JP2003142984A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave device which is less spurious with an excellent temperature characteristic and enlarges the electro-mechanical coupling coefficient K<2> of Rayleigh wave concerning the surface acoustic wave device with a piezoelectric film formed on a crystal substrate. SOLUTION: The device is provided with the crystal substrate 2 which has ranges of Euler angles (ϕ, θ, ψ) -19 deg.<ϕ<+15 deg., 107 deg.<θ<125 deg. and also -10 deg.<ψ<15 deg., the piezoelectric film 5 formed on the crystal substrate 2 and comb-line electrodes 3a-4b formed to be brought into contact with the film 5. When the thickness of the film 5 is defined as H and the wave length of a surface acoustic wave as λ, the film thickness H/λ standardized by the wave length λ of the piezoelectric film 5 is made to be >=0.05 in the surface acoustic wave device 1.

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 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) is formed by forming at least a pair of comb tooth electrodes 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, a SAW device using a surface wave substrate formed by forming a piezoelectric thin film on an elastic substrate such as a glass substrate or a piezoelectric substrate has been proposed.

In the structure using the surface wave substrate formed by laminating the piezoelectric thin film and the elastic substrate, the four kinds of structures shown in FIGS. 22 (a) and (b) and FIGS. 23 (a) and (b) are provided. Are known. That is, the SA shown in FIG.
In the W device 101, the piezoelectric thin film 10 is formed on the elastic substrate 102.
3 is formed, and the IDT 104 is formed on the piezoelectric thin film 103.
Are formed. On the other hand, in the SAW device 105 shown in FIG. 22B, the IDT 104 is formed on the lower surface of the piezoelectric thin film 103, that is, at the interface between the elastic substrate 102 and the piezoelectric thin film 103.

The SAW device 10 shown in FIG.
In No. 6, the short-circuit electrode 107 is formed on the elastic substrate 102, and the piezoelectric thin film 103 is laminated on the short-circuit electrode 107. The IDT 104 is formed on the piezoelectric thin film 103. That is, the SAW device 106 is shown in FIG.
This corresponds to the structure in which the short-circuit electrode 107 is inserted in the interface between the elastic substrate 102 and the piezoelectric thin film 103 in the SAW device 101 shown in (a).

In the SAW device 108 shown in FIG. 23 (b), the short-circuit electrode 107 is formed on the piezoelectric thin film 103. Further, the IDT 104 is formed on the interface between the elastic substrate 102 and the piezoelectric thin film 103. Therefore, SAW
The device 108 is the SAW device 105 shown in FIG.
2 corresponds to the structure in which the short-circuit electrode 107 is formed on the upper surface of the piezoelectric thin film 103.

The SAW devices 101, 105, 106,
108 to the formation position of the IDT 104 and the short-circuit electrode 107.
FIG. 24 shows the electromechanical coupling coefficient in the case of using the ZnO thin film as the piezoelectric thin film and the glass substrate as the elastic substrate, excepting the presence or absence of the above, the other configurations being the same.

FIG. 24 shows changes in the electromechanical coupling coefficient with respect to the normalized film thickness H / λ of the ZnO thin film in the above-mentioned four types of SAW devices. In the present specification, H represents the thickness of the piezoelectric thin film, and λ represents the wavelength of the surface wave excited (the units are the same).

The solid line A indicates the result of the SAW device 101, the broken line B indicates the result of the SAW device 105, the alternate long and short dash line C indicates the result of the SAW device 106, and the alternate long and two short dashes line D indicates the SAW device 1.
08 results are shown.

As is apparent from FIG. 24, by selecting H / λ, the SAW devices 105 and 108
It can be seen that a larger electromechanical coupling coefficient can be obtained as compared with the SAW devices 101 and 106.

Therefore, conventionally, Zn was formed on the glass substrate 102.
In the structure in which the O thin film 103 is formed, it is said that a larger electromechanical engagement coefficient can be obtained when the IDT 104 is formed at the interface between the glass substrate 102 and the ZnO thin film 103. The wave described as the Sezawa wave in FIG. 24 is a surface wave of a higher-order mode of Rayleigh wave.

In addition, IEEE ULTRASONICS
SYMPOSIUM (1997) pages 261 to 266 and Japan Society for the Promotion of Science, Acoustic Wave Device Technology 150th Committee 59th Research Material (1998) pages 23 to 28 [hereinafter abbreviated as reference 1]. ], Various characteristics of the surface acoustic wave in the case of using a surface wave substrate formed by forming a ZnO thin film on a quartz substrate are shown by the present inventor. This will be described with reference to FIGS. 25 (a) and 25 (b) and FIG. In this prior art, a frequency temperature coefficient TC is provided on a quartz substrate with a cut angle and a propagation direction in which the frequency time temperature characteristic TCF has a positive value.
By forming a ZnO thin film in which F is negative, TC
It has been confirmed by theory and experiment that a surface acoustic wave substrate having F of zero can be obtained.

The theory in this document 1 is based on the IEEE
E trans. Sonics & Ultrason.
vol. SU-15, No. 4 (1968) p. 209-.

