JPH10145170A - Surface acoustic wave converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide desired directivity for the distributed interdigital electrode of a double-electrode structure by setting the interdigital electrode to be double electrode structure, stacking the different kind of material on one electrode finger of the double electrodes and breaking the symmetry. SOLUTION: Large piezoelectric non-liner substrate materials Y-Z and LiNb O3 are set to be substrates, and the chirp-type interdigital electrodes 2a, 2b, 3a and 3b of λ/8 double-electrode structure and formed on the substrate by an Al film with a film thickness of 2000Å. Furthermore, Cr films 2c and 3c with the film thickness of 1500Å are stacked on the electrode fingers 2b and 3b, and a surface acoustic wave converter 10 is made. Thus, symmetry which a conventional double electrode structure chips electrode has is broken, and directivity can be given. When the electrode finger where the different type of metal such as the Cr film is stacked is changed, directivity is reversed. Thus, materials which are considered difficult to be obtained and which strongly excites with the chirp electrodes on the Y-Z and LiNb O3 substrates in the up direction can be obtained.

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

[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.

This surface acoustic wave convolver employs a distributed (chirp) interdigital transducer in which the width and the arrangement period of the positive and negative electrodes gradually change in the direction of propagation of the surface acoustic wave. The characteristics can be broadened more than those using the IDT electrodes.

A single electrode structure and a double electrode structure are known as the structure of an interdigital electrode. FIG. 15A is a plan view of a single electrode structure, and FIG. 15B is a cross-sectional view thereof. FIG. 16A is a plan view of a double electrode structure, and FIG. 16B is 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,
The directionality of the surface acoustic wave excitation was extremely weak, and had substantially bidirectional characteristics.

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 is impossible to form electrodes having a single electrode structure in which the chirp directions are opposite to each other with the same material on the same substrate with a certain type of substrate, and even if this is not the case, the steps are complicated and difficult to realize. . Therefore, in general, the second interdigital transducer having a bidirectional double electrode structure is used, and only the efficiency of the first interdigital transducer is improved.

On the other hand, as described in the above-mentioned Japanese Patent Application Laid-Open No. 4-331505, the ratio Zm of the characteristic acoustic impedance Zm of the excitation electrode portion to the characteristic acoustic impedance Zg of the gap portion in the single electrode structure in the electrode structure. When / Zg is smaller than 1 (Zm / Zg <1), the surface acoustic wave strongly excites in a direction in which the electrode cycle becomes shorter (down direction), and Zm / Zg is larger than 1 (Zm / Zg / Zg / Zg).
It is known that when Zg <1), the surface acoustic wave is strongly excited in the direction in which the electrode period becomes longer (up direction), and when Zm / Zg is equal to 1 (Zm / Zg = 1), bidirectionality is obtained. ing.

For example, YZ LiNbO _{3 is} used as a substrate material.
And using a Y-X 128 ° LiNbO _3, in the case of using the Al to the excitation electrode material, the ratio Zm / Zg with characteristic acoustic impedance Zg characteristic acoustic impedance Zm and the gap portion of the film thickness and the excitation electrode portion of the Al 17 is shown in FIG. 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 is, YZ LiNbO
_{In the case} where a substrate material is used to form a broadband distributed interdigital transducer having a single electrode structure,
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]

SUMMARY OF THE INVENTION The present invention is directed to a broadband dispersion type interdigital transducer having unidirectionality in an arbitrary direction on an arbitrary substrate, in particular, 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 of the electrodes and the period of the arrangement are provided. Has a distributed interdigital transducer that gradually changes in the direction of propagation of the surface acoustic wave, wherein the interdigital transducer has a double electrode structure, and one electrode finger of all the double electrodes. Is characterized by having a different substance on it.

[0012]

According to the above construction, in a double-structure wide-band dispersion type interdigital transducer which operates by alternately arranging positive and negative electrodes whose electrode width and period gradually become smaller on a piezoelectric or electrostrictive substrate, By disposing a dissimilar metal on one of the double electrodes and breaking the symmetry of the double-electrode dispersed IDT, the directionality can be controlled. As a result, it becomes possible to form a chirped electrode having the opposite direction on the same substrate, which cannot be conventionally performed depending on the characteristics of the substrate material.

