Method for manufacturing a channel cut X-ray optical element

Abstract

To provide a channel cut X-ray optical element including a refection surface opposite to a channel cut crystal, on which an affected layer introduced by machine work is removed by PCVM using high-pressure plasma at approximately atmospheric pressure and which has surface flatness of 1 μm or less, surface roughness of 1 nmRMS or less, and peak X-ray reflectance of 90% or more of a theoretical value, and its manufacturing method.SOLUTION: A pressure of a process gas which contains a reactant gas having an etching property relative to an optical element material is 10 kPa to 1000 kPa. A high-frequency voltage is applied to an electrode arranged opposite to an inner wall surface of a channel so that high-frequency plasma is locally generated. The local plasma relatively scans the inner wall surface so that an affected layer of the inner wall surface is removed.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a channel-cut X-ray optical element and a method for manufacturing the same, and more particularly to a channel-cut X-ray optical element with a high-quality X-ray Bragg reflection surface and a method for manufacturing the same.

Background Art

[0002] A channel-cut crystal uses the opposing two groove-shaped surfaces formed on the surface of a bulk single crystal material as Bragg reflection surfaces, and is an X-ray optical element that can extract a monochromatic X-ray beam in a direction parallel to the incident direction by reflection on the two surfaces (see Patent Document 1). Since this reflection surface is an inward-facing surface, it is difficult to perform high-precision polishing, and there has been a problem that it is difficult to completely remove the processing strain introduced during the formation of the groove by a mechanical processing method such as grinding. The presence of processing strain on the reflection surface means that there are a large number of defects in the crystal lattice microscopically, and the Bragg reflectivity will be greatly reduced from the theoretical value due to the lattice defects.

[0003] In general wet etching as a method for removing the processing strain of the reflection surface, since the etching rate depends on the crystal plane orientation, bunching occurs on the etching surface or etching pits are formed, so it is difficult to remove the processing strain while maintaining a good surface shape. The processing strain exists at a depth of several micrometers from the surface, and at most within 10 micrometers, and this is called a processed altered layer.

[0004] Therefore, the present inventors devised to remove the processed altered layer of the X-ray reflecting surface of the channel cut crystal by using PCVM (Plasma Chemical Vaporization Machining), which is a distortion-free etching technique using high-pressure plasma (see Non-Patent Documents 1 and 2). The basic principle of PCVM using high-pressure plasma near atmospheric pressure is disclosed in Patent Document 2, and various proposals have been made regarding the form of the electrode, such as a wire electrode, a rotating electrode, a nozzle electrode for ejecting a process gas, or a narrow blade electrode (see Patent Documents 2 to 4). Furthermore, a method of applying a pulse-modulated high-frequency voltage for controlling the plasma has also been proposed (see Patent Document 5).

[0005] However, since the FZ-Si single crystal used for the channel cut crystal has high purity and a large specific resistance, a sufficient electric field strength cannot be obtained near the electrode, and it is difficult to stably generate plasma. Furthermore, for the channel cut crystal, it is necessary to accurately process the two opposing surfaces. However, when the channel width is narrow and there is a portion (overlapping portion) where the normal direction of one of the paired reflecting surfaces intersects with the other surface, the conventional PCVM method cannot be used as it is.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] Therefore, in view of the above situation, the present invention aims to remove the processed affected layer introduced by machining on the opposing reflecting surfaces of a channel-cut crystal by PCVM using high-pressure plasma near atmospheric pressure, and provides a channel-cut X-ray optical element having a reflecting surface with a surface flatness of 1 μm or less, a surface roughness of 1 nm RMS or less, and a peak reflectivity for X-rays of 90% or more of the theoretical value, and a method for manufacturing the same.

Means for Solving the Problems

[0009] The present invention constitutes a channel-cut X-ray optical element and a method for manufacturing the same configured as follows in order to solve the above problems.

[0010] (1) A channel-cut X-ray optical element in which a channel having a pair of parallel inner wall surfaces facing at least a specific crystal plane is formed in an optical element material made of a single crystal with an impurity concentration of 0.1 ppm or less, and the inner wall surfaces of the channel are used as Bragg reflecting surfaces that reflect using the diffraction phenomenon of X-rays, wherein the Bragg reflecting surface has a surface flatness of 1 μm or less, a surface roughness of 1 nm RMS or less, and a peak reflectivity for X-rays of 90% or more of the theoretical value, Channel-cut X-ray optical element.

