JPH0918272A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE: To improve the unidirectionality of a surface acoustic wave, to miniaturize the device, and to reduce the loss at the time of using an internal reflection type unidirectional IDT having a floating electrode with respect to the surface acoustic wave device. CONSTITUTION: A piezoelectric substrate 1 and plural interdigital electrodes which have positive and negative exciting electrodes 11' for excitation and reception of the surface acoustic wave on the piezoelectric substrate are provided. Materials of the piezoelectric substrate 1 are X-112Y:LiTaO3 , and at least one interdigital electrode is the internal reflection type unidirectional interdigital electrode which excites the surface acoustic wave having unidirectionality, and the thickness of the electrode film of the internal reflection type unidirectional interdigital electrode is >=0.4% of the period λ of interdigital 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 device, and more particularly to a surface acoustic wave device composed of an internal reflection type unidirectional IDT having a floating electrode.

[0002]

[Prior Art and Problems to be Solved by the Invention]
With the miniaturization of mobile communication terminals such as car phones and mobile phones, there is a strong demand for miniaturization of electronic components used therein. For example, a surface acoustic wave filter has been used as a component of a duplexer instead of a dielectric filter that has been conventionally used, which greatly contributes to downsizing of a terminal.

However, the surface acoustic wave filter having a relatively low frequency in the tens to hundreds of tens of MHz band is still large in size, and the surface acoustic wave filter must be downsized in order to further downsize the terminal. At the same time, low loss is also required. Particularly in order to reduce the loss, in the conventional surface acoustic wave filter of several hundred MHz band or more, a multi-electrode configuration in which a large number of input electrodes and output electrodes are arranged or a surface acoustic wave resonator is used. .

Generally, a surface acoustic wave filter is one in which an input IDT (Inter Digital Transducer) and an output IDT are formed on the surface of a piezoelectric substrate which generates a strained surface acoustic wave when a voltage is applied. A multi-electrode structure is provided in which a plurality of electrodes each including an IDT and an output IDT are provided on a single piezoelectric substrate. Further, the IDT is shown in FIG.
As shown in FIG. 5, it is composed of a set of positive and negative excitation electrodes having interdigitally shaped branch portions.

However, since the distance between the branches of the positive and negative excitation electrodes of the IDT, that is, the electrode period, is inversely proportional to the frequency, the above multi-electrode configuration is applied to the surface acoustic wave filter in the tens to hundreds MHz band. Then, the size becomes enormous, and it is difficult to reduce the size. In order to satisfy the demand for miniaturization, a simple one-input one-output transversal type filter structure is proposed, and a surface acoustic wave filter using a unidirectional IDT that can reduce bidirectional loss as an IDT is proposed. Has been done.

For example, Japanese Patent Laid-Open No. 60-236312 discloses an internal reflection type unidirectional IDT having a floating electrode between positive and negative excitation electrodes. This has a feature that the manufacturing process is simple because an external phase circuit is not required and only one electrode film formation and patterning are required.

Further, Japanese Laid-Open Patent Publication No. 3-133209 discloses an internal reflection type unidirectional IDT in which there are two types of floating electrodes, an open type and a short type, and the line width of the open type floating electrode is λ. Wider than / 12 (λ is the period of the interdigital transducer),
It is described that the line width of the short-circuit type floating electrode is made narrower than λ / 12 to increase the unidirectionality.

FIG. 16 shows a conventionally used internal reflection type unidirectional IDT (FEUDT: Floati) having a floating electrode.
ng Electrode type Unidirectional Transducer). Here, IDT100
Is a positive / negative excitation electrode 101, a branch portion 102 of the positive / negative excitation electrode,
It is composed of a short circuit type floating electrode 103 and an open type floating electrode 104. 105 is an interval between the branch portions, that is, an electrode period.

In this conventionally used IDT, Al, Al-Cu, Cu or the like is generally used as an electrode material,
128 ° Y-X: LiNbO, where good unidirectionality is easily obtained
₃ was used as the material for the piezoelectric substrate.

