BAW resonator, BAW device, and electronic apparatus

Abstract

A bulk acoustic wave (BAW) resonator according to the present invention comprises: a first electrode, a piezoelectric layer, a second electrode, and a protective layer that are sequentially laminated on a support substrate; and an active region that causes the piezoelectric layer to resonate and a non-active region other than the active region. The protective layer has a first protective layer disposed at least in a position corresponding to the active region, and a second protective layer disposed in a position corresponding to at least a portion of the non-active region. The first protective layer has a positive frequency temperature coefficient, and the thermal conductivity of the second protective layer is higher than the thermal conductivity of the first protective layer.

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] A bulk acoustic wave (BAW) resonator has a piezoelectric layer made of a piezoelectric material between two electrode layers, and by utilizing the piezoelectric effect of the piezoelectric layer, it is used in electronic devices as electronic components such as pressure sensors, acceleration sensors, sensors such as AE (acoustic emission) sensors that detect elastic waves, radio frequency (RF) filters, and piezoelectric actuators.

[0003] As a BAW resonator, for example, an electronic device has been disclosed that has a passivation layer that covers a capacitor formed on a substrate and that includes a first electrode, a capacitor dielectric layer, and a second electrode, and the passivation layer is formed from an insulating material such as silicon oxide, silicon nitride, or aluminum oxide (see, for example, Patent Document 1).

[0004] Chinese Patent Application Publication No. 110649157

[0005] In conventional BAW resonators such as the electronic device of Patent Document 1, the protective layer provided on the surface thereof is generally formed of a uniform insulating material, and since piezoelectric materials have negative temperature coefficients of frequency (TCF), the protective layer is formed of an insulating material with a positive TCF, such as silicon oxide.

[0006] However, since insulating materials with a positive TCF have low thermal conductivity, protective layers formed using insulating materials with a positive TCF cannot exhibit sufficient heat dissipation, which has led to the problem that it is difficult for the protective layers used in conventional BAW resonators to achieve both a good TCF and high heat dissipation.

[0007] An object of one embodiment of the present invention is to provide a BAW resonator that exhibits a good TCF and has high heat dissipation properties.

[0008] One aspect of the present invention is a BAW resonator comprising: a first electrode, a piezoelectric layer, a second electrode, and a protective layer stacked in this order on a support substrate; an active region that resonates the piezoelectric layer; and an inactive region other than the active region, wherein the protective layer has a first protective layer disposed at a position corresponding to at least the active region, and a second protective layer disposed at a position corresponding to at least a portion of the inactive region, the first protective layer having a positive temperature coefficient of frequency, and the second protective layer having a higher thermal conductivity than the first protective layer.

[0009] One aspect of the BAW resonator according to the present invention can exhibit a good TCF and have high heat dissipation properties.

[0010] 1 is a schematic cross-sectional view showing a configuration of a BAW device including a BAW resonator according to an embodiment of the present invention, as viewed in the II direction of FIG. 1. FIG. 2 is a schematic cross-sectional view showing another example of the configuration of a BAW device, as viewed in the II direction of FIG. 2. FIG. 3 is a schematic cross-sectional view showing another example of the configuration of a BAW device, as viewed in the II direction of FIG.

[0011] 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 descriptions 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.

[0012] 1 is a schematic cross-sectional view showing the configuration of a BAW device including a BAW resonator according to this embodiment. As shown in FIG. 1, a BAW device 1A includes a support substrate 10 and a BAW resonator 20A provided on the support substrate 10.

[0013] 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 side 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.

[0014] [Support Base] As shown in FIG. 1, 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.

[0015] 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.

[0016] The metal plate may be made of a material such as aluminum, copper, stainless steel, or tantalum.

[0017] Examples of materials that may be used to form the inorganic dielectric substrate include MgO and sapphire.

[0018] 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. A thickness of 20 μm to 725 μm for the support substrate 10 allows for stable support of the BAW resonator 20A. Furthermore, warping of the support substrate 10 is suppressed, and the effect of warping on the piezoelectric characteristics of the support substrate 10 can be reduced, allowing the BAW resonator 20A to have desired piezoelectric characteristics. The piezoelectric characteristics include both the amount of voltage generated per applied stress of the BAW resonator 20A (positive piezoelectric effect) and the mechanical displacement rate per applied electric field (inverse 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] 1 , the BAW resonator 20A includes an acoustic mirror layer 21, a first electrode 22, a piezoelectric layer 23, a second electrode 24, and a protective layer 25. The BAW resonator 20A includes the acoustic mirror layer 21, the first electrode 22, the piezoelectric layer 23, the second electrode 24, and the protective layer 25, stacked in this order from the support substrate 10 side. The BAW resonator 20A includes a portion of the first electrode 22, a portion of an end of the piezoelectric layer 23, and the second electrode 24, which are covered with the protective layer 25.