[0011] (2) The channel-cut X-ray optical element according to (1), wherein the optical element material is single-crystalline silicon, single-crystalline germanium, or single-crystalline diamond.

[0012] (3) The channel-cut X-ray optical element according to (1) or (2), wherein the distance between the opposing Bragg reflection surfaces is 20 mm or less.

[0013] (4) The channel-cut X-ray optical element according to (3), wherein there is a portion where the normal direction of one surface of the pair of opposing Bragg reflection surfaces intersects the other surface, and the distance between the opposing Bragg reflection surfaces is 2 mm to 50 μm.

[0014] (5) A method for manufacturing a channel-cut X-ray optical element, in which a channel having at least a pair of parallel inner wall surfaces facing a specific crystal plane is formed in an optical element material made of a single crystal with an impurity concentration of 0.1 ppm or less, and the inner wall surfaces of the channel are used as Bragg reflection surfaces that reflect using the X-ray diffraction phenomenon, A pre-step of forming a structure including the channel in the optical element material, A process gas containing a reactive gas having etching properties with respect to the optical element material has a pressure of 10 kPa to 1000 kPa, a high-frequency voltage is applied to an electrode disposed opposite to the inner wall surface of the channel to locally generate high-frequency plasma, the local plasma is scanned relative to the inner wall surface to remove the processed altered layer of the inner wall surface, and a reflection surface high-quality improvement step of forming the Bragg reflection surface having a surface flatness of 1 μm or less, a surface roughness of 1 nm RMS or less, and a peak reflectivity with respect to X-rays of 90% or more of the theoretical value, A method for manufacturing a channel-cut X-ray optical element, including:

[0015] (6) The method for manufacturing a channel-cut X-ray optical element according to (5), wherein the pre-step includes a step of precisely polishing at least the inner wall surface of the channel.

[0016] (7) The channel cut X-ray optical element according to (5) or (6), wherein the optical element material is single crystal silicon, single crystal germanium or single crystal diamond.

[0017] (8) The method for manufacturing a channel cut X-ray optical element according to any one of (5) to (7), wherein the removal amount of the inner wall surface of the channel in the step of improving the quality of the reflection surface is 10 μm or more.

[0018] (9) The method for manufacturing a channel cut X-ray optical element according to any one of (5) to (8), wherein the electrode is a wire electrode in which a conductive wire is stretched on a support, a rotary electrode in which a conductive rotationally symmetric body is fixed to a rotation axis, a conductive nozzle electrode or blade electrode for ejecting a process gas.

[0019] (10) The method for manufacturing a channel cut X-ray optical element according to (9), wherein the electrode is the wire electrode, and a plasma generation mechanism for applying a pulse-modulated high-frequency voltage to the wire electrode is used.

[0020] (11) The method for manufacturing a channel cut X-ray optical element according to (9), wherein the electrode is the rotary electrode, the nozzle electrode or the blade electrode, and a plasma generation mechanism for arranging a ground auxiliary electrode on the back surface opposite to the inner wall surface of the channel and applying a high-frequency voltage to the electrode is used.

[0021] (12) The method for manufacturing a channel cut X-ray optical element according to any one of (9) to (11), wherein the distance between the opposing Bragg reflection surfaces is 20 mm or less.

[0022] (13) The channel cut X-ray optical element according to (10), wherein there is a portion where the normal direction of one surface of the pair of Bragg reflection surfaces intersects the other surface, and the distance between the opposing Bragg reflection surfaces is 2 mm to 50 μm.

Advantages of the Invention

[0023] The manufacturing method of the channel cut X-ray optical element of the present invention as described above can provide a channel cut X-ray optical element having a reflecting surface with a surface flatness of 1 μm or less, a surface roughness of 1 nm RMS or less, and a peak reflectivity for X-rays of 90% or more of the theoretical value, by removing the processed altered layer of the reflecting surface introduced by machining with PCVM using high-pressure plasma near atmospheric pressure.

