JPH08316779A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE: To obtain the surface acoustic wave element having provision for high frequencies by selecting a cut face of a single crystal sapphire substrate and a surface acoustic wave propagation direction to be a prescribed angle in Euler angle representation and a range substantially equivalent thereto so as to find out the cut face and the surface acoustic wave propagation direction at which a higher sound velocity than that of a conventional element. CONSTITUTION: A piezoelectric film 6 made of an AlN is formed to a cut face of a sapphire substrate 5 cut with ϕ=0 deg. and θ=50 deg. in the surface acoustic wave resonator. Interdigital electrodes 7, 8 are arranged to the surface of the piezoelectric film 6 and interdigital reflectors 9, 10 are arranged to both sides of them. The electrodes 7, 8 and the reflectors 9, 10 are arranged so that the line width and the line interval are both 1.0μm and the propagation direction ϕof a surface acoustic wave is 90 deg.. The piezoelectric film 6 is formed by sputtering and the film thickness is selected to be 4000Å. Furthermore, the electrodes 7, 8 and the reflectors 9, 10 are patterned by forming an Al film of 2000Å with a sputtering device to be formed interdigitally.

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 having a piezoelectric film formed on a single crystal sapphire substrate.

[0002]

2. Description of the Related Art In communication equipment such as portable telephones,
Surface acoustic wave devices are used as circuit devices such as resonator filters and signal processing delay lines. The surface acoustic wave element is one in which, for example, a comb-shaped electrode or a reflector is formed on the surface of a substrate having piezoelectricity, and an electric signal and a surface acoustic wave are mutually converted. Generally, the piezoelectric substrate of the surface acoustic wave device 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, there is an increasing need for a surface acoustic wave element that can be used in the gigahertz band. Center frequency f _{0 of the} surface acoustic wave element
Is expressed by the following equation in the relationship between the propagation velocity V of the surface acoustic wave and the wavelength λ.

F ₀ = V / λ where the wavelength λ is determined by the pitch d of the electrodes (λ
= 4d). Therefore, in order to cope with the higher frequency of the surface acoustic wave element, a method of using a material having a high sound velocity (surface acoustic wave propagation velocity) or a method of narrowing the electrode pitch can be considered.

Conventionally, lithium tantalate (LiTaO ₃ ) or lithium niobate (LiNbO ₃ ) has been used as a substrate material for surface acoustic wave devices. In the 36 ° Y-cut X propagation of the lithium tantalate substrate, the sound velocity is 41
The sound velocity is 4419 m / s in 32 m / s and 64 ° Y-cut X propagation of the lithium niobate substrate. The limit of patterning technology at the current mass production level is the electrode pitch of 0.6.
.mu.m, the center frequencies of the surface acoustic wave devices using these substrates have upper limits of 1.72 GHz and 1.84 GHz, respectively.

In order to meet the recent demands for higher frequency of surface acoustic wave devices, it is indispensable to develop a substrate with higher sound velocity. Therefore, studies have been conducted on surface acoustic wave devices using sapphire as a substrate, which has a higher acoustic velocity than lithium tantalate or lithium niobate. Since sapphire itself does not have piezoelectricity, one or several piezoelectric thin films such as zinc oxide (ZnO) and aluminum nitride (AlN) are formed on the substrate surface, and surface acoustic waves are excited by this piezoelectric thin film. . At present, the cut planes of single crystal sapphire used as the substrate of the surface acoustic wave device are mainly the C plane shown in FIG. 9 and the R plane shown in FIG.

[0006]

However, even in the conventional surface acoustic wave device using the sapphire substrate, the C
The maximum sound velocity on the surface and R surface is 5700 m / s and 5 respectively.
It is 880 m / s (see FIGS. 3 and 4), and there is a possibility that a cut surface having a higher sound velocity exists. Such an optimum cut surface and surface acoustic wave propagation direction have not been sufficiently studied. An object of the present invention is, in a surface acoustic wave element using a single crystal sapphire substrate, to find a cut surface and a surface acoustic wave propagation direction that can obtain a higher acoustic velocity than conventional ones, and thereby to detect a high frequency surface acoustic wave element. Is to provide.

[0007]

It is generally known that in a single crystal substrate, the speed of sound changes depending on the cut surface or the surface acoustic wave propagation direction, and also in the sapphire substrate, the C surface and the R surface. It is considered that there is a plane orientation with a higher sonic velocity than that. Therefore, in the present invention, the cut surface of the sapphire substrate and the surface acoustic wave propagation direction are variously changed,
The propagation velocity of each is theoretically calculated, and based on the result, the surface acoustic wave device having the optimum cut surface and the surface acoustic wave propagation direction is completed.

In the calculation of the surface acoustic wave propagation velocity, a generally known general solution method (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)) was adopted, and the optimum values of the cut surface and the surface acoustic wave propagation direction were obtained by computer simulation. Regarding the optimum cut surface and the surface acoustic wave propagation direction, when a surface acoustic wave element was actually prototyped and the characteristics thereof were actually measured, a measured value that coincided with the simulation result was obtained. This supports the validity of computer simulation.

