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 the main surface of the piezoelectric thin film. The length of the c-axis of the wurtzite structure is 0.500 nm or more.

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 a piezoelectric thin film made of AlN in which the c-axis length cL is less than 0.498 nm and the ratio cL / aL of the a-axis length aL to the c-axis length cL is less than 1.6.

[0005] Patent No. 5815329

[0006] It has been known that when the c-axis of AlN in a piezoelectric thin film is perpendicular to the main surface of the piezoelectric thin film, a decrease in cL / aL due to a decrease in cL improves the piezoelectric properties of the piezoelectric thin film. For example, tensile stress acts on the AlN crystal lattice in a direction parallel to the main surface of the piezoelectric thin film (perpendicular to the c-axis), causing the AlN to contract along the c-axis and decreasing cL. However, tensile stress limits the expansion and contraction of the crystal lattice in the c-axis direction. Therefore, even if cL and cL / aL decrease, the piezoelectric properties of the piezoelectric thin film do not improve as much as expected. Furthermore, when the AlN crystal lattice is distorted by tensile stress, defects such as cracks are likely to form in the piezoelectric thin film due to the expansion and contraction of the piezoelectric thin film caused by thermal shock (repeated rapid temperature changes of the piezoelectric thin film), which can easily degrade the piezoelectric properties of the piezoelectric thin film.

[0007] An object of one aspect of the present disclosure is to provide a piezoelectric thin film that has excellent piezoelectric properties and that suppresses deterioration of the piezoelectric properties due to thermal shock, and a piezoelectric thin film element that includes the piezoelectric thin film.

[0008] For example, one aspect of the present disclosure relates to a piezoelectric thin film according to any one of the following items [1] to [9], and a piezoelectric thin film element according to the following item

[10] .

[0009] [1] A piezoelectric thin film including aluminum nitride having a wurtzite structure, wherein the aluminum nitride includes 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, and a length of a c-axis of the wurtzite structure is 0.500 nm or more.

[0010] [2] The piezoelectric thin film according to [1], wherein the length of the a-axis of the wurtzite structure is 0.313 nm or more.

[0011] [3] The piezoelectric thin film according to [1] or [2], comprising: a plurality of first crystal grains containing the aluminum nitride; and a plurality of second crystal grains containing the aluminum nitride, wherein a length of the c-axis of the wurtzite structure in the plurality of first crystal grains is represented by cL1, and a length of the c-axis of the wurtzite structure in the plurality of second crystal grains is represented by cL2, and cL1 is longer than cL2.

[0012] [4] 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 comprising: a plurality of first crystal grains comprising the aluminum nitride; and a plurality of second crystal grains comprising the aluminum nitride, wherein a length of a c-axis of the wurtzite structure in the plurality of first crystal grains is represented by cL1, and a length of a c-axis of the wurtzite structure in the plurality of second crystal grains is represented by cL2, and cL1 is longer than cL2.

[0013] [5] The piezoelectric thin film according to [3] or [4], wherein cL1 / cL2 is 1.001 or more and 1.060 or less.

[0014] [6] The piezoelectric thin film according to any one of [1] to [5], wherein the additive element includes at least a divalent element and a tetravalent element.

[0015] [7] The piezoelectric thin film according to [6], 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.

[0016] [8] The piezoelectric thin film according to any one of [1] to [7], wherein the additive element includes at least a divalent element and a tetravalent element, wherein the ratio of the number of the divalent elements in the aluminum nitride to the total number of aluminum, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Ed] atomic %, wherein the ratio of the number of the tetravalent elements in the aluminum nitride to the total number of aluminum, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Et] atomic %, and wherein [Ed] / ([Ed]+[Et]) is 0.35 or more and 0.55 or less.

[0017] [9] The piezoelectric thin film according to any one of [1] to [8], wherein the additive element includes at least a divalent element and a tetravalent element, wherein a ratio of the number of aluminum elements in the aluminum nitride to the total number of aluminum elements, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Al] atomic %, wherein a ratio of the number of divalent elements in the aluminum nitride to the total number of aluminum elements, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Ed] atomic %, wherein a ratio of the number of tetravalent elements in the aluminum nitride to the total number of aluminum elements, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Et] atomic %, and wherein ([Ed] + [Et]) / ([Al] + [Ed] + [Et]) is 0.10 or more and 0.55 or less.

[0018]

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

[0019] According to one aspect of the present disclosure, there are provided a piezoelectric thin film that has excellent piezoelectric characteristics and that suppresses deterioration of the piezoelectric characteristics due to thermal shock, and a piezoelectric thin film element that includes the piezoelectric thin film.