[0024] Also, even in the case of a rotary electrode, a nozzle electrode, or a blade electrode, by disposing a ground auxiliary electrode on the back surface opposite to the inner wall surface of the channel, or by using a wire electrode with a high electric field strength, plasma can be stably generated and processed even with a high-resistance optical element material. In particular, by using a plasma generation mechanism in which a pulse-modulated high-frequency voltage is applied to the wire electrode, plasma can be stably generated and the processed altered layer can be removed with high efficiency even when the channel width is narrow or when there are overlapping portions on the reflecting surface. It should be noted that the wire electrode has advantages over other electrodes in that it can be applied even to small channel cut crystals that are expected to be developed in the future, particularly to high-quality channel cut X-ray optical elements with a channel width of 2 mm to 50 μm, and that stable processing is possible up to the crystal end.

Brief Description of the Drawings

[0025]

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Mode for Carrying Out the Invention

[0026] Next, based on the embodiments shown in the accompanying drawings, the present invention will be described in more detail. FIG. 1 is a schematic diagram of a channel cut X-ray optical element (channel cut crystal) of the present invention, FIG. 2 is a plan view thereof, and FIG. 3 shows an example of its use.

[0027] The channel cut X-ray optical element of the present invention is produced by cutting out from an optical element material made of single crystal silicon, single crystal germanium or single crystal diamond. This optical element material has an impurity concentration of 0.1 ppm or less and is highly resistive because of its high purity. Therefore, since it is more highly resistive than a silicon wafer for manufacturing general semiconductor elements, it is difficult to stably generate plasma between the electrodes. Currently, the available FZ-Si single crystal has a purity of 99.999999999% (impurity concentration of 0.001 ppm), and the FZ-Ge single crystal has a purity of 99.9999999% (impurity concentration of 0.1 ppm).

[0028] As shown in FIGS. 1 and 2, the channel cut X-ray optical element 1 of the present invention has a protrusion 4 having a first Bragg reflection surface 3 and a protrusion 6 having a second Bragg reflection surface 5 on the upper surface of a substantially rectangular parallelepiped base 2, and other portions interfering with the X-ray optical path are removed. Both Bragg reflection surfaces 3 and 5 are completely parallel and face a specific crystal plane. Further, the base 2 of the channel cut X-ray optical element 1 is fixed at an appropriate position of an X-ray generation facility (a synchrotron radiation facility (such as SPring-8), an X-ray free electron laser (XFEL) facility (such as SACLA)) during use. In order to prevent the strain from the fixed portion of the base 2 from propagating to the Bragg reflection surfaces 3 and 5, staggered slits 7 and 7 are formed from both side surfaces to form an S-shaped structure.

[0029] The manufacturing method first undergoes a pre - process of forming, by machining, a structure including a channel 8 having a pair of parallel inner wall surfaces 9, 9 facing at least a specific crystal plane in an optical element material. The inner wall surfaces 9, 9 of the channel 8 become Bragg reflection surfaces 3, 5 that reflect using the X - ray diffraction phenomenon. This pre - process usually includes a process of precisely polishing at least the inner wall surfaces 9, 9 of the channel. When normal mechanical precision polishing is impossible, polishing is done manually. Machining includes, in addition to precision cutting, processing techniques used in semiconductor processing such as dicing and wire sawing.

[0030] The distance (channel width) W between the opposing Bragg reflection surfaces is determined by the energy range of the X - ray and the allowable delay time during X - ray Bragg reflection. The channel width W is set narrower as the energy range of the X - ray is wider and as the allowable delay time is shorter. The channel width W at which the features of the present invention are exhibited is approximately 20 mm or less, which is difficult to manufacture, but it may also be 20 mm or more. In particular, when the channel width W is less than 2 mm, the manufacturing difficulty becomes extremely high. The lower limit of the channel width W is about 50 μm for reasons described later.

[0031] Next, the inner wall surfaces 9, 9 of the channel are refined using the PCVM technique. This PCVM method is a processing method that excites atoms with high chemical activity such as halogen in a high - pressure atmosphere (near atmospheric pressure) in a spatially - localized high - frequency (for example, 150 MHz) plasma to generate high - density neutral radicals, reacts with the workpiece, and removes it by changing it into a volatile substance (reaction product). By setting the processing atmosphere to high pressure, high - speed processing can be realized at a removal rate comparable to machining while enhancing the spatial resolution.

