a shallower angle than the critical angle. Is totally reflected, and otherwise it is hardly reflected. Such a critical angle is determined by the wavelength of X-rays and the substance forming the reflecting surface. The general tendency of the critical angle is that the shorter the wavelength, the substance forming the reflecting surface. The smaller the density of, the smaller.

When the complex refractive index n is set as n = 1-δ + iβ, the critical angle θc can be expressed as θc = (2δ) ^1/2 .

By the way, as one of the substances having a relatively large critical angle, there is tungsten, and even in the case of tungsten, the critical angle is X at a wavelength of 0.154 nm.
For a ray, it is extremely small at 0.55 degrees, and in order to obtain a reflection surface capable of total reflection of X-rays of that wavelength, a smooth surface of atomic size order is required. Such a reflective surface is often manufactured by depositing a material having a large critical angle such as tungsten or gold or platinum on a smooth polished glass surface. Then, conventionally, a device for condensing X-rays is constructed using the total reflection surface, but in this case, as described above, the critical angle is extremely small, for example, about 0.55 degrees, It is necessary to make the X-rays incident substantially parallel to the reflecting surface, which causes the size of the entire apparatus to be large and the distance between the reflecting surface and the condensing point to be very long. It has been a cause of various problems.

On the other hand, the reflection mechanism is not total reflection but X utilizing Bragg reflection due to a periodic crystal structure.
There is a line reflector. This Bragg reflection has a large wavelength dispersion characteristic, and X-ray reflection occurs only when the angle θ of the incident line with respect to the reflecting surface and the wavelength λ of the X-ray satisfy the Bragg condition. The Bragg condition of the reflection can be expressed as 2d Sin θ = mλ, ignoring the influence of the refractive index. Here, d is the period length of the periodic structure, and m is an integer. As the material of the reflecting surface of the Bragg reflection, a natural material such as LiF, graphite or Si is generally used. However, these materials have severe restrictions on the wavelength and reflection angle of X-rays because the lattice spacing cannot be freely selected.

Therefore, many attempts have been made to construct a new artificial crystal for the purpose of expanding the utilization limit of the crystal material. The artificial crystal for the X-ray reflecting mirror is also called a multi-layer film for X-rays, and two different types of layers are formed by using thin film forming techniques such as sputtering, vacuum deposition or MBE (molecular beam epitaxy). Å ~ several hundred Å
It is produced by laminating two to several hundred layers with a layer thickness of. In the multilayer film stacking technique, it is important to secure long-distance periodicity and to make the interface between adjacent layers steep, and the substance forming the layer is used. It is selected in consideration of the wavelength of the line.

[0008]

By the way, when an X-ray reflecting mirror surface is constructed by using the above-mentioned multilayer film, there is an advantage that X-rays of a short wavelength can be reflected by Bragg reflection, but long reflection by total reflection. There is a drawback that the reflection of X-rays on the wavelength side becomes small. Therefore, it is not appropriate to use this multilayer film as a reflecting mirror used in a wide wavelength range at a certain incident angle, and X-ray condensing Not suitable for devices.

The present invention has been made in view of the above circumstances, and an object of the present invention is to reflect X-rays having a wider wavelength range than both conventional single-layer reflecting mirrors and multilayer film reflecting mirrors. It is to provide an X-ray reflector capable of

[0010]

A structure for achieving the above object will be described with reference to FIG. 1 corresponding to an embodiment. According to the present invention, the refractive indexes in the X-ray wavelength region are different from each other. A multilayer film in which high-density layers and low-density layers are alternately laminated and at least 6 layers are stacked, and the third layer 3a and layers 3b, 3a below it are counted from the upper surface side of the multilayer film.
.. In order to satisfy the Bragg reflection condition, the high density layer 3a and the low density layer 3b are cyclically stacked with a cycle length d, and the second layer 2 as the upper layer is thick. Is formed to be (1/5) d to (1/2) d with respect to the above-mentioned cycle length, and the thickness of the first layer 1 is (1/5) d with respect to the above-mentioned cycle length.
2) d to (3/2) d.

[0011]

The short-wavelength X-rays are reflected by Bragg reflection by the periodic periodic layer 3 of the third layer and below, and in addition, the long-wavelength side X-rays are totally reflected by the first layer 1. To be done.
At this time, the second layer 2 is inserted so as not to impair Bragg reflection.

In the present invention, the above-mentioned total reflection and Bragg reflection are compatible with each other, that is, the two layers 1 and 2 of the first layer and the second layer are periodic layers of the third layer or less. 3 does not impair the X-ray interference effect of 3 and is the first layer 1
In order to increase the layer thickness of the first layer, the thickness of the layer 1 of the first layer is set to be 1/2 or more of the period length d of the periodic layer 3, and the layer 2 of the second layer. The thickness is 1/2 or less of the cycle length d. Furthermore, if the thickness of the first layer, Layer 1, is too thick, Bragg reflection is affected, so the upper limit is
(3/2) d, and if the layer thickness of the second layer 2 is too thin, Bragg reflection is affected, so the lower limit value is (1 /
5) d. The upper limit value and the lower limit value were determined based on the calculation described later.

