JP2022124810A - Piezoelectric thin-film element - Google Patents

Abstract

To provide a piezoelectric thin-film element with a piezoelectric thin film having excellent mechanical strength and piezoelectric characteristics.SOLUTION: A piezoelectric thin-film element 10 comprises: a substrate 6; a first electrode layer 4 laminated on the substrate 6; a piezoelectric thin film 3 laminated on the first electrode layer 4; and a second electrode layer 7 laminated on the piezoelectric thin film 3. The piezoelectric thin film 3 contains aluminum nitride and oxygen. An average concentration of oxygen in the piezoelectric thin film 3 is equal to or more than 0.001 atom% and equal to or less than 6 atom%.SELECTED DRAWING: Figure 1

Description

One aspect of the present invention relates to a piezoelectric thin film element.

In recent years, attention has been focused on MEMS (Micro Electro Mechanical Systems). A MEMS (micro-electro-mechanical system) is a device in which mechanical elements, electronic circuits, and the like are integrated on one substrate by microfabrication technology. Piezoelectric thin films are utilized in MEMS with functions such as sensors, transducers, filters, harvesters, or actuators. In manufacturing MEMS using piezoelectric thin films, a lower electrode layer, a piezoelectric thin film, and an upper electrode layer are laminated on a substrate such as silicon or sapphire. Through subsequent post-processes (microfabrication such as patterning, etching, and dicing), MEMS having arbitrary characteristics can be obtained. By selecting a piezoelectric thin film having excellent piezoelectric characteristics, the characteristics of the piezoelectric thin film element such as MEMS are improved, and the size of the piezoelectric thin film element can be reduced. The piezoelectric characteristics of a piezoelectric thin film are evaluated based on, for example, a positive piezoelectric constant (piezoelectric strain constant) d and a piezoelectric output coefficient g. g is equal to d/ε ₀ ε _r . _ε0 is the dielectric constant of the vacuum and _εr is the dielectric constant of the piezoelectric thin film. An increase in d and g improves the properties of the piezoelectric thin film element.

Examples of the piezoelectric composition constituting the piezoelectric thin film include Pb(Zr, Ti)O ₃ (lead zirconate titanate, abbreviation: PZT), LiNbO ₃ (lithium niobate), AlN (aluminum nitride), ZnO (oxide zinc) and CdS (cadmium sulfide) are known.

PZT and _LiNbO3 have a perovskite structure. The d of piezoelectric thin films with a perovskite structure is relatively large. However, when the piezoelectric thin film has a perovskite structure, d tends to decrease as the thickness of the piezoelectric thin film decreases. Therefore, a piezoelectric thin film having a perovskite structure is not suitable for microfabrication. Also, since the piezoelectric thin film having a perovskite structure has a relatively large _εr , the piezoelectric thin film having a perovskite structure tends to have a relatively small g.

AlN, ZnO and CdS, on the other hand, have a wurtzite structure. The d of the piezoelectric thin film with the wurtzite structure is smaller than that of the piezoelectric thin film with the perovskite structure. However, since the _εr of piezoelectric thin films with wurtzite structures is relatively small, piezoelectric thin films with wurtzite structures can have larger g than piezoelectric thin films with perovskite structures. Therefore, a piezoelectric composition having a wurtzite structure is a promising material for a piezoelectric thin film element that requires a large g.

When a piezoelectric thin film containing AlN is formed on the surface of a lower electrode layer made of metal, a tensile stress substantially parallel to the surface of the lower electrode layer is generated due to interaction between the AlN and the lower electrode layer. It tends to occur in thin films. Due to this tensile stress (residual stress), the piezoelectric thin film is easily destroyed. For example, when the piezoelectric thin film is machined (diced, etc.), cracks are likely to be formed in the piezoelectric thin film along a direction substantially perpendicular to the surface of the lower electrode layer due to tensile stress. Such breakage of the piezoelectric thin film degrades the piezoelectric properties and insulating properties of the piezoelectric thin film, thereby lowering the yield rate of the piezoelectric thin film element. Therefore, the tensile stress is reduced (relaxed), and high mechanical strength is required for the piezoelectric thin film element.

For example, the piezoelectric layer described in Patent Document 1 below has a laminated structure composed of a tensile stress layer and a compressive stress layer. By offsetting the tensile stress in the tensile stress layer with the compressive stress in the compressive stress layer, the mechanical strength of the piezoelectric layer is increased and cracks in the piezoelectric layer are suppressed.

The piezoelectric thin film described in Patent Document 2 below is composed of an AlN crystal, part of Al in the AlN crystal is replaced with a first element, and a second element having an ionic radius larger than that of Al is added to the AlN crystal. has been added. The doping of the first element and the second element reduces the residual stress in the piezoelectric thin film.

WO 2007/063842 pamphlet International Publication No. 2016/111280 pamphlet

The conventional method of reducing the tensile stress in the piezoelectric thin film deteriorates the orientation of the AlN crystal structure, resulting in deterioration of the piezoelectric properties of the piezoelectric thin film. Therefore, there is a need to reduce tensile stress in unconventional ways.

An object of one aspect of the present invention is to provide a piezoelectric thin film element having a piezoelectric thin film excellent in mechanical strength and piezoelectric properties.

A piezoelectric thin film element according to one aspect of the present invention includes a substrate, a first electrode layer laminated on the substrate, a piezoelectric thin film laminated on the first electrode layer, a second electrode layer laminated on the piezoelectric thin film, and the piezoelectric thin film contains aluminum nitride and oxygen, and the average oxygen concentration in the piezoelectric thin film is 0.001 atomic % or more and 6 atomic % or less.

The piezoelectric thin film may comprise a first piezoelectric layer laminated on the first electrode layer, and a second piezoelectric layer laminated on the first piezoelectric layer and in contact with the second electrode layer, wherein The concentration of oxygen may be lower than the concentration of oxygen in the second piezoelectric layer.

The ratio of the thickness of the second piezoelectric layer to the thickness of the piezoelectric thin film may be 0.1% or more and 60% or less, and the ratio of the total amount of oxygen in the second piezoelectric layer to the total amount of oxygen in the piezoelectric thin film is 60%. % and may be 100% or less.

The concentration of oxygen in the second piezoelectric layer may increase along a direction substantially perpendicular to the surface of the first electrode layer on which the piezoelectric thin film is laminated and from the first electrode layer to the second electrode layer.

the first electrode layer from platinum, iridium, gold, rhodium, palladium, silver, nickel, copper, aluminum, molybdenum, tungsten, vanadium, chromium, niobium, tantalum, ruthenium, zirconium, hafnium, titanium, yttrium, scandium and magnesium may contain at least one element selected from the group consisting of

According to one aspect of the present invention, a piezoelectric thin film element having a piezoelectric thin film excellent in mechanical strength and piezoelectric properties is provided.

