Large-size, fine-grained lanthanum metal targets and their preparation methods

Abstract

This application relates to the field of high-purity metal target preparation technology, and in particular to a large-size, fine-grained lanthanum metal target and its preparation method. The method includes the following steps: S1, high-purity lanthanum metal is subjected to suspension melting and ingot pulling under dual-frequency ultrasonic treatment conditions to obtain an ingot; the dual-frequency ultrasonic treatment conditions include high-frequency ultrasonic treatment at a frequency of 80-100 kHz and low-frequency ultrasonic treatment at a frequency of 45-55 kHz; during the ingot pulling process, at least three independently temperature-controlled crystallizers distributed along the ingot pulling direction are used to directionally solidify the melt; S2, the ingot is cut to obtain a target blank of a set thickness; S3, the target blank is sequentially rolled, corrected, subjected to a second annealing, and precision machining to obtain a lanthanum metal target of the target size. The above method can yield a large-size lanthanum metal target with uniform microstructure and a low average grain size.

Description

Technical Field

[0001] This application relates to the field of high-purity metal target preparation technology, and in particular to a large-size fine-grained lanthanum metal target and its preparation method. Background Technology

[0002] Lanthanum targets are important rare-earth metal sputtering targets. Lanthanum readily reacts with oxygen and nitrogen, making it an excellent "getter" that absorbs residual gases during coating, improving film purity and readily forming compound films. Lanthanum oxide films derived from lanthanum possess superior optical and electrical properties. Optically, lanthanum oxide films have high refractive index and a wide transmittance range, making them ideal materials for preparing high-performance antireflective coatings, reflective coatings, and other optical coatings. In microelectronics, lanthanum oxide has a high dielectric constant, making it an ideal "high-k gate dielectric" material to replace traditional silicon dioxide, used in the manufacture of more advanced and energy-efficient integrated circuit transistors. Furthermore, it can effectively modulate the electrical parameters of semiconductor devices. However, lanthanum targets oxidize rapidly in air, posing extremely stringent requirements for high-purity, large-size fabrication, and storage of the target material. Existing technologies have the following unresolved issues: 1. Long fabrication process and high cost. 2. Difficulty in controlling shape, size, and internal defects; existing lanthanum targets are relatively small, making it difficult to achieve diameters exceeding 350 mm while remaining defect-free. 3. The purity is low, making it difficult to reach 4N or higher.

[0003] In summary, lanthanum targets, with their unique and powerful functional properties, have become a high-value-added advanced material. However, the stringent process control requirements, such as easy oxidation, soft texture, and high cost, pose a key challenge for their widespread application. Summary of the Invention

[0004] Therefore, the purpose of this application is to overcome the shortcomings of the prior art and provide a lanthanum metal target and its preparation method. This solves the technical problem that traditional preparation methods are unable to prepare large-sized target blanks with uniform microstructure and no macroscopic defects.

[0005] To achieve the above objectives, this application adopts the following technical solution:

[0006] First, this application provides a method for preparing a large-size, fine-grained lanthanum metal target, comprising the following steps:

[0007] S1. Under dual-frequency ultrasonic treatment conditions, high-purity lanthanum metal is subjected to suspension melting and ingot pulling to obtain an ingot; the dual-frequency ultrasonic treatment includes high-frequency ultrasonic treatment with a frequency of 80~100kHz and low-frequency ultrasonic treatment with a frequency of 45~55kHz; during the ingot pulling process, at least three independently temperature-controlled crystallizers distributed along the ingot pulling direction are used to directionally solidify the melt, the three independently temperature-controlled crystallizers comprising, in sequence:

[0008] The air gap heat conduction slow cooling section crystallizer uses argon as the air gap medium and controls the air gap medium pressure to be 0.02~0.05MPa to allow the unsolidified melt to be slowly cooled.

[0009] The high-speed medium-cooled section crystallizer uses cooling water with a pressure of 0.5~0.7MPa and a flow rate of 8~10m / s for strong cooling;

[0010] An adjustable temperature homogenizing crystallizer is used to perform online annealing of solidified castings by heating.

[0011] S2. Cut the ingot to obtain a target blank of a set thickness;

[0012] S3. The target blank is rolled, corrected, annealed a second time, and precision machined in sequence to obtain a lanthanum metal target of the target size.

[0013] Preferably, the length of the air gap heat conduction slow cooling section crystallizer is 80~120mm; the length of the high-speed medium strong cooling section crystallizer is 150~250mm; and the length of the adjustable temperature uniform temperature section crystallizer is 100~150mm.

