JP2011071838A - Surface acoustic wave element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave element which is excellent in long term reliability and has high temperature characteristic improvement effect. <P>SOLUTION: The surface acoustic wave element is provided at least with: a piezoelectric substrate 2; a support substrate 3 stuck to the piezoelectric substrate 2 via an adhesive layer 4; and an interdigital electrode 5 which excites surface acoustic waves formed on a surface opposite to the bonding surface of the piezoelectric substrate 2, wherein a glass transition point Tg of the adhesive layer 4 is ≤200°C. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a surface acoustic wave device using a composite piezoelectric substrate.

A surface acoustic wave (SAW) element in which a comb-shaped electrode for exciting a surface acoustic wave is formed on a piezoelectric substrate is used as a frequency selection component in high-frequency communication such as a cellular phone. The piezoelectric substrate material used for this has high conversion efficiency (hereinafter referred to as electromechanical coupling coefficient) from electrical signals to mechanical vibrations, and the center frequency of filters, etc., determined by the electrode spacing of the comb electrodes and the acoustic velocity of the elastic waves. Is required not to vary with temperature (hereinafter referred to as temperature characteristics).
In other words, it is preferable to have a piezoelectric substrate having both a large electromechanical coupling coefficient and a small frequency temperature coefficient.

  As an example of a piezoelectric substrate that realizes such characteristics, there is a composite piezoelectric substrate in which a piezoelectric substrate and another substrate are bonded. As an example of such a composite piezoelectric substrate, the piezoelectric substrate and a support substrate are bonded with an adhesive layer, A surface acoustic wave element in which an electrode for exciting and detecting an acoustic wave is provided on the surface of a piezoelectric substrate is disclosed (see Patent Document 1). In addition, the surface acoustic wave device including the composite piezoelectric substrate as described above has a good temperature characteristic without changing various characteristics such as an electromechanical coupling coefficient, and increases the degree of freedom in designing the temperature characteristic. A surface acoustic wave element is disclosed in which the glass transition point Tg of the adhesive layer is set to −30 ° C. or more (see Patent Document 2).

  However, even when a composite piezoelectric substrate is produced using an adhesive having a glass transition point Tg of −30 ° C. or higher as described above, the effect of improving the temperature characteristics cannot be sufficiently obtained, Peeling or cracks on the surface of the piezoelectric substrate may occur, which causes a problem that the reliability of the surface acoustic wave element is inferior.

JP 2001-053579 A JP 2002-026684 A

  The present invention has been made in view of such problems, and an object of the present invention is to provide a surface acoustic wave device that has excellent long-term reliability and a high effect of improving temperature characteristics.

  In order to solve the above problems, in the present invention, at least a piezoelectric substrate, a support substrate bonded to the piezoelectric substrate via an adhesive layer, and an elastic surface formed on a surface opposite to the adhesive surface of the piezoelectric substrate There is provided a surface acoustic wave device including a comb-shaped electrode for exciting a wave, wherein a glass transition point Tg of the adhesive layer is 200 ° C. or lower.

  As described above, when the glass transition point Tg of the adhesive layer is 200 ° C. or lower, the surface acoustic wave element has excellent long-term reliability, and the temperature characteristics of the substrates bonded by the adhesive layer can be improved. .

Moreover, it is preferable that the glass transition point Tg of the said contact bonding layer is 100 degreeC or more and 200 degrees C or less.
As described above, when the glass transition point Tg of the adhesive layer is 100 ° C. or higher and 200 ° C. or lower, the effect of improving the temperature characteristics can be further stabilized.

  Moreover, it is preferable that the elasticity modulus in the room temperature of the said contact bonding layer is 1.5 GPa or more, and the elasticity modulus in 85 degreeC is 1 GPa or more.

  Thus, if an adhesive layer having an elastic modulus at room temperature of 1.5 GPa or more and an elastic modulus at 85 ° C. of 1 GPa or more is used, a material having a large thermal expansion coefficient among substrates made of different materials bonded together. It is possible to suppress the thermal expansion of the film, and the temperature characteristics can be improved.

  As described above, the surface acoustic wave device of the present invention can be a surface acoustic wave device having excellent long-term reliability and a high effect of improving temperature characteristics.

It is the cross-sectional schematic which showed an example of the composite piezoelectric substrate used for the surface acoustic wave element which concerns on this invention.

