JPH09167935A - Surface acoustic wave converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To impart a unidirectionality of an arbitrary direction to an interdigital electrode. SOLUTION: In a distributed type interdigital electrode which is composed by alternately arranging positive and negative electrodes 1 and 2 on a piezoelectric or electrostrictive substrate and in which the widths of the electrodes and the period of the arrangement are gradually changed toward the propagation direction, the structure is made a single electrode structure, the positive and negative electrodes 1 and 2 are buried in a substrate 3 and the third electrodes 5 which are not connected with both of these positive and negative electrodes, are mutually connected and are different from the positive and negative electrodes 1 and 2 in film thickness are provided on the gap part between the positive and negative electrodes.

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 converter having a distributed interdigital transducer in which the width and arrangement period of positive and negative electrodes gradually change in the propagation direction of surface acoustic waves. The present invention relates to a surface acoustic wave converter that can have directivity.

[0002]

2. Description of the Related Art A surface acoustic wave converter of this type is used, for example, for constructing a wideband surface acoustic wave convolver or surface acoustic wave filter.

The present inventors have firstly provided first and second interdigital electrodes for exciting surface acoustic waves on a piezoelectric substrate (including a piezoelectric thin film) or an electrostrictive substrate. An output electrode for detecting a surface wave and extracting a convolution output as an electric signal, wherein the first and second interdigital electrodes have a predetermined thickness, and the first interdigital electrode has a predetermined thickness. The electrodes are formed by alternately arranging positive and negative electrodes whose electrode width and period gradually become shorter toward the output electrode, and wherein the second IDT electrode gradually increases in electrode width and period toward the output electrode. A surface acoustic wave convolver in which positive and negative electrodes are alternately arranged has been proposed (JP-A-4-331505). The structure of the first IDT is called a down-chirp type, and the structure of the second IDT is called an up-chirp type.

Since this surface acoustic wave convolver uses a dispersion type (chirp type) interdigital transducer in which the width and arrangement period of the positive and negative electrodes gradually change in the propagation direction of the surface acoustic wave, the same period as before. The characteristics can be made wider than those using the interdigital transducer.

A single electrode structure and a double electrode structure are known as the structure of an interdigital electrode. 9A shows a plan view of the single electrode structure, and FIG. 9B shows a sectional view thereof. 10A shows a plan view of the double electrode structure, and FIG. 10B shows a sectional view thereof. In the figure,
Reference numerals 1 and 2 denote excitation electrodes (positive and negative electrodes), and reference numeral 3 denotes a substrate.

In the case of a single-electrode structure in a certain type of piezoelectric substrate, the dispersed interdigital transducer has a unidirectional property of strongly exciting a surface acoustic wave in a direction in which the electrode period is shortened (down direction). In the double electrode structure,
There was no directionality of surface acoustic wave excitation, and it had a bidirectional property.

Therefore, if the first interdigital transducer has a single electrode structure, it has unidirectionality in the direction of the output electrode and has high propagation efficiency (excitation efficiency) in the direction of the output electrode. If such unidirectionality in the direction of the output electrode can be given to the second IDT, the efficiency of the surface acoustic wave convolver can be further improved. But,
It was impossible to form electrodes of a single electrode structure made of the same material on the same substrate and having chirp directions opposite to each other, but it was difficult and difficult even if it was not so. In general, a bidirectional double electrode structure is used as the second interdigital transducer, and only the efficiency is improved by the first interdigital electrode.

On the other hand, as described in JP-A-4-331505, in the single electrode structure in the above electrode structure, the ratio Zm between the characteristic acoustic impedance Zm of the excitation electrode portion and the characteristic acoustic impedance Zg of the gap portion.
When / Zg is smaller than 1 (Zm / Zg <1), the surface acoustic wave is strongly excited in a direction (down direction) where the electrode period is shortened, and Zm / Zg is larger than 1 (Zm / Zg <
In the case of 1), the surface acoustic wave is strongly excited in the direction in which the electrode period becomes longer (up direction), and Zm / Zg is equal to 1 (Zm
When / Zg = 1), it is known that bidirectionality can be obtained.

For example, YZ LiNbO _{3 is} used as a substrate material.
And Y—X 128 ° LiNbO ₃ and using Al as the excitation electrode material, the ratio Zm / Zg between the film thickness of Al and the characteristic acoustic impedance Zm of the excitation electrode portion and the characteristic acoustic impedance Zg of the gap portion. FIG. 11 shows the relationship with. At this time, the film thickness of Al on the horizontal axis is normalized by the wavelength λ of the surface acoustic wave to be excited. In the case where the Y-X 128 ° LiNbO ₃ using a substrate material, Zm / Z by changing the thickness of the Al
g can be larger or smaller than 1. That is, YX
When a broadband dispersion type interdigital transducer having a single electrode structure is formed using 128 ° LiNbO ₃ as a substrate material, the direction in which surface acoustic waves are strongly excited is changed by changing the thickness of Al. You can change it freely. On the other hand, when YZ LiNbO ₃ is used, Zm / Zg cannot be made larger than 1 even if the film thickness of Al is changed. That Y-Z LiNbO ₃
When a wideband distributed interdigital transducer surface acoustic wave converter having a single electrode structure is constructed by using
Even if the film thickness of 1 is changed, the direction in which the surface acoustic wave is strongly excited cannot be changed, and the surface acoustic wave is always strongly excited only in the direction in which the electrode period becomes short (down direction).