[0032] Specifically, the pressure of the process gas containing a reactive gas having an etching property with respect to the optical element material is 10 kPa to 1000 kPa, and a high-frequency voltage is applied to the electrodes disposed opposite to the inner wall surfaces 9, 9 of the channel 8 to locally generate high-frequency plasma. The local plasma is scanned relative to the inner wall surfaces 9, 9 to remove the processed and modified layer on the inner wall surfaces 9, 9, and a reflective surface high-quality improvement process for forming the Bragg reflective surfaces 3, 5 with a surface flatness of 1 μm or less, a surface roughness of 1 nm RMS or less, and a peak reflectivity with respect to X-rays of 90% or more of the theoretical value is performed. More preferably, the peak reflectivity with respect to X-rays is 95% or more of the theoretical value.

[0033] Here, those that can be used as the electrodes are a wire electrode with a conductive wire stretched on a support, a rotary electrode with a conductive rotationally symmetric body fixed to a rotating shaft, a conductive nozzle electrode or blade electrode that ejects a process gas. The processing depth required to remove the processed and modified layer introduced during machining on the inner wall surfaces 9, 9 of the channel 8 is empirically sufficient at 10 μm, and most of it can be removed even at about 5 μm. Also, a predetermined gap is required between the electrode and the processing surface (inner wall surface 9). This gap depends on the frequency of the high-frequency power supply applied to the electrode, the type and pressure of the process gas, but conditions under which plasma can be stably generated are used. Usually, in PCVM, the gap between the electrode and the surface to be processed is set to 10 μm to several millimeters.

[0034] The process gas includes a reactive gas having an etching property with respect to the optical element material and a noble gas as a carrier gas, and the pressure is 10 kPa to 1000 kPa. This pressure corresponds to a range of about 1 / 10 to 10 atmospheres centered around the atmospheric pressure. When the pressure of the process gas is high, the plasma generation region is restricted near the electrode, enabling spatially controlled processing. On the other hand, it becomes difficult to generate and maintain the plasma.

[0035] As reaction gases in PCVM, halogen-based gases, oxygen gas, ozone gas, etc. are used. As halogen-based gases, there are fluorine-based gases and chlorine-based gases. Examples of fluorine-based gases include F2, CF4, C2F6, C3F6, C4F8, SF6, etc. Examples of chlorine-based gases include Cl2, BCl3, CCl4, etc. There is also CClF3 which contains both fluorine and chlorine. The reaction gas is appropriately selected according to the material of the object to be processed. For example, if the object to be processed is silicon or germanium, SF6 is selected, and if it is diamond, SF6 or O2 is selected.

[0036] As the carrier gas, an inert gas represented by noble gases is used. Examples of noble gases include He, Ne, Ar, Xe, etc.

[0037] In addition, there may be cases where a gas that contributes to the generation and stabilization of plasma is added.

[0038] The electrode material is made of a highly corrosion-resistant material such as stainless steel, nickel, aluminum, etc. that has excellent corrosion resistance according to the reaction gas. It is also preferable to coat the electrode with a material that has excellent corrosion resistance against the reaction gas. For example, the problem of corrosion can be solved by coating and dipping the surface with a material having corrosion resistance such as Al2O3, MgF2, Y2O3 (yttria), etc.

[0039] When the Bragg reflection surfaces 3 and 5 are located at positions separated in the groove direction P of the channel and there is no portion (overlapping portion) where the normal direction of one surface of the paired Bragg reflection surfaces 3 and 5 intersects the other surface, if the portions of the inner wall surfaces 9, 9 of the channel 8 that are not necessary in advance are removed during pre-processing, since there is no protrusion 4 or protrusion 6 behind one inner wall surface 9 to be processed, the restriction on the size of the electrode is relaxed and various electrodes can be used. However, when the channel width W is narrow and there is an overlapping portion of the Bragg reflection surfaces 3 and 5, the size of the electrode must be sufficiently smaller than the channel width W. The dimension of the electrode orthogonal to the inner wall surface 9 to be processed must be approximately half or less of the channel width W. When the channel width W becomes about 5 mm, it is difficult to use a rotary electrode or a nozzle electrode. Furthermore, when the channel width W becomes 2 mm or less, it becomes difficult to use a blade electrode. When the channel width W becomes 2 mm or less, only a wire electrode is used. Even when using a wire electrode, considering the diameter of the wire, the gap, and the removal amount of the processed heat-affected layer, the lower limit of the channel width W is approximately 50 μm, and groove formation can be achieved by a dicing blade in the previous process up to this extent.

