Piezoelectric thin film, and piezoelectric thin film element

Abstract

To provide a piezoelectric thin film with excellent piezoelectric properties.SOLUTION: A piezoelectric thin film 3 contains metal oxide. The piezoelectric thin film 3 has a first main surface s1 and a second main surface s2 located behind the first main surface s1. The metal oxide includes hafnium. The metal oxide contains a crystalline phase with a fluorite-type crystal structure. Crystalline phases include one or both of rectangular and tetragonal. The first lattice plane LP1 of the crystalline phase is the (111), (100), or (110) plane. The second lattice plane LP2 of the crystalline phase is a (111), (100), or (110) plane, and the second lattice plane LP2 is a different lattice plane from the first lattice plane LP1. A portion of the first lattice plane LP1 is oriented in a normal direction dn1 of the first main plane s1. A portion of the second lattice plane LP2 is oriented in a normal direction dn2 to the second main plane s2. The direction in which the first and second lattice planes LP1 and LP2 respectively are oriented varies within the crystalline phase.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present disclosure relates to a piezoelectric thin film and a piezoelectric thin film element. [Background technology]

[0002] As described in the following Patent Document 1, hafnium oxide (HfO 2 ) is widely known as a dielectric. As described in Patent Document 2 below, HfO 2 It is also known that a thin film containing a solid solution of HfO has ferroelectricity. Ferroelectricity is a physical property in which electric dipoles are aligned within a dielectric when there is no electric field outside the dielectric, and the direction of the electric dipoles changes reversibly along the electric field applied to the dielectric. 2 is believed to have a rectangular crystal structure. 2 The orthorhombic phase of HfO is an unstable phase at normal pressure. 2 It is not easy to form a ferroelectric thin film containing the orthorhombic phase of HfO. The thicker the ferroelectric thin film, the more difficult it is to maintain the orthorhombic phase in the piezoelectric thin film. For example, as described in Patent Document 2 below, when a ferroelectric thin film is formed on the surface of a crystalline substrate by epitaxial growth, the thickness of the piezoelectric thin film at which the orthorhombic phase is stably maintained is about 100 nm at most. Patent Document 3 below describes a method for forming a ferroelectric thin film containing HfO 2 and a thickness of more than 100 nm. The ferroelectric thin film described in the following Patent Document 3 contains an additive element such as yttrium, so that the orthorhombic crystal in the ferroelectric thin film is stabilized. Furthermore, in the ferroelectric thin film described in the following Patent Document 3, two types of lattice planes oriented in the normal direction to the surface of the ferroelectric thin film are mixed in the crystal phase. As a result, the stress in the ferroelectric thin film is relaxed, and cracks in the ferroelectric thin film are suppressed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2008-306036 A [Patent Document 2] International Publication No. 2016 / 031986 Brochure [Patent Document 3] Patent No. 7061752 Summary of the Invention [Problem to be solved by the invention]

[0004] The continuity of the crystal deformation in the piezoelectric response is lost due to the mixture of two types of lattice planes oriented in the normal direction of the piezoelectric thin film surface. 2 Conventional thin films containing such materials are unlikely to have sufficient piezoelectric properties.

[0005] An object of one aspect of the present invention is to provide a piezoelectric thin film having excellent piezoelectric characteristics, and a piezoelectric thin film element including the piezoelectric thin film. [Means for solving the problem]

[0006] For example, specific examples of one aspect of the present invention relate to a piezoelectric thin film element described in any one of [1] to [6] below, a method for producing a piezoelectric thin film element described in [7] below, and a piezoelectric thin film element described in [8] or [9] below.

[0007] [1] A piezoelectric thin film containing a metal oxide, the piezoelectric thin film has a first main surface and a second main surface located on the rear side of the first main surface, The metal oxide comprises hafnium; The metal oxide includes a crystalline phase having a fluorite-type crystal structure, The crystalline phase includes at least one type of crystals selected from the group consisting of orthorhombic and tetragonal crystals; the first lattice plane of the crystalline phase is a lattice plane selected from the group consisting of a (111) plane, a (100) plane, and a (110) plane; the second lattice plane of the crystalline phase is one lattice plane selected from the group consisting of a (111) plane, a (100) plane, and a (110) plane, and the second lattice plane is a lattice plane different from the first lattice plane; A part of the first lattice plane is oriented in a normal direction to the first main surface, A part of the second lattice plane is oriented in a normal direction to the second main surface, The orientation of the first lattice plane changes within the crystalline phase, The orientation of the secondary lattice planes changes within the crystalline phase. Piezoelectric thin film.

[0008] [2] A first lattice plane oriented in a normal direction to the first main surface is closer to the first main surface than a second lattice plane oriented in a normal direction to the second main surface; [1] The piezoelectric thin film according to claim 1.

[0009] [3] The metal oxide further contains at least one additional element X selected from the group consisting of zirconium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium; The piezoelectric thin film according to [1] or [2].

[0010] [4] The hafnium content in metal oxides is expressed as [Hf] atomic %. The content of the additive element X in the metal oxide is expressed as [X] atomic %. [X] / ([Hf]+[X]) is 0.05 or more and 0.70 or less. [3] The piezoelectric thin film according to the present invention.

[0011] [5] A portion of the crystalline phase is a tetragonal crystal including a first lattice plane oriented in a direction normal to the first principal surface; Another part of the crystalline phase is a rectangular crystal including a second lattice plane oriented in the normal direction of the second main surface. The piezoelectric thin film according to any one of [1] to [4].

[0012] [6] A portion of the crystalline phase is a rectangular crystal including a first lattice plane oriented in the normal direction of the first principal surface; Another part of the crystalline phase is another orthogonal crystal including a second lattice plane oriented in the normal direction of the second main surface. The piezoelectric thin film according to any one of [1] to [5].

[0013] [7] A method for producing the piezoelectric thin film according to any one of [1] to [6], forming a precursor film containing monoclinic crystals of an oxide containing hafnium; forming a piezoelectric thin film from the precursor film by heating the precursor film; Including, A method for manufacturing a piezoelectric thin film.

[0014] [8] A piezoelectric thin film according to any one of [1] to [6], A first electrode layer; A second electrode layer; and Including, a first main surface of the piezoelectric thin film directly or indirectly overlaps the first electrode layer; The second electrode layer directly or indirectly overlaps the second main surface of the piezoelectric thin film. Piezoelectric thin film element.

[0015] [9] The first principal surface of the piezoelectric thin film directly overlaps the first electrode layer; The first electrode layer comprises platinum or indium tin oxide; The piezoelectric thin film element according to [8]. Effect of the Invention

[0016] According to one aspect of the present invention, there is provided a piezoelectric thin film having excellent piezoelectric characteristics, and a piezoelectric thin film element including the piezoelectric thin film. [Brief description of the drawings]

[0017] [Figure 1] (a) in Figure 1 is a schematic cross-sectional view of a piezoelectric thin film element according to one embodiment of the present invention, and (b) in Figure 1 is an oblique exploded view of the piezoelectric thin film element shown in (a) in Figure 1, with the crystalline substrate 1 and second electrode layer 4 being omitted in (b) in Figure 1. [Diagram 2]FIG. 2(a) is a perspective view of a rectangular unit cell of the fluorite-type crystal structure (hafnium oxide), and FIG. 2(b) is a perspective view of a tetragonal unit cell of the fluorite-type crystal structure (hafnium oxide). [Diagram 3] (a) in Figure 3 is a schematic oblique view of a unit cell of a crystal phase (rectangular) having a fluorite-type crystal structure, showing the (111) plane of the crystal phase (rectangular), (b) in Figure 3 is a schematic oblique view of a unit cell of a crystal phase (rectangular) having a fluorite-type crystal structure, showing the (100) plane of the crystal phase (rectangular), and (c) in Figure 3 is a schematic oblique view of a unit cell of a crystal phase (rectangular) having a fluorite-type crystal structure, showing the (110) plane of the crystal phase (rectangular). [Figure 4] (a) in Figure 4 is a schematic oblique view of a unit cell of a crystal phase (tetragonal) having a fluorite-type crystal structure, showing the (111) face of the crystal phase (tetragonal), (b) in Figure 4 is a schematic oblique view of a unit cell of a crystal phase (tetragonal) having a fluorite-type crystal structure, showing the (100) face of the crystal phase (tetragonal), and (c) in Figure 4 is a schematic oblique view of a unit cell of a crystal phase (tetragonal) having a fluorite-type crystal structure, showing the (110) face of the crystal phase (tetragonal). [Diagram 5] FIG. 5 is a schematic diagram of a cross section of the crystal phase of a metal oxide contained in the piezoelectric thin film 3, and is an enlarged view of a part of the cross section of the piezoelectric thin film shown in (a) of FIG. [Figure 6] FIG. 6 is an image of a cross section of the piezoelectric thin film according to Example 1 of the present invention, which is an image taken by a transmission electron microscope. [Figure 7] (a) in Figure 7 is a fast Fourier transform pattern in the vicinity of the second main surface of the piezoelectric thin film in the cross section shown in Figure 6, and (b) in Figure 7 is a fast Fourier transform pattern in the vicinity of the first main surface of the piezoelectric thin film in the cross section shown in Figure 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Preferred embodiments of the present invention will be described below with reference to the drawings. In the drawings, equivalent components are given the same reference numerals. The present invention is not limited to the following embodiments. X, Y, and Z shown in Fig. 1(a), Fig. 1(b), Fig. 5, and Fig. 6 represent three mutually orthogonal coordinate axes. The directions indicated by X, Y, and Z are common to Fig. 1(a), Fig. 1(b), Fig. 5, and Fig. 6.

