JP2000236231A - Surface acoustic wave device - Google Patents

Abstract

(57) [Problem] To provide unevenness in propagation characteristics of surface acoustic waves due to uneven bonding strength of a substrate at a bonding surface. SOLUTION: A propagation substrate 11 which is a piezoelectric substrate, an auxiliary substrate 12 which is laminated on the propagation substrate 11 by direct bonding, and an elastic substrate which is formed on a surface of the propagation substrate 11 opposite to a surface where the auxiliary substrate 12 is bonded, are provided. A comb-shaped electrode 13 for exciting a wave, and the propagation substrate 11 and the auxiliary substrate 12 are arranged such that at least a region where the comb-shaped electrode 13 is formed or a region immediately below the surface acoustic wave propagation region including the region where the comb-shaped electrode 13 is formed, The surface acoustic wave element is not bonded to each other, and has a thermal expansion coefficient of the auxiliary substrate 12 in the propagation direction of the elastic wave smaller than that of the propagation substrate 11 in the propagation direction of the elastic wave.

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 used for mobile communication equipment and the like.

[0002]

2. Description of the Related Art In recent years, with the advancement and development of mobile communication technology, the performance of communication devices has been improved. These devices always require devices such as high-frequency filters and oscillators, and these devices are also required to have higher performance. Conventionally, surface acoustic wave elements have been widely used for these devices.

[0003] The characteristics of a surface acoustic wave element are mainly determined by the piezoelectric substrate on which the surface acoustic wave propagates. Important characteristics of the piezoelectric substrate are an electromechanical coupling coefficient and temperature dependency.
The electromechanical coupling coefficient is an amount related to the pass frequency bandwidth of a filter constituted by a surface acoustic wave element and the Q value of a resonator. And the amount of variation in the resonance frequency of the resonator. Further, the electromechanical coupling coefficient and the temperature dependency are specific to the material of the piezoelectric substrate and the substrate orientation.

The characteristics required for the piezoelectric substrate forming the surface acoustic wave element include, for example, a large electromechanical coupling coefficient in order to secure a wide pass band in a high frequency band.
Further, in order to suppress the frequency fluctuation, the temperature dependency is small. However, with respect to existing piezoelectric substrates, those having a large electromechanical coupling coefficient have a large temperature dependency. Therefore, realizing a surface acoustic wave device having a large electromechanical coupling coefficient and a small temperature dependency has been an issue in filter design.

[0005] A PCN system which is one of the mobile communication systems will be described as an example. In the PCN system, the frequency interval between the transmission frequency band and the reception frequency band in the high frequency band is as narrow as 20 MHz. In designing a filter in a high-frequency band, the frequency interval between the transmission frequency band filter and the reception frequency band filter is further narrowed in consideration of the manufacturing variation of the element and the amount of frequency fluctuation due to temperature change.

Therefore, it is difficult to secure the attenuation of the reception frequency band filter with respect to the transmission band and the attenuation of the transmission frequency band filter with respect to the reception band, that is, the so-called inter-band attenuation. For example, when a 36 ° rotation Y-cut X-propagating lithium tantalate is used as the piezoelectric substrate, the electromechanical coupling coefficient is 5% and the temperature dependency (delay time temperature coefficient; TCD) is 35 ppm / ° C. The appropriate band interval is several tens of MHz, and a sufficient inter-band attenuation cannot be secured. Therefore, the electromechanical coupling coefficient is about 5% or more for securing the frequency bandwidth, and the TCD is 35 ppm for securing the inter-band attenuation.
A piezoelectric substrate smaller than / ° C is desired.

[0007] However, a piezoelectric substrate having a large electromechanical coupling coefficient and a small TCD is sufficient, but no such substrate is available in existing substrates. Therefore, TC of existing board
Methods for reducing D have been proposed.

For example, IEE Transactions on Sonics and Ultrasonics, Volume SU-31, pp. 51-57
Page (1984) (IEEE Transactions on Sonics and U
ltrasonics, volume SU-31, pp. 51-57 (1984)), by forming a silicon oxide thin film layer on a lithium niobate substrate with the opposite sign of its material temperature coefficient, Methods for improving the temperature dependence of wave propagation characteristics are known.

However, in the above-mentioned IEE method, in order to obtain the effect of improving the temperature characteristics, the thickness of the silicon oxide thin film layer must be at most one wavelength with respect to the used surface acoustic wave wavelength. Need to be extremely thin. However, it is difficult to make the thickness and film quality of such a thin silicon oxide film layer uniform.

In order to solve these problems, there is a temperature characteristic improving method disclosed in JP-A-6-326553. This method uses a substrate in which substrates having different thermal expansion coefficients are laminated by direct bonding, and the substantial thermal expansion coefficient of the laminated substrate is reduced as compared with the case of a single substrate, and as a result, the elasticity is reduced. The temperature dependency of the surface acoustic wave element is improved.

A conventional surface acoustic wave device based on the temperature characteristic improving method disclosed in Japanese Patent Application Laid-Open No. 6-326553 will be described below.

FIG. 11 is a sectional view of a conventional surface acoustic wave device using a laminated substrate by direct bonding. In FIG. 11, 31 is a propagation substrate which is a piezoelectric substrate, 32 is an auxiliary substrate using a low thermal expansion coefficient material, and 33 is a comb-shaped electrode.

The propagation substrate 31 and the auxiliary substrate 32 are directly joined. As the propagation substrate 31, lithium tantalate or lithium niobate is used. The thickness of the propagation substrate 31 is usually at least five times the used wavelength. Since the thermal expansion coefficients of the propagation substrate 31 and the auxiliary substrate 32 are different, the substantial thermal expansion coefficients of the bonded substrates are different from the thermal expansion coefficients of those substrates, and as a result, the temperature dependence is also different. Become.

[0014]

However, the conventional surface acoustic wave device has the following problems.

In the conventional surface acoustic wave device, the propagation substrate and the auxiliary substrate are joined on the entire surface. In this case, it is sufficient that the substrate bonding is perfect, but if the substrate bonding is insufficient, the stress acting on the surface of the propagation substrate due to a temperature change will not be uniform. As a result, the speed of the surface acoustic wave changes while the surface acoustic wave is being transmitted on the substrate. In some cases, the surface acoustic wave propagation loss increases.

The present invention considers such a conventional surface acoustic wave element in consideration of the problem that the bonding strength of the substrate is non-uniform at the bonding surface, thereby causing variations in the propagation characteristics of the surface acoustic wave. It is an object of the present invention to provide a surface acoustic wave device having excellent temperature characteristics.

