Bulk acoustic wave device

Abstract

This BAW device includes a piezoelectric layer, a first acoustic Bragg reflector provided on one surface of the piezoelectric layer, and a second acoustic Bragg reflector provided on the other surface of the piezoelectric layer. The first acoustic Bragg reflector is formed by laminating a first high acoustic impedance layer and a first low acoustic impedance layer, each being formed of metal and having a thickness equating to an odd multiple of 1 / 4 of a wavelength corresponding to the resonant frequency. The second acoustic Bragg reflector is formed by laminating a second high acoustic impedance layer and a second low acoustic impedance layer. When the first high acoustic impedance layer is disposed at a position closest to the piezoelectric layer in the first acoustic Bragg reflector, the second high acoustic impedance layer is disposed at a position closest to the piezoelectric layer. When the first low acoustic impedance layer is disposed at a position closest to the piezoelectric layer in the first acoustic Bragg reflector, the second low acoustic impedance layer is disposed at a position closest to the piezoelectric layer.

Description

Bulk Acoustic Wave Devices

[0001] The present invention relates to a bulk acoustic wave (BAW) device used in a frequency filter or the like included in a communication device such as a smartphone.

[0002] A BAW device includes a resonator consisting of a piezoelectric layer sandwiched between electrodes. When an AC voltage is applied between the electrodes, vibrations with the same frequency as the AC voltage are generated in the piezoelectric layer. The vibrations generated in the piezoelectric layer are transmitted to the electrodes, causing the entire resonator to vibrate. When an integer multiple of half the wavelength λ of the vibration (nλ / 2, where n is a natural number) matches the total thickness of the resonator, which includes the piezoelectric layer and the electrodes, that is, when an AC voltage with a frequency (resonant frequency) corresponding to the vibration that satisfies this condition is applied between the electrodes, the resonator resonates.

[0003] There are several types of BAW devices, differing in the configuration of the member supporting the resonator. Among them, a solidly mounted resonator (SMR) type BAW device has the resonator mounted on an acoustic Bragg reflector composed of alternating layers with different acoustic impedances. Each layer of the acoustic Bragg reflector has a thickness that is an odd multiple of ¼ of the wavelength corresponding to the resonator's resonant frequency. Note that the wavelength here refers to the wavelength of each layer in the acoustic Bragg reflector, and has a different value for each layer. This acoustic Bragg reflector reflects vibrations having the resonant frequency generated in the resonator. As a result, the acoustic Bragg reflector in an SMR type BAW device serves as a support for the resonator while preventing the vibrations from leaking out of the resonator.

[0004] The materials used for the acoustic Bragg reflectors in SMR-type BAW devices are often metals with relatively large atomic weights, such as tungsten (W), molybdenum (Mo), platinum (Pt), or tantalum (Ta), or their oxides, for the layer with higher acoustic impedance (high acoustic impedance layer), and amorphous SiO2 for the layer with lower acoustic impedance (low acoustic impedance layer). In contrast, the SMR-type BAW device described in Non-Patent Document 1, written by the present inventor, uses a high acoustic impedance layer made of Pt and a low acoustic impedance layer made of titanium (Ti), which has a smaller atomic weight than Pt. A single electrode made of gold (Au) is provided on the piezoelectric layer. In the BAW device described in Non-Patent Document 1, both the high acoustic impedance layer and the low acoustic impedance layer of the acoustic Bragg reflector are made of metal, thereby improving heat dissipation.

[0005] Satoshi Tokai and Takahiko Yanagiya, "Fabrication of Fully Epitaxial SMR Using Acoustic Bragg Reflectors Composed of Epitaxial Metal Thin Films," Proceedings of the Piezoelectric Materials and Devices Symposium 2023, Published by the Piezoelectric Materials and Devices Symposium Organizing Committee, January 26, 2023, pp. 97-102

[0006] In mobile communications such as smartphones, higher frequency bands have been used as the technology transitions from fourth generation to the currently mainstream fifth generation. It is expected that even higher frequency bands will be used in the next generation (sixth generation). Therefore, there is a demand for increasing the resonant frequency of BAW devices. Increasing the resonant frequency of BAW devices requires thinning the piezoelectric layer and / or electrodes. However, making the piezoelectric layer too thin results in reduced voltage resistance, while making the electrodes too thin results in high electrical resistance.

[0007] The problem that the present invention aims to solve is to provide a BAW device having a configuration that is suitable for use at higher resonant frequencies.

[0008] The BAW device according to the present invention, which has been made to solve the above problems, comprises: a) a piezoelectric layer made of a piezoelectric material and having a thickness equal to an integral multiple of 1 / 2 the wavelength corresponding to the resonant frequency; b) a first acoustic Bragg reflector provided on one surface of the piezoelectric layer, the first acoustic Bragg reflector comprising: a first high acoustic impedance layer made of a metal and having a thickness equal to an odd multiple of 1 / 4 the wavelength corresponding to the resonant frequency; and a first low acoustic impedance layer made of a metal and having a smaller acoustic impedance than the first high acoustic impedance layer and having a thickness equal to an odd multiple of 1 / 4 the wavelength corresponding to the resonant frequency. a second acoustic Bragg reflector in which a second high acoustic impedance layer made of metal and having a thickness that is an odd multiple of 1 / 4 of the wavelength corresponding to the resonant frequency and a second low acoustic impedance layer made of metal and having an acoustic impedance smaller than that of the second high acoustic impedance layer and having a thickness that is an odd multiple of 1 / 4 of the wavelength corresponding to the resonant frequency are laminated and provided on the other surface of the piezoelectric layer, and when the first high acoustic impedance layer is arranged in a position closest to the piezoelectric layer in the first acoustic Bragg reflector, the second high acoustic impedance layer is arranged in a position closest to the piezoelectric layer, and when the first low acoustic impedance layer is arranged in a position closest to the piezoelectric layer in the first acoustic Bragg reflector, the second low acoustic impedance layer is arranged in a position closest to the piezoelectric layer.

