JPH0998058A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain a high acoustic velocity by using a surface acoustic wave device that has a specific surface directivity and a specific propagation directivity. SOLUTION: When the surface directivity and the surface acoustic wave propagation directivity of a monocrystal diamond layer are shown in Euler angles (0 deg., 0 deg., ψ) and also limited in a range substantially equal to these Euler angles, ψ is set in a range shown in the equation. Under such condition, a diamond layer 6 having its surface directivity (θ=0 deg., θ=0 deg.) is formed on the (100) surface of a silicone substrate 5. Then a piezoelectric film 7 of AIN(aluminum nitride) is formed on the surface of the layer 6. Furthermore, an interdigital electrode is provided on the surface of the film 7 together with interdigital reflectors placed at both sides of the interdigital electrode respectively. These electrode and reflectors have the line width and line space of 1.0μm and are set in the directions where the propagation directivity ψ of a surface acoustic wave is set at 45 deg..

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 single crystal diamond layer and a piezoelectric film laminated, and an electrode for propagating a surface acoustic wave formed on the front surface or the back surface of the piezoelectric film. It is a thing.

[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, one or several layers of piezoelectric film such as zinc oxide (ZnO) and aluminum nitride (AlN) are formed on the surface of a substrate such as sapphire or magnesium oxide, which has a higher acoustic velocity than lithium tantalate or lithium niobate. A surface acoustic wave element that excites a surface acoustic wave by means of this piezoelectric film has been studied. Further, in order to obtain a higher sound velocity, a surface acoustic wave device formed by laminating a diamond layer and a piezoelectric film has been studied (for example, WO89 / 08949).

[0006]

However, in the surface acoustic wave device formed by laminating the single crystal diamond layer and the piezoelectric film, the optimum surface orientation and surface acoustic wave propagation direction that can obtain the highest sound velocity are , Still not enough research done. An object of the present invention is to find a surface orientation and a surface acoustic wave propagation direction that can obtain the highest sound velocity in a surface acoustic wave device formed by laminating a single crystal diamond layer and a piezoelectric film, and by using this, an acoustic wave corresponding to a high frequency can be obtained. It is to provide a surface acoustic wave element.

[0007]

It is generally known that in a single crystal substrate, the sound velocity changes depending on the plane orientation or the surface acoustic wave propagation direction, and even the single crystal diamond layer has the highest sound velocity. It is considered that there is a plane orientation with. Therefore, in the present invention, the plane orientation of the single crystal diamond layer and the surface acoustic wave propagation direction are variously changed, the respective propagation velocities are theoretically calculated, and the optimum plane orientation and surface acoustic wave propagation are based on the results. A surface acoustic wave device having a direction is completed.

That is, in the surface acoustic wave device according to the present invention, the plane orientation of the single crystal diamond layer and the surface acoustic wave propagation direction are expressed as Euler angles (0 °, 0 °, ψ) and substantially equivalent thereto. In this range, ψ is set in the range of the following mathematical expression 4.

## EQU4 ## 36 ° + 90 ° × n ≦ ψ ≦ 54 ° + 90 ° × n where n is an integer (0, 1, 2, 3, ...).

Further, in the specific configuration, ψ is set to the value of the following expression 5.

## EQU5 ## ψ = 45 ° + 90 ° × n where n is an integer (0, 1, 2, 3, ...).

[0010]

According to the surface acoustic wave element having the surface orientation and the propagation direction expressed by the above mathematical expression 4, a higher sound velocity than that of the conventional surface acoustic wave element can be obtained. Further, according to the surface acoustic wave element having the surface orientation and the propagation direction expressed by the equation 5, the highest sound velocity can be obtained. Therefore, according to the present invention, it is possible to realize a surface acoustic wave device that can be used in a higher frequency band by using the conventional electrode fine processing technique and film forming technique.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be specifically described below with reference to the drawings. First, with reference to FIG. 10, Euler angles (φ, θ, ψ) for specifying the surface orientation and the surface acoustic wave propagation direction will be described. As shown in the figure, when the crystal axes of the single crystal diamond are the X axis, the Y axis, and the Z axis, the Z axis is the rotation axis and the X axis is the Y axis side.
Only this is made to be the first axis. Next, the Z axis is rotated counterclockwise by the angle θ with the first axis as the rotation axis, and this is set as the second axis. A diamond layer is grown by using the second axis as a normal line and growing in a plane orientation including the first axis. And
In the diamond layer grown in the plane orientation, an axis obtained by rotating the first axis counterclockwise about the second axis by an angle ψ is a third axis, and the third axis is a surface acoustic wave propagation direction. . At this time, the plane orientation and the surface acoustic wave propagation direction are expressed as Euler angles (φ, θ, ψ).