[0020] 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 a perspective view of a unit cell showing each primitive translational vector of aluminum nitride and the (0001) plane, Fig. 4(b) is a perspective view of a unit cell showing each primitive translational vector of aluminum nitride and the (0002) plane, and Fig. 4(c) is a perspective view of a unit cell showing each primitive translational vector of aluminum nitride and the (10-10) plane. 5A is a schematic diagram showing one specific example of the arrangement of aluminum and a plurality of additive elements in aluminum nitride, and FIG. 5B is a schematic diagram showing another specific example of the arrangement of aluminum and a plurality of additive elements in aluminum nitride. FIG. 6 is an electron beam diffraction pattern measured by irradiating an electron beam onto a cross section of a piezoelectric thin film according to one specific example (Example 13) of the present invention, and the cross section on which the electron beam diffraction pattern shown in FIG. 6 is measured is perpendicular to the first main surface of the piezoelectric thin film.

[0021] Preferred embodiments of the present invention will now be described with reference to the drawings. In the drawings, like components are designated by like reference numerals. The present invention is not limited to the following embodiments. X, Y, and Z shown in Figures 1 and 2 represent three coordinate axes that are orthogonal to one another. The directions of the X-axis, Y-axis, and Z-axis are common to Figures 1 and 2.

[0022] 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.

[0023] The piezoelectric thin film 3 includes aluminum nitride. The aluminum nitride includes at least one additive element. Doping the aluminum nitride with the additive element distorts the wurtzite structure of the aluminum nitride and changes the strength of the chemical bonds between atoms in the wurtzite structure. As a result, the wurtzite structure of the aluminum nitride is easily deformed when a voltage or external force is applied to the piezoelectric thin film 3, and the piezoelectric properties of the piezoelectric thin film 3 are easily improved. For example, the piezoelectric thin film 3 may be made only of AlN containing the additive element. In other words, the piezoelectric thin film 3 may be made only of Al, N, and the additive element. As described below, the piezoelectric thin film 3 may further include other elements in addition to Al, N, and the additive element.

[0024] The additive elements may include at least a divalent element Ed and a tetravalent element Et because this facilitates improving the piezoelectric properties of the piezoelectric thin film 3. The divalent element Ed may be rephrased as at least one of a Group 2 element and a Group 12 element. The tetravalent element Et may be rephrased as a Group 4 element. For example, the piezoelectric thin film 3 may be composed only of AlN containing the divalent element Ed and the tetravalent element Et. In other words, the piezoelectric thin film 3 may be composed only of Al, N, the divalent element Ed, and the tetravalent element Et. As described below, the piezoelectric thin film 3 may further include other elements in addition to Al, N, the divalent element Ed, and the tetravalent element Et.

[0025] The divalent element Ed may be at least one element selected from the group consisting of magnesium (Mg), calcium (Ca), zinc (Zn), strontium (Sr), and barium (Ba). The tetravalent element Et may be at least one element selected from the group consisting of zirconium (Zr), germanium (Ge), titanium (Ti), and hafnium (Hf). At least a portion of the divalent elements Ed may be magnesium, and at least a portion of the tetravalent elements Et may be at least one of zirconium and hafnium, since this facilitates improvement in the piezoelectric properties of the piezoelectric thin film 3.

[0026] 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, a trivalent 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 trivalent element may be at least one element selected from the group consisting of scandium (Sc), yttrium (Y), lanthanides, and indium (In). 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).

[0027] The aluminum nitride contained in the piezoelectric thin film 3 is crystalline and has a hexagonal wurtzite structure. For example, the aluminum nitride contained in the piezoelectric thin film 3 may be single crystal, polycrystalline, or imperfectly crystalline. FIG. 3 shows a unit cell uc (unit lattice) of the wurtzite structure of aluminum nitride. The unit cell uc of the wurtzite structure is a hexagonal prism. Some of the aluminum (Al) in the unit cell uc may be substituted with an additional element (for example, a divalent element Ed or a tetravalent element Et).

[0028] The unit cell uc shown in each of Figures 4(a), 4(b), and 4(c) is the same as the unit cell uc shown in Figure 3. Aluminum or an additional element may be placed at each of the 12 vertices of the unit cell uc (hexagonal column) shown in each of Figures 4(a), 4(b), and 4(c). However, the a of the wurtzite structure １ axis, a ２ axis, a ３ In order to show the a-axis, c-axis and each lattice plane, Al and additional elements are omitted in (a) of FIG. 4, (b) of FIG. 4 and (c) of FIG. 4. １ axis, a ２ axis, a ３The a and c axes are fundamental translation vectors that constitute the unit cell uc of the wurtzite structure. １ The axis orientation is [2-1-10]. ２ The axis orientation is [-12-10]. ３ The orientation of the axis is [-1-120]. The orientation of the c axis is

