Lithium niobate sputtering target and method for producing same

Abstract

The present invention addresses the problem of providing: a lithium niobate sputtering target which has a high density and a fine structure; and a method for producing the same. Provided is a lithium niobate sputtering target which has a relative density of 80% or more and an average crystal grain size of 40 µm or less. Provided is a method for producing a lithium niobate sputtering target, the method including a step for hot-press sintering LiNbO3 powder at 780°C to 1150°C inclusive in an inert gas atmosphere.

Description

Lithium niobate sputtering target and method for manufacturing the same

[0001] This disclosure relates to a lithium niobate sputtering target and a method for manufacturing the same.

[0002] Lithium niobate (LiNbO) ３ Single-crystal thin films of ) are widely used in piezoelectric materials and optical applications, and are particularly used in SAW (surface acoustic wave) filters. Conventionally, substrates sliced ​​from single-crystal ingots have been used to form thin films, but there have been problems with limitations on the thickness of the thin film and poor yield. Therefore, sputtering targets are being developed to improve yield and enable thin film formation.

[0003] For example, Patent Document 1 discloses a technique for improving conductivity through a raw material preparation process and oxygen deficiency treatment after sputtering target fabrication. Patent Document 2 also discloses a technique for making the electrical resistance in the thickness direction of a metal oxide sputtering target, measured at an applied voltage of 1.5V, in the range of 100 mΩ·cm to 10 Ω·cm by inducing oxygen deficiency during firing.

[0004] Chinese Patent Application Publication No. 116396076, Specification of Patent No. 3464251

[0005] When the sintering temperature of a lithium niobate sintered body is increased to increase its density, grain growth is promoted, and its strength decreases to the point where it cannot be processed into a sputtering target. Even if it can be processed into a sputtering target, cracks and fissures occur in the sputtering target during film deposition, making stable sputtering film deposition difficult. In view of the above problems, this disclosure aims to provide a lithium niobate sputtering target having high density and a fine structure, and a method for manufacturing the same.

[0006] The gist of this disclosure is as follows: [1] A lithium niobate sputtering target having a relative density of 80% or more and an average crystal grain size of 40 μm or less. [2] The lithium niobate sputtering target according to [1], having a flexural strength of 5 MPa or more. [3] The lithium niobate sputtering target according to [1] or [2], having a C content of 300 wt ppm or less. [4] LiNbO ３ A method for manufacturing a lithium niobate sputtering target, comprising the step of hot-press sintering the powder in an inert gas atmosphere at a temperature of 780°C or higher and 1150°C or lower. [5] The LiNbO ３ The powder has a particle size d ５０ A method for producing a lithium niobate sputtering target according to [4], wherein the median diameter (by volume) is 5 μm or less.

[0007] According to this disclosure, a lithium niobate sputtering target having a high density and fine structure, and a method for manufacturing the same can be provided.

[0008] The following describes specific embodiments of this disclosure, but each configuration and combination thereof in each embodiment is merely an example, and additions, omissions, substitutions, and other modifications can be made as appropriate without departing from the spirit of this disclosure.

[0009] An embodiment of the present invention (hereinafter also referred to as this embodiment) is a lithium niobate sputtering target having a relative density of 80% or more and an average crystal grain size of 40 μm or less. A high-density sputtering target is expected to suppress particles during film formation. Furthermore, a sputtering target with a fine structure is expected to suppress the occurrence of cracks and fissures.

[0010] The relative density of the lithium niobate sputtering target according to this embodiment is preferably 85% or more, more preferably 90% or more, and even more preferably 95% or more. In particular, it has been confirmed that crack resistance is high when the relative density is 90% or more during sputtering. Furthermore, the average crystal grain size of the lithium niobate sputtering target according to this embodiment is preferably 30 μm or less, more preferably 20 μm or less, even more preferably 10 μm or less, and most preferably 5 μm or less. In particular, it has been confirmed that crack resistance is high during the manufacturing process of the sputtering target when the average crystal grain size is 10 μm or less.