FIG. 1 is a schematic cross section of a piezoelectric thin film element according to one embodiment of the present invention, and the cross section shown in FIG. Vertical. FIG. 2 is a perspective view of a unit cell of the crystal structure (wurtzite structure) of aluminum nitride. FIG. 3 is an example of oxygen concentration distribution in a piezoelectric thin film. FIG. 4 is another example of the oxygen concentration distribution in the piezoelectric thin film. FIG. 5 is another example of the oxygen concentration distribution in the piezoelectric thin film. FIG. 6 is another example of the oxygen concentration distribution in the piezoelectric thin film. FIG. 7 shows the temperature profile of the substrate in the process of forming the piezoelectric thin film.

Preferred embodiments of the present invention will now be described, with occasional reference to the drawings. The present invention is not limited to the following embodiments. In each figure, the same reference numerals are given to the same or equivalent components. X, Y and Z shown in FIG. 1 mean three coordinate axes orthogonal to each other.

As shown in FIG. 1, the piezoelectric thin film element 10 according to the present embodiment includes a substrate 6, an adhesion layer 5 directly laminated on the surface of the substrate 6, and an adhesive layer 5 indirectly laminated on the surface of the substrate 6 via the adhesion layer 5. a first electrode layer 4 laminated on the first electrode layer 4, a piezoelectric thin film 3 directly or indirectly laminated on the surface of the first electrode layer 4, and a second electrode layer directly or indirectly laminated on the surface of the piezoelectric thin film 3 7 and . The lamination direction of the substrate 6, the adhesion layer 5, the first electrode layer 4, the piezoelectric thin film 3, and the second electrode layer 7 is substantially parallel to the Z-axis. The substrate 6, the adhesion layer 5, the first electrode layer 4, the piezoelectric thin film 3, and the second electrode layer 7 may each have a flat shape extending along the XY plane directions (X axis and Y axis). The thickness of each of the substrate 6, the adhesion layer 5, the first electrode layer 4, the piezoelectric thin film 3 and the second electrode layer 7 may be substantially uniform. The adhesion layer 5 may directly cover part or all of the surface of the substrate 6 . The first electrode layer 4 may directly cover part or all of the surface of the adhesion layer 5 . The piezoelectric thin film 3 may directly or indirectly cover part or all of the surface of the first electrode layer 4 . The second electrode layer 7 may directly or indirectly cover part or all of the surface of the piezoelectric thin film 3 . The adhesion layer 5 is not essential for the piezoelectric thin film element 10 , and the first electrode layer 4 may be laminated directly on the surface of the substrate 6 . Without the adhesion layer 5 , the first electrode layer 4 may directly cover part or all of the surface of the substrate 6 . The first electrode layer 4 may be called a lower electrode layer. The second electrode layer 7 may be called an upper electrode layer. A first intermediate layer (first buffer layer) may be provided between the first electrode layer 4 and the piezoelectric thin film 3 . For example, the first intermediate layer may consist of an electrical conductor or a piezoelectric material. The lattice constant of the first intermediate layer in the direction substantially parallel to the surface of the first electrode layer 4 may be less than or equal to the lattice constant of the first electrode layer 4 in the same direction and greater than or equal to the lattice constant of the piezoelectric thin film 3 in the same direction. can be A second intermediate layer (second buffer layer) may be provided between the piezoelectric thin film 3 and the second electrode layer 7 . For example, the second intermediate layer may consist of an electrical conductor or a piezoelectric material. The lattice constant of the second intermediate layer in the direction substantially parallel to the surface of the first electrode layer 4 may be less than or equal to the lattice constant of the second electrode layer 7 in the same direction and greater than or equal to the lattice constant of the piezoelectric thin film 3 in the same direction. can be

The piezoelectric thin film 3 contains crystalline aluminum nitride (AlN). The AlN forming the piezoelectric thin film 3 may be single crystal or polycrystal. The crystal structure of AlN is a hexagonal wurtzite structure. FIG. 2 shows the unit cell ucw of the wurtzite structure. The unit cell ucw is a regular hexagonal prism. The AlN in the piezoelectric thin film 3 may be columnar crystals extending in the direction normal to the surface of the first electrode layer 4 .

The (001) plane and (002) plane of AlN in the piezoelectric thin film 3 may be substantially parallel to the surface of the first electrode layer 4 on which the piezoelectric thin film 3 is laminated. In other words, the (001) plane and (002) plane of AlN in the piezoelectric thin film 3 may be oriented in the normal direction of the surface of the first electrode layer 4 . The (001) plane of AlN corresponds to the regular hexagonal crystal plane in the unit cell ucw shown in FIG. When the piezoelectric thin film 3 includes a plurality of AlN crystalline grains, the (001) planes and (002) planes of some or all of the crystal grains are substantially parallel to the surface of the first electrode layer 4. you can

The crystal orientation in which the piezoelectric properties of AlN are exhibited is [001] of AlN. Therefore, since the (001) plane and the (002) plane of AlN are substantially parallel to the surface of the first electrode layer 4, the piezoelectric thin film 3 can have excellent piezoelectric properties. For the same reason, the (001) plane and (002) plane of the wurtzite structure of aluminum nitride may be substantially parallel to the surface of the piezoelectric thin film 3 in contact with the first electrode layer 4 . In other words, the (001) plane and (002) plane of AlN in the piezoelectric thin film 3 may be oriented in the normal direction of the surface of the piezoelectric thin film 3 .

The piezoelectric thin film 3 contains oxygen (O) as well as AlN, and the average oxygen concentration in the piezoelectric thin film 3 is 0.001 atomic % or more and 6.000 atomic % or less. As a result, the piezoelectric thin film 3 can have high mechanical strength and excellent piezoelectric properties. The reason is as follows.