[0001] . １ axis, a ２ axis and a ３ The lengths of the axes may be approximately or exactly equal to each other. １ axis, a ２ axis and a ３ Any of the axes may be nearly or completely perpendicular to the c-axis. １ axis, a ２ axis and a ３ The angle between the axes may be approximately or completely 120°. FIG. 4(a) shows the (0001) plane of aluminum nitride. FIG. 4(b) shows the (0002) plane of aluminum nitride. FIG. 4(c) shows the (10-10) plane of aluminum nitride. The (0001) plane is approximately or completely parallel to the (0002) plane. The (10-10) plane is approximately or completely perpendicular to the (0001) plane and the (0002) plane. In the present disclosure, the length of the c-axis of the wurtzite structure is represented as cL. cL may be rephrased as the spacing between the (0001) planes. In the present disclosure, the length of the a-axis of the wurtzite structure is represented as aL. aL is the sum of the a １ axis, a ２ Axis or a ３ This can be rephrased as the length of the axis. １ axis, a ２ axis, and a ３ This may be rephrased as the average length of the axis.

[0029] 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 substantially 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, by orienting at least a portion of the (0001) plane of the aluminum nitride in the piezoelectric thin film 3 in the normal direction to the first principal 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 the aluminum nitride is likely to be oriented parallel to the first principal surface s31 (surface s1 of the first electrode layer 1) of the piezoelectric thin film 3.

[0030] The length cL of the c-axis of the wurtzite structure may be 0.500 nm or more. According to the present invention, in the process of forming the piezoelectric thin film 3, compressive stress, rather than tensile stress, is likely to act on the piezoelectric thin film 3 in a direction parallel to the first main surface s31 of the piezoelectric thin film 3 (a direction perpendicular to the c-axis). As the piezoelectric thin film 3 is compressed in a direction perpendicular to the c-axis, the wurtzite structure is likely to elongate along the c-axis, and the length cL of the c-axis is likely to become longer than the length of the c-axis of the wurtzite structure in conventional piezoelectric thin films. Because the piezoelectric thin film 3 is compressed in a direction parallel to the first main surface s31 to such an extent that the length cL of the c-axis is 0.500 nm or more, cracks extending in the direction perpendicular to the first main surface s31 (the c-axis direction) are less likely to form in the piezoelectric thin film 3. As a result, cracks due to expansion and contraction of the piezoelectric thin film 3 caused by thermal shock are less likely to form in the piezoelectric thin film. Therefore, deterioration of the piezoelectric properties of the piezoelectric thin film 3 due to thermal shock is suppressed. Furthermore, since the AlN in the piezoelectric thin film 3 contains an additive element, the length aL of the a-axis of the wurtzite structure is likely to increase. Therefore, even if cL is 0.500 nm or more, an increase in cL / aL is suppressed, and the piezoelectric thin film 3 can have excellent piezoelectric properties. If the length cL of the c-axis increases due to lattice matching between the piezoelectric thin film 3 and the layer below the piezoelectric thin film 3 (first electrode layer 1), AlN is likely to contract in the a-axis direction, and the length aL of the a-axis is likely to decrease. As a result, cL / aL is likely to increase, making it difficult to obtain sufficient piezoelectric properties. For example, the piezoelectric thin film 3 has a large e as a good piezoelectric property. ３１，ｆ e ３１，ｆ is 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 length cL of the c-axis of the wurtzite structure may be 0.500 nm or more and 0.534 nm or less, because this facilitates improvement of the piezoelectric characteristics and inhibits deterioration of the piezoelectric characteristics of the piezoelectric thin film 3 due to thermal shock.

[0031] The length aL of the a-axis of the wurtzite structure may be 0.313 nm or more because this facilitates improvement of the piezoelectric characteristics and inhibits deterioration of the piezoelectric characteristics of the piezoelectric thin film 3 due to thermal shock. For the same reason, the length aL of the a-axis of the wurtzite structure may be 0.313 nm or more and 0.338 nm or less.

[0032] cL / aL may be 1.53 or more and 1.60 or less, because the piezoelectric characteristics are likely to be improved and the deterioration of the piezoelectric characteristics of the piezoelectric thin film 3 due to thermal shock is likely to be suppressed.

[0033] The spacing of the (0002) planes of the wurtzite structure may be determined from an X-ray diffraction (XRD) pattern measured by out-of-plane diffraction on the first principal surface s31 or the second principal surface s32 of the piezoelectric thin film 3. Since the spacing of the (0002) planes is equal to half the spacing of the (0001) planes, twice the spacing of the (0002) planes may be considered to be the length cL of the c-axis of the wurtzite structure. The spacing of the (10-10) planes of the wurtzite structure may be determined from an XRD pattern measured by in-plane diffraction on the first principal surface s31 or the second principal surface s32 of the piezoelectric thin film 3. The length aL of the a-axis of the wurtzite structure may be calculated from the spacing of the (10-10) planes. For example, the length aL of the a-axis of the wurtzite structure may be 4 / 3 of the spacing of the (10-10) planes. －１／２ It may be double.

