JPH09130192A - Surface wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the temperature characteristic and also to increase the electricity-machine coupling coefficient for a surface wave device by setting the normalized thickness of a piezoelectric thin film at the specific value or more. SOLUTION: A ZnO thin film 12 is formed on the entire surface of a crystal substrate 10, and the IDT 13 and 14 are formed on the film 13 with a space secured between them. The IDT 13 and 14 include a pair of sawtooth electrodes 13a and 13b and 14a and 14b, respectively. When the film 12 is formed on the plate 10, the normalized thickness H/λ of the film 12 is desirably set at >=0.05 (H, thickness of film 12; λ, surface wavelength). The electricity-machine coupling coefficient is usually reduced when the DIT is formed between the no film and the crystal. However, a large electricity-machine coupling coefficient is secured when the DIT 13 and 14 are formed on the film 12 like this example. Furthermore, the electricity-machine coupling coefficient is further increased when a short circuit electrode is inserted into the interface between the plate 10 and the film 12.

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 using a quartz substrate, and more particularly to improvement of a surface acoustic wave device using a surface acoustic wave substrate having a piezoelectric thin film laminated on a quartz substrate.

[0002]

2. Description of the Related Art Conventionally, surface acoustic wave devices have been widely used, for example, as bandpass filters for mobile communication equipment. A surface wave (hereinafter, SAW) device has a structure in which at least one interdigital transducer (hereinafter, IDT) including at least one pair of comb-teeth electrodes is formed so as to be in contact with a piezoelectric body.

In recent years, various SAW devices using piezoelectric thin films have been proposed. That is, SA using a surface wave substrate formed by forming a piezoelectric thin film on a glass substrate or a piezoelectric substrate.
W devices have been proposed.

In the structure using the piezoelectric thin film and the glass substrate, four types of structures shown in FIGS. 1 (a) and 1 (b) and FIGS. 2 (a) and 2 (b) are known. That is, FIG.
In the SAW device 1 shown in (a), a ZnO thin film 3 is formed as a piezoelectric thin film on a glass substrate 2, and an IDT 4 is formed on the ZnO thin film 3. On the other hand, in the SAW device 5 shown in FIG. 1B, the IDT 4 is formed on the lower surface of the ZnO thin film 3, that is, at the interface between the glass substrate 2 and the ZnO thin film 3.

Further, in the SAW device 6 shown in FIG. 2 (a), the short-circuit electrode 7 is formed on the glass substrate 2,
The ZnO thin film 3 is laminated on the short-circuit electrode 7. ID
T4 is formed on this ZnO thin film 3. That is, the SAW device 6 is the SAW device 1 shown in FIG.
2 corresponds to the structure in which the short-circuit electrode 7 is inserted at the interface between the glass substrate 2 and the ZnO thin film 3.

In the SAW device 8 shown in FIG. 2B, the short-circuit electrode 7 is formed on the ZnO thin film 3. Also, ID
T4 is formed at the interface between the glass substrate 2 and the ZnO thin film 3. Therefore, the SAW device 8 corresponds to the structure in which the short-circuit electrode 7 is formed on the upper surface of the ZnO thin film 3 in the SAW device 5 shown in FIG.

The SAW devices 1, 5, 6 and 8 are replaced by IDTs.
FIG. 3 shows the electromechanical coupling coefficient when only the formation position of No. 4 and the presence or absence of the short-circuit electrode 7 are different, and the other configurations are the same. In FIG. 3, the four types of SAW devices 1, 5, 6, 8
The change in the electromechanical coupling coefficient with respect to the normalized film thickness H / λ of the ZnO thin film in FIG. In this specification, H represents the thickness of the ZnO thin film, and λ represents the wavelength of the surface wave excited. The solid line A shows the result of the SAW device 1, the broken line B shows the result of the SAW device 5, and the dashed-dotted line C shows the result of S.
The result of the AW device 6 is shown, and the chain double-dashed line D shows the result of the SAW device 8.

As is apparent from FIG. 3, by selecting H / λ, the SAW devices 5 and 8 can obtain a larger electromechanical coupling coefficient than the SAW devices 1 and 6. Therefore, in the conventional structure in which the ZnO thin film 3 is formed on the glass substrate 2, the IDT 4 is formed on the glass substrate 2 and the ZnO thin film 3.
It is said that a larger electromechanical coupling coefficient can be obtained by forming it at the interface with the O thin film 3. The wave described as Sezawa wave in FIG. 3 is a surface wave of a higher mode of Rayleigh wave.

