High-precision inner side surface processing method for a slotted monochromatic crystal

Abstract

This invention discloses a high-precision inner surface processing method for grooved monochromatic crystals. The steps include: 1) placing the grooved monochromatic crystal into a material tank and using a grinding head of a magnetron abrasive grinding device to coarsely grind the inner surface of the crystal; the magnetron abrasive grinding device includes a magnetron drive unit and the grinding head; 2) using the grinding head to finely grind the inner surface after step 1); 3) performing wet chemical etching on the inner surface after step 2) to remove the damaged layer; 4) using the grinding head to finely polish the inner surface after step 3); 5) cleaning the inner surface after step 4) to generate a soft hydration layer on the inner surface; 6) cleaning the grooved monochromatic crystal after step 5) and using the grinding head to perform stress-relieving polishing on the inner surface. This method can better transfer the heat generated at the interface during polishing, obtain a better surface structure, and is less restricted by the overlapping part of the primary and secondary crystals of the grooved crystal.

Description

Technical Field

[0001] This invention belongs to the field of synchrotron radiation technology and relates to a method for processing the inner surface of a channel-cut monochromatic crystal. Background Technology

[0002] Synchrotron radiation is the electromagnetic radiation emitted along the tangent of the orbit of high-speed charged particles (approaching the speed of light) moving in a curved path. Synchrotron radiation is an excellent X-ray source with many advantages, such as a wide and continuously tunable spectrum, high intensity, high brightness, and high collimation. Monochromatic crystals are the core spectroscopic elements in synchrotron radiation devices and are also known as the heart of the beamline.

[0003] Channel-cut crystal monochromators (CCMs) are made by cutting specific grooves into a single crystal according to different requirements, and using the two sides of the cut grooves as diffraction surfaces. Compared to double-crystal monochromators (DCMs) with a fixed high position, channel-cut crystal monochromators (CCMs) have excellent spot stability. Figure 1 As shown in Table 1, under the set initial conditions, the change in the spot position of the slotted crystal monochromator is only 1 / 5000 of that of the former. At the same time, the clamping mechanical structure required for the slotted crystal is also very simple and has higher reliability.

[0004] Table 1 compares the stability of CCM and DCM spot positions under specific conditions.

[0005]

[0006] For diffractive crystals, it is necessary to ensure the highest possible lattice perfection. When the diffractive crystal plane conditions (stress residue, surface shape, roughness, etc.) are the same, compared to two flat crystals, grooved crystals not only have excellent spot position stability but also higher energy resolution. However, due to limitations in processing technology, it is difficult to obtain stress-free inner surfaces with high surface shape accuracy for grooved crystals, which greatly limits their application range.

[0007] Currently, the main international research on grooved crystal polishing technology includes: APS in the United States, Osaka University in Japan, and Sring-8.

[0008] Ruben Khachatryan et al. of APS studied mechanochemical polishing of Z-type crystals, achieving high-precision crystal surfaces, but noticeable scratches remained on the processed crystal surfaces. Elina Kasman et al. used a thin-layer grinding disc that could penetrate narrow grooves to study mechanochemical polishing of channel-cut crystals; however, due to the stress deformation of the thin-layer grinding disc, the resulting crystal surface roughness was only [amount missing]. (Sa)@1.66*1.66mm 2 Meanwhile, the flatness is poor, even reaching 20μm (PV)@130mm (measured by coordinate measuring machine). References: Khachatryan R, Tkachuk A, Chu YS, et al. Open-faced Z-shaped channel-cut X-ray monochromator[J]. Proceedings of SPIE-The International Society for Optical Engineering, 2004, 5537; Khounsary AM, Macdonald CA, Kasman E, et al. SPIE Proceedings[SPIESPIE Optical Engineering+Applications-San Diego,California,United States(Sunday 9 August 2015)] Advances in Laboratory-based X-Ray Sources, Optics, and Applications IV-The best of both worlds: automated CMP polishing of channel-cut monochromators[J]. 2015; Kasman, Elina, Montgomery, et al. Strain-free polished channel-cut crystal monochromators: a new approach and results[C], 2017.

