JPH08139565A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE: To enlarge an electromechanical coupling coefficient by forming a piezoelectric thin film composed of a material for which the sound speed of surface acoustic waves is slower than an LiNbO3 substrate on a 41 deg. rotation Y plate X propagation LiNbO3 substrate with improved temperature characteristics. CONSTITUTION: On the 41 deg. rotation Y plate X propagation LiNbO3 substrate, the piezoelectric thin film for which the sound speed of Love waves is slower compared to the LiNbO3 substrate is formed. For the piezoelectric thin film, a ZnO thin film, a CdS thin film and a Ta2 O5 thin film are used. The film thickness H is turned to a value standardized by the wave lendth (a) of the surface acoustic waves and 0.045-0.09 and thus, the Love waves are stably excited. For a surface acoustic wave resonator 1, a pair of comb-line electrodes 3a and 3b provided with plural electrode fingers interposed each other are formed on the surface of a piezoelectric substrate 2 and thus, an interdigital transducer 3 is formed. On both sides of the transducer 3, grating reflectors 4a and 4b are arranged. By turning H/λto the range of 0.045-0.09, the electromechanical coupling coefficient is turned to be more than 0.43.

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 Love waves, and more particularly, by improving the piezoelectric substrate material, a large electromechanical coupling coefficient and a small temperature characteristic (T).
c) is realized.

[0002]

2. Description of the Related Art As a surface acoustic wave device constructed by using a piezoelectric substrate, various devices such as a surface acoustic wave resonator, a surface acoustic wave filter and an elastic convolver have been put into practical use. In order to obtain good characteristics in the surface acoustic wave device, it is required that the piezoelectric substrate used has a large electromechanical coupling coefficient Ks and a small temperature characteristic Tc. Therefore, in order to satisfy both of these, various piezoelectric materials have been studied conventionally.

As a piezoelectric substrate having a large electromechanical coupling coefficient Ks, a piezoelectric substrate is known in which a ZnO thin film having a surface wave propagation velocity slower than that of the substrate is formed on a Y-cut X-direction propagation LiNbO ₃ substrate. . It is known that a Love wave can be excited by using this piezoelectric substrate. For example,
It is known that the electromechanical coupling coefficient K of the piezoelectric substrate formed by forming the ZnO thin film on the Y-cut X-direction propagation LiNbO ₃ substrate is as large as about 0.47.

[0004]

However, in a piezoelectric substrate formed by forming a ZnO thin film on a Y-cut X-direction propagating LiNbO ₃ substrate, the temperature characteristic Tc is as large as 80 ppm / ° C., and therefore the piezoelectric material having a smaller temperature characteristic Tc is used. Substrates are needed.

In addition, Y-cut X-direction propagation LiNbO ₃
A piezoelectric substrate formed by forming a ZnO thin film on a substrate can excite a Love wave, but within a band capable of exciting a Love wave,
There is also a drawback that there is a Rayleigh wave component and the characteristics are deteriorated due to the influence of the Rayleigh wave.

It is an object of the present invention to provide a surface wave device using Love waves, which has a wide electro-mechanical coupling coefficient K and a small temperature characteristic Tc, and thus has a wide band and Love waves. An object of the present invention is to provide a surface wave device that can excite well.

[0007]

According to the present invention, a 41 ° rotation Y
A plate X propagation LiNbO ₃ substrate, a piezoelectric substrate having a piezoelectric thin film is formed on the LiNbO ₃ substrate, and the surface acoustic wave propagation velocity than that of the LiNbO ₃ substrate is slower material, provided on the piezoelectric substrate And a digital interdigital transducer (IDT), and is configured to utilize a Love wave.

That is, the inventor of the present application examined surface acoustic wave devices for exciting Love waves by using various piezoelectric substrate materials in order to achieve the above-mentioned object, and as a result, on a 41 ° rotating Y plate X propagating LiNbO ₃ substrate, It has been found that the electromechanical coupling coefficient K can be increased and the temperature characteristic Tc can be decreased by using the piezoelectric substrate formed by laminating the piezoelectric thin films as described above, and the present invention is achieved based on the finding. Came to.