[0017]

Means for Solving the Problems The first invention (claim 1)
Is formed on a surface of the propagation substrate that is a piezoelectric substrate, an auxiliary substrate that is stacked on the propagation substrate by direct bonding, and a surface of the propagation substrate that is opposite to a bonding surface of the auxiliary substrate.
A comb-shaped electrode that excites an elastic wave, wherein the propagation substrate and the auxiliary substrate are not joined to each other at least in a region immediately below the comb-shaped electrode forming region, and the propagation direction of the elastic wave of the auxiliary substrate is Is a surface acoustic wave element characterized by having a smaller thermal expansion coefficient in the propagation direction of the acoustic wave of the propagation substrate.

According to a second aspect of the present invention (corresponding to claim 2), a recess or a groove is formed on the propagation substrate side, and a region where the propagation substrate and the auxiliary substrate are not bonded due to the presence of the recess or the groove. The surface acoustic wave device according to the first aspect of the present invention is characterized in that

According to a third aspect of the present invention (corresponding to claim 3), a recess or a groove is formed on the auxiliary substrate side, and an area where the propagation substrate and the auxiliary substrate are not bonded due to the presence of the recess or the groove. 2. The surface acoustic wave device according to claim 1, wherein

According to a fourth aspect of the present invention (corresponding to claim 4), a region where the propagation substrate and the auxiliary substrate are joined extends over the entire periphery of the propagation substrate. ,
Or 3 is a surface acoustic wave device of the present invention.

According to a fifth aspect of the present invention (corresponding to claim 5), the propagation substrate and the auxiliary substrate have a substantially rectangular shape, and a region where the substrates are joined to each other is formed by any one of the propagation substrates. A surface acoustic wave device according to the first aspect of the present invention, wherein the surface acoustic wave device exists on a pair of two opposite sides.

According to a sixth aspect of the present invention (corresponding to claim 6), in any one of the pair of two opposing sides, the pair of two sides is substantially perpendicular to the propagation direction of the elastic wave. A surface acoustic wave element according to a fifth aspect of the present invention, characterized in that the two sides are a pair of two sides that receive more uniform stress in a plane.

According to a seventh aspect of the present invention (corresponding to claim 7), the elastic surface according to the second or third aspect of the present invention is characterized in that the groove is formed parallel to the direction of the fingers of the comb-shaped electrode. Wave element.

According to an eighth aspect of the present invention (corresponding to claim 8), the thermal expansion coefficient of the propagation substrate is anisotropic, and the groove is formed in a direction substantially larger than the thermal expansion coefficient of the propagation substrate. The surface acoustic wave device according to the second or third aspect of the present invention, which is formed in a vertical direction.

According to a ninth aspect of the present invention (corresponding to claim 9), in the direct bonding, the substrate surfaces of the propagation substrate and the auxiliary substrate are flattened, mirror-finished, cleaned, hydrophilized, and superposed. The surface acoustic wave device according to any one of the first to eighth aspects of the present invention, wherein the surface acoustic wave device is joined by heat treatment.

A tenth aspect of the present invention (corresponding to claim 10) is:
The surface acoustic wave device according to any one of the first to ninth aspects, an antenna, a transmission signal processing circuit, and a reception signal processing circuit, wherein the surface acoustic wave device is used as a filter or an oscillator. A mobile communication device characterized in that:

[0027]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) First, the first embodiment of the present invention will be described.
An embodiment will be described with reference to the drawings.

FIG. 1 is a partially cutaway perspective view of a surface acoustic wave device according to a first embodiment of the present invention, and FIG. 2 is a sectional view taken along the line aa 'of FIG. The surface acoustic wave element shown in FIGS. 1 and 2 is a rectangular propagation substrate 1 which is a piezoelectric substrate.
1, a rectangular auxiliary substrate 12 and a comb-shaped electrode 13.

The comb-shaped electrode 13 is formed on the surface of the propagation substrate 11 opposite to the joint surface with the auxiliary substrate 12, and
A groove 14 is formed on a surface facing one auxiliary substrate 12 in a direction substantially parallel to the comb-shaped electrode fingers constituting the comb-shaped electrode 13.

FIG. 3 is an exploded perspective view showing the propagation board 11 and the auxiliary board 12. In the surface acoustic wave device according to the present embodiment, the propagation substrate 11 and the auxiliary substrate 12 are stacked by direct bonding without an adhesive.

In the surface acoustic wave device according to the present embodiment, a 3 μm thick 100 μm
A 6 ° rotation Y-cut X-propagation lithium tantalate substrate is used, and a low thermal expansion glass substrate having a thickness of 300 μm is used as the auxiliary substrate 12.

Next, the operation of this embodiment will be described. By applying an alternating electric field to the comb-shaped electrode 13, a surface acoustic wave is excited, and the surface acoustic wave propagates along the surface of the propagation substrate 11. This surface acoustic wave is applied to the comb-shaped electrode 13.
Is converted into an electric signal again. As described above, the device functions as a surface acoustic wave device.

Although FIGS. 1 and 2 show the basic configuration of a surface acoustic wave device using the comb-shaped electrode 13, the number and the configuration of the comb-shaped electrode are required when a filter or a resonator is used. Change according to.

Next, a method of compensating for the temperature characteristic of the surface acoustic wave device according to the present embodiment will be described.

First, the temperature characteristics will be described. The delay time temperature coefficient (TCD) of the surface acoustic wave element is one of the quantities indicating the temperature characteristics, and approximately the thermal expansion coefficient α in the propagation direction of the acoustic wave of the propagation substrate and the temperature coefficient (TCC) of the surface acoustic wave velocity.
V). TCV mainly depends on the temperature coefficient of the elastic constant of the propagation substrate and the temperature coefficient of the density of the propagation substrate. For a substrate having a large electromechanical coupling coefficient such as lithium niobate or lithium tantalate, the TCV is a negative value. is there.

Most of the surface acoustic wave energy in the depth direction of the propagation substrate 11 is concentrated within about one wavelength of the surface acoustic wave wavelength. Propagation substrate 11 in the present embodiment
Is more than 10 times the surface acoustic wave wavelength,
V is determined by the substrate constant (material constant) of the propagation substrate 11,
Regardless of the substrate constant (material constant) of the auxiliary substrate 12.

In the present embodiment, the propagation substrate 11 has a larger thermal expansion coefficient in the surface acoustic wave propagation direction than the auxiliary substrate 12.

