JP2000013180A - Surface acoustic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave element having a satisfactory temperature characteristic without changing various characteristics such as the electro mechanical coupling coefficient of a propagation substrate and surface acoustic wave speed. SOLUTION: A transmission substrate 11 being a piezoelectric substrate, an auxiliary substrate 12 stacked to the propagation substrate 11 by direct junction and comb-line electrodes 13 which are formed on a face opposite to the junction face of the propagation substrate 11 and the auxiliary substrate 12 and which excite an acoustic wave are provided. The coefficient of thermal expansion in the propagation direction of the acoustic wave in the auxiliary substrate 12 is smaller than that of the propagation direction of the acoustic wave in the propagation substrate 11. A groove 14 is formed in a direction which is substantially vertical to comb-line electrode fingers constituting the comb-line electrodes 3 on the face of the auxiliary substrate 12 against the propagation substrate 11.

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 a transmitter and a high-frequency filter, 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 frequency bandwidth of a filter formed by a surface acoustic wave element and the Q value of a resonator. The temperature dependence is the amount of change in the center frequency of the filter with respect to temperature change. And the amount of fluctuation 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 for the transmission band and the attenuation of the transmission frequency band filter for 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 substantial band spacing is more than 10 MH
z, and a sufficient inter-band attenuation cannot be secured. Therefore, a piezoelectric substrate having an electromechanical coupling coefficient of about 5% or more and a TCD of less than 35 ppm / ° C. is desired. However, a piezoelectric substrate having a large electromechanical coupling coefficient and a small TCD only needs to be provided, but such an existing substrate does not have such a substrate. Therefore, the T
Methods for reducing CD have been proposed. For example, IEE Transactions on Sonics and Ultrasonics, Volume SU
-31, pp. 51-57 (1984) (IEEE Tran
sactions on Sonics and Ultrasonics, volume SU-31,
As shown in 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, the temperature dependence of surface acoustic wave propagation characteristics There are known ways to improve this. This method improves the temperature dependency and changes the propagation characteristics of the surface acoustic wave.

However, in the above-mentioned IEE method, the thickness of the silicon oxide thin film layer is made at most one wavelength less than the surface acoustic wave wavelength used in order to obtain the effect of improving the temperature characteristics. Although it is necessary, there is a problem in making the thickness and film quality of the silicon oxide film layer uniform in consideration of increasing the frequency of the surface acoustic wave element.

[0007] To solve these problems, there is a temperature characteristic improving method disclosed in Japanese Patent Application Laid-Open No. 6-326553. This method uses a substrate obtained by laminating substrates having different coefficients of thermal expansion by direct bonding. The thermal expansion coefficient of the laminated substrate is reduced as compared with the case of using only the substrate alone. Is improved in temperature dependency. According to this method, since the propagation characteristic of the surface acoustic wave does not depend on the thickness of the piezoelectric substrate, it is possible to increase the frequency of the surface acoustic wave element without making the piezoelectric substrate thin.

Hereinafter, 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.

FIG. 10 is a sectional view of a conventional surface acoustic wave device using a laminated substrate by direct bonding. In FIG. 10, 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 coefficient of the bonded substrate changes, and the temperature dependency also changes.

[0010]

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 joining is perfect, but if the joining is insufficient, the stress caused by the temperature change will not be uniform over the entire surface. In this case, the surface acoustic wave velocity changes while the surface acoustic wave is being transmitted on the substrate. In some cases, the surface acoustic wave propagation loss increases.

Further, in the conventional surface acoustic wave device, since the substrates are bonded by bonding the surfaces of two substrates to each other, the direction of the stress acting on the substrates as the temperature changes depends on the bonding orientation. Therefore, it is impossible to apply a stress substantially only in one direction in the plane of the substrate, which makes it difficult to control the temperature characteristics.

