JP3281510B2 - Surface acoustic wave device - Google Patents

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 lithium niobate or lithium tantalate as a piezoelectric material.

[0002]

2. Description of the Related Art In communication devices such as portable telephones,
Surface acoustic wave devices are widely applied as circuit devices such as resonator filters and signal processing delay lines. The surface acoustic wave element is, for example, a substrate having piezoelectricity as shown in FIG.
A screen-like electrode (2) or a grid-like reflector (3) (3) on the surface of (1)
And performs mutual conversion between an electric signal and a surface acoustic wave. In general, a piezoelectric substrate of a surface acoustic wave element is required to have a large electromechanical coupling coefficient and a small propagation loss.

By the way, with the recent increase in the frequency of communication equipment, the need for a surface acoustic wave element that can be used in the gigahertz band is increasing. Center frequency f _{0 of the} surface acoustic wave element
Is the propagation velocity V of the surface acoustic wave and the electrode finger period L (= wavelength λ)
Is expressed by the following equation.

F ₀ = V / L

[0004] Therefore, in order to cope with an increase in the frequency of the surface acoustic wave element, it is necessary to develop a piezoelectric substrate capable of obtaining a higher propagation velocity (phase velocity) V. This includes a method using a hard substrate material such as diamond and a method using a so-called leaky surface acoustic wave. Leaky surface acoustic waves are elastic waves that propagate on the surface while radiating energy in the depth direction of the elastic body.By appropriately selecting the cut surface and the surface acoustic wave propagation direction, the propagation loss is reduced, Furthermore,
It is possible to realize a higher propagation speed than a Rayleigh wave.

[0005] Surface acoustic wave devices using leaky surface acoustic waves include quartz LST cut, lithium niobate (LiNb).
There are known 41 ° YX cut, 64 ° YX cut of O ₃ ), and 36 ° YX cut of lithium tantalate (LiTaO ₃ ) (Yasutaka Shimizu, “Propagation properties of surface acoustic wave materials and Current state of use ”IEICE Transactions A Vol.J76-A,
2, pp129-137, 1993). Lithium tetraborate (Li ₂ B ₄ O ₇ )
On the substrate, leaky surface acoustic waves exceeding the fast shear wave phase velocity have been reported (Takahiro Sato, Hidenori Abe, "Longitudinal-type leaky waves propagating in lithium tetraborate substrate").
Committee 39th meeting (6.6.23). Since the phase velocity of this leaky surface acoustic wave is close to the phase velocity of the longitudinal wave, it is called a longitudinal wave type leaky wave. Furthermore, leaky surface acoustic waves propagating through a lithium niobate substrate having an arbitrary cut surface have already been reported (Yasutaka Shimizu, Koji Murakami, "LiNb
O ₃ substrate LSAW characteristics and New Cut, "Vol.J69-C, 1
0, pp1309-1318, 1986).

[0006]

However, the phase velocities obtained with the conventional lithium tantalate substrate and lithium niobate substrate are about 4000 m / s, and there is a possibility that a cut surface with a higher phase velocity may exist. . Also, regarding the optimal cut plane and the surface acoustic wave propagation direction, which have a small propagation loss and still obtain a large electromechanical coupling coefficient,
Not enough research has been done yet. An object of the present invention is to find a more suitable cut surface and surface acoustic wave propagation direction than ever before in a lithium niobate substrate and a lithium tantalate substrate, thereby providing a high performance surface acoustic wave element.

[0007]

Therefore, in the present invention, the propagation characteristics of leaky surface acoustic waves on a lithium niobate substrate and a lithium tantalate substrate are theoretically changed by variously changing the cut surface and the propagation direction of the surface acoustic wave. Researched. This results in two types of leaky surface acoustic waves:
Finding a first leaky surface wave (First Leaky Wave) having a phase velocity between a slow transverse wave and a fast transverse wave, and a second leaky surface wave (Second Leaky Wave) having a phase velocity exceeding a fast transverse wave,
The present invention has been completed.

In the evaluation of the characteristics of a surface acoustic wave element, a general solution known in the art (for example, JJ
Campbell, WRJones, "A Method for Estimating Optim
al Crystal Cuts and Propagation Directions for Exc
itation of Piezoelectric Surface Waves ", IEEE tran
saction on Sonics and Ultrasonics, vol.SU-15, No.
4, pp209-217, (1968)), and the phase velocity, the electromechanical coupling coefficient and the propagation loss were calculated by computer simulation. As for the optimal cut surface and the propagation direction of the surface acoustic wave, a prototype surface acoustic wave device was actually manufactured, and its characteristics were measured. As a result, measured values matching the simulation results were obtained. This confirms the validity of the computer simulation.

