JP2011254354A - Composite substrate and surface acoustic wave device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a noise to a surface acoustic wave when a composite substrate including a piezoelectric substrate and a support substrate joined together with a resin adhesion layer is used as a surface acoustic wave device.SOLUTION: An adhesion layer 13 joining a rear face of a piezoelectric substrate 11 and a support substrate 12 and comprising a resin adhesive composition contains particles comprising a substance having an elastic modulus higher than that of the adhesive composition. Accordingly, a noise to a surface acoustic wave can be reduced when the composite substrate 10 is used as a surface acoustic wave device.

Description

  The present invention relates to a composite substrate and a surface acoustic wave device using the same.

  2. Description of the Related Art Conventionally, surface acoustic wave devices that can function as filter elements or oscillators used in mobile phones and the like are known. As such a surface acoustic wave device, a support substrate and a piezoelectric substrate that propagates a surface acoustic wave (SAW) are bonded together, and a comb-shaped electrode capable of exciting a surface acoustic wave is provided on the surface of the piezoelectric substrate. It has been known. In such a surface acoustic wave device, when a wave (bulk wave) in the thickness direction of the piezoelectric substrate is reflected at the interface between the piezoelectric substrate and the support substrate, it becomes noise against the surface acoustic wave on the surface of the piezoelectric substrate. There is. In order to reduce this noise, it has been proposed to interpose an acoustic relaxation layer having an acoustic impedance intermediate between the piezoelectric substrate and the support substrate between the piezoelectric substrate and the support substrate (see, for example, Patent Document 1). ).

JP 2007-228225 A

  In the surface acoustic wave device described in Patent Document 1, when forming a composite substrate in which a piezoelectric substrate and a support substrate are laminated, each layer is formed by a plasma CVD method, or each layer is joined by direct joining. However, for example, when such a composite substrate cannot be formed, a composite substrate is formed by bonding a piezoelectric substrate and a support substrate with a resinous adhesive layer. In this case, since the resinous adhesive layer generally has a very low acoustic impedance compared to the piezoelectric substrate and the support substrate, the adhesive layer has an intermediate acoustic impedance between the piezoelectric substrate and the support substrate as in Patent Document 1. It cannot be a structure.

  The present invention has been made in order to solve such a problem, and in a composite substrate in which a piezoelectric substrate and a support substrate are joined by a resin adhesive layer, when the surface acoustic wave device is used as a surface acoustic wave device, The main purpose is to reduce noise.

  The composite substrate of the present invention employs the following means in order to achieve the main object described above.

The composite substrate of the present invention is
A piezoelectric substrate capable of propagating surface acoustic waves on the surface;
A support substrate;
A resin adhesive layer that joins the back surface of the piezoelectric substrate and the support substrate;
With
The adhesive layer contains particles made of a material having a higher elastic modulus than the resin.
Is.

  In this composite substrate, particles made of a material having a higher elastic modulus than the resin are contained in a resin adhesive layer that joins the back surface of the piezoelectric substrate and the support substrate. Thereby, when this composite substrate is used as a surface acoustic wave device, noise with respect to the surface acoustic wave can be reduced. In addition, although the mechanism by which noise is reduced is not certain, for example, it is considered as follows. In this composite substrate, the sound velocity of the adhesive layer increases due to the inclusion of particles with a high elastic modulus, and the acoustic impedance of the adhesive layer expressed by the product of the sound velocity and the density is larger than when no particles are contained. It may have become. As a result, the acoustic impedance between the piezoelectric substrate and the adhesive layer becomes a close value, and reflection of bulk waves at the interface between the piezoelectric substrate and the adhesive layer is reduced, so that the noise is reduced. Another possible mechanism is that the presence of particles in the adhesive layer causes irregular reflection of bulk waves, and the influence of the reflected bulk waves on the surface of the piezoelectric substrate is reduced. In the composite substrate of the present invention, the particles may be made of ceramics or carbon. The particles may be ceramics made of any one of silica, alumina, and zinc oxide.

  In the composite substrate of the present invention, the support substrate may have a smaller thermal expansion coefficient than the piezoelectric substrate. By doing so, since the change in size of the piezoelectric substrate when the temperature changes is suppressed by the support substrate, the change in characteristics due to the temperature of the surface acoustic wave device using this composite substrate can be suppressed.