FIG. 25 (a) is a diagram of a quartz substrate of 29 ° 45 'rotating Y plate 35 ° X propagation [Euler angle (0 °, 119 ° 45', 35 °)] described in the above-mentioned reference 1. 22 (a) shows the ZnO film thickness dependence of TCF of the SAW device of FIG. 22 (a), and FIG. 25 (b) shows the 42 ° 45 ′ rotating Y plate 35 ° X propagation [Euler angle (0
22 (a) in FIG. 22 (a), FIG.
3 shows the dependence of TCF of the SAW device on ZnO film thickness. Further, FIG. 26 shows electromechanical coupling coefficients of a Rayleigh wave and a Sezawa wave which is a spurious wave of a SAW device using a ZnO thin film as a piezoelectric thin film and a quartz substrate as an elastic substrate. Solid lines A to C in FIG. 26 are respectively shown in FIG.
The electromechanical coupling coefficient of the Rayleigh wave of the SAW device having the structure shown in FIGS.
C ″ and D ″ are shown in FIG. 22 (a), FIG. 23 (a) and FIG.
3 shows changes in the electromechanical coupling coefficient of the Sezawa wave that becomes spurious in the SAW device having the structure of FIG.

From FIGS. 25A and 25B to FIG.
It can be seen that in the SAW device of (a), the TCF becomes zero by selecting the standardized film thickness of the ZnO film. Table 1 below shows FIG. 22 described in the above-mentioned prior art.
(A) SAW device (Al / ZnO / quartz laminated structure)
And a comparison with a conventionally known SAW device with good TCF.

[0016]

[Table 1]

From FIG. 26 and Table 1, the SA of FIG.
The W device is an ST-X crystal substrate or La. ₃Ga_FiveSiO₁₄substrate
Electromechanical coupling coefficient K of about 1%, which is larger than²Got
And equivalent electromechanical engagement coefficient K²Li with₂B_FourO₇
It can be seen that the sound velocity is about 20% lower than that of the substrate. This
That is, the SAW device in FIG.
If a Versal SAW filter is configured, ST-X water
Crystal substrate and La₃Ga_FiveSiO₁₄Lower loss than the substrate
Yes, Li₂B_FourO₇Smaller and more temperature efficient than a board
This means that the frequency deviation due to it is small.

By the way, in FIG. 26, when a ZnO thin film is used as the piezoelectric thin film and a quartz substrate is used as the elastic substrate, the electromechanical coupling coefficient of the Rayleigh wave in the SAW device of FIG. 22 (b) is shown in FIG. 22 (a) and S in FIG. 23 (a)
It is shown that it is smaller than the electromechanical coupling coefficient of the Rayleigh wave in the AW device, and this tendency is contrary to the tendency when the glass substrate is used as the elastic substrate.

As described above, FIG. 22 (a) and FIG.
Since the SAW device of (a) has both a good TCF and a large electromechanical coupling coefficient, it is possible to improve the performance of a surface wave device such as a bandpass filter of mobile communication equipment by using this.

[0020]

However, as shown in FIG.
The SAW devices of FIGS. 23A and 23A also have a problem that the electromechanical coupling coefficient is insufficient and the characteristics required for the surface acoustic wave device cannot be sufficiently satisfied. In mobile communication, the conventional analog system, digital system, and code spreading system are shifting. For example, an intermediate frequency filter used in a digital system or a code spreading system requires low group delay deviation and low insertion loss. A transversal type filter is known as a bandpass filter using a surface wave device with a small group delay deviation.However, when a transversal type filter is configured with a conventional surface wave substrate, the electromechanical coupling coefficient is insufficient and the above requirements are met. I couldn't answer.

An object of the present invention is to provide a SAW device using a surface wave substrate formed by laminating a quartz substrate and a piezoelectric thin film,
An object of the present invention is to provide a SAW device having a small spurious characteristic, a good temperature characteristic, and a large electromechanical coupling coefficient.

[0022]

The present invention has been made to achieve the above object, and according to a broad aspect of the present application, at an Euler angle (φ, θ, ψ), −19 ° <
φ <+ 15 °, 107 ° <θ <125 °, and
The piezoelectric thin film is provided with a quartz substrate satisfying −10 ° <ψ <15 °, a piezoelectric thin film formed on the quartz substrate, and a comb electrode formed in contact with the piezoelectric thin film. Provided is a surface wave device, wherein a normalized film thickness H / λ of the piezoelectric thin film is 0.05 or more, where λ is a wavelength of the surface wave.

Preferably, as the crystal substrate, φ, θ and ψ at Euler angles (φ, θ, ψ) are φ = −2.5 ° ± 5 ° and θ = 116 ° ± 5 °, respectively. ψ
A quartz substrate in the range of ++ 2.5 ° ± 5 ° is used.

In a particular aspect of the present invention, the normalized film thickness H / λ of the piezoelectric thin film is set to 0.20 or more. In another specific aspect of the present invention, the piezoelectric thin film is configured so as to contact the substrate and / or the comb-teeth electrode on the negative side.

[0025] In still another specific aspect of the present invention, a short-circuit electrode formed on the piezoelectric thin film is further provided. In another specific aspect of the present invention, the Euler angle of the quartz substrate is the power flow angle PFA of the Rayleigh wave in FIG.
It is in the range of 5 °.

In another specific aspect of the present invention, the Euler angle of the quartz substrate is within a range in which the temperature coefficient of frequency TCF of the surface acoustic wave device in FIG. 7 is ± 25 ppm / ° C. In another particular aspect of the present invention, the Euler angle of the quartz substrate is
Temperature coefficient TCF of the surface acoustic wave device at ± 5pp
It is in the range of m / ° C.