[0019] The piezoelectric thin film element according to this embodiment may include a piezoelectric thin film, a first electrode layer, and a second electrode layer. For example, as shown in FIG. 1(a), a piezoelectric thin film element 10 may include a crystalline substrate 1, a first electrode layer 2 directly or indirectly overlapping the crystalline substrate 1, a piezoelectric thin film 3 directly or indirectly overlapping the first electrode layer 2, and a second electrode layer 4 directly or indirectly overlapping the piezoelectric thin film 3. The first electrode layer 2 may be a lower electrode layer. The second electrode layer 4 may be an upper electrode layer. A modified example of the piezoelectric thin film element 10 may not include the second electrode layer 4. For example, after a piezoelectric thin film element not including the second electrode layer is supplied as a product to an electronic device manufacturer, the second electrode layer may be added to the piezoelectric thin film element during the assembly and manufacturing process of the electronic device.

[0020] 1(a) and 1(b), the piezoelectric thin film 3 has a first main surface s1 and a second main surface s2 located on the rear side of the first main surface s1. The first main surface s1 faces the first electrode layer 2. The second main surface s2 faces the second electrode layer 4. The "principal surface" is the surface having the largest area among the multiple surfaces of the polyhedron. For example, the principal surface may be a surface that is substantially perpendicular to the thickness direction (Z-axis direction) of the piezoelectric thin film 3 among the multiple surfaces of the piezoelectric thin film 3. The thickness direction of the piezoelectric thin film 3 may be defined as a direction that is substantially perpendicular to the first principal surface s1 and the second principal surface s2 and extends from the first principal surface s1 to the second principal surface s2. The piezoelectric thin film 3 may have a pair of principal surfaces (the first principal surface s1 and the second principal surface s2) that are parallel to each other, and one or more end surfaces that are perpendicular to each principal surface. The shape of the piezoelectric thin film 3 (the shape of each principal surface) is not limited. For example, the shape of the piezoelectric thin film 3 may be a polygon such as a rectangle. For example, the piezoelectric thin film 3 may have a curved surface. For example, the piezoelectric thin film 3 may be a disk. The first principal surface s1 of the piezoelectric thin film 3 overlaps directly or indirectly with the first electrode layer 2. The second electrode layer 4 overlaps directly or indirectly with the second principal surface s2 of the piezoelectric thin film 3. The normal direction dn1 of the first principal surface s1 is perpendicular to the normal direction D N The normal direction dn2 of the second main surface s2 may be substantially parallel to the normal direction D N That is, both the first principal surface s1 and the second principal surface s2 may be substantially parallel to the principal surface of the first electrode layer 2. After the piezoelectric thin film 3 is formed on the principal surface of the first electrode layer 2, the second electrode layer 4 may be formed on the second principal surface s2 of the piezoelectric thin film 3.

[0021] The piezoelectric thin film 3 according to this embodiment includes a metal oxide. A part or the whole of the piezoelectric thin film 3 may be a metal oxide. The metal oxide includes at least hafnium (Hf). For example, a part or the whole of the metal oxide may be hafnium oxide (HfO 2 The piezoelectric thin film 3 may be ferroelectric. The piezoelectric thin film 3 may be an epitaxial thin film.

[0022] The metal oxide includes a crystal phase having a fluorite crystal structure. For example, the crystal phase of hafnium oxide has a fluorite crystal structure. The crystal phase having a fluorite crystal structure includes at least one of an orthorhombic crystal and a tetragonal crystal. The crystal phase may be composed of only an orthorhombic crystal. The crystal phase may be composed of only a tetragonal crystal. The crystal phase may include both an orthorhombic crystal and a tetragonal crystal. The crystal phase may be composed of only an orthorhombic crystal and a tetragonal crystal. In the present disclosure, an orthorhombic crystal may imply a rhombic crystal. The piezoelectric properties (ferroelectricity) of the piezoelectric thin film 3 are due to one or both of an orthorhombic crystal and a tetragonal crystal. In contrast, both a monoclinic crystal and a cubic crystal are unlikely to have piezoelectric properties. However, the metal oxide may further contain a small amount of monoclinic crystals as long as the piezoelectric characteristics of the piezoelectric thin film 3 are not impaired. The metal oxide may further contain a small amount of cubic crystals as long as the piezoelectric characteristics of the piezoelectric thin film 3 are not impaired.

[0023] For example, the unit cell ucо constituting an orthogonal crystal (orthogonal crystal of hafnium oxide) in the crystal phase is shown in (a) of FIG. 2, (a) of FIG. 3, (b) of FIG. 3, and (c) of FIG. 3. The unit cell ucо in each figure has a fluorite crystal structure. The ratio of the lengths of the primitive translation vectors a, b, and c constituting the unit cell ucо is not limited to the ratio shown in each figure. The primitive translation vectors a, b, and c of the orthogonal unit cell ucо are mutually orthogonal. The respective lengths of the primitive translation vectors a, b, and c of the orthogonal unit cell ucо are different from each other. In the case of an orthogonal crystal of hafnium oxide, the length of the primitive translation vector a (lattice constant a) may be about 5.30 Å, the length of the primitive translation vector b (lattice constant b) may be about 5.11 Å, and the length of the primitive translation vector c (lattice constant c) may be about 5.10 Å.

[0024] For example, the unit cell uct constituting the tetragonal crystal (tetragonal crystal of hafnium oxide) in the crystal phase is shown in (b) of FIG. 2, (a) of FIG. 4, (b) of FIG. 4, and (c) of FIG. 4. The unit cell uct in each figure has a fluorite crystal structure. The ratio of the lengths of the primitive translation vectors a, b, and c constituting the unit cell uct is not limited to the ratio shown in each figure. The primitive translation vectors a, b, and c of the tetragonal unit cell uct are mutually orthogonal, the length of the primitive translation vector a is equal to the length of the primitive translation vector b, and the length of the primitive translation vector a is different from the length of the primitive translation vector c. In the case of the tetragonal crystal of hafnium oxide, the length of the primitive translation vector a (lattice constant a) may be about 5.14 Å, the length of the primitive translation vector b (lattice constant b) may be about 5.14 Å, and the length of the primitive translation vector c (lattice constant c) may be about 5.25 Å.

[0025] The

[0111] in (a) of FIG. 3 and (a) of FIG. 4 is the orientation of the (111) plane (the normal direction of the (111) plane). The

[0100] in (b) of FIG. 3 and (b) of FIG. 4 is the orientation of the (100) plane (the normal direction of the (100) plane). The

[0110] in (c) of FIG. 3 and (c) of FIG. 4 is the orientation of the (110) plane (the normal direction of the (110) plane).

