JP2006253784A - Surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a means for strengthening heat-resistant cycle testing of a surface acoustic wave device using a positive/negative reflecting type reflector. <P>SOLUTION: The positive/negative reflecting type reflector is formed by arranging first electrodes with a width of λ/8 (λ is a wavelength of a surface wave) in periods of λ/2 and second electrodes with the width of λ/8 in the periods of λ/2 on the main surface of a piezoelectric substrate along in the propagation direction of the surface wave, by setting an intercentral distance between the center of the first electrode and that of the second electrode to λ/4, and by short-circuiting the first electrodes each other with a short circuit electrode. The surface acoustic wave device is composed by using an IDT electrode in which the positive/negative reflecting type reflector is arranged to a part of the IDT electrode. The width of the short circuit electrode is set to 2 μm or more. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a surface acoustic wave device, and more particularly to a surface acoustic wave device in which electrode breakage due to the pyroelectric effect of a SAW device using a positive / negative reflection type reflector as a part of an IDT electrode is improved.

In recent years, surface acoustic wave devices (SAW devices) have been widely used in the communication field, and are widely used especially for cellular phones, LANs, and the like because they have excellent characteristics such as high performance, small size, and mass productivity. Recently, SAW filters have also been used in highway automatic toll collection systems (ETCs), and many standards have a center frequency of 40 MHz and a bandwidth of 4 MHz to 5 MHz. Equipment mounted on the vehicle such as ETC has a severe use environment, and 3000 cycles of −55 ° C. (5 minutes) to 125 ° C. (5 minutes) are imposed as a heat cycle test.
Until now, many of these resonator type SAW filters have been adopted as these SAW filters. However, since the resonator type SAW filter has a defect that the group delay time characteristic is poor, recently, there is an increasing demand for a transversal type SAW filter having a good group delay time characteristic using a unidirectional converter. Yes.

FIG. 3 is a plan view showing the basic configuration of the transversal SAW filter. The IDT electrodes 12 and 13 are arranged on the main surface of the piezoelectric substrate 11 along the propagation direction of the surface wave at a predetermined interval. To do. The IDT electrodes 12 and 13 are each formed by a pair of comb electrodes having a plurality of electrode fingers interleaved with each other. One comb electrode of the IDT electrode 12 is connected to the input terminal IN, and the other comb electrode is grounded. Is done. Further, one comb-shaped electrode of the IDT electrode 13 is connected to the output terminal OUT, and the other comb-shaped electrode is grounded to constitute a transversal SAW filter.
In the IDT electrode as shown in FIG. 3, the excited surface wave propagates equally to both the left and right, so that a loss of 6 dB is essentially generated. Therefore, a surface acoustic wave transducer having unidirectionality has been developed and used to reduce insertion loss.

4A and 4B are schematic plan views showing the configuration of a strip-type grating reflector (hereinafter referred to as a grating reflector), in which a thin film strip 14 of metal or the like is placed on a substrate (not shown). Make it attached. When a piezoelectric substrate is used as the substrate, in addition to the elastic perturbation effect of the metal strip, electrical perturbation due to the electric field short-circuit effect of the metal strip is superimposed.
As shown in FIG. 4A, when a surface wave is perpendicularly incident on the grating reflector and reflected, the phase matching condition is expressed by the following equation from the Bragg condition, as is well known.
p = mλ / 2 (1)
Here, p is the period of the metal strip 14 (perturbation period), λ is the wavelength of the incident surface wave, and m is an integer. When Equation 1 is satisfied, that is, when the period p of the metal strip 11 is an integral multiple of half the wavelength of the surface wave, all the reflected surface waves are added in the same phase. It has been broken.
A grating reflector in which a metal strip 15 is short-circuited with a metal electrode 16 as shown in FIG. 4B is also often used in a SAW device.

FIG. 5 is a schematic plan view showing the configuration of a surface acoustic wave reflector having a positive / negative reflection coefficient (hereinafter referred to as a positive / negative reflection type reflector) disclosed in Japanese Patent Application Laid-Open No. 60-263505. A metal strip (metal electrode) 21 having a period of λ / 2 (λ is the wavelength of the surface wave) and a width of λ / 8 is disposed on the surface of the metal electrode 21, and the metal electrode 22 is interposed between these metal electrodes 21. Connecting. Further, a metal electrode 23 having a period of λ / 2 and an electrode width of λ / 8 is disposed at the center position of the paired electrodes 21. Since the electrode 21 is short-circuited by the metal electrode 22, if the phase of the reflected wave due to the piezoelectric action of this short-circuited floating electrode is −90 degrees, it is due to the piezoelectric action of the open floating electrode 23 that is not coupled to any of them. The phase of the reflected wave is +90 degrees.
When the incident surface wave is reflected, if the distance between the centers of the electrode 21 and the electrode 23 is set to λ / 4, the reflected wave from the electrode 21 and the reflected wave from the electrode 23 are in phase. Since the reflected waves are added together and have a large reflection coefficient, a strong reflected wave is obtained.

