BAW resonator, BAW device, and electronic device

Abstract

A BAW resonator according to the present invention comprises a first electrode, a piezoelectric layer having a Mg-doped piezoelectric material, and a second electrode which are laminated in this order on a support base material, wherein at least one among the first electrode and the second electrode is a Mg-containing alloy.

Description

BAW resonator, BAW device and electronic device

[0001] The present invention relates to a BAW resonator, a BAW device, and an electronic device.

[0002] Bulk acoustic wave (BAW) resonators utilize the piezoelectric effect of a piezoelectric layer disposed between two electrode layers and are used in electronic devices as electronic components such as pressure sensors, acceleration sensors, AE sensors that detect elastic waves, radio frequency (RF) filters, and piezoelectric actuators.

[0003] As a BAW resonator, for example, a ZnO buffer layer and an Mg ｘ Zn １－ｘ An acoustic device having a multilayer structure with an O film has been disclosed (see, for example, Patent Document 1).

[0004] US Patent Application Publication No. 2003 / 129307

[0005] However, in the acoustic device of Patent Document 1, Mg ｘ Zn １－ｘ Mg in the O film is Mg ｘ Zn １－ｘ The Mg concentration decreases as a result of diffusion to the electrodes located on both the top and bottom surfaces of the O film. ｘ Zn １－ｘ This causes a problem of fluctuation in the piezoelectric properties of the O film.

[0006] An object of one aspect of the present invention is to provide a piezoelectric element that can maintain its piezoelectric characteristics.

[0007] One aspect of the present invention is a BAW resonator comprising: a first electrode, a piezoelectric layer having a piezoelectric material doped with Mg, and a second electrode stacked in this order on a support substrate; and at least one of the first electrode and the second electrode is an alloy containing Mg.

[0008] A BAW resonator according to one aspect of the present invention can maintain its piezoelectric properties.

[0009] 1 is a plan view showing an example of a BAW device including a BAW resonator according to an embodiment of the present invention; FIG. 2 is a cross-sectional view taken along line II of FIG. 1; FIG. 3 is a schematic cross-sectional view showing another example of the configuration of a BAW device; and FIG. 4 is a schematic cross-sectional view showing another example of the configuration of a BAW device.

[0010] Hereinafter, embodiments of the present invention will be described in detail. To facilitate understanding of the description, the same components in each drawing are denoted by the same reference numerals, and duplicate explanations will be omitted. The scale of each component in the drawings may differ from the actual scale. In this specification, unless otherwise specified, the symbol "to" indicating a range of values ​​means that the values ​​before and after it are included as the lower and upper limits.

[0011] <BAW Device> Fig. 1 is a plan view of a BAW device including a BAW resonator according to an embodiment of the present invention (hereinafter, sometimes simply referred to as "this embodiment"), and Fig. 2 is a cross-sectional view taken along line II of Fig. 1, showing the configuration of a BAW device including a BAW resonator according to this embodiment. As shown in Fig. 2, a BAW device 1A includes a support substrate 10 and a BAW resonator 20A provided on the support substrate 10.

[0012] In this specification, a three-dimensional Cartesian coordinate system is used in three axial directions (X-axis, Y-axis, and Z-axis), and in a plane perpendicular to the height (thickness) direction (vertical direction) of the BAW resonator 20A, one of two mutually perpendicular directions is defined as the X-axis direction, the other as the Y-axis direction, and the thickness direction is defined as the Z-axis direction. The BAW resonator 20A in the Z-axis direction is defined as the +Z-axis direction, and the support substrate 10 side is defined as the −Z-axis direction. In the following description, for convenience of explanation, the +Z-axis direction will be referred to as up or upward, and the −Z-axis direction will be referred to as down or downward, but this does not represent a universal up-down relationship.

[0013] [Support Base] As shown in FIG. 2, the support base 10 is a substrate on which the BAW resonator 20A is mounted, and may be flexible so as to provide the BAW resonator 20A with flexibility.

[0014] The material for forming the support substrate 10 can be any material, regardless of type, as long as it can stably support the BAW resonator 20A. For example, a metal plate, a silicon (Si) substrate, an inorganic dielectric substrate, a glass substrate, etc. may be used.

[0015] The metal plate may be made of a material such as aluminum, copper, stainless steel, or tantalum.

[0016] The inorganic dielectric substrate may be made of, for example, MgO or sapphire.

[0017] The thickness of the support substrate 10 is not particularly limited and may be determined appropriately depending on the application of the BAW resonator 20A, the material of the support substrate 10, etc., and may be, for example, 20 μm to 725 μm. If the thickness of the support substrate 10 is 20 μm to 725 μm, the BAW resonator 20A can be stably supported. Furthermore, warping of the support substrate 10 is suppressed, and the influence of warping of the support substrate 10 on the piezoelectric characteristics can be reduced, allowing the BAW resonator 20A to have desired piezoelectric characteristics.

[0018] The piezoelectric characteristics include both the amount of voltage generated per applied stress (positive piezoelectric effect) and the rate of mechanical displacement per applied electric field (negative piezoelectric effect).

[0019] In this specification, the thickness of the support substrate 10 refers to the length in the direction perpendicular to the main surface of the support substrate 10. The method for measuring the thickness of the support substrate 10 is not particularly limited, and any measurement method can be used. The thickness of the support substrate 10 may be, for example, the thickness measured at an arbitrary location on the cross section of the support substrate 10, or may be the average value of the measured values ​​measured at several arbitrary locations. Hereinafter, the definition of thickness is similarly defined for other members.

[0020] [BAW Resonator] As shown in FIG. 2, the BAW resonator 20A includes an acoustic mirror layer 21, a first electrode 22, a piezoelectric layer 23, and a second electrode 24, which are laminated in this order from the support substrate 10 side.