Hereinafter, the present invention will be described more specifically.
As described above, in a surface acoustic wave element including a composite piezoelectric substrate in which a piezoelectric substrate and a support substrate are bonded via an adhesive layer, the glass transition point Tg of the adhesive layer is set to −30 ° C. or higher according to the prior art. In reality, however, various problems such as peeling at the bonding surface and cracking on the surface of LT (lithium tantalate), which is a piezoelectric substrate, occur, for example, when a heat cycle test or the like is performed. It was.

  As a result of diligent experiments and examinations on the above problem, the present inventor has reached the idea that when the adhesive having a high glass transition point Tg is used, the above-described problem occurs without releasing the stress. As a result of further experiments and the like by the present inventor based on this idea, if the glass transition point Tg of the adhesive is 200 ° C. or less, the above-described problem does not occur, and the surface acoustic wave device is obtained. The inventors have found that it is excellent in reliability and can improve temperature characteristics, and have completed the present invention.

Hereinafter, the present invention will be described in detail, but the present invention is not limited thereto.
FIG. 1 is a schematic cross-sectional view showing an example of a composite piezoelectric substrate used in a surface acoustic wave device according to the present invention.
The surface acoustic wave element 1 is formed on a side opposite to the adhesive surface of the piezoelectric substrate 2 and a composite piezoelectric substrate 6 in which the piezoelectric substrate 2 and the support substrate 3 are bonded together via an adhesive layer (adhesive) 4. And a comb-shaped electrode 5 for exciting a surface acoustic wave.
And in this invention, the glass transition point Tg of the contact bonding layer 4 is 200 degrees C or less, It is characterized by the above-mentioned.

Since the surface acoustic wave device 1 of the present invention has such a configuration, stress is generated in the piezoelectric substrate in accordance with a temperature change, and the temperature characteristics can be improved.
Further, since the substrates are bonded together via the adhesive layer 4, it can be made relatively inexpensive.
Since the glass transition point Tg of the adhesive layer 4 that bonds the piezoelectric substrate 2 and the support substrate 3 is 200 ° C. or less, the thermal expansion of a material having a large thermal expansion coefficient is suppressed and stress is appropriately released. And long-term reliability.

  In particular, when a glass transition point exists in the range of a typical operating temperature of −40 to + 85 ° C. of the surface acoustic wave element, the stress applied to the piezoelectric substrate is relaxed by the adhesive layer at a temperature exceeding the glass transition point. Therefore, since the effect of improving the temperature characteristics may change across the glass transition point, it is more preferable to use a glass transition point Tg of 100 ° C. or higher and 200 ° C. or lower.

Next, the components of the composite piezoelectric substrate 6 will be specifically described.
The composite piezoelectric substrate 6 shown in FIG. 1 is manufactured by bonding the piezoelectric substrate 2 and the support substrate 3 through the adhesive layer 4 as described above.
Specifically, it can be produced by, for example, applying an adhesive to one or both of the piezoelectric substrate 2 and the support substrate 3, bonding them together under vacuum, and firmly bonding them. At this time, it is preferable to clean the surface of each substrate before bonding so that no foreign matter is mixed into the adhesive surface, and the surface is subjected to a hydrophilic treatment with an ammonia-hydrogen peroxide aqueous solution or a plasma treatment. Thus, the adhesive strength may be increased.

The piezoelectric substrate 2 may be any material as long as it is made of a piezoelectric crystal material such as quartz, but if it is made of any one of lithium tantalate, lithium niobate, and lithium borate, these are electrically Since the crystal material has a large mechanical coupling coefficient, it can be a composite piezoelectric substrate capable of manufacturing a SAW device having a wide bandwidth as a frequency selective filter and a small insertion loss. A piezoelectric substrate made of these piezoelectric crystal materials can be obtained with a high quality by growing these single crystal rods by, for example, the Czochralski method and slicing them to a desired thickness.
The substrate orientation can also be appropriately selected according to the type of piezoelectric crystal material, the use of the SAW device, desired characteristics, etc., such as 36 ° rotation Y-cut, 41 ° rotation Y-cut, 45 ° rotation Y-cut, etc. .

  The material of the support substrate 3 is not particularly limited as long as it has a steel property such as ceramics and has high durability. For example, the main component is preferably alumina. As long as the main component is alumina, the composite piezoelectric substrate 6 is relatively inexpensive and hard, so that warpage is suppressed even when heated.

  The size of the substrate is not particularly limited. For example, the substrate can have a diameter of 100 mm, but may be larger or smaller.

  If the glass transition point Tg is 200 ° C. or less, the adhesive layer 4 is preferable because it has excellent long-term reliability and an effect of improving temperature characteristics is obtained. It is more preferable because the effect of improving the temperature characteristics can be further stabilized.