Since the FEUDT utilizes the piezoelectric reaction of the floating electrode, it is essentially effective for a substrate having a large electromechanical coupling coefficient K ^{2 in} order to obtain good unidirectionality. Are known. The electromechanical coupling coefficient K ² is one of the characteristic values of the piezoelectric substrate, and is a coefficient indicating the conversion efficiency when an electric signal is converted into a surface acoustic wave.
Therefore, the FEU used in the invention disclosed in the above publication
DT uses 128 ° Y—X: LiNbO ₃ having a relatively large value of K ² = 5.5% in any case as a substrate material.

However, 128 ° Y-X: LiNbO ₃
In the substrate using, the temperature coefficient of delay time, which is one characteristic value of the piezoelectric substrate, is as large as −74 ppm / C °, so that it is difficult to obtain stable temperature characteristics. Further, since K ² is large, it is necessary to increase the number of electrode pairs in the branch portion of the IDT when constructing a narrow band filter having good characteristics, which is disadvantageous for downsizing.

On the other hand, the temperature coefficient of delay time is -18 ppm / C °.
And as a substrate material exhibiting stable temperature characteristics, X-
112 ° Y: LiTaO ₃ is known. K ² is 0.76% when using this substrate material.
8 ° Y-X: It is smaller than that of LiNbO ₃ by about one digit, and a narrow band can be realized with a small number of electrodes, which is suitable for downsizing.

Further, the propagation velocity of the surface acoustic wave is 3960 m / s in the case of 128 ° Y-X: LiNbO ₃ , whereas it is 3230 m / s in the case of X-112 ° Y: LiTaO ₃ , which is slow. Therefore, it is advantageous for downsizing the narrow band filter. However, as described above, when K ² is small, the internal reflection type having a floating electrode as in the related art can obtain only a slight unidirectionality, and realizes a low-loss surface acoustic wave filter that can be practically used. I can't.

The present invention has been made in consideration of the above circumstances, and the material of the piezoelectric substrate is X-112 °.
Y: LiTaO ₃ is used, and when the internal reflection type unidirectional IDT is used as the IDT, the unidirectionality of the surface acoustic wave is increased to such an extent that it can be practically applied, and the surface acoustic wave device is stable, compact, and has low loss. The purpose is to provide.

[0015]

According to the present invention, there is provided a piezoelectric substrate, and a plurality of interdigital electrodes having positive and negative excitation electrodes for exciting or receiving surface acoustic waves on the piezoelectric substrate. The material is X-112 ° Y: LiTaO
₃ , the at least one interdigital electrode is an internal reflection type unidirectional blind electrode that excites a surface acoustic wave having unidirectionality, and the electrode film thickness of the internal reflection type unidirectional blind electrode is The present invention provides a surface acoustic wave device characterized in that it is 0.4% or more of the period λ of the interdigital transducer.

Here, the internal reflection type unidirectional interdigital transducer has an open floating electrode which is not electrically connected to the positive and negative excitation electrodes and a positive and negative excitation electrode between the positive and negative excitation electrodes. It may be provided with a short-circuit type floating electrode in which two floating electrodes are short-circuited without being physically connected.

Further, the film thickness of the electrode film of all or some of the interdigital electrodes is 0.4% of the period λ of the interdigital electrodes.
It may be configured as described above. Further, aluminum is usually used as a material for the electrode film of the interdigital transducer, but in addition to this, an alloy of aluminum and copper (for example, A
1-1% Cu, Al-2% Cu), gold, copper, titanium or the like may be used.

Further, the internal reflection type unidirectional interdigital transducer has one floating electrode which is not electrically connected to the positive and negative excitation electrodes, at a position deviated from the center between the positive and negative excitation electrodes, and You may make it have the structure which two adjacent floating electrodes were short-circuited. Further, one or more surface acoustic wave devices as described above may be used to form a surface acoustic wave filter.

Further, the present invention comprises a piezoelectric substrate and a plurality of interdigital electrodes having positive and negative excitation electrodes for exciting or receiving surface acoustic waves on the piezoelectric substrate, and at least one interdigital electrode is provided. , A floating electrode that is not electrically connected to the positive and negative excitation electrodes and is located between the positive and negative excitation electrodes and at a position separated from the excitation electrode by a distance of 1/6 of the period λ of the interdigital transducer. And the adjacent floating electrodes are short-circuited, the internal reflection type unidirectional blind electrodes, and the line width of the positive and negative excitation electrodes is λ / 8 or more and λ / 5 or less, It is intended to provide a surface acoustic wave device having a line width of λ / 12 or less.