[0009] In each layer of the BAW device according to the present invention, the "resonant frequency (f r ) corresponding to the wavelength (λ r "Let v be the speed of sound in that layer, and λ r =v / f r The thickness of each layer is allowed to have a slight error (for example, ±20%) from the value calculated based on the wavelength (an integer multiple of 1 / 2 of the wavelength, or an odd multiple of 1 / 4 of the wavelength).

[0010] The first high acoustic impedance layer and the first low acoustic impedance layer may be provided in a single layer (which allows them to function as acoustic Bragg reflectors), or three or more layers may be provided alternately, as is the case with the second high acoustic impedance layer and the second low acoustic impedance layer.

[0011] The number of first high acoustic impedance layers (or first low acoustic impedance layers) and the number of second high acoustic impedance layers (or second low acoustic impedance layers) may be the same or different. Furthermore, the first high acoustic impedance layers and the second high acoustic impedance layers may be made of the same material or different materials. The same applies to the first low acoustic impedance layers and the second low acoustic impedance layers.

[0012] In the BAW device according to the present invention, acoustic Bragg reflectors (first and second acoustic Bragg reflectors) in which both the high acoustic impedance layer and the low acoustic impedance layer are made of metal are provided on both surfaces of the piezoelectric layer. In other words, the BAW device according to the present invention is configured such that the piezoelectric layer is sandwiched in its thickness direction between two acoustic Bragg reflectors (same as above) made of metal.

[0013] In this BAW device, these two acoustic Bragg reflectors can be used as electrodes. When an AC voltage having the same frequency as the resonant frequency is applied between these electrodes, vibrations having the resonant frequency are generated in the piezoelectric layer. These vibrations are reflected by the electrodes (acoustic Bragg reflectors) on both surfaces of the piezoelectric layer. As a result, the electrodes do not vibrate, and only the piezoelectric layer vibrates. Furthermore, because the piezoelectric layer meets the resonance requirement of having a thickness that is an integer multiple of 1 / 2 the wavelength corresponding to the resonant frequency, the piezoelectric layer operates as a resonator.

[0014] In the BAW device according to the present invention, since only the piezoelectric layer operates as a resonator, the resonant frequency can be increased without thinning the piezoelectric layer compared to conventional BAW devices in which the combination of the piezoelectric layer and electrodes functions as a resonator. This prevents a decrease in voltage resistance. Furthermore, since the electrodes are not included in the resonator, there is no need to thin the electrodes to increase the resonant frequency, and therefore the electrical resistance of the electrodes is prevented from increasing.

[0015] In the BAW device described in Non-Patent Document 1, a metallic acoustic Bragg reflector is used as one of the electrodes, but the reason for using such an acoustic Bragg reflector is to improve heat dissipation, and Non-Patent Document 1 does not mention increasing the resonant frequency. Also, in the BAW device according to the present invention, by using metallic acoustic Bragg reflectors for both electrodes, both electrodes (acoustic Bragg reflectors) do not resonate at the resonant frequency (they reflect vibrations at that resonant frequency), and the resonant frequency is determined by the thickness of only the piezoelectric layer excluding the electrodes. Therefore, the BAW device according to the present invention can be used at a higher resonant frequency than conventional BAW devices.

[0016] In the first Bragg reflector, the first high acoustic impedance layer and the first low acoustic impedance layer may be arranged in any order, and either the first high acoustic impedance layer or the first low acoustic impedance layer may be arranged closest to the piezoelectric layer. In the second Bragg reflector, when the first high acoustic impedance layer is arranged closest to the piezoelectric layer in the first Bragg reflector, the second high acoustic impedance layer is arranged closest to the piezoelectric layer, and when the first low acoustic impedance layer is arranged closest to the piezoelectric layer in the first Bragg reflector, the second low acoustic impedance layer is arranged closest to the piezoelectric layer.

[0017] When the first high acoustic impedance layer and the second high acoustic impedance layer are positioned closest to the piezoelectric layer, both ends of the standing wave generated in the piezoelectric layer by resonance become fixed ends, and these ends become nodes. On the other hand, when the first low acoustic impedance layer and the second low acoustic impedance layer are positioned closest to the piezoelectric layer, both ends of the standing wave generated in the piezoelectric layer by resonance become free ends, and these ends become antinodes. In either case, the total length of the standing wave, i.e., the thickness of the piezoelectric layer, becomes an integer multiple of 1 / 2 the wavelength.

[0018] In the BAW device described in Non-Patent Document 1, the end of the standing wave generated in the piezoelectric layer that is in contact with the electrode consisting of only one layer is a free end. Therefore, when the metallic acoustic Bragg reflector is the fixed end, i.e., when a layer with a higher acoustic impedance is located closest to the piezoelectric layer in the Bragg reflector, the thickness of the piezoelectric layer must be an integral multiple of 1 / 4 of the wavelength corresponding to the resonant frequency (when a layer with a lower acoustic impedance is located closest to the piezoelectric layer, the thickness of the piezoelectric layer is the same as in the present invention). Therefore, in the BAW device described in Non-Patent Document 1, to increase the resonant frequency, the layer with a lower acoustic impedance must be located closest to the piezoelectric layer. In contrast, in the BAW device according to the present invention, since the resonant frequencies are the same, either the first (second) low acoustic impedance layer or the first (second) high acoustic impedance layer may be located closest to the piezoelectric layer. Therefore, which of these layers is located closest to the piezoelectric layer can be determined based on conditions other than the resonant frequency, such as which is more suitable for the crystal growth of the piezoelectric layer.

[0019] In the BAW device according to the present invention, it is permissible to add an electrode between the first acoustic Bragg reflector and / or the second acoustic Bragg reflector and the piezoelectric layer that is sufficiently thin (for example, a thickness of 1 / 10 or less of the wavelength corresponding to the resonance frequency) so as to have almost no effect on the resonance frequency of the BAW device. Since the electrode added in this way is electrically integrated with the first acoustic Bragg reflector or the second acoustic Bragg reflector and functions as a single electrode, electrical resistance does not pose a problem.

[0020] The BAW device according to the present invention may further include an insulating substrate on the surface of the first acoustic Bragg reflector opposite to the piezoelectric layer, thereby enabling the entire BAW device to be supported while being electrically insulated from the outside.

