Piezoelectric thin film and piezoelectric thin film INT

Abstract

To provide a piezoelectric thin film with reduced residual stress.SOLUTION: A piezoelectric thin film 3 contains aluminum nitride with oxygen and argon. The content of argon in the aluminum nitride is between 0.01 atom% and 0.27 atom%.SELECTED DRAWING: Figure 1

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) have attracted considerable attention. MEMS are devices in which mechanical components and electronic circuits are integrated onto a single substrate using microfabrication technology. MEMS with functions such as sensors, transducers, filters, harvesters, or actuators utilize piezoelectric thin films. In the manufacturing of 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. Subsequent post-processing (microfabrication such as patterning, etching, and dicing) allows for the creation of MEMS with desired characteristics. Selecting piezoelectric thin films with superior piezoelectric properties improves the characteristics of piezoelectric thin film elements such as MEMS, enabling miniaturization of piezoelectric thin film elements. For example, the piezoelectric properties of a piezoelectric thin film are evaluated based on the positive piezoelectric constant (piezoelectric strain constant) d and the piezoelectric power coefficient g. g is d / ε0ε r It is equal to ε0, where ε0 is the permittivity of vacuum, and ε r is the relative permittivity of the piezoelectric thin film. Increasing d and g improves the properties of the piezoelectric thin film element.

[0003] Examples of piezoelectric compositions that constitute piezoelectric thin films include Pb(Zr,Ti)O3 (lead zirconate titanate, abbreviated as PZT), LiNbO3 (lithium niobate), AlN (aluminum nitride), ZnO (zinc oxide), and CdS (cadmium sulfide).

[0004] The d of a piezoelectric thin film made of AlN is relatively small. However, the ε of AlN is small. rSince its saturation is relatively small, piezoelectric thin films composed of AlN can have a relatively large saturation. Furthermore, AlN is a relatively inexpensive material. For these reasons, AlN is attracting attention as a piezoelectric composition for piezoelectric thin films. (See Patent Documents 1-5 and Non-Patent Document 1 below.)

[0005] When a piezoelectric thin film made of AlN is laminated on the surface of a substrate, residual stress is likely to occur within the piezoelectric thin film due to the interaction between AlN and the substrate (e.g., lattice mismatch) and the interaction between AlN and the underlying electrode layer. Due to this residual stress, the piezoelectric thin film is prone to fracture. For example, mechanical processing of the piezoelectric thin film (such as dicing) can easily cause cracks to form in the piezoelectric thin film due to residual stress. Alternatively, mechanical processing of the piezoelectric thin film can easily cause the piezoelectric thin film to delaminate from the substrate due to residual stress. Fracture of the piezoelectric thin film can lead to a deterioration of its electrical and piezoelectric properties.

[0006] Conventionally, various methods for reducing residual stress in piezoelectric thin films are known. For example, the piezoelectric layer described in Patent Document 4 below has a laminated structure composed of a tensile stress layer and a compressive stress layer. By canceling out 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 cracking of the piezoelectric layer is suppressed.

[0007] The piezoelectric thin film described in Patent Document 5 below is composed of an AlN crystal, in which a portion of the Al in the AlN crystal is replaced by a first element. Furthermore, a second element having an ionic radius smaller than that of the first element and larger than that of Al is added to the AlN crystal. Doping with the first and second elements reduces residual stress in the piezoelectric thin film. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2020-65160 [Patent Document 2] Japanese Patent Application Laid-Open No. 2019-186691 [Patent Document 3] Japanese Patent Application Laid-Open No. 2020-77788 [Patent Document 4] International Publication No. 2007 / 063842<C [Patent Document 5] International Publication No. 2016 / 111280 [Non-Patent Document]

[0009] [Non-Patent Document 1] L. Vergara et al, Influence of oxygen and argon on the crystal quality and piezoelectric response of AlN sputtered thin films, Diamond and Related Materials 13 (2004) 839-842 [Summary of the Invention] [Problems to be Solved by the Invention]

[0010] In conventional methods for reducing residual stress in a piezoelectric thin film, the orientation of the crystal structure of AlN deteriorates, and the piezoelectric properties of the piezoelectric thin film deteriorate. Therefore, it is necessary to reduce residual stress by a method different from the conventional one.

[0011] An object of one aspect of the present invention is to provide a piezoelectric thin film with reduced residual stress and a piezoelectric thin film element including the piezoelectric thin film. [Means for Solving the Problems]

[0012] The piezoelectric thin film according to one aspect of the present invention includes aluminum nitride containing oxygen and argon. The content of argon in the aluminum nitride is 0.01 atomic % or more and 0.27 atomic % or less.

[0013] The content of oxygen in the aluminum nitride may be 0.10 atomic % or more and 1.05 atomic % or less.

[0014] The aluminum content in aluminum nitride may be expressed as [Al] atoms %. The nitrogen content in aluminum nitride may be expressed as [N] atoms %. The oxygen content in aluminum nitride may be expressed as [O] atoms %. [Al] / ([N]+[O]) may be between 0.90 and less than 1.00.

[0015] The absolute value of residual stress in a piezoelectric thin film may be between 0 MPa and 430 MPa.

[0016] A piezoelectric thin-film element according to one aspect of the present invention includes the above-mentioned piezoelectric thin film. [Effects of the Invention]

[0017] According to one aspect of the present invention, a piezoelectric thin film with reduced residual stress and a piezoelectric thin film element including the piezoelectric thin film are provided. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 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 Figure 1 is substantially perpendicular to the stacking direction of the substrate, adhesion layer, first electrode layer, piezoelectric thin film, and second electrode layer. [Figure 2] Figure 2 is a perspective view of the unit cell of the Wurtz crystal structure of aluminum nitride. [Figure 3] Figures 3(a), 3(b), and 3(c) are schematic cross-sectional views illustrating the process of forming a piezoelectric thin film. The cross-sections shown in Figures 3(a), 3(b), and 3(c) are approximately perpendicular to the stacking direction of the substrate, adhesion layer, first electrode layer, and piezoelectric thin film. [Modes for carrying out the invention]

[0019] Preferred embodiments of the present invention will be described below, with reference to the drawings as appropriate. The present invention is not limited to the embodiments described below. In each figure, the same or equivalent components are denoted by the same reference numerals. X, Y, and Z shown in Figures 1, 3(a), 3(b), and 3(c) represent three mutually orthogonal coordinate axes. The directions indicated by the coordinate axes in each figure are common to all figures.

