Piezoelectric thin film and piezoelectric thin film element

Abstract

This piezoelectric thin film includes a plurality of crystal grains that include aluminum nitride. The aluminum nitride includes an additive element. The plurality of crystal grains include a plurality of first crystal grains and a plurality of second crystal grains. The ratio of the additive element quantity in the first crystal grains to the total aluminum and additive element quantity in the first crystal grains is expressed as [Eadd]1 at%. The ratio of the additive element quantity in the second crystal grains to the total aluminum and additive element quantity in the second crystal grains g2 is expressed as [Eadd]2 at%. [Eadd]2 is higher than [Eadd]1.

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 indices 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 electric field strength (reception ability) generated 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.

[0004] For example, Patent Document 1 below discloses AlN containing, as additive elements, a Group 2 element or a Group 12 element and a Group 4 element or a Group 5 element.

[0005] Patent No. 6535637

[0006] It is known that the piezoelectric properties of AlN are improved by the addition of additional elements. However, as the content of the additional elements increases, it becomes more difficult to maintain the crystal structure (wurtzite structure) of AlN. As a result, the piezoelectric properties of AlN deteriorate. This tendency is particularly pronounced when the nitride of the additional element does not have piezoelectric properties. Furthermore, due to random segregation of the additional elements in AlN, heterogeneous phases that do not have piezoelectric properties are randomly formed in the AlN, and the heterogeneous phases tend to gather locally. As a result, the piezoelectric properties of AlN deteriorate.

[0007] An object of one aspect of the present disclosure is to provide a piezoelectric thin film having excellent piezoelectric properties, and a piezoelectric thin film element including the piezoelectric thin film.

[0008] For example, the present disclosure relates to a piezoelectric thin film according to any one of the following items [1] to

[10] , and a piezoelectric thin film element according to the following item

[11] .

[0009] [1] A piezoelectric thin film comprising a plurality of crystal grains containing aluminum nitride, wherein the aluminum nitride contains an additive element, wherein the plurality of crystal grains comprise a plurality of first crystal grains and a plurality of second crystal grains, wherein a ratio of the number of the additive element in the first crystal grains to a total number of aluminum and the additive element in the first crystal grains is expressed as [Eadd]1 atomic %, and a ratio of the number of the additive element in the second crystal grains to a total number of aluminum and the additive element in the second crystal grains is expressed as [Eadd]2 atomic %, wherein [Eadd]2 is higher than [Eadd]1.

[0010] [2] The piezoelectric thin film according to [1], wherein the additive element includes at least a divalent element and a tetravalent element.

[0011] [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.

[0012] [4] The piezoelectric thin film according to any one of [1] to [3], wherein the additive element includes at least a trivalent element.

[0013] [5] The piezoelectric thin film according to [4], wherein at least a part of the trivalent element is scandium.

[0014] [6] The additional elements are only divalent elements and tetravalent elements, or only trivalent elements, or the divalent elements, trivalent elements, and tetravalent elements, and the ratio of the number of divalent elements in the first crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Ed] 1 atomic %, the ratio of the number of trivalent elements in the first crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Etr] 1 atomic %, the ratio of the number of divalent elements in the second crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the second crystal grains is expressed as [Ed] 2 atomic %, and the ratio of the number of trivalent elements in the second crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the second crystal grains is expressed as [Etr] 2 atomic %, The piezoelectric thin film according to any one of [1] to [5], wherein ([Ed]2 + [Etr]2) / ([Ed]1 + [Etr]1) is greater than 1.000 and equal to or less than 3.000.

[0015] [7] The piezoelectric thin film according to any one of [1] to [6], wherein a ratio of the total volume of the plurality of first crystal grains in the piezoelectric thin film is expressed as A1 volume %, a ratio of the total volume of the plurality of second crystal grains in the piezoelectric thin film is expressed as A2 volume %, and A2 / (A1+A2) is 0.1% or more and 15.0% or less.

[0016] [8] The additive element includes at least a divalent element and a tetravalent element, wherein a ratio of the number of the divalent elements in the first crystal grains to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Ed]1 atomic %, wherein a ratio of the number of the tetravalent elements in the first crystal grains to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Et]1 atomic %, wherein a ratio of the number of the divalent elements in the second crystal grains to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the second crystal grains is expressed as [Ed]2 atomic %, wherein a ratio of the number of the tetravalent elements in the second crystal grains to a total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the second crystal grains is expressed as [Et]2 atomic %, and wherein [Ed]1 / ([Ed]1+[Et]1) is 0.40 or more and 0.58 or less, The piezoelectric thin film according to any one of [1] to [7], wherein [Ed]2 / ([Ed]2+[Et]2) is 0.40 or more and 0.58 or less.