For example, in a dispersion type interdigital transducer having a double electrode structure using YZ LiNbO ₃ , which is the most general substrate material among substrates having large piezoelectric nonlinearity, and using an aluminum film as an electrode material, When a chromium film is laminated on the down-side electrode finger, it can be excited strongly in the up direction, and when a chromium film is laminated on the up-side electrode finger, it can be strongly excited in the down direction. As described above, in the present invention, it has been impossible to obtain a material that strongly excites in the direction in which the electrode period becomes longer (up direction) by using Y-ZLiNbO ₃ as a substrate material.

As the foreign substance of the present invention, Al ₂ O ₃ , Ni, Ge, etc. can be used in addition to the chromium film.

The aluminum electrode and the foreign substance are made of a semiconductor I
A film can be formed by the same photolithographic technique (vacuum deposition, photoetching, lift-off, etc.) as in the case of manufacturing C and the like. In addition, when a portion to be selectively removed is a metal, an electrochemical effect is used. An anodizing method can be used, and a plating method can be used when a metal is to be selectively attached.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. Embodiment 1 FIGS. 1A and 1B show a configuration of a surface acoustic wave converter according to an embodiment of the present invention, wherein FIG. 1A is a plan view and FIG. The converter 10 shown in FIG. 1 has a chirped IDT 2 (2a, 2b) and 3 (2a, 2) having a λ / 8 double electrode structure made of an Al film having a thickness of 2000 ° on a YZ LiNbO ₃ substrate 1.
b) is formed, and a film thickness of 1500 .ANG. is formed on the electrode fingers 2b and 3b on the side (down side) where the period of the double electrode is shortened.
Of Cr films 2c and 3c.

FIG. 2 shows a conceptual diagram of the direction measurement. As shown in FIG. 2, a surface acoustic wave converter 20 to be measured is formed on the same substrate, and surface acoustic wave converters 21 and 31 of the same period type are formed on both sides sandwiching the same. , A high-frequency voltage is applied to the surface to excite the surface acoustic wave, which is received by the converters 21 and 22, and the voltage generated between the positive and negative electrodes of the converters 21 and 22 is measured. The ratio (dB) of the output voltage of each of the converters 21 and 22 to the input voltage to the converter 20 is defined as insertion loss.

FIG. 3 shows the insertion loss characteristics when the number of electrode pairs is 233, the center frequency is 200 MHz, the band is 40 MHz, and the cross width W is 50 μm (3λ) in the surface acoustic wave converter of FIG. In the converter of FIG. 1, the insertion loss characteristic is smaller in the direction in which the electrode period is longer (up direction) than in the direction in which the electrode period is shorter (down direction), and the surface acoustic wave is strongly excited in the up direction. It can be seen that the film has unidirectionality in the up direction.

Comparative Example 1 FIG. 4 shows a case where the Cr film is not formed in the state of FIG. 2 with respect to the surface acoustic wave converter of FIG. 1, that is, the conventional chirp of the λ / 8 double electrode structure shown in FIG. 4 shows the insertion loss characteristics of a surface acoustic wave converter having a IDT. It turns out that it has substantially bidirectionality.

Embodiment 2 FIGS. 5A and 5B show the configuration of a surface acoustic wave converter according to another embodiment of the present invention. FIG. 5A is a plan view and FIG. The converter shown in the figure has a 1500 .ANG.-thick Cr film laminated on the upper electrode fingers 2a and 3a of the double electrodes 2 and 3 in FIG. As shown in FIG. 6, the insertion loss characteristic is smaller in the down direction than in the up direction, and has a unidirectionality in the down direction.

As can be seen from the above Examples and Comparative Examples, as shown in FIG. 1, a different kind of metal is placed on one of the electrode fingers 2b and 3b of the excitation double electrodes 2 and 3 as shown in FIG. To break the symmetry of the conventional double electrode structure chirp electrode,
It is possible to give directionality. If the electrode finger on which the dissimilar metal is placed is changed as shown in FIG. 5, the direction is reversed. In consideration of this, a chirp electrode on a YZ LiNbO ₃ substrate that is strongly considered to be unable to be obtained in the past can be strongly excited in the up direction. According to the first and second embodiments, utilizing the fact that the conventional double-electrode-structured chirp electrode has bidirectionality, by disposing a dissimilar metal on one of the double electrodes, the symmetry is broken and the direction is given. It became possible.

