JP4075887B2 - Surface acoustic wave device - Google Patents

Description

  The present invention relates to a surface acoustic wave device used as, for example, a resonator or a bandpass filter. More specifically, a dielectric thin film is formed on an elastic substrate, and the elastic substrate is interposed between the elastic substrate and the dielectric thin film. The present invention relates to a surface acoustic wave device in which an IDT electrode and / or a reflector electrode is formed.

  Conventionally, surface acoustic wave devices have been widely used, for example, in band filters of mobile communication devices. In the surface acoustic wave device, at least one interdigital electrode (hereinafter referred to as an IDT electrode) including at least a pair of comb-tooth electrodes is formed so as to be in contact with the piezoelectric body.

  As the piezoelectric body, piezoelectric substrates made of various piezoelectric materials are used. Also, a surface acoustic wave device having a structure in which a dielectric thin film having piezoelectricity is formed on an elastic substrate such as a glass substrate or a piezoelectric substrate, or a structure in which a dielectric thin film not having piezoelectricity is formed on a piezoelectric substrate. Has also been proposed. For example, a surface acoustic wave device in which a dielectric thin film is formed on an elastic substrate is disclosed in Non-Patent Document 1 below.

  A conventional surface acoustic wave device using a dielectric thin film mainly uses a transversal structure having a plurality of IDT electrodes arranged at a predetermined distance. In the transversal structure, unnecessary ripples are generated due to the reflection of TTE (triple transit echo) by the strip on the IDT electrode. Therefore, in order to suppress such unnecessary ripples, it is desirable to suppress reflection on the strip of the electrode as much as possible. Therefore, as the IDT electrode, a double strip type IDT electrode that does not cause acoustic reflection, an IDT electrode having a single strip in which reflection is suppressed, and the like have been used.

  On the other hand, in recent years, various surface acoustic wave devices using unidirectional strips have been proposed in order to reduce loss. In a surface acoustic wave device having a structure using a unidirectional strip, it is necessary to increase the reflection in the strip in order to increase the directivity.

Further, in a reflector used in a surface acoustic wave device having a resonator structure disclosed in the following Non-Patent Document 2 or the like, a desired number of strips can be obtained by forming the reflector with strips having a large amount of reflection. It is said that the amount of reflection can be obtained. In addition, in the many-to-one IDT disclosed in the above-mentioned “many-to-one IDT coupling mode analysis”, it is shown that the conversion characteristic of the IDT can be made steep by increasing the amount of reflection of the strip constituting the IDT. .
"Practical research on the SAW filter for TV using ZnO film" (Kadota, 26th EM Symposium, 1997, pp. 83-90) "Binding mode analysis of many-to-one IDT" (Telecommunications Journal, 1977/9, Vol. J60-A No. 9)

  As described above, conventionally, various surface acoustic wave devices that need to enhance the reflection of strips in IDT electrodes and reflectors have been proposed. However, in these surface acoustic wave devices, IDT electrodes and reflector electrodes are formed on the piezoelectric substrate.

  On the other hand, a surface acoustic wave device using the dielectric thin film described above, that is, a surface acoustic wave device in which a dielectric thin film having piezoelectricity is formed on a glass substrate or a piezoelectric substrate, or a dielectric having no piezoelectricity on a piezoelectric substrate. In a surface acoustic wave device having a structure in which a thin film or a dielectric thin film having piezoelectricity is formed, the reflection coefficient of the IDT electrode or reflector strip is small, and the steepness of the conversion characteristics of the IDT is increased, and the size and the loss are reduced. It was difficult to make it easier.

  In the present invention, a dielectric thin film is formed on an elastic substrate in view of the current state of the prior art described above, and an IDT electrode and / or a reflector electrode is formed between the dielectric thin film and the elastic substrate. In the structure, the reflection coefficient of the strip of the IDT electrode and / or the reflector electrode can be increased, whereby the steepness of the conversion characteristic can be increased, and the size and the loss can be reduced. An object is to provide a surface acoustic wave device.

