Surface acoustic wave device

Abstract

A SAW device (10) has: a piezoelectric body plate (11) which is a plate material including a piezoelectric body, and includes a tilt polarization region (11X) including a plurality of small regions (11N) having substantially the same width in one direction parallel to a surface of the plate material, at each of the plurality of small regions, polarization of the piezoelectric body is tilted from a direction perpendicular to the surface of the plate material toward the one direction, and, at adjacent small regions, the polarization is alternately reversed; and a pair of electrodes (12) provided to cover the tilt polarization region, and sandwich a surface back of the piezoelectric body plate.

Description

Technical Field

[0001] This invention relates to surface acoustic wave (SAW) devices used in frequency filters and other components of communication devices such as smartphones. Background Technology

[0002] A surface acoustic wave (SAW) device is constructed by interlocking the teeth (hereinafter simply referred to as teeth) of two comb-shaped electrodes (also called interdigital electrodes, or IDEs) with gaps between them on the surface of a substrate including a piezoelectric element. When an alternating voltage is applied between the two comb-shaped electrodes, a surface acoustic wave propagating in the direction of the tooth arrangement of the comb-shaped electrodes is generated near the surface of the substrate. Several types of surface acoustic waves are known, but when the polarization of the piezoelectric element of the substrate is oriented in a direction perpendicular to the surface of the substrate, a Rayleigh wave resonance is formed. This Rayleigh wave is a wave formed by coupling a component of a longitudinal wave (L-wave) displaced in a direction parallel to the propagation direction with a component of a transverse wave (SV-wave) displaced in a direction perpendicular to the substrate (i.e., perpendicular to the propagation direction) (see, for example, Patent Document 1).

[0003] Prior art literature

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2005-045432 (

[0002] -

[0003] )

[0006] Non-patent literature

[0007] Non-patent document 1: Daiki Kishi et al., "C-axis tilt growth and transverse wave pressure of ScAlN MgZnO thin film used in AFC substrates" "Electrical Properties", Proceedings of Lectures at the Piezoelectric Materials / Devices Symposium 2023, published by the Piezoelectric Materials / Devices Symposium Operating Committee, January 26, 2023, pp. 177-182 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] The resonant frequency of a SAW device is inversely proportional to the spacing between adjacent teeth of the two comb-shaped electrodes (the spatial period of the teeth). Therefore, to increase the resonant frequency of a SAW device, the spacing between the teeth needs to be shortened, which in turn requires narrowing the width of each tooth. However, narrowing the tooth width reduces the power handling capacity and mechanical strength of the teeth. Consequently, conventional SAW devices using comb-shaped electrodes have struggled to increase their resonant frequency.

[0010] The problem to be solved by the present invention is to provide a SAW device that can improve the resonant frequency compared with conventional SAW devices that use comb-shaped electrodes.

[0011] Solution for solving the problem

[0012] The SAW device of the present invention, which was developed to solve the above-mentioned problems, is characterized in that...

[0013] The SAW device includes:

[0014] a) A piezoelectric plate comprising a piezoelectric element and a tilted polarization region comprising a plurality of small regions having approximately the same width in a direction parallel to the surface of the plate, wherein in each of the plurality of small regions, the polarization of the piezoelectric element tilts from a direction perpendicular to the surface of the plate toward said one direction, and in adjacent small regions, the polarization alternately reverses; and

[0015] b) A pair of electrodes configured to cover the tilted polarization region and sandwich the front and back surfaces of the piezoelectric plate.

[0016] In the SAW device according to this invention, when an AC voltage of a predetermined frequency is applied between a pair of electrodes, the polarization of each small region tilts from a direction perpendicular to the surface of the piezoelectric plate toward a certain direction (the direction of polarization of this small region is called the polarization direction). Therefore, within the piezoelectric plate, vibrations are generated that combine compressive expansion displacement parallel to the certain direction (parallel to the surface) with shear displacement perpendicular to the surface (i.e., perpendicular to the certain direction). Furthermore, by forming multiple small regions with alternately reversed polarization directions in the certain direction, the vibrations generated within the piezoelectric plate are out of phase with each other between adjacent small regions. Through these vibrations, Rayleigh waves are formed near the surface of the piezoelectric plate. These Rayleigh waves are waves propagating in the certain direction, and are waves resulting from the coupling of a longitudinal wave (L-wave) component displaced in a direction parallel to the propagation direction and a transverse wave (SV-wave) component displaced in a direction perpendicular to the surface of the piezoelectric plate (i.e., perpendicular to the propagation direction). Thus, surface acoustic waves are excited at the surface of the piezoelectric plate.

[0017] Since the SAW device involved in this invention has the above-described structure, when an AC voltage of approximately the same frequency as the Rayleigh wave (which depends on the speed of sound in the piezoelectric body) is applied between the electrodes, a Rayleigh wave resonance is formed in the tilted polarization region.