  The surface acoustic wave device of the present invention is formed using the composite substrate according to any one of the above-described aspects, and includes an electrode capable of exciting the surface acoustic wave on the surface of the piezoelectric substrate. According to this surface acoustic wave device, it is possible to obtain the same effect as that obtained with any of the composite substrates described above. Examples of the surface acoustic wave device include a resonator, a filter, and a convolver.

  In the surface acoustic wave device of the present invention, the peak value of the amplitude of spurious generated when the reflection characteristic (S11) is measured with a network analyzer is smaller than that in the case where the adhesive layer does not contain the particles. Also good. In this way, noise with respect to the surface acoustic wave is reliably reduced.

1 is a perspective view of a composite substrate 10. FIG. 4 is an explanatory view (cross-sectional view) schematically showing a manufacturing process of the composite substrate 10. FIG. 1 is a perspective view of a 1-port SAW resonator 30 manufactured using a composite substrate 10. FIG. 2 is a cross-sectional view showing a state in which a 1-port SAW resonator 30 is mounted on a ceramic substrate 40, sealed with resin, and mounted on a printed wiring board 60. FIG. 6 is a graph showing frequency characteristics of the 1-port SAW resonators of Example 1 and Comparative Example 1.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of a composite substrate 10 according to an embodiment of the present invention. The composite substrate 10 is used for a surface acoustic wave device, and is formed in a circular shape with one portion being flat. This flat portion is a portion called an orientation flat (OF), and is used for detecting the wafer position and direction when various operations are performed in the manufacturing process of the surface acoustic wave device. The composite substrate 10 of this embodiment includes a piezoelectric substrate 11, a support substrate 12, and an adhesive layer 13.

  The piezoelectric substrate 11 is a piezoelectric substrate capable of propagating surface acoustic waves. Examples of the material of the piezoelectric substrate 11 include lithium tantalate, lithium niobate, lithium niobate-lithium tantalate solid solution single crystal, lithium borate, and quartz. Although the magnitude | size of the piezoelectric substrate 11 is not specifically limited, For example, a diameter is 50-150 mm and thickness is 10-50 micrometers.

  The support substrate 12 is a substrate bonded to the back surface of the piezoelectric substrate 11 via the adhesive layer 13. The support substrate 12 is made of a material having a smaller thermal expansion coefficient than the piezoelectric substrate 11. Examples of the material of the support substrate 12 include silicon, sapphire, aluminum nitride, alumina, borosilicate glass, and quartz glass. Among these, silicon is the most practical material for semiconductor device fabrication, and is preferable because it is easy to composite the surface acoustic wave device fabricated using the composite substrate 10 and the semiconductor device. Although the magnitude | size of the support substrate 12 is not specifically limited, For example, a diameter is 50-150 mm and thickness is 100-500 micrometers. Further, the thermal expansion coefficient of the support substrate 12 is preferably 2 to 7 ppm / K when the thermal expansion coefficient of the piezoelectric substrate 11 is 13 to 20 ppm / K, for example. Note that the support substrate 12 is made of a material having a smaller thermal expansion coefficient than the piezoelectric substrate 11 in this way, so that the change in size of the piezoelectric substrate 11 when the temperature changes is suppressed by the support substrate 12. For this reason, the characteristic change by the temperature of the surface acoustic wave device using this composite substrate 10 can be suppressed.

  The adhesive layer 13 adheres the back surface of the piezoelectric substrate 11 and the front surface of the support substrate 12. This adhesive layer 13 is made by adding particles made of a material having a higher elastic modulus than the adhesive composition to a resin adhesive composition for bonding the piezoelectric substrate 11 and the support substrate 12. Examples of the material of the adhesive composition include an epoxy resin and an acrylic resin. Examples of the material of the particles include ceramics or carbon. In addition, examples of the ceramic include those made of any one of silica, alumina, and zinc oxide. The size of the particles is not particularly limited, and for example, the diameter is 0.05 to 1.0 μm. Further, the amount of particles in the adhesive layer 13 is not particularly limited, but is, for example, 5 to 15 wt% of the adhesive composition. Moreover, it is preferable that the thickness of the contact bonding layer 13 shall be 0.1-2 micrometers, for example. If the thickness of the adhesive layer 13 exceeds 2 μm, it is not preferable because the supporting substrate 12 cannot sufficiently obtain the effect of suppressing the characteristic change due to the temperature of the surface acoustic wave device using the composite substrate 10. Further, if the thickness of the adhesive layer 13 is less than 0.1 μm, it is not preferable because sufficient adhesive strength cannot be obtained. The thickness of the adhesive layer 13 is preferably 0.2 to 0.3 times the wavelength of the surface acoustic wave used in the surface acoustic wave device manufactured using the composite substrate 10, and is 1/4. A thickness of (= 0.25 times) is particularly preferable. In this way, when used as a surface acoustic wave device, bulk waves that reach the adhesive layer from the piezoelectric substrate are easily transmitted to the support substrate side, and reflection of bulk waves at the interface between the piezoelectric substrate 11 and the adhesive layer 13 is reduced. it can.

