JP2010178543A - Thin-film resonator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin-film resonator that excites lateral waves while at the same time suppressing the excitation of longitudinal waves at a predetermined frequency. <P>SOLUTION: The thin-film resonator 10 is such that n layers (n is a natural number of 3 or above) of piezoelectric layers (first piezoelectric layer 111, second piezoelectric layer 112, third piezoelectric layer...) are provided between a first electrode 121 and a second electrode 122. A polarization vector B in each piezoelectric layer is oriented to tilt about a normal line 13 of a plane α parallel to the thin-film resonator. The projection of the polarization vector P in each piezoelectric layer on the normal line 13 is in the same direction. The projections on the plane α of the polarization vectors P in odd-numberth piezoelectric layers counted from the first electrode 121 side orient in the opposite direction from the projections on the plane α of the polarization vectors P in the even-numberth piezoelectric layers counted from the first electrode 121 side. Due to this structure, excitation of a lateral wave having the largest intensity occurs at a different frequency from that of excitation of a longitudinal wave having the largest intensity, and hence excitation of longitudinal waves can be suppressed when lateral waves are excited. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a sliding mode type thin film resonator that converts a mechanical vibration of a transverse wave into an alternating voltage or converts an alternating voltage into a mechanical vibration of a transverse wave.

  The sliding mode thin film resonator is used as an element for transmitting / receiving a shear wave. Using such an element, for example, it is possible to measure a temporal change in the amount of solid deposited from a liquid sample. In this measurement, a transverse wave is transmitted to the wall of the container containing the liquid sample, and the resulting transverse wave vibration is received to determine the vibration frequency. Here, since the transverse wave propagates only in the solid and does not propagate in the liquid (and in the gas), only the vibration of the wall (solid) can be captured. Then, since the frequency of the received transverse wave vibration changes (slightly) in response to the change in the amount of solid deposited on the container wall, the change in frequency becomes a value reflecting the change in the amount of solid deposited. .

Patent Document 1 describes a thin film resonator (transducer) 90 shown in FIG. 1 as an example of a thin film resonator that transmits and receives a transverse wave. The thin film resonator 90 has two layers of a first piezoelectric layer 911 and a second piezoelectric layer 912 between a first electrode 921 and a second electrode 922. The first polarization vector P _{1 of} the first piezoelectric layer 911 and the second polarization vector P ₂ of the second piezoelectric layer 912 are respectively transducer surfaces (surfaces parallel to the first piezoelectric layer 911 and the second piezoelectric layer 912). It is oriented so as to be inclined with respect to the normal line 93 of α. The projections P _1S and P _2S of the first polarization vector P ₁ and the second polarization vector P ₂ onto the transducer surface α are in opposite directions, and the projection P onto the normal line 93 of the first polarization vector and the second polarization vector. _1L and _P2L are in the same direction. The first piezoelectric layer 911 and the second piezoelectric layer 912 have a wurtzite structure with high conversion efficiency between mechanical vibration and AC voltage (also called “wurtzite structure” or “wurtzite crystal structure”). A piezoelectric body having the above can be suitably used. In a piezoelectric body having a wurtzite structure, the direction of the polarization vector is the c-axis direction of the crystal axis.

JP 2008-182515 A

  In the thin film resonator described in Patent Document 1, a transverse wave having a predetermined resonance frequency is excited, and simultaneously, a longitudinal wave having the same resonance frequency as the transverse wave is excited. Such a characteristic is useful when a longitudinal wave and a transverse wave of the same frequency are used simultaneously, such as a vibration source for nondestructive inspection that detects cracks running in various directions inside the object to be measured. However, when it is desired to use only a transverse wave as in the measurement of the solid precipitation amount, a longitudinal wave having the same frequency as the transverse wave causes noise, which is not preferable. In the example of solid precipitation measurement, in addition to the transverse wave vibration of the container wall, the thin film resonator receives the longitudinal wave vibration that propagates the liquid in the container and the container wall at the same frequency as the transverse wave. The signal is a superposition of these two vibrations, making it difficult to extract only information about the solid.