The present invention includes an elastic substrate and a dielectric thin film formed on the elastic substrate, wherein at least one of the elastic substrate and the dielectric thin film is made of a piezoelectric material, and the elastic substrate and the dielectric An IDT electrode and / or reflector electrode formed between the body thin film and the IDT electrode and / or reflector electrode, wherein the IDT and / or reflector electrode has one strip disposed in a half-wavelength section of the surface wave in the surface wave propagation direction. It has at least one section, uses reflection by the strip , and when the wavelength of the surface wave is λ, the thickness of the strip is larger than 0.01λ, and the duty of the strip In the surface acoustic wave device in which the ratio is larger than 0.5 , the dielectric thin film covers the strip of the IDT electrode and / or the reflector electrode. The upper surface of the dielectric thin film portion on the trip is raised via an inclined surface with respect to the dielectric thin film portion away from the strip, and the inclined angle of the inclined surface with respect to the upper surface of the dielectric thin film portion on the strip is 40 degrees. wherein the range der Rukoto of 80 degrees.

  As described above, the present invention has a structure in which a dielectric thin film is formed on an elastic substrate, and an IDT electrode and / or a reflector electrode is formed between the elastic substrate and the dielectric thin film. Yes. Here, as the elastic substrate, an elastic substrate having no piezoelectricity such as a glass substrate or an alumina substrate, or a piezoelectric substrate made of piezoelectric ceramics or a piezoelectric single crystal is used. Further, as the dielectric thin film, when the elastic substrate does not have piezoelectricity, a dielectric thin film having piezoelectricity, such as a ZnO film, is used. However, when the elastic substrate has piezoelectricity, that is, when it is a piezoelectric substrate, the dielectric thin film may be a dielectric thin film having piezoelectricity, that is, a piezoelectric thin film, or a dielectric having no piezoelectricity. It may be a thin film.

  In order to constitute a surface acoustic wave device, at least one IDT electrode is usually formed. However, in the present invention, the IDT electrode and / or the reflector electrode has a half-surface wave in the surface wave propagation direction. There is at least one section in which one strip is arranged in the wavelength section, and reflection by the strip is used.

In the present invention, in the portion where the dielectric thin film covers the strip of the IDT electrode and / or the reflector electrode, the dielectric thin film portion on the strip passes through the inclined surface with respect to the dielectric thin film portion away from the strip. are raised, the inclination angle with respect to the upper surface of the dielectric thin film portion covering the strip of the inclined surface are in the range of 4 0 to 80 degrees, whereby the reflection coefficient of the strip is increased one layer.

  In the present invention, the density of the dielectric thin film is preferably higher than that of the IDT electrode and / or the reflector electrode, thereby effectively increasing the reflection coefficient of the strip.

In the present invention, as a material constituting the IDT electrode and / or reflector electrode, aluminum, an alloy obtained by adding copper, silicon, titanium or the like to aluminum, or various metals or alloys such as gold or copper are used. In addition, a ZnO thin film, a Ta ₂ O ₅ thin film, a CdS thin film, or the like is used as the dielectric thin film, and quartz, glass, sapphire, silicon, or the like is used as the elastic substrate material.

In a conventional configuration in which an IDT electrode and / or a reflector electrode is formed on a piezoelectric substrate such as quartz, LT, or LBO, even if the film thickness or line width of the electrode is changed, the sign of κ ₁₂ does not change, It is constant.

However, in a specific aspect of the present invention, when aluminum is used as a material constituting the IDT electrode and / or reflector electrode, and a ZnO thin film is used as the dielectric thin film, the IDT is determined by the density of the dielectric thin film. The density of the material constituting the electrode and / or reflector electrode is reduced. For this reason, the sign of κ ₁₂ changes from positive to zero to negative by adjusting the line width and film thickness of the strip constituting the IDT electrode and / or the reflector electrode, and κ when | κ ₁₂ | is the maximum value. The sign of ₁₂ is negative and therefore tends to be different from the conventional configuration.

Furthermore, under the condition that the sign of κ ₁₂ is negative, the phase of the acoustic reflected wave and the reflected wave due to electrical re-excitation is the same, so an open reflector constructed by electrically opening the strip is used. Then, a large reflection coefficient is obtained in accordance with the electro-mechanical coupling coefficient K ^2.

  In addition, a configuration using a glass substrate or a sapphire substrate is well known as the elastic substrate, but when a quartz substrate having a specific cutting orientation in which the frequency temperature coefficient is positive is used, the dielectric film has a negative polarity. A surface wave device excellent in temperature stability in which the frequency temperature coefficient is canceled and the frequency temperature coefficient becomes zero can be obtained.

