Piezoelectric thin film and piezoelectric thin film element

Abstract

A piezoelectric thin film contains aluminum nitride with a wurtzite structure. The aluminum nitride contains additive elements. The (0001) facet of the wurtzite structure is oriented in the normal direction of a first principal plane of the piezoelectric thin film. The piezoelectric thin film comprises a plurality of grains containing aluminum nitride. At least a portion of the plurality of grains are a plurality of columnar crystals extending along the normal direction (Z-axis direction) of the first principal plane of the piezoelectric thin film. The median grain size of the plurality of grains in a direction parallel to the first principal plane of the piezoelectric thin film is 90 nm or more and 500 nm or less.

Description

Technical Field

[0001] This disclosure relates to piezoelectric thin films and piezoelectric thin film elements. Background Technology

[0002] In recent years, MEMS (Micro Electro Mechanical Systems) has attracted much attention. MEMS are devices that integrate mechanical components, sensors, actuators, and electronic circuits onto a single substrate using microfabrication techniques. For example, piezoelectric thin films used in MEMS include aluminum nitride (AlN), zinc oxide (ZnO), lithium niobate (LiNbO3), and lead zirconate titanate (PZT).

[0003] For example, the performance indicators of piezoelectric thin films are the piezoelectric constant d (piezoelectric strain constant) and the piezoelectric constant g (voltage output constant). The piezoelectric constant d is an indicator of the strain per unit electric field (transmitting capability). The larger the piezoelectric constant d, the higher the performance of the piezoelectric element as an actuator (e.g., an inkjet recording head). On the other hand, the piezoelectric constant g is an indicator of the generated electric field strength per unit stress (receiving capability). The larger the piezoelectric constant g, the higher the performance of the piezoelectric element as a sensor. Among the piezoelectric thin films mentioned above, AlN has attracted much attention because it has a small piezoelectric constant d but a high piezoelectric constant g and is inexpensive. (See Patent Document 1 below.)

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent No. 7115257 Summary of the Invention

[0007] The technical problem that the invention aims to solve

[0008] It is known that the piezoelectric properties of AlN can be improved by adding elements. However, as the content of added elements in AlN increases, it becomes difficult to maintain the crystal structure (wurtzite structure) of AlN. As a result, the piezoelectric properties of AlN deteriorate. Furthermore, with the increase of added element content in AlN, the added elements tend to segregate at grain boundaries located between multiple grains within AlN. Due to the segregation of added elements at grain boundaries, the piezoelectric properties of AlN deteriorate, and the resistivity of AlN decreases. As the temperature of AlN containing added elements segregated at grain boundaries reaches high levels (e.g., 125°C), thermal diffusion of the added elements in AlN easily occurs. Due to the thermal diffusion of added elements, the dielectric loss (dielectric loss tangent) of AlN tends to increase.

[0009] One aspect of this disclosure is to provide a piezoelectric thin film and a piezoelectric thin film element comprising the piezoelectric thin film, wherein the piezoelectric thin film has excellent piezoelectric properties and suppresses the increase of dielectric loss as the temperature of the piezoelectric thin film increases.

[0010] Technical solutions for solving technical problems

[0011] This disclosure relates, for example, to the piezoelectric thin film described in any of [1] to [8] below, and the piezoelectric thin film element described in [9] below.

[0012] [1] A piezoelectric thin film, wherein it contains aluminum nitride having a wurtzite-type structure,

[0013] The aluminum nitride contains additive elements.

[0014] The (0001) facet of the wurtzite structure is oriented in the normal direction of the main surface of the piezoelectric film.

[0015] The piezoelectric thin film comprises a plurality of crystal grains containing the aluminum nitride.

[0016] At least a portion of the plurality of grains are a plurality of columnar crystals extending along the normal direction of the main surface of the piezoelectric film.

[0017] The median grain size of the plurality of grains in a direction parallel to the main surface of the piezoelectric film is greater than 90 nm and less than 500 nm.

[0018] [2] According to the piezoelectric thin film described in [1], wherein,

[0019] The added elements contain at least divalent and tetravalent elements.

[0020] [3] According to the piezoelectric thin film described in [2], wherein,

[0021] At least some of the divalent elements are magnesium.

[0022] At least a portion of the tetravalent elements are at least one of zirconium and hafnium.

[0023] [4] According to any one of [1] to [3], the piezoelectric thin film, wherein,

[0024] The added element contains at least a trivalent element.

[0025] [5] According to the piezoelectric thin film described in [4], wherein,

[0026] At least some of the trivalent elements are scandium.

[0027] [6] According to any one of [1] to [5], the piezoelectric thin film, wherein,

[0028] The piezoelectric thin film further includes at least one grain boundary located between the plurality of grains.

[0029] The added elements are either only divalent and tetravalent elements, or only trivalent elements, or a combination of the divalent, trivalent, and tetravalent elements.

[0030] The proportion of the divalent element in the grain relative to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain is expressed as [Ed] g atoms%.

[0031] The ratio of the amount of the trivalent element in the grain to the total amount of the aluminum, divalent element, trivalent element and tetravalent element in the grain is expressed as [Etr] g atoms%.

[0032] The ratio of the amount of the tetravalent element in the grain to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain is expressed as [Et] g atoms%.

[0033] The ratio of the amount of the divalent element in the grain boundary to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain boundary is expressed as [Ed]b atoms%.

[0034] The ratio of the amount of the trivalent element in the grain boundary to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain boundary is expressed as [Etr]b atoms%.

[0035] The ratio of the amount of the tetravalent element in the grain boundary to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain boundary is expressed as [Et]b atoms%.

[0036] [Ed]g + [Etr]g + [Et]g is represented as [Eadd]g.

[0037] [Ed]b + [Etr]b + [Et]b is represented as [Eadd]b.

[0038] [Eadd]b / [Eadd]g is greater than 0.70 and less than 1.60.

[0039] [7] According to any one of [1] to [6], the piezoelectric thin film, wherein,

[0040] The piezoelectric thin film further includes at least one grain boundary located between the plurality of grains.

[0041] The added elements are either only divalent and tetravalent elements, or only trivalent elements, or a combination of the divalent, trivalent, and tetravalent elements.

[0042] The proportion of the divalent element in the grain relative to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain is expressed as [Ed] g atoms%.

[0043] The ratio of the amount of the trivalent element in the grain to the total amount of the aluminum, divalent element, trivalent element and tetravalent element in the grain is expressed as [Etr] g atoms%.

[0044] The ratio of the amount of the tetravalent element in the grain to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain is expressed as [Et] g atoms%.

[0045] [Ed]g + [Etr]g + [Et]g is represented as [Eadd]g.

[0046] [Eadd]g is 4.5 atomic percent or more and 65.0 atomic percent or less.

[0047] [8] According to the piezoelectric thin film described in [7], wherein,

[0048] The [Eadd]g is 30.0 atomic% or more and 60.0 atomic% or less.