  The problem to be solved by the present invention is to provide a thin film resonator that excites a transverse wave while suppressing excitation of a longitudinal wave at a predetermined frequency.

The thin film resonator according to the present invention, which has been made to solve the above problems,
A thin film resonator in which an n layer (n is a natural number of 3 or more) piezoelectric layer is provided between a first electrode and a second electrode,
Oriented so that the polarization vector in each piezoelectric layer is inclined with respect to the normal of the plane parallel to the thin film resonator,
The projection of the polarization vector in each piezoelectric layer onto the normal is in the same direction,
The projection of the polarization vector in the odd-numbered piezoelectric layer counted from the first electrode side onto the surface is opposite to the projection of the polarization vector in the even-numbered piezoelectric layer counted from the first electrode side onto the surface. is there,
It is characterized by that.

According to the thin film resonator according to the present invention, the resonant frequency F _p for the strength of the transverse wave is maximum is to take the strength of the longitudinal wave is different from the resonant frequency F _s that maximizes the value, at the frequency F _p, the longitudinal wave The transverse wave can be excited while suppressing the excitation of. The reason will be described below.

In the thin film resonator according to the present invention, since the projection of the polarization vector in each piezoelectric layer to the normal line is in the same direction, when an AC voltage is applied between the first electrode and the second electrode, or externally When a longitudinal wave vibration is applied, all piezoelectric layers vibrate in the same phase in the normal direction. Therefore, the longitudinal wave generates a half-wave vibration over the entire laminate ( _total film thickness d _total ) of n piezoelectric layers, that is, the intensity of the first-order mode in which the wavelength λ _L is 2d _total. Is maximized.

On the other hand, since the projection of the polarization vector in the odd-numbered piezoelectric layer onto the surface (transducer surface) and the projection of the polarization vector in the even-numbered piezoelectric layer onto the transducer surface are in opposite directions, the first electrode and the second electrode When an AC voltage is applied between the electrodes, or when a lateral wave vibration is applied from the outside, the odd-numbered piezoelectric layers and the even-numbered piezoelectric layers are reversed in the direction parallel to the transducer surface. Vibrate. For this reason, the transverse wave has an n / 2 wavelength vibration over the entire laminated body, that is, the intensity of the nth mode in which the wavelength λ _S is (2 / n) d _total is maximized.

The longitudinal wave sound velocity V _L in the piezoelectric layer is approximately twice the transverse wave sound velocity V _{S in} the piezoelectric layer. Therefore, when the frequency f _L of the longitudinal wave with the maximum intensity and the frequency f _S of the transverse wave with the maximum intensity are represented by the sound velocity V _S ,
f _L = V _L / λ _L ≒ 2V _S / (2d _total ) = V _S / d _total … (1)
f _S = V _S / λ _S ≈V _S / ((2 / n) d _total ) = (n / 2) V _S / d _total … (2)
f _S ≒ (n / 2) f _L ... (3)
It becomes. That is, when n = 2, f _L and f _S are substantially equal, but when n is a natural number of 3 or more, f _L and f _S take different values. Therefore, by using three or more piezoelectric layers as in the present invention, the frequency at which the excitation with the maximum intensity occurs is different. In other words, in the thin film resonator according to the present invention, the maximum intensity transverse wave is excited at the predetermined frequency f _S without generating the maximum intensity longitudinal wave in the thin film resonator.

As described above, the resonance frequency of the longitudinal wave and the resonance frequency of the transverse wave are both determined by the _total film thickness d _total and the speed of sound in the piezoelectric layer. That is, the frequency of the longitudinal wave and the transverse wave hardly depends on the thickness d _k (k is a natural number of 1 to n) of each piezoelectric layer and the angle θ _k formed by the polarization vector and the normal line. Therefore, the thickness d _k and the angle θ _k may be different for each piezoelectric layer as long as the purpose of exciting the maximum intensity transverse wave without generating the maximum intensity longitudinal wave is achieved. However, if the thickness d _k and the angle θ _k are different for each piezoelectric layer, vibration modes other than the vibration mode having the maximum intensity are also generated in the longitudinal wave and the transverse wave. Therefore, it is desirable that the thickness d _k and the angle θ _k are the same in all layers.

