Epitaxial thin film and method for manufacturing the same

Abstract

An epitaxial thin film that does not have rotational twinning and exhibits properties comparable to those of an ideal perfect crystal is formed on the surface of an LN single crystal or LT single crystal. [Solution] A thin film is made using lithium niobate or lithium tantalate as the base material, to which one alkali metal element from sodium, potassium, rubidium, cesium, or francium is added, and this thin film is epitaxially grown on the surface of a substrate by high-frequency sputtering.

Description

Technical Field

[0006]

[0001] The present invention relates to an epitaxial thin film and a method for manufacturing the same.

Background Art

[0002] Lithium niobate (LiNbO3, hereinafter also referred to as LN) and lithium tantalate (LiTaO3, hereinafter also referred to as LT), which are oxide single crystals, are materials having excellent piezoelectric properties and electro-optic effects. In recent years, a technique for forming a thin film while maintaining the properties of these LN and LT single crystals has attracted attention from the viewpoint of improving the performance of communication and optical control elements using both materials.

[0003] When forming these LN and LT single crystal thin films by sputtering film formation, which is one of vapor phase growth methods, optimization of appropriate process conditions is required to reproduce the integrity of the crystal structure and the stoichiometric composition on the substrate.

[0004] By the way, the crystal structure of LN and LT single crystals is usually the R3c structure (also referred to as the LiNbO3 structure) of the normal space group, but theoretically, the space group is known as a crystal structure in which the R3- structure (also referred to as the ilmenite structure) can be formed. The difference between the LiNbO3 structure and the ilmenite structure lies in the stacking arrangement of cations along the polar axis in the crystal, and both share the corundum structure as the basic framework of the crystal structure and can coexist in the crystal. Therefore, the ilmenite structure can appear in the crystal as a stacking defect (also referred to as a non-stoichiometric defect when caused by a deviation from the stoichiometric composition) or a superstructure with respect to the LiNbO3 structure.

[0005] In the LiNbO3 structure and the ilmenite structure, the X-ray diffraction patterns are very similar. However, in Non-Patent Document 1, at the X-ray diffraction scattering angle 2θ of a lithium niobate crystal sample in which an ilmenite-type stacking defect was intentionally introduced, reflection peaks with large interplanar spacings that do not appear in the space group R3c were observed in the ranges of 11° to 22° and 25.3° to 31°.

[0006] Non-patent document 2 reports a method for obtaining lithium niobate single crystals with diametrically opposed chemical compositions by top-seeding solution growth using a flux containing potassium oxide (chemical formula: K2O) in the preparation of LN single crystals. In this case, it is known that the potassium element does not act as an impurity (dopant) and is not incorporated into the crystal.

[0007] Non-patent document 3 describes the importance of crystal growth and property control by adding various dopants to the crystal in the fabrication of LN single crystals, but it does not report the addition of alkali elements from the same group as lithium, such as potassium (K), rubidium (Rb), and cesium (Cs).

[0008] Patent Document 1 discloses a lithium niobate epitaxial thin film (ilmenite structure film) produced by sputtering. However, in its crystal structure, it does not distinguish between the ilmenite structure and the LiNbO3 structure, and it is stated that the film is not a single-structure epitaxial film but an epitaxial film containing rotational twins. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 5862364 [Non-patent literature]

[0010] [Non-Patent Document 1] AV Kadetova et al., Materialia 28 (2023) 101770. [Non-Patent Document 2] K. Polgar, et. al., J. Cryst. Growth 177 (1997) 211-216. [Non-Patent Document 3] O. Sanchez-Dena, et.al., Crystals 2020, 10(11), 990. [Overview of the project] [Problems that the invention aims to solve]

[0011] In recent years, with the remarkable progress in thin-film formation of LN and LT single crystals, a bottom-up approach based on film deposition is expected to be promising from the viewpoints of manufacturing cost, large-area production, and flexibility of the manufacturing process.

[0012] When forming a lithium niobate single crystal thin film by sputtering, the constituent elements are supplied to the substrate surface in an atomically decomposed state during the elementary process. This makes it easy to form lithium niobate thin films with compromised crystal integrity, such as those with rotational twinning. As a result, such thin films may exhibit discrepancies between their properties and those expected from an ideal, perfect crystal, potentially preventing the achieving of sufficient performance in communication and optical control elements.