[0034] As shown in FIG. 2 , the piezoelectric thin film 3 may include a plurality of first crystal grains g1 containing aluminum nitride and a plurality of second crystal grains g2 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. Some or all of the plurality of first crystal grains g1 may be columnar crystals extending in a direction perpendicular to the first principal surface s31 or the second principal surface s32 of the piezoelectric thin film 3. At least some or all of the plurality of second crystal grains g2 may also be columnar crystals extending in a direction perpendicular to the first principal surface s31 or the second principal surface s32 of the piezoelectric thin film 3. One or more first crystal grains g1 and one or more second crystal grains g2 may be arranged alternately in a direction parallel to the first principal surface s31 or the second principal surface s32.

[0035] The c-axis length of the wurtzite structure in the plurality of first crystal grains g1 is denoted by cL1. For example, cL1 may be the average value of the c-axis lengths of the wurtzite structure in the plurality of first crystal grains g1. The c-axis length of the wurtzite structure in the plurality of second crystal grains g2 is denoted by cL2. For example, cL2 may be the average value of the c-axis lengths of the wurtzite structure in the plurality of second crystal grains g2. cL1 may be longer than cL2. That is, the first crystal grains g1 and the second crystal grains g2 may have different c-axis lengths. In a piezoelectric thin film 3 including first crystal grains g1 and second crystal grains g2 having different c-axis lengths, the wurtzite structure is prone to local distortion due to local changes in the concentration of the additive element in AlN. For example, the concentration of the additive element is prone to change within the plurality of unit cells uc constituting AlN, and each unit cell uc is prone to distortion. As a result, the wurtzite structure is easily deformed when a voltage or external force is applied to the piezoelectric thin film 3, which easily improves the piezoelectric characteristics of the piezoelectric thin film 3. Furthermore, because the wurtzite structure is easily distorted, stress caused by the expansion and contraction of the piezoelectric thin film 3 due to thermal shock is easily alleviated within the piezoelectric thin film 3. As a result, defects caused by thermal shock are less likely to be formed in the piezoelectric thin film 3, which suppresses deterioration of the piezoelectric characteristics of the piezoelectric thin film 3 due to thermal shock.

[0036] cL1 / cL2 may be 1.002 or more and 1.061 or less, or 1.001 or more and 1.060 or less, because the piezoelectric characteristics are likely to be improved and the deterioration of the piezoelectric characteristics of the piezoelectric thin film 3 due to thermal shock is likely to be suppressed.

[0037] cL1 may be 0.503 nm or more and 0.539 nm or less because this facilitates improving the piezoelectric characteristics and inhibiting deterioration of the piezoelectric characteristics of the piezoelectric thin film 3 due to thermal shock. For the same reason, cL2 may be 0.500 nm or more and 0.533 nm or less.

[0038] The plurality of first crystal grains g1 and the plurality of second crystal grains g2, which differ from each other in the lengths of their c-axes, may be identified by an electron diffraction pattern. The electron diffraction pattern may be measured by irradiating an electron beam onto a cross section of the piezoelectric thin film 3. The cross section onto which the electron beam is incident may be perpendicular to the first principal surface s31 or the second principal surface s32 of the piezoelectric thin film 3. When the c-axes of the wurtzite structure in the piezoelectric thin film 3 have a uniform length, the electron diffraction pattern includes one diffraction spot, which is a spot of the electron beam diffracted by the (0002) plane of the wurtzite structure. On the other hand, when the piezoelectric thin film 3 includes a plurality of first crystal grains g1 and a plurality of second crystal grains g2, the electron diffraction pattern includes two spots, which are different in position, which are spots of the electron beam diffracted by the (0002) plane of the wurtzite structure. From one of the two spots, it is possible to identify the cL1 of the multiple first crystal grains g1 (the spacing between the (0002) planes of the multiple first crystal grains g1), and from the other of the two spots, it is possible to identify the cL2 of the multiple second crystal grains g2 (the spacing between the (0002) planes of the multiple second crystal grains g2).