[0020] (Piezoelectric thin films and piezoelectric thin film elements) As shown in Figure 1, the piezoelectric thin-film element 10 according to this embodiment includes a substrate 6, an adhesion layer 5 directly laminated on the surface of the substrate 6, a first electrode layer 4 directly laminated on the adhesion layer 5, a piezoelectric thin film 3 directly laminated on the surface of the first electrode layer 4, and a second electrode layer 7 directly laminated on the surface of the piezoelectric thin film 3. The lamination direction of the substrate 6, adhesion layer 5, first electrode layer 4, piezoelectric thin film 3, and second electrode layer 7 is substantially parallel to the Z axis. The substrate 6, adhesion layer 5, first electrode layer 4, piezoelectric thin film 3, and second electrode layer 7 may each have a flat shape extending along the XY plane direction (X axis and Y axis). The thickness of the substrate 6, adhesion layer 5, first electrode layer 4, piezoelectric thin film 3, and second electrode layer 7 may be substantially uniform. The adhesion layer 5 may cover part or all of the surface of the substrate 6. The first electrode layer 4 may cover part or all of the surface of the adhesion layer 5. The piezoelectric thin film 3 may cover part or all of the surface of the first electrode layer 4. The second electrode layer 7 may 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 directly laminated on the surface of the substrate 6. If the adhesion layer 5 is absent, the first electrode layer 4 may cover part or all of the surface of the substrate 6. The first electrode layer 4 may be referred to as the lower electrode layer. The second electrode layer 7 may be referred to as the 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 be made of a conductor or a piezoelectric material. 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 be made of a conductor or a piezoelectric material.

[0021] The piezoelectric thin film 3 contains crystalline aluminum nitride (AlN). The AlN constituting the piezoelectric thin film 3 may be single crystal or polycrystalline. The crystal structure of AlN is a hexagonal wurtzite structure. Figure 2 shows the unit cell ucw of the AlN crystal structure (wurtzite structure). The unit cell ucw is a 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.

[0022] The (001) and (002) planes 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) and (002) planes of AlN in the piezoelectric thin film 3 may be oriented in the direction normal to the surface of the first electrode layer 4. The (001) plane of AlN corresponds to the hexagonal crystal plane in the unit cell ucw shown in Figure 2. If the piezoelectric thin film 3 contains a plurality of AlN crystal grains, the (001) and (002) planes of some or all of the crystal grains may be substantially parallel to the surface of the first electrode layer 4.

[0023] The crystal orientation in which the piezoelectric properties of AlN are exhibited is the

[0001] plane of AlN. Therefore, by having the (001) and (002) planes of AlN substantially parallel to the surface of the first electrode layer 4, the piezoelectric thin film 3 can have excellent piezoelectric properties. For similar reasons, the (001) and (002) planes of the wurtzite-type 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) and (002) planes of AlN in the piezoelectric thin film 3 may be oriented in the direction normal to the surface of the piezoelectric thin film 3.

[0024] AlN contains oxygen and argon. The argon content in AlN is between 0.01 atomic% and 0.27 atomic%.

[0025] Since the piezoelectric thin film 3 is grown epitaxially on a crystalline substrate 6, a lattice mismatch occurs between the piezoelectric thin film 3 and the substrate 6, and also between the piezoelectric thin film 3 and the first electrode layer 4. As a result, residual stress is generated in the piezoelectric thin film 3. For example, the residual stress acts on the piezoelectric thin film 3 in a direction substantially parallel to the surface of the piezoelectric thin film 3 (a plane substantially perpendicular to the stacking direction). For example, tensile stress that expands the piezoelectric thin film 3 occurs in a direction substantially parallel to the surface of the piezoelectric thin film 3. Or, compressive stress that compresses the piezoelectric thin film 3 occurs in a direction substantially parallel to the surface of the piezoelectric thin film 3. However, if AlN contains oxygen and argon, and the argon content in AlN is between 0.01 atomic% and 0.27 atomic%, the crystal structure of AlN changes appropriately with the introduction of oxygen and argon. For example, the presence of oxygen and argon in AlN causes the lattice (unit cell ucw) of AlN to expand. In other words, the presence of oxygen and argon in AlN increases the lattice constant of AlN or the interatomic distance in AlN. As a result, the above-mentioned lattice mismatch is alleviated, and residual stress in the piezoelectric thin film 3 is reduced. For example, tensile stress is easily suppressed with the expansion of the AlN lattice (unit cell ucw).

[0026] The residual stress in the piezoelectric thin film 3 may be calculated based on Stoney's equation, which will be described later. Positive residual stress calculated based on Stoney's equation is tensile stress. Negative residual stress calculated based on Stoney's equation is compressive stress. It is preferable that the absolute value of the residual stress in the piezoelectric thin film 3 be as small as possible. For example, the absolute value of the residual stress in the piezoelectric thin film 3 may be 0 MPa to 430 MPa, 0 MPa to 403 MPa, 0 MPa to 350 MPa, 163 MPa to 403 MPa, or 163 MPa to 350 MPa. For example, the residual stress in the piezoelectric thin film 3 may be between -403 MPa and 387 MPa, between -385 MPa and 181 MPa, or between -350 MPa and 181 MPa.