[0017] [9] The added elements are only divalent elements and tetravalent elements, or only trivalent elements, or the divalent elements, trivalent elements, and tetravalent elements, and the ratio of the number of the divalent elements in the first crystal grains to the total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Ed] 1 atomic %, the ratio of the number of the trivalent elements in the first crystal grains to the total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Etr] 1 atomic %, the ratio of the number of the tetravalent elements in the first crystal grains to the total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Et] 1 atomic %, and the ratio of the number of the divalent elements in the second crystal grains to the total number of the aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the second crystal grains is expressed as [Ed] 2 atomic %, The piezoelectric thin film according to any one of [1] to [8], wherein a ratio of the number of trivalent elements in the second crystal grains to a total number of aluminum, divalent elements, trivalent elements, and tetravalent elements in the second crystal grains is expressed as [Etr]2 atomic %, a ratio of the number of tetravalent elements in the second crystal grains to a total number of aluminum, divalent elements, trivalent elements, and tetravalent elements in the second crystal grains is expressed as [Et]2 atomic %, [Ed]1 + [Etr]1 + [Et]1 is 9.0 atomic % or more and less than 61.0 atomic %, and [Ed]2 + [Etr]2 + [Et]2 is more than 9.0 atomic % and 61.0 atomic % or less.

[0018]

[10] The piezoelectric thin film according to any one of [1] to [9], wherein the first crystal grains extend along a direction perpendicular to a main surface of the piezoelectric thin film, and the second crystal grains extend along a direction perpendicular to the main surface of the piezoelectric thin film.

[0019]

[11] A piezoelectric thin film element comprising: the piezoelectric thin film according to any one of [1] to

[10] ; and an electrode layer, wherein the piezoelectric thin film directly or indirectly overlaps a surface of the electrode layer.

[0020] According to one aspect of the present disclosure, a piezoelectric thin film having excellent piezoelectric properties and a piezoelectric thin film element thereof are provided.

[0021] 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 perspective view of a unit cell of the crystal structure (wurtzite structure) of aluminum nitride contained in the piezoelectric thin film. Fig. 4(a) is an image of a cross section of a piezoelectric thin film according to one embodiment of the present invention, where the cross section shown in Fig. 4(a) 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. 4(b) is an image showing the distribution of magnesium in the cross section shown in Fig. 4(a).

[0022] Preferred embodiments of the present invention will now be described 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 Figures 1, 2, and 4(b) represent three mutually orthogonal coordinate axes. The directions of the X-axis, Y-axis, and Z-axis are common to Figures 1, 2, and 4(b).

[0023] 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.

[0024] The piezoelectric thin film 3 according to this embodiment includes a plurality of crystal grains containing aluminum nitride. The crystal structure of aluminum nitride is a hexagonal wurtzite structure. For example, the aluminum nitride contained in each of the plurality of crystal grains may be single crystal, polycrystalline, or imperfectly crystalline.

[0025] Aluminum nitride contains an additional element Eadd that is different from aluminum (Al) and nitrogen (N). For example, the additional element Eadd may include at least a divalent element Ed and a tetravalent element Et. For example, the additional element Eadd may include a trivalent element Etr. For example, the additional element Eadd may include only the divalent element Ed and the tetravalent element Etr, 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 additional element Eadd distorts the wurtzite structure of 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.

[0026] FIG. 3 shows the wurtzite structure of aluminum nitride contained in the piezoelectric thin film 3. The unit cell uc of 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 aluminum nitride in the piezoelectric thin film 3 may be oriented approximately or completely parallel to the first main 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, by orienting at least a portion of the (0001) plane of the aluminum nitride in the piezoelectric thin film 3 approximately or completely parallel to the first main surface s31 of the piezoelectric thin film 3, the piezoelectric properties of the piezoelectric thin film 3 are likely to be improved. During the manufacturing process of the piezoelectric thin film 3, the (0001) plane of aluminum nitride tends 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).

[0027] 2, the piezoelectric thin film 3 includes a plurality of first crystal grains g1 and a plurality of second crystal grains g2 as a plurality of crystal grains containing aluminum nitride. The piezoelectric thin film 3 may consist only of a plurality of first crystal grains g1 and a plurality of second crystal grains g2. One or more first crystal grains g1 may be in direct contact with one or more second crystal grains g2. One or more first crystal grains g1 may be in indirect contact with one or more second crystal grains g2 via a grain boundary phase.