[0018] In the SAW device according to this invention, the electrode for applying a voltage to excite vibration within the piezoelectric plate is configured to cover the entire tilted polarization region, which is composed of multiple small regions. Therefore, the problems of reduced power handling and mechanical strength of the teeth (electrodes) that occur when increasing the resonant frequency in conventional SAW devices using comb-shaped electrodes are not present in the SAW device according to this invention. That is, the SAW device according to this invention can be used at a higher resonant frequency.

[0019] In the SAW device of this invention, a reflector for reflecting surface acoustic waves can be provided on the outer side of the tilted polarization region in one direction. This facilitates the containment of surface acoustic waves within the tilted polarization region, thereby increasing the intensity of Rayleigh wave resonance. Furthermore, the reflector may be provided on only one side of the outer side in one direction, but it is preferable to provide it on both sides for easier containment of surface acoustic waves.

[0020] The SAW device involved in this invention can adopt the following structure:

[0021] The piezoelectric plate has two tilted polarization regions.

[0022] In each of the two tilted polarization regions, a pair of electrodes is provided.

[0023] SAW devices with two tilted polarization regions can be used as frequency filters in which an AC voltage signal with various overlapping frequencies is input to the electrodes disposed in one tilted polarization region, and an AC signal with a frequency corresponding to a Rayleigh wave whose width about the width of a small region in the other tilted polarization region is taken as half a wavelength is output from the electrodes disposed in the other tilted polarization region.

[0024] Invention Effects

[0025] According to the present invention, a SAW device with a higher resonant frequency can be obtained compared with conventional SAW devices using comb-shaped electrodes. Attached Figure Description

[0026] Figure 1 This is a longitudinal sectional view showing a first embodiment of the SAW device according to the present invention.

[0027] Figure 2 This is a top view showing the SAW device of the first embodiment.

[0028] Figure 3A This is a diagram showing the state of the substrate prepared in the manufacturing method of the SAW device according to the first embodiment.

[0029] Figure 3BThis is a diagram illustrating the process of fabricating a piezoelectric plate in the manufacturing method of the SAW device according to the first embodiment.

[0030] Figure 3C This is a diagram showing the state of the piezoelectric plate produced in the manufacturing method of the SAW device according to the first embodiment.

[0031] Figure 3D This is a diagram showing the state of the polarization-forming electrode produced in the manufacturing method of the SAW device according to the first embodiment.

[0032] Figure 3E This is a diagram illustrating the process of reversing the polarization of a piezoelectric material in a small region directly below the polarization-forming electrode in the manufacturing method of the SAW device according to the first embodiment.

[0033] Figure 3F This is a diagram showing the state in which the polarization-forming electrode has been removed during the manufacturing method of the SAW device in the first embodiment.

[0034] Figure 3G This is a diagram showing the state in which the first electrode is fabricated during the manufacturing method of the SAW device in the first embodiment.

[0035] Figure 3H This is a diagram showing the state of the reflector formed in the manufacturing method of the SAW device according to the first embodiment.

[0036] Figure 4 This is a diagram schematically illustrating the operation of the SAW device according to the first embodiment.

[0037] Figure 5 This is a schematic diagram illustrating the operation of a conventional SAW device.

[0038] Figure 6 This is a schematic diagram illustrating an example of a frequency filter formed by connecting multiple SAW devices according to the first embodiment.

[0039] Figure 7 This illustrates a SAW device fabricated according to a first embodiment, comprising a piezoelectric plate including PbTiO3, and the Si at each frequency measured using a network analyzer. 11 A graph of the parameter values.

[0040] Figure 8 This shows the Si of the SAW device for the first embodiment obtained by simulation at each frequency. 11 A graph of the parameter values.

[0041] Figure 9 This is a graph showing the absolute values ​​of the impedance at each frequency obtained through simulation for conventional SAW devices.

[0042] Figure 10 This illustrates how the wavelength λ of the elastic surface wave, the thickness h of the piezoelectric plate, and the electromechanical coupling coefficient K were determined through simulation in the SAW device of the first embodiment. 2 The chart shows the results obtained from the relationship.

[0043] Figure 11 This illustrates a SAW device for a first embodiment, achieved by using a piezoelectric plate with... Figure 8 A graph showing the absolute values ​​of impedance at each frequency obtained from simulations of different examples.

[0044] Figure 12 This demonstrates a method for using piezoelectric plates with conventional SAW devices. Figure 9 A graph showing the absolute values ​​of impedance at each frequency obtained from simulations of different examples.

[0045] Figure 13 It shows the manufacturing process including Sc 0.4 Al 0.6 A graph showing the absolute values ​​of impedance at each frequency, determined using a network analyzer, is presented for a first-implementation SAW device with a piezoelectric plate of N and a small region width of 4.5 μm.