[0027] When AlN contains O and the argon content in AlN is 0.01 atomic % or more and 0.27 atomic % or less, while maintaining the excellent characteristics of the piezoelectric thin film 3 (large absolute value of piezoelectric strain constant (d 33 ), high electrical resistivity (ρ), and small dielectric loss tangent (tanδ)), the residual stress in the piezoelectric thin film 3 can be reduced. That is, the residual stress in the piezoelectric thin film 3 can be reduced without impairing the piezoelectric and electrical characteristics of the piezoelectric thin film. When the argon content in AlN is less than 0.01 atomic %, it is difficult to reduce the residual stress in the piezoelectric thin film 3. Or when the argon content in AlN is higher than 0.27 atomic %, it is also difficult to reduce the residual stress in the piezoelectric thin film 3. When the argon content in AlN is higher than 0.27 atomic %, the absolute value of the piezoelectric strain constant (d 33 ) is small. When the argon content in AlN is less than 0.01 atomic %, the electrical resistivity (ρ) of the piezoelectric thin film 3 is low. When the argon content in AlN is less than 0.01 atomic %, the dielectric loss tangent (tanδ) of the piezoelectric thin film 3 is large.

[0028] For example, the absolute value of the piezoelectric strain constant (d 33 ) of the piezoelectric thin film 3 may be 6.7 pC / N or more and 7.1 pC / N or less. For example, the electrical resistivity (ρ) of the piezoelectric thin film 3 is 6.2×10 12 Ωcm or more and 3.0×10 13 Ωcm or less, 8.2×10 12 Ωcm or more and 3.0×10 13 Ωcm or less, or 1.1×10 13 Ωcm or more and 3.0×10 13 Ωcm or less. For example, the dielectric loss tangent (tanδ) of the piezoelectric thin film 3 may be 0.0% or more and 0.8% or less, 0.0% or more and 0.5% or less, 0.0% or more and 0.3% or less, 0.3% or more and 0.8% or less, or 0.3% or more and 0.5% or less.

[0029] The number of atoms (amount of substance) of aluminum (Al) in the piezoelectric thin film 3 is <al>It can be expressed as moles. The number of nitrogen (N) atoms (amount of substance) in the piezoelectric thin film 3 is: <n>It can be expressed as moles. The number of oxygen (O) atoms (amount of substance) in the piezoelectric thin film 3 is: <o>It can be expressed as moles. The number of argon (Ar) atoms (amount of substance) in the piezoelectric thin film 3 is: <ar>It can be expressed as moles. The aluminum content in aluminum nitride may be expressed as [Al] atoms in percent. The nitrogen content in aluminum nitride may be expressed as [N] atoms %. The oxygen content in aluminum nitride may be expressed as [O] atoms in percent. The argon content in aluminum nitride may be expressed as [Ar] atoms in percent. [Al] is 100× <al> / ( <al> + <n> + <o> + <ar>It can be defined as follows. [N] is 100× <n> / ( <al> + <n> + <o> + <ar>It can be defined as follows. [O] is 100× <o> / ( <al> + <n> + <o> + <ar>It can be defined as follows. [Ar] is 100× <ar> / ( <al> + <n> + <o> + <ar>It can be defined as follows.

[0030] For example, the amount of [Al] may be between 47.42 atomic% and 50.20 atomic%. For example, the amount of [N] may be between 49.15 atomic% and 52.00 atomic%. For example, the amount of [O] may be between 0.05 atoms and 1.37 atomic%. When the content of Al, N, and O is within the above ranges, residual stress in the piezoelectric thin film 3 is easily reduced, and the piezoelectric strain constant (d 33 It tends to have a large absolute value of ), high electrical resistivity (ρ), and a small dielectric loss tangent (tanδ).

[0031] The oxygen content ([O]) in AlN may be between 0.10 atomic% and 1.05 atomic%. The presence of argon in AlN causes the AlN lattice (unit cell ucw) to expand. This expansion of the AlN lattice (unit cell ucw) makes it easier for oxygen vacancies to be trapped within the AlN lattice (unit cell ucw). When the oxygen content is between 0.10 atomic% and 1.05 atomic%, residual stress in the piezoelectric thin film 3 is suppressed, and at the same time, high electrical resistivity and a small dielectric loss tangent are easily achieved due to the trapping of oxygen vacancies within the lattice (unit cell ucw). For example, when the oxygen content is between 0.10 atomic% and 1.05 atomic%, the piezoelectric thin film 3 has a residual stress of -403 Pa to 181 MPa and 8.2 × 10⁻¹⁰ 12 It tends to have an electrical resistivity of Ωcm or more and a dielectric loss tangent of 0.5% or less.

[0032] [Al] / ([N]+[O]) may be between 0.97 and 1.02, or between 0.90 and 1.00. [Al] / ([N]+[O]) is a value that indicates the balance between the cation (Al) and anions (N and O) in AlN. Theoretically, Al in AlN is a trivalent cation, N in AlN is a trivalent anion, and O in AlN is a divalent anion. When [Al] / ([N]+[O]) is 1.00 or less, the cation and anions are easily balanced electrically. When [Al] / ([N]+[O]) is greater than 1.02, there are more cations than anions, and the electrical resistivity tends to be low. When [Al] / ([N]+[O]) is less than 0.97, there are more anions than cations, and the electrical resistivity tends to be low. The piezoelectric thin film 3 tends to have high electrical resistivity due to a change in the balance of anions and cations such that [Al] / ([N]+[O]) is between 0.90 and less than 1.00. For example, when [Al] / ([N]+[O]) is between 0.90 and less than 1.00, the piezoelectric thin film 3 has a residual stress of -350 MPa to 181 MPa, and 1.1 × 10⁻¹⁰ 13 It is likely to have an electrical resistivity of Ωcm or more and a dielectric loss tangent of 0.5% or less. For similar reasons, [Al] may be between 47.42 atomic% and 49.69 atomic%, [N] may be between 49.20 atomic% and 52.00 atomic%, and [O] may be between 0.50 atomic% and 1.05 atomic%.