[0013] The problem that this invention aims to solve is to form an epitaxial thin film that does not have rotational twinning and exhibits properties comparable to those of an ideal perfect crystal on the surface of an LN single crystal or LT single crystal. [Means for solving the problem]

[0014] The present invention, which solves the above problems, is a method for manufacturing an epitaxial thin film (hereinafter referred to as the first invention), in which a thin film is deposited on the surface of a substrate by high-frequency sputtering, with lithium niobate as the base material and one alkali metal element of sodium, potassium, rubidium, cesium, or francium added to the base material, or a method for manufacturing an epitaxial thin film (hereinafter referred to as the second invention), in which a thin film is deposited on the surface of a substrate by high-frequency sputtering, with lithium tantalate as the base material and one alkali metal element of sodium, potassium, rubidium, cesium, or francium added to the base material.

[0015] In the above invention, the surface of the substrate may be a lithium niobate single crystal or a sapphire single crystal in the first invention, and a lithium tantalate single crystal or a sapphire single crystal in the second invention. Here, the substrate itself may be a lithium niobate single crystal, lithium tantalate single crystal or sapphire single crystal substrate, or only the surface may be a lithium niobate single crystal, lithium tantalate single crystal or sapphire single crystal substrate.

[0016] In the above invention, in the first invention, a target containing lithium, niobium, oxygen, and any one of the aforementioned alkali metal elements may be sputtered, or alternatively, a first target containing lithium, niobium, and oxygen and a second target containing any one of the aforementioned alkali metal elements may be sputtered simultaneously. Furthermore, in the second invention, a target containing lithium, tantalum, oxygen, and any one of the aforementioned alkali metal elements may be sputtered, or alternatively, a first target containing lithium, tantalum, and oxygen and a second target containing any one of the aforementioned alkali metal elements may be sputtered simultaneously.

[0017] In the above invention, it is more preferable that the composition formula Lix A(1-x) Nb / Ta Oy of the target containing lithium, niobium / tantalum, oxygen, and any one of the alkali metal elements is 0.920 ≤ x ≤ 0.995. However, niobium and tantalum are interchangeable, and A is any one of the alkali metal elements sodium, potassium, rubidium, cesium, or francium.

[0018] In the above invention, the temperature of the substrate may be set to less than 450°C, and epitaxial growth may be performed by the high-frequency sputtering method.

[0019] Furthermore, the above problems can also be solved by an epitaxial thin film manufacturing apparatus that uses the epitaxial thin film manufacturing method according to the above invention.

[0020] Further, the present invention for solving the above problems is an epitaxial thin film formed on the surface of a lithium niobate single crystal or a sapphire single crystal, which is an epitaxial thin film having lithium niobate as a base material and one of sodium, potassium, rubidium, cesium or francium added to the base material.

[0021] Further, the present invention for solving the above problems is an epitaxial thin film formed on the surface of a lithium tantalate single crystal or a sapphire single crystal, which is an epitaxial thin film having lithium tantalate as a base material and one of sodium, potassium, rubidium, cesium or francium added to the base material.

Advantages of the Invention

[0022] According to the present invention, an epitaxial thin film that does not have rotational twins and exhibits characteristics comparable to those of an ideal perfect crystal thin film can be formed on the surface of an LN single crystal or an LT single crystal.