As the piezoelectric thin film, one made of an appropriate piezoelectric material may be used as long as the material has a surface wave propagation velocity lower than that of the LiNbO ₃ substrate. For example, ZnO,
CdS or Ta ₂ O ₅ may, for example, be mentioned.

Further, it is preferable that H / λ where H is the thickness of the piezoelectric thin film and λ is the wavelength of the excited Love wave.
Is in the range of 0.045 to 0.09. Thus, the electromechanical coupling coefficient Ks is set to 0.43 by setting the film thickness H / λ normalized by the wavelength to be within the above specific range.
Thus, the Love wave can be excited more efficiently.

As described above, the present invention is intended to increase the electromechanical coupling coefficient and stabilize the temperature characteristics by using the piezoelectric substrate made of the above-mentioned specific piezoelectric substrate material. Can adopt an appropriate structure of a surface acoustic wave device using a conventionally known surface acoustic wave. For example, the IDT provided on the piezoelectric substrate may be formed on the piezoelectric substrate, or the LiN
It may be formed between the bO ₃ substrate and the piezoelectric thin film, and the number and shape of IDTs are not particularly limited.

The present invention can be applied to various surface wave devices such as a surface wave resonator utilizing a Love wave, a surface wave filter, an elastic convolver and the like.

[0013]

In the present invention, as described above, the 41 ° rotated Y plate X
A piezoelectric substrate formed by forming the above-mentioned piezoelectric thin film on a propagating LiNbO ₃ substrate is used, thereby increasing the electromechanical coupling coefficient and reducing the temperature characteristic Tc. The action of such a piezoelectric substrate has been experimentally confirmed by the inventor of the present application.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be clarified by describing embodiments with reference to the drawings.

41 ° rotated Y plate X propagating LiNbO ₃ substrate (hereinafter, simply referred to as 41 ° Y-X LiNbO ₃ substrate)
Is conventionally known to have a temperature characteristic of about 60 to 65 ppm / ° C. The LiNbO ₃ substrate is also known as a surface wave substrate that generates a leaky wave with attenuation.

When various piezoelectric thin films such as ZnO, CdS and Ta ₂ O ₅ were formed on the above 41 ° Y-X LiNbO ₃ substrate, it was confirmed that Love waves were effectively excited.

Hereinafter, the case where a ZnO thin film is formed on the 41 ° Y-X LiNbO ₃ substrate will be described as an example.
The following piezoelectric basic equation, equation of motion, and Maxell's equation hold for both the 41 ° Y—X LiNbO ₃ substrate and the ZnO thin film.

[0018]

[Equation 1]

To excite a Love wave, these equations are Z
41 ° Y-X LiNb with nO thin film surface and ZnO thin film
At the boundary with the O ₃ substrate, a sound velocity satisfying each boundary condition may be obtained. When the interdigital transducer is formed on the piezoelectric substrate, the electromechanical coupling is performed from the sound velocity V _m when electrically short-circuited and the sound velocity V _f when opened at the boundary where the interdigital electrode exists based on the following equation. The coefficient K is obtained.

[0020]

[Equation 2]

FIG. 1 shows that the LiNbO ₃ substrate + surface is
The results of measuring the acoustic velocities V _f and V _m of the surface wave when using piezoelectric substrates formed by forming ZnO thin films with various thicknesses are shown. In FIG. 1, ○ and ● are the above LiNb.
The sound velocity of the Love wave when the ZnO thin film is formed on the + surface of the O ₃ substrate is shown, and the sound velocity V _f when the boundary is free is shown by ○.
● indicates the sound velocity V _m when the boundary is metallized. On the other hand, Δ and ▲ indicate the sound velocity of the Rayleigh wave when the ZnO thin film is formed on the + surface of the LiNbO ₃ substrate, Δ is the sound velocity V _f when the boundary is free, and ▲ is the metallized boundary. In this case, the sound velocity is V _m .

FIG. 3 is a diagram showing the sound velocities of the Love wave and the Rayleigh wave when the ZnO thin film is formed on the negative surface of the LiNbO ₃ substrate, and ○, ●, Δ and ▲ are respectively.
It has the same meaning as in FIG.