[0028] The ratio of the number of additive elements Eadd in the first crystal grains g1 to the total number of aluminum and additive elements Eadd in the first crystal grains g1 is expressed as [Eadd]1 atomic %. The ratio of the number of additive elements Eadd in the second crystal grains g2 to the total number of aluminum and additive elements Eadd in the second crystal grains g2 is expressed as [Eadd]2 atomic %. Conventional piezoelectric thin films are prone to contain heterogeneous phases as the content of the additive element Eadd in the piezoelectric thin film increases. A heterogeneous phase is a phase (e.g., a crystal grain or grain boundary phase) in which the additive element Eadd segregates and which is different from aluminum nitride having a wurtzite structure. A heterogeneous phase is unlikely to have piezoelectric properties. On the other hand, in the piezoelectric thin film 3 according to this embodiment, [Eadd]2 is higher than [Eadd]1. In other words, the piezoelectric thin film 3 includes not only first crystal grains g1 having a relatively low ratio of the additive element Eadd, but also a plurality of second crystal grains g2 having a relatively high ratio of the additive element Eadd. In the growth process of the piezoelectric thin film 3 according to this embodiment, not only the first crystal grains g1, which have a relatively low ratio of the number of the additive element Eadd, but also the second crystal grains g2, which have a relatively high ratio of the number of the additive element Eadd, grow, so that random segregation of the additive element Eadd is suppressed in the growth process of the piezoelectric thin film 3. As a result, heterogeneous phases are unlikely to be randomly formed in the piezoelectric thin film 3, and heterogeneous phases are unlikely to gather locally in the piezoelectric thin film 3. For the above reasons, the piezoelectric thin film 3 is unlikely to contain heterogeneous phases that do not have piezoelectric properties, and the wurtzite structure of aluminum nitride in the piezoelectric thin film 3 is unlikely to be damaged by the additive elements. As a result, the piezoelectric properties of the piezoelectric thin film 3 are likely to be improved, and the relative dielectric constant and (ε ｒ For example, the piezoelectric thin film 3 has excellent piezoelectric properties such as a large e ３１，ｆ e ３１，ｆis the piezoelectric stress constant of the longitudinal vibration (in-plane vibration) of the piezoelectric thin film 3. ３１，ｆ The unit is [C / m ２ ]. The longitudinal lateral vibration is vibration (expansion and contraction) of the piezoelectric thin film 3 in a direction perpendicular to the polarization direction of the piezoelectric thin film 3 (for example, the thickness direction of the piezoelectric thin film 3). In other words, the longitudinal lateral vibration may be vibration (expansion and contraction) of the piezoelectric thin film 3 in a direction substantially or completely parallel to the first principal surface s31 or the second principal surface s32 of the piezoelectric thin film 3, and the longitudinal lateral vibration may be vibration (expansion and contraction) of the piezoelectric thin film 3 in a direction substantially or completely parallel to the surfaces of the first electrode layer 1 and the second electrode layer 2. The piezoelectric thin film 3 may contain a small amount of a different phase as long as the piezoelectric properties of the piezoelectric thin film 3 are not impaired.

[0029] 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), strontium (Sr), barium (Ba), and zinc (Zn). 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.

[0030] 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).

[0031] In the present disclosure, any element is represented as "X", the concentration of element X (unit: atomic %) in the first crystal grain g1 is represented as <X>1, and the concentration of element X (unit: atomic %) in the second crystal grain g2 is represented as <X>2. That is, the concentrations of Al, N, divalent element Ed, trivalent element Etr, and tetravalent element Et in the first crystal grain g1 are represented as <Al>1, <N>1, <Ed>1, <Etr>1, and <Et>1, respectively. The concentrations of Al, N, divalent element Ed, trivalent element Etr, and tetravalent element Et in the second crystal grain g2 are represented as <Al>2, <N>2, <Ed>2, <Etr>2, and <Et>2, respectively. The ratio of the number of aluminum atoms in the first crystal grains g1 to the total number of aluminum atoms, divalent element Ed, trivalent element Etr, and tetravalent element Et in the first crystal grains g1 is expressed as [Al]1 atomic %. [Al]1 may be equal to 100 × <Al>1 / (<Al>1 + <Ed>1 + <Etr>1 + <Et>1). The ratio of the number of divalent element Ed atoms in the first crystal grains g1 to the total number of aluminum atoms, divalent element Ed, trivalent element Etr, and tetravalent element Et in the first crystal grains g1 is expressed as [Ed]1 atomic %. [Ed]1 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 first crystal grains g1 to the total number of aluminum, divalent elements Ed, trivalent elements Etr, and tetravalent elements Et in the first crystal grains g1 is expressed as [Etr]1 atomic %. [Etr]1 may be equal to 100 × <Etr>1 / (<Al>1 + <Ed>1 + <Etr>1 + <Et>1). The ratio of the number of tetravalent elements Et in the first crystal grains g1 to the total number of aluminum, divalent elements Ed, trivalent elements Etr, and tetravalent elements Et in the first crystal grains g1 is expressed as [Et]1 atomic %. [Et]1 may be equal to 100 × <Et>1 / (<Al>1 + <Ed>1 + <Etr>1 + <Et>1). The ratio of the number of aluminum atoms in the second crystal grains g2 to the total number of aluminum atoms, the divalent element Ed, the trivalent element Etr, and the tetravalent element Et in the second crystal grains g2 is expressed as [Al]2 atomic %, and [Al]2 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 second crystal grains g2 to the total number of aluminum, divalent elements Ed, trivalent elements Etr, and tetravalent elements Et in the second crystal grains g2 is expressed as [Ed]2 atomic %. [Ed]2 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 second crystal grains g2 to the total number of aluminum, divalent elements Ed, trivalent elements Etr, and tetravalent elements Et in the second crystal grains g2 is expressed as [Etr]2 atomic %. [Etr]2 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 second crystal grains g2 to the total number of aluminum, divalent elements Ed, trivalent elements Etr, and tetravalent elements Et in the second crystal grains g2 is expressed as [Et]2 atomic %. [Et]2 may be equal to 100×<Et>2 / (<Al>2+<Ed>2+<Etr>2+<Et>2). The above-mentioned [Eadd]1 may be equal to [Ed]1+[Etr]1+[Et]1. The above-mentioned [Eadd]2 may be equal to [Ed]2+[Etr]2+[Et]2.

