JP7704332B2 - Method for manufacturing channel-cut X-ray optical element - Google Patents

Description

The present invention relates to a channel-cut X-ray optical element and a manufacturing method thereof, and more specifically to a channel-cut X-ray optical element with a high-quality X-ray Bragg reflection surface and a manufacturing method thereof.

A channel-cut crystal is an X-ray optical element that uses two opposing surfaces of a groove formed on the surface of a bulk single crystal material as Bragg reflecting surfaces, and can extract a monochromatic X-ray beam in a direction parallel to the direction of incidence by reflection on the two surfaces (see Patent Document 1). Because this reflecting surface faces inward, it is difficult to polish it with high precision, and there is an issue that it is difficult to completely remove the processing distortion introduced when forming the grooves by mechanical processing methods such as grinding. The presence of processing distortion on the reflecting surface means that microscopically there are many defects in the crystal lattice, and the lattice defects cause the Bragg reflectivity to be significantly reduced from the theoretical value.

In wet etching, a common method for removing processing distortion on a reflective surface, the etching speed depends on the crystal plane orientation, and bunching or etch pits are formed on the etched surface, making it difficult to remove the processing distortion while maintaining a good surface shape. Processing distortion exists several μm deep from the surface, and at most within 10 μm, and is called the processing-affected layer.

The inventors therefore devised a method of removing the altered layer on the X-ray reflecting surface of a channel-cut crystal by using PCVM (Plasma Chemical Vaporization Machining), a distortion-free etching technique that uses high-pressure plasma (see Non-Patent Documents 1 and 2). The basic principle of PCVM, which uses high-pressure plasma at near atmospheric pressure, is disclosed in Patent Document 2, and various electrode configurations have been proposed, including wire electrodes, rotating electrodes, nozzle electrodes that eject process gas, and narrow blade electrodes (see Patent Documents 2 to 4). Furthermore, a method of applying a pulse-modulated high-frequency voltage to control the plasma has also been proposed (see Patent Document 5).

However, the FZ-Si single crystal used in channel-cut crystals is highly pure and has a high resistivity, making it difficult to obtain sufficient electric field strength near the electrodes and stably generate plasma. Furthermore, channel-cut crystals require precise machining of the two opposing surfaces, but when the channel width is narrow and there is a portion where the normal direction of one of the pair of reflecting surfaces intersects with the other (overlapped portion), conventional PCVM techniques cannot be used as is.

Japanese Patent Application Publication No. 09-049899 Japanese Patent Application Publication No. 01-125829 Japanese Patent Application Publication No. 04-246184 Japanese Patent Application Publication No. 09-031670 JP 2014-177673 A

T Hirano, T Osaka, Y Sano, Y Inubushi, S Matsuyama, K Tono, T Ishikawa, M Yabashi, and K Yamauchi, Review of Scientific Instruments.87(2016)063118. T Hirano,Y Morioka,S Matsumura,Y Sano,T Osaka,S Matsuyama,M Yabashi,and K Yamauchi,Int.J.of Automation Technology.13(2019)246.

In view of the above situation, the present invention aims to provide a channel-cut X-ray optical element and its manufacturing method, in which the processing-induced alteration layer on the opposing reflective surfaces of the channel-cut crystal introduced by machining is removed by PCVM using high-pressure plasma at near atmospheric pressure, and the element has reflective surfaces with a surface flatness of 1 μm or less, a surface roughness of 1 nm RMS or less, and a peak reflectivity to X-rays of 90% or more of the theoretical value.

To solve the above-mentioned problems, the present invention provides a channel-cut X-ray optical element and a manufacturing method thereof as described below.

(1)
A channel-cut X-ray optical element, comprising an optical element material made of a single crystal with an impurity concentration of 0.1 ppm or less, a channel having a pair of parallel inner wall surfaces facing at least a specific crystal plane, the inner wall surfaces of the channel being Bragg reflecting surfaces that reflect X-rays by utilizing the diffraction phenomenon of the X-rays,
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 reflectance to X-rays of 90% or more of a theoretical value.
Channel-cut X-ray optics.

