JP2006303834A - Surface-acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To constitute a SAW device that endures a temperature change in heat cycle testing or the like, when the SAW device is constituted by using a piezoelectric substrate having strong pyroelectric effects. <P>SOLUTION: In a surface acoustic-wave device, a surface acoustic-wave device element, formed by arranging at least one IDT electrode on the piezoelectric substrate having the strong pyroelectric effect, is bonded and fixed to the internal bottom section of a package by using adhesive. In such a surface acoustic-wave device, the hardness of the adhesive is set to Shore 28 or smaller. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a surface acoustic wave device, and more particularly to a surface acoustic wave device that prevents electrode destruction due to the pyroelectric effect.

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 (ETC), 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 as a heat cycle test, 3000 cycles to 6000 cycles of −55 ° C. (5 minutes) to 125 ° C. (5 minutes) are imposed.
Until now, many of these SAW filters have been resonator-type SAW filters. However, the resonator-type SAW filters have the disadvantage of poor group delay time characteristics, and recently, unidirectional converters have been used. The demand for a transversal SAW filter having good group delay time characteristics has been increasing.

  FIG. 4A is a plan view showing the basic configuration of the transversal SAW filter element, and the IDT electrodes 21 and 22 are arranged on the main surface of the piezoelectric substrate 20 along the propagation direction of the surface wave at a predetermined interval. And the electrode 23 is disposed between the IDT electrodes 21 and 22. The IDT electrodes 21 and 22 are each formed by a pair of comb-shaped electrodes having a plurality of electrode fingers that are interleaved with each other. FIGS. 4B and 4C are a plan view in which the transversal SAW filter element shown in FIG. 4A is mounted on the inner bottom portion of the package 24 and a cross-sectional view at QQ. As shown in the plan view of FIG. 4B, terminal electrodes 25, 25 ′, 26, and 26 ′ that are electrically connected to the connecting terminal electrode (not shown) on the outer bottom portion at the peripheral stepped portion, and the grounding terminal electrode. As shown in the sectional view of FIG. 4C, the back surface of the piezoelectric substrate 20 is bonded and fixed to the inner bottom portion of the package 24 formed with the adhesive 27 using an adhesive 28. The comb electrodes of the IDT electrode 21 are connected to the input terminal electrodes 25 and 25 ′ by bonding wires 29, and the comb electrodes of the IDT electrode 22 are similarly connected to the output terminal electrodes 26 and 26 ′. The Further, the electrode 23 is connected to the ground terminal electrode 27 by a bonding wire 29, and a metal lid 31 is resistance-welded to a metallized portion formed on the upper peripheral portion of the package 24 to constitute a transversal SAW filter. .

When a regular IDT electrode is used as shown in FIG. 4 (a), the excited surface wave propagates equally to both the left and right, so that a loss of 6 dB is essentially generated. Therefore, it is common to use a surface acoustic wave transducer having unidirectionality to reduce insertion loss. In addition, the surface acoustic wave absorber (sound absorbing material) 30 may be applied to both ends of the piezoelectric substrate 20 in the long side direction (surface wave propagation direction) to suppress unnecessary reflected waves. Note that the action of the solid electrode 23 functions to shield direct waves between the input and output terminals.

Using a piezoelectric substrate with Y-cut lithium niobate (LiNbO ₃ ) rotated at 128 °, hardness Shore A80 as the adhesive, and a unidirectional surface acoustic wave transducer as part of the IDT electrode, the center frequency is 40 MHz, A transversal SAW filter with a width of 4 MHz was constructed, and good characteristics were obtained in both the passband and attenuation band. This transversal SAW filter was subjected to a heat cycle test in a temperature range of −55 ° C. (5 minutes) to 125 ° C. (5 minutes), 300 cycles and 3000 cycles. As a result, as shown in FIG. 5, 15% of electrode breakage occurred at 300 cycles and 87% at 3000 cycles. This electrode breakdown is thought to be mainly due to the pyroelectric effect of lithium niobate.

