JP3329115B2 - Surface wave device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a surface acoustic wave device, and more particularly to a surface acoustic wave device using a structure laminating a piezoelectric thin film on a piezoelectric substrate made of LiNbO ₃ piezoelectric single crystal.

[0002]

2. Description of the Related Art Conventionally, various surface acoustic wave devices, such as a surface acoustic wave resonator, a surface acoustic wave filter, and an elastic convolver, have been known as surface acoustic wave devices utilizing surface acoustic waves. An example of a conventional surface acoustic wave device will be described using an elastic convolver as an example.

[0003] An elastic convolver is a kind of signal processing device utilizing the nonlinear behavior of a piezoelectric material.
An arithmetic element that performs convolution integration of two input signals. As this elastic convolver, a structure incorporating a multistrip coupler (hereinafter abbreviated as MSC) is conventionally known. FIG. 6 shows an example of this known elastic convolver.

The elastic convolver 1 is formed by using a rectangular piezoelectric substrate 2 made of LiNbO ₃ piezoelectric single crystal as a piezoelectric body. Both end faces 2 of piezoelectric substrate 2
Input interdigital transducers (hereinafter abbreviated as IDTs) 3 are formed near a and 2b, respectively. Each input IDT 3 is constituted by a pair of comb-shaped electrodes having a plurality of electrode fingers interposed between each other.

At the center of the region between the input IDTs 3 and 3, a waveguide 4 extending parallel to the surface wave propagation direction is provided.
Are formed. An MSC 5 is formed between the waveguide 4 and each input IDT 3.

In the elastic convolver 1, when an input signal is applied to the input IDTs 3 and 3, the surface acoustic wave excited by the input signal is propagated along the directions of arrows A and B. Are respectively compressed in the MSC 5 and then superimposed in the waveguide 4 to extract the output signal.

[0007]

The performance of the convolver is as follows:
Generally, the efficiency F and the BT product (B indicates the bandwidth, T indicates the integration or the process time), and the efficiency F and the BT
Improvement of the product is desired.

Since the elastic convolver utilizes a surface acoustic wave, it is considered that the efficiency F can be increased by using the piezoelectric substrate 2 having a large electromechanical coupling coefficient and a large nonlinearity.

On the other hand, when an IDT is formed on a piezoelectric substrate made of a LiNbO ₃ piezoelectric single crystal to excite a surface acoustic wave, a ZnO piezoelectric thin film is further laminated on the surface of the piezoelectric substrate made of LiNbO _3, thereby increasing electric power. It is reported that a mechanical coupling coefficient can be obtained (A. Armstr.
ong. et al. Proc. 1972 IEEE
Ultrason. Symp. 370 pages to 372
Page (1972) and Nakamura et al .: Proceedings of the Acoustical Society of Japan,
October 1991, pages 953 to 954).

Therefore, in the elastic convolver 1 shown in FIG. 7, when a ZnO piezoelectric thin film is formed so as to cover the entire upper surface of the piezoelectric substrate 2, efficiency may be improved as compared with the conventional elastic convolver 1. I was assured.

[0011] However, even if the ZnO piezoelectric thin films are laminated as described above, the efficiency of the elastic convolver is not yet sufficiently high. Therefore, the appearance of an elastic convolver with higher efficiency is desired.

As described above, the elastic convolver is required to further improve the efficiency. Therefore, the appearance of a piezoelectric substrate having a larger electromechanical coupling coefficient is strongly desired. Similarly, in a surface acoustic wave device other than an elastic convolver, a surface acoustic wave substrate having a larger electromechanical coupling coefficient is similarly strongly demanded.

An object of the present invention is to provide a surface acoustic wave device using a surface acoustic wave substrate in which a piezoelectric thin film is formed on a piezoelectric substrate made of a LiNbO ₃ piezoelectric single crystal, and a larger electromechanical coupling coefficient can be obtained. An object of the present invention is to provide a surface acoustic wave device.

[0014]

Means for Solving the Problems The inventors of the present application have made intensive studies to achieve the above object, and as a result, have found that a specific LiNb
The present inventors have found that forming a specific piezoelectric thin film on a piezoelectric substrate made of O ₃ single crystal can effectively increase the electromechanical coupling coefficient, and have accomplished the present invention.