  Although the structure of the IDT electrode and / or reflector electrode is not particularly limited, in another specific aspect of the present invention, an IDT electrode is provided, and the IDT electrode is a unidirectional IDT electrode, and thus is reflected by the present invention. A unidirectional IDT electrode with an increased coefficient is constructed.

In the present invention, in the raised portion where the dielectric thin film covers the strip of the IDT electrode and / or the reflector electrode, the inclination angle is in the range of 40 degrees to 80 degrees . The reflection efficiency can be increased. Therefore, when the tilt angle is increased to 40 degrees or more in the portion where the IDT electrode is formed, the resonance efficiency can be improved, and a resonator type filter having a steep filter characteristic or a resonator having a high Q can be obtained. If the tilt angle is increased to 40 degrees or more in the portion where the reflector electrode is formed, the reflector can be downsized.

  Therefore, according to the present invention, the dielectric thin film is formed on the elastic substrate, and the IDT electrode and / or the reflector electrode are formed between the two, and the reflection by the strip of the IDT electrode and / or the reflector electrode. In the surface acoustic wave device using the surface acoustic wave device, since the reflection efficiency of the strip at the IDT electrode and / or the reflector electrode can be increased, the surface acoustic wave device can be reduced in size and reduced in loss.

  Hereinafter, the present invention will be described in more detail by describing specific embodiments of the present invention with reference to the drawings.

(First embodiment)
1A and 1B are a schematic plan view of a surface acoustic wave device according to a first embodiment of the present invention and a schematic enlarged cross section of a portion along the line BB in FIG. FIG.

  In the surface acoustic wave device 1 of the present embodiment, a ZnO thin film 3 is formed as a dielectric thin film on a quartz substrate 2 of a rotation 27 degree Y plate. Further, IDT electrodes 4 and 5 made of an aluminum thin film are provided between the quartz crystal substrate 2 and the ZnO thin film 3.

  In the surface acoustic wave device 1, when the IDT electrodes 4 and 5 are excited, a Rayleigh wave propagating in the X direction with a sound velocity of about 2500 to 3000 m / sec is generated.

  The ZnO thin film 3 is usually formed on the quartz substrate 2 by a thin film forming method such as sputtering. In this case, unevenness is generated on the upper surface of the ZnO thin film 3 in the portion where the strip 4 a of the IDT electrode 4 disposed on the quartz substrate 2 is provided. As shown in FIG. 1B, on the upper surface of the ZnO thin film 3, a portion covering the strip 4a is raised. That is, the raised portion 3a is formed on the other portion of the ZnO thin film 3 via the inclined surface 3b.

  The inclination angle θ between the inclined surface 3b and the upper surface of the raised portion 3a is usually 30 degrees (for example, “Reduction of variation in frequency characteristics of SAW filter by polishing ZnO thin film surface”) Vol. J96-A No. 10, 1993/10)).

The inventor of the present application has proposed a finite number proposed in pages 21 to 27 of “A Finite Element Analysis of Periodic Structure Piezoelectric Waveguide” (Electronic Communication Society Transactions, Vol. J68-C No. 1, 1985/1). Using the element method, in the structure in which one strip 4a shown in FIG. 1B is arranged in a half-wavelength section, the stop band frequencies of the open strip and the short strip and the sound velocity on the free surface were obtained. Further, based on the method proposed in “Introduction to Surface Acoustic Wave Device Simulation Technology” (Hashimoto, Realize Co., Ltd., page 233), κ ₁₂ / k ₀ representing the reflection amount in the Rayleigh wave strip was obtained. . κ ₁₂ represents an inter-mode coupling coefficient based on the mode coupling theory, and k ₀ represents λ / 2π.

  The results are shown in FIGS.

2A and 2B and FIGS. 3A and 3B, the film thickness T of the ZnO thin film 3 is 0.2λ to 0.5λ, and the film thickness H of the strip 4a is 0.01λ to 0.00. 04Ramuda, the inclination angle θ of the inclined surface 3b is a diagram showing the relationship between the κ ₁₂ / k ₀ and the duty ratio and strip thickness H in the case of 30 degrees.

