Surface acoustic wave device

Abstract

A SAW device (10) comprises: a piezoelectric body plate (11) that is a plate member made of a piezoelectric body, the piezoelectric body plate (11) including an inclined polarization region (11X) composed of a plurality of small regions (11N) that have substantially the same width in one direction parallel to the surface of the plate member, and being such that, in each of the plurality of small regions, the polarization of the piezoelectric body is inclined in the one direction from a direction perpendicular to the surface of the plate, and the polarization is alternately reversed in adjacent small regions; and a pair of electrodes (12) that cover the inclined polarization region and are provided so as to sandwich the front and rear of the piezoelectric body plate.

Description

Surface Acoustic Wave Device The present invention relates to a surface acoustic wave (SAW) device used in a frequency filter or the like included in a communication device such as a smartphone. An SAW device is formed by providing, on the surface of a substrate made of a piezoelectric material, two comb-shaped electrodes (also referred to as finger electrodes. The English name is interdigital electrode (IDE)), with the teeth (hereinafter simply referred to as teeth) of each comb-shaped electrode alternatingly meshed with each other with a gap therebetween. When an alternating voltage is applied between the two comb-shaped electrodes, a surface acoustic wave that propagates in the direction in which the teeth of the comb-shaped electrodes are arranged is generated near the surface of the substrate. Although several types of surface acoustic waves are known, when the polarization of the piezoelectric material of the substrate is directed in a direction perpendicular to the substrate surface, a resonance of Rayleigh waves, which is a wave formed by combining a component of a longitudinal wave (L wave) that is displaced in a direction parallel to the propagation direction and a component of a shear wave (SV wave) that is displaced in a direction perpendicular to the substrate (i.e., perpendicular to the propagation direction), is formed (see, for example, Patent Document 1). Japanese Patent Application Laid-Open No. 2005-045432 (

[0002] -

[0003] ) Daisuke Kishi et al., "c-Axis Inclined Epitaxial Growth of ScAlN and MgZnO Thin Films Using an Off-Angle Sapphire Substrate and Shear Piezoelectric Characteristics", Proceedings of the Symposium on Piezoelectric Materials and Devices 2023, published by the Organizing Committee of the Symposium on Piezoelectric Materials and Devices, January 26, 2023, pp. 177-182 The resonance frequency of an SAW device is inversely proportional to the interval between adjacent teeth of both comb-shaped electrodes (the spatial period for arranging the teeth). Therefore, in order to increase the resonance frequency of an SAW device, it is necessary to shorten the arrangement interval of the teeth, and accordingly, it is also necessary to narrow the width of each tooth. However, when the width of the tooth is narrowed, the breakdown voltage resistance and mechanical strength of the tooth decrease. Therefore, it is difficult for a conventional SAW device using comb-shaped electrodes to increase the resonance frequency. The problem to be solved by the present invention is to provide an SAW device capable of achieving a higher resonance frequency than a conventional SAW device using comb-shaped electrodes. The SAW device according to the present invention made to solve the above problems is a) A piezoelectric plate including an inclined polarization region composed of a plurality of small regions having substantially the same width in one direction parallel to the surface of the plate, wherein in each of the plurality of small regions, the polarization of the piezoelectric body is inclined from a direction perpendicular to the surface of the plate to the one direction, and the polarization is alternately inverted in adjacent small regions; b) A pair of electrodes covering the inclined polarization region and provided so as to sandwich the front and back of the piezoelectric plate; Characterized by comprising the above. In the SAW device according to the present invention, when an AC voltage having a predetermined frequency is applied between a pair of electrodes, since the polarization of each small region is inclined from a direction perpendicular to the surface of the piezoelectric plate to the one direction (the direction of polarization of this small region is referred to as the polarization direction), in the piezoelectric plate, a vibration combining a compressive expansion displacement in a direction parallel to the one direction (parallel to the surface) and a shear displacement in a direction perpendicular to the surface (i.e., perpendicular to the one direction) is generated. Further, since a plurality of small regions in which the polarization direction is alternately inverted in the one direction are formed, the vibrations generated in the piezoelectric plate are in antiphase with each other between adjacent small regions. Due to these vibrations, near the surface of the piezoelectric plate, a Rayleigh wave is formed, which is a wave in which a component of a longitudinal wave (L wave) propagating in the one direction and displacing in a direction parallel to the propagation direction and a component of a shear wave (SV wave) displacing in a direction perpendicular to the surface of the piezoelectric plate (i.e., perpendicular to the propagation direction) are combined. Thereby, a surface acoustic wave is excited on the surface of the piezoelectric plate. Since the SAW device according to the present invention has the above configuration, when an AC voltage having a frequency substantially