[0009] In 2016, Takashi Hirano et al. from Osaka University and Spring-8 University studied the fabrication process of channel-cut using plasma chemical vapor deposition (PCVM). The surface roughness of the zone-melted crystal obtained using this method reached 0.535 nm (Sa) @ 71 * 53 μm.2 Flatness is less than 200 nm (PV) @ 71 * 53 μm 2 The morphology photos of the processed sample show no scratches or defects. However, the processing technology has significant limitations. Because the process requires the rotating electrode to penetrate deep into the groove, the overlap size of the first and second crystals and the groove distance are greatly limited. The groove width mentioned in the literature also reaches 30 mm, which greatly limits the range of energy used. In 2018, Takashi Hirano et al. used the same process to fabricate crystals with a more compact size, but the overlap size of the first and second crystals of the fabricated grooved crystal was only 5 mm, and the overlapping part was still difficult to process (References: Hirano T, Osaka T, Sano Y, et al. Development of speckle-free channel-cut crystal optics using plasma chemical vaporization machining for coherent x-ray applications[J]. Review of Scientific Instruments, 2016, 87(6): 063118; Katayama T, Hirano T, Morioka Y, et al. X-ray optics for advanced ultrafast pump–probe X-ray experiments at SACLA[J]. Journal of Synchrotron (Radiation, 2019, 26) Furthermore, this method requires the crystal itself to be used as an electrode, which limits its application given that almost all crystal monochromators currently use high-resistivity zone-melted crystals (compared to other types of crystals, zone-melted crystals have higher purity and intrinsic lattice perfection, resulting in extremely high resistivity, typically around 10000 Ω·cm). 2 The above even reaches 3-50000 Ω·cm 2 (It can be considered an insulator and cannot be used as an electrode). Takashi Hirano et al. also conducted experiments using commercially available low-resistance Czochralski wafers in their article.

[0010] In summary, although the grooved crystals obtained using traditional CMP and PCVM techniques with thin-layer grinding discs have achieved certain results, it is difficult to obtain zone-melted crystals with extremely high lattice perfection. These two techniques still have significant limitations. Summary of the Invention

[0011] In view of the technical problems existing in the prior art, the purpose of this invention is to provide a stress-free magnetic polishing method suitable for the inner surface of grooved crystals.

[0012] The main technical contents of this invention include:

[0013] 1. Grind and polish the inner surface of the grooved crystal using a small magnetron grinding head with a coating layer;

[0014] 2. High-precision and stress-free polishing of the inner surface is achieved by using chemical mechanical polishing (CMP) technology.

[0015] The technical solution of this invention is as follows:

[0016] A method for high-precision machining of the inner surface of a grooved monochromatic crystal, comprising the following steps:

[0017] 1) The grooved monochromatic crystal is placed in a material tank, and the inner surface of the grooved monochromatic crystal is coarsely ground using the grinding head of a magnetron polishing device; wherein, the magnetron polishing device includes a magnetron drive unit and the grinding head, the magnetron drive unit is used to drive the grinding head, the magnetron drive unit is located outside the material tank, the grinding head is located inside the material tank and the cover layer of the grinding head is in contact with the inner surface; the material tank is filled with polishing fluid;

[0018] 2) Use the grinding head to finely grind the inner surface after the treatment in step 1);

[0019] 3) Perform wet chemical etching on the inner surface after step 2) to remove the damaged layer;

[0020] 4) Use the grinding head to perform fine polishing on the inner surface after step 3);

[0021] 5) After cleaning the inner surface treated in step 4), a soft hydration layer is formed on the inner surface.

[0022] 6) After cleaning the grooved monochromatic crystal processed in step 5), the inner surface is stress-relieved and polished using the grinding head.

[0023] Furthermore, the hardness of the coating layer in the coarse grinding stage is greater than that in the fine grinding stage, which is greater than that in the fine polishing stage, which is greater than that in the stress-relief polishing stage; the particle size of the abrasive particles in the grinding slurry in the coarse grinding stage is greater than that in the fine grinding stage, which is greater than that in the fine polishing stage; and no abrasive particles are added to the grinding slurry in the stress-relief polishing stage.

[0024] Furthermore, in the rough grinding stage, the material of the cover layer on the grinding head is the same as the material of the monochromatic crystal used for grooving; in the fine grinding and fine polishing stages, the cover layer material is polyurethane; and in the stress-relief polishing stage, the cover layer material is foamed rubber.

[0025] Furthermore, in the coarse grinding stage, the grinding fluid in the trough is an alumina solution with a particle size of 10μm, and stirring is maintained during the processing; the rotation speed of the grinding head is 220rpm, and the interfacial pressure between the grinding head and the inner surface is 22kPa.

