JP2003051734A - Surface acoustic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave element with a high impedance ratio being a ratio of the impedance at an anti-resonance frequency to the impedance at a resonance frequency, high weatherproof performance and a high degree of selection of types of material of the substrate. SOLUTION: The surface acoustic wave element includes a piezoelectric substrate. Interdigital electrodes are formed on the surface of the piezoelectric substrate. The piezoelectric substrate comprises a piezoelectric poly-crystal body and formed by layering a plurality of sheets comprising different piezoelectric ceramic compositions on the face of the substrate on which the interdigital electrodes are formed in a perpendicular direction to the face so as to differ in electric and mechanical characteristics, pressing the layers and sintering them.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device, and more particularly, to a comb-shaped crossed electrode formed on the surface of a piezoelectric substrate made of a piezoelectric polycrystal, which excites BGS waves to generate a high frequency resonator and a high frequency filter. The present invention relates to a surface acoustic wave device used in.

[0002]

2. Description of the Related Art Generally, in a surface acoustic wave device, a comb-shaped electrode pair having a large number of electrode fingers is arranged on a substrate having piezoelectricity (piezoelectric substrate) so that the electrode fingers cross each other.
The surface acoustic wave is excited by supplying an electric signal to the comb-shaped cross electrodes.

Rayleigh waves are the most common surface acoustic waves, but BGS (Bleustein-Gulyaev-Shimizu wave)
Also, applications such as piezoelectric surface slip waves) and Love waves are being studied.

These surface acoustic wave devices have a piezoelectric substrate material and a structure of a comb-shaped crossing electrode because of their electrical and mechanical characteristics such as resonance frequency, electrical or mechanical quality factor Q, and electromechanical coupling factor k. Can be used as a high frequency resonator or a high frequency filter.

By the way, the surface acoustic wave is literally a surface acoustic wave in which energy is concentrated and propagates near the surface of the substrate. A surface acoustic wave propagating on the surface of an isotropic substrate has displacement components only in the traveling direction of the wave and the depth direction of the substrate, which corresponds to a transverse wave and is in a direction perpendicular to the traveling direction of the wave and on the substrate surface. A surface wave (SH with only parallel components)
Wave; there is no horizontally-polarized shear wave).

However, since the piezoelectric substrate is anisotropic, it is possible to propagate SH waves (including pseudo SH waves) by concentrating energy near the surface thereof. This surface wave is called a BGS wave. Since the BGS wave is completely reflected by the end face in the propagation direction of the piezoelectric substrate, a reflector is formed on the substrate like a surface acoustic wave element that excites Rayleigh waves. Need not be, therefore
The BGS wave is more advantageous than the Rayleigh wave in terms of downsizing the surface acoustic wave device.

[0007]

However, the BGS
In the case of a wave, when a substrate having a small electromechanical coupling coefficient k is used, the impedance ratio Za / Zr, which is the ratio of the impedance Za at the antiresonance frequency to the impedance Zr at the resonance frequency, is small, forming a narrow band filter. In that case, there is a problem that sufficient filter characteristics cannot be obtained. On the other hand, when a substrate having a large electromechanical coupling coefficient k is used, the impedance ratio Za / Zr is large and a wide band filter can be formed, but there is a problem that the weather resistance is poor.

Further, in the surface acoustic wave device, it is desired that the degree of freedom of the material type of the substrate is high.

Therefore, the main object of the present invention is that the impedance ratio, which is the ratio of the impedance at the anti-resonance frequency to the impedance at the resonance frequency, is large, the weather resistance is good, and the degree of freedom of the material type of the substrate is high. An object is to provide a surface acoustic wave device.

[0010]

In a surface acoustic wave element according to the present invention, a comb-shaped cross electrode is formed on the surface of a piezoelectric substrate, and a surface acoustic wave is excited by supplying an electric signal to the comb-shaped cross electrode. In the surface acoustic wave device, the piezoelectric substrate is made of a piezoelectric polycrystal and has two or more layers laminated in a direction perpendicular to the surface on which the comb-shaped cross electrodes are formed so that the electrical and mechanical properties are different. It is a surface acoustic wave device characterized by being processed.

