JP3090219B2 - 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 a leaky surface acoustic wave (leaky wave). A surface acoustic wave device is a circuit element using a surface-acoustic wave generated on a piezoelectric substrate by forming a comb-shaped electrode or reflector of a fine metal film having a width of several μm on a piezoelectric substrate. As the surface acoustic wave, a surface wave called a Rayleigh wave that propagates along the surface of a semi-infinite elastic body is usually used.In addition, the surface of the piezoelectric substrate is radiated while emitting energy in the depth direction of the piezoelectric substrate. The existence of leaky surface acoustic waves (leaky waves) that propagate is known.

[0002]

2. Description of the Related Art Calculation results of propagation characteristics of a Rayleigh wave and a leaky surface acoustic wave on a (110) plane of a lithium tetraborate single crystal substrate have been reported (Murota, Shimizu,
"Li ₂ B ₄ O ₇ cut dependence of LSAW characteristics for propagating substrate" IEEE Ultrasonics, U
S-40, pp57-62, 1988). In this report,

[0003]

(Equation 1) When θ = 0 ° and the <001> direction is θ = 90 °
The propagation velocity, electromechanical coupling coefficient (k ² ), and propagation loss per wavelength of the surface acoustic wave in the propagation direction θ are calculated. According to this, it has been calculated that the leaky surface acoustic wave has a higher propagation velocity than the Rayleigh wave, and that the electromechanical coupling coefficient is larger than the Rayleigh wave in the range of θ = 55 ° to 80 °. For this reason, a surface acoustic wave device using a leaky surface acoustic wave as a surface acoustic wave for increasing the frequency and bandwidth is expected to be promising.

[0004]

However, in the above-mentioned report on the lithium tetraborate single crystal substrate, the electromechanical coupling coefficient is larger than that of the Rayleigh wave.
It is reported that in the range of 55 ° to 80 °, the propagation loss per wavelength becomes larger than that of Rayleigh waves. As described above, the leakage surface acoustic wave generally has a large propagation loss since energy leaks into the inside of the piezoelectric substrate, and has a problem that it is difficult to use the surface acoustic wave as a surface acoustic wave.

An object of the present invention is to provide a surface acoustic wave device having a large electromechanical coupling coefficient of a leaky surface acoustic wave and a small propagation loss.

[0006]

Means for Solving the Problems The present inventor has considered a method of reducing the propagation loss of a leaky surface acoustic wave, and as a result, in a layered medium having a surface layer formed on a semi-infinite base medium, a transverse wave of the base medium is obtained. If the speed of the transverse wave of the surface layer is slower than the speed of the surface layer, the waves of the base medium are pushed up by the waves of the surface layer, and energy is concentrated on the surface layer.
Waves). In other words, if a surface layer made of a material having a lower transverse wave speed is formed by a predetermined thickness on a piezoelectric substrate as a base medium, the energy leaking into the base medium as a leaky surface acoustic wave becomes a love wave is reduced. It was thought that the propagation loss could be reduced.

Accordingly, the object is to excite, receive, reflect, and apply surface acoustic waves to the surface of a lithium tetraborate single crystal substrate.
In the surface acoustic wave device in which a metal film for propagation is formed, the cutout angle of the lithium tetraborate single crystal substrate and the propagation direction of the surface acoustic wave are expressed by an Euler angle (130 ° to
140 °, 85 ° to 95 °, 55 ° to 80 °), and the thickness of the metal film is set to be equal to the surface of the lithium tetraborate single crystal substrate. The surface acoustic wave device is characterized in that the surface acoustic wave device has a predetermined value that becomes a Love wave propagating through the surface acoustic wave.

[0008]

According to the present invention, by setting the thickness of the metal film on the surface of the lithium tetraborate single crystal substrate to a predetermined thickness, the surface acoustic wave can be converted into a love wave to reduce the propagation loss.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface acoustic wave device according to a preferred embodiment of the present invention will be described with reference to FIGS. As shown in FIG. 1, the surface acoustic wave device of this embodiment has an input comb electrode 12 made of an interdigital electrode and an output electrode on a piezoelectric substrate 11 made of lithium tetraborate single crystal having a (110) surface. This is a surface acoustic wave filter in which a comb electrode 13 is formed, and a metal film 14 is formed in a propagation path region between the input comb electrode 12 and the output comb electrode 13. The input comb electrodes 12 and the output comb electrodes 13 are each 20.5 pairs and have a period of 10 μm.
(Electrode width 2.5 μm, wavelength 10 μm), aperture length 2,000
μm, and formed so that the propagation direction of the leaky surface acoustic wave becomes (135 °, 90 °, 65 °) in Eulerian angle representation.

