Piezoelectric element and MEMS device using said piezoelectric element

Abstract

[Problem] The purpose of the present invention is to provide: a piezoelectric element which does not have a buffer layer, has sufficient piezoelectric characteristics and stability, and can be further reduced in size as compared with conventional piezoelectric elements; and a MEMS device which uses the piezoelectric element. [Solution] The present invention comprises: a piezoelectric layer 10 that is formed of a nitride material which has a wurtzite crystal structure; a first electrode 20 that is provided on one surface of the piezoelectric layer; and a second electrode 30 that is provided on the other surface of the piezoelectric layer. The first electrode is formed of a first nitride material that has a wurtzite crystal structure and an electrical resistivity of 1.0 × 10-3 Ω∙cm or less. The second electrode is formed of a second nitride material that has a wurtzite crystal structure and an electrical resistivity of 1.0 × 10-3 Ω∙cm or less.

Description

Piezoelectric element and MEMS device using the same

[0001] The present invention relates to a piezoelectric element configured with electrodes and a piezoelectric body made of a material with a wurtzite crystal structure, and to a MEMS device using the piezoelectric element.

[0002] Devices utilizing the piezoelectric phenomenon are used in a wide range of fields, and their use is expanding in portable devices such as mobile phones, where there is a strong demand for miniaturization and power saving. One example is an FBAR filter using a film bulk acoustic wave resonator (FBAR).

[0003] FBAR filters are filters that use resonators that utilize the thickness-extensional vibration mode of a thin film that exhibits piezoelectric response, and have the characteristic of being able to resonate in the gigahertz band. FBAR filters with these characteristics have low loss and can operate over a wide frequency range, and are therefore expected to contribute to the further improvement of portable devices' high-frequency compatibility, miniaturization, and power savings.

[0004] As shown in FIGS. 17 and 18 , conventional piezoelectric elements used in such FBARs, particularly piezoelectric elements 200A and 200B in which the piezoelectric layer is made of a wurtzite crystal material, have been constructed by laminating a lower electrode 230, a piezoelectric layer 210, and an upper electrode 220 in this order from bottom to top on a buffer layer 250 provided on a substrate 300 (see Non-Patent Document 1), or by laminating a lower electrode 230, a buffer layer 250, a piezoelectric layer 210, and an upper electrode 220 in this order from bottom to top on the substrate 300.

[0005] Moreira et al, Vacuum, 86 (2011) 23

[0006] The reason why a buffer layer is provided in the conventional piezoelectric element described above is that providing a buffer layer can improve the crystallinity of the piezoelectric layer provided on the buffer layer, thereby enabling the formation of a stable piezoelectric layer.

[0007] However, providing a buffer layer increases the thickness of the piezoelectric element, which poses a problem in that it is not possible to further miniaturize the piezoelectric element.

[0008] Furthermore, since it is necessary to form a buffer layer, the manufacturing process of the piezoelectric element becomes complicated, resulting in problems such as reduced productivity and increased production costs.

[0009] In view of the above circumstances, the present invention aims to provide a piezoelectric element that does not require a buffer layer, has sufficient piezoelectric properties and stability, and can be made smaller than conventional piezoelectric elements, by fabricating an upper electrode or a lower electrode using an electrode material that is conductive and has a wurtzite crystal structure, thereby improving the crystallinity (crystallinity of the wurtzite crystal structure) of a piezoelectric layer provided (formed) on the upper electrode or lower electrode, and to provide a MEMS device using the piezoelectric element.

[0010] The inventors of the present invention have continued to conduct extensive research into the above-mentioned problems, and as a result have discovered the following revolutionary piezoelectric element and a MEMS device using the piezoelectric element.

[0011] A first aspect of the present invention for solving the above problems is a piezoelectric element comprising a piezoelectric layer including at least one piezoelectric material layer having a wurtzite crystal structure, a first electrode provided on one surface of the piezoelectric layer, and a second electrode provided on the other surface of the piezoelectric layer, wherein at least one of the first electrode and the second electrode has a wurtzite crystal structure and an electrical resistivity of 1.0×10 －３ The piezoelectric element is characterized by being made of a nitride material having a resistance of Ω·cm or less.

[0012] According to the first aspect, by fabricating the first electrode or the second electrode using a nitride material that is conductive and has a wurtzite crystal structure, it is possible to improve the crystallinity (crystallinity of the wurtzite crystal structure) of the piezoelectric layer provided (formed) on the first electrode or the second electrode, which results in a piezoelectric element that does not require a buffer layer, has sufficient piezoelectric properties and stability, and can be made smaller than conventional piezoelectric elements.

[0013] In a second aspect of the present invention, the first electrode material and the second electrode material have a wurtzite crystal structure and an electrical resistivity of 1.0×10 －３ The piezoelectric element according to the first aspect is characterized in that it is made of a nitride material having a resistivity of Ω·cm or less.