It is an object of the present invention to provide a conventional surface acoustic wave device in which the bonding strength of the substrate is not uniform at the bonding surface, thereby causing the propagation characteristics of the surface acoustic wave to vary. It is an object of the present invention to provide a surface acoustic wave device having good temperature characteristics without changing various characteristics such as the number of electromechanical couplings and the surface acoustic wave velocity of the piezoelectric substrate in consideration of the problem that the surface acoustic wave is difficult. is there.

[0014]

According to the present invention, there is provided a transmission substrate which is a piezoelectric substrate, an auxiliary substrate laminated by direct bonding to the transmission substrate, and an auxiliary substrate which is opposite to a bonding surface of the transmission substrate with the auxiliary substrate. A comb-shaped electrode formed on a surface of the propagation substrate, the surface of the propagation substrate facing the auxiliary substrate being partially joined directly to the auxiliary substrate, The coefficient of thermal expansion in the propagation direction of the surface acoustic wave element is smaller than the coefficient of thermal expansion of the propagation substrate in the propagation direction of the elastic wave.

[0015]

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 device shown in FIGS. 1 and 2 includes a propagation substrate 11, which is a piezoelectric substrate, an auxiliary substrate 12, and a comb-shaped electrode 13. Comb electrode 13
Are formed on the surface of the propagation substrate 11 opposite to the joint surface with the auxiliary substrate 12, and the surface of the auxiliary substrate 12 facing the propagation substrate 11 is substantially provided with the comb electrode fingers constituting the comb electrodes 13. A groove 14 is formed in a vertical direction. 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 thickness of 1
A 36-degree rotation Y-cut X-propagation lithium tantalate of 00 μm is used, and a 300 μm-thick low thermal expansion glass substrate is used as the auxiliary substrate 12.

Next, the operation of the 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 1
At 3 ', it 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 structure of a surface acoustic wave device using a comb-shaped electrode, the number and the structure of the comb-shaped electrode are required when a filter or a resonator is used. Change accordingly.

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 temperature coefficient of delay time (TCD) of the surface acoustic wave element is approximately represented by the difference between the thermal expansion coefficient α of the propagation substrate in the propagation direction of the acoustic wave and the temperature coefficient of surface acoustic wave velocity (TCV). Also, TC
V 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, T
CV is a negative value.

Most of the surface acoustic wave energy in the depth direction of the propagation substrate is concentrated within about one wavelength of the surface acoustic wave wavelength. Since the thickness of the propagation substrate in the present embodiment is at least 10 times the surface acoustic wave wavelength, TCV is determined by the substrate constant (material constant) of the propagation substrate, and is not related to the substrate constant (material constant) of the auxiliary substrate. .

In the present embodiment, a propagation substrate having a large thermal expansion coefficient in the direction of elastic wave propagation and an auxiliary substrate having a small thermal expansion coefficient in the direction of elastic wave propagation are directly joined to each other, so that a positive temperature change occurs. On the other hand, a compressive stress acts on the vicinity of the surface of the propagation substrate, and the extension of the propagation substrate is suppressed as compared with the case of the propagation substrate alone. Therefore, the density change of the propagation substrate is suppressed, but at the same time, the elastic constant changes (the elastic constant itself decreases and the elastic constant temperature coefficient decreases) due to the stress. The suppression of the density change contributes to the increase of the TCV, and the change of the elastic constant contributes to the decrease to the TCV. However, the influence of the change of the elastic constant is large, and as a result, the TCV is larger than that of the propagation substrate alone.
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 characteristic compensation of the surface acoustic wave device according to the present embodiment is performed.

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, that is, formation of a groove on an auxiliary substrate and direct bonding.

First, the formation of a groove on the auxiliary substrate will be described. The surface of the auxiliary substrate on which grooves are to be formed is mirror-polished. The groove is formed in a linear shape by using a dicing saw with a groove having a rectangular or rectangular shape. When a wide groove is formed, a desired groove width is obtained by inserting a dicing blade a plurality of times. In the present embodiment,
The depth of the groove is 80 μm, and the width of the groove is 400 μm.