The metal film 14 is formed of gold (Au). Since the transverse wave of the leaky surface acoustic wave is slower in the metal film than in the lithium tetraborate single crystal, it is possible to make the leaky surface acoustic wave into a love wave by setting the film thickness to a predetermined value. FIG. 2 shows measurement results of the propagation speed [m / sec] and the insertion loss [dB] when the thickness of the metal film 14 is changed. When the normalized thickness h / λ, which is obtained by dividing the thickness h of the metal film 14 by the wavelength λ of the leaky surface acoustic wave, is 0.5% or more, the insertion loss is reduced. It turned out to be. On the other hand, even if the normalized film thickness h / λ is increased, the propagation speed is not greatly reduced, and the advantage of using the leaky surface acoustic wave is not lost.

Referring to the above measurement results, the surface is (1)
10) Numerical calculation of the propagation velocity, electromechanical coupling coefficient, and insertion loss of a leaky surface acoustic wave when a metal film 14 containing gold as a main component is formed on a piezoelectric substrate 11 made of lithium tetraborate single crystal as a plane. Determined by In the piezoelectric substrate 11, displacement and potential are obtained by the equation of motion and the equation of piezoelectricity,
In the metal film 14, the displacement is obtained by the equation of motion,
The phase velocity was determined so as to satisfy the boundary condition established at the boundary between the piezoelectric substrate 11 and the metal film 14. The propagation direction of the leaky surface acoustic wave is represented by an Eulerian angle (135 °, 90 °, 6 °).
5 °), and the normalized thickness h / λ of the metal film 14 is 0.00
Simulation was performed for the range of 0 to 0.030.

FIGS. 3 and 4 show the simulation results.
Shown in In FIG. 3, the solid line shows the calculation result of the propagation speed when the metal film 14 is open (simulation using the metal film 14 as an insulator), and the dotted line shows the metal film 14.
Shows the calculation result of the propagation velocity when the short is (simulation using the metal film 14 as a conductor). In FIG. 4, the solid line indicates the calculation result of the electromechanical coupling coefficient, the broken line indicates the calculation result of the propagation loss per wavelength when the metal film 14 is short, and the dotted line indicates the one wavelength when the metal film 14 is open. The calculation result of the loss per hit is shown.

When the thickness of the metal film 14 is increased, the propagation speed of the leaky surface acoustic wave is reduced as shown in FIG. 3, but the electromechanical coupling coefficient is maintained at a high value as shown in FIG. It can be seen that the propagation loss per wavelength decreases. When the normalized thickness h / λ of the metal film 14 is about 0.5% or more, the propagation loss per wavelength becomes 0 dB. That is, the metal film 1
By setting the normalized film thickness h / λ of No. 4 to about 0.5% or more, a surface acoustic wave device having a small propagation loss per wavelength can be realized.

As a comparative example, a metal film 1 made of aluminum was formed on a (110) plane lithium tetraborate single crystal substrate 11.
5 and 6 show the simulation results in the case where No. 4 was formed. In FIG. 3, the solid line indicates that the metal film 14 is open.
The calculation results of the propagation velocity in the case of
Shows the calculation result of the propagation speed when is short. In FIG. 4, the solid line indicates the calculation result of the electromechanical coupling coefficient, the broken line indicates the calculation result of the propagation loss per wavelength when the metal film 14 is short, and the dotted line indicates the one wavelength when the metal film 14 is open. The calculation results of the propagation loss per hit are shown.

As the thickness of the metal film 14 increases, the propagation speed of the leaky surface acoustic wave increases as shown in FIG. 5, but the electromechanical coupling coefficient decreases as shown in FIG. It continues is a slight increase without propagation loss is reduced, there is no possibility that the propagation loss per wavelength is 0 dB. That is, when the metal film 14 is formed of aluminum, the propagation loss cannot be reduced no matter how thick the metal film 14 is, and a surface acoustic wave device using a leaky surface acoustic wave cannot be realized. .

3 and 4, the propagation direction of the leaky surface acoustic wave is represented by Euler angles (135 °, 90 °, θ) and θ = 6.
Calculation was performed for the case of 5 °, and it was found that the normalized thickness h / λ of the metal film 14 at which the propagation loss was 0 dB was 0.5%. The same calculation is performed for the case where θ in the Euler angle display (135 °, 90 °, θ) is changed, and the propagation loss is 0.
The normalized film thickness h / λ of the metal film 14 to be dB was calculated. The result is shown in FIG. 7, and as shown in FIG.
35 °, 90 °, the propagation mode of the following theta is 67 ° in theta), normalized film thickness h / lambda of the metal film 14 is the following equation h / λ ≧ -7.75 + 0.43θ- 4.57 × 10 - ³ θ
^In the range of ² (the shaded range in FIG. 7), the propagation loss of the leaky surface acoustic wave is 0 dB, and a surface acoustic wave device using the leaky surface acoustic wave can be realized.