As is clear from FIG. 1 and FIG.
It can be seen that the Love wave is excited due to the existence of the difference between V _f and V _m when the ZnO thin film is formed on either side of the + surface and the − surface of the iNbO ₃ substrate.

Further, in FIGS. 1 and 3, for the Rayleigh wave, there is almost no difference between the sound velocity V _f when the boundary is free and V _m when the boundary is metallized, and therefore the Rayleigh wave is hardly excited. I understand. Moreover, since the Rayleigh wave component does not exist between the sound velocities V _f and V _m of the Love wave, the above 41 ° Y-X LiNb.
It can be seen that the structure in which the ZnO thin film is formed on the O ₃ substrate is not easily affected by the Rayleigh wave component.

FIG. 2 shows the above 41 ° Y-X LiNbO ₃
FIG. 4 shows the electromechanical coupling coefficient K when ZnO thin films with various thicknesses are formed on the + surface of the substrate.
XLiNbO ₃ of the substrate - a diagram showing the electromechanical coupling coefficient in the case of forming a ZnO film in various thickness on the surface. As is clear from comparison between FIG. 2 and FIG. 4, the electromechanical coupling coefficient K becomes slightly larger in FIG. 2, that is, when the ZnO thin film is formed on the + surface of the LiNbO ₃ substrate. The maximum value of this electromechanical coupling coefficient K is
As is clear from FIG. 2, the film thickness H / λ = 0.06 of the ZnO piezoelectric thin film standardized by the wavelength λ of the Love wave is 0.491, which is very large. This electromechanical coupling coefficient K =
The value 0.491 is the electromechanical coupling coefficient K = when a ZnO thin film is formed on a Y-cut X-propagation LiNbO ₃ substrate.
Greater than 0.47, and ZnO / Y-XLiN
It can be seen that the film thickness H / λ = 0.11 at the maximum electromechanical coupling coefficient of the bO ₃ substrate is about half the film thickness.

That is, Zn was formed on the LiNbO ₃ substrate.
It can be seen that in the structure in which the O thin film is formed, the electromechanical coupling coefficient can be effectively increased even when the ZnO thin film is thin.

Further, as described above, in the ZnO / Y-cut X-propagation LiNbO ₃ substrate, the Rayleigh wave was present in the band in which the Love wave was excited.
In the structure in which the ZnO thin film is formed on the iNbO ₃ substrate, the Rayleigh thin film exists outside the band in which the Love wave is excited. Therefore, it can be seen that a surface wave device using Love waves with excellent characteristics can be provided.

Further, as described above, Y-XLiNbO ₃
Compared to the Tc of the substrate of 80 ppm / ° C, the above 41 ° Y-X
The LiNbO ₃ substrate has a temperature characteristic Tc of 60
~65ppm and is known to be small, therefore, by using a piezoelectric substrate formed by laminating a ZnO thin film on the LiNbO ₃ substrate, the thickness of ZnO is thin, the electromechanical coupling factor K is large, the temperature further It can be seen that since the surface acoustic wave device can be configured using the substrate having the small characteristic Tc, the surface acoustic wave device that can excite the Love wave satisfactorily regardless of the ambient temperature change can be provided.

[0029] The present invention is in the LiNbO ₃ substrate, but it has the characteristics that a piezoelectric substrate Love wave sound velocity compared to the LiNbO ₃ substrate by forming a slow piezoelectric thin film As the piezoelectric thin film, not only the above ZnO thin film but also other piezoelectric thin films such as CdS thin film and Ta ₂ O ₅ thin film may be used. Also, when a piezoelectric thin film made of CdS or Ta ₂ O ₅ is used, the film thickness H is 0.045 at a value standardized by the wavelength λ of the surface wave, as in the case of using the ZnO piezoelectric thin film. It suffices if it is set to about 0.09, whereby the Love wave can be stably excited.