[0011] In this embodiment, the lithium niobate sputtering target preferably has a flexural strength of 5 MPa or more. During film formation, the plasma concentrates on the target surface, causing a localized temperature increase in the sputtering target. Thermal stress in the target due to the temperature increase tends to concentrate at grain boundaries, leading to brittle fracture. In a sputtering target with a fine structure, thermal and mechanical stresses are dispersed without localized concentration, thus suppressing crack formation. A target that can suppress crack formation has high flexural strength. In this embodiment, the flexural strength is more preferably 10 MPa or more, even more preferably 20 MPa or more, and particularly preferably 30 MPa or more.

[0012] In this embodiment, the lithium niobate sputtering target preferably has a carbon (C) content of 300 wt ppm or less. If carbon from the manufacturing raw material remains in the sputtering target, it may be incorporated during film formation and worsen the film properties. Therefore, it is preferable to reduce the carbon content, which is an impurity, as much as possible. The carbon content is more preferably 200 wt ppm or less, and even more preferably 100 wt ppm or less.

[0013] The lithium niobate sputtering target according to this embodiment contains lithium (Li), niobium (Nb), and oxygen (O). The composition ratio of Li and Nb preferably satisfies the following formula. In the following formula, Li and Nb respectively represent the atomic percentages of lithium and niobium contained in the sputtering target. 0.90 ≤ Li / Nb ≤ 1.30 (formula) Li / Nb in the above formula is more preferably 0.95 or more, further preferably 0.98 or more, and more preferably 1.25 or less, further preferably 1.15 or less.

[0014] The lithium niobate sputtering target according to this embodiment may contain, as a dopant, B, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Y, Zr, Mo, Ag, Cd, In, Sn, Sb, Ba, Hf, Ta, W, Au, Tl, Pb, Bi, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. These dopants can contribute to improving the density of the sintered body and adjusting the optical properties. There is no particular limitation on the addition amount, but it can be added in an amount of 0.01 to 10 mol% in terms of oxide conversion.

[0015] Patent Document 1 discloses a sputtering target that causes oxygen deficiency to have a low resistance. In this case, there is a problem that an oxidation reaction occurs on the surface of the sputtering target during film formation, the film formation rate decreases, and stable film characteristics cannot be obtained. By setting the volume resistivity of the lithium niobate sputtering target to a predetermined value or more, stable film formation by RF sputtering is possible, and it is easier to adjust the film formation rate than DC (direct current) sputtering, and the film thickness uniformity can be improved.

[0016] From the above viewpoints, the lithium niobate sputtering target according to this embodiment preferably has a volume resistivity of 100 Ω·cm or more. More preferably 300 Ω·cm or more, further preferably 500 Ω·cm or more, and particularly preferably 1 kΩ·cm or more. There is no particular limitation on the upper limit value of the volume resistivity, but in practice, 1.0×10 ９It is preferably below Ω·cm.

[0017] The sputtering target of this embodiment can be in the shape of a disk flat plate, a rectangular flat plate, or a cylindrical shape, and can be joined by a backing plate and a bonding material. The thickness of the sputtering target is preferably 20 mm or less, more preferably 3.0 to 15 mm, and even more preferably 6.0 to 12 mm. Also, the area of the surface to be sputtered is preferably 81 cm ２ or more.

[0018] A method for manufacturing a lithium niobate sputtering target according to an embodiment of the present invention will be described below. However, it is obvious that the following manufacturing conditions, etc. are not limited to the disclosed range, and some omissions and changes may be made. In order to avoid the disclosed manufacturing method becoming unnecessarily unclear, detailed descriptions of well-known manufacturing processes and processing operations are omitted.

[0019] (1. Raw material powder) As the raw material powder, Li ２ CO ３ powder and Nb ２ O ５ powder can be used. The particle diameter d ２ O ５ (volume-based median diameter) of the Nb ５０ O ２ powder is preferably 5 μm or less. More preferably, it is 3 μm or less, and even more preferably 1 μm or less. When the particle diameter of the Nb ５ O ２ powder is large, the reaction may not proceed completely in the subsequent synthesis process (C may remain). When the particle diameter d ５ (volume-based median diameter) of the Nb ５０ O ２ powder is large, it is desirable to perform fine pulverization treatment. On the other hand, since Li ３ CO ２ becomes a liquid phase in the synthesis process and is easy to react, the particle diameter size does not affect the progress of the reaction so much. The particle diameter d ３ (volume-based median diameter) of the Li ５０ CO

[0020] (2. Mixing process) Li ２ CO ３ powder, Nb ２ O ５ The powders are weighed to achieve the desired composition ratio. After weighing, they are mixed to obtain a mixed powder. To obtain a dense and uniform sintered body, fine grinding and uniform mixing may be thoroughly performed using a mixer such as a ball mill or attritor.