  A method for manufacturing such a composite substrate 10 will be described below with reference to FIG. FIG. 2 is an explanatory view (cross-sectional view) schematically showing the manufacturing process of the composite substrate 10. First, a support substrate 12 having an OF and a predetermined diameter and thickness is prepared. In addition, a piezoelectric substrate 21 in a state before polishing the piezoelectric substrate 11 is prepared (see FIG. 2A). Then, an adhesive composition containing particles made of a material having a higher elastic modulus than that is uniformly applied to at least one of the front surface of the support substrate 12 and the back surface of the piezoelectric substrate 21. Thereafter, the two substrates are bonded together, and when the adhesive composition is a thermosetting resin, it is heated and cured, and when the adhesive composition is a photocurable resin, it is cured by irradiation with light. As a result, the adhesive composition containing the particles is cured to form the adhesive layer 13, and the bonded substrate 20 (composite substrate 10 before polishing) is obtained (see FIG. 2B). Thereafter, the piezoelectric substrate 21 is polished to a predetermined thickness by a polishing machine, and the surface of the piezoelectric substrate 21 is mirror-polished, so that the piezoelectric substrate 21 before polishing becomes the piezoelectric substrate 11 and the composite substrate 10 is obtained ( (Refer FIG.2 (c)).

  Thereafter, the composite substrate 10 obtained in this manner is made into a group of a large number of surface acoustic wave devices using a general photolithography technique, and then cut into individual surface acoustic wave devices by dicing. FIG. 3 shows a state in which the composite substrate 10 is an aggregate of 1-port SAW resonators 30 that are surface acoustic wave devices. The 1-port SAW resonator 30 is obtained by forming IDT (Interdigital Transducer) electrodes 32 and 34 (also referred to as comb-shaped electrodes and interdigital electrodes) and a reflective electrode 36 on the surface of the piezoelectric substrate 11 by photolithography. . The obtained 1-port SAW resonator 30 is mounted on the printed wiring board 60 as follows. That is, as shown in FIG. 4, after the IDT electrodes 32 and 34 and the pads 42 and 44 of the ceramic substrate 40 are connected via the gold balls 46 and 48, the resin 50 is sealed on the ceramic substrate 40. A lead-free solder paste is interposed between the electrodes 52 and 54 provided on the back surface of the ceramic substrate 40 and the pads 62 and 64 of the printed wiring board 60, and then mounted on the printed wiring board 60 by a reflow process. Is done. FIG. 4 shows the solders 66 and 68 after the solder paste is melted and re-solidified.

  A usage example of the 1-port SAW resonator 30 thus manufactured will be described. When a voltage is applied between the IDT electrodes 32 and 34, a surface acoustic wave is excited on the surface of the piezoelectric substrate 11, and the surface acoustic wave propagates on the surface of the piezoelectric substrate 11 from the IDT electrodes 32 and 24 toward the reflective electrodes 36 on both sides. To do. Then, the surface acoustic wave is reflected by the reflective electrode 36 and returns to the IDT electrodes 32 and 34. As a result, the 1-port SAW resonator 30 has a relationship of resonance frequency f = propagation velocity v / period λ, where λ is the period of the IDT electrodes 32 and 34 and v is the propagation velocity of the surface acoustic wave on the surface of the piezoelectric substrate 11. It operates as a resonator having a resonance frequency f derived from. Here, when a voltage is applied to the IDT electrodes 32 and 34, a wave (bulk wave) in the thickness direction of the piezoelectric substrate 11 is also generated in addition to the surface acoustic wave. When this bulk wave is reflected at the interface between the piezoelectric substrate 11 and the adhesive layer 13 and reaches the surface of the piezoelectric substrate 11, it becomes noise with respect to the surface acoustic wave, leading to deterioration of the characteristics of the one-port SAW resonator 30. In the composite substrate 10 of the present embodiment, noise caused by bulk waves in the 1-port SAW resonator 30 manufactured using the composite substrate 10 is reduced by including particles made of a material having a high elastic modulus in the adhesive layer 13. The