The propagation substrate 11 and the auxiliary substrate 12 are directly joined. As a result, a compressive stress acts on the surface layer of the propagation substrate 11 with respect to a positive temperature change, and the propagation substrate 11
The elongation of the propagation board 11 is suppressed as compared with the case of a single body.

As a result, a change in the density of the propagation substrate 11 is suppressed, but at the same time, a change in the elastic constant (a decrease in the elastic constant itself and a decrease in the temperature coefficient of the elastic constant) occurs due to the stress. The suppression of the density change contributes to an increase in TCV, and the change in elastic constant contributes to a decrease in TCV. However, the influence of the change in elastic constant is large, and as a result, T
CV decreases. Eventually, the TCD of the surface acoustic wave element decreases in accordance with the decrease in the coefficient of thermal expansion in the direction of propagation of the surface acoustic wave.

As described above, the temperature characteristics of the surface acoustic wave device of the present embodiment are compensated.

Next, the manufacturing process of the surface acoustic wave device according to the present embodiment will be described.

The manufacturing process of the surface acoustic wave device according to the present embodiment is roughly divided into two processes of forming the groove 14 on the propagation substrate 11 and direct bonding.

First, the formation of the groove 14 on the propagation substrate 11 will be described.

After cleaning the propagation substrate 11, the propagation substrate 1
A resist mask is formed at a place other than the surface where the first groove 14 is to be formed. Next, the propagation substrate 11 on which the resist mask is formed is etched with a mixed solution of hydrofluoric acid and nitric acid. In this embodiment, since the propagation substrate 11 is made of lithium tantalate, a mixed solution of hydrofluoric acid and nitric acid is used as an etchant, but an appropriate etchant is used according to the substrate material.
After the step of forming the groove 14, the resist mask is removed.

Alternatively, the groove 14 of the propagation substrate 11 may be formed as follows. The surface of the propagation substrate 11 other than the surface on which the groove 14 is to be formed is mirror-polished. The groove 14 is formed using a dicing saw so that the cross section of the groove 14 has a rectangular or rectangular shape. When the wide groove 14 is formed, a desired groove width is obtained by inserting a dicing blade a plurality of times.

Further, the groove 14 of the propagation substrate 11 may be formed as follows. After cleaning the propagation substrate 11, a resist mask is formed on a surface of the propagation substrate 11 other than the surface where the groove 14 is to be formed. Next, the surface of the propagation substrate 11 on which the resist mask is to be formed, on which the grooves 14 are to be formed, is sandblasted with blast abrasive grains. Finally, the resist mask is removed.

The formation of the groove 14 in the propagation substrate 11 is not limited to the above method, and any method may be used as long as the groove 14 is formed.

In this embodiment, the cross section of the groove 14 is rectangular or a shape similar to a rectangle, but may be V-shaped or another cross-sectional shape.

Next, the direct joining will be described.

First, the propagation substrate 11 to be directly joined
And the surface of the auxiliary substrate 12 on which the grooves 14 are formed are cleaned. Subsequently, the surface of the propagation substrate 11 and the auxiliary substrate 12
Is subjected to a hydrophilic treatment. Specifically, for example, by immersing the substrate in an ammonia-hydrogen peroxide solution, the hydroxyl group easily adheres to the surface of the substrate, and the substrate is hydrophilized. Next, it is sufficiently washed with pure water. Thereby, hydroxyl groups adhere to the substrate surface.

When the substrates are superposed in this state, the superposed substrates are mainly adsorbed by a hydrogen bond of a hydroxyl group. Thereby, the surface of the propagation substrate 11 and the surface of the auxiliary substrate 12 are bonded at the atomic level, and a direct bonding structure between the two substrates is realized. The above process is performed at room temperature.

Although a sufficient bonding strength is obtained as it is, in order to further increase the bonding strength, it is necessary to maintain the adsorption state at a temperature of 100 ° C. or more for several tens of minutes to several tens of minutes.
By performing the heat treatment for a long time, the water constituent components are removed from the bonding interface. In the present embodiment, at about 300 ° C.
Heat treatment for a long time.

By this heat treatment, the bonding involving oxygen, hydrogen, and the atoms constituting the substrate proceeds from the bond mainly composed of the hydrogen bond by the hydroxyl group, and the bonding between the atoms constituting the substrate starts gradually, and the bonding is strengthened. In particular, when silicon, carbon, or oxygen is present, covalent bonding proceeds and the bonding is strengthened.

On the bonding substrate obtained by the above process, a comb-shaped electrode 13 is formed on the bonding substrate by photolithography. In the present embodiment, the comb-shaped electrodes 13 are arranged so that the comb-shaped electrode fingers and the grooves 14 formed in the propagation substrate 11 are parallel. Through the above process, the surface acoustic wave device according to the present embodiment is manufactured.

Next, the groove 14 of the propagation substrate 11 of the surface acoustic wave device according to the present embodiment will be described. In the present embodiment, in the heat treatment step of bonding the substrates, the grooves 14 are formed for the purpose of relaxing the stress acting on the bonding substrate and uniformly applying the stress to the surface acoustic wave propagation region of the surface layer of the propagation substrate 14. Has formed.

Here, the relaxation of the stress is necessary to suppress the crushing of the bonded substrate, and the effect of the uniform stress on the surface acoustic wave propagation region of the surface layer of the propagation substrate 14 is good as a surface acoustic wave element. It is necessary to obtain various characteristics.

In order to alleviate the stress, it is sufficient to reduce the substrate bonding area. This is because when the substrate bonding area decreases, the thermal distortion of the bonding substrate also decreases, and as a result, the stress is reduced. Therefore, in the present embodiment, the groove 14 is formed on the back surface of the propagation substrate 11 as a method of reducing the substrate bonding area.

The degree of stress relaxation depends on the method of forming the groove 14,
In other words, it changes depending on the forming direction and the size of the groove 14. For example, when the groove 14 is formed perpendicularly to the surface acoustic wave propagation direction, the stress in the surface acoustic wave propagation direction is reduced.

On the other hand, when the grooves 14 are formed parallel to the surface acoustic wave propagation direction, the stress in the direction perpendicular to the surface acoustic wave propagation direction is reduced. In addition, since the magnitude of the stress is proportional to the difference in thermal expansion coefficient between the propagation substrate 11 and the auxiliary substrate 12 in the direction in which the stress acts, in the present embodiment, the groove 14 is perpendicular to the direction in which the difference in thermal expansion coefficient is large. When formed, stress relaxation is effectively performed. However, if the grooves 14 are formed perpendicular to the surface acoustic wave propagation direction, the bonding area of the substrate becomes small, and sufficient bonding strength may not be obtained.