Since the IDT electrode in which the positive / negative reflection type reflector shown in FIG. 5 is incorporated in a part of the electrode as shown in FIG. 6 has strong unidirectionality, it can be used as an IDT electrode of a transversal SAW filter.
FIG. 7 shows a transformer in which a split electrode is used for the IDT electrode 25 on the input side, a positive / negative reflection type reflector is used for a part of the IDT 26 on the output side, and a shielding electrode 27 is arranged between the IDT electrodes 25 and 26. This is a Versal type SAW filter.
However, if a positive / negative reflection type reflector is arranged inside the IDT electrode, as shown in FIG. 6, the width of the short-circuit electrode 22 ′ and the space between the short-circuit electrode 22 ′ and the bus bar 24 are required, and the surface wave In order to avoid this diffraction, the width of the short-circuit electrode 22 'is generally set to 1 μm or less. Incidentally, unidirectional converters are also disclosed in prior applications such as JP-A-2000-278075 and JP-A-2001-144573, and drawings are shown, but this is a conceptual diagram to the last, The width of the short-circuit electrode is generally set to 1 μm or less.
JP-A-60-263505 JP 2000-278075 A JP 2001-144573 A Mikio Shibayama et al. “Acoustic wave element technology handbook” published by Ohmsha, November 30, 1991

A 128-degree Y-cut LiNbO ₃ is used for the piezoelectric substrate, a positive / negative reflection type reflector is used for a part of the IDT electrode as shown in FIG. 7, and the short-circuit electrode width is 1 μm. The transversal SAW filter having a surface wave wavelength λ of about 100 μm and an electrode finger width of 12.5 μm and a bandwidth of 4 MHz has good characteristics in both passband and attenuation gradient.
However, when this transversal type SAW filter was subjected to a heat cycle test (-55 ° C. (5 minutes) to 125 ° C. (5 minutes)) of 1000 cycles, 3000 cycles, and 6000 cycles, 15 pieces as shown in FIG. Among them, there was a problem that seven, nine, and ten failures occurred, respectively.

Since the surface acoustic wave device of the present invention reduces electrode breakage due to the pyroelectric effect, the invention of claim 1 has a width when the wavelength of the surface wave excited by the IDT electrode arranged on the piezoelectric substrate is λ. Are arranged in order so that the center-to-center spacing is λ / 4, the first and third grating electrodes are open, and the second and second grating electrodes are λ / 8. 4 is a surface acoustic wave device having a structure in which a positive / negative reflection type reflector, which is short-circuited by electrically connecting four grating electrodes with a short-circuit electrode, is replaced with a part of electrode fingers constituting the IDT electrode. The surface acoustic wave device is characterized in that the short-circuit electrode has a width of 2 μm or more.
The invention according to claim 2 is the surface acoustic wave device according to claim 1, wherein both ends of the second and fourth grating electrodes are connected to each other by a short-circuit electrode.
A third aspect of the present invention is the surface acoustic wave device according to the first or second aspect, wherein the surface acoustic wave device is a transversal surface acoustic wave filter configured using the IDT electrode as an input or output IDT electrode.
According to a fourth aspect of the present invention, in the surface acoustic wave device according to any one of the first to third aspects, the electrode finger constituting the IDT electrode is a split electrode.
The invention according to claim 5 is the surface acoustic wave device according to any one of claims 1 to 4, characterized in that the piezoelectric substrate is either a rotated Y-cut lithium niobate or a rotated Y-cut lithium tantalate. .
According to the sixth aspect of the present invention, the first to fourth grating electrodes having a width of λ / 8 (λ is the wavelength of the surface acoustic wave propagating on the piezoelectric substrate) on the piezoelectric substrate have a center-to-center spacing of λ / 4. Are arranged in order along the propagation direction of the surface acoustic wave so that the first and third grating electrodes are open, and the second and fourth grating electrodes are electrically connected by a short-circuit electrode. A surface acoustic wave device including a positive and negative reflection type reflector that is a short-circuit type, wherein the short-circuit electrode has a width of 2 μm or more.

  Since the surface acoustic wave device of the present invention is designed so that the short-circuit electrode of the positive and negative reflection type reflector is wide, even if the SAW device is configured using a piezoelectric material having a large pyroelectric effect such as lithium niobate and lithium tantalate, There is an advantage that failure in the heat cycle test can be greatly reduced.