Since the piezoelectric thin film 3 grows epitaxially on the surface of the crystalline first electrode layer 4, lattice mismatch between the piezoelectric thin film 3 and the first electrode layer 4 is likely to occur. As a result, at the interface between the piezoelectric thin film 3 and the first electrode layer 4, stress that expands the wurtzite structure in the in-plane direction is likely to occur. That is, tensile stress (residual stress) substantially parallel to the surface of the first electrode layer 4 is likely to occur in the piezoelectric thin film 3 . If the piezoelectric thin film 3 does not contain oxygen, the piezoelectric thin film 3 is likely to tear in a direction substantially parallel to the surface of the first electrode layer 4 due to tensile stress. For example, due to tensile stress, cracks are likely to form in the piezoelectric thin film 3 along a direction substantially perpendicular to the surface of the first electrode layer 4 . As a result, the piezoelectric properties and insulating properties of the piezoelectric thin film 3 deteriorate.
However, since the piezoelectric thin film 3 according to the present embodiment contains oxygen, the crystal lattice of AlN forming the piezoelectric thin film 3 includes oxygen atoms. Due to inclusion of oxygen atoms in the crystal lattice of AlN, the crystal structure of AlN expands in a direction substantially parallel to the surface of the first electrode layer 4 . That is, the lattice constant of AlN increases in the direction substantially parallel to the surface of the first electrode layer 4 . As a result, the lattice mismatch between the piezoelectric thin film 3 and the first electrode layer 4 is reduced, the tensile stress in the piezoelectric thin film 3 is reduced (relaxed), and the piezoelectric thin film 3 can have high mechanical strength. That is, breakage of the piezoelectric thin film 3 due to residual stress in the piezoelectric thin film 3 is suppressed. As a result, the piezoelectric properties and insulating properties of the piezoelectric thin film 3 are improved. For the same reason as described above, breakage of the piezoelectric thin film 3 during the manufacturing process of the piezoelectric thin film element 10 according to this embodiment is suppressed, and the yield rate of the piezoelectric thin film element 10 increases.
As the average oxygen concentration in the piezoelectric thin film 3 decreases, the crystallinity of AlN and the orientation of AlN tend to improve, and the piezoelectric characteristics of the piezoelectric thin film 3 also tend to improve. However, when the average oxygen concentration in the piezoelectric thin film 3 is lower than 0.001 atomic %, the oxygen content in the piezoelectric thin film 3 is too small, and the tensile stress in the piezoelectric thin film 3 is not sufficiently reduced. It is difficult for 3 to have high mechanical strength.
As the average oxygen concentration in the piezoelectric thin film 3 increases, the tensile stress in the piezoelectric thin film 3 tends to decrease and the mechanical strength of the piezoelectric thin film 3 tends to increase. However, if the average oxygen concentration in the piezoelectric thin film 3 is higher than 6.000 atomic %, the oxygen content in the piezoelectric thin film 3 is too high, deteriorating the crystallinity and orientation of AlN. It is difficult to have good piezoelectric properties.
As the average oxygen concentration in the piezoelectric thin film 3 increases, the dielectric loss tangent (tan δ) of the piezoelectric thin film 3 tends to increase. In particular, when the average oxygen concentration in the piezoelectric thin film 3 is higher than 6.000 atomic %, the dielectric loss tangent of the piezoelectric thin film 3 is significantly large. It is preferable that the dielectric loss tangent of the piezoelectric thin film 3 is small. For example, the dielectric loss tangent of the piezoelectric thin film 3 may be 0% or more and 5% or less, 0% or more and 3% or less, 0% or more and 2% or less, or 0% or more and 1% or less.
The piezoelectric thin film 3 tends to have high mechanical strength and excellent piezoelectric characteristics, and the dielectric loss tangent of the piezoelectric thin film 3 tends to be reduced. .800 atomic % or less.

The piezoelectric thin film 3 consists of a first piezoelectric layer 1 directly laminated on the first electrode layer 4 and a second piezoelectric layer 2 directly laminated on the first piezoelectric layer 1 and in contact with the second electrode layer 7 . you can The oxygen concentration in the first piezoelectric layer 1 may be lower than the oxygen concentration in the second piezoelectric layer 2 . The average oxygen concentration in the first piezoelectric layer 1 may be lower than the average oxygen concentration in the second piezoelectric layer 2 . The first piezoelectric layer 1 may not contain oxygen. The first piezoelectric layer 1 may contain oxygen. The thickness T1 of the first piezoelectric layer 1 may be substantially uniform. The thickness T2 of the second piezoelectric layer 2 may be substantially uniform. T3 of the piezoelectric thin film 3 is equal to T1+T2.
In the process of forming the piezoelectric thin film 3 , after the first piezoelectric layer 1 grows on the surface of the first electrode layer 4 , the second piezoelectric layer 2 grows continuously on the first piezoelectric layer 1 . Therefore, the crystallinity and orientation of AlN contained in the second piezoelectric layer 2 depend on the crystallinity and orientation of AlN contained in the first piezoelectric layer 1 . That is, due to the improvement in the crystallinity of AlN contained in the first piezoelectric layer 1, the crystallinity of AlN contained in the second piezoelectric layer 2 is also improved, and the (001) plane of AlN contained in the first piezoelectric layer 1 Due to the orientation, the (001) plane of AlN included in the second piezoelectric layer 2 is also oriented in the same direction. Also, as the concentration of oxygen in the first piezoelectric layer 1 decreases, the crystallinity and orientation of AlN contained in the first piezoelectric layer 1 improve. For the above reasons, when the oxygen concentration in the first piezoelectric layer 1 is lower than the oxygen concentration in the second piezoelectric layer 2, the AlN contained in the second piezoelectric layer 2 is the AlN contained in the first piezoelectric layer 1 The excellent crystallinity and orientation of AlN are inherited, and the crystallinity and orientation of AlN in the entire piezoelectric thin film 3 are improved. As a result, the piezoelectric thin film 3 can have excellent piezoelectric properties.
Further, when the concentration of oxygen in the second piezoelectric layer 2 is higher than the concentration of oxygen in the first piezoelectric layer 1, the oxygen located near the surface of the second piezoelectric layer 2 in contact with the second electrode layer 7 becomes the second piezoelectric layer. It easily bonds with the elements (metal elements, etc.) forming the layer 2 . As a result, the second electrode layer 7 is easily adhered to the second piezoelectric layer 2, and peeling of the second electrode layer 7 from the second piezoelectric layer 2 (piezoelectric thin film 3) is suppressed.

The ratio (T2/T3) of the thickness T2 of the second piezoelectric layer 2 to the thickness T3 of the piezoelectric thin film 3 may be 0.1% or more and 60% or less. The percentage of the total amount of oxygen in 2 may be greater than 60% and less than or equal to 100%. The units for the total amount of oxygen may be moles. The unit of the ratio of the total amount of oxygen in the second piezoelectric layer 2 to the total amount of oxygen in the piezoelectric thin film 3 may be mol % or atomic %. When T2/T3 is 0.1% or more, the piezoelectric thin film 3 tends to have sufficiently high mechanical strength. When T2/T3 is 60% or less, the piezoelectric thin film 3 tends to have sufficiently high mechanical strength without impairing the piezoelectric properties of the piezoelectric thin film 3, and the dielectric loss tangent of the piezoelectric thin film 3 tends to be reduced. When the ratio of the total amount of oxygen in the second piezoelectric layer 2 to the total amount of oxygen in the piezoelectric thin film 3 is greater than 60%, the piezoelectric thin film 3 tends to have sufficiently high mechanical strength. Since the piezoelectric thin film 3 tends to have high mechanical strength and excellent piezoelectric properties, T2/T3 may be 1% or more and 60% or less. Since the piezoelectric thin film 3 tends to have high mechanical strength and excellent piezoelectric properties, the ratio of the total amount of oxygen in the second piezoelectric layer 2 to the total amount of oxygen in the piezoelectric thin film 3 is greater than 60% and 99.5%. It may be 9% or less, or 64% or more and 95% or less.

The thickness T3 of the piezoelectric thin film 3 may be, for example, 100 nm or more and 30000 nm or less, or 200 nm or more and 2000 nm or less. The thickness T2 of the second piezoelectric layer 2 may be (0.001×T3) nm or more and (0.600×T3) nm or less. The thickness T2 of the second piezoelectric layer 2 may be, for example, 10 nm or more and 1200 nm or less. The thickness T1 of the first piezoelectric layer 1 is equal to T3-T2. The thickness T1 of the first piezoelectric layer 1 may be, for example, 300 nm or more and 990 nm or less.