For example, for the region where the comb-shaped electrode 13 is formed,
This case corresponds to a case where the substrate dimension in the surface acoustic wave propagation direction is not large. At this time, the temperature characteristic compensation effect becomes extremely small. In such a case, the groove 14 may be formed parallel to the surface acoustic wave propagation direction.

Regarding the magnitude of the stress relaxation, the groove 14
The greater the depth of the groove, the greater the stress relaxation, and the greater the width of the groove 14, the greater the stress relaxation. In the present embodiment, the groove 1
The depth of the groove 4 is 30 μm on average and the width of the groove 14 is 400 μm. However, the stress is further relaxed by making the groove 14 deeper, and the width of the groove 14 is 400 μm. The stress is further alleviated by increasing the width.

In order to uniformly apply stress to the surface acoustic wave propagation region of the surface layer of the propagation substrate 11 in the state where the grooves 14 are formed, the direction in which the grooves 14 are formed and how the grooves 14 are formed are determined. is important. In the present embodiment, a piezoelectric material is used as the propagation substrate 11, but the piezoelectric material has anisotropic thermal expansion coefficient.

That is, the magnitude of thermal expansion and contraction in the direction of the propagation substrate 11 differs depending on the direction. Therefore, when the direction of formation of the groove 14 is oblique to the direction of propagation of the surface acoustic wave, a stress distribution occurs in the surface acoustic wave propagation region of the surface layer of the propagation substrate 11. Wave velocity is not uniform in the propagation region,
Not preferred.

Therefore, in order to uniformly apply stress to the surface acoustic wave propagation region on the surface layer of the propagation substrate 11, it is necessary to form the grooves 14 perpendicularly or parallel to the surface acoustic wave propagation direction. It is desirable that the groove 14 is formed at least immediately below the comb electrode 13 forming region or immediately below the surface acoustic wave propagation region including the comb electrode 13 in order to uniformly apply stress.

This is because, when the groove 14 and the bonding portion are provided directly below the surface acoustic wave propagation region of the surface layer of the propagation substrate 11, the stress in that region is not uniform due to the difference in thermal strain.

In order to confirm the effects of the surface acoustic wave device of the present embodiment, the surface acoustic wave device of the present embodiment and the conventional surface acoustic wave device were fabricated, and the electromechanical coupling coefficient and the TCD
The surface acoustic wave propagation characteristics were measured and compared.
The conventional surface acoustic wave devices to be compared have the same combinations of substrate materials and dimensions except that grooves are formed on the back surface of the propagation substrate 11 of the present embodiment.

In this embodiment, the propagation substrate 11 is made of anisotropic material lithium tantalate of 36 ° rotation Y-cut X propagation, and the auxiliary substrate 12 is made of glass of an isotropic material. 11 has a thermal expansion coefficient of 16 ppm / ° C. in the X direction, which is the surface acoustic wave propagation direction;
The thermal expansion coefficient of the auxiliary substrate 12 was 4 ppm / ° C.

In both the surface acoustic wave element of the present embodiment and the conventional surface acoustic wave element, the heat treatment condition of the heat treatment step for bonding the substrates is set to about 300 ° C. for 10 hours. The stress exceeded the elastic limit of the propagation substrate 11 or the auxiliary substrate 12, and a part of the bonded substrate was fractured.

On the other hand, in the present embodiment, the formation of the groove 14 in the propagation substrate 11 alleviates the stress acting on the bonding substrate, so that the fracture of the substrate in the heat treatment step can be suppressed. Next, the surface acoustic wave propagation characteristics were compared.

The TCD is obtained by measuring the temperature coefficient of frequency where the absolute value is the same as that of the TCD and having the same sign, and taking the sign of the difference as the TCD measurement value. Regarding the electromechanical coupling coefficient, there was no difference of about 5% in both surface acoustic wave devices.

On the other hand, the TCD of the conventional surface acoustic wave element is 25 ppm / ° C., while the surface acoustic wave element of the present embodiment is 28 ppm / ° C., and the propagation substrate (36 ° rotation Y cut X Propagating lithium tantalate)
TCD (36 ppm /
° C). In addition,
FIG. 12 is a sectional view of a surface acoustic wave device using the propagation substrate alone.

Further, in the embodiment of the present invention, it was confirmed that there was no change in the frequency response, and that there was no change in the surface acoustic wave velocity. Further, it was also confirmed that, compared to the conventional surface acoustic wave element, variations in characteristics due to defective bonding were reduced, and stable characteristics were obtained.

As described above, according to the present embodiment, crushing of the substrate in the heat treatment step of bonding the substrate is suppressed.
A surface acoustic wave device having good temperature characteristics can be obtained without changing various characteristics such as an electromechanical coupling coefficient and a surface acoustic wave propagation velocity.

In this embodiment, the propagation substrate 11 having a large thermal expansion coefficient in the direction of propagation of the surface acoustic wave and the auxiliary substrate 12 having a small thermal expansion coefficient in the direction of propagation of the surface acoustic wave are directly joined. Due to the temperature change, a compressive stress acts on the surface layer of the propagation substrate 11, and shows a value smaller than the thermal expansion coefficient of the propagation substrate 11 in the propagation direction of the original elastic wave, and the density change becomes small.

In the present embodiment, the stress acting in the direction perpendicular to the surface acoustic wave propagation direction is reduced as compared with the conventional one. On the other hand, in the direction parallel to the surface acoustic wave propagation direction, since the propagation substrate 11 and the auxiliary substrate 12 are directly joined, the stress acts almost the same as in the conventional case.

In this embodiment, the width of the groove 14 is 4
Although the thickness is set to 00 μm, the stress is further relaxed by increasing the width of the groove 14. Further, the depth of the groove 14 is set to 20 μm on average, but the stress is further reduced by making the groove 14 deeper. Further, in the present embodiment, the cross section of the groove 14 is a rectangle or a shape similar to a rectangle, but may be V-shaped or another cross section.

Further, in the direct bonding process, when foreign matter such as particles or dust is present on the bonding surface, bonding is not performed at a portion where the foreign matter is present, voids are generated, or the bonding strength is significantly reduced. Becomes In the case where the substrates are directly bonded, the smaller the continuous bonding area, the lower the possibility of poor bonding. However, by dividing the bonding surface by forming the groove 14, the continuous bonding area is reduced, and the possibility of poor bonding is reduced. be able to.

