Generating micro beam of X=rays for use in EXAFS

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© 1007118

<td>(2l) Patent application: 1007118 @ Filed: 25.09.97</td><td>© Int.CI.® G21K1 / 06, H05G2 / 00, G01N23 / 20</td>

<td>© Priority: 05.02.97 JP 2250697 27.09.96 JP 25601196 10.01.97 JP 294297 @ Signed up: 31.03.98 IE 98/06 © Date: 08.05.98</td><td>© Patent holder (s): NEC Corporation of Tokyo, Japan (JP). © Inventor (s): Koichi Izumi in Tokyo (JP) @ Authorized representative: Drs. F. Barendregt et al. At 2280 GE Rijswijk.</td>

<td>© Published: 01.07.98 IE 98/07</td><td></td>

(£ 4) Method of generating an X-ray microbeam and apparatus therefor.

© A method of generating an X-ray microbeam according to the present invention generates an X-ray microbeam with a limited divergence angle and a desired flatness in areas outside the focus. With this method, it is possible to compensate for a change in the degree of asymmetry due to a change in the wavelength of the X-rays chosen and therefore to keep the degree of asymmetry constant.

In addition, the condensing conditions, including the X-ray energy and beam size, can each be set independently of the others.

An apparatus for performing the above method has also been unfolded.

NL C 1007118

The content of this patent differs from the original filed description with claim (s) and possible drawing (s). The documents originally submitted can be viewed at the Industrial Property Office.

A method of generating an X-ray microbeam and apparatus therefor.

Background of the invention

The present invention relates to a method of generating an X-ray microbeam for various types of devices using X-rays and an apparatus for performing them.

X-ray devices are widely used today. X-rays for such an application must be condensed to form a microbeam with a small beam size. Various types of X-ray condensing technologies have been proposed in the past. For example, X-rays emitted from an X-ray generator or X-ray source can be condensed to a focal position or virtual light source by an X-ray Fresnel zone plate that plays the role of a condensing element. The Fresnel zone plate can be replaced by a mirror that totally reflects X-rays based on the fact that X-rays with a refractive index of less than 1 are totally reflected when they incident on the surface of an object at an angle less than the critical angle. Japanese Laid-Open Patent Publications 62-15014 and 4-43998 each teach an arrangement that includes an asymmetric reflection type crystal collimator mounted on an X-ray input path and a mirror. X-rays of a false emission point defined by the crystal collimator and the X-rays of the original emission point are reflected to the same point by asymmetric X-ray diffraction. Furthermore, a cross-section of an X-ray beam can be limited by a slit or pin hole to form a spatially limited X-ray beam.

However, the usual schemes described above leave the following problems unresolved. The problem with an X-ray Fresnel zone plate 10071 1 fl scheme is that the focal point changes with a change in the energy of the X-rays. The problem with the total reflection mirror scheme for X-rays is that it lacks a function of choosing energy. Furthermore, the problem with the asymmetric reflection type X-ray diffraction scheme is that the degree of asymmetry changes with a change in the wavelength ie the energy of the X-rays causing the condensation effectiveness to fluctuate.

Otherwise, a solar slit or dynamic diffraction using a perfect X-ray crystal is usually used to limit the angular divergence of an X-ray beam. However, the solar slit scheme can limit the divergence angle to only the order of minutes at most so that the microbeam obtained is too wide to be called a flat wave. With respect to the perfect X-ray crystal scheme, X-rays interact hardly with a substance so that a large number of diffraction surfaces interact. That is, a large number of reflected waves contribute to interference, which produces a noticeable interference effect.

This further limits the angular spread of the deflected wave and allows, under diffraction conditions, an angular divergence defined by the direction of the scattering planes defined by the direction of the radiated X-rays and the direction of the deflected X-rays in the order of seconds.

However, the condensation of X-rays and the limitation of the divergence angle of the X-rays are usually accomplished independently of each other, which prevents the formation of an X-ray microbeam with a limited divergence angle. This is due to the fact that condensation cannot be achieved without increasing the angular divergence and the angular divergence cannot be reduced without increasing the spatial spread. Furthermore, the spatial spread can be reduced by allowing a condensation element to be present only in the focal position; in the other positions, the beam size increases. Therefore, the microbeam spreads spatially with large values ​​due to the angular divergence as the distance from the focal point increases. That is, the micro beam cannot be used in positions other than the focal position.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method capable of forming an X-ray microbeam with a limited divergence angle and a desirable condensed flatness and an apparatus for performing it.

Another object of the invention is to provide a method capable of generating an X-ray microbeam while maintaining a constant degree of asymmetry and a constant condensation efficiency even when the X-ray wavelength is changed.