[0049] [9] A piezoelectric thin film element, wherein,

[0050] have:

[0051] The piezoelectric thin film described in any one of [1] to [8]; and

[0052] Electrode layer,

[0053] The piezoelectric film overlaps directly or indirectly with the surface of the electrode layer.

[0054] Invention Effects

[0055] According to one aspect of this disclosure, a piezoelectric film with excellent piezoelectric properties and suppressing the increase of dielectric loss as the temperature of the piezoelectric film rises, and a piezoelectric film element comprising the piezoelectric film, can be provided. Attached Figure Description

[0056] [ Figure 1 ] Figure 1This is a schematic exploded perspective view of a piezoelectric thin film element, which is a specific example of the present invention.

[0057] [ Figure 2 ] Figure 2 express Figure 1 A specific example of a schematic cross-section of the piezoelectric thin film element shown. Figure 2 The cross section shown is approximately perpendicular or completely perpendicular to the first principal surface of the piezoelectric film, and approximately parallel or completely parallel to the thickness direction of the piezoelectric film.

[0058] [ Figure 3 ] Figure 3 yes Figure 1 A schematic diagram of a portion of the first or second main surface of the piezoelectric thin film.

[0059] [ Figure 4 ] Figure 4 Image (a) is a schematic three-dimensional view of multiple grains (columnar crystals) contained in a piezoelectric thin film. Figure 4 (b) is a schematic diagram of the particle size distribution of multiple grains contained in the piezoelectric film.

[0060] [ Figure 5 ] Figure 5 It is a three-dimensional diagram of the unit cell of the aluminum nitride crystal structure (wurtzite structure) contained in the piezoelectric thin film.

[0061] [ Figure 6 ] Figure 6 Image (a) is an image of the main surface (second main surface) of a piezoelectric thin film in a specific example (Example 11) of the present invention. Figure 6 Image (b) is an image of the second principal surface (surface) of the piezoelectric film of Comparative Example 1. Detailed Implementation

[0062] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the drawings, the same reference numerals are used to denote the same constituent elements. The present invention is not limited to the embodiments described below. Figures 1-3 , Figure 6 (a) and Figure 6 In (b) above, X, Y, and Z refer to three mutually orthogonal coordinate axes. The directions of the X-axis, Y-axis, and Z-axis are respectively in... Figures 1-3 , Figure 6 (a) and Figure 6 The same as in (b) of the above. Figure 4 The directions of the X-axis and Z-axis shown in (a) are respectively related to... Figures 1-3 , Figure 6 (a) and Figure 6 In (b), the X-axis and Z-axis have the same direction. Figure 5 The direction of the Z-axis shown is... Figures 1-3 , Figure 4 (a) Figure 6 (a) and Figure 6 In (b), the Z-axis directions are all the same.

[0063] like Figure 1 As shown, the piezoelectric thin film element 10 of 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 also 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" is the surface with the largest area among the multiple surfaces of a polyhedron (e.g., a thin cuboid piezoelectric thin film 3). The area of ​​the first main surface s31 may be approximately equal to or exactly equal to the area of ​​the second main surface s32. The shape of the first main surface s31 may be approximately equal to or exactly equal to the shape of the second main surface s32. The first main surface s31 of the piezoelectric thin film 3 may directly overlap or indirectly overlap with the surface s1 of the first electrode layer 1. The surface s2 of the second electrode layer 2 may directly overlap or indirectly overlap with the second main surface s32 of the piezoelectric thin film 3. Figure 1 The X-axis and Y-axis are approximately parallel 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. Figure 1 The Z-axis is approximately perpendicular 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.

[0064] The piezoelectric thin film 3 comprises aluminum nitride with a wurtzite structure. The aluminum nitride contains at least one additive element, Eadd, which is different from aluminum (Al) and nitrogen (N). For example, the additive element Eadd may contain at least a divalent element Ed and a tetravalent element Et. Alternatively, the additive element Eadd may contain at least a trivalent element Etr. For example, the additive element Eadd may be only divalent Ed and tetravalent Et, or only trivalent Etr, or a combination of divalent Ed, trivalent Etr, and tetravalent Et. Due to the doping of aluminum nitride with the additive element Eadd, deformation of the wurtzite structure of aluminum nitride or changes in the strength of the chemical bonds between atoms in the wurtzite structure occurs. As a result, with the application of voltage or external force to the piezoelectric thin film 3, the wurtzite structure of aluminum nitride is easily deformed, and the piezoelectric properties of the piezoelectric thin film 3 are easily improved. The piezoelectric thin film 3 may also consist solely of AlN containing the additive element Eadd. As described later, the piezoelectric film 3 may contain other elements in addition to Al, N and the added element Eadd.

[0065] Figure 5This indicates 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 can be replaced by the additive element Eadd. For example, a portion of the Al in the unit cell uc can be replaced by the divalent element Ed, the trivalent element Etr, or the tetravalent element Et. At least a portion or all of the (0001) facet of the wurtzite structure (aluminum nitride) in the piezoelectric thin film 3 is oriented in the normal direction (Z-axis direction) of the first principal facet s31 (the 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) facet of the wurtzite structure in the piezoelectric thin film 3 can be substantially parallel or completely parallel to the first principal facet s31 (the 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) facet (i.e.,

[0001] ). Therefore, by aligning at least a portion of the (0001) facet of the aluminum nitride in the piezoelectric film 3 in the normal direction of the first principal surface s31 of the piezoelectric film 3, the piezoelectric properties of the piezoelectric film 3 are easily improved. During the manufacturing process of the piezoelectric film 3, the (0001) facet of the aluminum nitride is easily aligned parallel to the first principal surface s31 (surface s1 of the first electrode layer 1) of the piezoelectric film 3.

[0066] like Figure 2 and Figure 3 As shown, the piezoelectric thin film 3 comprises multiple grains g1 containing aluminum nitride. Each grain g1 can be a single crystal, polycrystalline, or incomplete crystal. Each grain g1 can consist solely of aluminum nitride containing the additive element Eadd. In addition to containing aluminum nitride with the additive element Eadd, each grain g1 may further contain other components. The piezoelectric thin film 3 may also contain one grain boundary g2 (grain boundary phase) or multiple grain boundaries g2 located between the multiple grains g1.

[0067] The median grain size (D) of multiple grains g1 in a direction parallel to the principal surface (first principal surface s31 or second principal surface s32) of the piezoelectric film 3 50 The wavelength range is 90nm or higher and 500nm or lower.

[0068] The method for determining the median particle size of multiple columnar crystals is as follows.