Here, the material of the piezoelectric substrate is X
-112 ° Y: LiTaO ₃ , the material of the electrode film of the interdigital transducer is aluminum, only the internal reflection type unidirectional interdigital electrode, or all or some of the interdigital electrodes on the piezoelectric substrate. The film thickness may be 0.4% or more of the wavelength λ of the surface acoustic wave to be excited.

When the material of the interdigital transducer is other than aluminum, the value obtained by multiplying the film thickness of the electrode film by the specific gravity of the material of the electrode film of the interdigital electrode with respect to aluminum is If it is 0.4% or more of the wavelength λ or if the electrode film of the interdigital transducer is composed of a plurality of materials formed in layers, the specific gravity of each material to aluminum is multiplied by the film thickness of each material The numerical value may be 0.4% or more of the wavelength λ of the surface acoustic wave to be excited. Further, one or more surface acoustic wave devices having such a configuration may be combined to form a surface acoustic wave filter.

[0022]

According to the present invention, X-112 ° Y: LiTa
A piezoelectric substrate made of O ₃ is used, an internal reflection type unidirectional interdigital electrode is used as the interdigital electrode for exciting surface acoustic waves, and the film thickness of the internal reflection type unidirectional interdigital electrode is the period of the interdigital electrode. Since it is 0.4% or more of λ, it is possible to increase the unidirectionality of the surface acoustic wave, and it is possible to provide a stable, small-sized and low-loss surface acoustic wave device.

Further, the internal reflection type unidirectional blind electrode is
Even if two types of floating electrodes, an open type floating electrode and a short type floating electrode, are provided, or if only a short type floating electrode is provided, by increasing the thickness of the electrode as described above,
The directionality of the internal reflection type unidirectional blind electrode can be improved.

Further, the internal reflection type unidirectional blind electrode in which the short-circuit type floating electrode is arranged at a position where the distance between the center of the floating electrode and the positive and negative excitation electrodes is ⅙ of the period λ of the interdigital electrode. In the electrodes, the line width of the positive and negative excitation electrodes is λ / 8 or more and λ / 5 or less, and the line width of the short-circuit type floating electrode is λ / 12.
Since it is as follows, it is possible to reduce the loss as well as increase the directionality.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. The present invention is not limited by this.

Embodiment 1 FIG. 1 shows a structural diagram of a substrate surface in an embodiment of the present invention. This drawing shows an example in which one set of input IDT and output IDT is formed on the piezoelectric substrate.

Reference numeral 1 denotes a piezoelectric substrate, which is X- in the present invention.
112 ° Y: A LiTaO ₃ substrate is used. X-112 °
The Y: LiTaO ₃ substrate has stable temperature characteristics and has a small electromechanical coefficient K ² , which is advantageous for downsizing a narrow band filter. Reference numeral 2 is an input IDT, for example, an internal reflection type unidirectional IDT (FEU) as shown in the figure.
DT) can be used.

Generally, the surface acoustic wave excited from the input IDT travels in the direction perpendicular to the electrodes of the IDT.
In FIG. 1, there are a surface acoustic wave traveling leftward and a surface acoustic wave traveling rightward. The FEUDT is an IDT devised so that the number of surface acoustic waves traveling toward the output side IDT in the two directions, that is, the right side direction is increased as much as possible, and in this sense, it is a unidirectional IDT. 3 is
This is an output IDT, and for example, a double electrode type IDT as shown in the figure can be used. Reference numeral 4 is a sound absorbing agent for suppressing reflection of SAW on the end surface of the piezoelectric substrate.

In the input IDT 2, 11 and 11 'are positive and negative excitation electrodes, 12 is a short circuit type floating electrode, and 13 is an open type floating electrode. Aluminum is used here as a material for these electrodes.