[0021] In the BAW resonator 20A, the piezoelectric layer 23 contains Mg, and at least one of the first electrode 22 and the second electrode 24 contains Mg. In the BAW resonator 20A, the piezoelectric layer 23 is sandwiched between the first electrode 22 and the piezoelectric layer 23 on both the upper and lower surfaces thereof, thereby preventing a decrease in the Mg concentration in the piezoelectric layer 23 due to diffusion of Mg in the piezoelectric layer 23 to at least one of the first electrode 22 and the second electrode 24. This prevents deterioration of the piezoelectric layer 23 in the BAW resonator 20A, thereby maintaining the piezoelectric characteristics.

[0022] 1, the BAW resonator 20A has an active region A1 and a non-active region A2 other than the active region A1. In FIG. 1, the active region A1 and the non-active region A2 are indicated by hatching.

[0023] The active region A1 is a region where the piezoelectric layer 23 resonates, and is a region where the first electrode 22, the piezoelectric layer 23, and the second electrode 24 overlap in a planar view. That is, the active region A1 is a region where the first electrode 22 and the second electrode 24 are disposed on both surfaces of the piezoelectric layer 23, and includes a region of the piezoelectric layer 23 sandwiched between the first electrode 22 and the second electrode 24. The shape of the active region A1 is generally rectangular in a planar view, but may also be generally circular, generally polygonal, or generally elliptical. Note that the active region A1 may include a region of the piezoelectric layer 23 sandwiched between the first electrode 22 and the second electrode 24 in a planar view, and may include a region corresponding to at least a portion of the second electrode 24 where the piezoelectric layer 23 can resonate.

[0024] [Acoustic Mirror Layer] As shown in Fig. 2, the acoustic mirror layer 21 is provided on the upper principal surface (top surface) 10a of the support substrate 10. The acoustic mirror layer 21 is composed of acoustic multilayer films with different specific acoustic impedances. The acoustic mirror layer 21 is a multilayer film in which high acoustic impedance layers 211 having a predetermined specific acoustic impedance are alternately stacked in one or more pairs (three pairs are stacked in Fig. 2) with low acoustic impedance layers 212 having a lower specific acoustic impedance than the high acoustic impedance layers 211 in the thickness direction of the acoustic mirror layer 21.

[0025] When resonant vibrations are transmitted from the piezoelectric layer 23 to the acoustic mirror layer 21, the vibration energy of the resonance is reflected at the interface between the high acoustic impedance layer 211 and the low acoustic impedance layer 212 in the acoustic mirror layer 21. The ease of propagation of vibration waves (elastic waves) generated by resonance of the piezoelectric layer 23 differs between the high acoustic impedance layer 211 and the low acoustic impedance layer 212. Due to the difference in propagation degree at the interface between the layers constituting the acoustic mirror layer 21, the elastic waves are reflected in the direction of the first electrode 22 located in the upper layer of the acoustic mirror layer 21, thereby returning the vibration energy of the resonance to the incident direction of the elastic waves without being affected by the support substrate 10, and dissipating heat energy in the direction of the support substrate 10.

[0026] The number of stacked high acoustic impedance layers 211 and low acoustic impedance layers 212 is not particularly limited, but may be, for example, 3 to 5. If the number of stacked high acoustic impedance layers 211 is 3, the elastic waves propagating from the piezoelectric layer 23 can be sufficiently reflected.

[0027] The high acoustic impedance layer 211 is made of W, Mo, Ta, ２ O ５ It is preferable that the insulating layer is made of a material having a high density or bulk modulus, such as ZnO.

[0028] The multiple high acoustic impedance layers 211 may be made of the same material or different materials.

[0029] The low acoustic impedance layer 212 is made of a material having a lower density or bulk modulus than the high acoustic impedance layer 211. Examples of materials having a lower density or bulk modulus than the high acoustic impedance layer 211 include SiO ２ The low acoustic impedance layer 212 may be an amorphous layer or a layer in which the amorphous phase is dominant. By making the low acoustic impedance layer 212 a layer in which the amorphous phase is dominant, stress in the high acoustic impedance layer 211 can be alleviated.

[0030] The high acoustic impedance layer 211 and the low acoustic impedance layer 212 may be formed on the support substrate 10 by sputtering or the like.

[0031] 2, the first electrode 22 is provided on the upper principal surface (top surface) 21a of the acoustic mirror layer 21. The first electrode 22 may be formed as a thin film on a part of or the entire top surface 21a of the acoustic mirror layer 21. The first electrode 22 may be provided so as to extend outward beyond the piezoelectric layer 23 so that a part of the first electrode 22 can come into contact with the outside.

[0032] The first electrode 22 is conductive and can be formed of an alloy containing Mg. By forming the first electrode 22 from an alloy containing Mg, even if Mg contained in the piezoelectric layer 23 escapes from the lower surface 23 b of the piezoelectric layer 23, the amount of Mg that has escaped can be supplied from the lower surface 23 b of the piezoelectric layer 23 into the piezoelectric layer 23.

[0033] As the alloy containing Mg, for example, an alloy containing at least one component selected from the group consisting of Al, Zn, Y, Gd, and Cu, or a compound containing the above component, doped with Mg, can be used.

[0034] The content of Mg in the first electrode 22 can be adjusted as desired, but is preferably 0.01 at% to 5 at%, for example, and the upper limit of the Mg content is more preferably 3 at% or less, and even more preferably 1 at% or less. If the Mg content is within the above-mentioned preferred range, the amount of Mg that has flowed out from the piezoelectric layer 23 can be supplied.