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

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

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

(5)
A method for manufacturing a channel-cut X-ray optical element, comprising forming a channel having a pair of parallel inner wall surfaces facing at least a specific crystal plane in an optical element material made of a single crystal having an impurity concentration of 0.1 ppm or less, and making the inner wall surfaces of the channel into Bragg reflecting surfaces that reflect X-rays by utilizing the diffraction phenomenon of X-rays, comprising:
a pre-step of forming a structure including the channel in the optical element material;
a reflection surface improvement process in which a process gas containing a reactive gas having an etching property for 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, and the localized plasma is scanned relatively to the inner wall surface to remove a processing-affected layer of the inner wall surface, thereby 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 to X-rays of 90% or more of the theoretical value;
A method for manufacturing a channel-cut X-ray optical element, comprising:

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

(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.

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

(9)
The method for manufacturing a channel-cut X-ray optical element described in any one of (5) to (8), wherein the electrode is a wire electrode in which a conductive wire is stretched across a support, a rotating electrode in which a conductive rotational symmetric body is fixed to a rotating shaft, or a conductive nozzle electrode or blade electrode that ejects a process gas.

(10)
The method for producing a channel-cut X-ray optical element according to (9), wherein the electrode is a wire electrode, and a plasma generating mechanism is used that applies a pulse-modulated high-frequency voltage to the wire electrode.

(11)
A method for manufacturing a channel-cut X-ray optical element as described in (9), wherein the electrode is the rotating electrode, the nozzle electrode or the blade electrode, an earth auxiliary electrode is placed on the back surface opposite the inner wall surface of the channel, and a plasma generating mechanism is used that applies a high-frequency voltage to the electrode.

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

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

The above-described manufacturing method of the channel-cut X-ray optical element of the present invention uses PCVM using high-pressure plasma at near atmospheric pressure to remove the process-affected layer on the reflecting surface introduced by machining, and can provide a channel-cut X-ray optical element with a reflecting surface having a surface flatness of 1 μm or less, a surface roughness of 1 nm RMS or less, and a peak reflectivity to X-rays of 90% or more of the theoretical value.

In addition, even if a rotating electrode, nozzle electrode, or blade electrode is used, by placing an auxiliary earth electrode on the back surface opposite the inner wall surface of the channel, or by using a wire electrode with a high electric field strength, it is possible to stably generate plasma and process even high-resistance optical element materials. In particular, by using a plasma generation mechanism that applies a pulse-modulated high-frequency voltage to the wire electrode, it is possible to stably generate plasma and remove the processing-affected layer with high efficiency even when the channel width is narrow or there is an overlapping portion on the reflective surface. Furthermore, wire electrodes have advantages over other electrodes, such as their applicability to small channel-cut crystals that are expected to be developed in the future, especially high-quality channel-cut X-ray optical elements with channel widths of 2 mm to 50 μm, and their ability to stably process up to the crystal ends.

1 is a simplified diagram of a channel cut X-ray optical element of the present invention. FIG. FIG. 13 is an explanatory diagram showing an example of use as a monochromator. FIG. 1 is a simplified explanatory diagram showing an example of machining using a wire electrode. 1A to 1C are schematic explanatory diagrams showing a processing example using a nozzle electrode. FIG. 13 is a simplified explanatory diagram showing an example of processing using a wire electrode when the channel width is narrow and there is an overlapping portion. FIG. FIG. 4 is a waveform diagram of a pulse-modulated high-frequency voltage. 1 is a graph showing the crystal plane dependency of the processing rate (MMR) when removing each plane orientation of Si by PCVM using a nozzle electrode. (a) is an X-ray topography image of the channel-cut crystal before PCVM treatment, and (b) is an X-ray topography image of the channel-cut crystal after PCVM treatment. 1 is a graph showing the incidence angle dependence of X-ray reflectivity of a Si(220) crystal before and after PCVM treatment. FIG. 1 is a diagram showing the layout of a PCVM processing experiment simulating a microchannel-cut crystal. FIG. 13 is an explanatory diagram showing a position for observing a profile of processing marks after removing the Si wafer processed according to FIG. 12 . 13A shows a profile of the Si wafer W1 along a profile observation line 32, and FIG. 13B shows a profile of the Si wafer W2 along a profile observation line 33. FIG. This is an experimental setup for investigating the machining characteristics of pulse-modulated high-frequency voltage in PCVM using a wire electrode. 13 is a graph showing the change in surface roughness (RMS) when the ON time of the high frequency voltage is fixed at 50 μs and the OFF time is changed to 200, 450, and 950 μs. 1 is a graph showing changes in surface roughness (RMS) depending on the duty ratio of ON time to OFF time. (a) shows the surface observation result before processing, (b) shows the surface observation result after processing with ON/OFF time = 50/200 [μs], and (c) shows the surface observation result after processing with ON/OFF time = 50/450 [μs].