[0014] According to the second aspect, since the piezoelectric layer is a wurtzite crystal, the crystallinity (crystallinity of the wurtzite crystal structure) of the second electrode or the first electrode provided (formed) on the piezoelectric layer can also be improved, thereby providing a piezoelectric element with higher piezoelectric properties and stability.

[0015] In a third aspect of the present invention, the nitride material is represented by the formula A1 α１ B1 β１ Al １－α１－β１ N or chemical formula A1 α１ B1 β１ Ga １－α１－β１ N, and α1 and β1 satisfy the following formula (1), or the chemical formula B2 β２ Al １－β２ N or chemical formula B2 β２ Ga １－β２ N, β2 satisfies the following formula (2), and B1 and B2 include monovalent ones: 0<α1 / β1<6 (1) 0<β2<0.52 (2) (0<α1<1, 0<β1<1, A1 is at least one of Mg, Zn, and Ni, and B1 and B2 are at least one of Au, Ag, and Cu.)

[0016] According to the third aspect, by fabricating the first electrode or the second electrode using a nitride material having a wurtzite crystal structure and higher electrical conductivity, the crystallinity (crystallinity of the wurtzite crystal structure) of the piezoelectric layer provided (formed) on the first electrode or the second electrode can be improved. In addition, because the piezoelectric layer has high crystallinity (crystallinity of the wurtzite crystal structure), the crystallinity (crystallinity of the wurtzite crystal structure) of the second electrode or the first electrode provided (formed) on the piezoelectric layer can also be improved. As a result, it is possible to provide a piezoelectric element that does not require a buffer layer, has sufficient piezoelectric properties and stability, and can be further miniaturized compared to conventional piezoelectric elements.

[0017] A fourth aspect of the present invention is the piezoelectric element according to the third aspect, characterized in that monovalent B1 is contained in the largest amount compared to B1 with a valence other than monovalent, or monovalent B2 is contained in the largest amount compared to B2 with a valence other than monovalent.

[0018] According to the fourth aspect, by fabricating the first electrode or the second electrode using a nitride material having a wurtzite crystal structure and further improving the crystallinity (crystallinity of the wurtzite crystal structure) of the piezoelectric layer provided (formed) on the first electrode or the second electrode, the crystallinity (crystallinity of the wurtzite crystal structure) of the piezoelectric layer can be improved. In addition, because the piezoelectric layer has high crystallinity (crystallinity of the wurtzite crystal structure), the crystallinity (crystallinity of the wurtzite crystal structure) of the second electrode or the first electrode provided (formed) on the piezoelectric layer can also be improved. As a result, it is possible to provide a piezoelectric element that does not require a buffer layer, has sufficient piezoelectric properties and stability, and can be further miniaturized compared to conventional piezoelectric elements.

[0019] A fifth aspect of the present invention is the piezoelectric element according to claim 3, characterized in that α1 and β1 satisfy the following formulas (3) and (4): 1≦α1 / β1≦5 (3) 0<α1+β1≦0.3 (4)

[0020] According to the fifth aspect, by fabricating the first electrode or the second electrode using a nitride material having a wurtzite crystal structure and higher electrical conductivity, the crystallinity (crystallinity of the wurtzite crystal structure) of the piezoelectric layer provided (formed) on the first electrode or the second electrode can be improved. In addition, because the piezoelectric layer has high crystallinity (crystallinity of the wurtzite crystal structure), the crystallinity (crystallinity of the wurtzite crystal structure) of the second electrode or the first electrode provided (formed) on the piezoelectric layer can also be improved. As a result, it is possible to provide a piezoelectric element that does not require a buffer layer, has sufficient piezoelectric properties and stability, and can be further miniaturized compared to conventional piezoelectric elements.

[0021] A sixth aspect of the present invention is the piezoelectric element according to the first aspect, wherein the Young's modulus of the first electrode and the second electrode is equal to or greater than the Young's modulus of the piezoelectric layer.

[0022] According to the sixth aspect, a high electromechanical coupling coefficient K ｅｆｆ ２ It is possible to provide a piezoelectric element having an effective electromechanical coupling coefficient.

[0023] A seventh aspect of the present invention is the piezoelectric element according to the first aspect, characterized in that the thickness t1 of the first electrode, the thickness d of the piezoelectric element, and the thickness t2 of the second electrode satisfy the following formula (5): 0.025≦(t1+t2) / d≦0.2 (5)

[0024] According to the seventh aspect, a higher K ｅｆｆ ２ It is possible to provide a piezoelectric element having the following.

[0025] An eighth aspect of the present invention is the piezoelectric element according to any one of the first to seventh aspects, characterized in that the first electrode or the second electrode is provided on the surface of the substrate.

[0026] According to the eighth aspect, it is possible to easily manufacture a piezoelectric element that does not have a buffer layer, has sufficient piezoelectric properties and stability, and can be made even smaller than conventional piezoelectric elements.