In addition, Jpn. J. Appl. Phys.
Vol. 32 (1993) pp. 2333-2336,
The change in the electromechanical coupling coefficient depending on the position of the IDT or the short-circuit electrode is shown when a surface wave substrate formed by forming a ZnO thin film on a LiNbO ₃ piezoelectric single crystal substrate is used. This will be described with reference to FIGS. 4 (a) and 4 (b) and FIGS. 5 (a) and 5 (b).

FIG. 4A shows the SAW shown in FIG.
The glass substrate 2 in the device 1 is replaced with LiNbO._Three Piezoelectric single crystal
ZnO thin for SAW device with structure replaced with substrate
Relative film thickness kh (k is 2π / λ, h is the ZnO thin film)
Film thickness) and electromechanical coupling coefficient K ^Two Shows the relationship with. In addition,
Solid line + is LiNbO_Three ZnO thin film is formed on the positive side of
In the case of doing, the broken line-is LiNbO._Three On the negative side of Z
It is a characteristic when an nO thin film is formed. Similarly, FIG.
(B) is a glass substrate of the SAW device 5 shown in FIG.
Plate 2 is LiNbO_Three When replaced with a piezoelectric single crystal substrate
Electromechanical coupling coefficient K^Two And a solid line is LiNb.
O_Three When a ZnO thin film is formed on the positive surface of
Shows the characteristics when a ZnO thin film is formed on the negative side
You. Further, FIG. 5A shows the SAW shown in FIG.
The glass substrate 2 of the device 6 is shown in FIG. 5 (b) in FIG. 2 (b).
The glass substrate 2 of the SAW device 8 is
bO_ThreeElectromechanical coupling when replaced with piezoelectric single crystal substrate
Coefficient K^Two FIG.

As is clear from FIGS. 4 and 5, LiN
Even in the structure in which the ZnO thin film is formed as the piezoelectric thin film on the bO ₃ piezoelectric single crystal substrate, as is apparent from FIGS. 4B and 5B, the IDT is provided between the piezoelectric single crystal substrate and the ZnO thin film. It can be seen that a larger electromechanical coupling coefficient K ² can be realized by forming it on the interface.

That is, conventionally, when a surface wave substrate having a ZnO thin film formed on a glass substrate or a piezoelectric substrate is used, the IDT should be formed between the piezoelectric thin film and the substrate in order to obtain a large electromechanical coupling coefficient. It was being done.

On the other hand, depending on the application, there is a demand for a surface acoustic wave substrate which has not only a large electromechanical coupling coefficient but also a good temperature characteristic, that is, a characteristic change caused by a temperature change is small. Quartz is known as a material. However, the quartz substrate has a problem that the electromechanical coupling coefficient is relatively small.

As described above, the quartz substrate has good temperature characteristics. For example, for a rotating Y-cut quartz substrate,
As shown in 0, it is known that the Euler angle and the TCD and the propagation loss have the relationship shown in the figure. Here, TCD indicates a rate of change of delay time with temperature (unit: ppm / ° C.).

In FIG. 10, the solid line A indicates the rotation Y.
T when a leaky surface acoustic wave is excited on a cut quartz substrate
CD, the solid line B shows the propagation loss, and the broken line C shows the TCD of the Rayleigh surface wave. However, the propagation loss of the Rayleigh wave is zero.

Since the TCD and propagation loss characteristics shown in FIG. 10 are known, the Euler angle shown in FIG.
A rotating Y-cut quartz substrate near 0 degrees was used. That is, a crystal substrate with a cut angle of TCD near 0 is used, and a SAW device in which an IDT is formed on the crystal substrate has been used as a SAW device having good temperature characteristics.

However, also in the SAW device using the quartz substrate constructed as described above, the electromechanical coupling coefficient is also small, and therefore, for example, when the SAW filter is constructed, a low insertion loss or a wide band filter is provided. It was difficult to obtain the characteristics.

[0018]

SUMMARY OF THE INVENTION An object of the present invention is to use a surface acoustic wave substrate having a quartz substrate and a piezoelectric thin film laminated on each other.
An AW device is provided with a structure capable of further increasing the electromechanical coupling coefficient.

Another object of the present invention is to provide an SA having good temperature characteristics and a larger electromechanical coupling coefficient.
It is to provide a W device.

[0020]

The present invention has been made to achieve the above object, and according to a broad aspect of the first invention of the present application, a quartz substrate and a quartz substrate are formed on the quartz substrate. And a comb-teeth electrode formed on the piezoelectric thin film, wherein the thickness of the piezoelectric thin film is H and the wavelength of the surface wave is λ.
, The normalized film thickness H / λ of the piezoelectric thin film is 0.05
A surface acoustic wave device is provided which is characterized as described above.