[0046] Figure 14 It shows the manufacturing process including Sc 0.4 Al 0.6 A graph showing the absolute values ​​of impedance at each frequency, determined using a network analyzer, of a first-implementation SAW device with a piezoelectric plate of N and a small region width of 3.6 μm.

[0047] Figure 15 This is a longitudinal sectional view showing a second embodiment of the SAW device according to the present invention.

[0048] Figure 16 This is a top view showing the SAW device according to the second embodiment. Detailed Implementation

[0049] use Figures 1 to 16 The embodiments of the SAW device involved in this invention will be described.

[0050] (1) First implementation method

[0051] (1-1) Structure of the SAW device in the first embodiment

[0052] exist Figure 1 and Figure 2 The structure of the SAW device 10 according to the first embodiment is shown. The SAW device 10 has a piezoelectric plate 11, an electrode 12, and a reflective portion 13.

[0053] The piezoelectric plate 11 is a plate including a piezoelectric body. In the piezoelectric plate 11, in one direction ( Figure 1 and Figure 2 the left - right direction), a plurality of imaginary small regions 111, 112, 113, 114... having substantially the same width in this one direction are arranged and formed. Hereinafter, these plurality of small regions are collectively referred to as "small region 11N". The width of the small region 11N related to the one direction is set to 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 sound velocity of the sound vibration in the piezoelectric plate 11. In addition, the width of each small region 11N in the direction perpendicular to the one direction is set to the same size in the present embodiment, but it may also be different for each small region 11N.

[0054] The small regions with odd symbols in the small region 11N (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". And the entire region in which these plurality of small regions 11N are formed is called "tilted polarization region 11X".

[0055] In any of the small regions 11N, the polarization P of the piezoelectric body inclines from the direction perpendicular to the surface of the plate of the piezoelectric plate 11 toward the one direction by substantially the same inclination angle θ. In addition, the polarization P is in a direction (diagonally upward or diagonally downward) that is 180° different 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 toward the one direction. In addition, in Figure 1 , it is depicted that the polarization P in the diagonally upward direction is formed in the small region 11N1, and the polarization P in the diagonally downward direction is formed in the small region 11N2, but the direction of the polarization P may also be opposite to this (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 it is sufficient that each has at least one.

[0056] Regarding the piezoelectric body as the material of the piezoelectric plate 11, any piezoelectric body can be used as long as the piezoelectric plate 11 can be fabricated to form the polarization P inclined from the direction perpendicular to the surface of the plate as described above. For example, PbTiO3 (lead titanate), LiNbO3 (lithium niobate), LiTaO3 (lithium tantalate), ZnO (zinc oxide), Mg 1-x Zn x O (0 < x < 1) (magnesium zinc oxide), AlN (aluminum nitride), Sc x Al 1-xN (0 < x < 1) (aluminum scandium nitride), etc. Among these piezoelectric bodies, LiNbO3, LiTaO3, and Sc x Al 1-x N is particularly suitable in that it can reverse polarization and has a high electromechanical coupling coefficient and Q value.

[0057] Each of the electrodes 12 includes a plate-like first electrode 121 and a second electrode 122, and they are arranged such that these first electrode 121 and second electrode 122 cover the inclined polarization region 11X and sandwich the piezoelectric body plate 11. An AC power supply 14 (the AC power supply 14 is not included in the SAW device 10) that applies an AC voltage is connected to the first electrode 121 and the second electrode 122.

[0058] In Figure 1 the example shown, the first electrode 121 and the second electrode 122 are both arranged only at positions facing the inclined polarization region 11X, but one of the first electrode 121 and the second electrode 122 can also be arranged in a range wider than the inclined polarization region 11X. For example, the second electrode 122 can be arranged on the entire surface of the piezoelectric body plate 11. In this case, using the conductive plate material of the second electrode 122 as the substrate, the piezoelectric body plate 11 and the first electrode 121 are sequentially formed on this substrate, whereby the SAW device 10 can be fabricated.

[0059] The reflection portions 13 are each provided on both outer sides ( Figure 1 and Figure 2 the left and right sides in Figure 1 in the direction perpendicular to the paper surface in Figure 2 in the up-down direction in

[0060] (1 - 2) Manufacturing method of the SAW device of the first embodiment

[0061] Referring to FIG. 3, an example of a method for manufacturing the SAW device 10 of the first embodiment will be described. In this example, the material of the piezoelectric body plate 11 is PbTiO3, and the piezoelectric body plate 11 is fabricated using the method described in Non-Patent Document 1. In addition, the second electrode 122 is arranged on the entire surface of the piezoelectric body plate 11.

[0062] First, prepare Sr 1-x La xTiO3 (a material obtained by doping carriers by replacing a portion of Sr with La in SrTiO3) single crystal substrate ( Figure 3A At this point, a substrate is cut from the single crystal such that the c-axis is tilted in one direction from a direction perpendicular to the surface of the substrate (e.g., at an angle in the range of 10 to 80°). This substrate becomes the second electrode 122 of the completed SAW device 10.