Next, the present invention will be described in more detail based on the embodiment shown in the attached drawings. Figure 1 is a schematic diagram of a channel-cut X-ray optical element (channel-cut crystal) of the present invention, Figure 2 is a plan view of the same, and Figure 3 shows an example of its use.

The channel cut X-ray optical element of the present invention is manufactured by cutting out 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 pure, so it has high resistance. Therefore, it has a higher resistance than silicon wafers used to manufacture general semiconductor elements, making it difficult to stably generate plasma between the electrodes. Currently available FZ-Si single crystals have a purity of 99.999999999% (impurity concentration of 0.001 ppm), and FZ-Ge single crystals have a purity of 99.9999999% (impurity concentration of 0.1 ppm).

As shown in Figs. 1 and 2, the channel cut X-ray optical element 1 of the present invention has a protrusion 4 with a first Bragg reflecting surface 3 and a protrusion 6 with a second Bragg reflecting surface 5 on the upper surface of a substantially rectangular parallelepiped base 2, and other parts that interfere with the optical path of X-rays have been removed. Both Bragg reflecting surfaces 3 and 5 are completely parallel and face a specific crystal plane. In addition, the base 2 of the channel cut X-ray optical element 1 is fixed in an appropriate position in an X-ray generating facility (a synchrotron radiation facility (SPring-8, etc.), an X-ray free electron laser (XFEL) facility (SACLA, etc.) during use, and slits 7, 7 are inserted alternately on both sides to form an S-shaped structure so that distortion from the fixed part of the base 2 does not propagate to the Bragg reflecting surfaces 3 and 5.

The manufacturing method first involves a pre-process in which an optical element material is machined to form a structure including a channel 8 having a pair of parallel inner wall surfaces 9, 9 facing at least a specific crystal plane. The inner wall surfaces 9, 9 of the channel 8 become Bragg reflecting surfaces 3, 5 that reflect X-rays using the diffraction phenomenon. This pre-process usually includes a step of precision polishing at least the inner wall surfaces 9, 9 of the channel. Polishing is performed manually when normal mechanical precision polishing is not possible. In addition to precision cutting, machining includes processing techniques used in semiconductor processing, such as dicing and wire sawing.

The distance (channel width) W between the opposing Bragg reflecting surfaces is determined by the energy range of X-rays and the allowable delay time during Bragg reflection of X-rays. The wider the X-ray energy range and the shorter the allowable delay time, the narrower the channel width W is set. 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 can be 20 mm or more. In particular, if the channel width W is less than 2 mm, the difficulty of manufacturing becomes extremely high. The lower limit of the channel width W is about 50 μm for reasons described below.

Next, the inner wall surfaces 9, 9 of the channel are made high quality using PCVM technology. This PCVM method is a processing method in which highly chemically active atoms such as halogens are excited in a spatially localized high frequency (e.g. 150 MHz) plasma in a high pressure atmosphere (near atmospheric pressure) to generate high density neutral radicals that react with the workpiece and turn them into volatile substances (reaction products), thereby removing the material. By using a high pressure processing atmosphere, it is possible to achieve high speed processing with a removal rate comparable to mechanical processing while increasing spatial resolution.

Specifically, a process gas containing a reactive gas having an etching property for the optical element material has a pressure of 10 kPa to 1000 kPa, and a high-frequency voltage is applied to electrodes arranged opposite the inner wall surfaces 9, 9 of the channel 8 to locally generate high-frequency plasma, and the localized plasma is scanned relatively to the inner wall surfaces 9, 9 to remove the processing-affected layer of the inner wall surfaces 9, 9, thereby performing a reflection surface improvement process to form the Bragg reflection surfaces 3, 5 having 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. More preferably, the peak reflectivity for X-rays is 95% or more of the theoretical value.