[0014] Preferably, the adjustable temperature uniform temperature section crystallizer is heated by resistance heating or induction coil heating, with a heating power of 5~15kW and a temperature control range of 200~600℃.

[0015] Preferably, the flow rate of the air gap medium in the air gap heat conduction and slow cooling section is 6~12 L / min.

[0016] Preferably, the temperature of the cooling water used in the high-speed medium cooling section is 16℃~25℃.

[0017] Preferably, the three crystallizer sections are isolated by high-temperature ceramic heat insulation rings. These rings reduce axial heat conduction between the crystallizer sections, maintaining the independent temperature control of each section. More preferably, the high-temperature ceramic heat insulation rings are made of boron nitride ceramic or zirconium oxide ceramic, with a thickness of 15-20 mm.

[0018] Preferably, the conditions for high-frequency ultrasonic processing also include an amplitude of 5~10μm, a coupling interval of 5~10ms, and an ultrasonic power of 32~40kW; the conditions for low-frequency ultrasonic processing also include an amplitude of 10~15μm, a coupling interval of 5~10ms, and an ultrasonic power of 32~40kW.

[0019] Preferably, the rate of the pull-down spindle is 3~6 mm / min.

[0020] Preferably, electromagnetic levitation melting is used for suspension melting, with a melting power of 360kW.

[0021] Preferably, a water-cooled copper crucible is used in the suspension melting process.

[0022] Preferably, the inner wall of the crystallizer is pre-coated with yttrium oxide. By utilizing its chemical inertness and physical isolation with the rare earth melt, it reduces secondary contamination of the target material by the crystallizer material on the one hand, and provides high-temperature lubrication on the other hand, suppressing defects such as tearing and pitting on the surface of the ingot caused by adhesion, thereby obtaining a smooth as-cast surface and reducing subsequent machining allowance.

[0023] Preferably, when the ingot length reaches 18-20mm, the ingot pulling is paused, and high-purity lanthanum metal is added to the suspension furnace for smelting. Then the ingot pulling continues, and the above steps are repeated until the total ingot length reaches 70-80mm. Then, the heating power of the suspension furnace is reduced to 0kW at 2-minute intervals by reducing the heating power of the suspension furnace by 50kW, and the ingot pulling speed is increased to 5-6mm / min. The ingot pulling ends, and the dual-frequency ultrasonic treatment is stopped.

[0024] Preferably, the ingot is cut into a target blank of a set thickness using multi-wire cutting, single-wire cutting, or a sawing machine.

[0025] Preferably, the rolling is carried out using a cross-shaped rolling method, and the rolling conditions include: 8 to 9 rolling passes, a reduction of 0.3 to 0.4 mm per pass, and a roller speed of 10 to 12 m / min per pass.

[0026] Preferably, the correction is performed at least three times, with each correction involving a reduction of 0.02 to 0.025 mm.

[0027] Preferably, the second annealing temperature is 400~450℃ and the time is 6~8h.

[0028] Preferably, the purity of the high-purity lanthanum metal is greater than 4N, and the suspension melting is carried out under a vacuum degree lower than 1×10⁻⁶. -2 The experiment was conducted under the condition of Pa.

[0029] Based on a general inventive concept, this application also provides a lanthanum metal target obtained according to the above preparation method, wherein the diameter of the lanthanum metal target is ≥350mm and the average grain size of the lanthanum metal target is ≤60μm.

[0030] Preferably, the purity of the lanthanum target is greater than 4N.

[0031] Compared with the prior art, this application has the following beneficial effects:

[0032] This application describes a method for directional solidification of ingots under dual-frequency ultrasonic treatment conditions, employing a combination of suspension melting and multi-stage temperature-controlled downward casting. Suspension melting avoids crucible contamination of the melt, ensuring the high purity of the target material. The downward casting process guides the melt to solidify sequentially from bottom to top, achieving dynamic compensation for solidification shrinkage and significantly reducing casting defects such as shrinkage cavities and porosity. Furthermore, the solidification process is controlled by three independently temperature-controlled crystallizers: the upper air-gap heat-conducting slow-cooling section regulates interfacial thermal resistance by controlling the air-gap medium pressure, maintaining slow cooling in the low solids region during the initial solidification stage to prevent thermal stress cracking; the middle high-speed medium strong cooling section uses high-pressure, high-speed cooling water for forced cooling, refining grains and eliminating microscopic defects; and the lower adjustable-temperature homogenization section uses resistance or induction heating for online annealing of the solidified ingot, eliminating radial temperature differences and preventing central tearing. Furthermore, dual-frequency ultrasonic synergistic treatment was introduced during the suspension melting and ingot pulling processes. The coupling effect of high and low frequency ultrasound generated enhanced cavitation and acoustic flow effects, effectively breaking up coarse columnar crystals, inhibiting the formation of microcracks and porosity, and promoting equiaxed crystal transformation. The resulting ingot was cut into target blanks and then subjected to multi-pass hot rolling. No edge cracks or deformation failures occurred during the hot rolling process, indicating that the ingot has good processing plasticity. The final lanthanum target material obtained had a diameter greater than 350 mm. Metallographic observation after etching revealed a uniform and fine equiaxed crystal structure with an average grain size of less than 60 μm, achieving efficient preparation of large-sized target blanks with uniform microstructure and no macroscopic defects. Attached Figure Description