[0013]

1 is a schematic sectional view showing the structure of an embodiment of the present invention. On the surface of the glass substrate S, the high density layers 1, 3a
.. 3a and low density layers 2, 3b .. 3b are alternately laminated in sequence, and a multilayer film for reflection is formed as a whole.

The layers 3a, 3b ... 3a, 3b, which are the third and subsequent layers from the upper surface side of the multilayer film, are periodically laminated so as to satisfy the Bragg condition. The number of layers of the third and subsequent periodic layers 3 is, for example, 20 layers.

On the other hand, the uppermost layer 1 of the multilayer film is a high density layer, and its layer thickness is (1/2) d to (3/2) with respect to the periodic length d of the periodic layer 3.
It is set in the range of d. Also, the second layer 2 below it is a low-density layer, and its layer thickness is set within the range of (1/5) d to (1/2) d with respect to the periodic length d of the periodic layer 3. ing.

Next, an example of a procedure for manufacturing the multilayer film having the above structure will be described. First, high density layer 3a and low density layer 3 are formed on the surface of glass substrate S by a high frequency magnetron sputtering method using tungsten as a material forming a high density layer and carbon as a material forming a low density layer.
b and b are alternately laminated to form the periodic layer 3. At this time, the number of periods of the periodic layer 3 is about 20 periods, and
The outermost layer of the periodic layer 3 is a high density layer 3a.

Next, a low density layer 2 having a thickness of (1/5) to (1/2) of the periodic length d of the periodic layer 3 is laminated on the surface of the periodic layer 3, and further, A high density layer 1 having a layer thickness of (1/2) d to (3/2) d is laminated thereon. The two layers are also formed by using the same material and film forming method as above.

Here, the period length d of the periodic layer 3 varies depending on the wavelength range of X-rays used in the light condensing device to which this multilayer film reflecting mirror is applied, but is typically about 2 nm to 20 nm. is there. Further, as the number of cycles of the periodic layer 3 is increased, the reflection characteristics thereof are improved, but in reality, the improvement of the characteristics is saturated at a certain number of cycles. About 20 cycles is sufficient. Furthermore, in designing those reflection characteristics, sufficiently complex data can be obtained by performing an operation based on Maxwell's electromagnetic wave theory using the complex refractive index n shown above. Furthermore, when designing the reflection characteristics, the required complex refractive index can be obtained by actual measurement, but the measured value and theoretical calculation value of the complex refractive index of each substance are widely published, You may use them.

In the multi-layered film reflecting mirror having the above-mentioned structure, the substrate material can be replaced with a material generally used as a substrate for this type of multi-layered film, such as silicon or plastic, in addition to glass.

Further, besides tungsten, a heavy metal such as gold, platinum, nickel or molybdenum may be used as the material forming the high density layer, and carbon or silicon may be used as the material forming the low density layer. And so on.

Further, as a film forming method adopted when laminating the multilayer films, other general thin film forming methods such as a vacuum evaporation method and an MBE method may be adopted in addition to the sputtering method.

Next, the reflection characteristics of the multilayer-film reflective mirror of the present invention will be described below based on the measurement results. First, for measurement, a carbon layer with a thickness of 5.8 nm and a thickness of 3.8 n were placed on a glass substrate.
A structure in which a 2.8 nm thick carbon layer and a 7.6 nm thick tungsten layer are sequentially stacked on the periodic layer 3 is formed by alternately stacking 10 tungsten layers each having a thickness of m. A multilayer film mirror was used. The reflectance was measured by using X-rays having a wavelength of 0.154 nm while changing the illumination angle of the X-rays (incident angle of incident angle measured from the reflection surface). The measurement result is shown in FIG. Further, FIG. 2 also shows the measurement results when the tungsten single-layer reflecting film having a sufficient thickness and the conventional multilayer-film reflecting mirror were measured by the same method as the above.

As is apparent from FIG. 2, the reflectance of the multilayer film A1 of the present invention shows a high reflectance up to 0.65 degrees except for a small part near the glancing angle of 0.55 degrees. In contrast,
The conventional multilayer film B exhibits a high reflectance near an illumination angle of 0.65 degrees, but has a low reflectance between 0.35 and 0.55 degrees. Further, from FIG. 2, it was found that the multilayer film A1 of the present invention has a higher reflectance in a wider range than the total reflection mirror C having a single tungsten layer. This means that when the incident angle (illumination angle) is fixed and the reflectance is measured by changing the wavelength, the multilayer film of the present invention reflects X-rays in a wide wavelength range from a shorter wavelength to a longer wavelength range. Shows that you do. The theoretical calculation value of the wavelength dependence of the reflectance is shown in the graph of FIG. As is clear from this graph, the multilayer film A1 of the present invention shows a high reflectance in a wide wavelength range except for a part thereof.
That is, the multilayer film A1 of the present invention maintains the reflectance equivalent to the reflectance in the short wavelength region of the conventional multilayer film B, and in addition to this, the total reflection mirror C of the tungsten single layer can reflect. It is shown that the X-ray reflection in the long wavelength region can be covered.