In another particular aspect of the present invention, the Euler angle of the quartz substrate is within a range in which the electromechanical coupling coefficient K ² of the Rayleigh wave in FIG. 8 is 0.8% or more. In another specific aspect of the present invention, the temperature coefficient of frequency TCF of the piezoelectric thin film has a negative value.

In another specific aspect of the present invention, the Euler angle of the quartz substrate is the difference ΔPFA between the power flow angles of the surface wave to be used and the unnecessary surface wave not to be used in FIG.
Is within ± 1 °.

In still another specific aspect of the present invention, the Euler angles (φ, θ, ψ) of the quartz substrate have a φ of −35 to +35.
It is said to be °. In the present invention, a crystal substrate having an Euler angle that is crystallographically equivalent to the above-mentioned specific Euler angle of the crystal substrate may be used.

In the present invention, the piezoelectric thin film is preferably made of one selected from the group consisting of ZnO, AlN, Ta ₂ O ₅ and CdS.

[0031]

1 (a) and 1 (b) are a plan view showing a surface acoustic wave device according to an embodiment of the present invention and a partially cutaway front sectional view showing an enlarged main part thereof.

In the surface acoustic wave device 1, interdigital transducers (IDTs) 3 and 4 are formed on a crystal substrate 2. Each of the IDTs 3 and 4 has a pair of comb electrodes 3a, 3b, 4a and 4b. The IDTs 3 and 4 are arranged at a predetermined distance in the surface wave propagation direction. That is, in the surface acoustic wave device 1 of this embodiment, the IDTs 3 and 4 are configured similarly to the known transversal type surface acoustic wave filter.

The feature of this embodiment is that the piezoelectric thin film 5 is laminated so as to cover the IDTs 3 and 4. When the film thickness of the piezoelectric thin film 5 is H and the surface wave wavelength is λ, the piezoelectric thin film 5 is formed. The standardized film thickness H / λ is set to 0.05 or more, whereby the electromechanical coupling coefficient K ² is increased and the temperature characteristic is also good. This will be explained in more detail below.

The inventors of the present application have used a quartz substrate as an elastic substrate, and a surface acoustic wave device using a surface acoustic wave substrate formed by forming a ZnO thin film as a piezoelectric thin film on the quartz substrate, in particular, FIG. ) And the characteristics of the surface acoustic wave device having the laminated structure shown in FIG. 23B, which is opposite to the tendency when the glass substrate is used as the elastic substrate, and is different from the above-mentioned prior art. Considered from the angle.

2 and 3 show a ZnO thin film as the piezoelectric thin film and an Euler angle (0, 119 ° 4) as the elastic substrate.
5 ', 35 °) X-propagation quartz substrate is used, as shown in FIG.
Zn in Rayleigh waves propagating through the SAW devices having the structures of 2 (a), (b) and FIGS. 23 (a), (b).
The relationship between the normalized thickness of the O thin film and the electromechanical coupling coefficient is shown. FIGS. 4 and 5 show the normalized thickness and electromechanical coupling coefficient of the ZnO thin film in spurious waves generated near the Rayleigh wave. Shows the relationship.

2 and 4, the Euler angles of the ZnO thin film are (0 °, 0 °, 0 °), and in FIGS. 3 and 5, the Euler angles of the ZnO thin film are (0 °, 180 °, 0
°) to invert the polarity of the ZnO thin film.

The results shown in FIGS. 2 to 5 are not obtained by the method of cambell et al. Described in the above-mentioned prior art, but in the literature (Journal of the Institute of Electronics and Communication Engineers, Vol. J68-C Nol (198).
5) Obtained using the finite element method proposed on pages 21 to 27), the electromechanical coupling coefficient K ² is obtained by deriving the sound velocity Vf on the free surface and the sound velocity Vm on the short-circuited surface by the finite element method. Was calculated by the following formula (1).

K ² = 2 × (Vf−Vm) / Vf (1) In FIGS. 2 to 5, the thickness of the Rayleigh wave ZnO thin film of the SAW device of FIG. 22B exceeds 0.05λ. And Z
Electromechanical coupling coefficient K compared to the case where no nO thin film is formed
² becomes larger.

The thickness of the ZnO thin film is 0.20λ.
When the range of 0.24λ is exceeded, the SAW of FIG.
The electromechanical coupling coefficient K ² of the Rayleigh wave of the device is shown in FIG.
It becomes larger than the electromechanical engagement decision number K ² of the Rayleigh wave of the SAW device of (a) and FIGS. 23 (a) and (b). In particular, when the thickness of the ZnO thin film is around 0.5λ, as shown in FIG.
It reaches three times the electromechanical coupling coefficient of the Rayleigh wave of the SAW device of (a).

The ZnO thin film has a thickness of 0.27λ
Beyond the range of 0.31λ, the electromechanical coupling coefficient of the Rayleigh wave in the SAW device of FIG. 23 (b) is greater than the electromechanical coupling coefficient of the Rayleigh wave in the SAW device of FIGS. 22 (a) and 23 (a). Also grows.

The above calculation results are similar to the results of FIG. 24 when a glass substrate is used as the elastic substrate.
The tendency is different from the theoretical value in Reference 1 shown in FIG.