  The material of each piezoelectric layer is not particularly limited as long as it is a piezoelectric body, but is desirably a piezoelectric body having a wurtzite structure with high conversion efficiency between mechanical vibration and AC voltage. Examples of the piezoelectric body having a wurtzite structure include zinc oxide (ZnO) and aluminum nitride (AlN).

When manufacturing a thin film resonator according to the present invention, a first piezoelectric layer is usually formed on a substrate (first electrode or second electrode, or a substrate different from the substrate), and the first piezoelectric layer is formed thereon. Second and subsequent piezoelectric layers are sequentially formed. In this case, the k-th (k is 2 to n) piezoelectric layer is affected by the crystal growth direction in the (k-1) -th piezoelectric layer directly below it, so that the crystal axis direction is predetermined. May deviate from direction. If the direction of the crystal axis is shifted in this way, the polarization vector is also shifted from a predetermined direction. Therefore, the thin film resonator according to the present invention prevents the orientation of the crystal axis of one piezoelectric layer from affecting the orientation of the crystal of the other piezoelectric layer between two adjacent piezoelectric layers. A buffer layer can be provided. For example, when zinc oxide is used as the material of each piezoelectric layer, silicon dioxide (SiO ₂ ) can be used as the material of the buffer layer. Also, if the material of the buffer layer has crystallinity, the direction of the crystal axis may affect the direction of the crystal axis of the piezoelectric layer, or unnecessary vibration may be generated when the thin film resonator is used. For this reason, it is desirable to use amorphous for the buffer layer. As the material of the amorphous buffer layer, for example, alumina, titanium dioxide, boron oxide, carbon, other glass materials or amorphous alloys can be used.

  According to the thin film resonator according to the present invention, it is possible to excite the maximum intensity transverse wave while suppressing the excitation of the longitudinal wave by preventing the occurrence of the maximum intensity longitudinal wave in the thin film resonator at a predetermined frequency. it can.

The longitudinal cross-sectional view which shows the conventional thin film resonator 90. FIG. 1 is a longitudinal sectional view showing a thin film resonator 10 according to an embodiment of the present invention. The figure which shows typically the longitudinal wave and transverse wave which arise in the thin film resonator 10 of a present Example. The schematic diagram which shows the crystal structure of the substance which has a wurtzite structure. (a) ZnO, (b) AlN, (c) LiNbO ₃ is a graph showing the relationship between the tilt angle ψ of the polarization vector and the electromechanical coupling constant k ₁₅ ′ (for reference, the tilt angle ψ and longitudinal wave vibration The relationship of the contributing electromechanical coupling constant k ₃₃ 'is also shown). The longitudinal cross-sectional view which shows the example (thin film resonator 10A) which provides a buffer layer between two adjacent piezoelectric material layers. Schematic which shows the RF magnetron sputtering apparatus 30 used for preparation of a piezoelectric material layer. The longitudinal cross-sectional view which shows the manufacturing method of 10 A of thin film resonators of a present Example. The pole figure which shows the result of the X-ray diffraction measurement performed with respect to the 1st piezoelectric material layer 111 and the 2nd piezoelectric material layer 112 in the thin film resonator 10A produced in the present Example. The graph which shows the measurement result of the conversion loss with respect to the transverse wave and longitudinal wave of 10 A of thin film resonators produced by the present Example. The longitudinal cross-sectional view which shows the other Example of this invention. The graph which shows the measurement result of the conversion loss with respect to the transverse wave and longitudinal wave of the thin film resonator 50 which is n = 6. The graph which shows the calculation result of the conversion loss with respect to the transverse wave and longitudinal wave of a thin film resonator in the case of n = 4 and 20, and n = 1 and 2 which are comparative examples.

  An embodiment of the thin film resonator according to the present invention will be described with reference to FIGS.