[0069] The median particle size can be calculated based on the individual particle size d of multiple grains g1 in a direction parallel to the main surface (first main surface s31 or second main surface s32) of the piezoelectric film 3. The particle size d of each grain g1 can be calculated based on the area of ​​the surface 3s of each grain g1 exposed on the main surface of the piezoelectric film 3. Figure 3 As shown in the figure. The area of ​​the surface 3s of each grain g1 exposed on the main surface of the piezoelectric film 3 is denoted as A. The grain size d (diameter) of each grain g1 is denoted as (4A / π). 1 / 2(4A / π) 1 / 2 This is equivalent to the diameter of a circle with area A (circular equivalent diameter). That is, the grain size d of each grain g1 is the Heywood diameter calculated based on the area A of the surface 3s of each grain g1. To determine the area A of the surface 3s of each grain g1, an image of the main surface of the piezoelectric thin film 3 is captured using a scanning electron microscope (SEM). An example of an image of the main surface of the piezoelectric thin film 3 is shown in... Figure 6 (a) is shown in the figure. Next, the image of the main surface of the piezoelectric film 3 is binarized. For example, in the binarized image of the main surface of the piezoelectric film 3, the white part corresponds to the surface 3s of the grain g1 exposed on the main surface of the piezoelectric film 3. For example, in the binarized image of the main surface of the piezoelectric film 3, the black part corresponds to the grain boundary g2. The area of ​​a closed region (white region) surrounded by the grain boundary g2 is measured as the area A of the surface 3s of a grain g1. Grains g1 that are not clearly defined by the grain boundary g2 are excluded from the objects whose area A is measured. The binarization of the image of the main surface of the piezoelectric film 3 can be performed manually or by image analysis software. The area A of the surface 3s of the grain g1 can be measured by image analysis software. For example, the number n (number of samples) of multiple grains g1 whose area A is measured can be more than 300 and less than 500. For example, as the image analysis software, image analysis software manufactured by TDK Corporation (not for sale) can be used. For example, as image analysis software, Mac-View, manufactured by Mountech Co., Ltd. of Japan, can be used.

[0070] Alternatively, the particle size d of multiple grains g1 in a direction parallel to the main surface of the piezoelectric film 3 can be determined not on the main surface of the piezoelectric film 3, but in a cross section parallel to the main surface of the piezoelectric film 3.

[0071] Based on the grain size d of the n grains g1 calculated using the method described above, the grain size distribution of grain g1 is obtained. An example of the grain size distribution of grain g1 is shown in... Figure 4 As shown in (b) of the diagram, the grain size distribution G of grain g1 is a number distribution (a number-based grain size distribution). That is, the horizontal axis of the grain size distribution G is the grain size d of grain g1, and the vertical axis of the grain size distribution G is the number N of grains g1 with grain size d. Based on this grain size distribution G, the median grain size D of n grains g1 is determined. 50 . Figure 4 The particle size distribution G shown in (b) is a frequency distribution, but the particle size distribution can also be a cumulative distribution.

[0072] like Figure 2As shown, some or all of the multiple grains g1 are multiple columnar crystals extending along the normal direction (Z-axis direction) of the first principal surface s31 of the piezoelectric thin film 3. For example, the multiple columnar crystals may extend substantially perpendicular or completely perpendicular to the first principal surface s31 of the piezoelectric thin film 3. Figure 4 In (a), a single grain g1, which is a columnar crystal, is approximated as a cylinder with a diameter equal to the grain size d (Heywood diameter). In other words, a single grain g1, which is a columnar crystal, is approximated as a cylinder with an end face (circle) area of ​​A. The length (height) of the columnar crystal in the direction perpendicular to the first principal surface s31 of the piezoelectric film 3 is denoted as H. A columnar crystal is defined as a grain g1 that extends along the normal direction of the first principal surface s31 of the piezoelectric film 3 and has an aspect ratio d / H greater than 0 and less than 1. For example, the length H of the columnar crystal can be equal to the thickness T of the piezoelectric film 3. The thickness T of the piezoelectric film 3 can also be referred to as the distance between the first principal surface s31 and the second principal surface s32 of the piezoelectric film 3. The (0001) plane of the wurtzite-type structure in the columnar crystal is easily oriented in the normal direction of the first principal surface s31 of the piezoelectric film 3. Therefore, by including columnar crystals in the piezoelectric film 3, the piezoelectric film 3 easily has excellent piezoelectric properties.

[0073] Median grain size D' of multiple columnar crystals 50 It can be above 90nm and below 500nm. This is in addition to determining D' solely based on multiple columnar crystals. 50 In addition, D' 50 The definition and determination method can be compared with D 50 The definition and determination method are the same. That is, the median grain size D' of multiple columnar crystals 50 In the method for determining the size of grains, the grain size d of grains g1 with an aspect ratio d / H greater than 1 is excluded from the grain size distribution G.

[0074] The median grain size (D) of multiple grains g1 in a direction parallel to the principal surface (first principal surface s31 or second principal surface s32) of the piezoelectric film 3 50 When the diameter is above 90 nm and below 500 nm, the piezoelectric properties of the piezoelectric film 3 are improved, the dielectric loss is suppressed, and the resistivity of the piezoelectric film 3 increases with the increase of temperature. The median grain size D' of the multiple columnar crystals in the direction parallel to the main surface of the piezoelectric film 3 is... 50 The same applies to the case where the wavelength is 90nm or higher and 500nm or lower. The mechanism by which the dielectric loss of the piezoelectric film 3 increases and the resistivity of the piezoelectric film 3 increases with the increase of the temperature of the piezoelectric film 3 is as follows. Furthermore, "the dielectric loss increases with the increase of the temperature of the piezoelectric film 3" means that when the temperature of the piezoelectric film 3 decreases to room temperature after the temperature of the piezoelectric film 3 has increased, the dielectric loss of the piezoelectric film 3 is greater than that of the piezoelectric film 3 before the temperature increase.

[0075] As the content of Eadd in AlN increases, it becomes difficult to maintain the wurtzite structure of AlN. Therefore, due to the excessive Eadd content in AlN, the piezoelectric properties of the piezoelectric film 3 deteriorate. Furthermore, with increasing Eadd content in AlN, Eadd tends to segregate at grain boundaries. This segregation at grain boundaries further deteriorates the piezoelectric properties of AlN. The Eadd segregated at grain boundaries becomes a path for leakage current, thus reducing the resistivity of the piezoelectric film 3 in the grain boundary extension direction (thickness direction of the piezoelectric film 3). As the temperature of AlN containing Eadd segregated at grain boundaries reaches high levels (e.g., 125°C), thermal diffusion of Eadd easily occurs. This thermal diffusion of Eadd easily increases the dielectric loss of the piezoelectric film 3.

[0076] For the reasons mentioned above, in order to improve piezoelectric properties, suppress the increase in dielectric loss with increasing temperature, and reduce the increase in resistivity, it is desirable to reduce the total area (total volume of grain boundary phase) of grain boundary g2 in piezoelectric thin film 3.

[0077] The total area of ​​grain boundaries g2 in the piezoelectric thin film 3 is approximately equal to or exactly equal to the surface area of ​​all grains g1 in the piezoelectric thin film 3. Therefore, as the specific surface area of ​​each grain g1 decreases, the surface area of ​​each grain g1 decreases, and the total area of ​​grain boundaries g2 also decreases.