[0026] The cross section of the piezoelectric thin film 3 shown in (a) in Fig. 1 is substantially parallel to the thickness direction of the piezoelectric thin film 3. Fig. 5 is a schematic diagram of a cross section 3cs of the crystal phase of a metal oxide contained in the piezoelectric thin film 3, and is an enlarged view of a part of the cross section of the piezoelectric thin film 3 shown in (a) in Fig. 1. In other words, the cross section 3cs of the crystal phase of the metal oxide shown in Fig. 5 is also substantially parallel to the thickness direction of the piezoelectric thin film 3. The first lattice plane LP1 of the crystal phase (fluorite crystal structure) is one lattice plane selected from the group consisting of a (111) plane, a (100) plane, and a (110) plane. For example, the first lattice plane LP1 may be a (111) plane, a (100) plane, or a (110) plane included in a rectangular crystal or a tetragonal crystal. The second lattice plane LP2 of the crystal phase (fluorite crystal structure) is one lattice plane selected from the group consisting of a (111) plane, a (100) plane, and a (110) plane, and the second lattice plane LP2 is a lattice plane different from the first lattice plane LP1. For example, the second lattice plane LP2 may be a (111) plane, a (100) plane, or a (110) plane included in a rectangular crystal or a tetragonal crystal. For example, the first lattice plane LP1 may be a (111) plane, and the second lattice plane may be a (100) plane or a (110) plane. When the first lattice plane LP1 is a (111) plane and the second lattice plane is a (100) plane, the piezoelectric thin film 3 has excellent piezoelectric properties (for example, a large piezoelectric strain constant d 33,f ) is likely to be obtained. For example, the first lattice plane LP1 may be a (100) plane, and the second lattice plane may be a (111) plane or a (110) plane. For example, the first lattice plane LP1 may be a (110) plane, and the second lattice plane may be a (111) plane or a (100) plane. Since the crystalline phase has a periodic structure, it goes without saying that the crystalline phase includes a plurality of first lattice planes LP1 and a plurality of second lattice planes LP2.

[0027] As shown in FIG. 5, the crystalline phase has all of the following characteristics 1, 2, 3, and 4.

[0028] [Feature 1] is that "a part of the plurality of first lattice planes LP1 is oriented in the normal direction dn1 of the first main surface s1 of the piezoelectric thin film 3." In other words, the angle θ1 between at least one lattice plane (lattice plane LP11) of the plurality of first lattice planes LP1 and the first main surface s1 may be smaller than the angle between the other lattice planes and the first main surface s1. For example, a part of the first lattice planes LP1 (lattice plane LP11) may be substantially parallel to the first main surface s1.

[0029] [Feature 2] is that "a part of the plurality of second lattice planes LP2 is oriented in the normal direction dn2 of the second main surface s2 of the piezoelectric thin film 3." In other words, the angle θ2 between at least one lattice plane (lattice plane LP21) of the plurality of second lattice planes LP2 and the second main surface s2 may be smaller than the angle between the other lattice planes and the second main surface s2. For example, a part of the second lattice planes LP2 (lattice plane LP21) may be substantially parallel to the second main surface s2.

[0030] [Feature 3] is that "the direction in which the first lattice plane LP1 is oriented changes (continuously or gradually) in the crystalline phase (piezoelectric thin film 3)." In other words, the angle θ1 between the first principal plane s1 and the first lattice plane LP1 changes (continuously or gradually) in the crystalline phase. For example, as shown in FIG. 5, the angle θ1 between the first principal plane s1 and the first lattice plane LP1 may increase (continuously or gradually) along the thickness direction (Z-axis direction) of the piezoelectric thin film 3.

[0031] [Feature 4] is that "the direction in which the second lattice plane LP2 is oriented changes (continuously or gradually) in the crystal phase (piezoelectric thin film 3)." In other words, the angle θ2 between the second main surface s2 and the second lattice plane LP2 changes (continuously or gradually) in the crystal phase. For example, as shown in FIG. 5, the angle θ2 between the second main surface s2 and the second lattice plane LP2 may decrease (continuously or gradually) along the thickness direction (Z-axis direction) of the piezoelectric thin film 3.

[0032] In the present disclosure, "variable crystal orientation" refers to all of the above characteristics 1 to 4.

[0033] The crystalline phase may be an incomplete crystal having all of the above characteristics 1 to 4. As long as the crystalline phase has all of the above characteristics 1 to 4, the crystalline phase may be a single crystal or a polycrystal. For example, as long as the crystalline phase has all of the above characteristics 1 to 4, the crystalline phase may include a plurality of columnar crystals. The columnar crystals are crystals that extend from the first main surface s1 toward the second main surface s2.

[0034] The crystal phase includes not only the first lattice plane LP1 oriented in the normal direction dn1 of the first main surface s1 but also another lattice plane (second lattice plane LP2) oriented in the normal direction dn2 of the second main surface s2, so that the stress acting on the crystal phase is relaxed. As a result, the piezoelectric thin film 3 has sufficient mechanical strength, and cracks in the piezoelectric thin film 3 are suppressed. For example, when the crystal phase includes not only the first lattice plane LP1 oriented in the normal direction dn1 of the first main surface s1 but also the second lattice plane LP2 oriented in the normal direction dn2 of the second main surface s2, the stress acting on the orthogonal crystals in the piezoelectric thin film 3 is easily relaxed. As a result, the orthogonal crystals in the piezoelectric thin film 3 are easily stabilized under normal pressure. The stabilization of the orthogonal crystals makes it easy for the piezoelectric thin film 3 to have excellent piezoelectric properties and high mechanical strength. According to this embodiment, even if the thickness of the piezoelectric thin film 3 is greater than 100 nm, the piezoelectric thin film 3 can have sufficient mechanical strength, and cracks in the piezoelectric thin film 3 can be suppressed. However, when the crystal phase in a conventional piezoelectric thin film includes two types of lattice planes oriented in the normal direction of one of the main surfaces of the piezoelectric thin film, continuous deformation of the crystal phase (fluorite-type crystal structure) accompanying application of a voltage or external force to the piezoelectric thin film is difficult to occur. As a result, the conventional piezoelectric thin film is difficult to have sufficient piezoelectric characteristics. In contrast, the piezoelectric thin film 3 according to this embodiment has not only the above features 1 and 2 but also the above features 3 and 4. That is, in the case of this embodiment, the orientation directions of the first lattice plane LP1 and the second lattice plane LP2 change within the crystal phase. Therefore, continuous deformation of the crystal phase (fluorite-type crystal structure) accompanying application of a voltage or external force to the piezoelectric thin film 3 is easy to occur, and the piezoelectric thin film 3 has excellent piezoelectric characteristics (for example, a large piezoelectric strain constant d 33,f ).

[0035] The first lattice plane LP1 (lattice plane LP11) oriented in the normal direction dn1 of the first main surface s1 may be closer to the first main surface s1 than the second lattice plane LP2 (lattice plane LP21) oriented in the normal direction dn2 of the second main surface s2, because the crystal phase is likely to have "changing crystal orientation" and the piezoelectric thin film 3 is likely to have excellent piezoelectric characteristics. In other words, the second lattice plane LP2 oriented in the normal direction dn2 of the second main surface s2 may be closer to the second main surface s2 than the first lattice plane LP1 oriented in the normal direction dn1 of the first main surface s1. For example, the first lattice plane LP1 oriented in the normal direction dn1 of the first main surface s1 may be the first lattice plane LP1 (lattice plane LP11) whose distance from the first main surface s1 is smaller than its distance from the second main surface s2. For example, the second lattice plane LP2 oriented in the normal direction dn2 of the second main surface s2 may be a second lattice plane LP2 (lattice plane LP21) whose distance from the second main surface s2 is smaller than its distance from the first main surface s1.