In the surface acoustic wave element according to the present invention, the piezoelectric substrate may be laminated so that the electric characteristics and the mechanical characteristics are inclined.

In the surface acoustic wave device according to the present invention, BGS waves may be used as the surface acoustic waves.

Further, in the surface acoustic wave element according to the present invention, a piezoelectric substrate is formed by laminating two or more sheets of piezoelectric ceramic materials having different compositions into a laminated body and sintering the laminated body. May be.

Further, in the surface acoustic wave element according to the present invention, a piezoelectric substrate having inclined electric and mechanical characteristics may be formed by thermally diffusing a part of the sheet between adjacent sheets. .

As a result of diligent research into the main factors of the above problems, it was revealed that the degree of surface concentration of wave energy is involved in these problems. The BGS wave is
It is known that the surface concentration of the wave is smaller than that of the Rayleigh wave. The surface concentration of BGS wave energy is substantially determined by the electromechanical coupling coefficient k and the dielectric constant ε of the piezoelectric substrate. Especially when there are electrodes on the surface of the piezoelectric substrate,
It has been clarified that the electromechanical coupling coefficient k is dominant, and that the larger the electromechanical coupling coefficient k, the higher the surface concentration of the BGS wave energy. Therefore, when a piezoelectric substrate having a small electromechanical coupling coefficient k is used, the surface concentration of BGS wave energy is low, and the wave energy is distributed with a certain width in the thickness direction of the piezoelectric substrate. The decrease in the surface concentration causes a loss of waves in the piezoelectric substrate. Therefore, it is considered that this loss is the main cause of the decrease in the impedance ratio.

To solve this problem, even if the piezoelectric substrate has a small electromechanical coupling coefficient k, if the energy of the BGS wave can be concentrated near the surface, the decrease in impedance ratio can be suppressed. Therefore, in the surface acoustic wave element, by arranging piezoelectric media having different electromechanical coupling coefficients k in multiple layers or in an inclined manner in the thickness direction of the piezoelectric substrate, only the electromechanical coupling coefficient k near the surface is lowered. A structure in which a piezoelectric medium having a high electromechanical coupling coefficient k is arranged in the thickness direction from there to concentrate the energy of the BGS wave near the surface, and even in a surface acoustic wave element having a low electromechanical coupling coefficient k, the impedance ratio is It has been found that a relatively high surface acoustic wave device can be constructed.

On the other hand, when a piezoelectric substrate having a high electromechanical coupling coefficient k is used, there is a problem that the weather resistance, particularly the humidity resistance is deteriorated and the frequency change due to the humidity resistance test becomes large.
This problem is due to the fact that the piezoelectric substrate having a large electromechanical coupling coefficient k has a high degree of surface concentration of BGS waves as described above, and therefore the waves propagate only near the surface most susceptible to deterioration due to humidity and temperature. Therefore, contrary to the above case, the piezoelectric medium having a high electromechanical coupling coefficient k is provided only near the surface, and the piezoelectric medium having a low electromechanical coupling coefficient k is arranged in the thickness direction from the piezoelectric medium.
It has been found that a surface acoustic wave device having a high electromechanical coupling coefficient k with excellent moisture resistance can be configured by suppressing excessive surface concentration of GS wave energy.

By the way, the invention from the viewpoint of improving the characteristics by controlling the degree of surface concentration of the surface wave is completely new, but a similar configuration can be made by using the conventional technique. For example, JP-A-9-208399
Japanese Patent No. 3242187 discloses a structure in which piezoelectric single crystals made of different materials are laminated. The invention disclosed in Japanese Unexamined Patent Publication No. 9-208399 aims to improve the characteristics of a surface acoustic wave device while maintaining good crystallinity and bondability. However, when the surface concentration of surface waves is controlled by such a laminated structure of piezoelectric single crystals as in the present invention, it is considered that there are restrictions on the material type and it is difficult to obtain desired characteristics. Furthermore, there are concerns about problems such as difficult workability, increased processing cost, and defective bonding.