[0021] (3. Synthesis process) Li ２ CO ３ Powder and Nb ２ O ５ The mixed powder is calcined in air at a temperature between 720°C and 1100°C to produce LiNbO ３ Synthesize the powder.

[0022] (4. Grinding process) After synthesis, LiNbO ３ The powder is dry-ground or wet-ground. Particle size d ５０ It is preferable to grind the material until the median diameter (by volume) is 5 μm or less. In the case of wet grinding, after weighing, the amount of media is determined according to the amount of raw material. The diameter of the media is 0.5 mm or 1.0 mm, and the material of the media is alumina (Al ２ O ３ ) and zirconia (ZrO ２ ) can be used. The dispersion medium is water or ethanol, and when water is used, it is preferable to recover the entire amount when taking out the mixed slurry. The grinding method is wet ball mill grinding or wet bead mill grinding, and the grinding time can be 1 hour or more. Using the grinding method described above, the particle size d ５０ It is preferable to mix and grind until the median diameter (by volume) is 5 μm or less. After wet grinding, it is dried, and then crushed and sieved.

[0023] (5. Sintering process) LiNbO ３A lithium niobate sintered body is produced by hot-press sintering the powder in an inert gas atmosphere such as argon or nitrogen at a sintering holding temperature of 780°C or higher and 1150°C or lower. If the sintering holding temperature is below 780°C, the density of the sintered body may not increase sufficiently. On the other hand, if the sintering holding temperature exceeds 1150°C, the average grain size of the sputtering target becomes coarser. The sintering time is preferably 3 hours or more and 7 hours or less. Furthermore, the heating rate is preferably 0.5 to 5°C / min or less.

[0024] (6. Finishing Process) The sintered body obtained through the above sintering process can be used to produce a sputtering target with a desired shape using a surface grinder, cylindrical grinder, machining center, or other processing machine as needed. The sputtering target is produced by machining the sintered body, and its relative density and flexural strength are substantially the same as those measured on the sintered body.

[0025] The following explanation is based on examples and comparative examples. However, these examples are merely illustrative and do not limit the invention in any way. That is, the present invention is limited only by the claims and encompasses various variations other than those included in the examples of this disclosure.

[0026] The evaluation methods used in the examples and comparative examples are as follows. Since sputtering targets are processed by grinding, polishing, etc., the surface of the sintered body after polishing is substantially the same as the sputtering surface of the sputtering target. Therefore, the various evaluation results of the sintered body can be considered identical to the characteristics of the sputtering target. Furthermore, although various evaluations are performed on a representative part (sample) of the sintered body in this disclosure, if the various evaluation results of a representative sample fall within the scope of this disclosure, the sintered body is included in the present invention. In other words, if the various physical properties of a representative sample fall within the scope of this disclosure, even if a specific, exceptional, or partial sample that does not represent the sintered body is measured and it deviates from the scope of this disclosure, the sintered body is still included in the present invention.

[0027] (Regarding component analysis) Quantitative analysis of Li and Nb was performed using the following equipment: Equipment: SPS3500DD manufactured by SII Corporation Method: ICP-OES (Inductively coupled plasma emission spectrometry) Quantitative analysis of C was performed using the following equipment: Equipment: CSLS600 manufactured by LECO Corporation Method: Infrared absorption in oxygen stream

[0028] (Regarding relative density) The relative density was calculated using the following formula: Relative density (%) = Archimedes density / Theoretical density × 100 Archimedes density: A sample (approximately 20 × 20 × 10 mm) was cut from the sintered body, and the density was calculated from the sample mass in air and the sample mass in water (Archimedes method). Theoretical density: 4.65 g / cm³ ３

