JP2002350368A - Method and apparatus for measuring x-ray absorption fine structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to carry out a measurement with a high signal to noise ratio for a short time using a method and an apparatus for measuring the X-ray absorption fine structure(XAFS) of a specified element in a sample. SOLUTION: An X-ray waveguide, at least the portion of which is composed of a sample to be measured, is formed with a narrow space such as a wedge or the like. X-ray is entered the waveguide, and the attenuation of the X-ray in the waveguide is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fine structure (X-ray Absorption F) appearing near the absorption edge of an X-ray absorption spectrum.
ine Structure (hereinafter referred to as XAFS)).

[0002]

2. Description of the Related Art The X-ray absorption coefficient of a substance rapidly increases when the energy of incident X-rays reaches the binding energy (absorption edge) of the inner shell electrons of the atoms constituting the substance. Decreasing. Such vibration of the X-ray absorption spectrum is called X-ray absorption fine structure (XAFS). Among them, the structure appearing in the range of several tens of eV from the absorption edge is XANES (X-Ray Absorption Near Edge Structur
It is called e) and contains information such as electronic states and bond angles. The fine vibration structure that appears over about 1 keV on the higher energy side than XANES is EXAFS (Extended X-Ray
It is called Absorption Fine Structure, and by analyzing this, it is possible to determine the local horizontal structure (interatomic distance, coordination number, fluctuation) around the absorbing atom.

FIGS. 9 (a) to 9 (d) show XAFS measurement method 4
It is a block diagram of two main methods (A, B, C, and D, respectively). Of these, A and B are for the purpose of measuring bulk physical properties, and C and D are for the purpose of measuring physical properties of the surface. In the method A of FIG. 9A, the X-ray 4 is transmitted through the thin film sample 1 and the amount of X-ray absorption in the sample is measured by the first ion chamber and the second ion chamber (2 and 3 respectively). Things. Method B is applied when it is difficult to use a thin film sample as the measurement target, and irradiates the sample 1 with X-rays and measures the generated fluorescent X-rays 5 with a semiconductor detector (SSD) 6. is there. Methods C and D use X-rays 4
Is made to enter under conditions close to total reflection to obtain information of several nm near the surface. Among them, method C measures the amount of X-ray attenuation due to reflection, and method D detects fluorescent X-rays 5 generated from near the surface.

FIG. 10 is a characteristic graph obtained by measuring the XAFS of the Ni—P layer formed on the aluminum substrate by electroless plating according to the method B. The energy of the incident X-ray to measure the output Io and SSD output I of the first ion Chillan bar 2 is scanned, and the I / I ₀ and fluorescence intensity. The time required for the measurement is 84 minutes. At an energy of 8.35 eV (Ni absorption edge), the fluorescence intensity sharply increased, and XAFS on the high energy side.
Vibration has been observed. By analyzing this signal, for example, a radial structure function indicating the distribution of interatomic distance can be determined.

[0005]

The above-mentioned conventional XAFS
The problems of the measuring method and the measuring device are as follows. First, methods A and B are suitable for measuring physical properties of bulk, but cannot be used for surface evaluation. In contrast,
The problem with the methods C and D for measuring the physical properties of the surface is that it is difficult to obtain a sufficiently large S / N.

As described above, in the methods C and D, X-rays are incident under conditions close to total reflection, so that the amount of attenuation of X-rays due to reflection is very small. Therefore, in the method C, the change in the output I due to the change in the absorption coefficient μ of the sample is very small, and averaging of signals over a long time is indispensable to obtain a sufficient S / N ratio. Since the amount of X-ray absorption due to total reflection is small, the intensity of fluorescent X-rays is also small. Therefore, even in the method D, an averaging operation for a long time to increase the S / N ratio is required.

Although the XAFS oscillation is certainly observed in the measurement example (according to the method B) shown in FIG. 10, the S / N ratio is not sufficient. Since the fluorescence intensity is further reduced in the method D, it is apparent that the measurement time is longer than that in the method B. The present invention has been made to solve the problems in the conventional XAFS measurement method and measurement apparatus as described above, and an object of the invention is to provide a total reflection XAFS measurement method capable of obtaining a sufficient S / N ratio in a short time. It is to provide a measuring device.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a total reflection XAFS measurement method capable of obtaining a sufficient S / N ratio in a short time. This is achieved by having a waveguide, by introducing X-rays into the waveguide, and measuring the attenuation of the X-rays in the waveguide.