That is, the first invention of the present application provides a piezoelectric substrate made of a single crystal of LiNbO ₃ propagated in the Y-cut Z direction, and at least one I-type substrate formed on the + Y plane of the piezoelectric substrate.
DT, and are formed on the + Y surface of the piezoelectric substrate.
This is a surface wave device including a + surface piezoelectric thin film made of one of nO, Ta ₂ O ₅ and CdS.

According to a second aspect of the present invention, there is provided a piezoelectric substrate made of a LiNbO ₃ single crystal propagated in a Y-cut Z direction, and at least one IDT formed on a −Y plane of the piezoelectric substrate.
And on the -Y surface of the piezoelectric substrate, Zn
O, consisting of one of Ta ₂ O ₅ and CdS - and a plane piezoelectric film, a surface acoustic wave device.

As described above, the LiN
When a + plane or-plane piezoelectric thin film is formed on a + Y plane or -Y plane of a piezoelectric substrate made of a bO ₃ single crystal, the electromechanical coupling coefficient is effectively increased. Also, preferably,
Assuming that the thickness of the piezoelectric thin film is H and the wavelength of the propagating surface wave is λ, H / λ is in the range of 0.08 to 0.3.

The present invention is characterized by the piezoelectric substrate constituting the surface acoustic wave substrate and the piezoelectric thin film formed on the piezoelectric substrate as described above, and other configurations are not particularly limited. . That is, an appropriate number of IDTs may be formed on the piezoelectric substrate according to the intended surface acoustic wave device, and the structure of each IDT is not particularly limited. Further, the IDT is preferably formed between the piezoelectric substrate and the piezoelectric thin film, but may be formed on the piezoelectric thin film. That is, the IDT formed on the + Y or -Y surface of the piezoelectric substrate
May be formed directly on the + Y plane or the −Y plane, or may be formed via a piezoelectric thin film.

[0019]

As described above, the inventor of the present application has made LiNbO ₃
+ Z on the + Y plane or -Y plane of the piezoelectric substrate made of a single crystal
It has been found that the electromechanical coupling coefficient can be effectively increased in a surface wave substrate formed with a piezoelectric thin film having a surface or a −Z surface. As described above, the electromechanical coupling coefficient can be increased by using the + Y plane or the −Y plane of the piezoelectric substrate made of LiNbO ₃ single crystal and combining the piezoelectric thin film with the + Z plane or the −Z plane. Has been experimentally confirmed by the inventor of the present application.
+ Y plane and-of LiNbO ₃ single crystal propagating in cut Z direction
It has not been reported that the difference between the Y plane and the difference between the + Z plane and the −Z plane of the piezoelectric thin film affects the electromechanical coupling coefficient.

Preferably, in the present invention, the thickness of the piezoelectric thin film is set within the above specific range, whereby the electromechanical coupling coefficient is further increased.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be clarified by describing non-limiting embodiments of the present invention with reference to the drawings.

FIG. 1 is a plan view showing an elastic convolver according to one embodiment of the present invention. The elastic convolver 11 is configured using a piezoelectric substrate 12 having a square planar shape. The piezoelectric substrate 12 is composed of a Y-cut Z-direction propagating LiNbO ₃ single crystal substrate, but the surface shown, that is, the upper surface is a + Y surface.

Input IDTs 13 and 13 are formed near the end faces 12a and 12b of the piezoelectric substrate 12, respectively. Each of the input IDTs 13 includes a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween. In the center between the input IDTs 13, 13,
A waveguide 14 extending parallel to the surface wave propagation direction is formed. MS between the waveguide 14 and each input IDT 13
C15 and 15 are formed.

Further, on the upper surface of the piezoelectric substrate 12, ZnO piezoelectric thin films 16, 16 are coated so as to cover the portions where the input IDTs 13, 13 and the MSCs 15, 15 are formed. In this embodiment, this ZnO piezoelectric thin film
The upper surface is a + Z plane. In the present embodiment, although Y-cut Z-propagation LiNbO ₃ ZnO piezoelectric thin film on the + Y surface on the + Z surface of the single crystal substrate as described above is formed, on the contrary, Y-cut Z-propagation LiNbO ₃ single A -Z plane ZnO piezoelectric thin film may be formed on the -Y plane of the crystal substrate.

