JP5874738B2 - Surface acoustic wave device - Google Patents

Description

The present invention relates to a surface acoustic wave device using a LiNbO ₃ substrate, and more particularly to a surface acoustic wave device using a Love wave propagating through a LiNbO ₃ substrate.

Conventionally, various surface acoustic wave devices using a Love wave propagating through a LiNbO ₃ substrate have been proposed. For example, Patent Document 1 below discloses an example of such a surface acoustic wave device.

In Patent Document 1, an IDT electrode made of Au is formed on a LiNbO ₃ substrate. A SiO ₂ film is formed so as to cover the IDT electrode. In Patent Document 1, it is possible to use a love wave propagating through a LiNbO ₃ substrate by using an IDT electrode made of Au having a large mass.

JP-A-5-335879

  However, the surface acoustic wave device described in Patent Document 1 has a problem that loss is large because an IDT electrode made of Au having a large mass is used. That is, in a structure using an IDT electrode made of Au having a large mass, the thickness of the IDT electrode is relatively thin. For this reason, there is a problem that the electrical resistance increases and the loss increases.

An object of the present invention is to provide a surface acoustic wave device that uses a love wave propagating through a LiNbO ₃ substrate and can reduce loss.

The surface acoustic wave device according to the present invention includes a LiNbO ₃ substrate, an IDT electrode formed on the LiNbO ₃ substrate, and a dielectric layer provided so as to cover the IDT electrode. In the present invention, the IDT electrode is composed mainly of Al, and the LiNbO ₃ are grooves formed between the electrode fingers of the IDT electrode in the upper surface of the substrate, are used Love wave propagating LiNbO ₃ substrate. That the IDT electrode is mainly composed of Al means that Al exceeds 50% by mass of the entire IDT electrode. The IDT electrode may include an electrode layer made of Al or an alloy containing Al as a main component as a main electrode layer, and another electrode layer may be formed as a subordinate electrode layer. On the other hand, when the main electrode layer is composed of a plurality of electrode layers, it means one or more electrode layers exceeding 50% by weight of the entire IDT electrode in weight ratio.

  In another specific aspect of the surface acoustic wave device according to the present invention, the IDT electrode is made of Al or an alloy containing Al as a main component. Thus, in the present invention, the IDT electrode may be a single layer electrode made of Al or an alloy containing Al as a main component.

  The alloy containing Al as a main component means an alloy containing Al in excess of 50% by weight.

  In still another specific aspect of the surface acoustic wave device according to the present invention, the dielectric layer is made of a silicon oxide film. In the case of a silicon oxide film, the frequency temperature characteristic can be improved.

  In the surface acoustic wave device according to the present invention, a second dielectric layer formed on the silicon oxide film may be further formed. When such a second dielectric layer is formed, the frequency can be adjusted by adjusting the thickness of the second dielectric layer.

  As the second dielectric layer, preferably, at least one dielectric selected from the group consisting of silicon nitride, silicon oxynitride, and aluminum nitride can be used.

In still another specific aspect of the surface acoustic wave device according to the present invention, the surface acoustic wave device further includes an adhesion layer formed between the IDT electrode and the LiNbO ₃ substrate. By forming the adhesion layer, the bonding strength of the IDT electrode to the LiNbO ₃ substrate can be increased.

As the adhesion layer, Ti, Ni or an alloy containing these as a main component is preferably used. An alloy containing these as a main component means an alloy in which the content ratio of these metals is 50% by weight. When the adhesion layer is made of Ti, Ni or an alloy containing these metals as a main component, the adhesion of the IDT electrode to the LiNbO ₃ substrate can be further enhanced.

In the present invention, a protective layer may be provided on the upper surface of the IDT electrode. As a result, the IDT electrode can be protected. Preferably, the protective layer is made of a material that is harder to etch than Al. Accordingly, when etching is performed when forming a groove in the LiNbO ₃ substrate or when etching is performed when forming an IDT electrode mainly composed of Al, a metal portion mainly composed of Al constituting the IDT electrode. Damage can be suppressed.

In still another specific aspect of the surface acoustic wave device according to the present invention, the second Euler angle θ of the LiNbO ₃ substrate is in the range of 70 ° to 105 °.