[0009] The present invention is based on the experimental fact that X-rays easily pass through minute gaps. For example, when two planes are overlapped and seem to be in close contact with each other, and the end is irradiated with X-rays, a considerable amount of X-rays passes through the mating surface and is emitted from the other. The inventor interpreted this phenomenon as follows. It is known that the refractive index of X-rays in the atmosphere is larger than that in a solid, contrary to visible light. Therefore, when X-rays enter the solid from the atmosphere, the glancing angle φ
(= 90 ° one incident angle θ) is smaller than the critical angle φc,
Total reflection occurs at the interface. Therefore, the gap between the mating surfaces is an excellent waveguide for X-rays. In general, light cannot pass through a waveguide if its width is less than about the wavelength of light. However, even with a smooth surface, the roughness and undulations are affected by the X-ray wavelength (~
0.1 nm), so that the X-rays can pass through the gap even if the mating surfaces are practically in close contact. The present invention has been born from such considerations.

In view of the above considerations, the present invention provides a method for measuring the X-ray absorption fine horizontal structure (XAFS) of a target element in a measurement sample, the method comprising: It is assumed that a line waveguide is provided, X-rays are incident on the waveguide, and the attenuation of the X-rays in the waveguide is measured. The measurement device has an X-ray source and two detectors, and is a device for measuring X-ray absorption fine horizontal structure (XAFS). At least a part of the measurement device is a material containing a measurement sample between the two detectors. It is assumed that an X-ray waveguide to be laid horizontally is provided, X-rays are incident on the waveguide, and the attenuation of the X-rays in the waveguide is measured.

[0011] In the first embodiment of the present invention, two coplanar samples are arranged in parallel to form a gap, and X-rays are injected into the gap. X-rays travel by repeating total reflection between the two planes. Even under total reflection conditions, light enters the solid side for a short distance (a few nm for X-rays) and then reverses. X-rays are slightly attenuated with each total reflection. Light that penetrates into a solid in total reflection is called evanescent light. Since the X-rays that have passed through the gap in this way exhibit attenuation according to the absorption coefficient of the sample, the absorption spectrum of the sample can be obtained by scanning the energy of the incident X-rays.

In the second embodiment of the present invention, a wedge-shaped gap is formed between two coplanar samples, and X-rays are obliquely incident on the wedge ridge (the bottom of a V-shaped valley). As the X-ray travels inside the gap, the direction of travel is reversed while repeating reflection in a short cycle, and is emitted outside through a U-turn. In this case, since the glancing angle φ becomes larger in the gap, the wedge angle and the incident direction are adjusted so as not to exceed a predetermined angle at the maximum φ.

[0013] The third and fourth embodiments of the present invention are as follows.
Instead of using a single planar sample, a reflective surface formed of a substance that does not substantially absorb X-rays at the target wavelength is used. In the third embodiment, two planes are arranged in parallel, and in the fourth embodiment, two planes are arranged in a wedge shape. The principle and concept are the same as those of the first and second embodiments, respectively. .

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments. In addition, the following example was implemented by the synchrotron radiation. [First Embodiment] FIG. 2 is a configuration diagram showing a device configuration for obtaining X-rays used in an experiment in a synchrotron radiation facility. The emitted light is taken out of the bending magnet 7, and is guided into the experimental hatch as incident X-rays 4 for the experiment via the two-crystal monochromatic device 9 and the condenser mirror 10. 8 in the light path is 4
This is a quadrant slit for cutting unnecessary light such as scattered X-rays to obtain a small beam with a small divergence angle.

FIG. 1 is a block diagram of a first embodiment of the present invention. The measured sample is a Ni-P layer formed by electroless plating on an aluminum substrate as in FIG. As shown in the figure, two identical samples 1 were prepared, a gap of about 200 μm was formed by sandwiching the spacer 11, and X-rays 4 were incident from the end face. First and second ion chambers 2 and 3 were inserted on the incident side and the output side, respectively, and the intensity of X-rays (Io and I, respectively) was measured. The glancing angle of the incident X-ray was 0.3 to 0.5 degrees.