In the surface acoustic wave device according to the first invention of the present application, the piezoelectric thin film is formed on the quartz substrate as described above, and the comb electrode is formed on the piezoelectric thin film. That is, conventionally, when a surface acoustic wave substrate formed by laminating a non-piezoelectric substrate or a piezoelectric substrate and a piezoelectric thin film is used, the IDT or the comb-teeth electrode should be formed between the substrate and the piezoelectric thin film, which is the electromechanical method. It was thought that the coupling coefficient could be increased. However, in the case of a quartz substrate which is a piezoelectric substrate, the inventor of the present application has found that forming a piezoelectric thin film on the quartz substrate and forming a comb electrode on the piezoelectric thin film can increase the electromechanical coupling coefficient. The inventors have found that the electromechanical coupling coefficient can be further increased if the piezoelectric thin film in that case has the above-mentioned specific thickness, and the first invention of the present application has been achieved.

The surface acoustic wave device preferably further includes a short-circuit electrode formed between the quartz substrate and the piezoelectric thin film. According to a wide aspect of the second invention of the present application, a quartz substrate, a piezoelectric thin film formed on the quartz substrate, and a comb-teeth electrode formed so as to be in contact with the piezoelectric thin film, the quartz substrate, There is provided a surface acoustic wave device characterized in that a crystal substrate having a cut angle and a propagation direction having a negative delay time temperature characteristic TCD is used.

In the second invention, as is clear from the description of the embodiments below, as the crystal substrate, a crystal substrate having a cut angle and a propagation direction in which the delay time temperature characteristic TCD has a negative value is used. , A piezoelectric thin film is laminated on a quartz substrate. The piezoelectric thin film usually has a delay time temperature characteristic T
CD has a positive value. Therefore, in the above-described surface acoustic wave device of the present invention, the delay time temperature characteristic TCD is canceled by the crystal substrate and the piezoelectric thin film, so that a good temperature characteristic is realized.

On the other hand, as will be described later, a crystal substrate having a cut angle and a propagation direction in which the delay time temperature characteristic TCD has a negative value is compared with a crystal substrate having a cut angle and a propagation direction in which the delay time temperature characteristic TCD is close to zero. And has a large electromechanical coupling coefficient. Therefore, according to the second invention, it is possible to provide a surface acoustic wave device having good temperature characteristics and having a larger electromechanical coupling coefficient.

According to another specific aspect of the second invention,
The comb electrode is formed on the piezoelectric thin film, and thus the first
As in the case of the invention of 1), an even larger electromechanical coupling coefficient can be realized.

Further, according to another specific aspect of the second invention, a short-circuit electrode formed between the quartz substrate and the piezoelectric thin film is further provided, whereby the electromechanical coupling coefficient is increased.

According to still another aspect of the second invention, when the thickness H of the piezoelectric thin film and the wavelength λ of the surface wave are:
The normalized film thickness H / λ of the piezoelectric thin film is set to 0.03 or more, whereby the electromechanical coupling coefficient is further increased.

In the first and second inventions of the present application, the piezoelectric thin film may be composed of one selected from the group consisting of ZnO, AlN, Ta ₂ O ₅ and CdS. However, other piezoelectric thin films may be used. The piezoelectric thin film made of the above material has a positive delay time temperature characteristic TCD. Therefore, in the second invention of the present application, by forming the piezoelectric thin film from the above material, it is possible to configure the SAW device having good temperature characteristics as described above.

The SAW device according to the first and second inventions of the present application can be applied to various SAW devices such as a SAW filter, a SAW resonator and a SAW delay line.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a technique on which the present invention is based will be first described as Reference Examples 1 and 2 with reference to the drawings.

Reference Example 1 First, an ST having dimensions of 76.2 mm diameter × 0.5 mm
Using a cut X-direction propagation quartz substrate, the following three types of S
An AW device was produced.

SAW device 11 ... ZnO on the quartz substrate
A thin film was formed on the entire surface, two IDTs were formed on the thin film at a predetermined distance, and a SAW filter was produced. That is, as shown in FIG. 9A, the IDTs 13 and 14 were formed on the piezoelectric thin film 12 at a predetermined distance. I
The DTs 13 and 14 have a pair of comb electrodes 13 respectively.
a, 13b and 14a, 14b. Also, this S
In the AW device 11, as shown in FIG. 9B, the ZnO thin film 12 is formed on the quartz substrate 10.