A first surface acoustic wave element according to the present invention is one in which electrodes are formed on a piezoelectric substrate made of lithium niobate, and the cut surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are controlled by a right-handed system. In the Euler angle display of (φ, θ, ψ) and a range substantially equivalent thereto, φ is 0 ° to 180 °.
° range, theta and 90 °, the ψ is set to 0 °, depending on which
The first having a phase velocity between the slow and fast transverse waves
It is characterized by generating a leaky surface wave .

More specifically, φ is 20 °, 40 °,
The angle is set to 80 °, 100 °, 140 °, 160 °, or an angle substantially equivalent to these angles.

A second surface acoustic wave element according to the present invention is one in which electrodes are formed on a piezoelectric substrate made of lithium tantalate, and the cut surface of the piezoelectric substrate and the direction of propagation of the surface acoustic wave are defined by an Euler angle. When (φ, θ, ψ) and a range substantially equivalent thereto are shown, φ is 90 ° and θ is 90
°, the ψ set in the range of 0 ° to 180 °, depending on which
And a second leakage surface having a higher phase velocity than the fast shear wave
It is characterized by generating waves .

More specifically, ψ is 31 °, 164
° or an angle substantially equivalent to these angles.

A third surface acoustic wave element according to the present invention is one in which electrodes are formed on a piezoelectric substrate made of lithium niobate, and the cut surface of the piezoelectric substrate and the direction of surface acoustic wave propagation are defined by an Euler angle. When (φ, θ, ψ) and a range substantially equivalent thereto are shown, φ is 90 °, θ is 90 °,
ψ is set in the range of 0 ° to 180 ° , thereby
Generates a second leaky surface wave having a higher phase velocity than the shear wave
Wake up .

More specifically, ψ is 37 °, 164
° or an angle substantially equivalent to these angles.

[0015]

In the first elastic surface element, when the lithium niobate substrate is cut (φ, 90 °, 0 °),
A first leaky surface wave faster than the Rayleigh wave is generated over the entire range of φ from 0 ° to 180 °. The propagation loss of the first leaky surface wave when the surface is electrically open is φ
Becomes substantially zero around 20 °, 40 °, 80 °, 100 °, 140 ° and 160 °, and the electromechanical coupling coefficient K ² is 1
A high value of 6.2% is obtained.

In the second surface acoustic wave element, when the lithium tantalate substrate is cut at (90 °, 90 °, ψ), ψ is approximately equal to 0 ° to 180 °, A second leaky surface wave having twice the phase velocity is generated. Further, when ψ is 31 °, the electromechanical coupling coefficient K ² is 2.14%, and the propagation loss of the second leaky surface wave is such that the surface is electrically open and electrically short-circuited.
ψ becomes substantially zero at 164 °.

In the third surface acoustic wave device, in the (90 °, 90 °, （) cut of the lithium niobate substrate, the phase velocity is about 0 ° to 180 ° over the entire range of 0 ° to 180 °. A very high second leaky surface wave of 7000 m / s is generated. When ψ is 37 °, the electromechanical coupling coefficient becomes 12.9%, and the propagation loss of the second leaky surface acoustic wave becomes substantially zero when ψ is 164 °.

[0018]

According to the present invention, in the lithium niobate substrate and the lithium tantalate substrate, the cut surface and the surface acoustic wave propagation direction are appropriately set, so that a higher phase velocity can be obtained and the propagation loss can be reduced. It is possible to provide a surface acoustic wave element that can obtain a small and large electromechanical coupling coefficient.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings. First, based on FIG. 8, the Euler angles (φ, θ,
ψ) will be described. As shown, the crystal axes are X, Y, Z
In this case, the X axis is rotated around the Z axis by the angle φ toward the Y axis, and this is defined as the A1 axis. Next, the Z axis is rotated counterclockwise about the A1 axis by the angle θ, and this is defined as the A2 axis. The substrate is cut in a plane orientation including the A1 axis with the A2 axis as a normal line. Then, on the substrate cut in the plane orientation, an axis obtained by rotating the A1 axis counterclockwise around the A2 axis by the angle ψ is set as the A3 axis,
The three axes are the surface acoustic wave propagation directions. At this time, the cut plane and the surface acoustic wave propagation direction are represented as Euler angles (φ, θ, ψ).