  In still another specific aspect of the surface acoustic wave device according to the present invention, when the wavelength determined by the pitch of the electrode fingers of the IDT electrode is λ, the depth of the groove is 0.04λ or more.

  In still another specific aspect of the surface acoustic wave device according to the present invention, when the wavelength determined by the pitch of the electrode fingers of the IDT electrode is λ, the thickness of the IDT electrode is 0.125λ or more.

  In still another specific aspect of the surface acoustic wave device according to the present invention, when the wavelength determined by the pitch of the electrode fingers of the IDT electrode is λ, the thickness of the dielectric layer is 0.1λ or less.

In the surface acoustic wave device according to the present invention, a groove is formed between the electrode fingers of the IDT electrode on the upper surface of the LiNbO ₃ substrate, and the IDT electrode is mainly composed of Al. Therefore, a love wave propagating through the LiNbO ₃ substrate is generated. A surface acoustic wave device can be provided. In addition, since the IDT electrode is mainly Al, the loss can be reduced as compared with the surface acoustic wave device described in Patent Document 1.

FIG. 1A and FIG. 1B are a partially cutaway front sectional view of a surface acoustic wave device according to an embodiment of the present embodiment and a schematic front sectional view showing an enlarged electrode structure. FIG. 2 is a schematic plan view showing an electrode structure of the surface acoustic wave device according to one embodiment of the present invention. FIG. 3 is a diagram illustrating impedance characteristics of the surface acoustic wave device according to the embodiment of the present invention. FIG. 4 is a diagram showing the relationship between the depth of the groove and the width of the stop band in the surface acoustic wave device according to one embodiment of the present invention. FIG. 5 is a schematic front cross-sectional view showing a main part of a surface acoustic wave device according to a modification of one embodiment of the present invention. FIG. 6 is a diagram showing the relationship between the depth of the groove and the sound velocity of the excited love wave in the surface acoustic wave device according to the embodiment of the present invention. FIG. 7 is a diagram showing the relationship between the thickness of the IDT electrode and the sound speed of the excited love wave in the surface acoustic wave device according to one embodiment of the present invention. FIG. 8 is a diagram showing the relationship between the thickness of the dielectric layer in the surface acoustic wave device according to one embodiment of the present invention and the sound speed of the higher-order mode of the excited love wave. FIG. 9 is a diagram showing the relationship between the Euler angle θ of the LiNbO ₃ substrate and the specific bandwidth in the surface acoustic wave device according to one embodiment of the present invention.

  Hereinafter, the present invention will be clarified by describing specific embodiments of the present invention with reference to the drawings.

  FIG. 1A is a partially cutaway front sectional view of a surface acoustic wave device according to the present embodiment, and FIG. 1B is a schematic front sectional view showing an enlarged main part thereof.

The surface acoustic wave device 1 has a LiNbO ₃ substrate 2. An IDT electrode 3 is formed on the LiNbO ₃ substrate 2. In FIG. 1A, only the portion where the IDT electrode 3 is formed is shown. However, in the present embodiment, the electrode structure shown in FIG. 2 is formed on the LiNbO ₃ substrate 2. That is, an IDT electrode 3 and reflectors 6 and 7 arranged on both sides of the IDT electrode 3 in the surface acoustic wave propagation direction are provided. Thereby, a surface acoustic wave resonator is formed.

As shown in FIG. 1A, the IDT electrode 3 has a plurality of electrode fingers 3a. A groove 2a is formed on the upper surface of the LiNbO ₃ substrate 2 between the adjacent electrode fingers 3a.

The groove 2 a can be formed by etching the LiNbO ₃ substrate 2.

  In the present embodiment, the IDT electrode 3 is made of Al or an alloy containing Al as a main component. However, the IDT electrode 3 is not particularly limited as long as it is mainly composed of Al. “Al mainly” means an IDT electrode in which Al exceeds 50% by weight in the entire IDT electrode 3. For example, when the IDT electrode 3 is made of a single-layer metal film, it is made of Al or an alloy containing Al as a main component. An alloy containing Al as a main component refers to an alloy containing Al at a ratio exceeding 50% by weight of Al.