FIG. 3 shows the relationship between Io and I obtained by the experiment.
This is the result of obtaining y = ln (Io / I) and plotting it against the energy of incident X-rays. y rapidly increases near the absorption edge, and thereafter decreases while repeating XAFS oscillation. The time required for the measurement was 24 minutes. Comparing this result with the measurement result by the conventional method (FIG. 10), it can be seen that in this example, the XAFS oscillation appears very clearly, and the S / N ratio is clearly higher than that of FIG. Although the measurement time was as short as 24 minutes (84 minutes in FIG. 10), a high S / N ratio was obtained, which clearly showed the superiority of the present invention. This is due to the fact that the method according to the present invention measures with light having much higher intensity than fluorescence, and that the amount of X-ray attenuation can be increased by multiple reflections in the guided wave.

Further, when comparing the result of this embodiment with FIG. 10, it is noticed that the curve shape is greatly different. In the measurement by the conventional method, the absorption increases almost stepwise at the absorption edge, whereas the measurement result according to the present invention shows that the X-ray attenuation gradually increases as approaching the absorption edge, and shows a peak at the absorption edge. It has since declined. This is considered to be because a kind of reflection spectrum that can be measured by the method according to the present invention is related to a change in refractive index (abnormal dispersion) near the absorption edge. Hereinafter, this point will be considered.

The reflectivity of X-rays is expressed by Fresnel's equation:

[0019]

[Equation 1] Can be represented by Here, n is the refractive index, and is expressed as n = 1-δ-iβ (3). The imaginary part β in the equation (3) is a part related to the absorption coefficient μ, and is expressed as β = μλ / 4π (1-δ) (4). φ _c is the critical angle, and has the relationship φ _c = √2δ (5). Using the equations (1) to (5), assuming the values of δ and β for the energy, the reflectance R
The relationship between (E, φ) and energy E can be obtained.

FIG. 4 shows the energy dependence of δ and β used in the calculation.
Expresses its existence. These values are _ThreeThe experimental data of P
It is decided to match. As this figure shows
Β is greatly affected by μ (see equation (4)),
Delta is V-shaped near the absorption edge
It can be seen that the shape changes. By δ and β in FIG.
(E, φ) was calculated. The curve of the embodiment shown in FIG.
Assuming that the number of reflections in the wave path is m, ln (1 / R)^m= -mlnR
Therefore, as a countermeasure,
-LnR was plotted against energy.

FIG. 5 shows the results. The three curves are calculated on the assumption that φ = 0.25 °, 0.30 °, and 0.35 ° from below. It can be seen that the calculation result of φ = 0.30 ° is in good agreement with the experimental result. That is, it can be interpreted that the curve of the embodiment in FIG. 4 is obtained by superimposing the XAFS vibration on the reflectance curve. In contrast to the conventional fluorescence method, a result substantially corresponding to a change in β is obtained, whereas the method according to the present invention has a peak near the absorption edge mainly due to the influence of δ due to the relationship of equation (5). It can be considered that a curve is obtained.

[Embodiment 2] FIG. 6 is a block diagram of a second embodiment of the present invention. In the first embodiment of FIG. 5, the smaller the distance between the two samples, the smaller the amount of X-rays that can be injected into the gap. Therefore, in the second embodiment shown in FIG. 6, a wedge-shaped gap is formed between two samples, and X-rays are incident obliquely on the ridge line (the bottom of the V-shaped valley) of the wedge.

Reference numeral 11 denotes a spacer for forming a gap. As the X-ray travels inside the gap, the direction of travel is reversed while repeating reflection in a short cycle, and is emitted outside through a U-turn. Therefore, when the ray is viewed from the E direction, FIG.
The route shown in the plan view of (c) will be followed. In this case, since the viewing angle φ becomes larger in the gap, the wedge angle and the incident direction are adjusted so as not to exceed a predetermined angle at the maximum φ.

According to the present embodiment, a curve having a finer structure than that of the first embodiment was obtained, and more accurate measurement was possible. This is because X-rays can be injected from a large gap, and the amount of X-rays that can be injected can be increased. [Embodiment 3] Instead of using the two flat samples of Embodiments 1 and 2, it is also possible to use a plane formed of a substance which does not substantially absorb X-rays at a target wavelength.