[0032] ([Ed]2 + [Etr]2) / ([Ed]1 + [Etr]1) may be greater than 1.000 and not greater than 3.000, because this facilitates improving the piezoelectric characteristics of the piezoelectric thin film 3 and reducing the dielectric loss of the piezoelectric thin film 3. For the same reason, ([Ed]2 + [Etr]2) / ([Ed]1 + [Etr]1) may be 1.005 or greater and 2.262 or less, or 1.010 or greater and 2.250 or less.

[0033] The total volume ratio of the plurality of first crystal grains g1 in the piezoelectric thin film 3 is expressed as A1 vol %. The total volume ratio of the plurality of second crystal grains in the piezoelectric thin film 3 is expressed as A2 vol %. A2 / (A1+A2) may be 0.1% or more and 15.0% or less because the piezoelectric characteristics of the piezoelectric thin film 3 are likely to improve and the relative dielectric constant of the piezoelectric thin film 3 is likely to decrease. The volume ratio a1 of one first crystal grain g1 in the piezoelectric thin film 3 is calculated by multiplying the area a1' (unit: (nm)) of one first crystal grain g1 in the cross section of the piezoelectric thin film 3. ２The volume ratio a2 of one second crystal grain g2 in the piezoelectric thin film 3 tends to be equal to the area a2' (unit: (nm) ２ ) ratio. Therefore, one a2' / (a1'+a2') may be calculated from the area of ​​each pair of adjacent first and second crystal grains g1 and g2 in the cross section of the piezoelectric thin film 3, and the average value of a2' / (a1'+a2') of multiple pairs of first and second crystal grains g1 and g2 may be regarded as A2 / (A1+A2). The cross sections on which a1' and a2' are measured may be approximately or completely perpendicular to the first main surface s31 or the second main surface s32 of the piezoelectric thin film 3.

[0034] In the first crystal grains g1 and the second crystal grains g2, 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, so [Ed]1 / ([Ed]1+[Et]1) may be 0.40 or more and 0.58 or less, and [Ed]2 / ([Ed]2+[Et]2) may be 0.40 or more and 0.58 or less. For the same reasons, [Ed]1 / ([Ed]1+[Et]1) may be 0.35 or more and 0.58 or less, and [Ed]2 / ([Ed]2+[Et]2) may be 0.29 or more and 0.65 or less.

[0035] Because the piezoelectric properties of the piezoelectric thin film 3 are easily improved, [Ed]1 + [Etr]1 + [Et]1 may be 9.0 atomic % or more and less than 61.0 atomic %, and [Ed]2 + [Etr]2 + [Et]2 may be greater than 9.0 atomic % and less than 61.0 atomic %. That is, [Eadd]1 may be 9.0 atomic % or more and less than 61.0 atomic %, and [Eadd]2 may be greater than 9.0 atomic % and less than 61.0 atomic %. For the same reason, [Ed]1 + [Etr]1 + [Et]1 may be 9.1 atomic % or more and less than 59.4 atomic %, and [Ed]2 + [Etr]2 + [Et]2 may be 9.5 atomic % or more and less than 60.5 atomic %. That is, [Eadd]1 may be 9.1 atomic % or more and 59.4 atomic % or less, and [Eadd]2 may be 9.5 atomic % or more and 60.5 atomic % or less.

[0036] For the reason that the piezoelectric characteristics of the piezoelectric thin film 3 are easily improved, some or all of the multiple first crystal grains g1 may extend along a direction perpendicular to the first main surface s31 or the second main surface s32 of the piezoelectric thin film 3. For the same reason, at least some or all of the multiple second crystal grains g2 may extend along a direction perpendicular to the first main surface s31 or the second main surface s32 of the piezoelectric thin film 3. For the same reason, one or more first crystal grains g1 extending along a direction perpendicular to the first main surface s31 or the second main surface s32 of the piezoelectric thin film 3 and one or more second crystal grains g2 extending along a direction perpendicular to the first main surface s31 or the second main surface s32 of the piezoelectric thin film 3 may be alternately arranged in a direction parallel to the first main surface s31 or the second main surface s32. In other words, the difference in concentration of the additional element Eadd between the first crystal grains g1 and the second crystal grains g2 may be repeated in the direction parallel to the first main surface s31 or the second main surface s32. As a result, the piezoelectric characteristics of the piezoelectric thin film 3 are more likely to improve, and the relative permittivity and dielectric loss of the piezoelectric thin film 3 are more likely to decrease, compared to a piezoelectric thin film having a concentration difference of the added element Eadd in the thickness direction of the piezoelectric thin film.