In the surface acoustic wave device according to the present invention, the cut surface of the single crystal sapphire substrate and the surface acoustic wave propagation direction are (0 °, θ, ψ) in the Euler angle display and a range substantially equivalent thereto. At this time, θ and ψ are set in the range of the following Expression 4.

32 ° + 90 ° × m ≦ θ ≦ 72 ° + 90 ° × m 63 ° + 180 ° × n ≦ ψ ≦ 117 ° + 180 ° × n where m and n are integers (0, 1, 2, 3, 3) …)

Further, in the concrete construction, θ and ψ are set to the values of the following mathematical expression 5.

## EQU5 ## θ = 50 ° + 90 ° × m ψ = 90 ° + 180 ° × n where m and n are integers (0, 1, 2, 3, ...).

[0011]

According to the surface acoustic wave element having the cut surface and the propagation direction represented by the above-mentioned equation 4, a sound velocity exceeding the maximum value (5880 m / s) of the sound velocity on the conventional R surface can be obtained. Further, according to the surface acoustic wave element having the cut surface and the propagation direction expressed by the above-mentioned equation 5, the maximum sound velocity (60
60 m / s) will be obtained. Therefore, according to the present invention, it is possible to realize a surface acoustic wave element that can be used in a higher frequency band by using the conventional electrode fine processing technology and the piezoelectric material.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. First, based on FIG. 6, the Euler angles (φ, θ, for identifying the cut surface and the surface acoustic wave propagation direction)
ψ) will be described. As shown in the figure, when the crystal axes of the sapphire single crystal are the a ₁ , a ₂ and c axes, the c 1 axis is rotated about the a ₁ axis toward the a ₂ axis, and the One axis. Next, with the first axis as the rotation axis, the c-axis is rotated counterclockwise by the angle θ, and this is designated as the second axis.
A substrate is obtained by cutting the second axis as a normal line in a plane orientation including the first axis. Then, in the substrate cut in the plane orientation, an axis obtained by rotating the first axis counterclockwise by an angle ψ about the second axis is set as a third axis, and the third axis is set as a surface acoustic wave propagation direction. . At this time, the cut surface and the surface acoustic wave propagation direction are represented by Euler angles (φ, θ, ψ).

Next, the simulation result will be described. In the calculation of the sound velocity, the existence of the piezoelectric film formed on the surface of the sapphire substrate was ignored, and it was assumed that the surface of the sapphire substrate was a free surface. The validity of this assumption will be described later.

FIG. 1 shows the sound velocity of the sapphire substrate of the present invention cut at φ = 0 ° and θ = 50 ° as a function of the propagation direction ψ. The sound velocity fluctuates with a cycle of 180 °, and the maximum sound velocity is 6060 m / s at ψ = −90 ° and 90 °. On the other hand, FIG. 3 and FIG. 4 respectively show the sound velocity in the conventional sapphire C plane and R plane as a function of the propagation direction ψ. The maximum values of sound velocity on the C and R surfaces are 5700 m / s and 5 respectively.
It is 880 m / s.

From the comparison between the present invention (FIG. 1) and the conventional one (FIGS. 3 and 4), in the sapphire substrate having the cut surface of the present invention, the maximum value of the R surface is 58 in the range of 63 ° ≦ ψ ≦ 117 °.
It can be seen that a sound velocity higher than 80 m / s can be obtained.
As is clear from FIG. 1, since the sound velocity on the cut surface of the present invention has a periodicity of 180 ° with respect to the propagation direction ψ, the optimum value of ψ should be represented by the range of the above formulas 4 or 5. Can be done.

FIG. 2 shows the condition of φ = 0 ° and ψ = 90 °.
The speed of sound is expressed as a function of θ. Speed of sound is 90
Fluctuates with a cycle of °, and θ = -130 °, -4
Maximum speed of sound is 6060m at 0 °, 50 ° and 140 °
/ S.

Further, it can be seen that a sound velocity larger than the maximum value of 5880 m / s on the R surface can be obtained within the range of 32 ° ≦ θ ≦ 72 °. As is clear from FIG. 2, the sound velocity is φ = 0.
Under the condition of ° and ψ = 90 °, since it has a periodicity of 90 ° with respect to θ, the optimum value of θ can be expressed by the range of the above-mentioned formula 4 or formula 5.

Finally, an embodiment in which the present invention is applied to a surface acoustic wave resonator will be described. As shown in FIG. 7, the surface acoustic wave resonator has aluminum nitride (AlN) on the cut surface of the sapphire substrate (5) cut at φ = 0 ° and θ = 50 °.
To form a piezoelectric film (6). As shown in FIG. 8, comb electrodes (7) and (8) are arranged on the surface of the piezoelectric film (6), and comb reflectors (9) and (10) are arranged on both sides thereof.
These electrodes and reflectors have a line width and line spacing of 1.0.
The surface acoustic wave is arranged so that the propagation direction ψ of the surface acoustic wave is 90 °.