As shown in the figure, the positive and negative excitation electrodes 11 and 11 'are electrodes facing each other, and the branch portions 14 of the positive and negative excitation electrodes protrude in a comb shape at a predetermined constant interval. The branch portion 14 from above and the branch portion 14 from below constitute a pair of electrodes, and the IDT is generally composed of a plurality of pairs of branch portions. The interval between the branch portions 14 (hereinafter referred to as the electrode period) is set to be substantially equal to the wavelength λ of the surface acoustic wave to be excited most strongly. For example, when the propagation velocity of surface acoustic waves is 3230 m / s, X
-112 ° Y: When a LiTaO ₃ substrate is used and the center frequency of the filter is 40 MHz, the wavelength λ is 80.
The length is 75 μm, and the electrode period is substantially the same.

All the line widths of the positive and negative excitation electrodes 11 and 11 ', the short-circuit type floating electrode 12 and the open type floating electrode 13 of the input IDT (FEUDT) are made equal to λ / 12. Further, when the electrode material is aluminum, the input I
The film thickness of all the electrode films of DT is set to 0.4% or more of the wavelength λ of the surface acoustic wave. For example, when the period of the interdigital transducer is λ = 82 μm, the film thickness of aluminum is 0.3.
28 μm or more. In the embodiment of FIG. 1, the electrode period is 82 μm, the number of electrode pairs of the input IDT is 20, and the number of electrode pairs of the output IDT is 28.

FIG. 2 shows an input IDT (FEU) of the present invention.
FIG. 6 shows a perspective view of another embodiment of DT). Here, H represents the film thickness, We is the line width of the positive and negative excitation electrodes, and Wf is the line width of the short circuit type floating electrode. In FEUDT in FIG. 1, We
= Wf.

Next, it will be shown that the unidirectionality of the surface acoustic wave device of the present invention is improved by increasing the thickness of the electrode film. In order to measure this unidirectional characteristic, FIG.
The measurement sample shown in was used. This is X-112 °
Y: Input IDT in the center on LiTaO ₃ substrate
(FEUDT), a double electrode type IDT which is a bidirectional IDT is arranged on both sides thereof, and the input IDT
Double electrode type IDT on both sides of surface acoustic wave excited by
The difference in loss when received at was measured.

Here, the direction in which the surface acoustic wave excited from the FEUDT is desired to travel is called the forward direction, and the direction in which it is not desired to travel is called the reverse direction. Also, the numerical value obtained by subtracting the loss in the double electrode type IDT existing in the reverse direction from the loss in the double electrode type IDT existing in the forward direction as viewed from the FEUDT is called directionality. It can be said that the sex is good. The measurement of the degree of directivity was performed by changing the electrode film thickness of the input IDT of the measurement sample of FIG.

4 and 5 are frequency-loss characteristic diagrams of the FEUDT when aluminum is used for the electrodes.
FIG. 4 is a characteristic diagram when the film thickness of the aluminum electrode film is 0.1% λ, and FIG. 5 is a characteristic diagram when the film thickness is 0.4% λ. The horizontal axis is the frequency of the surface acoustic wave (M
Hz), and the vertical axis represents loss (dB).

FIGS. 4 (a) and 5 (a) show the loss in the direction in which the surface acoustic wave of the FEUDT is desired to be excited, that is, the forward direction loss, and FIGS. 4 (b) and 5 (b) are the directions in which the excitation is not desired. , Ie, the loss in the opposite direction. 4 (a) and (b)
It can be seen that the loss in the forward direction is smaller than that in the reverse direction by about 3.37 dB. That is, it can be said that the directivity is 3.37 dB when the film thickness of the aluminum electrode film is 0.1% λ.

Similarly, comparing FIGS. 5A and 5B, when the thickness of the aluminum electrode film is 0.4% λ,
It can be said that the degree of directivity is 4.10 dB. Therefore, when the electrode film is aluminum, the film thickness is 0.1.
It can be seen that the directionality is better when the thickness is 0.4% λ than the thickness of% λ.

FIG. 6 is a graph showing the relationship between the thickness of the electrode film and the directivity of the input IDT. The horizontal axis represents the electrode film thickness normalized by the wavelength λ, and the vertical axis represents the degree of directivity.
Graph A in FIG. 6 shows an FE having only short-circuited floating electrodes.
In the case of UDT, graph B shows the change in the case of FEUDT having two types of floating electrodes, a short-circuit type and an open type.