FIG. 7 is a block diagram of the third embodiment. In this embodiment, as the reflection plate 12, a quartz substrate coated with gold is used. Reference numeral 11 denotes a spacer for forming the gap 13 by causing the sample 1 and the reflection plate 12 to face each other in parallel. Compared with the first embodiment, the absorption is reduced by half, so that the accuracy is also reduced by half. However, the present invention can be applied to a case where the sample is a soft substance and a waveguide cannot be constituted by itself.

[Embodiment 4] FIG. 8 is a horizontal sectional view of a fourth embodiment of the present invention. In this embodiment, the reflection plate 12 has a roof shape, and the sample 1 is placed on the roof 12 so that the sample 1
A wedge-shaped gap 13 is formed between the gap 12 and the gap 12. The material of the reflection plate 12 is a quartz substrate coated with gold, as in the third embodiment.

[0027]

As described above, according to the present invention, in the total reflection XAFS measurement, a much larger amount of light can be detected as compared with the conventional fluorescence method. Ratio, high accuracy and high reliability total reflection X
AFS measurement becomes possible.

[Brief description of the drawings]

FIG. 1A is a configuration diagram of a first embodiment of the present invention, and FIG. 1B is a cross-sectional view taken along line AA in FIG.

FIG. 2 is a device configuration diagram for obtaining X-rays to be used.

FIG. 3 is a characteristic graph showing a measurement result of XAFS according to Example 1.

FIG. 4 is a graph showing the energy dependence of δ and β used for calculating the reflectance.

FIG. 5 is a graph showing a simulation result of a measurement curve according to the present invention.

6A is a configuration diagram of a second embodiment of the present invention, FIG. 6B is a cross-sectional view taken along line BB of FIG. 6A, and FIG. 6C is a plan view of FIG.

7A is a configuration diagram of a third embodiment of the present invention, and FIG. 7B is a cross-sectional view taken along line CC of FIG.

8A is a configuration diagram of a fourth embodiment of the present invention, and FIG. 8B is a cross-sectional view taken along line DD of FIG. 8A.

9A to 9D are configuration diagrams of a conventional XAFS measurement method.

FIG. 10 is a characteristic graph of the XAFS measurement result according to the conventional method B.

[Explanation of symbols]

 1 Sample 2, 3 First and second ion chamber 4 Incident X-ray S Fluorescent X-ray 6 Semiconductor detector (SSD) 7 Bending magnet 8 ‥‥‥ 4 quadrant slit 9 ‥‥‥ 2 crystal monochromator 10 ‥‥‥ Condenser mirror 11 ‥‥‥ Spacer 12 ‥‥‥ Reflector 13 ‥‥‥ Gap

Claims (7)

[Claims] 1. A method for measuring X-ray absorption fine structure (XAFS) of a target element in a measurement sample, comprising: providing an X-ray waveguide at least a part of which is laterally formed of a material containing the measurement sample. A XAFS measurement method, comprising: X-rays being incident on a waveguide, and the amount of attenuation of the X-rays in the waveguide is measured. 2. An apparatus for measuring X-ray absorption fine structure (XAFS), comprising an X-ray source and two detectors, wherein at least a part of the X-ray absorption fine horizontal structure (XAFS) is a material containing a measurement sample between the two detectors. An XAFS measurement apparatus, comprising: a horizontal X-ray waveguide; and X-rays incident on the waveguide and measuring the amount of X-ray attenuation in the waveguide. 3. An X-ray waveguide according to claim 2, wherein the same two materials are opposed to each other in parallel to constitute an X-ray waveguide.
AFS measurement device. 4. The XAFS measuring apparatus according to claim 2, wherein the wedge-shaped gap formed by the same two samples is an X-ray waveguide. 5. An X-ray waveguide in which a plane formed of a substance having substantially no absorption of X-rays at a target wavelength is opposed to a sample surface in parallel.
2. The XAFS measurement device according to 1. 6. The X-ray waveguide according to claim 2, wherein a wedge-shaped gap formed between a plane formed by a substance having substantially no absorption of X-rays at a target wavelength and a sample surface is used as an X-ray waveguide. XAFS measurement device. 7. The XAFS measuring apparatus according to claim 4, wherein the wedge angle and the incident direction are adjusted so as not to exceed a predetermined angle at the maximum glancing angle φ.