Piezoelectric thin film and piezoelectric thin film element

Abstract

This piezoelectric thin film includes aluminum nitride having a wurtzite structure. The aluminum nitride includes an additive element. The (0001) plane of the wurtzite structure faces in a direction normal to a first main surface of the piezoelectric thin film. This piezoelectric thin film includes a plurality of crystal grains that include aluminum nitride. At least some of the plurality of crystal grains are a plurality of columnar crystals extending along a direction (Z-axis direction) normal to the first main surface of the piezoelectric thin film. The median size of the plurality of crystal grains in a direction parallel to the first main surface of the piezoelectric thin film is 90-500 nm inclusive.

Description

Piezoelectric thin film and piezoelectric thin film element

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

[0002] In recent years, MEMS (Micro Electro Mechanical Systems) have been attracting attention. MEMS (Micro Electro Mechanical Systems) are devices in which mechanical components, sensors, actuators, electronic circuits, etc. are integrated on a single substrate using microfabrication technology. For example, piezoelectric thin films used in MEMS include aluminum nitride (AlN), zinc oxide (ZnO), and lithium niobate (LiNbO ３ ), and lead zirconate titanate (PZT), etc. are known.

[0003] For example, the indexes that indicate the performance of a piezoelectric thin film are the piezoelectric constant d (piezoelectric strain constant) and the piezoelectric constant g (voltage output constant). The piezoelectric constant d is an index of the amount of strain (transmission ability) per unit electric field. The larger the piezoelectric constant d, the higher the performance of the piezoelectric body as an actuator (e.g., an inkjet recording head). On the other hand, the piezoelectric constant g is an index of the generated electric field strength (reception ability) per unit stress. The larger the piezoelectric constant g, the higher the performance of the piezoelectric body as a sensor. Among the above piezoelectric thin films, AlN has attracted attention because, although its piezoelectric constant d is small, it has a high piezoelectric constant g and is inexpensive. (See Patent Document 1 below.)

[0004] Patent No. 7115257

[0005] It is known that the piezoelectric properties of AlN are improved by the addition of additive elements. However, as the content of additive elements in AlN increases, it becomes more difficult to maintain the crystal structure (wurtzite structure) of AlN. As a result, the piezoelectric properties of AlN deteriorate. Furthermore, as the content of additive elements in AlN increases, the additive elements tend to segregate at the grain boundaries between multiple crystal grains in AlN. Due to the segregation of additive elements at the grain boundaries, the piezoelectric properties of AlN deteriorate and the resistivity of AlN decreases. As the temperature of AlN containing additive elements segregated at the grain boundaries reaches a high temperature (e.g., 125°C), thermal diffusion of the additive elements in AlN tends to occur. Due to the thermal diffusion of the additive elements, the dielectric loss (dielectric loss tangent) of AlN tends to increase.

[0006] An object of one aspect of the present disclosure is to provide a piezoelectric thin film that has excellent piezoelectric characteristics and suppresses an increase in dielectric loss that accompanies a temperature rise in the piezoelectric thin film, and a piezoelectric thin film element that includes the piezoelectric thin film.

[0007] For example, the present disclosure relates to a piezoelectric thin film according to any one of the following items [1] to [8], and a piezoelectric thin film element according to the following item [9].

[0008] [1] A piezoelectric thin film comprising aluminum nitride having a wurtzite structure, wherein the aluminum nitride comprises an additive element, a (0001) plane of the wurtzite structure is oriented in a normal direction to a main surface of the piezoelectric thin film, the piezoelectric thin film comprises a plurality of crystal grains comprising the aluminum nitride, at least some of the plurality of crystal grains are a plurality of columnar crystals extending along the normal direction to the main surface of the piezoelectric thin film, and a median diameter of the plurality of crystal grains in a direction parallel to the main surface of the piezoelectric thin film is 90 nm or more and 500 nm or less.

[0009] [2] The piezoelectric thin film according to [1], wherein the additive element includes at least a divalent element and a tetravalent element.

[0010] [3] The piezoelectric thin film according to [2], wherein at least a portion of the divalent elements is magnesium, and at least a portion of the tetravalent elements is at least one of zirconium and hafnium.

[0011] [4] The piezoelectric thin film according to any one of [1] to [3], wherein the additive element includes at least a trivalent element.

[0012] [5] The piezoelectric thin film according to [4], wherein at least a part of the trivalent element is scandium.

[0013] [6] The piezoelectric thin film further includes at least one grain boundary located between the plurality of crystal grains, and the additive element is only a divalent element and a tetravalent element, or only a trivalent element, or the divalent element, the trivalent element, and the tetravalent element, and the ratio of the number of the divalent elements in the crystal grains to the total number of aluminum, the divalent element, the trivalent element, and the tetravalent element in the crystal grains is expressed as [Ed] g atomic %, and the ratio of the number of the trivalent elements in the crystal grains to the total number of aluminum, the divalent element, the trivalent element, and the tetravalent element in the crystal grains is expressed as [Etr] g atomic %, and the ratio of the number of the tetravalent elements in the crystal grains to the total number of aluminum, the divalent element, the trivalent element, and the tetravalent element in the crystal grains is expressed as [Et] g atomic %, The piezoelectric thin film according to any one of [1] to [5], wherein a ratio of the number of divalent elements in the grain boundary to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the grain boundary is expressed as [Ed]b atomic %, a ratio of the number of trivalent elements in the grain boundary to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the grain boundary is expressed as [Etr]b atomic %, a ratio of the number of tetravalent elements in the grain boundary to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the grain boundary is expressed as [Et]b atomic %, [Ed]g + [Etr]g + [Et]g is expressed as [Eadd]g, [Ed]b + [Etr]b + [Et]b is expressed as [Eadd]b, and [Eadd]b / [Eadd]g is 0.70 or more and 1.60 or less.

[0014] [7] The piezoelectric thin film further includes at least one grain boundary located between the plurality of crystal grains, and the additional element is only a divalent element and a tetravalent element, or only a trivalent element, or the divalent element, the trivalent element, and the tetravalent element, and the ratio of the number of the divalent elements in the crystal grains to the total number of aluminum, the divalent element, the trivalent element, and the tetravalent element in the crystal grains is expressed as [Ed]g atomic %, and the ratio of the number of the trivalent elements in the crystal grains to the total number of aluminum, the divalent element, the trivalent element, and the tetravalent element in the crystal grains is expressed as [Etr]g atomic %, and the ratio of the number of the tetravalent elements in the crystal grains to the total number of aluminum, the divalent element, the trivalent element, and the tetravalent element in the crystal grains is expressed as [Et]g atomic %, and [Ed]g + [Etr]g + [Et]g is expressed as [Eadd]g, The piezoelectric thin film according to any one of [1] to [6], wherein [Eadd]g is 4.5 atomic % or more and 65.0 atomic % or less.

[0015] [8] The piezoelectric thin film according to [7], wherein the [Eadd]g is 30.0 atomic % or more and 60.0 atomic % or less.

[0016] [9] A piezoelectric thin film element comprising: the piezoelectric thin film according to any one of [1] to [8]; and an electrode layer, wherein the piezoelectric thin film directly or indirectly overlaps a surface of the electrode layer.

[0017] According to one aspect of the present disclosure, there are provided a piezoelectric thin film that has excellent piezoelectric characteristics and suppresses an increase in dielectric loss that accompanies a temperature rise in the piezoelectric thin film, and a piezoelectric thin film element that includes the piezoelectric thin film.