matching the frequency corresponding to a Rayleigh wave having a half-wavelength equal to the width of the small region (this frequency depends on the speed of sound in the piezoelectric body) is applied between the electrodes, resonance of the Rayleigh wave is formed in the inclined polarization region. In the SAW device according to the present invention, the electrodes for applying a voltage for exciting vibrations in the piezoelectric plate are provided so as to cover the entire inclined polarization region that combines a plurality of small regions. Therefore, the problem of the reduction in the dielectric strength and mechanical strength of the teeth (electrodes) that occurred when trying to increase the resonance frequency in a SAW device using conventional comb-shaped electrodes does not occur in the SAW device according to the present invention. That is, the SAW device according to the present invention can be used at a high resonance frequency. Furthermore, in the SAW device according to the present invention, a reflecting portion for reflecting surface acoustic waves can be provided outside the inclined polarization region in the one direction. Thereby, it becomes easier to confine the surface acoustic waves within the inclined polarization region, and the intensity of the resonance of the Rayleigh waves can be increased. Note that the reflecting portion may be provided only on one side of the outside in the one direction, but it is desirable to provide it on both sides in order to more easily confine the surface acoustic waves. In the SAW device according to the present invention, the piezoelectric plate has two inclined polarization regions, and the electrodes are provided in pairs for each of the two inclined polarization regions. It is possible to adopt such a configuration. The SAW device having two inclined polarization regions in this way inputs a signal having an alternating voltage with various frequencies superimposed between the electrodes provided in one inclined polarization region, and outputs from between the electrodes provided in the other inclined polarization region an alternating current signal having a frequency corresponding to a Rayleigh wave having a width of a small region related to the one direction as a half wavelength. It can be used as a frequency filter. According to the present invention, it is possible to obtain a SAW device that can have a higher resonance frequency than a conventional SAW device using comb-shaped electrodes. Longitudinal cross-sectional view showing the first embodiment of the SAW device according to the present invention. Top view showing the SAW device of the first embodiment. Diagram showing the state where a substrate is prepared in the manufacturing method of the SAW device of the first embodiment. Diagram showing the process of fabricating a piezoelectric plate in the manufacturing method of the SAW device of the first embodiment. Diagram showing the state where a piezoelectric plate is fabricated in the manufacturing method of the SAW device of the first embodiment. Diagram showing the state where a polarization forming electrode is fabricated in the manufacturing method of the SAW device of the first embodiment. Diagram showing the process of inverting the polarization of the piezoelectric body in a small region directly below the polarization forming electrode in the manufacturing method of the SAW device of the first embodiment. Diagram showing the state where the polarization forming electrode is removed in the manufacturing method of the SAW device of the first embodiment. Diagram showing the state where the first electrode is fabricated in the manufacturing method of the SAW device of the first embodiment. Diagram showing the state where a reflection portion is fabricated in the manufacturing method of the SAW device of the first embodiment. Diagram schematically showing the operation of the SAW device of the first embodiment. Diagram schematically showing the operation of a conventional SAW device. Schematic diagram showing an example of a frequency filter formed by connecting a plurality of SAW devices of the first embodiment. A SAW device of the first embodiment including a piezoelectric plate made of PbTiO3 was fabricated, and the S 11 Graph showing the values of the parameters. For the SAW device of the first embodiment, S for each frequency obtained by simulation 11 Graph showing the values of the parameters. Graph showing the absolute value of the impedance for each frequency obtained by simulation for a conventional SAW device. In the SAW device of the first embodiment, the wavelength λ of the surface acoustic wave, the thickness h of the piezoelectric plate, and the electromechanical coupling coefficient K 2 Graph showing the results obtained by simulation of the relationship. For the SAW device of the first embodiment, a graph showing the absolute value of the impedance for each frequency obtained by simulation in an example where the conditions of the piezoelectric plate are different from those in FIG. 8. For a conventional SAW device, a graph showing the absolute value of the impedance for each frequency obtained by simulation in an example where the conditions of the piezoelectric plate are different from those in FIG. 9. Sc 0.4 Al 0.6A graph showing the absolute value of impedance for each frequency measured with a network analyzer, for a SAW device of the first embodiment including a piezoelectric plate made of N with a width of the small region of 4.5 μm. Sc 0.4 Al 0.6 A graph showing the absolute value of impedance for each frequency measured with a network analyzer, for a SAW device of the first embodiment including a piezoelectric plate made of N with a width of the small region of 3.6 μm. A longitudinal sectional view showing a second embodiment of