[0033] The piezoelectric thin film 3 may consist only of AlN containing O and Ar. The AlN containing O and Ar may consist only of Al, N, O, and Ar. However, the piezoelectric thin film 3 may further contain other components other than AlN containing O and Ar, provided that residual stress is suppressed and the piezoelectric and electrical properties are not impaired. For example, the AlN containing O and Ar may further contain at least one additive element selected from the group consisting of monovalent metal element Mm, divalent metal element Md, trivalent metal element Mtr, tetravalent metal element Mt, and pentavalent metal element Mp. Doping AlN with the above additive elements may distort the wurtzite structure of AlN or change the strength of the interatomic chemical bonds in the wurtzite structure. As a result, the piezoelectric properties of the piezoelectric thin film 3 may be improved. The monovalent metallic 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 metallic element Md may be at least one selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The trivalent metallic element Mtr may be at least one element selected from the group consisting of Sc (scandium), Y (yttrium), lanthanides, and In (indium). The tetravalent metallic element Mt may be at least one selected from the group consisting of germanium (Ge), titanium (Ti), zirconium (Zr), and hafnium (Hf). The pentavalent metallic 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 valence of the additive elements contained in the piezoelectric thin film 3 may be 3. The total content of monovalent metal element Mm in aluminum nitride may be expressed as [Mm] atoms %. The total content of pentavalent metal element Mp in aluminum nitride may be expressed as [Mp] atoms %. [Mm] / [Mp] may be approximately 1.0. The total content of the divalent metal element Md in aluminum nitride may be expressed as [Md] atoms %. The total content of the tetravalent metal element Mt in aluminum nitride may be expressed as [Mt] atoms %. [Md] / [Mt] may be approximately 1.0.

[0034] For example, the substrate 6 may be a semiconductor substrate (such as a silicon substrate, gallium arsenide substrate, or SOI (Silicon-on-Insulator) substrate), an optical crystal substrate (such as a sapphire substrate), an insulating substrate (such as a glass substrate or a ceramic substrate), or a metal substrate (such as a stainless steel plate). The substrate 6 may be crystalline.

[0035] For example, the first electrode layer 4 may contain at least one element selected from the group consisting 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 be a single metal. The first electrode layer 4 may also be an alloy containing at least two elements selected from the above group.

[0036] The first electrode layer 4 may have a crystalline structure. That is, the first electrode layer 4 may be crystalline. For example, if the metal contained in the first electrode layer 4 is at least one element selected from the group consisting of Pt, Ir, Au, Rh, Pd, Ag, Ni, Cu, and Al, the first electrode layer 4 is likely to have a face-centered cubic (fcc) structure, and the (111) plane of the fcc structure is likely to be oriented in the direction normal to the surface of the first electrode layer 4. If the metal contained in the first electrode layer 4 is at least one element selected from the group consisting of Mo, W, V, Cr, Nb, and Ta, the first electrode layer 4 is likely to have a body-centered cubic (bcc) structure, and the (110) plane of the bcc structure is likely to be oriented in the direction normal to the surface of the first electrode layer 4. When the metal contained in the first electrode layer 4 is at least one element selected from the group consisting of Ru, Zr, Hf, Ti, Y, Sc, and Mg, the first electrode layer 4 is likely to have a hexagonal close-packed (hcp) structure, and the (001) plane of the hcp structure is likely to be oriented in the direction normal to the surface of the first electrode layer 4.

[0037] In a direction substantially parallel to the surface of the first electrode layer 4, the lattice length of the crystal structure of the first electrode layer 4 may be larger than the lattice length of conventional AlN (e.g., pure AlN). For example, in a direction substantially parallel to the surface of the first electrode layer 4, the interatomic spacing in the crystal structure of the first electrode layer 4 may be larger than the interatomic spacing in conventional AlN. Alternatively, in a direction substantially parallel to the surface of the first electrode layer 4, the lattice constant of the crystal structure of the first electrode layer 4 may be larger than the lattice constant of conventional AlN. For example, the lattice length in the crystal structure of molybdenum is larger than the lattice length in AlN. If the lattice length in the crystal structure of the first electrode layer 4 is larger than the lattice length in conventional AlN in a direction substantially parallel to the surface of the first electrode layer 4, tensile stress is likely to occur in the conventional piezoelectric thin film in a direction substantially parallel to the surface of the first electrode layer 4 due to lattice mismatch between the first electrode layer 4 and the conventional piezoelectric thin film (AlN). However, if the piezoelectric thin film 3 contains aluminum nitride containing oxygen and argon, and the argon content in the aluminum nitride is between 0.01 atomic% and 0.27 atomic%, the tensile stress in the piezoelectric thin film 3 is reduced.

[0038] The adhesion layer 5 may contain at least one element selected from the group consisting of Al (aluminum), Si (silicon), Ti (titanium), Zn (zinc), Y (yttrium), Zr (zirconium), Cr (chromium), Nb (niobium), Mo (molybdenum), Hf (hafnium), Ta (tantalum), W (tungsten), Pt (platinum), Ru (ruthenium), and Ce (cerium). The adhesion layer 5 may be a single metal, an alloy, or a compound (oxide, nitride, etc.). The adhesion layer 5 may also be composed of another piezoelectric thin film (e.g., AlN without O and Ar), a polymer, or a ceramic. The adhesion layer 5 has the function of suppressing the peeling of the first electrode layer 4 due to mechanical shock, etc. The adhesion layer 5 may be referred to as an interface layer, a support layer, a buffer layer, or an intermediate layer.

[0039] The second electrode layer 7 may contain at least one element selected from the group consisting of Pt, Ir, Au, Rh, Pd, Ag, Ni, Cu, Al, Mo, W, V, Cr, Nb, Ta, Ru, Zr, Hf, Ti, Y, Sc, and Mg. The second electrode layer 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.