Further, FIG. 4 shows a case where the ZnO thin film thickness T = 0.3λ, the strip 4a thickness H = 0.01λ to 0.08λ, and the inclination angle θ = 90 degrees and κ ₁₂ / k ₀ and the duty It is a figure which shows the relationship between ratio and the film thickness H of a strip.

As is clear from FIGS. 2A to 4, it can be seen that the absolute value of κ ₁₂ is small in the region where the duty ratio is small, and κ ₁₂ shows a positive value in the region where the film thickness H of the strip is small. . Further, as the duty ratio increases, κ ₁₂ shows a negative value. This phenomenon, than the density of the strips 4a made of aluminum (= 2.69g / cm ^3), is believed to occur because the density of the ZnO thin film ³ (= 5.68g / cm 3) is large.

Therefore, although a slight variation is seen depending on the inclination angle θ, the absolute value of κ ₁₂ can be increased if the duty ratio is larger than 0.5, compared to the case where the duty ratio is 0.5. It can be seen that the amount of reflection by the strip can be increased. More preferably, if the duty ratio is 0.6 or more, the maximum value of the absolute value of κ ₁₂ can be obtained.

FIG. 5 shows, as a comparative example, the electromechanical coupling coefficient K ² when the film thickness T of the ZnO thin film is 0.3λ, the film thickness H of the strip 4a is 0.02λ, and the inclination angle θ = 30 degrees, It is a figure which shows the relationship between a duty ratio and the film thickness H of a strip. As is apparent from FIG. 5, it can be seen that the electromechanical coupling coefficient K ² becomes maximum when the duty ratio is near 0.5, and decreases when it deviates from 0.5. Therefore, for example, when configuring a resonator or a resonator type filter, since the bandwidth is strongly correlated with the electromechanical coupling coefficient, in this embodiment, the duty ratio can be selected according to the desired bandwidth. I know it ’s good.

(Second embodiment)
FIG. 6 is a schematic plan view showing an electrode structure of the surface acoustic wave device according to the second embodiment of the present invention.

  In the surface acoustic wave device 10, a reflector 13 is disposed between the IDT electrodes 11 and 12. The reflector 13 has a structure in which a section A in which one strip 13a is arranged in a half-wavelength section is continuous in the 300 section surface wave propagation direction. The IDT electrodes 11 and 12 are double strip type IDT electrodes having 20 pairs of electrode fingers. The strip arrangement pitch of the IDT electrodes 11 and 12 and the reflector electrode 13 is the same.

  Further, as the elastic substrate, a quartz substrate with a 27-degree rotation Y plate is used. After the IDT electrodes 11 and 12 and the reflector electrode 13 are formed on the quartz substrate, a ZnO thin film is formed (not shown). did. That is, as in the case of the first embodiment, the electrode structure shown in FIG. 6 is formed between the quartz substrate and the ZnO thin film. The IDT electrodes 11 and 12 and the reflector electrode 13 are made of aluminum.

In the frequency transmission characteristic of the surface acoustic wave device 10 shown in FIG. 6, the surface wave propagating from the IDT electrode 11 to the IDT electrode 12 does not propagate in the reflection band of the reflector 13 but propagates outside the reflection band of the reflector. . Therefore, in the frequency transmission characteristics of the surface acoustic wave device 10, a stop band is generated near the center frequency. Based on the bandwidth of this stop band, κ ₁₂ / k ₀ was measured based on the following equation (1).

κ ₁₂ / k ₀ = (bandwidth of stopband / center frequency) / 2 (1)
Table 1 below shows the value of κ ₁₂ / k ₀ measured as described above.

  As can be seen from Table 1, it can be seen that the values shown in FIG.

(Third embodiment)
The surface acoustic wave device of the third example was configured in the same manner except that a glass substrate was used instead of the quartz crystal substrate in the first example. In this case, a Rayleigh wave is generated in the vicinity of the sound speed of 2500 to 2900 / s.

FIGS. 7A and 7B and FIGS. 8A and 8B show the ZnO thin film thickness T = 0.1λ to 0.7λ and the strip thickness H = 0 in the third embodiment. The relationship between κ ₁₂ / k ₀ , the duty ratio, and the film thickness H of the strip when .01λ to 0.35λ and the inclination angle θ of the inclined surface is 30 degrees is shown.