[0039] In the present disclosure, any element is represented as "X," and the concentration of element X in aluminum nitride (unit: atomic %) is represented as <X>. For example, the concentrations of Al, N, divalent element Ed, and tetravalent element Et in aluminum nitride are represented as <Al>, <N>, <Ed>, and <Et>, respectively. The ratio of the number of aluminum atoms in aluminum nitride to the total number of aluminum atoms, divalent element Ed, and tetravalent element Et in aluminum nitride is represented as [Al] atomic %. [Al] may be equal to 100 × <Al> / (<Al> + <Ed> + <Et>). The ratio of the number of divalent element Ed in aluminum nitride to the total number of aluminum atoms, divalent element Ed, and tetravalent element Et in aluminum nitride is represented as [Ed] atomic %. [Ed] may be equal to 100 × <Ed> / (<Al> + <Ed> + <Et>). The ratio of the number of tetravalent elements Et in aluminum nitride to the total number of aluminum, divalent elements Ed, and tetravalent elements Et in aluminum nitride is expressed as [Et] atomic %. [Et] may be equal to 100 × <Et> / (<Al> + <Ed> + <Et>). Because this facilitates improvement of piezoelectric properties and inhibits deterioration of the piezoelectric properties of the piezoelectric thin film 3 due to thermal shock, [Ed] / ([Ed] + [Et]) may be 0.33 to 0.59 or 0.35 to 0.55. For similar reasons, ([Ed] + [Et]) / ([Al] + [Ed] + [Et]) may be 0.07 to 0.61 or 0.10 to 0.55.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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. 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 raw material gas in the atmosphere, the duration of sputtering of each target, the temperature of the substrate surface, and the substrate bias may also be controlling factors for the composition and thickness of each of the adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer. A piezoelectric thin film having a desired shape or pattern may be formed by etching (e.g., plasma etching).

[0048] A specific example of a method for forming a piezoelectric thin film in which the (0001) plane of the wurtzite structure is parallel to the main surface of the piezoelectric thin film and the length of the c-axis of the wurtzite structure is 0.500 nm or more may be as follows.

[0049] While the substrate is heated at a film-forming temperature T1 (e.g., 200°C), 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. Because the film-forming temperature T1 is relatively low, the crystallinity of the adhesion layer made of AlN is intentionally suppressed.

[0050] After the adhesion layer is formed, it may be annealed in a vacuum at an annealing temperature (e.g., 600°C) higher than the film formation temperature T1. Because the annealing temperature is higher than the film formation temperature T1, the growth of crystal grains in the adhesion layer is promoted. Because the portion of the adhesion layer in contact with the surface of the substrate is constrained by the crystalline structure of Si, the crystallinity of AlN in the portion of the adhesion layer in contact with the main surface of the substrate is unlikely to change during annealing. In contrast, the crystallization of AlN near the exposed outer surface of the adhesion layer (i.e., the portion not in contact with the main surface of the substrate) is likely to be promoted by annealing.

[0051] After annealing the adhesion layer, the substrate on which the adhesion layer is laminated may be cooled to room temperature. As the adhesion layer cools, tensile stress due to the growth of AlN crystal grains acts near the outer surface of the adhesion layer. The tensile stress is a stress that expands the adhesion layer in a direction parallel to the major surface of the substrate. In contrast, as the adhesion layer cools, compressive stress due to lattice mismatch between the substrate and the adhesion layer acts on the portion of the adhesion layer that contacts the major surface of the substrate. The compressive stress is a stress that compresses the adhesion layer in a direction parallel to the major surface of the substrate.

[0052] After the adhesion layer is cooled, the substrate on which the adhesion layer is laminated may be heated at a film formation temperature T2 (e.g., 600°C) to form a first metal layer made of Mo on the outer surface (main surface) of the adhesion layer. In a state in which the outer surface (main surface) of the adhesion layer is expanded by heating at the film formation temperature T2, the first metal layer is formed on the outer surface (main surface) of the adhesion layer.

[0053] After the first metal layer is formed, the substrate on which the first metal layer is laminated may be cooled to room temperature. As the substrate is cooled, the adhesion layer (particularly the portion in contact with the main surface of the substrate) shrinks, and compressive stress due to the shrinkage of the adhesion layer acts on the first electrode layer.

[0054] After the first metal layer is cooled, the piezoelectric thin film may be formed on the surface (main surface) of the first metal layer while the substrate on which the first metal layer is laminated is heated at a film formation temperature T3 (e.g., 250° C.). After the piezoelectric thin film is formed, the substrate on which the piezoelectric thin film is laminated may be cooled to room temperature.

[0055] As described above, during the process of laminating the adhesion layer, the first electrode layer, and the piezoelectric thin film, compressive stress is likely to act on the piezoelectric thin film due to repeated heating and cooling of the substrate (thermal expansion and contraction of each layer). As a result, the wurtzite structure is likely to elongate along the c-axis, and the c-axis length cL is likely to be 0.500 nm or more. Furthermore, the additive elements contained in AlN in the piezoelectric thin film are likely to increase the a-axis length aL of the wurtzite structure.