[0037] R(Eadd) is defined by the following formula 1. R(Eadd) may be 1.01 or more and 1.49 or less because this tends to improve the piezoelectric characteristics of the piezoelectric thin film 3 and tends to reduce the relative permittivity and dielectric loss of the piezoelectric thin film 3. R(Eadd) greater than 1.00 means that the proportion of the additional element Eadd in the plurality of second crystal grains g2 is higher than the proportion of the additional element Eadd in the plurality of first crystal grains g1. R(Eadd) = [Eadd]2 / [Eadd]1 = ([Ed]2 + [Etr]2 + [Et]2) ÷ ([Ed]1 + [Etr]1 + [Et]1) (Formula 1)

[0038] R(Ed+Etr) is defined by the following formula 2. R(Ed+Etr) may be 1.005 or more and 2.262 or less because this tends to improve the piezoelectric characteristics of the piezoelectric thin film 3 and to reduce the relative permittivity and dielectric loss of the piezoelectric thin film 3. R(Ed+Etr) greater than 1.000 means that the proportion of the divalent element Ed and the trivalent element Etr in the plurality of second crystal grains g2 is higher than the proportion of the divalent element Ed and the trivalent element Etr in the plurality of first crystal grains g1. R(Ed+Etr)=([Ed]2+[Etr]2)÷([Ed]1+[Etr]1) (Formula 2)

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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 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.

[0045] 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.

[0046] 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.

[0047] In conventional RF magnetron sputtering for forming piezoelectric thin films, 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 the target. ＋ ) 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 the 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). In conventional RF magnetron sputtering, N ２ Since power supply to the target begins in a state where N is present in the deposition chamber, the inner wall of the deposition chamber is covered with nitride of atoms (at least one of Al and Eadd) derived from the target. In other words, in conventional RF magnetron sputtering, the piezoelectric thin film grows in a state where the inner wall of the deposition chamber is covered with nitride (insulator). In contrast, in the RF magnetron sputtering for forming the piezoelectric thin film according to this embodiment, N ２ Before supplying the gas into the deposition chamber, the deposition chamber was filled with a gas consisting of only Ar (substantially N ２ The deposition chamber is filled with only Ar, and power is supplied to the target. ２ The supply of power to the target is started in a state where Ar and N are not substantially present in the deposition chamber. As a result, the inner wall of the deposition chamber is covered with a conductor (metal) consisting of atoms derived from the target, rather than a nitride (insulator). After the inner wall of the deposition chamber is covered with the conductor, the deposition chamber is filled with Ar and N. ２The deposition chamber is filled with a mixture of Ar and N ２ By supplying power to the target in a state where the chamber is filled with the mixed gas, the piezoelectric thin film according to this embodiment grows on the surface of the substrate (the surface of the first electrode layer laminated on the substrate). That is, in RF magnetron sputtering according to this embodiment, the piezoelectric thin film grows in a state where the inner wall of the deposition chamber is covered with a conductor. Because the potential of the inner wall covered with a conductor is more stable than the potential of the inner wall covered with a nitride (insulator), the distribution of plasma (Ar ions) in the deposition chamber whose inner wall is covered with a conductor differs from the distribution of plasma in the deposition chamber whose inner wall is covered with a nitride. As a result, in the deposition chamber whose inner wall is covered with a conductor, random segregation of the additive element Eadd is suppressed, and the multiple seed crystals of the first crystal grains (crystals with a relatively low concentration of the additive element) and the multiple seed crystals of the second crystal grains (crystals with a relatively high concentration of the additive element) are likely to be uniformly dispersed on the surface of the substrate (the surface of the first electrode layer laminated on the substrate). As a result, a heterogeneous phase in which the additive element segregates is unlikely to form in the piezoelectric thin film, and multiple first crystal grains and multiple second crystal grains are likely to grow on the surface of the substrate (the surface of the first electrode layer laminated on the substrate). In contrast, the potential of an inner wall covered with a nitride is more unstable than the potential of an inner wall covered with a conductor. Therefore, in a deposition chamber whose inner walls are covered with a nitride, the distribution of Al and the additive element Eadd on the surface of the substrate (the surface of the first electrode layer laminated on the substrate) is likely to be nonuniform and random. As a result, the additive element is likely to segregate randomly on the surface of the substrate (the surface of the first electrode layer laminated on the substrate), and heterogeneous phases are likely to form randomly in the piezoelectric thin film. In other words, in a deposition chamber whose inner walls are covered with a nitride, it is difficult to form first crystal grains and second crystal grains with a controlled concentration of the additive element.

[0048] 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).

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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 3.72 W / cm ２ The temperature of the substrate during the formation of the adhesion layer was maintained at 300° C. The substrate bias was 30 W. The thickness of the adhesion layer was uniform. The thickness of the adhesion layer was adjusted to 30 nm.