In the present embodiment, since no substrate bonding is performed in a region immediately below the surface layer of the propagation substrate 11 through which the surface acoustic wave propagates, no bonding failure occurs in this region. Therefore, there is no change or deterioration in the surface acoustic wave propagation characteristics.

Further, in this embodiment, the 36 ° rotation Y-cut X-propagation lithium tantalate is used as the propagation substrate 11. However, the present invention is not limited to this. The same effect can be obtained by using a substrate whose thermal expansion coefficient in the elastic wave propagation direction is smaller than that of the propagation substrate 11 in the elastic wave propagation direction.

In the present embodiment, glass is used as the auxiliary substrate 12, but the present invention is not limited to this, and another low thermal expansion material such as silicon may be used. Auxiliary substrate 12
When glass is used as the material, due to its non-crystallinity,
Bonding with the propagation substrate 11 which is a single crystal becomes easy. Further, in the case of glass, materials having various mechanical properties can be obtained depending on the composition, and the control of temperature characteristics becomes easy.

Further, immediately below the surface acoustic wave propagation region of the propagation substrate 11 of the present embodiment, there is no mirror-bonded interface between the propagation substrate 31 and the auxiliary substrate 32 of the conventional surface acoustic wave element. When the groove 14 is formed, the portion immediately below the surface acoustic wave propagation region of the propagation substrate 11 is substantially in the same state as when roughening is performed. There is also an effect of suppressing unnecessary spurious in the frequency response of the surface acoustic wave element due to the above.

Further, in the present embodiment, the grooves 14
Although the portion is hollow, the groove 14 may be filled with a resin using a capillary phenomenon, for example, after bonding the substrates. Groove 1
By filling the resin 4, the reflection of the bulk wave on the back surface of the propagation substrate 11 is further suppressed, and the unnecessary spurious can be more effectively suppressed.

Further, in the present embodiment, the grooves 14 are formed in the propagation substrate 11, but the effects described so far are essentially caused by the presence of the grooves 14 in the substrate structure of the present embodiment. The same effect can be obtained by forming the groove 14 in the auxiliary substrate 12 as shown in FIG. However, the spurious suppression effect cannot be obtained.

(Second Embodiment) Next, a second embodiment of the present invention will be described.
An embodiment will be described with reference to the drawings. This embodiment is the same as the configuration of the surface acoustic wave element according to the first embodiment described above, except for the groove 14 of the present invention. Therefore, in the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted. Unless otherwise described, it is the same as the first embodiment.

FIG. 4 is a partially cutaway perspective view of a surface acoustic wave device according to a second embodiment of the present invention, and FIG. 5 is a sectional view taken along the line bb 'of FIG. FIG. 6 shows the propagation substrate 1
FIG. 2 is an exploded perspective view showing an auxiliary substrate 1 and an auxiliary substrate 12. FIG. 4-
The surface acoustic wave device shown in FIG. 6 is the same as the surface acoustic wave device shown in FIGS. The groove 14 of the surface acoustic wave element in the present embodiment is
2 are formed in a direction substantially perpendicular to the comb-shaped electrode fingers constituting the comb-shaped electrode 13.

Next, the manufacturing process of the surface acoustic wave device according to the present embodiment will be described. The manufacturing process of the surface acoustic wave device according to the present embodiment
The process is roughly divided into two processes of forming the upper groove 14 and direct bonding.

First, the formation of the groove 14 on the propagation substrate 11 will be described. After cleaning the propagation substrate 11, the propagation substrate 1
A resist mask is formed on the surface where the one groove 14 is to be formed. Next, the propagation substrate 11 on which the resist mask is formed is etched with a mixed solution of hydrofluoric acid and nitric acid.

In this embodiment, since the propagation substrate 11 is made of lithium tantalate, a mixed solution of hydrofluoric acid and nitric acid is used as an etchant, but an appropriate etchant is used according to the substrate material. After the step of forming the groove 14, the resist mask is removed. In the present embodiment, the groove 14
Has an average depth of 30 μm and the width of the groove 14 is 400 μm.

The groove 14 of the propagation substrate 11 may be formed as follows. The surface of the propagation substrate 11 where the grooves 14 are to be formed is mirror-polished. Groove 14 is formed using a dicing saw.
The groove 14 has a rectangular or rectangular shape, and is formed in a linear shape. When the wide groove 14 is formed, a desired groove width is obtained by inserting a dicing blade a plurality of times.

The groove 14 of the propagation substrate 11 may be formed as follows. First, after the propagation substrate 11 is washed, a resist mask is formed on the surface of the propagation substrate 11 where the groove 14 is to be formed. Next, the surface of the propagation substrate 11 on which the resist mask is to be formed, on which the grooves 14 are to be formed, is sandblasted with blast abrasive grains. Finally, the resist mask is removed.

As for the substrate bonding, the direct bonding between the propagation substrate 11 and the auxiliary substrate 12 is performed similarly to the first embodiment.

The comb-shaped electrode 13 is formed on the bonding substrate obtained by the above process by photolithography. In the present embodiment, the angle between the comb-shaped electrode 13 and the groove 14 formed in the auxiliary substrate 12 is 0 ° (the longitudinal direction of the comb-shaped electrode finger coincides with the main direction of the groove 14). Have been placed. Through the above process, the surface acoustic wave device according to the present embodiment is manufactured.

In order to confirm the effects of the surface acoustic wave element of the present embodiment, the surface acoustic wave element of the present embodiment and a conventional surface acoustic wave element were manufactured, and the electromechanical coupling coefficient and the TCD
The surface acoustic wave propagation characteristics were measured and compared.
The conventional surface acoustic wave devices to be compared have the same combinations of substrate materials and dimensions except that grooves are formed on the back surface of the propagation substrate 11 of the present embodiment.

In this embodiment, an X-cut 112 ゜ Y-propagating lithium tantalate of an anisotropic material is used as the propagation substrate 11, and an isotropic material glass is used as the auxiliary substrate 12. The thermal expansion coefficient of the auxiliary substrate 12 is 4 ppm / ° C.
The thermal expansion coefficient of the propagation substrate 11 in the direction of propagation of the surface acoustic wave is 6 ppm / ° C., and the thermal expansion coefficient in the direction perpendicular to the direction of propagation of the surface acoustic wave is 14 ppm / ° C. The forming direction was set to the direction of surface acoustic wave propagation.

In both the surface acoustic wave element of the present embodiment and the conventional surface acoustic wave element, the heat treatment condition in the heat treatment step for bonding the substrate is set to about 200 ° C. for 10 hours. The acting thermal stress is applied to the propagation substrate 31.
Or, the elastic substrate exceeded the elastic limit of the auxiliary substrate 32, and a part of the bonded substrate was fractured.