[0040] As an example of the use of the channel-cut X-ray optical element of the present invention, the one shown in FIG. 3 is a monochromator in which two line-symmetric elements 1A and 1B are arranged at a Bragg reflection angle with respect to the X-ray beam B. The incident X-rays are reflected by the first Bragg reflection surface 3 of the first channel-cut X-ray optical element 1A, then reflected by the second Bragg reflection surface 5, and further reflected by the second Bragg reflection surface 5 of the second channel-cut X-ray optical element 1B, and then reflected by the first Bragg reflection surface 3. The outgoing X-rays return on the extension line of the incident X-rays after four reflections. With this monochromator, even if the wavelength (energy) of the incident X-rays has a spread, the wavelength (energy) of the outgoing X-rays after four reflections is monochromatized to match the ideal Bragg reflection angle. In FIG. 3, the solid line represents the case where a channel-cut X-ray optical element with a wide channel width W is used, and the imaginary line represents the case where a channel-cut X-ray optical element with a narrow channel width W is used. Since the delay time of the X-rays is caused by this increase in the optical path length, if a channel-cut X-ray optical element with a narrow channel width W is used, the delay time will also be shortened.

[0041] FIG. 4 shows the case where the inner wall surfaces 9, 9 of the channel 8 are processed using the wire electrode 10, and FIG. 5 shows the case where they are processed using the nozzle electrode 20. As shown in FIG. 4, the wire electrode 10 has a structure in which a conductive wire 12 is stretched between the tips of an arm-shaped support 11 with a predetermined tension. Then, the pre-processed optical element material is held on a moving stage 14 disposed inside the chamber 13, the wire 12 of the wire electrode 10 is arranged in parallel with one inner wall surface 9 with a predetermined gap, and the wire 12 is scanned in the width direction of the Bragg reflection surface 3 or the Bragg reflection surface 5. Further, a process gas supply system 15 and an exhaust system 16 are connected to the chamber 13. After evacuating the inside of the chamber 13 with the exhaust system 16, a process gas at a predetermined pressure is filled from the process gas supply system 15. Then, a high-frequency voltage is applied to the wire electrode 10 from a high-frequency power supply 17, and a high-frequency plasma is generated around the wire 12 for processing.

[0042] As shown in Fig. 5, the nozzle electrode 20 is arranged with a predetermined gap with respect to one inner wall surface 9, and while ejecting a process gas from the nozzle electrode 20, a high-frequency voltage is applied to the nozzle electrode 20 to generate plasma in the gap. In this case, the ground auxiliary electrode 21 is arranged and grounded on the side surface of the protruding portion 6 corresponding to the back surface on the opposite side of the inner wall surface 9, so that plasma can be stably generated in the gap even if the optical element material has a high resistance. In addition, even in the case of processing by the nozzle electrode 20, as shown in Fig. 4, the chamber 13, the moving stage 14, the process gas supply system 15, the exhaust system 16, and the high-frequency power supply 17 are provided. Also, the same applies when a rotating electrode or a blade electrode is used instead of the nozzle electrode 20.

[0043] Figs. 6 to 8 show processing examples using the wire electrode 10 in the case where the channel width W is narrower than 2 mm and there is a portion (overlapping portion) where the normal direction of one surface of the pair of Bragg reflecting surfaces 3 and 5 intersects the other surface. In this case, the wires 12 are arranged at equal distances from both inner wall surfaces 9, 9 of the channel 8 to process both inner wall surfaces 9, 9 simultaneously. Here, if a high-frequency voltage is continuously applied to the wire electrode 10, plasma maintenance will eventually become impossible in the overlapping portion of both inner wall surfaces 9, 9. This is presumably because the plasma generation region changes from the overlapping portion where plasma maintenance is difficult after plasma generation to only the non-overlapping portion where it is relatively easy. It has been found that this problem can be solved by pulse-modulating the high-frequency voltage as shown in Fig. 8 and repeating the process of turning off the plasma and then turning it on again before the plasma generation region changes.