The Euler angle of the ZnO thin film is (0 °,
0 °, 0 °) has a larger electromechanical coupling coefficient K ² of the Rayleigh wave than the case where the Euler angle of the ZnO thin film is (0 °, 180 °, 0 °), and electromechanical coupling of spurious The coefficient K ² tends to be small.

By the above calculation, FIG. 22 (b) and FIG.
A SAW surface acoustic wave device having a structure of 3 (b) is mounted on a quartz substrate by Z
It is understood that the electromechanical coupling coefficient K ² can be significantly increased by using the surface acoustic wave substrate formed with the nO thin film.

By the way, representative evaluation items of the surface acoustic wave substrate include not only the electromechanical coupling coefficient K ^2, but also the power flow angle PFA and the frequency temperature characteristic TCF.
Further, in the SAW device having each laminated structure shown in FIGS. 22A to 23B in which the ZnO thin film is used as the piezoelectric thin film and the quartz substrate is used as the elastic substrate, the Rayleigh wave of 11
A spurious wave having a sound velocity of about 0% is generated. Therefore,
The electromechanical coupling coefficient K _SP ^{2 of} spurious waves is also an important evaluation item.

That is, in the surface wave substrate, PFA, TC
F and K_SP ²Is desired to be small. So the elastic body
A quartz substrate is used as the substrate, and a ZnO thin film is used as the piezoelectric thin film.
The structure of FIG. 22 (b) using the same as that of FIG. 1 (b).
In the SAW device of the same structure, the Euler angle of the quartz substrate
(0 °, θ, ψ) and ZnO thin film thickness, Rayleigh wave
Power flow angle PFA, frequency temperature coefficient TCF, Rayleigh
-Wave electromechanical coupling coefficient K ²And a spurious wave electric machine
Mechanical coupling coefficient K_SP ²Calculate the relationship with and using the finite element method
did. The results are shown in FIG. 6 to FIG. 9, FIG. 10 to FIG. 13 and FIG.
~ It demonstrates with reference to FIG.

FIGS. 6 to 9 are views showing the substrate orientation dependence of the surface wave propagating in the ZnO / Al / quartz substrate. In this case, the normalized film thickness of ZnO is 0.20λ. 6 shows the power flow angle of the Rayleigh wave, FIG. 7 shows the frequency temperature coefficient TCF of the Rayleigh wave, FIG. 8 shows the electromechanical coupling coefficient K ² of the Rayleigh wave, and FIG. 9 shows the direction of the electromechanical coupling coefficient K _SP ² of the spurious wave. Show each dependency.

The contour lines in FIGS. 6 to 9 mean that the power flow angle PFA, frequency temperature coefficient TCF, electromechanical coupling coefficients K ² and K _SP ² are the same portion, respectively. 10 to 13 and FIGS. 14 to 17 show similar results, the normalized film thickness of the ZnO film is 0.25λ in FIGS. 10 to 13, and in FIGS. ,
The normalized film thickness of the ZnO film is 0.30λ.

In FIGS. 6 to 17, the area indicated by each arrow X indicated by hatching where the diagonal lines intersect is a condition in which the calculated value cannot be obtained initially because of the small electromechanical coupling coefficient K ^2. Is. PFA and TCF in FIG. 6, FIG. 10, FIG. 14 and FIG. 7, FIG. 11, and FIG. 15 are values obtained by the following equations (2) and (3), respectively.

PFA = tan ⁻¹ (Vf ⁻¹ × ∂Vf / ∂ψ) Equation (2) TCF = Vf ⁻¹ × ∂Vf / ∂T−α Equation (3) In Equation (2), ψ is The propagation direction (degrees) of the surface wave, T is the temperature (° C.), and α is the coefficient of thermal expansion in the propagation direction of the surface wave.

From FIG. 6, FIG. 10 and FIG. 14, the condition that the power flow angle PFA is as small as −2.5 to + 2.5 ° is
It can be seen that the region is a hatched region surrounded by thick lines Y1 and Y2 in FIG.

Further, from FIGS. 7, 11 and 15, the condition that the frequency temperature coefficient TCF is as small as −25 to +25 ppm / ° C. is surrounded by lines T1 and T2 in FIG. 7 and hatched. It can be seen that it is the area indicated by the attachment.
Especially, -5 to +5 ppm, which is smaller than that of the Li ₂ B ₄ O ₇ substrate.
/ ° C., the conditions are lines T3 and T4 in FIG. 7 and line T in FIG.
It can be seen that the region is surrounded by 5, T6 and lines T7, T8 in FIG.

Further, the electromechanical coupling coefficient K ² can be increased as compared with the case where the ZnO thin film is not formed when the ZnO film thickness is 0.05λ or more as described above, but is further improved by adjusting the Euler angle. Is possible. For example, in FIG. 8, the coupling coefficient is 0.8% in the range surrounded by the thick line K.
As described above, a value equal to or higher than the coupling coefficient of the conventional SAW device of FIG. Although the coupling coefficient increases at all Euler angles as the ZnO film thickness increases, the range where the coupling coefficient is large is the range surrounded by the thick line K in FIG. 8 at a specific film thickness.