[0027] A ninth aspect of the present invention is a MEMS device using the piezoelectric element according to the first aspect.

[0028] Here, the term "MEMS device" is not particularly limited as long as it is a microelectromechanical system, and examples include physical sensors and actuators such as high-frequency filters, pressure sensors, acceleration sensors, and gyro sensors, microphones, fingerprint authentication sensors, vibration power generators, transistors, inverters, transducers, SAW devices, ferroelectric memories, diodes, and batteries.

[0029] According to the ninth aspect, a smaller MEMS device can be provided.

[0030] The specification of Japanese Patent Application No. 2024-31861 is incorporated herein by reference.

[0031] FIG. 1 is a schematic cross-sectional view of a piezoelectric element according to embodiment 1. FIG. 2 is a table showing the compositions of the examples. FIG. 3 is a graph showing the results of measurements taken with an X-ray diffractometer (XRD) for examples 5, 7, and 8. FIG. 4 is a graph showing the results of measurements of the valence of Mg and Au for examples 1-5 and 1-8. FIG. 5 is a graph showing the results of measurements of Mg and Au for examples 1-5 and 1-8. ０．０５０ Au ０．０３７ Al ０．９１３ N (top row), Mg ０．０７３ Au ０．０３６ Al ０．８９１ N (second from the top), Mg ０．１０１ Au ０．０３３ Al ０．８６６ N (third from the top) and Mg ０．３８８ Au ０．０８１ Al ０．５３１ 6 is a graph showing the results of measuring the valence of Mg in N. ０．０５０ Au ０．０３７ Al ０．９１３FIG. 7 is a diagram showing the results of measuring the atomic valence of Au in N. FIG. 7 is a table showing the electrical resistivity of the thin films of Examples 1-5, 1-7, 1-8, and 1-13 to 1-27. FIG. 8 is a schematic cross-sectional view of an electrode-equivalent thin film and a piezoelectric-equivalent thin film prepared to examine the crystallinity of a semiconductor element. FIG. 9 is a table showing the results of measuring the half-width of the rocking curve. FIG. 10 is a graph showing the results of measuring the maximum value of the diffraction intensity of the crystal plane (002) when the molar ratio of Mg to Au is changed. FIG. 11 is a graph showing the results of measuring the maximum value of the diffraction intensity of the crystal plane (002) when the molar ratio of Mg to Au is changed. FIG. 12 is a table showing the compositions of Examples 2-1 to 2-11. FIG. 13 is a graph showing the results of measuring the X-ray diffractometer (XRD) of Examples 2-1 to 2-3. FIG. 14 is a graph showing the results of measuring the Au ０．０２ Al ０．９８ FIG. 15 is a table showing the compositions and electrical resistivities of Examples 2-5 to 2-11. FIG. 16 is a graph showing the results of measuring the maximum diffraction intensity of the crystal plane (002) when the Au concentration is changed. FIG. 17 is a schematic cross-sectional view of a conventional piezoelectric element. FIG. 18 is a schematic cross-sectional view of a conventional piezoelectric element.

[0032] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A piezoelectric element according to the present invention and a MEMS device using the piezoelectric element will be described below with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments.

[0033] (Embodiment 1) Fig. 1 is a schematic cross-sectional view of a piezoelectric element according to this embodiment. As shown in this figure, a piezoelectric element 1 according to this embodiment is provided directly on the surface of a substrate 100, and is composed of a second electrode 30 (lower electrode), a piezoelectric layer 10 provided directly on the upper surface of the second electrode 30, and a first electrode 20 (upper electrode) provided directly on the piezoelectric layer 10. In other words, unlike the conventional piezoelectric elements described above, the piezoelectric element 1 does not have a buffer layer. Note that the terms "first" and "second" used for the electrodes are used for ease of explanation, and "first" may be referred to as "second" and "second" may be referred to as "first," and are not terms that particularly limit the positional relationship, etc.

[0034] The piezoelectric layer 10 is not particularly limited as long as it has piezoelectricity and includes at least one piezoelectric material layer having a wurtzite crystal structure. That is, the piezoelectric layer 10 may be composed of one or more piezoelectric material layers having a wurtzite crystal structure, or may be composed of one or more piezoelectric material layers having a wurtzite crystal structure and one or more other piezoelectric material layers.