[0044] Ultra-narrow channel-cut crystals (μCCs) with a channel width of sub-mm are key devices for the advancement of XFEL light sources. In X-ray spectroscopy experiments that require high energy resolution, probe X-rays with a narrow bandwidth are essential. However, there has been a dilemma in that the light intensity decreases drastically (typically by two to three orders of magnitude) as the bandwidth is restricted. Conventional self-seeding techniques utilize a transmission monochromator based on forward Bragg diffraction from a diamond crystal. However, there are problems such as low monochromatic light generation efficiency, strong transmitted light coaxial with the transmitted light, resulting in a poor signal-to-noise ratio and an XFEL intensity smaller than expected. The self-seeding technique using ultra-narrow channel-cut crystals is a useful method to resolve this dilemma. By improving the monochromaticity of the oscillating XFEL itself, it is possible to generate high-intensity monochromatic XFELs without using a transmission monochromator that causes intensity loss. However, it has been extremely difficult with conventional techniques to remove the processed altered layer on the Bragg reflecting surfaces 3, 5 in such ultra-narrow channel-cut crystals.

[0045] Figure 9 shows the crystal plane dependence of the machining rate (MMR) when each crystal orientation of Si was removed by PCVM using a nozzle electrode. As samples, FZ-Si (100), (110), (111), (422), (511) wafers were prepared. Plasma was generated between the electrode tip and the wafer in an atmospheric pressure He environment, and the process gas was directly supplied to the plasma region through the flow path inside the electrode. Uniaxial scanning machining was performed six times for each wafer, and the MRR was derived from the area of each cross-sectional profile. The machining conditions were a pressure of 100 kPa, a process gas (He:SF6 = 99.5:0.5), a flow rate of 100 sccm, a scanning speed of 10 mm / min, a gap of 0.3 mm, a power of 150 MHz, and 17 W.

[0046] An X-ray crystal optical element is required to be set in various plane orientations according to requirements. However, Si crystals are anisotropic materials with significantly different atomic densities for each crystal plane, and there are many processing methods that exhibit characteristics dependent on the crystal plane. When the crystal plane dependence is strong, it is necessary to optimize the conditions for each crystal plane, and it becomes difficult to obtain a flat and smooth surface due to the generation of etch pits. As shown in Fig. 9, the ratio of MMR is (100):(110):(111):(422):(511) = 1.03:1.04:1.00:1.01:0.95, and it can be said that the difference in the processing amounts of each crystal plane is at the error level. In the plasma process using F radicals, it has been shown that equivalent processing characteristics can be obtained without dependence on any crystal plane, and a flat and smooth surface without etch pits can be obtained.

[0047] As shown in Fig. 5, typical examples of the double reflection image of the X-ray beam when the processing distortion of the reflection surface of the Si channel-cut crystal was removed by PCVM using a small plasma generation electrode (nozzle electrode 20) accessible to the inner wall surface 9 of the channel 8 are shown in Figs. 10(a) and (b). The scratch-like dark lines that existed innumerably before PCVM processing have disappeared due to PCVM processing, indicating that the processed altered layer existing on the reflection surface of the channel-cut crystal can be removed by PCVM. Also, as a result of measuring the reflectivity from the values of the intensity monitors arranged before and after the channel-cut crystal, it was confirmed that the reflectivity improved from the initial 71.5% to 84.1% by PCVM processing (the theoretical value is 84.4%), and the effect was confirmed.

Example

[0048] A channel-cut X-ray optical element (microchannel-cut crystal (μCC)) is manufactured in which the interval (channel width) between the opposing Bragg reflection surfaces 3 and 5 is 100 μm and which has a portion (overlapping portion) where the normal direction of one surface of the pair of Bragg reflection surfaces 3 and 5 intersects the other surface.