[0035] The content of Mg contained in the first electrode 22 may be measured by, for example, Rutherford backscattering spectroscopy (RBS) using a Pelletron 3SDH (manufactured by NEC Corporation) as a measuring device, or by secondary ion mass spectrometry using dynamic SIMS (D-SIMS) or the like. Furthermore, the content of Mg contained in the first electrode 22 may be measured using an analytical device commonly used for composition analysis, such as an electron microscope for chemical analysis (ESCA).

[0036] When the second electrode 24 is formed of an alloy containing Mg, the first electrode 22 may be formed of any conductive material, such as metals such as Pt, Au, Ag, Cu, Al, Zn, Y, Gd, Ti, Cr, Zr, Nb, Mo, Rh, Pd, Ru, Ir, Ta, and W.

[0037] The first electrode 22 may be formed of a single layer of an alloy containing Mg or any conductive material other than an alloy containing Mg.

[0038] The thickness of the first electrode 22 can be appropriately designed and may be, for example, 30 nm to 300 nm. If the thickness of the first electrode 22 is 30 nm to 300 nm, the first electrode 22 can function as an electrode, and the BAW resonator 20A can be made thinner while suppressing the BAW resonator 20A from shifting to the low frequency side.

[0039] 2 , the piezoelectric layer 23 is provided on the upper surface 21 a of the acoustic mirror layer 21 and on the main surface (upper surface) 22 a above the first electrode 22, and is disposed between the acoustic mirror layer 21, the first electrode 22, and the second electrode 24. Note that when the first electrode 22 is provided on the entire upper surface 21 a of the acoustic mirror layer 21, the piezoelectric layer 23 may be provided only on the upper surface 22 a of the first electrode 22.

[0040] The piezoelectric layer 23 is preferably provided so as to be in contact with the upper surface 22 a and the side surface 22 b of the first electrode 22. When the piezoelectric layer 23 is in contact with the upper surface 22 a and the side surface 22 b of the first electrode 22, even if Mg in the piezoelectric layer 23 is extracted from the lower surface 23 b of the piezoelectric layer 23, Mg can be supplied into the piezoelectric layer 23 from the upper surface 22 a and the side surface 22 b of the first electrode 22 through the lower surface 23 b of the piezoelectric layer 23.

[0041] The piezoelectric layer 23 preferably contains a piezoelectric material doped with Mg as an additive element at a predetermined ratio (hereinafter also referred to as an Mg-containing piezoelectric material), and may be composed of the Mg-containing piezoelectric material as a main component. The term "main component" means that the content of the Mg-containing piezoelectric material is 95 at% or more, preferably 98 at% or more, and more preferably 99 at% or more.

[0042] As the piezoelectric material, a piezoelectric material having a perovskite crystal structure (perovskite crystal material) or a piezoelectric material having a wurtzite crystal structure (wurtzite crystal material) can be used.

[0043] The wurtzite crystal structure has the general formula AB, where A is an electropositive element and B is an electronegative element. Wurtzite crystal materials have a hexagonal unit cell with a polarization vector parallel to the c-axis.

[0044] Wurtzite crystal materials contain Zn, Al, Ga, Cd, etc. as the electropositive element A represented by the general formula AB. Examples of wurtzite crystal materials that can be used include zinc oxide (ZnO), aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), indium phosphide (InP), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), cadmium sulfide (CdS), cadmium selenide (CdSe), and cadmium telluride (CdTe). Among these, ZnO is preferred as the wurtzite crystal material because it is relatively easy to achieve good c-axis orientation even in low-temperature processes. These materials may be used alone or in combination. When two or more wurtzite crystal materials are used in combination, one or more of these components may be included as the main component, with other components included as optional components. The respective materials may be laminated or formed as a single layer.

[0045] The wurtzite crystal material preferably contains ZnO, more preferably consists essentially of ZnO, and even more preferably consists solely of ZnO. Here, "substantially" means that the wurtzite crystal material may contain, in addition to ZnO, inevitable impurities that may be inevitably contained during the manufacturing process.

[0046] Mg, which is doped into the piezoelectric material at a predetermined ratio, is a metal element that does not exhibit conductivity when added, and does not exhibit conductivity even when it invades the site of an electropositive element A such as Zn, thereby improving the value of the electromechanical coupling coefficient k. Mg may be contained in the elemental state or in the oxide state. As an Mg-containing piezoelectric material, a wurtzite crystal material doped with Mg is preferred. In particular, the square value of the electromechanical coupling coefficient k of the piezoelectric material (k ２ It is preferable to use Mg-doped ZnO (MgZnO), which is ZnO doped with Mg, as the Mg-containing piezoelectric material, because this allows the Q value, which is an index of the steepness of the resonance characteristics, to be maintained while controlling the Mg-doped ZnO (MgZnO), thereby exhibiting excellent resonance characteristics.

[0047] The k of the Mg-containing piezoelectric material contained in the piezoelectric layer 23 2 The k value indicates the energy conversion efficiency of the Mg-containing piezoelectric material. The higher the energy conversion efficiency of the electric energy, the better the operating efficiency of the BAW resonator 20A including the piezoelectric layer 23, and the BAW resonator 20A has excellent piezoelectric characteristics. For the same material and composition, the smaller the disorder in the crystal orientation of the Mg-containing piezoelectric material contained in the piezoelectric layer 23, the higher the k value of the Mg-containing piezoelectric material. 2 The value increases and then gradually becomes constant. That is, as the disorder in the crystal orientation of the Mg-containing piezoelectric material decreases, the energy conversion efficiency of the Mg-containing piezoelectric material increases and then gradually becomes constant, and the piezoelectricity becomes constant. 2 The larger the value, the higher the energy conversion efficiency of the Mg-containing piezoelectric material, which means that the piezoelectric properties are improved.