Further, from FIG. 9, FIG. 13 and FIG.
Electromechanical coupling coefficient K for pre-stress_SP ²Is as small as 0-0.1
The condition is that the area is surrounded by thick lines S1, S2, S3.
I understand that

When the IDT is composed of only double electrodes that do not generate surface wave reflection, in order to suppress the spurious response below the relative insertion loss of 30 dB required by the CDMA-IF filter or the like, the electromechanical coupling coefficient K _SP ² Must be 0.1% or less as described above. Here, the relative insertion loss is the difference between the insertion loss of the Rayleigh wave response and the insertion loss of the spurious wave response.

Further, the inventors of the present application have confirmed by experiments that the reflection coefficient of spurious waves is 10 times or more the reflection coefficient of Rayleigh waves. Therefore, when the IDT is composed of a single electrode or a unidirectional electrode that causes reflection of surface waves, spurious waves have a large reflection coefficient of surface waves.
Resonance operation occurs in the IDT alone, and even a small electromechanical coupling coefficient K _SP ² causes a large spurious wave outside the band.

FIG. 28 shows US Pat. No. 4,162.
Λ / 16 width strip and 3 disclosed in US Pat.
A unidirectional electrode in which two strips each having a λ / 16λ width are arranged in a half-wavelength section is provided with 100
The relationship between the electromechanical coupling coefficient K _SP ² of spurious waves and the relative insertion loss of spurious waves when arranged in pairs is shown. At this time, the thickness of the ZnO thin film is 0.3λ, the thickness of the electrode made of Al is 0.02λ, and the crystal substrate Euler angles are (0 °, 116 °, 0 °).

From FIG. 28, in order to suppress the spurious response to a relative insertion loss of 30 dB or less, the electromechanical coupling coefficient K _SP
It is understood that it is necessary to suppress ² to 0.008% or less. Therefore, the inventor of the present application experimentally confirmed the range in which spurious waves can be suppressed by variously changing the Euler angles (φ, θ, ψ) of the quartz substrate. The results are shown in FIGS.
Will be described with reference to. Here, the thickness of the ZnO thin film was 0.3λ and the thickness of the electrode made of Al was 0.02λ.

FIG. 29 shows the relationship between the electromechanical coupling coefficient K _SP ^{2 of} spurious waves and the substrate orientation θ of the aluminum film thickness.
Spurious wave electromechanical coupling coefficient K _SP ² is 0.008%
In order to set the following, from FIG. 29, the substrate orientation θ is set to 107 °
It can be seen that it is sufficient to set <θ <125 °. Further, preferably, θ is set to 116 ° ± 5 °, and in that case,
Electromechanical coupling coefficient K _SP ^{2 of} spurious waves is 0.005%
It becomes the following.

FIG. 30 shows the relationship between the electromechanical coupling coefficient K _SP ² of spurious waves and the substrate orientation φ. From FIG. 30, the electromechanical coupling coefficient K _SP ² of the spurious wave is 0.00
In order to make it 8% or less, the substrate orientation φ should be −19 ° to +15.
It turns out that the range should be in the range of °. Further, it is preferable that the electromechanical coupling coefficient K _SP ² of spurious waves can be 0.005% or less when φ is set in the range of −2.5 ° ± 5 °.

FIG. 31 shows the relationship between the electromechanical coupling coefficient K _SP ^{2 of} spurious waves and the propagation direction ψ. From FIG. 31, it can be seen that the propagation direction ψ may be set to −10 ° to + 15 ° in order to set the electromechanical coupling coefficient K _SP ² of the spurious wave to 0.008% or less. Also, preferably, ψ is +
It is understood that the electromechanical coupling coefficient K _SP ² of spurious waves can be set to 0.005% or less when the range is 2.5 ° ± 5 °.

Table 2 shows a strip of λ / 16 width and 3λ /
The experimental values are shown when 40 pairs of unidirectional electrodes each having two strips of 16λ width arranged in a half wavelength section are arranged on the input side and the output side respectively.

[0062]

[Table 2]

The effect of lowering the electromechanical coupling coefficient K _SP ² of spurious waves as shown in FIGS. 28 to 31 is as follows.
It has been confirmed that not only the structure shown in FIG. 1B but also the structure in which a comb electrode is formed on a piezoelectric thin film as shown in FIG. Therefore, in the present invention, the comb-teeth electrode may be formed in contact with the piezoelectric thin film.

Therefore, in the surface acoustic wave device in which the piezoelectric thin film is formed on the quartz substrate and the comb-teeth electrode is formed so as to be in contact with the piezoelectric thin film, the Euler angles of the quartz substrate are set to the above-mentioned ranges. By setting, PFA,
It can be seen that a surface wave device having a small TCF and K _SP ² and a large electromechanical coupling coefficient K ^{2 for} Rayleigh waves can be provided.

Next, a concrete experimental example will be described. By the way, the power flow angle is an angle showing the difference between the direction of the phase velocity of the surface wave and the direction of the group velocity, and when the power flow angle exists, the energy of the surface wave is in the direction normal to the electrode finger of the comb electrode. On the other hand, they propagate with a power flow angle offset. The surface wave energy loss L _{PFA at} this time is expressed by the following equation (4).

L _PFA = 10 × log ₁₀ [{W-tan (PFA)} / W] (dB / λ) Equation (4) In Equation (4), W is the wavelength of the surface wave normalized by λ. The cross width of the comb-teeth electrode is shown.

