JP2001111378A - Surface acoustic wave element and surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave element for preventing the deterioration of temperature frequency characteristics of a surface acoustic wave element in which a piezoelectric substrate is constituted of piezoelectric materials whose electro-mechanical coefficient is larger than that of crystals such as an LN or LT, and for preventing the increase of a loss due to the deterioration of an excitation level generated at the time of preventing the deterioration of the temperature frequency characteristics by forming a thin film such as an SiO2 on the piezoelectric substrate including electrode fingers. SOLUTION: In a surface acoustic wave element formed by forming a comb- shaped electrode 13 on one face of a piezoelectric substrate 11 constituted of piezoelectric materials, an expansion controlling plate 14 constituted of materials whose thermal expansion coefficient is smaller than that of the piezoelectric materials constituting the piezoelectric substrate is connected and integrated with the other face of the piezoelectric substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a surface acoustic wave device, and more particularly to a reduction in temperature-frequency characteristics which is a drawback of a surface acoustic wave device using a piezoelectric material having a larger electromechanical coefficient than quartz as a piezoelectric substrate. The present invention relates to a surface acoustic wave device that solves the problem.

[0002]

2. Description of the Related Art A surface acoustic wave element has a structure in which comb-teeth-shaped electrode fingers are arranged on a piezoelectric substrate made of quartz or the like. For example, a high-frequency electric field is applied to these electrodes to excite a surface acoustic wave to generate a surface acoustic wave. The filter characteristic is obtained by converting a surface wave into a high-frequency electric field by a piezoelectric reaction.
Although the surface acoustic wave element using quartz is excellent in temperature characteristics, it has a disadvantage that the loss is large because the electromechanical coupling coefficient is small. In other words, quartz has a small coupling coefficient between the electrical and mechanical actions, so that the necessary and sufficient characteristics can be secured only by arranging a large number of electrode fingers on the quartz substrate. As a result, if the area of the quartz substrate is large, Inevitably, miniaturization was limited. On the other hand, a surface acoustic wave element using a piezoelectric material such as lithium niobate (LiNbO ₃ , hereinafter referred to as LN) or lithium tantalate (TiTaO ₃ , hereinafter referred to as LT) has a small loss. On the other hand, it has the disadvantage of poor temperature-frequency characteristics. That is, since LN and LT have a very large coupling coefficient between the electric action and the mechanical action, a sufficiently strong excitation can be ensured only by providing a small number of electrodes on the substrate. In addition, while the surface acoustic wave device using this element can be downsized, the distance between the electrode fingers fluctuates on the substrate due to the fact that the LN substrate easily expands and contracts in the plane direction due to heat. And
It has the disadvantage that the resulting frequency varies.

[0003] Elasticity using a piezoelectric substrate made of LN or LT
Poor temperature-frequency characteristics, which is the above disadvantage of surface acoustic wave elements
In order to improve the efficiency, for example, conventionally, as shown in FIG.
Cover the electrode fingers 2 formed on the surface of the piezoelectric substrate 1 made of N.
As shown in FIG._TwoCoated with thin film 3
Integration was being done. That is, SiO_TwoLike
A substance having a small coefficient of thermal expansion is formed on the piezoelectric substrate 1
Heat by mechanically fixing the surface and the electrode fingers 2
Measures have been taken to forcibly suppress the expansion and contraction caused by this.
However, the mass is increased by forming the thin film 3 on the substrate.
Is added, this is the free movement of the substrate surface including the electrode fingers
Is a factor that hinders the development of the LT board.
The intensity of the vibration intensity is reduced, the excitation level is reduced, and the loss is
Increase, resulting in a loss of the benefits of LN and LT
I was In other words, SiO _TwoThin film 3 is an electrode
Since the substrate surface including the finger 2 is pressed down rigidly, the LT base
The plate 1 does not expand and contract due to heat, and the temperature-frequency characteristics are stabilized.
On the other hand, the thin film 3 also suppresses free excitation of the substrate surface.
Results in intrusion. In addition, SiO_TwoEtc. thin film
Reference numeral 3 denotes a liquid phase growth or a vapor phase growth on the piezoelectric substrate 1.
Deposits using these techniques require a long time.
As a result, there is a limit in increasing productivity.

[0004]

SUMMARY OF THE INVENTION The problem to be solved by the present invention is the temperature, which is a defect of a surface acoustic wave element using a piezoelectric material such as LN and LT having a larger electromechanical coefficient than quartz as a piezoelectric substrate. At the same time as solving the problem of the deterioration of the frequency characteristics, SiO ₂ was placed on the piezoelectric substrate including the electrode fingers.
It is an object of the present invention to provide a surface acoustic wave device that can solve the problem of an increase in loss due to a decrease in excitation level, which is a problem that occurs when a decrease in temperature-frequency characteristics is prevented by forming a thin film such as the above.