the SAW device according to the present invention. A top view showing the SAW device of the second embodiment. Embodiments of the SAW device according to the present invention will be described with reference to FIGS. 1 to 16. (1) First Embodiment (1-1) Configuration of the SAW Device of the First Embodiment The configurations of the SAW device 10 of the first embodiment are shown in FIGS. 1 and 2. This SAW device 10 has a piezoelectric plate 11, an electrode 12, and a reflection part 13. The piezoelectric plate 11 is a plate made of a piezoelectric material. A plurality of virtual small regions 111, 112, 113, 114... having substantially the same width in one direction (the left-right direction in FIGS. 1 and 2) are formed side by side in the one direction on the piezoelectric plate 11. Hereinafter, these plurality of small regions are collectively referred to as "small region 11N". The width of the small region 11N in the one direction is set to be 1 / 2 (half wavelength) of the wavelength of the surface acoustic wave generated in the SAW device 10 of the present embodiment. Here, the wavelength of the surface acoustic wave is determined by the speed of sound of the acoustic vibration in the piezoelectric plate 11. In addition, although the width in the direction perpendicular to the one direction of each small region 11N is the same size in the present embodiment, it may be different for each small region 11N. Among the small regions 11N, the small regions with odd symbols (small regions 111, 113, 115...) are called "small region 11N1", and the small regions with even symbols (small regions 112, 114, 116...) are called "small region 11N2". Further, the entire region in which these plurality of small regions 11N are formed is called "tilted polarization region 11X". In any of the small regions 11N, the polarization P of the piezoelectric body is inclined by substantially the same inclination angle θ in the one direction from the direction perpendicular to the surface of the plate material of the piezoelectric plate 11. Further, the polarizations P are oriented in directions that are 180° different from each other (diagonally upward or diagonally downward) between the small region 11N1 and the small region 11N2. That is, each small region 11N is formed such that the polarization P alternates and reverses in the one direction. In FIG. 1, the polarization P that is diagonally upward is formed in the small region 11N1, and the polarization P that is diagonally downward is formed in the small region 11N2, but the direction of the polarization P may be the opposite (diagonally downward in the small region 11N1 and diagonally upward in the small region 11N2). The number of the small regions 11N1 and the small regions 11N2 is arbitrary, and at least one of each is sufficient. As the piezoelectric body that is the material of the piezoelectric plate 11, any piezoelectric body can be used as long as the piezoelectric plate 11 can be manufactured such that the polarization P inclined from the direction perpendicular to the surface of the plate material is formed as described above. For example, PbTiO3 (lead titanate), LiNbO3 (lithium niobate), LiTaO3 (lithium tantalate), ZnO (zinc oxide), Mg 1-x Zn x ZnO (0 < x < 1) (magnesium zinc oxide), AlN (aluminum nitride), Sc x Al 1-x AlN (0 < x < 1) (scandium aluminum nitride), etc. can be used. Among these piezoelectric bodies, in particular, LiNbO3, LiTaO3, and Sc x Al 1-x AlN are suitable in that they can be polarization-reversed and have a high electromechanical coupling coefficient and Q value. Each of the electrodes 12 is composed of a plate-shaped first electrode 121 and a second electrode 122, and the first electrode 121 and the second electrode 122 are provided so as to cover the inclined polarization region 11X and sandwich the piezoelectric plate 11. An AC power source 14 for applying an AC voltage between them is connected to the first electrode 121 and the second electrode 122 (the AC power source 14 is not included in the SAW device 10). In the example shown in FIG. 1, both the first electrode 121 and the second electrode 122 are provided only at positions facing the inclined polarization region 11X. However, one of the first electrode 121 and the second electrode 122 may be provided over a range wider than the inclined polarization region 11X. For example, the second electrode 122 may be provided over the entire surface of the piezoelectric plate 11. In that case, a conductive plate material serving as the second electrode 122 is used as a substrate, and the piezoelectric plate 11 and the first electrode 121 are sequentially formed on the substrate, whereby the SAW device 10 can be manufactured. The reflection portions 13 are provided one by one on both outer sides (the left side and the right side in FIGS. 1 and 2) of the inclined polarization region 11X with respect to the one direction on one surface of the piezoelectric plate 11. Each reflection portion 13 is formed by arranging a plurality of rod-shaped members extending in a direction parallel to the surface of the piezoelectric plate 11 and perpendicular to the one direction (the direction perpendicular to the paper surface in FIG. 1 and the vertical direction in FIG. 2) at substantially equal intervals. The interval for arranging the rod-shaped members is set to be an integral multiple of the half wavelength of the surface acoustic wave generated in the SAW device 10 of the present embodiment. (1-2) Manufacturing method of the SAW device according to the first embodiment With reference to FIG. 3, an example of a method for manufacturing the SAW device 10 according to the first embodiment will be described. In this example, PbTiO3 is used as the material of the piezoelectric plate 11, and the piezoelectric plate 11 is manufactured using