SAW device 12 ... Unlike the SAW device 11, the SAW device 2 is the same as the SAW device 1 except that the IDT is formed between the quartz substrate and the ZnO thin film.
Was formed.

SAW device 13 ... In the SAW device 11, a short-circuit electrode made of Al was formed on the entire interface between the ZnO thin film and the quartz substrate, and the others were the same as in the SAW device 11.

In the SAW devices 11 to 13 described above, Zn
The thickness of the O thin film was variously changed, and the relationship between the thickness of the ZnO thin film and the electromechanical coupling coefficient was measured. FIG. 6 shows the results. In FIG. 6, a broken line D, a solid line E, and an alternate long and short dash line F show the results of the SAW devices 11, 12, and 13, respectively. In addition,
The horizontal axis of FIG. 6 shows the normalized film thickness H / λ of the ZnO thin film.

As is apparent from FIG. 6, by adjusting the film thickness of the ZnO thin film, the largest electromechanical coupling coefficient is obtained in the SAW device 13, and then the SAW device 1 is obtained.
It can be seen that a large electromechanical coupling coefficient is obtained in No. 1 and the electromechanical coupling coefficient is reduced in the SAW device 12 by forming a ZnO thin film. That is, FIG.
The results shown in are for Z-axis propagation in the ST-cut X-direction crystal substrate.
When the IDT is formed between the ZnO film and the crystal, the electromechanical coupling coefficient becomes smaller than that of the crystal in which nO is not formed. On the other hand, when the IDT is formed on the ZnO thin film, a large electromechanical coupling coefficient is obtained. More preferably, it is shown that a larger electromechanical coupling coefficient can be obtained by further inserting a short circuit electrode at the interface between the quartz substrate and the ZnO thin film.

Reference Example 2 165 ° rotating Y plate having dimensions of 76.2 mm diameter × 0.5 mm Y-plane propagation in the X direction (0 °, 75 °, 0 ° at Euler angles) using a quartz substrate and the following SAW devices 14 to. 16 was produced.

SAW device 14 ... The SAW device 11 was constructed in the same manner as the SAW device 11 of Reference Example 1 described above except that the substrate material was the rotating Y-plate X-direction propagation quartz substrate. SAW device 15 ... The SAW device 12 was constructed in the same manner as the SAW device 12 manufactured in Reference Example 1 except that the rotating Y-plate X-direction propagation crystal substrate was used as the crystal substrate.

SAW device 16 ... A SAW device 16 was constructed in the same manner as the SAW device 13 produced in Reference Example 1 except that the rotating Y-plate X-direction propagation quartz substrate was used. In the SAW devices 14 to 16, the electromechanical coupling coefficient was measured while varying the film thickness of the ZnO thin film. FIG. 7 shows the results.

In FIG. 7, the broken line G shows the characteristics of the SAW device 14, the solid line H shows the result of the SAW device 15, and the dashed-dotted line I.
Shows the result of the SAW device 16. As is apparent from FIG. 7, even when the rotating Y plate X-direction propagation crystal substrate is used, as in the case of Reference Example 1, an IDT, that is, a comb electrode is formed between the ZnO thin film and the crystal substrate. It can be seen that a larger electromechanical coupling coefficient can be obtained when the ZnO thin film is formed on the ZnO thin film. Further, as is clear by comparing the characteristics of the one-dot chain line I and the broken line G, it can be seen that an even larger electromechanical coupling coefficient can be obtained by further forming a short-circuit electrode between the quartz substrate and the ZnO thin film. .

From the results of Reference Examples 1 and 2 described above, even when the crystal substrates having different cut angles are used, when the surface acoustic wave substrate formed by forming the ZnO thin film on the crystal substrate is used, the comb It can be seen that a large electromechanical coupling coefficient can be obtained by forming the tooth electrode on the ZnO thin film. Based on the above-mentioned Reference Examples 1 and 2, experimental examples of the present invention which can further increase the electromechanical coupling coefficient will be described.

Experimental Example 1 First, 11 having a size of 76.2 mm diameter × 0.5 mm
A quartz substrate made of a 5 ° rotated Y plate was used. The quartz substrate with this cut angle is one of the substrate materials known to obtain the largest electromechanical coupling coefficient in leaky surface acoustic waves. The propagation direction of the leaky surface acoustic wave is the X direction.

SAW device 17 using the following leaky surface acoustic wave using the 115 ° rotated Y plate X direction propagation quartz substrate.
~ 19 were produced. SAW device 17 ... The SAW device 11 was constructed in the same manner as the SAW device 11 except that the above-mentioned crystal substrate was used.