A direction substantially perpendicular to the surface of the first electrode layer 4 and directed from the first electrode layer 4 to the second electrode layer 7 is referred to as a thickness direction D of the piezoelectric thin film 3 . The [001] (crystal orientation) of AlN contained in the piezoelectric thin film 3 may be substantially parallel to the thickness direction D. Specific examples of the oxygen concentration distribution in the piezoelectric thin film 3 along the thickness direction D are shown in FIGS. 3, 4, 5 and 6. FIG. The horizontal axis shown in each figure indicates the distance d (unit: nm) from the surface of the first electrode layer 4 in the thickness direction D and corresponds to an arbitrary position in the piezoelectric thin film 3 . The vertical axis of the graph shown in each figure indicates the oxygen concentration [O] (unit: atomic %) at the distance d from the surface of the first electrode layer 4 .

The concentration of oxygen in the second piezoelectric layer 2 may increase along the thickness direction D gradually or stepwise. That is, the oxygen concentration distribution in the second piezoelectric layer 2 may have a gradient that increases along the thickness direction D. As shown in FIG. When the oxygen concentration in the second piezoelectric layer 2 increases along the thickness direction D, the tensile stress in the piezoelectric thin film 3 tends to be reduced, and the piezoelectric thin film 3 tends to have high mechanical strength.

As shown in FIG. 3, the oxygen concentration distribution in the second piezoelectric layer 2 may be represented by a straight line.

As shown in FIG. 4, the oxygen concentration distribution in the second piezoelectric layer 2 may be represented by a downward convex curve. In the case of the oxygen concentration distribution shown in FIG. 4, oxygen is localized in the vicinity of the surface of the second piezoelectric layer 2 in contact with the second electrode layer 7 . As a result, the piezoelectric thin film 3 tends to have sufficiently high mechanical strength without impairing the piezoelectric properties of the piezoelectric thin film 3 , and the second electrode layer 7 easily adheres to the second piezoelectric layer 2 .

As shown in FIG. 5, the oxygen concentration distribution in the second piezoelectric layer 2 may be represented by an upward convex curve.

As shown in FIG. 6, the concentration of oxygen in the second piezoelectric layer 2 may be substantially uniform.

As shown in FIGS. 3, 4, 5 and 6, the concentration of oxygen in the first piezoelectric layer 1 may be substantially uniform. When the oxygen concentration in the first piezoelectric layer 1 is substantially uniform, the crystallinity and orientation of AlN in the entire piezoelectric thin film 3 tend to improve. However, along the thickness direction D, the concentration of oxygen in the first piezoelectric layer 1 may increase.

The average oxygen concentration [O] ₁ in the first piezoelectric layer 1 may be, for example, 0.000 atomic % or more and 5.000 atomic % or less, or 0.001 atomic % or more and 5.000 atomic % or less. . [O] ₁ may be, for example, a value obtained by dividing the integral of the oxygen concentration distribution in the first piezoelectric layer 1 along the thickness direction D by the thickness T1 of the first piezoelectric layer 1 .

The average oxygen concentration [O] ₂ in the second piezoelectric layer 2 may be, for example, 2.000 atomic % or more and 6.000 atomic % or less. [O] ₂ may be, for example, a value obtained by dividing the integral of the oxygen concentration distribution in the second piezoelectric layer 2 along the thickness direction D by the thickness T2 of the second piezoelectric layer 2 .

As described above, the average oxygen concentration [O] ₃ in the piezoelectric thin film 3 is 0.001 atomic % or more and 6.000 atomic % or less. [O] ₃ may be, for example, a value obtained by dividing the integral of the oxygen concentration distribution in the piezoelectric thin film 3 along the thickness direction D by the thickness T 3 of the piezoelectric thin film 3 . [O] ₃ is equal to the weighted average of [O] ₁ and [O] ₂ based on T1 and T2. That is, [O] ₃ is equal to (T1/T3)*[O] ₁₊ (T2/T3)*[O] ₂ .

The first piezoelectric layer 1 and the second piezoelectric layer 2 may be distinguished from each other on the basis of different concentrations of oxygen. For example, the interface between the first piezoelectric layer 1 and the second piezoelectric layer 2 may be a plane including a bending point or an inflection point of a line indicating the oxygen concentration distribution in the piezoelectric thin film 3 along the thickness direction D.

The oxygen concentration may be substantially uniform throughout the piezoelectric thin film 3 . The piezoelectric thin film 3 may not be composed of the first piezoelectric layer 1 and the second piezoelectric layer 2, and the oxygen concentration in the piezoelectric thin film 3 may increase continuously along the thickness direction D, and The line indicating the oxygen concentration distribution in the piezoelectric thin film 3 does not have to have a bending point and an inflection point.

The piezoelectric thin film 3 may consist of Al, N and O only. However, the piezoelectric thin film 3 may further contain other elements in addition to Al, N and O as long as the mechanical strength and piezoelectric properties of the piezoelectric thin film 3 are not impaired. For example, the piezoelectric thin film 3 includes a monovalent metal element Mm, a divalent metal element Md, a trivalent metal element Mtr, a tetravalent metal element Mt, and a pentavalent metal element Mt. It may further contain at least one additional element selected from the group consisting of the (pentavalent) metal elements Mp. The doping of AlN with the additive elements described above distorts the wurtzite structure of AlN and changes the strength of chemical bonds between atoms in the wurtzite structure. As a result, the piezoelectric characteristics of the piezoelectric thin film 3 may be improved.
The monovalent metal element Mm may be at least one element selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs).
The divalent metal element Md may be at least one selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba).
The trivalent metal element Mtr may be at least one element selected from the group consisting of Sc (scandium), Y (yttrium), lanthanides and In (indium).
The tetravalent metal element Mt may be at least one selected from the group consisting of germanium (Ge), titanium (Ti), zirconium (Zr) and hafnium (Hf).
The pentavalent metal element Mp may be at least one element selected from the group consisting of Cr (chromium), V (vanadium), Nb (niobium) and Ta (tantalum).
The average value of the valences of the additive elements contained in the piezoelectric thin film 3 may be three.
The total content of monovalent metal elements Mm in aluminum nitride may be expressed as [Mm] atomic %. The total content of pentavalent metal elements Mp in aluminum nitride may be expressed as [Mp] atomic %. [Mm]/[Mp] may be approximately 1.0.
The total content of the divalent metal element Md in aluminum nitride may be expressed as [Md] atomic %. The total content of the tetravalent metal element Mt in aluminum nitride may be expressed as [Mt] atomic %. [Md]/[Mt] may be approximately 1.0.

The substrate 6 is, for example, a semiconductor substrate (silicon substrate, gallium arsenide substrate, etc.), an optical crystal substrate (sapphire substrate, etc.), an insulator substrate (glass substrate, ceramic substrate, etc.), or a metal substrate (stainless steel plate, etc.). It's okay.