the method described in Non-Patent Document 1. Further, the second electrode 122 is provided over the entire surface of the piezoelectric plate 11. First, a substrate made of a single crystal of Sr 1-x La x TiO3 (a material obtained by doping carriers by substituting a part of Sr in SrTiO3 with La) is prepared (FIG. 3A). At that time, the substrate is cut out from the single crystal so that the c-axis is inclined in the one direction from a direction perpendicular to the surface of the substrate (for example, at an angle within the range of 10 to 80°). This substrate becomes the second electrode 122 after the completion of the SAW device 10. Next, the piezoelectric plate 11 is manufactured (FIG. 3C) by epitaxially growing PbTiO3 by causing sputter particles SP to be incident on the upper surface of the second electrode (substrate) 122 using a magnetron sputtering apparatus (FIG. 3B). PbTiO3 is Sr 1-x La xSince the piezoelectric plate 11 has the same structure (perovskite structure) as TiO3, the c-axis of the piezoelectric plate 11 epitaxially grows so as to face the same direction as the c-axis of Sr 1-x La x TiO3. As a result, the piezoelectric plate 11 is formed such that the c-axis is inclined in the one direction from the direction perpendicular to its surface. At this time, the direction of the polarization P is obliquely upward as a whole within the piezoelectric plate 11 (from the second electrode 122 side toward the opposite side). Next, by depositing a conductive material on the surface of the piezoelectric plate 11 by sputtering, a plurality of polarization formation electrodes 31 having the same width W as the small region 11N in the one direction are formed at a period 2W that is twice the width W (FIG. 3D). Note that this polarization formation electrode 31 is not the first electrode 121 after the completion of the SAW device 10, but is used only for the operation of polarization formation as described below, and thus does not require the mechanical strength as high as that of the comb-shaped electrode in a conventional SAW device. Further, the polarization formation electrode 31 may be formed by another method such as vapor deposition. Next, a DC voltage having a strength such that the side of the polarization formation electrode 31 is positive and an electric field larger than the coercive field is formed in the piezoelectric plate 11 is applied between each polarization formation electrode 31 and the second electrode (substrate) 122 (FIG. 3E). Thereby, the polarization P is inverted and obliquely downward directly below the polarization formation electrode 31 in the piezoelectric plate 11. On the other hand, in the portion between the polarization formation electrodes 31, the polarization P of the piezoelectric plate 11 directly below does not invert. As a result, small regions 11N1 in which the polarization P is obliquely upward and small regions 11N2 in which the polarization P is obliquely downward are formed to be alternately arranged in the one direction within the piezoelectric plate 11. After that, the polarization formation electrode 31 is removed (FIG. 3F) by a method such as chemical etching. Then, a conductive material is deposited by sputtering (or other methods such as vapor deposition) over the entire inclined polarization region 11X, which is the region on the upper surface of the piezoelectric plate 11 where the small regions 11N1 and 11N2 are formed, to fabricate the first electrode 121 (FIG. 3G). Also, the reflective portion 13 is formed by the same sputtering method (as above) (FIG. 3H). Note that since the conductivity of the material of the reflective portion 13 is not required, when the same material as that of the first electrode 121 is used for the material of the reflective portion 13, the first electrode 121 and the reflective portion 13 may be fabricated simultaneously. Through the above steps, the SAW device 10 is completed. In the case of fabricating the SAW device 10 in which the second electrode 122 is formed only in the portion corresponding to the inclined polarization region 11X, among the surfaces of a single crystal plate made of an insulator, where the c-axis is inclined by 1 degree from the direction perpendicular to the surface of the plate, the second electrode 122 is fabricated by depositing a conductive material within the range of the shape corresponding to the inclined polarization region 11X. Then, the piezoelectric plate 11, the polarization formation electrode 31, the inversion of polarization in the small region 11N2, the removal of the polarization formation electrode 31, and the fabrication of the first electrode 121 may be performed in the same manner as above. In this case, in order to make the inclination of the c-axis of the substrate easily reflected in the inclination of the c-axis of the piezoelectric plate 11, it is desirable that the second electrode 122 be as thin as possible. The method of fabricating the piezoelectric plate 11 such that the c-axis is inclined by the 1 degree from the direction perpendicular to the surface is not limited to the above method. For example, when fabricating the piezoelectric plate 11 made of ZnO, Mg 1-x Zn x O, AlN, Sc x Al 1-x N, etc., the target piezoelectric plate 11 can be fabricated by causing sputtered particles to enter from a direction inclined by 1 degree from the direction perpendicular to the surface of the substrate (the c-axis of the substrate does not need to be inclined and further, the substrate does not need to be a single crystal). When fabricating the piezoelectric plate 11 by this method, the second electrode 122 may cover the entire substrate or may be formed only in the portion corresponding to the inclined polarization region 11X of the substrate made of an insulator. (1-3) Operation of the SAW device of the