[0078] For example, when each grain g1 in the piezoelectric thin film 3 is approximated to have a diameter similar to the aforementioned median grain size D... 50 In the case of equal spheres, the specific surface area SSA of each grain g1 is expressed by the following mathematical formula 1.

[0079] Mathematical formula 1:

[0080]

[0081] For example, when each grain g1 in the piezoelectric thin film 3 is approximated as having a diameter (coarseness) similar to the aforementioned median grain size D' 50 In the case of columnar crystals of equal height H, the surface area S of the columnar crystal (cylinder) is represented by the following mathematical formula 2A.

[0082] Mathematical expression 2A:

[0083]

[0084] The volume V of a columnar crystal (cylinder) is represented by the following mathematical formula 2B.

[0085] Mathematical expression 2B:

[0086]

[0087] The specific surface area SSA' of a columnar crystal (cylinder) is (the surface area S of the columnar crystal) / (the volume V of the columnar crystal). Based on the above mathematical formulas 2A and 2B, the specific surface area SSA' of the columnar crystal is expressed by the following mathematical formula 2C.

[0088] Mathematical expression 2C:

[0089]

[0090] As shown in mathematical formulas 1 and 2C above, with the median grain size (D) of multiple grains g1... 50 or D' 50 As the specific surface area (SSA or SSA') of each grain g1 increases, the surface area of ​​each grain g1 decreases, and the total area of ​​the grain boundary g2 also decreases. With the increase of the median grain size (D) of multiple grains g1... 50 or D' 50 With the increase of Eadd, the total area of ​​grain boundary g2 (total volume of grain boundary phase) decreases. Therefore, the segregation of the added element Eadd in grain boundary g2 of the piezoelectric film 3 is suppressed, as is the thermal diffusion of the added element Eadd through grain boundary g2 in the piezoelectric film 3. As a result, the piezoelectric properties of the piezoelectric film 3 are improved, the increase in dielectric loss of the piezoelectric film 3 with increasing temperature is suppressed, and the increase in resistivity of the piezoelectric film 3 is also suppressed.

[0091] The inventors discovered for the first time that, especially when the median grain size of multiple grains g1 is above 90 nm and below 500 nm, the piezoelectric properties are easily improved, the increase in dielectric loss with increasing temperature is easily suppressed, and the resistivity is easily increased.

[0092] However, the technical scope of this disclosure is not limited by the above-described mechanism.

[0093] Based on the reasons that piezoelectric properties are easily improved, dielectric loss is easily suppressed as temperature increases, and resistivity easily increases, the median grain size (D) of multiple grains g1 is... 50 or D' 50 It can also be above 91nm and below 495nm.

[0094] The divalent element Ed in aluminum nitride can be at least one element selected from magnesium (Mg), calcium (Ca), zinc (Zn), strontium (Sr), and barium (Ba). The trivalent element Etr in aluminum nitride can be at least one element selected from scandium (Sc), yttrium (Y), lanthanides, and indium (In). The tetravalent element Et in aluminum nitride can be at least one element selected from zirconium (Zr), germanium (Ge), titanium (Ti), and hafnium (Hf).

[0095] Based on the reason that the piezoelectric properties of the piezoelectric film 3 can be easily improved, at least a portion of the divalent element Ed can be magnesium, and at least a portion of the tetravalent element Et can be at least one of zirconium and hafnium. For the same reason, at least a portion of the trivalent element Etr can be scandium.

[0096] The aluminum nitride contained in the piezoelectric film 3 may also contain at least one element selected from monovalent and pentavalent elements. The monovalent element may be at least one element selected from lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). The pentavalent element may be at least one element selected from chromium (Cr), vanadium (V), niobium (Nb), and tantalum (Ta). The aluminum nitride contained in the piezoelectric film 3 may also contain other elements such as oxygen (O) and argon (Ar).

[0097] In this disclosure, any element is denoted as "X", the concentration (in atomic %) of element X in grain g1 is denoted as <X>1, and the concentration (in atomic %) of element X in grain boundary g2 is denoted as <X>2.

[0098] That is, the concentrations of Al, N, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain g1 are respectively expressed as <Al>1, <N>1, <Ed>1, <Etr>1, and <Et>1.

[0099] The concentrations of Al, N, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain boundary g2 are represented as <Al>2, <N>2, <Ed>2, <Etr>2, and <Et>2, respectively.

[0100] The proportion of aluminum in grain g1 relative to the total amount of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain g1 is expressed as [Al]g atomic%. [Al]g can be equal to 100 × <Al>1 / (<Al>1 + <Ed>1 + <Etr>1 + <Et>1).

[0101] The proportion of the amount of divalent element Ed in grain g1 relative to the total amount of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain g1 is expressed as [Ed]g atomic%. [Ed]g can be equal to 100 × <Ed>1 / (<Al>1 + <Ed>1 + <Etr>1 + <Et>1).

[0102] The proportion of the trivalent element Etr in grain g1 relative to the total amount of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain g1 is expressed as [Etr]g atomic%. [Etr]g can be equal to 100 × <Etr>1 / (<Al>1 + <Ed>1 + <Etr>1 + <Et>1).

[0103] The proportion of the tetravalent element Et in grain g1 relative to the total amount of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain g1 is expressed as [Et]g atomic%. [Et]g can be equal to 100 × <Et>1 / (<Al>1 + <Ed>1 + <Etr>1 + <Et>1).

[0104] The proportion of aluminum in grain boundary g2 relative to the total amount of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain boundary g2 is expressed as [Al]b atomic percent. [Al]b can be equal to 100 × <Al>2 / (<Al>2 + <Ed>2 + <Etr>2 + <Et>2).

[0105] The proportion of the divalent element Ed in grain boundary g2 relative to the total amount of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain boundary g2 is expressed as [Ed]b atomic percent. [Ed]b can be equal to 100 × <Ed>2 / (<Al>2 + <Ed>2 + <Etr>2 + <Et>2).

[0106] The proportion of the trivalent element Etr in grain boundary g2 relative to the total amount of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain boundary g2 is expressed as [Etr]b atomic percent. [Etr]b can be equal to 100 × <Etr>2 / (<Al>2 + <Ed>2 + <Etr>2 + <Et>2).

[0107] The proportion of the tetravalent element Et in grain boundary g2 relative to the total amount of aluminum, divalent element Ed, trivalent element Etr, and tetravalent element Et in grain boundary g2 is expressed as [Et]b atomic percent. [Et]b can be equal to 100 × <Et>2 / (<Al>2 + <Ed>2 + <Etr>2 + <Et>2).

[0108] [Ed]g + [Etr]g + [Et]g is represented as [Eadd]g.

[0109] [Ed]b + [Etr]b + [Et]b is represented as [Eadd]b.

[0110] Based on the reason that it is easy to suppress the increase of dielectric loss of the piezoelectric film 3 as the temperature of the piezoelectric film 3 increases, [Eadd]b / [Eadd]g can be 0.70 or more and 1.60 or less, or 0.70 or more and 1.59 or less.