Here, the electrodes that can be used are wire electrodes with conductive wires stretched over a support, rotating electrodes with a conductive rotational symmetric body fixed to a rotating shaft, and conductive nozzle or blade electrodes that spray process gas. Empirically, a machining depth of 10 μm is sufficient to remove the machining-affected layer introduced into the inner wall surfaces 9, 9 of the channel 8 during machining, and even a depth of about 5 μm can remove most of it. In addition, a certain gap is required between the electrode and the machining surface (inner wall surface 9). This gap depends on the frequency of the high-frequency power source applied to the electrode and the type and pressure of the process gas, but conditions that allow stable generation of plasma are used. Normally, in PCVM, the gap between the electrode and the surface to be machined is set to 10 μm to several mm.

The process gas contains a reactive gas that has etching properties for the optical element material and a rare gas as a carrier gas, and has a pressure of 10 kPa to 1000 kPa. This pressure corresponds to a range of approximately 1/10 to 10 atmospheres around atmospheric pressure. If the process gas pressure is high, the plasma generation area is limited to the vicinity of the electrode, making it possible to perform spatially controlled processing, but it becomes difficult to generate and maintain the plasma.

Reactive gases used in PCVM include halogen-based gases, oxygen gases, ozone gases, etc. Halogen-based gases include fluorine-based gases and chlorine-based gases. Fluorine-based gases include _F2 , _CF4 , _C2F6 , _C3F6 , _C4F8 , _SF6 , _etc. Chlorine- _based gases include _Cl2 , _BCl3 , _CCl4 , _{etc. CClF3} contains both fluorine and chlorine. The reactive gas is appropriately selected according to the material of the workpiece. For example, if the workpiece is silicon or germanium, _SF6 is selected, and if the workpiece is diamond, _SF6 or _O2 is selected.

As the carrier gas, an inert gas, such as a rare gas, is used. Examples of rare gases include He, Ne, Ar, and Xe.

In addition, gases that contribute to plasma generation and stabilization may also be added.

The electrode material is made of a material having high corrosion resistance such as stainless steel, nickel, aluminum, etc., which has excellent corrosion resistance according to the reaction gas. It is also preferable to coat the electrode with a material having excellent corrosion resistance against the reaction gas. For example, the corrosion problem can be solved by _coating or dipping the surface with a material _having corrosion resistance such as _Al2O3 , _MgF2 , _Y2O3 (yttria), etc.

If the Bragg reflecting surfaces 3, 5 are located at positions apart in the groove direction P of the channel and there is no portion (overlap) where the normal direction of one of the paired Bragg reflecting surfaces 3, 5 intersects with the other surface, if unnecessary portions of the inner wall surfaces 9, 9 of the channel 8 are removed in advance during pre-processing, there will be no protrusions 4 or 6 behind one of the inner wall surfaces 9 of the processing target, and restrictions on the size of the electrode will be relaxed, making it possible to use various electrodes. However, if the channel width W is narrow and there is an overlapping portion of the Bragg reflecting surfaces 3, 5, the size of the electrode must be sufficiently smaller than the channel width W. The dimension of the electrode perpendicular to the inner wall surface 9 of the processing target must be approximately half or less of the channel width W. When the channel width W is about 5 mm, it is difficult to use a rotating electrode or a nozzle electrode. Furthermore, when the channel width W is 2 mm or less, it is difficult to use a blade electrode. When the channel width W is 2 mm or less, a wire electrode is exclusively used. Even when using wire electrodes, the lower limit of the channel width W is approximately 50 μm, taking into account the wire diameter, gap, and amount of removed layer that has been damaged by processing, and up to this size, grooves can be formed using a dicing blade in the pre-processing step.

As an example of the use of the channel cut X-ray optical element of the present invention, FIG. 3 shows a monochromator in which two linearly symmetrical elements 1A and 1B are arranged at a Bragg reflection angle with respect to the X-ray beam B. The incident X-ray is 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 then 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, so that the outgoing X-ray returns on the extension line of the incident X-ray after four reflections. With this monochromator, even if the wavelength (energy) of the incident X-ray spreads, the wavelength (energy) of the outgoing X-ray after four reflections is monochromatized to match the ideal Bragg reflection angle. In FIG. 3, the solid line shows the case where a channel cut X-ray optical element with a wide channel width W is used, and the imaginary line shows the case where a channel cut X-ray optical element with a narrow channel width W is used. The delay time of X-rays is caused by this increase in optical path length, so if a channel-cut X-ray optical element with a narrow channel width W is used, the delay time will also be shortened.