first embodiment While referring to FIG. 4, the operation of the SAW device 10 of the first embodiment will be described in conjunction with the operation of a SAW device using a conventional comb-shaped electrode. FIG. 4 is a diagram schematically showing the operation of the SAW device 10 of the present embodiment. For reference, FIG. 5 schematically shows the operation of a SAW device 90 using a conventional comb-shaped electrode. In the conventional SAW device 90, on one surface of a piezoelectric plate 91 in which an upward polarization P is formed throughout, a first comb-shaped electrode 921 and a second comb-shaped electrode 922 are alternately provided so as to mesh with each other with a gap therebetween. First, while referring to FIG. 5, the operation of the conventional SAW device 90 will be described. Generally, in a piezoelectric plate made of a piezoelectric material plate with polarization perpendicular to the plate, (i) vertical compression and expansion displacement due to an alternating electric field in the direction perpendicular to the piezoelectric plate (vertical direction), (ii) compression and expansion displacement in the direction parallel to the piezoelectric plate (parallel direction) due to an alternating electric field in the vertical direction, and (iii) shear displacement in the vertical direction due to an alternating electric field in the parallel direction (displacement where the surface in the parallel direction slides in the vertical direction) can occur. In the conventional SAW device 90, as described below, displacements (ii) and (iii) among these are utilized. In the conventional SAW device 90, when an alternating voltage is applied between the first comb-shaped electrode 921 and the second comb-shaped electrode 922, an alternating electric field having a strong component in the direction perpendicular to the piezoelectric plate 91 is formed in the piezoelectric plate 91 in the vicinity directly below the first comb-shaped electrode 921 and the second comb-shaped electrode 922, whereby a compression and expansion displacement in the direction parallel to the piezoelectric plate 91 occurs. Also, in the vicinity directly below between the first comb-shaped electrode 921 and the second comb-shaped electrode 922, an alternating electric field having a strong component in the direction parallel to the piezoelectric plate 91 is formed in the piezoelectric plate 91, whereby a shear displacement in the direction perpendicular to the piezoelectric plate 91 is formed. Due to these displacements, a resonance of a Rayleigh wave, which is a wave propagating in the direction parallel to the piezoelectric plate 91 and is a wave in which a component of a shear wave (SV wave) that is displaced in the direction perpendicular to the propagation direction and a component of a longitudinal wave (L wave) that is displaced in the direction parallel to the propagation direction are combined, is formed. Thereby, a surface acoustic wave is excited on the surface of the piezoelectric plate 91. In response to the operation of the conventional SAW device 90, the SAW device 10 of the present embodiment operates as follows (see FIG. 4). In the SAW device 10 of the present embodiment, when an alternating voltage is applied between the first electrode 121 and the second electrode 122, an alternating electric field in a direction perpendicular to the piezoelectric plate 91 is formed across the entire inclined polarization region 11X in the piezoelectric plate 11. Since the polarization P in the piezoelectric plate 11 is inclined in the one direction from a direction perpendicular to the piezoelectric plate 11, a component P of the polarization P in a direction perpendicular to the piezoelectric plate 11 ｚ and a compressive expansion displacement in a direction parallel to the piezoelectric plate 11 are generated by the alternating electric field, and a component P of the polarization P in the one direction (a direction parallel to the piezoelectric plate 11) ｘ and a slip displacement in a direction perpendicular to the piezoelectric plate 11 are formed by the alternating electric field. Due to these displacements, a resonance of a Rayleigh wave, which is a wave propagating in the one direction and is a combined wave of a component of a longitudinal wave (L wave) that is displaced in a direction parallel to the one direction (i.e., the propagation direction) and a component of a shear wave (SV wave) that is displaced in a direction perpendicular to the piezoelectric plate 11 (i.e., perpendicular to the propagation direction), is formed. As a result, a surface acoustic wave is excited on the surface of the piezoelectric plate 11. In the SAW device 90 using the conventional comb-shaped electrodes 911 and 912, in order to increase the resonance frequency, it is necessary to shorten the arrangement interval of the teeth, and accordingly, it is also necessary to narrow the width of each tooth. Therefore, there is a problem that the breakdown voltage resistance and mechanical strength of the teeth (electrodes) are reduced. On the other hand, according to the SAW device 10 of the present embodiment, since the first electrode 121 and the second electrode 122 may be provided so as to cover the entire inclined polarization region 11X that combines a plurality of small regions 11N, the problem of reduction in the breakdown voltage resistance and mechanical strength of the electrodes does not occur. Therefore, the SAW device 10 of the present embodiment can be used at a high resonance frequency. This SAW device 10 functions as a resonator alone, but by connecting a plurality of them in a "ladder type" as shown in FIG. 6 by a known method related to resonators, it can be used as a frequency filter (band filter) 50 that passes a signal current within