[0018] FIG. 1 is a schematic exploded perspective view of a piezoelectric thin film element according to one embodiment of the present invention. FIG. 2 shows one embodiment of a schematic cross section of the piezoelectric thin film element shown in FIG. 1, where the cross section shown in FIG. 2 is approximately or completely perpendicular to the first main surface of the piezoelectric thin film and approximately or completely parallel to the thickness direction of the piezoelectric thin film. FIG. 3 is a schematic view of a portion of the first or second main surface of the piezoelectric thin film shown in FIG. 1. FIG. 4(a) is a schematic perspective view of multiple crystal grains (columnar crystals) contained in the piezoelectric thin film, and FIG. 4(b) is a schematic view of the particle size distribution of multiple crystal grains contained in the piezoelectric thin film. FIG. 5 is a perspective view of a unit cell of the crystal structure (wurtzite structure) of aluminum nitride contained in the piezoelectric thin film. FIG. 6(a) is an image of the main surface (second main surface) of a piezoelectric thin film according to one embodiment of the present invention (Example 11), and FIG. 6(b) is an image of the second main surface (front surface) of a piezoelectric thin film according to Comparative Example 1.

[0019] Preferred embodiments of the present invention will be described below with reference to the drawings. In the drawings, equivalent components are designated by equivalent reference numerals. The present invention is not limited to the following embodiments. X, Y, and Z shown in FIGS. 1 to 3, 6(a), and 6(b) refer to three mutually orthogonal coordinate axes. The directions of the X, Y, and Z axes are common to FIGS. 1 to 3, 6(a), and 6(b). The directions of the X and Z axes shown in FIG. 4(a) correspond to the directions of the X and Z axes in FIGS. 1 to 3, 6(a), and 6(b). The direction of the Z axis shown in FIG. 5 corresponds to the direction of the Z axis in FIGS. 1 to 3, 4(a), 6(a), and 6(b).

[0020] As shown in FIG. 1 , the piezoelectric thin film element 10 according to this embodiment includes at least a first electrode layer 1 (e.g., a lower electrode layer) and a piezoelectric thin film 3. The piezoelectric thin film element 10 may further include a second electrode layer 2 (e.g., an upper electrode layer). The piezoelectric thin film 3 has a first main surface s31 and a second main surface s32 located on the back side of the first main surface s31. A "main surface" refers to the surface with the largest area among multiple surfaces of a polyhedron (e.g., the piezoelectric thin film 3, which is a thin rectangular parallelepiped). The area of ​​the first main surface s31 may be approximately or completely equal to the area of ​​the second main surface s32. The shape of the first main surface s31 may be approximately or completely equal to the shape of the second main surface s32. The first main surface s31 of the piezoelectric thin film 3 may directly or indirectly overlap the surface s1 of the first electrode layer 1. The surface s2 of the second electrode layer 2 may directly or indirectly overlap the second main surface s32 of the piezoelectric thin film 3. 1 are substantially or completely parallel to the first main surface s31 of the piezoelectric thin film 3 and the surface s1 of the first electrode layer 1. The Z axis in FIG. 1 is substantially or completely perpendicular to the first main surface s31 of the piezoelectric thin film 3 and the surface s1 of the first electrode layer 1.

[0021] The piezoelectric thin film 3 includes aluminum nitride having a wurtzite structure. The aluminum nitride includes at least one additive element Eadd that is different from aluminum (Al) and nitrogen (N). For example, the additive element Eadd may include at least a divalent element Ed and a tetravalent element Et. For example, the additive element Eadd may include at least a trivalent element Etr. For example, the additive element Eadd may include only the divalent element Ed and the tetravalent element Et, or only the trivalent element Etr, or the divalent element Ed, the trivalent element Etr, and the tetravalent element Et. Doping aluminum nitride with the additive element Eadd distorts the wurtzite structure of the aluminum nitride or changes the strength of the chemical bonds between atoms in the wurtzite structure. As a result, the wurtzite structure of aluminum nitride is easily deformed when a voltage or external force is applied to the piezoelectric thin film 3, which improves the piezoelectric properties of the piezoelectric thin film 3. The piezoelectric thin film 3 may be made of only AlN containing the additional element Eadd. As will be described later, the piezoelectric thin film 3 may further contain other elements in addition to Al, N, and the additional element Eadd.

[0022] FIG. 5 shows the wurtzite structure of aluminum nitride contained in the piezoelectric thin film 3. The unit cell uc of the aluminum nitride (wurtzite structure) is a hexagonal prism. A portion of the aluminum in the unit cell uc may be substituted with an additional element Eadd. For example, a portion of the Al in the unit cell uc may be substituted with a divalent element Ed, a trivalent element Etr, or a tetravalent element Et. At least a portion or all of the (0001) plane of the wurtzite structure (aluminum nitride) in the piezoelectric thin film 3 is oriented in the normal direction (Z-axis direction) to the first principal surface s31 (surface s1 of the first electrode layer 1) of the piezoelectric thin film 3. In other words, at least a portion or all of the (0001) plane of the wurtzite structure in the piezoelectric thin film 3 may be approximately or completely parallel to the first principal surface s31 (surface s1 of the first electrode layer 1) of the piezoelectric thin film 3. Aluminum nitride is polarized in a crystal orientation perpendicular to the (0001) plane (i.e.,

[0001] ). Therefore, the piezoelectric characteristics of the piezoelectric thin film 3 are likely to be improved by orienting at least a part of the (0001) plane of aluminum nitride in the piezoelectric thin film 3 in the normal direction to the first main surface s31 of the piezoelectric thin film 3. During the manufacturing process of the piezoelectric thin film 3, the (0001) plane of aluminum nitride is likely to be oriented parallel to the first main surface s31 of the piezoelectric thin film 3 (the surface s1 of the first electrode layer 1).

[0023] As shown in Figures 2 and 3, the piezoelectric thin film 3 includes a plurality of crystal grains g1 containing aluminum nitride. Each crystal grain g1 may be single crystal, polycrystalline, or imperfectly crystalline. Each crystal grain g1 may consist solely of aluminum nitride containing the additional element Eadd. Each crystal grain g1 may further include other components in addition to aluminum nitride containing the additional element Eadd. The piezoelectric thin film 3 may further include one grain boundary g2 (grain boundary phase) or multiple grain boundaries g2 located between the plurality of crystal grains g1.

[0024] The median diameter (D ５０ ) is 90 nm or more and 500 nm or less.

[0025] The method for measuring the median diameter of the plurality of columnar crystals is as follows.