[0048] In addition, k ２The k value can be obtained, for example, by using a network analyzer to apply an AC voltage to a sample of the piezoelectric device and measure the conversion loss of the piezoelectric layer 23. Specifically, the tip of a probe connected to the terminal of the network analyzer is pressed against the second electrode 24 on the upper surface of the sample, and an AC voltage is applied, and the conversion loss is measured by the network analyzer based on longitudinal acoustic waves (ultrasonic waves) generated inside the piezoelectric layer 23. By comparing the measured conversion loss with a theoretical curve based on Mason's equivalent circuit model, the k value of the vibration in the thickness direction of the piezoelectric layer 23 can be obtained. ２ Measure the value.

[0049] The conversion loss is expressed as the ratio (dB) of the power of the output frequency to the power of the input frequency. The electromechanical coupling coefficient k is expressed as the square root of the ratio of the mechanical energy to the supplied electrical energy, so there is a correlation between the electromechanical coupling coefficient k and the conversion loss.

[0050] The smaller the conversion loss, and the greater the difference between the resonant frequency and the half-resonant frequency, the larger the electromechanical coupling coefficient k in the thickness direction. If the resonant frequency is fr and the half-resonant frequency is fa, then k is the square of the electromechanical coupling coefficient k. 2 The value is expressed, for example, by the following formula (1): ２ Value = (π / 2) × (fr / fa) cot[(π / 2) × (fr / fa)] ... (1)

[0051] The Q value is a value that represents the sharpness (sharpness) of frequency characteristics. The larger the Q value, the sharper the frequency characteristics.

[0052] The piezoelectric material may be doped with other metal elements as additive elements in addition to Mg. Examples of other metal elements that may be doped into the piezoelectric material include alkaline earth metals such as Ca and Sr, as well as Sc, Ti, Zr, Si, Sr, and Li. These other metal elements may be contained in the elemental state or in the form of an oxide.

[0053] The content of additive elements, including Mg and other metal elements, in the piezoelectric layer 23 is not particularly limited and may be adjusted as appropriate. The method for measuring the content of additive elements in the piezoelectric layer 23 is not particularly limited as long as it is a measurable method. The content of additive elements in the piezoelectric layer 23 may be measured, for example, by Rutherford backscattering spectroscopy (RBS) using a Pelletron 3SDH (manufactured by NEC Corporation) as a measuring device, or by secondary ion mass spectrometry using dynamic SIMS (D-SIMS) or the like. The content of additive elements in the piezoelectric layer 23 may also be measured using an analytical device commonly used for composition analysis, such as an electron microscope for chemical analysis (ESCA).

[0054] The thickness of the piezoelectric layer 23 is not particularly limited, and may be any thickness that provides sufficient piezoelectric characteristics, i.e., polarization characteristics proportional to pressure, and that can stably exhibit piezoelectric characteristics by reducing the occurrence of cracks in the piezoelectric layer 23. The thickness of the piezoelectric layer 23 may be, for example, 50 nm to 5 μm. If the thickness of the piezoelectric layer 23 is 50 nm to 5 μm, the occurrence of cracks is suppressed and sufficient resonance characteristics can be exhibited.

[0055] The crystal orientation of the piezoelectric layer 23 is preferably 5° or less. If the crystal orientation is 5° or less, the crystal orientation in the c-axis direction (c-axis orientation) of the piezoelectric material contained in the piezoelectric layer 23 is good, and the energy conversion efficiency is improved, thereby improving the resonance characteristics in the thickness direction of the piezoelectric layer 23. If the piezoelectric layer 23 contains ZnO as the piezoelectric material, ZnO has a wurtzite crystal structure, and there is a higher correlation between the crystal orientation and the resonance characteristics than with piezoelectric materials having other crystal structures. If the crystal orientation of ZnO is 5° or less, it is easier to increase the energy conversion efficiency, thereby improving the resonance characteristics of the BAW resonator 20A.

[0056] The crystalline orientation of the piezoelectric layer 23 can be evaluated by the full width at half maximum (FWHM) obtained when the surface of the piezoelectric layer 23 is measured by the X-ray rocking curve (XRC) method. That is, the crystalline orientation of the piezoelectric layer 23 is represented by the FWHM of the peak waveform of the rocking curve obtained when the diffraction from the (0002) plane of the crystal of the piezoelectric material contained as a main component in the piezoelectric layer 23 is measured by the XRC method. When the piezoelectric material contained in the piezoelectric layer 23 has a wurtzite crystal structure such as ZnO, the FWHM indicates the degree of parallelism of the c-axis direction of the crystals constituting the piezoelectric material. Therefore, the FWHM of the peak waveform of the rocking curve obtained by the XRC method can be used as an indicator of the c-axis orientation of the piezoelectric layer 23. Therefore, the smaller the FWHM of the rocking curve, the better the crystalline orientation of the piezoelectric layer 23 in the c-axis direction can be evaluated.

[0057] The crystal orientation of the piezoelectric layer 23 may be evaluated using the XRC method to measure diffraction from a specific crystal plane of the piezoelectric material (e.g., the (0002) plane of a ZnO crystal) in the piezoelectric layer 23, and may also be evaluated using the peak intensity as well as the FWHM of the rocking curve. That is, the crystal orientation of the piezoelectric layer 23 may be evaluated using the value obtained by dividing the integrated value of the peak intensity by the FWHM as an evaluation value. For example, the larger the evaluation value obtained by dividing the integrated value of the peak intensity by the FWHM, the better the crystal orientation of the piezoelectric layer 23 can be evaluated to be.

[0058] When two or more types of piezoelectric materials are used in combination, the piezoelectric layer 23 may be formed by laminating piezoelectric layers made of the respective piezoelectric materials.