[0021] Fig. 2 is a plan view of the BAW resonator as viewed in the II direction of Fig. 1. As shown in Fig. 2, the BAW resonator 20A has an active region A1 and an inactive region A2 other than the active region A1. In Fig. 2, the active region A1 and the inactive region A2 are indicated by hatching.

[0022] The active region A1 is a region that causes the piezoelectric layer 23 to resonate, 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 is sandwiched between the first electrode 22 and the second electrode 24 in the piezoelectric layer 23. The active region A1 is substantially rectangular in a planar view, but may be substantially circular, substantially polygonal, substantially elliptical, or the like.

[0023] In the BAW resonator 20A, in a plan view, the protective layer 25 arranged in the region corresponding to the active region A1 is configured to have a positive TCF, and the protective layer 25 arranged in the region corresponding to the inactive region A2 is configured to have heat dissipation properties, thereby allowing the protective layer 25 to exhibit a good TCF and high heat dissipation properties.

[0024] The TCF represents the amount of frequency variation with respect to temperature changes, and the smaller the TCF, the more stable the frequency is with respect to temperature changes.

[0025] 1, 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 may be composed of acoustic multilayer films with different specific acoustic impedances. The acoustic mirror layer 21 is a multilayer film in which two or more pairs of high acoustic impedance layers having a predetermined specific acoustic impedance and low acoustic impedance layers having a lower specific acoustic impedance than the high acoustic impedance layers are alternately stacked.

[0026] When resonant vibrations are transmitted to the acoustic mirror layer 21, the vibration energy of the resonance is reflected by the acoustic mirror layer 21. The speed at which vibration waves (elastic waves) propagate through high acoustic impedance layers is different from the speed at which they propagate through low acoustic impedance layers. By designing the thickness of the acoustic mirror layer 21 so that the reflected waves reinforce each other due to interference at the interfaces of the layers that make up the layer 21, the vibration energy of the resonance is returned in the direction of incidence of the elastic waves without being affected by the support substrate 10, while the thermal energy is released in the direction of the support substrate 10.

[0027] The high acoustic impedance layer is made of W, Mo, or Ta. ２ O ５ and ZnO, or other materials with high density or bulk modulus.

[0028] The low acoustic impedance layer is formed of a material having a lower density or bulk modulus than the high acoustic impedance layer. Examples of materials having a lower density or bulk modulus than the high acoustic impedance layer include SiO ２ The low acoustic impedance layer may be an amorphous layer or a layer in which the amorphous phase is predominant. By using an amorphous layer as the low acoustic impedance layer, stress in the high acoustic impedance layer can be alleviated.

[0029] The high acoustic impedance layer and the low acoustic impedance layer may be formed on the support substrate 10 by sputtering or the like.

[0030] (First Electrode) As shown in FIG. 1 , the first electrode 22 is provided on the upper principal surface (top surface) 21 a 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 21 a of the acoustic mirror layer 21. When the first electrode 22 is formed as a thin film on a part of the top surface 21 a of the acoustic mirror layer 21, a plurality of first electrodes 22 may be provided in parallel in a stripe pattern. The first electrode 22 is provided so as to extend outward beyond the piezoelectric layer 23. Note that "extending the first electrode 22 outward beyond the piezoelectric layer 23" refers to a state in which the first electrode 22 is also disposed outside the piezoelectric layer 23 in a plan view seen from the stacking direction of the first electrode 22.

[0031] Any conductive material can be used for the first electrode 22. Examples of such a material include metals such as Pt, Au, Ag, Cu, Al, Ti, Cr, Zr, Nb, Mo, Rh, Pd, Ru, Ir, Ta, and W.

[0032] The thickness of the first electrode 22 can be appropriately designed and may be, for example, 40 nm to 300 nm. If the thickness of the first electrode 22 is 40 nm to 300 nm, the first electrode 22 can function as an electrode and the BAW resonator 20A can be made thinner.

[0033] 1, the piezoelectric layer 23 is provided on the upper principal surface (top surface) 22a of the first electrode 22. The piezoelectric layer 23 preferably contains an inorganic material as a main component. The term "main component" means that the content of the inorganic material is 95 atomic % or more, preferably 98 atomic % or more, and more preferably 99 atomic % or more.

[0034] As the inorganic material, a piezoelectric material having a perovskite crystal structure (perovskite crystal material), a piezoelectric material having a wurtzite crystal structure (wurtzite crystal material), or the like can be used.

[0035] The wurtzite crystal structure is represented by 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.