The groove of the auxiliary substrate may be formed as follows. First, after cleaning the auxiliary substrate, a photoresist mask is formed on a surface of the auxiliary substrate where a groove is to be formed. Next, the auxiliary substrate on which the photoresist mask is formed is etched with a hydrofluoric acid-based aqueous solution. In this embodiment mode, since the auxiliary substrate contains silicon oxide as a main component, a hydrofluoric acid-based aqueous solution is used as an etchant, but an appropriate etchant is used depending on a substrate material. After the groove forming step, the photoresist mask is removed.

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

The formation of the groove in the auxiliary substrate is not limited to the above method, and any method may be used as long as it forms a linear groove.

Next, the direct joining will be described.

First, the surface of the propagation substrate to be directly bonded and the groove forming surface of the auxiliary substrate are cleaned. Subsequently, the surface of the propagation substrate and the groove forming surface of the auxiliary substrate are 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 and the surface of the auxiliary substrate 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 strengthen 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 200 ° 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 forming the substrate gradually starts, thereby strengthening the bonding. 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 is formed on the bonding substrate by photolithography. In the present embodiment, the comb-shaped electrodes are arranged such that the angle between the comb-shaped electrode fingers and the groove formed in the auxiliary substrate is 90 °. Through the above process, the surface acoustic wave device according to the present embodiment is manufactured.

Here, the effect of the groove of the auxiliary substrate in the present embodiment will be described. In the case of direct joining, if foreign matter such as particles or dust is present on the joining surface, the portion where the foreign matter is present is not joined and voids are formed, or the joining strength becomes extremely weak, resulting in poor joining.
When the substrates are directly joined, the smaller the continuous joining area, the lower the possibility of poor joining. However, if the joining surface is divided by forming grooves, the continuous joining area decreases, and the possibility of poor joining can be reduced. . In addition, by forming the groove, stress is uniformly applied in a region where bonding is good, and a surface acoustic wave device having good characteristics can be obtained.

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 a surface acoustic wave device having a substrate made of lithium tantalate alone, which is a 36 ° rotation Y-cut X-propagation substrate, are described. , Electromechanical coupling coefficient and delay time temperature coefficient (TCD) were measured for comparison. The surface acoustic wave element having a substrate made of a single 36 ° rotation Y-cut X-propagation lithium tantalate as a comparison object has the same dimensions as the surface acoustic wave element according to the present embodiment. As for TCD, a frequency temperature coefficient at a place where the absolute value is the same as that of TCD with a different sign is measured, and
The opposite sign is used as the measured value of TCD. Regarding the electromechanical coupling coefficient, there was no difference between them in about 5%. On the other hand, for TCD, 36 ° rotation Y
It was confirmed that the surface acoustic wave element using cut lithium X-propagation lithium tantalate alone as a substrate was improved to 35 ppm / ° C., whereas the surface acoustic wave element in the present embodiment was improved to 25 ppm / ° C. This is almost the same characteristic as the conventional surface acoustic wave device shown in FIG. Further, as compared with the conventional surface acoustic wave device shown in FIG. 10, it was also confirmed that the variation in characteristics due to poor bonding was reduced and stable characteristics were obtained.

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. .

In this embodiment, a 36 ° rotation Y-cut X-propagation lithium tantalate is used as the propagation substrate. 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 wave is smaller than that of the propagation substrate in the propagation direction of the elastic wave.

In this embodiment, a propagation substrate 11 having a large thermal expansion coefficient in the propagation direction of an elastic wave,
The auxiliary substrate 12 which is thicker and has a small thermal expansion coefficient in the direction of propagation of the elastic wave is directly joined. It shows a value smaller than the thermal expansion coefficient in the propagation direction, and the change in density is also small.

In this embodiment, the stress acting in the direction parallel to the comb-shaped electrode fingers is reduced as compared with the conventional one. The stress in this direction becomes uniform. On the other hand, in the direction perpendicular to the comb-shaped electrode fingers, the stress works effectively because the propagation substrate and the auxiliary substrate are directly bonded. As a result, the main direction of the stress acting on the substrate is a direction perpendicular to the comb-shaped electrode fingers, so that the stress can have directionality and the stress can be easily controlled.