[0026] The median diameter may be calculated from the grain size d of each of the multiple crystal grains g1 in a direction parallel to the main surface (first main surface s31 or second main surface s32) of the piezoelectric thin film 3. The grain size d of each crystal grain g1 may be calculated from the area of ​​the surface 3s of each crystal grain g1 exposed to the main surface of the piezoelectric thin film 3. The surface 3s of each crystal grain g1 exposed to the main surface of the piezoelectric thin film 3 is shown in FIG. 3. The area of ​​the surface 3s of each crystal grain g1 exposed to the main surface of the piezoelectric thin film 3 is represented as A. The grain size d (diameter) of each crystal grain g1 is expressed as (4A / π) １／２ It is expressed as: (4A / π) １／２ corresponds to the diameter (equivalent circle diameter) of a circle having an area A. In other words, the grain size d of each crystal grain g1 is the Heywood diameter calculated from the area A of the surface 3s of each crystal grain g1. To measure the area A of the surface 3s of each crystal grain g1, an image of the main surface of the piezoelectric thin film 3 is captured using a scanning electron microscope (SEM). An example of the image of the main surface of the piezoelectric thin film 3 is shown in (a) of FIG. 6. Next, the image of the main surface of the piezoelectric thin film 3 is binarized. For example, white areas in the binarized image of the main surface of the piezoelectric thin film 3 correspond to the surfaces 3s of the crystal grains g1 exposed on the main surface of the piezoelectric thin film 3. For example, black areas in the binarized image of the main surface of the piezoelectric thin film 3 correspond to the grain boundaries g2. The area of ​​one closed region (white region) surrounded by the grain boundaries g2 is measured as the area A of the surface 3s of one crystal grain g1. Crystal grains g1 that are not clearly defined by the grain boundaries g2 are excluded from the measurement of the area A. The binarization of the image of the main surface of the piezoelectric thin film 3 may be performed manually or by image analysis software. The measurement of the area A of the surface 3s of the crystal grain g1 may be performed by image analysis software. For example, the number n (number of samples) of the multiple crystal grains g1 for which the area A is measured may be 300 or more and 500 or less. For example, image analysis software (not for sale) manufactured by TDK Corporation may be used as the image analysis software. For example, Mac-View manufactured by Mountec Co., Ltd. may be used as the image analysis software. The grain size d of each of the multiple crystal grains g1 in a direction parallel to the main surface of the piezoelectric thin film 3 may be determined not on the main surface of the piezoelectric thin film 3 but on a cross section parallel to the main surface of the piezoelectric thin film 3.

[0027] The particle size distribution of the crystal grains g1 can be obtained from the particle size d of the n crystal grains g1 calculated by the above method. An example of the particle size distribution of the crystal grains g1 is shown in FIG. 4(b). The particle size distribution G of the crystal grains g1 is a number distribution (particle size distribution based on the number). That is, the horizontal axis of the particle size distribution G is the particle size d of the crystal grains g1, and the vertical axis of the particle size distribution G is the number N of crystal grains g1 having the particle size d. Based on this particle size distribution G, the median diameter D of the n crystal grains g1 can be calculated. ５０ The particle size distribution G shown in (b) of FIG. 4 is a frequency distribution, but the particle size distribution may be an integral distribution.

[0028] As shown in FIG. 2 , some or all of the multiple crystal grains g1 are multiple columnar crystals extending along the normal direction (Z-axis direction) of the first main surface s31 of the piezoelectric thin film 3. For example, the multiple columnar crystals may extend substantially or completely perpendicular to the first main surface s31 of the piezoelectric thin film 3. In (a) of FIG. 4 , each columnar crystal grain g1 is approximated by a cylinder whose thickness is equal to the grain size d (Heywood diameter). In other words, each columnar crystal grain g1 is approximated by a cylinder whose end face (circle) has an area A. The length (height) of the columnar crystal in the direction perpendicular to the first main surface s31 of the piezoelectric thin film 3 is represented as H. A columnar crystal is defined as a crystal grain g1 extending along the normal direction of the first main surface s31 of the piezoelectric thin film 3 and having an aspect ratio d / H greater than 0 and less than 1. For example, the length H of the columnar crystal may be equal to the thickness T of the piezoelectric thin film 3. The thickness T of the piezoelectric thin film 3 may be rephrased as the distance between the first main surface s31 and the second main surface s32 of the piezoelectric thin film 3. Since the (0001) plane of the wurtzite structure in the columnar crystals is likely to be oriented in the normal direction of the first main surface s31 of the piezoelectric thin film 3, the piezoelectric thin film 3 including the columnar crystals is likely to have excellent piezoelectric properties. ５０ However, D' may be 90 nm or more and 500 nm or less. ５０ D' is determined solely on the basis of a plurality of columnar crystals. ５０ The definition and specification method is D ５０ That is, the definition and specification method of the median diameter D' of the plurality of columnar crystals may be the same as that of the ５０In the method of specifying (2), the grain size d of the crystal grain g1 having an aspect ratio d / H of 1 or more is excluded from the grain size distribution G.

[0029] The median diameter (D ５０ When the thickness D' of the piezoelectric thin film 3 is 90 nm or more and 500 nm or less, the piezoelectric characteristics of the piezoelectric thin film 3 are improved, an increase in dielectric loss due to a rise in temperature of the piezoelectric thin film 3 is suppressed, and the resistivity of the piezoelectric thin film 3 is increased. ５０ The same applies when the thickness is 90 nm or more and 500 nm or less. The mechanism by which the piezoelectric thin film 3 is improved, the increase in dielectric loss due to a rise in temperature of the piezoelectric thin film 3 is suppressed, and the resistivity of the piezoelectric thin film 3 increases is as follows. Note that the increase in dielectric loss due to a rise in temperature of the piezoelectric thin film 3 means that the dielectric loss of the piezoelectric thin film 3 at the point when the temperature of the piezoelectric thin film 3 has decreased to room temperature after the temperature rise of the piezoelectric thin film 3 is greater than the dielectric loss of the piezoelectric thin film 3 before the temperature rise.