As shown in FIG. 2, the thin-film resonator 10 of this example includes four piezoelectric layers (first piezoelectric layer 111, in order from the first electrode 121 side) between the first electrode 121 and the second electrode 122. A second piezoelectric layer 112, a third piezoelectric layer 113, and a fourth piezoelectric layer 114) are provided. The polarization vectors P _{1 to} P ₄ of the first piezoelectric layer 111 to the fourth piezoelectric layer 114 are all inclined with respect to the normal 13 of the plane α parallel to the piezoelectric layer (parallel to the normal 13). But not vertical). Further, the projections P _{1L to} P _4L of the polarization vectors P _{1 to} P ₄ onto the normal line 13 are all in the same direction. On the other hand, the projections P _{1s to} P _4s of the polarization vectors P _{1 to} P ₄ onto the plane α are odd-numbered piezoelectric layers counted from the first electrode 121 side (that is, the first piezoelectric layer 111 and the third piezoelectric layer). 113) (P _1s , P _3s ) and those in the even-numbered piezoelectric layers (second piezoelectric layer 112 and fourth piezoelectric layer 114) (P _2s , P _4s ) are in opposite directions. . In this embodiment, the thicknesses of the first piezoelectric layer 111 to the fourth piezoelectric layer 114 are d ₁ to d _4, and the total thickness of these four layers is d _total .

  The operation of the thin film resonator 10 will be described with reference to FIG. When an AC voltage is applied between the first electrode 121 and the second electrode 122, the first piezoelectric layer 111 and the second piezoelectric layer 112 are compressed in the normal direction, which contributes to the generation of longitudinal waves ( Both vibration (longitudinal wave vibration) and sliding vibration (transverse wave vibration) in the direction parallel to the transducer surface that contribute to the generation of the transverse wave are generated.

Regarding the longitudinal wave vibration, since the projections P _{1L to} P _4L onto the normal line 13 are all in the same direction, all the piezoelectric layers vibrate in the same phase (FIG. 3A). Accordingly, the upper surface of the first piezoelectric layer 111 and the lower surface of the fourth piezoelectric layer 114 have a phase that is 180 ° different. Therefore, the longitudinal wave has an odd-order vibration mode. It is the primary mode that has the maximum intensity among the odd-order vibration modes. The wavelength λ _L of the longitudinal wave vibration in the primary mode is 2d _total .

On the other hand, for the transverse wave vibration, the projection of the polarization vectors P _{1 to} P ₄ onto the plane α is reversed between the odd-numbered piezoelectric layer and the even-numbered piezoelectric layer, so the odd-numbered piezoelectric layer and the even-numbered piezoelectric layer The piezoelectric layer vibrates in the opposite phase (FIG. 3B). Accordingly, the upper surface of the odd-numbered piezoelectric layer and the lower surface of the even-numbered piezoelectric layer have the same phase, and the lower surface of the odd-numbered piezoelectric layer and the upper surface of the even-numbered piezoelectric layer have the same phase. Therefore, the transverse wave has a vibration mode in which an integral multiple of the thickness of two piezoelectric layers is one wavelength. Among such vibration modes, the intensity is maximized in a vibration mode in which the thickness (one time) of two piezoelectric layers is one wavelength, which is a quartic mode in this embodiment. The wavelength λ _S of the fourth-order mode transverse wave vibration is 0.5d _total .

As described above, since the acoustic velocity V _L of the longitudinal wave in the piezoelectric layer is approximately twice the acoustic velocity V _S of the transverse wave in the piezoelectric layer, the frequencies f _L and 4 of the longitudinal wave vibration of the primary mode are 4 The frequency f _S of the transverse wave vibration of the next mode is
f _L = V _L / λ _L ≒ 2V _S / (2d _total ) = V _S / d _total … (4)
f _S = V _S / λ _S = V _S /(0.5d _total ) = 2V _S / d _total … (5)
∴f _S ≒ 2f _L (6)
It becomes. That is, to have a frequency f _S is different from the frequency f _L value (twice the frequency f _L), in the thin film resonator 10 of the present embodiment, at a frequency f _S, prevents the longitudinal wave of the maximum intensity occurs It is possible to excite a transverse wave having the maximum intensity while suppressing the excitation of the longitudinal wave.