[0040] The thickness of the substrate 6 may be, for example, 50 μm or more and 10,000 μm or less. The thickness of the adhesion layer 5 may be, for example, 0.003 μm or more and 2 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.

[0041] The piezoelectric thin-film element according to this embodiment has a wide range of applications. For example, the piezoelectric thin-film element may be a piezoelectric microphone, harvester, oscillator, resonator, acoustic multilayer film, or filter. For example, the piezoelectric thin-film element may be a piezoelectric actuator. A piezoelectric actuator may be used in haptics. That is, a piezoelectric actuator may be used in various devices that require tactile feedback. Devices that require tactile feedback may be wearable devices, touchpads, displays, or game controllers. For example, a piezoelectric actuator may be used in a head assembly, head stack assembly, or hard disk drive. For example, a piezoelectric actuator may be used in a printer head or inkjet printer. For example, a piezoelectric actuator may be used in a piezoelectric switch. For example, the piezoelectric thin-film element may be a piezoelectric sensor. A piezoelectric sensor may be used in, for example, a gyro sensor, pressure sensor, pulse wave sensor, ultrasonic sensor, ultrasonic transducer such as a piezoelectric micromachined ultrasonic transducer (PMUT), or shock sensor. Products utilizing piezoelectric micromechanical ultrasonic transducers may include, for example, biometric authentication sensors, medical / healthcare sensors (fingerprint sensors, or ultrasonic vascular authentication sensors), or ToF (Time of Flight) sensors. Filters may include, for example, BAW (Bulk Acoustic Wave) filters or SAW (Surface Acoustic Wave) filters. Each of the piezoelectric thin-film elements described above may be part or all of a MEMS.

[0042] The crystal structures of the substrate 6, adhesion layer 5, first electrode layer 4, piezoelectric thin film 3, and second electrode layer 7 may be determined by X-ray diffraction (XRD). The composition of each layer and the piezoelectric thin film 3 may be determined by at least one of the following analytical methods: X-ray fluorescence analysis (XRF), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray analysis (EDX), inductively coupled plasma mass spectrometry (ICP-MS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and electron beam microanalysis (EPMA). The thicknesses of the substrate 6, adhesion layer 5, first electrode layer 4, piezoelectric thin film 3, and second electrode layer 7 may be measured by scanning electron microscope (SEM) in a cross-section of the piezoelectric thin film element 10 parallel to the stacking direction.

[0043] (Manufacturing methods for piezoelectric thin films and piezoelectric thin film elements) For example, a method for manufacturing the piezoelectric thin film element 10 may include the steps of forming an adhesion layer 5 on the surface of a substrate 6, forming a first electrode layer 4 on the surface of the adhesion layer 5, forming a piezoelectric thin film 3 on the surface of the first electrode layer 4, and forming a second electrode layer 7 on the surface of the piezoelectric thin film 3. However, as described above, the adhesion layer 5 is not essential for the piezoelectric thin film element 10, and the first electrode layer 4 may be formed directly on the surface of the substrate 6.

[0044] 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 in a vacuum chamber. For example, the piezoelectric thin film 3 is formed in a vacuum chamber by RF (Radio-Frequency) magnetron sputtering using a target made of a single aluminum element. The adhesion layer 5, the first electrode layer 4, and the second electrode layer 7 may each be formed by sputtering using at least one target. The adhesion layer 5, the first electrode layer 4, and the second electrode layer 7 may each be formed by sputtering using multiple targets. The target may contain at least one of the elements that constitute each layer. By selecting and combining targets having a predetermined composition, each layer having the desired composition can be formed. For example, the target may be a single metal, an alloy, or an oxide.

[0045] The composition of the sputtering atmosphere may be a control factor for the composition of the piezoelectric thin film 3. For example, the sputtering atmosphere (deposition atmosphere) for forming the piezoelectric thin film 3 may contain nitrogen gas (N2), argon gas (Ar), and oxygen gas (O2). The deposition atmosphere may consist only of nitrogen gas, argon gas, and oxygen gas. The nitrogen, argon, and oxygen in the piezoelectric thin film 3 originate from the nitrogen, argon, and oxygen in the deposition atmosphere. During the formation process of the piezoelectric thin film 3, nitrogen gas, argon gas, and oxygen gas are continuously supplied into the vacuum chamber. The partial pressure or concentration of oxygen gas in the deposition atmosphere may depend on the vacuum level (airtightness) of the vacuum chamber. The composition of the sputtering atmosphere may be a control factor for the composition of the adhesion layer 5, the first electrode layer 4, and the second electrode layer 7, respectively.

[0046] The input power (power density) applied to each target is a control factor for the composition and thickness of the adhesion layer 5, the first electrode layer 4, the piezoelectric thin film 3, and the second electrode layer 7, respectively. The total pressure of the sputtering atmosphere, the partial pressure or concentration of the raw material gas (e.g., nitrogen) in the atmosphere, the duration of sputtering for each target, the temperature of the substrate 6, and the substrate bias (power applied to the substrate 6) are also control factors for the composition and thickness. The piezoelectric thin film 3 having a desired shape or pattern may be formed by etching (e.g., plasma etching).

[0047] The process of forming the piezoelectric thin film 3 (method for manufacturing the piezoelectric thin film 3) includes a first film formation step, a second film formation step, and a heat treatment step.

[0048] As shown in Figure 3(a), in the first film deposition step, the Ar-rich layer 3A is formed directly on the surface of the first electrode layer 4. The Ar-rich layer 3A is formed by RF magnetron sputtering using a target made of pure aluminum in a deposition atmosphere containing nitrogen gas, argon gas, and oxygen gas. Therefore, the Ar-rich layer 3A contains aluminum nitride containing argon and oxygen. The Ar-rich layer 3A may consist only of aluminum nitride containing argon and oxygen.