[0111] Based on the reason that the piezoelectric properties of the piezoelectric film 3 can be easily improved, [Eadd]g can be 4.5 atomic% or more and 65.0 atomic% or less, 4.6 atomic% or more and 65.0 atomic% or less, or 30.0 atomic% or more and 60.0 atomic% or less.

[0112] In grain g1, based on the reasons that the average valence of aluminum, divalent element Ed, trivalent element Etr and tetravalent element Et is easily balanced with the valence of nitrogen, and that the wurtzite structure of aluminum nitride is easily maintained, and the piezoelectric properties of the piezoelectric film 3 are easily improved, [Ed]g / ([Ed]g+[Et]g) can be 0.29 or more and 0.61 or less, or 0.29 or more and 0.59 or less.

[0113] Based on the reasons that piezoelectric properties are easy to improve, dielectric loss is suppressed as temperature increases, and resistivity is easy to increase, [Ed]b / ([Ed]b+[Et]b) can be above 0.44 and below 0.55.

[0114] The first electrode layer 1 may contain at least one element selected from 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 metallic element or an alloy.

[0115] The second electrode layer 2 may contain at least one element selected from 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 metallic element or an alloy. The composition of the second electrode layer 2 may be the same as that of the first electrode layer 1. Alternatively, the composition of the second electrode layer 2 may differ from that of the first electrode layer 1.

[0116] The piezoelectric thin film element 10 may further include a substrate. The first electrode layer 1 may directly overlap or indirectly overlap with the substrate. The piezoelectric thin film element 10 may further include an adhesive layer. The adhesive layer may be disposed between the substrate and the first electrode layer 1. That is, the first electrode layer 1 may indirectly overlap with the substrate via the adhesive layer.

[0117] For example, the substrate can be a semiconductor substrate (silicon substrate, or gallium arsenide substrate, etc.), an optical crystal substrate (sapphire substrate, etc.), an insulator substrate (glass substrate, or ceramic substrate, etc.), a metal substrate (stainless steel plate, etc.), or an SOI (Silicon-on-Insulator) substrate. The substrate can be crystalline. For example, the substrate can be monocrystalline or polycrystalline. The substrate can also be amorphous.

[0118] For example, the adhesive layer may contain at least one element selected from 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 adhesive layer can be a metallic element, alloy, or compound (oxide, etc.). The adhesive layer can also be other piezoelectric films (e.g., aluminum nitride), polymers, or ceramics. The adhesive layer functions to suppress the peeling of the first electrode layer caused by mechanical impact, etc. The adhesive layer may also be referred to as an interface layer, support layer, buffer layer, or intermediate layer.

[0119] For example, the thickness of the substrate can be 50 μm or more and 10,000 μm or less. For example, the thickness of the adhesive layer can be 0.003 μm or more and 2 μm or less. For example, the thickness of the first electrode layer 1 can be 0.01 μm or more and 1 μm or less. For example, the thickness T of the piezoelectric film 3 can be 100 nm or more and 30,000 nm or less. For example, the thickness of the second electrode layer 2 can be 0.01 μm or more and 1 μm or less. The thicknesses of the substrate, adhesive layer, first electrode layer 1, piezoelectric film 3, and second electrode layer 2 can be substantially uniform or completely uniform.

[0120] The crystal structures of the substrate, adhesive layer, first electrode layer, piezoelectric thin film, and second electrode layer can be determined by X-ray diffraction and electron beam diffraction. The compositions of the substrate, adhesive layer, first electrode layer, piezoelectric thin film, and second electrode layer can be determined by at least one of the following analytical methods: X-ray fluorescence (XRF), X-ray photoelectron spectrometry (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 microanalysis (EPMA). The thicknesses of the substrate, adhesive layer, first electrode layer, piezoelectric thin film, and second electrode layer can be measured in cross-section of the piezoelectric thin film element using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), or scanning electron microscopy (SEM). The crystallinity and crystal structure of each grain can be analyzed and determined based on the electron beam diffraction patterns measured within the cross-section of the piezoelectric thin film (parallel to the stacking direction) of each exposed grain.

[0121] The bonding layer, the first electrode layer, the piezoelectric thin film, and the second electrode layer can be sequentially stacked on the surface of the substrate by vapor phase growth methods such as sputtering. In particular, the piezoelectric thin film is formed by RF (Radio Frequency) magnetron sputtering. The bonding layer, the first electrode layer, the piezoelectric thin film, and the second electrode layer can each be formed by sputtering using at least one target material. Alternatively, the bonding layer, the first electrode layer, the piezoelectric thin film, and the second electrode layer can be formed separately by sputtering using multiple targets with different compositions. The target material can contain at least one element selected from the elements constituting each layer or the piezoelectric thin film. By selecting and combining targets with a predetermined composition, the compositions of the bonding layer, the first electrode layer, the piezoelectric thin film, and the second electrode layer can be controlled to a target composition. For example, the target material can be a metallic element, an alloy, or an oxide. For example, when the piezoelectric thin film is aluminum nitride containing divalent and tetravalent elements, a target material made of aluminum, a target material made of a divalent element, and a target material made of a tetravalent element can be used. Alloys composed of two or more elements selected from aluminum, divalent elements, and tetravalent elements can also be used as the target material. For example, when the piezoelectric thin film is aluminum nitride containing a trivalent element, targets made of aluminum and targets made of a trivalent element can be used. The composition of the sputtering atmosphere can be a control factor of the composition of each of the sealing layer, the first electrode layer, the piezoelectric thin film, and the second electrode layer. For example, nitrogen is used as the raw material for the piezoelectric thin film (aluminum nitride). The input power (power density) applied to the cathode on which each target material is disposed can be a control factor of the composition and thickness of each of the sealing layer, the first electrode layer, the piezoelectric thin film, and the second electrode layer. The total pressure of the sputtering atmosphere, the partial pressure or concentration of the raw gas in the atmosphere, the sputtering duration of each target, the temperature of the substrate surface, and the substrate bias voltage can also be control factors for the composition and thickness of the bonding layer, the first electrode layer, the piezoelectric thin film, and the second electrode layer. Piezoelectric thin films with desired shapes or patterns can also be formed by etching (e.g., plasma etching).

[0122] A specific example of a method for forming a piezoelectric thin film containing multiple grains with a median particle size of 90 nm or more and 500 nm or less can be as follows.

[0123] A buffer layer composed of AlN can be directly formed on the main surface of the substrate while the substrate is being heated. The substrate can be made of a single crystal of Si. The main surface of the substrate can be parallel to the (100) plane of Si.

[0124] A first metal layer composed of Mo can be formed on the surface (main surface) of the adhesive layer while the substrate with the adhesive layer is heated. During the formation of the first metal layer, Mo grains grow in the first metal layer.