Here, the most important thing in practicing the present invention is the thickness of each of the first layer 1 and the second layer 2 for achieving both a wide total reflection area and Bragg reflection by the periodic layer. The most desirable layer thickness depends on the material constituting the layer, the wavelength range used, and the like. However, generally, according to the policy of increasing the thickness of the first layer 1 without impairing the X-ray interference effect of the third and subsequent periodic layers 3, the first layer 1 Is 1/2 or more of the period length d of the periodic layer, and the layer thickness of the second layer 2 is 1/2 the period length.
It becomes the following. Then, the specific optimum values of the period length d of the periodic layer and the layer thickness of each layer could be determined by a simulation calculation based on the electromagnetic wave theory. From the calculation result, the optimum thickness of the layer 1 of the first layer is in the range of 1/2 to 3/2 of the cycle length d of the periodic layer 3, and the optimum thickness of the layer of the second layer. It has been found that the length is in the range of 1/5 to 1/2 of the period length d of the periodic layer 3.

FIG. 4 shows an example of the calculation result. In this example, the material forming the high-density layer is tungsten, the material forming the low-density layer is carbon, and the periodic layer has a thickness of 3.8 n.
a high density layer of m and a low density layer of 5.8 nm each
The structure is such that 0 layers are stacked, and the thickness of the second low-density layer 2 made of carbon is 2.8 nm. Then, the thickness of the first high-density layer 1 made of tungsten is set to three kinds of 4.9 nm, 7.6 nm and 12.6 nm, and X-rays are irradiated onto these three kinds of multilayer films at an angle of 0.6 degree. Simulation calculation was performed for the case of incidence.

As is clear from the graph of FIG. 4, as the layer thickness of the first high-density layer 1 becomes thicker, that is, as the curve in the graph shifts from A3 to A1 to A2, the long wavelength becomes longer. Although the total reflection appearing on the side tends to extend to the short wavelength side, on the contrary, the peak value of Bragg reflection tends to decrease. However, in all of these three kinds of multilayer films, Bragg reflection and total reflection in the wavelength region of 0.18 nm or more are simultaneously realized.

Here, in this example, the period length d of the periodic layer 3 is (5.8 + 3.8 = 9.6 nm), and the ratio of the layer thickness of the first high-density layer 1 to the period length d is , A3: (4.9nm → 0.51d),
A1: (7.6nm → 0.79d), A2: (12.6nm → 1.31d),
Therefore, it has been confirmed that optimum reflection characteristics can be obtained by setting the thickness of the first high-density layer 1 within the range of 1/2 to 3/2 with respect to the periodic length d. The lower limit of the layer thickness of the second low-density layer 2 is preferably about 1/5 of the cycle length d in consideration of the film-forming property of this layer and the characteristics on the interface with an adjacent layer.

[0028]

As described above, according to the present invention,
A structure in which low-density layers and high-density layers that do not impede the X-ray interference effect of the periodic layers are alternately laminated on the surface of the periodic layers in which the high-density layers and the low-density layers are periodically laminated. Therefore, it is possible to realize both Bragg reflection and total reflection with a single reflecting mirror, which enables the conventional total reflection mirror to be used for reflection under conditions where the incident angle (illumination angle) is limited. With the (single-layer film reflecting mirror), it becomes possible to reflect X-rays in the short wavelength range, which could not be reflected, in addition to X-rays in the long wavelength range. Alternatively, it is possible to obtain a multilayer-film reflective mirror having a critical angle of total reflection that has a high reflectance, which cannot be obtained by the conventional multilayer-film reflective mirror.
It is possible to realize a multilayer-film reflective mirror that can be applied to a line focusing device.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing the structure of an embodiment of the present invention.

FIG. 2 is a graph showing the measurement results of the reflectances of the multi-layered film reflecting mirror of the present invention, the conventional multi-layered film reflecting mirror, and the tungsten single-layer film reflecting mirror.

FIG. 3 is a graph showing simulation calculation values of respective reflectances of the multilayer mirror of the present invention, the conventional multilayer mirror and the tungsten single-layer mirror.

FIG. 4 is a graph showing simulation calculation values of reflectance of the multilayer-film reflective mirror of the present invention.

[Explanation of symbols]

 1 ... High-density layer (first layer) 2 ... Low-density layer (second layer) 3 ... Periodic layer 3a ... 3a ... High-density layer 3b ... 3b・ ・ ・ Low-density layer d ・ ・ ・ ・ ・ ・ Cycle length S ・ ・ ・ ・ ・ ・ Glass substrate

Claims (1)

[Claims] 1. A high-density layer and a low-density layer having different refractive indices in the X-ray wavelength region are alternately arranged and at least 6
In the multilayer film in which the layers are stacked, the high-density layer and the low-density layer have cycle lengths in order to satisfy the Bragg reflection condition in the third layer and the layer below the third layer when counted from the upper surface side of the multilayer film. d
In addition to being periodically laminated with, the thickness of the second layer above it is (1/5) d ~ (1/2) with respect to the above cycle length.
d, and the thickness of the first layer is (1/2) d to (3/2) d with respect to the above-mentioned cycle length. Reflector.