[0021] In the BAW device according to the present invention, it is preferable that the electrical impedance of a capacitor formed by the piezoelectric layer, the first acoustic Bragg reflector, and the second acoustic Bragg reflector is 50Ω at the resonant frequency, thereby enabling electrical impedance matching when the BAW device according to the present invention is incorporated into an electric circuit.

[0022] The capacitance of a capacitor is proportional to the area of ​​the electrodes (first acoustic Bragg reflector and second acoustic Bragg reflector) and inversely proportional to the thickness of the piezoelectric layer, which is made of a piezoelectric material, a type of dielectric. Furthermore, since the capacitive reactance is inversely proportional to the capacitance, it is inversely proportional to the area of ​​the electrodes and proportional to the thickness of the piezoelectric layer. In a BAW device according to the present invention, the thickness of the piezoelectric layer is determined by the resonant frequency, so the capacitive reactance can be set to 50 Ω at the resonant frequency by adjusting the area of ​​the electrodes.

[0023] In a BAW device of the present invention in which the electrical impedance of the capacitor is 50 Ω at the resonant frequency, the area (first area) of the entirety of one of the first acoustic Bragg reflector and the second acoustic Bragg reflector, a portion of the other acoustic Bragg reflector in the thickness direction, and the piezoelectric layer can be smaller than the area (second area) of the other acoustic Bragg reflector other than the portion in the thickness direction. As described above, the first area is set so that the electrical impedance of the capacitor is 50 Ω at the resonant frequency. This BAW device can be manufactured by fabricating the first acoustic Bragg reflector, the piezoelectric layer, and the second acoustic Bragg reflector to have the second area, and then etching the portion to be the first area while leaving only the first area. Whereas fabricating a first acoustic Bragg reflector or the like having the first area from the beginning requires fabricating multiple layers with the same area in the same position, this BAW device allows for easy area adjustment by fabricating the portion to be the first area by etching.

[0024] According to the present invention, a BAW device having a configuration suitable for use at higher resonance frequencies can be obtained, in that it is possible to prevent a decrease in voltage resistance and an increase in the electrical resistance of the electrodes.

[0025] 1 is a longitudinal sectional view showing an embodiment of a BAW device according to the present invention. FIG. 2 is a longitudinal sectional view showing a modified BAW device. FIG. 3 is a top view of the modified BAW device shown in FIG. 2. FIG. 4 is a longitudinal sectional view showing another modified BAW device. FIG. 5 is a longitudinal sectional view showing an example of a state in which the BAW device of the present embodiment is used. FIG. 6 is a schematic view showing an example of a frequency filter (bandpass filter) formed by connecting a plurality of BAW devices of the present embodiment. FIG. 7 is a longitudinal sectional view showing a state in which a substrate is prepared in a method for manufacturing the BAW device of the present embodiment. FIG. 8 is a longitudinal sectional view showing a state in which a first high acoustic impedance layer is formed on a substrate in a method for manufacturing the BAW device of the present embodiment. FIG. 9 is a longitudinal sectional view showing a state in which a first low acoustic impedance layer is formed on a first high acoustic impedance layer in a method for manufacturing the BAW device of the present embodiment. FIG. 10 is a longitudinal sectional view showing a state in which a first acoustic Bragg reflector is formed in a method for manufacturing the BAW device of the present embodiment. 1 is a longitudinal cross-sectional view showing a state in which a second low acoustic impedance layer is fabricated on a piezoelectric layer in a method for fabricating a BAW device according to the present embodiment. 2 is a longitudinal cross-sectional view showing a state in which a second high acoustic impedance layer is fabricated on a second low acoustic impedance layer in a method for fabricating a BAW device according to the present embodiment. 3 is a longitudinal cross-sectional view showing a state in which a piezoelectric layer of the fabricated BAW device is made of Mg 1-x Zn x A scanning electron microscope (SEM) photograph of a MgZnO-BAW device made of ScZnO was taken from above. A magnified SEM photograph of a MgZnO-BAW device taken from diagonally above. x Al 1-x1 shows an SEM photograph of a longitudinal cross section of an ScAlN-BAW device made of N. 2 shows an SEM photograph of a longitudinal cross section of an MgZnO-BAW device. 3 shows a graph showing the results of a 2θ-ω scan by X-ray diffraction measurement of an ScAlN-BAW device. 4 shows a graph showing the results of a 2θ-ω scan by X-ray diffraction measurement of an MgZnO-BAW device. 5 shows a graph showing the results of an ω scan of the Ti(0002) plane by X-ray diffraction measurement of an ScAlN-BAW device. 6 shows a graph showing the results of an ω scan of the Pt(111) plane by X-ray diffraction measurement of an ScAlN-BAW device. 7 shows a graph showing the results of a φ scan around an axis perpendicular to the Ti(10-12) plane by X-ray diffraction measurement of an ScAlN-BAW device. 8 shows a graph showing the results of a φ scan around an axis perpendicular to the Pt(002) plane by X-ray diffraction measurement of an ScAlN-BAW device. Pole figures obtained for the Ti(10-12) plane of the ScAlN-BAW device. Pole figures obtained for the Pt(002) plane of the ScAlN-BAW device. For the ScAlN-BAW device, X-ray diffraction measurements were performed to determine the Sc x Al 1-x Graph showing the results of ω scan on the N(0002) plane. 1-x Zn x Graph showing the results of ω scan on the O(0002) plane. x Al 1-x The graph shows the results of φ scanning around the axis perpendicular to the N(10-12) plane. 1-x Zn x Graph showing the results of φ scanning around the axis perpendicular to the O(10-12) plane. x Al 1-x Pole figure obtained for the N(10-12) plane. 1-x Zn xPole figures obtained with respect to the O(10-12) plane. Electron diffraction images of the uppermost (closest to the piezoelectric layer) of the first acoustic impedance layer of the ScAlN-BAW device. Electron diffraction images of the uppermost (closest to the piezoelectric layer) of the first high acoustic impedance layer of the ScAlN-BAW device. Electron diffraction images of the piezoelectric layer of the ScAlN-BAW device. Electron diffraction images of the uppermost (farthest from the piezoelectric layer) of the second high acoustic impedance layer of the ScAlN-BAW device. Graph showing the results of measuring the frequency dependence of the electrical impedance of the ScAlN-BAW device. Graph showing the results of measuring the frequency dependence of the electrical impedance of the MgZnO-BAW device.