[0059] 2, the second electrode 24 is provided on the upper principal surface (top surface) 23a of the piezoelectric layer 23, and is arranged to face the first electrode 22 and the acoustic mirror layer 21. Similar to the first electrode 22, the second electrode 24 may be formed as a thin film on a part of or the entire top surface 23a of the piezoelectric layer 23. Furthermore, the second electrode 24 may be formed in any appropriate shape when viewed in plan of the BAW resonator 20A.

[0060] The second electrode 24 is preferably provided so as to be in contact with the upper surface 23 a of the piezoelectric layer 23. When the second electrode 24 is in contact with the upper surface 23 a of the piezoelectric layer 23, even if Mg in the piezoelectric layer 23 is removed from the upper surface 23 a of the piezoelectric layer 23, Mg can be supplied from the second electrode 24 into the piezoelectric layer 23 from the upper surface 23 a of the piezoelectric layer 23.

[0061] The second electrode 24 can be made of the same material as the first electrode 22. That is, the second electrode 24 has conductivity and can be made of an alloy containing Mg. By making the second electrode 24 of an alloy containing Mg, even if Mg contained in the piezoelectric layer 23 escapes from the upper surface 23 a of the piezoelectric layer 23, the amount of Mg that has escaped from the upper surface 23 a of the piezoelectric layer 23 into the piezoelectric layer 23 can be supplied.

[0062] As the alloy containing Mg, for example, an alloy containing at least one component selected from the group consisting of Al, Zn, Y, Gd, and Cu, or a compound containing the above component, doped with Mg, can be used.

[0063] The Mg content of the second electrode 24 may be in the same range as the Mg content of the first electrode 22. The Mg content of the second electrode 24 can be measured using the same method as the Mg content of the first electrode 22.

[0064] When the first electrode 22 is formed of an alloy containing Mg, the second electrode 24 may be formed of any conductive material, similar to the first electrode 22. As the conductive material may be the same material as the first electrode 22, a detailed description thereof will be omitted.

[0065] Like the first electrode 22, the second electrode 24 may be formed of a single layer of an alloy containing Mg or any conductive material other than an alloy containing Mg.

[0066] The thickness of the second electrode 24 can be appropriately designed, and is preferably, for example, 30 nm to 300 nm. If the thickness of the second electrode 24 is within the above-mentioned preferred range, the function of the electrode can be exhibited, and the BAW resonator 20A can be made thinner while suppressing the BAW resonator 20A from shifting to the lower frequency side.

[0067] The BAW device 1A may have a protective layer on the surface of the BAW resonator 20A to protect the BAW resonator 20A. The material for forming the protective layer is not particularly limited, and may be Al. ２ O ３ , SiO ２ , SiON, Si ３ N ４ The protective layer may be formed by a commonly used method such as vapor deposition, coating, or sputtering.

[0068] [Manufacturing Method of BAW Device] The manufacturing method of the BAW device 1A is not particularly limited, and any appropriate manufacturing method can be used. An example of the manufacturing method of the BAW device 1A will be described.

[0069] First, a cycle of forming a high acoustic impedance layer 211 and a low acoustic impedance layer 212 is repeated multiple times (three times in FIG. 2 ) on the upper surface 10 a of a support substrate 10 formed to a predetermined size. In this way, an acoustic mirror layer 21 is formed in which the high acoustic impedance layer 211 and the low acoustic impedance layer 212 are stacked alternately in multiple pairs.

[0070] The method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212 is not particularly limited, and may be either a dry process or a wet process. If a dry process is used as the method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212, thin high acoustic impedance layer 211 and low acoustic impedance layer 212 can be easily formed. Furthermore, the high acoustic impedance layer 211 and the low acoustic impedance layer 212 may be formed using the same method or different methods.

[0071] Examples of dry processes include sputtering and vapor deposition, and examples of wet processes include plating.

[0072] As the sputtering, for example, a sputtering method such as DC (direct current) or RF (radio frequency) magnetron sputtering can be used.

[0073] Sputtering is a preferred method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212. By using sputtering, it is possible to easily form the thin high acoustic impedance layer 211 and the low acoustic impedance layer 212 that have a high density or bulk modulus.

[0074] The high acoustic impedance layer 211 may be made of, for example, W, Mo, or Ta. ２ O ５ A thin film formed of a material with a high density or bulk modulus, such as ZnO, can be used.

[0075] The low acoustic impedance layer 212 is, for example, SiO ２ The oxides may be used.

[0076] Next, the first electrode 22 is formed on the upper surface 21a of the acoustic mirror layer 21. The method for forming the first electrode 22 is not particularly limited, and either a dry process or a wet process may be used, similar to the method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212. Details of the dry process and the wet process are similar to the method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212, and therefore will not be described here. By using a sputtering method to form the first electrode 22, a uniform film with strong adhesion can be formed. Furthermore, the first electrode 22 can be formed with a desired thickness with high precision simply by controlling the time.

[0077] The first electrode 22 may be formed on a part of or the entire upper surface 21 a of the acoustic mirror layer 21. The first electrode 22 may also be formed into any appropriate shape by processing it into a pattern having a predetermined shape by etching or the like.

[0078] Next, a piezoelectric layer 23 is formed on the upper surface 22 a of the first electrode 22 and the upper surface 21 a of the acoustic mirror layer 21. For example, using a target containing elements constituting the Mg-containing piezoelectric material, the Mg-containing piezoelectric material may be deposited by DC or RF magnetron sputtering in a mixed gas atmosphere containing an inert gas such as Ar and a trace amount of oxygen. The piezoelectric layer 23 is deposited by sputtering the Mg-containing piezoelectric material on the first electrode 22 and the acoustic mirror layer 21. Note that the Mg-containing piezoelectric material may be deposited by placing a mask or the like on the first electrode 22 and the acoustic mirror layer 21 so that the piezoelectric layer 23 is not formed in any area other than predetermined regions on the first electrode 22 and the acoustic mirror layer 21.