FIG. 1 shows the structure of a surface acoustic wave resonator embodying the present invention. Silicon substrate as shown
In the (100) plane of (5), the plane orientation of the surface is φ = 0 °, θ = 0
Forming a diamond layer (6) whose surface is
A piezoelectric film (7) made of aluminum nitride (AlN) is formed. Regarding the plane orientation and the propagation direction of the diamond layer (6), Euler angle notation (0 °, 0 °, ψ) means ψ with respect to the X axis in the plane represented by Miller index notation (001). This means that the surface acoustic wave is propagated in the direction inclined by only. Further, since the (100) plane of the silicon substrate (5) is equivalent to the (001) plane of the substrate, the silicon substrate (5) and the diamond layer (6) in this embodiment have the same plane orientation. Can be said to be doing.

Further, as shown in FIG. 2, comb electrodes (8) and (9) are arranged on the surface of the piezoelectric film (7), and comb reflectors (10) and (11) are arranged on both sides thereof. There is. These electrodes and reflectors have a line width and a line spacing of 1.0 μm, and are arranged so that the propagation direction ψ of the surface acoustic wave is 45 °.

The diamond layer (6) is formed by chemical vapor deposition (C
VD) technology or epitaxial growth technology,
The crystal is grown in a plane orientation of φ = 0 ° and θ = 0 °, and then polished to form a thickness of 2 to 10 μm. Also, piezoelectric film (7)
Is formed by using an ECR dual ion beam sputtering apparatus to a film thickness of 0.2 to 3 μm. Further, the electrodes (8) (9) and the reflectors (10) (11) are patterned by photolithography after forming an aluminum film having a thickness of 0.2 μm using the same device as the piezoelectric film (7). , Comb shape.

Next, the results of the simulations shown in FIGS. 3 to 7 will be described. Here, for the calculation of the speed of sound, a generally known general solution method (for example, JJCampbel
l, WRJones, "A Method for Estimating Optimal Crys
tal Cuts and Propagation Directions for Excitation
of Piezoelectric Surface Waves ", IEEE transaction
on Sonics and Ultrasonics, vol.SU-15, No.4, pp209
-217, (1968)), the optimum values of the surface orientation and the surface acoustic wave propagation direction were obtained. However, in the calculation of sound velocity,
The existence of the piezoelectric film and the silicon substrate was ignored, and the surface of the diamond layer was assumed to be a free surface. The validity of this assumption will be described later.

FIG. 3 shows the sound velocity of the diamond layer of the present invention having a plane orientation of φ = 0 ° and θ = 0 ° as a function of the propagation direction ψ. As shown, 36 ° ≤ ψ
A sound velocity exceeding 11100 m / s is obtained in the range of ≤54 °, and the maximum sound velocity is 11120 m / s at ψ = 45 ° and 135 °.

In addition, FIGS. 4 to 7 show the sound velocity as a function of the propagation direction ψ when φ is maintained at 0 ° and θ is changed to 10 °, 20 °, 30 ° and 45 °. It is a representation.

As is apparent from FIGS. 3 to 7, the value of ψ at which the sound velocity peaks gradually deviates from 45 ° and 135 ° in FIG. 3 as θ increases from 0 °. However, the peak value of the sound velocity is the largest when θ in FIG. 3 is 0 °, and the peak value decreases when θ is larger than 0 °. Therefore, it can be said that 0 ° is the optimum value for θ. Also, from the result of the same simulation (not shown) in which φ was changed from 0 ° to 45 °, it was found that 0 ° was also the optimum value for φ.

From the simulation results of FIG. 3 in which φ and θ are set to the optimum values of 0 °, in general, in the single crystal diamond layer, the plane orientation and the propagation direction are represented by Euler angles (0 °, 0). It can be said that the sound velocity can be maximized by setting the range of (°, 36 to 54 °), more preferably (0 °, 0 °, 45 °). Note that the sound velocity in the diamond layer has a periodicity of 90 ° with respect to the propagation direction ψ, as is clear from the crystal structure of diamond and further as confirmed by the graph of FIG. The optimum value of the direction ψ can be represented by the range of the above-mentioned formula 4 or formula 5.

Next, the result of the simulation considering the existence of the piezoelectric film in the calculation of the sound velocity will be described. 8 and 9 show the sound velocity as a function of the propagation direction ψ when an aluminum nitride thin film having a (001) plane is formed as a piezoelectric film on the diamond layer of the present invention. If the KH parameter is 0.4,
Is when the KH parameter is 0.6. Where KH
The parameter is given by the product of the wave number K (K = 2π / λ) of the surface acoustic wave and the thickness H (μm) of the piezoelectric film.