According to FIG. 6, in both graphs A and B, increasing the electrode film thickness on the horizontal axis increases the directivity. For example, in graph A, when the film thickness is 0.12% λ, the directivity is 3.3 dB, and when the film thickness is 0.4% λ, the directivity is 4 dB. Directivity is improved. In addition, the degree of directivity tends to saturate as the electrode film thickness increases,
At 0.4% λ or more, the degree of directivity is about 4 dB or more, and the effect of increasing the film thickness can be obtained.
In particular, it is more preferable that the film thickness is 1% λ or more. Moreover, according to the graph of FIG. 6, 0.4%
If the thickness is λ or more, good directivity is obtained, so there is no need to specify the upper limit value, but from the above viewpoint, it is preferably several μm or less.

From the above, by setting the thickness of the FEUDT electrode film to 0.4% λ or more, X-11
A 2 ° Y: Surface acoustic wave device having stable temperature characteristics and low loss, which can improve the unidirectionality of the surface acoustic wave when FEUDT is formed using a LiTaO ₃ substrate to a practically sufficient degree. can do.

Although aluminum is used as the electrode material in the embodiment of FIG. 1, in addition to this, an alloy of aluminum and copper (Al-1% Cu, Al-2% C) is used.
u), copper, gold, titanium, etc. can also be used.

When an electrode material other than aluminum is used as described above, when the electrode film thickness is h and the specific gravity of the electrode material with respect to aluminum is m, h × m ≧ 0.
The electrode film thickness is set so as to be 4% λ. For example,
When copper is used as the electrode material, the electrode period λ = 82 μm
At this time, the film thickness h of copper is preferably 0.12 μm (120 nm) or more. Here, the specific gravity of copper to aluminum is 2.73.

The electrode material may be formed as a layered structure in which a plurality of materials are combined. In this case, h × m is calculated for each of the plurality of materials, and the sum is 0.
Each electrode film thickness may be set so as to be 4% or more. For example, when aluminum and copper are formed in layers, the thickness of aluminum can be 50 nm and the thickness of copper can be 600 nm when λ = 82 μm. Further, in FIG. 1, a set of input IDT and output ID
Although the example in which T and T are formed is shown, a plurality of IDTs may be formed on one piezoelectric substrate.

In FIG. 7, Cu600 is used as a material for the electrode film.
nm and Al-1% Cu 50 nm (Al equivalent, 2% λ) are used, and a frequency-loss characteristic diagram of an example in which these are formed in a layered form is shown. Comparing (a) and (b) of FIG. 7 reveals that the degree of directivity is 4.02 dB. Therefore, even when the electrode film is formed by using a plurality of materials, the FEUDT directivity shows a good value when the film thickness is considerably increased to 2% λ by converting to Al. You can In the embodiment described above, an example in which one set of input IDT and output IDT is formed is shown, but a plurality of IDTs are formed on one piezoelectric substrate.
May be formed.

Generally, a surface acoustic wave filter is constructed by using a plurality of input IDTs and output IDTs as shown in FIG. Further, the surface acoustic wave filter is used as a component forming a duplexer of a small mobile terminal such as a mobile phone. Therefore, if a surface acoustic wave filter is constructed by using the surface acoustic wave device of the present invention as shown in FIG. 1 as a basic element, a smaller surface acoustic wave filter having a good loss characteristic and a small portable terminal are provided. can do.

Further, although the example in which the FEUDT is used as the input IDT has been shown, the surface acoustic wave filter is formed by using a plurality of IDTs, and when aluminum is used for the electrode film, the present invention is applied to all the IDTs. By applying FEUDT, the film thickness of the electrode film of the IDT may be 0.4% λ. Further, also in the input IDT or the output IDT which does not adopt the FEUDT, the film thickness of the aluminum electrode film may be 0.4% λ as in the present invention.

Example 2: Next, the FEUD of FIG. 1 or FIG.
An embodiment in the case of a surface acoustic wave device having a structure in which the open floating electrode is removed from T is shown in FIG. Here, a FEUDT having only a short-circuit type floating electrode as a floating electrode
In the same manner as in the above example, it is shown that the degree of directivity is improved.