The present invention is not limited to the above embodiment, and various modifications are possible. For example, in the above-described embodiment, a lithium tetraborate single crystal substrate whose surface acoustic wave propagation direction is (135 °, 90 °, θ) in Eulerian angle display is used, but about 5 ° from the above Eulerian angle display. It may be shifted to the extent. In the above embodiment, a metal film made of gold is used. However, a metal thin film such as titanium, tungsten, molybdenum, or aluminum is formed on the surface of the piezoelectric substrate, and a metal made of an alloy containing gold as a main component is formed on the metal thin film. A film may be formed. The adhesion of the metal film to the piezoelectric substrate can be improved.

Further, in the above embodiment, the surface acoustic wave filter has been described as an example, but other surface acoustic wave devices may be used. For example, the present invention may be applied to a surface acoustic wave resonator in which a terminal electrode formed of an interdigital electrode is sandwiched between a pair of grating reflectors on a piezoelectric substrate. In the present invention, since the lithium tetraborate single crystal used for the piezoelectric substrate has a symmetry of a point group of 4 mm, various characteristics of the leaky surface acoustic wave have a predetermined symmetry in Euler angle display (λ, μ, θ). Show. For example, the characteristics of leaky surface acoustic waves at θ = 80 ° and θ = 100 ° in the Eulerian angle display (λ, μ, θ) are the same.

[0019]

As described above, according to the present invention, the metal film on the surface of the lithium tetraborate single crystal substrate is formed of a metal containing gold as a main component, and the film thickness is set to a predetermined thickness, whereby the elasticity is improved. The propagation loss can be reduced by changing the surface wave into a love wave, and a surface acoustic wave device using a leaky surface acoustic wave can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a surface acoustic wave device according to an embodiment of the present invention.

FIG. 2 is a graph showing measurement results of a propagation velocity and an insertion loss when a normalized thickness of a metal film is changed in a surface acoustic wave device according to one embodiment of the present invention.

FIG. 3 is a graph showing a simulation result of a propagation velocity when the normalized thickness of the metal film is changed in the surface acoustic wave device according to one embodiment of the present invention.

FIG. 4 is a graph showing a simulation result of an electromechanical coupling coefficient and a propagation velocity per wavelength when a normalized thickness of a metal film is changed in a surface acoustic wave device according to an embodiment of the present invention.

FIG. 5 is a graph showing a simulation result of a propagation speed when a standardized thickness of a metal film is changed in a comparative example in which a metal film is formed of aluminum.

FIG. 6 is a graph showing a simulation result of an electromechanical coupling coefficient and a propagation loss per wavelength when a normalized thickness of a metal film is changed in a comparative example in which a metal film is formed of aluminum.

[7] a graph showing the normalized film thickness range propagation loss <br/> loss is 0dB in the case of changing the propagation direction of the leaky surface acoustic wave in the surface acoustic wave device according to an embodiment of the present invention is there.

[Explanation of symbols]

 11 piezoelectric substrate 12 input comb electrode 13 output comb electrode 14 metal film

Claims (2)

(57) [Claims] 1. A surface acoustic wave device in which a metal film for exciting, receiving, reflecting, and propagating a surface acoustic wave is formed on a surface of a lithium tetraborate single crystal substrate. And the propagation direction of the surface acoustic wave is expressed by an Euler angle (130 ° to 140 °).
°, 85 ° to 95 °, 55 ° to 80 °), and the thickness of the metal film is such that the surface acoustic wave is applied to the surface of the lithium tetraborate single crystal substrate. A surface acoustic wave device having a predetermined value to be a propagating love wave. 2. The surface acoustic wave device according to claim 1, wherein the propagation direction of the leaky surface acoustic wave is represented by an Euler angle (130 ° to 130 °).
140 °, 85 ° to 95 °, θ), θ = 55 ° to
The metal film is formed so as to be within a range of 67 °, the metal film is formed of a metal containing gold as a main component, and the thickness h of the metal film is standardized by a wavelength λ of a surface acoustic wave. The thickness h / λ is calculated by the following equation: h / λ ≧ −7.75 + 0.43θ−4.57 × 10 ⁻³ θ
^2. A surface acoustic wave device having a range of ² .