[0026] Embodiments of the BAW device according to the present invention will be described with reference to FIGS. 1 to 17.

[0027] (1) Configuration of the BAW device according to the present embodiment FIG. 1 shows a longitudinal sectional view of the schematic configuration of the BAW device 10 according to the present embodiment. This BAW device 10 includes a piezoelectric layer 11, a first acoustic Bragg reflector 121, a second acoustic Bragg reflector 122, and a substrate 13.

[0028] The piezoelectric layer 11 is made of a piezoelectric material. For the piezoelectric material of the piezoelectric layer 11, ZnO (zinc oxide), Mg 1-x Zn x O (magnesium zinc oxide, 0 <x <1), AlN (aluminum nitride), Sc x Al 1-x N (scandium aluminum nitride, 0 <x <1), PbTiO3 (lead titanate), LiNbO3 (lithium niobate), LiTaO3 (lithium tantalate), etc. can be used. In the present embodiment, the piezoelectric layer 11 is made of a single crystal produced by an epitaxial method, but a single crystal produced by another method may be used, or a polycrystal may be used as long as the polarization directions (specific crystal axes) are aligned in a specific direction.

[0029] The thickness t p of the piezoelectric layer 11 r is an integer multiple of 1 / 2 of the resonance wavelength λ r corresponding to the resonance frequency f p , that is, t r / 2, where the resonant wavelength λ r is expressed as λ using the sound velocity v in the piezoelectric layer 11. r =v / f r It can be calculated by the following equation: p An error of about ±20% from the value calculated in this way is allowed.

[0030] The first acoustic Bragg reflector 121 is provided below the piezoelectric layer 11. Note that the up-down relationship here is defined for convenience in order to explain the relative positional relationship of each component, with the thickness direction of the piezoelectric layer 11 being the up-down direction, and does not limit the up-down orientation of the BAW device 10 when in use (the same applies hereinafter).

[0031] The first acoustic Bragg reflector 121 is formed by alternately laminating first low acoustic impedance layers 1211 and first high acoustic impedance layers 1212. The first low acoustic impedance layers 1211 and the first high acoustic impedance layers 1212 are both made of metal, with the first high acoustic impedance layer 1212 having a higher acoustic impedance than the first low acoustic impedance layer 1211. Since the acoustic impedance is determined by the product of the speed of sound and density, the metal materials for both layers are selected so that the product of the speed of sound and density of the material for the first high acoustic impedance layer 1212 is higher than that of the material for the first low acoustic impedance layer 1211.

[0032] The thickness t of the first low acoustic impedance layer 1211 b11 and the thickness t of the first high acoustic impedance layer 1212 b12 and the resonant frequency f of the piezoelectric layer 11. r The thickness of each layer is determined to be an odd multiple of 1 / 4 of the wavelength. b11 , t b12 An error of about ±20% from the calculated value is allowed.

[0033] The first low acoustic impedance layer 1211 and the first high acoustic impedance layer 1212 can be made of, for example, titanium (Ti) and platinum (Pt). The sound velocity of Ti is slightly less than twice that of Pt (sound velocity of longitudinal waves: Ti: 6100 m / s, Pt: 3260 m / s), and the density of Pt is more than four times higher than that of Ti (Ti: 4.475 g / cm). 3 , Pt: 21.40 g / cm 3 ). As described above, acoustic impedance is determined by the product of the sound velocity and density. In this example, however, the density ratio is more significantly different than the sound velocity ratio, so the first high acoustic impedance layer 1212, which has a higher density than the first low acoustic impedance layer 1211, is thicker. Since the thicknesses of both layers depend on the sound velocity (but not on the density) as described above, the first high acoustic impedance layer 1212, which has a faster sound velocity than the first high acoustic impedance layer 1211, is thicker (t b11 >t b12 ).

[0034] Note that Ti and Pt listed here are merely examples of materials for the first low acoustic impedance layer 1211 and the first high acoustic impedance layer 1212, and various metal materials can be used. In addition to Ti, the first low acoustic impedance layer 1211 can be made of elements such as Sc (scandium), V (vanadium), Cr (chromium), Mn (manganese), Fe (iron), Co (cobalt), Ni (nickel), Cu (copper), Zn (zinc), Ge (germanium), Al (aluminum), Si (silicon), and carbon (C), as well as alloys of these elements (including Ti). For the elemental C, graphene, which has metallic thermal conductivity, can be used. In addition to Pt, the material for the first high acoustic impedance layer 1212 can be, for example, Mo (molybdenum), Ta (tantalum), Hf (hafnium), Ir (iridium), Ru (ruthenium), W (tungsten), Au (gold), Zr (zirconium), Nb (niobium), or other elemental substance or alloys of these (including Pt).

[0035] In this embodiment, the first low acoustic impedance layer 1211 and the first high acoustic impedance layer 1212 are made of single crystals produced by an epitaxial method, but they may also be made of single crystals produced by other methods, or may be made of something other than a single crystal.

[0036] Although FIG. 1 shows an example in which five first low acoustic impedance layers 1211 and five first high acoustic impedance layers 1212 are alternately stacked, the number of layers of each of these layers is not limited to five, and it is sufficient that there is at least one layer of each.

[0037] In the example shown in Figure 1, of the layers constituting the first acoustic Bragg reflector 121, the first low acoustic impedance layer 1211 is positioned closest to the piezoelectric layer 11, but the second low acoustic impedance layer 1221 may also be positioned closest to the piezoelectric layer 11.

[0038] The second acoustic Bragg reflector 122 is provided on the upper side of the piezoelectric layer 11 (the opposite side of the first acoustic Bragg reflector 121 when viewed from the piezoelectric layer 11). Therefore, the piezoelectric layer 11 is sandwiched between the first acoustic Bragg reflector 121 and the second acoustic Bragg reflector 122 in the thickness direction.