[0036] As the wurtzite crystal material, it is preferable to use a material that exhibits piezoelectric properties above a certain value and can be crystallized in a low-temperature process at 200°C or less. The wurtzite crystal material is represented by the general formula AB, and contains Zn, Al, Ga, Cd, Si, etc. as the electropositive element A. Examples of wurtzite crystal materials that can be used include zinc oxide (ZnO), aluminum nitride (AlN), and gallium nitride (GaN). Among these, ZnO is preferred as the wurtzite crystal material because it tends to achieve relatively 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 may be included as the main component, with other components included as optional components. Furthermore, each material may be stacked, or multiple targets may be used to form a single layer.

[0037] The wurtzite crystal material preferably contains ZnO, more preferably consists essentially of ZnO, and even more preferably consists only of ZnO. "Substantially" means that the wurtzite crystal material may contain, in addition to ZnO, inevitable impurities that may be inevitably contained during the manufacturing process.

[0038] In addition to the above-mentioned ZnO and AlN, inorganic materials such as wurtzite crystal materials may contain alkaline earth metals such as Mg, Ca, and Sr, or metals such as V, Ti, Zr, Si, Sr, and Li, in a predetermined range of proportions. These components may be contained in the elemental state or in the oxide state. Among these, Mg-doped ZnO (MgZnO), in which ZnO is doped with Mg, is preferred as the inorganic material, as it controls the K value, which is an index of the piezoelectric properties of the piezoelectric layer 23, while maintaining the Q value, which is an index of the steepness of the piezoelectric properties, and exhibits excellent piezoelectric properties.

[0039] The K value is the value of the electromechanical coupling constant k. The square value (k 2The k value) indicates the energy conversion efficiency of the electrical energy determined for the piezoelectric material. The higher the energy conversion efficiency of the electrical 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 piezoelectric material contained in the piezoelectric layer 23, the higher the k value of the piezoelectric material. 2 The value increases and then gradually becomes constant. That is, as the disorder of the crystal orientation of the piezoelectric material decreases, the energy conversion efficiency of the piezoelectric material increases and then gradually becomes constant, and the piezoelectricity becomes constant. Therefore, the larger the electromechanical coupling constant k, the 2 The larger the value of k, the higher the energy conversion efficiency of the piezoelectric material, which means that the piezoelectric properties are improved. Also, the larger the electromechanical coupling constant k, the smaller the disturbance in the crystal orientation, which means that the crystal orientation is improved.

[0040] The Q value is a value that represents the sharpness (sharpness) of frequency characteristics. The larger the Q value, the sharper the frequency characteristics.

[0041] The content of the additive element in the piezoelectric layer 23 is not particularly limited as long as it is within a range that allows the piezoelectric layer 23 to have a wurtzite crystal structure. The method for measuring the content of the additive element contained in the piezoelectric layer 23 is not particularly limited as long as it is a measurable method. The content of the additive element contained in the piezoelectric layer 23 may be measured by, for example, Rutherford backscattering spectroscopy (RBS) using a Pelletron 3SDH (manufactured by NEC Corporation) as a measuring device, or may be measured by secondary ion mass spectrometry using dynamic SIMS (D-SIMS) or the like.

[0042] 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 or the like 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 piezoelectric characteristics can be exhibited.

[0043] 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 piezoelectric 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 piezoelectric 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 piezoelectric characteristics of the BAW resonator 20A.

[0044] 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.

[0045] The crystal orientation of the piezoelectric layer 23 may be evaluated by measuring the diffraction from a specific crystal plane of the piezoelectric material (e.g., the (0002) plane of a ZnO crystal) of the piezoelectric layer 23 using an XRC method, 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.

[0046] When two or more inorganic materials are used in combination, the piezoelectric layer 23 may be formed by laminating piezoelectric layers made of the respective inorganic materials.

[0047] 1, the second electrode 24 is provided on the upper main surface (top surface) 23a of the piezoelectric layer 23, and is arranged so as to face the first electrode 22. The second electrode 24 can be formed of any conductive material, and the same material as the first electrode 22 can be used.

[0048] The second electrode 24 may be formed as a thin film on a part of or the entire upper surface 23a of the piezoelectric layer 23, similarly to the first electrode 22, or may be formed in any suitable shape. For example, when a plurality of first electrodes 22 are provided in parallel stripes, a plurality of second electrodes 24 may be provided in parallel stripes in a direction perpendicular to the direction in which the stripes of the first electrodes 22 extend in a plan view.

[0049] The thickness of the second electrode 24 can be appropriately designed, and is preferably, for example, 40 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.