  Further, in the thin film resonator 10 of the present embodiment, the thickness of the stacked body used for exciting the transverse wave of the same frequency can be made larger than in the case where there are only two piezoelectric layers. As a result, the mechanical strength and the voltage resistance can be increased, and the area of the electrode, that is, the area for transmitting / receiving the transverse wave can be increased. The reason why the area of the electrode can be increased is as follows. The thin film resonator is connected to an AC circuit in both cases of transmission and reception of sound waves, and acts as a capacitor in the AC circuit. Therefore, in order to achieve impedance matching in the AC circuit, the reactance of the thin film resonator needs to be a predetermined value (usually 50Ω). Since the reactance Xc of the capacitor is proportional to the distance d between the electrodes and inversely proportional to the area S of the electrode (XcSd / S), the distance d can be increased, that is, the thickness of the laminate can be increased. The area S can be increased.

The material of the first piezoelectric layer 111 to the fourth piezoelectric layer 114 is not particularly limited as long as it is a piezoelectric body, but a wurtzite structure such as ZnO or AlN that has high conversion efficiency between mechanical vibration and AC voltage. It is desirable to have As shown in FIG. 4, the piezoelectric body having a wurtzite structure has a hexagonal unit cell, and is composed of alternating layers of A ^{n +} (A layer) and B ⁿ⁻ (B layer). The B layer is disposed at a position shifted in one direction of the c-axis from a position equidistant from the two A layers above and below the B layer. Due to this crystal structure, a piezoelectric body having a wurtzite structure has spontaneous polarization (polarity) in a direction parallel to the c-axis even when no external electric field is applied.

  As the material of the first electrode 121 and the second electrode 122, a normal electrode material such as aluminum or gold can be used.

The inclination angle ψ of the polarization vector (defined by the angle with respect to the normal 13) is not particularly limited, but it is desirable to determine the electromechanical coupling constant k ₁₅ ′ contributing to the vibration of the transverse wave as large as possible. As shown in FIG. 5, the electromechanical coupling constant k ₁₅ ′ is maximized at ψ = 28 ° for ZnO, at ψ = 30 ° for AlN, and at ψ = 168 ° for lithium niobate (LiNbO ₃ ). Here, LiNbO ₃ is a piezoelectric body having a structure similar to a trigonal ilmenite structure (other than the wurtzite structure).

  Next, a thin film resonator 10A that is a modification of the thin film resonator 10 will be described. As illustrated in FIG. 6, the thin film resonator 10 </ b> A includes a first buffer layer 141 between the first piezoelectric layer 111 and the second piezoelectric layer 112. When the first buffer layer 141 is manufactured, when one of the first piezoelectric layer 111 and the second piezoelectric layer 112 is manufactured after the first piezoelectric layer 111 is manufactured, This is for producing the other piezoelectric layer without being influenced by the direction of the crystal axis (which affects the direction of the polarization vector). Similarly, a second buffer layer 142 is provided between the second piezoelectric layer 112 and the third piezoelectric layer 113, and a third buffer layer 143 is provided between the third piezoelectric layer 113 and the fourth piezoelectric layer 114, respectively. Have.

An example of a method for manufacturing the thin film resonator 10A will be described with reference to FIGS. Here, ZnO is used as the material of the first piezoelectric layer 111 to the fourth piezoelectric layer 114, the amorphous SiO ₂ is used for the first buffer layer 141 to the third buffer layer 143, and the first electrode 121 and the second electrode 122 are formed. The case where aluminum is used as an example is taken as an example.

  First, the RF magnetron sputtering apparatus 30 (FIG. 7) used for producing the first piezoelectric layer 111 to the fourth piezoelectric layer 114 will be described. The RF magnetron sputtering apparatus 30 has a pair of plate-like anodes 31 (upper side) and a cathode 32 (lower side), and has a magnetron circuit 33 below the cathode 32. Between the anode 31 and the cathode 32, a substrate table 34 is provided that can fix the substrate 21 in a direction inclined with respect to the anode 31 and the cathode 32. In the present embodiment, the substrate table 34 has a fixed surface 341 inclined by 80 ° with respect to the anode 31 and the cathode 32. Further, a heater and a cooling water circulation pipe (not shown) are built in the substrate stand 34, and the temperature of the substrate 21 can be adjusted by them. A target T made of a film forming material is placed on the cathode 32. The upper end of the substrate table 34 and the lower surface of the anode 31 are in contact with each other, and the lower end of the substrate table 34 and the upper surface of the target T are separated by 3 mm. The distance between the upper end and the lower end of the substrate base 34 is 45 mm. These parts are arranged in a vacuum vessel (not shown).