[0026] Furthermore, in the fine grinding stage, the grinding fluid in the material tank is an alumina suspension with a particle size of 3μm and a pH of 10; the grinding head rotates at 250rpm and the interfacial pressure between the grinding head and the inner surface is 12.7kPa.

[0027] Furthermore, in the fine polishing stage, the polishing liquid in the material tank is a silica suspension with a particle size of 50nm and a pH of 11; the grinding head rotates at 350rpm and the interfacial pressure between the grinding head and the inner surface is 6.3kPa.

[0028] Furthermore, the grooved monochromatic crystal is placed in an alkaline chemical solution and kept at a constant temperature to generate a soft hydrated layer on the inner surface.

[0029] Furthermore, the grooved monochromatic crystal is placed in an alkaline chemical solution with pH=13 for 48 hours and kept at a constant temperature of 30°C; the chemical components of the alkaline chemical solution are KOH and NH2OH.

[0030] Furthermore, in the stress-relief polishing stage, the polishing liquid in the material tank is deionized water, the rotation speed of the grinding head is 200 rpm, and the interface pressure between the grinding head and the inner surface is 1.4 kPa.

[0031] Furthermore, the magnetically controlled drive unit includes a drive device and a magnet.

[0032] Traditional small grinding head or grinding disc technologies share a common characteristic: they are all mechanically rigidly connected, resulting in a wide speed adjustment range, typically from 0 to the maximum motor speed. Magnetic control, however, differs. It has its own suitable speed range: too low a speed can cause the grinding head to bounce, while too high a speed can lead to unstable operation. The experimentally obtained stable speed range is shown in the table below. A relatively high speed is selected, while allowing a certain margin from the maximum value to ensure long-term stability and high removal efficiency.

[0033] Processing steps coarse grinding stage Fine grinding stage Fine polishing Final stress-relief polishing Speed ​​range (RPM) 50-260 70-300 120-420 50-250

[0034] Compared with the prior art, the positive effects of the present invention are as follows:

[0035] 1) This invention uses a small grinding head driven by a magnetic field for polishing, which can process narrow grooves in a confined space. Currently, it can complete the processing of crystals with the narrowest groove of 3mm.

[0036] 2) In this invention, the small grinding head is immersed in the grinding and polishing fluid during processing. Compared with traditional mechanical and chemical grinding and polishing, it can better transfer away the heat generated at the interface during polishing and obtain a better surface structure.

[0037] 3) Compared to the traditional mechanochemical polishing technology of Japanese APS using thin-layer grinding discs, this invention uses a small grinding head that can completely and effectively contact the crystal surface with minimal force (unlike large grinding discs which require greater force to make the entire surface adhere to the crystal surface) and can achieve deterministic removal, obtaining a high-precision surface (surface shape and roughness). Submicron-level (surface shape) and subnanometer-level (roughness) processing have been achieved.

[0038] 4) Compared with the technology of Osaka University in Japan and Spring-8PCVM, this technology has a lower cost, less technical difficulty, and is less restricted by the overlapping part of the first and second crystals of the grooved crystal.

[0039] 5) Compared with Osaka University in Japan and Spring-8PCVM technology, this invention is not limited by crystal type and can process high-resistivity zone melt crystals suitable for high-quality monochromatic devices. Attached Figure Description

[0040] Figure 1 For comparison of the effects;

[0041] (a) Schematic diagram of the stability change of the light emission position of the CCM, (b) Schematic diagram of the stability change of the light emission position of the DCM.

[0042] Figure 2 This is a schematic diagram of a grooved crystal.

[0043] Figure 3 This is a three-dimensional structural diagram of the present invention.

[0044] Figure 4 This is a schematic diagram of the small grinding head structure.

[0045] Attached reference numerals: 1-Large magnet, 2-Aluminum alloy insert plate, 3-Grooving crystal, 4-Polishing small grinding head, 5-Acrylic material tank, 6-Magnet of small grinding head, 7-Covering layer, 11-Single crystal, 12-Double crystal. Detailed Implementation

[0046] The invention will now be described in further detail with reference to specific experiments, simulations, and accompanying drawings.

[0047] Figure 2This is a schematic diagram of a grooved crystal.

[0048] Figure 3 This is a schematic diagram of the processing, including an external driving large magnet 1, whose rotation can generate a gradient magnetic field to drive the small grinding head to rotate; an aluminum alloy insert plate 2, used to bond the grooving crystal 3, to realize the quick replacement of the grooving crystal; a polishing small grinding head 4; an acrylic material tank 5; a magnet for the small grinding head 6; and a cover layer 7, with different cover layer materials selected for different processing stages.