  Note that it is also conceivable to reduce noise by irregularly reflecting bulk waves by roughening the back surface of the piezoelectric substrate 11 instead of containing particles made of a material having a high elastic modulus in the adhesive layer 13. However, when the back surface of the piezoelectric substrate 11 is roughened, a portion deviating from the preferable thickness range (0.1 to 2 μm) of the adhesive layer 13 is generated, and the effect of suppressing the characteristic change due to temperature and the piezoelectric substrate In some cases, the adhesive force between the support 11 and the support substrate 12 becomes insufficient. In the composite substrate 10 of this embodiment, the adhesive layer 13 contains particles made of a material having a high elastic modulus, thereby reducing noise in the 1-port SAW resonator 30 without roughening the back surface of the piezoelectric substrate 11. It is possible to obtain the effect of

  In general, the thicker the piezoelectric substrate 11 is, the more difficult it is for the reflected bulk wave to reach the surface of the piezoelectric substrate 11, thereby reducing noise with respect to the surface acoustic wave. However, the thicker the piezoelectric substrate 11 is, the lower the effect that the above-described support substrate 12 suppresses the characteristic change due to temperature. In the composite substrate 10 of the present embodiment, noise is reduced by including particles made of a material having a high elastic modulus in the adhesive layer 13, so that the piezoelectric substrate is less than that in which the adhesive layer does not contain particles. 11 can be made thinner, and the support substrate 12 can increase the effect of suppressing the characteristic change due to the temperature of the 1-port SAW resonator 30 by that much.

  According to the composite substrate 10 of the present embodiment described in detail above, the adhesive layer 13 made of a resin adhesive composition that joins the back surface of the piezoelectric substrate 11 and the support substrate 12 has an elastic modulus higher than that of the adhesive composition. It contains particles made of high-quality substances. Thereby, when this composite board | substrate 10 is utilized as a surface acoustic wave device, the noise with respect to a surface acoustic wave can be reduced. In addition, since the support substrate 12 has a smaller thermal expansion coefficient than the piezoelectric substrate 11, it is possible to suppress changes in characteristics due to temperature of the surface acoustic wave device using the composite substrate 10.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  For example, in the above-described embodiment, a 1-port SAW resonator has been described as a surface acoustic wave device manufactured using the composite substrate 10, but the same thing can be achieved by manufacturing other surface acoustic wave devices using the composite substrate 10. An effect can be obtained. Examples of other surface acoustic wave devices include a 2-port SAW resonator, a transversal SAW filter, a ladder SAW filter, and a convolver.

[Example 1]
As Example 1, the composite substrate 10 shown in FIG. 1 was produced, and the 1-port SAW resonator 30 shown in FIG. 3 was produced using the composite substrate 10.

  Specifically, first, a lithium tantalate substrate (hereinafter referred to as an LT substrate) having a thickness of 250 μm and a diameter of 100 mm serving as the piezoelectric substrate 11 and a silicon substrate having a thickness of 250 μm and a diameter of 100 mm serving as the support substrate 12 were prepared. . Here, as the LT substrate, a 42 ° Y-cut X-propagation LT substrate in which the propagation direction of the surface acoustic wave is X and the cutting angle is a rotational Y-cut is used. The back surface of the LT substrate 11 was not roughened. Subsequently, a mixed adhesive in which silica ceramic particles having an average particle diameter of 50 nm were mixed with the epoxy resin adhesive in an amount of 5 wt% of the epoxy resin adhesive was prepared. Specifically, the epoxy resin adhesive was put into a beaker together with a stirrer and stirred using a stirrer. At this time, ceramic particles were put into the beaker and stirred for 30 minutes to obtain a mixed adhesive. . This mixed adhesive was applied to the surface of the silicon substrate by spin coating. And it bonded so that the back surface of LT board | substrate might contact the side which apply | coated the mixed adhesive of the silicon substrate, it heated at 160 degreeC, and the bonded substrate from which the thickness of the contact bonding layer 13 after hardening was set to 1 micrometer was formed.