[0010]

DISCLOSURE OF THE INVENTION The present invention is directed to a wideband distributed interdigital array having a unidirectionality in an arbitrary direction on an arbitrary substrate, in particular, a unidirectionality opposite to the unidirectionality derived from a normal chirp direction. An object is to provide an electrode surface acoustic wave converter.

[0011]

In order to achieve the above object, according to the present invention, positive and negative electrodes are alternately arranged on a piezoelectric or electrostrictive substrate, and the width and the arrangement of the electrodes are changed. In a surface acoustic wave converter having a distributed interdigital transducer having a period that gradually changes in the propagation direction of the surface acoustic wave, the interdigital transducer has a single electrode structure, and the positive and negative electrodes (excitation electrodes) are formed on a substrate. Embedded in
It is characterized in that a third electrode (short-circuit electrode) which is not connected to the positive and negative electrodes but is connected to each other and has a different film thickness from the positive and negative electrodes is provided in the gap portion.

[0012]

[Function] In a single electrode structure wide band type interdigital transducer that operates by alternately arranging positive and negative electrodes whose electrode width and period are gradually shortened on a piezoelectric or electrostrictive substrate, the excitation electrode portion is formed on the substrate. By embedding or attaching a short-circuit electrode to the gap portion, it is possible to change the ratio of the characteristic acoustic impedance Zm of the excitation electrode portion and the characteristic acoustic impedance Zg of the gap portion in any piezoelectric or electrostrictive substrate. . That is, the direction in which the surface acoustic wave is strongly excited can be controlled. As a result, it has become possible to form a chirp-type electrode having the opposite directionality on the same substrate, which has been conventionally impossible depending on the characteristics of the substrate material. For example,
With the use of YZ LiNbO ₃ as a substrate material, a material that strongly excites in the direction in which the electrode period becomes longer has not been obtained, but has become available in reality.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. Example 1 FIG. 1 shows an electrode structure according to an example of the present invention,
(A) is a plan view and (b) is a sectional view. As shown in the figure, by embedding the excitation electrodes 1 and 2 in the substrate 3, it is possible to greatly change the characteristic acoustic impedance Zm of the excitation electrode. At that time, the value of Zm / Zg is determined by the depth of the groove 4 and the thickness of the electrodes 1 and 2 embedded therein. FIG. 2 shows the groove depth (H when the excitation electrode having the structure shown in FIG. 1 was made of Al on a YZ LiNbO ₃ substrate.
_groove ) and Zm / Zg. This figure shows the case where the film thickness H _Al of the Al electrode is normalized by the wavelength λ of the surface acoustic wave to be excited, and the value is H _Al /λ=0.01175. The depth H _groove of the horizontal axis is also normalized by the wavelength λ of the surface acoustic wave to be excited and H /
It is represented by λ (= H _groove / λ). As can be seen from FIG. 2, when the groove depth is approximately H / λ <0.03, Zm / Zg <1. That is, it can be seen that the electrode is strongly excited in the direction in which the electrode period becomes shorter (down direction). Further, it can be seen that when the groove depth is approximately H / λ = 0.03, Zm / Zg = 1. That is, it can be seen that the directionality is lost. Furthermore, the depth of the groove is about H.
It is understood that when /λ>0.03, Zm / Zg> 1. That is, it can be seen that the electrode is strongly excited in the direction in which the electrode period becomes longer (up direction). As described above, it can be seen that when the groove depth is set to an appropriate value, the electrode is strongly excited in the direction in which the electrode period becomes longer (up direction).

According to the present embodiment, the characteristic acoustic impedance Zm of the excitation electrode can be largely changed, whereby only Zm / Zg <1 can be conventionally achieved, and strong excitation can be made only in the down direction. Not Y-Z L
In the combination of iNbO ₃ substrate and Al electrode, Zm
It became possible to make / Zg larger than 1 and to strongly excite in the up direction.