In the present embodiment, the depth of the groove is set to 80 μm. However, the stress is further reduced by making the groove deeper. Although the width of the groove is set to 400 μm, the stress is further reduced by increasing the width of the groove.

Further, in the present embodiment, the groove cross section is rectangular or a shape conforming to a rectangle, but may be V-shaped or another cross section.

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.

(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 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. Also, unless otherwise specified,
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 parallel 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 is roughly divided into two processes of forming a groove on an auxiliary substrate and directly bonding.

First, the formation of a groove on the auxiliary substrate will be described. The surface of the auxiliary substrate on which grooves are to be formed is mirror-polished. The groove is formed in a linear shape with a dicing saw using a rectangular or rectangular shape. When a wide groove is formed, a desired groove width is obtained by inserting a dicing blade a plurality of times. In the present embodiment, the depth of the groove is 80 μm on average, and the width of the groove is about 800 μm.

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

On the bonding substrate obtained by the above process, a comb-shaped electrode is formed on the bonding substrate by photolithography. In the present embodiment, the comb-shaped electrodes are arranged such that the angle between the comb-shaped electrode fingers and the grooves formed in the auxiliary substrate is 0 ° (the longitudinal direction of the comb-shaped electrode fingers and the main direction of the grooves match). . Through the above process, the surface acoustic wave device according to the present embodiment is manufactured.

Here, the effect of the groove of the auxiliary substrate in the present embodiment will be described. In the case of direct joining, if foreign matter such as particles or dust is present on the joining surface, the portion where the foreign matter is present is not joined and voids are formed, or the joining strength becomes extremely weak, resulting in poor joining.
When the substrates are directly joined, the smaller the continuous joining area, the lower the possibility of poor joining. However, if the joining surface is divided by forming grooves, the continuous joining area decreases, and the possibility of poor joining can be reduced. . In addition, since the stress is uniformly applied to the region where the bonding is good by forming the groove, a surface acoustic wave device having good characteristics can be obtained.

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 a surface acoustic wave device using a single substrate of lithium tantalate of 36 ° rotation Y-cut X propagation are used. , Electromechanical coupling coefficient and delay time temperature coefficient (TCD) were measured for comparison. The surface acoustic wave element having a substrate made of a single 36 ° rotation Y-cut X-propagation lithium tantalate as a comparison object has the same dimensions as the surface acoustic wave element according to the present embodiment. As for TCD, a frequency temperature coefficient at a place where the absolute value is the same as that of TCD with a different sign is measured, and
The opposite sign is used as the measured value of TCD. Regarding the electromechanical coupling coefficient, there was no difference between them in about 5%. On the other hand, for TCD, 36 ° rotation Y
It was confirmed that the surface acoustic wave device using cut lithium X-propagation lithium tantalate alone as a substrate had an improvement of 35 ppm / ° C., whereas the surface acoustic wave device of the present embodiment had an improved performance of 29 ppm / ° C. This is almost the same characteristic as the conventional surface acoustic wave device shown in FIG. Further, as compared with the conventional surface acoustic wave device shown in FIG. 10, it was also confirmed that the variation in characteristics due to poor bonding was reduced and stable characteristics were obtained.

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 the number of electromechanical coupling systems and the surface acoustic wave propagation velocity. .

In this embodiment, the 36 ° -rotation Y-cut X-propagation lithium tantalate is used as the propagation substrate. 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 wave is smaller than that of the propagation substrate in the propagation direction of the elastic wave.

In the present embodiment, a propagation substrate 11 having a large thermal expansion coefficient in the propagation direction of an elastic wave,
The auxiliary substrate 12 which is thicker and has a small thermal expansion coefficient in the direction of propagation of the elastic wave is directly joined. It shows a value smaller than the thermal expansion coefficient in the propagation direction, and the change in density is also small.