The first electrode layer 4 is composed of Pt (platinum), Ir (iridium), Au (gold), Rh (rhodium), Pd (palladium), Ag (silver), Ni (nickel), Cu (copper), Al (aluminum ), Mo (molybdenum), W (tungsten), V (vanadium), Cr (chromium), Nb (niobium), Ta (tantalum), Ru (ruthenium), Zr (zirconium), Hf (hafnium), Ti (titanium ), Y (yttrium), Sc (scandium), and Mg (magnesium). The first electrode layer 4 may consist of only one kind of metal Me. The first electrode layer 4 may consist of only at least two types of metal Me. That is, the first electrode layer 4 may consist only of an alloy composed of at least two kinds of metals Me. The first electrode layer 4 may further contain an element other than the metal Me.
The metal Me forming the first electrode layer 4 may have a crystal structure. That is, the first electrode layer 4 may be crystalline. For example, when the metal Me is at least one element selected from the group consisting of Pt, Ir, Au, Rh, Pd, Ag, Ni, Cu and Al, the metal Me has a face-centered cubic (fcc) structure. The (111) plane of the fcc structure is easily oriented in the direction normal to the surface of the first electrode layer 4 . When the metal Me is at least one element selected from the group consisting of Mo, W, V, Cr, Nb and Ta, the metal Me tends to have a body-centered cubic (bcc) structure, and the (110) The plane is easily oriented in the direction normal to the surface of the first electrode layer 4 . When the metal Me is at least one element selected from the group consisting of Ru, Zr, Hf, Ti, Y, Sc and Mg, the metal Me tends to have a hexagonal close-packed (hcp) structure. The (001) plane tends to be oriented in the normal direction of the surface of the first electrode layer 4 .

In the direction substantially parallel to the surface of the first electrode layer 4, the lattice length of the crystal structure of the metal Me may be longer than the lattice length of AlN. For example, in the direction substantially parallel to the surface of the first electrode layer 4, the atomic spacing in the crystal structure of metal Me may be larger than the atomic spacing in AlN. Alternatively, in the direction substantially parallel to the surface of the first electrode layer 4, the lattice constant of the crystal structure of the metal Me may be larger than the lattice constant of AlN. For example, the lattice length in the crystal structure of molybdenum is larger than that in AlN. In the direction substantially parallel to the surface of the first electrode layer 4, when the lattice length in the crystal structure of the metal Me is larger than the lattice length in AlN, the first electrode layer 4 (metal Me) and the conventional piezoelectric thin film (AlN) Tensile stress tends to occur in a conventional piezoelectric thin film in a direction substantially parallel to the surface of the first electrode layer 4 due to the lattice mismatch between the . However, since the piezoelectric thin film 3 according to this embodiment contains oxygen, the tensile stress in the piezoelectric thin film 3 is reduced, and the piezoelectric thin film 3 can have high mechanical strength.

The adhesion layer 5 is made of Al (aluminum), Si (silicon), Ti (titanium), Zn (zinc), Y (yttrium), Zr (zirconium), Cr (chromium), Nb (niobium), Mo (molybdenum), At least one element selected from the group consisting of Hf (hafnium), Ta (tantalum), W (tungsten) and Ce (cerium) may be included. The adhesion layer 5 may be a single metal, an alloy, or a compound (such as an oxide). The adhesion layer 5 may be composed of another piezoelectric thin film, polymer, or ceramics. The interposition of the adhesion layer 5 facilitates orientation of the (111) plane of the fcc structure of the first electrode layer 4 in the normal direction of the surface of the first electrode layer 4 . Alternatively, the (110) plane of the bcc structure of the first electrode layer 4 is easily oriented in the normal direction of the surface of the first electrode layer 4 due to the presence of the adhesion layer 5 . Alternatively, the interposition of the adhesion layer 5 facilitates orientation of the (001) plane of the hcp structure of the first electrode layer 4 in the normal direction of the surface of the first electrode layer 4 . The adhesion layer 5 also has a function of suppressing peeling of the first electrode layer 4 due to mechanical impact or the like. The adhesion layer 5 may be called an interface layer, a support layer, a buffer layer, or an intermediate layer.

The second electrode layer 7 contains Pt, Ir, Au, Rh, Pd, Ag, Ni, Cu, Al, Mo, W, V, Cr, Nb, Ta, Ru, Zr, Hf, Ti, Y, Sc and Mg. It may contain at least one element selected from the group consisting of The second electrode layer 7 may be a single metal. The second electrode layer 7 may be an alloy containing at least two elements selected from the above group.

The thickness of the substrate 6 may be, for example, 50 μm or more and 10000 μm or less. The thickness of the adhesion layer 5 may be, for example, 0.003 μm or more and 1 μm or less. The thickness of the first electrode layer 4 may be, for example, 0.01 μm or more and 1 μm or less. The thickness of the second electrode layer 7 may be, for example, 0.01 μm or more and 1 μm or less.

The adhesion layer 5, the first electrode layer 4, the piezoelectric thin film 3, and the second electrode layer 7 may be laminated on the surface of the substrate 6 by vapor deposition such as sputtering. The adhesion layer 5, the first electrode layer 4, the piezoelectric thin film 3 and the second electrode layer 7 may each be formed by sputtering using at least one kind of target. The adhesion layer 5, the first electrode layer 4, the piezoelectric thin film 3, and the second electrode layer 7 may be formed by sputtering using multiple targets. The target may contain at least one of the elements constituting each layer or piezoelectric thin film. Each layer and piezoelectric thin film 3 each having a target composition can be formed by selecting and combining targets having a predetermined composition. For example, the target may be an elemental metal, an alloy, or an oxide. The composition of the sputtering atmosphere may be the controlling factor for the composition of each layer and the piezoelectric thin film 3 respectively. The input power (power density) applied to each target is a control factor for the composition and thickness of each layer and piezoelectric thin film 3 respectively. The total pressure of the sputtering atmosphere, the partial pressure or concentration of the source gas (e.g., nitrogen) in the atmosphere, the duration of sputtering for each target, the temperature of the substrate surface on which the piezoelectric thin film is formed, the substrate bias, etc. It is a control factor for the composition and thickness of each thin film 3 . Etching (eg, plasma etching) may form a piezoelectric thin film having a desired shape or pattern.

A sputtering atmosphere (film formation atmosphere) for forming the piezoelectric thin film 3 contains nitrogen gas (N ₂ ) and oxygen gas (O ₂ ). Oxygen contained in the piezoelectric thin film 3 is derived from oxygen in the film formation atmosphere. For example, the film formation atmosphere may be a mixed gas containing a rare gas (such as argon), a nitrogen gas, and an oxygen gas. The partial pressure of oxygen gas in the film formation atmosphere may be 5.11×10 ⁻⁷ Pa, for example. When the oxygen gas partial pressure in the film-forming atmosphere is higher than 5.11×10 ⁻⁷ Pa, the average oxygen concentration in the piezoelectric thin film 3 tends to be higher than 6 atomic %. If the oxygen gas partial pressure in the film-forming atmosphere is too low, the average oxygen concentration in the piezoelectric thin film 3 tends to be lower than 0.001 atomic %.