[0036] Because the crystalline phase is likely to have "variable crystal orientation" and the piezoelectric thin film 3 is likely to have excellent piezoelectric properties, a part of the crystalline phase may be a tetragonal crystal (first crystal c1) including a first lattice plane LP1 (lattice plane LP11) oriented in the normal direction dn1 of the first principal surface s1, and another part of the crystalline phase may be a rectangular crystal (second crystal c2) including a second lattice plane LP2 (lattice plane LP21) oriented in the normal direction dn2 of the second principal surface s2. For a similar reason, the symmetry of a first crystal c1 (tetragonal) including a first lattice plane LP1 (lattice plane LP11) oriented in the normal direction dn1 of the first principal surface s1 of the crystalline phase may be higher than the symmetry of a second crystal c2 (rectangular) including a second lattice plane LP2 (lattice plane LP21) oriented in the normal direction dn2 of the second principal surface s2 of the crystalline phase. For a similar reason, the symmetry of the first crystal c1 (tetragonal) of the crystal phase, which is closer to the first main surface s1 than to the second main surface s2, may be higher than the symmetry of the second crystal c2 (rectangular) of the crystal phase, which is closer to the first main surface s1 than to the second main surface s2. A part of the crystalline phase may be a cubic crystal whose distance from the first main surface s1 is smaller than that from the second main surface s2. In other words, a part of the first crystals c1 whose distance from the first main surface s1 is smaller than that from the second main surface s2 may be a cubic crystal. The first crystal c1 including the first lattice plane LP1 (lattice plane LP11) oriented in the normal direction dn1 of the first principal surface s1 may be exposed at the first principal surface s1 (the interface between the piezoelectric thin film 3 and the first electrode layer 2). In other words, the first principal surface s1 may be a surface of the first crystal c1. The second crystal c2 including the second lattice plane LP2 (lattice plane LP21) oriented in the normal direction dn2 of the second principal surface s2 may be exposed at the second principal surface s2 (the interface between the piezoelectric thin film 3 and the second electrode layer 4). In other words, the second principal surface s2 may be a surface of the second crystal c2.

[0037] A portion of the crystalline phase may be a rectangular crystal including a first lattice plane LP1 oriented in the normal direction dn1 of the first principal surface s1, and another portion of the crystalline phase may be another rectangular crystal including a second lattice plane LP2 oriented in the normal direction dn2 of the second principal surface s2.

[0038] The metal oxide contained in the piezoelectric thin film 3 may be composed of only hafnium oxide. The metal oxide may further contain at least one additive element X selected from the group consisting of zirconium (Zr), yttrium (Y), and lanthanoids. That is, the metal oxide may further contain at least one additive element X selected from the group consisting of zirconium (Zr), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The metal oxide may contain a plurality of additive elements X (two or more kinds of additive elements X). The metal oxide may further contain at least one element selected from the group consisting of zirconium (Zr), cerium (Ce), and yttrium (Y) as an additive element X. When the metal oxide contains at least one additive element X in addition to hafnium, the metal oxide is likely to contain one or both of an orthorhombic crystal and a tetragonal crystal, the crystal phase of the metal oxide is likely to have "variable crystal orientation", and the piezoelectric thin film 3 is likely to have excellent piezoelectric properties.

[0039] The content of hafnium in the metal oxide may be expressed as [Hf] atomic %. The total content of the additive element X in the metal oxide may be expressed as [X] atomic %. [X] / ([Hf]+[X]) may be 0.05 to 0.70, 0.05 to 0.65, 0.05 to 0.60, 0.05 to 0.59, 0.05 to 0.55, 0.05 to 0.50, 0.05 to 0.45, 0.05 to 0.40, 0.05 to 0.35, 0.05 to 0.30, 0.05 to 0.25, 0.05 to 0.20, or 0.17 to 0.20. When [X] / ([Hf]+[X]) is within the above range, the metal oxide is likely to contain one or both of orthorhombic and tetragonal crystals, the crystal phase of the metal oxide is likely to have "variable crystal orientation", and the piezoelectric thin film 3 is likely to have excellent piezoelectric properties. [X] / ([Hf]+[X]) is <x>The metal oxide contained in the piezoelectric thin film 3 may be expressed as HfO 2 or Hf 1- <x>< / x> X <x>< / x> O 2 It may be expressed as:

[0040] The metal oxide may further contain an additional element E other than the additional element X. For example, the additional element E may be at least one element selected from the group consisting of aluminum (Al), silicon (Si), bismuth (Bi), and rare earth elements (excluding Y and Ce). For example, the content of the additional element E in the metal oxide may be 0 atomic % or more and 10 atomic % or less with respect to the total amount of Hf, the additional element X, and the additional element E.

[0041] The thickness of the piezoelectric thin film 3 may be substantially uniform. There is no particular limitation on the thickness of the piezoelectric thin film 3. For example, the thickness of the piezoelectric thin film 3 may be 30 nm or more and 3000 nm or less, or 30 nm or more and 1000 nm or less.

[0042] For example, the composition of the piezoelectric thin film 3 may be analyzed by X-ray fluorescence (XRF) or inductively coupled plasma (ICP) emission spectroscopy. For example, the crystal structure of the piezoelectric thin film 3 (crystalline phase) may be identified by X-ray diffraction (XRD). For example, the thickness of the piezoelectric thin film 3 may be measured in a cross section of the piezoelectric thin film 3 using a transmission electron microscope (TEM) or a scanning electron microscope (SEM).

[0043] For example, a method for analyzing the "changing crystal orientation" of a crystalline phase may be the following method. An image of the cross section of the piezoelectric thin film 3 is taken by a transmission electron microscope (TEM). The cross section of the piezoelectric thin film 3 taken is substantially parallel to the thickness direction of the piezoelectric thin film (i.e., the direction from the first main surface s1 to the second main surface s2). Three or more arbitrary measurement points are selected from the image of the cross section along the thickness direction of the piezoelectric thin film 3. However, one measurement point 1 is located near the first main surface s1, and the distance between the measurement point 1 and the first main surface s1 is shorter than the distance between the other measurement points and the first main surface s1. One measurement point 2 is located near the second main surface s2, and the distance between the measurement point 2 and the second main surface s2 is shorter than the distance between the other measurement points and the second main surface s2. One or more measurement points 3 are located between the measurement points 1 and 2. A fast Fourier transform (FFT) pattern of each measurement point is obtained by image processing of each measurement point. Each FFT pattern includes a first spot indicating the direction in which the first lattice plane LP1 is oriented and a second spot indicating the direction in which the second lattice plane LP2 is oriented. That is, the orientation direction of each of the first lattice plane LP1 and the second lattice plane LP2 at each measurement point is identified from the FFT pattern of each measurement point. The type and structure of the crystal present at each measurement point may be identified from the FFT pattern of each measurement point. When the orientation direction of the first lattice plane LP1 changes within the crystalline phase, the positions of the first spot in each of the three or more FFT patterns are different from each other. When the orientation direction of the second lattice plane LP2 changes within the crystalline phase, the positions of the second spot in each of the three or more FFT patterns are different from each other. The "changing crystal orientation" of the crystalline phase may be analyzed in a manner similar to that described above, except that an electron diffraction pattern is measured at each measurement point instead of an FFT pattern.

[0044] <Method of manufacturing piezoelectric thin film> The method for manufacturing the piezoelectric thin film 3 includes a step of forming a precursor film containing monoclinic crystals of an oxide containing hafnium, and a step of forming the piezoelectric thin film 3 from the precursor film by heating (annealing) the precursor film.

[0045] The precursor film is formed from a raw material (target) made of an oxide containing hafnium. The composition of the raw material may be the same as the composition of the piezoelectric thin film 3.

[0046] By the vapor phase growth method, the elements constituting the above-mentioned raw materials are evaporated. The evaporated elements are attached and deposited on the surface of the first electrode layer 2 or the surface of the crystalline substrate 1. As a result, a precursor film is epitaxially grown on the surface of the first electrode layer 2 or the surface of the crystalline substrate 1. For example, the vapor phase growth method may be a sputtering method, an electron beam evaporation method, a chemical vapor deposition (CVD) method, or a pulsed-laser deposition (PLD) method. The excitation source differs depending on the type of vapor phase growth method. For example, the excitation source of the sputtering method is Ar plasma. The excitation source of the electron beam evaporation method is an electron beam. The excitation source of the PLD method is a laser beam (for example, an excimer laser). When the raw materials are irradiated with these excitation sources, the elements constituting the raw materials are evaporated. The atmosphere of the vapor phase growth method may be a vacuum or an inert gas. For example, the inert gas may be nitrogen gas or a rare gas. For example, the pressure of the atmosphere for the vapor deposition process may be 1 Torr or less, or 10 mTorr or less.