SAW device 18 ... A SAW device 18 was produced in the same manner as the SAW device 12 except that the above-mentioned crystal substrate was used. SAW device 19 ... A SAW device 19 was produced in the same manner as the SAW device 13 except that the above-mentioned specific crystal substrate was used.

The thicknesses of the ZnO thin films of the SAW devices 17 to 19 were varied and the electromechanical coupling coefficient was measured. The results are shown in Fig. 8. In FIG. 8, the broken line J is SAW
The solid line K shows the result of the apparatus 17, the solid line K shows the result of the SAW apparatus 18, and the chain line L shows the experimental result of the SAW apparatus 19.

Further, in the characteristics shown in FIG. 8, a distinction between the case where the excited surface wave is a leaky surface acoustic wave and the case where it is a normal Rayleigh wave is shown in FIG. That is,
As is clear from FIG. 8, when the normalized film thickness of the ZnO thin film is less than H / λ = 0.14 (the position indicated by the chain double-dashed line M), the leaky surface acoustic wave with attenuation is excited and propagates. It was found that it cannot be used for the transversal type SAW device used. However, it may be used for a SAW element that does not require propagation. Therefore, Zn
By setting the standardized thickness of the O thin film to 0.14 or more, a SAW with a large electromechanical coupling coefficient without attenuation
It can be seen that the device can be obtained.

Further, as is clear from FIG. 8, in the SAW device 18 in which the comb-teeth electrode is formed between the quartz substrate and the ZnO thin film, it is understood that the leaky surface acoustic wave cannot be effectively excited. On the other hand, in the SAW devices 17 and 19 in which the IDT, that is, the comb-teeth electrode is formed on the ZnO thin film, by setting the above H / λ to 0.14 or more, the electromechanical coupling coefficient is large and the attenuation is high. It can be seen that it is possible to provide a SAW device using leaky surface acoustic waves. In the case of a leaky surface acoustic wave, as shown in FIG. 10B, the propagation constant indicating attenuation differs depending on the cut angle of the substrate, and the 115 ° rotating Y-plate X propagation shown here has an attenuation of 0.2 dB / λ. However, depending on the cut angle, there is a point where attenuation is zero (for example, X propagation of 105 ° rotated Y plate or 30 ° rotated Y plate). At such a cut angle, the ZnO film thickness H /
When λ is larger than 0.05, there is no attenuation and a large electromechanical coupling coefficient is obtained.

As is clear from comparison of the results shown in FIGS. 6 to 8, the standardized film thickness H of the ZnO thin film capable of exciting the cut angle of the quartz substrate used and the surface wave used.
/ Λ is different, and in the case of Rayleigh wave surface wave like the ST cut X-direction propagation crystal substrate and the 165 ° rotated Y plate X-direction propagation crystal substrate described above, H / λ is set to 0.05.
In the case of leaky surface acoustic waves, H / λ is 0.14 or more in the 115 ° rotated Y-plate X-propagation crystal substrate, and H / λ in the 105 ° -rotated Y-plate X-propagation crystal substrate.
/ Λ may be 0.05 or more. Therefore, when the ZnO thin film is formed on the quartz substrate, the normalized film thickness H / λ of the ZnO thin film is preferably 0.05 or more.

As described above, as the crystal substrate used,
Since various rotating Y plates with different Euler angles were used,
Many types of SAW devices using quartz substrates with different angle
I created a kind. The above-mentioned oy of the SAW device thus obtained
Rahler angle and electromechanical coupling coefficient K ^Two Figure 16 (a)
Is marked with a circle.

In FIG. 16A, the solid line N shows the leaky surface wave, and the broken line P shows the electromechanical engagement coefficient of the rotating Y-plate X-propagating crystal substrate itself for the Rayleigh wave. In the figure, ○ and ● indicate the square value K ² of the electromechanical coupling coefficient of the Rayleigh wave and the leaky surface acoustic wave in the structure in which the ZnO film is formed on the crystal and the IDT is formed on the ZnO film. ing. On the other hand, the △ and ▲ marks are the Z
The square value K ² of the electromechanical coupling coefficient of the Rayleigh wave and the leaky surface acoustic wave when a short-circuit electrode is provided at the boundary between nO and crystal is shown. As described above, K ² is slightly different depending on the cut angle. 16 (b) is the same as FIG. 10, but is shown again immediately below FIG. 16 (a) for comparison with FIG. 16 (a).