  Moreover, when it consists of a some metal film, the ratio which occupies 50 weight% of the whole should just consist of an alloy which has Al or Al as a main component. Even in this case, Al is contained in a ratio exceeding 50 wt% of the entire IDT electrode 3.

  Cu, Si, Mg, Mn, Fe, Zn etc. can be mentioned as a metal which forms the alloy which has Al as a main component with Al.

A silicon oxide film 4 is formed as a first dielectric layer so as to cover the upper surface of the LiNbO ₃ substrate 2. A silicon nitride film 5 is formed as a second dielectric layer so as to cover the silicon oxide film 4.

Note that the portion where the reflectors 6 and 7 are formed also has the same structure as that shown in FIG. That is, a groove is formed on the LiNbO ₃ substrate 2 between adjacent electrode fingers of the reflectors 6 and 7.

The surface acoustic wave device 1 according to this embodiment is characterized by using the LiNbO ₃ substrate 2 and using the IDT electrode 3 made of Al having a low mass or an alloy containing Al as a main component, but the groove 2a. This is because it is possible to use love waves. This will be described based on a specific experimental example.

As the LiNbO ₃ substrate 2, Euler angles (0 °, 85 °, 0 °) was prepared LiNbO ₃ substrate. A groove 2a having a depth of 0.1λ was formed on the upper surface of the LiNbO ₃ substrate 2 when the surface acoustic wave wavelength was λ. The IDT electrode 3 is made of Al and has a thickness of 0.2λ. The duty of the IDT electrode 3 was 0.7. Therefore, the pitch between the electrode fingers 3a and 3a of the IDT electrode 3 is λ / 2, but the width of the groove 2a is 0.15λ, and the width of the electrode finger 3a of the IDT electrode 3 is 0.35λ.

As the silicon oxide film 4, an SiO ₂ film having a thickness of 0.1λ was formed. The thickness referred to here is a distance H from the upper surface of the IDT electrode 3 to the upper surface of the silicon oxide film 4 as shown in FIG. The silicon oxide film 4 has a positive frequency temperature characteristic. On the other hand, LiNbO ₃ has negative frequency temperature characteristics. Therefore, the absolute value of the frequency temperature coefficient of the surface acoustic wave device 1 can be reduced by forming the silicon oxide film 4.

A SiN film having a thickness of 0.2λ was formed as the silicon nitride film 5 as the second dielectric layer. The SiO ₂ film and the SiN film were formed by a sputtering method. As the sputtering method, since the quality of the SiO ₂ film is particularly improved, the bias sputtering method is desirable. Then, etching back or the like is performed after sputtering to flatten the surface of the SiO ₂ film. The IDT electrode 3 was formed by photolithography on the piezoelectric substrate on which the groove 2a was formed. FIG. 3 shows the resonance characteristics of the surface acoustic wave device 1 obtained as described above. Moreover, the speed of sound at the resonance frequency of the surface acoustic wave device 1 was 3800 m / sec. Therefore, it can be seen that the love wave is excited because it is slower than the slow transverse sound velocity propagating through the LiNbO ₃ substrate.

  In addition, as shown in FIG. 3, the specific bandwidth is about 5% of the resonance frequency, and the impedance ratio is good at about 60 dB. That is, a stop band having a width of 5% or more is obtained, and a sufficiently high reflection coefficient is obtained.

FIG. 4 shows the relationship between the depth of the groove and the width of the stop band in the surface acoustic wave device 1. The width of the stop band is expressed as a percentage (%) with respect to the resonance frequency, and the depth (%) of the groove is a depth from the upper surface of the LiNbO ₃ substrate, and is expressed as a ratio (%) with respect to λ. As shown in FIG. 4, when no groove is formed between the IDT electrodes, only a stop band of about 2% can be obtained, and a band higher than the stop band cannot be obtained. It cannot be made. However, by providing a groove between the IDT electrodes, a stop band larger than 2%, that is, a sufficiently high reflection coefficient can be obtained. Therefore, according to the present invention, a wide band can be achieved. The reason why a sufficiently high reflection coefficient can be obtained by this structure will be described later.