On the other hand, in the present embodiment, the formation of the groove 14 in the propagation substrate 11 alleviates the stress acting on the bonded substrate, so that the substrate can be prevented from being fractured in the heat treatment step. Next, the surface acoustic wave propagation characteristics were compared. As for TCD, the frequency temperature coefficient where the absolute value was the same as that of the TCD with the opposite sign was measured, and the opposite sign was taken as the TCD measurement value. Regarding the electromechanical coupling coefficient, there was no difference of about 0.7% in both surface acoustic wave devices.

On the other hand, the TCD of the conventional surface acoustic wave element is 11 ppm / ° C., whereas the surface acoustic wave element of the present embodiment is 14 ppm / ° C., and the propagation substrate (X cut 112 ゜ Y propagation It was confirmed that the TCD (lithium tantalate) alone was improved with respect to TCD (18 ppm / ° C.). Further, it was also confirmed that, compared to the conventional surface acoustic wave element, variations in characteristics due to defective bonding were reduced, and stable characteristics were obtained. Also, there was no change in the frequency response, and it was confirmed that there was no change in the surface acoustic wave sound velocity.

As described above, according to the present embodiment, it is possible to suppress the fracture of the substrate in the heat treatment step of bonding the substrate.
A surface acoustic wave device having good temperature characteristics can be obtained without changing various characteristics such as an electromechanical coupling coefficient and a surface acoustic wave propagation velocity.

In this embodiment, the propagation substrate 11 having a large thermal expansion coefficient in the direction of propagation of the surface acoustic wave and the auxiliary substrate 12 having a small thermal expansion coefficient in the direction of propagation of the surface acoustic wave are directly joined. In the vicinity of the surface of the propagation substrate, a compressive stress acts on the temperature change, and the temperature change shows a value smaller than the thermal expansion coefficient of the propagation surface wave in the propagation direction of the surface acoustic wave.

In the present embodiment, the stress acting in the direction parallel to the surface acoustic wave propagation direction is reduced as compared with the conventional one. On the other hand, in the direction perpendicular to the surface acoustic wave propagation direction, since the propagation substrate 11 and the auxiliary substrate 12 are directly joined, the stress acts almost the same as in the conventional case.

In this embodiment, the X-cut 112 ゜ Y-propagating lithium tantalate is used as the propagation substrate 11. However, the present invention is not limited to this. The same effect can be obtained by using a substrate whose thermal expansion coefficient in the propagation direction of the surface acoustic wave is smaller than that of the propagation substrate 11 in the propagation direction of the elastic wave.

In the present embodiment, the depth of the groove 14 is 30 μm on average, but the stress is further reduced by making the groove 14 deeper. Although the width of the groove 14 is set to 400 μm, the stress is further reduced by increasing the width of the groove 14.

In the direct bonding process, when foreign matter such as particles or dust is present on the bonding surface, bonding is not performed at a portion where the foreign matter exists, resulting in voids or a significant decrease in bonding strength. . In the case where the substrates are directly bonded, the smaller the continuous bonding area, the lower the possibility of poor bonding. However, by dividing the bonding surface by forming the groove 14, the continuous bonding area is reduced, and the possibility of poor bonding is reduced. be able to.

In the present embodiment, since no substrate bonding is performed in a region immediately below the surface layer of the propagation substrate 11 where the surface acoustic wave propagates, no bonding failure occurs in this region. Therefore, there is no change or deterioration in the surface acoustic wave propagation characteristics.

In the present embodiment, glass is used as the auxiliary substrate 12, but the present invention is not limited to this, and another low thermal expansion material such as silicon may be used. Auxiliary substrate 12
When glass is used as the material, due to its non-crystallinity,
Bonding with the propagation substrate 11 which is a single crystal becomes easy.

Further, in the case of glass, materials having various mechanical properties can be obtained depending on the composition, and the control of the temperature characteristics becomes easy.

In addition, immediately below the surface acoustic wave propagation region of the propagation substrate 11 of the present embodiment, there is no mirror-bonded interface between the propagation substrate 31 and the auxiliary substrate 32 of the conventional surface acoustic wave element. When the groove 14 is formed, the portion immediately below the surface acoustic wave propagation region of the propagation substrate 11 is substantially in the same state as when roughening is performed. There is also an effect of suppressing unnecessary spurious in the frequency response of the surface acoustic wave element.

Further, in the present embodiment, the grooves 14
Although the portion is hollow, the groove 14 may be filled with a resin using a capillary phenomenon, for example, after bonding the substrates. Groove 1
By filling the resin 4, the reflection of the bulk wave on the back surface of the propagation substrate 11 is further suppressed, and the unnecessary spurious can be more effectively suppressed.

Although the grooves 14 are formed in the propagation substrate 11 in the present embodiment, the effects described so far are essentially caused by the presence of the grooves 14 in the substrate structure of the present embodiment. FIG. 14 shows the auxiliary substrate 12
As shown in FIG. 7, the same effect can be obtained by forming the groove 14.

(Third Embodiment) Next, a third embodiment of the present invention will be described.
An embodiment will be described with reference to the drawings. The present embodiment is the same as the configuration of the surface acoustic wave element according to the first embodiment described above, except that the concave portion 15 of the present invention is provided instead of the groove 14 of the present invention. Therefore, in the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted. Unless otherwise described, it is the same as the first embodiment.

FIG. 7 is a partially cutaway perspective view of a surface acoustic wave device according to a third embodiment of the present invention, and FIG. 8 is a sectional view taken along the line cc 'in FIG. FIG. 9 is a cross-sectional view taken along the line dd ′ in FIG. FIG.
FIG. 3 is an exploded perspective view showing the auxiliary substrate 12 and FIG.

The surface acoustic wave device shown in FIGS. 7 to 10 has the same structure as that shown in FIGS.
This is the same as the surface acoustic wave device shown in FIG. The concave portion 15 of the surface acoustic wave element according to the present embodiment
It is formed so as to include at least a position immediately below the comb-shaped electrode 13.

Next, the manufacturing process of the surface acoustic wave device according to the present embodiment will be described. The manufacturing process of the surface acoustic wave device according to the present embodiment is roughly divided into two processes of forming a concave portion on the auxiliary substrate and directly bonding.

First, the formation of the recess 15 on the propagation substrate 11 will be described. The surface of the propagation substrate 11 where the concave portion 15 is to be formed is mirror-polished. After cleaning the propagation substrate 11, a resist mask is formed on the surface of the propagation substrate 11 where the concave portion 15 is to be formed. Next, the propagation substrate 11 on which the resist mask is formed is etched with a mixed solution of hydrofluoric acid and nitric acid.