Experimental Example 2 As described with reference to FIG. 10, when a SAW device using a quartz substrate is conventionally constructed, in order to improve temperature characteristics, TCD has a cut angle near zero. Although a quartz substrate was used, a sufficiently large electromechanical coupling coefficient could not be obtained in this case. In the relationship between Euler angle and TCD shown in FIG. 10, the present inventor has found that a good TCD and a large electromechanical coupling coefficient can be obtained by combining a quartz substrate having a negative TCD and a ZnO thin film. I found it.

The relationship between the cut angle and the propagation direction of the crystal substrate and the TCD is as follows.
There is a region where TCD is negative. This is shown in FIG.
This will be described with reference to FIG.

FIG. 11 shows the relationship between the angle θ from the X axis in the propagation direction in the ST cut quartz substrate and TCD, and FIG. 12 shows the angle θ from the Y axis in the propagation direction in the X cut quartz substrate.
And TCD, FIG. 13 shows the relationship between the angle θ from the X axis in the propagation direction of the Y-cut crystal substrate and TCD, and FIG. 14 shows the angle θ from the X axis in the propagation direction of the Z-cut crystal substrate and TC.
The relationship with D is shown. As is clear from FIGS. 11 to 14, even in these quartz substrates, there is a region where the TCD becomes negative by changing the propagation direction angle θ.

The second invention of the present application is the TCD as described above.
By combining a quartz substrate having a cut angle or a propagation direction showing a negative value with a piezoelectric thin film having a positive TCD value, a good TCD and a large electromechanical coupling coefficient are realized. This will be described with reference to FIG. 15, taking a quartz substrate made of a rotating Y plate as an example.

First, as a quartz substrate, a diameter of 76.2 mm ×
A substrate having a size of 0.5 mm was prepared. The crystal substrate is a 165 ° rotated Y plate X propagation and ST cut 35.
° Two types of propagation were prepared.

A ZnO film was formed on the above-mentioned quartz substrate so that H / λ = 0 to 0.
A film having a film thickness of up to 0.5 was formed, and an IDT was further formed thereon so as to match the propagation direction. The SAW is the temperature characteristic of the delay time of the characteristic (temperatures of 15, 25, 35 ℃ 3
Point) was measured. The result is shown in FIG. For the 165 ° rotated Y-plate X propagation, H / λ = 0.35, and for the ST cut 35 ° propagation, the ZnO film thickness H / λ = 0.16 near T.
You can see that CD = 0. Further, the electromechanical coupling coefficient at this film thickness shows a large value as described above. That is,
In the case of 165 ° rotation Y-plate X propagation, the electromechanical coupling coefficient K ² was 0.2% and TCD = −30 ppm / ° C in the past, but ZnO of H / λ = 0.35 was applied to the quartz substrate. By forming the film and the IDT, K ² = 1.
04% and TCD = 0 ppm / ° C were obtained. In addition, Zn
By providing a short-circuit electrode between the O film and the quartz substrate,
TCD = 0 ppm / ° C., and K ² is larger.
35% is obtained.

On the other hand, in the case of ST cut propagation of 35 °,
Conventionally, K ² = 0.14% and TCD = −20 ppm / ° C., but K ² is 4.8 by forming a ZnO film with H / λ = 0.16 and an IDT on a quartz substrate. Double K ²
= 0.77% and TCD = 0 ppm / ° C. are obtained. If a short-circuit electrode is provided between the ZnO film and the quartz substrate,
Further, K ² = 0.89% and TCD = 0 ppm / ° C. are obtained.

Therefore, as is clear from this experimental example, even if a crystal substrate having an Euler angle with a large electromechanical coupling coefficient is used as the crystal substrate and the TCD has a negative value in that case, the ZnO thin film As a crystal substrate TCD
Zn showing a positive TCD that can offset the negative value of
It can be seen that a SAW device having a very large electromechanical coupling coefficient and good temperature characteristics can be constructed by forming an O thin film and then forming an IDT thereon.

In the above description, two types of substrates have been described, but the same applies to the case where a rotating Y plate or a quartz substrate having another cut angle shown in FIGS. 11 to 14 is used.
Even if is negative, it can be seen that a surface acoustic wave device having a large electromechanical coupling coefficient and good temperature characteristics can be formed by using a quartz substrate having an Euler angle having a large electromechanical coupling coefficient. That is, as shown in FIG. 10, the Euler angles (0,0,0) to (0,135,0) range for Rayleigh waves in the rotating Y-plate X propagation, and the Euler angles (0,0) for leaky surface waves. , 0) to (0, 20, 0),
(0,45,0) to (0,65,0), (0,135,
The range of 0) to (0,180,0) is a negative range of TCD. As shown in FIG. 11, when the ST cut quartz substrate is used, the Euler angle (0, 132.75 ±
5,0) to (0,132.75 ± 5,50) and (0,132.75 ± 5,130) to (0,13)
2.75 ± 5, 180), the TCD is in the negative range.