Figure 4 shows the case where the inner wall surfaces 9, 9 of the channel 8 are processed using a wire electrode 10, and Figure 5 shows the case where the inner wall surfaces 9, 9 of the channel 8 are processed using a nozzle electrode 20. As shown in Figure 4, the wire electrode 10 has a structure in which a conductive wire 12 is stretched with a predetermined tension between the tips of an arm-shaped support 11. Then, the optical element material after pre-processing is held on a moving stage 14 arranged inside a chamber 13, and the wire 12 of the wire electrode 10 is arranged parallel to one of the inner wall surfaces 9 with a predetermined gap, and the wire 12 is scanned in the width direction of the Bragg reflecting surface 3 or the Bragg reflecting surface 5. In addition, a process gas supply system 15 and an exhaust system 16 are connected to the chamber 13, and after the inside of the chamber 13 is evacuated by the exhaust system 16, a process gas of 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 source 17, and high-frequency plasma is generated around the wire 12 for processing.

As shown in FIG. 5, the nozzle electrode 20 is placed with a predetermined gap from one of the inner wall surfaces 9, and while the process gas is being ejected 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, an earth auxiliary electrode 21 is placed and grounded on the side of the protrusion 6 corresponding to the back surface on the opposite side of the inner wall surface 9, so that plasma is generated stably in the gap even if the optical element material has high resistance. Even in the case of processing using 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. The same applies when a rotating electrode or a blade electrode is used instead of the nozzle electrode 20.

Figures 6 to 8 show an example of processing using a wire electrode 10 in a case where the channel width W is narrower than 2 mm and there is a portion (overlapped portion) where the normal direction of one of the paired Bragg reflecting surfaces 3, 5 intersects with the other surface. In this case, a wire 12 is placed at an equal distance from both inner wall surfaces 9, 9 of the channel 8, and both inner wall surfaces 9, 9 are processed simultaneously. Here, if a high-frequency voltage is continuously applied to the wire electrode 10, plasma cannot be maintained at the overlapping portion of both inner wall surfaces 9, 9 in due course. This is thought to be because the plasma generation area changes from the overlapping portion, where it is difficult to maintain plasma, to only the non-overlapping portion, where it is relatively easy, after plasma generation. It has been found that this problem can be solved by pulse-modulating the high-frequency voltage as shown in Figure 8, and repeatedly turning off and on the plasma before the plasma generation area changes.

Ultra-narrow channel cut crystals (μCCs) with a channel width of sub-mm are key devices for the sophistication of XFEL light sources. In X-ray spectroscopy experiments that require high energy resolution, narrow-bandwidth probe X-rays are essential, but the more the bandwidth is limited, the more drastic the reduction in light intensity (typically by 2-3 orders of magnitude) is. Conventional CEFC technology uses a transmission monochromator based on forward Bragg diffraction from diamond crystals, but this technology has problems such as low monochromatic light generation efficiency and the presence of strong transmitted light on the same axis as the transmitted light, resulting in a poor S/N ratio and lower XFEL intensity than expected. CEFC technology using ultra-narrow channel cut crystals is a useful method for solving this dilemma, and by improving the monochromaticity of the oscillating XFEL itself, it is possible to generate high-intensity monochromatic XFEL without using a transmission monochromator that causes intensity loss. However, it is extremely difficult to remove the processing-induced layers on the Bragg reflection surfaces 3 and 5 of such ultra-narrow channel cut crystals using conventional technology.

Figure 9 shows the crystal plane dependence of the machining rate (MMR) when removing each surface orientation of Si by PCVM using a nozzle electrode. FZ-Si (100), (110), (111), (422), and (511) wafers were prepared as samples. Plasma was generated between the tip of the electrode and the wafer under atmospheric pressure He environment, and the process gas was directly supplied to the plasma region through a flow path inside the electrode. One-axis scanning was performed six times for each wafer, and the MRR was derived from the area of each cross-sectional profile. The processing conditions were pressure 100 kPa, process gas (He:SF ₆ = 99.5:0.5), flow rate 100 sccm, scanning speed 10 mm/min, gap 0.3 mm, power 150 MHz, 17 W.