In this embodiment, since the propagation substrate 11 is made of lithium tantalate, a mixed solution of hydrofluoric acid and nitric acid is used as an etchant, but an appropriate etchant is used according to the substrate material. After the step of forming the concave portion 15, the resist mask is removed. The recess 15 of the propagation substrate 11 may be formed as follows. The surface of the propagation substrate 11 where the groove 14 is to be formed is mirror-polished.

For the concave portion 15, first, after cleaning the auxiliary substrate 12, a resist mask is formed on the surface of the propagation substrate 11 where the concave portion 15 is to be formed. Next, the surface of the propagation substrate 11 on which the resist mask is formed, on which the concave portions 15 are to be formed, is sandblasted with blast abrasive grains. Finally, the resist mask is removed.

Note that the formation of the recess 15 in the propagation substrate 11 is not limited to the above method, and any method may be used as long as the recess 15 is formed.

As for the substrate bonding, the direct bonding between the propagation substrate 11 and the auxiliary substrate 12 is performed as in the first embodiment.

On the bonding substrate obtained by the above process, the comb-shaped electrode 13 is formed on the bonding substrate by photolithography. In the present embodiment, the comb-shaped electrodes 13 are arranged so as to be directly above the concave portions 15. Through the above process, the surface acoustic wave device according to the present embodiment is manufactured.

Here, the auxiliary substrate 1 in the present embodiment
The effect of the second recess 15 will be described. As in the first embodiment, the formation of the concave portion 15 reduces the substrate bonding area, so that the thermal distortion of the bonded substrate is also reduced. As a result, the stress is relaxed, and the crushing of the substrate in the heat treatment step of direct bonding is suppressed. Can be.

In the case of direct joining, if foreign matter such as particles or dust is present on the joining surface, the joining is not performed at the portion where the foreign matter is present, resulting in voids or a significant decrease in joining strength. . In the case where the substrates are directly joined, the possibility of poor joining decreases as the continuous joining area decreases, but the possibility of joining failure can be reduced by reducing the joining area where the joining is continued by forming the recess 15.

In this embodiment, the stress acting in the direction perpendicular to and in the direction parallel to the comb-shaped electrode fingers is smaller than that of the conventional surface acoustic wave device. However, since there is no substrate bonding region immediately below the surface acoustic wave propagation region, that is, immediately below the region where the comb-shaped electrode is formed, non-uniform stress due to poor bonding in the surface acoustic wave propagation region does not occur.

Further, by forming the concave portion 15, it is possible to obtain a surface acoustic wave element in which a uniform stress acts on the surface acoustic wave propagation region of the surface layer of the propagation substrate 11.

In order to confirm the effect of the surface acoustic wave device according to the present embodiment, the surface acoustic wave device according to the present embodiment and the conventional surface acoustic wave device were manufactured, and the surface acoustic wave propagation such as the electromechanical coupling coefficient and the TCD was performed. The characteristics were measured and compared.

The conventional surface acoustic wave device to be compared is the same as the surface acoustic wave device of the present embodiment except that the recess 15 is formed on the back surface of the propagation substrate 11 of the surface acoustic wave device. Is the same. In the present embodiment, a 36 ° rotation Y-cut X-propagation lithium tantalate is used as the propagation substrate 11, and glass is used as the auxiliary substrate 12.

Although the heat treatment conditions of the heat treatment step for bonding the substrates are set to about 200 ° C. for 10 hours for both the surface acoustic wave element of the present embodiment and the conventional surface acoustic wave element, the conventional surface acoustic wave element The acting thermal stress is applied to the propagation substrate 31.
Or, the elastic substrate exceeded the elastic limit of the auxiliary substrate 32, and a part of the bonded substrate was fractured.

On the other hand, in the present embodiment, the formation of the recess 15 in the propagation substrate 11 alleviates the stress acting on the bonding substrate, so that the fracture of the substrate in the heat treatment step can be suppressed. Next, the surface acoustic wave propagation characteristics were compared.
As for TCD, the frequency temperature coefficient where the absolute value was the same as that of the TCD with the opposite sign was measured, and the opposite sign was taken as the TCD measurement value. Regarding the electromechanical coupling coefficient, there was no difference of about 5% in both surface acoustic wave devices.

On the other hand, the TCD of the conventional surface acoustic wave device was 25 ppm / ° C., whereas the surface acoustic wave device of the present embodiment was 30 ppm / ° C. Although the degree of improvement in the temperature characteristics is small, variation in characteristics due to defective bonding is reduced as compared with the conventional surface acoustic wave device shown in FIG. 11, and there is an effect that stable characteristics can be obtained.

Further, when the surface acoustic wave device using the 36 ° rotation Y-cut X-propagation lithium tantalate simple substance substrate has a TCD of 36 ppm / ° C., the temperature characteristics of the surface acoustic wave device of this embodiment are Has been improved.
Also, there was no change in the frequency response, and it was confirmed that there was no change in the surface acoustic wave sound velocity.

As described above, according to the present embodiment, a surface acoustic wave having good temperature characteristics can be obtained without changing various characteristics such as an electromechanical coupling coefficient and a surface acoustic wave propagation velocity. .

Further, in this embodiment, the propagation substrate 11 having a large thermal expansion coefficient in the direction of elastic wave propagation and the auxiliary substrate 12 having a small thermal expansion coefficient in the direction of elastic wave propagation are directly joined. In the vicinity of the surface of the propagation substrate, a compressive stress acts on the change in temperature, and the value of the thermal expansion coefficient is smaller than the thermal expansion coefficient of the propagation substrate in the propagation direction of the elastic wave.

In the present embodiment, the stress acting in the direction parallel to and perpendicular to the surface acoustic wave propagation direction is reduced as compared with the conventional one.

In this embodiment, the size of the recess 15 is 800 μm × 400 μm, and the depth of the recess is 20 μm on average. Although the width of the groove 14 is 400 μm, the degree of relaxation of the stress can be changed by changing the area size and the depth of the recess 15.

Further, in this embodiment, the 36 ° -rotated Y-cut X-propagation lithium tantalate is used as the propagation substrate 11. However, the present invention is not limited to this. The same effect can be obtained by using a substrate whose thermal expansion coefficient in the elastic wave propagation direction is smaller than that of the propagation substrate 11 in the elastic wave propagation direction.