Further, as is clear from FIG. 12, the rotation X
Euler angles are (90, 90, 0) for cut quartz substrates
Range of (90, 90, 35) and (90, 90, 14)
TCD is negative between 5) to (90, 90, 180).

Similarly, as shown in FIG. 13, the Euler angles are (0, 90,
0) to (0, 90, 35) and (0, 90, 1
TCD is negative between 45) to (0, 90, 180). Further, as is clear from FIG. 14, in the Z-cut quartz substrate, when the Euler angle (0, 0, φ) φ is a propagation direction other than 0, 60, 120 and 180, T
The value of CD is a negative value.

Therefore, in each crystal substrate within the above range,
Although TCD shows a negative value, a cut angle with a large electromechanical coupling coefficient is selected within that range, and a ZnO thin film having a positive TCD value is set on the upper surface of each quartz substrate so as to cancel the negative value of TCD. Formed on the IDT
By forming the above, it is possible to configure a surface acoustic wave device having a large electromechanical coupling coefficient and good temperature characteristics as described above.

When a single crystal material such as a quartz substrate is used, due to the anisotropy of the cut angle, the direction of the phase velocity vector from the IDT to the IDT coincides with the actual energy propagation direction. This phenomenon is called power flow, and the angle generated at this time is called the power flow angle. In consideration of this power flow, the cut angle of the quartz substrate is preferably such that the power flow angle is 0 and at the same time, the angle at which TCD shows a negative value is 0. This preferable cut angle is between (0, 25, 0) and (0, 105, 0) ((0, 25, 0)).
0,15) to (0,45,35), (0,10,6)
0) to (0, 20, 70), (0, 90, 30) to
During (0,180,45), (0,0,85) to (0,
0, 90), (90, 90, 25) to (90, 9)
0, 31) to (0, 90, -3) to (0, 90,
3), where TCD is negative and the power flow angle is zero.

Therefore, Z is formed on each quartz substrate within the range described above.
By forming the nO thin film and further forming the IDT, it is possible to construct a surface acoustic wave device having a large electromechanical coupling coefficient, good temperature characteristics, and no bias in the propagation direction.

In the above experimental example, Zn was used as the piezoelectric thin film.
The case where an O thin film is formed has been described, but in addition to the ZnO thin film, a positive TCD such as AlN, Ta ₂ O ₅ or CdS is used.
An appropriate piezoelectric thin film having a value may be used.

Further, it should be pointed out that the crystal substrate may use either the plus side or the minus side.

[0067]

As described above, according to the first invention of the present application,
Since the comb-teeth electrode is formed on the piezoelectric thin film formed on the quartz substrate and the normalized film thickness H / λ of the piezoelectric thin film is set to 0.05 or more, a surface acoustic wave device having a large electromechanical coupling coefficient is obtained. Can be provided. That is,
The electromechanical coupling coefficient can be remarkably increased as compared with the structure in which the comb-teeth electrode is formed between the quartz substrate and the piezoelectric thin film, which is said to have a large electromechanical coupling coefficient.

According to the second invention of the present application, a crystal substrate having a cut angle and a propagation direction having a negative TCD value is used as the crystal substrate, and the piezoelectric thin film is formed on the crystal substrate. Therefore, the piezoelectric thin film is usually a positive TC
Since it has a D value, the TCD value of the quartz substrate is the T value of the piezoelectric thin film.
It is possible to obtain a surface acoustic wave device having a good temperature characteristic, which is offset by the CD value. Therefore, even if the TCD value is negative, by using a crystal substrate having a large cut angle and propagation direction with a large electromechanical coupling coefficient, good temperature characteristics, which could not be realized conventionally, and a large electric machine can be realized. It becomes possible to provide a surface acoustic wave device having a coupling coefficient.

[Brief description of the drawings]

1A and 1B are cross-sectional views showing a laminated structure of a glass substrate, a piezoelectric thin film, and an IDT in a conventional surface acoustic wave device, respectively.

2A and 2B are respectively a conventional SAW.
Glass substrate, short-circuit electrode, piezoelectric thin film and ID in device
FIG. 6 is a cross-sectional view for explaining the T stacked structure.

FIG. 3 is a diagram showing a relationship between a standardized film thickness of a piezoelectric thin film in the SAW device shown in FIGS. 1 and 2 and an electromechanical coupling coefficient.