[0033] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some implementation methods of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0034] Figure 1 This is a structural diagram of the crystallizer used in the embodiments of this application.

[0035] Figure 2 The image shows the ultrasonic flaw detection pattern of the target material prepared in Example 1.

[0036] Figure 3 Metallographic image of the lanthanum target blank prepared in Example 1.

[0037] Figure 4 Metallographic image of the lanthanum metal target prepared in Example 1.

[0038] Figure 5 A cross-sectional view of the ingot prepared for Comparative Example 1 after being cut to a depth of 10 mm.

[0039] Figure 6 A cross-sectional view of the ingot prepared for Comparative Example 1 after being cut to a depth of 20 mm.

[0040] Figure 7 Metallographic image of the target material prepared in Comparative Example 2.

[0041] Explanation of reference numerals in the attached figures

[0042] 1. Air gap heat conduction and slow cooling section crystallizer; 11. Air gap medium inlet; 12. Air gap medium outlet;

[0043] 2. High-speed medium-cooled section crystallizer; 21. Cooling water inlet; 22. Cooling water outlet;

[0044] 3. Adjustable temperature uniform temperature section crystallizer;

[0045] 4. Heat insulation ring. Detailed Implementation

[0046] The embodiments described in this specification are merely for explaining this application and are not intended to limit this application.

[0047] For simplicity, this paper only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range, just as any upper limit can be combined with any other upper limit to form an undefined range. Furthermore, although not explicitly stated, every point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can serve as its own lower or upper limit and be combined with any other point or individual value, or with other lower or upper limits, to form an undefined range.

[0048] Those skilled in the art will understand that the order in which the steps are written in the various embodiments or examples does not imply a strict execution order and does not limit the implementation process in any way. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but sequentially is preferred.

[0049] The present application is further illustrated below with reference to embodiments. It should be understood that these embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0050] See Figure 1In this embodiment, the three independently temperature-controlled crystallizers used in the casting pull-down process include, sequentially along the casting pull-down direction, an air-gap heat-conducting slow-cooling section crystallizer 1, a high-speed medium strong-cooling section crystallizer 2, and an adjustable-temperature homogenizing section crystallizer 3. The air-gap heat-conducting slow-cooling section crystallizer 1 is equipped with an air-gap medium inlet 11 and an air-gap medium outlet 12, through which the undried melt is slowly cooled. The high-speed medium strong-cooling section crystallizer 2 is equipped with a cooling water inlet 21 and a cooling water outlet 22. In this high-speed medium strong-cooling section, cooling water is used for strong cooling, allowing the melt to cool and solidify relatively quickly. The adjustable-temperature homogenizing section crystallizer 3 has an induction coil on its outer ring, which heats the solidified casting for online annealing. Simultaneously, to reduce temperature crosstalk caused by axial heat conduction between the crystallizer sections and to ensure the temperature control independence of each crystallizer section, the three crystallizer sections are physically separated by a heat-insulating ring 4 for thermal isolation; the heat-insulating ring is made of high-temperature resistant boron nitride ceramic.

[0051] Example 1

[0052] I. Preparation of Lanthanum Target Material

[0053] Step 1: Place 16 kg of high-purity lanthanum metal (purity > 4N) into a water-cooled copper crucible in a suspension furnace, then begin evacuating the vacuum until the vacuum level is less than 1 × 10⁻⁶. -2 Pa, start the heating power of the suspension furnace, and gradually increase it to 360kW at a rate of 50kW every 3 minutes.

[0054] Step 2: Start high-frequency pulse stirring: frequency 75kHz, power 40kW, amplitude 6μm; and start low-frequency ultrasonic treatment: frequency 50kHz, ultrasonic power 35kW, amplitude 10μm, coupling interval 6ms; after the raw materials are completely melted, keep warm for 8 minutes.