As in the case of FIG. 1, a surface acoustic wave having a center frequency of 40 MHz is excited, and the piezoelectric substrate is X-.
112 ° Y: LiTaO ₃ is used, and the electrode film is Cu
And Al-1% Cu formed in layers are used. In addition, the positive and negative excitation electrodes 11 and 11 'and the short-circuit type floating electrode 12
The line widths (We, Wf) of both are λ / 12. Further, the film thickness of the electrode film is Cu 600 nm, Al-1% Cu
Of 50 nm and 2% λ in terms of Al. As shown in Example 1, the film thickness does not need to be limited to 2% λ and is 0.4% in terms of Al.
It is sufficient to use one having a value of about λ or more.

FIG. 9 shows the FEUD having the above structure.
The frequency-loss characteristic figure of T is shown. Also, for comparison,
FIG. 10 shows a frequency-loss characteristic diagram of the FEUDT having a thin film thickness. In FIG. 10, the electrode film is made of Al-1% Cu.
The film having a thickness of 0 nm (Al equivalent, 0.12% λ) was used.

First, comparing FIGS. 9A and 9B,
The loss in the forward direction was around 4.8 dB in the vicinity of 40 MHz. Therefore, the directivity is about 4.8 dB.
It can be said. Further, in FIG. 10, the degree of directivity was about 3.3 dB. From the above, it can be said that the thicker film shown in FIG. 9 has better directivity of about 1.5 dB.

Further, in the case of the forward direction shown in FIG. 9A, the loss is about 8.75 dB in the vicinity of 40 MHz, whereas in the case of the forward direction shown in FIG. It was 10.45 dB. Therefore, it can be seen that the thicker film shown in FIG. 9 reduces the loss by 1.7 dB.

As described above, even if the FEUDT is provided with the short-circuit type floating electrode and does not have the open type floating electrode, by increasing the film thickness of the electrode film, the unidirectionality and The loss can be improved. In the case where the surface acoustic wave filter is composed of a plurality of IDTs as in the case of the first embodiment, the film thickness of the aluminum electrode film of only the FEUDT or all the IDTs or a part of the IDTs is 0.4% λ. The above may be applied.

Example 3 Next, an example in which the line widths of the positive and negative excitation electrodes and the line width of the short-circuit type floating electrode in the FEUDT are changed will be described. Here, it is shown that the insertion loss can be reduced when the line width of the positive and negative excitation electrodes is made thick and the line width of the short-circuit type floating electrode is made thin.

First, as a general theory, the relationship between the line width of the FEUDT electrode, the directivity and the insertion loss will be described. Using the above-described measurement sample shown in FIG. 3, the change in the degree of directivity due to the change in line width was measured. Here, X-112 ° Y: LiTaO ₃ was used as the piezoelectric substrate. Further, as the central FEUDT, an open type floating electrode was removed and a short type floating electrode was used. In addition, the line width of the positive and negative excitation electrodes of the central FEUDT is λ
The directional degree was measured by fixing the line width of the short-circuit type floating electrode while fixing at / 12.

FIG. 11 is a graph showing changes in the degree of directivity with respect to the line width of the short circuit type floating electrode. According to the figure, the line width of the short-circuit type floating electrode is from λ / 8 to λ which is larger than λ / 12.
It can be seen that the degree of directivity becomes maximum at about / 6. The degree of directivity is improved by optimizing the line width of the floating electrode, which has been conventionally known.

FIG. 12 is a graph showing changes in the insertion loss of the FEUDT in the forward direction with respect to the line width of the short circuit type floating electrode. From the figure, it can be seen that the insertion loss in the forward direction tends to increase as the line width of the short-circuit type floating electrode increases.

Therefore, from the measurement results of FIG. 11 and FIG. 12, a large degree of directivity can be obtained by thickening the line width of the short-circuit type floating electrode to some extent, but as a side effect of thickening the line width of the floating electrode. It can be seen that the propagation loss of the surface acoustic wave increases, and the insertion loss increases substantially by making the floating electrode thick.

Next, FIG. 13 and FIG. 14 show measurement results of the degree of directivity and insertion loss when the line width of the positive and negative excitation electrodes of the FEUDT is changed. According to FIG. 13, the directivity becomes maximum when the line width of the positive and negative excitation electrodes is in the vicinity of λ / 8 to λ / 5. According to FIG. 14, the line width of the positive and negative excitation electrodes is
The insertion loss becomes the minimum in the vicinity of λ / 8 to λ / 5. From the above, when the line width of the positive and negative excitation electrodes is changed from λ / 8 to λ / 5 thicker than λ / 12,
It can be seen that unlike the change in the line width of the floating electrode shown in, there is no side effect of increase in propagation loss, and insertion loss can be substantially reduced.