FIG. 6 shows the relationship between the groove depth in the surface acoustic wave device 1 and the sound speed of the excited love wave. As shown in FIG. 6, by setting the depth of the groove to 4% (0.04λ) or more, the sound speed of the excited love wave becomes slower than 4050 m / sec, which is the slow transverse wave speed of the LiNbO ₃ substrate. . For this reason, a love wave is hard to leak and a love wave can be confined effectively.

FIG. 7 shows the relationship between the thickness of the IDT electrode and the speed of sound of the excited love wave in the surface acoustic wave device 1. As shown in FIG. 7, by setting the thickness of the IDT electrode to 12.5% (0.125λ) or more, the sound velocity of the excited love wave is higher than 4050 m / sec, which is the slow transverse wave velocity of the LiNbO ₃ substrate. Become slow. For this reason, a love wave is hard to leak and a love wave can be confined effectively.

FIG. 8 shows the relationship between the thickness of the dielectric layer in the surface acoustic wave device 1 and the sound speed of the higher-order mode of the excited love wave. As shown in FIG. 8, when the thickness of the dielectric layer was 8%, 6%, 4%, 2%, higher order modes were not observed. From this result, it can be seen that by setting the thickness of the dielectric layer to 10% (0.1λ) or less, the occurrence of spurious due to the higher order mode can be suppressed. In addition, when the thickness of the dielectric layer is 10% (0.1λ) or less, the higher-order mode was not observed because the sound speed of the higher-order mode was faster than the fast shear wave sound speed of the LiNbO ₃ substrate. This is considered to be due to leakage to the LiNbO ₃ substrate side.

FIG. 9 shows the relationship between the Euler angle θ of the LiNbO ₃ substrate and the specific bandwidth in the surface acoustic wave device 1. As shown in FIG. 9, by setting Euler angle θ of LiNbO ₃ substrate to 70 ° to 105 °, the specific bandwidth of Rayleigh wave is suppressed to 0.3% or less while keeping the specific bandwidth of Love wave wide. I understand that I can do it.

The data shown in FIGS. 6 to 9 is data under the following conditions.
Dielectric layer material: SiO ₂
IDT electrode material: Al
Second dielectric layer: SiN film having a thickness of 2% (0.02λ) Duty ratio: 0.7
Temperature: 25 ° C
Euler angle θ in FIGS. 6 to 8: 85 °
Groove depth in FIGS. 7-9: 10% (0.1λ)
6, 8 and 9 IDT electrode thickness: 20% (0.2λ)
The thickness of the dielectric layer in FIGS. 6, 7, and 9: 10% (0.1λ)

As described above, conventionally, when a Love wave is excited using a LiNbO ₃ substrate, Au having a high mass has to be used as an IDT electrode. Therefore, there is a problem that the loss increases. On the other hand, according to the present embodiment, the IDT electrode 3 mainly composed of light Al may be used. Therefore, the thickness of the IDT electrode 3 can be increased and the duty ratio can be further increased to reduce the loss. Here, even if the IDT electrode has a thickness of 0.2λ and a duty ratio of 0.70, when the groove is not formed, the anti-resonance frequency is cut off, which is favorable. It has been confirmed by simulation of FEM that impedance characteristics cannot be obtained. On the other hand, in this embodiment, as shown in FIG. 3, a favorable impedance characteristic can be obtained.

In addition, when a dielectric layer, particularly a SiO ₂ film, is formed so as to cover the IDT electrode, it is necessary to use a high-mass Au electrode as described above in order to obtain a sufficient reflection coefficient. Therefore, there is a problem that the electrode film thickness and the duty ratio are limited and the loss becomes large. When a material such as Al is used as the IDT electrode in order to prevent loss deterioration, a high reflection coefficient can be obtained because the density of the dielectric layer formed on the electrode, for example, the SiO ₂ film and the Al electrode is almost the same. Absent. For this reason, in the conventional structure, it was impossible to achieve both reduction in loss and increase in the reflection coefficient. On the other hand, according to the present invention, by forming the groove between the IDT electrodes, the factor given to the reflection coefficient is not the difference in density between the IDT electrode and the SiO ₂ film, but between the LiNbO ₃ substrate and the SiO ₂ film. It becomes density difference. Therefore, a sufficiently high reflection coefficient can be obtained. In addition, since an Al electrode is used, a reduction in loss can be achieved.