In accordance with the present invention, a method of forming a flat wave X-ray microbeam comprises the steps of condensing X-rays emitted from an X-ray source to a focal point, causing diffraction with scattering planes perpendicular to each other to occur simultaneously and limiting the divergence angle of the condensed X-ray beam thereby separating a portion of the X-ray beam that may be considered to be a flat wave.

Also, in accordance with the present invention, an apparatus for generating a flat-wave X-ray microbeam has an X-ray source, a condensing element for condensing X-rays emitted from the X-ray source to a focus, and an optical element placed in a focus to limit the divergence angle of a condensed X-ray beam.

Furthermore, in accordance with the present invention, in a method of forming an X-ray microbeam by an asymmetric reflection type X-ray diffraction method using a diffraction plane that is not parallel to a crystal surface, a crystal is rotated about an axis perpendicular to the diffraction plane about the maintain angle of incidence and angle of failure of the crystal surface while maintaining a Bragg state.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, aspects and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawing in which:

FIG. 1 is a schematic view showing a conventional apparatus for condensing an X-ray beam using an X-ray Fresnel zone plate;

FIG. 2 is a schematic view showing a conventional device for condensing an X-ray beam using a total reflection mirror;

FIG. 3 is a schematic view showing a conventional apparatus for condensing an X-ray beam using a slit or a pin hole;

Fig. 4 is a sketchy image for describing a Laue-state diffraction;

Figures 5A and 5B show simultaneous reflection or multi-beam 30 diffraction in which a large number of lattice planes cooperate;

FIG. 6 is a sketchy image showing an X-ray microwave generating device embodying the present invention;

Fig. 7 is a sketchy image showing an alternative embodiment of the present invention; and Figures 8A and 8B are schematic images showing another alternative embodiment of the present invention.

H0071 18

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To better understand the present invention, a brief reference will now be made to a conventional X-ray condensing device as shown in Fig. 1. As shown, device 10 generally includes an X-ray generator or X-ray source for emitting X-rays. 14. The X-rays 14 exiting the X-ray generator 12 are condensed by an X-ray Fresnel zone plate 16 into a focal point or virtual light source 18. The X-rays Fresnel zone plate 16 is a Fresnel zone plate originally made for visible radiation and used for X-rays.

Fig. 2 shows another conventional X-ray beam condensing device. As shown, the device 10A generally includes a mirror 20 for totally reflecting X-rays instead of the Fresnel zone plate 16. This arrangement is based on the fact that since X-rays 14 have a refractive index of less than 1, they are totally reflected when they incident on the surface of the mirror 20 at an angle less than the critical angle.

Fig. 3 shows yet another conventional X-ray beam condenser. As shown, the device 10B generally reduces the cross-sectional area of ​​the X-ray beam 14 spatially using a pin hole or slit 22.

The conventional devices shown in Figures 1-3 leave some problems unresolved as discussed previously.

Basically, in accordance with the present invention, X-rays are condensed to form a microbeam. After that, part of the microbeam that can be considered a flat wave is separated. Specifically, a flat wave X-ray microbeam forming apparatus according to the present invention includes an X-ray generator or an X-ray source and a condensing element. A Borrmann element for simultaneous reflection is arranged in the focus of the condensing element. X-rays exiting the X-ray generator are limited in their divergence angle by the Borrmann element. The X-ray generator can be formed by synchrotron radiation or an X-ray tube. In a diffraction state in which divergence planes defined by the direction of the incident X-rays and that of the deflected X-rays are perpendicular to each other, the angular divergence in the direction lying in the divergent planes can be limited to the order of seconds. When the divergence angle is limited by such a dynamic diffraction, not only can a wave that is deflected in the direction of reflection but also a wave that is deflected in the direction of the transmission in divergence angle.

Fig. 4 shows a Laue state diffraction. Assume, as shown, that a single crystal of silicon 24 has a sufficient thickness. Laue state diffraction increases the X-ray beam transmission in the transmission direction compared to the case without diffraction and further limits the angular divergence. Such an anomalous transmission phenomenon is called the Borrmann effect. When a large number of lattice planes interacting in the diffraction are present, a wave appears in the transmission direction and the same number of waves as lattice planes in the reflection direction (simultaneous reflection or multi-beam diffraction). The simultaneous reflection indicates a state in which when the diffraction ^ satisfying the Bragg state ^ occurs for a given grating plane (h, k, 1) it also satisfies the Bragg state for another grating plane (m, n, o) at the same time.