[0053] 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 600° C. The thickness of the first electrode layer was uniform. The thickness of the first electrode layer was adjusted to 0.2 μm.

[0054] Before placing the substrate on which the first electrode layer was formed in a film-forming chamber for the piezoelectric thin film, a gas consisting of only Ar (substantially N ２A gas containing no Ar was supplied into the deposition chamber for the piezoelectric thin film. With the deposition chamber filled with only Ar, power was supplied to multiple targets for the piezoelectric thin film. The multiple targets used were a target made of Al (metal), a target made of the divalent element Ed (metal), and a target made of the tetravalent element Et (metal). In other words, by RF magnetron sputtering in Ar gas using multiple metal targets, the inner wall of the deposition chamber was covered with a metal film derived from the multiple targets. The Ed of Example 1 is shown in Table 1 below. The Et of Example 1 is shown in Table 1 below. The input power per unit area (cathode power) of each sputtering target was 3.72 W / cm ２ The atmospheric pressure in the deposition chamber was 0.2 Pa. No substrate bias was applied. The duration of RF magnetron sputtering in Ar gas was 10 minutes.

[0055] After the inner wall of the deposition chamber was covered with a metal film, a piezoelectric thin film was formed directly on the entire surface of the first electrode layer by RF magnetron sputtering in the deposition chamber using multiple targets. During the formation of the piezoelectric thin film, the atmosphere in the deposition 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 film formation pressure (air pressure inside the film formation chamber) was 0.2 Pa. The substrate bias was 20 W. The thickness of the piezoelectric thin film was adjusted to about 1000 nm.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] <Composition of Piezoelectric Thin Film> The overall average composition of the piezoelectric thin film was analyzed by X-ray fluorescence spectroscopy (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 the additional element Eadd (i.e., Ed and Et).

[0061] <Analysis of the Cross Section of the Piezoelectric Thin Film> The cross section of the piezoelectric thin film was observed using a scanning transmission electron microscope (STEM). The cross section observed using the STEM was perpendicular to the first principal surface (the surface of the first electrode layer) of the piezoelectric thin film. The cross-sectional composition of the piezoelectric thin film was analyzed using an energy dispersive X-ray spectrometer (EDS) attached to the STEM. A Titan G2 manufactured by Thermo Fisher Scientific Inc. (formerly FEI company) was used as the STEM. Five observation areas randomly selected within the same cross section of the piezoelectric thin film were analyzed. The distance from the center of the main surface (circle) of the substrate to each observation area was 10 mm. The concentration (unit: atomic %) of each element in each measurement area was mapped. Each measurement area contained multiple crystal grains with different concentrations of the added element Edd. The composition of each crystal grain was identified by area analysis within the cross section of each crystal grain. The area where the area analysis was performed was rectangular, and the dimensions of the area where the area analysis was performed were approximately 100 nm long x 100 nm wide. Of the multiple crystal grains contained in each observation area, one crystal grain (first crystal grain) with the smallest concentration of the added element Edd was identified, and of the multiple crystal grains contained in each observation area, one crystal grain (second crystal grain) with the largest concentration of the added element Edd was identified. The average value of the concentration of each element in the total of five first crystal grains identified in the total of five observation areas was calculated. The average value of the concentration of each element in the total of five second crystal grains identified in the total of five observation areas was calculated.

[0062] The results of the above analysis using STEM-EDS were as follows: The piezoelectric thin film included a plurality of first crystal grains and a plurality of second crystal grains. Each crystal grain was a columnar crystal extending substantially or completely perpendicular to the first main surface (surface of the first electrode layer) of the piezoelectric thin film. Each crystal grain was made of aluminum nitride containing the added element Eadd. [Eadd]2 was higher than [Eadd]1. Details of the results of the analysis using STEM-EDS are shown in Tables 1 and 2 below.

[0063] <Measurement of the area of ​​crystal grains> Five pairs of first and second crystal grains exposed in the cross section of the piezoelectric thin film were identified by the STEM-EDS. The first and second crystal grains were identified based on the contrast in the image of the cross section taken by the STEM and the concentration difference of the added element Eadd between the multiple crystal grains. The concentration difference of the added element Eadd was identified by mapping the added element Eadd in the cross section. The mapping of the added element Eadd was performed by STEM-EDS. Using the image of the cross section, the area a1' (unit: (nm)) of each first crystal grain was measured. ２ ), and the area a2' of each second crystal grain (unit: (nm) ２ ) was measured. Image analysis software (not for sale) manufactured by TDK Corporation was used to measure the area of ​​each crystal grain. Mac-View manufactured by Mountec Co., Ltd. may also be used as the image analysis software. a2' / (a1'+a2') was calculated from the area of ​​each pair of first and second crystal grains. The average value of a2' / (a1'+a2') for five pairs of first and second crystal grains was considered to be A2 / (A1+A2). A2 / (A1+A2) for Example 1 is shown in Table 2 below.