[0029] (Regarding average grain size) The average grain size was measured using the Code method. The equipment and measurement conditions used were as follows. The Code method involves drawing a straight line of arbitrary length from grain boundary to grain boundary, counting the number of intersections with the grain boundaries, and dividing the length of the line by the number of particles to calculate the average grain size. An observation sample (approximately 10 × 15 × 5 mm) was cut from the sintered body, and the cross-section of the observation sample (a cross-section perpendicular to the sputtering surface) was mirror-polished. Next, three fields of view of the central part of the observation sample cross-section were taken using a scanning electron microscope (FE-SEM). Five straight lines (in the direction of the long side of the image) were drawn on the captured image, and the length of the line segments that intersected the crystal particles was measured, and the number of particles was counted. At this time, the magnification was adjusted so that the number of particles crossed by each line in the field of view was between 10 and 80. The value obtained by dividing the length of the line segment by the number of particles was taken as the grain size, and the arithmetic mean of the grain sizes for the three fields of view was taken as the average grain size. If pores or localized ultrafine crystalline regions were present in the sintered body structure, the average grain size was calculated from the length of the straight line excluding the pores or localized ultrafine crystalline regions and the intersection points. Equipment used: JXA-8500F (manufactured by JEOL Ltd.) Acceleration voltage: 15.0 kV Beam current: 2.0 × 10⁻¹⁶ －８ A

[0030] (Regarding flexural strength) The flexural strength was measured using the following apparatus. A sample for measurement (approximately 3 x 4 x 40 mm) was cut from the sintered body, the surface of the cut sample (corresponding to the sputtered surface) was polished, and the flexural strength of the polished surface was measured under the conditions in Table 1 in accordance with JIS R 1601:2008. Test method: Three-point bending test, distance between supports: 30 mm, head speed: 0.5 mm / min. Measurements were performed on 10 test pieces, and the average value was calculated.

[0031]

[0032] (Regarding volume resistivity) Volume resistivity was measured using the following apparatus. A sintered body sample was processed into a square shape, and the volume resistivity was measured near the intersection of the diagonals of the surface ground 2 mm from the surface. The thickness of the sample was 3.0 to 10.0 mm. When the volume resistivity of the sintered body exceeded 1000 Ω·cm, the following apparatus was used for measurement. Apparatus: High Resistivity Meter High Resistar-UX MCP-HT800 manufactured by Nitto Seiko Analytech Co., Ltd. Method: Constant voltage application / leakage current measurement method Method: Double ring method Measurement temperature: Room temperature (20 to 25°C) Applied voltage: 1 to 1000V The applied voltage was varied from 1 to 1000V, and the volume resistivity was measured.

[0033] When the volume resistivity of the sintered body was 1000 Ω·cm or less, the following equipment was used for measurement: Equipment: NPS Resistivity Meter Σ-5+ Method: Constant current application method Method: DC 4-probe method Measurement temperature: Room temperature (20-25°C)

[0034] (Example 1) Li ２ CO ３ powder, Nb ２ O ５ The powders were weighed to a ratio of Li:Nb = 1:1 (atomic percent), and then mixed in a rotary-blade dry mixer. After that, the mixed powder was calcined at 1000°C to produce LiNbO2. ３ The powder was synthesized. Next, LiNbO ３ The powder is finely ground, resulting in a particle size d ５０ The median diameter was set to 0.97 μm. After grinding, LiNbO ３The powder was packed into a carbon die and sintered in an argon atmosphere at a temperature of 980°C and a surface pressure of 250 kgf / cm². ２ Under the condition of a heating rate of 3°C / min, sintering was performed for 3 hours to obtain LiNboO ３ A sintered body was prepared. The obtained LiNboO ３ The sintered body was machined to produce a lithium niobate sputtering target. The resulting lithium niobate sputtering target was measured for various properties, and the results showed a Li / Nb composition ratio of 1.0, a relative density of 97.3%, and a volume resistivity of 5.19 × 10⁻¹⁴ when measured at an applied voltage of 50V. ７ The average crystal grain size measured at Ω·cm and FE-SEM magnification of 1000x was 2.64 μm, the flexural strength was 78.6 MPa, and the carbon content was 60 wt ppm, thus obtaining the desired results.