Therefore, as described above, the power flow angle PFA is preferably 0 ° in the surface acoustic wave substrate. However, in the presence of PFA, the degree of difficulty in designing the comb-teeth electrode increases, but as shown in FIG.
Electrode finger arrangement angle θ of the comb-teeth electrode 12 on 1
By matching _strip and PFA, θ _strip
It is possible to suppress the deterioration of the insertion loss due to the difference between PFA and PFA. On the contrary, if the arrangement angle of the electrode fingers and the PFA are different, the insertion loss will be deteriorated.

In the surface acoustic wave substrate according to the present invention, there are spurious waves having a sound velocity of about 110% of Rayleigh waves. In the above comb-shaped electrode in which the electrode finger arrangement angle θ _strip is matched with the power flow angle PFA of the Rayleigh wave, if the difference between the power flow angles of the Rayleigh wave and the spurious wave is ΔPFA, the loss L of the spurious wave due to ΔPFA is L. _PFAS
P is represented by the following equation (5), like the equation (4).

L _PFASP = 10 × log ₁₀ [{W-tan (ΔPFA)} / W] (dB / λ) Equation (5) From Equation (5), by increasing ΔPFA, the response of the spurious wave is increased. It turns out that it can be suppressed. For example, W
When the surface wave propagates 200λ in the comb electrode of = 10λ, the spurious wave can be suppressed by about 1.5 dB when ΔPFA is 1 °.

18 to 20 are diagrams showing the substrate dependence of ΔPFA. Similar to FIGS. 6 to 17, FIGS. 18 to 20 show ZnO film thicknesses of 0.20λ and 0.25λ in a SAW device having a ZnO / Al / quartz substrate structure.
And 0.30λ respectively show the substrate dependence of ΔPFA. The contour lines in FIGS. 18 to 20 are ΔPF.
It is shown that A has the same value, and from FIGS.
It can be seen that the condition under which PFA is ± 1 ° or more is the line P in FIG.

In FIG. 27, the normalized film thickness of ZnO is 0.3λ,
It is the substrate orientation φ dependence of the electromechanical coupling coefficient K ² of the Rayleigh wave and the electromechanical coupling coefficient K _sp ² of the spurious wave when the Euler angle is (φ, 117 °, 0 °). From FIG. 27, the electromechanical coupling coefficient K _sp ^{2 of} spurious waves is 0 to 0.1%.
For small conditions, φ is -35 ° to + 35 °, + 85 ° to +
It can be seen that it is 155 °.

The basic principle of the present invention is, as described in the above-mentioned document 1, a piezoelectric thin film having a negative value of TCF on a quartz substrate having a cut angle and a propagation direction in which the frequency-temperature characteristic TCF has a positive value. By forming the, the temperature characteristic of the quartz substrate and the temperature characteristic of the piezoelectric thin film are canceled to obtain a good frequency temperature characteristic. Therefore, as the piezoelectric thin film, one having a negative TCF value is preferably used.

In the above embodiment, as the piezoelectric thin film,
Although the case of forming a ZnO thin film has been described, in addition to the ZnO thin film, a piezoelectric thin film having a positive TCF, for example, Al
A piezoelectric thin film made of N, Ta ₂ O ₅ or CdS may be used.

Further, in the quartz substrate, the surface on which the piezoelectric thin films are laminated may be either a positive surface or a negative surface. Further, the surface acoustic wave when the piezoelectric thin film is formed on the quartz substrate may have a slight displacement component in the SH direction, but in the present specification, for the sake of convenience, the surface acoustic wave in which the Rayleigh wave is deformed in this way is used. It is pointed out that it is called a Rayleigh wave including.

Further, the calculated values shown in the present application are calculated under the condition that the denseness of the ZnO thin film is good. It should be pointed out that when the density of the ZnO film is low, the value is close to the calculated value when the film thickness of the ZnO film is thin.

[0076]

In the surface acoustic wave device according to the present invention, the piezoelectric thin film is formed on the quartz substrate having the Euler angles (φ, θ, ψ) within the above specific range, and the piezoelectric thin film is in contact with the piezoelectric thin film. Since the comb-teeth electrode is formed and the standardized film thickness H / λ of the piezoelectric thin film is 0.05 or more, the electromechanical coupling coefficient of spurious is small, and therefore spurious can be suppressed effectively. It is possible to provide a surface wave device having a large electromechanical coupling coefficient K ^{2 of} Rayleigh waves.

In the present invention, in particular, φ is −2.5 ° ± 5 ° and θ is 11 at the Euler angle of the quartz substrate.
When 6 ° ± 5 ° and ψ are in the range of + 2.5 ° ± 5 °, the electromechanical coupling coefficient of spurious can be set to 0.005% or less, and spurious can be suppressed more effectively. be able to.

In particular, the normalized film thickness H / λ of the piezoelectric thin film is 0.
In case of 20 or more, the electromechanical coupling coefficient K of Rayleigh wave
² can be made even larger. When the piezoelectric thin film has a negative surface and is in contact with the quartz substrate and / or the comb-teeth electrode, that is, by configuring the positive surface of the piezoelectric thin film to be the upper surface, the electromechanical coupling coefficient K ² is further increased. be able to.

Further, a short-circuit electrode may be further formed on the piezoelectric thin film, and even in that case, a surface acoustic wave device having a large electromechanical coupling coefficient K ² can be constructed according to the present invention.