[0035] The material constituting the piezoelectric material layer used in the piezoelectric layer 10 is not particularly limited as long as it has piezoelectricity and a wurtzite crystal structure. Examples of materials constituting the piezoelectric material layer used in the piezoelectric layer 10 include AlN, GaN, InN, AlGaN, ScAlN, ScGaN, ScAlGaN, YAlN, YbAlN, YbGaN, YbAlGaN, BAlN, BGaN, BAlGaN, ZnO, LiZnO, and CeMnZnO, as well as nitride materials described in the specification of Japanese Patent Application No. 2020-212574, Japanese Patent Application No. 2021-027147, and Japanese Patent Application No. 2021-501 Examples of the nitride material include the nitride material described in the specification of Japanese Patent Application No. 655, the nitride material described in the specification of Japanese Patent Application No. 2023-188952, the nitride material described in the specification of Japanese Patent Application No. 2020-168462, the nitride material described in the specification of Japanese Patent Application No. 2020-168463, the nitride material described in the specification of Japanese Patent Application No. 2018-231681, and the nitride material described in the specification of Japanese Patent Application No. 2019-020273. The thickness of the piezoelectric layer 10 is not particularly limited.

[0036] Next, the first electrode 20 and the second electrode 30 will be described. The first electrode 20 has a wurtzite crystal structure and an electrical resistivity of 1.0×10 －３ There are no particular limitations on the nitride material (first nitride material, first electrode material) as long as it has an electrical resistivity of Ω cm or less. －６ Ω・cm or more, 1.0×10 －３ Ω cm or less is preferable, and 1.47 × 10 －６Ω・cm or more, 1.47×10 －４ It is more preferable that the resistivity is Ω·cm or less.

[0037] The first nitride material may be, for example, a compound represented by the chemical formula A1 α１ B1 β１ Al １－α１－β１ N or chemical formula A1 α１ B1 β１ Ga １－α１－β１ N, α1 and β1 satisfy the following formula (1), and B1 may be monovalent. 0<α1 / β1<6 (1) Furthermore, it is more preferable that the monovalent B1 is contained in the largest amount compared to B1 with a valence other than monovalent. Furthermore, it is more preferable that α1 and β1 satisfy the following formulas (3) and (4), since this reduces structural mismatch with the piezoelectric layer 10. 1≦α1 / β1≦5 (3) 0<α1+β1≦0.3 (4) Note that A1 is at least one of magnesium (Mg), zinc (Zn), and nickel (Ni), and may be any one of Mg, Zn, and Ni, or a combination of Mg, Zn, and Ni. Furthermore, B1 is at least one of gold (Au), silver (Ag), and copper (Cu), and may be any one of Au, Ag, and Cu, or may be a combination of Au, Ag, and Cu. Furthermore, when A1 and B1 are a combination of a plurality of elements, the ratio of these elements is not particularly limited.

[0038] Here, one method generally known for improving the conductivity of a material is to add (do) an element having a smaller valence than a predetermined element constituting the material. The valence of aluminum and gallium in the first nitride material is 3 (Al ３＋ , Ga ３＋ ), so it is a divalent element (Mg ２＋ , Ca ２＋ , Sr ２＋ , Ba ２＋ , Zn ２＋ , Ni ２＋ , Pt ２＋ , Cd ２＋ , Mn ２＋ , Ru ２＋ , Ir ２＋ ) and monovalent elements (Li ＋ , Na＋ , K ＋ , Au ＋ , Ag ＋ , Cu ＋ According to band theory, when an element such as ZnO is added (doped), these elements act as acceptors, forming an acceptor level just above the valence band. As a result, carriers can move, improving conductivity (decreasing electrical resistivity).

[0039] From this, as will be described later, Mg α１ Au β１ Al １－α１－β１ N, Zn α１ Ag β１ Al １－α１－β１ N, Mg α１ Au β１ Ga １－α１－β１ N, Mg α１ Cu β１ Ga １－α１－β１ N, Mg α１ Ag β１ Al １－α１－β１ N, Mg α１ Cu β１ Al １－α１－β１ N and Ni α１ Cu β１ Al １－α１－β１ The electrical resistivity of the first nitride material composed of N is 1.0×10 －３ If the resistance is Ω cm or less, the above-mentioned chemical formula A1 α１ B1 β１ Al １－α１－β１ N or chemical formula A1 α１ B1 β１ Ga １－α１－β１ N, the electrical resistivity of the first nitride material composed of a combination of other elements is 1.0 × 10 －３ It can be estimated that the resistivity will be Ω·cm or less.

[0040] Furthermore, when the Young's modulus of the first electrode 20 is equal to or greater than the Young's modulus of the piezoelectric layer 10, the electromechanical coupling coefficient K ｅｆｆ ２Furthermore, when the piezoelectric layer 10 is made of aluminum scandium nitride, it is more preferable that the Young's modulus of the first electrode 20 is equal to or greater than the Young's modulus of the piezoelectric layer 10 and is in the range of 50 GPa to 500 GPa, and it is particularly preferable that the Young's modulus of the first electrode 20 is equal to or greater than the Young's modulus of the piezoelectric layer 10 and is in the range of 200 GPa to 450 GPa. The thickness t1 of the first electrode 20 is not particularly limited.