[0050] 1 , the protective layer 25 is provided so as to cover a part of the upper surface 22 a of the first electrode 22, a part of the end of the piezoelectric layer 23, and the second electrode 24. That is, the protective layer 25 is provided on a part of the upper surface 22 a of the first electrode 22, an exposed portion 23 a′ and side surface 23 b of the upper surface 23 a of the piezoelectric layer 23, and a main surface (upper surface) 24 a and side surface 24 b of the second electrode 24.

[0051] The top surfaces of the first electrode 22, the piezoelectric layer 23, and the second electrode 24 are the surfaces of these layers facing the protective layer 25, and are the main surfaces of these layers that come into contact with the layers stacked on top of them. For example, the top surface 23a of the piezoelectric layer 23 is the main surface that comes into contact with the second electrode 24. The side surfaces 24b of the piezoelectric layer 23 and the second electrode 24 are the surfaces of these layers in the X-axis direction or the Y-axis direction.

[0052] The protective layer 25 has the functions of both providing a good TCF and heat dissipation. The protective layer 25 has a first protective layer 251 and a second protective layer 252, which can be formed of different materials.

[0053] The first protective layer 251 is a layer having a positive TCF and is disposed, for example, at a position corresponding to the active region A1. For example, as shown in Figure 2, the first protective layer 251 is disposed at a position corresponding to the active region A1 in a plan view. Note that since the first protective layer 251 is disposed at a position corresponding to the active region A1, in Figure 2, the region of the first protective layer 251 corresponds to the hatched region of the active region A1.

[0054] The material for forming the first protective layer 251 may be any material that can exhibit a positive TCF. Examples of the material for forming the first protective layer 251 include SiO ２ Since the first protective layer 251 exhibits a positive TCF, the material for forming the first protective layer 251 can be SiO. ２ and a material containing SiO as a main component, such as SiON.

[0055] The second protective layer 252 has a higher thermal conductivity than the first protective layer 251 and is disposed, for example, at a position corresponding to the inactive region A2. For example, as shown in FIG. 2 , the second protective layer 252 is disposed at a position corresponding to the inactive region A2 in a plan view. Since the second protective layer 252 is disposed at a position corresponding to the inactive region A2, the region of the second protective layer 252 corresponds to the hatched region of the inactive region A2 in FIG. 2 . Furthermore, if a hole or the like is provided in a portion of the second protective layer 252 to contact the second electrode 24 in the inactive region A2, the second protective layer 252 need not be provided in the portion of the hole.

[0056] As long as the protective layer 25 can provide both good TCF and heat dissipation, the first protective layer 251 may be disposed at a position corresponding to at least the active region A1, and the second protective layer 252 may be disposed at a position corresponding to at least a portion of the non-active region A2. That is, the first protective layer 251 may be disposed at a position corresponding to the active region A1 and a portion of the non-active region A2, and the second protective layer 252 may be disposed at a position corresponding to only a portion of the non-active region A2. In this embodiment, the second protective layer 252 is also disposed in the non-active region A2 between two active regions A1 on the second electrode 24. However, forming the second protective layer 252 may be difficult when the spacing between the first electrodes 22 is narrow or the shape of the gap between the first electrodes 22 in a plan view is complex. Therefore, the first protective layer 251 may be disposed instead of the second protective layer 252 in the non-active region A2 between two active regions A1.

[0057] The second protective layer 252 may be formed from AlN, Al ２ O ３ , Si ３ N ４ and SiC, and these may be used alone or in combination of two or more.

[0058] The protective layer 25 may be formed by a commonly used method such as vapor deposition, coating, or sputtering.

[0059] The thickness of the protective layer 25 is preferably 200 nm or more, more preferably 300 nm or more, and even more preferably 400 nm or more, as long as it can exhibit good TCF and heat dissipation properties. If the thickness of the protective layer 25 is 200 nm or more, good TCF and heat dissipation properties can be exhibited.

[0060] The thickness of the first protective layer 251 may be the same as or different from the thickness of the second protective layer 252 .

[0061] [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.

[0062] First, high acoustic impedance layers and low acoustic impedance layers are stacked alternately on the upper surface 10a of a support substrate 10 formed to a predetermined size, forming an acoustic mirror layer 21.

[0063] The method for forming the high acoustic impedance layer and the low acoustic impedance layer 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 and the low acoustic impedance layer, thin high acoustic impedance layer and low acoustic impedance layer can be easily formed.

[0064] Examples of dry processes include sputtering and vapor deposition, and examples of wet processes include plating.

[0065] As the sputtering, for example, a sputtering method such as DC (direct current) or RF (radio frequency) magnetron sputtering can be used.

[0066] By using sputtering as a method for forming the high acoustic impedance layer and the low acoustic impedance layer, it is possible to easily form thin high acoustic impedance layer and low acoustic impedance layer with high density, and therefore sputtering is preferred as a method for forming the high acoustic impedance layer and the low acoustic impedance layer.