[0049] Figure 4 This is a schematic diagram of a small grinding head, which includes a magnet 6 and a covering layer 7.

[0050] An external large magnet 1 is mounted on the motion mechanism, so in addition to its own rotational motion, it can also have three-dimensional motion in space (X / Y / Z). The Y / Z axes can realize the free processing of the small grinding head on the crystal surface, and the polishing pressure (affected by the distance between the two magnets) can be achieved by controlling the movement of the X axis.

[0051] Different coating layers are selected for different processing stages. At the beginning of processing, because the crystal surface after the shape cutting has a thick damaged layer, a high removal efficiency is required to achieve rapid removal. Therefore, cast iron is selected as the coating layer material at this stage. In the fine grinding stage, polyurethane with a high hardness value is selected. On the one hand, it can ensure the removal efficiency, and on the other hand, compared with cast iron, polyurethane has a much lower hardness, which causes less damage to the surface. After the crystal is etched, the damaged layer on the surface can be completely removed, but the surface roughness and shape will be destroyed after etching.

[0052] Taking a specific processing example, a cylinder with a diameter of 60mm and a height of 60mm is selected for the large magnet, and a cylinder with a diameter of 10mm and a height of 5mm is selected for the small magnet. The specific processing flow of this invention is as follows:

[0053] 1) Coarse Grinding Stage. The coating on the small grinding head is made of monocrystalline silicon material (2mm thick circular wafer with a ground surface); the grinding fluid is an alumina solution with a particle size of 10μm, which is stirred during processing. The rotation speed is 220rpm, removing approximately 120μm of material. The distance between the magnets is 25mm, and the interface pressure during processing is 22kPa. In this stage, the coating and the workpiece are made of the same material. The advantages of this are: ① Non-magnetic. In contrast, when using a cast iron grinding disc, the cast iron itself is magnetic, which can magnetize and disrupt the magnetic field, causing instability in the grinding head. ② Because the coating and the workpiece are made of the same material, the effective abrasive particles penetrate to the same shallower depth between them, resulting in a smaller damaged layer. In traditional techniques using cast iron or other materials, the abrasive particles penetrate to a deeper depth in the workpiece. ③ Using the same material does not introduce additional impurity elements, and the processed surface is not contaminated. Analysis of experimental results shows that the achieved results are very good.

[0054] 2) Fine grinding stage. A polyurethane material with high hardness (Shore hardness 90) is selected for the coating layer. The grinding slurry is replaced with an alumina suspension with a particle size of 3μm (pH=10). The rotation speed is 250rpm, the thickness removed is 55μm, the distance between the two magnets is 30mm, and the interfacial pressure is 12.7kPa.

[0055] 3) Wet chemical etching stage. The crystal is immersed in HNO3 / HF (10:3) for etching for more than 12 minutes to remove all damaged layers, with a etch thickness of 35μm. Generally, after a 3μm grinding stage, the remaining damaged layer thickness is less than 20μm. The processing stress applied in the fine grinding stage of this invention is lower than that of traditional processes, so the remaining damaged layer thickness will be thinner. At the same time, the thickness removed by this invention during the etching process is 35μm, so it can be determined that the damaged layer is completely removed. Unlike the commonly used 5:3 or 10:1 ratio, this invention, through multiple experimental comparisons, has determined that an HNO3 / HF ratio of 10:3 can ensure the etching rate while maintaining the surface accuracy as much as possible.

[0056] 4) Fine polishing stage. The coating on the small grinding head is replaced with a polyurethane material with a lower hardness value (Shore hardness 66). The polishing slurry is a silica suspension with a particle size of 50nm (pH=11). The rotation speed is 350rpm, the thickness removed is about 8μm, the distance is 40mm, and the interfacial pressure is 6.3kPa.

[0057] 5) In the soft hydration layer formation stage, after thorough ultrasonic cleaning in step 4), the material is placed in an alkaline chemical solution with pH=13 for 48 hours, while maintaining a constant temperature of 30℃. The alkaline chemical components generated in this stage are KOH and NH2OH. The main purpose of this stage is similar to the corrosion in step 3), but the corrosion depth is smaller, while maintaining the original surface accuracy to a great extent. This allows the stress layer generated in step 4) to undergo a chemical reaction, forming a soft hydration layer. The immersion at pH=13 and a constant temperature of 30℃ for 48 hours is a conclusion drawn from experiments. The chemical reaction is most active under these conditions; too low a temperature results in a slow reaction, while too high a temperature decreases the reaction rate.