Brief Description of the Drawings

[0023] [Figure 1] It is a diagram showing an embodiment of an apparatus for manufacturing an epitaxial thin film of the present invention. [Figure 2] It is a diagram showing another embodiment of an apparatus for manufacturing an epitaxial thin film of the present invention. [Figure 3] It is a cross-sectional view showing an embodiment of an epitaxial thin film of the present invention. [Figure 4] It is a diagram showing X-ray diffraction results of an epitaxial thin film of lithium niobate according to a comparative example of the present invention and an epitaxial thin film with addition of a different alkali metal element according to an example of the present invention. [Figure 5] It is a pole figure showing the results of XRD pole measurement (2θ = 23.715) of an epitaxial thin film of lithium niobate according to a comparative example of the present invention and an epitaxial thin film with addition of a different alkali metal element according to an example of the present invention. [Figure 6]This is a pole diagram showing the results of XRD pole measurements (2θ=19.200) of a 128Y-cut lithium niobate substrate according to a comparative example of the present invention, an epitaxial thin film of lithium niobate according to a comparative example, and an epitaxial thin film doped with heterogeneous alkali metal elements according to an example. [Figure 7] This figure shows the results of piezoelectric response microscopy (PFM) observations of a 128Y-cut lithium niobate substrate according to a comparative example of the present invention, an epitaxial thin film of lithium niobate according to a comparative example, and an epitaxial thin film doped with heterogeneous alkali metal elements according to an example. [Modes for carrying out the invention]

[0024] Figure 1 is a cross-sectional view showing one embodiment of the epitaxial thin film manufacturing apparatus 1 according to the present invention. The epitaxial thin film manufacturing apparatus 1 of this embodiment is a sputtering apparatus that, when the surface of the substrate 2 is an LN single crystal, LT single crystal, or sapphire single crystal, deposits a thin film by epitaxial growth on the surface of the substrate 2 having an LN single crystal, LT single crystal, or sapphire single crystal surface by high-frequency sputtering, using lithium niobate as the base material and adding one alkali metal element from sodium, potassium, rubidium, cesium, or francium to the base material. When the surface of the substrate is an LT single crystal or sapphire single crystal, deposits a thin film by epitaxial growth using lithium tantalate as the base material and adding one alkali metal element from sodium, potassium, rubidium, cesium, or francium to the base material.

[0025] In this embodiment, the epitaxial thin film manufacturing apparatus 1 uses lithium niobate as the base material when the surface of the substrate 2 is an LN single crystal or a sapphire single crystal, and targets a powder sintered body of heteroalkali metal-added lithium niobate, to which one alkali metal element from sodium, potassium, rubidium, cesium, or francium is added. Hereinafter, this target will also be referred to as heteroalkali metal-added LN powder sintered body. Alternatively, when the surface of the substrate 2 is an LT single crystal or a sapphire single crystal, the target uses lithium tantalate as the base material, and targets a powder sintered body of heteroalkali metal-added lithium tantalate, to which one alkali metal element from sodium, potassium, rubidium, cesium, or francium is added. Hereinafter, this target will also be referred to as heteroalkali metal-added LT powder sintered body.

[0026] Then, high-frequency power is applied to the target 121, which is made of a LN powder sintered body or a LT powder sintered body with dissimilar alkali metal elements, to form a plasma. Positive ions accelerated by the DC negative bias generated on the surface of the target 121 collide with the target 121, and the particles sputtered from the target surface are used as the material for the epitaxial thin film. The substrate 2 has a surface made of LiNbO3 single crystal, LiTaO3 single crystal, or sapphire single crystal (α-Al2O3), and the LiNbO3 single crystal and LiTaO3 single crystal may have been subjected to a reduction reaction treatment.

[0027] As shown in Figure 1, the epitaxial thin film manufacturing apparatus 1 of this embodiment comprises a chamber 11 that can be set to a predetermined vacuum level, a sputtering electrode 12 provided in the chamber 11, and a susceptor 13 also provided in the chamber 11.

[0028] The chamber 11 according to this embodiment includes a film deposition chamber 111, into which a target 121 mounted on a sputtering electrode 12 faces, and a substrate 2 mounted on a susceptor 13 faces. The film deposition chamber 111 can be set to a predetermined vacuum level by an exhaust device 113 provided at the exhaust port 112 of the chamber 11.

[0029] The sputtering electrode 12 of this embodiment supplies high-frequency power to the target 121 attached to its tip. The frequency of this high-frequency power is not limited, but may include a frequency of 13.56 MHz. Furthermore, if the surface of the substrate is an LN single crystal or a sapphire single crystal, a target made of LN powder sintered with heterogeneous alkali metal elements is used for the target 121, and if the surface of the substrate is an LT single crystal or a sapphire single crystal, a target made of LT powder sintered with heterogeneous alkali metal elements is used.