[0067] The high acoustic impedance layer may be, for example, a W, Mo, or Ta film formed by DC or RF magnetron sputtering. ２ O ５ A thin film formed of a material with a high density or bulk modulus, such as ZnO, can be used.

[0068] The low acoustic impedance layer may be, for example, a SiO film formed by DC or RF magnetron sputtering. ２ An oxide such as a film can be used.

[0069] When using DC or RF magnetron sputtering, the high acoustic impedance layer or the low acoustic impedance layer is Ta. ２ O５ , ZnO and SiO ２ For example, when the high acoustic impedance layer is a ZnO film formed by DC or RF magnetron sputtering, either a Zn sputtering target or a ZnO sputtering target may be used for sputtering, depending on the gas atmosphere.

[0070] Next, the first electrode 22 is deposited (formed) on the upper surface 21 a 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 and the low acoustic impedance layer. Details of the dry process and the wet process are similar to the method for forming the high acoustic impedance layer and the low acoustic impedance layer, and therefore will not be described here.

[0071] The first electrode 22 may be formed on the entire upper surface 21 a of the acoustic mirror layer 21. Alternatively, the first electrode 22 may be formed into an arbitrary shape by processing the first electrode 22 into a pattern having a predetermined shape by etching or the like. For example, the first electrode 22 may be patterned into stripes and a plurality of first electrodes 22 may be arranged in the form of stripes.

[0072] Next, a piezoelectric layer 23 is formed on the upper surface 22a of the first electrode 22. For example, the piezoelectric layer 23 may be formed by sputtering, such as DC or RF magnetron sputtering, in a mixed gas atmosphere containing an inert gas such as Ar and a trace amount of oxygen, using a target containing elements constituting the piezoelectric material. The piezoelectric layer 23 is formed by sputtering the piezoelectric material onto the first electrode 22. Note that the piezoelectric material may be formed by placing a mask or the like on the first electrode 22 so that the piezoelectric layer 23 is not formed in any area other than a predetermined area on the first electrode 22.

[0073] The laminate including the support substrate 10, the acoustic mirror layer 21, and the first electrode 22 may be placed on a deposition plate, which 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 batchwise manner.

[0074] A target containing the elements that make up the piezoelectric material is used as a cathode and is placed opposite the deposition plate of the sputtering device with a gap between them.

[0075] When the piezoelectric material includes, for example, a wurtzite crystalline material, a target containing the wurtzite crystalline material may be used as the target. As the target containing the wurtzite crystalline material, a single or multiple targets containing a material constituting the wurtzite crystalline material contained as a main component in the piezoelectric layer 23 may be used. The piezoelectric layer 23 containing the wurtzite crystalline material can be formed by using a multi-target sputtering method when multiple targets are used as cathodes, or by using a one-dimensional sputtering method when a single target is used as the cathode.

[0076] When multiple targets are used as cathodes, each target contains a material that constitutes the wurtzite crystal material contained as a main component in the piezoelectric layer 23. When multiple targets are used, for example, a target containing Zn, a target containing Si or Sn, and a target containing Al or Mg may be used. Note that each target may be a metal oxide target containing oxygen. The multiple targets may be arranged in the film formation chamber at intervals. During sputtering, the power applied to each target is adjusted depending on the type of wurtzite crystal material contained in the piezoelectric layer 23, thereby adjusting the atomic ratio between the materials that constitute the piezoelectric layer 23.

[0077] When a single target is used as the cathode, the single target contains the wurtzite crystalline material contained in the piezoelectric layer 23. When a single target is used, an alloy target may be used in which the atomic ratio of the wurtzite crystalline material contained in the piezoelectric layer 23 is adjusted. For example, an alloy target containing Zn, Si or Sn, and Al or Mg may be used. The alloy target may be a target of a wurtzite crystalline material and a metal oxide containing oxygen.

[0078] For example, when the piezoelectric material is a wurtzite crystal material made of ZnO, a ZnO sintered compact target may be used. The ZnO sintered compact target 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 ZnO sintered compact target in an atmosphere of the mixed gas containing the inert gas and oxygen, the piezoelectric layer 23 can be obtained on the acoustic mirror layer 21 and the first electrode 22 while suppressing the amount of inert gas that enters during ZnO film formation.

[0079] For example, when the piezoelectric material is a wurtzite crystal material made of MgZnO containing ZnO and MgO in a predetermined mass ratio, a multi-sputtering method using a target made of ZnO sintered body and a target made of MgO sintered body may be used. Alternatively, a one-dimensional sputtering method using an alloy target containing ZnO and MgO, such as a ZnO sintered body target to which MgO has been added in advance at a predetermined ratio, may be used.