[0041] The second electrode 30 has a wurtzite crystal structure and an electrical resistivity of 10 －３ There are no particular limitations on the nitride material (second nitride material, second electrode material) as long as it has a resistivity of Ω cm or less. The electrical resistivity of the second nitride material is 1.47×10, similar to that of the first nitride material. －６ Ω・cm or more, 1.0×10 －３ Ω cm or less is preferable, and 1.47 × 10 －６ Ω・cm or more, 1.47×10 －４ It is more preferable that the resistivity is Ω·cm or less.

[0042] As the second nitride material, similarly to the first nitride material, for example, a nitride material represented by the chemical formula A1′ α１’ B1' β１’ Al １－α１’－β１’ N or chemical formula A1′ α１’ B1' β１’ Ga １－α１’－β１’ N, α1′ and β1′ satisfy the following formula (6), and B1′ includes a monovalent one: 0<α1′ / β1′<6 (6)

[0043] It is more preferable that the monovalent B1' is contained in the largest amount compared to the B1's with valences other than monovalent. Furthermore, it is more preferable that α1' and β1' satisfy the following formulas (7) and (8), since this reduces the structural mismatch with the piezoelectric layer 10: 1≦α1' / β1'≦5 (7) 0<α1'+β1'≦0.3 (8)

[0044] Note that A1' is at least one of magnesium (Mg), zinc (Zn), and nickel (Ni), and may be any one of Mg, Zn, and Ni, or may be a combination of multiple elements of Mg, Zn, and Ni. Also, B1' is at least one of gold (Au), silver (Ag), and copper (Cu), and may be any one of Au, Ag, and Cu, or may be a combination of multiple elements of Au, Ag, and Cu. Note that when A1' and B1' are multiple elements, the ratio of these elements is not particularly limited.

[0045] Here, as with the first electrode 20, Mg α１’ Au β１’ Al １－α１’－β１’ N, Zn α１’ Ag β１’ Al １－α１’－β１’ N, Mg α１’ Au β１’ Ga １－α１’－β１’ N, Mg α１’ Cu β１’ Ga １－α１’－β１’ N, Mg α１’ Ag β１’ Al １－α１’－β１’ N, Mg α１’ Cu β１’ Al １－α１’－β１’ N and Ni α１’ Cu β１’ Al １－α１’－β１’ The electrical resistivity of the second nitride material composed of N is 1.0×10 －３ If the resistivity is Ω·cm or less, the above-mentioned chemical formula A1′ α１’ B1' β１’ Al １－α１’－β１’ N or chemical formula A1′ α１’ B1' β１’ Ga １－α１’－β１’ N, the electrical resistivity of the second nitride material composed of a combination of other elements is 1.0 × 10 －３ It can be estimated that the resistivity will be Ω·cm or less.

[0046] Similarly to the first electrode 20, when the Young's modulus of the second electrode 30 is equal to or greater than the Young's modulus of the piezoelectric layer 10, the electromechanical coupling coefficient K ｅｆｆ ２ The thickness t2 of the second electrode 30 is not particularly limited.

[0047] Here, the first nitride material and the second nitride material may be made of the same nitride material or different nitride materials, or may be made of the same nitride material and have the same thickness (t1=t2).

[0048] In addition, in the piezoelectric element 1 satisfying the above formulas (1), (3), (4), and (6) to (8), the second electrode 30 has even higher crystallinity (crystallinity of a wurtzite crystal structure), which can further improve the crystallinity (crystallinity of a wurtzite crystal structure) of the piezoelectric layer 10 provided (formed) on the second electrode 30. In addition, because the piezoelectric layer 10 has even higher crystallinity (crystallinity of a wurtzite crystal structure), it can also further improve the crystallinity (crystallinity of a wurtzite crystal structure) of the first electrode 20 provided (formed) on the piezoelectric layer 10. As a result, a piezoelectric element having such a structure does not require a buffer layer, has sufficient piezoelectric properties and high stability, and can be made smaller than conventional piezoelectric elements.

[0049] The piezoelectric element according to the present invention, including this embodiment, can be manufactured by a known manufacturing method.

[0050] By configuring the piezoelectric element 1 as described above, the electrodes (first electrode 20 or second electrode 30) are fabricated using a nitride material having a wurtzite crystal structure, which makes it possible to improve the crystallinity (crystallinity of the wurtzite crystal) of the piezoelectric layer 10 provided (formed) on the electrodes. Furthermore, because the piezoelectric layer 10 is a wurtzite crystal, it is also possible to improve the crystallinity (crystallinity of the wurtzite crystal structure) of the second electrode 30 or first electrode 20 provided (formed) on the piezoelectric layer 10. As a result, it is possible to provide a piezoelectric element that does not require a buffer layer, has sufficient piezoelectric properties and stability, and can be made smaller than conventional piezoelectric elements.