When the Euler angle of the quartz substrate is in the range surrounded by the lines Y1 and Y2 in FIG. 6, the power flow angle can be set to ± 2.5 °. When the Euler angle of the quartz substrate is in the range surrounded by the lines T1 and T2 in FIG. 7, the frequency temperature coefficient TCF of the surface acoustic wave device is ± 25 ppm.
It is possible to provide a surface acoustic wave device having a low temperature dependency. Especially when the Euler angle of the quartz substrate is in the range surrounded by the lines T3 and T4 in FIG.
The frequency temperature coefficient TCF of the surface acoustic wave device can be ± 5 ppm / ° C.

Further, when the Euler angle of the quartz substrate is in the range surrounded by the line K in FIG. 8, the electromechanical coupling coefficient K ² of the Rayleigh wave can be 0.8% or more.
When the frequency temperature coefficient TCF of the piezoelectric thin film has a negative value, it is offset by the frequency temperature coefficient of the quartz substrate,
It is possible to easily configure a surface acoustic wave device having little temperature dependence.

When the Euler angle of the quartz substrate is in the range surrounded by the line P in FIG. 18, the difference ΔPF in power flow angle between the surface wave to be used and the unnecessary surface wave not to be used.
Since A is in the range of ± 1 °, it is possible to provide a surface acoustic wave device having excellent characteristics. In particular, when the distance L ₁₁ between adjacent IDTs is W where the electrode finger crossing width of the IDT is,
When L ₁₁ > W / tan (ΔPFA), the influence of unnecessary surface waves can be suppressed more effectively.

[Brief description of drawings]

1A and 1B are schematic configuration diagrams for explaining a surface acoustic wave device according to an embodiment of the present invention, and FIG.
Is a plan view and (b) is a cross-sectional view of a main part.

FIG. 2 is a graph showing a ZnO film having Euler angles (0 °, 0 °, 0 °) formed on a quartz substrate having Euler angles (0 °, 119.75 °, 35 °) in SAW devices having various laminated structures. ZnO standardized film thickness of Rayleigh wave and electromechanical coupling coefficient K
^The figure which shows the relationship with ² .

FIG. 3 shows ZnO films having Euler angles (0 °, 180 °, 0 °) formed on quartz substrates having Euler angles (0 °, 119.75 °, 35 °) in SAW devices having various laminated structures. diagram showing the relationship between the electromechanical coupling coefficient K ² of ZnO normalized film thickness of the Rayleigh wave and the Rayleigh wave.

FIG. 4 is a diagram illustrating a case where a ZnO film having Euler angles (0 °, 0 °, 0 °) is formed on a quartz substrate having Euler angles (0 °, 119.75 °, 35 °) in SAW devices having various laminated structures. diagram showing the relationship between the electromechanical coupling coefficient K _Sp ² of ZnO normalized film thickness and the spurious wave of the Rayleigh wave.

FIG. 5 is a graph showing a ZnO film having Euler angles (0 °, 180 °, 0 °) formed on a quartz substrate having Euler angles (0 °, 119.75 °, 35 °) in SAW devices having various laminated structures. diagram showing the relationship between the electromechanical coupling coefficient K _Sp ² of ZnO normalized film thickness and the spurious wave of the Rayleigh wave.

FIG. 6 is a diagram showing the substrate orientation dependence of the power flow angle of a Rayleigh wave when the normalized film thickness of ZnO is 0.20λ.

FIG. 7 is a diagram showing the substrate orientation dependence of the Rayleigh wave frequency temperature coefficient TCF when the normalized film thickness of ZnO is 0.20λ.

FIG. 8 is a diagram showing the substrate orientation dependence of the electromechanical coupling coefficient K ² of the Rayleigh wave when the normalized film thickness of ZnO is 0.20λ.

FIG. 9 is a diagram showing the substrate orientation dependence of the electromechanical coupling coefficient K _SP ² of spurious waves when the normalized film thickness of ZnO is 0.20λ.

FIG. 10 is a diagram showing the substrate orientation dependence of the power flow angle of Rayleigh waves when the normalized film thickness of ZnO is 0.25λ.

FIG. 11 is a diagram showing the substrate orientation dependence of the frequency temperature coefficient TCF of a Rayleigh wave when the normalized film thickness of ZnO is 0.25λ.

FIG. 12 is a diagram showing the substrate orientation dependence of the electromechanical coupling coefficient K ² of the Rayleigh wave when the normalized film thickness of ZnO is 0.25λ.

FIG. 13 is a diagram showing the substrate orientation dependence of the electromechanical coupling coefficient K _SP ² of spurious waves when the normalized film thickness of ZnO is 0.25λ.

FIG. 14 is a diagram showing the substrate orientation dependence of the power flow angle of a Rayleigh wave when the normalized film thickness of ZnO is 0.30λ.

FIG. 15 is a diagram showing the substrate orientation dependence of the frequency temperature coefficient TCF of the Rayleigh wave when the normalized film thickness of ZnO is 0.30λ.

FIG. 16 is a diagram showing the substrate orientation dependence of the electromechanical coupling coefficient K ² of the Rayleigh wave when the normalized film thickness of ZnO is 0.30λ.

FIG. 17 is a diagram showing the substrate orientation dependence of the electromechanical coupling coefficient K _SP ² of spurious waves when the normalized film thickness of ZnO is 0.30λ.

FIG. 18 is a diagram showing the substrate orientation dependence of ΔPFA when the normalized film thickness of ZnO is 0.20λ.