X-ray crystal optical elements are required to be set to various plane orientations according to requirements, but Si crystals are an anisotropic material with atomic densities that vary greatly for each crystal plane, and many processing techniques exhibit characteristics that depend on the crystal plane. If the crystal plane dependence is strong, conditions must be optimized for each crystal plane, and etch pits occur, making it difficult to obtain a flat and smooth surface. As shown in Figure 9, the MMR ratio is (100):(110):(111):(422):(511) = 1.03:1.04:1.00:1.01:0.95, and the difference in the amount of processing for each crystal plane can be said to be within the margin of error. It has been shown that the plasma process using F radicals can obtain the same processing characteristics regardless of the crystal plane, and can obtain a flat and smooth surface without etch pits.

Figures 10(a) and (b) show typical examples of double-reflection images of X-ray beams when processing distortions on the reflective surface of a Si channel-cut crystal are removed by PCVM using a small plasma generation electrode (nozzle electrode 20) that can access the inner wall surface 9 of the channel 8 as shown in Figure 5. The numerous scratch-like dark lines that existed before PCVM processing have disappeared by PCVM processing, demonstrating that PCVM can remove the processing-affected layer on the reflective surface of the channel-cut crystal. In addition, the reflectance was measured from the values of intensity monitors placed before and after the channel-cut crystal, and it was confirmed that the reflectance increased from the initial 71.5% to 84.1% by PCVM processing (theoretical value is 84.4%), confirming its effectiveness.

A channel-cut X-ray optical element (microchannel-cut crystal (μCC)) is manufactured in which the distance (channel width) between the opposing Bragg reflecting surfaces 3, 5 is 100 μm, and the normal direction of one of the pair of Bragg reflecting surfaces 3, 5 intersects with the other surface (overlap).

As pre-processing for PCVM, the FZ-Si crystal block was cut and a channel with a width of 60 to 70 μm and a depth of 0.6 mm was formed using a dicing blade, after which the surface was finished by solution etching and hand polishing. The inner wall surface of the channel was set to Si(220). To keep the channel width to 100 μm, the channel width in pre-processing was set to 80 μm, and 10 μm of each inner wall surface 9, 9 was removed using PCVM with a wire electrode with a diameter of 50 μm.

The prepared microchannel-cut crystal was evaluated using synchrotron radiation X-rays. Monochromatic light of 10 keV supplied from a two-crystal spectrometer was cut into a 50 x 500 μm area smaller than the aperture of the crystal with 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 reflectance were obtained under conditions close to the actual usage conditions, and the effectiveness of PCVM was investigated. Figure 11 shows the incidence angle dependence of the reflectance of a Si (220) crystal before and after PCVM treatment. Although intensity modulation was observed in the profile even after PCVM treatment from the reflection profile not shown, the PCVM treatment clarified the edge cut out by the aperture and increased the reflection intensity. In addition, although the peak value of the reflectance characteristic of the crystal after treatment 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 reflectance improved from 32% before PCVM treatment to 38% after treatment (theoretical value 40%). This value corresponds to a peak reflectivity for X-rays of 95% of the theoretical value.

In the case of a channel-cut X-ray optical element (microchannel-cut crystal) in which the Bragg reflection surfaces 3 and 5 have an overlap and the channel width is sub-mm, it is difficult to directly observe the reflection surface after processing by PCVM. Therefore, as shown in Fig. 12, two Si wafers W1 and W2 were fixed to a support table 30 with a gap of 100 μm and an overlap of 2 mm, and processing was performed in a stationary state with a wire electrode 10 equipped with a wire 12 having a diameter of 50 μm. The processing conditions were a pressure of 100 kPa, process gas (He:SF ₆ = 99:1), wire diameter of 50 μm, gap of 30-40 μm, power of 150 MHz, ON time of 2 μs, and OFF time of 40 μs.

As shown in Figure 13, after PCVM processing, the Si wafers W1 and W2 were removed from the support table 30 and the processed surfaces were observed. Figure 13 shows the positions of the processing marks 31 and the profile observation lines 32 and 33 when the Si wafer W1 is viewed from the processed surface side and the Si wafer W2 is viewed from the back side (opposite the processed surface). The profile observation lines 32 and 33 are also at approximately the same position in the overlap region 34.