[0079] The laminate including the support substrate 10, the acoustic mirror layer 21, and the first electrode 22 may be placed on a deposition plate that serves as an anode in a deposition chamber of a sputtering apparatus. The deposition plate may be rotatable, for example. By placing the laminate including the support substrate 10, the acoustic mirror layer 21, and the first electrode 22 on the deposition plate, the piezoelectric layer 23 can be deposited on the first electrode 22 in a batch process.

[0080] A target containing elements constituting the Mg-containing piezoelectric material is used as a cathode and is disposed opposite, with a gap therebetween, a film formation plate that serves as the anode of the sputtering device.

[0081] The target containing the elements constituting the Mg-containing piezoelectric material may be a target containing Mg and a piezoelectric material, or a target containing the Mg-containing piezoelectric material. The target containing the piezoelectric material may be a single or multiple targets containing the materials constituting the piezoelectric material. The target containing the Mg-containing piezoelectric material may be a single or multiple targets containing the Mg-containing piezoelectric material.

[0082] When a plurality of targets containing a piezoelectric material or a target containing an Mg-containing piezoelectric material are used as cathodes, a multi-target sputtering method is used, and when a single target is used as the cathode, a single-target sputtering method is used. When a multi-target sputtering method is used, a multi-target sputtering device may be used as the sputtering device.

[0083] A target used in a multi-target sputtering method or a single-target sputtering method is placed in a sputtering apparatus, and a mixed gas containing an inert gas such as Ar and oxygen is supplied into the sputtering apparatus. By sputtering using the placed target in an atmosphere of the mixed gas containing the inert gas and oxygen, a piezoelectric layer 23 containing an Mg-containing piezoelectric material can be formed on the first electrode 22 and the acoustic mirror layer 21.

[0084] When multiple targets are used as the cathode, a Mg target and a target of the piezoelectric material contained in the piezoelectric layer 23, or a target for each material constituting the piezoelectric material contained in the piezoelectric layer 23, are used. Each target may be a target of the above-mentioned Mg or a metal oxide containing the piezoelectric material and oxygen. The multiple targets may be placed in the film formation chamber at intervals. During sputtering, the power applied to each target is adjusted depending on the type of Mg and piezoelectric material contained in the piezoelectric layer 23, thereby adjusting the atomic ratio between the materials constituting the piezoelectric layer 23.

[0085] When a single target is used as the cathode, an alloy target containing a piezoelectric material doped with Mg at a predetermined ratio is used. The single target may be an alloy target in which the atomic ratio between Mg and the piezoelectric material contained in the piezoelectric layer 23 is adjusted. The alloy target may also be a metal oxide target containing Mg, a piezoelectric material, and oxygen.

[0086] The targets may be a target of sintered MgO and a target of sintered piezoelectric material. By placing the target of sintered MgO and the target of sintered piezoelectric material in a sputtering apparatus and performing sputtering in a mixed gas atmosphere containing an inert gas such as Ar and oxygen, it is possible to obtain the piezoelectric layer 23 containing the Mg-containing piezoelectric material on the first electrode 22 and the acoustic mirror layer 21 while suppressing the amount of inert gas entering during the deposition of MgZnO.

[0087] For example, when the Mg-containing piezoelectric material is MgZnO containing ZnO and MgO in a predetermined mass ratio, the target may be a ZnO sintered body target or an MgO sintered body target. Alternatively, the target may be an alloy target containing ZnO and MgO, such as an alloy target of a ZnO sintered body to which MgO has been added in advance at a predetermined ratio.

[0088] The gas atmosphere used during sputtering is not limited to a mixed gas atmosphere containing an inert gas and oxygen, but may be an inert gas atmosphere.

[0089] The pressure in the gas atmosphere during sputtering may be appropriately determined depending on the type of piezoelectric material, the sputtering method, etc., and may be set to, for example, 0.1 Pa to 2.0 Pa.

[0090] The deposition temperature for the piezoelectric layer 23 is not particularly limited and may be appropriately selected depending on the layer structure of the BAW resonator 20A, and may be, for example, 150° C. or lower.

[0091] By using a sputtering method to form the piezoelectric layer 23, it is possible to form a uniform film with strong adhesion while maintaining the composition ratio of the compound target. Furthermore, by simply controlling the time, it is possible to form the piezoelectric layer 23 with a desired thickness with high precision.

[0092] The piezoelectric layer 23 may be formed by laminating a plurality of thin films made of an Mg-containing piezoelectric material.

[0093] Next, the end faces of the piezoelectric layer 23 may be processed to form side faces.

[0094] The processing method is Cl ２ , C.F. ４ or CHF ３ Dry etching using reactive gases such as HCl or HNO ３ A common method such as wet etching using an acid solution such as

[0095] Next, a second electrode 24 having a predetermined shape is formed on the upper surface 23a of the piezoelectric layer 23. The second electrode 24 can be formed using the same method as that used for forming the first electrode 22.

[0096] The thickness of the second electrode 24 can be designed appropriately and may be, for example, 30 nm to 300 nm.

[0097] The second electrode 24 is formed on the entire surface or a part of the upper surface 23a of the piezoelectric layer 23, and may be formed in any appropriate shape.

[0098] The BAW resonator 20A is obtained by forming a second electrode 24 having a predetermined shape on the upper surface 23a of the piezoelectric layer 23. This results in a BAW device 1A in which the BAW resonator 20A is stacked on the support substrate 10.