The thickness H of the ZnO piezoelectric thin films 16, 16 is such that when the wavelength of the surface wave to be excited is λ, H / λ is 0.1.
It is selected to be in the range of 08 to 0.3. Therefore, as is apparent from an experimental example described later, by forming the + Y-plane ZnO piezoelectric thin film 16 having a film thickness in the above specific range as compared with the case where the ZnO piezoelectric thin films are simply formed by lamination,
Further, the IDT 13 and the piezoelectric thin film 16 are formed on the + Y surface of the piezoelectric substrate.
The efficiency of the elastic convolver 11 is effectively increased by forming the above.

Further, in the elastic convolver 11 of this embodiment, the ZnO piezoelectric thin films 16
Since no coating is formed thereon, the propagation loss on the waveguide 14 is reduced. Therefore, not only the efficiency is improved as compared with the conventional elastic convolver 1, but also the bandwidth is not narrowed.

Next, in the above embodiment, each of the above structures is formed on the + Y plane of the piezoelectric substrate made of LiNbO ₃ single crystal propagated in the Y cut Z direction, and the + Z plane ZnO piezoelectric thin film 16 is formed.
The reason why the electromechanical coupling coefficient is increased by setting the thickness of the film to the above specific range will be described based on specific experimental examples.

As shown in FIG. 2, the propagation L in the Y cut Z direction
A first surface wave substrate 19 having a + Z plane ZnO piezoelectric thin film 18 formed on a + Y plane of a piezoelectric substrate 17 made of an iNbO ₃ piezoelectric single crystal was prepared. Further, the first surface acoustic wave substrate 19
In addition, a second surface wave substrate in which a ZnO piezoelectric thin film having a -Z plane was formed on a + Y plane of a piezoelectric substrate composed of a YN-cut Z-direction propagation LiNbO ₃ piezoelectric single crystal was prepared. IDT is Z
The structure exists at the boundary between the nO piezoelectric film and the LN substrate.

The electromechanical coupling coefficient ks is obtained from the difference ΔV between the phase speed Vf when a certain boundary of the IDT is electrically open and the phase speed Vm when the boundary is short-circuited.

[0030]

(Equation 1)

[0031] FIG. 3 shows the phase velocity of the Rayleigh wave propagating in the Z direction on the first surface acoustic wave substrate. In FIG. 3, the broken line indicates Vf and the solid line indicates Vm. At the time of the above measurement, calculation was performed from the center frequency of a SAW filter having 12.5 pairs of normal IDTs.

FIG. 4 shows the phase velocity of the Z-direction propagating Rayleigh wave when the second surface acoustic wave substrate is used. Next, FIG. 5 shows the relationship between the electromechanical coupling coefficient of the first and second surface wave substrates obtained from the above equation (1) and the value H / λ standardized by the wavelength of the surface wave of the ZnO piezoelectric thin film. Show. In FIG. 5, the solid line A indicates the characteristic on the first surface acoustic wave substrate, and the broken line B indicates the characteristic on the second surface acoustic wave substrate.

As can be seen from FIG. 5, a larger electromechanical coupling coefficient ks can be obtained when the first surface acoustic wave substrate is used than when the second surface acoustic wave substrate is used. . That is, as is apparent from FIG. 5, the electromechanical coupling coefficient ks = 0.0 when no ZnO piezoelectric thin film is formed.
22 to, when the electromechanical coupling coefficient ks with the first surface acoustic wave substrate is at most 0.32, the electromechanical coupling coefficient ks is 1.43 times, be increased to 2.04 times the ks ² Understand.

As described above, the Y-cut Z direction propagation LiN
Experiments show that the electromechanical coupling coefficient can be effectively increased when the first surface wave substrate 19 formed by forming a + Z plane ZnO piezoelectric thin film on the + Y plane of a piezoelectric substrate made of a bO ₃ piezoelectric single crystal is used. I was assured. Further, according to the present inventor's experiments, Y cut Z-propagation LiNbO ₃ piezoelectric single crystal made of a piezoelectric substrate -Y plane, even in the case of using a surface wave substrate with the -Z plane ZnO piezoelectric thin film It has been confirmed that the electromechanical coupling coefficient can be effectively increased as in the above embodiment.