As apparent from FIGS. 7 and 8, when the thickness of the ZnO thin film is 0.2λ or more and the thickness H of the strip is 0.01λ, almost no reflection occurs near the duty ratio = 0.5, with the duty ratio is out of 0.5, it can be seen that the kappa ₁₂ increases. Further, when the thickness H of the strip is greater than 0.01λ, for example, 0.35λ, when the duty ratio is larger than 0.5, κ ₁₂ is greater than when the duty ratio is 0.5. It can be seen that the absolute value of increases and the reflection coefficient increases. Further, when the duty ratio is 0.6 or more, kappa ₁₂ takes a maximum value, it can be seen that the reflection coefficient is further improved.

A surface acoustic wave device similar to that shown in FIG. 1 was constructed. However, the film thickness T of the ZnO thin film 3 is set to 0.3λ, the film thickness H of the strip 4a is set to 0.02λ, and the inclination angle θ described above is changed in a range of 20 degrees to 80 degrees, and κ ₁₂ / k ₀ , The relationship with the duty ratio was investigated. The results are shown in FIG.

As is apparent from FIG. 9, when the duty ratio is optimized, the absolute value of κ ₁₂ varies depending on the tilt angle θ, and when θ is increased compared to the conventional tilt angle θ = 30 degrees, κ _It can be seen that the absolute value of ₁₂ also increases.

FIG. 10 shows the relationship between the duty ratio at which the absolute value of κ ₁₂ is maximized and the value of κ ₁₂ / k _{0 in} that case for each inclination angle θ. As is apparent from FIG. 10, when the tilt angle θ is larger than 30 degrees, κ ₁₂ increases, and preferably 40 degrees or more can stabilize κ ₁₂ against changes in the tilt angle θ. It can be seen that a surface acoustic wave device having a large κ ₂ can be obtained.

  The tilt angle θ can be easily controlled by adjusting the incident angle of the film-forming particles when the ZnO thin film is formed by sputtering. For example, by adjusting the mounting angle between the planetary and the substrate shown in “Practical research on SAW filter for TV using ZnO thin film” (Kadoda, 26th EM Symposium, 1997, pp. 84, FIG. 3), By making the incident angle of the film-forming particles close to vertical, the inclination angle θ can be easily made larger than 30 degrees.

  Further, the inclination angle θ can be increased by making the side surface in the cross section of the electrode strip closer to the direction orthogonal to the elastic substrate surface, that is, by making the side surface appearing in the cross section steep. Thus, steepening the side surface appearing in the cross-sectional shape of the electrode strip can be easily achieved by selecting a method of forming the IDT electrode or the reflector electrode. For example, in order to sharpen the cross-sectional shape of the electrode strip, the dry etching method is most preferable, the lift-off method is next preferable, and the wet etching method is next preferable.

In the first to fourth embodiments described above, the case where the ZnO thin film is used as the dielectric thin film has been described as an example. However, in addition to the ZnO thin film, the SiO ₂ thin film, the AlN thin film, the Ta ₂ O ₅ thin film, and the CdS Various dielectric thin films such as a thin film can be used. That is, even when an appropriate dielectric thin film is used in which the tilt angle at the raised portion on the surface of the dielectric thin film affects the amount of reflection of the surface wave, the same effect as in the above embodiment can be obtained.

  Moreover, in the said Example, although the IDT electrode and / or reflector electrode which consist of aluminum were formed, also when a surface acoustic wave apparatus is comprised using common metal materials, such as copper and gold | metal | money, the above-mentioned The same effect as the embodiment can be obtained.

  Further, in the first embodiment and the second embodiment, a quartz substrate with a 27-degree rotation Y plate is used, but a quartz substrate with another crystal orientation may be used. Further, the glass substrate used in the third embodiment can be made of various glasses such as borosilicate glass or Pyrex (registered trademark) glass. Furthermore, in the present invention, the elastic substrate is not limited to such a quartz substrate or glass substrate, but may be composed of another insulating substrate, or a piezoelectric substrate made of a piezoelectric single crystal or piezoelectric ceramic. It may be.

  The surface acoustic wave device according to the present invention has at least one section in which one strip is disposed in a half-wavelength section as described above, but the half-wavelength section is continuously disposed. When the configuration is adopted in the reflector, the number of the strips of the reflector can be reduced as compared with the conventional case. In addition, when a structure in which half-wavelength sections are continuously arranged is adopted for the IDT electrode, the resonance efficiency inside the IDT electrode can be increased, and a good resonator type filter and a resonator having a high Q are configured. can do.