Therefore, in the present invention, the piezoelectric polycrystalline material, which has a high degree of freedom in the kind of material to be laminated or arranged in a tilted manner and is easy to process, is used to form a laminated or tilted structure. The main effect is to control the surface concentration of surface waves and improve the characteristics accordingly.

The above objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of the embodiments of the invention with reference to the drawings.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION (Embodiment) FIG. 1 shows a sample manufacturing process and an evaluation process in an embodiment of the present invention. The details will be described below.

As raw materials, Pb ₃ O ₄ , ZrO ₂ , Ti
O ₂ , MnCO ₃ , NiO, and Nb ₂ O ₅ were weighed so as to obtain the piezoelectric ceramic compositions A, B, and C having the composition formula shown in Table 1, and the respective mixtures were mixed in a ball mill for 4 to 32 hours. Crushed.

[0023]

[Table 1]

Then, the respective mixtures are dehydrated, dried and calcined at a temperature of 800 to 900 ° C. for 2 hours, and then the obtained powder is mixed with 3 to 10% by weight of a binder, a dispersant and an antifoaming agent. In addition, a slurry was prepared by pulverizing for 8 to 16 hours, and a ceramic sheet having a thickness of 50 to 150 μm was formed or prepared by the doctor blade method.

Sheets A, B and C made of the piezoelectric ceramic compositions A, B and C were overlaid and pressure bonded under the respective conditions shown in FIGS.
Calcination is performed at 0 to 1250 ° C. for 1 to 3 hours, one main surface of the obtained calcination body is subjected to # 800 to # 8000 polishing in order and mirror-polished, and the thickness of the first layer on the surface is 10 to 100 μm. A piezoelectric substrate having a total element thickness of 300 to 700 μm was obtained.

Then, comb-shaped polarization electrodes are formed by Ag vapor deposition electrodes on both main surfaces of each piezoelectric substrate, and the piezoelectric substrates are polarized so that the polarization direction is parallel to the surface of the piezoelectric substrates. Was applied. Polarization was performed by applying an electric field of 2.0 to 4.0 kV / mm in oil at 60 to 120 ° C. for 30 to 60 minutes. Then, the Ag vapor deposition electrode was removed with an etching solution to obtain a polarized piezoelectric substrate.

Then, in a process similar to the conventional process, an Al electrode film is formed on one principal surface of the above-mentioned piezoelectric substrate by a sputter deposition method and is patterned by using a photolithography technique to form a comb-shaped cross electrode. . In this case, the comb-shaped cross electrode was formed in a direction perpendicular to the direction in which the above-mentioned comb-shaped polarization electrode was formed. Then, by cutting the piezoelectric substrate into a desired size, the piezoelectric substrate having each structure shown in FIGS.
The surface acoustic wave device (sample) shown in (1) was obtained.

Each surface acoustic wave element was fixed to a jig with a terminal, the surface acoustic wave element and the terminal were connected by a wire, and the characteristics were evaluated by an impedance analyzer.
The evaluated items are the impedance ratio Za / Zr between the anti-resonance point and the resonance point, the electromechanical coupling coefficient k _{BGS of} the BGS wave, the mechanical quality coefficient Qm _BGS , and the shift amount Δfr / fr of the resonance frequency before and after leaving in humidity. is there. The results are summarized in Table 2.

[0029]

[Table 2]

In Table 2, sample A-1 is a sample having a piezoelectric substrate having a multilayer structure shown in FIG.
Sample A-2 is a sample having a piezoelectric substrate having an inclined structure shown in FIG. 2B, and Sample A-3 is a sample having a piezoelectric substrate having a single-layer structure shown in FIG. 2C. , Sample C-1 is a sample having a piezoelectric substrate having a multilayer structure shown in FIG. 2D, and Sample C-2 is a sample having a piezoelectric substrate having an inclined structure shown in FIG. 2E. , Sample C-3 is a sample having a piezoelectric substrate having a single-layer structure shown in FIG. 2F, and Samples A-3 and C-3 marked with *
Is a comparative example and is outside the scope of the present invention. The same applies to the graphs shown in FIGS. 4 and 5.