Fig. 5A and 5B show simultaneous reflection to which a large number of grid surfaces are related. Fig. 5A and 5B are sectional views perpendicular to each other; Fig. 5B is a sectional view in the direction of an arrow B as shown in Fig. 5A. While the lattice plane 1007118 and the direction of the deflected X-rays, which are indicated by broken lines, are representative of diffraction that may be permissible due to the symmetry of a single crystal of silicon

26, they are not relevant to the present invention. Since the two diffraction planes are perpendicular to each other, the divergence angle of the X-ray beam in the transmission direction is limited by diffraction in the direction located in the separate scattering plane. As a result, an X-ray beam limited in two different directions is available. A slit is arranged after the Borrmann diffraction element. A portion of the X-rays transmitted and diffracted by an optical element, ie, meeting the diffraction conditions, are selectively formed on the exit side of the above slit. This successfully generates a flat-wave X-ray microbeam.

The prerequisite of the above arrangement is that the X-ray generator, condensing element, Borrmann element for simultaneous reflection and slit are arranged in sequence. If the condensing element were arranged after the Borrmann element, the divergence angle would increase and an X-ray beam with a small beam size and a small divergence angle would be prevented.

Referring to Fig. 6, an X-ray microbeam generating device embodying the present invention will be described. As shown, the mi30 includes crobeam generation device, generally indicated as

30, an X-ray generator 32 capable of X-ray. 2 gene beams with a size of 3 mm, a divergence angle of 4 mrad and a number of photons of

-9 ...

/ sec. A condensing element is designed as a

Fresnel zone plate 34. A Borrmann element 36 for simultaneous reflection comprises a single crystal of silicon that is 2 mm thick (1.4 mm or higher) and has a (001) face. A tantalum plate 38 having a thickness of 1 to 5 mm is spaced about 5 cm from the diffraction element 36 and is provided with an opening with a diameter of 5 mm. If desired, the Fresnel zone plate 34 can be replaced with a mirror that totally reflects X-rays or a Bragg-Fresnel lens which is a reflection type Fresnel lens.

In the aforementioned device 30, the X-rays exiting the X-ray generator 32 are spatially limited by the Fresnel zone plate 34 to deliver an X-ray beam. The divergence angle of the X-ray beam is limited by the Borrmann 10 diffraction element 36 arranged at the focal point of the Fresnel zone plate 34 (focal length 1 m). As a result, a flat-wave X-ray microbeam is generated. Subsequently, the diffraction element 36 causes 333, 333, 333 and 333 reflections to occur simultaneously for X-rays with a wavelength of 0.12 nm. Waves that are bent 70 ° from the incident direction are excluded by a slit 38a formed in the tantalum plate 38 so that only one wave deflected in the transmission direction is separated. Tests showed that the transmitted wave had a divergence angle of 1 second to 2 seconds and a beam diameter of up to about 10 mm.

The illustrated embodiment is not limited to the above parameters but allows to select all suitable lattice planes that fit a wavelength. For example, when X-rays with a wavelength of 0.36 mm perpendicularly incident on a silicon (001) plane, 111, 111, ΐΐΐ and 11T reflections are caused to occur simultaneously. Likewise, for 0.0-72 nm or 0.052 nm X-rays, 555, 555, 555 and 555 reflections can be used, or

777, 777, 777 and 777 reflections. Furthermore, silicon playing the role of a diffraction element can be replaced with, for example, germanium or a crystal to change the distance between the lattice planes. Such an alternative crystal is applicable for a different wavelength.

Suppose the slit 38a of the tantalum plate 38 is replaced with a pin hole. The pin hole is then located at a position where the X-rays are incident on the diffraction element, since the size of the X-ray beam at the pin hole is minimal. For this purpose, metal or the like is deposited on the incident surface of the silicon crystal of the diffraction element 36, Fig. 6, and a pin hole (up to 1 µm) is formed by a laser at the incident point. With this configuration, it is also possible to form a flat wave X-ray microbeam. As long as the silicon crystal has a sufficient thickness, the flatness of the wave is not affected by the Borrmann effect, although the intensity of the emerging beam is reduced.

As indicated above, the indicated embodiment is capable of forming an X-ray microbeam with a limited divergence angle and desired flatness in areas beyond the focus. This allows the use of a flat wave X-ray microbeam which has a sufficiently small spatial spread. Therefore, limitations imposed hitherto on the working area due to the focal point and on the working distance are overcome so that the fine structure of a fabric can be easily analyzed with, for example, X-ray analysis.