[0125] In RF magnetron sputtering for forming piezoelectric thin films, the film-forming chamber (vacuum chamber) is filled with a mixture of Ar and N2 gases, and then an electric current is supplied to all the target materials. Ar ions (Ar...) within the film-forming chamber... ＋ The atoms collide with the surface of the target material supplied with electricity. Atoms that have gained kinetic energy from the Ar ions detach from the surface of each target material 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).

[0126] During the formation of the piezoelectric thin film, a substrate bias voltage (electricity) of 20W is continuously supplied to the substrate. As a result, the number of atoms reaching the surface of the first electrode layer from each target per unit time is reduced compared to the formation process of the piezoelectric thin film with a substrate bias voltage of zeroW, and the number of seed crystals (nuclei) formed on the surface of the first electrode layer is also reduced compared to the formation process of the piezoelectric thin film with a substrate bias voltage of zeroW. The fewer the number of seed crystals (nuclei), the larger the space that each seed crystal can grow, and the easier it is to increase the median grain size of the multiple grains formed by the growth of multiple seed crystals.

[0127] Based on the power supplied to each target material, the film formation rate (the rate at which the thickness of the piezoelectric film increases) is adjusted to a value lower than the existing film formation rate (0.8 nm / s) (0.35 nm / s). A lower film formation rate allows for a longer migration time for individual atoms (Al, Eadd, and N) on the surface of the first electrode layer, facilitating seed growth. Therefore, a lower film formation rate makes it easier to increase the median grain size of multiple grains formed from multiple seeds.

[0128] The substrate temperature during the formation of the piezoelectric thin film is 150°C, which is higher than the existing temperature (77°C). The higher the substrate temperature, the easier it is to promote the growth of seed crystals, and the median grain size of multiple grains formed by multiple seed crystals is more likely to increase.

[0129] As described above, by adjusting the substrate bias, the film formation rate of the piezoelectric film, and the substrate temperature during the formation process of the piezoelectric film, it is possible to form a piezoelectric film containing multiple grains with a median particle size of 90 nm or more and 500 nm or less.

[0130] The piezoelectric thin-film element of this embodiment has applications in various aspects. For example, the piezoelectric thin-film element can be a piezoelectric microphone, a sensor, an oscillator, a resonator, an acoustic multilayer film, or a filter. For example, the piezoelectric thin-film element can also be a piezoelectric actuator. Piezoelectric actuators can be used for haptics. That is, piezoelectric actuators can be used in various devices that require feedback from skin sensation (tactile feedback). For example, devices requiring feedback from skin sensation can be wearable devices, touchpads, displays, or game controllers. For example, piezoelectric actuators can be used in magnetic head assemblies, magnetic head cantilever assemblies, or hard disk drives. For example, piezoelectric actuators can also be used in printheads or inkjet printer devices. Piezoelectric actuators can also be used in piezoelectric switches. For example, the piezoelectric thin-film element can also be a piezoelectric sensor or a piezoelectric transducer. For example, piezoelectric sensors or piezoelectric transducers can be used in gyroscope sensors, pressure sensors, pulse wave sensors, ultrasonic sensors, ultrasonic transducers, or impact sensors. The ultrasonic transducer can be a piezoelectric micromachined ultrasonic transducer (PMUT). Products using piezoelectric micromachined ultrasonic transducers can be biometric sensors such as fingerprint sensors and ultrasonic vascular authentication sensors, medical or healthcare sensors, or Time of Flight (ToF) sensors. For example, the filter can be a BAW (Bulk Acoustic Wave) filter or a SAW (Surface Acoustic Wave) filter. The aforementioned piezoelectric thin-film elements can be part of or integral into a Micro Electro Mechanical System (MEMS).

[0131] This invention is not necessarily limited to the embodiments described above. Various modifications can be made to this invention without departing from its spirit, and these modifications are also included in this invention.

[0132] Example

[0133] The present invention will be described in detail through the following embodiments and comparative examples. The present invention is not limited to the following embodiments.

[0134] (Example 1)

[0135] A wafer made of single-crystal silicon (Si) was used as the substrate. The substrate has a diameter of 8 inches and a thickness of 725 μm. The substrate has a uniform thickness. The main surface of the substrate is parallel to the (100) plane of the Si.

[0136] An adhesive layer was directly formed on the entire main surface of the substrate using RF magnetron sputtering within a vacuum chamber. The adhesive layer consisted of aluminum nitride without any additives. Elemental Al was used as the sputtering target. The atmosphere within the vacuum chamber was a mixture of Ar and N₂. The input power per unit area of ​​the sputtering target was 0.74 W / cm². 2 The substrate temperature was maintained at 200°C during the formation of the adhesive layer. The film formation pressure (gas pressure within the vacuum chamber) was 0.4 Pa. No substrate bias voltage was applied. The adhesive layer thickness was uniform. The film formation rate was controlled at 0.09 nm / s. The adhesive layer thickness was adjusted to approximately 30 nm.

[0137] A first electrode layer composed of Mo was formed integrally on the surface of the sealing layer using RF magnetron sputtering within a vacuum chamber. Elemental Mo was used as the sputtering target. The atmosphere within the vacuum chamber was argon. The input power per unit area of ​​the sputtering target was 0.93 W / cm². 2 The temperature of the substrate and the bonding layer is maintained at 450°C during the formation of the first electrode layer. The thickness of the first electrode layer is uniform. The thickness of the first electrode layer is adjusted to 0.2 μm.

[0138] A piezoelectric thin film was directly formed on the entire surface of the first electrode layer by RF magnetron sputtering within a vacuum chamber. Al, Ed (divalent), and Et (tetravalent) were used as sputtering targets; that is, three metal targets were used. The divalent Ed and tetravalent Et used in Example 1 are shown in Table 1 below. The atmosphere within the vacuum chamber was a mixture of Ar and N2. The input power per unit area (cathode power) of each sputtering target was 3.72 W / cm². 2 The substrate temperature was maintained at 150°C during the formation of the piezoelectric thin film. The film formation pressure (gas pressure in the vacuum chamber) was 1 Pa. The substrate bias voltage during the formation of the piezoelectric thin film was 20 W. The film formation rate of the piezoelectric thin film was controlled at 0.35 nm / s. The thickness of the piezoelectric thin film was adjusted to approximately 1000 nm.

[0139] A second electrode layer composed of Mo was formed on the entire second main surface of the piezoelectric thin film using the same method as the first electrode layer. That is, the composition of the second electrode layer is the same as that of the first electrode layer.

[0140] The laminated structure is patterned on the substrate using photolithography. After patterning, the laminate is cut as a whole to obtain a quadrilateral piezoelectric thin film element.

[0141] The piezoelectric thin film element of Example 1 is composed of a substrate, an adhesive layer directly stacked on the surface of the substrate, a first electrode layer directly stacked on the surface of the adhesive layer, a piezoelectric thin film directly stacked on the surface of the first electrode layer, and a second electrode layer directly stacked on the surface (second main surface) of the piezoelectric thin film.

[0142] The following analyses and measurements were performed during or after the fabrication of the piezoelectric thin film element. Multiple identical piezoelectric thin film elements were fabricated as samples for the following analyses and measurements.