[0080] When the multi-target sputtering method is used, a multi-target sputtering apparatus is used as the sputtering apparatus, and a mixed gas containing an inert gas such as Ar and oxygen is supplied into the multi-target sputtering apparatus. In an atmosphere of the mixed gas containing the inert gas and oxygen, a ZnO sintered compact target and an MgO sintered compact target are simultaneously and independently sputtered onto the first electrode 22, thereby forming the piezoelectric layer 23 made of MgZnO on the first electrode 22.

[0081] When the one-dimensional sputtering method is used, a sputtering apparatus is used to perform sputtering in a mixed gas atmosphere containing an inert gas such as Ar and oxygen, using, for example, a target of ZnO sintered body to which MgO has been added in advance at a predetermined ratio, thereby forming a piezoelectric layer 23 made of MgZnO on the first electrode 22.

[0082] 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.

[0083] 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.

[0084] The deposition temperature of the piezoelectric layer 23 is not particularly limited and may be appropriately selected depending on the layer structure of the BAW resonator 20A, for example, the piezoelectric layer 23 may be deposited at 150° C. or lower.

[0085] By using a sputtering method to form the first electrode 22 and 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 first electrode 22 and the piezoelectric layer 23 with a desired thickness with high precision.

[0086] The piezoelectric layer 23 may be formed by laminating a plurality of thin films made of piezoelectric materials.

[0087] Next, the end faces of the piezoelectric layer 23 are processed to form side faces 23b.

[0088] The processing method is Cl ２ , C.F. ４ and CHF ３ Dry etching using reactive gases such as HCl and HNO ３ A common method such as wet etching using an acid solution such as HCl or HClCl can be used.

[0089] 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.

[0090] The thickness of the second electrode 24 can be appropriately designed and may be, for example, 40 nm to 300 nm.

[0091] The second electrode 24 may be formed in any suitable shape on the entire or part of the upper surface 23 a of the piezoelectric layer 23. For example, when the first electrode 22 is formed in a stripe pattern, the second electrode 24 may be formed in a stripe pattern in a direction perpendicular to the direction in which the stripes of the first electrode 22 extend in a plan view of the BAW resonator 20A.

[0092] Next, a protective layer 25 is formed to cover a portion of the upper surface 22a of the first electrode 22, the end portion of the piezoelectric layer 23 (a portion of the upper surface 23a and side surface 23b of the piezoelectric layer 23), and the upper surface 24a and side surface 24b of the second electrode 24.

[0093] The first protective layer 251 constituting the protective layer 25 is formed on the active region A1 in a plan view, and the second protective layer 252 constituting the protective layer 25 is formed on the non-active region A2 in a plan view.

[0094] The first protective layer 251 and the second protective layer 252 that constitute the protective layer 25 may be formed separately or simultaneously.

[0095] The method for forming the first protective layer 251 and the second protective layer 252 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 and the low acoustic impedance layer. Details of the dry process and the wet process are similar to the method for forming the high acoustic impedance layer and the low acoustic impedance layer, and therefore will not be repeated here.

[0096] The BAW resonator 20A is obtained by forming a protective layer 25 on a part of the upper surface 22a of the first electrode 22, the end of the piezoelectric layer 23, and the upper surface 24a and side surface 24b of the second electrode 24. This results in a BAW device 1A in which the BAW resonator 20A is stacked on the support substrate 10.

[0097] After the protective layer 25 is formed, the entire BAW resonator 20A may be heat-treated at a temperature (e.g., 130° C.) lower than the melting point or glass transition point of the support substrate 10. This heat treatment crystallizes the first electrode 22 and the second electrode 24, thereby reducing their resistance. The heat treatment is not essential, and may not be performed after the formation of the BAW resonator 20A, for example, when the support substrate 10 is made of a material that is not heat-resistant.

[0098] Alternatively, the BAW device 1A may be manufactured by forming the 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.

[0099] As described above, the BAW resonator 20A according to this embodiment includes a first electrode 22, a piezoelectric layer 23, a second electrode 24, and a protective layer 25 on a support substrate 10. The protective layer 25 includes a first protective layer 251 having a positive TCF at least on the active region A1, and a second protective layer 252 having a higher thermal conductivity than the first protective layer 251 on at least a portion of the non-active region A2. The active region A1 is a region where the piezoelectric layer 23 resonates, and the non-active region A2 is a region that does not include the region where the piezoelectric layer 23 resonates. The first protective layer 251 is positioned at a position corresponding to at least the active region A1 where the piezoelectric layer 23 resonates, and therefore has a large effect on the frequency of vibration generated by the piezoelectric layer 23. The second protective layer 252 is positioned at a position corresponding to at least a portion of the non-active region A2, which is outside the region where the piezoelectric layer 23 resonates, in a plan view, and therefore has a small effect on the frequency. The first protective layer 251 exhibits a positive TCF, thereby enabling the active region A1 to exhibit a positive TCF, and the second protective layer 252 has a higher thermal conductivity than the first protective layer 251, thereby enabling the non-active region A2 to exhibit higher heat dissipation than the active region A1. Thus, the protective layer 25 can exhibit both good TCF and high heat dissipation properties.