Further, in Table 2, Δfr / fr (%)
Has an ambient temperature of 60 ° C. and a humidity of 95% R.V. H. Is the rate of change of the resonance frequency fr after 1000 hours of the humidity resistance test in which the sample was left in the constant temperature and humidity chamber.

Separately from this, the distribution of the energy of the BGS wave near the surface of each surface acoustic wave element (sample) was obtained by simulation by the finite element method (FEM). The results are shown in FIGS. 4 and 5.

In Table 2, samples A-1, A-2 and A
-3, the electromechanical coupling coefficient k for all samples
_{Although the BGS} is very low at about 20%, the impedance ratio Za / Zr is 45d in the samples A-1 and A-2 according to the present invention, as compared with the sample A-3 in the comparative example.
It can be seen that the impedance ratio Za / Zr capable of forming a good filter can be ensured because it is B or more.

Further, from the simulation result shown in FIG. 4, in the samples A-1 and A-2 according to the present invention, the energy of the BGS wave was concentrated near the surface as compared with the sample A-3 of the comparative example. You can see that This is because in the surface acoustic wave device, piezoelectric media having different electromechanical coupling coefficients k in the thickness direction of the piezoelectric substrate are arranged in multiple layers or in an inclined manner to lower only the electromechanical coupling coefficient k near the surface, By adopting a structure in which a piezoelectric medium having a high electromechanical coupling coefficient k is arranged in the thickness direction from that, the BGS
It shows that the wave energy can be concentrated near the surface. Therefore, even with a surface acoustic wave element having a low electromechanical coupling coefficient k, it is possible to suppress the wave loss in the piezoelectric substrate due to a decrease in the surface concentration of the wave, and to configure a surface acoustic wave element with a small decrease in impedance ratio.

Next, when the samples C-1, C-2 and C-3 in Table 2 are compared, the electromechanical coupling coefficient k _BGS of each sample is 50% or more, which is very high, but a comparative example. Sample C-according to the present invention as compared with Sample C-3 of
1 and C-2 show that the shift amount of the resonance frequency after 1000 hours of the humidity resistance test can be reduced to half or less.

From the simulation result of FIG. 4,
It can be seen that the samples C-1 and C-2 according to the present invention can suppress excessive surface concentration of BGS wave energy as compared with the sample C-3 of the comparative example.

When a piezoelectric substrate having a high electromechanical coupling coefficient k is used, the weather resistance, particularly the humidity resistance is deteriorated and the frequency change due to the humidity resistance test becomes large. This is because a large piezoelectric substrate has a high degree of surface concentration of BGS waves, and therefore waves propagate only near the surface that is most susceptible to deterioration due to humidity and temperature. Therefore, as described above, the piezoelectric medium having a high electromechanical coupling coefficient k is formed only in the vicinity of the surface, and the piezoelectric medium having a low electromechanical coupling coefficient k is arranged in the thickness direction from the surface, whereby the energy of the BGS wave is increased. It is possible to configure a surface acoustic wave element having a high electromechanical coupling coefficient k, which suppresses excessive surface concentration of the element, has excellent moisture resistance.

In this example, a multilayer structure or a gradient structure obtained by firing a molded body obtained by stacking two or more layers of compositions having different compositions of piezoelectric ceramic composition and electrical and mechanical characteristics. Piezoelectric substrate is used.
In this case, it is considered that each piezoelectric ceramic composition of each layer has a multilayer structure in which a part of the piezoelectric ceramic composition is thermally diffused in the vicinity of the interface, or a pseudo graded structure. However, the present invention is not limited to this embodiment. That is, the present invention configures a surface acoustic wave element having a piezoelectric substrate having a multilayer structure or a gradient structure composed of piezoelectric media having different compositions or electrical and mechanical characteristics, so that surface energy of surface acoustic waves is concentrated on the surface. The present invention provides a technique for controlling the degree and improves the characteristics of the surface acoustic wave device. Therefore, a surface acoustic wave element having a multi-layered structure with almost no mutual diffusion between the layers, for example, mutual diffusion is suppressed by low-temperature sintering of a piezoelectric ceramic composition, or a thin film forming method such as a CVD method is used. Also in the above, it is obvious that the same effect can be obtained by changing the layer thickness and the number of layers.