[0099] The manufacturing method of the BAW device 1A is not limited to the above method, and the BAW device 1A may also be manufactured by forming a BAW resonator 20A on a substrate, and then placing the BAW resonator 20A formed on the substrate on the upper surface of the support base 10.

[0100] As described above, the BAW resonator 20A includes an acoustic mirror layer 21, a first electrode 22, a piezoelectric layer 23, and a second electrode 24 on a support substrate 10. The piezoelectric layer 23 has a piezoelectric material doped with Mg, and the first electrode 22 and the second electrode 24 contain Mg and are provided so as to sandwich the piezoelectric layer 23 on both the upper and lower surfaces thereof. As a result, even if Mg contained in the piezoelectric layer 23 sublimes and is lost from the piezoelectric layer 23, the first electrode 22 and the second electrode 24 can supply the lost Mg into the piezoelectric layer 23 from the upper surface 23a and the lower surface 23b of the piezoelectric layer 23. Therefore, even if Mg contained in the piezoelectric layer 23 is lost, it can be replenished, and therefore, a decrease in the Mg concentration in the piezoelectric layer 23 due to diffusion of Mg in the piezoelectric layer 23 to the first electrode 22 and the second electrode 24 side can be suppressed, and deterioration of the piezoelectric layer 23 can be suppressed. Therefore, the BAW resonator 20A can suppress the degradation of the piezoelectric characteristics of the piezoelectric layer 23, and can maintain the piezoelectric characteristics for a long period of time.

[0101] The piezoelectric characteristics of the BAW resonator 20A are determined by the piezoelectric constant d ３３ The piezoelectric constant d (unit: pC / N) can be measured and evaluated. ３３ is a value that represents the expansion / contraction mode in the polarization direction, and is expressed as the amount of polarization charge per unit pressure applied in the polarization direction. ３３represents the expansion / contraction mode in the film thickness direction of the BAW resonator 20A, that is, in the c-axis direction.

[0102] Piezoelectric constant d ３３ The piezoelectric constant d is evaluated by the following procedure. The BAW resonator 20A is placed on a stage with the first electrode 22 facing downward, and a predetermined pressure is applied from the upper surface of the BAW resonator 20A with an indenter, and the charge generated by polarization in the c-axis (film thickness) direction is measured. The amount of charge generated when the applied load is changed from 5N to 6N is divided by the load difference of 1N, and the value is calculated as the piezoelectric constant d ３３ value.

[0103] In the BAW resonator 20A, the first electrode 22 and the second electrode 24 are preferably disposed to sandwich the piezoelectric layer 23, with the top surface 22a and the side surface 22b of the first electrode 22 in contact with the bottom surface 23b of the piezoelectric layer 23 and the second electrode 24 in contact with the top surface 23a of the piezoelectric layer 23. This prevents Mg from being released from the top surface 23a and the bottom surface 23b of the piezoelectric layer 23, and prevents Mg in the piezoelectric layer 23 from diffusing toward the first electrode 22 and the second electrode 24, thereby more reliably maintaining the effect of suppressing deterioration of the piezoelectric layer 23.

[0104] In the BAW resonator 20A, the first electrode 22 and the second electrode 24 are preferably made of at least one component selected from the group consisting of Al, Zn, Y, Gd, and Cu, or a compound containing the component doped with Mg. This allows the first electrode 22 and the second electrode 24 to more reliably supply Mg to areas where Mg has been lost from the piezoelectric layer 23, thereby more reliably preventing defects from occurring on the surface of the piezoelectric layer 23. This allows the BAW resonator 20A to better maintain its piezoelectric characteristics.

[0105] In the BAW resonator 20A, the Mg content of either the first electrode 22 or the second electrode 24 is preferably 0.01 at % to 5 at %. This allows the first electrode 22 and the second electrode 24 to reliably supply Mg to the portions of the piezoelectric layer 23 where Mg has been lost, thereby preventing defects from occurring on the surface of the piezoelectric layer 23 and more reliably preventing deterioration of the piezoelectric layer 23. This allows the BAW resonator 20A to more reliably maintain its piezoelectric characteristics for a long period of time.

[0106] The BAW resonator 20A can include MgZnO as a piezoelectric material in the piezoelectric layer 23. Generally, the k of a piezoelectric layer formed by doping a piezoelectric material with another metal element is ２ There is a trade-off between the k value and the Q value. For example, if a piezoelectric element is used in a high-frequency filter that extracts only signals in a high-frequency range such as the 5G band and removes signals in other frequency bands, the k value required in the high-frequency range will be ２ When the piezoelectric layer 23 contains MgZnO as a piezoelectric material, the k value of MgZnO increases with increasing Mg concentration. ２ There is no trade-off between the k and Q values, and even in the high frequency range, ２ The Q value can be maintained while controlling the Q value. Therefore, by including MgZnO as the piezoelectric material in the piezoelectric layer 23, the BAW resonator 20A can stably exhibit piezoelectric characteristics even in the high frequency range of applications such as high frequency filters. However, MgZnO is a composite oxide, and Mg is prone to sublimation during use, which can easily cause characteristic fluctuations. In this embodiment, as described above, even when the BAW resonator 20A includes MgZnO as the piezoelectric material in the piezoelectric layer 23, the first electrode 22 and the second electrode 24 can compensate for Mg atoms lost from the piezoelectric layer 23, so that the BAW resonator 20A can stably exhibit piezoelectric characteristics even in the high frequency range of applications such as high frequency filters.

[0107] The BAW device 1A includes the BAW resonator 20A, and thus can exhibit excellent piezoelectric characteristics over a long period of time in the high frequency band.