[0055] Step 3: After the heat preservation is completed, the ingot pulling begins at a speed of 3 mm / min. The total length of the crystallizer in the ingot pulling equipment is 400 mm (40 mm in total height of the heat insulation ring). The upper air gap heat conduction and slow cooling section of the crystallizer is 100 mm long, purged with argon gas at a pressure of 0.05 MPa and a flow rate of 8 L / min to slowly cool the melt. The middle high-speed medium strong cooling section of the crystallizer is 150 mm long, cooled by cooling water at a pressure of 0.7 MPa and a flow rate of 10 m / s. The lower adjustable temperature homogenization section of the crystallizer is 110 mm long, heated by resistance with a heating power of 15 kW, maintaining the temperature of the lower crystallizer at 300℃.

[0056] Once the total length of the ingot reaches 20mm, stop the ingot pulling process. Begin adding 10kg of high-purity lanthanum metal to the water-cooled copper crucible through the feeding hopper of the suspension furnace. After the metal has completely melted, hold the mixture at that temperature for 8 minutes. After holding, continue pulling the ingot at a speed of 3mm / min until the length reaches 40mm. Repeat this process until three feedings are completed. Once the total length of the ingot reaches 70mm, reduce the heating power of the suspension furnace by 50kW every 2 minutes until the heating power reaches 0kW. Increase the pulling speed to 6mm / min, and the pulling process ends. Turn off the high-frequency pulse stirring and low-frequency ultrasonic treatment.

[0057] Step 4: One hour after the ingot pulling is completed, open the furnace door, remove the ingot and measure its dimensions. The ingot diameter is 295mm and the total height is 102mm. The shrinkage cavity depth of the ingot is measured to be 6mm. Use a multi-wire cutting device to slice the ingot, starting from the shrinkage cavity. The slice thickness is 10mm, and a total of 9 slices are made to obtain a lanthanum target billet with no surface cracks or other defects.

[0058] Step 5: Place the lanthanum target billet with dimensions of 295×10mm under an argon atmosphere and hold it at 430℃ for 20 minutes. After holding, quickly place the target billet into the rolling mill roller table and roll it using a cross-shaped rolling method. The roller table speed is 12m / min, the reduction per pass is 0.4mm, and the reduction for each correction is 0.025mm. A total of 9 passes are performed, with a total deformation of 29%. After completing the 9 passes, the target billet is corrected three times to minimize the bending degree. After correction, annealing is performed at 420℃ for 8 hours. The dimensions of the annealed target material are measured to be 356×7.2mm.

[0059] Step 6: Perform precision machining on the annealed target material to obtain a lanthanum metal target material with a final size of 350×7.0mm.

[0060] III. Performance Testing

[0061] (1) Take a sample of the lanthanum target blank obtained in step 4 and perform metallographic testing. The metallographic image is shown below. Figure 3 As shown in the figure, the grain size is approximately 300 μm.

[0062] (2) The target material obtained after annealing was subjected to ultrasonic C-scan, and its ultrasonic flaw detection images are as follows: Figure 2 As shown, from Figure 2 It can be seen that the internal defect rate of the target material is <0.2%.

[0063] (3) Metallographic samples were cut from the precision-machined lanthanum target, and after mounting, grinding, and polishing to a mirror finish, metallographic etching was performed. The etchant formula was concentrated sulfuric acid: acetic acid: phosphoric acid: nitric acid = 2 mL: 18 mL: 10 mL: 30 mL. The above mixed acid was stirred evenly, and then diluted 10 times with deionized water before metallographic etching. The etching time was 5 s. The grain boundary morphology of the sample was observed using a metallographic microscope, and the results are as follows: Figure 4 As shown in the figure. The results show that the grain size of the lanthanum metal target is less than 55 μm.

[0064] (4) Samples of the lanthanum metal target were taken and tested by GDMS. The results showed that the purity of the lanthanum metal target was greater than 4N.

[0065] Comparative Example 1

[0066] The preparation method of this comparative example is basically the same as that of Example 1. The main difference is that in this comparative example, conventional ingot pulling is used during the ingot pulling process, and the crystallizer is cooled by water throughout the process. The cooling water pressure is 0.7 MPa and the flow rate is 10 m / s. Moreover, ultrasonic treatment is not performed during the batching, melting, and ingot pulling processes. Other conditions and steps are the same as in Example 1.

[0067] The ingot was cut using single-wire cutting, with a cutting thickness of 10mm. The surface of the ingot cross-section after cutting is as follows. Figure 5 As shown, its surface has obvious, relatively long cracks; the ingot was further cut using a single-wire cutting device, with a cutting thickness of 10mm; the surface of the cut ingot cross-section is as follows. Figure 6 As shown, it can be seen that the ingot has deep shrinkage cavities and cracks inside after cutting, resulting in low ingot utilization.