[0039] The second acoustic Bragg reflector 122 is formed by alternately laminating second low acoustic impedance layers 1221 and second high acoustic impedance layers 1222. The second low acoustic impedance layers 1221 and the second high acoustic impedance layers 1222 are both made of metal, and the second high acoustic impedance layers 1222 have a higher acoustic impedance than the second low acoustic impedance layers 1221. The thickness t b21 and the thickness t of the second high acoustic impedance layer 1222 b22 and the resonant frequency f of the piezoelectric layer 11. r The thickness of each layer is determined to be an odd multiple of 1 / 4 of the wavelength. b21 , t b22 An error of about ±20% from the calculated value is allowed.

[0040] 1 , the second acoustic Bragg reflector 122 has the same configuration (in terms of material, number of layers, etc.) as the first acoustic Bragg reflector 121. However, as long as the above requirements for the second acoustic Bragg reflector 122 are met, the second acoustic Bragg reflector 122 may have a different configuration (in terms of material and / or number of layers, etc.) from the first acoustic Bragg reflector 121.

[0041] 1 , the second low acoustic impedance layer 1221 is disposed in the position closest to the piezoelectric layer 11 in the second acoustic Bragg reflector 122, but this arrangement is applied when the first low acoustic impedance layer 1211 is disposed in the position closest to the piezoelectric layer 11 in the first acoustic Bragg reflector 121 as described above. When the first high acoustic impedance layer 1212 is disposed in the position closest to the piezoelectric layer 11 in the first acoustic Bragg reflector 121, the second high acoustic impedance layer 1222 is disposed in the position closest to the piezoelectric layer 11 in the second acoustic Bragg reflector 122.

[0042] The substrate 13 is made of an insulating material and serves to support the first acoustic Bragg reflector 121 (and the piezoelectric layer 11 and second acoustic Bragg reflector 122 provided thereon) and to electrically insulate it from the outside. In this embodiment, the substrate 13 uses a plate material made of a single crystal having a crystal structure and lattice constant similar to that of each layer of the first acoustic Bragg reflector 121, the piezoelectric layer 11, and the second acoustic Bragg reflector 122 (made of the same material as the first acoustic Bragg reflector 121). This makes it possible to fabricate the first acoustic Bragg reflector 121, the piezoelectric layer 11, and the second acoustic Bragg reflector 122 in this order on the substrate 13 by an epitaxial method. As in the above example, Ti is used as the material for the first low acoustic impedance layer 1211, Pt is used as the material for the first high acoustic impedance layer 1212, and ZnO and Mg are used as the material for the piezoelectric layer 122. 1-x Zn x O, AlN, Sc x Al 1-x When using a material having a wurtzite structure such as N, the substrate 13 can be suitably made of a single crystal of sapphire.

[0043] The material of the substrate 13 can be single crystal sapphire, as well as single crystals of SiC (silicon carbide), Si (silicon), MgO (magnesium oxide), SrTiO3 (strontium titanate), GaAs (gallium arsenide), Ge (germanium), etc.

[0044] In this BAW device 10, the first acoustic Bragg reflector 121 and the second acoustic Bragg reflector 122 are both made of metal, and therefore are electrically conductive, and function as electrodes. Since the piezoelectric layer 11, which is made of a piezoelectric material, a type of dielectric, is sandwiched between these electrodes, the BAW device 10 is electrically a capacitor. When incorporating such a BAW device 10 into an AC electric circuit, it is necessary to set the area of ​​one (or both) of the electrodes so that the electrical impedance becomes a predetermined value (usually 50 Ω at the resonant frequency of the BAW device 10) in order to achieve electrical impedance matching when incorporated into the electric circuit (note that the impedance of the BAW device 10 is determined by the thickness t of the piezoelectric layer 11). p Although it also depends on the thickness t p is the resonant frequency f r (It is determined by the following and cannot be changed to set the electrical impedance.)

[0045] 2 (longitudinal cross-sectional view) and 3 (top view), in the BAW device 10 of this embodiment, portions of the first acoustic Bragg reflector 121 and the second acoustic Bragg reflector 122 may be etched from above together with the piezoelectric layer 11 to form grooves 14 in the shape of a closed curve when viewed from the top, and the area of ​​the region 15 surrounded by the grooves 14 (the first area described above) may be set to correspond to the area of ​​the electrode at which the impedance becomes a predetermined value. In this way, only the region inside the closed curve of the second acoustic Bragg reflector 122 functions as one of the electrodes, and impedance matching can be achieved by adjusting the area of ​​that electrode. Here, the groove 14 penetrates the entire second acoustic Bragg reflector 122 and the piezoelectric layer 11 in the thickness direction, but it is sufficient to provide the groove 14 only in a portion near the top of the first acoustic Bragg reflector 121.

[0046] In the examples shown in FIGS. 2 and 3, the portion outside the groove 14 is left, but as shown in FIG. 5, the entire portion outside the region 15 may be removed.

[0047] Alternatively, first acoustic Bragg reflector 121 and second acoustic Bragg reflector 122 may be fabricated from the beginning so that they have an area that will give BAW device 10 an electrical impedance of 50 Ω at the resonant frequency. However, while it is relatively easy to fabricate electrodes made of a single layer of metal to have a predetermined area in conventional BAW devices, in BAW device 10 of this embodiment, first acoustic Bragg reflector 121 and second acoustic Bragg reflector 122, which are electrodes, are formed by stacking multiple layers, and therefore advanced technology is required to fabricate these multiple layers in the same position and with the same area. For this reason, it is easier to fabricate first acoustic Bragg reflector 121 and second acoustic Bragg reflector 122 with a larger area (the above-mentioned second area) as described above, and then etch them to give the first area.

[0048] (2) Operation of the BAW Device of This Embodiment The operation of the BAW device 10 of this embodiment will be described using Figures 5 and 6. When the BAW device 10 is used, the first terminal 211 is connected to the first acoustic Bragg reflector 121, and the second terminal 212 is connected to the second acoustic Bragg reflector 122, as shown in Figure 5. As described above, these acoustic Bragg reflectors are made of metal, and therefore both function as electrodes.