The present invention relates to a surface acoustic wave device using the above-mentioned piezoelectric substrate, and the specific structure of the surface acoustic wave device, that is, the number and shape of interdigital transducers, is not particularly limited. Instead, a conventionally known structure can be appropriately adopted. As an example, for example, in the surface acoustic wave resonator 1 shown in FIG. 5, by using the piezoelectric substrate of the above embodiment as the piezoelectric substrate 2, a Love wave having good temperature characteristics and excellent resonance characteristics is used. A surface acoustic wave resonator can be provided. In FIG. 5, the surface acoustic wave resonator 1 has a pair of comb-teeth electrodes 3a and 3b having a plurality of electrode fingers to be inserted into each other on the surface of the piezoelectric substrate 2 to which the present invention is applied. Interdigital transducer 3
Is configured. In addition, grating reflectors 4a, 4 are provided on both sides of the interdigital transducer 3.
b is arranged.

[0031]

INDUSTRIAL APPLICABILITY According to the present invention, the temperature characteristics Tc of 41 are excellent.
Rotating Y-plate X-propagating LiNbO ₃ on the LiNbO ₃ substrate
Since the piezoelectric substrate formed by forming the piezoelectric thin film made of a material having a surface acoustic wave slower than the _three substrates is used, the electromechanical coupling coefficient of the piezoelectric substrate has a sufficiently large value. Therefore,
In the surface acoustic wave device of the present invention using the above-mentioned piezoelectric substrate, the temperature characteristic Tc is excellent, and the Love wave is effectively excited. Therefore, it is possible to provide a surface acoustic wave device that has a wide band and can excite a Love wave satisfactorily regardless of changes in ambient temperature.

Further, as described in claim 3, the ratio H of the film thickness H of the piezoelectric thin film to the wavelength λ of the excited Love wave is set.
By setting / λ in the range of 0.045 to 0.09,
Since the electromechanical coupling coefficient can be set to 0.43 or more, it is possible to provide a surface acoustic wave device having more excellent temperature characteristics and electrical characteristics.

[Brief description of drawings]

FIG. 1 ZnO in a piezoelectric substrate formed by forming a ZnO thin film on the + surface of a 41 ° rotated Y plate X-propagation LiNbO ₃ substrate.
The figure which shows the relationship between the relative film thickness H / (lambda) of a thin film, and the speed of sound of various surface waves excited.

FIG. 2 shows a relationship between a relative film thickness H / λ of a ZnO thin film of a piezoelectric substrate formed by forming a ZnO thin film on the + surface of a 41 ° rotated Y plate X-propagation LiNbO ₃ substrate and an electromechanical coupling coefficient. Fig.

FIG. 3 shows a relative film thickness H / λ of a ZnO thin film and a sound velocity of a surface wave when a piezoelectric substrate in which a ZnO thin film is formed on a − surface of a 41 ° rotated Y plate X propagation LiNbO ₃ substrate is used. FIG.

FIG. 4 is a diagram showing the relationship between the relative film thickness H / λ of a ZnO thin film and the electromechanical coupling coefficient when the ZnO thin film is formed on the negative surface of a 41 ° rotated Y plate X-propagation LiNbO ₃ substrate.

FIG. 5 is a perspective view showing a surface acoustic wave resonator as an example of a surface acoustic wave device to which the present invention is applied.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Surface wave resonator as a surface wave device 2 ... Piezoelectric substrate 3a, 3b ... Comb tooth electrode 4a, 4b ... Reflector

Claims (3)

[Claims] 1. A 41 ° rotated Y plate X propagating LiNbO₃substrate
And the LiNbO ₃Formed on the board, and before
LiNbO₃A material whose surface wave propagation velocity is slower than that of the substrate
A piezoelectric substrate having a piezoelectric thin film made of, and an interdigital transformer provided on the piezoelectric substrate.
And configured to utilize Love waves.
A surface acoustic wave device, characterized in that 2. The surface acoustic wave device according to claim 1, wherein the piezoelectric thin film is made of ZnO, CdS or Ta ₂ O ₅ . 3. When the thickness of the piezoelectric thin film is H and the wavelength of the excited Love wave is λ, H / λ is 0.045 to
The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device has a range of 0.09.