  The thin film resonator 10A is manufactured as follows. First, the first electrode 121 is fabricated by vapor-depositing aluminum on a quartz substrate 21 having a long side of 45 mm, a short side of 25 mm, and a thickness of 1 mm (FIG. 8A). Next, the substrate 21 is fixed to the fixing surface 341 of the substrate table 34 with the first electrode 121 facing down and the long side facing up and down, and the target T made of ZnO is placed on the cathode 32. (FIG. 7). Then, after evacuating the vacuum chamber, a mixed gas of argon and oxygen is introduced into the vacuum chamber at a mixing ratio of 3: 1 so that the pressure in the vacuum chamber becomes 1.0 Pa. In this state, the ZnO target T is sputtered for 1 hour by applying 200 W of high frequency power to the magnetron circuit 33 while maintaining the temperature of the substrate 21 at 400 ° C. Thereby, the sputtered ZnO particles are deposited on the surface of the first electrode 121, and the first piezoelectric layer 111 made of ZnO is formed (b). Here, since the fixed surface 341 is inclined with respect to the anode 31 and the cathode 32, the direction of the c-axis (direction of the polarization vector) in ZnO constituting the first piezoelectric layer 111 is relative to that layer. Tilt.

Next, the vacuum in the vacuum chamber is once broken, and the target T is exchanged from ZnO to SiO ₂ . Then, after evacuating the vacuum chamber, a mixed gas of argon and oxygen is introduced into the vacuum chamber at a mixing ratio of 1: 1 so that the pressure in the vacuum chamber becomes 1.0 Pa. In this state, while maintaining the temperature of the substrate 21 at 100 ° C., 200 W high frequency power is applied to the magnetron circuit 33 to sputter the SiO ₂ target T for 10 minutes. As a result, the first buffer layer 141 made of amorphous SiO ₂ is formed on the surface of the first piezoelectric layer 111 (c).

Subsequently, the vacuum in the vacuum chamber is broken, and the target T is exchanged from SiO ₂ to ZnO. At the same time, the substrate 21 is rotated 180 ° around an axis 342 perpendicular to the fixed surface 341 (d). Then, a second piezoelectric layer 112 is formed on the surface of the first buffer layer 141 by sputtering a ZnO target T under the same conditions as those for manufacturing the first piezoelectric layer 111 (e). Here, the c-axis (polarization vector) in ZnO constituting the second piezoelectric layer 112 is obtained by rotating the substrate 21 around the axis 342 as described above and the presence of the first buffer layer 141. ), The component parallel to the second piezoelectric layer 112 faces in the opposite direction to that of the first piezoelectric layer 111.

  Thereafter, the second buffer layer 142, the third piezoelectric layer 113, the third buffer layer 143, and the fourth piezoelectric layer 114 are formed in this order. Here, the third piezoelectric layer 113 and the fourth piezoelectric layer 114 are the same method as the second piezoelectric layer 112, and the second buffer layer 142 and the third buffer layer 143 are the same method as the first buffer layer 141. Respectively. Finally, aluminum is vapor-deposited on the fourth piezoelectric layer 114 to produce the second electrode 122 (f), whereby the thin film resonator 10A is obtained.

  Various measurements were performed on the thin film resonator 10A manufactured using the above method. First, X-ray diffraction measurement was performed on the first piezoelectric layer 111 and the second piezoelectric layer 112, and a pole figure relating to the (0002) plane of the ZnO crystal was created (FIG. 9). The (0002) plane is a plane perpendicular to the c-axis (that is, the polarization vector). According to this pole figure, the detected values of X-ray diffraction are distributed with a full width at half maximum of about 7 ° centered around 25 ° with respect to the elevation angle ψ in both the first piezoelectric layer 111 and the second piezoelectric layer 112. ing. This indicates that the polarization vectors of both the first piezoelectric layer 111 and the second piezoelectric layer 112 are inclined by about 25 ° with respect to the normal line 13. On the other hand, the azimuth angle φ is approximately 180 ° symmetrical between the first piezoelectric layer 111 and the second piezoelectric layer 112. This indicates that the projection of the polarization vector onto the plane α is reversed between the first piezoelectric layer 111 and the second piezoelectric layer 112.