[0049] When AC voltages with various frequencies superimposed are applied between the first terminal 211 and the second terminal 212, vibrations having those frequencies are generated in the piezoelectric layer 11. Here, the thickness of the piezoelectric layer 11 is determined by the resonant frequency f r The resonant wavelength λ corresponding to r An integer multiple of 1 / 2 of (λ r / 2, λ r , 3λ r / 2, 2λ r By setting the frequency to the resonant frequency f rWhen the first high acoustic impedance layer 1212 and the second high acoustic impedance layer 1222 are arranged in positions closest to the piezoelectric layer 11, both ends of the standing wave formed in the thickness direction of the piezoelectric layer 11 become nodes, and when the first low acoustic impedance layer 1211 and the second low acoustic impedance layer 1221 are arranged in the same positions, they become antinodes. Furthermore, the vibration generated in the piezoelectric layer 11 reaches the first acoustic Bragg reflector 121 and the second acoustic Bragg reflector 122, but when the thicknesses of the layers of these acoustic Bragg reflectors (the first low acoustic impedance layer 1211, the first high acoustic impedance layer 1212, the second low acoustic impedance layer 1221, and the second high acoustic impedance layer 1222) are different from each other in terms of the resonance frequency f r The resonant frequency f is an odd multiple of 1 / 4 of the wavelength for each layer corresponding to r is reflected at the boundary between the piezoelectric layer 11 and the acoustic Bragg reflectors. As a result, only the piezoelectric layer 11 acts as a resonator.

[0050] According to the BAW device 10 of this embodiment, since only the piezoelectric layer 11 operates as a resonator, the resonant frequency can be increased without thinning the piezoelectric layer 11, compared to conventional BAW devices in which the combination of the piezoelectric layer and the electrodes functions as a resonator. This prevents a decrease in voltage resistance. Furthermore, since the first acoustic Bragg reflector 121 and the second acoustic Bragg reflector 122, which are electrodes, are not included in the resonator, there is no need to thin the electrodes in order to increase the resonant frequency, and therefore an increase in the electrical resistance of the electrodes is also prevented.

[0051] Furthermore, in the BAW device 10 of this embodiment, the resonator (piezoelectric layer 11) is sandwiched between the first acoustic Bragg reflector 121 and the second acoustic Bragg reflector 122 made of metal on the top and bottom, and therefore heat dissipation from the resonator is also excellent.

[0052] In the BAW device 10 of this embodiment, the piezoelectric layer 11 functions as a resonator by itself, but by connecting a plurality of BAW devices 10 in a "ladder configuration" using a known technique related to resonators as shown in Fig. 6, the device can be used as a frequency filter (bandpass filter) 50 that passes a signal current within a frequency band having a certain width. Note that in Fig. 6, each BAW device 10 has the first acoustic Bragg reflector 121 on the upper or left side of the figure and the second acoustic Bragg reflector 122 on the lower or right side, but the first acoustic Bragg reflector 121 and the second acoustic Bragg reflector 122 may be arranged in the opposite direction. Furthermore, these orientations do not need to be uniform across all BAW devices 10 and may differ for each BAW device 10.

[0053] (3) Example of Fabrication of BAW Device of the Present Embodiment and Experimental Results of the Fabrication Example Next, the BAW device 10 of the present embodiment was fabricated, and the results of an experiment conducted on the obtained BAW device 10 will be described. In this example, Sc was used as the material for the piezoelectric layer 11. x Al 1-x N (x=0.12) and Mg 1-x Zn x Two types of BAW devices 10 were fabricated, one using Ti and the other using O (x=0.70). In both of these two examples, the first low acoustic impedance layer 1211 and the second low acoustic impedance layer 1221 were made of Ti, the first high acoustic impedance layer 1212 and the second high acoustic impedance layer 1222 were made of Pt, and the substrate 13 was made of sapphire.

[0054] These BAW devices 10 were manufactured by forming the first acoustic Bragg reflector 121, the piezoelectric layer 11, and the second acoustic Bragg reflector 122 under the conditions shown in Table 1 using a magnetron sputtering apparatus.

[0055] The following is a detailed description of a method for fabricating the BAW device 10. First, a substrate 13 is fabricated by cutting a single crystal of sapphire into a plate shape with the (0001) plane of the hexagonal crystal structure as the surface (FIG. 7A).

[0056] Next, a first high acoustic impedance layer 1212 made of a plate-shaped single crystal of Pt with a face-centered cubic structure and a (111) plane as its surface is fabricated on the substrate 13 by epitaxially growing Pt to a thickness of 550 nm under the conditions shown in Table 1 ( FIG. 7B ). Subsequently, a first low acoustic impedance layer 1211 made of a plate-shaped single crystal of Ti with a hexagonal structure and a (0001) plane as its surface is fabricated on the first high acoustic impedance layer 1212 by epitaxially growing Ti to a thickness of 900 nm under the conditions shown in Table 1 ( FIG. 7C ). Subsequently, the first high acoustic impedance layer 1212 and the first low acoustic impedance layer 1211 are alternately epitaxially grown by a similar method to fabricate the first acoustic Bragg reflector 121 ( FIG. 7D ).

[0057] Next, Sc was applied to the first acoustic Bragg reflector 121 under the conditions shown in Table 1. x Al 1-x N or Mg 1-x Zn x By epitaxially growing O, Sc with a hexagonal crystal structure (0001) plane becomes the surface. x Al 1-x N or Mg 1-x Zn x A piezoelectric layer 11 made of a plate-shaped single crystal of O is fabricated (FIG. 7E).

[0058] Next, a single layer of a second low acoustic impedance layer 1221 is epitaxially grown on the piezoelectric layer 11 under the same conditions as the first low acoustic impedance layer 1211 (FIG. 7F). Subsequently, a single layer of a second high acoustic impedance layer 1222 is epitaxially grown on the second low acoustic impedance layer 1221 under the same conditions as the first high acoustic impedance layer 1212 (FIG. 7G). Subsequently, the second low acoustic impedance layers 1221 and the second high acoustic impedance layers 1222 are epitaxially grown alternately by the same method to produce a second acoustic Bragg reflector 122 (see FIG. 1).