In this embodiment, glass is used as the auxiliary substrate. However, the present invention is not limited to this, and another low thermal expansion material such as silicon may be used. When glass is used as the auxiliary substrate, its non-crystallinity facilitates bonding with a single crystal propagation substrate. Further, in the case of glass, materials having various mechanical properties can be obtained depending on the composition, and the control of temperature characteristics becomes easy.

In addition, there is no mirror joining interface between the propagation substrate 31 and the auxiliary substrate 32 of the conventional surface acoustic wave element immediately below the surface acoustic wave propagation region of the propagation substrate 11 of the present embodiment. When the concave portion 15 is formed, the portion immediately below the surface acoustic wave propagation region of the propagation substrate 11 is substantially in the same state as when roughening is performed. There is also an effect of suppressing unnecessary spurious in the frequency response of the surface acoustic wave element.

Further, in the present embodiment, the recess 15 is formed in the propagation substrate 11, but the effect described so far is essentially caused by the presence of the recess 15 in the substrate structure of the present embodiment. The same effect can be obtained by forming the concave portion 15 in the auxiliary substrate 12 as shown in FIG.

Further, a mobile communication device such as a mobile phone according to the present invention is characterized in that the surface acoustic wave device according to the present invention is used as a filter or an oscillator. The mobile communication device of the present invention includes, as components other than the filter or the oscillator, for example, an antenna, a transmission signal processing circuit, a reception signal processing circuit, and the like, of course, known components required for each mobile communication device. Is provided.

The shape of the non-bonding region between the propagation substrate and the auxiliary substrate according to the present invention is not limited to the above-described groove and concave portion, but includes any shape such as a curved groove, a concave portion having a different depth, and a polygon. .

The shape of the bonding region between the propagation substrate and the auxiliary substrate according to the present invention includes an arbitrary shape such as a partially broken circumferential shape, a dotted shape, etc., in addition to the above-mentioned band shape and circumferential shape.

The bonding region of the present invention does not need to be located at the outermost end of the substrate, but may be located slightly inside.

[0143]

As is apparent from the above description, the present invention can provide a surface acoustic wave device having good temperature characteristics by suppressing variations in the propagation characteristics of surface acoustic waves.

[Brief description of the drawings]

FIG. 1 is a partially cutaway perspective view of a surface acoustic wave device according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along the line a-a 'in FIG.

FIG. 3 is an exploded perspective view of the surface acoustic wave device according to the first embodiment of the present invention.

FIG. 4 is a partially cutaway perspective view of a surface acoustic wave device according to a second embodiment of the present invention.

FIG. 5 is a cross-sectional view taken along the line b-b 'of FIG.

FIG. 6 is an exploded perspective view of a surface acoustic wave device according to a second embodiment of the present invention.

FIG. 7 is a partially cutaway perspective view of a surface acoustic wave device according to a third embodiment of the present invention.

8 is a cross-sectional view taken along the line c-c 'in FIG.

FIG. 9 is a cross-sectional view taken along the line d-d 'of FIG.

FIG. 10 is an exploded perspective view of a surface acoustic wave device according to a third embodiment of the present invention.

FIG. 11 is a sectional view of a conventional surface acoustic wave element.

FIG. 12 is a cross-sectional view of a conventional surface acoustic wave element using a single propagation substrate.

FIG. 13 is an exploded perspective view of a surface acoustic wave device according to another embodiment of the present invention.

FIG. 14 is an exploded perspective view of a surface acoustic wave device according to another embodiment of the present invention.

FIG. 15 is an exploded perspective view of a surface acoustic wave device according to another embodiment of the present invention.

[Explanation of symbols]

 11, 31 Propagation substrate 12, 32 Auxiliary substrate 13, 33 Comb-shaped electrode 14 Groove 15 Recess

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akihiko Nanba 1006 Kadoma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. 72) Inventor Yoshihiro Tomita 1006 Kadoma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (10)

[Claims] An acoustic wave is formed on a surface of a propagation substrate that is a piezoelectric substrate, an auxiliary substrate that is laminated on the propagation substrate by direct bonding, and a surface of the propagation substrate that is opposite to a bonding surface of the auxiliary substrate with the auxiliary substrate. And the auxiliary substrate is not joined to each other at least in a region immediately below the comb-electrode forming region, and the heat of the auxiliary substrate in the propagation direction of the elastic wave is provided. The expansion coefficient is
A surface acoustic wave element having a coefficient of thermal expansion smaller than that of the propagation substrate in a propagation direction of the acoustic wave. 2. The transmission substrate according to claim 1, wherein a recess or a groove is formed on the side of the propagation substrate, and the presence of the recess or the groove forms a region where the propagation substrate and the auxiliary substrate are not bonded. 2. The surface acoustic wave device according to claim 1. 3. The device according to claim 1, wherein a concave portion or a groove is formed on the auxiliary substrate side, and a region where the propagation substrate and the auxiliary substrate are not bonded is formed by the presence of the concave portion or the groove. 2. The surface acoustic wave device according to claim 1. 4. The surface acoustic wave device according to claim 1, wherein a region where the propagation substrate and the auxiliary substrate are joined extends over the entire periphery of the propagation substrate. 5. The propagation substrate and the auxiliary substrate have a substantially rectangular shape, and a region where the substrates are bonded to each other is located on one of a pair of two opposite sides of the propagation substrate. The surface acoustic wave device according to claim 1, wherein: 6. The pair of two opposing sides is defined as one of two pairs of sides that receives more uniform stress in a plane substantially perpendicular to the propagation direction of the elastic wave. 6. The surface acoustic wave device according to claim 5, wherein the surface acoustic wave device comprises a pair of two sides. 7. The comb-shaped electrode according to claim 2, wherein the groove is formed in parallel with a direction of a finger of the comb-shaped electrode.
The surface acoustic wave device as described in the above. 8. The propagation substrate has an anisotropic thermal expansion coefficient, and the groove is formed in a direction substantially perpendicular to a direction in which the thermal expansion coefficient of the propagation substrate is larger. The surface acoustic wave device according to claim 2 or 3, wherein 9. The direct bonding flattens and mirrors the surface of each of the propagation substrate and the auxiliary substrate.
The surface acoustic wave device according to any one of claims 1 to 8, wherein the surface acoustic wave device is cleaned, hydrophilized, superposed, and then bonded by heat treatment. 10. A surface acoustic wave element according to claim 1, comprising an antenna, a transmission signal processing circuit, and a reception signal processing circuit, wherein said surface acoustic wave element is a filter or an oscillator. A mobile communication device characterized by being used as a mobile communication device.