The full width at half maximum Δψ and the thickness d of the inclination angles ψ and ψ in each of the first piezoelectric layer 111 to the fourth piezoelectric layer 114 are as shown in the following table.
The thicknesses of the first buffer layer 141, the second buffer layer 142, and the third buffer layer 143 were 0.13 μm, 0.12 μm, and 0.13 μm in this order.

Further, when the conversion loss relating to the transverse wave and the longitudinal wave was measured, as shown in FIG. 10, the conversion loss was minimum, that is, the excitation intensity was maximized at 280 MHz for the transverse wave and 147 MHz for the longitudinal wave. This result almost satisfies the above-mentioned relationship (5). On the other hand, when the conversion loss of the transverse wave and the longitudinal wave is compared at f _S = 280 MHz, which is the frequency at which the excitation intensity of the transverse wave is maximum, the former is about 5 dB and the latter is about 22 dB. This means that at frequency f _S , about 30% of the input energy is output in the transverse wave, whereas only about 0.6% of the input energy is output in the longitudinal wave. Therefore, the manufactured thin film resonator 10A can transmit and receive a transverse wave at the frequency f _S with almost no influence of the longitudinal wave.

  Up to this point, the case where there are four piezoelectric layers has been described as an example, but the number of piezoelectric layers may be three or five or more. In the thin film resonator 40 shown in FIG. 11A, a first piezoelectric layer 411, a second piezoelectric layer 412, and a third piezoelectric layer are sequentially arranged between the first electrode 421 and the second electrode 422 from the first electrode 421 side. Three layers of the piezoelectric layer 413 are provided. In addition, the thin film resonator 50 illustrated in FIG. 11B includes a first piezoelectric layer 511, a second piezoelectric layer 512, and a first piezoelectric layer 511 in order from the first electrode 521 side between the first electrode 521 and the second electrode 522. ... n layers (5 layers or more) of the nth piezoelectric layer 51n are provided. In any of the thin film resonators of these examples, as in the case of the thin film resonator 10, all the polarization vectors are inclined with respect to the normal of the plane α parallel to the piezoelectric layer. Similarly to the case of the thin film resonator 10, the projections of the polarization vectors onto the normal line are all in the same direction, while the projections of the polarization vectors onto the plane α are the same as those in the odd-numbered piezoelectric layers. The one in the second piezoelectric layer is in the opposite direction.

In the thin film resonator 40 and the thin film resonator 50, as in the case of the thin film resonator 10, the longitudinal wave vibration has the intensity of the first-order mode in which the sum d _total of the thicknesses of all the piezoelectric layers is 1/2 wavelength. The maximum of the transverse wave vibration is the intensity of the nth-order mode in which the thickness of two piezoelectric layers is one wavelength. The frequency f _L of the longitudinal wave having the maximum intensity is V _S / d _total and the frequency f _S of the transverse wave is (n / 2) V _S / d _total (the above formulas (1) and (2)). In both the thin film resonator 40 and the thin film resonator 50, since n is larger than 3, f _L and f _S take different values.

  Also in the thin film resonator 40 and the thin film resonator 50, a buffer layer can be provided between adjacent piezoelectric layers.

A thin film resonator 50 having six piezoelectric layers (n = 6) made of ZnO was manufactured, and conversion loss with respect to a transverse wave and a longitudinal wave was measured. The thickness of each piezoelectric layer and the inclination angle of the polarization vector are the same as those measured when the piezoelectric layer has four layers (the _total thickness d _total of the piezoelectric layers is “four layers”). 1.5 times). As a result, as shown in FIG. 12, the frequency f _S at which the excitation intensity of the transverse wave is maximum (conversion loss is minimum) is 280 MHz, whereas the frequency f _{L at} which the excitation intensity of the longitudinal wave is maximum is 84 MHz. Met. This substantially satisfies the relationship of the above-mentioned expression (3) (when f _S ≈3f _L : n = 6). By increasing the piezoelectric layer, the difference between f _S and f _L can be increased.