FIGS. 4A and 4B show conventional SAWs, respectively.
In the device, the piezoelectric thin film and the ID are formed on the LiNbO ₃ substrate.
In the structure obtained by laminating an IDT and a piezoelectric thin film in this order on the formed structure and LiNbO ₃ substrate to T, shows the relationship between the normalized thickness and the electromechanical coupling coefficient of the ZnO film.

5 (a) and (b) are LiNbO ₃ respectively.
Zn when the IDT and the short-circuit electrode are formed at different positions on the surface acoustic wave substrate formed by forming a ZnO thin film on the substrate
The figure which shows the relationship between the normalized film thickness of an O thin film, and an electromechanical coupling coefficient.

FIG. 6 is a diagram showing a relationship between a standardized film thickness and an electromechanical coupling coefficient of ZnO thin films of three types of surface acoustic wave devices in which a piezoelectric thin film and a comb-teeth electrode are formed on an ST-cut quartz substrate.

FIG. 7: Zn on 165 ° rotated Y plate X direction propagation quartz substrate
The figure which shows the relationship between the standardized film thickness and the electromechanical coupling coefficient of the ZnO thin film of the surface acoustic wave device of 3 types of structures which formed the O thin film and the comb-teeth electrode.

FIG. 8: ZnO on a quartz substrate composed of a 115 ° rotated Y plate
The figure which shows the relationship between the standardized film thickness of a ZnO thin film and the electromechanical coupling coefficient in the surface acoustic wave device which comprised the thin film and the comb-teeth electrode in various forms.

9A and 9B are a schematic plan view of a SAW device configured as a reference example and an embodiment of the present invention, and a partially cutaway cross-sectional view of a portion where a comb electrode is formed.

FIG. 10: Euler angle and TC on a rotating Y-plate crystal substrate
The figure which shows the relationship with D and a propagation loss.

FIG. 11: Propagation direction and TC in ST-cut quartz substrate
The figure which shows the relationship with D.

FIG. 12 is a diagram showing the relationship between the propagation direction of an X-cut crystal substrate and TCD.

FIG. 13 is a diagram showing the relationship between the propagation direction of a Y-cut crystal substrate and TCD.

FIG. 14 is a diagram showing the relationship between the propagation direction of a Z-cut crystal substrate and TCD.

FIG. 15: 165 ° rotation Y plate X propagation and ST cut 35 °
The figure which shows the relationship between ZnO film thickness and TCD when a propagation quartz substrate is used.

16 (a) and 16 (b) are respectively the second of the present application.
FIG. 3 is a diagram showing the relationship between the Euler angle and the electromechanical coupling coefficient of the rotating Y-plate quartz crystal substrate according to the embodiment of FIG.

[Explanation of reference numerals] 10 ... Quartz substrate 11 ... SAW device 12 ... ZnO thin film 13, 14 ... IDT 13a, 13b, 14a, 14b ... Comb-shaped electrode

Claims (7)

[Claims] 1. A quartz substrate, a piezoelectric thin film formed on the quartz substrate, and a comb-teeth electrode formed on the piezoelectric thin film, wherein the thickness of the piezoelectric thin film is H and the wavelength of the surface wave is λ.
, The normalized film thickness H / λ of the piezoelectric thin film is 0.05
A surface acoustic wave device characterized by the above. 2. The surface acoustic wave device according to claim 1, further comprising a short-circuit electrode formed between the quartz substrate and the piezoelectric thin film. 3. A quartz substrate, a piezoelectric thin film formed on the quartz substrate, and a comb-teeth electrode formed in contact with the piezoelectric thin film, wherein the quartz substrate has a negative group delay time temperature characteristic TCD. A surface acoustic wave device characterized in that a crystal substrate having a cut angle and a propagation direction having a value of is used. 4. The surface acoustic wave device according to claim 3, wherein the comb-teeth electrode is formed on the piezoelectric thin film. 5. The surface acoustic wave device according to claim 4, further comprising a short-circuit electrode formed between the quartz substrate and the piezoelectric thin film. 6. The normalized film thickness H / λ of the piezoelectric thin film, where H is the film thickness of the piezoelectric thin film and λ is the wavelength of the surface wave.
Is 0.03 or more, The surface acoustic wave device according to any one of claims 3 to 5. 7. The piezoelectric thin film is ZnO, AlN, Ta
The surface acoustic wave device according to any one of claims 1 to 6, which is a piezoelectric thin film made of one kind selected from the group consisting of ₂ O ₅ and CdS.