A temperature profile of the substrate 6 in the process of forming the piezoelectric thin film 3 is shown in FIG. The horizontal axis of FIG. 7 indicates time (unit: seconds). The vertical axis in FIG. 7 indicates the temperature of the substrate 6 (unit: degrees Celsius).

When forming the piezoelectric thin film 3 having a substantially uniform oxygen concentration, the process of forming the piezoelectric thin film 3 includes a first temperature raising step of raising the temperature of the substrate 6 from the initial temperature T0 to the initial film forming temperature TI, and a film forming atmosphere. and a first film forming step in which the temperature of the substrate 6 is maintained at the initial film forming temperature TI. The first temperature raising step may be performed before starting the sputtering of the Al-containing target. That is, the first film forming step (growing the piezoelectric thin film 3 in the film forming atmosphere) may start with the start of sputtering of the target. When forming the piezoelectric thin film 3 with a substantially uniform oxygen concentration, the initial film formation temperature TI is, for example, higher than 300°C and lower than 400°C, preferably 350°C. As the initial film formation temperature TI increases, the oxygen concentration in the piezoelectric thin film 3 tends to increase. When the initial film formation temperature TI is lower than 350° C., the average oxygen concentration in the piezoelectric thin film 3 tends to be lower than 0.001 atomic %. When the initial film formation temperature TI is higher than 350° C., the average oxygen concentration in the piezoelectric thin film 3 tends to be higher than 6 atomic %. When forming the piezoelectric thin film 3 with a substantially uniform oxygen concentration, the film formation time t _TOTAL is equal to the duration of the first film formation process (first film formation time ti).

When the piezoelectric thin film 3 composed of the first piezoelectric layer 1 and the second piezoelectric layer 2 is formed, the step of forming the piezoelectric thin film 3 further includes a first temperature raising step and a second temperature raising step following the first film forming step. . The second temperature raising step is a step of raising the temperature of the substrate 6 from the initial film formation temperature TI to the final film formation temperature TF. When the piezoelectric thin film 3 composed of the first piezoelectric layer 1 and the second piezoelectric layer 2 is formed, the step of forming the piezoelectric thin film 3 may further include a second film forming step following the above-described second temperature raising step. The second film forming process is a process of maintaining the temperature of the substrate 6 at the final film forming temperature TF in the film forming atmosphere. The first piezoelectric layer 1 may be formed in the first film forming step. In the second heating step and the second film forming step, the second piezoelectric layer 2 having a gradient of oxygen concentration distribution may be formed. The inventors have found that the second piezoelectric layer 2 having a high oxygen concentration is formed because the oxygen in the film formation atmosphere is easily ionized as the temperature of the substrate 6 rises in the second temperature raising step. Guess. When the piezoelectric thin film 3 composed of the first piezoelectric layer 1 and the second piezoelectric layer 2 is formed, the initial film formation temperature TI may be, for example, 200° C. or higher and 300° C. or lower. As the initial film formation temperature TI increases, the oxygen concentration in the first piezoelectric layer 1 tends to increase. The temperature increase rate (second temperature increase rate R _T ) in the second temperature increase step may be, for example, 1.0° C./second or more and 1.9 seconds or less. The final film forming temperature TF may be, for example, 230° C. or higher and 350° C. or lower. As the final film forming temperature TF increases, the oxygen concentration in the second piezoelectric layer 2 tends to increase. When the final film forming temperature TF is higher than 350° C., the average oxygen concentration in the piezoelectric thin film 3 tends to be higher than 6 atomic %. When forming the piezoelectric thin film 3 composed of the first piezoelectric layer 1 and the second piezoelectric layer 2, the film forming time t _TOTAL is the first film forming time ti, the time required for the second temperature rising step, and the second film forming step is the sum of the duration of (second film formation time tf). When forming the piezoelectric thin film 3 composed of the first piezoelectric layer 1 and the second piezoelectric layer 2, the first film formation time ti is the time from the start of the first film formation step to the start of the second temperature raising step. rephrased.

The units of the film formation time t _TOTAL , the first film formation time ti, and the second film formation time tf may be seconds. As the film formation time t _TOTAL increases, the thickness T3 of the piezoelectric thin film 3 tends to increase. As the first film forming time ti increases, the thickness T1 of the first piezoelectric layer 1 tends to increase. The thickness T2 of the second piezoelectric layer 2 tends to increase as the time required for the second heating step and the second film formation time tf increase.

The crystal structure of each of the adhesion layer 5, the first electrode layer 4, the piezoelectric thin film 3 and the second electrode layer 7 may be specified by an X-ray diffraction (XRD) method. The composition of each layer and the piezoelectric thin film 3 is determined by X-ray fluorescence analysis (XRF method), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray analysis (EDX), inductively coupled plasma mass spectrometry (ICP-MS), and laser ablation. It may be specified for at least one analysis method among inductively coupled plasma-mass spectrometry (LA-ICP-MS) and electron probe microanalyzer (EPMA). The respective thicknesses of the adhesion layer 5, the first electrode layer 4, the piezoelectric thin film 3, and the second electrode layer 7 are measured in a cross section of the piezoelectric thin film element 10 parallel to the thickness direction D with a scanning electron microscope (SEM). good. Al, N and O concentration distributions in the piezoelectric thin film 3 along the thickness direction D may be measured by photoelectron spectroscopy (XPS), for example. A cross section of the piezoelectric thin film 3 parallel to the thickness direction D may be observed by SEM, and line analysis using EDX in the cross section of the piezoelectric thin film 3 reveals Al, N and O in the piezoelectric thin film 3 along the thickness direction D. Individual concentration distributions may be measured. Based on the O concentration distribution in the piezoelectric thin film 3 along the thickness direction D, the thickness T1 of the first piezoelectric layer 1 and the thickness T2 of the second piezoelectric layer 2 may be specified.

Applications of the piezoelectric thin film element according to the present embodiment are diverse. For example, the piezoelectric thin film element may be a piezoelectric actuator. Piezoelectric actuators may be used for haptics. In other words, piezoelectric actuators may be used in various devices that require cutaneous (tactile) feedback. The device for which tactile feedback is desired may be a wearable device, touchpad, display, or game controller. Piezoelectric actuators may be used in head assemblies, head stack assemblies, or hard disk drives. Piezoelectric actuators may be used in printer heads or in inkjet printer devices. Piezoelectric actuators may be used in piezoelectric switches. The piezoelectric thin film element may be a piezoelectric sensor or a piezoelectric transducer. Piezoelectric sensors or piezoelectric transducers may be used, for example, as gyro sensors, pressure sensors, pulse wave sensors, ultrasonic sensors, ultrasonic transducers, or shock sensors. The ultrasonic transducer may be a Piezoelectric Micromachined Ultrasonic Transducer (PMUT). Products that apply piezoelectric micromechanical ultrasonic transducers can be, for example, biometric sensors, medical/healthcare sensors (fingerprint sensors or ultrasonic blood vessel authentication sensors), or Time of Flight (ToF) sensors. . Piezoelectric thin film elements may be used in piezoelectric microphones, filters (SAW filters or BAW filters), harvesters, oscillators, resonators, or acoustic multilayer films. Each piezoelectric thin film element described above may be part or all of a MEMS.