Further, in this embodiment, the stress acting in the direction perpendicular to the comb-shaped electrode fingers is reduced as compared with the conventional one. The stress in this direction becomes uniform. On the other hand, in the direction parallel to the comb-shaped electrode fingers, the stress works effectively because the propagation substrate and the auxiliary substrate are directly bonded. As a result, the main direction of the stress acting on the substrate is a direction parallel to the comb-shaped electrode fingers, so that the stress can be given a directionality, and the stress can be easily controlled.

In the present embodiment, the depth of the groove is 80 μm on average, but the stress is further reduced by making the groove deeper. In addition, the width of the groove is 800 μ
The stress is further reduced by increasing the width of the groove.

Further, in the present 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.

(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 of the present invention is provided instead of the groove 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 shows the propagation substrate 1
FIG. 2 is an exploded perspective view showing an auxiliary substrate 1 and an auxiliary substrate 12. FIG.
The surface acoustic wave device shown in FIG.
This is the same as the surface acoustic wave device shown in FIGS. The concave portion 15 of the surface acoustic wave element according to the present embodiment is formed on the auxiliary substrate 12 so as to include at least a position immediately below the comb-shaped electrode.

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 groove on an auxiliary substrate and directly bonding.

First, the formation of a groove on the auxiliary substrate will be described. The surface of the auxiliary substrate on which grooves are to be formed is mirror-polished. For the concave portion, first, the auxiliary substrate is washed, and then a photoresist mask is formed on the surface of the auxiliary substrate where the concave portion is to be formed. Next, the surface of the auxiliary substrate on which the photoresist mask is to be formed is sandblasted with blast abrasive grains. Finally, the photoresist mask is removed.

The formation of the concave portion of the auxiliary substrate is not limited to the above method, and any method may be used as long as the concave portion is formed.

In this embodiment, the shape of the concave portion is rectangular, and the size of the concave region is 800 μm × 400 μm. The depth of the recess is set to 80 μm on average.

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

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

Here, the effect of the groove of the auxiliary substrate in the present embodiment will be described. In the case of direct joining, if foreign matter such as particles or dust is present on the joining surface, the portion where the foreign matter is present is not joined and voids are formed, or the joining strength becomes extremely weak, resulting in poor joining.
In the case where the substrates are directly bonded, the possibility of poor bonding decreases as the continuous bonding area decreases, but the possibility of poor bonding can be reduced by reducing the continuous bonding area by forming recesses.

In the present embodiment, the stress acting in the direction perpendicular to and parallel to the comb-shaped electrode fingers is almost the same as 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, 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.
Therefore, it is possible to obtain a surface acoustic wave element in which a uniform stress acts on the surface acoustic wave propagation region.

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 a surface acoustic wave device using a single substrate of lithium tantalate of 36 ° rotation Y-cut X propagation as a substrate will be described. , Electromechanical coupling coefficient and delay time temperature coefficient (TCD) were measured for comparison. The surface acoustic wave element having a substrate made of a single 36 ° rotation Y-cut X-propagation lithium tantalate as a comparison object has the same dimensions as the surface acoustic wave element according to the present embodiment. As for TCD, a frequency temperature coefficient at a place where the absolute value is the same as that of TCD with a different sign is measured, and
The opposite sign is used as the measured value of TCD. Regarding the electromechanical coupling coefficient, there was no difference between them in about 5%. On the other hand, for TCD, 36 ° rotation Y
It was confirmed that the surface acoustic wave element using cut lithium X-propagation lithium tantalate alone as a substrate had an improvement of 35 ppm / ° C., while the surface acoustic wave element of the present embodiment had an improved 22 ppm / ° C. It was also confirmed that this was improved as compared with the conventional surface acoustic wave device shown in FIG. Further, as compared with the conventional surface acoustic wave device shown in FIG. 10, it was also confirmed that variations in characteristics due to defective bonding were reduced and stable characteristics were obtained.

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 the number of electromechanical coupling systems and the surface acoustic wave propagation velocity. .