First Leakage Table for Lithium Niobate Substrate
Surface wave FIGS. 1 and 2, the lithium niobate substrate (phi, 90
(°, 0 °) cut shows the propagation characteristics of the first leaky surface wave as a function of the angle φ in both cases where the surface is electrically open and electrically short.

As shown in FIG. 1, the phase velocity of the first leaky surface wave is larger than the phase velocity of the Rayleigh wave regardless of whether the surface is open or short-circuited. In particular, when the surface is open, the value is close to the speed of a fast shear wave. However, in the angle range of φ from 0 ° to 180 °, there is a region on the way where no solution can be obtained. On the other hand, when the surface is short-circuited, the speed of the first leaky surface wave is a value close to the speed of the slow shear wave.

As described above, the speed difference between the case where the surface is open and the case where the surface is short-circuited is large, and as a result, the electromechanical coupling coefficient becomes large. FIG. 2 shows the electromechanical coupling coefficient K ² and the propagation loss per wavelength as a function of the angle φ. As shown, φ = 0 °, 60 °, 120 ° and 1
Around 80 °, the electromechanical coupling coefficient K ² has a maximum value of 25.1.
%.

On the other hand, when the surface is open, φ = 20 °, 40 °, 80 °, 100 °, 140 °
° and 160 °, it becomes substantially zero, and the electromechanical coupling coefficient K ²
Is a large value of 16.2% in these cuts. However, when the surface is short-circuited, the propagation loss is large.

Second Leakage in Lithium Tantalate Substrate
Surface wave diagrams 3 and 4 show that the lithium tantalate substrate (90 °,
In the case of a 90 °, 開放) cut, the propagation characteristic of the second leaky surface wave is expressed as a function of the angle ψ in both cases where the surface is electrically open and electrically short-circuited.

As shown in FIG. 3, the phase velocity of the second leaky surface wave is about 6000 m in both cases where the surface is open or short-circuited.
/ S, which is about twice as high as the Rayleigh wave, and is very close to that of the longitudinal wave (Longitudinal).

FIG. 4 shows the electromechanical coupling coefficient and the propagation loss per wavelength. As shown, ψ is 31 °
, The electromechanical coupling coefficient K ² has a maximum value of 2.14%. In addition, the propagation loss when the surface is electrically open is much smaller than the propagation loss when the surface is electrically short-circuited. And in both cases where the surface is open and short circuit,
The propagation loss is substantially zero when ψ is 164 °.

Second Leakage Table for Lithium Niobate Substrate
The surface wave diagrams 5 and 6 show (90 °, 9 °) of the lithium niobate substrate.
FIG. 7 shows the propagation characteristics of the second leaky surface acoustic wave as a function of the angle 、 when the surface is both electrically open and electrically short in the 0 °, ψ) cut.

As shown in FIG. 5, the phase velocity of the second leaky surface wave is very high, about 7000 m / s, which is about twice the phase velocity of the Rayleigh wave. Further, the phase velocity of the second leaky surface acoustic wave shows a different change between the case of electrical opening and the case of electrical shorting.
There is a difference of 0 m / s, and as a result, a large electromechanical coupling coefficient is obtained.

FIG. 6 shows the electromechanical coupling coefficient K ² and the propagation loss per wavelength as a function of the angle ψ. As shown in the drawing, when ψ is 37 °, the electromechanical coupling coefficient K ² has a maximum value of 12.9%, which is a large value.
In addition, the propagation loss when the surface is electrically open is much smaller than the propagation loss when the surface is electrically short-circuited. In both cases where the surface is open and short-circuited, the propagation loss is substantially zero when １６ is 164 °.

The characteristics shown in FIGS. 1 to 6 are based on computer simulations. However, the above-described characteristic evaluation method employed in this embodiment has some errors due to, for example, modeling of a surface acoustic wave element. Even if there is, it is considered that the error hardly occurs in the horizontal axis direction of the graphs of FIGS. Needless to say, in comparing the first leaky surface wave and the second leaky surface wave according to the present invention with the conventional Rayleigh wave, both have the same magnitude of error, so that the above-mentioned comparison result is not affected. It can be said.