[0056] 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 stacked on the substrate). As a result, a piezoelectric thin film grows on the surface of the substrate (the surface of the first electrode layer stacked on the substrate). When a piezoelectric thin film including a plurality of first crystal grains g1 and a plurality of second crystal grains g2 that differ from each other in c-axis length is formed, the power supplied to each target (input power per unit area of ​​each target) during RF magnetron sputtering (growth of the piezoelectric thin film) changes. That is, for example, when the piezoelectric thin film is made of AlN containing a divalent element Ed and a tetravalent element Et, a cycle consisting of the following steps 1 to 3 is repeated during RF magnetron sputtering. Step 1 is a step in which the power supplied to one target (first target) of the Al target, the Ed target, and the Et target is made greater than the power supplied to each of the other two targets. In step 1, atoms derived from the first target are more likely to reach the surface of the substrate (the surface of the first electrode layer stacked on the substrate) than atoms derived from the other two targets. Step 2 follows step 1. Step 2 is a step of increasing the power supplied to one target (second target) other than the first target among the target made of Al, the target made of Ed, and the target made of Et, compared to the power supplied to each of the other two targets. In step 2, atoms derived from the second target are more likely to reach the surface of the substrate (the surface of the first electrode layer stacked on the substrate) than atoms derived from the other two targets. Step 3 follows step 2. Step 3 is a step of increasing the power supplied to one target (third target) other than the first and second targets among the target made of Al, the target made of Ed, and the target made of Et, compared to the power supplied to each of the other two targets. In step 3, atoms derived from the third target are more likely to reach the surface of the substrate (the surface of the first electrode layer stacked on the substrate) than atoms derived from the other two targets.As shown in (a) of FIG. 5 , the repetition of the cycle consisting of steps 1 to 3 tends to cause the concentrations of Al and the additive elements (Ed and Et) to change locally within the AlN crystal lattice. In other words, as shown in (a) of FIG. 5 , the repetition of the cycle consisting of steps 1 to 3 tends to cause Al and the additive elements (Ed and Et) to be irregularly arranged within the crystal lattice. As a result, the AlN crystal lattice tends to be distorted, the c-axis length tends to change depending on the concentration of the additive element, and multiple first crystal grains g1 and multiple second crystal grains g2 with different c-axis lengths tend to grow. In contrast to the repetition of the above cycle, when the power supplied to each target is constant during the growth of the piezoelectric thin film, as shown in (b) of FIG. 5 , the Al and the additive elements (Ed and Et) tend to be regularly and periodically arranged within the crystal lattice. As a result, the AlN crystal lattice tends to be less distorted, and multiple crystal grains with uniform c-axis lengths tend to grow.

[0057] 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).

[0058] 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.

[0059] 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.

[0060] 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.

[0061] An adhesion layer was formed directly on the entire surface of Si(100) 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 deposition 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 deposition rate was controlled at 0.09 nm / sec. The thickness of the adhesion layer was adjusted to approximately 30 nm. Before forming the first electrode layer, the substrate on which the adhesion layer was formed was annealed in vacuum at 600°C for 20 minutes, and after annealing, it was cooled to room temperature.

[0062] 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. Before the formation of the piezoelectric thin film, the substrate on which the first electrode layer was formed was cooled to room temperature.

[0063] 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 200°C. The film formation pressure (air pressure inside the vacuum chamber) was 1 Pa. The substrate bias was 32 W. The film formation rate was controlled to 0.35 nm / sec. The thickness of the piezoelectric thin film was adjusted to about 1000 nm.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] <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). The details of the composition of the piezoelectric thin film of Example 1 are shown in Table 1 below.

[0069] <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 principal surface) of the piezoelectric thin film using the X-ray diffractometer. The 2θ-θ scan was a measurement in the out-of-plane direction of the second principal surface of the piezoelectric thin film, and the 2θχ-φ scan was a measurement in the in-plane direction of the second principal surface of the piezoelectric thin film.

[0070] The XRD pattern obtained by XRD showed that the piezoelectric thin film (i.e., aluminum nitride containing an additive element) had a wurtzite structure. Furthermore, the XRD pattern showed that the (0002) plane (and the (0001) plane) of the wurtzite structure were parallel to the surfaces of the first electrode layer (the first and second principal surfaces of the piezoelectric thin film). The XRD pattern obtained by 2θ-θ scanning included a diffracted X-ray peak originating from the (0002) plane. The c-axis length cL of the wurtzite structure was determined from the diffracted X-ray peak originating from the (0002) plane. The XRD pattern obtained by 2θχ-φ scanning included a diffracted X-ray peak originating from the (10-10) plane. The a-axis length aL of the wurtzite structure was determined from the diffracted X-ray peak originating from the (10-10) plane. The cL, aL, and cL / aL of Example 1 are shown in Table 1 below.

[0071] <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. A Titan G2 manufactured by Thermo Fisher Scientific Inc. (formerly FEI company) was used as the STEM. The piezoelectric thin film contained multiple crystal grains (columnar crystals) extending in a direction perpendicular to the first principal surface of the piezoelectric thin film. Electron diffraction patterns were measured at three measurement points randomly selected from the cross section observed using the STEM. In all three electron diffraction patterns, only one electron beam spot originated from the (0002) plane. In other words, the c-axis lengths of the wurtzite structures in the multiple crystal grains contained in the piezoelectric thin film of Example 1 were uniform.