[0100] Therefore, by providing the protective layer 25, the BAW resonator 20A can exhibit a good TCF and have high heat dissipation properties.

[0101] Generally, the first electrode 22, the piezoelectric layer 23, and the second electrode 24 have negative TCFs. Therefore, during use, as the temperature of the BAW resonator 20A increases, the frequency of vibrations generated from the piezoelectric layer 23 tends to shift toward lower frequencies. In this embodiment, the first protective layer 251 of the BAW resonator 20A exhibits a positive TCF at a position corresponding to the active region A1, thereby preventing the frequency of vibrations generated from the piezoelectric layer 23 from shifting toward lower frequencies even when the temperature of the BAW resonator 20A increases during use. Furthermore, the second protective layer 252 of the BAW resonator 20A exhibits high heat dissipation properties at a position corresponding to the inactive region A2, thereby preventing temperature changes in the BAW resonator 20A during use. Therefore, the BAW resonator 20A can effectively suppress fluctuations in the frequency of vibrations generated from the piezoelectric layer 23.

[0102] In the BAW resonator 20A, the active region A1 can be a region of the piezoelectric layer 23 sandwiched between the first electrode 22 and the second electrode 24. This allows the active region A1 to be the region where the second electrode 24 is disposed and where the piezoelectric layer 23 resonates in a plan view, so that the first protective layer 251 can exert a positive TCF only in the active region A1. Therefore, the BAW resonator 20A can more reliably exert a good TCF.

[0103] In the BAW resonator 20A, the first protective layer 251 and the second protective layer 252 can be formed of different materials. This allows the BAW resonator 20A to avoid overlapping functions between the first protective layer 251 and the second protective layer 252, while allowing the first protective layer 251 to exhibit a positive TCF and the second protective layer 252 to exhibit heat dissipation properties.

[0104] The first protective layer 251 of the BAW resonator 20A can be configured to include a material containing SiO as a main component, such as SiO or SiON. Since SiO exhibits a positive TCF, the first protective layer 251 of the BAW resonator 20A can reliably exhibit a positive TCF in the active region A1.

[0105] The BAW resonator 20A has a second protective layer 252 made of AlN, Al ２ O３ , Si ３ N ４ and SiC. ２ O ３ , Si ３ N ４ Since SiC has excellent heat dissipation properties, the BAW resonator 20A can reliably exhibit heat dissipation properties in the inactive region A2 due to the second protective layer 252.

[0106] The BAW resonator 20A can include MgZnO as a piezoelectric material in the piezoelectric layer 23. Generally, there is a trade-off between the K value and the Q value of a piezoelectric layer formed by doping a piezoelectric material with other elements. For example, when a BAW resonator is used in a high-frequency filter that extracts only signals in a high-frequency range, such as the 5 GHz band, and removes signals in other frequency bands, the Q value tends to decrease once the required K value is achieved in the high-frequency range. By including MgZnO as a piezoelectric material in the piezoelectric layer 23, there is no trade-off between the K value and the Q value of MgZnO with respect to the Mg concentration, and the Q value can be maintained while controlling the K value even in the high-frequency range. Therefore, by including MgZnO as a piezoelectric material in the piezoelectric layer 23, the BAW resonator 20A can stably exhibit piezoelectric characteristics even in the high-frequency range of a high-frequency filter, etc.

[0107] The BAW device 1A includes the BAW resonator 20A, and thus can exhibit piezoelectric characteristics in the high frequency band, as well as a good TCF and high heat dissipation performance.

[0108] As described above, the BAW resonator 20A can exhibit good TCF and high heat dissipation, and therefore can be used in various electronic devices as an electronic component utilizing the piezoelectric effect. The BAW resonator 20A can be used in applications requiring high piezoelectric characteristics, particularly in the high-frequency range, and therefore can be suitably used, for example, in high-frequency filters. High-frequency filters include SAW filters using surface acoustic waves (SAW) and BAW filters using BAW. The BAW resonator 20A can exhibit good TCF and high heat dissipation while exhibiting piezoelectric characteristics in the high-frequency band, and therefore can be effectively used as a BAW filter.

[0109] (Modification) In this embodiment, the BAW device 1A is not limited to the above configuration, and may have another configuration as long as it can exhibit good TCF and high heat dissipation properties. An example of another configuration of the BAW device 1A is shown below.