a frequency band having a certain width. (1-4) Experimental and calculation results regarding the SAW device of the first embodiment Next, the results of fabricating and experimenting with the SAW device 10 of the first embodiment, as well as the results of simulation, will be described. In this experiment, a SAW device 10 including a piezoelectric plate 11 made of PbTiO3 with a c-axis tilt angle θ of 35° and a thickness of 500 nm was fabricated by the above-described manufacturing method. The width of the small region 11N in the one direction was set to 10 μm. For the fabricated SAW device 10, S 11 parameters were obtained for each frequency using a network analyzer. The S 11 parameter is a parameter indicating the reflection characteristics on the input side, and indicates that resonance occurs in the SAW device 10 at the frequency at which the frequency change of this value becomes a minimum value. In the simulation, the finite element method was used, and the material of the piezoelectric plate 11, the c-axis tilt angle θ, and the width of the small region 11N were set to the same values as those of the SAW device 10 fabricated in the experiment, and the thickness of the piezoelectric plate 11 was set to 500 nm. Fig. 7 shows the S 11 parameters obtained in the experiment, and Fig. 8 shows the S 11 parameters obtained in the simulation. In the experiment, at a frequency near 200 MHz (indicated by a thick arrow in Fig. 7), the S 11 parameter takes a slightly minimum value. Also, when the frequency range near 200 MHz is enlarged in the simulation (the inset in Fig. 8), a clear minimum value is obtained at 203 MHz. Thus, a resonance at a frequency of about 200 MHz was confirmed without contradiction between the experiment and the simulation. Note that the minimum values observed at around 330 MHz in the experiment and at 370 MHz in the simulation are considered not to be due to surface acoustic waves because when the width (period) of the small region 11N is halved in the simulation, the minimum value appears at around 650 MHz (not around 740 MHz which is twice the frequency). For comparison, for the SAW device 90 using a conventional comb-shaped electrode, the absolute value of the impedance for each frequency was obtained by simulation. In this simulation, in accordance with the parameters in the SAW device 10 of the present embodiment, a piezoelectric plate 91 made of PbTiO3 with a thickness of 500 nm and a c-axis inclined by 35° from the direction perpendicular to the piezoelectric plate was used, and the distance between the adjacent first comb-shaped electrode 921 and the second comb-shaped electrode 922 was set to 10 μm. Note that this distance corresponds to the width of the small region 11N in the SAW device 10 of the present embodiment. The results obtained from this simulation are shown in FIG. 9. At a frequency of about 200 MHz, a signal indicating resonance due to an elastic surface wave can be seen. By comparing this result with the result obtained with the SAW device 10, it was confirmed that, similar to the conventional SAW device 90, resonance due to an elastic surface wave at a frequency of about 200 MHz is formed in the SAW device 10 of the present embodiment. In the experimental results shown in FIG. 7, the signal of resonance due to the elastic surface wave is small, but this is considered to be because the electromechanical coupling coefficient K in the thickness direction of the piezoelectric plate 11 in the SAW device 10 fabricated in the experiment 2 is small. In the configuration of the SAW device 10 of the present embodiment, when the relationship between the wavelength of the elastic surface wave to be formed (twice the width of the small region 11N in the one direction), the thickness h of the piezoelectric plate 11, and the electromechanical coupling coefficient K 2 was simulated using the finite element method, a graph as shown in FIG. 10 was obtained with the horizontal axis being h / λ and the vertical axis being K 2 . According to this graph, when h / λ = 0.4375, K 2 takes a maximum value of 10.99%. On the other hand, in the SAW device 10 fabricated in this experiment, h / λ = 0.025, and when this value is applied to the graph of FIG. 10, K 2 is 0.1% or less, which is 1 / 100 or less of the above maximum value. Therefore, it is considered that a stronger resonance than that obtained in this experiment can be obtained by appropriately determining the thickness h of the piezoelectric plate 11 according to the wavelength λ of the elastic surface wave to be formed so that the electromechanical coupling coefficient K 2 approaches the maximum value. Therefore, further for the SAW device 10 of the present embodiment, the electromechanical coupling coefficient K 2 was simulated by the finite element method under the condition that it is close to the above maximum value. In this simulation, the thickness of the piezoelectric plate 11 was set to 1 μm (1000 nm). Further, the width of the small region 11N in the one direction was set to 1 μm so that the wavelength λ of the surface elastic wave was 2 μm. The material of the piezoelectric plate 11 and the inclination angle θ of the c-axis were the same as those in the previous example (PbTiO3, θ = 35°). The value of h / λ under this condition is 0.5, and the electromechanical coupling coefficient K 2 is 10.9% (at a frequency of 1000 MHz). When the absolute value of the impedance for each frequency was obtained under the above conditions, as shown in FIG. 11, a clear peak was observed near the frequency of 1000 MHz. Although a peak is also observed near the frequency of 2000 MHz, this is considered to be due to the harmonic wave when the resonance at the frequency of 1000 MHz is used as the fundamental wave. For comparison, in the SAW device 90 using the conventional