Next, the result of calculating the conversion loss for the transverse wave and the longitudinal wave of the thin film resonator in the case of n = 4 and 20 and the comparative example n = 1 and 2 will be described with reference to FIG. In this calculation, the material of the piezoelectric layer is ZnO, the angle between the polarization vector and the normal 13 is 25 °, the thickness of each piezoelectric layer is 5.54 μm, the electromechanical coupling constant k ₁₅ ′ is 0.23, k ₃₃ 'was set to 0.15. As indicated by the vertical dashed line in the figure, the transverse wave and longitudinal wave have the maximum excitation intensity at the same frequency, whereas when n = 4 and 20, the frequency at which the transverse wave has the maximum excitation intensity. In f _S (vertical broken line in the figure), the longitudinal wave does not have the maximum excitation intensity. In particular, when n = 20, the longitudinal wave has a conversion loss exceeding 60 dB at the frequency f _S and can be almost ignored. Note that even in the case of n = 1 as a comparative example, the frequency having the maximum excitation intensity is different between the longitudinal wave and the transverse wave, but the conversion loss of the longitudinal wave at the frequency f _S is higher than that in the case of n = 4 and 20. small.

  Up to this point, it has been described from the viewpoint of suppressing the excitation of the longitudinal wave at the frequency at which the transverse wave of the maximum intensity is excited (the longitudinal wave is not used), but in the thin film resonator according to the present invention, Alternatively, a longitudinal wave having a maximum intensity with a different frequency may be used together.

10, 10A, 40, 50, 90 ... thin film resonators 111, 411, 511, 911 ... first piezoelectric layers 112, 412, 512, 912 ... second piezoelectric layers 113, 413, 513 ... third piezoelectric layers 114, 514 ... 4th piezoelectric layer 51n ... nth piezoelectric layer 121, 421, 521, 921 ... 1st electrode 122, 422, 522, 922 ... 2nd electrode 13, 93 ... normal 141 of surface (alpha) ... 1st DESCRIPTION OF SYMBOLS 1 Buffer layer 142 ... 2nd buffer layer 143 ... 3rd buffer layer 21 ... Substrate 31 ... Anode 32 ... Cathode 33 ... Magnetron circuit 34 ... Substrate stand 341 ... Fixed surface 342 ... Axis perpendicular to fixed surface 341

Claims (7)

A thin film resonator in which an n layer (n is a natural number of 3 or more) piezoelectric layer is provided between a first electrode and a second electrode,
Oriented so that the polarization vector in each piezoelectric layer is inclined with respect to the normal of the plane parallel to the thin film resonator,
The projection of the polarization vector in each piezoelectric layer onto the normal is in the same direction,
The projection of the polarization vector in the odd-numbered piezoelectric layer counted from the first electrode side onto the surface is opposite to the projection of the polarization vector in the even-numbered piezoelectric layer counted from the first electrode side onto the surface. is there,
A thin film resonator characterized by that.   2. The thin film resonator according to claim 1, wherein each piezoelectric layer has the same thickness.   3. The thin film resonator according to claim 1, wherein a polarization vector of each piezoelectric layer and an angle formed by the normal line are the same. 4.   4. The thin film resonator according to claim 1, wherein the material of each piezoelectric layer has a wurtzite structure.   The buffer layer for preventing the direction of the crystal axis of one piezoelectric layer from affecting the direction of the crystal axis of the other piezoelectric layer is provided between two adjacent piezoelectric layers. The thin film resonator according to any one of 1 to 4.   6. The thin film resonator according to claim 5, wherein the material of the two adjacent piezoelectric layers is zinc oxide, and the material of the buffer layer is silicon dioxide.   The thin film resonator according to claim 5 or 6, wherein the buffer layer is amorphous.