Manufacturing Example FIG. 7 shows an example of a manufacturing process of a surface acoustic wave converter having the structure of the first embodiment. First, Al is applied to the entire surface of the substrate 1.
Then, Cr is deposited at 1500 ° and 1500 ° is deposited, and a pattern as shown in FIG. Next, a cathode of a DC power supply (not shown) is connected to the bus bars 5 and 6, an anode is connected to a separately prepared Pt plate (not shown), and the substrate 1 and the Pt plate are dipped in an etching solution of Cr. Due to the electrochemical effect, Cr on electrode fingers that are not electrically connected is etched (FIG. 7).
(B)). Further, a resist is applied, and a window for connecting the floating electrode and the bus bar portion is opened by exposure. Cr in the portion where the window is opened is removed by etching. Further A
Then, the floating electrode is connected to the bus bar by a lift-off process (FIG. 7C). As a result, as shown in FIG. 1, it is possible to obtain an interdigital electrode in which the Cr films 2c and 3c are placed on the down-side electrode fingers 2b and 3b.

On the other hand, in FIG. 7A, when it is desired to leave the floating electrode with Cr remaining, the anodes are connected to the bus bars 5 and 6, and
The cathode is connected to the t-plate and immersed in the electrolyte used for anodic oxidation. Then, the Cr of the electrode connected to the anode melts, and a pattern opposite to that of FIG. 7B can be obtained. Thereafter, by repeating the same operation as described above, FIG.
As shown in FIG. 7, an interdigital electrode in which Cr films 2d and 3d are placed on up-side electrode fingers 2a and 3a opposite to the arrangement of FIG. 7C or FIG. 1 can be obtained. The reverse of the electrode arrangement of 4-3 is obtained.

As described above, if the electrochemical effect is used,
A desired pattern can be obtained without precise overlay exposure. In addition, by slightly changing the process with one mask, a pattern having a reversed pattern, that is, a pattern having a reversed direction can be obtained.

Example 3 and Comparative Example 2 A surface acoustic wave convolver having the structure shown in FIG. 8 was prepared using a chirp type surface acoustic wave converter. The surface acoustic wave convolver of FIG. 8 is different from the surface acoustic wave convolver disclosed in Japanese Patent Application Laid-Open No. 4-331505 as an up-chirp type blind having a λ / 8 double electrode structure of FIG. In this case, an electrode having a shape electrode is used. Japanese Patent Application Laid-Open No. 4-33 discloses a converter having a single electrode structure.
As shown in FIG. 15, the one having a down-chirp type interdigital transducer having a λ / 4 single electrode structure as shown in FIG. 15 was used. The number of electrode pairs, center frequency, band and cross width are the same as the transducer on the double electrode structure side, 233 pairs, 200 MHz, 40 MHz and = 50
μm (3λ), and the input / output ports were sufficiently matched with the electric circuit. The width (length in the propagation direction) of the output electrode was 4.9 μS, which is the time required for the surface acoustic wave to propagate in the output electrode.

As Comparative Example 2, as shown in FIG.
The convolver disclosed in Japanese Patent Application Laid-Open No. 4-331505, that is, the one shown in FIG. 8 has a double electrode structure as a conventional bidirectional converter shown in FIG. 5 (C in FIG. 1).
r film is not formed). The number of electrode pairs, the center frequency, the band, the intersection width, and the like were the same as those in FIG.

FIGS. 10 and 11 show the insertion loss characteristics of the convolvers of FIGS. 8 and 9, respectively. FIG. 12 shows the frequency characteristics of the F value (convolution efficiency) of both convolvers, and FIGS. 13 and 14 show the output waveforms of both convolvers. As shown in FIG. 12, in the conventional convolver of FIG. 9, the maximum value of the F value (broken line) is -58.5 dB.
In contrast, in the Al / Cr double-layer convolver according to the present invention shown in FIG. 8, the maximum value of the F value (solid line) is −5.
6.5 dBm, which is an improvement of about 2 dB. This F value-
56.5 dBm is higher than the highest currently reported for elastic convolvers. The output waveforms of FIGS. 13 and 14 also show that the amplitude of the waveform of FIG. 8 is clearly larger than that of the conventional example of FIG.