[0047] It is preferable to form the precursor film by the PLD method because the precursor film is likely to contain monoclinic crystals and is likely to change into the piezoelectric thin film 3 (a crystalline phase having "variable crystal orientation") by annealing the precursor film. In the PLD method, the elements constituting the raw material (target) can be instantly and uniformly plasmatized by a pulsed laser. Therefore, it is easy to form a piezoelectric thin film 3 having substantially the same composition as the raw material. According to the PLD method, it is easy to control the thickness, growth rate, and crystallinity of the piezoelectric thin film 3 by adjusting various parameters such as the oscillation frequency of the laser pulse, the number of shots, the pulse energy, and the distance between the first electrode layer 2 or the crystalline substrate 1 and the target. It is preferable that the oscillation frequency of the pulsed light (e.g., KrF laser light) irradiated to the target in the PLD method is 3 Hz or less because the precursor film is likely to contain monoclinic crystals and the formation of an amorphous phase in the precursor film is suppressed. The growth rate of the precursor film decreases with the decrease in the oscillation frequency of the pulsed light. As the growth rate of the precursor film decreases, time is ensured for the plasmatized raw material to migrate on the surface of the first electrode layer 2 or the surface of the crystalline substrate 1. In other words, sufficient time is ensured for crystals to grow in the precursor film. As a result, the precursor film tends to be in the monoclinic phase rather than the amorphous phase. As the pulse energy decreases, the growth rate of the precursor film tends to decrease, and the precursor film tends to be in the monoclinic phase.

[0048] The temperature of the surface of the first electrode layer 2 or the surface of the crystalline substrate 1 in the vapor phase growth method is room temperature or normal temperature (20°C±15°C). In other words, there is no need to heat the first electrode layer 2 or the crystalline substrate 1 during the formation of the precursor film, and the precursor film is formed at room temperature or normal temperature. Most of the precursor films formed at room temperature or normal temperature are monoclinic. The lengths of the primitive translation vectors a, b, and c constituting the unit cell of the monoclinic crystal are different from each other. The primitive translation vectors a and b constituting the unit cell of the monoclinic crystal are perpendicular to each other, the primitive translation vectors b and c constituting the unit cell of the monoclinic crystal are also perpendicular to each other, and the primitive translation vectors c and a constituting the unit cell of the monoclinic crystal are not perpendicular to each other. For example, the precursor film may be made of only monoclinic crystals. However, a part of the precursor film may be one or both of an orthorhombic crystal and a tetragonal crystal. If the precursor film is a non-crystalline (amorphous) film, it is difficult to form a piezoelectric thin film 3 including a crystalline phase having a "variable crystal orientation" by annealing the precursor film.

[0049] The monoclinic crystals contained in the precursor film may have a fluorite crystal structure. The first lattice plane (e.g., (111) plane) of the monoclinic crystals (fluorite crystal structure) in the precursor film may be substantially parallel to the surface of the first electrode layer 2 or the surface of the crystalline substrate 1. However, the precursor film containing the orthorhombic crystals does not have "variable crystal orientation" and piezoelectric properties.

[0050] After the precursor film is formed at room temperature or normal temperature, the precursor film is heated (annealed). By annealing, a part or all of the monoclinic crystals in the precursor film are changed to a crystal phase containing one or both of an orthorhombic crystal and a tetragonal crystal. The heating rate of the heated precursor film may be 30°C / sec or more and 50°C / sec or less. A crystal phase having a "changing crystal orientation" is formed by rapidly increasing the temperature of the precursor film during annealing. If the heating rate of the precursor film is less than the lower limit, it is difficult to form a crystal phase having a "changing crystal orientation". In other words, if the heating rate of the precursor film is less than the lower limit, it is difficult for the orientation directions of the first lattice plane LP1 and the second lattice plane LP2 to change within the crystal phase. The orthorhombic crystal is unstable under normal pressure. Therefore, in the process of annealing the precursor film, the orthorhombic layer tends to easily return to the monoclinic crystal, and the orthorhombic crystal tends to be more difficult to maintain than the tetragonal crystal. However, by cooling, which will be described later, following the annealing, the orientation direction of some of the lattice planes contained in the orthogonal crystals changes, and the orthogonal crystals in the piezoelectric thin film 3 become stable under normal pressure.

[0051] For example, the temperature of the precursor film during annealing (annealing temperature) may be 500°C or more and 1200°C or less, 500°C or more and 1000°C or less, or 800°C or more and 1000°C or less. By having the temperature of the precursor film within the above range, a part or all of the monoclinic crystals in the precursor film are likely to change to one or both of the orthorhombic crystals and the tetragonal crystals, and a crystal phase having a "changing crystal orientation" is likely to be formed. For example, the time during which the annealing temperature is maintained (annealing time) may be 5 seconds or more and 1200 seconds or less. The annealing of the precursor film may be carried out in an inert gas or an oxidizing gas. For example, the inert gas may be nitrogen gas or a rare gas. For example, the oxidizing gas may be air or oxygen.

[0052] Following the annealing, the precursor film may be cooled at a predetermined cooling rate. The piezoelectric thin film 3 is completed by cooling the precursor film following the annealing. In the process of cooling the precursor film following the annealing, the orientation direction of some lattice planes contained in the orthogonal crystal changes, and the orthogonal crystal in the piezoelectric thin film 3 is stabilized under normal pressure. For example, the cooling rate may be 45°C / min or more and 200°C / min or less. By having the cooling rate within the above range, the orientation direction of some lattice planes contained in the orthogonal crystal is likely to change. If the cooling rate is too small, the orthogonal crystal in the precursor film is likely to change to a monoclinic crystal. If the cooling rate is too large, stress caused by the sudden contraction of the precursor film is likely to act on the precursor film, and cracks are likely to form in the piezoelectric thin film 3 obtained after cooling. The precursor film may be cooled in an inert gas or an oxidizing gas. For example, the inert gas may be nitrogen gas or a rare gas. For example, the oxidizing gas may be air or oxygen.

[0053] The mechanisms of crystal phase generation, phase transition, and crystal orientation during the manufacturing process of the piezoelectric thin film are not limited to the above mechanisms.

[0054] <Crystalline Substrate, First Electrode Layer, and Second Electrode Layer> The crystalline substrate 1 may be single crystal or polycrystalline. For example, the crystalline substrate 1 may have one crystal structure selected from the group consisting of a fluorite structure, a bixbite structure, a pyrochlore structure, a diamond structure, a zincblende structure, a face-centered cubic structure, a hexagonal structure, a perovskite structure, a NaCl structure, a rutile structure, a spinel structure, and a corundum structure.

[0055] For example, the crystalline substrate 1 having a fluorite structure may be yttria-stabilized zirconia (YSZ), cerium oxide, or calcium fluoride.

[0056] For example, the crystalline substrate 1 having a bixbyite structure may be indium oxide, indium tin oxide, scandium oxide, yttrium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

[0057] For example, the crystalline substrate 1 having a pyrochlore structure is made of bismuth ruthenium oxide (Bi 2 Ru 2 O 7 ), rare earth ruthenium oxide (R 2 Ru 2 O 7 , R is a rare earth element. ), Bismuth iridium oxide (Bi 2 Ir 2 O 7 ), or rare earth iridium oxide (R 2 Ir 2 O 7 , R is a rare earth element.

[0058] For example, the crystalline substrate 1 having a diamond structure may be a semiconductor such as silicon or germanium. When the crystalline substrate 1 is silicon or germanium, an adhesion layer made of titanium, chromium, or the like may be formed on the surface of the crystalline substrate 1 in order to improve the adhesion of the first electrode layer 2.

[0059] For example, the crystalline substrate 1 having a zinc blende structure may be a compound semiconductor such as gallium arsenide, aluminum arsenide, gallium phosphide, aluminum phosphide, beta zinc sulfide, or zinc selenide.

[0060] For example, the crystalline substrate 1 having a face-centered cubic structure may be platinum, iridium, or gold.

[0061] For example, the crystalline substrate 1 having a hexagonal crystal structure may be titanium, zirconium, hafnium, zinc, zinc oxide, or boron nitride.

[0062] For example, the crystalline substrate 1 having a perovskite structure is made of strontium ruthenium oxide (SrRuO 7 ), calcium ruthenium oxide (CaRuO 3 ), strontium iridium oxide (SrIrO 3 ), lanthanum nickel oxide (LaNiO 3 ), lanthanum alkaline earth manganese oxide (La 1-x Ae x MnO 3 ), or lanthanum alkaline earth cobalt oxide (La 1-x Ae x Chief of Staff 3 Ae is at least one element selected from the group consisting of Ca, Sr and Ba.