Reference is made to Fig. 7 to describe an alternative embodiment of the present invention. As shown, in this embodiment, the size of the X-ray beam is reduced by asymmetric reflection using a reflection plane that is not parallel to a crystal surface

42, ie, a lattice plane 44. A crystal 40 is rotated about an axis 46 perpendicular to the lattice plane 44 to vary the angle of incidence and the angle of projection of the crystal surface 42. This allows the asymmetric factor, ie the degree of asymmetry attributable to a change in the energy of the X-rays, to remain constant and thereby effect X-ray energy scanning without affecting the condensation efficiency. Assuming that the asymmetry is factor b, then b is expressed as follows in terms of an angle θθ between the crystal surface 42 and the incident X-rays and an angle θθ between the surface 42 and the incident X-rays:

b = είηθθ / sin θθ (equation 1).

Bending with the above degree of asymmetry increases the spatial distribution of the incident X-rays in the scattering plane by 1 / b times in terms of the X-rays falling out while increasing the angular divergence by b times. A Bragg10 corner θ_β Assuming an angle α between the lattice plane 44 relating to the diffraction and the crystal surface 42, the degree of asymmetry b is formed by:

b = sin (ö_B + a) / sin (ög-a) (equation 2) where a can run from -θ_β to θ_β.

When a large number of crystals are used to effect sequential reflection, the beam size can be further reduced. In the asymmetric reflection, by rotating the crystal 40 about the axis 46 perpendicular to the grating plane 44, it is possible to vary the angles of the X-rays incident on the crystal surface and emerging X-rays. Consequently, in the range of rotation from 0 degrees to 180 degrees, the asymmetry factor can be varied from b to 1 / b including b = 1 which applies when the rotation angle is 90 ° (a = 0). The rotation of the crystal 40 therefore compensates for a change in the wavelength (or energy) of the incident X-rays and thus for a change in the degree of asymmetry, ie the Bragg angle at which the degree of asymmetry remains constant. Furthermore, any desired condensing conditions or values ​​can be selected based on the degree of asymmetry b so that the beam size can be varied.

Fig. 8A and 8B show another alternative embodiment of the present invention. Briefly, this embodiment sequentially uses perpendicular scattering planes for reflection to reduce beam size. Furthermore, the embodiment reduces the angular width, with respect to the diffraction of incident X-rays, to the order of secon1007118 den in which an X-ray beam is generated with a limited angular width.

As shown in Figs. 8A and 8B, an X-ray beam 52 emitted from an X-ray beam generator 50 in its beam size is limited by a silicon single crystal 54 that produces an asymmetric Bragg reflection. The X-ray generator 50 is constituted by a rotary anode type X-ray generator; the bundle size is 1 x 1 mm. For X-rays of 0.05 nm, the Bragg angle for the 422 reflection is 13.0 degrees. When the crystal 54 is cut so that the angle between the (422) plane and the crystal surface is 12.0 degrees, the degree of asymmetry b is 24.3.

The X-rays diffracted by the crystal 54 are further deflected by any crystal 56 so that the beam size can be further reduced to about 10 μη as determined by experiments. Crystals 58 and 60 are arranged to define a scattering plane perpendicular to the scattering plane of crystals 54 and 56. As a result, the beam size is reduced to about 10 mm in both the horizontal and vertical directions as also determined by tests. The angular divergence of the curved X-rays was found to be 10 seconds. Then, the crystals 54-60 are rotated to emit X-rays whose wavelength is 0.15 nm. In this case, the Bragg angle and the asymmetry factor are 42.6 degrees and 57.0, respectively. Tests showed that under the above conditions, the condensing conditions changed significantly and achieved their beam size of about 5 μη.

Tests found that when the axis 46 of the individual crystal was rotated at an angle of

2.3 degrees between the emerging X-rays and the crystal surface and to achieve an asymmetry degree of about 2.4, the beam size of about 10 μη remained a wavelength despite a change. Furthermore, by varying the angle between the emerging X-rays and the crystal surface, it was possible to vary the beam 1007118 size steplessly from 10 mm to several centimeters.

As noted above, in the embodiments shown in Figures 7, 8A and 8B, the energy of a small diameter X-ray beam can be sensed over a wide range without affecting the condensing conditions. This allows EXAFS (Extended X-ray Absorption Fine Structure) or any experiment to be easily performed with a small beam size. Furthermore, the beam size can be freely varied through the condensing conditions to perform analysis of local stresses of a sample or the analysis of a fine structure. Specifically, it is possible to compensate for a change in the asymmetry degree due to a change in the wavelength of the selected X-rays, and therefore to keep the asymmetry degree constant. In addition, the condensing conditions including the X-ray energy and the beam size can each be set independently of the others. Various modifications will become possible to those skilled in the art after receiving the teachings of the present specification without going beyond the scope thereof.