[0143] <Composition of piezoelectric thin films>

[0144] The composition of the piezoelectric thin film was analyzed using X-ray fluorescence (XRF). A wavelength dispersive X-ray fluorescence apparatus (RIGAKU AZX-400) manufactured by Rigaku Corporation, Japan, was used in the XRF method. The results showed that the piezoelectric thin film consisted of aluminum nitride containing additive elements (Ed and Et).

[0145] <Crystal Structure of Piezoelectric Thin Films>

[0146] The crystal structure of the piezoelectric thin film was analyzed by X-ray diffraction (XRD). A multi-object X-ray diffraction apparatus (SmartLab) manufactured by Rigaku Corporation of Japan was used in the XRD method. 2θ-θ scans, ω scans, and 2θχ-φ scans were performed on the surface (second principal plane) of the piezoelectric thin film using the aforementioned X-ray diffraction apparatus.

[0147] The XRD pattern obtained by the XRD method shows that the piezoelectric film (i.e., aluminum nitride containing additive elements) has a wurtzite structure. The XRD pattern also shows that the (0002) plane (and (0001) plane) of the wurtzite structure is parallel to the first and second principal planes of the piezoelectric film.

[0148] <Analysis of the cross-section of piezoelectric thin films>

[0149] The cross-section of the piezoelectric thin film was analyzed using scanning transmission electron microscopy (STEM). A Titan G2 microscope manufactured by ThermoFisher Scientific Inc. (formerly FEI company) was used for STEM. The cross-section analyzed by STEM is perpendicular to the first principal surface of the piezoelectric thin film (the surface of the first electrode layer).

[0150] The cross-section of the piezoelectric thin film contains multiple grains. Each grain is a columnar crystal extending along the normal direction of the first principal plane of the piezoelectric thin film. In addition, the cross-section of the piezoelectric thin film contains multiple grain boundaries, each located between an adjacent pair of columnar crystals.

[0151] Five observation areas were randomly selected from the upper cross section analyzed by STEM. The distance from the center of the main surface (circle) of the substrate to each observation area was 10 mm. The concentration of each element in a randomly selected grain from each observation area was determined. The concentration of each element in the grain boundaries of five randomly selected locations from each observation area was determined. Energy-dispersive spectrometry (EDS), an accessory to STEM, was used to analyze the composition of the grain boundaries and grains.

[0152] The [Ed]g, [Etr]g, [Et]g, and [Al]g values ​​for each of the five grains were determined using the method described above. The average values ​​of [Ed]g, [Etr]g, [Et]g, and [Al]g for each of the five grains were calculated. These average values ​​are shown in Table 1 below.

[0153] The [Ed]b, [Etr]b, [Et]b, and [Al]b values ​​for a total of 25 grain boundaries were determined using the method described above. The average values ​​of [Ed]b, [Etr]b, [Et]b, and [Al]b for each of the 25 grain boundaries were calculated. These average values ​​are shown in Table 1 below.

[0154] <Determination of Median Particle Size D50>

[0155] To prevent the piezoelectric film from becoming charged, the entire second principal surface of the piezoelectric film was covered with a thin film made of Pt. Secondary electron images of three randomly selected locations within the second principal surface of the piezoelectric film covered by the Pt film were captured using a scanning transmission electron microscope (SEM). An S-4700 from Hitachi High-Technologies Co., Ltd. was used as the SEM. Each secondary electron image measures 880 nm in height and 1260 nm in width.

[0156] The secondary electron images show that multiple columnar crystals extending along the normal direction of the first principal surface of the piezoelectric film are exposed on the second principal surface of the piezoelectric film.

[0157] Monochrome images of each secondary electron image were obtained through image processing (binarization). The grain size d (Heywood diameter) of multiple grains within each monochrome image was measured. Image analysis software was used for the Heywood diameter measurement. The image analysis software used was manufactured by TDK Corporation (not for sale).

[0158] The median grain size D is calculated based on the grain size distribution obtained by the above method. 50 Table 2 below shows the D of Example 1. 50The result.

[0159] <Determination of resistivity ρ>

[0160] The resistivity ρ (unit: Ωcm) of the piezoelectric thin film in the direction perpendicular to the first and second principal surfaces (thickness direction) was measured. An ADVANTEST measuring apparatus (R8340A) manufactured by ADVANTEST Co., Ltd. was used for the resistivity ρ measurement. An electric field of 1 V / μm was applied to the piezoelectric thin film during the resistivity ρ measurement. The area of ​​the portion of the first and second electrode layers where the electric field was applied was 600 × 600 (μm). 2 The resistivity ρ of Example 1 is shown in Table 2 below. A higher resistivity ρ is preferred. In Table 2, "E+m" refers to "×10"... m (m is any positive integer.)

[0161] <piezoelectric constant d 33 The determination of >

[0162] The piezoelectric constant d of the piezoelectric thin film was determined. 33 (Unit: pC / N). Piezoelectric constant d 33 The details of the measurement are as follows. Table 2 below shows the piezoelectric constant d measured at three points. 33 The average value. Large d 33 This implies excellent piezoelectric properties.

[0163] Measuring apparatus: d-meter manufactured by Piezotest 33 Instrument (PM200)

[0164] Frequency: 110Hz

[0165] Clamping pressure: 0.25N

[0166] <Determination of Dielectric Loss Tangent>

[0167] Prior to the high-temperature test described later, the dielectric loss tangent (tanδ) of each of the three piezoelectric thin-film elements of Example 1 was measured. The tanδ (unit: %) was measured using a measuring apparatus (E4980A) manufactured by Agilent Technologies, Inc. An electric field of 1 V / μm was applied to the piezoelectric thin film during the tanδ measurement. The areas of the portions in the first and second electrode layers where the electric field was applied were each 600 × 600 (μm). 2 .

[0168] In the high-temperature test, the three piezoelectric thin film elements were kept in a constant temperature bath at 125°C for 1000 hours. After the high-temperature test, the tanδ of each of the three piezoelectric thin film elements was measured.

[0169] The average value of tanδ of the three piezoelectric thin film elements measured at room temperature before the high-temperature test is expressed as tanδ. 前 .

[0170] The average value of tanδ for the three piezoelectric thin film elements measured at room temperature after the high-temperature test is expressed as tanδ. 后 .

[0171] According to tanδ 前 and tanδ 后 Calculate the rate of change of tanδ as defined by the following mathematical formula A. (unit:%). The lower the dielectric value, the better it can suppress the increase in dielectric loss as the temperature of the piezoelectric film rises. That is, preferably... Low. As shown in Table 2 below. The tanδ shown in Table 2 below is tanδ 前 .

[0172] Mathematical formula A:

[0173]

[0174] (Examples 2-12)

[0175] As raw materials for the piezoelectric films of Examples 2, 3 and 5 to 12, targets composed of divalent element Ed as shown in Table 1 below and targets composed of tetravalent element Et as shown in Table 1 below were used.