[0030] As the content of the additive element Eadd in AlN increases, it becomes difficult to maintain the wurtzite structure of AlN. Therefore, the piezoelectric characteristics of the piezoelectric thin film 3 deteriorate due to the excessive content of the additive element Eadd in AlN. Furthermore, as the content of the additive element Eadd in AlN increases, the additive element Eadd becomes more likely to segregate at the grain boundaries in AlN. The segregation of the additive element Eadd at the grain boundaries deteriorates the piezoelectric characteristics of AlN. The additive element Eadd segregated at the grain boundaries serves as a leakage current path, and therefore reduces the resistivity of the piezoelectric thin film 3 in the direction in which the grain boundaries extend (the thickness direction of the piezoelectric thin film 3). As the temperature of AlN containing the additive element Eadd segregated at the grain boundaries reaches a high temperature (e.g., 125°C), thermal diffusion of the additive element Eadd in AlN is likely to occur. The dielectric loss of the piezoelectric thin film 3 is likely to increase due to thermal diffusion of the added element Eadd. For the above reasons, it is desirable to reduce the total area of ​​the grain boundaries g2 in the piezoelectric thin film 3 (total volume of the grain boundary phase) in order to improve the piezoelectric characteristics, suppress the increase in dielectric loss with increasing temperature, and increase the resistivity. The total area of ​​the grain boundaries g2 in the piezoelectric thin film 3 is approximately or completely equal to the surface area of ​​all the crystal grains g1 in the piezoelectric thin film 3. Therefore, as the specific surface area of ​​each crystal grain g1 decreases, the surface area of ​​each crystal grain g1 also decreases, and the total area of ​​the grain boundaries g2 also decreases. For example, when each crystal grain g1 in the piezoelectric thin film 3 has a diameter equal to the above-mentioned median diameter D ５０ When the crystal grain g1 is approximated by a sphere equal to 1 / 2, the specific surface area SSA of each crystal grain g1 is expressed by the following formula 1: Specific surface area SSA = (surface area of ​​sphere) ÷ (volume of sphere) = 4π(D ５０ / 2) ２ ÷{4π(D ５０ / 2) ３ / 3} = 6 / D ５０ (Equation 1) For example, when the diameter (thickness) of each crystal grain g1 in the piezoelectric thin film 3 is equal to the median diameter D', ５０ and a height H, the surface area S of the columnar crystal (cylinder) is expressed by the following formula 2A: Surface area S = [surface area of ​​the side surface (cylinder) of the cylinder] + [area of ​​a pair of end faces (circles) of the cylinder] = [2π(D' ５０ / 2)×H]+[π(D' ５０ / 2) ２ × 2] = πD' ５０H+π(D' ５０ ) ２ / 2 (Formula 2A) The volume V of the columnar crystal (cylinder) is expressed by the following formula 2B: Volume V = π(D' ５０ / 2) ２ × H = π(D' ５０ ) ２ H / 4 (Formula 2B) The specific surface area SSA' of a columnar crystal (cylinder) is (surface area S of columnar crystal) / (volume V of columnar crystal). Based on the above formulas 2A and 2B, the specific surface area SSA' of a columnar crystal is expressed by the following formula 2C. Specific surface area SSA'=S / V={πD' ５０ H+π(D' ５０ ) ２ / 2}÷(π(D' ５０ ) ２ H / 4) = 4 / D' ５０ +2 / H (Formula 2C) As shown in the above formula 1 and formula 2C, the median diameter (D ５０ or D' ５０ As the median diameter (D ５０ or D' ５０ ), the total area of ​​the grain boundaries g2 (total volume of the grain boundary phase) decreases, so that segregation of the additive element Eadd at the grain boundaries g2 in the piezoelectric thin film 3 is suppressed, and thermal diffusion of the additive element Eadd through the grain boundaries g2 in the piezoelectric thin film 3 is also suppressed. As a result, the piezoelectric properties of the piezoelectric thin film 3 are improved, an increase in the dielectric loss of the piezoelectric thin film 3 with an increase in temperature of the piezoelectric thin film 3 is suppressed, and the resistivity of the piezoelectric thin film 3 increases. The inventors have found for the first time that, particularly when the median diameter of the plurality of crystal grains g1 is 90 nm or more and 500 nm or less, the piezoelectric properties are likely to improve, the increase in dielectric loss with an increase in temperature is likely to be suppressed, and the resistivity is likely to increase. However, the technical scope of the present disclosure is not limited by the above mechanism. For the reasons that the piezoelectric properties are likely to improve, the increase in dielectric loss with an increase in temperature is likely to be suppressed, and the resistivity is likely to increase, the median diameter (D ５０ or D' ５０) may be 91 nm or more and 495 nm or less.

[0031] The divalent element Ed contained in the aluminum nitride may be at least one element selected from the group consisting of magnesium (Mg), calcium (Ca), zinc (Zn), strontium (Sr), and barium (Ba). The trivalent element Etr contained in the aluminum nitride may be at least one element selected from the group consisting of scandium (Sc), yttrium (Y), lanthanides, and indium (In). The tetravalent element Et contained in the aluminum nitride may be at least one element selected from the group consisting of zirconium (Zr), germanium (Ge), titanium (Ti), and hafnium (Hf). Because this facilitates improvement in the piezoelectric properties of the piezoelectric thin film 3, at least a portion of the divalent element Ed may be magnesium, and at least a portion of the tetravalent element Et may be at least one of zirconium and hafnium. For the same reason, at least a portion of the trivalent element Etr may be scandium.

[0032] The aluminum nitride contained in the piezoelectric thin film 3 may further contain at least one element selected from the group consisting of a monovalent element and a pentavalent element. The monovalent element may be at least one element selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). The pentavalent element may be at least one element selected from the group consisting of chromium (Cr), vanadium (V), niobium (Nb), and tantalum (Ta). The aluminum nitride contained in the piezoelectric thin film 3 may further contain other elements such as oxygen (O) and argon (Ar).

[0033] In the present disclosure, any element is represented as "X", the concentration of element X in crystal grain g1 (unit: atomic %) is represented as <X>1, and the concentration of element X in grain boundary g2 (unit: atomic %) is represented as <X>2. That is, the concentrations of Al, N, divalent element Ed, trivalent element Etr, and tetravalent element Et in crystal grain g1 are represented as <Al>1, <N>1, <Ed>1, <Etr>1, and <Et>1. The concentrations of Al, N, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain boundary g2 are represented as <Al>2, <N>2, <Ed>2, <Etr>2, and <Et>2. The ratio of the number of aluminum atoms in crystal grain g1 to the total number of aluminum atoms, divalent element Ed, trivalent element Etr, and tetravalent element Et in crystal grain g1 is represented as [Al]g atomic %. [Al]g may be equal to 100×<Al>1 / (<Al>1+<Ed>1+<Etr>1+<Et>1). The ratio of the number of divalent elements Ed in the crystal grains g1 to the total number of aluminum, divalent elements Ed, trivalent elements Etr, and tetravalent elements Et in the crystal grains g1 is expressed as [Ed]g atomic %. [Ed]g may be equal to 100×<Ed>1 / (<Al>1+<Ed>1+<Etr>1+<Et>1). The ratio of the number of trivalent elements Etr in the crystal grains g1 to the total number of aluminum, divalent elements Ed, trivalent elements Etr, and tetravalent elements Et in the crystal grains g1 is expressed as [Etr]g atomic %. [Etr]g may be equal to 100×<Etr>1 / (<Al>1+<Ed>1+<Etr>1+<Et>1). The ratio of the number of tetravalent element Et in crystal grain g1 to the total number of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in crystal grain g1 is expressed as [Et]g atomic %. [Et]g may be equal to 100×<Et>1 / (<Al>1+<Ed>1+<Etr>1+<Et>1). The ratio of the number of aluminum in grain boundary g2 to the total number of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain boundary g2 is expressed as [Al]b atomic %. [Al]b may be equal to 100×<Al>2 / (<Al>2+<Ed>2+<Etr>2+<Et>2).The ratio of the number of divalent elements Ed in the grain boundary g2 to the total number of aluminum, divalent elements Ed, trivalent elements Etr, and tetravalent elements Et in the grain boundary g2 is expressed as [Ed]b atomic %. [Ed]b may be equal to 100 × <Ed>2 / (<Al>2 + <Ed>2 + <Etr>2 + <Et>2). The ratio of the number of trivalent elements Etr in the grain boundary g2 to the total number of aluminum, divalent elements Ed, trivalent elements Etr, and tetravalent elements Et in the grain boundary g2 is expressed as [Etr]b atomic %. [Etr]b may be equal to 100 × <Etr>2 / (<Al>2 + <Ed>2 + <Etr>2 + <Et>2). The ratio of the number of tetravalent elements Et in the grain boundary g2 to the total number of aluminum, divalent elements Ed, trivalent elements Et, and tetravalent elements Et in the grain boundary g2 is expressed as [Et]b atomic %, which may be equal to 100 × <Et>2 / (<Al>2 + <Ed>2 + <Etr>2 + <Et>2).

[0034] [Ed]g + [Etr]g + [Et]g is expressed as [Eadd]g. [Ed]b + [Etr]b + [Et]b is expressed as [Eadd]b. Since an increase in the dielectric loss of the piezoelectric thin film 3 due to an increase in the temperature of the piezoelectric thin film 3 is easily suppressed, [Eadd]b / [Eadd]g may be 0.70 to 1.60, or 0.70 to 1.59. Since the piezoelectric properties of the piezoelectric thin film 3 are easily improved, [Eadd]g may be 4.5 atomic % to 65.0 atomic %, 4.6 atomic % to 65.0 atomic % or 30.0 atomic % to 60.0 atomic %.