[0064] <Crystalline Structure of Piezoelectric Thin Film> The crystalline structure of the piezoelectric thin film was analyzed by the following X-ray diffraction (XRD) method. For the XRD method, a multipurpose X-ray diffractometer (SmartLab) manufactured by Rigaku Corporation was used. 2θ-θ scan, ω scan, and 2θχ-φ scan were performed on the surface of the piezoelectric thin film using the above X-ray diffractometer.

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

[0066] <Piezoelectric constant e ３１，ｆ Measurement of the piezoelectric constant e of the piezoelectric thin film of Example 1 ３１，ｆ (unit: C / m ２ ) was measured. ３１，ｆA device combining a laser Doppler vibrometer and an oscilloscope was used to measure the vibration intensity. The laser Doppler vibrometer used was the LV-1800 manufactured by Ono Sokki Co., Ltd. The oscilloscope used was the wavepro 960 manufactured by Teledyne Technologies Inc. (formerly Teledyne LeCroy Inc.). ３１，ｆ The frequency of the AC voltage in the measurement was 500 Hz. ３１，ｆ The voltage applied to the piezoelectric thin film during the measurement was 20 V / μm. ３１，ｆ The average values ​​of the three measurement points are shown in Table 2 below.

[0067] <Relative permittivity ε ｒ Measurement of relative dielectric constant ε of the piezoelectric thin film of Example 1 ｒ (unit: none) was measured. ｒ A measuring device (E4980A) manufactured by Agilent Technologies, Inc. was used to measure ε. ｒ In the measurement, an electric field of 1 V / μm was applied to the piezoelectric thin film. The area of ​​the first electrode layer and the second electrode layer to which the electric field was applied was 600 × 600 (μm). ２ The ε in Example 1 ｒ are shown in Table 2 below.

[0068] <Measurement of Dielectric Loss Tangent> The dielectric loss tangent (unit: %) of the piezoelectric thin film of Example 1 was measured. The above-mentioned E4980A was used to measure the dielectric loss tangent (tan δ). In measuring tan δ, 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 tan δ of Example 1 is shown in Table 2 below.

[0069] (Examples 2 to 20, Comparative Examples 1 and 2) A target made of Al, a target made of the divalent element Ed, and a target made of the tetravalent element Et were used as raw materials for the piezoelectric thin film in each of Examples 2, 3, 5 to 20, and Comparative Example 2. Ed and Et for each of Examples 2, 3, 5 to 20, and Comparative Example 2 are shown in Table 1 below. A target made of Al and a target made of the trivalent element Etr were used as raw materials for the piezoelectric thin film in Example 3. Etr in Example 3 is shown in Table 1 below. A target made of the divalent element Ed and a target made of the tetravalent element Et were not used as raw materials for the piezoelectric thin film in Example 3. A target containing the additional element Eadd was not used as a raw material for the piezoelectric thin film in Comparative Example 1.

[0070] The compositions of the piezoelectric thin films of Examples 5 to 10 and 13 to 16 were adjusted to be different from each other by changing the input power of each target. The compositions of the piezoelectric thin films of Examples 11, 12 and 17 to 20 were adjusted to be different from each other by changing the input power of each target.

[0071] Before forming the piezoelectric thin film in each of Comparative Examples 1 and 2, the inner wall of the deposition chamber was not covered with a metal film.

[0072] Except for the above-mentioned points, the piezoelectric thin film elements of Examples 2 to 20 and Comparative Examples 1 and 2 were fabricated in the same manner as in Example 1. Analysis and measurement were carried out on the piezoelectric thin film elements of Examples 2 to 20 and Comparative Examples 1 and 2 in the same manner as in Example 1. The results of the analysis and measurement of Examples 2 to 20 and Comparative Examples 1 and 2 are shown in Tables 1 and 2 below.

[0073] Except for the differences shown in Tables 1 and 2 below, the piezoelectric thin films of Examples 2 to 20 each had the same characteristics as Example 1.

[0074] As shown in Table 1 below, the composition of the first crystal grains in Comparative Example 1 was the same as the composition of the second crystal grains in Comparative Example 1. In other words, the composition of the multiple crystal grains included in the piezoelectric thin film of Comparative Example 1 was uniform.

[0075] As shown in Table 1 below, the second crystal grains of Comparative Example 2 contained an excess of the additional element Eadd, did not have a wurtzite structure, and were not aluminum nitride. That is, an excess of the additional element segregated in the second crystal grains of Comparative Example 2. In Comparative Example 2, the composition of the multiple first crystal grains made of aluminum containing the additional element Eadd was uniform.

[0076] An image of a cross section of the piezoelectric thin film of Example 15 is shown in FIG. 4(a). The image shown in FIG. 4(a) was taken by STEM. The distribution of magnesium in the cross section shown in FIG. 4(a) is shown in FIG. 4(b). FIG. 4(b) was obtained by mapping using STEM-EDS. The white areas in FIG. 4(b) are areas where magnesium is present.