[0030] Furthermore, it is more preferable that the composition formula Lix A(1-x) Nb Oy of the target of the LN powder sintered body with dissimilar alkali metal elements is 0.920 ≤ x ≤ 0.995, and it is more preferable that the composition formula Lix A(1-x) Ta Oy of the target of the LT powder sintered body with dissimilar alkali metal elements is 0.920 ≤ x ≤ 0.995. In other words, it is more preferable that the addition ratio of dissimilar alkali metal elements is 0.5 atomic% to 8 atomic%. In addition, lithium oxide Li2O may be contained to some extent in the target of the LN powder sintered body with dissimilar alkali metal elements or the LT powder sintered body with dissimilar alkali metal elements.

[0031] The susceptor 13 of this embodiment is made of a conductive material, holds the substrate 2, and can be rotated at a predetermined rotational speed by a drive device 132 connected to a rotating shaft 131. The susceptor 13 may be provided with a counterbore (a recess corresponding to the outer shape of the substrate 2) so that deposition processing can be performed on multiple substrates 2 in a single process. The material constituting the susceptor 13 is not particularly limited, but examples include Inconel, silicon carbide, graphite, and silicon. The conductivity of the susceptor 13 does not necessarily have to be exhibited at room temperature, and may be exhibited when the substrate 2 is heated to below 450°C.

[0032] Furthermore, a heater 133 is provided at the bottom of the susceptor 13 to heat the substrate 2 to a predetermined temperature. The heating method of the heater 133 is not limited, but examples include a resistance heating method using graphite or silicon carbide as a resistor, and a photothermal heating method using a halogen lamp. By depositing an epitaxial thin film by high-frequency sputtering while maintaining the substrate 2 at a temperature of less than 450°C, it is possible to promote the epitaxial growth of the deposited thin film while preventing the re-oxidation of the LN single crystal or LT single crystal that has been treated with a reduction reaction.

[0033] The rotation axis 131 of the susceptor 13 is maintained in an electrically levitated state. By electrically levitating the rotation axis 131, it becomes possible to adjust the impact of positive ions. By depositing a thin film while rotating the substrate 2 in an electrically levitated state, it is possible to uniformly deposit a thin film over the entire surface of the substrate 2 while satisfying the requirement to mitigate the impact of negative ions.

[0034] The epitaxial thin film manufacturing apparatus 1 of this embodiment includes a high-frequency matching unit 134 connected to the rotating shaft 131 of the susceptor 13, which is electrically levitated and capable of adjusting the reactance of the rotating shaft 131. Here, reactance corresponds to the capacitance and inductance within the high-frequency matching unit 134. This adjustment by the high-frequency matching unit 134 is performed by measuring the DC voltage generated within the high-frequency matching unit 134 during deposition, thereby diagnosing the state of electrical coupling of the substrate 2 to the high-frequency plasma.

[0035] Here, when the DC voltage reaches its negative maximum value, it indicates that the capacitive substrate sheath and capacitance are in a series resonance state, attracting positive ions to the substrate 2 and causing reverse sputtering. Conversely, when the DC voltage reaches its positive maximum value, it corresponds to a parallel resonance state between the substrate 2 and the stray capacitance of the high-frequency matching unit 134 including the rotating shaft 131, attracting negatively charged particles to the substrate 2. In this embodiment, by adjusting and maintaining the reactance to the latter, the re-evaporation of Li atoms deposited on the substrate 2 due to reverse sputtering can be suppressed, and the epitaxial growth of the deposited thin film can be promoted.

[0036] The epitaxial thin film manufacturing apparatus 1 of this embodiment further includes a DC power supply 135 connected to the rotating shaft 131, which applies a DC voltage to the electrically floating rotating shaft 131. The DC power supply 135 may apply either a negative voltage or a positive voltage, but it is desirable to apply a positive voltage from the viewpoint of promoting the epitaxial growth of the deposited thin film.