[0059] Thereafter, a groove 14 is formed by a focused ion beam so as to surround the region 15 throughout the entire thickness of the second acoustic Bragg reflector 122 and the piezoelectric layer 11, and to a depth equivalent to one layer of the first low acoustic impedance layer 1211 and the first high acoustic impedance layer 1212 that are the uppermost layers of the first acoustic Bragg reflector 121 (see FIGS. 2 and 3). By the above operations, the BAW device 10 was fabricated.

[0060] Next, the fabricated BAW device 10 was evaluated by the following experiment. x Al 1-x The BAW device 10 in which the piezoelectric layer 11 is Mg is called an "ScAlN-BAW device." 1-x Zn x The BAW device 10 in this case is called an "MgZnO-BAW device."

[0061] First, scanning electron microscope (SEM) photographs were taken of the MgZnO-BAW device. Fig. 8A shows a photograph taken from above, and Fig. 8B shows an enlarged photograph taken obliquely from above. Fig. 8A shows that rectangular grooves 14 are formed when viewed from above, and that regions 15 are surrounded by the grooves 14. Fig. 8B shows that multiple layers are stacked in region 15.

[0062] Next, SEM photographs of longitudinal cross sections are shown in Fig. 8C for the ScAlN-BAW device and Fig. 8D for the MgZnO-BAW device. In both, it can be seen that multiple first low acoustic impedance layers 1211, multiple first high acoustic impedance layers 1212, one piezoelectric layer 11, multiple second low acoustic impedance layers 1221, and multiple second high acoustic impedance layers 1222 are stacked.

[0063] Next, measurements were performed on the ScAlN-BAW device and the MgZnO-BAW device using an X-ray diffractometer (X'Pert PRO (manufactured by PANalytical)) at the stage when the first acoustic Bragg reflector 121 and the piezoelectric layer 11 were fabricated ( FIG. 7E ). These measurements included X-ray diffraction measurements using 2θ-ω scans, ω scans, and φ scans. In the 2θ-ω scan, ω is the angle of incidence of the incident X-rays on the surface of each layer, and 2θ is the angle between the incident X-rays and the outgoing X-rays detected by the detector. The measurement was performed while varying ω and 2θ while maintaining ω = θ. The ω scan was performed while fixing 2θ (the detector position) and varying ω. The φ scan was performed while fixing ω and 2θ so that ω = θ, and rotating the sample around an axis perpendicular to a specific crystal plane.

[0064] The results of the 2θ-ω scan are shown in Figure 9A for the ScAlN-BAW device and in Figure 9B for the MgZnO-BAW device. In both cases, diffraction of the (0002) plane of Ti by the first low acoustic impedance layer 1211 and diffraction of the (111) plane of Pt by the first high acoustic impedance layer 1212 are observed. In Figure 9A, diffraction of the ScAlN-BAW device by the piezoelectric layer 11 is observed. x Al 1-x Diffraction of Mg by the piezoelectric layer 11 is shown in FIG. 1-x Zn x Diffraction from the O (0002) plane can be seen.

[0065] Next, ω scans were performed on the ScAlN-BAW device, with 2θ fixed at the (0002) plane of Ti and 2θ fixed at the (111) plane of Pt. The ω scan results are shown in Figure 10A for Ti and Figure 10B for Pt. In both cases, peaks with full widths at half maximum of 0.42° (Ti) and 0.96° (Pt) were observed.

[0066] For this ScAlN-BAW device, phi scans were performed around an axis perpendicular to the (10-12) plane of Ti and around an axis perpendicular to the (002) plane of Pt. Furthermore, pole figures were obtained for the (10-12) plane of Ti and the (002) plane of Pt, respectively. The phi scan results are shown in Figure 11A for Ti and Figure 11B for Pt. The pole figures are also shown in Figure 12A for Ti and Figure 12B for Pt. Both of these scans exhibit six-fold rotational symmetry, corresponding to the crystal structure of single crystals of Ti and Pt, indicating that single crystals, rather than polycrystals, were obtained.

[0067] Next, for each of the ScAlN-BAW device and the MgZnO-BAW device, x Al 1-x N and Mg 1-x Zn x The ω scan was performed with the 2θ value fixed at the value corresponding to the (0002) plane of ScAlN. The results are shown in Figure 13A for the ScAlN-BAW device and in Figure 13B for the MgZnO-BAW device. In both cases, the full width at half maximum was 2.4° (Sc x Al 1-x N) and 1.9° (Mg 1-x Zn x O) peaks are observed.

[0068] For each of these ScAlN-BAW devices and MgZnO-BAW devices, x Al 1-x N and Mg 1-x Zn x The φ scan was performed around the axis perpendicular to the (10-12) plane of O, and the pole figure was obtained. x Al 1-x Regarding N, see Figure 14A. 1-x Zn x The pole figures for Sc are shown in Fig. 14B. x Al 1-x Regarding N, see Figure 15A. 1-x Zn x The graphs for Sc and O are shown in Figure 15B. x Al 1-x N and Mg 1-x Znx The six-fold rotational symmetry corresponding to the crystal structure of O is observed, indicating that a single crystal, not a polycrystal, was obtained.

[0069] Up to this point, the crystallinity of the first acoustic Bragg reflector 121 and the piezoelectric layer 11 has been evaluated by X-ray diffraction measurement at the stage when the first acoustic Bragg reflector 121 and the piezoelectric layer 11 have been fabricated (before the second acoustic Bragg reflector 122 is fabricated). As for the second acoustic Bragg reflector 122, even if X-ray diffraction measurement is performed after fabricating it on the piezoelectric layer 11, X-ray diffraction originating from the first acoustic Bragg reflector 121 is also observed, and therefore it is not possible to evaluate the crystallinity of the second acoustic Bragg reflector 122 alone. Therefore, the crystallinity of the second acoustic Bragg reflector 122 was evaluated using a transmission electron microscope (TEM, JEM-2010 (manufactured by JEOL Ltd.)) as follows.