[0063] Next, sputtered particles SP are incident onto the upper surface of the second electrode (substrate) 122 using a magnetron sputtering device. Figure 3B PbTiO3 was epitaxially grown to fabricate piezoelectric plate 11. Figure 3C PbTiO3 has similar properties to Sr. 1-x La x TiO3 has the same structure (perovskite structure), therefore, the piezoelectric plate 11 is epitaxially grown such that its c-axis is oriented in the same direction as the Sr of the second electrode (substrate) 122. 1- x La x The c-axis of TiO3 is in the same direction. As a result, the piezoelectric plate 11 is formed such that the c-axis is tilted from the direction perpendicular to its surface toward the same direction. At this time, the direction of polarization P is obliquely upward in the whole within the piezoelectric plate 11 (from the side of the second electrode 122 toward its opposite side).

[0064] Next, conductive material is deposited on the surface of the piezoelectric plate 11 by sputtering, thereby forming a plurality of polarization forming electrodes 31 having the same width W as the small region 11N in the said one direction at a period 2W twice the width W. Figure 3D Furthermore, this polarization forming electrode 31 is not the first electrode 121 after the completion of the SAW device 10. As described below, it is only used for the polarization forming operation, and therefore, it does not require the mechanical strength of the comb electrode in conventional SAW devices. In addition, the polarization forming electrode 31 can also be fabricated by other methods such as vapor deposition.

[0065] Next, a DC voltage with a positive polarization electrode 31 and an electric field strength greater than the coercive electric field is applied between each polarization forming electrode 31 and the second electrode (substrate) 122. Figure 3E Therefore, directly below the polarization-forming electrode 31 in the piezoelectric plate 11, the polarization P is reversed, becoming obliquely downward. On the other hand, in the portion between the polarization-forming electrodes 31, the polarization P of the piezoelectric plate 11 directly below it is not reversed. As a result, within the piezoelectric plate 11, small regions 11N1 with polarization P obliquely upward and small regions 11N2 with polarization P obliquely downward are alternately formed in one direction.

[0066] Subsequently, the polarization forming electrode 31 was removed by methods such as chemical etching. Figure 3F Then, the entire region 11X, which has small regions 11N1 and 11N2 formed throughout the upper surface of the piezoelectric plate 11, i.e., the tilted polarization region 11X, is deposited with conductive material by sputtering (or other methods such as vapor deposition), thereby fabricating the first electrode 121. Figure 3G Additionally, the reflective part 13 is formed using the same sputtering method (as above). Figure 3H Furthermore, since the material of the reflective portion 13 is not limited to whether it needs to be conductive, the first electrode 121 and the reflective portion 13 can be fabricated simultaneously if the material of the reflective portion 13 is the same as that of the first electrode 121. Through the above processes, the SAW device 10 is completed.

[0067] Furthermore, in the case of fabricating a SAW device 10 in which the second electrode 122 is formed only in the portion corresponding to the tilted polarization region 11X, the second electrode 122 is fabricated by vapor-depositing a conductive material within a region corresponding to the tilted polarization region 11X on the surface of a substrate that is a single-crystal substrate including an insulator and whose c-axis is tilted in one direction from a direction perpendicular to the surface of the substrate. Based on this, the fabrication of the piezoelectric plate 11, the fabrication of the polarization-forming electrode 31, the reversal of polarization in the small region 11N2, the removal of the polarization-forming electrode 31, and the fabrication of the first electrode 121 are performed using the same method as described above. In this case, in order to make the tilt of the c-axis of the substrate easily reflected in the tilt of the c-axis of the piezoelectric plate 11, it is desirable that the second electrode 122 be as thin as possible.

[0068] The method of fabricating the piezoelectric plate 11 such that its c-axis is tilted from a direction perpendicular to the surface toward the aforementioned direction is not limited to the method described above. For example, in the fabrication of materials including ZnO and Mg... 1-x Zn x O, AlN, Sc x Al 1-x When fabricating a piezoelectric plate 11 of type N, sputtering particles are incident from a direction perpendicular to the surface of the substrate (the c-axis does not need to be tilted and may not be a single crystal) in a direction tilted in one direction, thereby enabling the fabrication of the target piezoelectric plate 11. In the case of fabricating the piezoelectric plate 11 by this method, the second electrode 122 may be formed throughout the entire substrate or only in the portion of the substrate including the insulator corresponding to the tilted polarization region 11X.

[0069] (1-3) Operation of the SAW device in the first embodiment

[0070] Reference Figure 4 The operation of the SAW device 10 of the first embodiment will be described in conjunction with the operation of a conventional SAW device using comb-shaped electrodes. Figure 4 This is a diagram schematically illustrating the operation of the SAW device 10 of this embodiment. For reference, in Figure 5 The schematic diagram illustrates the operation of a SAW device 90 using conventional comb-shaped electrodes. In the conventional SAW device 90, on one surface of a piezoelectric plate 91 with an upwardly polarized P formed throughout, a first comb-shaped electrode 921 and a second comb-shaped electrode 922 are configured to engage alternately with a gap between them.