[0037] In this embodiment, the target 121 is positioned so as not to directly face the substrate 2. That is, as shown in Figure 1, the target 121 is positioned offset with respect to the rotation axis 131, and the surface normal PL1 of the target 121 is set to have an inclination α of 15° to 75° with respect to the substrate normal PL2 of the substrate 2. The DC negative bias generated on the surface of the target 121 accelerates the negative ions in the plasma and causes these negative ions to be emitted along the surface normal L1 of the target 121. Therefore, by not positioning the substrate 2 and the target 121 to face each other, direct collision of negative ions with the substrate 2 can be avoided, and the impact caused by negative ions can be mitigated. This can promote the epitaxial growth of the deposited thin film.

[0038] The epitaxial thin film manufacturing apparatus 1 of this embodiment includes a shielding plate 122 that mitigates the impact of negative ions emitted from a target 121 along the surface normal PL1 of the target 121. The shielding plate 122 of this embodiment is positioned in front of the target 121 so as to shield the portion where the group of surface normals PL1 of the target 121 intersects with the surface of the substrate 2. This suppresses the impact of negative ions emitted along the surface normal PL1 of the target 121 on the substrate 2, and further promotes the epitaxial growth of the deposited thin film.

[0039] The epitaxial thin film manufacturing apparatus 1 of this embodiment includes a porous metal plate 123 and a porous quartz plate 124. As shown in Figure 1, the porous metal plate 123 and the porous quartz plate 124 of this embodiment are provided on the inner wall of the deposition chamber 111, in front of the target 121, with the porous quartz plate 124 positioned in front of the porous metal plate 123. The porous metal plate 123 is electrically coupled to the plasma and supplies electrons to the plasma. At this time, the potential of the porous metal plate 123 may be ground, or a controlled negative potential may be supplied to more actively supply electrons. Furthermore, in order to increase the power efficiency of the high-frequency power supplied to the target 121 that is consumed in the actual sputtering phenomenon, the surface area of ​​the porous metal plate 123 may be increased by stacking multiple plates. The porous quartz plate 124 prevents the deposition of sputtered particles on the porous metal plate 123 and maintains a constant supply of electrons. Furthermore, the porous quartz plate 124 also has the function of preventing the reverse deposition of the material of the porous metal plate 123, which is sputtered, onto the substrate 2 when a negative potential is supplied.

[0040] The epitaxial thin film manufacturing apparatus 1 of this embodiment further includes a gas supply device 125 that supplies argon gas and oxygen gas to the film deposition chamber 111 as process gases during film deposition.

[0041] Figure 2 shows another embodiment of the epitaxial thin film manufacturing apparatus 1 of the present invention. The epitaxial thin film manufacturing apparatus 1 of the embodiment shown in Figure 2 differs from the epitaxial thin film manufacturing apparatus 1 of the embodiment shown in Figure 1 in that it includes two sputtering electrodes 12A and 12B and two targets 121A and 121B. Since the components other than those related to these differences are the same as those of the epitaxial thin film manufacturing apparatus 1 of the embodiment shown in Figure 1, the same reference numerals are used in Figure 2 and their descriptions are incorporated herein by reference.

[0042] The epitaxial thin film manufacturing apparatus 1 of the embodiment shown in Figure 2 is equipped with two sputtering electrodes 12A and 12B. One target 121A is mounted on one sputtering electrode 12A, and the other target 121B is mounted on the other sputtering electrode 12B. Here, one of the targets 121A is an LN powder sintered body target when the substrate surface is an LN single crystal or a sapphire single crystal, and an LT powder sintered body target is used when the substrate surface is an LT single crystal or a sapphire single crystal. These LN powder sintered body or LT powder sintered body targets may contain lithium oxide Li2O to some extent.

[0043] In contrast, the other target 121B uses a powder sintered body of one of the alkali metal elements, sodium, potassium, rubidium, cesium, or francium, regardless of whether the substrate surface is an LN single crystal, LT single crystal, or sapphire single crystal.