[0035] In the crystal grains g1, [Ed]g / ([Ed]g+[Et]g) may be 0.29 or more and 0.61 or less, or 0.29 or more and 0.59 or less, because the average valences of aluminum, the divalent element Ed, the trivalent element Etr, and the tetravalent element Et are easily balanced with the valence of nitrogen, the wurtzite structure of aluminum nitride is easily maintained, and the piezoelectric properties of the piezoelectric thin film 3 are easily improved. [Ed]b / ([Ed]b+[Et]b) may be 0.44 or more and 0.55 or less, because the piezoelectric properties are easily improved, the increase in dielectric loss with temperature rise is easily suppressed, and the resistivity is easily increased.

[0036] The first electrode layer 1 may contain at least one element selected from the group consisting of platinum (Pt), iridium (Ir), gold (Au), rhodium (Rh), palladium (Pd), silver (Ag), nickel (Ni), copper (Cu), aluminum (Al), molybdenum (Mo), tungsten (W), vanadium (V), chromium (Cr), niobium (Nb), tantalum (Ta), ruthenium (Ru), zirconium (Zr), hafnium (Hf), titanium (Ti), yttrium (Y), scandium (Sc), and magnesium (Mg). The first electrode layer 1 may be a metal element or an alloy.

[0037] The second electrode layer 2 may contain at least one element selected from the group consisting of Pt, Ir, Au, Rh, Pd, Ag, Ni, Cu, Al, Mo, W, V, Cr, Nb, Ta, Ru, Zr, Hf, Ti, Y, Sc, and Mg. The second electrode layer 2 may be a metal element or an alloy. The composition of the second electrode layer 2 may be the same as the composition of the first electrode layer 1. The composition of the second electrode layer 2 may be different from the composition of the first electrode layer 1.

[0038] The piezoelectric thin film element 10 may further include a substrate. The first electrode layer 1 may directly or indirectly overlap the substrate. The piezoelectric thin film element 10 may further include an adhesion layer. The adhesion layer may be interposed between the substrate and the first electrode layer 1. In other words, the first electrode layer 1 may indirectly overlap the substrate via the adhesion layer.

[0039] For example, the substrate may be a semiconductor substrate (such as a silicon substrate or a gallium arsenide substrate), an optical crystal substrate (such as a sapphire substrate), an insulator substrate (such as a glass substrate or a ceramic substrate), a metal substrate (such as a stainless steel plate), or an SOI (Silicon-on-Insulator) substrate. The substrate may be crystalline. For example, the substrate may be single crystal or polycrystalline. The substrate may also be amorphous.

[0040] For example, the adhesion layer may contain at least one element selected from the group consisting of aluminum (Al), silicon (Si), titanium (Ti), zinc (Zn), yttrium (Y), zirconium (Zr), chromium (Cr), molybdenum (Mo), hafnium (Hf), tantalum (Ta), tungsten (W), platinum (Pt), and ruthenium (Ru). The adhesion layer may be a metal element, an alloy, or a compound (oxide, etc.). The adhesion layer may be another piezoelectric thin film (e.g., aluminum nitride), a polymer, or a ceramic. The adhesion layer has the function of suppressing peeling of the first electrode layer due to mechanical impact, etc. The adhesion layer may also be referred to as an interface layer, a support layer, a buffer layer, or an intermediate layer.

[0041] For example, the thickness of the substrate may be 50 μm or more and 10,000 μm or less. For example, the thickness of the adhesion layer may be 0.003 μm or more and 2 μm or less. For example, the thickness of the first electrode layer 1 may be 0.01 μm or more and 1 μm or less. For example, the thickness T of the piezoelectric thin film 3 may be 100 nm or more and 30,000 nm or less. For example, the thickness of the second electrode layer 2 may be 0.01 μm or more and 1 μm or less. The thicknesses of the substrate, adhesion layer, first electrode layer 1, piezoelectric thin film 3, and second electrode layer 2 may each be approximately or completely uniform.

[0042] The crystal structures of the substrate, adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer may be identified by X-ray diffraction and electron diffraction. The compositions of the substrate, adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer may be identified by at least one analytical method including X-ray fluorescence spectroscopy (XRF), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), inductively coupled plasma mass spectrometry (ICP-MS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and electron probe microanalyzer (EPMA). The thicknesses of the substrate, adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer may be measured in the cross section of the piezoelectric thin film element using a transmission electron microscope (TEM), a scanning transmission electron microscope (STEM), or a scanning electron microscope (SEM). The crystallinity and crystal structure of each crystal grain may be analyzed and identified based on the electron diffraction pattern measured within the cross section of each crystal grain exposed in the cross section (cross section parallel to the stacking direction) of the piezoelectric thin film.

[0043] The adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer may be sequentially stacked on the surface of the substrate by a vapor deposition method such as sputtering. In particular, the piezoelectric thin film is formed by RF (Radio Frequency) magnetron sputtering. The adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer may each be formed by sputtering using at least one target. The adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer may each be formed by sputtering using multiple targets with different compositions. The target may contain at least one element from among the elements constituting each layer or piezoelectric thin film. By selecting and combining targets with predetermined compositions, the compositions of the adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer can be controlled to the desired compositions. For example, the target may be an elemental metal, an alloy, or an oxide. For example, when the piezoelectric thin film is aluminum nitride containing a divalent element and a tetravalent element, a target made of aluminum, a target made of a divalent element, and a target made of a tetravalent element may be used. One or more alloys composed of two or more elements selected from the group consisting of aluminum, divalent elements, and tetravalent elements may be used as the target. For example, when the piezoelectric thin film is aluminum nitride containing a trivalent element, a target composed of aluminum and a target composed of a trivalent element may be used. For example, when the piezoelectric thin film is aluminum nitride containing a divalent element, a trivalent element, and a tetravalent element, a target composed of aluminum, a target composed of a divalent element, a target composed of a trivalent element, and a target composed of a tetravalent element may be used. One or more alloys composed of two or more elements selected from the group consisting of aluminum, divalent elements, trivalent elements, and tetravalent elements may be used as the target. The composition of the sputtering atmosphere may be a controlling factor for the composition of each of the adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer. For example, nitrogen gas is a raw material for the piezoelectric thin film (aluminum nitride). The input power (power density) applied to the cathode on which each target is placed may be a controlling factor for the composition and thickness of each of the adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer.The total pressure of the sputtering atmosphere, the partial pressure or concentration of the source gas in the atmosphere, the duration of sputtering of each target, the temperature of the substrate surface, and the substrate bias may also be factors for controlling the composition and thickness of each of the adhesion layer, the first electrode layer, the piezoelectric thin film, and the second electrode layer. Etching (e.g., plasma etching) may be used to form a piezoelectric thin film having a desired shape or pattern.

[0044] A specific example of a method for forming a piezoelectric thin film containing a plurality of crystal grains with a median diameter of 90 nm to 500 nm may be as follows.

[0045] While the substrate is heated, an adhesion layer (buffer layer) made of AlN may be formed directly on the main surface of the substrate. The substrate may be made of single crystal Si. The main surface of the substrate may be parallel to the (100) plane of Si.

[0046] While the substrate on which the adhesion layer is laminated is heated, a first metal layer made of Mo may be formed on the surface (main surface) of the adhesion layer. During the process of forming the first metal layer, crystal grains made of Mo grow in the first metal layer.