The invention is not limited to the embodiments described above. For example, the oxygen concentration in the first piezoelectric layer 1 may be higher than the oxygen concentration in the second piezoelectric layer 2 .

The present invention will be described in detail below with reference to examples and comparative examples. The present invention is by no means limited by these examples.

(Example 1)
A wafer made of a single crystal of Si was used as the substrate. The surface of the substrate was parallel to the (100) plane of Si. The thickness of the substrate was 725 μm. The substrate diameter was about 8 inches. The thickness of the substrate was uniform.

A first electrode layer (lower electrode layer) made of Mo was directly formed on the entire surface of the adhesion layer by RF magnetron sputtering in a vacuum chamber. Mo alone was used as a 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 substrate temperature was kept at 600° C. during the formation of the first electrode layer. The thickness of the first electrode layer was uniform. The thickness of the first electrode layer was 0.2 μm.

In the piezoelectric thin film forming process, the piezoelectric thin film was directly formed on the entire surface of the first electrode layer by RF magnetron sputtering in a vacuum chamber. A simple substance of Al was used as a sputtering target. The input power per unit area of the sputtering target was 3.72 W/cm ² . In the piezoelectric thin film forming process, the above-described first temperature raising process and first film forming process were performed. In the case of Example 1, the above-described second heating step and second film forming step were not performed. The film-forming atmosphere of the piezoelectric thin film was a mixed gas consisting of Ar, _N2 and _O2 . The total pressure of the film-forming atmosphere was 0.4 Pa. The partial pressure of O ₂ in the deposition atmosphere was maintained at 5.11×10 ⁻⁷ Pa.

The film formation time t _TOTAL and the initial film formation temperature TI of Example 1 are shown in Table 1 below. The thickness T3 of the piezoelectric thin film of Example 1 was adjusted to the values shown in Tables 1 and 2 below. The thickness T3 of the piezoelectric thin film was uniform.

A second electrode layer made of Mo was formed directly over the entire surface of the piezoelectric thin film in the same manner as the first electrode layer. The thickness of the second electrode layer was the same as the thickness of the first electrode. The thickness of the second electrode layer was uniform.

The laminate produced by the above procedure includes a substrate, a first electrode layer directly laminated on the substrate, a piezoelectric thin film directly laminated on the first electrode layer, and a second electrode directly laminated on the piezoelectric thin film. , consisted of Subsequent photolithography patterned the layered structure on the substrate. After patterning, the entire laminate was cut by dicing to obtain a rectangular piezoelectric thin film element of Example 1. FIG. The piezoelectric thin film element is composed of a substrate, a first electrode layer directly laminated on the substrate, a piezoelectric thin film directly laminated on the first electrode layer, and a second electrode layer directly laminated on the piezoelectric thin film. rice field.

A plurality of piezoelectric thin film elements of Example 1 were produced by the above method. The following analyzes and measurements were performed on these piezoelectric thin film elements.

<Composition of entire piezoelectric thin film>
The composition of the entire piezoelectric thin film was analyzed by X-ray fluorescence spectroscopy (XRF method). For the XRF method, a wavelength dispersive fluorescent X-ray device (RIGAKU AZX-400) manufactured by Rigaku Corporation was used. The concentrations of Al, N and O (unit: atomic %) in the piezoelectric thin film were quantified by the XRF method.

<Concentration Distribution of Elements in Piezoelectric Thin Film in Thickness Direction D>
Al, N and O concentration distributions in the piezoelectric thin film along the thickness direction D were measured by X-ray photoelectron spectroscopy (XPS). That is, XPS was continuously performed on the surface of the piezoelectric thin film while the thickness T3 of the piezoelectric thin film was gradually and uniformly reduced by sputtering the surface of the piezoelectric thin film directly laminated on the first electrode layer. The XPS method includes ULVAC-PHI, Inc. was used.

<Crystal structure of piezoelectric thin film>
The crystal structure of the piezoelectric thin film directly formed on the first electrode layer was analyzed by X-ray diffraction (XRD) method. A multi-purpose X-ray diffractometer (SmartLab) manufactured by Rigaku Corporation was used for the XRD method. The surface of the piezoelectric thin film directly formed on the first electrode layer was subjected to 2θ-θ scan, ω scan and 2θχ-φ scan using the above X-ray diffraction apparatus. As a result of analysis based on the XRD method, it was confirmed that the piezoelectric thin film had a wurtzite structure. The (002) plane of the wurtzite structure was parallel to the surface of the first electrode layer in contact with the piezoelectric thin film. The (110) plane of Mo (body-centered cubic structure) constituting the first electrode layer was oriented in the normal direction of the surface of the first electrode layer in contact with the piezoelectric thin film.

From the above analysis, it was confirmed that the piezoelectric thin film of Example 1 has the following features.

The piezoelectric thin film of Example 1 was made of crystalline AlN containing oxygen, and AlN had a wurtzite structure. The (002) plane of AlN was oriented in the thickness direction D of the piezoelectric thin film (normal direction to the surface of the first electrode layer). The oxygen concentration in the piezoelectric thin film of Example 1 was uniform in the thickness direction D. The average oxygen concentration [O] ₃ in the piezoelectric thin film of Example 1 is shown in Table 2 below.

<Dielectric loss tangent>
The dielectric loss tangent (tan δ) of the piezoelectric thin film of Example 1 was measured. An Agilent LCR meter (E4980A) was used to measure tan δ. For tan δ measurements, an electric field of 1 V/μm was applied to the piezoelectric thin film. The area of the portion to which the electric field was applied in each of the first electrode layer and the second electrode layer was 600×600 μm ² . The tan δ of Example 1 is shown in Table 2 below.

<piezoelectric constant _d33 >
The piezoelectric constant d ₃₃ (unit: pC/N) of the piezoelectric thin film of Example 1 was measured. The details of the measurement of the piezoelectric constant _d33 were as follows. The piezoelectric constant d ₃₃ (3-point measurement point average value) of Example 1 is shown in Table 2 below.
Measuring device: d ₃₃ meter (PM200) manufactured by Piezotest
Frequency: 110Hz
Clamp pressure: 0.25N

<Crack rate Rc>
100 samples were produced by dicing the piezoelectric thin film elements in the stacking direction. The dimensions of each sample are uniform, and each sample is a chip-shaped piezoelectric thin film element. By applying an electric field of 0.4 V/μm to the piezoelectric thin film of each sample, the presence or absence of continuity in the piezoelectric thin film of each sample was examined. A commercially available multimeter was used to check continuity. Among the 100 samples, the number n of samples in which continuity in the piezoelectric thin film was detected was counted. The crack rate Rc is defined as n%. When cracks due to the dicing described above are formed in the piezoelectric thin film of each sample, the cracks function as conduction paths, so conduction in the piezoelectric thin film of each sample is detected. As the mechanical strength of the piezoelectric thin film increases, cracks due to dicing are suppressed and the crack rate Rc decreases. The crack rate Rc of Example 1 is shown in Table 2 below.