FIG. 19 is a diagram showing the substrate orientation dependency of ΔPFA when the normalized film thickness of ZnO is 0.25λ.

FIG. 20 is a diagram showing the substrate orientation dependence of ΔPFA when the normalized film thickness of ZnO is 0.30λ.

FIG. 21 is a schematic diagram for explaining the relationship between the electrode finger arrangement angle θ _strip of the comb-teeth electrode and the power flow angle PFA in the surface acoustic wave device.

22A and 22B are schematic cross-sectional views for explaining an example of a laminated structure of a substrate, a piezoelectric thin film, and a comb electrode in a surface acoustic wave device.

23A and 23B are schematic cross-sectional views for explaining an example of a laminated structure of a substrate, a piezoelectric thin film, and a comb electrode in a surface acoustic wave device.

FIG. 24 is a diagram showing a relationship between a standardized film thickness of a ZnO thin film in a conventional surface acoustic wave device and an electromechanical coupling coefficient Ks.

25A and 25B are diagrams showing the dependence of the frequency temperature coefficient TCF on the ZnO film thickness of the surface acoustic wave device described in the prior art.

FIG. 26 is a diagram showing the relationship between the normalized film thickness H / λ of ZnO and the electromechanical coupling coefficient described in the prior art.

FIG. 27 shows the electromechanical coupling coefficient K ² of the Rayleigh wave and the electromechanical coupling coefficient K of the spurious wave when the normalized film thickness of ZnO is 0.3λ and the Euler angle (φ, 117 °, 0 °).
shows a substrate azimuth φ dependence of the _sp ^2.

FIG. 28 shows the relationship between the electromechanical coupling coefficient K _SP ² of spurious waves and the insertion loss of spurious waves.

FIG. 29 shows the relationship between the electromechanical coupling coefficient K _SP ^{2 of} spurious waves, the aluminum film thickness, and the substrate orientation θ.

FIG. 30 shows the relationship between the electromechanical coupling coefficient K _SP ^{2 of} spurious waves and the substrate orientation φ.

FIG. 31 shows the relationship between the electromechanical coupling coefficient K _SP ^{2 of} spurious waves and the propagation direction ψ.

[Explanation of symbols]

1. Surface wave device 2 ... Crystal substrate 3, 4 ... IDT 3a, 3b, 4a, 4b ... Comb tooth electrodes 5 ... Piezoelectric thin film

Claims (14)

[Claims] 1. At Euler angles (φ, θ, ψ), −
19 ° <φ <+ 15 ° and 107 ° <θ <125
Of the piezoelectric thin film, the quartz substrate having a temperature of -10 ° <ψ <15 °, a piezoelectric thin film formed on the quartz substrate, and a comb-teeth electrode formed in contact with the piezoelectric thin film. A surface acoustic wave device, wherein a normalized film thickness H / λ of the piezoelectric thin film is 0.05 or more, where H is the film thickness and λ is the wavelength of the surface wave. 2. The Euler angles (φ, θ,
ψ), φ is −2.5 ° ± 5 °, θ is 116 ° ±
5 ° and ψ are in the range of + 2.5 ° ± 5 °.
The surface wave device according to. 3. The normalized film thickness H / λ of the piezoelectric thin film is 0.
The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device has 20 or more. 4. The surface acoustic wave device according to claim 1, wherein the piezoelectric thin film is in contact with the substrate and / or the comb-teeth electrode on the minus side. 5. The surface acoustic wave device according to claim 1, further comprising a short circuit electrode formed on the piezoelectric thin film. 6. The surface acoustic wave device according to claim 1, wherein the Euler angle of the quartz substrate is within a range in which the power flow angle PFA of the Rayleigh wave in FIG. 6 is ± 2.5 °. 7. The Euler angle of the quartz substrate has a frequency temperature coefficient TCF of the surface acoustic wave device of FIG. 7 of ± 25 ppm.
The surface acoustic wave device according to any one of claims 1 to 6, which is in a range of / ° C. 8. The Euler angle of the quartz substrate has a frequency temperature coefficient TCF of ± 5 ppm / of the surface acoustic wave device in FIG.
The surface acoustic wave device according to claim 7, which is in a range of ° C. 9. The surface acoustic wave device according to claim 1, wherein an Euler angle of the quartz substrate is within a range in which a Rayleigh wave electromechanical coupling coefficient K ² in FIG. 8 is 0.8% or more. . 10. A frequency temperature coefficient TCF of the piezoelectric thin film.
The surface acoustic wave device according to any one of claims 1 to 9, wherein has a negative value. 11. The Euler angle of the quartz substrate is as shown in FIG.
The surface acoustic wave device according to any one of claims 1 to 10, wherein a difference ΔPFA between the power flow angles of the surface wave to be used and the unnecessary surface wave that is not used is within a range of ± 1 °. 12. The Euler angles (φ, θ,
φ of φ) is −35 to + 35 °,
The surface acoustic wave device according to claim 1. 13. The Euler angle of the crystal substrate according to claim 5.
A surface acoustic wave device which is crystallographically equivalent to the Euler angles according to 9 to 11 or 12. 14. The piezoelectric thin film comprises ZnO, AlN, T
The surface acoustic wave device according to claim 1, comprising one selected from the group consisting of a ₂ O ₅ and CdS.