The piezoelectric film (6) was formed by using an ECR dual ion beam sputtering apparatus, and the film thickness was 4000 Å in this embodiment. Also, the electrodes (7) (8) and the reflector
(9) (10) is 2000 Å using the same device as the piezoelectric film (6)
After the aluminum film was formed, it was patterned by photolithography and formed into a comb shape.

For comparison between the present invention and the prior art, a conventional surface acoustic wave resonator having an aluminum nitride piezoelectric film and a comb-shaped electrode having the same structure as described above formed on the sapphire C surface was prepared, and the pass characteristics of both were made. Actually measured.

FIG. 5 shows the pass characteristics of the surface acoustic wave resonator using the substrate of the present invention and the pass characteristics of the conventional surface acoustic wave resonator. The surface acoustic wave resonator of the present invention is
The center frequency is 1.56 GHz, and the center frequency of the conventional surface acoustic wave resonator using the sapphire C plane is 1.
It is higher than 43 GHz, and it can be seen that it can be used in a higher frequency band.

From the results shown in FIGS. 1 and 2, (0 °,
50 °, 90 °) cut sapphire substrate has a sound velocity of 60
It is 60 m / s, and the center frequency is 1.5 from this value.
Although it is 2 GHz, it can be seen that there is no large error compared with the actually measured value. This supports the validity of the above computer simulation ignoring the existence of the piezoelectric film.

Further, in the theoretical calculation by computer simulation, even if there is some error due to modeling of the surface acoustic wave element, the error is hardly generated in the horizontal axis direction of the graphs of FIGS. 1 to 4. Conceivable. However, when comparing FIG. 1 of the present invention with FIGS. 3 and 4 of the related art, it can be said that there is no influence on the above-mentioned conclusion regarding the angle range, because both have the same magnitude of error.

The above description of the embodiments is for explaining the present invention, and should not be construed as limiting the invention described in the claims or limiting the scope. Further, the configuration of each part of the present invention is not limited to the above embodiment, but various modifications can be made within the technical scope described in the claims. For example, it goes without saying that the present invention is not limited to the surface acoustic wave resonators shown in FIGS. 7 and 8 and can be implemented in surface acoustic wave elements of any structure such as filters and delay lines.

[Brief description of drawings]

FIG. 1 is a graph showing a sound velocity of a sapphire substrate in a surface acoustic wave device of the present invention as a function of a propagation direction ψ.

FIG. 2 is a graph showing the sound velocity of a sapphire substrate in the surface acoustic wave device of the present invention as a function of plane orientation θ.

FIG. 3 is a graph showing the speed of sound on the C-plane of sapphire as a function of the propagation direction ψ.

FIG. 4 is a graph showing the sound velocity on the sapphire R surface as a function of the propagation direction ψ.

FIG. 5 is a characteristic of the surface acoustic wave resonator according to the present invention,
It is a graph showing the passage characteristic of the conventional surface acoustic wave resonator.

FIG. 6 is a diagram illustrating Euler angle display.

FIG. 7 is a perspective view showing a structure of a surface acoustic wave resonator for implementing the present invention.

FIG. 8 is a plan view showing patterns of comb-shaped electrodes and reflectors.

FIG. 9 is a perspective view showing a sapphire C surface.

FIG. 10 is a perspective view showing a sapphire R surface.

[Explanation of symbols]

 (5) Sapphire substrate (6) Piezoelectric film (7) Comb-shaped electrode (8) Comb-shaped electrode

Front page continuation (72) Kenichi Shibata 2-5-5 Keihan Hondori, Moriguchi City, Osaka Prefecture Sanyo Electric Co., Ltd. (72) Inventor Taizo Kobayashi 2-5-5 Keihan Hondori, Moriguchi City, Osaka Prefecture Sanyo Electric Co., Ltd.

Claims (2)

[Claims] 1. In a surface acoustic wave device having a piezoelectric film formed on a single crystal sapphire substrate, the cut surface of the single crystal sapphire substrate and the surface acoustic wave propagation direction are expressed as (0 °, θ , Ψ) and a range substantially equivalent thereto, a surface acoustic wave device in which θ and ψ are set in the range of the following mathematical expression 1. 32 ° + 90 ° × m ≦ θ ≦ 72 ° + 90 ° × m 63 ° + 180 ° × n ≦ ψ ≦ 117 ° + 180 ° × n where m and n are integers (0, 1, 2, 3, …) 2. The surface acoustic wave device according to claim 1, wherein θ and ψ are set to values of the following mathematical expression 2. ## EQU2 ## θ = 50 ° + 90 ° × m ψ = 90 ° + 180 ° × n where m and n are integers (0, 1, 2, 3, ...)