[0005]

According to a first aspect of the present invention, there is provided a surface acoustic wave device in which a comb-shaped electrode is formed on one surface of a piezoelectric substrate made of a piezoelectric material. An expansion / contraction regulating plate made of a material having a smaller coefficient of thermal expansion than the piezoelectric material constituting the piezoelectric substrate is joined and integrated on the other surface of the substrate. According to a second aspect of the present invention, there is provided a surface acoustic wave device in which a surface acoustic wave element having a comb-shaped electrode formed on one surface of a piezoelectric substrate made of a piezoelectric material is housed in a package. In addition, the expansion and contraction restricting plate made of a material having a smaller coefficient of thermal expansion than the piezoelectric material constituting the piezoelectric substrate and having a larger area than the piezoelectric substrate is joined and integrated, and the extension of the expansion and contraction restricting plate protruding from the peripheral edge of the piezoelectric substrate is formed. By fixing the bottom of the metal cap, the upper surface of the expansion / contraction regulating plate including the piezoelectric substrate is sealed by the metal cap.
The invention according to claim 3 is characterized in that the piezoelectric material is lithium niobate or lithium tantalate.
The invention of claim 4 is characterized in that the expansion / contraction regulating plate is made of SiO ₂ .

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. FIG. 1 is a cross-sectional view of an example of a surface acoustic wave device according to the present invention. The surface acoustic wave device includes lithium niobate (LiNbO3, hereinafter referred to as LN) or lithium tantalate (TiTaO3, hereinafter).
A comb-shaped electrode 1 composed of a plurality of electrode fingers 12 for excitation is provided on a surface of a piezoelectric substrate 11 made of a piezoelectric material such as LT.
3, and a structure in which a material having a smaller coefficient of thermal expansion than the material forming the piezoelectric substrate, for example, an expansion / contraction regulating plate 14 made of SiO ₂ glass is joined and integrated on the back surface of the piezoelectric substrate 11. By integrating the expansion / contraction regulating plate 14 on the back surface of the piezoelectric substrate 11, the thermal expansion of the piezoelectric substrate 11 is suppressed and the rate of change in frequency is reduced without any mechanical constraint on the substrate surface including the electrode fingers 12. It was made possible. Note that the expansion / contraction regulating plate 14 may be any expansion / contraction regulating plate made of a material having a smaller coefficient of thermal expansion than the material of the piezoelectric substrate. For example, instead of SiO ₂ glass,
Quartz glass may be used. The use of an adhesive as a means for integrally joining the expansion / contraction regulating plate 14 to the back surface of the piezoelectric substrate 11 is not a very desirable choice for the purpose of the present invention. It is because the adhesive is interposed between the back surface of the piezoelectric substrate 11 and the expansion / contraction regulating plate 14,
This is because the adhesive acts as a buffer and the expansion / contraction restricting plate 14 cannot restrict expansion and contraction of the piezoelectric substrate due to heat. Therefore, in the present invention, a direct joining method is employed.
The direct bonding method employed in the present invention means that the back surface of the piezoelectric substrate and the bonding surface of the expansion / contraction regulating plate 14 are each finished to an ultra-clean surface, and then subjected to a hydrogenation treatment (hydrophilic treatment) so that they are brought into close contact with each other in a surface contact state. Thereafter, it is a process of joining and integrating by heating. Specifically, for example, both bonding surfaces are each polished to a super-mirror shape, and then both surfaces are respectively cleaned and activated with a hydrogen peroxide solution to be activated and then adhered to each other to bond hydrogen and oxygen at a molecular level. Can be exemplified. The bonding is completed by performing a heat treatment after the hydrogenation treatment. As described above, the expansion and contraction restricting plate 14 is directly adhered to the back surface of the piezoelectric substrate 11 by the direct bonding method without the interposition of an adhesive. The expansion and contraction of the piezoelectric substrate is restricted by the expansion and contraction restricting plate 14 made of a material having a small expansion coefficient, so that the pitch between the electrode fingers on the surface of the piezoelectric substrate does not change. Therefore, the excitation of the piezoelectric substrate 1 is not hindered, and a desired frequency can be output stably.