[0047] In RF magnetron sputtering for forming a piezoelectric thin film, the deposition chamber (vacuum chamber) is filled with Ar and N ２ After the chamber is filled with the mixed gas, power is supplied to all targets. ＋ ) collide with the surface of the target to which power is supplied. Atoms that gain kinetic energy from the Ar ions are desorbed from the surface of each target and reach the surface of the substrate (the surface of the first electrode layer laminated on the substrate). As a result, a piezoelectric thin film grows on the surface of the substrate (the surface of the first electrode layer laminated on the substrate).

[0048] During the piezoelectric thin film formation process, a substrate bias (power) of 20 W is continuously supplied to the substrate. As a result, the number of atoms reaching the surface of the first electrode layer from each target per unit time is reduced compared to the piezoelectric thin film formation process in which the substrate bias is zero W, and the number of seed crystals (crystal nuclei) formed on the surface of the first electrode layer is also reduced compared to the piezoelectric thin film formation process in which the substrate bias is zero W. The fewer the number of seed crystals (crystal nuclei), the more space there is for various crystals to grow, and the more likely it is that the median diameter of the multiple crystal grains formed by the growth of the multiple seed crystals will increase.

[0049] Based on the power supplied to each target, the deposition rate of the piezoelectric thin film (the rate at which the thickness of the piezoelectric thin film increases) is adjusted to a value (0.35 nm / sec) lower than the conventional deposition rate (0.8 nm / sec). The lower the deposition rate, the longer the time allowed for migration of the atoms (Al, the additive element Eadd, and N) on the surface of the first electrode layer, and the easier it is to promote the growth of the seed crystals. Therefore, the lower the deposition rate, the easier it is to increase the median diameter of the multiple crystal grains formed from the multiple seed crystals.

[0050] The substrate temperature during the piezoelectric thin film formation process is 150° C., which is higher than the conventional temperature (77° C.). The higher the substrate temperature, the more likely it is that the seed crystal growth is promoted and the more likely it is that the median diameter of the multiple crystal grains formed from the multiple seed crystals increases.

[0051] As described above, by adjusting the substrate bias, the deposition rate of the piezoelectric thin film, and the temperature of the substrate during the piezoelectric thin film formation process, it is possible to form a piezoelectric thin film containing a plurality of crystal grains with a median diameter of 90 nm or more and 500 nm or less.

[0052] The piezoelectric thin film element according to this embodiment has a wide range of applications. For example, the piezoelectric thin film element may be a piezoelectric microphone, a harvester, an oscillator, a resonator, an acoustic multilayer, or a filter. For example, the piezoelectric thin film element may be a piezoelectric actuator. The piezoelectric actuator may be used in haptics. That is, the piezoelectric actuator may be used in various devices requiring cutaneous (tactile) feedback. For example, devices requiring cutaneous feedback may be wearable devices, touchpads, displays, or game controllers. 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. The piezoelectric actuator may be used in a piezoelectric switch. For example, the piezoelectric thin film element may be a piezoelectric sensor or a piezoelectric transducer. 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, or a shock sensor. The ultrasonic transducer may be a piezoelectric micromachined ultrasonic transducer (PMUT). Products applying the piezoelectric micromechanical ultrasonic transducer may be biometric sensors such as fingerprint sensors and ultrasonic blood vessel authentication sensors, medical or healthcare sensors, or ToF (Time of Flight) sensors. For example, the filter may be a Bulk Acoustic Wave (BAW) filter or a Surface Acoustic Wave (SAW) filter. Each of the above-mentioned piezoelectric thin film elements may be a part or the entirety of a Micro Electro Mechanical System (MEMS).

[0053] The present invention is not necessarily limited to the above-described embodiments. 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.

[0054] The present invention will be described in detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples.

[0055] Example 1 A wafer made of single crystal silicon (Si) was used as the substrate. The diameter of the substrate was 8 inches, and the thickness of the substrate was 725 μm. The thickness of the substrate was uniform. The main surface of the substrate was parallel to the (100) plane of Si.

[0056] An adhesion layer was formed directly on the entire main surface of the substrate by RF magnetron sputtering in a vacuum chamber. The adhesion layer was made of aluminum nitride containing no additive elements. The sputtering target was aluminum. The atmosphere in the vacuum chamber was Ar and N. ２ The input power per unit area of ​​the sputtering target was 0.74 W / cm ２ The temperature of the substrate during the formation of the adhesion layer was maintained at 200°C. The film formation pressure (air pressure inside the vacuum chamber) was 0.4 Pa. No substrate bias was applied. The thickness of the adhesion layer was uniform. The film formation rate was controlled to 0.09 nm / sec. The thickness of the adhesion layer was adjusted to approximately 30 nm.

[0057] A first electrode layer made of Mo was formed on the entire surface of the adhesive layer by RF magnetron sputtering in a vacuum chamber. Mo alone was used as the sputtering target. The atmosphere in the vacuum chamber was Ar gas. The input power per unit area of ​​the sputtering target was 0.93 W / cm. ２ The temperatures of the substrate and the adhesive layer during the formation of the first electrode layer were maintained at 450° C. The thickness of the first electrode layer was uniform. The thickness of the first electrode layer was adjusted to 0.2 μm.

[0058] A piezoelectric thin film was formed directly on the entire surface of the first electrode layer by RF magnetron sputtering in a vacuum chamber. As sputtering targets, Al, a divalent element Ed, and a tetravalent element Et were used. In other words, three types of metal targets were used. The divalent element Ed and the tetravalent element Et in Example 1 are shown in Table 1 below. The atmosphere in the vacuum chamber was Ar and N ２ The input power per unit area of ​​each sputtering target (cathode power) was 3.72 W / cm ２ The temperature of the substrate during the formation of the piezoelectric thin film was maintained at 150°C. The film formation pressure (air pressure inside the vacuum chamber) was 1 Pa. The substrate bias during the formation of the piezoelectric thin film was 20 W. The film formation rate of the piezoelectric thin film was controlled to 0.35 nm / sec. The thickness of the piezoelectric thin film was adjusted to about 1000 nm.

[0059] A second electrode layer made of Mo was formed on the entire second main surface of the piezoelectric thin film in a manner similar to that of the first electrode layer, i.e., the composition of the second electrode layer was the same as that of the first electrode layer.

[0060] The laminated structure formed on the substrate was patterned by photolithography, and after patterning, the entire laminate was cut by dicing to obtain rectangular piezoelectric thin film elements.

[0061] The piezoelectric thin film element of Example 1 was composed of a substrate, an adhesive layer laminated directly on the surface of the substrate, a first electrode layer laminated directly on the surface of the adhesive layer, a piezoelectric thin film laminated directly on the surface of the first electrode layer, and a second electrode layer laminated directly on the surface (second main surface) of the piezoelectric thin film.

[0062] The following analyses and measurements were carried out during or after the fabrication of the piezoelectric thin film elements: A plurality of identical piezoelectric thin film elements were fabricated as samples for the following analyses and measurements.

[0063] <Composition of Piezoelectric Thin Film> The composition of the piezoelectric thin film was analyzed by X-ray fluorescence analysis (XRF). A wavelength dispersive X-ray fluorescence spectrometer (RIGAKU AZX-400) manufactured by Rigaku Corporation was used for the XRF. The analysis results showed that the piezoelectric thin film was made of aluminum nitride containing additive elements (Ed and Et).