As described above, according to the present invention, as a result of theoretically studying the first leaky surface wave and the second leaky surface wave on the lithium niobate substrate and the lithium tantalate substrate, the optimum cut surfaces of these substrates are respectively determined. In addition, the direction of surface acoustic wave propagation was found, and thereby, a surface acoustic wave device capable of handling a higher frequency band than before was completed.

The description of the above embodiments is for the purpose of illustrating the present invention, and should not be construed as limiting the invention described in the claims or reducing the scope thereof. Further, the configuration of each part of the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made within the technical scope described in the claims.

[Brief description of the drawings]

FIG. 1 is a graph showing characteristics of phase velocity of a surface acoustic wave device having a (φ, 90 °, 0 °) cut lithium niobate substrate.

FIG. 2 is a graph showing characteristics of an electromechanical coupling coefficient and a propagation loss according to the first embodiment.

FIG. 3 is a graph showing phase velocity characteristics of a surface acoustic wave device having a (90 °, 90 °, ψ) cut lithium tantalate substrate.

FIG. 4 is a graph showing characteristics of an electromechanical coupling coefficient and a propagation loss according to the first embodiment.

FIG. 5 is a graph showing phase velocity characteristics of a surface acoustic wave device having a (90 °, 90 °, ψ) cut lithium niobate substrate.

FIG. 6 is a graph showing characteristics of an electromechanical coupling coefficient and a propagation loss according to the first embodiment.

FIG. 7 is a plan view showing a configuration example of a surface acoustic wave element.

FIG. 8 is a diagram illustrating Euler angle display.

[Explanation of symbols]

 (1) Substrate (2) Electrode (3) Reflector

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-54-97346 (JP, A) JP-A-62-171215 (JP, A) Yasutaka Shimizu, Koji Murakami, Characteristics of leaky surface acoustic waves on LiNbO3 substrate New Cut, IEICE Transactions C, Japan, IEICE, October 25, 1986, Vol. J69-C, NO. 10, 1309-1318 Yasutaka Shimizu, Yousuke Edo and Takaya Watanabe, A New Cut of LiTaO3 with the Zero Slope Theory Lifetime, USA. 1, 253-256 (58) Field surveyed (Int. Cl. ⁷ , DB name) H03H 9/25

Claims (6)

(57) [Claims] 1. A surface acoustic wave device in which electrodes for transmitting surface acoustic waves are formed on a piezoelectric substrate made of lithium niobate, wherein the cut surface of the piezoelectric substrate and the direction of surface acoustic wave propagation are adjusted by an oiler. When the angle is represented by (φ, θ, ψ) and a range substantially equivalent thereto, φ is 0 ° to 180 °.
° range, theta and 90 °, the ψ is set to 0 °, depending on which
The first having a phase velocity between the slow and fast transverse waves
A surface acoustic wave device that generates a leaky surface wave . 2. φ is 20 °, 40 °, 80 °, 100 °
The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device is set at an angle of 140, 140, or 160 °. 3. A surface acoustic wave device in which an electrode for transmitting a surface acoustic wave is formed on a piezoelectric substrate made of lithium tantalate, wherein a cut surface of the piezoelectric substrate and a propagation direction of the surface acoustic wave are adjusted by an oiler. When the angle is represented by (φ, θ, 実 質) and a range substantially equivalent thereto, φ is set to 90 °, θ is set to 90 °, and ψ is set to a range of 0 ° to 180 ° .
Thus, the second leakage table having a higher phase velocity than the fast shear wave
A surface acoustic wave device that generates a surface wave. 4. The surface acoustic wave device according to claim 3, wherein ψ is set to 31 ° or 164 °. 5. A surface acoustic wave device in which electrodes for transmitting surface acoustic waves are formed on a piezoelectric substrate made of lithium niobate, wherein the cut surface of the piezoelectric substrate and the direction of propagation of the surface acoustic waves are adjusted by an oiler. When the angle is represented by (φ, θ, 実 質) and a range substantially equivalent thereto, φ is set to 90 °, θ is set to 90 °, and ψ is set to a range of 0 ° to 180 ° .
Thus, the second leakage table having a higher phase velocity than the fast shear wave
A surface acoustic wave device that generates a surface wave. 6. The surface acoustic wave device according to claim 5, wherein ψ is set to 37 ° or 164 °.