[0049] As a pre-treatment of PCVM, it was cut out from an FZ-Si crystal block, a channel with a width of 60 to 70 μm and a depth of 0.6 mm was formed by a dicing blade, then solution etching was carried out, and the surface was finished by hand polishing. The inner wall surface of the channel was set to Si(220). In order to suppress the channel width to 100 μm, the channel width in the pre-treatment was set to 80 μm, and in PCVM using a wire electrode with a diameter of 50 μm, 10 μm was removed from each of the inner wall surfaces 9, 9.

[0050] The fabricated microchannel-cut crystal was evaluated using synchrotron radiation X-rays. Monochromatic light of 10 keV supplied from a two-crystal spectrometer was cut out to 50×500 μm smaller than the aperture of the crystal by a slit and irradiated onto the microchannel-cut crystal. The transmitted light was blocked using a beam stop. As described above, the reflection profile and reflectivity were obtained in a form close to the actual use conditions, and the effectiveness of PCVM was investigated. Fig. 11 shows the incident angle dependence of the reflectivity of the Si(220) crystal before and after PCVM treatment. From the reflection profile (not shown), although intensity modulation was observed in the profile even after PCVM treatment, clarification of the edge part cut out at the aperture and an increase in the reflection intensity were recognized due to PCVM treatment. Also, although the peak value of the reflectivity characteristics of the treated crystal was slightly smaller than the calculated value, it was clearly improved compared to before treatment, and diffraction characteristics close to the theoretical value were obtained. Specifically, the reflectivity was improved from 32% before PCVM treatment to 38% (theoretical value 40%) after treatment. This value corresponds to 95% of the theoretical peak reflectivity for X-rays.

[0051] In the case of a channel-cut X-ray optical element (micro-channel cut crystal) with a Bragg reflecting surface 3,5 having an overlapping portion and a channel width of sub-mm, it is difficult to directly observe the reflecting surface after processing by PCVM. Therefore, as shown in Fig. 12, two Si wafers W1 and W2 were fixed to a support base 30 with a spacing of 100 μm and an overlapping portion of 2 mm, and wire electrode 10 equipped with a wire 12 with a diameter of 50 μm was processed in a stationary state. The processing conditions were a pressure of 100 kPa, a process gas (He:SF6 = 99:1), a wire diameter of 50 μm, a gap of 30 - 40 μm, a power of 150 MHz, an ON time of 2 μs, and an OFF time of 40 μs.

[0052] As shown in Fig. 13, after the PCVM treatment, the Si wafers W1 and W2 were removed from the support base 30, and the processed surfaces were observed. In Fig. 13, the positions of the processing marks 31 and the profile observation lines 32 and 33 are shown as seen from the processed surface side for the Si wafer W1 and from the back side (opposite side to the processed surface) for the Si wafer W2. Also, the profile observation lines 32 and 33 are at approximately the same position in the overlapping region 34.

[0053] Fig. 14(a) shows the profile at the profile observation line 32 of the Si wafer W1, and Fig. 14(b) shows the profile at the profile observation line 33 of the Si wafer W2. As a result, it was confirmed that even with a channel width of sub-mm, it is possible to simultaneously process the overlapping portion of the reflecting surface by using a wire electrode. The slight difference between the two profiles is presumably due to the wire electrode being slightly displaced from the center between the two Si wafers W1 and W2, and the decrease in the removal amount at the center of the profile is presumably due to the supply of the process gas being stagnant in the gap. However, in the actual processing of the reflecting surface, the wire electrode is scanned in the width direction of the reflecting surface, so the processing amount is averaged and there is no problem.

[0054] Finally, in PCVM using the wire electrode 10, the machining characteristics by a pulse-modulated high-frequency voltage were investigated. The experimental setup is shown in Fig. 15. A sample 41 made of an Si wafer was fixed on the upper surface of a support base 40 fixed to a moving stage 14, and the wire 12 of the wire electrode 10 was arranged parallel to the surface of the sample 41. The machining conditions were a pressure of 100 kPa, a process gas (He:SF6 = 99.5:0.5), a wire diameter of 50 μm, a gap of 100 μm, a power of 150 MHz, an ON time of 25 - 50 μs, and an OFF time of 200 - 950 μs.