[0064] <Crystalline structure of piezoelectric thin film> The crystalline structure of the piezoelectric thin film was analyzed by X-ray diffraction (XRD). A multipurpose X-ray diffractometer (SmartLab) manufactured by Rigaku Corporation was used for the XRD. 2θ-θ scan, ω scan, and 2θχ-φ scan were performed on the surface (second main surface) of the piezoelectric thin film using the X-ray diffractometer.

[0065] The XRD pattern obtained by the XRD method showed that the piezoelectric thin film (i.e., aluminum nitride containing the additive element) has a wurtzite structure, and the (0002) plane (and the (0001) plane) of the wurtzite structure are parallel to the first and second principal surfaces of the piezoelectric thin film.

[0066] <Analysis of the Cross Section of the Piezoelectric Thin Film> The cross section of the piezoelectric thin film was analyzed using a scanning transmission electron microscope (STEM). A Titan G2 manufactured by Thermo Fisher Scientific Inc. (formerly FEI company) was used as the STEM. The cross section analyzed using the STEM was perpendicular to the first main surface (the surface of the first electrode layer) of the piezoelectric thin film. The cross section of the piezoelectric thin film included multiple crystal grains. Each crystal grain was a columnar crystal extending along the normal direction of the first main surface of the piezoelectric thin film. Furthermore, the cross section of the piezoelectric thin film included multiple grain boundaries, and each grain boundary was located between a pair of adjacent columnar crystals.

[0067] Five observation areas were randomly selected from the upper cross section analyzed by STEM. The distance from the center of the main surface (circle) of the substrate to each observation area was 10 mm. The concentration of each element in one crystal grain randomly selected from each observation area was measured. The concentration of each element in five grain boundaries randomly selected from each observation area was measured. Energy dispersive spectroscopy (EDS) attached to the STEM was used to analyze the composition of each grain and grain. Using the above method, [Ed]g, [Etr]g, [Et]g, and [Al]g were measured for each of the five crystal grains. The average values ​​of [Ed]g, [Etr]g, [Et]g, and [Al]g were calculated from [Ed]g, [Etr]g, [Et]g, and [Al]g for each of the five crystal grains. The average values ​​are shown in Table 1 below. Using the above method, [Ed]b, [Etr]b, [Et]b, and [Al]b were measured for each of the 25 grain boundaries. Average values ​​of [Ed]b, [Etr]b, [Et]b, and [Al]b were calculated from [Ed]b, [Etr]b, [Et]b, and [Al]b for each of the 25 grain boundaries. The average values ​​are shown in Table 1 below.

[0068] <Measurement of Median Diameter D50> To prevent charging of the piezoelectric thin film, the entire second principal surface of the piezoelectric thin film was covered with a thin film made of Pt. Secondary electron images of three randomly selected locations on the second principal surface of the piezoelectric thin film covered with the thin film made of Pt were taken using a scanning transmission electron microscope (SEM). An S-4700 SEM manufactured by Hitachi High-Technologies Corporation was used as the SEM. The dimensions of each secondary electron image were 880 nm long x 1260 nm wide. Each secondary electron image showed that numerous columnar crystals extending along the normal direction of the first principal surface of the piezoelectric thin film were exposed on the second principal surface of the piezoelectric thin film. Monochrome images of each secondary electron image were obtained by image processing (binarization processing). The grain size d (Heywood diameter) of each of the multiple crystal grains in each monochrome image was measured. Image analysis software was used to measure the Heywood diameter. As the image analysis software, image analysis software (not for sale) manufactured by TDK Corporation was used. Based on the grain size distribution of the crystal grains obtained by the above method, the median diameter D ５０was calculated. ５０ The results are shown in Table 2 below.

[0069] <Measurement of Resistivity ρ> The resistivity ρ (unit: Ωcm) of the piezoelectric thin film was measured in the direction (thickness direction) perpendicular to the first and second principal surfaces of the piezoelectric thin film. A measuring device (R8340A) manufactured by ADVANTEST Co., Ltd. was used to measure the resistivity ρ. During the measurement of the resistivity ρ, an electric field of 1 V / μm was applied to the piezoelectric thin film. The area of ​​the portion of the first electrode layer and the second electrode layer to which the electric field was applied was 600 × 600 (μm). ２ The resistivity ρ of Example 1 is shown in Table 2 below. It is preferable that the resistivity ρ is high. In Table 2, "E+m" (m is any positive integer) means "×10 ｍ " means.

[0070] <Piezoelectric constant d ３３ Measurement of the piezoelectric constant d of the piezoelectric thin film ３３ (unit: pC / N) was measured. ３３ The details of the measurement were as follows: The piezoelectric constant d was measured at three points. ３３ The average values ​​of d are shown in Table 2 below. ３３ means excellent piezoelectric properties. Measuring device: d manufactured by Piezotest ３３ Meter (PM200) Frequency: 110Hz Clamping pressure: 0.25N

[0071] <Measurement of Dielectric Loss Tangent> Before the high-temperature test described below, the dielectric loss tangent (tanδ) of each of the three piezoelectric thin film elements of Example 1 was measured. A measuring device (E4980A) manufactured by Agilent Technologies, Inc. was used to measure tanδ (unit: %). In measuring tanδ, an electric field of 1 V / μm was applied to the piezoelectric thin film. The area of ​​the portion of the first electrode layer and the second electrode layer to which the electric field was applied was 600 × 600 (μm). ２ In the high-temperature test, the three piezoelectric thin film elements were held in a thermostatic chamber at 125°C for 1000 hours. After the high-temperature test, the tan δ of each of the three piezoelectric thin film elements was measured. The average value of tan δ of the three piezoelectric thin film elements measured at room temperature before the high-temperature test was tan δ ＢＥＦＯＲＥThe average value of tan δ of the three piezoelectric thin film elements measured at room temperature after the high temperature test is tan δ ＡＦＴＥＲ It is expressed as tan δ ＢＥＦＯＲＥ and tan δ ＡＦＴＥＲ From this, the rate of change in tan δ, Δtan δ (unit: %), defined by the following mathematical formula A, was calculated. The lower Δtan δ, the more the increase in dielectric loss due to the temperature rise of the piezoelectric thin film is suppressed. In other words, it is preferable that Δtan δ is low. Δtan δ is shown in Table 2 below. The tan δ shown in Table 2 below is ＢＥＦＯＲＥ Δtanδ=100×(tanδ ＡＦＴＥＲ -tan δ ＢＥＦＯＲＥ ) / tanδ ＢＥＦＯＲＥ (Formula A)

[0072] Examples 2 to 12 As raw materials for the piezoelectric thin films of Examples 2, 3, and 5 to 12, a target consisting of the divalent element Ed and a target consisting of the tetravalent element Et shown in Table 1 below were used. The compositions of the piezoelectric thin films of Examples 3 and 5 to 12 were adjusted to be different from each other by changing the input power of each target. As raw materials for the piezoelectric thin film of Example 4, a target consisting of the trivalent element Etr was used instead of the target consisting of Ed and the target consisting of Et. The trivalent element Etr in Example 4 is shown in Table 1 below.

[0073] Except for the above-mentioned points, the piezoelectric thin film elements of each of Examples 2 to 12 were fabricated in the same manner as in Example 1. Analysis and measurement were carried out on the piezoelectric thin film elements of each of Examples 2 to 12 in the same manner as in Example 1. The results of the analysis and measurement of each of Examples 2 to 12 are shown in Tables 1 and 2 below. A secondary electron image of the second main surface of the piezoelectric thin film of Example 11 is shown in Figure 6(a).