In consideration of the above points, a FEUDT having the following structure was created. That is, the FEU shown in FIG.
In DT, the line width We of the positive and negative excitation electrodes is set to λ / 5,
The line width Wf of the short circuit type floating electrode is set to λ / 12. Further, from the center of the branch portion 14 of the positive and negative excitation electrodes to the electrode period, that is, 1/6 of the wavelength λ of the surface acoustic wave at the filter center frequency.
The short-circuit type floating electrode 12 is arranged so that the center of the short-circuiting type floating electrode is located at a position separated by only.

In FIG. 8, on the right side of the positive and negative excitation electrodes 11 and 11 ', at the position where the distance between the centers of the excitation electrodes 11 and 11' is λ / 6,
The structure which has arrange | positioned the short circuit type floating electrode is shown. A three-layer film of Al-1% Cu / Cu / Al-1% Cu is used as the electrode film, and the film thickness is 50 nm / 580 nm / 50 nm.

FIG. 15 shows the frequency-loss characteristic of the FEUDT in the third embodiment having the above configuration.
In FIG. 15, the insertion loss in the forward direction was about 7.9 dB and the directivity was about 4.9 dB. Therefore, comparing with Example 2 (FIG. 11) in which the line widths of the positive and negative excitation electrodes and the floating electrode are the same λ / 12, as in Example 3,
It can be seen that the insertion loss and the directivity are improved by changing the line width.

From the above, the line width of the positive and negative excitation electrodes is larger than the line width (λ / 12) of the short-circuit type floating electrode by λ /
By setting it to about 8 to λ / 5, the directivity of the FEUDT can be improved and the loss as a surface acoustic wave device can also be improved.

The line width Wf of the short circuit type floating electrode is λ / 1.
It may be smaller than 2. This is because even if it is smaller than λ / 12, characteristics such as unidirectionality of FEUDT are hardly affected. Further, as the piezoelectric substrate 1, similar to the first and second embodiments, X-112 ° Y:
LiTaO ₃ was used, but it is not necessary to limit it to this in Example 3, and other substrate materials such as 128 ° Y-X: L are used.
iNbO ₃ may be used.

Further, as shown in Examples 1 and 2,
When using aluminum as the electrode film, FEUDT
Has a film thickness H of 0.4% λ or more, and when a material other than aluminum is used, a value obtained by multiplying the electrode film thickness H by the specific gravity of aluminum is 0.4% λ or more, and a plurality of materials are layered. When combined, the film thickness of the electrode film may be set so that the sum of the film thickness of each material other than aluminum and the specific gravity of aluminum is 0.4% λ or more. preferable.

Also in the third embodiment, a plurality of IDTs are used.
When the surface acoustic wave filter is composed of FEUDT
The film thickness of the aluminum electrode film of all or some of the IDTs may be 0.4% λ or more.
In this case, when the material of the electrode film is other than aluminum, the respective film thicknesses converted into aluminum may be used as described above.

[0066]

According to the present invention, X-11 is formed on the piezoelectric substrate.
2 ° Y: LiTaO ₃ is used, and the internal reflection unidirectional interlace electrode is used as the interdigital electrode that excites the surface acoustic wave. The film thickness of the internal reflection unidirectional interlace electrode is 0.4% of the wavelength λ of the surface acoustic wave. Because of the above, it is possible to provide a stable, small-sized, low-loss surface acoustic wave device that enhances the unidirectionality of the surface acoustic wave to a practically sufficient extent.

Further, the line width of the positive and negative excitation electrodes is set to λ / 8 or more and λ / 5 or less thicker than the line width of the floating electrode, and the line width of the floating electrode is set to λ / 12 or less. And the loss can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment of a surface acoustic wave device of the present invention.

FIG. 2 is a perspective view of an embodiment of the FEUDT of the present invention.