  The adhesive layer 4 preferably has an elastic modulus of, for example, 1.5 GPa or more at room temperature and 1 GPa or more at 85 ° C. If the adhesive layer 4 has such an elastic modulus, it becomes possible to suppress thermal expansion of a substrate having a large thermal expansion coefficient among substrates of different materials bonded together, and the temperature characteristics of the bonded substrate can be reduced. Can be improved.

  Although it will not specifically limit as an adhesive agent which comprises the contact bonding layer 4, if the above prescriptions are satisfy | filled, For example, the adhesive agent etc. which have an epoxy as a main component are mentioned.

The comb-shaped electrode 5 is a pair of electrodes including a plurality of electrode fingers and a bus bar that commonly connects the electrode fingers, and can be formed using a conventional photolithography method or the like.
FIG. 1 shows the basic configuration of the surface acoustic wave element 1 using the comb-shaped electrode 5, but the number and configuration of the comb-shaped electrodes 5 may be changed according to the desired function and application of the surface acoustic wave element 1. Of course, it can be changed.

EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not limited to these.
(Examples 1-5, Comparative Examples 1-2)
As shown in Table 1 below, commercially available epoxy UV adhesives A to G having different glass transition points Tg are used, and lithium tantalate as a piezoelectric substrate and alumina as a support substrate are passed through each adhesive. The composite piezoelectric substrate was manufactured by bonding.
Each produced composite piezoelectric substrate was cut into 1 mm square chips, and a reflow test (260 ° C.) was performed three times. Thereafter, 100 cycles of a heat cycle test (−40 ° C. to 125 ° C.) were performed to inspect the appearance of the composite piezoelectric substrate.

  Next, comb electrodes were formed on the above composite piezoelectric substrate and processed into a chip shape in the same manner, and the temperature coefficient of the antiresonance frequency and the variation of the temperature coefficient in the substrate were examined.

The results are shown in Table 1.

As shown in Table 1, 22 Examples 1 to 5 (adhesives A to E) were evaluated and the number of defects was zero. In Examples 1 to 5, a temperature characteristic improvement effect was high, and a surface acoustic wave device having good temperature characteristics could be obtained. In particular, in Examples 3 to 5, the temperature coefficient is as small as about −25 to −18 ppm / ° C., the variation of the temperature coefficient in the substrate surface is smaller, the stability is higher, and the temperature characteristics are better. ing.
On the other hand, in Comparative Examples 1 and 2 (adhesives F to G), although the variation of the frequency temperature coefficient and the temperature coefficient within the substrate surface was small, about 3 to 10 out of 22 LT surfaces were cracked.
From these results, it can be said that the present invention in which the glass transition point Tg of the adhesive layer is 200 ° C. or less has been proved to be sufficiently significant.

(Examples 6 to 9)
As shown in Table 2 below, a glass transition point Tg of 132 ° C. is used to fabricate a composite piezoelectric substrate using adhesives H to K having different elastic moduli, and after forming a comb-shaped electrode, it is processed into a chip shape. The temperature coefficient of the resonance frequency and the variation of the temperature coefficient in the substrate were examined.
The results are shown in Table 2.

As shown in Table 2, it was found that in Example 6, the frequency temperature coefficient was particularly small as compared with Examples 7 to 9, and the temperature characteristic improving effect was higher.
That is, it is confirmed that the temperature characteristics are further improved when the elastic modulus at room temperature of the adhesive having a glass transition point Tg of 200 ° C. or lower is 1.5 GPa or more and the elastic modulus at 85 ° C. is 1 GPa or more. did it.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits the same function and effect. Are included in the technical scope.

DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave element, 2 ... Piezoelectric substrate, 3 ... Support substrate, 4 ... Adhesive layer (adhesive), 5 ... Comb-shaped electrode, 6 ... Composite piezoelectric substrate.

Claims (3)

  At least a piezoelectric substrate, a support substrate bonded to the piezoelectric substrate via an adhesive layer, and a comb-shaped electrode for exciting a surface acoustic wave formed on a surface opposite to the adhesive surface of the piezoelectric substrate, A surface acoustic wave device having a glass transition point Tg of an adhesive layer of 200 ° C. or lower.   The surface acoustic wave device according to claim 1, wherein a glass transition point Tg of the adhesive layer is 100 ° C. or higher and 200 ° C. or lower.   3. The surface acoustic wave device according to claim 1, wherein the adhesive layer has an elastic modulus at room temperature of 1.5 GPa or more and an elastic modulus at 85 ° C. of 1 GPa or more.

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