[0051] <Example of electrode> Using the following equipment and sputtering target, a nitride material thin film (Mg) with a thickness of 0.05 μm to 2 μm, to which magnesium, zinc or nickel (A1=Mg, Zn or Ni) and gold, silver or copper (B1=Au, Ag or Cu) are added, is deposited on an n-type silicon substrate with a specific resistance (electrical resistivity) of 0.02 Ωcm. α１ Au β１ Al １－α１－β１ N, Zn α１ Ag β１ Al １－α１－β１ N, Mg α１ Au β１ Ga １－α１－β１ N, Mg α１ Cu β１ Ga １－α１－β１ N, Mg α１ Ag β１ Al １－α１－β１ N, Mg α１ Cu β１ Al １－α１－β１ N and Ni α１ Cu β１ Al １－α１－β１ A multi-target simultaneous sputtering deposition apparatus (manufactured by Kenix) Magnesium sputtering target material (purity: 99.99%) Zinc sputtering target material (purity: 99.9%) Nickel sputtering target material (purity: 99.9%) Gold sputtering target material (purity: 99.9%) Silver sputtering target material (purity: 99.9%) Copper sputtering target material (purity: 99.9%) Aluminum sputtering target material (purity: 99.999%) Gallium nitride sputtering target material (purity: 99.99%) Gas: Mixed gas of nitrogen (purity: 99.99995% or more) and argon gas (purity: 99.9999% or more) (mixing ratio (nitrogen:argon) 30:70) Substrate heating temperature: 500°C

[0052] The deposition experiment was carried out with the pressure in the sputtering chamber at 10 －５ The experiment was carried out after evacuation with a vacuum pump to a high vacuum of less than 1 Pa. In addition, to prevent the inclusion of impurities such as oxygen, the target surface was cleaned immediately after the target was attached and immediately before each film formation experiment.

[0053] The composition of each of the obtained nitride material thin films is shown in Figure 2, and the results of measurement by an X-ray diffractometer (XRD) for Examples 1-5, 1-7, and 1-8 are shown in Figure 3. As can be seen from this figure, each of the obtained aluminum nitride thin films was found to have a wurtzite crystal structure.

[0054] Next, the results of measuring the valence of Mg and Au for Examples 1-5 and 1-8 using an X-ray photoelectron spectroscopy (XPS) device are shown in FIG. ０．０５０ Au ０．０３７ Al ０．９１３ N (top row), Mg ０．０７３ Au ０．０３６ Al ０．８９１ N (second from the top), Mg ０．１０１ Au ０．０３３ Al ０．８６６ N (third from the top) and Mg ０．３８８ Au ０．０８１ Al ０．５３１ The results of measuring the valence of Mg in N (bottom row) using an X-ray diffraction (XRD) device are shown in FIG. ０．０５０ Au ０．０３７ Al ０．９１３ The results of measuring the valence of Au in N are shown in Figure 6. The peaks shown in these figures indicate the binding energy of each element. From Figures 4 and 5, it was found that the binding energy of Mg2p coincides with that when the valence of Mg is 2 (+2). Similarly, from Figures 4 and 6, it was found that the binding energy of Au4f coincides with that when the valence of Au is 1 (+1). Therefore, the thin films of Examples 1-5 and 1-8, as well as the Mg ０．０５０ Au ０．０３７ Al ０．９１３ N and Mg ０．１０１ Au ０．０３３ Al ０．８６６ The valence of Mg contained in N is 2 (+2), and the thin films of Examples 1-5 and 1-8 and Mg ０．０５０ Au ０．０３７ Al ０．９１３ Au with a valence of N of 1 (+1) is replaced by Au with a valence of 0 (Au ０ ) and found to contain more of it than the

[0055] Furthermore, the electrical resistivity of the thin films of Examples 1-5, 1-7, 1-8, and 1-13 to 1-27 was measured using a resistivity / Hall measurement system (Resist Test 8300, manufactured by Toyo Corporation). The results are shown in Figure 7. As can be seen from this figure, the electrical resistivity of the obtained nitride thin films was 7.8 x 10 －４ Ω cm or less, and the electrical resistivity of the obtained nitride thin film was 1.47 × 10 Ω cm or less, except for Examples 1-25 and 1-26. －４ It was found that the resistivity was Ω·cm or less.

[0056] <Crystalline Properties of Semiconductor Element> As shown in FIG. 8, a Zn layer corresponding to the second electrode is formed on a silicon substrate. ０．０３ Ag ０．０２ Al ０．９５ An AlN thin film corresponding to the piezoelectric layer was formed on the N thin film. ０．０３ Ag ０．０２ Al ０．９５ The half-widths of the rocking curves of the crystal plane (002) of the N thin film and the AlN thin film were measured, and the results are shown in FIG.

[0057] As can be seen from this figure, the half-width of the rocking curve of the AlN thin film corresponding to the piezoelectric layer is sufficiently small, which means that a piezoelectric layer with high wurtzite crystallinity can be formed even without a buffer layer.