[0072] <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，ｆ A 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 (average value of three measurement points) is shown in Table 1 below. ３１，ｆ is the e described later. ＢＥＦＯＲＥ is equivalent to

[0073] <Thermal Shock Test> The following thermal shock test was carried out. Three piezoelectric thin film elements were placed in a thermostatic chamber. The thermostatic chamber had a first thermostatic chamber and a second thermostatic chamber arranged above and below. The first thermostatic chamber and the second thermostatic chamber were connected via an elevator, and the piezoelectric thin film elements could be moved back and forth between the first and second thermostatic chambers by the elevator. The temperature of the first thermostatic chamber was kept at -40°C, and the temperature of the second thermostatic chamber was kept at 125°C. A cycle consisting of the following step A and step B following step A was repeated 700 times. Step A was a step in which the piezoelectric thin film elements were kept in the first thermostatic chamber for 30 minutes. Step B was a step in which the piezoelectric thin film elements were kept in the second thermostatic chamber for 30 minutes.

[0074] Before the thermal shock test, the e of the three piezoelectric thin film elements ３１，ｆ The average value of e ＢＥＦＯＲＥ After the above cycle was repeated 700 times, the e of the three piezoelectric thin film elements was measured. ３１，ｆ The average value of e ＡＦＴＥＲ was measured. ＢＥＦＯＲＥ and e ＡＦＴＥＲ From the above, e defined by the following formula 1 ３１，ｆ The rate of change Δe (unit: %) of the piezoelectric constant was calculated. The smaller the absolute value of Δe, the more the deterioration of the piezoelectric characteristics due to the thermal shock is suppressed. In other words, it is preferable that the absolute value of Δe is small. Δe in Example 1 is shown in Table 1 below. Δe=100×(e ＡＦＴＥＲ -e ＢＥＦＯＲＥ ) / e ＢＥＦＯＲＥ (Equation 1)

[0075] (Examples 2 to 18, Comparative Examples 1 and 2) Targets consisting of the divalent element Ed shown in Table 1 below and targets consisting of the tetravalent element Et shown in Table 1 below were used as raw materials for the piezoelectric thin film in each of Examples 2 to 18 and Comparative Example 2. As raw materials for the piezoelectric thin film in Comparative Example 1, a target consisting of Ed and no target consisting of Ed were used.

[0076] The piezoelectric thin film of Example 4 was formed by RF magnetron sputtering, in which a cycle consisting of the following steps 1 to 3 was repeated: In step 1, the input power per unit area of ​​the Al target (cathode power) was 3.72 W / cm ２ In step 1, the input power per unit area of ​​each of the other two targets was maintained at 0.74 W / cm ２ Step 2 was continued from Step 1. In Step 2, the input power per unit area of ​​the target made of Ed was maintained at 3.72 W / cm ２ In step 2, the input power per unit area of ​​each of the other two targets was maintained at 0.74 W / cm ２ Step 3 was continued from Step 2. In Step 3, the input power per unit area of ​​the target made of Et was maintained at 3.72 W / cm. ２ In step 3, the input power per unit area of ​​each of the other two targets was maintained at 0.74 W / cm ２ Each of steps 1 to 3 lasted for 2 seconds. The cycle consisting of steps 1 to 3 was repeated until the thickness of the piezoelectric thin film reached 1000 nm.

[0077] The temperature of the substrate during the formation of the piezoelectric thin film of Example 4 was maintained at 200° C. The deposition pressure (air pressure inside the vacuum chamber) of the piezoelectric thin film of Example 4 was 1 Pa. The substrate bias during the formation of the piezoelectric thin film of Example 4 was 32 W. The deposition rate of the piezoelectric thin film of Example 4 was controlled to 0.35 nm / sec.

[0078] Except for changing the maximum or minimum value of the input power per unit area of ​​each sputtering target, the piezoelectric thin films of Examples 5 to 18 were also formed by RF magnetron sputtering, in which the cycle consisting of steps 1 to 3 described above was repeated.

[0079] In the step of forming the adhesive layer of Comparative Example 2, the input power per unit area of ​​Al alone (sputtering target) was 3.72 W / cm ２The temperature of the substrate during the formation of the adhesion layer of Comparative Example 2 was maintained at 300°C. The substrate bias during the formation of the adhesion layer of Comparative Example 2 was 30 W. The deposition rate of the adhesion layer of Comparative Example 2 was controlled to 0.71 nm / sec. Before forming the first electrode layer, the substrate on which the adhesion layer of Comparative Example 2 was formed was annealed at 450°C in a vacuum, and after annealing, was cooled to room temperature. The deposition pressure (air pressure in the vacuum chamber) during the formation of the piezoelectric thin film of Comparative Example 2 was 0.2 Pa. The substrate bias during the formation of the piezoelectric thin film of Comparative Example 2 was 20 W.