[0049] As shown in Figure 3(b), in the second deposition step, the Ar-poor layer 3B is formed directly on the surface of the Ar-rich layer 3A. The Ar-poor layer 3B is also formed by RF magnetron sputtering using a target made of pure aluminum in a deposition atmosphere containing nitrogen gas, argon gas, and oxygen gas. Therefore, the Ar-poor layer 3B also contains aluminum nitride containing argon and oxygen. The Ar-poor layer 3B may consist only of aluminum nitride containing argon and oxygen.

[0050] The argon contained in the Ar-rich layer 3A and the Ar-poor layer 3B originates from the argon in the deposition atmosphere. The partial pressure of argon in the deposition atmosphere during the first deposition process is adjusted to a higher value than the partial pressure of argon in the deposition atmosphere during the second deposition process. As a result, the argon content (in atomic percent) in the Ar-rich layer 3A is controlled to a higher value than the argon content in the Ar-poor layer 3B.

[0051] In the first film deposition process, the flow rate of nitrogen gas supplied into the vacuum chamber may be 40 sccm. In the first film deposition process, the flow rate of argon gas supplied into the vacuum chamber may be 20 sccm. In the second film deposition process, the flow rate of nitrogen gas supplied into the vacuum chamber may be 40 sccm. In the second film deposition process, the flow rate of argon gas supplied into the vacuum chamber may be 20 sccm. The total pressure of the deposition atmosphere in each of the first and second film deposition processes may be controlled by the opening degree of a valve located between the vacuum chamber and the vacuum pump. The total pressure of the deposition atmosphere in the first film deposition process may be approximately 1.0 Pa. The partial pressure of nitrogen gas in the deposition atmosphere in the first film deposition process may be approximately 0.66 Pa. The partial pressure of argon gas in the deposition atmosphere in the first film deposition process may be approximately 0.33 Pa. The total pressure of the deposition atmosphere in the second film deposition process may be approximately 0.4 Pa. The partial pressure of nitrogen gas in the deposition atmosphere of the second deposition process may be approximately 0.26 Pa. The partial pressure of argon gas in the deposition atmosphere of the second deposition process may be approximately 0.13 Pa. The partial pressure of oxygen in the deposition atmosphere of the first and second deposition processes is 4.98 × 10⁻⁶. -7 Pa may be used. Because the deposition atmosphere in the first and second deposition processes satisfies the above conditions, the content of each element in the aluminum nitride contained in the piezoelectric thin film 3 can be easily controlled within the range described above.

[0052] For example, the substrate bias in the first and second film deposition processes may be between 30W and 50W. The argon content in the aluminum nitride contained in the piezoelectric thin film 3 tends to increase with increasing substrate bias. For example, the temperature of the substrate 6 in the first and second film deposition processes may be between 140°C and 220°C. The oxygen content in the aluminum nitride contained in the piezoelectric thin film 3 tends to increase with increasing substrate temperature.

[0053] As shown in (c) of Figure 3, in the heat treatment process, the Ar-rich layer 3A and the Ar-poor layer 3B are heated in a vacuum chamber filled with argon gas. In the heat treatment process, due to the difference in Ar content between the Ar-rich layer 3A and the Ar-poor layer 3B, argon in the Ar-rich layer 3A thermally diffuses into the Ar-poor layer 3B. As a result, a piezoelectric thin film 3 in which argon is dispersed substantially uniformly is formed from the Ar-rich layer 3A and the Ar-poor layer 3B. In the heat treatment process, the Ar-rich layer 3A and the Ar-poor layer 3B may be heated to 600°C. That is, the temperature of the substrate 6 in the heat treatment process may be 600°C. The pressure of the argon gas in the heat treatment process may be 0.4 Pa.

[0054] For example, the thickness of the piezoelectric thin film 3 may be between 100 nm and 30,000 nm, or between 200 nm and 2,000 nm. The thickness of the piezoelectric thin film 3 may be rephrased as the width of the piezoelectric thin film 3 in the stacking direction (the Z-axis direction in Figure 1). The thickness of the piezoelectric thin film 3 may be expressed as T. The thickness of the Ar-rich layer 3A is T A It can be expressed as follows: The thickness of the Ar poor layer 3B is T B This can be expressed as follows: Thickness T of Ar-rich layer 3A A T may be between 0.001T and 0.020T. A +T B It is approximately equal to . Therefore, the thickness T of the Ar poor layer 3B B , TT A That's fine. [Examples]

[0055] The present invention will be described in detail below with reference to examples and comparative examples. The present invention is not limited in any way by these examples.

[0056] (Example 1) A wafer made of a single crystal of silicon was used as the substrate. The surface of the substrate was parallel to the (100) plane of silicon. The thickness of the substrate was 725 μm. The diameter of the substrate was approximately 8 inches. The thickness of the substrate was uniform.

[0057] An adhesion layer was directly formed across the entire surface of the substrate by RF magnetron sputtering in a vacuum chamber. The adhesion layer consisted of aluminum nitride without added elements. Elemental aluminum (Al) was used as the sputtering target. The atmosphere inside the vacuum chamber was a mixed gas 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 300°C during the adhesion layer formation process. The substrate bias was 30W. The adhesion layer thickness was uniform. The adhesion layer thickness was adjusted to 30nm.

[0058] A first electrode layer (lower electrode layer) made of Mo was directly formed across the entire surface of the adhesion layer by RF magnetron sputtering in a vacuum chamber. A single Mo material was used as the sputtering target. The atmosphere inside the vacuum chamber was Ar gas. The input power per unit area of ​​the sputtering target was 0.93 W / cm². 2 The temperature of the substrate and adhesion layer was maintained at 600°C during the formation process 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.