[0176] By changing the input power of each target, the composition of the piezoelectric films in Examples 3 and 5-12 was adjusted to be different from each other.

[0177] The piezoelectric film of Example 4 used a target material composed of the trivalent element Etr, which replaced the target material composed of Ed and the target material composed of Et. The trivalent element Etr of Example 4 is shown in Table 1 below.

[0178] In addition to the matters described above, piezoelectric thin film elements of Examples 2 through 12 were fabricated using the same method as in Example 1. Analysis and measurements related to the piezoelectric thin film elements of Examples 2 through 12 were performed using the same method as in Example 1. The results of the analysis and measurements for Examples 2 through 12 are shown in Tables 1 and 2 below. Figure 6 (a) shows a secondary electron image of the second principal surface of the piezoelectric thin film of Example 11.

[0179] Apart from the differences shown in Tables 1 and 2 below, the piezoelectric films of Examples 2 to 12 each have the same characteristics as those of Example 1.

[0180] (Comparative Example 1)

[0181] An adhesive layer was directly formed on the entire main surface of the substrate using RF magnetron sputtering within a vacuum chamber. The adhesive layer was composed of Ti. Elemental Ti was used as the sputtering target. The atmosphere within the vacuum chamber was argon. The input power per unit area of ​​the sputtering target was 9.87 W / cm². 2 The substrate temperature was maintained at 300°C during the formation of the adhesive layer. The film formation pressure (gas pressure within the vacuum chamber) was 0.7 Pa. No substrate bias was applied. The adhesive layer thickness was uniform. The adhesive layer thickness was adjusted to approximately 30 nm.

[0182] A first electrode layer composed of Cr was formed integrally on the surface of the sealing layer using RF magnetron sputtering within a vacuum chamber. Elemental Cr was used as the sputtering target. The atmosphere within the vacuum chamber was argon. The input power per unit area of ​​the sputtering target was 9.87 W / cm². 2 The temperature of the substrate and the bonding layer is maintained at 500°C during the formation of the first electrode layer. The thickness of the first electrode layer is uniform. The thickness of the first electrode layer is adjusted to 0.2 μm.

[0183] Before forming the piezoelectric thin film, the first electrode layer inside the vacuum chamber was annealed at 600°C. The annealing lasted for 10 minutes.

[0184] A piezoelectric thin film was directly formed on the entire surface of the first electrode layer by RF magnetron sputtering within a vacuum chamber. Only elemental Al (metal) was used as the sputtering target. That is, the piezoelectric thin film of Comparative Example 1 was composed of aluminum nitride without any additives. The atmosphere within the vacuum chamber was a mixture of Ar and N2. The input power per unit area (cathode power) of the sputtering target was 9.87 W / cm². 2 The substrate temperature was maintained at 300°C during the formation of the piezoelectric thin film. The film formation pressure (gas pressure within the vacuum chamber) was 0.3 Pa. The substrate bias voltage was zero W during the formation of the piezoelectric thin film. The film formation rate was controlled at 0.71 nm / s. The thickness of the piezoelectric thin film was adjusted to approximately 1300 nm.

[0185] Apart from the matters described above, the piezoelectric thin film element of Comparative Example 1 was fabricated using the same method as in Example 1.

[0186] The analyses and measurements related to the piezoelectric thin-film element of Comparative Example 1 were performed using the same methods as in Example 1. The results of the analyses and measurements of Comparative Example 1 are shown in Tables 1 and 2 below. Figure 6 (b) shows a secondary electron image of the second principal surface of the piezoelectric thin film of Comparative Example 1. Except for the differences shown in Tables 1 and 2 below (composition of the piezoelectric thin film and D...),... 50Apart from the piezoelectric film of Comparative Example 1, it has the same features as that of Example 1.

[0187] In Table 1 below, r(Ed)g refers to [Ed]g / ([Ed]g+[Et]g).

[0188] In Table 1 below, r(Ed)b refers to [Ed]b / ([Ed]b+[Et]b).

[0189] In Table 2 below, R(Eadd) refers to [Eadd]b / [Eadd]g.

[0190] [Table 1]

[0191]

[0192] [Table 2]

[0193]

[0194] Industrial availability

[0195] For example, the piezoelectric film of one aspect of this disclosure can be used in microphones, sensors, transducers, filters, acquisition devices, or actuators.

[0196] Explanation of reference numerals in the attached figures

[0197] 1: First electrode layer, 2: Second electrode layer, 3: Piezoelectric thin film, 10: Piezoelectric thin film element, s1: Surface of the first electrode layer, s2: Surface of the second electrode layer, s31: First principal surface of the piezoelectric thin film, s32: Second principal surface of the piezoelectric thin film, uc: Unit cell of wurtzite structure (aluminum nitride), Ed: Divalent element, Etr: Trivalent element, Et: Tetravalent element, g1: Grain, g2: Grain boundary.

Claims

1. A piezoelectric thin film, wherein, Contains aluminum nitride with a wurtzite-type structure. The aluminum nitride contains additive elements. The (0001) facet of the wurtzite structure is oriented in the normal direction of the main surface of the piezoelectric film. The piezoelectric thin film comprises: a plurality of grains containing the aluminum nitride. At least a portion of the plurality of grains are a plurality of columnar crystals extending along the normal direction of the main surface of the piezoelectric film. The median grain size of the plurality of grains in a direction parallel to the main surface of the piezoelectric film is greater than 90 nm and less than 500 nm.

2. The piezoelectric thin film according to claim 1, wherein, The added elements contain at least divalent and tetravalent elements.

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

4. The piezoelectric thin film according to claim 1, wherein, The added element contains at least a trivalent element.

5. The piezoelectric thin film according to claim 4, wherein, At least some of the trivalent elements are scandium.

6. The piezoelectric thin film according to claim 1, wherein, The piezoelectric thin film further includes at least one grain boundary located between the plurality of grains. The added elements are either only divalent and tetravalent elements, or only trivalent elements, or a combination of the divalent, trivalent, and tetravalent elements. The proportion of the divalent element in the grain relative to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain is expressed as [Ed] g atoms%. The ratio of the amount of the trivalent element in the grain to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain is expressed as [Etr] g atoms%. The proportion of the tetravalent element in the grain relative to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain is expressed as [Et] g atoms%. The ratio of the amount of the divalent element in the grain boundary to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain boundary is expressed as [Ed]b atoms%. The ratio of the amount of the trivalent element in the grain boundary to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain boundary is expressed as [Etr]b atoms%. The ratio of the amount of the tetravalent element in the grain boundary to the total amount of aluminum, the divalent element, the trivalent element, and the tetravalent element in the grain boundary is expressed as [Et]b atoms%. [Ed]g + [Etr]g + [Et]g is represented as [Eadd]g. [Ed]b + [Etr]b + [Et]b is represented as [Eadd]b. [Eadd]b / [Eadd]g is greater than 0.70 and less than 1.

60.

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

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

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

Images