[0063] For example, the crystalline substrate 1 having a NaCl structure may be magnesium oxide or titanium nitride.

[0064] For example, the crystalline substrate 1 having a rutile structure may be titanium oxide, iridium oxide, or ruthenium oxide.

[0065] For example, the crystalline substrate 1 having a spinel structure is made of magnesium aluminum spinel oxide (Mg 2 AlO 4 ) or triiron tetroxide.

[0066] For example, the crystalline substrate 1 having a corundum structure may be aluminum oxide or iron trioxide.

[0067] The thickness of the crystalline substrate 1 may be substantially uniform. For example, the thickness of the crystalline substrate 1 may be 10 μm or more and 1000 μm or less. When the crystalline substrate 1 is conductive, the crystalline substrate 1 functions as an electrode, and the first electrode layer 2 may not be required. In other words, when the crystalline substrate 1 is conductive, the piezoelectric thin film 3 may be directly overlapped with the crystalline substrate 1.

[0068] For example, the crystalline substrate 1 may be a uniaxially oriented substrate. For example, among the multiple lattice planes contained in the crystalline substrate 1, a lattice plane that is substantially parallel to the surface (main surface) of the crystalline substrate 1 may be one lattice plane selected from the group consisting of a (111) plane, a (100) plane, a (001) plane, a (110) plane, and a (101) plane.

[0069] Because the crystalline phase contained in the piezoelectric thin film 3 is likely to have “changing crystal orientation,” the first lattice plane LP1 oriented in the normal direction dn1 of the first main surface s1 of the piezoelectric thin film 3 may be a (111) plane, and the first lattice plane LP1 oriented in the normal direction dn1 of the first main surface s1 may be substantially parallel to the surface (main surface) of the crystalline substrate 1, and the (111) plane in the crystalline substrate 1 may be substantially parallel to the surface (main surface) of the crystalline substrate 1.

[0070] For example, the first electrode layer 2 may be made of at least one metal selected from the group consisting of Pt (platinum), Pd (palladium), Rh (rhodium), Au (gold), Ru (ruthenium), Ir (iridium), Mo (molybdenum), Ti (titanium), Ta (tantalum), and Ni (nickel). The first electrode layer 2 may contain a conductive composition among the compositions listed above as specific examples of the crystalline substrate 1.

[0071] Because the crystal phase contained in the piezoelectric thin film 3 is likely to have "variable crystal orientation", the first main surface s1 of the piezoelectric thin film 3 may directly overlap the first electrode layer 2, and the first electrode layer 2 may contain platinum or indium tin oxide (ITO). The first electrode layer 2 may be made of only platinum or indium tin oxide.

[0072] The first electrode layer 2 may be crystalline. The first electrode layer 2 may be single crystal or polycrystalline. For example, among the multiple lattice planes included in the first electrode layer 2, a lattice plane that is substantially parallel to the main surface of the first electrode layer 2 may be one lattice plane selected from the group consisting of a (111) plane, a (100) plane, a (001) plane, a (110) plane, and a (101) plane.

[0073] The first electrode layer 2 may have a substantially uniform thickness. For example, the first electrode layer 2 may have a thickness of 1 nm or more and 1.0 μm or less.

[0074] For example, the second electrode layer 4 may be made of at least one metal selected from the group consisting of Pt, Pd, Rh, Au, Ru, Ir, Mo, Ti, Ta, and Ni. The second electrode layer 4 may contain a conductive composition among the compositions listed above as specific examples of the crystalline substrate 1. The first electrode layer 2 may contain platinum or indium tin oxide. The second electrode layer 4 may be crystalline. The second electrode layer 4 may be single crystal or polycrystalline. The thickness of the second electrode layer 4 may be substantially uniform. For example, the thickness of the second electrode layer 4 may be 1 nm or more and 1.0 μm or less.

[0075] The piezoelectric thin film element 10 may further include at least one intermediate layer (buffer layer). For example, a substrate-side intermediate layer may be interposed between the crystalline substrate 1 and the first electrode layer 2. For example, a first intermediate layer may be interposed between the first electrode layer 2 and the piezoelectric thin film 3. For example, a second intermediate layer may be interposed between the piezoelectric thin film 3 and the second electrode layer 4. For example, each intermediate layer may include at least one of the compositions listed above as specific examples of the crystalline substrate 1. Each intermediate layer may be crystalline. Each intermediate layer may be single crystal or polycrystalline.

[0076] The first electrode layer 2, the second electrode layer 4, and each intermediate layer may be formed by a PLD method, a sputtering method, a vacuum deposition method, a printing method, a spin coating method, or a sol-gel method. Each layer may be annealed to improve the crystallinity of each layer.

[0077] At least a part or the entire surface of the piezoelectric thin film element 10 may be covered with a protective film. Covering with a protective film improves the durability (for example, moisture resistance) of the piezoelectric thin film element 10.

[0078] <Examples of piezoelectric thin film elements> The piezoelectric thin film element according to the present embodiment has a wide variety of applications. For example, the piezoelectric thin film element may be a part or the whole of one device selected from the group consisting of a piezoelectric actuator, a piezoelectric sensor, a piezoelectric transducer, a piezoelectric microphone, a harvester, an oscillator, a resonator, an acoustic multilayer film, a pyroelectric element, and a filter. For example, the piezoelectric actuator may be used for haptics. That is, the piezoelectric actuator may be used in various devices that require feedback by skin sensation (tactile sensation). For example, the device that requires feedback by skin sensation may be a wearable device, a touch pad, a display, or a game controller. For example, the piezoelectric actuator may be used in a head assembly, a head stack assembly, or a hard disk drive. For example, the piezoelectric actuator may be used in a printer head or an inkjet printer device. For example, the piezoelectric actuator may be used in a piezoelectric switch. For example, the piezoelectric sensor or the piezoelectric transducer may be used in a gyro sensor, a pressure sensor, a pulse wave sensor, an ultrasonic sensor, an ultrasonic transducer, an infrared sensor, or a shock sensor. The ultrasonic transducer may be a Piezoelectric Micromachined Ultrasonic Transducer (PMUT). The product to which the piezoelectric micromachined ultrasonic transducer is applied may be a biometric sensor such as a fingerprint sensor and an ultrasonic blood vessel authentication sensor, a sensor for medical or health care, or a Time of Flight (ToF) sensor. For example, the filter may be a BAW (Bulk Acoustic Wave) filter or a SAW (Surface Acoustic Wave) filter. Each of the piezoelectric thin film elements described above may be a part or the whole of a Micro Electro Mechanical System (MEMS). Each of the piezoelectric thin film elements described above may be a wearable device or a portable device.

[0079] Although the preferred embodiment of the present invention has been described above, the present invention is not necessarily limited to the above-mentioned embodiment. Various modifications of the present invention are possible without departing from the spirit of the present invention, and these modifications are also included in the present invention. EXAMPLES

[0080] The present invention is not limited to the following examples.

[0081] Example 1 A single crystal of yttria-stabilized zirconia (YSZ) was used as the crystalline substrate. The (111) plane of the single crystal was parallel to the surface (main surface) of the crystalline substrate. The crystalline substrate was rectangular, with dimensions of 5 mm length x 5 mm width. The thickness of the crystalline substrate was 500 μm. The thickness of the crystalline substrate was uniform.

[0082] A first electrode layer (lower electrode layer) made of indium tin oxide (ITO) was formed directly on the entire surface of the crystalline substrate by magnetron sputtering in a vacuum chamber. ITO itself was used as the raw material (target) for the first electrode layer. The input power to the target was 40 W. The atmosphere in the vacuum chamber was Ar and O. 2 The gas was a mixture of Ar and O 2 The ratio of the volumes (unit: sccm) is Ar:O 2 =20:1. The temperature of the crystalline substrate during the formation of the first electrode layer was maintained at 700° C. The thickness of the first electrode layer was uniform. The thickness of the first electrode layer was adjusted to 0.05 μm.