[0110] In this embodiment, the BAW device 1A includes a BAW resonator 20A having an acoustic mirror layer 21 formed of an acoustic multilayer film. However, the acoustic mirror layer 21 may be formed of a space. For example, as shown in FIG. 3 , the BAW device 1B includes a BAW resonator 20B having a recess (depression) 11 formed on the upper surface 10a of the support substrate 10, with the space S formed between the recess 11 and the first electrode 22 of the support substrate 10 functioning as the acoustic mirror layer 21. Because the space S can function as the acoustic mirror layer, the BAW resonator 20B can have the first electrode 22 directly formed on the upper surface 10a of the support substrate 10. This allows the overall thickness of the BAW resonator 20B to be reduced, thereby enabling miniaturization.

[0111] In this embodiment, the BAW device 1A does not need to include the acoustic mirror layer 21 of the BAW resonator 20A. As shown in Fig. 4, the BAW device 1C may include a BAW resonator 20B stacked on the support substrate 10 side.

[0112] 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, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the inventions and their equivalents as set forth in the claims.

[0113] Note that aspects of the present invention are as follows, for example: <1> A BAW resonator comprising: a first electrode, a piezoelectric layer, a second electrode, and a protective layer sequentially stacked on a support substrate; an active region that resonates the piezoelectric layer; and an inactive region other than the active region, wherein the protective layer has a first protective layer disposed at a position corresponding to at least the active region and a second protective layer disposed at a position corresponding to at least a part of the inactive region, wherein the first protective layer has a positive temperature coefficient of frequency, and the thermal conductivity of the second protective layer is higher than that of the first protective layer. <2> The BAW resonator according to <1>, wherein the active region is a region of the piezoelectric layer sandwiched between the first electrode and the second electrode. <3> The first protective layer is made of SiO ２ <4> The BAW resonator according to <1> or <2>, wherein the second protective layer is made of AlN, Al ２ O ３ , Si ３ N ４ and SiC. <5> The BAW resonator according to any one of <1> to <3>, wherein the piezoelectric layer contains ZnO doped with Mg. <6> The BAW resonator according to any one of <1> to <5>, wherein the piezoelectric layer contains ZnO doped with Mg. <7> A BAW device comprising: a support substrate; and the BAW resonator according to any one of <1> to <6>, provided on the support substrate. <8> The BAW device according to <7>, wherein the support substrate has a recess on its surface; and wherein a gap formed between the recess of the support substrate and the first electrode functions as an acoustic mirror layer. <9> An electronic device comprising the BAW device according to <7> or <8>.

[0114] This application claims priority based on Japanese Patent Application No. 2023-162443 filed with the Japan Patent Office on September 26, 2023, and incorporates the entire contents of said application by reference.

[0115] 1A to 1C BAW device 10 Support substrate 11 Recess (hollow portion) 20A, 20B BAW resonator 21 Acoustic mirror layer 22 First electrode 23 Piezoelectric layer 24 Second electrode 25 Protective layer 251 First protective layer 252 Second protective layer 10a, 21a, 22a, 23a, 24a Main surface (upper surface) 23b, 24b Side surface A1 Active region A2 Non-active region S Space

Claims

1. A BAW resonator comprising: a first electrode, a piezoelectric layer, a second electrode and a protective layer laminated in that order on a supporting substrate; an active region that causes the piezoelectric layer to resonate; and an inactive region other than the active region, wherein the protective layer has a first protective layer arranged at a position corresponding to at least the active region, and a second protective layer arranged at a position corresponding to at least a portion of the inactive region, wherein the first protective layer has a positive frequency temperature coefficient, and the thermal conductivity of the second protective layer is higher than that of the first protective layer.

2. A BAW resonator as claimed in claim 1, wherein the active region is a region of the piezoelectric layer sandwiched between the first electrode and the second electrode.

3. The first protective layer is made of SiO ２ 2. The BAW resonator according to claim 1, wherein the main component is SiO or SiON.

4. The second protective layer is made of AlN, Al ２ O ３ , Si ３ N ４ 2. The BAW resonator of claim 1, comprising at least one of:

5. The BAW resonator of claim 1, wherein the piezoelectric layer comprises Mg-doped ZnO.

6. A BAW resonator as described in claim 1, further comprising an acoustic mirror layer between the support substrate and the first electrode, 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.

7. A BAW device comprising: a support substrate; and a BAW resonator according to claim 1 provided on the support substrate.

8. A BAW device as described in claim 7, wherein the support substrate has a recess on its surface, and a gap formed between the recess of the support substrate and the first electrode functions as an acoustic mirror layer.

9. An electronic device comprising the BAW device according to claim 8.

Images