FIG. 1 is a view showing an embodiment of a SAW device according to the present invention, and is a schematic plan view showing a main part of an IDT electrode. The main part of the IDT electrode includes a split electrode α in which two positive electrode fingers 2 and a negative electrode finger 2 are arranged in 1λ (λ is the wavelength of a surface wave), and a positive / negative reflection type reflector indicated by β. I have. The configuration of the positive / negative reflection type reflector β is the first to fourth grays having a width of λ / 8, where λ is the wavelength of the surface wave excited by the IDT electrode disposed on the piezoelectric substrate (not shown). The first and third grating electrodes 1 and 3 are opened, and the second and fourth gray electrodes are arranged in order such that the spacing between the centers is λ / 4. The positive and negative reflection type reflectors are configured by conducting the connecting electrodes 2 and 4 through the short-circuit electrode 5.
When the phase of the reflected wave due to the piezoelectric action of the short-circuit type floating electrode 2 is set to −90 degrees by short-circuiting the second electrode 2, the phase of the reflected wave due to the piezoelectric action of the open type floating electrode 1 of the first electrode is +90. Since the reflected wave from the first electrode 1 and the reflected wave from the second electrode 2 are in phase, a strong reflected wave is obtained. As shown in FIG. 1, by providing a positive / negative reflection type reflector on a part of the IDT electrode, the IDT electrode functions as a unidirectional converter, and a transversal SAW filter is configured using the IDT electrode. Then, a filter with a small insertion loss can be obtained.

Lithium niobate and lithium tantalate, which have a large electromechanical coupling coefficient and a ferroelectric piezoelectric material, also have a large pyroelectric effect. It was presumed that the occurrence of a spark in between and a part of the electrode was caused by the failure of the transversal SAW filter shown in FIG.
Therefore, the width of the short-circuit electrode 3 shown in FIG. 1 was widened to 2 μm and 4 μm, a transversal SAW filter was prototyped using the electrode pattern shown in FIG. 7, and a heat cycle test was performed. FIG. 2 shows the results of a heat cycle test of 15 transversal SAW filters in which the width of the short-circuit electrode 3 is increased. When the width of the short-circuit electrode 3 was 2 μm and 4 μm, there was no failure in any case after the heat cycle test of 1000 cycles, 3000 cycles, and 6000 cycles.

In the above description, an embodiment in which a positive / negative reflection type reflector is provided on a part of the IDT electrode of a transversal SAW filter has been described. If a SAW device is configured by arranging positive and negative reflection type reflectors on a part of the electrodes, the loss of the SAW device can be reduced, and a SAW device that is resistant to the pyroelectric effect can be configured.

When the thickness of the short-circuiting electrode 3 shown to h, and width w 1, the product h · w is 0.8 [mu] m ² or more, by a 408Myuemu ² or less, since the short-circuiting electrode 3 is enhanced pyroelectric The damage of the short-circuit electrode 3 which is an influence of the effect could be avoided.

In the above, an example using 128-degree Y-cut LiNbO ₃ as the piezoelectric substrate has been described, but the same applies to other rotated Y-cut lithium niobate. Further, the present invention can be similarly applied to other piezoelectric materials such as rotated Y-cut lithium tantalate having a large pyroelectric effect.

It is the schematic block diagram which showed the structure of the principal part of the IDT electrode which concerns on this invention. It is a result of the heat cycle test of the transversal type SAW filter concerning the present invention. It is a top view which shows the structure of the conventional transversal type | mold SAW filter. (A), (b) is a figure explaining the conventional reflector. It is a figure explaining the principle of a positive / negative reflection type reflector. It is a top view of the principal part of the electrode pattern provided with the positive / negative reflection type reflector in a part of IDT electrode. It is a schematic plan view of a transversal type SAW filter in which a positive / negative reflection type reflector is arranged on a part of an IDT electrode. It is a result of the heat cycle test of the conventional transversal type SAW filter.

Explanation of symbols

1, 2, 3, 4 Electrode finger 5 Short-circuit electrode 6 Bus bar α Split electrode β Positive / negative reflection type reflector

Claims (6)

When the wavelength of the surface wave excited by the IDT electrode disposed on the piezoelectric substrate is λ, the first to fourth grating electrodes having a width of λ / 8 are arranged in order so that the center-to-center spacing is λ / 4. A positive and negative reflection type reflector in which the first and third grating electrodes are open-type, and the second and fourth grating electrodes are electrically connected by a short-circuit electrode to form a short-circuit type, the IDT electrode A surface acoustic wave device having a structure in which the electrode finger is replaced with a part of the electrode finger, wherein the short-circuit electrode has a width of 2 μm or more. 2. The surface acoustic wave device according to claim 1, wherein both ends of the second and fourth grating electrodes are connected to each other by a short-circuit electrode. The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device is a transversal surface acoustic wave filter configured using the IDT electrode as an input or output IDT electrode. The surface acoustic wave device according to claim 1, wherein the electrode fingers constituting the IDT electrode are split electrodes. 5. The surface acoustic wave device according to claim 1, wherein the piezoelectric substrate is either a rotated Y-cut lithium niobate or a rotated Y-cut lithium tantalate. Surface acoustic waves are formed on the piezoelectric substrate so that the distance between the centers of the first to fourth grating electrodes having a width of λ / 8 (λ is the wavelength of the surface acoustic wave propagating on the piezoelectric substrate) is λ / 4. Positive and negative reflection type in which the first and third grating electrodes are open-type, and the second and fourth grating electrodes are conductively connected by a short-circuit electrode. A surface acoustic wave device including a reflector, wherein the short-circuit electrode has a width of 2 μm or more.

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