[0044] Then, high-frequency power is simultaneously applied to one sputtering electrode 12A, which is fitted with one target 121A, and to the other sputtering electrode 12B, which is fitted with the other target 121B, to form a plasma. Positive ions accelerated by the DC negative bias generated on the surface of the target collide with the target, and the particles sputtered from the surfaces of the respective targets 121A and 121B are used as material for the epitaxial thin film. [Examples]

[0045] Figure 3 is a cross-sectional view showing an embodiment of the epitaxial thin film according to the present invention. In the illustrated example, an epitaxial thin film 22 of potassium-doped lithium niobate is formed on an LN single crystal 21. In the deposition (epitaxial growth) of the epitaxial thin film of this embodiment using the manufacturing apparatus 1 described above, a potassium-doped LN powder sintered body prepared with the composition formula Lix K(1-x) Nb Oy, where x = 0.950, was used for the target 121 shown in Figure 1. The sputtering rate was 0.3 angstroms / second, the flow rate ratio of argon gas to oxygen gas was 50:50, the pressure in the deposition chamber 111 was 0.3 to 0.5 Pa, and the thickness of the epitaxial thin film was 250 to 300 nm. In addition, the heating temperature of the substrate 2 by the heater 133 was arbitrarily set to a temperature of less than 400°C as shown below.

[0046] Figures 4 to 6 are graphs showing the X-ray diffraction (XRD) measurement results of epitaxial thin films of potassium-doped lithium niobate deposited using the epitaxial thin film manufacturing apparatus 1 of this embodiment. The X-ray diffractometer used was a Rigaku Smart Lab, and the out-of-plane crystallinity was evaluated by measuring ω-2θ. In order to verify the effect of potassium doping, the XRD results of an epitaxial thin film of lithium niobate deposited using a target of a sintered lithium niobate powder without potassium doping are also shown. A 128Y cut lithium niobate substrate 2 was used for forming the epitaxial thin film.

[0047] Figure 4 is a graph showing the X-ray diffraction (XRDω-2θ measurement) results for a comparative example (Figures 4(a) and (b)) in which an epitaxial thin film of lithium niobate was formed on a 128Y-cut lithium niobate substrate with the heating temperature of substrate 2 set to 300°C, and an example (Figures 4(c) and (d)) in which an epitaxial thin film of potassium-doped lithium niobate was formed. Figures 4(b) and (d) are magnified graphs of the diffraction peaks of the (014) plane in Figures 4(a) and (c), respectively.

[0048] The results in Figures 4(a) and (c) confirm that each epitaxial thin film has no diffraction peaks other than the diffraction peak corresponding to the 128Y cut plane (lattice plane (104)). The results in Figures 4(b) and (d) allow for the identification of the diffraction peak of the substrate 21 and the diffraction peak originating from the thin film accompanied by intensity vibrations (fringes), confirming that each thin film 22 (Epilayer) is an epitaxial thin film. At the same time, the fringes indicate the presence of an interface between the substrate 21 and the thin film 22, suggesting that the crystal structures of the substrate 21 and the thin film 22 are strictly different. This is likely due to the difference in the content of ilmenite structure and LiNbO3 structure between the substrate 21 and the thin film 22. The clarity of the fringes differs in each case, which is presumed to be largely due to the surface roughness of the thin film 22. Surface roughness was evaluated using an atomic force microscope (AFM, Dimension Icon, Bruker). The surface roughness of the lithium niobate epitaxial thin film was Ra=0.8 nm, and the surface roughness of the potassium-doped lithium niobate epitaxial thin film was Ra=1.6 nm.

[0049] Figure 5 is a pole diagram showing the results of XRD pole measurements (2θ=23.715) for comparative examples ((a) and (b)) in which an epitaxial thin film of lithium niobate was formed on a 128Y-cut lithium niobate substrate with the heating temperature of substrate 2 set to 250°C and 300°C, and for examples ((c) and (d)) in which an epitaxial thin film of potassium-doped lithium niobate was formed. The diffraction spots shown on the pole diagram correspond to diffraction from the crystal lattice plane of LiNbO3(012).

[0050] Let's compare the results. First, comparing Figure 5(a), which is the diffraction pattern of an epitaxial thin film of lithium niobate deposited by setting the heating temperature of substrate 2 to 300°C, with Figure 5(c), which is the pattern of an epitaxial thin film of potassium-doped lithium niobate, we see that both show two-fold symmetric spots, which indicate a single single-crystal structure. When combined with the results in Figure 4, it is confirmed that a single-crystal film without a twinning structure has been formed.