(Examples 2-6)
In the steps of forming the piezoelectric thin film in each of Examples 2 to 6, the above-described second temperature raising step and second film forming step were performed following the first temperature raising step and first film forming step.

The film formation time t _TOTAL , the initial film formation temperature TI, the final film formation temperature TF, the first film formation time ti, the second film formation time tf, and the second temperature rise rate R _T for each of Examples 2 to 6 are as follows. It is shown in Table 1. The thickness T3 of each of the piezoelectric thin films of Examples 2 to 6 was adjusted to the values shown in Tables 1 and 2 below. The thickness T3 of each of the piezoelectric thin films of Examples 2 to 6 was uniform.

Piezoelectric thin film elements of Examples 2 to 6 were produced in the same manner as in Example 1 except for the step of forming the piezoelectric thin film. Analysis and measurement of the piezoelectric thin film elements of Examples 2 to 6 were carried out in the same manner as in Example 1. As a result of the analysis, it was confirmed that the piezoelectric thin films of Examples 2 to 6 had the following characteristics.

The piezoelectric thin film was made of crystalline AlN containing oxygen, and AlN had a wurtzite structure. The (002) plane of AlN was oriented in the thickness direction D of the piezoelectric thin film (normal direction to the surface of the first electrode layer). The piezoelectric thin film consisted of a first piezoelectric layer directly laminated to the first electrode layer and a second piezoelectric layer directly laminated to the first piezoelectric layer and in contact with the second electrode layer. The oxygen concentration in the first piezoelectric layer was lower than the oxygen concentration in the second piezoelectric layer. The concentration of oxygen in the first piezoelectric layer was uniform in the thickness direction D. The concentration of oxygen in the second piezoelectric layer gradually increased along the thickness direction D. The thickness T1 of the first piezoelectric layer was uniform. The thickness T2 of the second piezoelectric layer was also uniform.

The average oxygen concentration [O] ₃ in each of the piezoelectric thin films of Examples 2 to 6 is shown in Table 2 below.
The average oxygen concentration [O] ₁ in the first piezoelectric layer of each of Examples 2 to 6 is shown in Table 2 below.
The average oxygen concentration [O] ₂ in the second piezoelectric layer of each of Examples 2 to 6 is shown in Table 2 below.
The thickness T1 of the first piezoelectric layer in each of Examples 2-6 is shown in Table 2 below.
The thickness T2 of the second piezoelectric layer for each of Examples 2-6 is shown in Table 2 below.
T2/T3 in Table 2 below is the ratio of the thickness T2 of the second piezoelectric layer to the thickness T3 of the piezoelectric thin film. The T2/T3 for each of Examples 2-6 is shown in Table 2 below.
R[O] ₂ in Table 2 below means the ratio of the total amount of oxygen in the second piezoelectric layer to the total amount of oxygen in the piezoelectric thin film. R[O] ₂ is equal to {(T2/T3)*[O] ₂ }/[O] ₃ . The R[O] ₂ for each of Examples 2-6 is shown in Table 2 below.
Tan δ, d ₃₃ and Rc for each of Examples 2-6 are shown in Table 2 below.

(Comparative Examples 1 and 2)
The film formation time t _TOTAL and the initial film formation temperature TI of Comparative Examples 1 and 2 are shown in Table 1 below. In the case of Comparative Examples 1 and 2, the above-described second heating step and second film forming step were not performed. The thickness T3 of the piezoelectric thin film of each of Comparative Examples 1 and 2 was adjusted to the values shown in Tables 1 and 2 below. The thickness T3 of each of the piezoelectric thin films of Comparative Examples 1 and 2 was uniform.

Piezoelectric thin film elements of Comparative Examples 1 and 2 were produced in the same manner as in Example 1 except for the step of forming the piezoelectric thin film. Analysis and measurement of the piezoelectric thin film elements of Comparative Examples 1 and 2 were carried out in the same manner as in Example 1.

The piezoelectric thin film of Comparative Example 1 did not contain oxygen. The piezoelectric thin film of Comparative Example 1 consisted of only crystalline AlN, and AlN had a wurtzite structure. The (002) plane of AlN of Comparative Example 1 was oriented in the thickness direction D of the piezoelectric thin film (normal direction to the surface of the first electrode layer).

The piezoelectric thin film of Comparative Example 2 was made of crystalline AlN containing oxygen, and AlN had a wurtzite structure. The (002) plane of AlN of Comparative Example 2 was oriented in the thickness direction D of the piezoelectric thin film (normal direction to the surface of the first electrode layer). The oxygen concentration in the piezoelectric thin film of Comparative Example 2 was uniform in the thickness direction D.

The average oxygen concentration [O] ₃ in the piezoelectric thin films of Comparative Examples 1 and 2 are shown in Table 2 below. The tan δ, d ₃₃ and Rc for Comparative Examples 1 and 2, respectively, are shown in Table 2 below.

For example, piezoelectric thin film elements according to one aspect of the present invention may be used in microphones, sensors, transducers, filters, harvesters, or actuators.

DESCRIPTION OF SYMBOLS 1... First piezoelectric layer 2... Second piezoelectric layer 3... Piezoelectric thin film 4... First electrode layer 5... Adhesion layer 6... Substrate 7... Second electrode layer 10... Piezoelectric thin film element ucw... A unit cell of a wurtzite structure.

Claims (5)

a substrate;
a first electrode layer laminated on the substrate;
a piezoelectric thin film laminated on the first electrode layer;
a second electrode layer laminated on the piezoelectric thin film;
with
the piezoelectric thin film contains aluminum nitride and oxygen;
The average oxygen concentration in the piezoelectric thin film is 0.001 atomic % or more and 6 atomic % or less.
Piezoelectric thin film element. the piezoelectric thin film comprises a first piezoelectric layer laminated on the first electrode layer and a second piezoelectric layer laminated on the first piezoelectric layer and in contact with the second electrode layer,
the concentration of oxygen in the first piezoelectric layer is lower than the concentration of oxygen in the second piezoelectric layer;
The piezoelectric thin film element according to claim 1. A ratio of the thickness of the second piezoelectric layer to the thickness of the piezoelectric thin film is 0.1% or more and 60% or less,
A ratio of the total amount of oxygen in the second piezoelectric layer to the total amount of oxygen in the piezoelectric thin film is more than 60% and 100% or less.
3. The piezoelectric thin film element according to claim 2. The concentration of oxygen in the second piezoelectric layer increases along a direction that is substantially perpendicular to the surface of the first electrode layer on which the piezoelectric thin film is laminated and that extends from the first electrode layer to the second electrode layer. do,
4. The piezoelectric thin film element according to claim 2 or 3. The first electrode layer is platinum, iridium, gold, rhodium, palladium, silver, nickel, copper, aluminum, molybdenum, tungsten, vanadium, chromium, niobium, tantalum, ruthenium, zirconium, hafnium, titanium, yttrium, scandium and magnesium Containing at least one element selected from the group consisting of
The piezoelectric thin film element according to any one of claims 1 to 4.

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