[0058] Next, an aluminum nitride thin film (Mg α１ Au β１ Al １－α１－β１ The maximum diffraction intensity of the crystal plane (002) measured by an X-ray diffractometer (XRD) when the molar ratio (α1 / β1) of Mg to Au in aluminum nitride thin film (N) was changed is shown in Figure 10. As can be seen from this figure, when α1 / β1 is in the range of 0 < α1 / β1 < 6, the aluminum nitride thin film has a wurtzite crystal structure, and when α / β is in the range of 1 ≦ α / β ≦ 5, the aluminum nitride thin film has a wurtzite crystal structure with higher crystallinity.

[0059] Furthermore, aluminum nitride thin films (Mg２＋ α１ Au １＋ β１ Al １－α１－β１ The maximum value of the diffraction intensity of the crystal plane (002) of each aluminum nitride thin film was measured by an X-ray diffractometer (XRD), and the measurement results are shown in FIG.

[0060] This figure shows that aluminum nitride thin films in the range of 0 < α1 + β1 < 0.3 have a highly crystalline wurtzite crystal structure. Although it is difficult to see in this figure, the maximum diffraction intensity of the (002) crystal plane of the aluminum nitride thin film when α1 + β1 = 0.3 is greater than that when α1 + β1 = 0.4.

[0061] (Embodiment 2) In Embodiment 1, aluminum nitride (AlN) or gallium nitride (GaN) doped with two elements is used as the first electrode material and the second electrode material, but the present invention is not limited thereto. For example, aluminum nitride doped with only one element or gallium nitride doped with only one element may be used as the first electrode material and the second electrode material.

[0062] Specifically, the first electrode material may be, for example, a compound represented by the chemical formula B2 β２ Al １－β２ N or chemical formula B2 β２ Ga １－β２ N, β2 satisfies the following formula (2), and B2 includes a monovalent one: 0<β2<0.52 (2) It is more preferable that β2 is in the range of 0<β2<0.2.

[0063] And, it is more preferable that the amount of monovalent B2 is the largest compared to B2 with a valence other than monovalent.

[0064] B2 is at least one of gold (Au), silver (Ag), and copper (Cu), and may be any one of Au, Ag, and Cu, or may be a combination of two or more elements selected from Au, Ag, and Cu. When B2 is a combination of two or more elements, the ratio of these elements is not particularly limited.

[0065] As the second electrode material, for example, a compound represented by the chemical formula B2' is used, similarly to the first electrode material. β２’ Al １－β２’ N or chemical formula B2' β２’ Ga １－β２’ N, β2' satisfies the following formula (9), and B2' includes a monovalent one: 0<β2'<0.52 (9) It is more preferable that β2' is in the range of 0<β2'<0.2.

[0066] And, it is more preferable that the amount of monovalent B2' is the largest compared with B2' having a valence other than monovalent.

[0067] Note that B2' is at least one of gold (Au), silver (Ag), and copper (Cu), and may be any one of Au, Ag, and Cu, or may be a combination of Au, Ag, and Cu. Furthermore, when B2' is a combination of multiple elements, the ratio of these elements is not particularly limited. Even when a piezoelectric element is constructed using such an electrode material, the same effects as those of the first embodiment can be obtained.

[0068] <Electrode Example> A nitride material thin film with a thickness of 0.05 μm to 2 μm doped with gold or copper (B2 = Au, Cu) was fabricated on an n-type silicon substrate with a specific resistance (electrical resistivity) of 0.02 Ωcm using the following equipment and sputtering targets: Multi-target simultaneous sputtering deposition equipment (Kenix) Gold sputtering target material (purity: 99.9%) Copper sputtering target material (purity: 99.9%) Aluminum sputtering target material (purity: 99.999%) Gallium nitride sputtering target material (purity: 99.99%) Gas: Mixture of nitrogen (purity: 99.99995% or higher) and argon gas (purity: 99.9999% or higher) (mixture ratio (nitrogen:argon) 30:70) Substrate heating temperature: 500°C

[0069] The deposition experiment was carried out with the pressure in the sputtering chamber at 10 －５ The experiment was carried out after evacuation with a vacuum pump to a high vacuum of less than 1 Pa. In addition, to prevent the inclusion of impurities such as oxygen, the target surface was cleaned immediately after the target was attached and immediately before each film formation experiment.

[0070] The composition of each of the obtained nitride material thin films is shown in Figure 12, and the measurement results of Examples 2-1 to 2-3 using an X-ray diffractometer (XRD) are shown in Figure 13. As can be seen from this figure, each of the obtained aluminum nitride thin films of Examples 2-1 to 2-3 was found to have a wurtzite crystal structure.