[0083] A precursor film was formed directly on the entire main surface of the first electrode layer by PLD in a vacuum chamber. The precursor film source (target) was HfO containing Ce as an additive element X. 2 The [X] / ([Hf]+[X]) ratio in the precursor film raw material was 0.20. <x>means [X] / ([Hf]+[X]). The crystal structure of the precursor film was analyzed using an X-ray diffraction apparatus described later. The precursor film was a monoclinic crystal having a fluorite-type crystal structure. The input power to the target was 200 mJ. In the PLD method, a pulse of KrF laser light was irradiated onto the target. The pulse oscillation frequency was 3 Hz. The atmosphere in the vacuum chamber was O 2 The gas was a gas. The pressure in the vacuum chamber was 10 mTorr. In the process of forming the precursor film, the crystalline substrate and the lower electrode layer were not heated. That is, the precursor film was formed at room temperature or normal temperature. The thickness of the precursor film was adjusted to 500 nm. The thickness of the precursor film substantially coincided with the thickness of the piezoelectric thin film formed from the precursor film. The film formation time was 40 hours.

[0084] The precursor film was annealed in nitrogen gas. The heating rate during annealing was 30° C. / sec. The temperature of the precursor film during annealing (annealing temperature) was 1000° C. The annealing time was 10 seconds.

[0085] Following the annealing, the precursor film was cooled in nitrogen gas to form a piezoelectric thin film. The cooling was carried out for 30 minutes. The temperature of the precursor film was decreased from 1000°C to 25°C by cooling. The cooling rate was adjusted to 32.5°C / min. The thickness of the piezoelectric thin film corresponded to that of the precursor film.

[0086] A second electrode layer (upper electrode layer) made of Pt was formed on the entire surface (second main surface) of the piezoelectric thin film by electron beam deposition in a vacuum chamber. Pt was used as the raw material. The crystalline substrate was not heated during the process of forming the second electrode layer. The thickness of the second electrode layer was uniform. The thickness of the second electrode layer was adjusted to 0.17 μm.

[0087] The laminate produced by the above procedure was composed of a crystalline substrate, a first electrode layer formed directly on a main surface of the crystalline substrate, a piezoelectric thin film laminated directly on the main surface of the first electrode layer, and a second electrode layer laminated directly on a second main surface of the piezoelectric thin film.

[0088] The laminate structure on the crystalline substrate was patterned by photolithography. After patterning, the entire laminate was diced to obtain a plurality of piezoelectric thin film elements of Example 1. The piezoelectric thin film elements had the same surface roughness as the laminate. The piezoelectric thin film elements were rectangular in shape. The following analyses and measurements were performed during or after the fabrication of the piezoelectric thin film elements.

[0089] <Overall composition of piezoelectric thin film> The composition of the entire piezoelectric thin film was analyzed by X-ray fluorescence analysis (XRF method). For the XRF method, a wavelength dispersive X-ray fluorescence device (RIGAKU AZX-400) manufactured by Rigaku Corporation was used. As a result of the analysis, the composition of the piezoelectric thin film of Example 1 was consistent with the composition of the raw material of the precursor film. In other words, the piezoelectric thin film was Ce-doped HfO 2 The Ce content in the piezoelectric thin film was the same as the Ce content in the target for the precursor film.

[0090] <Crystal structure and lattice plane orientation> The X-ray diffraction (XRD) pattern of the piezoelectric thin film was measured by 2θ-θ measurement. A Philips X-ray diffractometer (X'Pert MRD) was used for the measurement. The piezoelectric thin film consisted of a crystalline phase with a fluorite-type crystal structure. The crystalline phase included orthorhombic and tetragonal crystals.

[0091] As described in the above embodiment, the piezoelectric thin film of Example 1 was analyzed based on the analysis method of "changing crystal orientation". That is, an image of the cross section of the piezoelectric thin film of Example 1 was taken by a transmission electron microscope (TEM). Titan G2 manufactured by Thermo Fisher Scientific Inc. (formerly FEI company) was used for the TEM. The cross section of the photographed piezoelectric thin film 3 was parallel to the thickness direction of the piezoelectric thin film. The image of the cross section of the piezoelectric thin film of Example 1 is shown in FIG. 6.

[0092] Three measurement points 1 to 3 were selected from the cross section shown in FIG. 6 along the thickness direction (Z-axis direction) of the piezoelectric thin film 3. Measurement point 1 was located near the first main surface s1 of the piezoelectric thin film 3, and the distance between measurement point 1 and the first main surface s1 was shorter than the distance between the other measurement points and the first main surface s1. The first main surface s1 is a surface that directly overlaps the main surface of the first electrode layer. Measurement point 2 was located near the second main surface s2 of the piezoelectric thin film 3, and the distance between measurement point 2 and the second main surface s2 was shorter than the distance between the other measurement points and the second main surface s2. The second main surface s2 is a surface that directly overlaps the main surface of the second electrode layer. Measurement point 3 was located in the center between measurement point 1 and measurement point 2. An FFT pattern for each measurement point was obtained by image processing of each measurement point.

[0093] The FFT pattern of the measurement point 2 located in the vicinity of the second principal surface s2 is shown in FIG. The FFT pattern of the measurement point 1 located in the vicinity of the first main surface s1 is shown in FIG. The spot (220) in each FFT pattern originates from the (220) plane (a lattice plane parallel to the (110) plane) of the crystal phase (fluorite crystal structure). The arrow pointing from the central spot to the spot (220) in each FFT pattern corresponds to the orientation direction of the (220) plane (and the (110) plane). The spot (111) in each FFT pattern is derived from the (111) plane of the crystal phase (fluorite crystal structure). The arrow pointing from the center spot to the spot (111) in each FFT pattern corresponds to the orientation direction of the (111) plane. The positions of the spot (220) in the FFT patterns of the measurement points 1 to 3 were different from each other. The positions of the spot (111) in the FFT patterns of the measurement points 1 to 3 were also different from each other. The FFT pattern at measurement point 1 located in the vicinity of the first principal surface s1 contained a spot derived from a tetragonal crystal. The FFT pattern at measurement point 2 located in the vicinity of the second principal surface s2 contained a spot derived from a rectangular crystal. The FFT pattern of measurement point 3, which is located in the middle between measurement point 1 and measurement point 2, contained a diffraction spot derived from a tetragonal crystal.

[0094] As a result of the analysis of the crystal structure and the orientation of the lattice plane, the crystal phase contained in the piezoelectric thin film of Example 1 had all of the following characteristics 1 to 6. In other words, the crystal phase contained in the piezoelectric thin film of Example 1 had "variable crystal orientation" (see FIG. 5). [Feature 1]: A part of the first lattice plane LP1 was oriented in the normal direction dn1 of the first principal surface s1 of the piezoelectric thin film 3. At a measurement point 1 located in the vicinity of the first principal surface s1, the first lattice plane LP1 (lattice plane LP11) was substantially parallel to the first principal surface s1. [Feature 2]: A part of the second lattice plane LP2 was oriented in the normal direction dn2 of the second principal surface s2 of the piezoelectric thin film 3. At the measurement point 2, the second lattice plane LP2 (lattice plane LP21) was substantially parallel to the second principal surface s2. [Feature 3]: The orientation direction of the first lattice plane LP1 changes (continuously or gradually) in the crystal phase (piezoelectric thin film 3). The angle θ1 between the first principal surface s1 and the first lattice plane LP1 increases (continuously or gradually) along the thickness direction (Z-axis direction) of the piezoelectric thin film 3. [Feature 4]: The orientation direction of the second lattice plane LP2 changes (continuously or gradually) in the crystal phase (piezoelectric thin film 3). The angle θ2 between the second principal surface s2 and the second lattice plane LP2 decreases (continuously or gradually) along the thickness direction (Z-axis direction) of the piezoelectric thin film 3. [Feature 5]: A first crystal c1 including a first lattice plane LP1 (lattice plane LP11) oriented in the normal direction dn1 of the first principal surface s1 was detected at a measurement point 1 located in the vicinity of the first principal surface s1. [Feature 6]: A second crystal c2 including a second lattice plane LP2 (lattice plane LP21) oriented in the normal direction dn2 of the second principal surface s2 was detected at measurement point 2 located in the vicinity of the second principal surface s2. The above features 5 and 6 mean that the first lattice plane LP1 oriented in the normal direction dn1 of the first main surface s1 is closer to the first main surface s1 than the second lattice plane LP2 oriented in the normal direction dn2 of the second main surface s2.