[0071] First, refer to Figure 5 The operation of a conventional SAW device 90 will be explained below. Typically, in a piezoelectric plate comprising a piezoelectric material and polarized perpendicularly to that plate, (i) a vertical compressive expansion displacement caused by an alternating current field perpendicular to the piezoelectric plate (vertical direction), (ii) a vertical compressive expansion displacement caused by an alternating current field parallel to the piezoelectric plate (parallel direction), and (iii) a vertical shear displacement (displacement of a parallel surface shearing in the vertical direction) caused by an alternating current field in the parallel direction. In the conventional SAW device 90, displacements (ii) and (iii) are utilized as described below.

[0072] 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 with a strong component in the direction perpendicular to the piezoelectric plate 91 is formed in the vicinity directly below the first comb-shaped electrode 921 and the second comb-shaped electrode 922, thereby generating a compressive expansion displacement in the direction parallel to the piezoelectric plate 91. Furthermore, in the vicinity directly below the first comb-shaped electrode 921 and the second comb-shaped electrode 922, an alternating electric field with a strong component in the direction parallel to the piezoelectric plate 91 is formed in the piezoelectric plate 91, thereby generating a shear displacement in the direction perpendicular to the piezoelectric plate 91. These displacements create a Rayleigh wave resonance, which is a wave propagating in a direction parallel to the piezoelectric plate 91, and is a wave resulting from the coupling of a transverse wave (SV wave) component displaced in a direction perpendicular to the propagation direction and a longitudinal wave (L wave) component displaced in a direction parallel to the propagation direction. Thus, surface acoustic waves are excited on the surface of the piezoelectric plate 91.

[0073] Corresponding to the operation of the conventional SAW device 90, the SAW device 10 of this embodiment operates as follows (see reference). Figure 4In the SAW device 10 of this embodiment, when an alternating voltage is applied between the first electrode 121 and the second electrode 122, an alternating electric field is formed throughout the tilted polarization region 11X within the piezoelectric plate 91, in a direction perpendicular to the piezoelectric plate 91. The polarization P within the piezoelectric plate 11 tilts from the direction perpendicular to the piezoelectric plate 11 toward the aforementioned direction, thereby passing through the component P of the polarization P in the direction perpendicular to the piezoelectric plate 11. z The alternating electric field creates a compressive expansion displacement in a direction parallel to the piezoelectric plate 11, and the component P of polarization P in that direction (parallel to the piezoelectric plate 11) x The alternating electric field creates a shear displacement in a direction perpendicular to the piezoelectric plate 11. These displacements generate a Rayleigh wave resonance, which is a wave propagating in the stated direction and is a coupled wave consisting of a longitudinal wave (L-wave) component displaced in the stated direction (i.e., the direction parallel to the propagation direction) and a transverse wave (SV-wave) component displaced in the direction perpendicular to the piezoelectric plate 11 (i.e., perpendicular to the propagation direction). Thus, surface acoustic waves are excited on the surface of the piezoelectric plate 11.

[0074] In the conventional SAW device 90 using comb-shaped electrodes 911 and 912, to increase the resonant frequency, the spacing between the teeth needs to be shortened, which also requires narrowing the width of each tooth. Therefore, there is a problem of reduced power handling capacity and mechanical strength of the teeth (electrodes). In contrast, in the SAW device 10 of this embodiment, the first electrode 121 and the second electrode 122 are configured to cover the entire tilted polarization region 11X formed by a combination of multiple small regions 11N. Therefore, the problem of reduced power handling capacity and mechanical strength of the electrodes is not generated. Thus, the SAW device 10 of this embodiment can be used at a higher resonant frequency.

[0075] The SAW device 10 functions as a resonator on its own, but can also be used via known methods associated with resonators, such as... Figure 6 As shown, a frequency filter (band filter) 50 is used to allow signal currents with a certain width of frequency band to pass through by connecting multiple filters in a "ladder" configuration.

[0076] (1-4) Experimental and calculation results related to the SAW device of the first embodiment

[0077] Next, the results obtained from experiments and simulations of fabricating the SAW device 10 according to the first embodiment will be explained. In this experiment, a SAW device 10 was fabricated using the above-described manufacturing method, comprising a piezoelectric plate 11 including PbTiO3 with a c-axis tilt angle θ of 35° and a thickness of 500 nm. The width of the small region 11N in one direction is 10 μm. For the fabricated SAW device 10, the SS at each frequency was calculated using a network analyzer. 11 Parameter. S 11 The parameters are those that indicate the reflection characteristics on the input side, showing the frequency at which the frequency variation of this value becomes a minimum, at which the SAW device 10 resonates. In the simulation, the finite element method is used, and the material of the piezoelectric plate 11, the tilt angle θ of the c-axis, and the width of the small region 11N are set to the same values ​​as the experimentally fabricated SAW device 10, and the thickness of the piezoelectric plate 11 is set to 500 nm.