  Further, when the half-wavelength section is used as a reflective element of a unidirectional IDT electrode such as a reflective bank type unidirectional electrode, FEUDT or EWC electrode, the unidirectionality can be further enhanced.

(A) And (b) is a schematic plan view of the surface acoustic wave device according to the first embodiment, and a cross-sectional view showing an enlarged portion along the line BB in (a). (A) and (b), in the first embodiment, the κ ₁₂ / k ₀ indicating the reflection efficiency, shows a relationship between the duty ratio and strip thickness H. (A) and (b), in the first embodiment, the κ ₁₂ / k ₀ indicating the reflection efficiency, shows a relationship between the duty ratio and strip thickness H. In the first embodiment, the ZnO thin film thickness T = 0.3λ, the strip thickness H = 0.01λ to 0.08λ, and the inclination angle θ = 90 °, κ ₁₂ / k ₀ , the duty ratio, The figure which shows the relationship with the film thickness H of a strip. In the second embodiment, the film thickness T = 0.1λ to 0.7λ of the ZnO thin film formed on the glass substrate, the film thickness H of the strip H = 0.01λ to 0.35λ, and the inclination angle θ = 30 degrees. It shows the κ ₁₂ / k ₀ in the case of the relationship between the duty ratio and strip thickness H. The typical top view which shows the electrode structure of the surface acoustic wave apparatus which concerns on the 3rd Example of this invention. (A) and (b), in the first embodiment, the κ ₁₂ / k ₀ indicating the reflection efficiency, shows a relationship between the duty ratio and strip thickness H. (A) and (b), in the first embodiment, the κ ₁₂ / k ₀ indicating the reflection efficiency, shows a relationship between the duty ratio and strip thickness H. In the third embodiment, when the ZnO thin film thickness T = 0.3λ, the strip thickness H = 0.02λ, and the inclination angle θ = 20 degrees to 80 degrees, κ ₁₂ / k ₀ and the duty ratio The figure which shows a relationship. In the third embodiment, shows a duty ratio absolute value of kappa ₁₂ is maximized, the inclination angle dependence of the value of κ ₁₂ / k ₀ in this case.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave apparatus 2 ... Quartz substrate 3 ... ZnO thin film 3a ... Raised part 3b ... Inclined surface 10 ... Surface acoustic wave apparatus 11, 12 ... IDT
14: Reflector electrode θ: Tilt angle

Claims (7)

An elastic substrate;
A dielectric thin film formed on the elastic substrate, and at least one of the elastic substrate and the dielectric thin film is made of a piezoelectric material,
An IDT electrode and / or a reflector electrode formed between the elastic substrate and the dielectric thin film;
The IDT and / or reflector electrode has at least one section in which one strip is disposed in a half-wave section of the surface wave in the surface wave propagation direction, and utilizes reflection by the strip , In the surface acoustic wave device in which the film thickness of the strip is larger than 0.01λ and the duty ratio of the strip is larger than 0.5 when the wavelength is λ ,
In the portion where the dielectric thin film covers the strip of the IDT electrode and / or the reflector electrode, the upper surface of the dielectric thin film portion on the strip is inclined with respect to the dielectric thin film portion away from the strip. are raised Te, the inclination angle with respect to the upper surface of the dielectric thin film portion on the strip of the inclined surface is characterized the scope der Rukoto of 40 to 80 degrees, the surface acoustic wave device.   The surface acoustic wave device according to claim 1, wherein an inclination angle of the inclined surface is larger than 40 degrees.   The surface acoustic wave device according to claim 1, wherein a density of the dielectric thin film is larger than a density of the IDT electrode and / or a reflector electrode.   The surface acoustic wave device according to claim 1, wherein the IDT electrode and / or the reflector electrode is made of aluminum.   The surface acoustic wave device according to claim 1, wherein the dielectric thin film is a ZnO thin film.   The surface acoustic wave device according to claim 1, wherein the elastic substrate is made of quartz or glass.   The surface acoustic wave device according to claim 1, comprising an IDT as the IDT electrode and / or reflector electrode, wherein the IDT electrode is a unidirectional IDT electrode.

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