[0051] Next, Figure 5(b), which shows the diffraction pattern of an epitaxial thin film of lithium niobate deposited by setting the heating temperature of substrate 2 to 250°C, shows "additional" spots (rotation twin parts) indicating the presence of rotational twins in the thin film. In contrast, Figure 5(d), which shows the diffraction pattern of an epitaxial thin film of potassium-doped lithium niobate, shows a two-fold symmetric pattern, similar to the case where the heating temperature of substrate 2 was set to 300°C, and does not show "additional" spots (rotation twin parts) indicating the presence of rotational twins in the thin film. Therefore, it was confirmed that when a film is deposited using a sputtering target made of potassium-doped lithium niobate, an epitaxial film without rotational twins can be formed at a lower temperature compared to when a film is deposited using a sputtering target made of lithium niobate alone.

[0052] Figure 6 shows the results of XRD pole point measurements (2θ=19.200) observing diffraction spots on the polar axis of the lithium niobate substrate 2 with a 128Y cut (Figure 6(a)), comparative examples ((b) and (c)) in which an epitaxial thin film of lithium niobate was formed on the lithium niobate substrate with a 128Y cut by setting the heating temperature of the substrate 2 to 250°C and 300°C, and examples ((d) and (e)) in which an epitaxial thin film of potassium-doped lithium niobate was formed.

[0053] In the case of lithium niobate with a LiNbO3 structure, diffraction at 2θ = 19.200, which is (003) reflection, is prohibited, and diffraction spots are not normally observed. However, since diffraction spots were observed in Figure 6(a) when the substrate itself was measured, it was confirmed that the 128Y-cut lithium niobate substrate used in this embodiment and comparative example is lithium niobate containing an ilmenite structure internally.

[0054] Next, in Figure 6(b), which shows the diffraction spots of an epitaxial thin film of lithium niobate deposited by setting the heating temperature of substrate 2 to 300°C, it was observed that the change in the broadening of the diffraction spots was small, confirming that the ilmenite structure content in the thin film was low. In contrast, in Figure 5(c), which shows the diffraction spots of an epitaxial thin film of lithium niobate deposited by setting the heating temperature of substrate 2 to 250°C, the broadening of the diffraction spots was larger, confirming that the thin film contained a larger amount of ilmenite structure compared to the diffraction spots at 300°C shown in Figure 6(b).

[0055] Next, Figures 6(d) and (e) show the diffraction spots of epitaxial thin films of potassium-doped lithium niobate deposited at heating temperatures of 300°C and 250°C for substrate 2. Since the diffusion of the diffraction spots was observed in both cases, it was confirmed that the ilmenite structure was present. However, since the diffusion of the diffraction spots was similar for both films with different heating temperatures, it was confirmed that the ilmenite structure content was similar. In other words, it was confirmed that the epitaxial thin film of potassium-doped lithium niobate yielded the same crystal quality even at lower substrate temperatures compared to the epitaxial thin film of lithium niobate without potassium doping.

[0056] Figure 6 shows the results of observing the surfaces of the lithium niobate substrate 2 with a 128Y cut (Figures 7(a) and (b)), the lithium niobate epitaxial thin film deposited on this substrate 2 with the substrate 2 heated to 300°C (Figures 7(c) and (d)), and the potassium-doped lithium niobate epitaxial thin film deposited with the substrate 2 heated to 300°C (Figures 7(e) and (f)) using a piezoelectric response microscope (PFM). A Dimension Icon piezoelectric response microscope manufactured by Burker was used. Figures 7(b), (d), and (f) are histograms of the piezoelectric responses of Figures 7(a), (c), and (e), respectively, and Figures 7(d) and (f) are histograms of the piezoelectric response amplitude normalized by the mode of the piezoelectric response amplitude signal of the substrate itself shown in Figure 7(b).