Therefore, the improved lithium niobate disclosed in Japanese Patent Application Laid-Open No. 2004-254114 or the like is used, which is less likely to generate static electricity due to temperature change and can quickly self-neutralize even when charges are generated. This improved lithium niobate is a piezoelectric crystal in which iron, copper, manganese, molybdenum, cobalt, nickel, zinc, or the like is contained in a lithium niobate single crystal at a ratio of 0.01 wt% to 1.00 wt%. A failure rate shown in FIG. 6 is obtained by making a prototype using this piezoelectric substrate in the same manner as in FIG. 5 and conducting a heat cycle test. In the 3000 cycle heat cycle test, no electrode destruction occurred, but in the 6000 cycle, about 4% electrode destruction occurred.
JP 2004-254114 A

The problem to be solved is that the transversal type SAW filter is formed using the improved lithium niobate as described above in order to prevent the electrode destruction due to the pyroelectric effect, but the temperature range is -55 ° C ( In the heat cycle test of 5 minutes) to 125 ° C. (5 minutes) and 6000 cycles, there is a problem that about 4% of electrode destruction occurs.

In the surface acoustic wave device according to the first aspect of the present invention, an adhesive is used for the surface acoustic wave device element formed by disposing at least one IDT electrode on the piezoelectric substrate in order to improve the guaranteed attenuation. In the surface acoustic wave device bonded and fixed to the inner bottom portion of the package, the surface acoustic wave device is configured by setting the hardness of the adhesive to Shore 50 or less.
According to a second aspect of the present invention, there is provided a surface acoustic wave device in which a surface acoustic wave device element formed by disposing at least one IDT electrode on a piezoelectric substrate is bonded and fixed to an inner bottom portion of a package with an adhesive. The surface acoustic wave device is configured with a hardness of shore 28 or less.
According to a third aspect of the present invention, in the piezoelectric substrate according to the first or second aspect, the piezoelectric substrate is a piezoelectric substrate cut out from a lithium niobate single crystal having charge neutralization characteristics that self-neutralizes and removes surface charges. It is a surface acoustic wave device of description.

  Since the surface acoustic wave device according to the present invention is configured using a soft adhesive as an adhesive used for bonding and fixing the back surface of the piezoelectric substrate and the bottom of the package, the difference in thermal expansion coefficient between the package and the piezoelectric substrate. Therefore, the surface acoustic wave device that can sufficiently withstand the heat cycle test can be constructed.

  FIG. 1 is a diagram showing an embodiment of a transversal type surface acoustic wave filter according to the present invention. FIG. 1 (a) is a transversal type surface acoustic wave filter element, and FIG. 1 (b) is a transversal type elastic wave filter. The top view of what mounted the surface wave filter element in the package, the figure (c) is a sectional view in QQ. The basic configuration of the transversal SAW filter element is that the IDT electrodes 2 and 3 are arranged on the main surface of the piezoelectric substrate 1 along the propagation direction of the surface wave at a predetermined interval and the IDT electrodes 2 and The electrode 4 is disposed between the three. IDT electrodes The IDT electrodes 2 and 3 are each formed of a pair of comb-shaped electrodes having a plurality of electrode fingers that are interleaved with each other. As shown in the plan view of FIG. 1B, terminal electrodes 6, 6 ′, 7, 7 ′ which are electrically connected to connection terminal electrodes (not shown) on the outer bottom portion at the stepped portion on the periphery of the package 5, and grounding Terminal electrode 8 is formed. As shown in the sectional view of FIG. 1C, the back surface of the piezoelectric substrate 1 is bonded and fixed to the inner bottom portion of the package 5 using an adhesive 9. The comb electrodes of the IDT electrode 2 are connected to the terminal electrodes 6 and 6 ′ by the bonding wire 10, and the comb electrodes of the IDT electrode 3 are similarly connected to the input terminal electrodes 7 and 7 ′. The Further, the electrode 4 is connected to the grounding terminal electrode 8 by a bonding wire 10, and a metal lid 12 is resistance-welded to a metallized portion (not shown) formed on the upper peripheral edge of the package 5, thereby transversal SAW. A filter is configured. Note that the sound absorbing material 11 may be applied to both ends of the piezoelectric substrate 1 to suppress unnecessary reflected waves.

  Even if a transversal SAW filter is constructed using a modified lithium niobate that self-neutralizes, a temperature range of −55 ° C. (5 minutes) to 125 ° C. (5 minutes), 4% in a heat cycle test of 6000 cycles Some degree of electrode destruction occurred. The reason for this is that there is a difference between the thermal expansion coefficient of the piezoelectric substrate and the thermal expansion coefficient of the package. If the piezoelectric substrate is firmly bonded and fixed to the inner bottom of the package, the piezoelectric substrate is distorted with respect to temperature changes. It was presumed that the electric charge generated by the distortion was superimposed on the electric charge due to the pyroelectric effect, and electrode destruction occurred.