[0059] After the formation of the first electrode layer, the following first film deposition process and second film deposition process following the first film deposition process were carried out. In the first and second film deposition processes, the partial pressure of oxygen in the deposition atmosphere was 4.98 × 10⁻⁶ -7 The pressure was maintained at Pa. During the first and second film deposition processes, the substrate temperature (temperature of the substrate, adhesion layer, and first electrode layer) was maintained at the values ​​shown in Table 1 below. During the first and second film deposition processes, the substrate bias was maintained at the values ​​shown in Table 1 below.

[0060] In the first deposition process, an Ar-rich layer was directly formed across the entire surface of the first electrode layer by RF magnetron sputtering in a vacuum chamber. A single Al material was used as the sputtering target. The input power per unit area of ​​the sputtering target was 5.58 W / cm². 2 The deposition atmosphere for the first deposition process was a mixed gas consisting of Ar, N2, and O2. The total pressure of the deposition atmosphere for the first deposition process was maintained at 1.0 Pa. In the first deposition process, the flow rate of nitrogen gas supplied into the vacuum chamber was maintained at 40 sccm. In the first deposition process, the flow rate of argon gas supplied into the vacuum chamber was maintained at 20 sccm. The thickness of the Ar-rich layer was adjusted to 6 nm. RF magnetron sputtering in the first deposition process was continued for 10 seconds.

[0061] In the second deposition process, an Ar-poor layer was directly formed across the entire surface of the Ar-rich layer by RF magnetron sputtering in a vacuum chamber. A single Al material was used as the sputtering target. The input power per unit area of ​​the sputtering target was 3.72 W / cm². 2 The deposition atmosphere for the second deposition process was a mixed gas consisting of Ar, N2, and O2. The total pressure of the deposition atmosphere for the second deposition process was maintained at 0.4 Pa. In the second deposition process, the flow rate of nitrogen gas supplied into the vacuum chamber was maintained at 40 sccm. In the second deposition process, the flow rate of argon gas supplied into the vacuum chamber was maintained at 20 sccm. The thickness of the Ar poor layer was adjusted to 994 nm. RF magnetron sputtering in the second deposition process was continued for 1713 seconds.

[0062] The second film deposition process and the heat treatment process were carried out. In the heat treatment process, the Ar-rich layer and the Ar-poor layer were heated in a vacuum chamber filled with argon gas. During the heat treatment process, the substrate temperature (temperature of the substrate, adhesion layer, first electrode layer, Ar-rich layer, and Ar-poor layer) was maintained at 600°C. The atmospheric pressure inside the vacuum chamber during the heat treatment process was maintained at 0.4 Pa. During the heat treatment process, the Ar-rich layer and the Ar-poor layer were heated for 320 seconds.

[0063] Through the first film formation process, the second film formation process, and the heat treatment process described above, a piezoelectric thin film was formed over the entire surface of the first electrode layer. The thickness of the piezoelectric thin film was 1000 nm.

[0064] A second electrode layer made of Mo was formed directly onto the entire surface of the piezoelectric thin film using the same method as for the first electrode layer. The thickness of the second electrode layer was the same as that of the first electrode. The thickness of the second electrode layer was uniform.

[0065] The laminate fabricated using the above procedure consisted of a substrate, an adhesion layer directly laminated to the substrate, a first electrode layer directly laminated to the adhesion layer, a piezoelectric thin film directly laminated to the first electrode layer, and a second electrode directly laminated to the piezoelectric thin film. The laminated structure on the substrate was patterned by photolithography. After patterning, the laminate was cut by dicing to obtain a rectangular piezoelectric thin-film element of Example 1. The piezoelectric thin-film element consisted of a substrate, an adhesion layer directly laminated to the substrate, a first electrode layer directly laminated to the adhesion layer, a piezoelectric thin film directly laminated to the first electrode layer, and a second electrode layer directly laminated to the piezoelectric thin film.

[0066] The piezoelectric thin film and piezoelectric thin film element of Example 1 were fabricated using the method described above. The following analyses and measurements were performed on the piezoelectric thin film and piezoelectric thin film element.

[0067] <Composition of piezoelectric thin film> Prior to the formation of the second electrode layer, the composition of the piezoelectric thin film was analyzed by X-ray fluorescence analysis (XRF). A wavelength-dispersive X-ray fluorescence spectrometer (RIGAKU ZSX-Primus IV) manufactured by Rigaku Corporation was used for the XRF analysis. The content (in atomic %) of each element in the piezoelectric thin film was quantified by the XRF method. The content of each element in the piezoelectric thin film, [Al] / ([N]+[O]), [Al] / ([N]+[O]+[Ar]) and [O] / ([N]+[O]+[Ar]), is shown in Table 1 below.

[0068] <Crystal structure of piezoelectric thin film> Prior to the formation of the second electrode layer, the crystal structure of the piezoelectric thin film was analyzed by X-ray diffraction (XRD). A multi-purpose X-ray diffractometer (SmartLab) manufactured by Rigaku Corporation was used for the XRD method. 2θ-θ scans, ω scans, and 2θχ-φ scans were performed on the surface of the piezoelectric thin film directly formed on the first electrode layer using the above X-ray diffractometer. The results of the analysis based on the XRD method indicated that the piezoelectric thin film had a wurtzite-type structure. The (002) plane of the wurtzite-type 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 direction normal to the surface of the first electrode layer in contact with the piezoelectric thin film.

[0069] From the above analysis, it was confirmed that the piezoelectric thin film of Example 1 has the following characteristics.

[0070] The piezoelectric thin film of Example 1 consisted of crystalline AlN containing oxygen and argon. The (002) plane of the AlN was oriented in the thickness direction of the piezoelectric thin film (normal direction to the surface of the first electrode layer).