In this embodiment, a 36 ° rotation Y-cut X-propagation lithium tantalate is used as a propagation substrate. 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 wave is smaller than that of the propagation substrate in the propagation direction of the elastic wave.

In this embodiment, the propagation substrate 11 having a large thermal expansion coefficient in the direction of propagation of the elastic wave and the auxiliary substrate 12 which is thicker than the propagation substrate and has a small coefficient of thermal expansion in the direction of propagation of the elastic wave are directly joined. In the vicinity of the surface of the propagation substrate due to a positive temperature change, compressive stress acts, showing a value smaller than the thermal expansion coefficient of the propagation direction of the elastic wave inherent in the propagation substrate,
The density change is also small.

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 the first to third embodiments described above, the grooves or recesses of the present invention are formed on the auxiliary substrate of the present invention. However, the present invention is not limited to this. When the opposing surfaces of the substrate and the auxiliary substrate are partially joined directly, compared to the case where the surfaces are joined directly over the entire surface, the effect of reducing the possibility of joint failure by reducing the continuous joining area is reduced. Is obtained.

In the first to third embodiments, the thickness of the auxiliary substrate of the present invention is described as being larger than the thickness of the propagation substrate of the present invention. However, the present invention is not limited to this. At least, the thermal expansion coefficient of the auxiliary substrate in the propagation direction of the elastic wave may be smaller than the thermal expansion coefficient of the propagation substrate in the propagation direction of the elastic wave.

[0073]

As is apparent from the above description, the present invention provides a surface acoustic wave having good temperature characteristics without changing various characteristics of the piezoelectric substrate such as the number of electromechanical couplings and the surface acoustic wave velocity. An element can be provided.

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

FIG. 10 is a sectional view of a conventional surface acoustic wave element.

[Explanation of symbols]

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

Continued on the front page (72) Inventor Akihiko Nanba 1006 Kadoma Kadoma, Osaka Pref. Matsushita Electric Industrial Co., Ltd. (72) Inventor Tetsuyoshi Kotera 1006 Odakadoma Kadoma, Osaka Pref. Matsushita Electric Industrial Co., Ltd. (72) Inventor Yutaka Taguchi 1006 Kazuma Kadoma, Kadoma City, Osaka Pref. Matsushita Electric Industrial Co., Ltd. (72) Yoshihiro Tomita 1006 Kadoma Kadoma, Kadoma City, Osaka Pref. KK09

Claims (8)

[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. A comb-shaped electrode for exciting the auxiliary substrate, the surface of the propagation substrate facing the auxiliary substrate is partially directly joined to the auxiliary substrate, and the thermal expansion coefficient of the auxiliary substrate in the elastic wave propagation direction is A surface acoustic wave element having a thermal expansion coefficient smaller than that of the propagation substrate in a propagation direction of the acoustic wave. 2. The surface acoustic wave device according to claim 1, wherein a thickness of the auxiliary substrate is larger than a thickness of the propagation substrate. 3. The surface acoustic wave device according to claim 1, wherein the auxiliary substrate has a groove formed substantially in one direction on a surface facing the propagation substrate. 4. The surface acoustic wave device according to claim 3, wherein the groove is formed in a direction substantially perpendicular or parallel to a comb-shaped electrode finger constituting the comb-shaped electrode. 5. The surface acoustic wave device according to claim 1, wherein the auxiliary substrate has a concave portion formed at least at a position directly below the comb electrode on a surface facing the propagation substrate. 6. The direct bonding flattens and mirrors the substrate surfaces of the propagation substrate and the auxiliary substrate, respectively.
The surface acoustic wave device according to any one of claims 1 to 5, wherein the surface acoustic wave device is cleaned, hydrophilized, superposed, and then joined by heat treatment. 7. The propagation substrate according to claim 1, wherein the propagation substrate is a piezoelectric single crystal made of any one of lithium niobate, lithium tantalate, lithium borate, and langasite. The surface acoustic wave device as described in the above. 8. The surface acoustic wave device according to claim 1, wherein the auxiliary substrate contains silicon or silicon oxide as a main component.