Example 2 FIG. 3 (a) shows a plane of an electrode structure which makes it possible to greatly change the characteristic acoustic impedance Zg of the gap between the excitation electrodes, thereby making Zm / Zg larger than 1. The figure is shown in FIG. 3 (b). Thus, by disposing the short-circuit electrode 5 in the gap between the excitation electrodes 1 and 2, it is possible to greatly change the characteristic acoustic impedance Zg of the gap between the excitation electrodes. The value of Zm / Zg is determined by the electrode material and film thickness of the excitation electrode and the short-circuit electrode at that time. As an example,
FIG. 4 shows the relationship between the thickness of the short-circuit electrode and Zm / Zg when the electrode was formed on a YZ LiNbO ₃ substrate and Al was used as the excitation electrode and the short-circuit electrode. At this time, the film thickness H _Al of Al of the excitation electrode uses a value normalized by the wavelength λ of the surface acoustic wave to be excited, and the value is H / λ =
It was set to 0.01175. Further, the film thickness H _{Al-short of the short-} circuit electrode, which is the horizontal axis, is also normalized by the wavelength λ of the surface acoustic wave to be excited and expressed as H / λ (= H _Al-short / λ). As can be seen from FIG. 4, when the thickness of the short-circuit electrode is approximately H / λ <0.015, Zm / Zg <1.
That is, it can be seen that the electrode is strongly excited in the direction in which the electrode period becomes shorter (down direction). Further, when the film thickness of the short-circuit electrode is approximately H / λ = 0.015, Zm / Zg = 1. That is, it can be seen that the directionality is lost. Furthermore, when the thickness of the short-circuit electrode is approximately H / λ> 0.015, Zm /
It can be seen that Zg> 1. That is, it can be seen that the electrode is strongly excited in the direction in which the electrode period becomes longer (up direction).
As described above, in the related art, only Ym / Zg <1 can be achieved, and YZ can be strongly excited only in the down direction.
In the combination of LiNbO ₃ substrate and Al electrode,
It can be seen that by setting the film thickness of the short-circuit electrode to an appropriate value, the electrode is strongly excited in the direction in which the electrode period becomes longer (up direction).

Example 3 FIG. 5 shows Y-X 128 ° LiNbO as an electrostrictive substrate.
The relationship between the film thickness of the short-circuiting electrode and Zm / Zg in the surface acoustic wave transducer in which the above-mentioned electrode was manufactured in exactly the same manner as in Example 2 except that ₃ was used is shown. The film thickness H _Al of the excitation electrode Al is a value normalized by the wavelength λ of the surface acoustic wave H / λ =
It was set to 0.012. For comparison, the broken line shows the relationship between the film thickness (H / λ) of the excitation electrode Al and Zm / Zg (the same as the broken line in FIG. 11) when the short-circuit electrode is not provided. It can be seen that Zm / Zg can be freely adjusted to be positive or negative as in the second embodiment. Further, it is understood that the change of Zm / Zg with respect to the change of the Al film thickness is opposite when the film thickness of the short-circuit electrode is changed and when the film thickness of the excitation electrode is changed.

Example 4 FIG. 6 (a) shows that the characteristic acoustic impedance Zg of the gap between the excitation electrodes can be greatly changed, and further the characteristic acoustic impedance Zm of the excitation electrode can be greatly changed to Zm / A plan view of an electrode structure capable of making Zg larger than 1 is shown in FIG. 6B. In this way, by embedding the excitation electrodes 1 and 2 in the substrate 3, it is possible to greatly change the characteristic acoustic impedance Zm of the excitation electrodes, and further by arranging the short-circuit electrode 5 in the gap between the excitation electrodes, It is possible to greatly change the characteristic acoustic impedance Zg of the gap between the electrodes. The value of Zm / Zg is determined by the depth of the groove 4 and the electrode material and film thickness of the excitation electrodes 1 and 2 and the short-circuit electrode 5 at that time. As an example, Y
FIG. 7 shows the relationship between the groove depth and Zm / Zg when the electrode was formed on a —Z LiNbO ₃ substrate and Al was used as the excitation electrode and the short-circuit electrode. At this time, as the film thicknesses H _Al and H _Al-short of Al of the excitation electrode and the short-circuit electrode, the values standardized by the wavelength λ of the surface acoustic wave to be excited are used, and all the values are H / λ = 0.01175. And
The depth H _groove of the horizontal axis is also standardized by the wavelength λ of the surface acoustic wave to be similarly excited. In the case of this embodiment, as can be seen from FIG. 7, regardless of the depth of the groove,
Zm / Zg> 1. That is, it can be seen that the electrode is strongly excited in the direction in which the electrode period becomes longer (up direction). Thus, with this electrode structure, in the conventional combination of the YZ LiNbO ₃ substrate and the Al electrode, which could only achieve Zm / Zg <1, and which could be strongly excited only in the down direction, the electrode period It can be seen that it is possible to strongly excite in the direction in which becomes longer (up direction).

In the above description, the groove is provided in the substrate and the excitation electrode is formed in the groove so that the excitation electrode is embedded in the substrate. However, conversely, the gap of the substrate is raised by vapor deposition or the like. The same state may be created by.