Further, although the BGS wave is used in the present embodiment, also in other surface acoustic waves such as Rayleigh waves, the surface concentration of the wave influences the electrical or mechanical characteristics of the surface acoustic wave element. Therefore, it can be expected that the same effect can be obtained.

A surface acoustic wave device having a laminated structure similar to that of the present invention is disclosed in JP-A-9-31546, JP-A-10-107581 and JP-A-9-208546.
Although disclosed in JP-A-10-1357773 and the like, JP-A-9-208399 and JP-A-9-31254 are also disclosed.
No. 6, JP-A-10-107581 uses a single crystal substrate and aims at the effect of improving the crystallinity (lattice defects, cracks) of the single crystal. The difference is clear. Further, Japanese Patent Laid-Open No. 10-1357773 improves the stress migration resistance of the comb-shaped electrode, and its structure is completely different from the surface acoustic wave device according to the present invention including a non-piezoelectric single crystal, and the effect of the present invention is also present. It is obvious that this is completely different from the control of the surface concentration of the surface acoustic wave energy.

[0041]

According to the present invention, in a surface acoustic wave element for exciting BGS waves, its piezoelectric substrate is made of a piezoelectric polycrystal, and in a direction perpendicular to the surface on which the comb-shaped crossing electrodes are formed. , A surface acoustic wave element in which two or more layers are laminated so as to have different electrical characteristics and mechanical characteristics, for example, to be inclined, and the surface concentration of the surface acoustic wave is controlled by controlling the degree of surface concentration of the surface acoustic wave energy. Degree, that is, when a piezoelectric substrate having a small electromechanical coupling coefficient k is used, the degree of surface concentration of surface acoustic wave energy is reduced and the impedance ratio is reduced, and the electromechanical coupling coefficient k is reduced. When a high piezoelectric substrate is used, it is possible to solve the problem that the moisture resistance deteriorates due to excessive surface energy concentration of surface acoustic waves. Furthermore, by constructing the above-mentioned multilayer structure or inclined structure using a piezoelectric substrate made of porcelain as in the present invention, a desired surface acoustic wave device can be constructed relatively easily and at low cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a sample manufacturing process and an evaluation process in an example of the present invention.

FIG. 2 is an illustrative view showing a structure of a sample piezoelectric substrate in an example of the present invention.

FIG. 3 is a perspective view showing a sample in an example of the present invention.

FIG. 4 shows the distribution of BGS wave energy near the surface of the sample in the example of the present invention by the finite element method (FEM).
5 is a graph showing a result obtained by a simulation by.

FIG. 5: BG for other samples in the example of the present invention
The finite element method (FE
It is a graph which shows the result calculated | required by the simulation by M).

[Explanation of symbols]

A sheet made of piezoelectric ceramic composition A B Sheet made of piezoelectric ceramic composition B C Sheet made of piezoelectric ceramic composition C

Claims (5)

[Claims] 1. A surface acoustic wave device in which a comb-shaped cross electrode is formed on a surface of a piezoelectric substrate and a surface acoustic wave is excited by supplying an electric signal to the comb-shaped cross electrode. A surface acoustic wave, which is made of a polycrystal and is laminated in two or more layers in a direction perpendicular to a surface on which the comb-shaped crossing electrodes are formed so as to have different electric characteristics and mechanical characteristics. element. 2. The surface acoustic wave device according to claim 1, wherein the piezoelectric substrate is laminated such that electric characteristics and mechanical characteristics are inclined. 3. The surface acoustic wave device according to claim 1, wherein a BGS wave is used as the surface acoustic wave. 4. The piezoelectric substrate is formed by laminating two or more sheets of piezoelectric ceramic materials having different compositions to form a laminated body, and sintering the laminated body. The surface acoustic wave device according to any one of claims 1 to 3. 5. The piezoelectric substrate having inclined electric and mechanical characteristics is formed by thermally diffusing a part of the sheet between the adjacent sheets. The surface acoustic wave device described in 1.