Figure 14(a) shows the profile at profile observation line 32 of Si wafer W1, and Figure 14(b) shows the profile at profile observation line 33 of 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 reflective surface by using a wire electrode. The slight difference between the two profiles is presumed to be due to the wire electrode being slightly offset from the center between the two Si wafers W1 and W2, and the amount of removal is small in the center of the profile is presumed to be due to a stagnation in the supply of process gas in the gap. However, in actual processing of the reflective surface, the amount of processing is averaged by scanning the wire electrode in the width direction of the reflective surface, so there is no problem.

Finally, the processing characteristics of a pulse-modulated high-frequency voltage were investigated in a PCVM using a wire electrode 10. The experimental arrangement is shown in Fig. 15. A sample 41 made of a Si wafer was fixed on the upper surface of a support table 40 fixed to a moving stage 14, and the wire 12 of the wire electrode 10 was placed parallel to the surface of the sample 41. The processing conditions were a pressure of 100 kPa, process gas (He: _SF6 = 99.5:0.5), wire diameter of 50 µm, gap of 100 µm, power of 150 MHz, ON time of 25-50 µs, and OFF time of 200-950 µs.

When the ON/OFF time was 50/450 μs, the machining depth at the center of the sample in the static machining marks 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 by using a wire electrode, the same amount of machining can be obtained at the ends of the reflective surface as at the center.

Next, FIG. 16 shows the change in surface roughness (RMS) when the ON time of the high frequency voltage is fixed at 50 μs and the OFF time is changed to 200, 450, and 950 μs. FIG. 17 shows the surface roughness versus duty ratio for the results of FIG. 16. As a result, if the OFF time is 200 μs (duty ratio 20%), the surface roughness worsens, but it is presumed that this is due to the reaction products re-adhering to the surface. It was also found that the deterioration of surface roughness due to PCVM can be prevented by lengthening the OFF time to 450 μs (duty ratio 10%) or 950 μs (duty ratio 5%), in other words, by reducing the duty ratio. Note that FIG. 18(a) shows the surface observation result of the sample before PCVM processing, (b) shows the surface observation result after processing with an OFF time of 200 μs, and (c) shows the surface observation result after processing with an OFF time of 450 μs.

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 table 31 Processing mark 32 Profile observation line 33 Profile observation line 34 Overlap region 40 Support table 41 Sample B X-ray beam P Groove direction W Channel width W1 First Si wafer W2 Second Si wafer

Claims (5)

A method for manufacturing a channel-cut X-ray optical element, comprising forming a channel having a pair of parallel inner wall surfaces facing at least a specific crystal plane in an optical element material made of a single crystal having an impurity concentration of 0.1 ppm or less, and making the inner wall surfaces of the channel into Bragg reflecting surfaces that reflect X-rays by utilizing the diffraction phenomenon of X-rays, comprising:
the channel-cut X-ray optical element has a portion where a normal direction of one of the pair of Bragg reflecting surfaces intersects with a direction of the other of the pair of Bragg reflecting surfaces, and the distance between the opposing Bragg reflecting 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 reflection surface improvement process in which a process gas containing a reactive gas having an etching property for the optical element material has a pressure of 10 kPa to 1000 kPa, a pulse-modulated high-frequency voltage is applied to a wire electrode arranged opposite to the inner wall surface of the channel to locally generate high-frequency plasma, and the localized plasma is scanned relatively to the inner wall surface to remove a processing-affected layer of the inner wall surface, thereby 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 to X-rays of 90% or more of the theoretical value;
A method for manufacturing a channel-cut X-ray optical element, comprising: The method for manufacturing a channel-cut X-ray optical element according to claim 1 , wherein the pre-processing step includes a step of precision polishing at least an inner wall surface of the channel. 3. The method for producing a channel-cut X-ray optical element according to claim 1 , wherein the optical element material is single crystal silicon, single crystal germanium or single crystal diamond. 4. The method for producing a channel-cut X-ray optical element according to claim 1 , wherein the amount of removal of the inner wall surface of the channel in the reflecting surface quality improvement step is 10 μm or more. 2. The method for producing a channel-cut X-ray optical element according to claim 1 , wherein the distance between the opposing Bragg reflecting surfaces is 2 mm to 50 μm.

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