[0077] In the following Table 1, r(Ed)1 means [Ed]1 / ([Ed]1+[Et]1). In the following Table 1, r(Ed)2 means [Ed]2 / ([Ed]2+[Et]2). In the following Table 2, R(Eadd) and R(Ed+Etr) are defined in the above embodiment.

[0078]

[0079]

[0080] 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.

[0081] 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 aluminum nitride (wurtzite structure), Ed...divalent element, Etr...trivalent element, Et...tetravalent element, g1...first crystal grain, g2...second crystal grain.

Claims

1. A piezoelectric thin film comprising a plurality of crystal grains containing aluminum nitride, wherein the aluminum nitride contains an additive element, the plurality of crystal grains comprising a plurality of first crystal grains and a plurality of second crystal grains, wherein a ratio of the number of the additive element in the first crystal grains to a total number of aluminum and the additive element in the first crystal grains is expressed as [Eadd]1 atomic %, and a ratio of the number of the additive element in the second crystal grains to a total number of aluminum and the additive element in the second crystal grains is expressed as [Eadd]2 atomic %, and [Eadd]2 is higher than [Eadd]1.

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 added elements are only divalent elements and tetravalent elements, or only trivalent elements, or the divalent elements, trivalent elements, and tetravalent elements, and the ratio of the number of divalent elements in the first crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Ed] 1 atomic %, the ratio of the number of trivalent elements in the first crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Etr] 1 atomic %, the ratio of the number of divalent elements in the second crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the second crystal grains is expressed as [Ed] 2 atomic %, and the ratio of the number of trivalent elements in the second crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the second crystal grains is expressed as [Etr] 2 atomic %, The piezoelectric thin film according to claim 1 , wherein ([Ed] 2 + [Etr] 2 ) / ([Ed] 1 + [Etr] 1 ) is greater than 1.000 and equal to or less than 3.

000.

7. The piezoelectric thin film according to claim 1, wherein the total volume ratio of the plurality of first crystal grains in the piezoelectric thin film is expressed as A1 volume %, the total volume ratio of the plurality of second crystal grains in the piezoelectric thin film is expressed as A2 volume %, and A2 / (A1+A2) is 0.1% or more and 15.0% or less.

8. The added element includes at least a divalent element and a tetravalent element, wherein the ratio of the number of divalent elements in the first crystal grains to the total number of aluminum, divalent elements, trivalent elements, and tetravalent elements in the first crystal grains is expressed as [Ed]1 atomic %, the ratio of the number of tetravalent elements in the first crystal grains to the total number of aluminum, divalent elements, trivalent elements, and tetravalent elements in the first crystal grains is expressed as [Et]1 atomic %, the ratio of the number of divalent elements in the second crystal grains to the total number of aluminum, divalent elements, trivalent elements, and tetravalent elements in the second crystal grains is expressed as [Ed]2 atomic %, the ratio of the number of tetravalent elements in the second crystal grains to the total number of aluminum, divalent elements, trivalent elements, and tetravalent elements in the second crystal grains is expressed as [Et]2 atomic %, and [Ed]1 / ([Ed]1+[Et]1) is 0.40 or more and 0.58 or less, The piezoelectric thin film according to claim 1 , wherein [Ed] 2 / ([Ed] 2 + [Et] 2 ) is 0.40 or more and 0.58 or less.

9. The added elements are only divalent elements and tetravalent elements, or only trivalent elements, or the divalent elements, trivalent elements, and tetravalent elements; the ratio of the number of divalent elements in the first crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Ed] 1 atomic %; the ratio of the number of trivalent elements in the first crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Etr] 1 atomic %; the ratio of the number of tetravalent elements in the first crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the first crystal grains is expressed as [Et] 1 atomic %; the ratio of the number of divalent elements in the second crystal grains to the total number of aluminum, the divalent elements, the trivalent elements, and the tetravalent elements in the second crystal grains is expressed as [Ed] 2 atomic %; 2. The piezoelectric thin film according to claim 1, wherein a ratio of the number of trivalent elements in the second crystal grains to a total number of aluminum, divalent elements, trivalent elements, and tetravalent elements in the second crystal grains is expressed as [Etr]2 atomic %, a ratio of the number of tetravalent elements in the second crystal grains to a total number of aluminum, divalent elements, trivalent elements, and tetravalent elements in the second crystal grains is expressed as [Et]2 atomic %, [Ed]1 + [Etr]1 + [Et]1 is 9.0 atomic % or more and less than 61.0 atomic %, and [Ed]2 + [Etr]2 + [Et]2 is more than 9.0 atomic % and 61.0 atomic % or less.

10. The piezoelectric thin film according to claim 1, wherein the first crystal grains extend in a direction perpendicular to the main surface of the piezoelectric thin film, and the second crystal grains extend in a direction perpendicular to the main surface of the piezoelectric thin film.

11. A piezoelectric thin film element comprising: a piezoelectric thin film according to any one of claims 1 to 10; and an electrode layer, wherein the piezoelectric thin film directly or indirectly overlaps a surface of the electrode layer.

Images