As is clear from FIGS. 8 and 9, the peak value of the sound velocity decreases as the thickness of the piezoelectric film increases, but the optimum values of ψ for obtaining the peak value are 45 ° and 135 °. That is, it is the same as the optimum value when the piezoelectric film shown in FIG. 3 is ignored. From this, it can be said that the influence of the piezoelectric film appears on the sound velocity on the vertical axis of the graph, but has no effect on the propagation direction ψ on the horizontal axis. Therefore, in the simulation for obtaining the optimum value of ψ, the piezoelectric film can be ignored, and the validity of the simulation results shown in FIGS. 3 to 7 is supported.

Although the silicon substrate is arranged on the back surface of the diamond layer, as described above, the silicon substrate and the diamond layer have the same plane orientation. Is considered to have little effect on.

From the above discussion, generally, in a surface acoustic wave device in which a single crystal diamond layer and a piezoelectric film are laminated on a substrate, the plane orientation and the propagation direction of the single crystal diamond layer are represented by Euler angles (0 It can be said that the speed of sound can be maximized by setting the range of (°, 0 °, 36 to 54 °), more preferably (0 °, 0 °, 45 °).
Further, in both of FIG. 8 and FIG.
Since it appears in a cycle of 0 °, the optimum value of the propagation direction ψ can be expressed by the range of the above-mentioned formula 4 or formula 5.

Incidentally, (0 °, 90 °, ψ), (90 °, 0
When the same simulation was performed for diamond layers having plane orientations of (°, ψ) and (90 °, 90 °, ψ), the same results as in FIG. 3 were obtained. When these plane orientations are represented by the Miller index display, the (010) plane and the (00
1) plane and (100) plane, which are the same as or equivalent to the (001) plane. Therefore, the same effect can be obtained by growing the diamond layer in these plane directions.

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 reducing the scope.
Further, the configuration of each part of the present invention is not limited to the above embodiment, and various modifications can be made within the technical scope described in the claims. For example, although a silicon substrate is used as the substrate in the above embodiment, a GaAs substrate can also be used. Moreover, although a piezoelectric film made of AlN as a piezoelectric film, ZnO, LiTaO _3, LiNbO _3, or it is also possible to use a piezoelectric film made of Li ₂ B ₄ O ₇ and the like.

Needless to say, the present invention is not limited to the surface acoustic wave resonators shown in FIGS. 1 and 2, but can be applied to surface acoustic wave devices of any structure such as filters, delay lines, convolvers and the like.

[Brief description of drawings]

FIG. 1 is a perspective view showing a structure of a surface acoustic wave resonator for implementing the present invention.

FIG. 2 is a plan view showing a pattern of comb electrodes and a reflector.

FIG. 3 is a graph showing the sound velocity in the surface acoustic wave device of the present invention as a function of the propagation direction ψ.

FIG. 4 is a graph showing the sound velocity of a diamond layer with the surface orientation φ = 0 ° and θ = 10 ° as a function of the propagation direction ψ.

FIG. 5 is a graph of the same as above, where the plane orientation of the surface is φ = 0 ° and θ = 20 °.

FIG. 6 is a graph of the same as above, where the plane orientation of the surface is φ = 0 ° and θ = 30 °.

FIG. 7 is a graph of the same as above, where the plane orientation of the surface is φ = 0 ° and θ = 45 °.

FIG. 8 shows the sound velocity in the surface acoustic wave device of the present invention, in which the KH parameter of the piezoelectric film is 0.4 and the propagation direction ψ.
Is a graph represented as a function of.

FIG. 9 is a graph showing the sound velocity in the surface acoustic wave device of the present invention when the KH parameter of the piezoelectric film is set to 0.6.

FIG. 10 is a diagram illustrating Euler angle display.

[Explanation of symbols]

 (5) Silicon substrate (6) Diamond layer (7) Piezoelectric film (8) Comb-shaped electrode (9) Comb-shaped electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Shibata 2-5-5 Keihan Hondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd.

Claims (2)

[Claims] 1. A surface acoustic wave device comprising a single crystal diamond layer and a piezoelectric film laminated, and an electrode for propagating a surface acoustic wave formed on the front surface or the back surface of the piezoelectric film. When the plane orientation of the layer surface and the surface acoustic wave propagation direction are set to (0 °, 0 °, ψ) in Euler angles and a range substantially equivalent thereto, ψ is set to the range of the following mathematical formula 1 Surface wave element. ## EQU1 ## 36 ° + 90 ° × n ≦ ψ ≦ 54 ° + 90 ° × n where n is an integer (0, 1, 2, 3, ...) 2. The value of ψ is set to the value of the following expression 2.
The surface acoustic wave device described in 1. ## EQU2 ## ψ = 45 ° + 90 ° × n where n is an integer (0, 1, 2, 3, ...)