[0058] 6) Final stress-relief polishing stage. After thorough ultrasonic cleaning of the crystal from step 5), the polishing fluid is replaced with high-purity deionized water, and the grinding head material is changed to a particularly soft foamed rubber material (Shore hardness 20). The polishing speed is 200 rpm, the distance is 55 mm, and the interfacial pressure is 1.4 kPa. The main purpose of this stage is to remove the hydration layer while generating the smallest possible strain. Therefore, the softest foamed rubber currently available is selected as the covering layer. This material is being proposed for the first time in this invention and has not been used in previous related technologies. The use of pure water is to avoid introducing new abrasive particles and preventing new invasive damage.

[0059] 7) In step 5), the stress layer may not be able to completely react to form a soft hydration layer. Therefore, steps 5)-6) 2-3 times can be repeated to ensure that the stress layer residue is completely eliminated.

[0060] Although specific embodiments of the invention have been disclosed for illustrative purposes to aid in understanding and implementing the invention, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the content disclosed in the preferred embodiments, and the scope of protection claimed by the invention is defined by the claims.

Claims

1. A method for high-precision machining of the inner surface of a grooved monochromatic crystal, comprising the following steps: 1) The grooved monochromatic crystal is placed in a material tank, and the inner surface of the grooved monochromatic crystal is coarsely ground using the grinding head of a magnetron polishing device; wherein, the magnetron polishing device includes a magnetron drive unit and the grinding head, the magnetron drive unit is used to drive the grinding head, the magnetron drive unit is located outside the material tank, the grinding head is located inside the material tank and the cover layer of the grinding head is in contact with the inner surface; the material tank is filled with polishing fluid; 2) Use the grinding head to finely grind the inner surface after the treatment in step 1); 3) Perform wet chemical etching on the inner surface after step 2) to remove the damaged layer; 4) Use the grinding head to perform fine polishing on the inner surface after the treatment in step 3); 5) After cleaning the inner surface treated in step 4), a soft hydration layer is formed on the inner surface. 6) After cleaning the grooved monochromatic crystal processed in step 5), the inner surface is stress-relieved and polished using the grinding head. In the rough grinding stage, the material of the cover layer on the grinding head is the same as the material of the monochromatic crystal used for grooving; in the fine grinding and fine polishing stages, the material of the cover layer is polyurethane; and in the stress-relief polishing stage, the material of the cover layer is foamed rubber. The grooved monochromatic crystal is placed in an alkaline chemical solution and kept at a constant temperature to generate a soft hydrated layer on the inner surface.

2. The method according to claim 1, characterized in that, The hardness of the coating layer in the coarse grinding stage is greater than that in the fine grinding stage, which is greater than that in the fine polishing stage, which is greater than that in the stress-relief polishing stage. The particle size of the abrasive particles in the grinding slurry in the coarse grinding stage is greater than that in the fine grinding stage, which is greater than that in the fine polishing stage. No abrasive particles are added to the grinding slurry in the stress-relief polishing stage.

3. The method according to claim 1 or 2, characterized in that, In the coarse grinding stage, the grinding fluid in the trough is an alumina solution with a particle size of 10μm, and stirring is maintained during the processing; the rotation speed of the grinding head is 220rpm, and the interfacial pressure between the grinding head and the inner surface is 22kPa.

4. The method according to claim 1 or 2, characterized in that, In the fine grinding stage, the grinding liquid in the material tank is an alumina suspension with a particle size of 3 μm and a pH of 10; the grinding head rotates at 250 rpm and the interfacial pressure between the grinding head and the inner surface is 12.7 kPa.

5. The method according to claim 1 or 2, characterized in that, In the fine polishing stage, the polishing liquid in the material tank is a silica suspension with a particle size of 50nm and a pH of 11; the grinding head rotates at 350rpm and the interfacial pressure between the grinding head and the inner surface is 6.3kPa.

6. The method according to claim 1, characterized in that, The grooved monochromatic crystal was placed in an alkaline chemical solution with pH=13 for 48 hours and kept at a constant temperature of 30°C; the chemical components of the alkaline chemical solution were KOH and NH2OH.

7. The method according to claim 1 or 2, characterized in that, During the stress-relief polishing stage, the polishing liquid in the material tank is deionized water, the rotation speed of the grinding head is 200 rpm, and the interface pressure between the grinding head and the inner surface is 1.4 kPa.

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