[0095] In the case of Example 1, the first lattice plane LP1 oriented in the normal direction dn1 of the first main surface s1 was the (111) plane. In the case of Example 1, the first crystal c1 detected at the measurement point 1 located in the vicinity of the first main surface s1 was a tetragonal crystal. In the case of Example 1, the second lattice plane LP2 oriented in the normal direction dn2 of the second main surface s2 was the (110) plane. In the case of Example 1, the second crystal c2 detected at the measurement point 2 located in the vicinity of the second main surface s2 was an orthorhombic crystal.

[0096] <Piezoelectric strain constant d 33,f > The piezoelectric strain constant d of the piezoelectric thin film of Example 1 33,f (unit: pm / V) was measured. d 33,f For the measurement of d, an apparatus combining a ferroelectric evaluation system and a laser Doppler vibrometer was used. The ferroelectric evaluation system was an FCE manufactured by Toyo Technica Co., Ltd., and the laser Doppler vibrometer was manufactured by Polytec GmbH. The frequency of the alternating voltage in the measurement was 10 kHz. During the measurement of d 33,f the maximum value of the voltage applied to the piezoelectric thin film was 150 V. d of Example 1 33,f is shown in Table 1 below.

[0097] (Examples 2 to 7, Comparative Examples 1 and 2) In the raw materials (targets) of the precursor films of Examples 2 to 7 and Comparative Example 1 respectively <x>were adjusted to the values ​​shown in Table 1 below. <x>The definition is as described above.

[0098] The thickness of each of the piezoelectric thin films in Examples 6 and 7 was adjusted to the value shown in the following Table 1. The raw material (target) of the precursor film in each of Examples 6 and 7 contained an element shown in the following Table 1 as the additive element X in place of Ce.

[0099] In the case of Comparative Example 1, annealing and cooling of the precursor film was not performed.

[0100] In the step of forming the precursor film in Comparative Example 2, the pulsed oscillation frequency of the KrF laser light with which the target was irradiated was 5 Hz.

[0101] Except for the above points, the piezoelectric thin films and piezoelectric thin film elements of Examples 2 to 7, and the thin films and thin film elements of Comparative Examples 1 and 2 were produced in the same manner as in Example 1. Analysis and measurement were carried out in the same manner as in Example 1 for Examples 2 to 7 and Comparative Examples 1 and 2.

[0102] The crystalline phases contained in the piezoelectric thin films of Examples 2 and 3 had all of the above characteristics 1 to 6, similar to Example 1. In other words, the crystalline phases contained in the piezoelectric thin films of Examples 2 and 3 had "variable crystal orientation."

[0103] In the cases of Examples 4 to 7, the first crystals c1 detected at the measurement point 1 located near the first principal surface s1 were orthorhombic rather than tetragonal. In the cases of Examples 4 to 7, the second crystals c2 detected at the measurement point 2 located near the second principal surface s2 were also orthorhombic. In other words, the entire crystal phase contained in the piezoelectric thin film in each of Examples 4 to 7 was orthorhombic. In the case of Example 6, the second lattice plane LP2 oriented in the normal direction dn2 of the second main surface s2 was not the (110) plane but the (100) plane. Except for the above-mentioned points, like Example 1, the crystal phases contained in the piezoelectric thin films of Examples 4 to 7 had all of the above-mentioned features 1 to 6. In other words, the crystal phases contained in the piezoelectric thin films of Examples 4 to 7 had "variable crystal orientation."

[0104] d of each of Examples 2 to 7 33,f is shown in Table 1 below.

[0105] In the case of Comparative Example 1, both the precursor film and the thin film were monoclinic. In the thin film of Comparative Example 1, the direction in which the lattice planes were oriented did not change. In other words, the thin film of Comparative Example 1 did not have "variable crystal orientation". The thin film of Comparative Example 1 did not have piezoelectric properties.

[0106] The precursor film of Comparative Example 2 was amorphous. The thin film of Comparative Example 2 was monoclinic. In the thin film of Comparative Example 2, the direction in which the lattice planes were oriented did not change. In other words, the thin film of Comparative Example 2 did not have "variable crystal orientation." The thin film of Comparative Example 2 did not have piezoelectric properties.

[0107] The "first lattice plane LP1" in Table 1 below is the first lattice plane LP1 oriented in the normal direction dn1 of the first main surface s1, and means the first lattice plane LP1 detected at the measurement point 1. "First crystal c1" in Table 1 below refers to the crystal detected at measurement point 1. The "second lattice plane LP2" in Table 1 below is the second lattice plane LP2 oriented in the normal direction dn2 of the second principal surface s2, and means the second lattice plane LP2 detected at the measurement point 2. "Second crystal c2" in Table 1 below refers to the crystal detected at measurement point 2.

[0108] [Table 1] [Industrial Applicability]

[0109] For example, the piezoelectric thin film according to the present disclosure may be used in a piezoelectric thin film element, a pyroelectric thin film element, or a MEMS. [Explanation of symbols]

[0110] 1...crystalline substrate, 2...first electrode layer, 3...piezoelectric thin film, 3cs...cross section of crystalline phase, 4...second electrode layer, 10...piezoelectric thin film element, c1...first crystal, c2...second crystal, s1...first main surface of piezoelectric thin film, s2...second main surface of piezoelectric thin film, dn1...normal direction of the first main surface of piezoelectric thin film, dn2...normal direction of the second main surface of piezoelectric thin film, ucо...rectangular unit cell, uct...tetragonal unit cell, θ1...angle between the first main surface and the first lattice plane, θ2...angle between the second main surface and the second lattice plane.< / x> < / x> < / x> < / x>

Claims

1. A piezoelectric thin film including a metal oxide, the piezoelectric thin film has a first main surface and a second main surface located on a back side of the first main surface, the metal oxide comprises hafnium; The metal oxide includes a crystalline phase having a fluorite crystal structure, The crystalline phase includes at least one type of crystals selected from the group consisting of orthorhombic and tetragonal crystals, the first lattice plane of the crystalline phase is a lattice plane selected from the group consisting of a (111) plane, a (100) plane, and a (110) plane; a second lattice plane of the crystal phase is one lattice plane selected from the group consisting of a (111) plane, a (100) plane, and a (110) plane, and the second lattice plane is a lattice plane different from the first lattice plane; a portion of the first lattice plane is oriented in a normal direction of the first main surface, a portion of the second lattice plane is oriented in a normal direction of the second main surface, the orientation of the first lattice plane changes within the crystalline phase; The orientation of the second lattice plane changes within the crystalline phase. Piezoelectric thin film.

2. The first lattice plane oriented in the normal direction of the first main surface is closer to the first main surface than the second lattice plane oriented in the normal direction of the second main surface. The piezoelectric thin film according to claim 1 .

3. The metal oxide further contains at least one additional element X selected from the group consisting of zirconium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium; The piezoelectric thin film according to claim 1 .

4. The content of the hafnium in the metal oxide is expressed as [Hf] atomic %, The content of the additive element X in the metal oxide is expressed as [X] atomic %, [X] / ([Hf]+[X]) is 0.05 or more and 0.70 or less; The piezoelectric thin film according to claim 3 .

5. a portion of the crystalline phase is a tetragonal crystal including the first lattice plane oriented in the normal direction of the first main surface, Another part of the crystalline phase is a rectangular crystal including the second lattice plane oriented in the normal direction of the second main surface. The piezoelectric thin film according to claim 1 .

6. a part of the crystalline phase is a rectangular crystal including the first lattice plane oriented in the normal direction of the first main surface, Another part of the crystalline phase is another orthogonal crystal including the second lattice plane oriented in the normal direction of the second main surface. The piezoelectric thin film according to claim 1 .

7. A method for producing the piezoelectric thin film according to any one of claims 1 to 6, comprising the steps of: forming a precursor film containing monoclinic crystals of the hafnium-containing oxide; forming the piezoelectric thin film from the precursor film by heating the precursor film; Equipped with A method for manufacturing a piezoelectric thin film.

8. The piezoelectric thin film according to any one of claims 1 to 6, A first electrode layer; A second electrode layer; Equipped with the first principal surface of the piezoelectric thin film directly or indirectly overlaps the first electrode layer; The second electrode layer directly or indirectly overlaps the second main surface of the piezoelectric thin film. Piezoelectric thin film element.

9. the first major surface of the piezoelectric thin film directly overlies the first electrode layer; The first electrode layer comprises platinum or indium tin oxide; 9. The piezoelectric thin film element according to claim 8.

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