[0071] Next, Au ０．０２ Al ０．９８ The results of measuring the valence of Au for N using an X-ray photoelectron spectroscopy (XPS) device are shown in Figure 14. The peaks shown in this figure indicate the binding energy of each element. It was found that the binding energy of Au4f coincides with that of Au with a valence of 1 (+1). Therefore, Au ０．０２ Al ０．９８ In the N thin film, Au with a valence of 1 (+1) is replaced by Au with a valence of 0 (Au ０ ) and found to contain more of it than the

[0072] Furthermore, using a resistivity / Hall measurement system (manufactured by Toyo Corporation), ０．１０ Al ０．９０ When the electrical resistivity of the SiO2 film was measured, it was found to be 5.48×10 －４ The value was Ω·cm.

[0073] Furthermore, the electrical resistivity of each of Examples 2-5 to 2-11 was measured using a resistivity measuring device (Resist Test 8300, manufactured by Toyo Corporation). The results are shown in Figure 15. From this figure, it can be seen that the electrical resistivity of each of Examples 2-5 to 2-11 was 1.0 x 10 －３ It was found to be smaller than Ω·cm.

[0074] Next, an aluminum nitride thin film (Au β２ Al １－β２ The maximum diffraction intensity of the (002) crystal plane measured by an X-ray diffractometer (XRD) when the Au concentration in aluminum nitride (N) was changed is shown in Figure 16. As can be seen from this figure, it was found that the aluminum nitride thin film has a wurtzite crystal structure when β2 is in the range of 0 < β2 < 0.2.

[0075] (Embodiment 3) In Embodiments 1 and 2, the first electrode and the second electrode are formed using the nitride material described above, but the present invention is not limited thereto. For example, either the first electrode or the second electrode may be formed using a known electrode material. The electrode material is not particularly limited, and examples thereof include Pt.

[0076] (Other Embodiments) In the first embodiment, the thickness of the piezoelectric element is not limited, but the thickness of the piezoelectric element may be limited. Specifically, the piezoelectric element may be configured so that the thickness t1 of the first electrode, the thickness t2 of the second electrode, and the thickness d of the piezoelectric element satisfy the following formula (5): 0.025≦(t1+t2) / d≦0.2 (5) A piezoelectric element configured in this manner has a high electromechanical coupling coefficient K ｅｆｆ ２ It will have the following characteristics.

[0077] It goes without saying that a piezoelectric element can be fabricated using the first electrode and second electrode described above, and that the piezoelectric element can also be used to fabricate a MEMS device.

[0078] 1, 200A, 200B Piezoelectric element 10, 210 Piezoelectric layer 20, 220 First electrode 30, 230 Second electrode 100, 300 Substrate 250 Buffer layer

Claims

1. A piezoelectric element comprising: a piezoelectric layer including at least one piezoelectric material layer having a wurtzite crystal structure; a first electrode provided on one surface of the piezoelectric layer; and a second electrode provided on the other surface of the piezoelectric layer, wherein at least one of the first electrode and the second electrode has a wurtzite crystal structure and an electrical resistivity of 1.0×10 －３ A piezoelectric element comprising a nitride material having a resistivity of Ω·cm or less.

2. The first electrode material and the second electrode material have a wurtzite crystal structure and an electrical resistivity of 1.0×10 －３ 2. The piezoelectric element according to claim 1, wherein the piezoelectric element is made of the nitride material having a resistivity of Ω·cm or less.

3. The nitride material has the formula A1 α１ B1 β１ Al １－α１－β１ N or chemical formula A1 α１ B1 β１ Ga １－α１－β１ N, and α1 and β1 satisfy the following formula (1), or chemical formula B2 β２ Al １－β２ N or chemical formula B2 β２ Ga １－β２ 3. The piezoelectric element according to claim 1, wherein β2 satisfies the following formula (2): 0<α1 / β1<6 (1) 0<β2<0.52 (2) (0<α1<1, 0<β1<1, A1 is at least one of Mg, Zn, and Ni, and B1 and B2 are at least one of Au, Ag, and Cu.) 4. The piezoelectric element according to claim 3, characterized in that monovalent B1 is contained in the largest amount compared to B1 with a valence other than monovalent, or monovalent B2 is contained in the largest amount compared to B2 with a valence other than monovalent.

5. The piezoelectric element according to claim 3, wherein α1 and β1 satisfy the following formulas (3) and (4): 1≦α1 / β1≦5 (3) 0<α1+β1≦0.3 (4) 6. The piezoelectric element according to claim 1, wherein the Young's modulus of the first electrode and the second electrode is equal to or greater than the Young's modulus of the piezoelectric layer.

7. The piezoelectric element according to claim 1, wherein the thickness t1 of the first electrode, the thickness d of the piezoelectric element, and the thickness t2 of the second electrode satisfy the following formula (5): 0.025≦(t1+t2) / d≦0.2 (5) 8. The piezoelectric element according to claim 1, wherein the first electrode or the second electrode is provided on the surface of a substrate.

9. A MEMS device using the piezoelectric element according to claim 1.

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