[0035] (Example 2) A lithium niobate sputtering target was fabricated using a method almost identical to that of Example 1, except that the sintering temperature was changed to 800°C. The physical properties of the obtained lithium niobate sputtering target were measured, and the results showed a Li / Nb composition ratio of 1.0, a relative density of 84.7%, and a volume resistivity of 1.59 × 10⁻¹⁴ when measured at an applied voltage of 50V. ８ The average crystal grain size measured at Ω·cm and FE-SEM magnification of 5000x was 0.64 μm, the flexural strength was 31.2 MPa, and the carbon content was 110 wt ppm, thus obtaining the desired results.

[0036] (Example 3) Of the conditions in Example 1, LiNbO ３ The fine grinding of the powder is enhanced, and the particle size d ５０ The median diameter (by volume) was set to 0.59 μm. After grinding, LiNbO ３ The powder was packed into a carbon die and sintered in an argon atmosphere at a temperature of 850°C and a surface pressure of 250 kgf / cm². ２ Under the condition of a heating rate of 1°C / min, sintering was performed for 3 hours to obtain LiNboO ３A sintered body was fabricated. The obtained lithium niobate sputtering target was measured for various physical properties. The results showed a Li / Nb composition ratio of 1.0, a relative density of 91.7%, and a volume resistivity of 9.59 × 10⁻¹⁴ when measured at an applied voltage of 50V. ７ The average crystal grain size measured at Ω·cm and FE-SEM magnification of 5000x was 0.89 μm, the flexural strength was 48.5 MPa, and the carbon content was 100 wt ppm, thus obtaining the desired results.

[0037] (Example 4) A lithium niobate sputtering target was fabricated using a method almost identical to that of Example 3, except that the sintering temperature was changed to 980°C. The obtained lithium niobate sputtering target was measured for various physical properties, and the results showed a Li / Nb composition ratio of 1.0, a relative density of 98.9%, and a volume resistivity of 4.90 × 10⁻¹⁰ when measured at an applied voltage of 50V. ７ The average crystal grain size, measured in Ω·cm and with an FE-SEM magnification of 1000x, was 3.11 μm, the flexural strength was 92.4 MPa, and the carbon content was 60 wt ppm, thus obtaining the desired results.

[0038] (Comparative Example 1) LiNbO was prepared in substantially the same manner as in Example 1, except that the particle size of the synthesized powder after pulverization and the sintering temperature were changed to 1200°C. ３ A sintered body was fabricated. Because the shape of this sintered body was distorted upon removal from the furnace, it was not possible to measure its relative density, flexural strength, etc. A portion of the distorted sintered body was embedded in resin and polished. Two fields of view were captured of the polished cross-section of the sintered body using a scanning electron microscope (magnification: 50x). Five straight lines (along the longer side of the image) were drawn on the captured image. Excluding the portion overlapping with the resin, the length of the line segments intersecting the crystal grains was measured, and the number of particles was counted. The crystal grain size was then calculated by dividing the length of the line segment by the number of particles. The average crystal grain size was calculated from the arithmetic mean of the crystal grain sizes from the two fields of view. The result showed that the average crystal grain size was 156.5 μm, indicating grain coarsening.

[0039]

[0040] According to this disclosure, a lithium niobate sputtering target with high density and a fine structure can be obtained, which may contribute to the advancement of sputtering thin film formation technology used in the manufacture of piezoelectric materials and optical devices. This disclosure may contribute to Goal 9 of the United Nations Sustainable Development Goals (SDGs), "Build resilient infrastructure, promote inclusive and sustainable industrialization and foster technological innovation."

[0041] According to this disclosure, a lithium niobate sputtering target having high density and a fine structure can be obtained. Furthermore, thin films formed using the lithium niobate sputtering target according to this disclosure are useful as intermediate layers in piezoelectric materials or as optical thin films.

Claims

1. A lithium niobate sputtering target having a relative density of 80% or more and an average crystal grain size of 40 μm or less.

2. The lithium niobate sputtering target according to claim 1, wherein the flexural strength is 5 MPa or more.

3. A lithium niobate sputtering target according to claim 1 or 2, wherein the C content is 300 wt ppm or less.

4. LiNbo ３ A method for manufacturing a lithium niobate sputtering target, comprising the step of hot-press sintering a powder in an inert gas atmosphere at a temperature of 780°C or higher and 1150°C or lower.

5. The LiNbO ３ Powder particle size d ５０ A method for producing a lithium niobate sputtering target according to claim 4, wherein the median diameter (by volume) is 5 μm or less.

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