[0057] The mode of the piezoelectric response amplitude of the potassium-doped lithium niobate epitaxial thin film shown in Figure 7(f) was found to be larger than that of the lithium niobate epitaxial thin film shown in Figure 7(d), and also larger than that of the substrate itself shown in Figure 7(b). Therefore, it was confirmed that the dissimilar alkali metal element-doped lithium niobate thin films deposited in the examples are thin films with excellent piezoelectric response. [Explanation of symbols]

[0058] 1…Equipment for manufacturing epitaxial thin films 11... Chamber 111…Deposition chamber 112... Exhaust port 113... Exhaust system 12, 12A, 12B... Sputtering electrodes 121, 121A, 121B… Target 122...Shielding plate 123...Porous metal plate 124...Porous quartz plate 125... Gas supply device 13… Susceptor 131... Rotation axis 132... Drive unit 133... Heater 134... High-frequency matching unit 135…DC power supply PL1... Target surface normal PL2... Surface normal of the circuit board α...Angle between the surface normal of the target and the surface normal of the substrate. 2… Circuit board 21…LN single crystal 22…Epitaxial thin film

Claims

1. A method for manufacturing an epitaxial thin film, comprising depositing a thin film on the surface of a substrate by high-frequency sputtering, wherein the thin film is made of lithium niobate as the base material, and one of the alkali metal elements (sodium, potassium, rubidium, cesium, or francium) is added to the base material.

2. The method for manufacturing an epitaxial thin film according to claim 1, wherein the surface of the substrate is the surface of a lithium niobate single crystal or a sapphire single crystal.

3. A method for producing an epitaxial thin film according to claim 1 or 2, comprising sputtering a target containing lithium, niobium, oxygen, and any one of the aforementioned alkali metal elements.

4. A method for producing an epitaxial thin film according to claim 1 or 2, comprising simultaneously sputtering a first target containing lithium, niobium, and oxygen, and a second target containing any one of the aforementioned alkali metal elements.

5. A method for producing an epitaxial thin film according to claim 3, wherein the composition formula Lix A(1-x) Nb Oy of the target containing lithium, niobium, oxygen, and any one of the alkali metal elements is 0.920 ≤ x ≤ 0.995, wherein A is any one of the alkali metal elements sodium, potassium, rubidium, cesium, or francium.

6. A method for manufacturing an epitaxial thin film, comprising depositing a thin film on the surface of a substrate by high-frequency sputtering, wherein the thin film is made using lithium tantalate as the base material, to which one alkali metal element from sodium, potassium, rubidium, cesium, or francium is added, and the base material is then deposited.

7. The method for manufacturing an epitaxial thin film according to claim 6, wherein the surface of the substrate is the surface of a lithium tantalate single crystal or a sapphire single crystal.

8. A method for producing an epitaxial thin film according to claim 6 or 7, comprising sputtering a target containing lithium, tantalum, oxygen, and any one of the aforementioned alkali metal elements.

9. A method for producing an epitaxial thin film according to claim 5 or 6, comprising simultaneously sputtering a first target containing lithium, tantalum, and oxygen, and a second target containing any one of the aforementioned alkali metal elements.

10. A method for producing an epitaxial thin film according to claim 7, wherein the composition formula Lix A(1-x) Ta Oy of the target containing lithium, tantalum, oxygen, and any one of the alkali metal elements is 0.920 ≤ x ≤ 0.

995.

11. A method for producing an epitaxial thin film according to any one of claims 1 to 10, wherein the temperature of the substrate is set to less than 450°C and epitaxial growth is performed by the high-frequency sputtering method.

12. An apparatus for manufacturing an epitaxial thin film using the method for manufacturing an epitaxial thin film according to any one of claims 1 to 11.

13. An epitaxial thin film formed on the surface of a lithium niobate single crystal or a sapphire single crystal, An epitaxial thin film having lithium niobate as the base material, to which one alkali metal element from sodium, potassium, rubidium, cesium, or francium is added.

14. An epitaxial thin film formed on the surface of a lithium tantalate single crystal or a sapphire single crystal, An epitaxial thin film comprising lithium tantalate as the base material, to which one alkali metal element, either sodium, potassium, rubidium, cesium, or francium, is added.

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