[0078] In respectively Figure 7 The figure shows the S obtained through experiments. 11 Parameters, in Figure 8 The figure shows S obtained through simulation. 11 Parameters. In the experiment, at around 200MHz ( Figure 7 At the frequency indicated by the medium-thick arrow, S 11 The parameters are taken to a slightly minimum value. Additionally, in the simulation, when the frequency is amplified around 200MHz ( Figure 8 (As shown in the inset), a clear minimum was obtained at 203 MHz. Thus, there is no contradiction between the experiment and the simulation, confirming the resonance frequency at approximately 200 MHz. Furthermore, regarding the minimum observed experimentally near 330 MHz and in the simulation at 370 MHz, when the width (period) of the small region 11N was halved in the simulation, a minimum was observed at a frequency (not near 740 MHz as twice the value). Therefore, it is considered that this is not a minimum caused by surface acoustic waves.

[0079] For comparison, the absolute value of the impedance at each frequency was calculated through simulation for a SAW device 90 using conventional comb-shaped electrodes. In this simulation, in accordance with the parameters in the SAW device 10 of this embodiment, a piezoelectric plate 91 comprising PbTiO3 with a thickness of 500 nm and whose c-axis is tilted at 35° from the direction perpendicular to the piezoelectric plate was used, and the spacing between adjacent first comb-shaped electrodes 921 and second comb-shaped electrodes 922 was set to 10 μm. Furthermore, this spacing corresponds to the width of the small region 11N in the SAW device 10 of this embodiment. Figure 9The results obtained through this simulation are shown. A signal representing the resonance caused by the surface wave was observed at a frequency of approximately 200 MHz. By comparing this result with the result obtained in the SAW device 10, it was confirmed that, like the conventional SAW device 90, the SAW device 10 of this embodiment exhibits a resonance caused by the surface wave at a frequency of approximately 200 MHz.

[0080] In addition, Figure 7 In the experimental results shown, the resonance signal caused by the elastic surface wave is relatively small. However, this is believed to be due to the thickness-dependent electromechanical coupling coefficient K of the piezoelectric plate 11 in the experimentally fabricated SAW device 10. 2 Smaller. In the structure of the SAW device 10 in this embodiment, the finite element method is used to determine the wavelength λ (twice the width of the small region 11N in one direction) of the elastic surface wave to be formed, the thickness h of the piezoelectric plate 11, and the electromechanical coupling coefficient K. 2 After simulating the relationship, as follows: Figure 10 Thus, we obtain h / λ on the horizontal axis and K on the vertical axis. 2 The chart shows that when h / λ = 0.4375, K... 2 We take the maximum value of 10.99%. On the other hand, in the SAW device 10 fabricated through this experiment, h / λ = 0.025. When this value is applied... Figure 10 When K is in the chart 2 The value becomes less than 0.1%, which is less than 1 / 100 of the aforementioned maximum value. Therefore, it is assumed that if the thickness h of the piezoelectric plate 11 is appropriately determined according to the wavelength λ of the elastic surface wave to be formed, the electromechanical coupling coefficient K will be such that... 2 Approaching the maximum value will result in a stronger resonance than that obtained through this experiment.

[0081] Therefore, further regarding the SAW device 10 of this embodiment, in terms of the electromechanical coupling coefficient K... 2 A simulation based on the finite element method was performed under conditions close to the aforementioned maximum value. In this simulation, the thickness of the piezoelectric plate 11 was set to 1 μm (1000 nm). Furthermore, the width of the small region 11N in one direction was set to 1 μm, such that the wavelength λ of the elastic surface wave is 2 μm. The material of the piezoelectric plate 11 and the c-axis tilt angle θ were set to the same as in the previous example (PbTiO3, θ = 35°). Under these conditions, the value of h / λ is 0.5, and the electromechanical coupling coefficient K... 2 It is 10.9% (at a frequency of 1000MHz). After calculating the absolute value of the impedance at each frequency under the above conditions, as follows... Figure 11As shown, a distinct peak is observed near 1000 MHz. Furthermore, although a peak is also observed near 2000 MHz, this is believed to be a harmonic peak based on the resonance at 1000 MHz as the fundamental frequency.

[0082] For comparison, in the SAW device 90 using conventional comb-shaped electrodes, the same conditions as for piezoelectric plate 11 were applied to piezoelectric plate 91. For the case where the spacing between adjacent first comb-shaped electrodes 921 and second comb-shaped electrodes 922 was set to 1 μm, making the wavelength λ of the surface wave 2 μm, the absolute value of the impedance at each frequency was calculated. The result is as follows: Figure 12 As shown, the resonance peaks caused by the elastic surface waves of the fundamental frequency were observed near 1000 MHz, and the resonance peaks caused by the elastic surface waves of the harmonics were observed near 2000 MHz. Thus, Figure 11 The results shown are consistent with Figure 12 The results shown correspond well, so it can be said that in the SAW device 10 of this embodiment in the simulation, the resonance caused by the elastic surface wave with a frequency of about 1000MHz is formed, just like the conventional SAW device 90.