[0055] The machining depth at the center of the sample in the stationary machining mark with an ON / OFF time = 50 / 450 μs was 2.67 μm, and the machining depths at both ends of the sample were 2.77 μm and 3.01 μm, respectively. As a result, it was found that when using a wire electrode, a machining amount equivalent to that at the center can be obtained even at the end of the reflecting surface.

[0056] Next, Fig. 16 shows the change in the surface roughness (RMS) when the ON time of the high-frequency voltage was fixed at 50 μs and the OFF time was changed to 200, 450, and 950 μs. Fig. 17 shows the surface roughness with respect to the duty ratio for the results of Fig. 16. As a result, when the OFF time is 200 μs (duty ratio 20%), the surface roughness deteriorates, but it is presumed that this is due to the reattachment of reaction products to the surface. And when the OFF time is lengthened to 450 μs (duty ratio 10%) and 950 μs (duty ratio 5%), in other words, when the duty ratio is decreased, it was found that the deterioration of the surface roughness by PCVM can be prevented. Incidentally, Fig. 18(a) shows the surface observation result of the sample before PCVM machining, (b) shows the surface observation result after machining with an OFF time of 200 μs, and (c) shows the surface observation result after machining with an OFF time of 450 μs, respectively.

Explanation of Symbols

[0057] 1, 1A, 1B Channel-cut X-ray optical element (channel-cut crystal) 2 Base 3 First Bragg reflecting surface 4 Protrusion 5 Second Bragg reflecting surface 6 Protrusion 7 Slit 8 Channel 9 Inner wall surface 10 Wire electrode 11 Support 12 Wire 13 Chamber 14 Moving stage 15 Process gas supply system 16 Exhaust system 17 High-frequency power supply 20 Nozzle electrode 21 Earth auxiliary electrode 30 Support base 31 Processing marks 32 Profile observation line 33 Profile observation line 34 Overlap region 40 Support base 41 Sample B X-ray beam P Groove direction W Channel width W1 First Si wafer W2 Second Si wafer

Claims

1. A method for manufacturing a channel-cut X-ray optical element, wherein a channel having a pair of parallel inner wall surfaces facing at least a specific crystal plane is formed in an optical element material made of a single crystal with an impurity concentration of 0.1 ppm or less, and the inner wall surfaces of the channel are used as Bragg reflection surfaces that reflect using the X-ray diffraction phenomenon, wherein in the channel-cut X-ray optical element, there is a portion where the normal direction of one surface of the pair of Bragg reflection surfaces intersects the other surface, and the distance between the opposing Bragg reflection surfaces is 50 μm or more and 20 mm or less, a pre-step of forming a structure including the channel in the optical element material, a process gas containing a reactive gas having etching properties with respect to the optical element material has a pressure of 10 kPa to 1000 kPa, a high-frequency voltage with pulse modulation is applied to a wire electrode disposed opposite to the inner wall surface of the channel to locally generate a high-frequency plasma, the local plasma is scanned relative to the inner wall surface to remove a processed altered layer of the inner wall surface, and a reflection surface high-quality improvement step of forming the Bragg reflection surface having a surface flatness of 1 μm or less, a surface roughness of 1 nm RMS or less, and a peak reflectivity with respect to X-rays of 90% or more of the theoretical value, A method for manufacturing a channel-cut X-ray optical element, including the above steps.

2. The method for manufacturing a channel-cut X-ray optical element according to claim 1, wherein the pre-step includes a step of precisely polishing at least the inner wall surface of the channel.

3. The method for manufacturing a channel-cut X-ray optical element according to claim 1 or 2, wherein the optical element material is single crystal silicon, single crystal germanium, or single crystal diamond.

4. The method for manufacturing a channel-cut X-ray optical element according to any one of claims 1 to 3, wherein the removal amount of the inner wall surface of the channel in the reflection surface high-quality improvement step is 10 μm or more.

5. The method for manufacturing a channel-cut X-ray optical element according to claim 1, wherein the distance between the opposing Bragg reflection surfaces is 2 mm to 50 μm.

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