Accordingly, LiNbO ₃ propagating in the Y-cut Z-direction
By using a surface wave substrate in which a + Z-plane or -Z-plane ZnO piezoelectric thin film is formed on a + Y-plane or a -Y-plane of a piezoelectric substrate made of a piezoelectric single crystal, for example, when used in an elastic convolver, The efficiency of the stick convolver can be effectively increased. Also, in a surface acoustic wave device other than the elastic convolver, sufficient resonance characteristics and filter characteristics can be obtained.

The above-mentioned IEEE Ultraso
n. Symp. And the ASJ Lecture Papers describe the Y-cut Z-direction propagation, LiNbO ₃ substrate and Z
Although the + and-planes were not considered for either of the nO thin films, the maximum value of Ks shown in both figures is kh = 1.
0 (corresponding to 0.16 when converted to H / λ),
These figures show the + Y plane LiNbO ₃ and -Z in the present invention.
It is pointed out that this corresponds to a combination with a plane ZnO thin film, which is different from the scope of the present invention.

[0037]

As described above, according to the present invention, the specific piezoelectric thin film having the + Z plane or the -Z plane is provided on the + Y plane or the -Y plane of the piezoelectric substrate made of the specific LiNbO ₃ single crystal. Since it is formed, the electromechanical coupling coefficient is effectively increased, and therefore, it is possible to provide a surface acoustic wave device having excellent characteristics such as efficiency.

[Brief description of the drawings]

FIG. 1 is a plan view showing an elastic convolver as a surface acoustic wave device according to one embodiment of the present invention.

FIG. 2 is a partially cutaway sectional view for explaining a surface acoustic wave substrate.

FIG. 3 is a diagram showing a phase velocity of a Rayleigh wave propagating in the Z direction on a first surface acoustic wave substrate.

FIG. 4 is a diagram showing the phase velocity of a Rayleigh wave propagating in the Z direction on a second surface acoustic wave substrate.

FIG. 5 is a graph showing the relationship between the electromechanical coupling coefficient of the first and second surface wave substrates and the value H / standardized by the wavelength of the surface wave of the ZnO piezoelectric thin film.
The figure which shows the relationship with (lambda).

FIG. 6 is a plan view showing an elastic convolver as an example of a conventional surface acoustic wave device.

[Explanation of symbols]

11 Elastic convolver as surface wave device 12 Piezoelectric substrate made of Y-cut Z-direction propagating LiNbO ₃ single crystal substrate 13, 13 Input IDT 16 Piezoelectric thin film 17 Piezoelectric substrate 18 Piezoelectric thin film 19 Surface acoustic wave substrate

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-260882 (JP, A) JP-A-63-10210 (JP, A) JP-A-4-258011 (JP, A) JP-A-6-260 268469 (JP, A) PHILIPPE DEFRANOULD, CHARLES MAERFEL D, A SAW Planar Piezoelectric Electric Convolver, PROCEEDINGS OF THE IEEE, May 1976, VOL. 64, NO. 5, p. 748-751 (58) Field surveyed (Int.Cl. ⁷ , DB name) H03H 9/25 H03H 9/145 H03H 9/72

Claims (3)

(57) [Claims] 1. A piezoelectric substrate made of a Y-cut Z-direction propagating LiNbO ₃ single crystal, at least one interdigital transducer formed on a + Y surface of the piezoelectric substrate, and formed on a + Y surface of the piezoelectric substrate. Yes, ZnO, T
a surface wave device comprising: a + surface piezoelectric thin film made of one of a ₂ O ₅ and CdS. 2. A piezoelectric substrate made of a Y-cut Z-direction propagating LiNbO ₃ single crystal, at least one interdigital transducer formed on a −Y surface of the piezoelectric substrate, and formed on a −Y surface of the piezoelectric substrate. And ZnO, T
A surface acoustic wave device comprising: a surface piezoelectric thin film comprising one of a ₂ O ₅ and CdS. 3. Assuming that the thickness 5 of the piezoelectric thin film is H and the wavelength of a propagating surface wave is λ, H / λ is 0.08-0.
3. The surface acoustic wave device according to claim 1, wherein the range is 3.