[0083] When, in a conventional SAW device 90, adjacent first comb-shaped electrodes 921 and second comb-shaped electrodes 922 are actually fabricated with a narrow spacing of 1 μm, as in this simulation, the width of the teeth of these comb-shaped electrodes must be less than 1 μm, which would reduce the power handling capacity and mechanical strength of the teeth. In contrast, in the SAW device 10 of this embodiment, comb-shaped electrodes are not required, thus preventing a decrease in the power handling capacity and mechanical strength of the electrodes.

[0084] Next, the production of equipment including Sc x Al 1-x The results obtained from experiments on a SAW device 10 with an N-type piezoelectric plate 11 are explained. In this experiment, a piezoelectric plate 11 with x=0.4, a thickness h of 1.8 μm, and a c-axis (and polarization P) tilted 1° in one direction from the direction perpendicular to the piezoelectric plate 11 was fabricated. Based on this, two types of SAW devices 10 were fabricated with the polarization P alternately reversed, resulting in the widths of small regions 111, 112... in the piezoelectric plate 11 being 4.5 μm (wavelength λ=9.0 μm, h / λ=0.20) and 3.6 μm (λ=7.2 μm, h / λ=0.25).

[0085] The results obtained by measuring the absolute value of the impedance at each frequency using a network analyzer for these two types of SAW devices 10 are as follows: Figure 13An example is shown where the width of the small region is 4.5 μm. Figure 14 An example with a small region width of 3.6 μm is shown. Resonance of the elastic surface wave was observed at 600 MHz in the former and at 690 MHz in the latter. Based on these resonant frequencies and wavelengths λ, the sound velocity of the elastic surface wave was calculated to be 5400 m / s in both cases. Additionally, the electromechanical coupling coefficient K... 2 The former is 8.7% and the latter is 6.0%, both of which are relatively high values.

[0086] (2) SAW device of the second embodiment

[0087] (2-1) Structure of the SAW device in the second embodiment

[0088] Reference Figure 15 and Figure 16 The SAW device 20 of the second embodiment will now be described. This SAW device 20 includes a piezoelectric plate 21, an input-side electrode 221, and an output-side electrode 222. Furthermore, the structure corresponding to the reflective portion 13 of the SAW device 10 of the first embodiment is not included in the SAW device 20 of the second embodiment.

[0089] Piezoelectric plate 21 is a plate including a piezoelectric element. An input-side tilted polarization region 211X is provided on piezoelectric plate 21, and this input-side tilted polarization region 211X is formed in one direction (…). Figure 15 and Figure 16 Multiple hypothetical input-side small regions 2111, 2112, 2113, 2114... (collectively referred to as "input-side small regions 211N") having approximately the same width in the left-right direction. Additionally, on the piezoelectric plate 21, an output-side tilted polarization region 212X is provided separately from the input-side tilted polarization region 211X in the same direction. This output-side tilted polarization region 212X has multiple hypothetical output-side small regions 2121, 2122, 2123, 2124... (collectively referred to as "output-side small regions 212N") having approximately the same width in the same direction as the input-side small regions 211N.

[0090] In either the input-side small region 211N or the output-side small region 212N, the polarization P of the piezoelectric element is tilted by approximately the same tilt angle θ from a direction perpendicular to the surface of the piezoelectric plate 21 towards said one direction. Between the input-side small region 211N1 and the output-side small region 212N1 with odd-numbered symbols (symbols 2111, 2113... and 2121, 2123...) and the input-side small region 211N2 and the output-side small region 212N2 with even-numbered symbols (symbols 2112, 2114... and 2122, 2124...), the polarization P is oriented in a direction that differs from each other by 180° (slanted upwards or downwards).

[0091] Each input-side electrode 221 includes a plate-shaped first input-side electrode 2211 and a second input-side electrode 2212, which are configured to cover the input-side tilted polarization region 211X and sandwich the piezoelectric plate 21. Similarly, each output-side electrode 222 includes a plate-shaped first output-side electrode 2221 and a second output-side electrode 2222, which are configured to cover the output-side tilted polarization region 212X and sandwich the piezoelectric plate 21.

[0092] (2-2) Method for manufacturing SAW device according to the second embodiment

[0093] The SAW device 20 of the second embodiment can be fabricated using the same method as the case of "fabricating a SAW device 10 in the first embodiment where the second electrode 122 is formed only in the portion corresponding to the tilted polarization region 11X" except for the positions where the tilted polarization regions (input-side tilted polarization region 211X and output-side tilted polarization region 212X) and electrodes (input-side electrode 221 and output-side electrode 222) are formed.