[0108] The BAW device 1A can have excellent piezoelectric characteristics for a long period of time, and can therefore be used in electronic devices for a variety of purposes as an electronic component that utilizes the piezoelectric effect.

[0109] As described above, the BAW resonator 20A can suppress the degradation of the piezoelectric characteristics of the piezoelectric layer 23, and therefore can be used in various electronic devices as an electronic component utilizing the piezoelectric effect. The BAW resonator 20A can also be used in applications requiring high filter characteristics, particularly in the high frequency range, and therefore can be suitably used in, for example, high frequency filters such as SAW filters using surface acoustic waves (SAW) and BAW filters using bulk acoustic waves (BAW), and timing devices such as MEMS resonators. In particular, the BAW resonator 20A can suppress the degradation of the piezoelectric characteristics of the piezoelectric layer 23 and can be used in high frequency bands, and therefore can be effectively used as a BAW filter.

[0110] (Modification) In this embodiment, the BAW device 1A is not limited to the above configuration, and may have other configurations. An example of another configuration of the BAW device 1A is shown below.

[0111] In this embodiment, the BAW resonator 20A has an acoustic mirror layer 21 formed of an acoustic multilayer film, but the acoustic mirror layer 21 may also be formed of a space. For example, as shown in FIG. 3 , in the BAW device 1B, the BAW resonator 20B may have a recess 11 formed on the upper surface 10 a of the support substrate 10, and the space S formed between the recess 11 and the first electrode 22 of the support substrate 10 may function as an acoustic mirror layer. In the BAW resonator 20B, since the space S can function as an acoustic mirror layer, the first electrode 22 can be provided directly on the upper surface 10 a of the support substrate 10. This allows the overall thickness of the BAW resonator 20B to be reduced, thereby enabling miniaturization.

[0112] 4, in the BAW device 1C, the BAW resonator 20C may have a recess 221 formed in the lower surface 22c of the first electrode 22, and the space S formed between the support substrate 10 and the recess 221 of the first electrode 22 may function as an acoustic mirror layer. Since the space S can function as an acoustic mirror layer in the BAW resonator 20C, the first electrode 22 can be provided directly on the upper surface 10a of the support substrate 10. This allows the overall thickness of the BAW resonator 20C to be reduced, thereby enabling miniaturization.

[0113] Although the embodiments have been described above, they are presented as examples and the present invention is not limited to the above embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, or modifications can be made without departing from the spirit of the invention. The above embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as set forth in the claims.

[0114] The present embodiment disclosed above can be specified, for example, by the following aspects. [1] A BAW resonator comprising a support substrate and a first electrode, a piezoelectric layer having a piezoelectric material doped with Mg, and a second electrode stacked in this order, wherein at least one of the first electrode and the second electrode is an alloy containing Mg. [2] The BAW resonator according to [1], wherein the first electrode and the second electrode are made of at least one component selected from the group consisting of Al, Zn, Y, Gd, and Cu, doped with Mg. [3] The BAW resonator according to [1] or [2], wherein the Mg content of either the first electrode or the second electrode is 0.01 at% to 5 at%. [4] The BAW resonator according to any one of [1] to [3], wherein the piezoelectric material contains ZnO. [5] A BAW resonator according to any one of [1] to [4], having an acoustic mirror layer between a support substrate on which the first electrode is provided and the first electrode, the acoustic mirror layer being a laminate formed of one or more pairs of alternating high acoustic impedance layers and low acoustic impedance layers. [6] A BAW device comprising: a support substrate; and the BAW resonator according to any one of [1] to [5] provided on the support substrate. [7] The BAW device according to [6], having an acoustic mirror layer between a support substrate on which the first electrode is provided and the first electrode, the acoustic mirror layer being a gap formed between the support substrate and the first electrode. [8] Electronic equipment comprising the BAW device according to [6] or [7].

[0115] This application claims priority based on Japanese Patent Application No. 2024-147297, filed with the Japan Patent Office on August 29, 2024, and incorporates the entire contents of said application by reference.

[0116] 1A, 1B, 1C BAW device 10 Support substrate 11, 221 Recess (hollow portion) 20A, 20B, 20C BAW resonator 21 Acoustic mirror layer 22 First electrode 23 Piezoelectric layer 24 Second electrode 10a, 21a, 22a, 23a Main surface (upper surface) 211 High acoustic impedance layer 212 Low acoustic impedance layer S Space A1 Active region A2 Non-active region

Claims

a first electrode, a piezoelectric layer including a piezoelectric material doped with Mg, and a second electrode laminated in this order on a support substrate; At least one of the first electrode and the second electrode is an alloy containing Mg.

2. The BAW resonator according to claim 1, wherein the first electrode and the second electrode are made of at least one element selected from the group consisting of Al, Zn, Y, Gd, and Cu, doped with Mg.

3. The BAW resonator according to claim 1, wherein the Mg content of either the first electrode or the second electrode is 0.01 at % to 5 at %.

3. The BAW resonator of claim 1, wherein the piezoelectric material comprises ZnO.

3. The BAW resonator according to claim 1, further comprising an acoustic mirror layer between the first electrode and a support substrate on which the first electrode is disposed, the acoustic mirror layer being a laminate in which one or more pairs of high acoustic impedance layers and low acoustic impedance layers are alternately stacked. 　 A supporting substrate; The BAW resonator according to claim 1 or 2, which is provided on a support substrate; A BAW device comprising: 　 an acoustic mirror layer is provided between a support substrate on which the first electrode is provided and the first electrode; The BAW device of claim 6 , wherein the acoustic mirror layer is an air gap formed between the support substrate and the first electrode. 　 An electronic device comprising the BAW device according to claim 6.

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