  Next, this bonded substrate was polished with a polishing machine until the thickness of the LT substrate became 10 μm. As a polishing machine, a machine that first thins the LT substrate and then performs mirror polishing is used. When the thickness is reduced, a bonded substrate is sandwiched between the polishing platen and the pressure plate, and a slurry containing abrasive grains is supplied between the bonded substrate and the polishing platen, and the pressure plate is used for bonding. A pressure plate that rotates while pressing the substrate against the surface plate was used. Subsequently, when performing mirror polishing, the polishing surface plate is assumed to have a pad attached to the surface and the abrasive grains are changed to a high count, and by giving rotation and revolution motion to the pressure plate, The surface of the LT substrate was mirror-polished. Specifically, first, the surface of the LT substrate of the bonded substrate was pressed against the surface of the platen, and polishing was performed with a rotation speed of 100 rpm and a polishing duration of 60 minutes. Subsequently, the polishing surface plate is assumed to have a pad attached to the surface and the abrasive grains are changed to a higher one, the pressure for pressing the bonded substrate against the surface plate surface is 0.2 MPa, the rotational speed of the rotation motion Was polished at 100 rpm, the revolution speed was 100 rpm, and the polishing time was 60 minutes. As a result, the LT substrate before polishing became the polished piezoelectric substrate 11, and the composite substrate 10 of Example 1 was completed.

  Subsequently, the IDT electrodes 32 and 34 and the reflective electrode 36 were formed on the fabricated composite substrate 10 using a general photolithography technique to form an assembly of 4000 one-port SAW resonators 30. Then, each one-port SAW resonator 30 was cut out by dicing. The IDT electrodes 32 and 34 are formed to have a line width of 0.55 μm and a period λ of 2.2 μm, and the reflective electrode 36 has a stripe line width of 0.55 μm and a period p of 1 / 2λ. It formed so that it might become.

[Comparative Example 1]
A composite substrate 10 was produced in the same manner as in Example 1 except that the adhesive layer 13 was formed only with an epoxy resin adhesive without mixing silica ceramic particles, and a 1-port SAW as in Example 1. A resonator 30 was produced.

[Evaluation of reflection characteristics]
The reflection characteristics (S11) of the 1-port SAW resonator 30 of Example 1 and Comparative Example 1 were measured using a network analyzer. The results are shown in FIG. As shown in FIG. 5, in Comparative Example 1, spurious was generated in a higher frequency region than the resonance frequency (about 1920 MHz), and the peak value of the amplitude of this spurious was 2 dB. On the other hand, in Example 1, the peak value of the amplitude of spurious generated in a higher frequency region than the resonance frequency is 0.7 dB, and the peak value of the amplitude of spurious is suppressed to a value smaller than that of Comparative Example 1. From this, in Example 1, since the adhesive layer 13 contains ceramic particles made of silica, the occurrence of spurious (that is, noise with respect to the surface acoustic wave) is reduced as compared with Comparative Example 1. Recognize.

  DESCRIPTION OF SYMBOLS 10 Composite substrate, 11 Piezoelectric substrate, 12 Support substrate, 13 Adhesive layer, 20 Bonded substrate, 21 Piezoelectric substrate (before polishing), 30 1-port SAW resonator, 32, 34 IDT electrode, 36 Reflective electrode, 40 Ceramic substrate, 42,44 pad, 46,48 gold ball, 50 resin, 52,54 electrode, 60 printed wiring board, 62,64 pad, 66,68 solder.

Claims (6)

A piezoelectric substrate capable of propagating surface acoustic waves on the surface;
A support substrate;
A resin adhesive layer that joins the back surface of the piezoelectric substrate and the support substrate;
With
The adhesive layer contains particles made of a material having a higher elastic modulus than the resin.
Composite board. The particles are made of ceramics or carbon.
The composite substrate according to claim 1. The particles are ceramics made of either silica, alumina, or zinc oxide.
The composite substrate according to claim 2. The support substrate has a smaller thermal expansion coefficient than the piezoelectric substrate,
The composite substrate according to claim 1.   A surface acoustic wave device formed using the composite substrate according to any one of claims 1 to 4 and having an electrode capable of exciting the surface acoustic wave on a surface of the piezoelectric substrate. Compared to the case where the adhesive layer does not contain the particles, the peak value of the amplitude of spurious generated when the reflection characteristic (S11) is measured with a network analyzer becomes a small value.
The surface acoustic wave device according to claim 5.