  Therefore, a transversal SAW filter was constructed by changing the hardness of the adhesive that fixes the piezoelectric substrate to the inner bottom of the package, and a heat cycle test was performed. The piezoelectric substrate used was a 128 ° rotated Y-cut cut from ordinary lithium niobate, and the adhesive hardness of the piezoelectric substrate was Shore A50, Shore A35, Shore A28, and a temperature range of −55 ° C. (5 minutes) to 125 ° C. (5 minutes), 300 cycles and 3000 cycles of heat cycle test. The results are shown in FIGS. 2 (a) and 2 (b). When the Shore A50 adhesive was used, 0% electrode breakage occurred in the 300-cycle heat cycle test and 55% electrode breakage occurred in the 3000-cycle heat cycle test. When the Shore A35 adhesive was used, electrode breakage was 0% in the 300-cycle heat cycle test and 30% electrode breakage in the 3000-cycle heat cycle test. When the Shore A28 adhesive was used, electrode breakage was 0% in the 300-cycle heat cycle test, and 2% electrode breakage occurred in the 3000-cycle heat cycle test. From this result, it was found that when ordinary lithium niobate is used, the electrode breakage can be reduced by using Shore A28 having a low hardness as the adhesive for fixing the piezoelectric substrate.

  Next, a transversal-type SAW filter element is formed by using improved lithium niobate for the piezoelectric substrate, and Shore A50, Shore A35, and Shore A28 are used as the hardness of the adhesive of the piezoelectric substrate, respectively. And a heat cycle test of a temperature range of −55 ° C. (5 minutes) to 125 ° C. (5 minutes), 3000 cycles, and 6000 cycles was performed. The results are shown in FIGS. 3 (a) and 3 (b). It has been found that no electrode breakage occurs even when an adhesive having any hardness of Shore A50, Shore A35, or Shore A28 is used.

Although the transversal SAW filter using the lithium niobate and the improved lithium niobate as the piezoelectric substrate has been described above, the present invention relates to a resonator type SAW filter using lithium niobate, an improved lithium niobate, Needless to say, the present invention can be applied to SAW devices such as SAW resonators.
The present invention can also be applied to lithium tantalate, improved lithium tantalate, and the like, which are strong pyroelectric effect crystals.

BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic block diagram which showed the structure of the transversal type | mold SAW filter based on this invention, (a) is a top view of an element, (b) is the top view mounted in the package, (c) is sectional drawing. (A), (b) is a figure which shows the relationship between the hardness of an adhesive agent, and an electrode destruction rate in a heat cycle test. (A), (b) is a figure which shows the relationship between the hardness of an adhesive agent, and an electrode destruction rate in a heat cycle test. It is a figure which shows the structure of the conventional transversal type | mold SAW filter, (a) is a top view of an element, (b) is the top view mounted in the package, (c) is sectional drawing. It is a figure which shows the relationship between the hardness of the adhesive agent, and an electrode destruction rate in a heat cycle test. It is a figure which shows the relationship between the hardness of the adhesive agent, and an electrode destruction rate in a heat cycle test.

Explanation of symbols

1 Piezoelectric substrate 2, 3 IDT electrode 4 Electrode (solid electrode)
5 Package 6, 6 ', 7, 7' Terminal electrode 8 Ground electrode 9 Adhesive 10 Bonding wire 11 Sound absorbing material 12 Metal lid

Claims (3)

In a surface acoustic wave device in which a surface acoustic wave device element formed by arranging at least one IDT electrode on a piezoelectric substrate is bonded and fixed to an inner bottom portion of a package using an adhesive,
A surface acoustic wave device characterized in that the adhesive has a hardness of Shore 50 or less. In a surface acoustic wave device in which a surface acoustic wave device element formed by arranging at least one IDT electrode on a piezoelectric substrate is bonded and fixed to an inner bottom portion of a package with an adhesive,
A surface acoustic wave device characterized in that the adhesive has a hardness of Shore 28 or less. 3. The surface acoustic wave device according to claim 1, wherein the piezoelectric substrate is a piezoelectric substrate cut out from a lithium niobate single crystal having charge neutralization characteristics for self-neutralizing and removing surface charges.

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