[0071] <residual stress σ> The residual stress σ (unit: MPa) in the piezoelectric thin film was calculated using the following procedure. First, the radius of curvature R of the substrate immediately before the piezoelectric thin film was formed was calculated. Before The radius of curvature R of the substrate (in μm) was measured. The substrate immediately before the piezoelectric thin film is formed refers to the laminate consisting of the substrate, adhesion layer, and first electrode layer. Subsequently, the radius of curvature R of the substrate after the piezoelectric thin film is formed was measured. After (Unit: μm) was measured. The substrate after the piezoelectric thin film is formed refers to a laminate consisting of the substrate, adhesion layer, first electrode layer, and piezoelectric thin film. R Before and R After A KLA-Tencor P-16 profiler was used for each measurement. The residual stress σ was then calculated based on Equation 1 (Stony's equation) below.

number

[0072] In equation 1, E represents the Young's modulus (in GPa) of the silicon substrate. s t is the Poisson's ratio of a silicon substrate. sub. (Unit: μm) represents the thickness of a silicon substrate. film (Unit: μm) represents the thickness of the piezoelectric thin film. Positive residual stress σ is tensile stress. Negative residual stress σ is compressive stress. The residual stress σ for Example 1 is shown in Table 2 below.

[0073] <Piezoelectric constant d> 33 > Piezoelectric constant d of the piezoelectric thin film in Example 1 33 The piezoelectric constant d was measured (in units: pC / N). 33 The details of the measurement were as follows: Piezoelectric constant d in Example 1 33 The (average of the three measurement points) is shown in Table 2 below. Measurement device: d made by Piezotest 33 Meter (PM200) Frequency: 110Hz Clamping pressure: 0.25N

[0074] <Electrical resistivity ρ> The electrical resistivity ρ of the piezoelectric thin film in Example 1 was measured. An ADVANTEST R8340A measuring device was used to measure the electrical resistivity ρ. For the measurement of electrical resistivity ρ, an electric field of 1 V / μm was applied to the piezoelectric thin film located between the first and second electrode layers. The area of ​​the portion of the first and second electrode layers to which the electric field was applied was 600 × 600 μm. 2 The electrical resistivity ρ of Example 1 is shown in Table 2 below. In Table 2 below, "E+n" (where n is a positive integer) is expressed as "×10 n It means "...".

[0075] <Dielectric loss tangent> The dielectric loss tangent (tanδ) of the piezoelectric thin film in Example 1 was measured. An Agilent LCR meter (E4980A) was used to measure tanδ. For the tanδ measurement, an electric field of 1 V / μm was applied to the piezoelectric thin film located between the first and second electrode layers. The area of ​​the region where the electric field was applied in both the first and second electrode layers was 600 × 600 μm. 2 The tanδ for Example 1 is shown in Table 2 below.

[0076] (Examples 2-10 and Comparative Examples 1-4) In Examples 2-10 and Comparative Examples 1-4, the substrate temperature (temperature of the substrate, adhesion layer, and first electrode layer) was maintained at the values ​​shown in Table 1 below during the first and second film deposition processes. In Examples 2-10 and Comparative Examples 1-4, the substrate bias was maintained at the values ​​shown in Table 1 below during the first and second film deposition processes.

[0077] Piezoelectric thin films and piezoelectric thin film elements for Examples 2-10 and Comparative Examples 1-4 were fabricated in the same manner as in Example 1, except for the substrate and substrate bias.

[0078] Analysis and measurements of the piezoelectric thin films and piezoelectric thin film elements of Examples 2-10 and Comparative Examples 1-4 were performed using the same method as in Example 1.

[0079] In Examples 2-10 and Comparative Examples 3 and 4, the piezoelectric thin film consisted of crystalline AlN containing oxygen and argon. In Comparative Examples 1 and 2, the piezoelectric thin film was made of crystalline AlN containing oxygen, and the piezoelectric thin film did not contain argon. In all of Examples 2-10 and Comparative Examples 1-4, the (002) plane of AlN was oriented in the thickness direction of the piezoelectric thin film (normal direction to the surface of the first electrode layer). The compositions of the piezoelectric thin films for Examples 2-10 and Comparative Examples 1-4 are shown in Table 1 below. σ and d for Examples 2-10 and Comparative Examples 1-4, respectively. 33 ρ and tanδ are shown in Table 2 below.

[0080] [Table 1]

[0081] [Table 2] [Industrial applicability]

[0082] For example, a piezoelectric thin film according to one aspect of the present invention may be used in a microphone, sensor, transducer, filter, harvester, or actuator. [Explanation of Symbols]

[0083] 3... Piezoelectric thin film, 3A... Ar-rich layer, 3B... Ar-poor layer, 4... First electrode layer, 5... Adhesion layer, 6... Substrate, 7... Second electrode layer, 10... Piezoelectric thin film element, ucw... Unit cell with wurtzite structure.< / ar> < / o> < / n> < / al> < / ar> < / ar> < / o> < / n> < / al> < / o> < / ar> < / o> < / n> < / al> < / n> < / ar> < / o> < / n> < / al> < / al> < / ar> < / o> < / n> < / al>

Claims

1. It comprises aluminum nitride containing oxygen and argon, The argon content in the aluminum nitride is 0.01 atomic percent or more and 0.27 atomic percent or less. The oxygen content in the aluminum nitride is 0.10 atomic percent or more and 1.05 atomic percent or less. Piezoelectric thin film.

2. The aluminum content in the aluminum nitride is expressed as [Al] atomic %, The nitrogen content in the aluminum nitride is expressed as [N] atomic percent, The oxygen content in the aluminum nitride is expressed as [O] atomic %, [Al] / ([N]+[O]) is 0.90 or greater and less than 1.

00. The piezoelectric thin film according to claim 1.

3. The absolute value of the residual stress in the piezoelectric thin film is 0 MPa or more and 430 MPa or less. The piezoelectric thin film according to claim 1 or 2.

4. A piezoelectric thin film according to any one of claims 1 to 3, Piezoelectric thin-film element.

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