[0094] (2-3) Operation of the SAW device in the second embodiment

[0095] When using the SAW device 20 of the second embodiment, an AC voltage with multiple frequencies is applied between the first input-side electrode 2211 and the second input-side electrode 2212. This forms a Rayleigh wave resonance within the input-side tilted polarization region 211X, with a length corresponding to half a wavelength of the width of one direction of the input-side small region 211N among these multiple frequencies (wavelengths). The principle for forming the Rayleigh wave resonance is the same as in the case of the SAW device 10 of the first embodiment.

[0096] A surface acoustic wave, including a Rayleigh wave formed within the input-side tilted polarization region 211X, propagates near the surface of the piezoelectric plate 21 and reaches the output-side tilted polarization region 212X. In the output-side tilted polarization region 212X, by resonating the piezoelectric plate 21 with a length corresponding to half the wavelength of the width of the output-side small region 212N in one direction, an AC voltage with a frequency corresponding to the resonant frequency is generated between the first output-side electrode 2221 and the second output-side electrode 2222. This AC voltage is taken from the output terminals provided at the first output-side electrode 2221 and the second output-side electrode 2222. Through the above operation, the SAW device 20 of the second embodiment functions as a frequency filter that extracts a specific frequency AC voltage signal from a signal of AC voltages with multiple frequencies mixed from the input-side electrode 221. Such a frequency filter is called a transverse frequency filter.

[0097] According to the second embodiment of the SAW device 20, similarly to the first embodiment, the input-side electrode 221 and the output-side electrode 222 are respectively configured to cover the entirety of the tilted polarization region (input-side tilted polarization region 211X and output-side tilted polarization region 212X) formed by summing up multiple small regions (input-side small region 211N and output-side small region 212N). Therefore, problems such as a decrease in the power withstand capability and mechanical strength of the electrodes are not generated. Therefore, the SAW device 20 of the second embodiment can be used at a higher resonant frequency.

[0098] This invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the spirit of this invention.

[0099] Explanation of reference numerals in the attached figures

[0100] 10, 20, 90… SAW devices;

[0101] 11, 21, 91… piezoelectric plates;

[0102] Small regions: 111, 112, 113, 114, 115, 116, 11N1, 11N2, 11N…

[0103] 11X… tilted polarization region;

[0104] 12…electrodes;

[0105] 121…First electrode;

[0106] 122…Second electrode;

[0107] 13…reflective part;

[0108] 14… AC power supply;

[0109] 21…Piezoelectric plate;

[0110] 2111, 2112, 2113, 2114, 211N1, 211N2, 211N… Input-side small area;

[0111] 211X…Input-side tilted polarization region;

[0112] 2121, 2122, 2123, 2124, 212N1, 212N2, 212N… small area on the output side;

[0113] 212X… Output-side tilted polarization region;

[0114] 221…Input side electrode;

[0115] 2211…First input side electrode;

[0116] 2212…Second input side electrode;

[0117] 222… Output side electrode;

[0118] 2221…First output side electrode

[0119] 2222…Second output side electrode;

[0120] 31…Electrode for polarization formation;

[0121] 50… Frequency filter (band filter);

[0122] 911…comb-shaped electrode;

[0123] 921…First comb-shaped electrode;

[0124] 922…Second comb-shaped electrode;

[0125] P…polarization;

[0126] Px… is the polarized component in one direction parallel to the piezoelectric plate;

[0127] Pz…the polarized component in the direction perpendicular to the piezoelectric plate;

[0128] SP...sputtering particles;

[0129] W...the width of the small region in one direction;

[0130] θ… Inclination angle.

Claims

1. A surface acoustic wave device, characterized in that, The surface acoustic wave device comprises: a) A piezoelectric plate comprising a piezoelectric element and a tilted polarization region comprising a plurality of small regions having approximately the same width in a direction parallel to the surface of the plate, wherein in each of the plurality of small regions, the polarization of the piezoelectric element tilts from a direction perpendicular to the surface of the plate toward said one direction, and in adjacent small regions, the polarization alternately reverses; and b) A pair of electrodes configured to cover the tilted polarization region and sandwich the front and back surfaces of the piezoelectric plate.

2. The surface acoustic wave device according to claim 1, characterized in that, The surface acoustic wave device also has a reflective part for reflecting surface acoustic waves on the outer side of the tilted polarization region in one direction.

3. The surface acoustic wave device according to claim 1, characterized in that, The piezoelectric plate has two tilted polarization regions. In each of the two tilted polarization regions, a pair of electrodes is provided.

4. The surface acoustic wave device according to any one of claims 1 to 3, characterized in that, The piezoelectric body is one of PbTiO3, LiNbO3, LiTaO3, ZnO, Mg 1-x Zn x O (0 < x < 1), AlN, and Sc x Al 1-x N (0 < x < 1).

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