comb-shaped electrodes, the same conditions as those for the piezoelectric plate 11 were applied to the piezoelectric plate 91, and the distance between the adjacent first comb-shaped electrode 921 and the second comb-shaped electrode 922 was set to 1 μm so that the wavelength λ of the surface elastic wave was 2 μm. As a result, as shown in FIG. 12, a resonance peak due to the fundamental wave surface elastic wave is observed near the frequency of 1000 MHz, and a resonance peak due to the harmonic wave surface elastic wave is observed near the frequency of 2000 MHz. Since the results shown in FIG. 11 and the results shown in FIG. 12 correspond well in this way, it can be said that in the SAW device 10 of the present embodiment in this simulation, resonance due to the surface elastic wave at a frequency of about 1000 MHz is formed, similar to the conventional SAW device 90. When the comb-shaped electrodes in which the adjacent first comb-shaped electrode 921 and the second comb-shaped electrode 922 are arranged at a narrow interval of 1 μm in the conventional SAW device 90 as in this simulation are actually manufactured, the width of the teeth of those comb-shaped electrodes must be less than 1 μm, and the breakdown voltage resistance and mechanical strength of the teeth will decrease. On the other hand, in the SAW device 10 of the present embodiment, since it is not necessary to use comb-shaped electrodes, it is possible to prevent the breakdown voltage resistance and mechanical strength of the electrodes from decreasing. Next, the results of experiments conducted by fabricating a SAW device 10 including a piezoelectric plate 11 composed of Sc x Al 1-x N will be described. In this experiment, x = 0.4, the thickness h was 1.8 μm, and a piezoelectric plate 11 was fabricated in which the c-axis (and polarization P) was inclined by 1° in one direction from the direction perpendicular to the piezoelectric plate 11. Moreover, two types of SAW devices 10 were fabricated in which the polarization P was alternately inverted so that the widths of the small regions 111, 112,... in the piezoelectric plate 11 were 4.5 μm (the wavelength λ of the elastic surface wave to be formed was 9.0 μm, h / λ = 0.20) and 3.6 μm (λ = 7.2 μm, h / λ = 0.25). For these two types of SAW devices 10, the results of measuring the absolute value of the impedance for each frequency with a network analyzer are shown in FIG. 13 for the example where the width of the small region is 4.5 μm and in FIG. 14 for the example where the width of the small region is 3.6 μm, respectively. In the former, resonance of the elastic surface wave is observed at 600 MHz, and in the latter, at 690 MHz. From these resonance frequencies and the wavelength λ, the sound velocity of the elastic surface wave is determined to be 5400 m / s for both the former and the latter. Also, the electromechanical coupling coefficient K 2 was found to be as high as 8.7% for the former and 6.0% for the latter. (2) SAW device of the second embodiment (2-1) Configuration of the SAW device of the second embodiment The SAW device 20 of the second embodiment will be described with reference to FIGS. 15 and 16. This SAW device 20 includes a piezoelectric plate 21, an input-side electrode 221, and an output-side electrode 222. Note that the configuration corresponding to the reflection portion 13 included in the SAW device 10 of the first embodiment is not included in the SAW device 20 of the second embodiment. The piezoelectric plate 21 is a plate made of a piezoelectric material. In the piezoelectric plate 21, an input-side inclined polarization region 211X is provided, in which a plurality of virtual input-side small regions 2111, 2112, 2113, 2114... (collectively referred to as "input-side small regions 211N") having substantially the same width in one direction (the left-right direction in FIGS. 15 and 16) are formed. Further, in the piezoelectric plate 21, an output-side inclined polarization region 212X is provided, in which a plurality of virtual output-side small regions 2121, 2122, 2123, 2124... (collectively referred to as "output-side small regions 212N") having substantially the same width as the input-side small regions 211N in the one direction are formed, and the output-side inclined polarization region 212X is provided at a distance from the input-side inclined polarization region 211X in the one direction. In both the input-side small regions 211N and the output-side small regions 212N, the polarization P of the piezoelectric material is inclined by substantially the same inclination angle θ in the one direction from the direction perpendicular to the surface of the plate material of the piezoelectric plate 21. Between the input-side small regions 211N1 and the output-side small regions 212N1 with odd signs (signs 2111, 2113... and 2121, 2123...) and the input-side small regions 211N2 and the output-side small regions 212N2 with even signs (signs 2112, 2114... and 2122, 2124...) among the input-side small regions 211N and the output-side small regions 212N, the polarization P faces in directions that are 180° different from each other (diagonally up or diagonally down). The input-side electrodes 221 are all composed of plate-shaped first input-side electrodes 2211 and second input-side electrodes 2212, and the first input-side electrodes 2211 and the second input-side electrodes 2212 are provided so as to cover the input-side inclined polarization region 211X and sandwich the piezoelectric plate 21. Similarly, the output-side electrodes 222 are all composed of plate-shaped first output-side electrodes 2221 and second output-side electrodes 2222, and the first output-side electrodes 2221 and the second output-side electrodes 2222 are provided so as to cover the output-side inclined polarization region 212X