FIG. 3 is a configuration diagram of a sample for measuring the degree of directivity of the FEUDT of the present invention.

FIG. 4 is a frequency-loss characteristic diagram of FEUDT (film thickness: 0.1% λ) in Example 1 of the present invention.

FIG. 5 is a frequency-loss characteristic diagram of FEUDT (film thickness: 0.4% λ) in Example 1 of the present invention.

FIG. 6 is a graph showing the relationship between the normalized electrode film thickness and the degree of directivity in Example 1 of the present invention.

FIG. 7 is a schematic view of Embodiment 1 of the present invention in which an electrode film is Cu +.
It is a frequency-loss characteristic figure in case of Al-1% Cu.

FIG. 8 is a perspective view of an embodiment of FEUDT of Embodiment 2 of the present invention.

FIG. 9 is a frequency-loss characteristic diagram (Cu + Al-1% Cu) of the FEUDT in the case of the second embodiment of the present invention.

FIG. 10 is a frequency-loss characteristic diagram (Al-1% Cu) of FEUDT in the case of Example 2 of the present invention.

FIG. 11 is a graph showing the relationship between the standardized floating electrode width and the directionality in Example 3 of the present invention.

FIG. 12 is a graph showing the relationship between the standardized floating electrode width and the insertion loss in Example 3 of the present invention.

FIG. 13 is a graph showing the relationship between the normalized excitation electrode width and the degree of directivity in Example 3 of the present invention.

FIG. 14 is a graph showing the relationship between the normalized excitation electrode width and the insertion loss in Example 3 of the present invention.

FIG. 15 is a frequency-loss characteristic diagram of FEUDT in Embodiment 3 of the present invention.

FIG. 16 is a configuration diagram of a conventional internal reflection type unidirectional blind electrode.

[Explanation of symbols]

 1 Piezoelectric Substrate 2 Input IDT (FEUDT) 3 Output IDT 4 Sound Absorber 11, 11 'Positive / Negative Excitation Electrode 12 Short-Circuit Floating Electrode 13 Open Floating Electrode 14 Branch of Positive / Negative Excitation Electrode H Film Thickness We Positive / Negative Excitation Electrode Line Width Wf Line width of short circuit type floating electrode

Claims (5)

[Claims] 1. A piezoelectric substrate, and a plurality of interdigital electrodes having positive and negative excitation electrodes for exciting or receiving surface acoustic waves on the piezoelectric substrate, wherein the material of the piezoelectric substrate is X-112 ° Y. : LiTaO ₃ , at least one interdigital electrode is an internal reflection type unidirectional interdigital electrode that excites a surface acoustic wave having unidirectionality, and an electrode film of the internal reflection type unidirectional interdigital electrode. A surface acoustic wave device having a thickness of 0.4% or more of a period λ of the interdigital transducer. 2. The internal reflection type unidirectional interdigital transducer has an open floating electrode that is not electrically connected to the positive and negative excitation electrodes and a positive and negative excitation electrode between the positive and negative excitation electrodes. The surface acoustic wave device according to claim 1, further comprising: a short-circuit type floating electrode in which two floating electrodes are not electrically connected to each other. 3. The internal reflection type unidirectional interdigital transducer has one floating electrode which is not electrically connected to the positive and negative excitation electrodes at a position displaced from the center between the positive and negative excitation electrodes, and The surface acoustic wave device according to claim 1, wherein two adjacent floating electrodes are short-circuited. 4. A piezoelectric substrate, and a plurality of interdigital electrodes having positive and negative excitation electrodes for exciting or receiving surface acoustic waves on the piezoelectric substrate, wherein at least one interdigital electrode has positive and negative electrodes. Between the excitation electrodes, and 1 / of the period λ from the excitation electrode to the interdigital electrode.
One floating electrode that is not electrically connected to the positive and negative excitation electrodes is provided at a position separated by a distance of 6
An internal reflection type unidirectional blind electrode having a structure in which two floating electrodes are short-circuited, and the line widths of the positive and negative excitation electrodes are λ / 8 or more and λ / 5.
In the following, the surface acoustic wave device is characterized in that the floating electrode has a line width of λ / 12 or less. 5. A surface acoustic wave filter comprising one or a plurality of the surface acoustic wave devices according to claim 1 or 4.