Next, FIG. 2 is an exploded perspective view and a longitudinal sectional view showing a second embodiment of the present invention. A surface acoustic wave device having a composite substrate structure as in the embodiment of FIG. 1 is packaged. This is an example of the configuration when performing. That is, in the first embodiment, the planar shape of the expansion / contraction regulating plate 14 matches the bottom surface shape of the piezoelectric substrate 11, so that when the two are joined, the expansion / contraction regulating plate 14 does not protrude from the peripheral edge of the piezoelectric substrate 11. However, in the second embodiment, the expansion and contraction regulating plate 14 is used.
Are set longer than the lengths of the four sides of the piezoelectric substrate, the piezoelectric substrate 11
Are configured so that the expansion / contraction regulating plate 14 protrudes from the peripheral edge of the piezoelectric substrate 11 when they are directly joined. In the present embodiment, the expansion / contraction regulating plate 1 further protruding from the peripheral edge of the piezoelectric substrate 11.
The upper surface of the overhanging portion 15 is used as a space for adhesively fixing the skirt of the metal cap 16. That is,
By fixing the skirt portion of the metal cap 16 to the upper surface of the overhang portion 15 with an adhesive or the like, a packaged surface acoustic wave device is completed, and the height is reduced. By the way, in the conventional surface acoustic wave device,
After mounting the piezoelectric substrate on the inner bottom surface of a box-shaped lower package with an open top using a special package from the piezoelectric substrate, fix the metal lid around the opening edge of the lower package and seal the opening However, when this package structure is applied to the surface acoustic wave device according to the present invention, there is a problem that the height of the package is increased by the thickness of the expansion / contraction regulating plate 14. On the other hand, in the present embodiment, since the expansion / contraction regulating plate 14 joined to the bottom surface of the piezoelectric substrate is also used as the bottom plate of the package, it is possible to realize the same low profile as the conventional package. Since the surface acoustic wave device shown in FIG. 2 is a surface mount type, it goes without saying that an external electrode (not shown) connected to an electrode finger or the like is provided on the bottom surface of the expansion / contraction regulating plate 14.

[0008]

As described above, according to the present invention, LN, LN
Since the expansion / contraction regulating plate made of a material having a smaller thermal expansion coefficient than the piezoelectric substrate material is directly bonded to the back side of the piezoelectric substrate made of T or the like, the excitation of the piezoelectric substrate surface is prevented, and the electrode finger pitch expands / contracts due to heat, etc. The problem described above is not caused. That is, it is possible to solve the problem of a decrease in temperature-frequency characteristics, which is a defect of a surface acoustic wave element using a piezoelectric material having a larger electromechanical coefficient than quartz, such as LN and LT, as a piezoelectric substrate. On a piezoelectric substrate containing
It is possible to solve the problem of an increase in loss due to a decrease in excitation level, which is a problem that occurs when a decrease in temperature-frequency characteristics is prevented by forming a thin film such as iO ₂ . In addition, since the composite surface acoustic wave element in which the expansion and contraction restricting plate is joined to the back surface of the piezoelectric substrate has a larger thickness by the amount of the expansion and contraction restricting plate, if this is accommodated in a conventional package, Although the overall height is inevitably large, the use of the expansion / contraction regulating plate as the bottom plate of the package can effectively prevent the height of the device using the surface acoustic wave element from increasing.

[Brief description of the drawings]

FIG. 1 is a sectional view of a surface acoustic wave device according to an embodiment of the present invention.

2A and 2B are an exploded perspective view and a sectional view of a surface acoustic wave device according to an embodiment of the present invention.

FIG. 3 is an explanatory view of a conventional example.

[Explanation of symbols]

11 piezoelectric substrate, 12 electrode finger, 13 comb-shaped electrode, 14
Telescopic control plate, 15 overhang, 16 metal cap.

Claims (4)

[Claims] 1. A surface acoustic wave device comprising a piezoelectric substrate made of a piezoelectric material and a comb-shaped electrode formed on one surface of the piezoelectric substrate, wherein the surface of the piezoelectric substrate is more thermally expanded than the piezoelectric material constituting the piezoelectric substrate on the other surface of the piezoelectric substrate. A surface acoustic wave device characterized in that an expansion / contraction regulating plate made of a material having a small coefficient is joined and integrated. 2. A surface acoustic wave device in which a surface acoustic wave element formed by forming a comb-shaped electrode on one surface of a piezoelectric substrate made of a piezoelectric material is housed in a package. An expansion / contraction restricting plate made of a material having a smaller coefficient of thermal expansion than the piezoelectric material constituting the piezoelectric substrate and having a larger area than the piezoelectric substrate is joined and integrated, and a metal cap is provided on a protruding portion of the expansion / contraction restricting plate which protrudes from the periphery of the piezoelectric substrate. A surface acoustic wave device characterized in that an upper surface of an expansion / contraction regulating plate including a piezoelectric substrate is sealed with a metal cap by fixing a bottom portion of the surface acoustic wave device. 3. The method according to claim 1, wherein the piezoelectric material is lithium niobate or lithium tantalate.
Or the surface acoustic wave device and the surface acoustic wave device according to 2. 4. The surface acoustic wave element and the surface acoustic wave device according to claim 1, wherein the expansion / contraction regulating plate is made of SiO ₂ .