[0028]

The surface acoustic wave converter of the present invention can be applied to a filter. A conventional surface acoustic wave converter having a single electrode structure is arranged so that the directions thereof are opposed to each other, and one is used for excitation and the other is used for reception, whereby a high-efficiency filter can be formed. However, the delay time of the filter having this configuration differs depending on the frequency, and a filter having a constant delay time regardless of the frequency cannot be obtained. When trying to keep the delay time constant, the directionality of excitation and the directionality of reception become opposite, and there is a problem that the efficiency of the convolver is low as described above. Even in such a case, the present invention can reduce the loss of the filter.

[0029]

As described above, according to the present invention,
Positive and negative electrodes are alternately arranged on a piezoelectric or electrostrictive substrate, and have a distributed interdigital electrode in which the width and arrangement period of the electrodes gradually change in the direction of propagation of surface acoustic waves In the surface acoustic wave converter, the interdigital transducer has a double electrode structure, and a dissimilar substance is placed on one electrode finger of all the double electrodes to break its symmetry, so that the double electrode which is originally bidirectional is formed. It becomes possible to give a desired directionality to the dispersed IDT having the structure. 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.

FIG. 2 is a conceptual diagram of a directional characteristic device for measuring the insertion loss of the converter of FIG.

FIG. 3 is a graph showing the insertion loss of the converter of FIG.

FIG. 4 is a graph showing insertion loss of a conventional converter.

FIG. 5 is a plan view (a) and a sectional view (b) of a surface acoustic wave converter according to another embodiment of the present invention.

FIG. 6 is a graph showing the insertion loss of the converter of FIG.

FIG. 7 is an explanatory view showing an example of a method of manufacturing the converter of FIG.

FIG. 8 is a plan view showing a configuration of a surface acoustic wave convolver using the converter of FIG. 1;

FIG. 9 is a plan view showing a configuration of a conventional surface acoustic wave convolver.

FIG. 10 is a graph showing insertion loss of the convolver of FIG. 8;

FIG. 11 is a graph showing insertion loss of the convolver of FIG. 9;

FIG. 12 is a graph showing a frequency characteristic of an F value of the convolver shown in FIGS. 8 and 9;

FIG. 13 is an output waveform diagram of the convolver of FIG. 8;

FIG. 14 is an output waveform diagram of the convolver of FIG. 9;

15A and 15B are a plan view (a) and a cross-sectional view (b) of a conventional surface acoustic wave converter having an interdigital transducer having a single electrode structure.

16A and 16B are a plan view (a) and a cross-sectional view (b) of a conventional surface acoustic wave converter having a double-electrode interdigital transducer.

FIG. 17 is a graph showing the relationship between the thickness of the excitation electrode and Zm / Zg in the converter of FIG.

[Explanation of symbols]

1: substrate, 2, 3: interdigital electrode, 2a, 2b, 3a,
3b: electrode finger, 2c, 2d, 3c, 3d: Cr film, 1
0, 20: surface acoustic wave converter.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Seiichiro Takahashi 27 201 5-5 Sunflat, 5-5 Suwa-cho, Taishiro-ku, Sendai City, Miyagi Prefecture (72) Inventor Shunji Kato 5-30-3 Nakazuma, Ageo City, Saitama Prefecture Oak Hills Issue 202

Claims (2)

[Claims] 1. A distributed type in which positive and negative electrodes are alternately arranged on a piezoelectric or electrostrictive substrate, and the width and the arrangement period of the electrodes gradually change in the direction of propagation of the surface acoustic wave. A surface acoustic wave transducer having interdigital transducers, wherein the interdigital transducer has a double electrode structure, and a heterogeneous substance is placed on one of the electrode fingers constituting each double electrode. Wave converter. 2. The surface acoustic wave converter according to claim 1, wherein the interdigital transducer is made of an aluminum film, and a chromium film is laminated as the dissimilar substance.