[0070] First, using a composite beam processing and observation system (JIB-4700, manufactured by JEOL Ltd.), a small piece was cut out from the surface of the second acoustic Bragg reflector 122 of the ScAlN-BAW device using a focused ion beam, and a longitudinal cross-sectional image and an electron diffraction image were obtained using a TEM. The electron diffraction image was obtained for the uppermost second high acoustic impedance layer 1222 (the layer that was fabricated last and was the uppermost layer in the entire ScAlN-BAW device). In addition, electron diffraction images were also obtained for the piezoelectric layer 11 and the uppermost first low acoustic impedance layer 1211 and first high acoustic impedance layer 1212 of the first acoustic Bragg reflector 121 (the layer closest to the piezoelectric layer 11). The obtained electron diffraction images are shown in Fig. 16A for the first low acoustic impedance layer 1211, Fig. 16B for the first high acoustic impedance layer 1212, Fig. 16C for the piezoelectric layer 11, and Fig. 16D for the second low acoustic impedance layer 1221. Similar diffraction patterns were obtained for all of them, and it can be seen that the layers were obtained as single crystals up to the second high acoustic impedance layer 1222, which was the last layer to be fabricated, at the top.

[0071] Next, the resonance characteristics of the fabricated ScAlN-BAW device and MgZnO-BAW device were measured. A network analyzer (E5071C, manufactured by Keysight Technologies) was used to measure the change in the electrical impedance of the BAW device with frequency. The measurement results are shown in Figure 17A for the ScAlN-BAW device and in Figure 17B for the MgZnO-BAW device. A clear resonance peak was observed in the ScAlN-BAW device. Although not as clear as in the ScAlN-BAW device, a resonance peak was observed around 1.8 GHz in the MgZnO-BAW device.

[0072] For the fabricated ScAlN-BAW device, the resonant frequency f s , anti-resonant frequency f p , electromechanical coupling coefficient k 2 , the Q value calculated from the half-width of the peak (anti-resonance peak) of the real part of the impedance a , and the Q value calculated from the half-width of the peak (resonance peak) of the real part of the admittance r When the value was calculated, f s =1.809GHz, f p =1.826GHz, k 2 = 2.3%, Q a =270, Q r =410.

[0073] The electrical resistance of the fabricated ScAlN-BAW device was measured in the frequency range of 1.6 to 1.7 GHz, where no resonance or antiresonance occurs, and was found to be approximately 0.9 Ω. For comparison, an ScAlN-BAW device (the first acoustic Bragg reflector 121 and piezoelectric layer 11 have the same configuration as the fabricated ScAlN-BAW device and correspond to the configuration of the BAW device described in Non-Patent Document 1) was fabricated in which an electrode consisting of only a single layer of Au with a thickness of approximately 100 nm was provided instead of the second acoustic Bragg reflector 122. The electrical resistance of this device was measured in the frequency range of 1.6 to 1.7 GHz and was found to be approximately 2 Ω. It can be seen that the electrode consisting of the second acoustic Bragg reflector 122 uses Pt (electrical resistivity: 1.06×10 Ωm) and Ti (electrical resistivity: 4.20×10 Ωm), which have higher electrical resistivity than a conventional single-layer Au electrode (electrical resistivity: 2.44×10 Ωm), and is thicker overall, yet has a lower overall electrical resistance than the conventional electrode. This result shows that the ScAlN-BAW device fabricated in this embodiment has a lower electrode electrical resistance than the conventional BAW device described in Non-Patent Document 1, thereby reducing the impedance of the BAW device.

[0074] Although the embodiments of the BAW device according to the present invention have been described above, it goes without saying that the present invention is not limited to the above embodiments and various modifications are possible.

[0075] REFERENCE SIGNS LIST 10 BAW device 11 piezoelectric layer 121 first acoustic Bragg reflector 1211 first low acoustic impedance layer 1212 first high acoustic impedance layer 122 second acoustic Bragg reflector 1221 second low acoustic impedance layer 1222 second high acoustic impedance layer 13 substrate 14 groove 15 area surrounded by groove 211 first terminal 212 second terminal

Claims

1. a) A piezoelectric layer made of a piezoelectric material and having a thickness that is an integer multiple of 1 / 2 of the wavelength corresponding to the resonance frequency; b) a first high acoustic impedance layer made of a metal and having a thickness that is an odd multiple of 1 / 4 of the wavelength corresponding to the resonance frequency; and a first low acoustic impedance layer made of a metal and having a thickness that is an odd multiple of 1 / 4 of the wavelength corresponding to the resonance frequency and having a lower acoustic impedance than the first high acoustic impedance layer, are laminated, and a first acoustic Bragg reflector provided on one surface of the piezoelectric layer; c) a second high acoustic impedance layer made of a metal and having a thickness that is an odd multiple of 1 / 4 of the wavelength corresponding to the resonance frequency; and a second low acoustic impedance layer made of a metal and having a thickness that is an odd multiple of 1 / 4 of the wavelength corresponding to the resonance frequency and having a lower acoustic impedance than the second high acoustic impedance layer, are laminated, and is provided on the other surface of the piezoelectric layer. When the first high acoustic impedance layer is disposed at the position closest to the piezoelectric layer in the first acoustic Bragg reflector, the second high acoustic impedance layer is disposed at the position closest to the piezoelectric layer, and when the first low acoustic impedance layer is disposed at the position closest to the piezoelectric layer in the first acoustic Bragg reflector, the second low acoustic impedance layer is disposed at the position closest to the piezoelectric layer. A bulk acoustic wave device characterized by comprising a second acoustic Bragg reflector.

2. Further, the bulk acoustic wave device according to claim 1, characterized in that a substrate made of an insulator is provided on the surface of the first acoustic Bragg reflector opposite to the side of the piezoelectric layer.

3. The bulk acoustic wave device according to claim 1 or 2, characterized in that the electrical impedance of a capacitor formed by the piezoelectric layer, the first acoustic Bragg reflector, and the second acoustic Bragg reflector is 50 Ω at the resonance frequency.

4. The bulk acoustic wave device according to claim 3, characterized in that the area of the whole of either one of the first acoustic Bragg reflector and the second acoustic Bragg reflector, a part in the thickness direction of the other one, and the piezoelectric layer is smaller than the area of the other one other than the part in the thickness direction.

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