Manufacturing Example FIG. 8 shows an example of a manufacturing process of a surface acoustic wave converter having the structure of the above-mentioned fourth embodiment.

First, Al is applied to the entire surface of the substrate 3 by 2000 Å
Evaporation is performed, and the Al film 5 and the resist 6 are left only in the portion to be the short-circuit electrode using a photoetching method (a). Next, the side surface of the Al film 5 is anodized (b), and then the substrate is dug into the groove 4 by dry etching (c).
Further, 2000 Å Al is vapor-deposited to form the excitation electrodes 1 and 2 (d), the resist 6 is lifted off, and the excess Al film 7 is removed at the same time (e). As a result, a surface acoustic wave converter as shown in FIG. 6 or FIG. 8 (f) 4 can be manufactured.

In the above description, the side surface of the Al film is anodized because the side surface of the Al film is made into an insulator as shown in FIG.
The short-circuiting electrode (Al
This is because the side surface of the film) is not electrically connected to the excitation electrodes 1 and 2. When etching the Al film, the anodic oxidation may be omitted by overetching to remove the portion to be oxidized.

[0022]

As described above, according to the present invention, a single electrode structure in which positive and negative electrodes whose electrode width and period gradually change are alternately arranged on a piezoelectric or electrostrictive substrate is used. In the wideband distributed interdigital transducer, by embedding the excitation electrode part in the substrate or attaching a short-circuit electrode in the gap part, the characteristics of the excitation electrode part and the characteristics of the gap part in the arbitrary piezoelectric and electrostrictive substrate It is possible to change the ratio of the acoustic impedances, which makes it possible to change the direction in which the surface acoustic wave is strongly excited. By applying this to a surface acoustic wave convolver, higher efficiency can be achieved. Further, by applying to a broadband filter, it is possible to reduce the loss.

[Brief description of the drawings]

FIG. 1 is a plan view (a) and a sectional view (b) of a surface acoustic wave converter according to one embodiment of the present invention.

2 is the groove depth and Zm / Zg in the transducer of FIG.
6 is a graph showing the relationship of.

FIG. 3 is a plan view (a) and a sectional view (b) of a surface acoustic wave converter according to another embodiment of the present invention.

4 is a film thickness and Zm of a short-circuit electrode in the converter of FIG.
It is a graph which shows the relationship of / Zg.

FIG. 5: The substrate material in the converter of FIG.
The film thickness and Zm of the short-circuit electrode in the case of 28 ° LiNbO ₃
It is a graph which shows the relationship of / Zg.

FIG. 6 is a plan view (a) and a sectional view (b) of a surface acoustic wave converter according to still another embodiment of the present invention.

FIG. 7: Groove depth and Zm / Zg in the transducer of FIG.
6 is a graph showing the relationship of.

FIG. 8 is a diagram showing a manufacturing process of the converter shown in FIG. 6;

FIG. 9 is a plan view (a) and a sectional view (b) of a surface acoustic wave converter having a conventional interdigital transducer having a single electrode structure.

FIG. 10 is a plan view (a) and a sectional view (b) of a surface acoustic wave converter having a conventional interdigital transducer having a double electrode structure.

11 is a diagram illustrating a film thickness and Z of an excitation electrode in the converter of FIG.
It is a graph which shows the relationship of m / Zg.

[Explanation of symbols]

 1, 2: excitation electrodes, 3: substrate, 4: groove, 5: short-circuit electrode.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Seiichiro Takahashi 52 Funacho, Wakabayashi-ku, Sendai, Miyagi Prefecture Pansion Aihara 101 (72) Inventor Shunji Kato 5-30-3 Nakatsuma, Ageo-shi, Saitama No. 202 Oak Hills 202

Claims (3)

[Claims] 1. Dispersion in which positive and negative electrodes are alternately arranged on a piezoelectric or electrostrictive substrate, and the width and the cycle of the electrodes gradually change in the propagation direction of surface acoustic waves. In a surface acoustic wave converter having a type interdigital transducer, the interdigital electrode has a single electrode structure, and the surface of each positive and negative electrode is formed lower than the surface of the substrate. converter. 2. Elasticity in which positive and negative electrodes are alternately arranged on a piezoelectric or electrostrictive substrate, and the widths of the electrodes and the arrangement period gradually change in the propagation direction of the surface acoustic wave. In the surface acoustic wave converter, the interdigital electrodes have a single electrode structure, and are connected to each other between the positive and negative electrodes without being connected to any of the positive and negative electrodes, and the positive and negative electrodes are different in film thickness. A surface acoustic wave converter, wherein a different third electrode is provided. 3. A surface acoustic wave converter having the positive and negative electrodes according to claim 1 and the third electrode according to claim 2.