[0074] Except for the differences shown in Tables 1 and 2 below, the piezoelectric thin films of Examples 2 to 12 each had the same characteristics as Example 1.

[0075] Comparative Example 1: An adhesion layer was formed directly on the entire main surface of a substrate by RF magnetron sputtering in a vacuum chamber. The adhesion layer was made of Ti. Ti alone was used as the sputtering target. The atmosphere in the vacuum chamber was Ar gas. The input power per unit area of ​​the sputtering target was 9.87 W / cm. ２ The temperature of the substrate during the formation of the adhesion layer was maintained at 300°C. The film formation pressure (air pressure inside the vacuum chamber) was 0.7 Pa. No substrate bias was applied. The thickness of the adhesion layer was uniform. The thickness of the adhesion layer was adjusted to about 30 nm.

[0076] A first electrode layer made of Cr was formed on the entire surface of the adhesive layer by RF magnetron sputtering in a vacuum chamber. Cr alone was used as the sputtering target. The atmosphere in the vacuum chamber was Ar gas. The input power per unit area of ​​the sputtering target was 9.87 W / cm. ２ The temperatures of the substrate and the adhesive layer during the formation of the first electrode layer were maintained at 500° C. The thickness of the first electrode layer was uniform. The thickness of the first electrode layer was adjusted to 0.2 μm.

[0077] Before the formation of the piezoelectric thin film, the first electrode layer was annealed in a vacuum chamber at 600° C. The duration of annealing was 10 minutes.

[0078] A piezoelectric thin film was formed directly on the entire surface of the first electrode layer by RF magnetron sputtering in a vacuum chamber. Only aluminum (metal) was used as the sputtering target. In other words, the piezoelectric thin film of Comparative Example 1 was made of aluminum nitride containing no additive elements. The atmosphere in the vacuum chamber was Ar and N ２ The input power per unit area of ​​the sputtering target (cathode power) was 9.87 W / cm ２The temperature of the substrate during the formation of the piezoelectric thin film was maintained at 300°C. The deposition pressure (air pressure inside the vacuum chamber) was 0.3 Pa. The substrate bias during the formation of the piezoelectric thin film was zero W. The deposition rate of the piezoelectric thin film was controlled to 0.71 nm / sec. The thickness of the piezoelectric thin film was adjusted to about 1300 nm.

[0079] A piezoelectric thin film element of Comparative Example 1 was fabricated in the same manner as in Example 1, except for the above points.

[0080] Analysis and measurement were carried out on the piezoelectric thin film element of Comparative Example 1 in the same manner as in Example 1. The results of the analysis and measurement of Comparative Example 1 are shown in Tables 1 and 2 below. A secondary electron image of the second main surface of the piezoelectric thin film of Comparative Example 1 is shown in (b) of FIG. 6. The differences shown in Tables 1 and 2 below (composition of the piezoelectric thin film and D ５０ ), the piezoelectric thin film of Comparative Example 1 had the same characteristics as those of Example 1.

[0081] In the following Table 1, r(Ed)g means [Ed]g / ([Ed]g+[Et]g). In the following Table 1, r(Ed)b means [Ed]b / ([Ed]b+[Et]b). In the following Table 2, R(Eadd) means [Eadd]b / [Eadd]g.

[0082]

[0083]

[0084] For example, a piezoelectric thin film according to one aspect of the present disclosure may be used in a microphone, a sensor, a transducer, a filter, a harvester, or an actuator.

[0085] 1...first electrode layer, 2...second electrode layer, 3...piezoelectric thin film, 10...piezoelectric thin film element, s1...surface of first electrode layer, s2...surface of second electrode layer, s31...first main surface of piezoelectric thin film, s32...second main surface of piezoelectric thin film, uc...unit cell of wurtzite structure (aluminum nitride), Ed...divalent element, Etr...trivalent element, Et...tetravalent element, g1...crystal grain, g2...grain boundary.

Claims

1. A piezoelectric thin film comprising aluminum nitride having a wurtzite structure, wherein the aluminum nitride comprises an additive element, the (0001) plane of the wurtzite structure is oriented in the normal direction of a main surface of the piezoelectric thin film, the piezoelectric thin film comprises a plurality of crystal grains comprising the aluminum nitride, at least some of the plurality of crystal grains are a plurality of columnar crystals extending along the normal direction of the main surface of the piezoelectric thin film, and the median diameter of the plurality of crystal grains in the direction parallel to the main surface of the piezoelectric thin film is 90 nm or more and 500 nm or less.

2. The piezoelectric thin film according to claim 1, wherein the additive element includes at least a divalent element and a tetravalent element.

3. The piezoelectric thin film according to claim 2, wherein at least a portion of the divalent elements is magnesium, and at least a portion of the tetravalent elements is at least one of zirconium and hafnium.

4. The piezoelectric thin film according to claim 1, wherein the additive element includes at least a trivalent element.

5. The piezoelectric thin film according to claim 4, wherein at least a portion of the trivalent element is scandium.

6. The piezoelectric thin film further includes at least one grain boundary located between the plurality of crystal grains, and the additive element is only a divalent element and a tetravalent element, or only a trivalent element, or the divalent element, the trivalent element, and the tetravalent element, and the ratio of the number of the divalent elements in the crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the crystal grains is expressed as [Ed] g atomic %, and the ratio of the number of the trivalent elements in the crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the crystal grains is expressed as [Etr] g atomic %, and the ratio of the number of the tetravalent elements in the crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the crystal grains is expressed as [Et] g atomic %, 2. The piezoelectric thin film according to claim 1, wherein a ratio of the number of divalent elements in the grain boundary to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the grain boundary is expressed as [Ed]b atomic %, a ratio of the number of trivalent elements in the grain boundary to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the grain boundary is expressed as [Etr]b atomic %, a ratio of the number of tetravalent elements in the grain boundary to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the grain boundary is expressed as [Et]b atomic %, [Ed]g + [Etr]g + [Et]g is expressed as [Eadd]g, [Ed]b + [Etr]b + [Et]b is expressed as [Eadd]b, and [Eadd]b / [Eadd]g is 0.70 or more and 1.60 or less.

7. The piezoelectric thin film further includes at least one grain boundary located between the plurality of crystal grains, and the added element is only a divalent element and a tetravalent element, or only a trivalent element, or the divalent element, the trivalent element, and the tetravalent element, and the ratio of the number of the divalent elements in the crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the crystal grains is expressed as [Ed]g atomic %, and the ratio of the number of the trivalent elements in the crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the crystal grains is expressed as [Etr]g atomic %, and the ratio of the number of the tetravalent elements in the crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the crystal grains is expressed as [Et]g atomic %, and [Ed]g + [Etr]g + [Et]g is expressed as [Eadd]g, The piezoelectric thin film according to claim 1 , wherein [Eadd]g is 4.5 atomic % or more and 65.0 atomic % or less.

8. The piezoelectric thin film according to claim 7, wherein the [Eadd]g is 30.0 atomic % or more and 60.0 atomic % or less.

9. A piezoelectric thin film element comprising: a piezoelectric thin film according to any one of claims 1 to 8; and an electrode layer, wherein the piezoelectric thin film directly or indirectly overlaps the surface of the electrode layer.

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