and sandwich the piezoelectric plate 21. (2-2) Manufacturing method of the SAW device of the second embodiment The SAW device 20 of the second embodiment can be manufactured in the same manner as in the case of "manufacturing the SAW device 10 in which the second electrode 122 is formed only in the portion corresponding to the inclined polarization region 11X in the first embodiment", except for the positions where the inclined polarization regions (the input-side inclined polarization region 211X and the output-side inclined polarization region 212X) and the electrodes (the input-side electrode 221 and the output-side electrode 222) are formed. (2-3) Operation of the SAW device of the second embodiment When using the SAW device 20 of the second embodiment, an alternating voltage in which a plurality of frequencies are mixed is applied between the first input-side electrode 2211 and the second input-side electrode 2212. Then, among those plurality of frequencies (wavelengths), resonance of Rayleigh waves having a length corresponding to the width in the one direction of the input-side small region 211N as the half wavelength is formed within the input-side inclined polarization region 211X. Here, the principle by which the resonance of Rayleigh waves is formed is the same as in the case of the SAW device 10 of the first embodiment. The surface acoustic wave composed of Rayleigh waves formed within the input-side inclined polarization region 211X propagates near the surface of the piezoelectric plate 21 and reaches the output-side inclined polarization region 212X. In the output-side inclined polarization region 212X, due to the resonance of the piezoelectric plate 21 having a length corresponding to the width in the one direction of the output-side small region 212N as the half wavelength, an alternating voltage having a frequency corresponding to the frequency of the resonance is generated between the first output-side electrode 2221 and the second output-side electrode 2222. This alternating voltage is taken out from the output terminals provided on the first output-side electrode 2221 and the second output-side electrode 2222. By the above operation, the SAW device 20 of the second embodiment functions as a frequency filter that extracts a signal of an alternating voltage of a specific frequency from a signal of an alternating voltage in which a plurality of frequencies mixed in from the input-side electrode 221. Such a frequency filter is called a transverse type frequency filter. According to the SAW device 20 of the second embodiment, similar to the case of the first embodiment, the input-side electrode 221 and the output-side electrode 222 may be provided so as to cover the entire inclined polarization regions (input-side inclined polarization region 211X and output-side inclined polarization region 212X) that combine a plurality of small regions (input-side small regions 211N and output-side small regions 212N). Therefore, problems such as a decrease in the breakdown voltage resistance and mechanical strength of the electrodes do not occur. Therefore, the SAW device 20 of the second embodiment can be used at a high resonance frequency. Needless to say, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the present invention. 10, 20, 90... SAW device 11, 21, 91... Piezoelectric plate 111, 112, 113, 114, 115, 116, 11N1, 11N2, 11N... Small region 11X... Inclined polarization region 12... Electrode 121... First electrode 122... Second electrode 13... Reflecting portion 14... AC power supply 21... Piezoelectric plate 2111, 2112, 2113, 2114, 211N1, 211N2, 211N... Input-side small region 211X... Input-side inclined polarization region 2121, 2122, 2123, 2124, 212N1, 212N2, 212N... Output-side small region 212X... Output-side inclined polarization region 221... Input-side electrode 2211... First input-side electrode 2212... Second input-side electrode 222... Output-side electrode 2221... First output-side electrode 2222... Second output-side electrode 31... Electrode for polarization formation 50... Frequency filter (band filter) 911... Comb-shaped electrode 921... First comb-shaped electrode 922... Second comb-shaped electrode P... Polarization Px... Component in one direction parallel to the polarized piezoelectric plate Pz... Component in the direction perpendicular to the polarized piezoelectric plate SP... Sputtered particles W... Width of the small region in the one direction θ... Angle of inclination

Claims

1. a) A piezoelectric plate comprising an inclined polarization region composed of a plurality of small regions having substantially the same width in one direction parallel to the surface of the plate, wherein in each of the plurality of small regions, the polarization of the piezoelectric body is inclined from a direction perpendicular to the surface of the plate to the one direction, and the polarization is alternately reversed in adjacent small regions; and b) A pair of electrodes provided so as to cover the inclined polarization region and sandwich the front and back of the piezoelectric plate, the surface acoustic wave device being characterized by comprising the same.

2. Further, the surface acoustic wave device according to claim 1, characterized by comprising a reflection portion for reflecting a surface acoustic wave outside the one direction of the inclined polarization region.

3. The surface acoustic wave device according to claim 1, characterized in that the piezoelectric plate has two inclined polarization regions, and the electrodes are provided in pairs for each of the two inclined polarization regions.

4. The piezoelectric material is PbTiO3, LiNbO3, LiTaO3, ZnO, Mg 1-x Zn x O (0 <x<1)、AlN及びSc x Al 1-x N (0 <x<1)のうちの1つであることを特徴とする請求項1~3のいずれかに記載の表面弾性波デバイス。

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