FIG. 5 is a schematic front sectional view showing the main part of a surface acoustic wave device according to a modification of the embodiment. In this modification, an adhesion layer 8 is formed between the IDT electrode 3 and the LiNbO ₃ substrate 2. The adhesion layer 8 is not an essential component in the present invention. However, the adhesion strength of the IDT electrode 3 to the LiNbO ₃ substrate 2 can be increased by providing the adhesion layer 8. Such an adhesion layer 8 can be formed of, for example, Ni, Cr, or an alloy mainly composed of these.

  Furthermore, in this modification, the protective layer 9 is formed on the upper surface of the IDT electrode 3. As the protective layer 9, a material that is more difficult to etch than Al is preferably used. Accordingly, when the groove 2a is formed during the etching for forming the IDT electrode 3 or after the IDT electrode 3 is formed, the IDT electrode 3 can be protected during the etching for forming the groove 2a.

  As a material constituting the protective layer 9, Cu, Ti, etc., which are more difficult to etch than Al, can be suitably used.

Although the surface acoustic wave resonator has been described in the above embodiment and the modification, the present invention can be applied to various surface acoustic wave devices using a LiNbO ₃ substrate and using a love wave such as a filter. .

1 ... the surface acoustic wave device 2 ... LiNbO ₃ substrate 2a ... groove 3 ... IDT electrode 3a ... electrode fingers 4 ... silicon oxide film 5 ... silicon nitride film 6 and 7 ... reflector 8 ... adhesion layer 9 ... protective layer

Claims (13)

A LiNbO ₃ substrate;
An IDT electrode formed on the LiNbO ₃ substrate;
A dielectric layer provided to cover the IDT electrode;
With
The IDT electrode is mainly composed of Al, and the IDT electrode is formed on the upper surface of the LiNbO ₃ substrate.
A groove is formed between electrode fingers of the T electrode, and a Love wave propagating through the LiNbO ₃ substrate is used,
The dielectric layer is located in the groove, and the depth of the groove is 0.04λ or more when the wavelength determined by the pitch of the electrode fingers of the IDT electrode is λ. Surface wave device. The surface acoustic wave device according to claim 1, wherein the IDT electrode includes an electrode layer made of Al or an alloy containing Al as a main component as a main electrode layer. The surface acoustic wave device according to claim 1, wherein the IDT electrode is made of Al or an alloy containing Al as a main component. It said dielectric layer comprises a silicon oxide film, a surface acoustic wave device according to any one of claims 1-3. The surface acoustic wave device according to claim 4 , further comprising a second dielectric layer formed on the silicon oxide film. The surface acoustic wave device according to claim 5 , wherein the second dielectric layer is made of at least one dielectric selected from the group consisting of silicon nitride, silicon oxynitride, and aluminum nitride. Further comprising an adhesion layer formed between the IDT electrode and the LiNbO ₃ substrate, a surface acoustic wave device according to any one of claims 1-6. The surface acoustic wave device according to claim 7 , wherein the adhesion layer is made of Ti, Ni, or an alloy containing these as a main component. Further comprising a protective layer provided on the upper surface of the IDT electrode, the surface acoustic wave device according to any one of claims 1-8. The surface acoustic wave device according to claim 9 , wherein the protective layer is made of Cu or Ti. The surface acoustic wave device according to any one of claims 1 to 10 , wherein a second Euler angle θ of the LiNbO ₃ substrate is in a range of 70 ° to 105 °. The wavelength determined by the pitch of electrode fingers of the IDT electrode is taken as lambda, the thickness of the IDT electrode is not less than 0.125Ramuda, surface acoustic wave according to any one of claims 1 to 1 1 apparatus. Wherein the wavelength determined by the pitch of electrode fingers of IDT electrodes when a lambda, the dielectric layer thickness is less than or equal to 0.1 [lambda], a surface acoustic according to any one of claims 1 to 1 2 Wave equipment.

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