[0068] Comparative Example 2

[0069] The preparation method of this comparative example is basically the same as that of Example 1. The main difference is that this comparative example uses only a single high-frequency ultrasonic treatment during the melting and casting process, with an ultrasonic frequency of 80 kHz. Other preparation conditions are the same as those of Example 1.

[0070] The resulting ingots, obtained after drawing, have higher hardness, which increases the difficulty of cutting and rolling. Microscopic analysis of ingot samples yields metallographic images as shown below. Figure 7 As shown in the metallographic diagram, the grain size in the ingot reaches 400 μm.

[0071] Comparative Example 3

[0072] The preparation method of this comparative example is basically the same as that of Example 1. The main difference is that this comparative example uses only a single low-frequency ultrasonic treatment during the melting and casting process, with an ultrasonic frequency of 50 kHz. Other preparation conditions are the same as those of Example 1.

[0073] When the ingot prepared in this comparative example is hot rolled, the rolling is difficult and the grain size of the ingot reaches 400μm.

[0074] Comparative Example 4

[0075] The preparation method of this comparative example is basically the same as that of Example 1. The main difference is that online annealing was not performed in the lower section of the crystallizer, while other conditions are the same as those of Example 1.

[0076] After cutting the ingot prepared in this comparative example to a depth of 20 mm, a crack was found at the center of the ingot cross-section surface.

[0077] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing a large-size, fine-grained lanthanum metal target, characterized in that, Includes the following steps: S1. Under dual-frequency ultrasonic treatment conditions, high-purity lanthanum metal is subjected to suspension melting and ingot pulling to obtain an ingot; the dual-frequency ultrasonic treatment includes high-frequency ultrasonic treatment at a frequency of 75kHz and low-frequency ultrasonic treatment at a frequency of 45~55kHz; the conditions for the high-frequency ultrasonic treatment also include an amplitude of 5~10μm and an ultrasonic power of 32~40kW, and the conditions for the low-frequency ultrasonic treatment also include an amplitude of 10~15μm and an ultrasonic power of 32~40kW; during the ingot pulling process, at least three independently temperature-controlled crystallizers distributed along the ingot pulling direction are used to directionally solidify the melt, the three independently temperature-controlled crystallizers comprising, in sequence: The air gap heat conduction slow cooling section crystallizer uses argon as the air gap medium and controls the air gap medium pressure to be 0.02~0.05MPa to allow the unsolidified melt to be slowly cooled. The high-speed medium-cooled section crystallizer uses cooling water with a pressure of 0.5~0.7MPa and a flow rate of 8~10m / s for strong cooling; An adjustable temperature uniform temperature section crystallizer is used to perform online annealing of solidified castings by heating; the adjustable temperature uniform temperature section crystallizer is heated by resistance heating or induction coil heating, with a heating power of 5~15kW and a temperature control range of 200~600℃. The rate of the pull-down ingot is 3~6 mm / min, and the inner wall of the crystallizer is pre-coated with yttrium oxide. S2. Cut the ingot to obtain a target blank of a set thickness; S3. The target blank is rolled, corrected, annealed a second time, and precision machined in sequence to obtain a lanthanum metal target of the target size; the temperature of the second annealing is 400~450℃ and the time is 6~8h.

2. The preparation method according to claim 1, characterized in that, The length of the air gap heat conduction slow cooling section crystallizer is 80~120mm; the length of the high-speed medium strong cooling section crystallizer is 150~250mm; and the length of the adjustable temperature uniform temperature section crystallizer is 100~150mm.

3. The preparation method according to claim 1, characterized in that, The rolling process is carried out using a cross-shaped rolling method. The rolling conditions include: 7 to 9 rolling passes, with a reduction of 0.3 to 0.5 mm per pass, and a roller speed of 10 to 12 m / min per pass.

4. The preparation method according to claim 1, characterized in that, The correction is performed at least twice, with each correction involving a reduction of 0.02 to 0.025 mm.

5. The preparation method according to claim 1, characterized in that, The purity of the high-purity metallic lanthanum is greater than 4N, and the suspension melting is carried out under a vacuum degree lower than 1×10⁻⁶. -2 The experiment was conducted under the condition of Pa.

6. A lanthanum metal target obtained by the preparation method according to any one of claims 1 to 5, characterized in that, The diameter of the lanthanum metal target is ≥350mm, and the average grain size of the lanthanum metal target is ≤60μm.

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