[0080] Except for the above-mentioned points, the piezoelectric thin film elements of Examples 2 to 18 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 18 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 18 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0081] Except for the differences shown in Table 1 below, the piezoelectric thin films of Examples 2 and 3 and Comparative Examples 1 and 2 had the same characteristics as Example 1. In all of Examples 2 and 3 and Comparative Examples 1 and 2, the c-axes of the wurtzite structures in the multiple crystal grains contained in the piezoelectric thin films had uniform lengths.

[0082] In each of the three electron beam diffraction patterns for Examples 4 to 18, the electron beam spot originating from the (0002) plane was split into two. That is, the piezoelectric thin film for each of Examples 4 to 18 included a plurality of first crystal grains and a plurality of second crystal grains that differed from each other in c-axis length. In each of Examples 4 to 18, the average value cL1 of the c-axis length of the wurtzite structure in the plurality of first crystal grains was determined from the three electron beam diffraction patterns. In each of Examples 4 to 18, the average value cL2 of the c-axis length of the wurtzite structure in the plurality of second crystal grains was determined from the three electron beam diffraction patterns. The cL1, cL2, and cL1 / cL2 values ​​for each of Examples 4 to 18 are shown in Table 1 below. The electron beam diffraction pattern for Example 13 is shown in FIG. 6 . Except for the above-described differences in the c-axis length and the differences shown in Table 1 below, the piezoelectric thin film for each of Examples 4 to 18 had the same characteristics as Example 1.

[0083] In the following Table 1, r(Edt) means ([Ed] + [Et]) / ([Al] + [Ed] + [Et]). In the following Table 1, r(Ed) means [Ed] / ([Ed] + [Et]).

[0084]

[0085] 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.

[0086] 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, Et...tetravalent element, g1...first crystal grain, g2...second crystal grain.

Claims

1. A piezoelectric thin film comprising aluminum nitride having a wurtzite structure, wherein the aluminum nitride contains an additive element, the (0001) plane of the wurtzite structure is oriented in the normal direction to a major surface of the piezoelectric thin film, and the length of the c-axis of the wurtzite structure is 0.500 nm or more.

2. The piezoelectric thin film according to claim 1, wherein the length of the a-axis of the wurtzite structure is 0.313 nm or more.

3. The piezoelectric thin film according to claim 1, comprising: a plurality of first crystal grains containing the aluminum nitride; and a plurality of second crystal grains containing the aluminum nitride, wherein the length of the c-axis of the wurtzite structure in the plurality of first crystal grains is represented by cL1, and the length of the c-axis of the wurtzite structure in the plurality of second crystal grains is represented by cL2, and cL1 is longer than cL2.

4. 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 comprising: a plurality of first crystal grains comprising the aluminum nitride; and a plurality of second crystal grains comprising the aluminum nitride, wherein the length of the c-axis of the wurtzite structure in the plurality of first crystal grains is represented by cL1, and the length of the c-axis of the wurtzite structure in the plurality of second crystal grains is represented by cL2, and cL1 is longer than cL2.

5. The piezoelectric thin film according to claim 3 or 4, wherein cL1 / cL2 is 1.001 or more and 1.060 or less.

6. The piezoelectric thin film according to claim 1 or 4, wherein the additive element includes at least a divalent element and a tetravalent element.

7. The piezoelectric thin film according to claim 6, 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.

8. The piezoelectric thin film according to claim 1 or 4, wherein the added element includes at least a divalent element and a tetravalent element, the ratio of the number of divalent elements in the aluminum nitride to the total number of aluminum, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Ed] atomic %, the ratio of the number of tetravalent elements in the aluminum nitride to the total number of aluminum, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Et] atomic %, and [Ed] / ([Ed]+[Et]) is 0.35 or more and 0.55 or less.

9. The piezoelectric thin film according to claim 1 or 4, wherein the additive element includes at least a divalent element and a tetravalent element, wherein the ratio of the number of aluminum elements in the aluminum nitride to the total number of aluminum elements, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Al] atomic %, wherein the ratio of the number of divalent elements in the aluminum nitride to the total number of aluminum elements, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Ed] atomic %, wherein the ratio of the number of tetravalent elements in the aluminum nitride to the total number of aluminum elements, the divalent elements, and the tetravalent elements in the aluminum nitride is expressed as [Et] atomic %, and wherein ([Ed] + [Et]) / ([Al] + [Ed] + [Et]) is 0.10 or more and 0.55 or less.

10. A piezoelectric thin film element comprising: the piezoelectric thin film according to claim 1 or 4; and an electrode layer, wherein the piezoelectric thin film directly or indirectly overlaps the surface of the electrode layer.

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